TWI856274B - Molten Al-Zn-Si-Mg plated steel, surface treated steel and painted steel - Google Patents

Abstract

The object of the present invention is to provide a molten Al-Zn-Si-Mg coated steel sheet which is stable and has excellent corrosion resistance. To achieve the above object, the present invention is a molten Al-Zn-Si-Mg coated steel sheet having a coating film, wherein the coating film 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 diffraction intensity of _Mg2Si and _MgZn2 in the coating film by X-ray diffraction method satisfies the following relationship (1): _Mg2Si (111)/ _MgZn2 (100)≦2.0…(1).

Description

Molten Al-Zn-Si-Mg plated steel, surface treated steel and painted steel

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

The molten Al-Zn type plated steel sheet represented by the 55% Al-Zn type has high corrosion resistance among the molten galvanized steel sheets because it can combine the corrosion resistance of Zn and the high corrosion resistance of Al. Therefore, the molten Al-Zn plated steel sheet is 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 and other civil engineering fields due to its excellent corrosion resistance. In recent years, the demand for molten Al-Zn coated steel plates has increased, especially in acid rain caused by air pollution, de-icing agents for roads in snowy areas, and development in coastal areas, as there is an increasing demand for materials with excellent corrosion resistance or maintenance-free materials in harsher operating environments.

The characteristics of the coating film of the molten Al-Zn system plated steel plate are that it is composed of the dendrite-shaped part (α-Al phase) of supersaturated and Zn-containing Al solidified and the Zn-Al eutectic structure existing in the dendrite gap (inter-dendrite). The α-Al phase is a multi-layered structure in the thickness direction of the coating film. Due to this characteristic film structure, the corrosion path from the surface becomes complicated, so corrosion becomes difficult to proceed. The molten Al-Zn system plated steel plate is also known to achieve better corrosion resistance than the molten zinc plated steel plate with the same coating film thickness.

For this type of molten Al-Zn coated steel plate, attempts have been made to further extend the life of the plate, and molten Al-Zn-Si-Mg coated steel plates with Mg added have been put into practical use.

As such a molten Al-Zn-Si-Mg coated steel plate, for example, Patent Document 1 discloses a molten Al-Zn-Si-Mg coated steel plate, wherein the coating film contains an Al-Zn-Si alloy containing Mg, wherein the Al-Zn-Si alloy is an alloy containing 45-60 wt% of elemental aluminum, 37-46 wt% of elemental zinc, and 1.2-2.3 wt% of Si, and the concentration of Mg is 1-5 wt%.

Furthermore, Patent Document 2 discloses a molten Al-Zn-Si-Mg coated steel plate, the purpose of which is to improve corrosion resistance by containing at least one of 2-10% Mg and 0.01-10% Ca in the coating film, and to improve the protective effect after the base steel plate is exposed.

Furthermore, Patent Document 3 discloses a molten Al-Zn-Si-Mg based plated steel plate, which is formed by a coating layer containing, by mass%, Mg: 1-15%, Si: 2-15%, Zn: 11-25%, and the remainder being Al and inevitable impurities. By making the size of intermetallic compounds such as _Mg2Si phase or _MgZn2 phase present in the plated film less than 10 μm, the corrosion resistance of the plate and the end surface is improved.

The above-mentioned molten Al-Zn coated 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 requirements for its appearance are still strong. Therefore, a technology for improving the appearance of molten Al-Zn coated steel sheet is also developed.

For example, Patent Document 4 discloses a molten Al-Zn-Si-Mg coated steel sheet that suppresses wrinkle-shaped concave-convex defects by containing 0.01-10% Sr in the coating film.

In addition, Patent Document 5 also discloses a molten Al-Zn-Si-Mg plated steel plate that suppresses the plaque defect by containing 500-3000 ppm of Sr in the coating film.

In addition, Patent Document 6 discloses a molten Al-Zn-Si-Mg plated steel sheet having both surface appearance and corrosion resistance due to the inclusion of 0.001~1.0% Sr in the plated film.

In addition, Patent Document 7 discloses a molten Al-Zn-Si-Mg-based plated steel sheet that has both surface appearance and corrosion resistance of the flat plate portion and the processed portion by containing 0.001 to 1.0% Sr in the plated film.

In addition, Patent Document 8 also discloses a molten Al-Zn-Si-Mg plated steel sheet having both surface appearance and corrosion resistance by containing 0.01-0.2% Sr in the plated film.

In addition, Patent Document 9 discloses a molten Al-Zn-Si-Mg plated steel sheet with improved corrosion resistance by controlling the Si and Mg concentrations in the plated film at a specific ratio.

In addition, regarding the above-mentioned molten Al-Zn-based plated steel sheet, when used in a severely corrosive environment, there is a problem of white rust due to corrosion of the plated film. Since the white rust degrades the appearance of the steel sheet, the development of plated steel sheet with improved white rust resistance is being carried out.

For example, Patent Document 10 discloses a molten Al-Zn-Si-Mg coated steel sheet having an appropriate mass ratio of Mg in the Si-Mg phase to the total Mg in the coating layer for the purpose of improving the rust resistance of the processed portion.

In addition, Patent Document 11 discloses a technology for improving black discoloration resistance and rust resistance by forming a chemical conversion film containing a urethane resin on the coating film of a molten Al-Zn-Si-Mg system coated steel plate.

In addition, coated steel sheets having chemical conversion films, bottom coating films, top coating films, etc. formed on the surface of molten Al-Zn-based plated steel sheets are required to be subjected to various processing such as 90-degree bending or 180-degree bending by press forming, roll forming, or embossing, and thus require long-term coating durability. In order to meet these requirements, molten Al-Zn-based plated steel sheets are known to have a coated steel sheet formed by forming a chemical conversion film containing chromate, a base coating film also containing a chromate-based rust-proof pigment, and a top coating film with excellent weather resistance such as a thermosetting polyester resin film or a fluorine resin film formed thereon.

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 to meet these requirements, for example, Patent Document 12 discloses a surface treated molten metal-plated steel material, which is to plate an aluminum-zinc alloy coating (α) containing Al, Zn, Si and Mg and adjusting the content of these elements on the surface of the steel material, and then form a film (β) with at least one compound (A) selected from titanium compounds and zirconium compounds as a film-forming component as an upper layer, and the mass ratio of the Si-Mg phase in the aluminum-zinc alloy coating (α) relative to the total amount of Mg in the coating is adjusted to be 3% or more.

[Prior Art Literature] [Patent Literature]

Patent document 1: Japanese Patent No. 5020228

Patent document 2: Japanese Patent No. 5000039

Patent document 3: Japanese Patent Publication No. 2002-12959

Patent document 4: Japanese Patent No. 3983932

Patent document 5: Japanese Patent Publication No. 2011-514934

Patent Document 6: International Publication No. 2020/179147

Patent document 7: International Publication No. 2020/179148

Patent document 8: Japanese Patent Publication No. 2020-143370

Patent Document 9: International Publication No. 2016/140370

Patent document 10: Japanese Patent No. 5751093

Patent document 11: Japanese Patent Publication No. 2019-155872

Patent document 12: Japanese Patent Publication No. 2005-169765

However, as disclosed in patent documents 1 to 3, the technology of adding Mg to the coating film may not necessarily significantly improve the corrosion resistance.

The molten Al-Zn-Si-Mg coated steel sheets disclosed in patent documents 1 to 3 achieve improved corrosion resistance only by containing Mg in the coating composition, but the characteristics of the metal phase and intermetallic compound phase constituting the coating film are not considered, and it is impossible to discuss the corrosion resistance in general. Therefore, even if the same coating bath composition is used to manufacture molten Al-Zn-Si-Mg coated steel sheets, there are differences in corrosion resistance if the corrosion promotion test is performed, and it may not be superior to Al-Zn coated steel sheets without Mg addition, which is problematic.

Similarly, in improving the appearance of coating, simply adding Sr to the coating film does not necessarily mean that wrinkle-shaped concave-convex defects can be eliminated. The molten Al-Zn-Si-Mg-based plated steel sheets disclosed in Patent Documents 4 to 8 also fail to have both corrosion resistance and appearance. In addition, since Mg is an easily oxidized element, the Mg contained in the coating bath generates oxides (scum) near the bath surface, or during molten coating, FeAl-based compounds (bottom scum) containing iron exist in the coating bath or locally at the bottom over time. Such slag adheres to the surface of the coating film, causing convex defects and also has the risk of damaging the appearance of the coating film surface.

Furthermore, when a steel sheet is plated in a bath containing Mg added to a molten Al-Zn-Si bath, it is known that in addition to the α-Al phase, _Mg2Si phase, _MgZn2 phase, and Si phase are precipitated in the coating. However, the effect of the precipitation amount or existence ratio of each phase on corrosion resistance is still unclear.

The molten Al-Zn-Si-Mg coated steel plate disclosed in Patent Document 9 achieves improved corrosion resistance by managing the concentrations of Si and Mg at a specific ratio so that no Si phase precipitates in the coating film. However, it cannot be said that the Si phase can be suppressed. Even if the formation of the Si phase in the coating film can be suppressed, excellent corrosion resistance may not be obtained. This is technically incomplete.

In addition, no technology can achieve sufficient improvement in rust resistance. Regarding the molten Al-Zn-Si-Mg coated steel sheet of Patent Document 10, although the rust resistance of the processed part and the heated flat part is described to be improved, the rust resistance of the unheated flat part is not considered, and achieving stable rust resistance is still a problem. Moreover, regarding the molten Al-Zn-Si-Mg coated steel sheet of Patent Document 11, it is expected not only to obtain stable and excellent corrosion resistance and rust resistance, but also to further improve.

Furthermore, as mentioned above, the coated steel plate is required to have long-term coating durability in various processing states such as 90-degree bending or 180-degree bending by press forming, roll forming, embossing forming, etc. However, the technology of Patent Document 12 cannot necessarily and stably obtain corrosion resistance and surface appearance after processing.

The corrosion resistance of the coated steel sheet will, of course, affect the corrosion resistance of the base coated steel sheet. As for the surface appearance, since the height difference of the wrinkle defects is tens of μm, even if the surface is smoothed by the coating, the unevenness cannot be completely eliminated, and it is not expected to improve the appearance of the coated steel sheet. In addition, since the coating becomes thinner at the convex part, there is also a concern that the corrosion resistance will be reduced locally. Therefore, in order to obtain a coated steel sheet with excellent corrosion resistance and surface appearance, it is important to improve the corrosion resistance and surface appearance of the base coated steel sheet.

