TWI911599B - Hot-dip coated Al-Zn series steel sheet and its manufacturing method - Google Patents

Abstract

The purpose of this invention is to provide a coated steel sheet with good adhesion between the chemically treated film and the primer coating, and excellent corrosion resistance of the processed and cut ends. In order to achieve the above purpose, the coating of this invention is characterized by having: containing Al: 45~65% by mass and Si: 1.0~3.0% by mass, with the remainder being composed of Zn and unavoidable impurities, and the Mg content relative to the total mass of the aforementioned coating is 0.3% by mass _or less, and the diffraction intensity of _MgZn2 and _Mg2Zn11 in the aforementioned coating obtained by X-ray diffraction satisfies the following relationships (1) and (2). MgZn ₂ (100)=0 ・・・(1), Mg ₂ Zn ₁₁ (321)=0 ・・・(2)

Description

Hot-dip coated Al-Zn series steel sheet and its manufacturing method

This invention relates to a hot-dip galvanized Al-Zn steel sheet with stable and excellent workability and corrosion resistance of the processed parts, and a method for manufacturing the same.

Hot-dip galvanized Al-Zn steel sheets, represented by those with 55% Al-Zn content, are known to exhibit high corrosion resistance even among hot-dip galvanized steel sheets, as shown in Patent 1, due to the combination of the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al. Therefore, the exceptional corrosion resistance of hot-dip Al-Zn steel sheets makes them primarily used in building materials such as roofs and walls exposed to the elements for extended periods, as well as in civil engineering applications such as railings, wiring and piping, and soundproofing. In particular, the demand for corrosion-resistant or maintenance-free materials in more demanding environments, such as acid rain caused by air pollution, the application of de-icing agents to prevent road frost in snow-covered areas, and coastal development, has been steadily increasing. Therefore, the need for hot-dip galvanized Al-Zn steel sheets has also been gradually increasing in recent years.

The coating of hot-dip Al-Zn steel sheets is characterized by a structure consisting of supersaturated Al containing Zn solidified into dendritic crystals (α-Al phase) and Zn-Al eutectic structure existing in the interdendritic spaces (interdendritic spaces). The α-Al phase has a complex layered structure along the thickness direction of the coating. It is also known that this characteristic coating structure makes corrosion more difficult due to the more complex corrosion path from the surface. Furthermore, hot-dip Al-Zn steel sheets achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same coating thickness.

Generally, hot-dip galvanized Al-Zn steel sheets are manufactured by using thin steel sheets, formed by hot-rolling or cold-rolling steel billets, as the base steel sheet. This base steel sheet is then subjected to recrystallization annealing and hot-dip galvanizing in the annealing furnace of a continuous hot-dip galvanizing machine. Furthermore, in addition to a specified concentration of Al or Zn in the coating bath, Si is usually added to suppress excessive growth of the interfacial alloy layer formed at the base metal (base steel sheet)-coating interface. Through the action of Si, the thickness of the interfacial alloy layer in the hot-dip galvanized Al-Zn steel sheet can be controlled to approximately 1~5 μm. It is known that for a given coating thickness, a thinner interfacial alloy layer results in a thicker main layer exhibiting high corrosion resistance. Therefore, controlling the growth of the interfacial alloy layer is related to improving corrosion resistance.

Furthermore, it is known that when hot-dip galvanized Al-Zn steel sheets are subjected to bending or other processing, cracks will occur in the coated film of the processed area depending on the degree of processing (processing degree). In hot-dip galvanized Al-Zn steel sheets, the aforementioned thick interfacial alloy layer becomes the starting point for cracking, and mainly the interdentrites of the coated film become the propagation path for cracks. Therefore, even when bending processing with the same degree of processing is performed, compared with hot-dip galvanized steel sheets with the same coating thickness, there is a relatively large tendency for cracking and the appearance of openings. Therefore, even in applications requiring high processing precision, there are still issues such as damage to the appearance caused by large cracks that can be confirmed by the naked eye, or a significant reduction in corrosion resistance of the cracked portion of the exposed base steel plate compared to the uncracked portion (reduced corrosion resistance of the processed portion).

To address the issues of processability and corrosion resistance of the processed parts, for example, Patent 2 or Patent 3 discloses a method for manufacturing hot-dip galvanized Al-Zn steel sheets to improve processability by applying a specified heat treatment to the hot-dip galvanized Al-Zn steel sheet. [Prior Art Documents] [Patent Documents]

[Patent Document 1] Japanese Patent Publication No. 46-7161 [Patent Document 2] Japanese Patent Publication No. 61-28748 [Patent Document 3] Japanese Patent Application Publication No. 2002-275646 [Patent Document 4] Japanese Patent Application Publication No. 2004-285387 [Patent Document 5] Japanese Patent Publication No. 2016-540885

[Problem to be Solved by the Invention] However, even when heat treatment as disclosed in Patent 2 or Patent 3 is applied, impurities may still be present in the coated film, and hot-dip galvanized Al-Zn steel sheets may not necessarily exhibit excellent workability. Further improvements are desired to consistently achieve excellent workability and corrosion resistance of the processed areas.

