TWI878770B - Hot-dip plated Al-Zn-Si-Mg steel plate and its manufacturing method, surface treated steel plate and its manufacturing method, and coated steel plate and its manufacturing method - Google Patents

Abstract

The present invention provides a hot-dip plated Al-Zn-Si-Mg steel sheet with stable and excellent corrosion resistance. In order to achieve the above object, the present invention is a hot-dip plated Al-Zn-Si-Mg steel sheet with a coating, characterized in that the coating has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities, the Co content of the inevitable impurities is 0.080 mass% or less relative to the total mass of the coating, and the diffraction intensity of Si and _Mg2Si in the coating satisfies the following relationship (1) by X-ray diffraction method. Si(111): diffraction intensity of Si (111) plane (interval d = 0.3135nm), Mg ₂ Si(111): diffraction intensity of Mg ₂ Si (111) plane (interval d = 0.3668nm)

Description

Hot dip plated Al-Zn-Si-Mg steel plate and its manufacturing method, surface treated steel plate and its manufacturing method, coated steel plate and its manufacturing method

The present invention relates to a hot-dip plated Al-Zn-Si-Mg steel plate having stable and excellent corrosion resistance and a manufacturing method thereof, a surface treated steel plate and a manufacturing method thereof, and a coated steel plate and a manufacturing method thereof.

Hot-dip Al-Zn steel sheets, represented by 55% Al-Zn, are known to have high corrosion resistance among hot-dip galvanized steel sheets because they have both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al. Therefore, hot-dip Al-Zn steel sheets are mainly used in the field of building materials such as roofs or walls that are exposed to the outdoors for a long time, guardrails, wiring and piping, sound insulation walls, etc. due to their excellent corrosion resistance. In particular, the demand for materials with excellent corrosion resistance or maintenance-free materials in harsher operating environments such as acid rain caused by air pollution, de-icing agents for roads in snowy areas, and development in coastal areas has increased. Therefore, the demand for hot-dip plated Al-Zn steel plates has increased in recent years.

The characteristics of the coating of hot-dip Al-Zn steel plates are that the supersaturated and Zn-containing Al solidifies into a dendrite-like part (α-Al phase) and the Zn-Al eutectic structure existing in the inter-dendrite gap, and has a structure in which the α-Al phase is layered multiple times in the thickness direction of the coating. Due to this characteristic coating structure, the corrosion path from the surface becomes complicated, so corrosion becomes difficult to proceed. Hot-dip Al-Zn steel plates are also known to achieve better corrosion resistance than hot-dip galvanized steel plates with the same coating thickness.

Attempts have been made to further extend the service life of such hot-dip plated Al-Zn-based steel sheets, and hot-dip plated Al-Zn-Si-Mg-based steel sheets to which Mg is added have been put into practical use. As such hot-dip plated Al-Zn-Si-Mg-based steel sheets, for example, Patent Document 1 discloses a hot-dip plated Al-Zn-Si-Mg-based steel sheet, which contains an Al-Zn-Si alloy containing Mg in the coating film, and the Al-Zn-Si alloy is an alloy containing 45 to 60% by weight of elemental aluminum, 37 to 46% by weight of elemental zinc, and 1.2 to 2.3% by weight of Si, and the concentration of Mg is 1 to 5% by weight. In addition, Patent Document 2 discloses a hot-dip plated Al-Zn-Si-Mg steel plate, the purpose of which is to improve the corrosion resistance by containing one or more of 2-10% Mg and 0.01-10% Ca in the coating film, and at the same time improve the protective effect after the base steel plate is exposed. Furthermore, Patent Document 3 discloses a hot-dip plated Al-Zn-Si-Mg steel plate, which forms a coating layer containing Mg: 1-15%, Si: 2-15%, Zn: 11-25% by mass, and the remainder is Al and inevitable impurities, and by making the size of the intermetallic compound such as _Mg2Si phase or MgZn2 _phase existing in the coating film less than 10μm, the corrosion resistance of the flat plate and the end surface is improved.

The above-mentioned hot-dip Al-Zn steel sheet has a beautiful appearance with a white metallic luster and a shiny pattern, so it is often used without being painted. In reality, the demand for its appearance is still strong. Therefore, a technology for improving the appearance of hot-dip Al-Zn steel sheets has also been developed. For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg steel sheet in which wrinkle-like concave-convex defects are suppressed by containing 0.01 to 10% Sr in the coating film. In addition, Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg steel sheet in which spot defects are suppressed by containing 500 to 3000 ppm Sr in the coating film.

Furthermore, when the hot-dip plated Al-Zn steel sheet is used in a severely corrosive environment, there is a problem of rusting due to corrosion of the plated film. Since the rusting reduces the appearance of the steel sheet, the development of plated steel sheets with improved rust resistance is being pursued. For example, Patent Document 6 discloses a hot-dip plated Al-Zn-Si-Mg steel sheet for the purpose of improving the rust resistance of the processed portion, which is to make the mass ratio of the Mg phase in the Si-Mg phase to the total amount of Mg in the plated layer appropriate. In addition, Patent Document 7 discloses a technology for improving blackening resistance and rust resistance by forming a chemical film containing a urethane resin on the coating film of a hot-dip plated Al-Zn-Si-Mg steel sheet.

In addition, the coated steel plate formed with a chemical film, a primer coating, an upper coating film, etc. on the surface of the hot-dip Al-Zn steel plate is subjected to various processing such as 90-degree bending or 180-degree bending by press forming, roll forming or embossing, and requires long-term coating durability. In order to meet such requirements, the hot-dip Al-Zn steel plate is known to have a coated steel plate formed with a chemical film containing chromate, a chromate-based rust-proof pigment in the primer coating, and a thermosetting polyester resin coating or a fluorine resin coating having excellent weather resistance. However, for these recently coated steel sheets, the use of chromate, which is a substance that causes environmental load, is considered to be a problem, and there is a strong desire to develop a coated steel sheet that can improve corrosion resistance or surface appearance even without chromate. As a technology corresponding to such requirements, for example, Patent Document 8 discloses a surface treated hot-dip plated steel material, which is a steel material having an aluminum-zinc alloy coating (α) containing Al, Zn, Si and Mg and the contents of these elements being adjusted, and a film (β) having at least one compound (A) selected from titanium compounds and zirconium compounds as a film-forming component is formed as an upper layer thereof, and the mass ratio of the Si-Mg phase in the aluminum-zinc alloy coating (α) to the total amount of Mg in the coating is adjusted to 3% or more. Prior Art Documents Patent Documents

Patent document 1: Japanese Patent Publication No. 5020228 Patent document 2: Japanese Patent Publication No. 5000039 Patent document 3: Japanese Patent Publication No. 2002-12959 Patent document 4: Japanese Patent Publication No. 3983932 Patent document 5: Japanese Patent Publication No. 2011-514934 Patent document 6: Japanese Patent Publication No. 5751093 Patent document 7: Japanese Patent Publication No. 2019-155872 Patent document 8: Japanese Patent Publication No. 2005-169765

Invention to solve the problem

However, the technology of adding Mg to the coating film disclosed in Patent Documents 1 to 3 may not necessarily significantly improve the corrosion resistance. Although the hot-dip plated Al-Zn-Si-Mg steel sheets disclosed in Patent Documents 1 to 3 seek to improve the corrosion resistance only by adding Mg to the coating composition, the effects of components other than the above four elements (Al, Zn, Si, Mg) and the characteristics of the metal phase and intermetallic compound phase constituting the coating film are not considered, and the corrosion resistance cannot be generally discussed. Therefore, even if a hot-dip Al-Zn-Si-Mg steel sheet is produced using a coating bath composition having the same content of the above four elements, there is a difference in corrosion resistance when a corrosion promotion test is performed, and it may not be superior to the Al-Zn coated steel sheet without Mg addition, which is problematic. Similarly, in improving the coating appearance, when Sr is added to the coating film alone, it may not be possible to eliminate the wrinkle-like uneven defects. Regarding the hot-dip Al-Zn-Si-Mg steel sheets disclosed in Patent Documents 4 and 5, it is also impossible to take both corrosion resistance and appearance into consideration. In addition, since Mg is an easily oxidized element, the Mg contained in the plating bath generates oxides (scum) near the bath surface, or during hot dip plating, FeAl-based compounds (bottom dross) containing iron that are present in the plating bath or at the bottom will occur over time. These drosses will adhere to the surface of the coating film, causing convex defects and possibly damaging the appearance of the coating film surface. In addition, when a steel sheet is plated with a bath in which Mg is added to a hot dip Al-Zn-Si bath, it is known that in addition to the α-Al phase, _Mg2Si phase, _MgZn2 phase, and Si phase are also precipitated in the coating film. However, the effect of the precipitation amount or existence ratio of each phase on corrosion resistance is still unclear.

Furthermore, no single technology can achieve sufficient improvement in rust resistance. Regarding the hot-dip Al-Zn-Si-Mg steel plate of Patent Document 6, although it is described that the rust resistance of the processed portion and the heated flat portion is improved, the rust resistance of the unheated flat portion is not considered, and the realization of stable rust resistance is still a problem. Moreover, regarding the hot-dip Al-Zn-Si-Mg steel plate of Patent Document 7, it is also hoped that not only stable and excellent corrosion resistance and rust resistance can be obtained, but also further improvement can be expected.

In addition, as mentioned above, the coated steel plate is required to have long-term coating durability in the state of being subjected to various processing such as 90-degree bending or 180-degree bending by press forming, roll forming, embossing forming, etc., but the corrosion resistance and surface appearance after processing may not be stably obtained in the technology of Patent Document 8. The corrosion resistance of the coated steel plate will of course affect the corrosion resistance of the coated steel plate as the base. Regarding the surface appearance, since the height difference of the wrinkle-like defects reaches tens of μm, even if the surface is smoothed by coating, the unevenness cannot be completely eliminated, and it is considered that it is not possible to expect improvement in the appearance of the coated steel plate. In addition, since the coating becomes thinner at the protruding part, there is also a concern that the corrosion resistance may be partially reduced. Therefore, in order to obtain a coated steel plate with excellent corrosion resistance and surface appearance, it is important to improve the corrosion resistance and surface appearance of the underlying coated steel plate.

In view of the above circumstances, the object of the present invention is to provide a hot-dip plated Al-Zn-Si-Mg steel plate with stable and excellent corrosion resistance and a method for manufacturing the same. In addition, the object of the present invention is to provide a surface-treated steel plate with stable and excellent corrosion resistance and rust resistance and a method for manufacturing the same. Furthermore, the object of the present invention is to provide a coated steel plate with stable and excellent corrosion resistance and corrosion resistance of processed parts and a method for manufacturing the same. Means for solving the problem

The inventors of the present invention have conducted research to solve the above-mentioned problems and have focused on the composition of the coating of hot-dip plated Al-Zn-Si-Mg steel sheets. It is important to control not only the concentrations of Al, Zn, Si and Mg, but also the concentrations of elements contained as impurities. They have found that by properly controlling the Co content therein, the deterioration of corrosion resistance can be effectively suppressed. In addition, it was found that the _Mg2Si phase and Si phase formed in the coating film of the hot-dip Al-Zn-Si-Mg system steel sheet increase or decrease in precipitation amount and their existence ratio changes depending on the balance of each component in the coating film or the formation conditions of the coating film. Depending on the balance of the composition, if any phase does not precipitate, the corrosion resistance of the hot-dip Al-Zn-Si-Mg system steel sheet changes with the existence ratio of these phases. In particular, when the Si phase is less than the _Mg2Si phase or the Si phase, the corrosion resistance is steadily improved. However, it is known that it is very difficult to distinguish the difference between the _Mg2Si phase and the Si phase even when observing the coating film from the surface or cross-section by using a general method such as a scanning electron microscope, using secondary electron imaging or reflected electron imaging. Although microscopic information can be obtained by observation using a transmission electron microscope, the existence ratio of the _Mg2Si and Si phases that affects the macroscopic information of corrosion resistance or appearance cannot be grasped.

Therefore, the inventors of the present invention have further repeated their dedicated research and found that by focusing on the X-ray diffraction method, the intensity ratio of the specific diffraction peaks of the Mg ₂ Si phase and the Si phase can be used to quantitatively determine the existence ratio of the phases. Furthermore, if the Mg ₂ Si phase and the Si phase in the coating film meet the specific existence ratio, stable and excellent corrosion resistance can be achieved. In addition, the inventors of the present invention also found that after controlling the Co content or the existence ratio of the Mg ₂ Si phase and the Si phase in the coating film, by controlling the Sr concentration in the coating bath, the occurrence of wrinkle-like uneven defects can be reliably suppressed, and a coated steel sheet with excellent surface appearance can be obtained.

Furthermore, the inventors of the present invention have also examined the chemical film formed on the aforementioned coating film, and have found that by forming the chemical film with a specific resin and a specific metal compound, the affinity between the chemical film and the coating film and the rust-proof effect can be improved, and the stability of rust resistance can be improved. Furthermore, the inventors of the present invention have also examined the chemical film and the primer coating formed on the aforementioned coating film, and have found that by forming the chemical film with a specific resin and a specific inorganic compound, and by forming the primer coating with a specific polyester resin and an inorganic compound, the barrier property and adhesion of the coating can be improved, and excellent post-processing corrosion resistance can be achieved even without chromate.