In view of the above situation, the purpose of the present invention is to provide a stable molten Al-Zn-Si-Mg coated steel plate with excellent corrosion resistance.

The purpose of the present invention is to provide a surface-treated steel plate that is stable and has excellent corrosion resistance and rust resistance.

Furthermore, the purpose of the present invention is to provide a stable coated steel plate with excellent corrosion resistance and corrosion resistance of processed parts.

As a result of the active research conducted by the inventors to solve the above problems, it was found that the _Mg2Si phase, _MgZn2 phase and Si phase formed in the coating film of the molten Al-Zn-Si-Mg system coated steel sheet can change the existence ratio by increasing or decreasing the precipitation amount according to the balance of each component in the coating film or the formation conditions of the coating film, and that there is a situation where a certain phase does not precipitate according to the balance of the composition. It was also found that the corrosion resistance of the molten Al-Zn-Si-Mg system coated steel sheet changes with the existence ratio of these phases, and in particular, when the MgZn2 _phase is more than the _Mg2Si phase or the Si phase, the corrosion resistance is stably improved.

However, it is known that it is very difficult to distinguish the difference between the _Mg2Si phase, _MgZn2 phase and Si phase even when observing the coating film from the surface or cross section using a general method such as a scanning electron microscope, taking secondary electron images or reflected electron images. Observation using a transmission electron microscope, which is a method that can provide more detailed analysis, can obtain microscopic information, but it is impossible to grasp the existence ratio of the _Mg2Si , _MgZn2 and Si phases that affects the macroscopic information of corrosion resistance or appearance.

Therefore, the inventors of the present invention have conducted further active research and found that by focusing on the X-ray diffraction method, the intensity ratio of the specific diffraction peaks of the _Mg2Si phase, _MgZn2 phase and Si phase can be used to quantitatively determine the existence ratio of the phases. In addition, if the _Mg2Si phase and the _MgZn2 phase in the coating film meet the specific existence ratio, in addition to achieving stable and excellent corrosion resistance, the generation of slag can also be suppressed, and a good surface appearance can be ensured.

Furthermore, the inventors have also found that by controlling the existence ratio of _Mg2Si phase, _MgZn2 phase, Si phase, etc. in the molten Al-Zn-Si-Mg coated steel plate, the occurrence of wrinkle-like concave-convex defects can be reliably suppressed by controlling the Sr concentration in the bath, and a coated steel plate with excellent surface appearance can be obtained.

In addition, the inventors of the present invention have also examined the chemical conversion film formed on the aforementioned coating film, and have found that by making the chemical conversion film consist of a specific resin and a specific metal compound, the affinity between the chemical conversion 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 conversion film and the base coating film formed on the aforementioned coating film, and have found that by making the chemical conversion film consist of a specific resin and a specific inorganic compound, and the base coating film consist of a specific polyester resin and an inorganic compound, the barrier property and adhesion of the coating film can be improved, and even if it is free of chromate, excellent post-processing corrosion resistance can be achieved.

This invention is completed based on the above insights, and its gist is as follows.

1. A molten Al-Zn-Si-Mg coated steel plate having a coating film, wherein the coating film 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, a portion of the coating film is mechanically cut until the base steel plate is exposed and then measured in powder state by X-ray diffraction method, and the diffraction intensity of _Mg2Si and _MgZn2 in the coating film by X-ray diffraction method satisfies the following relationship (1): _Mg2Si (111)/ _MgZn2 (100)≦2.0...(1)

Mg ₂ Si (111): diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm), MgZn ₂ (100): diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm).

2. The molten Al-Zn-Si-Mg coated steel plate 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)=0...(2)

Si(111): Diffraction intensity of Si (111) plane (plane spacing d=0.3135nm).

3. The molten Al-Zn-Si-Mg coated steel plate as described in 1 or 2 above, wherein the coating further contains Sr: 0.01~1.0 mass %.

4. A molten Al-Zn-Si-Mg coated steel plate as described in any one of 1 to 3 above, wherein the Al content in the coating is 50-60% by mass.

5. A molten Al-Zn-Si-Mg coated steel plate as described in any one of 1 to 4 above, wherein the Si content in the coating is 1.0 to 3.0 mass %.

6. A molten Al-Zn-Si-Mg coated steel plate as described in any one of 1 to 5, wherein the Mg content in the coating is 1.0 to 5.0 mass %.

7. A surface treated steel plate having a coating film as described in any one of 1 to 6 above and a chemical conversion film formed on the coating film, wherein the chemical conversion film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyurethane 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.

8. A coated steel plate, wherein a coating is formed directly or via a chemical conversion coating on the coating film of any one of 1 to 6 above, wherein the chemical conversion coating contains: a resin component, which contains 30 to 50% by weight of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, and the content ratio of (a) to (b) is ((a): (b)) with a mass ratio in the range of 3:97~60:40; and an inorganic compound, which contains 2~10 mass% of a vanadium compound, 40~60 mass% of a zirconium compound and 0.5~5 mass% of a fluorine compound, The aforementioned coating film has at least a base coating film, and the base 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.

According to the present invention, a stable molten Al-Zn-Si-Mg plated steel sheet with excellent corrosion resistance can be provided.

Furthermore, according to the present invention, a surface-treated steel plate with stable properties and excellent corrosion resistance and rust resistance can be provided.

Furthermore, according to the present invention, a stable coated steel plate with excellent corrosion resistance and corrosion resistance of processed parts can be provided.

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

(Molten Al-Zn-Si-Mg plated steel plate)

The molten Al-Zn-Si-Mg coated steel plate of the present invention has a coating film on the surface of the steel plate. The coating film has the following composition: Al: 45~65 mass%, Si: 1.0~4.0 mass% and Mg: 1.0~10.0 mass%, and the rest is composed of Zn and unavoidable impurities.

The Al content in the aforementioned coating film is 45-65% by mass, preferably 50-60% by mass, based on the balance between corrosion resistance and working surface. The reason is that if the Al content in the aforementioned coating film is at least 45% by mass, Al dendritic solidification occurs, and a coating film 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 film, the corrosion path becomes complex, thereby improving the corrosion resistance of the coating film itself. 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 corrosion resistance, the Al content is preferably set 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, which reduces the corrosion resistance of the Al-Zn-Si-Mg coating. Therefore, the Al content in the aforementioned coating must be less than 65% by mass, preferably less than 60% by mass.

The main purpose of adding Si to the aforementioned coating 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 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. 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 needs to be 1.0 mass% or more. On the other hand, when the Si content in the aforementioned coating exceeds 4.0% by mass, not only the growth inhibition effect of the aforementioned interface alloy layer is saturated, but also corrosion is promoted due to the presence of excessive Si phase in the coating, so the Si content is set to 4.0% or less. In addition, the Si content in the aforementioned coating is preferably set to 3.0% or less from the perspective of inhibiting the presence of excessive Si phase. In addition, in terms of the relationship with the Mg content described later, the Si content is preferably set to 1.0~3.0% by mass from the perspective of easily satisfying the relationship (1) described later.

The coating film contains 1.0-10.0% of Mg. Since the coating film contains Mg, the Si can exist in the form of an intermetallic compound of a Mg ₂ Si phase, which can inhibit the promotion of corrosion.

Furthermore, when the aforementioned coating film contains Mg, an intermetallic compound _MgZn2 phase is also formed in the coating film, and the corrosion resistance can be further improved. When the Mg content in the aforementioned coating film is less than 1.0 mass%, sufficient corrosion resistance cannot be ensured because the solid solution of the main phase α-Al phase mainly uses Mg due to the formation of the aforementioned intermetallic compound ( _Mg2Si , _MgZn2 ). On the other hand, when the Mg content in the aforementioned coating film increases, in addition to the saturation of the corrosion resistance improvement effect, the α-Al phase is weakened and the workability is reduced, so the content is set to 10.0% or less. In addition, 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 slag during coating formation and facilitating the coating bath management. In relation to the Si content, the Mg content is preferably 3.0 mass % from the viewpoint of easily satisfying the relational expression (1) described later. Considering the compatibility with slag suppression, the Mg content is more preferably 3.0-5.0 mass %.

Furthermore, in the molten Al-Zn-Si-Mg based plated steel sheet of the present invention, the diffraction intensity of _Mg2Si and _MgZn2 in the above-mentioned coating film by X-ray diffraction method needs to satisfy the following relationship (1).

Mg ₂ Si(111)/MgZn ₂ (100)≦2.0...(1)

Mg ₂ Si (111): diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm), MgZn ₂ (100): diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm).

As mentioned above, the important thing in the present invention is to control the existence ratio of the intermetallic compounds such as _Mg2Si and _MgZn2 generated in the coating film to a specific ratio by containing the aforementioned Mg. The influence of these on corrosion resistance is still under investigation and there are still many unknowns, but the following mechanism is speculated.

When the molten Al-Zn-Si-Mg plated steel sheet is exposed to a corrosive environment, the above-mentioned intermetallic compound dissolves preferentially over the α-Al phase, and the area around the formed corrosion products becomes a Mg-rich environment. It is presumed that the formed corrosion products are not easily decomposed in such a Mg-rich environment, and as a result, the protective effect of the coating film is improved. Moreover, the protective effect of the coating film is improved because _MgZn2 is greater than _Mg2Si , so it is believed that increasing the abundance ratio of _MgZn2 in the intermetallic compound present in the above-mentioned coating film is effective.

The important thing about the ratio of _Mg2Si to _MgZn2 in the aforementioned coating is that the diffraction peak intensity obtained by X-ray diffraction method satisfies the relationship (1): _Mg2Si (111)/ _MgZn2 (100)≦2.0. However, when the ratio of _Mg2Si to _MgZn2 in the aforementioned coating does not satisfy the relationship (1), that is, when _Mg2Si (111)/ _MgZn2 (100)>2.0, since there is more _Mg2Si in the intermetallic compound present in the aforementioned coating, the aforementioned Mg-rich environment cannot be obtained near the corrosion product, and it is difficult to obtain the protective effect enhancement effect of the aforementioned coating.

Furthermore, regarding the abundance ratio of _Mg2Si and _MgZn2 in the aforementioned coating film, assuming that the composition of the coating film satisfies the range of the present invention (containing Al: 45-65 mass %, Si: 1.0-4.0 mass % and Mg: 1.0-10.0 mass %, with the remainder being Zn and inevitable impurities), when the abundance ratio of _Mg2Si and _MgZn2 does not satisfy relationship (1), the protective effect of the coating film cannot be fully obtained as a result of the present invention.