It is known that impurities are unavoidably introduced into the coating bath during the hot-dip plating process, and this is no exception for Al-Zn hot-dip plating. Examples of impurities mixed into the coated film include those contained in the plating raw materials or those precipitated from the substrate steel plate or the equipment in the plating bath, such as Fe, Cr, Ni, Cu, Co, W, Mg, and Ca. These components are inevitably included in the coated film. In particular, the production of hot-dip galvanized Zn-Al-Mg or Al-Zn-Si-Mg steel sheets with high corrosion resistance has gradually increased in recent years. With the expansion of the circulation of Zn raw materials containing high Mg concentrations, produced through the regeneration of slag generated during manufacturing, Mg often becomes mixed in as an impurity during the plating bath and even in the plating film.

As mentioned above, unavoidable impurities in the coating can degrade the appearance, corrosion resistance, and processability of hot-dip galvanized steel sheets. The extent of this impurity effect depends on the composition and concentration of the coating. That is, even impurities of the same composition can be harmful or harmless to the properties of the galvanized steel sheet. Therefore, it is necessary to investigate the impurities affecting the properties of various hot-dip galvanized steel sheets and develop techniques to control impurity concentrations in order to consistently obtain the necessary properties. For example, Patent 4 discloses a hot-dip galvanized steel sheet with excellent appearance, which is composed of Al: 0.10~0.6%, Bi: 0.03~0.3% by mass, with the remainder being Zn and unavoidable impurities, and the contents of Pb, Sn, and Cd, which are the aforementioned unavoidable impurities, are controlled at 0.002% each. Furthermore, Patent 5 discloses a hot-dip Zn-Al-Mg series steel sheet with excellent corrosion resistance, which is composed of Al: 4.4~5.6%, Mg: 0.3~0.56%, with the remainder being Zn and unavoidable impurities, and is controlled to exclude Ni, which is the aforementioned unavoidable impurity.

However, the technology disclosed in patents 4 or 5 focuses on improving corrosion resistance. For hot-dip galvanized Zn-Al steel plates without Mg or hot-dip galvanized Al-Zn steel plates with high Al concentration, the impact of unavoidable impurities on processability or corrosion resistance of the processed parts is not fully considered. Therefore, it is hoped that a technology can be developed that can more reliably and stably achieve excellent processability and corrosion resistance of the processed parts.

In view of this situation, the present invention aims to provide a hot-dip galvanized Al-Zn steel sheet with reliable and consistently excellent workability and corrosion resistance of the processed parts, and a method for manufacturing the same. [Means for solving the problem]

The inventors, through their review of the above-mentioned problems, focused on controlling the concentration and state of Mg, which is an arbitrary or unavoidable impurity in the hot-dip Al-Zn steel sheet. They also found that by reducing the Mg content and simultaneously removing _MgZn2 and _Mg2Zn11 from the coated film, excellent processability and corrosion resistance _of the processed parts can be reliably and stably obtained.

This invention is based on the above views, and its main points are as follows. 1. A hot-dip galvanized Al-Zn steel sheet, which is a hot-dip galvanized Al-Zn steel sheet with a galvanized coating, characterized in that the galvanized coating contains Al: 45~65% by mass and Si: 1.0~3.0% by mass, with the remainder being composed of Zn and unavoidable impurities, and the Mg content relative to the total mass of the galvanized coating is 0.3% by mass _or less, and the diffraction intensities of _MgZn2 and _Mg2Zn11 in the galvanized coating obtained by X-ray diffraction satisfy the following relationships (1) and (2): _MgZn2 ( ₁₀₀ )=0 ・・・(1) _Mg2Zn11 (321)=0 ・・・(2) _MgZn2 (100): Diffraction intensity of the (100) plane (plane spacing d = 0.4510 nm) of MgZn ₂ , _and (321): Diffraction intensity of the _{(321) plane (plane spacing d = 0.2290 nm) of Mg 2 Zn 11.} 2. The hot-dip galvanized Al-Zn steel sheet as described in 1 above, wherein the aforementioned coating film contains one or more of B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi in a total of 0.01 to 3.0% by mass. 3. The hot-dip galvanized Al-Zn steel sheet as described in 1 or 2 above, wherein the unavoidable impurity content in the aforementioned coating film is less than 5.0% by mass. 4. The hot-dip galvanized Al-Zn steel sheet as described in any of 1 to 3 above, wherein the Mg content in the unavoidable impurities of the aforementioned galvanized film is less than 0.3% by mass relative to the total mass of the aforementioned galvanized film.