The present invention is completed based on the above-mentioned knowledge and insights, and its gist is as follows. 1. A hot-dip plated Al-Zn-Si-Mg steel plate, which is a hot-dip plated Al-Zn-Si-Mg steel plate having a coating, characterized in that: the coating has the following composition: containing Al: 45-65 mass%, Si: 1.0-4.0 mass% and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities, relative to the total mass of the coating, the Co content in the inevitable impurities is 0.080 mass% or less, the diffraction intensity of Si and _Mg2Si in the coating by X-ray diffraction method satisfies the following relationship (1), Si(111): diffraction intensity of the (111) plane of Si (plane spacing d=0.3135nm), Mg ₂ Si(111): diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d=0.3668nm).

2. The hot-dip plated Al-Zn-Si-Mg steel sheet as described in 1 above, wherein the diffraction intensity of Si in the coating film as measured by X-ray diffraction method satisfies the following relationship (2): Si(111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135nm).

3. The hot-dip plated Al-Zn-Si-Mg steel sheet as described in 1 or 2 above, wherein the coating further contains Sr: 0.01 to 1.0 mass %.

4. The hot-dip plated Al-Zn-Si-Mg steel sheet as described in any one of 1 to 3 above, wherein the Al content in the plated film is 50 to 60 mass %.

5. The hot-dip plated Al-Zn-Si-Mg steel sheet as described in any one of 1 to 4 above, wherein the Si content in the above-mentioned coating is 1.0 to 3.0 mass %.

6. The hot-dip plated Al-Zn-Si-Mg steel sheet as described in any one of 1 to 5 above, wherein the content of Mg in the plated film is 1.0 to 5.0 mass %.

7. A method for manufacturing a hot-dip plated Al-Zn-Si-Mg steel plate, which is a method for manufacturing a hot-dip plated Al-Zn-Si-Mg steel plate with a coating film, characterized in that: The formation of the coating film comprises a hot-dip plating step of immersing the base steel plate in a plating bath, the plating bath having the following composition: containing Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities, Relative to the total mass of the plating bath, the Co content in the inevitable impurities of the plating bath is controlled to be less than 0.080 mass%.

8. The method for producing a hot-dip plated Al-Zn-Si-Mg steel plate as described in 7 above, wherein the coating bath further contains Sr: 0.01 to 1.0 mass %.

9. A surface treated steel plate having a coating film of a hot dip plated Al-Zn-Si-Mg steel plate as described in any one of 1 to 6 above and a chemical film formed on the coating film, characterized in that: The chemical film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound.

10. A method for manufacturing a surface-treated steel sheet, which comprises a coating film formed by the method for manufacturing a hot-dip Al-Zn-Si-Mg steel sheet as described in 7 or 8 above and a chemical film formed on the coating film, wherein the chemical film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound.

15. A coated steel plate, which is a coated steel plate formed with a coating directly or through a chemical film on the coating film of the hot-dip plated Al-Zn-Si-Mg steel plate as described in any one of 1 to 6 above, and is characterized by: The aforementioned chemical film contains a resin component and an inorganic compound, the resin component contains 30 to 50% by mass of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 by mass ratio, the inorganic compound contains 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound and 0.5 to 5% by mass of a fluorine compound, The aforementioned coating film has at least a primer coating film, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide.

16. A method for manufacturing a coated steel plate, wherein a coating film is formed directly or via a chemically formed film on a coating film formed by the method for manufacturing a hot-dip Al-Zn-Si-Mg steel plate as described in 7 or 8 above, and the characteristics are: The aforementioned chemical film contains a resin component and an inorganic compound, the resin component contains a total of 30 to 50 mass % of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 in terms of mass ratio, the inorganic compound contains 2 to 10 mass % of a vanadium compound, 40 to 60 mass % of a zirconium compound and 0.5 to 5 mass % of a fluorine compound, The aforementioned coating film has at least a primer coating film, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide. Effect of the invention

According to the present invention, a hot-dip plated Al-Zn-Si-Mg steel sheet with stable and excellent corrosion resistance can be provided. In addition, according to the present invention, a surface-treated steel sheet with stable and excellent corrosion resistance and rust resistance and a method for manufacturing the same can be provided. Furthermore, according to the present invention, a coated steel sheet with stable and excellent corrosion resistance and corrosion resistance of processed parts and a method for manufacturing the same can be provided.

(Hot-dip plated Al-Zn-Si-Mg steel plate)

The hot-dip plated Al-Zn-Si-Mg steel sheet of the present invention has a coating film on the surface of the steel sheet. The coating film has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities.

From the perspective of the balance between corrosion resistance and working surface, the Al content in the aforementioned coating is 45-65% by mass, preferably 50-60% by mass. This is because if the Al content in the aforementioned coating is at least 45% by mass, Al dendritic solidification occurs, and a coating structure with a dendritic solidification structure of the α-Al phase as the main body can be obtained. By adopting a structure in which the dendritic solidification structure is layered in the thickness direction of the coating, the corrosion path becomes complex, and the corrosion resistance of the coating itself is improved. In addition, the more the dendrites of the α-Al phase are layered, the more complicated the corrosion path is, and the less likely it is for corrosion to reach the base steel plate. Therefore, in order to improve the corrosion resistance, it is better to set the Al content to 50% by mass or more. On the other hand, when the Al content in the aforementioned coating exceeds 65% by mass, Zn almost changes to a structure that is solid-dissolved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg coating system deteriorates. Therefore, the Al content in the aforementioned coating must be 65% by mass or less, preferably 60% by mass or less.

The main purpose of adding Si in the aforementioned coating film is to suppress the growth of the Fe-Al and/or Fe-Al-Si interface alloy layer generated at the interface with the base steel plate, so as not to deteriorate the adhesion between the coating film and the steel plate. In fact, if the steel plate is immersed in an Al-Zn coating bath containing Si, the Fe on the surface of the steel plate reacts with the Al or Si in the bath to form an Fe-Al and/or Fe-Al-Si intermetallic compound layer at the interface between the base steel plate and the coating film. At this time, the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy. Therefore, the higher the ratio of the Fe-Al-Si alloy, the more the growth of the interface alloy layer can be suppressed. Therefore, the Si content in the aforementioned coating film must be 1.0 mass% or more. On the other hand, when the Si content in the above-mentioned coating exceeds 4.0 mass%, not only the growth suppression effect of the above-mentioned interface alloy layer is saturated, but also corrosion is promoted due to the presence of excess Si phase in the coating, so the Si content is set to 4.0 mass% or less. Furthermore, from the viewpoint of suppressing the presence of excess Si phase, the Si content in the above-mentioned coating is preferably set to 3.0 mass% or less. Moreover, in terms of the relationship with the Mg content described later, from the viewpoint of easily satisfying the relational expression (1) described later, it is also preferred that the Si content is set to 1.0 to 3.0 mass%.

The coating film contains 1.0 to 10.0 mass % of Mg. By containing Mg in the coating film, the Si can exist in the form of an intermetallic compound of Mg ₂ Si phase, which can inhibit the promotion of corrosion. In addition, when the coating film contains Mg, an intermetallic compound of MgZn ₂ phase is also formed in the coating film, which can further improve the corrosion resistance. When the Mg content in the coating film is less than 1.0 mass %, Mg is used for solid solution of the main phase α-Al phase due to the formation of the intermetallic compound (Mg ₂ Si, MgZn ₂ ), so sufficient corrosion resistance cannot be ensured. On the other hand, when the Mg content in the aforementioned coating film increases, in addition to the saturation of the effect of improving the corrosion resistance, the workability is reduced due to the embrittlement of the α-Al phase, so the content is set to 10.0 mass% or less. Furthermore, the Mg content in the aforementioned coating film is preferably set to 5.0 mass% or less from the viewpoint of suppressing the generation of dross during coating formation and making the coating bath management easy. Moreover, in terms of the relationship with the aforementioned Si content, from the viewpoint of easily satisfying the relationship (1) described later, it is preferred to set the aforementioned Mg content to 3.0 mass%, and when considering the compatibility with dross suppression, it is more preferred to set the aforementioned Mg content to 3.0 to 5.0 mass%.

Furthermore, the aforementioned coating film contains Zn and inevitable impurities. Among them, the aforementioned inevitable impurities contain Fe. The Fe is inevitably contained in the coating film due to the dissolution of the steel plate or the equipment in the bath into the coating bath, and is inevitably contained in the coating film as a result of the diffusion from the base steel plate when the interface alloy layer is formed. The Fe content in the aforementioned coating film is generally about 0.3 to 2.0 mass%. As other inevitable impurities, Cr, Ni, Cu, Co, W, etc. can be cited. These components are contained as impurities in the metal blocks that become the raw materials of the coating bath due to the dissolution of the W-C or Co-Cr-W series sprayed coatings on the base steel plate or stainless steel bath equipment or the coating equipment applied to the bath equipment into the coating bath. Moreover, they are inevitably contained in the coating film when the tank or bath equipment used in the production of the coated steel plate to which these components are intentionally added is used.

Furthermore, the hot-dip Al-Zn-Si-Mg-based steel sheet of the present invention is characterized in that the Co content in the aforementioned inevitable impurities is 0.080 mass% or less relative to the total mass of the aforementioned coating. Since the Co contained in the aforementioned coating may deteriorate the corrosion resistance of the hot-dip Al-Zn-Si-Mg-based steel sheet, after appropriately controlling the contents of Al, Zn, Si and Mg in the aforementioned coating, the Co content as an inevitable impurity is further suppressed, thereby suppressing the deterioration of the corrosion resistance. Based on the same viewpoint, the Co content in the aforementioned inevitable impurities is preferably set to 0.020 mass% or less, more preferably set to 0.001 mass% or less, and most preferably set to 0 mass% relative to the total mass of the aforementioned coating.

Furthermore, the Co contained in the aforementioned coating film exists mainly as a solid solution element in α-Al. When Co is contained at a concentration exceeding the solid solution limit, it may precipitate as a Fe-Co-Al compound or a Fe-Co-Al-Si compound in the alloy layer formed at the interface between the base iron and the coating. Also, although the mechanism of the deterioration of corrosion resistance due to the inclusion of Co as an impurity has not been determined, it is inferred that: the corrosion rate increases due to the decrease in the barrier property of the α-Al phase in which Co is solid-dissolved, or the compound containing Co functions as a cathode in a corrosive environment, forming a solidified structure and a local battery in the surrounding area, thereby increasing the corrosion rate.

There is no particular limitation on the total content of inevitable impurities in the coating film, but excessive impurities may affect various properties of the coated steel sheet, so the total content is preferably set to 5.0 mass % or less.

Furthermore, in the hot-dip plated Al-Zn-Si-Mg steel sheet of the present invention, the diffraction intensity of Si and Mg ₂ Si in the coating film by X-ray diffraction method must satisfy the following relationship (1). Si(111): diffraction intensity of Si (111) plane (inter-plane spacing d = 0.3135nm), Mg ₂ Si(111): diffraction intensity of Mg ₂ Si (111) plane (inter-plane spacing d = 0.3668nm)

As mentioned above, in the hot-dip plated Al-Zn-Si-Mg steel sheet of the present invention, it is important to control the existence ratio of the _Mg2Si phase and the Si phase generated in the coating film to a specific ratio by containing the aforementioned Mg and Si. The influence of these on corrosion resistance is still under investigation and there are many unknowns, but it is inferred to be the following mechanism.

When the hot-dip plated Al-Zn-Si-Mg steel sheet is exposed to a corrosive environment, the above-mentioned intermetallic compound dissolves preferentially over the α-Al phase, resulting in a Mg-rich environment near the formed corrosion products. It is inferred that in such a Mg-rich environment, the formed corrosion products are not easily decomposed, resulting in an enhanced protective effect of the coating. In addition, the enhanced protective effect of the coating is more reliably exhibited when Si in the coating exists as a _Mg2Si phase instead of a Si phase, so it is considered effective to reduce the existence ratio of the Si phase relative to the _Mg2Si phase.

The abundance ratio of Si and _Mg2Si in the coating is determined by using the diffraction peak intensity obtained by X-ray diffraction method, and needs to satisfy the relationship (1): Si(111)/ _Mg2Si (111)≦0.8. When the abundance ratio of Si and _Mg2Si in the coating does not satisfy the relationship (1), that is, when Si(111)/ _Mg2Si (111)＞0.8, since the Si phase in the coating increases, the Mg-rich environment cannot be obtained near the corrosion product, and it becomes difficult to obtain the protective effect of the coating. From the same viewpoint, the abundance ratio of Si to Mg ₂ Si (Si(111)/Mg ₂ Si(111)) is preferably 0.5 or less, more preferably 0.3 or less, and particularly preferably 0.2 or less.

Here, in the above relationship (1), Si(111) is the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135nm), and _Mg2Si (111) is the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d = 0.3668nm). As a method for measuring Si(111) and _Mg2Si (111) by the above X-ray diffraction, a part of the above-mentioned coating film can be mechanically cut off and X-ray diffraction can be performed in a powder state (powder X-ray diffraction measurement method) to calculate. Regarding the measurement of diffraction intensity, the diffraction peak intensity of Si corresponding to the plane spacing d = 0.3135nm and the diffraction peak intensity of _Mg2Si corresponding to the plane spacing d = 0.3668nm are measured, and by calculating these ratios, Si(111)/ _Mg2Si (111) can be obtained. In addition, the amount of coating film required for the powder X-ray diffraction measurement (the amount of coating film cut out) is sufficient from the viewpoint of accurately measuring Si(111) and _Mg2Si (111), and is preferably 0.1g or more, and 0.3g or more is preferred. When the coating is cut, the powder may contain steel plate components other than the coating, but the intermetallic compound phase is contained only in the coating and does not affect the peak intensity. Furthermore, when the coating is powdered and X-ray diffracted, it is difficult to calculate the correct phase ratio because the coating formed on the coated steel plate is affected by the plane orientation of the solidified structure of the coating, which makes it difficult to calculate the correct phase ratio.