Here, in the above relationship (1), Mg ₂ Si (111) is the diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm), and MgZn ₂ (100) is the diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm).

As a method for measuring _Mg2Si (111) and _MgZn2 (100) by the aforementioned X-ray diffraction, a portion of the aforementioned 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 diffraction intensity measurement, the diffraction peak intensity of _Mg2Si corresponding to the plane spacing d=0.3668nm and the diffraction peak intensity of _MgZn2 corresponding to the plane spacing d=0.4510nm are measured, and by calculating these ratios, _Mg2Si (111)/ _MgZn2 (100) can be obtained.

In addition, the amount of coating required for powder X-ray diffraction measurement (the amount of coating cut out) is preferably 0.1 g or more, preferably 0.3 g or more, from the viewpoint of accurately measuring Mg ₂ Si (111) and MgZn ₂ (100). In addition, when the coating is cut out, steel plate components other than the coating may be included in the powder. These intermetallic compound phases are only contained in the coating and do not affect the peak strength. The reason for performing X-ray diffraction by making the coating film into powder is that when performing X-ray diffraction on the coating film formed on the coated steel plate, it is difficult to calculate the correct phase ratio due to the influence of the surface orientation of the solidified structure of the coating film.

In addition, the molten Al-Zn-Si-Mg coated steel sheet of the present invention is based on the viewpoint of more stably improving the corrosion resistance, and the diffraction intensity of Si in the aforementioned coating film by X-ray diffraction method better satisfies the following relationship (2).

Si(111)=0...(2)

Si(111): Diffraction intensity of Si (111) plane (plane spacing d=0.3135nm).

In the dissolution reaction of general Al alloys in aqueous solution, it is known that Si phase exists as a cathode site and promotes the dissolution of the surrounding α-Al phase. Therefore, the viewpoint of reducing Si phase to inhibit the dissolution of α-Al phase is also effective. Among them, as in relationship (2), the absence of Si phase film (the aforementioned Si (111) diffraction peak intensity is zero) is the best for stabilizing corrosion resistance.

The diffraction peak intensity of the Si(111) plane by X-ray diffraction can be measured by the same method as the above-mentioned method for measuring Mg ₂ Si(111) and MgZn ₂ (100).

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 the relationship (1) and relationship (2), by adjusting the balance of the Si content, the Mg content and the Al content in the aforementioned coating film, the existence ratio of _Mg2Si , _MgZn2 and Si (the diffraction intensity of _Mg2Si (111), _MgZn2 (100) and Si(111)) can be controlled. In addition, if the balance of the Si content, the Mg content and the Al content in the aforementioned coating film is necessarily set to a certain content ratio, it is not interpreted as satisfying the relationship (1) and relationship (2). For example, it is necessary to change the content ratio of Mg and Al according to the Si content (mass %).

In addition to adjusting the balance of Si content, Mg content and Al content in the aforementioned coating film, the diffraction intensity of _Mg2Si (111), _MgZn2 (100) and Si(111) can also be controlled by adjusting the conditions during coating film formation (e.g. cooling conditions after coating) to satisfy relations (1) and (2).

In addition, the molten Al-Zn-Si-Mg coated steel plate of the present invention contains Zn and inevitable impurities.

Among them, the aforementioned unavoidable impurities contain Fe. The Fe is inevitably contained in the coating bath due to the dissolution of the steel plate or the machine in the bath. When the interface alloy layer is formed, it is inevitably contained in the coating film due to the diffusion from the base steel plate. The Fe content in the coating film is usually about 0.3~2.0 mass%. Examples of other inevitable impurities are Cr, Ni, Cu, etc. There is no special restriction on the total content of the aforementioned unavoidable impurities. When contained in excess, it is possible to affect various properties of the coated steel plate, so it is better to be less than 5.0 mass% in total.

In addition, in the molten Al-Zn-Si-Mg steel sheet of the present invention, it is preferred that the aforementioned coating film contains 0.01 to 1.0 mass % Sr. By containing Sr in the aforementioned coating film, the occurrence of surface defects such as wrinkle-shaped concave-convex defects can be more reliably suppressed, and good surface appearance can be achieved.

In addition, the aforementioned wrinkle defects are wrinkle-shaped uneven defects formed on the surface of the aforementioned coating film, and white stripes are observed on the surface of the aforementioned coating film. Such wrinkle defects are easy to occur when more Mg is added to the aforementioned coating film. Therefore, the aforementioned molten-coated steel plate, by containing Sr in the aforementioned coating film, makes the Sr in the surface layer of the aforementioned coating film oxidized before Mg, and by inhibiting the oxidation reaction of Mg, the occurrence of the aforementioned wrinkle defects can be inhibited.

Furthermore, in the molten Al-Zn-Si-Mg steel sheet of the present invention, it is preferred that the ratio of _Mg2Si to _MgZn2 in the coating film satisfies the relation (1), and the coating film contains 0.01-1.0 mass % Sr. Thus, the surface appearance improvement effect of Sr can be more enjoyed. Although the reason is not clear, it is speculated that if the coating film contains more _Mg2Si , the oxidation of the coating surface layer is not easily suppressed, and the effect of improving the appearance when Sr is added is affected. Furthermore, when the Sr content in the aforementioned coating film is less than 0.01 mass %, it is difficult to obtain the effect of suppressing the occurrence of the above-mentioned wrinkle defects. If the Sr content in the aforementioned coating film exceeds 1.0 mass %, Sr is excessively incorporated into the interface alloy layer, and there is a risk that the influence on the coating adhesion, etc. will be greater than the effect of improving the appearance. Therefore, the Sr content in the aforementioned coating film is preferably 0.01~1.0 mass %.

In addition, the aforementioned coating film can improve the stability of corrosion products and play a role in delaying the progress of corrosion, just like the above-mentioned Mg, and preferably contains 0.01-10% by mass of one or more selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B. The reason why the total content of the above-mentioned components is set to 0.01-10% by mass is that a sufficient corrosion delay effect can be obtained, and the effect will not be saturated.

In addition, the coating weight of the above-mentioned coating is preferably 45-120 g/m ² per side from the viewpoint of satisfying various characteristics. 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 coating cracking 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 45-100 g/m ² .

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

The composition of the coating film can be determined by, for example, dissolving the coating film in hydrochloric acid, and then confirming the solution by ICP emission spectrometry or atomic absorption spectrometry. This method is only an example, and any method can be used without any particular limitation as long as the composition of the coating film can be accurately quantified.

Furthermore, the coating film of the molten Al-Zn-Si-Mg-based plated 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 good precision.

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

In addition, there is no particular limitation on the method of obtaining the aforementioned base steel plate. For example, in the case of the aforementioned hot-rolled steel plate, a steel plate that has undergone a hot-rolling step and a pickling step may be used, and in the case of the aforementioned cold-rolled steel plate, a cold-rolling step may be further applied to produce the steel plate. Furthermore, in order to obtain the characteristics of the steel plate, a recrystallization annealing step may also be performed before the melt-plating step.

In addition, the method for manufacturing the molten Al-Zn-Si-Mg system coated steel plate of the present invention is not particularly limited. For example, the base steel plate can be cleaned, heated, and immersed in a coating bath by a continuous molten coating device. In the heating step of the steel plate, recrystallization annealing is performed for the structural control of the base steel plate itself, and heating in a reducing environment such as a nitrogen-hydrogen environment is effective for preventing oxidation of the steel plate and reducing the trace oxide film on the surface.

Furthermore, regarding the coating bath used in manufacturing the molten Al-Zn-Si-Mg coated steel sheet of the present invention, as described above, since the composition of the coating film is generally equal to that of the coating bath, a coating bath containing 45-65% by mass of Al, 1.0-4.0% by mass of Si, and 1.0-10.0% by mass of Mg, with the remainder being Zn, Fe, and unavoidable impurities, can be used.

In addition, the bath temperature of the aforementioned coating bath is not particularly limited, but is preferably within the temperature range of (melting point + 20°C) ~ 650°C.

The reason why the lower limit of the bath temperature is set to the melting point + 20°C is that in order to carry out the melt plating treatment, the bath temperature must be above the solidification point, and the reason why it is set to the melting point + 20°C is to prevent solidification caused by a local drop in the bath temperature of the plating bath. On the other hand, the reason why the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, the plating film will be difficult to cool rapidly, and there is a risk that the interface alloy layer formed between the plating film and the steel plate will become thicker.

Furthermore, there is no particular restriction on the temperature of the base steel plate immersed in the coating bath (immersion plate temperature), but from the perspective of ensuring the coating characteristics in the aforementioned continuous melt coating operation and preventing the bath temperature from changing, it is preferably controlled within ±20°C relative to the temperature of the aforementioned coating bath.

In addition, the immersion time of the steel plate in the aforementioned coating bath is more than 0.5 seconds. If it is less than 0.5 seconds, there is a risk that a sufficient coating film cannot be formed on the surface of the aforementioned base steel plate. There is no particular upper limit on the immersion time, but since the interface alloy layer formed between the coating film and the steel plate may become thicker when the immersion time is longer, it is better to be within 8 seconds.

In addition, the molten Al-Zn-Si-Mg plated steel plate can form a coating directly or through an intermediate layer on the aforementioned plated film, depending on 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. Examples include roller coating, curtain coating, spray coating, etc. After coating the coating 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, regarding the aforementioned intermediate layer, there is no particular limitation as long as it is a layer formed between the coating film of the molten-coated steel plate and the aforementioned 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 conversion film formed on the coating film.

Among them, the composition of the aforementioned coating is the same as the coating of the molten Al-Zn-Si-Mg system coated steel plate of the present invention.

The surface treated steel plate of the present invention forms a chemical conversion film on the aforementioned film.

Furthermore, the aforementioned chemical conversion 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 chemical conversion film is characterized by containing at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyurethane 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 chemical conversion film on the coating film, in addition to improving the affinity with the coating film and uniformly forming the chemical conversion film on the coating film, the anti-rust effect and barrier effect of the chemical conversion film can also be improved. As a result, the stable corrosion resistance and rust resistance of the surface treated steel plate of the present invention can be achieved.

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

As the aforementioned epoxy resin, for example, bisphenol A type, bisphenol F type, novolac type epoxy resins, etc., 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 for the aforementioned 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 aforementioned acrylic resins include polyacrylic acid and its copolymers, polyacrylate and its copolymers, polymethacrylic acid and its copolymers, polymethacrylate and its copolymers, urethane-acrylic acid copolymers (or urethane-modified acrylic resins), styrene-acrylic acid copolymers, etc., and these resins modified by other alkyd resins, epoxy resins, phenolic resins, etc. may be used.

As the aforementioned acrylic silicone resin, for example, a resin having a hydrolyzable alkoxysilyl group on the side chain or at the end of an acrylic copolymer as the main agent is added with a hardener. In addition, when using acrylic silicone resin, in addition to corrosion resistance, excellent weather resistance can also be expected.

As for the aforementioned alkyd resin, examples include 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, etc.

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. are used. Specifically, the aforementioned polyester is exemplified by polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc. And these polyester resins modified with acrylic acid can also be used.

Examples of the aforementioned polyalkane resins include ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, vinyl copolymers of carboxyl-modified polyolefin resins, ethylene-unsaturated carboxylic acid copolymers, vinyl isomers, etc. Furthermore, these resins modified with other alkyd resins, epoxy resins, phenolic resins, etc. may be used.

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

Examples of the aforementioned fluororesins include fluoroolefin polymers, or copolymers of fluoroolefins and alkyl vinyl ethers, cycloalkyl vinyl ethers, carboxylic acid-modified vinyl esters, hydroxy alkyl allyl ethers, tetrafluoropropyl vinyl ethers, etc. When using such fluororesins, not only corrosion resistance but also excellent weather resistance and excellent hydrophobicity can be expected.

Furthermore, in order to improve corrosion resistance and processability, it is particularly preferred to use a hardener. As a hardener, urea resins (butylated urea resins, etc.), melamine resins (butylated melamine resins, butylated melamine resins, etc.), butylated urea melamine resins, benzoguanamine resins, etc. amino resins, blocked isocyanates, oxazoline compounds, phenol resins, etc. can be appropriately 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 conversion film, corrosion resistance and sweat resistance can be improved. The aforementioned P compound is a compound containing P, and may contain, for example, one or more selected from inorganic phosphoric acid, organic phosphoric acid and salts thereof.

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 at least one selected from phosphoric acid, dihydrogen phosphate, hydrogen phosphate, phosphate, pyrophosphoric acid, pyrophosphate, tripolyphosphoric acid, tripolyphosphate, phosphorous acid, phosphite, hypophosphorous acid, and hypophosphite. And as the aforementioned organic phosphoric acid, it is preferred to use phosphonic acid (phosphonic acid compound). In addition, as the aforementioned phosphonic acid, it is preferred to use at least one 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 preferably at least one selected from an alkali metal salt and an alkaline earth metal salt.

When the chemical conversion treatment liquid containing the P compound is applied to the molten Al-Zn-Si-Mg 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, which are 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 aforementioned concentrated layer, the chemical conversion film and the surface of the plated film are firmly bonded, and the adhesion of the chemical conversion film is improved.

The concentration of the P compound in the chemical conversion treatment solution is not particularly limited and can be set to 0.25 mass% to 5 mass%. When the concentration of the P compound is less than 0.25 mass%, not only the etching effect is insufficient, the adhesion with the coating interface is reduced, the corrosion resistance of the plane part is reduced, but also the corrosion resistance and sweat resistance of the defective part, the cut end face part, and the damaged part of the coating film caused by processing are also 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 P compound exceeds 5 mass%, not only the life of the chemical conversion treatment solution is shortened, but also the appearance of the film formed is prone to be uneven, and the amount of P eluted from the chemical conversion film increases, and there is a possibility that the blackening resistance is reduced. Based on the same viewpoint, the concentration of the P compound is preferably 3.5 mass% or less, and more preferably 2.5 mass% or less. Regarding the content of the P compound in the chemical conversion film, for example, a chemical conversion treatment solution with a P compound concentration of 0.25 mass% to 5 mass% can be applied and dried, so that the P adhesion amount in the chemical conversion film after drying is 5 to 100 mg/ ^m2 .

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

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

As the trialkoxysilane, any one can be used without particular limitation. Preferably, 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) is used. Examples of the trialkoxysilane include trimethoxysilane, triethoxysilane, methyltriethoxysilane, and the like.

As the aforementioned tetraalkoxysilane, any one can be used without particular limitation. Preferably, 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) is used. Examples of such tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane.

As the aforementioned silane coupling agent, any one can be used without particular limitation. Examples include γ-glycidyloxypropyl trimethoxysilane, γ-glycidyloxypropyl methyldiethoxysilane, γ-glycidyloxypropyl triethoxysilane, γ-aminopropyl trimethoxysilane, γ-aminopropyl methyldiethoxysilane, γ-aminopropyl triethoxysilane, γ-methacryloxypropyl trimethoxysilane, γ-methacryloxypropyl triethoxysilane, γ-butyl propyl methyldimethoxysilane and γ-butyl propyl trimethoxysilane, vinyl triethoxysilane, γ-isocyanate propyl triethoxysilane, etc.

Furthermore, by containing the aforementioned Si compound in the chemical conversion film, the Si compound is dehydrated and condensed to form an amorphous chemical conversion film with a siloxane bond having a high barrier effect for shielding corrosion factors. Furthermore, by combining with the aforementioned resin, a chemical conversion film with 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 produced by processing, and the composite effect with the aforementioned coating film also has the effect of inhibiting the corrosion of the base steel plate. Based on the viewpoint of a higher effect of forming a stable corrosion product, it is preferred to use at least one of colloidal silicon oxide and dry silicon oxide as the aforementioned Si compound.

The concentration of the aforementioned Si compound in the chemical conversion treatment liquid used to form the aforementioned chemical conversion film is 0.2 mass% to 9.5 mass%. If the concentration of the Si compound in the aforementioned chemical conversion treatment liquid is 0.2 mass% or more, a barrier effect caused by siloxane bonds can be obtained, and as a result, in addition to the corrosion resistance of the planar part, the corrosion resistance of the defective part, the cut part, and the damaged part caused by processing, etc. and the sweat resistance are improved. Moreover, if the concentration of the aforementioned Si compound is 9.5 mass% or less, the life of the chemical conversion treatment liquid can be extended. The concentration of Si compound is set to 0.2 mass%~9.5 mass% of chemical conversion treatment solution. By coating and drying, the Si adhesion amount in the dried chemical conversion film can be 2~95mg/ ^m2 .

By including the aforementioned Co compound and the aforementioned Ni compound in the aforementioned chemical conversion film, the blackening resistance can be improved. This is believed to be because Co and Ni have the effect of slowing down the dissolution of water-soluble components from the film in a corrosive environment. Moreover, the aforementioned Co and the aforementioned Ni are elements that are more difficult to oxidize than Al, Zn, Si, and Mg. Therefore, by concentrating at least one of the aforementioned Co compound and the aforementioned Ni compound at the interface between the aforementioned chemical conversion film and the aforementioned coating film (forming a concentrated layer), the concentrated layer becomes a barrier to corrosion, and the blackening resistance can be improved.

By using a chemical conversion treatment solution containing the aforementioned Co compound, Co can be contained in the aforementioned chemical conversion film and incorporated 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.

Furthermore, by using a chemical conversion treatment solution containing the aforementioned Ni compound, Ni can be contained in the aforementioned chemical conversion film and incorporated into the aforementioned concentration 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 chemical conversion treatment solution is not particularly limited, but the total can be 0.25 mass% to 5 mass%. When the concentration of the 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 plane portion is reduced, but also the corrosion resistance of the defective portion, the cut end portion, and the coating film damaged due to processing, etc. may 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 Co compound and/or the Ni compound exceeds 5 mass%, the appearance of the film when it is formed is prone to be uneven, and there is a risk of reduced corrosion resistance. Based on the same viewpoint, it is preferably 4.0 mass % or less, and more preferably 3.0 mass % or less. The chemical conversion treatment solution having a total concentration of 0.25 mass % to 5 mass % of the Co compound and/or Ni compound can be applied and dried to make the total amount of Co and Ni in the dried chemical conversion film 5 to 100 mg/m ² .

The aforementioned Al compound, the aforementioned Zn compound, and the aforementioned Mg compound can be contained in the chemical conversion treatment solution to form a concentration layer containing at least one selected from Al, Zn, and Mg on the coating film side of the aforementioned chemical conversion film. The formed concentration layer can improve corrosion resistance.

Furthermore, the aforementioned Al compound, the aforementioned Zn compound, and the aforementioned Mg compound are not particularly limited if they are compounds containing Al, Zn, and Mg, 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 are given.

As the aforementioned Zn compound, for example, one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide and zinc hydroxide.

As the aforementioned Mg compound, for example, one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide and magnesium hydroxide.

The total concentration of Al compounds, Zn compounds and/or Mg compounds in the chemical conversion treatment solution used to form the aforementioned chemical conversion film is preferably 0.25 mass% to 5 mass%. If the aforementioned total concentration is 0.25 mass% or more, the aforementioned concentration layer can be formed more effectively, and the corrosion resistance can be further improved as a result. On the other hand, if the aforementioned total concentration is less than 5 mass%, the appearance of the chemical conversion film will be more uniform, and the corrosion resistance of the damaged part of the plated film generated by the plane part or defect part, processing, etc. will be further improved.

Since the aforementioned V compound is contained in the aforementioned chemical conversion film, V can be appropriately dissolved in a corrosive environment, and combined with zinc ions of the coating 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 of not only the flat part of the steel plate, but also the defective part, the damaged part of the coating film caused by processing, and the corrosion from the cut end face to the flat part.

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

The V compound in the chemical conversion treatment solution used to form the aforementioned chemical conversion film is preferably 0.05 mass% to 4 mass%. If the concentration of the aforementioned 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 the coating film damaged parts caused by defects, cut end faces, and processing is improved. On the other hand, when the concentration of the aforementioned V compound exceeds 4 mass%, the appearance of the chemical conversion film is prone to be 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 plate can be improved. The aforementioned Mo compound is a compound containing Mo, which can be obtained by adding one or both of molybdenum acid and molybdenum salt to the chemical conversion treatment solution.

The aforementioned molybdate salt may be, for example, one or more selected from sodium molybdate, potassium molybdate, magnesium molybdate, and zinc molybdate.