Furthermore, in the hot-dip plated Al-Zn-Si-Mg steel sheet of the present invention, after controlling the concentrations of the above-mentioned Al, Zn, Si, Mg and Co as an inevitable impurity, from the point of view of being able to more stably improve the corrosion resistance, the diffraction intensity of Si in the above-mentioned coating by X-ray diffraction method preferably satisfies the following relationship (2). Si(111): The diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135nm) is generally known to exist as a cathode site in the dissolution reaction of Al alloy in aqueous solution, and promotes the dissolution of the surrounding α-Al phase. Therefore, reducing the Si phase is also effective in suppressing the dissolution of the α-Al phase. Among them, as in relationship (2), a film without Si phase (the aforementioned diffraction peak intensity of Si(111) is zero) is the best for stabilizing corrosion resistance. In addition, the method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction is as described above.

Here, there is no particular limitation on the method for satisfying the above-mentioned relationship (1) and relationship (2). For example, in order to satisfy relationship (1) or relationship (2), the existence ratio of Si and Mg2Si (the diffraction intensity of Si( ₁₁₁ ) and _Mg2Si (111)) can be controlled by adjusting the balance of the content of Si, the content of Mg, and the content of Al in the above-mentioned coating film. The balance of the content of Si, the content of Mg, and the content of Al in the above-mentioned coating film does not necessarily satisfy relationship (1) or relationship (2) by setting it to a certain content ratio. For example, it is necessary to change the content ratio of Mg and Al according to the content of Si (mass %). In addition to adjusting the balance of the Si content, Mg content and Al content in the coating film, the diffraction intensity of Si(111) and _Mg2Si (111) can be controlled by adjusting the conditions during the coating film formation (e.g., cooling conditions after coating) to satisfy the relationship (1) or the relationship (2).

In addition, in the hot-dip Al-Zn-Si-Mg steel sheet of the present invention, the coating preferably contains 0.01 to 1.0 mass % of Sr. Since the coating contains Sr, the occurrence of surface defects such as wrinkle-like concave-convex defects can be more reliably suppressed, and good surface appearance can be achieved. Furthermore, the wrinkle-like defects are wrinkle-like concave-convex defects formed on the surface of the coating, and white stripes are observed on the surface of the coating. Such wrinkle-like defects are easy to occur when more Mg is added to the coating. Therefore, in the aforementioned hot-dip plated steel sheet, by including Sr in the aforementioned plated film, Sr in the surface layer of the aforementioned plated film is oxidized preferentially over Mg, thereby suppressing the oxidation reaction of Mg and suppressing the occurrence of the aforementioned wrinkle-like defects.

In addition, the above-mentioned coating film preferably further contains 0.01 to 10 mass% of one or more selected from Cr, Mn, V, Mo, Ti, Ca, Sb and B in total from the viewpoint of improving the stability of corrosion products and achieving the effect of delaying the progress of corrosion in the same way as the above-mentioned Mg. The reason why the total content of the above-mentioned components is set to 0.01 to 10 mass% is that a sufficient corrosion delay effect can be obtained, and at the same time, the effect will not be saturated.

Furthermore, the coating weight of the above-mentioned coating is preferably 45 to 120 g/m ² per side from the viewpoint of satisfying various characteristics. This is because when the coating weight of the above-mentioned coating is 45 g/m ² or more, sufficient corrosion resistance can be obtained for applications requiring long-term corrosion resistance such as building materials, and when the coating weight of the above-mentioned coating is 120 g/m ² or less, the occurrence of coating cracks during processing can be suppressed, and excellent corrosion resistance can be achieved. Based on the same viewpoint, the coating weight of the above-mentioned coating is more preferably 45 to 100 g/m ² .

The amount of the coating film can be derived by, for example, dissolving and peeling off a specific area of the coating film with a mixture of hydrochloric acid and hexamethylenetetramine as shown in JIS H 0401: 2013, and calculating the difference in weight of the steel plate before and after peeling. The amount of coating film on each side can be obtained by sealing with tape so that the coating surface of the non-target side is not exposed and performing the dissolution.

The composition of the coating film is the same as the content of Co, and the coating film can be dissolved by immersing it in hydrochloric acid or the like, and the solution can be confirmed by ICP emission spectrometry or atomic absorption analysis. This method is only an example, and any method can be used without particular limitation as long as it can accurately quantify the composition of the coating film.

Furthermore, the coating film of the hot-dip plated Al-Zn-Si-Mg steel sheet obtained by the present invention is substantially the same as the composition of the coating bath. Therefore, by controlling the composition of the coating bath, the composition of the coating film can be controlled with high precision.

In addition, there is no particular limitation on the base steel plate constituting the hot-dip plated Al-Zn-Si-Mg steel plate of the present invention, and a cold-rolled steel plate or a hot-rolled steel plate can be appropriately used according to the required performance or specifications.

Furthermore, there is no particular limitation on the method for obtaining the aforementioned base steel sheet. For example, in the case of the aforementioned hot-rolled steel sheet, a steel sheet that has been subjected to a hot-rolling step and a pickling step may be used, and in the case of the aforementioned cold-rolled steel sheet, a cold-rolling step may be further applied for production. Furthermore, before the hot-dip coating step in order to obtain the characteristics of the steel sheet, a recrystallization annealing step may also be performed.

(Method for manufacturing hot-dip plated Al-Zn-Si-Mg steel plate) The method for manufacturing hot-dip plated Al-Zn-Si-Mg steel plate of the present invention is a method for manufacturing hot-dip plated Al-Zn-Si-Mg steel plate having a coating film, wherein the coating film is formed by a hot-dip plating treatment step of immersing a base steel plate in a plating bath, wherein the plating bath has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and unavoidable impurities. Moreover, there is no particular limitation on the hot-dip plating treatment step except for the coating bath conditions described later. For example, the base steel plate can be manufactured by washing, heating, and dipping it into a coating bath using a continuous hot dip coating device. In the heating step of the steel plate, recrystallization annealing is performed to control the structure of the base steel plate itself. At the same time, heating in a reducing environment such as a nitrogen-hydrogen environment is effective to prevent oxidation of the steel plate and reduce the trace oxide film on the surface.

Furthermore, regarding the plating bath used in the aforementioned hot dip plating treatment step, as described above, since the composition of the aforementioned plating film is substantially the same as that of the plating bath, a composition containing 45 to 65 mass % of Al, 1.0 to 4.0 mass % of Si, and 1.0 to 10.0 mass % of Mg, with the remainder being Zn and unavoidable impurities, can be used.

Furthermore, in the manufacturing method of the hot-dip plated Al-Zn-Si-Mg steel sheet of the present invention, the Co content in the inevitable impurities of the aforementioned coating bath is controlled to be less than 0.080 mass % relative to the total mass of the aforementioned coating bath. As mentioned above, since the Co contained in the aforementioned coating film has the effect of deteriorating the corrosion resistance of the hot-dip plated Al-Zn-Si-Mg steel sheet, after appropriately controlling the contents of Al, Zn, Si and Mg in the coating bath, the deterioration of the corrosion resistance can be suppressed by further suppressing the Co content as an inevitable impurity. Furthermore, the content of Co as an inevitable impurity in the aforementioned coating bath needs to be controlled below 0.080 mass %, preferably below 0.020 mass %, and more preferably below 0.001 mass %. If the Co content in the aforementioned coating bath is below 0.080 mass %, the hot-dip plated Al-Zn-Si-Mg steel sheet produced can have sufficiently excellent corrosion resistance. Furthermore, if the Co content in the aforementioned coating bath is below 0.020 mass %, it can have more excellent corrosion resistance, and if it is below 0.001 mass %, it can have particularly excellent corrosion resistance. As such, the lower the Co content in the aforementioned coating bath, the better the corrosion resistance of the hot-dip plated Al-Zn-Si-Mg steel sheet. Therefore, there is no particular lower limit on the Co content, and the optimal Co content in the aforementioned coating bath is 0%.

Here, the means for reducing the Co content in the aforementioned coating bath is not particularly limited. For example, from the perspective of effectively suppressing the dissolution of the Co-Cr-W system sprayed film applied to the equipment in the bath into the aforementioned coating bath, it is preferable to use a sprayed film that does not contain Co.

As another means of reducing the Co content in the aforementioned coating bath, it is preferable to use a metal block with a low Co content in the impurities as a raw material for the coating bath. Furthermore, it is also effective not to use a tank or bath equipment used in the production of hot-dip plated Al-Zn-Si-Mg steel plates to which Co is intentionally added. This is because the Co-containing metal block attached to the aforementioned tank or the aforementioned bath equipment can be suppressed from dissolving and mixing into the coating bath.

Furthermore, the bath temperature of the aforementioned coating bath is not particularly limited, but is preferably set to a temperature range of (melting point + 20°C) to 650°C. The reason why the lower limit of the aforementioned bath temperature is set to melting point + 20°C is that in order to perform hot dip coating treatment, the aforementioned bath temperature must be set above the solidification point. By setting it to melting point + 20°C, solidification caused by a local decrease in the bath temperature of the aforementioned coating bath can be prevented. On the other hand, the reason why the upper limit of the aforementioned bath temperature is set to 650°C is that if it exceeds 650°C, the aforementioned coating film becomes difficult to cool rapidly, and there is a risk that the interface alloy layer formed between the coating film and the steel plate will become thicker.

In addition, there is no particular limitation on the temperature of the base steel plate immersed in the plating bath (immersion plate temperature), but from the perspective of ensuring the plating characteristics in the aforementioned continuous hot dip plating operation and preventing bath temperature changes, it is preferably controlled within ±20°C relative to the temperature of the aforementioned plating bath.

In addition, the immersion time of the base steel plate in the plating bath is preferably 0.5 seconds or longer. If it is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. There is no particular upper limit to the immersion time, but if the immersion time is prolonged, there is a risk that the interface alloy layer formed between the plating film and the steel plate may become thicker, so it is more preferably set to within 8 seconds.

Furthermore, the hot-dip plated Al-Zn-Si-Mg steel sheet can also be coated directly or via an intermediate layer on the aforementioned coating film according to the required performance.

Furthermore, there is no particular limitation on the method for forming the aforementioned coating film, and it can be appropriately selected according to the required performance. For example, the forming methods include roller coating, curtain coating, spray coating, etc. After coating the coating material containing the organic resin, the coating film can be formed by heating and drying by means of hot air drying, infrared heating, induction heating, etc.

Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the coating film of the hot-dip plated steel sheet and the coating film.

(Surface treated steel plate) The surface treated steel plate of the present invention has a coating film on the surface of the steel plate and a chemical coating film formed on the coating film. The composition of the coating film is the same as the coating film of the hot dip plated Al-Zn-Si-Mg steel plate of the present invention.

The surface treated steel plate of the present invention forms a chemical film on the aforementioned film. Moreover, the aforementioned chemical film only needs to be formed on at least one side of the surface treated steel plate, and can also be formed on both sides of the surface treated steel plate according to the application or required performance.

Moreover, in the surface treated steel sheet of the present invention, the aforementioned chemical film is characterized by containing at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound. By forming the aforementioned chemical film on the plated film, in addition to improving the affinity with the plated film, the chemical film can be uniformly formed on the aforementioned plated film, and the anti-rust effect and barrier effect of the chemical film can also be improved. As a result, the surface treated steel sheet of the present invention can achieve stable corrosion resistance and rust resistance.

Here, from the viewpoint of improving corrosion resistance, the resin constituting the aforementioned chemical film is at least one selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin. Based on the same viewpoint, the aforementioned resin preferably contains at least one of urethane resin and acrylic resin. Moreover, the resin constituting the aforementioned chemical film also includes addition polymers of the above resins.

As the epoxy resin, for example, bisphenol A type, bisphenol F type, novolac type, etc. epoxy resins which are etherified with glycidyl, bisphenol A type epoxy resins which are etherified with glycidyl by adding propylene oxide, ethylene oxide or polyalkylene glycol, aliphatic epoxy resins, alicyclic epoxy resins, polyether epoxy resins, etc. can be used.

As the urethane resin, for example, oil-modified polyurethane resin, alkyd polyurethane resin, polyester polyurethane resin, polyether polyurethane resin, polycarbonate polyurethane resin, etc. can be used.

Examples of the acrylic resin include polyacrylic acid and copolymers thereof, polyacrylate and copolymers thereof, polymethacrylic acid and copolymers thereof, polymethacrylate and copolymers thereof, urethane-acrylic acid copolymers (or urethane-modified acrylic resins), styrene-acrylic acid copolymers, and the like. Furthermore, these resins modified with other alkyd resins, epoxy resins, phenolic resins, and the like may be used.

Examples of the acrylic silicone resin include a resin having a hydrolyzable alkoxysilyl group at the side chain or the end of an acrylic copolymer as a main agent and a hardener added thereto. When the acrylic silicone resin is used, in addition to corrosion resistance, excellent weather resistance can be expected.