The concentration of 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 can be further inhibited, 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 aforementioned Zr compound and the aforementioned Ti compound in the aforementioned chemical conversion film, the chemical conversion film can be prevented from becoming porous and the film can be made dense. Therefore, it is difficult for corrosion factors to penetrate the aforementioned chemical conversion film, and the corrosion resistance can be improved.

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

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

By including the aforementioned Ca compound in the aforementioned chemical conversion film, the corrosion rate can be reduced.

The aforementioned Ca compound is a compound containing Ca, and examples thereof include Ca oxide, Ca nitrate, Ca sulfate, and intermetallic compounds containing Ca. More specifically, examples of the aforementioned 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 aforementioned Ca compound in the aforementioned chemical conversion film is not particularly limited.

Furthermore, the aforementioned chemical conversion film may contain various known ingredients commonly used in the field of coatings as needed. Examples include 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, various additives, coloring pigments, physical pigments, brighteners, etc., hardening catalysts, organic solvents, lubricants, etc.

In addition, the surface treated steel plate of the present invention preferably has the aforementioned chemical conversion film that does not contain harmful components such as hexavalent chromium, trivalent chromium, and fluorine. Since the chemical conversion treatment liquid used to form the aforementioned chemical conversion film does not contain such harmful components, it is highly safe and has a relatively small environmental load.

The amount of the chemical conversion film is not particularly limited. For example, from the viewpoint of ensuring more reliable corrosion resistance and preventing the chemical conversion film from peeling, the amount of the chemical conversion film is preferably 0.1 to 3.0 g/m ² , and more preferably 0.5 to 2.5 g/m ² . When the amount of the chemical conversion film is 0.1 g/m ² or more, corrosion resistance can be more reliably ensured, and when the amount of the chemical conversion film is 3.0 g/m ² or less, cracking and peeling of the chemical conversion film can be prevented.

The amount of the chemically converted film can be determined by appropriately selecting a method from known methods such as analyzing the film with fluorescent X-rays to determine the amount of an element with a known content in the film.

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

In addition, the surface treated steel plate of the present invention can also form a coating on the aforementioned chemical conversion film as needed.

(Coated steel plate)

The coated steel plate of the present invention is a coated steel plate formed by directly or through a chemical conversion film on a plated film.

Among them, the composition of the aforementioned coating is the same as the coating of the molten Al-Zn-Si-Mg system coated steel plate of the present invention.

The coated steel plate of the present invention can form a chemical conversion film on the aforementioned coating film.

Furthermore, the aforementioned chemical conversion 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 application and required performance.

Furthermore, the coated steel plate of the present invention is characterized in that the chemical conversion film contains: a resin component, which contains 30-50% by mass of (a): anionic polyurethane resin having an ester bond and (b): epoxy resin having a bisphenol skeleton, and the content ratio of (a) to (b) ((a): (b)) is in the range of 3:97-60:40 by mass; and an inorganic compound, which contains 2-10% by mass of a vanadium compound, 40-60% by mass of a zirconium compound and 0.5-5% by mass of a fluorine compound.

By forming the above-mentioned chemical conversion film on the plated film, the strength and adhesion of the chemical conversion film can be improved, and the corrosion resistance can also be improved.

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

Regarding the aforementioned (a) anionic polyurethane resin having an ester bond, an example is a resin obtained by copolymerizing dihydroxymethyl alkanoic acid with a reaction product of polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups. Furthermore, by dispersing in a liquid such as water by a known method, a chemical conversion treatment liquid can be obtained.

Examples of the aforementioned polyester polyol include polyesters obtained by dehydration condensation reaction of acid components such as diol components and ester-forming derivatives of hydroxycarboxylic acids, polyesters obtained by ring-opening polymerization of cyclic ester compounds such as ε-caprolactone, and copolymerized polyesters thereof.

Examples of the aforementioned polyisocyanate include aromatic polyisocyanate, aliphatic polyisocyanate, alicyclic polyisocyanate, etc. Examples of the aforementioned 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 their derivatives (e.g., prepolymers obtained by reaction with polyols, modified polyisocyanates of carbodiimide compounds of diphenylmethane diisocyanate, etc.), etc.

Furthermore, when the aforementioned polyester polyol reacts with the aforementioned diisocyanate or polyisocyanate to form urethane, the aforementioned (a) anionic polyurethane resin having an ester bond can be obtained by, for example, copolymerizing dihydroxymethylalkanoic acid, self-emulsifying and water-soluble (water-dispersing). In this case, the dihydroxymethylalkanoic acid is exemplified by 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, known epoxy resins can be used. Examples include, for example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, bisphenol S type epoxy resin, etc. 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 known method to obtain a chemical conversion treatment liquid.

The aforementioned resin component functions as an adhesive for the aforementioned chemical conversion film. The aforementioned (a) anionic polyurethane resin with an ester bond constituting the adhesive has flexibility, so it can play an effect of making the chemical conversion film difficult to be destroyed (peeled off) during processing. The aforementioned (b) epoxy resin with a bisphenol skeleton can play an effect of improving the adhesion with the underlying zinc-coated steel plate and the upper base coating film.

The aforementioned resin components contain 30-50% by mass in total in the aforementioned chemical conversion film. When the content of the aforementioned resin components is less than 30% by mass, the adhesive effect of the chemical conversion film is reduced, and when it exceeds 50% by mass, the functions caused by the inorganic components shown below, such as the inhibitor effect, are reduced. Based on the same viewpoint, the content of the aforementioned resin components in the aforementioned chemical conversion film is preferably 35-45% by mass.

Furthermore, the aforementioned resin component must be a ratio of the aforementioned (a) anionic polyurethane resin having an ester bond to the aforementioned (b) epoxy resin having a bisphenol skeleton ((a):(b)) in the range of 3:97~60:40 by mass ratio. If the aforementioned (a):(b) is outside the above range, the flexibility or adhesion of the chemically treated film will decrease, and sufficient corrosion resistance cannot be obtained. Based on the same viewpoint, the aforementioned (a):(b) is preferably 10:90~55:45.

In addition, the aforementioned resin component may include resins other than the aforementioned (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton (other resin components) according to the required performance. The aforementioned other resin components are not particularly limited, and for example, at least one selected from acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyurethane resins, amino resins and fluororesins 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 conversion treatment film can be more reliably obtained.

Furthermore, the chemical conversion film contains 2-10% by mass of a vanadium compound, 40-60% by mass of a zirconium compound, and 0.5-5% by mass of a fluorine compound as inorganic compounds.

By including these compounds, the corrosion resistance of the chemical conversion film can be improved.

The aforementioned vanadium compound is added to the chemical conversion treatment solution to act as a rustproofing agent (inhibitor). Since the aforementioned vanadium compound is contained in the aforementioned chemical conversion 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.

Examples of the vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, oxyvanadium trichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadium acetylacetonate, vanadium acetylacetonate, etc. Among them, it is particularly desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

Furthermore, the content of the vanadium compound in the chemical conversion treatment film is 2-10% by mass. When the content of the vanadium compound in the chemical conversion treatment film is less than 2% by mass, the corrosion resistance is reduced due to insufficient inhibitor effect. 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 chemical conversion film can be expected to improve the strength and corrosion resistance of the chemical conversion film by reacting with the plated metal and coexisting with the resin component. Furthermore, the zirconium compound itself helps to form a dense chemical conversion film, and since it is rich in coating properties, a barrier effect can be expected.

Examples of the aforementioned zirconium compound include zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride, and neutralized salts thereof.

In addition, the content of zirconium compound in the aforementioned chemical conversion treatment film is 40~60% by mass. The reason is that when the content of 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. When the content of 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 strict processing.

The aforementioned fluorine compound is contained in the aforementioned chemical conversion film and acts as an adhesion imparting agent with the coating film. As a result, the corrosion resistance of the aforementioned chemical conversion film can be improved.

As the aforementioned fluorine compound, 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, ammonium fluoride, or fluoride salts such as sodium fluoride and potassium fluoride are preferably used.

In addition, the content of the fluorine compound in the aforementioned chemical conversion treatment film is 0.5~5% by mass. The reason is that 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 when the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion treatment film is reduced.

In addition, the amount of the chemical conversion film is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance and improving the adhesion of the chemical conversion film, the amount of the chemical conversion film is preferably 0.025 to 0.5 g/m ² . When the amount of the chemical conversion film is 0.025 g/m ² or more, corrosion resistance can be more reliably ensured, and when the amount of the chemical conversion film is 0.5 g/m ² or less, peeling of the chemical conversion film can be suppressed.

The amount of the chemically converted film can be obtained by appropriately selecting a method from existing methods such as analyzing the film with fluorescent X-rays to determine the amount of an element with a known content in the film.

In addition, the method for forming the aforementioned chemical conversion film is not particularly limited and can be appropriately selected according to the required performance or manufacturing equipment. For example, the chemical conversion treatment liquid is continuously applied to the aforementioned coating film by a roller coater, and then dried at a plate temperature of about 60 to 200°C (peak metal temperature: PMT) using hot air or induction heating. The aforementioned chemical conversion treatment liquid can be applied by known methods such as windless spray, electrostatic spray, curtain coater, etc. in addition to the roller coater. In addition, the aforementioned chemical conversion film can be either a single-layer film or a multi-layer film without any special restrictions as long as it contains the aforementioned resin and the aforementioned metal compound.

The coated steel plate of the present invention, as described above, forms a coating film on the coating film directly or through a chemical conversion film, and the coating film has at least a base coating film.

Furthermore, the aforementioned base coating film in the present invention 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 base coating film can improve the adhesion of the coating film and improve the corrosion resistance by containing the aforementioned polyester resin having urethane bonds and the aforementioned inorganic compound.

The aforementioned base coating film 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 base coating film is not easy to crack during processing, and because of its high affinity with the chemical conversion treatment film containing the urethane resin, it is particularly helpful to improve the corrosion resistance of the processed part.

In addition, the "main component" here refers to the component with the highest content among all the components in the base coating film.

As the aforementioned polyester resin having urethane bonds, known resins such as resins obtained by reacting polyester polyol with diisocyanate or polyisocyanate having two or more isocyanate groups can be used. In addition, a resin obtained by reacting the aforementioned polyester polyol with the aforementioned diisocyanate or polyisocyanate in a state where the hydroxyl group is excessive (urethane-modified polyester resin) and curing with blocked polyisocyanate can also be used.

Furthermore, the aforementioned polyester polyol can be obtained by a known method utilizing the dehydration condensation reaction of a polyol component and a polyacid component.