As the alkyd resin, for example, there can be mentioned oil-modified alkyd resin, rosin-modified alkyd resin, phenol-modified alkyd resin, styrenated alkyd resin, silicon-modified alkyd resin, acrylic acid-modified alkyd resin, oil-free alkyd resin, high molecular weight oil-free alkyd resin, and the like.

The aforementioned polyester resin is a polycondensate synthesized by dehydrating and condensing a polycarboxylic acid and a polyol to form an ester bond. As the polycarboxylic acid, for example, terephthalic acid, 2,6-naphthalene dicarboxylic acid, etc. are used, and as the polyol, for example, ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, etc. can be cited. Specifically, the aforementioned polyester can be polyethylene terephthalate, polypropylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc. In addition, these polyester resins modified with acrylic acid can also be used.

As for the polyalkylene resin, for example, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, vinyl copolymers of carboxyl-modified polyolefin resins, ethylene-unsaturated carboxylic acid copolymers, vinyl ionomers, etc. can be cited. Furthermore, these resins modified with other alkyd resins, epoxy resins, phenolic resins, etc. can be used.

The amino resin is a thermosetting resin generated by the reaction of an amine or amide compound with an aldehyde, and examples thereof include melamine resin, guanamine resin, thiourea resin, etc. From the viewpoint of corrosion resistance, weather resistance, adhesion, etc., melamine resin is preferably used. The melamine resin is not particularly limited, but examples thereof include butylated melamine resin, methylated melamine resin, and water-based melamine resin.

As the above-mentioned fluororesin, there can be cited fluoroolefin polymers, or copolymers of fluoroolefin and alkyl vinyl ether, cycloalkyl vinyl ether, carboxylic acid-modified vinyl ester, hydroxyalkyl allyl ether, tetrafluoropropyl vinyl ether, etc. When such fluororesins are used, not only corrosion resistance but also excellent weather resistance and excellent hydrophobicity can be expected.

Furthermore, the resin constituting the chemical film preferably uses a hardener in order to improve corrosion resistance and processability. As the hardener, urea resins (butylated urea resins, etc.), melamine resins (butylated melamine resins, butylated melamine resins, etc.), butylated urea/melamine resins, amino resins such as benzoguanamine resins, blocked isocyanates, oxazoline compounds, phenol resins, etc. can be suitably used.

In addition, as for the metal compound constituting the aforementioned chemical conversion film, at least one selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds and Ca compounds can be used. Based on the same viewpoint, the aforementioned metal compound preferably contains at least one of P compounds, Si compounds and V compounds.

Here, by including the aforementioned P compound in the aforementioned chemical film, the corrosion resistance and sweat resistance can be improved. The aforementioned P compound is a compound containing P, for example, it can contain one or more selected from inorganic phosphoric acid, organic phosphoric acid and their salts.

As the aforementioned inorganic phosphoric acid, organic phosphoric acid and salts thereof, any compound can be used without particular limitation. For example, as the aforementioned inorganic phosphoric acid, it is preferred to use one or more selected from phosphoric acid, dihydrogen phosphate, hydrogen phosphate, phosphate, pyrophosphoric acid, pyrophosphate, tripolyphosphoric acid, tripolyphosphate, phosphorous acid, phosphite, hypophosphorous acid, and hypophosphite. Moreover, as the aforementioned organic phosphoric acid, it is preferred to use phosphonic acid (phosphonic acid compound). Furthermore, as the aforementioned phosphonic acid, it is preferred to use one or more selected from nitrogen trimethylenephosphonic acid, phosphonobutane tricarboxylic acid, methyl diphosphonic acid, methylenephosphonic acid and ethylene diphosphonic acid. Furthermore, when the aforementioned P compound is a salt, the salt is preferably a salt of an element from Group 1 to Group 13 in the periodic table, more preferably a metal salt, and more preferably one or more selected from an alkali metal salt and an alkali earth metal salt.

When the chemical conversion treatment liquid containing the above-mentioned P compound is applied to the plated steel plate, the surface of the plated film is etched by the action of the P compound, and a concentrated layer of Al, Zn, Si and Mg, the constituent elements of the plated film, is formed on the side of the plated film in front of the chemical conversion film. By forming the above-mentioned concentrated layer, the bonding between the chemical conversion film and the surface of the plated film becomes stronger, and the adhesion of the chemical conversion film is improved. The concentration of the P compound in the above-mentioned chemical conversion treatment liquid is not particularly limited, but can be set to 0.25 mass% to 5 mass%. When the concentration of the aforementioned P compound is less than 0.25 mass%, the etching effect is insufficient, the adhesion with the coating interface is reduced, and not only the corrosion resistance of the flat surface is reduced, but also the corrosion resistance and sweat resistance of the defective part, the cut end surface part, and the damaged part of the coating film caused by processing may be reduced. Based on the same viewpoint, the concentration of the P compound is preferably 0.35 mass% or more, and more preferably 0.50 mass% or more. On the other hand, when the concentration of the aforementioned P compound exceeds 5 mass%, not only the life of the chemical treatment solution is shortened, but also the appearance of the film when formed is easily uneven, and the amount of P eluted from the chemical film increases, and there is also a possibility of reduced blackening resistance. From the same viewpoint, the concentration of the P compound is preferably 3.5 mass % or less, more preferably 2.5 mass % or less. The content of the P compound in the chemical conversion film can be adjusted by, for example, applying a chemical conversion treatment solution having a P compound concentration of 0.25 mass % to 5 mass % and drying the solution to obtain a P content of 5 to 100 mg/m ² in the dried chemical conversion film.

The Si compound is a component that forms the skeleton of the chemical film together with the resin, and can improve the affinity with the coating film and form the chemical film uniformly. The Si compound is a compound containing Si, and is preferably one or more selected from silicon dioxide, trialkoxysilane, tetraalkoxysilane and silane coupling agent.

As the aforementioned silica, any one can be used without particular limitation. As the aforementioned silica, for example, at least one of wet silica and dry silica can be used. As colloidal silica, which is one of the aforementioned wet silica, for example, Snowtex O, C, N, S, 20, OS, OXS, NS, etc. manufactured by Nissan Chemical Co., Ltd. can be appropriately used. In addition, as the aforementioned dry silica, for example, AEROSIL50, 130, 200, 300, 380, etc. manufactured by Nippon Aerosil Co., Ltd. can be appropriately used.

Any trialkoxysilane may be used without particular limitation. For example, it is preferred to use a trialkoxysilane represented by the general formula: R ₁ Si(OR ₂ ) ₃ (wherein R ₁ is hydrogen or an alkyl group having 1 to 5 carbon atoms, and R ₂ is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such trialkoxysilane include trimethoxysilane, triethoxysilane, and methyltriethoxysilane.

As the aforementioned tetraalkoxysilane, any one can be used without particular limitation. For example, it is preferable to use a tetraalkoxysilane represented by the general formula: Si(OR) ₄ (wherein R is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such a tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane.

As the silane coupling agent, any one can be used without particular limitation. For example, γ-glycidoxypropyl trimethoxysilane, γ-glycidoxypropyl methyldiethoxysilane, γ-glycidoxypropyl triethoxysilane, γ-aminopropyl trimethoxysilane, γ-aminopropyl methyldiethoxysilane, γ-aminopropyl triethoxysilane, γ-methacryloxypropyl trimethoxysilane, γ-methacryloxypropyl triethoxysilane, γ-butylpropyl methyldimethoxysilane, γ-butylpropyl trimethoxysilane, vinyl triethoxysilane, γ-isocyanate propyl triethoxysilane, and the like can be mentioned.

Furthermore, by including the aforementioned Si compound in the chemical film, the Si compound is dehydrated and condensed to form an amorphous chemical film with a siloxane bond having a high barrier effect for shielding corrosion factors. Furthermore, by combining with the aforementioned resin, a chemical film having higher barrier properties is formed. In addition, in a corrosive environment, a dense and stable corrosion product is formed in a defective part or a damaged part of the coating film caused by processing, etc., and the corrosion of the base steel plate is also suppressed by the composite effect with the aforementioned coating film. From the viewpoint of the high effect of forming a stable corrosion product, it is preferred to use at least one of colloidal silica and dry silica as the aforementioned Si compound.

The concentration of the Si compound in the chemical conversion treatment liquid used to form the chemical conversion film is 0.2 mass % to 9.5 mass %. If the concentration of the Si compound in the chemical conversion treatment liquid is 0.2 mass % or more, a barrier effect due to siloxane bonds can be obtained, and as a result, in addition to the corrosion resistance of the planar portion, the corrosion resistance of the defective portion, the cut portion, and the damaged portion caused by processing, etc., and the sweat resistance are improved. Moreover, if the concentration of the Si compound is 9.5 mass % or less, the life of the chemical conversion treatment liquid can be extended. By applying and drying the chemical conversion treatment liquid having a concentration of 0.2 mass % to 9.5 mass %, the amount of Si adhesion in the chemical conversion film after drying can be made 2 to 95 mg/m ² .

By including the aforementioned Co compound and the aforementioned Ni compound in the aforementioned chemical film, the blackening resistance can be improved. It is believed that this is because Co and Ni have the effect of slowing down the dissolution of water-soluble components from the film in a corrosive environment. In addition, compared with Al, Zn, Si and Mg, the aforementioned Co and the aforementioned Ni are elements that are not easily oxidized. Therefore, by making at least one of the aforementioned Co compound and the aforementioned Ni compound concentrate (form a concentrated layer) at the interface between the aforementioned chemical film and the aforementioned coating film, the concentrated layer becomes a barrier to corrosion, and as a result, the blackening resistance can be improved.

By using a chemical treatment solution containing the aforementioned Co compound, Co can be contained in the aforementioned chemical film and can be absorbed into the aforementioned concentrated layer. As the aforementioned Co compound, it is preferred to use a cobalt salt. As the aforementioned cobalt salt, it is more preferred to use one or more selected from cobalt sulfate, cobalt carbonate and cobalt chloride. In addition, by using a chemical treatment solution containing the aforementioned Ni compound, Ni can be contained in the aforementioned chemical film and can be absorbed into the aforementioned concentrated layer. As the aforementioned Ni compound, it is preferred to use a nickel salt. As the aforementioned nickel salt, it is more preferred to use one or more selected from nickel sulfate, nickel carbonate and nickel chloride.

The concentration of the Co compound and/or the Ni compound in the aforementioned chemical conversion treatment solution is not particularly limited, but can be set to a total of 0.25 mass% to 5 mass%. When the concentration of the aforementioned Co compound and/or the Ni compound is less than 0.25 mass%, the interface concentration layer becomes uneven, and not only the corrosion resistance of the flat portion is reduced, but also the corrosion resistance of the defective portion, the cut end surface portion, and the coating film damaged by processing, etc. is likely to be reduced. Based on the same viewpoint, it is preferably 0.5 mass% or more, and more preferably 0.75 mass% or more. On the other hand, when the concentration of the aforementioned Co compound and/or the Ni compound exceeds 5 mass%, the appearance of the film when it is formed tends to become uneven, and there is a risk of reduced corrosion resistance. From the same viewpoint, it is preferably 4.0 mass % or less, and more preferably 3.0 mass % or less. By applying and drying the chemical conversion treatment solution having a total concentration of 0.25 mass % to 5 mass % of the Co compound and/or Ni compound, the total amount of Co and Ni in the chemical conversion film after drying can be made 5 to 100 mg/m ² .

The Al compound, Zn compound and Mg compound mentioned above can be contained in the chemical treatment solution to form a concentrated layer containing at least one selected from Al, Zn and Mg on the coating film side of the chemical film. The formed concentrated layer can improve the corrosion resistance. Moreover, the Al compound, Zn compound and Mg compound mentioned above are not particularly limited as long as they are compounds containing Al, Zn and Mg respectively, but are preferably inorganic compounds, preferably salts, chlorides, oxides or hydroxides.

As the aforementioned Al compound, for example, one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, and aluminum hydroxide can be cited. As the aforementioned Zn compound, for example, one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide, and zinc hydroxide can be cited. As the aforementioned Mg compound, for example, one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium hydroxide can be cited.

The total concentration of the Al compound, Zn compound and/or Mg compound in the chemical treatment solution used to form the above-mentioned chemical film is preferably 0.25 mass% to 5 mass%. If the above-mentioned total concentration is 0.25 mass% or more, the above-mentioned concentration layer can be formed more effectively, and as a result, the corrosion resistance can be further improved. On the other hand, if the above-mentioned compound concentration is 5 mass% or less, the appearance of the chemical film changes more uniformly, and the corrosion resistance of the flat part or defective part, and the damaged part of the coating film caused by processing, etc. is further improved.

Since the aforementioned V compound is contained in the aforementioned chemical film, V can be appropriately dissolved in a corrosive environment, and combined with zinc ions of the plating component dissolved in the same corrosive environment to form a dense protective film. The formed protective film can further improve the corrosion resistance not only for the flat part of the steel plate, but also for defects, damaged parts of the plating film caused by processing, and corrosion from the cut end surface to the flat part.

The aforementioned V compound is a compound containing V, and examples thereof include one or more selected from sodium metavanadate, vanadium sulfate, and vanadium acetylacetonate.

The V compound in the chemical treatment solution used to form the above-mentioned chemical film is preferably 0.05 mass% to 4 mass%. If the concentration of the above-mentioned V compound is 0.05 mass% or more, it is easy to dissolve in a corrosive environment to form a protective film, and the corrosion resistance of defective parts, cut end faces, and parts of the coating film damaged by processing is improved. On the other hand, if the concentration of the above-mentioned V compound exceeds 4 mass%, the appearance of the chemical film when it is formed tends to become uneven, and the blackening resistance is also reduced.

By including the aforementioned Mo compound in the aforementioned chemical conversion film, the blackening resistance of the surface treated steel sheet can be improved. The aforementioned Mo compound is a compound containing Mo, and can be obtained by adding one or both of molybdenum acid and a molybdate salt to a chemical conversion treatment solution. Furthermore, as the aforementioned molybdate salt, for example, one or more selected from sodium molybdate, potassium molybdate, magnesium molybdate, and zinc molybdate can be cited.

The concentration of the Mo compound in the chemical conversion treatment solution used to form the chemical conversion film is preferably 0.01 mass% to 3 mass%. If the concentration of the Mo compound is 0.01 mass% or more, the generation of oxygen-deficient zinc oxide is further suppressed, and the blackening resistance can be further improved. On the other hand, if the concentration of the Mo compound is 3 mass% or less, in addition to further extending the life of the chemical conversion treatment solution, the corrosion resistance can also be further improved.

By including the Zr compound and the Ti compound in the chemical film, the chemical film can be prevented from becoming porous and the film can be made dense. As a result, corrosion factors are less likely to penetrate the chemical film, and corrosion resistance can be improved.

The Zr compound is a compound containing Zr, and for example, one or more selected from zirconium acetate, zirconium sulfate, zirconium potassium carbonate, zirconium sodium carbonate, and zirconium ammonium carbonate can be used. Among them, organic titanium chelate compounds are preferred because they can densify the film when the chemical treatment liquid is dried to form a film, thereby obtaining better corrosion resistance.

The Ti compound is a compound containing Ti, and for example, one or more selected from titanium sulfate, titanium chloride, titanium hydroxide, titanium acetylacetonate, titanium octanediol, and titanium ethylacetylacetonate can be used.

The concentration of Zr compound and/or Ti compound in the chemical conversion treatment solution used to form the chemical conversion film is preferably 0.2 mass% to 20 mass%. If the total concentration of the Zr compound and/or Ti compound is 0.2 mass% or more, the effect of inhibiting the penetration of corrosion factors is improved, and not only the corrosion resistance of the flat part, but also the corrosion resistance of the defective part, the cut end face part, and the coating film damaged by processing can be improved. On the other hand, if the total concentration of the Zr compound and/or Ti compound is 20 mass% or less, the life of the chemical conversion treatment solution can be further extended.

By including the aforementioned Ca compound in the aforementioned chemical conversion film, the effect of reducing the corrosion rate can be exhibited.

The Ca compound is a compound containing Ca, and examples thereof include Ca oxides, Ca nitrates, Ca sulfates, and intermetallic compounds containing Ca. More specifically, examples of the Ca compound include CaO, CaCO ₃ , Ca(OH) ₂ , Ca(NO ₃ ) ₂ ·4H ₂ O, and CaSO ₄ ·2H ₂ O. The content of the Ca compound in the chemical film is not particularly limited.

Furthermore, the chemical film may contain various well-known components commonly used in the field of coatings as needed. For example, various surface conditioners such as leveling agents and defoaming agents, dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, titanium salt coupling agents and other additives, coloring pigments, physical pigments, brighteners and other pigments, curing catalysts, organic solvents, lubricants, etc. can be cited.

Furthermore, in the surface treated steel sheet of the present invention, the chemical film preferably does not contain harmful components such as hexavalent chromium, trivalent chromium, fluorine, etc. Since the chemical treatment solution used to form the chemical film does not contain such harmful components, it is highly safe or has a small environmental load.

Furthermore, the amount of the chemical film is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance and preventing peeling of the chemical film, it is preferred to set the amount of the chemical film to 0.1 to 3.0 g/m ² , and more preferably to 0.5 to 2.5 g/m ^2. By setting the amount of the chemical film to 0.1 g/m ² or more, corrosion resistance can be more reliably ensured, and by setting the amount of the chemical film to 3.0 g/m ² or less, cracking or peeling of the chemical film can be prevented. The amount of the chemical film can be obtained by a method appropriately selected from existing methods, such as a method of analyzing the film by fluorescent X-rays to determine the amount of an element whose content is known in advance.

Furthermore, the method for forming the aforementioned chemical film is not particularly limited and can be appropriately selected according to the required performance or manufacturing equipment. For example, the aforementioned coating film can be continuously coated with a chemical treatment liquid by a roller coater, and then dried by hot air or induction heating at a plate temperature of about 60 to 200°C (peak metal temperature, Peak Metal Temperature: PMT). In the coating of the aforementioned chemical treatment liquid, in addition to the roller coater, well-known methods such as airless spray, electrostatic spray, curtain coater, etc. can also be appropriately adopted. Furthermore, if the chemically-formed film comprises the resin and the metal compound, it may be a single-layer film or a multi-layer film without any particular limitation.

Furthermore, the surface treated steel sheet of the present invention may also form a coating on the aforementioned chemically formed film as needed.

(Manufacturing method of surface-treated steel sheet) The manufacturing method of the surface-treated steel sheet of the present invention is a manufacturing method of a surface-treated steel sheet having a coating film and a chemical film formed on the coating film. In addition, in the manufacturing method of the present invention, the chemical film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound, The formation of the aforementioned coating film comprises a hot dip coating treatment step of immersing the base steel plate in a coating bath, wherein the coating bath has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities, and the Co content in the inevitable impurities of the aforementioned coating bath is controlled to be less than 0.080 mass%.

As mentioned above, Co contained in the aforementioned coating film may deteriorate the corrosion resistance of the surface treated steel sheet. Therefore, after appropriately controlling the contents of Al, Zn, Si and Mg in the coating bath, the content of Co as an inevitable impurity is further suppressed, thereby suppressing the deterioration of corrosion resistance.

Moreover, the conditions of the aforementioned hot dip coating treatment step are the same as those described in the coating film of the surface treated steel plate of the present invention. In addition, the composition of the aforementioned chemical coating is also the same as that described in the chemical coating film of the surface treated steel plate of the present invention.

(Coated steel plate) The coated steel plate of the present invention is a coated steel plate having a coating film formed on the coating film directly or via a chemical film. The composition of the coating film is the same as the coating film of the hot-dip coated Al-Zn-Si-Mg steel plate of the present invention.

The coated steel plate of the present invention can form a chemical film on the aforementioned coating film. Moreover, the aforementioned chemical film only needs to be formed on at least one side of the coated steel plate, and can also be formed on both sides of the coated steel plate according to the purpose or required performance.

・Chemical film In the coated steel sheet of the present invention, the chemical film is characterized by containing a resin component and an inorganic compound, the resin component containing 30 to 50% by mass of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 in terms of mass ratio, and the inorganic compound contains 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. By forming the chemical film on the coating film, the strength and adhesion of the chemical film can be improved while the corrosion resistance can be improved.

Here, the resin component constituting the chemical film comprises (a) an anionic polyurethane resin having an ester bond and (b) an epoxy resin having a bisphenol skeleton.

As for the anionic polyurethane resin (a) having an ester bond, there can be mentioned a resin obtained by copolymerizing a reaction product of a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups with dihydroxymethyl alkanoic acid. Alternatively, the resin can be dispersed in a liquid such as water by a well-known method to obtain a chemical treatment liquid.

Examples of the aforementioned polyester polyol include polyesters obtained by dehydration condensation reaction of an acid component such as an ester derivative of a diol component and a hydroxycarboxylic acid, polyesters obtained by ring-opening polymerization of a cyclic ester compound such as ε-caprolactone, and copolymerized polyesters thereof. Examples of the aforementioned polyisocyanate include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and the like. Examples of the aromatic polyisocyanate include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, m-xylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, triphenylmethane triisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and derivatives thereof (e.g., prepolymers obtained by reaction with polyols, modified polyisocyanates of carbodiimide compounds of diphenylmethane diisocyanate, etc.).

Furthermore, when the polyester polyol is reacted with the diisocyanate or polyisocyanate to synthesize urethane, the anionic polyurethane resin (a) having an ester bond can be obtained by copolymerizing, for example, dihydroxymethylalkanoic acid, self-emulsifying and water-dispersing. In this case, examples of the dihydroxymethylalkanoic acid include dihydroxymethylalkanoic acid having 2 to 6 carbon atoms, and more specifically, dihydroxymethylacetic acid, dihydroxymethylpropionic acid, dihydroxymethylbutyric acid, dihydroxymethylheptanoic acid, and dihydroxymethylhexanoic acid.

In addition, regarding the aforementioned (b) epoxy resin having a bisphenol skeleton, well-known epoxy resins can be used. For example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, bisphenol S type epoxy resin, etc. can be cited. Such epoxy resins can be obtained by reacting bisphenol compounds such as bisphenol A, bisphenol F, bisphenol AD, bisphenol S, etc. with epichlorohydrin in the presence of an alkaline catalyst. Among them, component [A] preferably includes bisphenol A type epoxy resin or bisphenol F type epoxy resin, and more preferably includes bisphenol A type epoxy resin. The (b) epoxy resin having a bisphenol skeleton can be dispersed in a liquid such as water by a well-known method to obtain a chemical treatment liquid.

The resin component mentioned above acts as a binder for the chemical film. The anionic polyurethane resin (a) having an ester bond constituting the binder has flexibility, so the chemical film is not easily damaged (peeled off) during processing. The epoxy resin (b) having a bisphenol skeleton can improve the adhesion with the zinc-coated steel plate of the base and the primer coating of the upper layer. The resin component is contained in the chemical film in a total of 30 to 50% by mass. When the content of the resin component is less than 30% by mass, the binder effect of the chemical film is reduced. When it exceeds 50% by mass, the functions of the inorganic components shown below, such as the inhibitor effect, are reduced. Based on the same viewpoint, the content of the resin component in the chemical film is preferably 35-45% by mass.

Furthermore, the resin component must have a content ratio ((a):(b)) of the anionic polyurethane resin (a) having an ester bond and the epoxy resin (b) having a bisphenol skeleton in a mass ratio of 3:97 to 60:40. This is because when the (a):(b) ratio is outside the above range, the flexibility of the film after the chemical treatment is reduced or the adhesion is reduced, and sufficient corrosion resistance cannot be obtained. Based on the same viewpoint, the (a):(b) ratio is preferably 10:90 to 55:45.

Furthermore, the resin component mentioned above may include resins other than the above-mentioned (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton (other resin components) according to the required performance. There is no particular limitation on the above-mentioned other resin components, and for example, at least one selected from acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin or a combination of two or more thereof may be used. When the resin component contains other resins, the total content of the anionic polyurethane resin (a) having an ester bond and the epoxy resin (b) having a bisphenol skeleton is preferably 50% by mass or more, and more preferably 75% by mass or more. This is because the flexibility reduction or adhesion of the chemical treatment film can be more reliably obtained.

Furthermore, the aforementioned chemical film contains 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound as inorganic compounds. By including these compounds, the corrosion resistance of the chemical film can be improved.

The aforementioned vanadium compound is added to the chemical treatment solution to act as a rust preventer (inhibitor). Since the aforementioned vanadium compound is contained in the aforementioned chemical film, the vanadium compound can be appropriately dissolved in a corrosive environment, and combined with zinc ions of the plating component that are also dissolved in a corrosive environment to form a dense protective film. The formed protective film can further improve the corrosion resistance not only for the flat part of the steel plate, but also for the defective part, the damaged part of the plating film caused by processing, and the corrosion from the cut end face to the flat part. As for the above-mentioned vanadium compounds, for example, vanadium pentoxide, metavanadic acid, ammonium metavanadate, oxyvanadium trichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadium acetylacetonate, vanadium acetylacetonate, etc. can be cited. In particular, among these, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

The content of the vanadium compound in the chemical conversion treatment film is 2 to 10% by mass. When the content of the vanadium compound in the chemical conversion treatment film is less than 2% by mass, the inhibitor effect is insufficient, resulting in reduced corrosion resistance. On the other hand, when the content of the vanadium compound exceeds 10% by mass, the moisture resistance of the chemical conversion treatment film is reduced.

The zirconium compound contained in the aforementioned chemical film can be expected to improve the strength and corrosion resistance of the chemical treatment film by reacting with the plated metal and coexisting with the resin component. In addition, the zirconium compound itself helps to form a dense chemical treatment film, and since it is rich in coating properties, a barrier effect can be expected. As the aforementioned zirconium compound, there can be cited zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride and other neutralized salts.

Furthermore, the content of the zirconium compound in the aforementioned chemical conversion treatment film is 40 to 60% by mass. This is because when the content of the zirconium compound in the aforementioned chemical conversion treatment film is less than 40% by mass, the strength or corrosion resistance of the chemical conversion treatment film is reduced. If the content of the zirconium compound exceeds 60% by mass, the chemical conversion treatment film becomes brittle, and the chemical conversion treatment film is damaged or peeled off when subjected to severe processing.

The fluorine compound is contained in the chemical film and acts as an adhesion imparting agent with the coating film. As a result, the corrosion resistance of the chemical film can be improved. As the fluorine compound, for example, fluoride salts such as ammonium salts, sodium salts, potassium salts, or fluoride compounds such as ferrous fluoride and ferric fluoride can be used. Among them, it is preferred to use ammonium fluoride, or fluoride salts such as sodium fluoride and potassium fluoride.