Examples of the aforementioned polyols include diols and trivalent or higher polyols. 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, etc. Moreover, examples of the aforementioned trivalent or higher polyols include glycerol, trihydroxymethylethane, trihydroxymethylpropane, pentaerythritol, dipentaerythritol, etc. These polyols may be used alone or in combination of two or more.

The aforementioned polyacid uses a common polycarboxylic acid, but a monofatty acid may 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, glutaric 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, hexahydroterephthalic acid, and the like. These polyacids may be used alone or in combination of two or more.

Examples of the aforementioned polyisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethyl hexamethylene diisocyanate, dimer acid diisocyanate, and aromatic diisocyanates such as xylene diisocyanate (XDI), meta-xylene diisocyanate, toluene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), and cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, hydrogenated MDI, and their adducts, urea bodies, isocyanurate bodies, etc. These polyisocyanates may be used alone or in combination of two or more.

In addition, the hydroxyl group value of the aforementioned polyester resin having a urethane bond is not particularly limited. Based on the viewpoints of solvent resistance, processability, etc., it is preferably 5~120mgKOH/g, more preferably 7~100mgKOH/g, and even more preferably 10~80mgKOH/g.

Furthermore, the number average molecular weight of the aforementioned polyester resin having urethane bonds is preferably 500-15,000, more preferably 700-12,000, and even more preferably 800-10,000, based on the viewpoints of solvent resistance and processability.

In the aforementioned base coating film, the content of the aforementioned polyester resin having a urethane bond is preferably 40-88% by mass. When the content of the aforementioned polyester resin having a urethane bond is less than 40% by mass, there is a possibility that the adhesive function of the base coating film is reduced. On the other hand, when the content of the aforementioned polyester resin having a urethane bond exceeds 88% by mass, there is a possibility that the functions of the inorganic substances shown below, such as the inhibitor effect, are 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. Among them, tetravalent vanadium compounds or tetravalent vanadium compounds obtained by reduction or oxidation are particularly preferred.

The vanadium compound added to the aforementioned base coating film may be the same or different from the vanadium compound added to the aforementioned chemical conversion treatment film. The vanadium acid compound is believed to be a vanadium acid ion that slowly dissolves due to moisture intruding from the outside and reacts with ions on the surface of the zinc-plated steel plate to form a passive film with good adhesion, protecting the exposed metal part and exhibiting a rust-proof effect.

The content of the aforementioned vanadium compound in the aforementioned base coating film is not particularly limited, but based on the viewpoint of both corrosion resistance and moisture resistance, it is preferably 4 to 20% by mass. When the content of the aforementioned vanadium compound is less than 4% by mass, there is a possibility that the inhibitor effect is reduced, resulting in reduced corrosion resistance. When the content of the aforementioned vanadium compound exceeds 20% by mass, there is a possibility that the moisture resistance of the base coating film is reduced.

Regarding the phosphoric acid compound which is one of the aforementioned inorganic compounds, it also functions as an inhibitor. As the aforementioned phosphoric acid compound, for example, phosphoric acid, ammonium salt of phosphoric acid, alkaline metal salt of phosphoric acid, alkaline earth metal salt of phosphoric acid, etc. can be used. In particular, alkaline metal salt of phosphoric acid such as calcium phosphate can be preferably used.

The content of the aforementioned phosphate compound in the aforementioned base coating film is not particularly limited, but based on the viewpoint of both corrosion resistance and moisture resistance, it is preferably 4 to 20% by mass. When the content of the aforementioned phosphate compound is less than 4% by mass, there is a risk that the inhibitor effect is reduced, resulting in reduced corrosion resistance. When the content of the aforementioned phosphate compound exceeds 20% by mass, there is a risk that the moisture resistance of the base coating film is reduced.

Magnesium oxide, one of the aforementioned inorganic compounds, has the effect of generating a product containing Mg as a sparingly soluble magnesium salt during initial corrosion, thereby achieving stabilization and improving corrosion resistance.

The content of the aforementioned magnesium oxide in the aforementioned base coating film is not particularly limited, but based on the viewpoint of both corrosion resistance and corrosion resistance of the processed part, it is preferably 4 to 20% by mass. When the content of the aforementioned magnesium oxide is less than 4% by mass, there is a risk that the above-mentioned effect is reduced, resulting in reduced corrosion resistance. When the content of the aforementioned magnesium oxide exceeds 20% by mass, there is a risk that the flexibility of the aforementioned base coating film is reduced, resulting in reduced corrosion resistance of the processed part.

Furthermore, the aforementioned base coating film may also contain ingredients other than the aforementioned polyester resin having urethane bonds and inorganic compounds.

For example, a crosslinking agent is used when forming a base coating film. The crosslinking agent is a crosslinking coating film formed by reacting with the polyester resin having a urethane bond, 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 be used in combination. Among them, based on the viewpoint of the corrosion resistance of the processed part of the obtained coated steel plate, it is preferred to use a blocked polyisocyanate compound, etc. Examples of the blocked polyisocyanate include polyisocyanate compounds 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 base coating film may also contain various known ingredients commonly used in the coating field as needed. Specific examples include 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 aforementioned base coating film is preferably 1.5μm or more. The reason is that by making the thickness of the aforementioned base coating film 1.5μm or more, the corrosion resistance can be improved more reliably, and the adhesion of the top coating film formed on the chemical conversion treatment film or the base coating film can be improved.

There is no particular restriction on the method of forming the aforementioned base coating film. In addition, regarding the coating method of the coating composition constituting the aforementioned base coating film, it is preferably coated by a roller coating, a curtain coating, or the like. After the aforementioned coating composition is coated, it is baked by heating means such as hot air heating, infrared heating, and induction heating to obtain a base coating film. The aforementioned baking process usually sets the maximum plate temperature to about 180~270℃ and can be performed within this temperature range for about 30 seconds to 3 minutes.

Furthermore, regarding the coating film constituting the coated steel plate of the present invention, it is preferred that the coating film is formed on the aforementioned bottom coating film, and then a top coating film is formed.

The aforementioned top coating film can not only give the coated 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.

There is no particular restriction on the composition of the aforementioned top coating film, and the material or thickness can be appropriately selected according to the required performance.

For example, the top coating film can be formed using polyester resin coating, silicone polyester resin coating, polyurethane resin coating, acrylic resin coating, fluororesin coating, etc.

Furthermore, the aforementioned top 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 top coating film is preferably 5~30μm. When the thickness of the top coating film is 5μm or more, the color tone appearance can be more reliably stabilized, and when the thickness of the top coating film is 30μm or less, the reduction in processability (occurrence of cracks in the top coating film) can be more reliably suppressed.

The coating method of the coating composition used to form the aforementioned top coating film is not particularly limited. The aforementioned coating composition can be coated by methods such as roller coating, curtain coating, etc. After the aforementioned coating composition is coated, it is baked by heating means such as hot air heating, infrared heating, induction heating, etc. to form a top coating film. The aforementioned baking treatment usually sets the maximum plate temperature to about 180~270℃ and is carried out within this temperature range for about 30 seconds to 3 minutes.

[Implementation example]

<Example 1: Samples 1~44>

A cold-rolled steel plate with a thickness of 0.8 mm manufactured by a common method was used as a base steel plate. Annealing and plating treatments were performed in a melt-plated plating simulator manufactured by RHESCA (Co., Ltd.) to produce samples 1 to 44 of melt-plated steel plates under the conditions shown in Table 1.

In addition, regarding the composition of the coating bath used for the production of the molten-coated steel plate, the composition of the coating bath was varied in the range of Al: 30~75 mass%, Si: 0.5~4.5 mass%, Mg: 0~10 mass%, and Sr: 0.00~0.15 mass% in order to obtain the coating film composition of each sample shown in Table 1. Moreover, the bath temperature of the coating bath was 590°C when Al: 30~60 mass%, and 630°C when Al: exceeded 60 mass%, and the coating immersion plate temperature of the base steel plate was controlled to be the same temperature as the coating bath temperature. In addition, the coating treatment was performed under the condition that the plate temperature was cooled to a temperature range of 520~500°C within 3 seconds.

In addition, the adhesion amount of the coating film was controlled to be 85±5 g/m ² per single side for samples 1 to 41, and to be 51 to 125 g/m ² per single side for samples 42 to 44.

(Evaluation)

The following evaluations were performed on each sample of the molten-coated steel plate obtained as described above. The evaluation results are shown in Table 1.

(1) Composition of the coating film (adhesion amount, composition, X-ray diffraction intensity)

，以膠帶密封非測定面後，以JIS H 0401：2013所示之鹽酸與六亞甲基四胺之混合液溶解剝離鍍敷，自剝離前後之樣品質量差，算出鍍敷皮膜之附著量。算出結果、所得鍍敷皮膜之附著量示於表1。 For each sample after coating, punch 100mm

After sealing the non-measurement surface with tape, the coating was stripped 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 difference in sample mass before and after stripping. The calculation results and the adhesion amount of the coating film obtained are shown in Table 1.

Then, the stripping liquid is filtered and the filtrate and solid content are analyzed separately. Specifically, the filtrate is analyzed by ICP emission spectrometry to quantify the components other than insoluble Si.

After the solids were dried and ashed in a heating furnace at 650°C, sodium carbonate and sodium tetraborate were added to melt. The melt was then dissolved with hydrochloric acid, and the insoluble Si was quantified by ICP emission spectrometry analysis of the solution. The Si concentration in the coating film is the sum of the soluble Si concentration obtained by the filter solution analysis and the insoluble Si concentration obtained by the solid content analysis. The calculated results and the composition of the coating film are shown in Table 1.

Furthermore, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film on the evaluation symmetric surface was mechanically cut 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.) using X-ray: Cu-Kα (wavelength = 1.54178Å), kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, 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 base intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d = 0.3668nm), the diffraction intensity of the (100) plane of _MgZn2 (plane spacing d = 0.4510nm), and the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135nm) were measured. The measurement results are shown in Table 1.

(2) Corrosion resistance evaluation

Each sample of the obtained molten-coated steel plate was cut into a size of 120mm×120mm, and the range of 10mm from each edge of the evaluation target surface and the end face of the sample and the evaluation non-target surface were sealed with tape, and the evaluation target surface was exposed in a size of 100mm×100mm, and used as the evaluation sample. In addition, the evaluation sample was made of 3 identical ones.

For the three evaluation samples prepared as above, the corrosion promotion test was carried out using the cycle shown in Figure 1. The corrosion promotion test started from wetting and continued for 300 cycles. The corrosion reduction of each sample was measured using the method described in JIS Z 2383 and ISO8407, and the evaluation was performed based on the following criteria. The evaluation results are shown in Table 1.