Furthermore, the content of the fluorine compound in the aforementioned chemical conversion treatment film is 0.5 to 5% by mass. This is because when the content of the fluorine compound in the aforementioned chemical conversion treatment film is less than 0.5% by mass, the adhesion of the processed part cannot be fully obtained, and if the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion treatment film is reduced.

Furthermore, the amount of the chemical film is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance and improving the adhesion of the chemical film, it is preferred to set the amount of the chemical film to 0.025 to 0.5 g/m ^2. By setting the amount of the chemical film to 0.025 g/m ² or more, corrosion resistance can be more reliably ensured, and by setting the amount of the chemical film to 0.5 g/m ² or less, peeling of the chemical film can be suppressed. The amount of the chemical film can be obtained by a method appropriately selected from existing methods, such as a method of analyzing the film by fluorescent X-rays to determine the amount of an element whose content is known in advance.

Furthermore, the method for forming the aforementioned chemical film is not particularly limited and can be appropriately selected according to the required performance or manufacturing equipment. For example, the aforementioned coating film can be continuously coated with a chemical treatment liquid by a roller coater, and then dried at a peak metal temperature (PMT) of about 60 to 200°C using hot air or induction heating. In the coating of the aforementioned chemical treatment liquid, in addition to the roller coater, well-known methods such as airless spray, electrostatic spray, curtain coater, etc. can also be appropriately used. Furthermore, if the chemically-formed film comprises the resin and the metal compound, it may be a single-layer film or a multi-layer film without any particular limitation.

・Coating film The coated steel plate of the present invention is formed on the coating film directly or via a chemical film as described above, and the coating film has at least a primer coating film.

Moreover, in the present invention, the aforementioned primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide. The aforementioned primer coating film can improve the adhesion of the coating film and improve the corrosion resistance by containing the aforementioned polyester resin having a urethane bond and the aforementioned inorganic compound.

The aforementioned primer coating contains a polyester resin having a urethane bond as a main component. The aforementioned polyester resin having a urethane bond has both flexibility and strength, so the primer coating is less likely to crack during processing, and because of its high affinity with the chemical treatment film containing the urethane resin, it is particularly helpful to improve the corrosion resistance of the processed part. Moreover, the "main component" here refers to the component with the largest content among the components in the primer coating.

As the polyester resin having a urethane bond, a well-known resin such as a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups can be used. In addition, a resin obtained by reacting the polyester polyol with the diisocyanate or polyisocyanate in the presence of an excess of hydroxyl groups (urethane-modified polyester resin) and curing the resin with a blocked polyisocyanate can also be used.

Moreover, the aforementioned polyester polyol can be obtained by a well-known method utilizing a dehydration condensation reaction of a polyol component and a polyacid component. As the aforementioned polyol, diols and trivalent or higher polyols can be cited. Examples of the aforementioned diols include ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, hexanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol, methyl propanediol, cyclohexanedimethanol, 3,3-diethyl-1,5-pentanediol, and the like. In addition, the aforementioned trivalent or higher polyols include, for example, glycerol, trihydroxymethylethane, trihydroxymethylpropane, pentaerythritol, dipentaerythritol, etc. These polyols can be used alone or in combination of two or more. The aforementioned polyacids are usually polycarboxylic acids, but monovalent fatty acids can be used in combination as needed. Examples of the aforementioned polycarboxylic acid include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, and their anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, and hexahydroterephthalic acid. These polycarboxylic acids may be used alone or in combination of two or more.

As for the aforementioned polyisocyanate, for example, there can be mentioned aliphatic diisocyanates such as hexamethylene diisocyanate, trimethyl hexamethylene diisocyanate, dimer acid diisocyanate, and aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, toluene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), and cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, hydrogenated MDI, and adducts, biuret forms, isocyanurate forms, etc. These polyisocyanates may be used alone or in combination of two or more.

In addition, the hydroxyl value of the aforementioned polyester resin having a urethane bond is not particularly limited, but from the viewpoint of solvent resistance, processability, etc., it is preferably 5 to 120 mgKOH/g, more preferably 7 to 100 mgKOH/g, and particularly preferably 10 to 80 mgKOH/g. Furthermore, the number average molecular weight of the aforementioned polyester resin having a urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and particularly preferably 800 to 10,000 from the viewpoint of solvent resistance, processability, etc.

In the primer coating, the content of the polyester resin having a urethane bond is preferably 40 to 88% by mass. When the content of the polyester resin having a urethane bond is less than 40% by mass, the function as a binder of the primer coating may be reduced. On the other hand, when the content of the polyester resin having a urethane bond exceeds 88% by mass, the function of the inorganic substance shown below, such as the inhibitor effect, may be reduced.

The vanadium compound, which is one of the aforementioned inorganic compounds, acts as an inhibitor. Examples of the aforementioned vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, oxyvanadium trichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadium acetylacetonate, vanadium acetylacetonate, etc. In particular, among these, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation. The vanadium compound added to the aforementioned primer coating may be the same as the vanadium compound added to the aforementioned chemical treatment film, or may be a different type. It is believed that vanadium acid compounds are slowly dissolved by moisture intruding from the outside, and the vanadium acid ions react with the ions on the surface of the zinc-based steel plate to form a passive film with good adhesion, which protects the exposed metal parts and has an anti-rust effect.

The content of the vanadium compound in the primer coating is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of both corrosion resistance and moisture resistance. When the content of the vanadium compound is less than 4% by mass, the inhibitor effect may be reduced, resulting in a decrease in corrosion resistance. When the content of the vanadium compound exceeds 20% by mass, the moisture resistance of the primer coating may be reduced.

Phosphoric acid compounds, which are one of the aforementioned inorganic compounds, also function as inhibitors. Examples of the aforementioned phosphoric acid compounds include phosphoric acid, ammonium salts of phosphoric acid, alkaline metal salts of phosphoric acid, and alkaline earth metal salts of phosphoric acid. In particular, alkaline metal salts of phosphoric acid such as calcium phosphate are preferably used.

The content of the phosphate compound in the primer coating is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of both corrosion resistance and moisture resistance. When the content of the phosphate compound is less than 4% by mass, the inhibitor effect may be reduced, resulting in reduced corrosion resistance. When the content of the phosphate compound exceeds 20% by mass, the moisture resistance of the primer coating may be reduced.

Magnesium oxide, one of the inorganic compounds, generates a product containing Mg by initial corrosion, and stabilizes as a sparingly soluble magnesium salt, thereby having the effect of improving corrosion resistance.

The content of the magnesium oxide in the primer coating is not particularly limited, but from the perspective of both corrosion resistance and corrosion resistance of the processed portion, it is preferably 4 to 20% by mass. When the content of the magnesium oxide is less than 4% by mass, the above effect may be reduced, resulting in a decrease in corrosion resistance. If the content of the magnesium oxide exceeds 20% by mass, the flexibility of the primer coating may be reduced, resulting in a decrease in corrosion resistance of the processed portion.

Furthermore, the aforementioned primer coating may also contain components other than the aforementioned polyester resin having urethane bonds and inorganic compounds. For example, a crosslinking agent used in forming the primer coating can be cited. The aforementioned crosslinking agent is one that reacts with the aforementioned polyester resin having urethane bonds to form a crosslinked coating, and examples thereof include oxazoline compounds, epoxy compounds, melamine compounds, isocyanate compounds, carbodiimide compounds, silane coupling compounds, etc., and two or more crosslinking agents may also be used in combination. Among them, from the perspective of the corrosion resistance of the processed portion of the obtained coated steel plate, it is preferred to use a blocked polyisocyanate compound, etc. As the blocked polyisocyanate, for example, there can be cited a polyisocyanate compound in which the isocyanate group is blocked by alcohols such as butanol, oximes such as methyl ethyl ketone oxime, lactamides such as ε-caprolactam, diketones such as acetoacetic acid diester, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

Furthermore, the aforementioned primer coating may contain various well-known ingredients commonly used in the field of coatings as needed. Specifically, for example, various surface conditioners such as leveling agents and defoaming agents, dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, titanium salt coupling agents and other additives, various pigments such as coloring pigments and physical pigments, brighteners, hardening catalysts, organic solvents, etc.

The thickness of the primer coating is preferably 1.5 μm or more. This is because by setting the thickness of the primer coating to 1.5 μm or more, the corrosion resistance can be improved more reliably, or the adhesion of the coating formed on the chemical treatment film or the primer coating can be improved.

There is no particular limitation on the method of forming the aforementioned primer coating film. In addition, regarding the coating method of the coating composition constituting the aforementioned primer coating film, it is preferably applied by a roller coating machine, a curtain coating, or the like. After the aforementioned coating composition is applied, it is baked by heating means such as hot air heating, infrared heating, induction heating, etc. to obtain a primer coating film. The aforementioned baking treatment can usually set the maximum plate temperature to about 180 to 270°C and be carried out within this temperature range for about 30 seconds to 3 minutes.

In addition, regarding the coating film constituting the coated steel plate of the present invention, it is preferred to further form an upper coating film on the aforementioned primer coating film. The aforementioned upper coating film can not only give the painted steel plate beauty such as color, gloss, and surface condition, but also improve various properties such as processability, weather resistance, chemical resistance, pollution resistance, water resistance, and corrosion resistance.

The composition of the aforementioned top coating film is not particularly limited, and the material or thickness can be appropriately selected according to the required performance. For example, the aforementioned top coating film can be formed using polyester resin coatings, silicone polyester resin coatings, polyurethane resin coatings, acrylic resin coatings, fluororesin coatings, etc. Furthermore, the above-mentioned coating film may contain appropriate amounts of titanium oxide, red iron oxide, mica, carbon black or other various coloring pigments; metal pigments such as aluminum powder or mica; physical pigments formed by carbonates or sulfates; various microparticles such as silica microparticles, nylon resin beads, acrylic resin beads, etc.; curing catalysts such as p-toluenesulfonic acid, dibutyltin dilaurate, etc.; wax; and other additives.

In addition, from the perspective of both appearance and processability, the thickness of the upper coating film is preferably 5 to 30 μm. When the thickness of the upper coating film is 5 μm or more, the color tone appearance can be more reliably stabilized, and when the thickness of the upper coating film is 30 μm or less, the reduction in processability (occurrence of cracks in the upper coating film) can be more reliably suppressed.

There is no particular limitation on the coating method of the coating composition used to form the upper coating film. For example, the coating composition can be applied by roller coating, curtain coating, etc. After the coating composition is applied, the upper coating film can be formed by baking by heating means such as hot air heating, infrared heating, induction heating, etc. The baking process can usually set the maximum plate temperature to about 180 to 270°C and perform it within this temperature range for about 30 seconds to 3 minutes.

(Manufacturing method of coated steel plate) The manufacturing method of the coated steel plate of the present invention is a manufacturing method of a coated steel plate in which a coating film is formed directly on a coating film or through a chemically formed film. Moreover, in the manufacturing method of the present invention, the aforementioned chemical film contains a resin component and an inorganic compound, the resin component contains a total of 30 to 50 mass % of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 in terms of mass ratio, the inorganic compound contains 2 to 10 mass % of a vanadium compound, 40 to 60 mass % of a zirconium compound and 0.5 to 5 mass % of a fluorine compound, the aforementioned coating film has at least a primer coating film, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide, The formation of the aforementioned coating film comprises a hot dip coating treatment step of immersing the base steel plate in a coating bath, wherein the coating bath has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities, and the Co content in the inevitable impurities of the aforementioned coating bath is controlled to be less than 0.080 mass%.

As mentioned above, Co contained in the aforementioned coating film may deteriorate the corrosion resistance of the coated steel sheet. Therefore, after appropriately controlling the contents of Al, Zn, Si and Mg in the coating bath, the content of Co as an inevitable impurity is further suppressed, thereby suppressing the deterioration of corrosion resistance.

Moreover, the conditions of the aforementioned hot dip coating treatment step are the same as those described in the coating film of the coated steel plate of the present invention. Moreover, the composition of the aforementioned chemical conversion film and the aforementioned coating film is also the same as that described in the chemical conversion film of the coated steel plate of the present invention. Example

[Example 1: Samples 1 to 55] A cold rolled steel plate with a thickness of 0.8 mm manufactured by a conventional method was used as a base steel plate, and annealing treatment and plating treatment were performed using a hot dip plating simulator manufactured by RHESCA to manufacture samples 1 to 55 of hot dip plated steel plates under the conditions shown in Table 1. Furthermore, regarding the composition of the plating bath used for manufacturing the hot dip plated steel plates, the composition of the plating bath was variously changed within the range of Al: 5 to 75 mass%, Si: 0.0 to 4.5 mass%, Mg: 0 to 10 mass%, and Co: 0.000 to 0.120 mass% so as to obtain the coating film composition of each sample shown in Table 1. In addition, the bath temperature of the plating bath is set to 450°C in the case of Al: 5 mass%, 480°C in the case of Al: 15 mass%, 590°C in the case of Al: 30-60 mass%, and 630°C in the case of Al: more than 60 mass%, and the plating immersion plate temperature of the base steel plate is controlled to be the same temperature as the plating bath temperature. Furthermore, in the case of Al: 30-60 mass%, the plating treatment is carried out under the condition that the plate temperature is cooled to a temperature range of 520-500°C within 3 seconds. In addition, the coating film adhesion amount was controlled to be: 85±5 g/m ² per side in samples 1 to 52, 50±5 g/m ² per side in sample 53, 100±5 g/m ² per side in sample 54, and 125±5 g/m ² per side in sample 55.