◎: The corrosion loss of the three samples is less than 45g/ ^m2

○: The corrosion loss of the three samples is less than 90g/ ^m2

×: The corrosion loss of more than one sample exceeds 90g/ ^m2

(3) Surface appearance

For each sample of the obtained molten-coated steel plate, the surface of the coating film was visually observed.

Next, the observation results were evaluated based on the following benchmarks. The evaluation results are shown in Table 1.

◎: No wrinkle defects were observed

○: Wrinkle defects are only observed within 50mm from the edge

×: Wrinkle defects were observed outside the range of 50mm from the edge

(4) Processability

Each sample of the obtained molten-coated steel plate was cut into a size of 70mm×150mm, and then 8 plates of the same thickness were sandwiched inside and bent 180° (8T bending). After bending, SELLOTAPE (registered trademark) glass tape was strongly adhered to the outer surface of the bent part and then peeled off. The surface state of the coating film on the outer surface of the bent part and the presence of coating film adhesion (peeling) on the surface using tape were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 1.

○: No cracking or peeling was observed on the coated film

△: There are cracks in the coating film, but no peeling is observed

×: Cracks and peeling were observed on the coating film at the same time

(5) Bath stability

When manufacturing various samples of molten-coated steel plates, the bath surface state of the coating bath was visually confirmed and compared with the bath surface of the coating bath used in manufacturing molten Al-Zn-based coated steel plates (bath surface without Mg oxide). The evaluation was based on the following criteria, and the evaluation results are shown in Table 1.

○: Same as the molten Al-Zn plating bath (55 mass% Al-balance Zn- 1.6 mass% bath)

△: Compared with the molten Al-Zn coating bath (55 mass% Al-the rest Zn-1.6 mass% bath), there are more white oxides

×: Black oxide formation was observed in the plating bath

From the results in Table 1, it can be seen that the samples of the present invention show a good balance in terms of corrosion resistance, surface appearance, processability and bath stability compared with the samples of the comparative examples.

<Implementation Example 2: Samples 1~112>

(1) A cold-rolled steel plate with a thickness of 0.8 mm manufactured by a common method was used as the base steel plate. The annealing and plating treatments were performed using a melt-plated plating simulator manufactured by RHESCA (Co., Ltd.) to produce melt-plated steel plate samples with the coating film conditions shown in Tables 3 and 4.

In addition, regarding the composition of the coating bath used for the production of the molten-coated steel plate, the coating bath composition was varied in the range of Al: 30~75 mass%, Si: 0.5~4.5 mass%, Mg: 0~10 mass%, and Sr: 0.00~0.15 mass% in order to obtain the coating film composition of each sample shown in Table 2. Moreover, the bath temperature of the coating bath was set to 590°C when Al: 30~60 mass%, and to 630°C when Al: exceeded 60 mass%, and the coating immersion plate temperature of the base steel plate was controlled to be the same as the coating bath temperature. In addition, the coating treatment was carried out under the condition that the plate temperature was cooled to a temperature range of 520~500°C within 3 seconds.

In addition, the adhesion amount of the coating film was controlled to be 85±5 g/m ² per single side for samples 1 to 82 and 95 to 112, and was controlled to be 51 to 125 g/m ² per single side for samples 83 to 94.

(2) Subsequently, the chemical conversion treatment liquid was applied on the coating film of each sample of the prepared molten-coated steel plate using a rod coater and dried in a hot air furnace (heating rate: 60℃/s, PMT: 120℃) to form a chemical conversion film, thereby preparing the various samples of surface-treated steel plates shown in Tables 3 and 4.

In addition, the chemical conversion treatment liquid is prepared by dissolving each component in water as a solvent to prepare surface treatment liquid A~F. The types of components (resins, metal compounds) contained in the surface treatment liquid are as follows.

(resin)

Urethane resin: SUPERFLEX 130, SUPERFLEX 126 (First Industrial Pharmaceutical Co., Ltd.)

Acrylic resin: BONCOAT EC-740EF (DIC Co., Ltd.)

(Metal compounds)

P compound: aluminum dihydrogen tripolyphosphate

Si compound: silicon oxide

V compound: sodium metavanadate

Mo compound: molybdenum acid

Zr compound: potassium zirconium carbonate

Table 2 shows the composition of the prepared chemical conversion treatment solutions A~F and the amount of chemical conversion film formed. In addition, the concentration of each component in Table 2 of this manual is the solid concentration (mass %).

(Evaluation)

The following evaluations were performed on the samples of the molten-coated steel plates and surface-treated steel plates obtained as described above. The evaluation results are shown in Tables 3 and 4.

(1) Composition of the coating film (adhesion amount, composition, X-ray diffraction intensity)

，以膠帶密封非測定面後，以JIS H 0401：2013所示之鹽酸與六亞甲基四胺之混合液溶解剝離鍍敷，自剝離前後之樣品質量差，算出鍍敷皮膜之附著量。算出結果、所得鍍敷皮膜之附著量示於表3及4。 For each sample of molten-coated steel plate, punch 100mm

After sealing the non-measurement surface with tape, the coating was stripped by dissolving 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 difference in sample mass before and after stripping. The calculation results and the adhesion amount of the coating film obtained are shown in Tables 3 and 4.

Then, the stripping liquid is filtered and the filtrate and solid content are analyzed separately. Specifically, the filtrate is analyzed by ICP emission spectrometry to quantify the components other than insoluble Si.

After the solids were dried and ashed in a heating furnace at 650°C, sodium carbonate and sodium tetraborate were added to melt. The melt was then dissolved with hydrochloric acid, and the insoluble Si was quantified by ICP emission spectrometry analysis of the solution. 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 content analysis. The calculation results and the composition of the obtained coating film are shown in Tables 3 and 4.

Furthermore, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film on the evaluation symmetric surface was mechanically cut 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.) using X-rays: Cu-Kα (wavelength = 1.54178Å), kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, 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 after subtracting the base intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d = 0.3668nm), the diffraction intensity of the (100) plane of _MgZn2 (plane spacing d = 0.4510nm), and the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135nm) were measured. The measurement results are shown in Tables 3 and 4.

(2) Corrosion resistance evaluation

For each sample of the molten-coated steel plate and the surface-treated steel plate, cut it into a size of 120mm×120mm, seal the area 10mm 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 evaluation target surface exposed in a size of 100mm×100mm as the evaluation sample. In addition, the evaluation sample is made of 3 identical ones.

For the three evaluation samples prepared as above, the corrosion promotion test was carried out using the cycle shown in Figure 1. The corrosion promotion test started from wetting and continued for 300 cycles. The corrosion reduction of each sample was measured using the method described in JIS Z 2383 and ISO8407, and the evaluation was performed based on the following criteria. The evaluation results are shown in Tables 3 and 4.

◎: The corrosion loss of the three samples is less than 30g/ ^m2

○: The corrosion loss of the three samples is less than 70g/ ^m2

×: The corrosion loss of more than one sample exceeds 70g/ ^m2

(3) Rust resistance

For each sample of the molten-coated steel plate and the surface-treated steel plate, cut it into a size of 120mm×120mm, seal the area 10mm away from each edge of the evaluation object surface and the end face of the sample and the evaluation non-object surface with tape, and use the evaluation object surface exposed in a size of 100mm×100mm as the evaluation sample.

The above evaluation samples were subjected to the salt water spray test described in JIS Z 2371 for 90 hours and evaluated according to the following criteria. The evaluation results are shown in Tables 3 and 4.

◎: No rust on the flat plate

○: The rust area of the flat plate is less than 10%

×: The rust area of the flat plate is more than 10%

(4) Surface appearance

For each sample of the molten-coated steel plate, the coating film surface was visually observed.

Next, the observation results were evaluated based on the following benchmarks. The evaluation results are shown in Tables 3 and 4.

◎: No wrinkle defects were observed

○: Wrinkle defects are only observed within 50mm from the edge

×: Wrinkle defects were observed outside the range of 50mm from the edge

(5) Processability

For each sample of the molten-coated steel plate, cut it into a size of 70mm×150mm, then sandwich 8 plates of the same thickness inside and perform 180° bending (8T bending). After bending, strongly adhere SELLOTAPE (registered trademark) glass tape to the outer surface of the bend and then peel it off. Visually observe the surface condition of the coating film on the outer surface of the bend and whether the coating film is attached (peeled off) on the surface using the tape, and evaluate the processability according to the following criteria. The evaluation results are shown in Tables 3 and 4.

○: No cracking or peeling was observed on the coated film

△: There are cracks in the coating film, but no peeling is observed

×: Cracks and peeling were observed on the coating film at the same time

(6) Bath stability

During molten plating, the bath surface of the plating bath was visually checked and compared with the bath surface of the plating bath used in the manufacture of molten Al-Zn-based plated steel sheets (bath surface without Mg oxide). The evaluation was based on the following criteria, and the evaluation results are shown in Tables 3 and 4.

○: Same as the molten Al-Zn plating bath (55 mass% Al-balance Zn-1.6 mass% bath)

△: Compared with the molten Al-Zn coating bath (55 mass% Al-the rest Zn-1.6 mass% bath), there are more white oxides

×: Black oxide formation was observed in the plating bath

From the results in Tables 3 and 4, it can be seen that the samples of the present invention have better balance in corrosion resistance, rust resistance, surface appearance, processability and bath stability than the samples of the comparative examples.

Furthermore, from the results in Table 4, it can be seen that the rust resistance of the samples subjected to chemical conversion treatments A to D shows particularly excellent results.

<Example 3: Samples 1~44>

(1) A cold-rolled steel plate with a thickness of 0.8 mm manufactured by a common method was used as the base steel plate. The molten-coated steel plate samples with the coating film conditions shown in Table 6 were prepared by annealing and coating simulator manufactured by RHESCA (Co., Ltd.).

In addition, regarding the composition of the coating bath used for the production of the molten-coated steel plate, the coating bath composition was varied in the range of Al: 30~75 mass%, Si: 0.5~4.5 mass%, Mg: 0~10 mass%, Sr: 0.00~0.15 mass% in order to obtain the coating film composition of each sample shown in Table 6. Moreover, the bath temperature of the coating bath was set to 590°C when Al: 30~60 mass%, and to 630°C when Al: exceeded 60 mass%, and the coating immersion plate temperature of the base steel plate was controlled to be the same as the coating bath temperature. In addition, the coating treatment was carried out under the condition that the plate temperature was cooled to a temperature range of 520~500°C within 3 seconds.