(Evaluation) The following evaluations were performed on each sample of the hot-dip plated steel sheet obtained as described above. The evaluation results are shown in Table 1.

(1) Coating film (composition, adhesion amount, X-ray diffraction intensity) For each sample after coating, a 100 mm φ punch was applied, and the non-measurement surface was sealed with tape. The coating layer was then stripped off by dissolving it with a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The adhesion amount of the coating film was calculated from the mass difference of the sample before and after stripping. Table 1 shows the calculated results and the adhesion amount of the coating film obtained. Then, the stripping liquid was filtered, and the filter liquid and solid components were analyzed separately. Specifically, the filter liquid was analyzed by ICP emission spectrometry to quantify the components other than insoluble Si. In addition, the solid components were dried and ashed in a heating furnace at 650°C, and then sodium carbonate and sodium tetraborate were added to melt. Furthermore, the melt was dissolved with hydrochloric acid, and the dissolved solution was analyzed by ICP emission spectrometry to quantify the insoluble Si. The Si concentration in the coating film was the sum of the soluble Si concentration obtained by the filter solution analysis and the insoluble Si concentration obtained by the solid component analysis. Table 1 shows the calculated results and the composition of the obtained coating film. In addition, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film on the evaluation object surface was mechanically cut out until the base steel plate was exposed. After the obtained powder was fully mixed, 0.3 g was taken out and the powder was qualitatively analyzed using an X-ray diffraction device ("SmartLab" manufactured by RIGAKU Co., Ltd.) under the conditions of using X-ray: Cu-Kα (wavelength = 1.54178Å), Kβ ray removal: Ni filter, tube voltage: 40kV, tube current: 30mA, scanning speed: 4°/min, sampling interval: 0.020°, divergent slit: 2/3°, parallel slit (Soller slit): 5°, detector: high-speed one-dimensional detector (D/teX Ultra). The intensity obtained by subtracting the background intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of Si (plane spacing d=0.3135nm) and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d=0.3668nm) were measured. Table 1 shows the measurement results.

(2) Corrosion resistance evaluation For each sample of the obtained hot-dip coated steel plate, cut it into a size of 120 mm × 120 mm, and seal the area 10 mm away from each edge of the evaluation target surface and the end surface of the sample with the evaluation non-target surface with tape, and use the sample with the evaluation target surface exposed in a size of 100 mm × 100 mm as the evaluation sample. In addition, three identical evaluation samples were prepared. For the three evaluation samples prepared as described above, the corrosion promotion test was carried out using the cycle shown in Figure 1. The corrosion promotion test was carried out from the start of the wet test to 300 cycles. The corrosion loss of each sample was measured according to the method described in JIS Z 2383 and ISO8407, and the evaluation was based on the following criteria. The evaluation results are shown in Table 1. ◎: The corrosion loss of all three samples was less than 45g/ ^m2 ○: The corrosion loss of all three samples was less than 70g/ ^m2 ×: The corrosion loss of more than one sample exceeded 70g/ ^m2

(3) Surface appearance For each sample of the obtained hot-dip coated steel sheet, the surface of the coating film was visually observed. Furthermore, the observation results were evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: No wrinkle defects were observed at all ○: Wrinkle defects were observed only within a range of 50 mm from the edge ×: Wrinkle defects were observed outside a range of 50 mm from the edge

(4) Processability For each sample of the obtained hot-dip coated steel plate, cut into a size of 70 mm × 150 mm, sandwich 8 plates of the same thickness on the inside, and apply 180° bending processing (8T bending). After bending, Cellotape (registered trademark: transparent tape) is strongly adhered to the outer surface of the bent part and then torn off. The surface state of the coating film outside the bent part and the surface of the used tape are visually observed to see if the coating film is attached (peeled off), and the processability is evaluated using the following criteria. The evaluation results are shown in Table 1. ○: No cracks or peeling are observed on the coating film △: There are cracks on the coating film, but no peeling is observed ×: Both cracks and peeling are observed on the coating film

(5) Bath stability When manufacturing various samples of hot-dip plated steel sheets, the state of the bath surface of the coating bath was visually confirmed and compared with the bath surface of the coating bath used when manufacturing hot-dip Al-Zn steel sheets (bath surface without Mg oxide). The evaluation was performed based on the following criteria, and the evaluation results are shown in Table 1. 0: The same level as the hot-dip Al-Zn coating bath (55 mass% Al-remainder Zn-1.6 mass% bath) △: Compared with the hot-dip Al-Zn coating bath (55 mass% Al-remainder Zn-1.6 mass% bath), there are more white oxides ×: The formation of black oxides is seen in the coating bath

From the results in Table 1, it can be seen that compared with the samples of the comparative examples, the samples of the present invention examples are well-balanced and excellent in corrosion resistance, surface appearance, processability and bath stability.

[Example 2: Samples 1 to 134] (1) A cold rolled steel plate with a thickness of 0.8 mm manufactured by a conventional method was used as a base steel plate, and annealing treatment and plating treatment were performed using a hot dip plating simulator manufactured by RHESCA to manufacture samples of hot dip plated steel plates having the coating film conditions shown in Tables 3 and 3. Furthermore, regarding the composition of the plating bath used for manufacturing the hot dip plated steel plates, the composition of the plating bath was varied in various ways within the ranges of Al: 5 to 75 mass%, Si: 0.0 to 4.5 mass%, Mg: 0 to 10 mass%, and Co: 0.000 to 0.120 mass% so as to obtain the coating film compositions of the samples shown in Tables 3 and 3. In addition, the bath temperature of the plating bath is set to 450°C in the case of Al: 5 mass%, 480°C in the case of Al: 15 mass%, 590°C in the case of Al: 30-60 mass%, and 630°C in the case of Al: more than 60 mass%, and the plating immersion plate temperature of the base steel plate is controlled to be the same temperature as the plating bath temperature. Furthermore, in the case of Al: 30-60 mass%, the plating treatment is carried out under the condition that the plate temperature is cooled to a temperature range of 520-500°C within 3 seconds. Moreover, the adhesion amount of the coating film was controlled to be: 85±5g/m ² per side for samples 1-104 and 117-134, 50±5g/m ² per side for samples 105-106, 100±5g/m ² per side for samples 107-108, 125g/m ² ±5g/m ² per side for samples 109-110, and 70±5g/m ² per side for samples 111-116.

(2) Then, a chemical conversion treatment liquid was applied on the coating film of each sample of the hot-dip plated steel plate by a rod coater, and dried by a hot air furnace (heating rate: 60°C/s, PMT: 120°C) to form a chemical conversion film, and each sample of the surface-treated steel plate shown in Tables 3 and 3 was prepared. The chemical conversion treatment liquid is prepared by dissolving each component in water as a solvent to form surface treatment liquids A to F. The types of each component (resin, metal compound) contained in the surface treatment liquid are as follows. (Resin) Urethane resin: Superflex 130, Superflex 126 (Daiichi Kogyo Seiyaku Co., Ltd.) Acrylic resin: Boncoat EC-740EF (DIC Co., Ltd.) (Metallic compound) P compound: dihydrogen aluminum tripolyphosphate Si compound: silicon dioxide V compound: sodium metavanadate Mo compound: molybdenum acid Zr compound: potassium zirconium carbonate Table 2 shows the composition of the prepared chemical treatment solutions A to F and the adhesion amount of the formed chemical film. In addition, the concentration of each component in Table 2 of this manual is the solid component concentration (mass %).

(Evaluation) The following evaluations were performed on each sample of the hot-dip plated steel plate and the surface-treated steel plate obtained as described above. The evaluation results are shown in Tables 3 and 4.

(1) Coating (composition, adhesion amount, X-ray diffraction intensity) For each sample of hot-dip coated steel plate, a 100 mm φ punch was applied, and the non-measurement surface was sealed with tape. The coating layer was then stripped off by dissolving it with a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The adhesion amount of the coating was calculated from the mass difference of the sample before and after stripping. Tables 3 and 4 show the calculated results and the adhesion amount of the coating obtained. Then, the stripping liquid was filtered, and the filter liquid and solid components were analyzed separately. Specifically, the filter liquid was analyzed by ICP emission spectrometry to quantify the components other than insoluble Si. In addition, the solid components were dried and ashed in a heating furnace at 650°C, and then sodium carbonate and sodium tetraborate were added to melt. Furthermore, the melt was dissolved with hydrochloric acid, and the dissolved solution was analyzed by ICP emission spectrometry to quantify the insoluble Si. The Si concentration in the coating film was the sum of the soluble Si concentration obtained by the filtrate analysis and the insoluble Si concentration obtained by the solid component analysis. Tables 3 and 4 show the calculated results and the composition of the obtained coating film. In addition, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film on the evaluation object surface was mechanically cut out until the base steel plate was exposed. After the obtained powder was fully mixed, 0.3 g was taken out and the above powder was qualitatively analyzed using an X-ray diffraction device ("SmartLab" manufactured by RIGAKU Co., Ltd.) under the conditions of using X-ray: Cu-Kα (wavelength = 1.54178Å), Kβ ray removal: Ni filter, tube voltage: 40kV, tube current: 30mA, scanning speed: 4°/min, sampling interval: 0.020°, divergent slit: 2/3°, parallel slit: 5°, detector: high-speed one-dimensional detector (D/teX Ultra). The intensity obtained by subtracting the background intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of Si (plane spacing d=0.3135nm) and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d=0.3668nm) were measured. Tables 3 and 4 show the measurement results.

(2) Corrosion resistance evaluation For each sample of hot-dip coated steel plate and surface treated steel plate, cut into a size of 120 mm × 120 mm, and seal the area 10 mm away from each edge of the evaluation target surface and the end face of the sample with the evaluation non-target surface with tape, and use the sample with the evaluation target surface exposed in a size of 100 mm × 100 mm as the evaluation sample. In addition, three identical evaluation samples were made. For the three evaluation samples made as described above, the corrosion promotion test was carried out using the cycle shown in Figure 1. The corrosion promotion test was carried out from the start of the wet test to 300 cycles. The corrosion loss of each sample was measured according to the method described in JIS Z 2383 and ISO8407, and the evaluation was based on the following criteria. The evaluation results are shown in Tables 3 and 4. ◎: The corrosion loss of all three samples was less than 30 g/ ^m2 ○: The corrosion loss of all three samples was less than 50 g/ ^m2 ×: The corrosion loss of more than one sample exceeded 50 g/ ^m2

(3) Rust resistance For each sample of hot-dip coated steel plate and surface treated steel plate, cut into a size of 120 mm × 120 mm, seal the area 10 mm away from each edge of the evaluation target surface and the end face of the sample with the non-evaluation target surface with tape, and use the sample with the evaluation target surface exposed in a size of 100 mm × 100 mm as the evaluation sample. Using the above evaluation samples, the salt water spray test described in JIS Z 2371 was carried out for 90 hours and evaluated using the following criteria. The evaluation results are shown in Tables 3 and 4. ◎: No rust on the flat plate ○: The rust occurrence area of the flat plate is less than 10% ×: The rust occurrence area of the flat plate is more than 10%

(4) Surface appearance For each sample of hot-dip coated steel plate, the surface of the coated film was visually observed. Then, the observation results were evaluated according to the following criteria. The evaluation results are shown in Tables 3 and 4. ◎: No wrinkle defects were observed at all ○: Wrinkle defects were observed only within a range of 50 mm from the edge ×: Wrinkle defects were observed outside a range of 50 mm from the edge

(5) Processability For each sample of hot-dip coated steel plate, cut it into a size of 70 mm × 150 mm, sandwich 8 plates of the same thickness on the inside, and apply 180° bending processing (8T bending). After bending, Cellotape (registered trademark: transparent tape) is firmly attached to the outer surface of the bent part and then torn off. Visually observe the surface condition of the coating film outside the bent part and the surface of the tape used for the coating film to see if it is attached (peeled off), and evaluate the processability using the following criteria. The evaluation results are shown in Tables 3 and 4. ○: No cracks or peeling are observed on the coating film △: There are cracks on the coating film, but no peeling is observed ×: Both cracks and peeling are observed on the coating film

(5) Bath stability During hot dip plating, the state of the bath surface of the plating bath was visually confirmed and compared with the bath surface of the plating bath used when manufacturing hot dip Al-Zn steel sheets (bath surface without Mg oxide). The evaluation was performed based on the following criteria, and the evaluation results are shown in Tables 3 and 4. ○: The same level as the hot dip Al-Zn plating bath (55 mass% Al-remainder Zn-1.6 mass% bath) △: Compared with the hot dip Al-Zn plating bath (55 mass% Al-remainder Zn-1.6 mass% bath), there are more white oxides ×: The formation of black oxides is seen in the plating bath

From the results of Table 3 and Table 4, it can be seen that compared with the samples of the comparative examples, the samples of the present invention examples are well-balanced and excellent in corrosion resistance, rust resistance, surface appearance, processability and bath stability. In addition, from the results of Table 4, it can be seen that the rust resistance of the samples subjected to the chemical conversion treatments A to D is particularly excellent.