In addition, the adhesion amount of the coating film was controlled to be 85±5g/m ² per single side for samples 1 to 41, and was controlled to be 42 to 125g/m ² per single side for samples 42 to 44.

(2) Subsequently, the chemical conversion treatment liquid shown in Table 5 was applied to the coating film of each sample of the prepared molten-coated steel plate using a rod coater and dried in a hot air drying furnace (reaching plate temperature: 90°C) to form a chemical conversion treatment film with an adhesion amount of 0.1 g/ ^m2 .

In addition, the chemical conversion treatment liquid used is a chemical conversion treatment liquid with a pH of 8 to 10 prepared by dissolving each component in water as a solvent. The types of components (resin components, inorganic compounds) contained in the chemical conversion treatment liquid are as follows.

(Resin ingredients)

Resin A: (a) anionic polyurethane resin having an ester bond ("SUPERFLEX210" 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.) are mixed at a mass ratio of (a): (b) = 50:50.

Resin B: Acrylic resin ("BONCOAT EC-740EF" manufactured by DIC Corporation)

(Inorganic compounds)

Vanadium compounds: organic vanadium compounds chelated with acetylacetone

Zirconium compounds: ammonium zirconium carbonate

Fluorine compounds: ammonium fluoride

(3) Next, a base coating material was applied on the chemical conversion film formed as described above using a rod coater, and the steel plate was baked at a reaching temperature of 230°C for 35 seconds to form a base coating film having the composition shown in Table 5. Subsequently, a top coating material composition was applied on the base coating film formed as described above using a rod coater, and the steel plate was baked at a reaching temperature of 230°C to 260°C for 40 seconds to form a top 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 base coating material is obtained by mixing the components and stirring them with a ball mill for about 1 hour. The resin components and inorganic compounds constituting the base coating film are as follows.

(Resin ingredients)

Resin α: Urethane-modified polyester resin (obtained by reacting 455 parts by mass of polyester resin and 45 parts by mass of isophorone diisocyanate, resin acid value 3, number average molecular weight 5,600, hydroxyl value 36) cured with blocked isocyanate.

In addition, the urethane modified polyester resin is prepared under the following conditions. 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 to a flask equipped with a stirrer, a distillation tower, a water separator, a cooling tube, and a thermometer. The mixture was heated and stirred. The generated condensation water was distilled out of the system while the temperature was raised from 160°C to 230°C at a constant speed over 4 hours. After reaching 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 was below 5. After cooling to 100°C, SOLVESSO 100 (EXXON 120 parts by mass of butyl solvent (produced by MOBILE, trade name, high boiling point aromatic hydrocarbon solvent) and 100 parts by mass of butyl solvent were added to obtain a polyester resin solution.

Resin β: Urethane-cured polyester resin ("EVERCLAD 4900" manufactured by Kansai Coatings Co., Ltd.)

(Inorganic compounds)

Vanadium compounds: magnesium vanadate

Phosphate compounds: calcium phosphate

Magnesium oxide compounds: magnesium oxide

The resin used for the top coating is the following coating.

Resin I: Melamine-cured polyester coating ("PRECOLOR HD0030HR" manufactured by BASF JAPAN Co., Ltd.)

Resin II: Organic sol-fired fluororesin coating with polyvinylidene fluoride and acrylic resin in a mass ratio of 80:20 ("PRECOLOR No.8800HR" manufactured by BASF JAPAN Co., Ltd.)

(Evaluation)

The following evaluations were performed on each sample of the coated steel plate obtained as described above. The evaluation results are shown in Table 6.

(1) Composition of the coating film (adhesion amount, composition, X-ray diffraction intensity)

，以膠帶密封非測定面後，以JIS H 0401：2013所示之鹽酸與六亞甲基四胺之混合液溶解剝離鍍敷，自剝離前後之樣品質量差，算出鍍敷皮膜之附著量。算出結果、所得鍍敷皮膜之附著量示於表6。 For each sample of molten-coated steel plate, punch 100mm

After sealing the non-measurement surface with tape, the coating was stripped by dissolving 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 difference in sample mass before and after stripping. The calculated results and the adhesion amount of the coating film obtained are shown in Table 6.

Then, the stripping liquid is filtered and the filtrate and solid content are analyzed separately. Specifically, the filtrate is analyzed by ICP emission spectrometry to quantify the components other than insoluble Si.

After the solids were dried and ashed in a heating furnace at 650°C, sodium carbonate and sodium tetraborate were added to dissolve them. Furthermore, the melt was dissolved with hydrochloric acid, and the insoluble Si was quantified by ICP emission spectrometry analysis of the solution. The Si concentration in the coating film is the sum of the soluble Si concentration obtained by the filter solution analysis and the insoluble Si concentration obtained by the solid content analysis. The calculated results and the composition of the coating film are shown in Table 6.

Furthermore, for each sample, after cutting into a size of 100 mm × 100 mm, the coating film on the evaluation symmetric surface was mechanically cut 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.) using X-ray: Cu-Kα (wavelength = 1.54178Å), kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, 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 base intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane of _Mg2Si (plane spacing d = 0.3668nm), the diffraction intensity of the (100) plane of _MgZn2 (plane spacing d = 0.4510nm), and the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135nm) were measured. The measurement results are shown in Table 6.

(2) Corrosion resistance evaluation

For each sample of coated steel plate, cut it into a size of 120mm×120mm, seal the three edges of the sample with a distance of 10mm from the evaluation target surface and the three edges of the sample with the non-evaluation target surface with tape, and use the evaluation target surface exposed in a size of 100mm×100mm as the evaluation sample. In addition, three identical evaluation samples are made.

For the three evaluation samples prepared as above, the corrosion promotion test was carried out in the cycle shown in Figure 1. The corrosion promotion test started from wetting, and the samples were taken out every 20 cycles, washed and dried, and then visually observed to confirm the occurrence of red rust on the shear end surface of one side that was not sealed with the tape.

Next, the number of cycles when red rust was detected was evaluated according to the following criteria. The evaluation results are shown in Table 6.

◎: The number of cycles of rusting in 3 samples is ≥ 600 cycles

○: 600 cycles > 3 samples had red rust in ≥ 400 cycles

×: The number of red rust occurrence cycles of at least one sample is <400 cycles

(3) Appearance after painting

For each sample of coated steel plate, the surface was visually inspected.

Next, the observation results were evaluated based on the following benchmarks. The evaluation results are shown in Table 6.

◎: No wrinkle defects were observed

○: Wrinkle defects are only observed within 50mm from the edge

×: Wrinkle defects were observed outside the range of 50mm from the edge

(4) Processability after painting

For each sample of coated steel plate, cut it into 70mm×150mm size, insert 8 plates of the same thickness inside and perform 180° bending (8T bending). After bending, strongly adhere SELLOTAPE (registered trademark) glass tape to the outer surface of the bend and peel it off. Visually observe the surface condition of the coating on the outer surface of the bend and whether the coating is attached (peeled) on the surface using the tape, and evaluate the processability according to the following criteria. The evaluation results are shown in Table 6.

○: No cracking or peeling was observed on the coated film

△: There are cracks in the coating film, but no peeling is observed

×: Cracking and peeling were observed on the coating film at the same time

(5) Bath stability

During molten plating, the bath surface of the plating bath was visually checked and compared with the bath surface of the plating bath used in the manufacture of molten Al-Zn-based plated steel sheets (bath surface without Mg oxide). The evaluation was based on the following criteria, and the evaluation results are shown in Table 6.

○: Same as the molten Al-Zn plating bath (55 mass% Al-balance Zn-1.6 mass% bath)

△: Compared with the molten Al-Zn coating bath (55 mass% Al-the rest Zn-1.6 mass% bath), there are more white oxides

×: Black oxide formation was observed in the plating bath

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

[Industrial availability]

According to the present invention, a stable molten Al-Zn-Si-Mg plated steel sheet with excellent corrosion resistance can be provided.

Furthermore, according to the present invention, a surface-treated steel plate with stable properties and excellent corrosion resistance and rust resistance can be provided.

Furthermore, according to the present invention, a stable coated steel plate with excellent corrosion resistance and corrosion resistance of the processed part can be provided.

Claims (8)

A molten Al-Zn-Si-Mg coated steel plate having a coating film, wherein the coating film 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, a portion of the coating film is mechanically cut until the base steel plate is exposed and then measured in powder state by X-ray diffraction method, and the diffraction intensity of _Mg2Si and _MgZn2 in the coating film by X-ray diffraction method satisfies the following relationship (1): _Mg2Si (111)/ _MgZn2 (100)≦2.0...(1) _Mg2 Si(111): diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d=0.3668nm), MgZn ₂ (100): diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d=0.4510nm). For example, the molten Al-Zn-Si-Mg coated steel plate of claim 1, wherein the diffraction intensity of Si in the aforementioned coating film by X-ray diffraction method satisfies the following relationship (2): Si(111)=0...(2)Si(111): the diffraction intensity of the (111) plane of Si (plane spacing d=0.3135mm). For example, the molten Al-Zn-Si-Mg coated steel plate of claim 1 or 2, wherein the coating film further contains Sr: 0.01~1.0 mass %. For the molten Al-Zn-Si-Mg coated steel plate of claim 1 or 2, the Al content in the coating is 50-60% by mass. For the molten Al-Zn-Si-Mg coated steel plate of claim 1 or 2, the Si content in the aforementioned coating is 1.0~3.0 mass %. For the molten Al-Zn-Si-Mg coated steel plate of claim 1 or 2, the Mg content in the aforementioned coating is 1.0~5.0 mass %. A surface treated steel plate having a coating film as in any one of claims 1 to 6 and a chemical conversion film formed on the coating film, wherein the chemical conversion film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyurethane 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, wherein a coating is formed on a coating film as in any one of claims 1 to 6, directly or via a chemical conversion film, wherein the chemical conversion film contains: a resin component, which contains 30-50% by weight of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, and the content ratio of (a) to (b) is ( (a): (b)) with a mass ratio in the range of 3:97~60:40; and an inorganic compound, which contains 2~10 mass% of a vanadium compound, 40~60 mass% of a zirconium compound and 0.5~5 mass% of a fluorine compound, the aforementioned coating film has at least a base coating film, and the base coating film contains a polyester resin with a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide.

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