[Example 3: Samples 1 to 55] A cold rolled steel plate with a thickness of 0.8 mm manufactured by a conventional method was used as a base steel plate, and annealing treatment and plating treatment were performed using a hot dip plating simulator manufactured by RHESCA to manufacture samples 1 to 55 of hot dip plated steel plates under the conditions shown in Table 5. Furthermore, regarding the composition of the plating bath used for manufacturing the hot dip plated steel plates, the composition of the plating bath was variously changed within the range of Al: 5 to 75 mass%, Si: 0.0 to 4.5 mass%, Mg: 0 to 10 mass%, and Co: 0.000 to 0.120 mass% so as to obtain the composition of the plating film of each sample shown in Table 6. In addition, the bath temperature of the plating bath is set to 450°C in the case of Al: 5 mass%, 480°C in the case of Al: 15 mass%, 590°C in the case of Al: 30-60 mass%, and 630°C in the case of Al: more than 60 mass%, and the plating immersion plate temperature of the base steel plate is controlled to be the same temperature as the plating bath temperature. Furthermore, in the case of Al: 30-60 mass%, the plating treatment is carried out under the condition that the plate temperature is cooled to a temperature range of 520-500°C within 3 seconds. In addition, the coating film adhesion amount was controlled to be: 85±5 g/m ² per side in samples 1 to 52, 50±5 g/m ² per side in sample 53, 100±5 g/m ² per side in sample 54, and 125±5 g/m ² per side in sample 55.

(2) Thereafter, the chemical treatment solution shown in Table 5 was applied to the coating film of each sample of the hot-dip plated steel plate by a rod coater, and dried in a hot air drying furnace (reaching plate temperature: 90°C) to form a chemical treatment film with an adhesion amount of 0.1 g/ ^m2 . The chemical treatment solution used was a chemical treatment solution prepared by dissolving each component in water as a solvent and adjusting the pH to 8 to 10. The types of each component (resin component, inorganic compound) contained in the chemical treatment solution are as follows. (Resin components) Resin A: (a) anionic polyurethane resin having an ester bond ("Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and (b) epoxy resin having a bisphenol skeleton ("Yuka Resin RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd.) mixed at a mass ratio of (a):(b) = 50:50 Resin B: acrylic resin ("Boncoat EC-740EF" manufactured by DIC Co., Ltd.) (Inorganic compounds) Vanadium compound: organic vanadium compound chelated with acetylacetone Zirconium compound: zirconium carbonate Ammonium fluoride compound: ammonium fluoride

(3) Then, a primer coating was applied on the chemical film formed as described above by a rod coater, and the film was baked at a steel plate reaching temperature of 230°C and a baking time of 35 seconds to form a primer coating film having the component composition shown in Table 5. Then, a coating composition was applied on the primer coating film formed as described above by a rod coater, and the film was baked at a steel plate reaching temperature of 230°C to 260°C and a baking time of 40 seconds to form an upper coating film having the resin conditions and film thickness shown in Table 5, and coated steel plates of various samples were prepared. In addition, the primer coating was obtained by mixing the components and stirring them with a ball mill for about 1 hour. The resin components and inorganic compounds constituting the primer coating film are as follows. (Resin components) Resin α: Urethane-modified polyester resin (resin 455 parts by mass of polyester resin and 45 parts by mass of isophorone diisocyanate are reacted, resin acid value is 3, number average molecular weight is 5,600, hydroxyl value is 36) cured by blocked isocyanate. The urethane-modified polyester resin is prepared under the following conditions. In a flask equipped with a stirrer, a rectifier, a water separator, a cooling tube and a thermometer, 320 parts by mass of isophthalic acid, 200 parts by mass of adipic acid, 60 parts by mass of trihydroxymethylpropane and 420 parts by mass of cyclohexanedimethanol were added, heated and stirred, and the generated condensation water was removed from the system while the temperature was raised from 160°C to 230°C at a constant speed over 4 hours. After reaching the temperature of 230°C, 20 parts by mass of xylene were slowly added. The condensation reaction was continued while the temperature was maintained at 230°C. The reaction was terminated when the acid value became less than 5. After cooling to 100°C, Solvesso 100 (EXXON MOBILE Co., Ltd., trade name, high boiling point aromatic hydrocarbon solvent) 120 parts by mass, butyl solvent 100 parts by mass, and a polyester resin solution was obtained. Resin β: Urethane curing polyester resin (Kansai Paint Co., Ltd. "Everclad 4900") (Inorganic compound) Vanadium compound: Magnesium vanadate Phosphoric acid compound: Calcium phosphate Magnesium oxide compound: Magnesium oxide In addition, regarding the resin used for the top coating film shown in Table 5, the following coating was used. Resin I: Melamine-cured polyester coating ("Precolor HD0030HR" manufactured by BASF Japan) Resin II: Organic sol-based baking fluororesin coating with a mass ratio of 80:20 of polyvinylidene fluoride and acrylic resin ("Precolor No.8800HR" manufactured by BASF Japan)

(Evaluation) The following evaluation was performed on each sample of the coated steel plate obtained as described above. The evaluation results are shown in Table 6.

(1) Coating (composition, adhesion amount, X-ray diffraction intensity) For each sample of hot-dip coated steel plate, a 100 mm φ punch was applied, and the non-measurement surface was sealed with tape. The coating layer was then stripped off by dissolving it with a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The adhesion amount of the coating was calculated from the mass difference of the sample before and after stripping. Table 6 shows the calculated results and the adhesion amount of the coating obtained. Then, the stripping liquid was filtered, and the filter liquid and solid components were analyzed separately. Specifically, the filter liquid was analyzed by ICP emission spectrometry to quantify the components other than insoluble Si. In addition, the solid components were dried and ashed in a heating furnace at 650°C, and then sodium carbonate and sodium tetraborate were added to melt. Furthermore, the melt was dissolved with hydrochloric acid, and the dissolved solution was analyzed by ICP emission spectrometry to quantify the insoluble Si. The Si concentration in the coating film was the sum of the soluble Si concentration obtained by the filtrate analysis and the insoluble Si concentration obtained by the solid component analysis. Table 6 shows the calculated results and the composition of the obtained coating film. In addition, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film of the evaluation symmetry surface was mechanically cut out until the base steel plate was exposed. After the obtained powder was fully mixed, 0.3 g was taken out and the above powder was qualitatively analyzed using an X-ray diffraction device ("SmartLab" manufactured by RIGAKU Co., Ltd.) under the conditions of using X-ray: Cu-Kα (wavelength = 1.54178Å), Kβ ray removal: Ni filter, tube voltage: 40kV, tube current: 30mA, scanning speed: 4°/min, sampling interval: 0.020°, divergent slit: 2/3°, parallel slit: 5°, detector: high-speed one-dimensional detector (D/teX Ultra). The intensity obtained by subtracting the background intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of Si (plane spacing d=0.3135nm) and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d=0.3668nm) were measured. Table 6 shows the measurement results.

(2) Corrosion resistance evaluation For each sample of coated steel plate, cut it into a size of 120 mm × 120 mm, seal the area 10 mm away from each edge of the evaluation target surface and the end face of the sample with the evaluation non-target surface with tape, and use the sample with the evaluation target surface exposed in a size of 100 mm × 100 mm as the evaluation sample. In addition, three identical evaluation samples were made. For the three evaluation samples made as described above, the corrosion promotion test was carried out using the cycle shown in Figure 1. The corrosion promotion test starts from the wet state. After every 20 cycles, the sample is taken out, washed and dried, and then visually observed to confirm the occurrence of rust on the shear end surface of one side not sealed by the tape. Then, the number of cycles when rust is seen is evaluated according to the following criteria. The evaluation results are shown in Table 6. ◎: The number of cycles when rust occurs in 3 samples ≧600 cycles ○: 600 cycles > 3 samples The number of cycles when rust occurs ≧500 cycles ×: The number of cycles when rust occurs in at least 1 sample <500 cycles

(3) Appearance after coating For each sample of coated steel plate, the surface was visually observed. Then, the observation results were evaluated according to the following criteria. The evaluation results are shown in Table 6. ◎: No wrinkle defects were observed at all ○: Wrinkle defects were observed only within 50 mm from the edge ×: Wrinkle defects were observed outside the range of 50 mm from the edge

(4) Processability after coating For each sample of coated steel plate, cut into a size of 70 mm × 150 mm, sandwich 8 plates of the same thickness on the inside, and apply 180° bending processing (8T bending). After bending, Cellotape (registered trademark: transparent tape) is strongly adhered to the outer surface of the bend and then torn off. Visually observe the surface condition of the coating film outside the bend and the surface of the tape used for whether the coating film is attached (peeled off), and evaluate the processability using the following criteria. The evaluation results are shown in Table 6. ○: No cracks or peeling are observed on the coating film △: There are cracks on the coating film, but no peeling is observed ×: Both cracks and peeling are observed on the coating film

(5) Bath stability During hot dip plating, the state of the bath surface of the plating bath was visually confirmed and compared with the bath surface of the plating bath used when manufacturing hot dip Al-Zn steel sheets (bath surface without Mg oxide). The evaluation was performed based on the following criteria, and the evaluation results are shown in Table 6. ○: The same level as the hot dip Al-Zn plating bath (55 mass% Al-remainder Zn-1.6 mass% bath) △: Compared with the hot dip Al-Zn plating bath (55 mass% Al-remainder Zn-1.6 mass% bath), there are more white oxides ×: The formation of black oxides is seen in the plating bath

From the results in Table 6, it can be seen that compared with the samples in the comparative examples, the samples in the examples of the present invention are well-balanced and excellent in terms of corrosion resistance, appearance after painting, processability after painting, and bath stability. Possibility of industrial application

According to the present invention, a hot-dip plated Al-Zn-Si-Mg steel sheet having stable and excellent corrosion resistance and a method for manufacturing the same can be provided.

[Figure 1] is a diagram used to explain the process of the Japanese Automobile Standards Combined Cycle Test (JASO-CCT).

Claims (9)

A hot-dip plated Al-Zn-Si-Mg steel sheet having a coating film, characterized in that: the coating film has the following composition: Al: 50-60 mass%, Si: 1.0-3.0 mass%, Mg: 1.0-10.0 mass% and Sr: 0.01-1.0 mass%, and the remainder is composed of Zn and inevitable impurities; the Co content of the inevitable impurities is 0.080 mass% or less relative to the total mass of the coating film; the diffraction intensity of Si and _Mg2Si in the coating film by X-ray diffraction method satisfies the following relationship (1): Si(111)/ _Mg2Si (111)≦0.8・・・(1) Si(111): diffraction intensity of Si (111) plane (inter-plane spacing d=0.3135nm), Mg ₂ Si(111): diffraction intensity of Mg ₂ Si (111) plane (inter-plane spacing d=0.3668nm). For the hot-dip coated Al-Zn-Si-Mg steel sheet of claim 1, the diffraction intensity of Si in the coating film by X-ray diffraction method satisfies the following relationship (2): Si(111)=0 ···(2) Si(111): diffraction intensity of Si (111) plane (plane spacing d=0.3135nm). The hot-dip plated Al-Zn-Si-Mg steel sheet of claim 1 or 2, wherein the content of Mg in the coating is 1.0 to 5.0 mass %. A method for manufacturing a hot-dip plated Al-Zn-Si-Mg steel plate having a coating film is provided, wherein the coating film is formed by a hot-dip plating step of immersing a base steel plate in a coating bath, wherein the coating bath has the following composition: Al: 45-65 mass%, Si: 1.0-4.0 mass%, and Mg: 1.0-10.0 mass%, and the remainder is composed of Zn and inevitable impurities; the Co content in the inevitable impurities of the coating bath is controlled to be less than 0.080 mass% relative to the total mass of the coating bath, and the Si and Mg in the coating film _{are 2} The diffraction intensity of Si by X-ray diffraction method satisfies the following relationship (1), Si(111)/Mg ₂ Si(111)≦0.8 ···(1) Si(111): diffraction intensity of Si (111) plane (plane spacing d=0.3135nm), Mg ₂ Si(111): diffraction intensity of Mg ₂ Si (111) plane (plane spacing d=0.3668nm). The method for producing a hot-dip plated Al-Zn-Si-Mg steel plate as claimed in claim 4, wherein the coating bath further contains Sr: 0.01 to 1.0 mass %. A surface treated steel sheet having a coating film as claimed in claim 1 or 2 and a chemical film formed on the coating film, characterized in that: The chemical film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound. A method for manufacturing a surface-treated steel sheet, which comprises a coating film formed by the method for manufacturing a hot-dip Al-Zn-Si-Mg steel sheet as in claim 4 or 5 and a chemical film formed on the coating film, wherein the chemical film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound and Ca compound. A coated steel plate, which is a coated steel plate formed with a coating film directly or via a chemical film on a coating film as claimed in claim 1 or 2, characterized in that: The chemical film contains a resin component and an inorganic compound, the resin component contains a total of 30 to 50 mass % of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 in terms of mass ratio, the inorganic compound contains 2 to 10 mass % of a vanadium compound, 40 to 60 mass % of a zirconium compound and 0.5 to 5 mass % of a fluorine compound, The aforementioned coating film at least has a primer coating film, and the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide. A method for manufacturing a coated steel plate, wherein a coating film is formed directly or via a chemically formed film on a coating film formed by a method for manufacturing a hot-dip Al-Zn-Si-Mg steel plate as in claim 4 or 5, and the characteristics are: The aforementioned chemical film contains a resin component and an inorganic compound, the resin component contains 30 to 50% by mass of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97 to 60:40 by mass ratio, the inorganic compound contains 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound and 0.5 to 5% by mass of a fluorine compound, The aforementioned coating film has at least a primer coating film, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide.

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