5. A method for manufacturing a hot-dip galvanized Al-Zn steel sheet, characterized in that: the aforementioned galvanized film is formed on a substrate steel sheet using a galvanizing bath, the galvanizing bath comprising: Al: 45-65% by mass and Si: 1.0-3.0% by mass, with the remainder consisting of Zn and unavoidable impurities; and the Mg content relative to the total mass of the aforementioned galvanizing bath is controlled to be below 0.3% by mass. The heating temperature T at which the steel sheet with the aforementioned coating is reheated reaches its highest temperature is set to 130~300°C. After heating, the average cooling rate C (°C/hr) from the aforementioned heating temperature T (°C) to 100°C satisfies the following relationship (3): C≦(T-100)／2　・・・(3). 6. The hot-dip galvanized Al-Zn steel sheet manufacturing method as described in 5 above, wherein the aforementioned coating bath further contains one or more of B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi in a total of 0.01~3.0% by mass. 7. The hot-dip galvanized Al-Zn steel sheet as described in 5 or 6 above, wherein the unavoidable impurity content in the aforementioned plating bath is 5.0% by mass or less in total. 8. The hot-dip galvanized Al-Zn steel sheet as described in any one of 5 to 7 above, wherein the Mg content among the unavoidable impurities in the aforementioned plating bath, relative to the total mass of the aforementioned plating bath, is 0.3% by mass or less. [Invention Effects]

According to the present invention, a hot-dip galvanized Al-Zn steel sheet with excellent workability and corrosion resistance of the processed parts and a method thereof can be reliably and reliably provided.

(Hot-dip Al-Zn steel sheet) As shown in Figure 1, the hot-dip Al-Zn steel sheet of the present invention has a coating film 20 on the surface of a base steel sheet 10. Furthermore, the aforementioned coating film 20 has a main layer 21 and an interface alloy layer 22 formed at the interface between the main layer and the base steel sheet 10. Furthermore, the aforementioned coating film 20 has the following composition: containing Al: 45~65% by mass and Si: 1.0~3.0% by mass, with the remainder consisting of Zn and unavoidable impurities.

From the perspective of balancing corrosion resistance and operational performance, the Al content in the aforementioned coated film is 45-65% by mass, preferably 50-60% by mass. This is because if the Al content in the aforementioned coated film is at least 45% by mass, dendritic Al crystals will solidify, resulting in a coated film structure with dendritic crystal solidification of the α-Al phase as the main component. By adopting this dendritic crystal solidification structure to accumulate in the thickness direction of the coated film, the corrosion path becomes more complex, thereby improving the corrosion resistance of the coated film itself. Furthermore, the more dendritic layers of the α-Al phase accumulate, the more complex the corrosion path becomes, making it harder for corrosion to reach the base steel plate, thus improving corrosion resistance. Therefore, the Al content in the aforementioned coating is preferably 50% by mass or more. On the other hand, when the Al content in the aforementioned coating exceeds 65% by mass, it changes into a structure where Zn is almost completely dissolved in α-Al, failing to inhibit the dissolution reaction of the α-Al phase, leading to a deterioration in the corrosion resistance of the hot-dip Al-Zn system. Therefore, the Al content in the aforementioned coating needs to be below 65% by mass, preferably below 60% by mass.

Furthermore, the Si in the aforementioned coating is primarily added to suppress the growth of Fe-Al and/or Fe-Al-Si interfacial alloy layers formed at the interface with the substrate steel plate, and to prevent deterioration of the adhesion between the coating and the steel plate. In practice, when the steel plate is immersed in an Al-Zn plating bath containing Si, the Fe on the steel plate surface undergoes an alloying reaction with the Al or Si in the bath, resulting in the formation of Fe-Al and/or Fe-Al-Si intermetallic compound layers at the substrate steel plate/coating film interface. However, since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, a higher proportion of the Fe-Al-Si alloy suppresses the overall growth of the interfacial alloy layer. Therefore, the Si content in the aforementioned coating needs to be at least 1.0% by mass. On the other hand, when the Si content in the aforementioned coated film exceeds 3.0% by mass, not only will the growth inhibition effect of the aforementioned interface alloy layer become saturated, but there will also be an excess Si phase in the coated film, which will reduce the processability. Therefore, the Si content is made to be less than 3.0%.

Furthermore, the aforementioned coated film contains Zn and unavoidable impurities. Among these unavoidable impurities is Fe. This Fe is unavoidably included due to precipitation from the steel plate or bath equipment into the coating bath, and also as a result of diffusion from the base steel plate during the formation of the interfacial alloy layer, thus becoming unavoidably included in the aforementioned coated film. The Fe content in the aforementioned coated film is typically in the range of 0.3 to 2.0% by mass. Other unavoidable impurities include Cr, Ni, Cu, Co, W, Mg, and Ca. These components are included as impurities in the metal blocks of the plating bath raw material due to the precipitation of W-C or Co-Cr-W based melt spray coatings from the base steel sheet or stainless steel bath equipment, or the coating applied to the bath equipment, into the plating bath. They are also unavoidably included in the aforementioned plating coating due to the use of tanks or bath equipment in the manufacture of plating sheets with the intentional addition of these components.

There is no particular limitation on the total content of unavoidable impurities in the aforementioned coating. However, if they are present in excess, they may affect the various properties of the coated steel sheet. Therefore, it is preferable to make the total content less than 5.0% by mass, and even better to make it less than 1.0% by mass.

Furthermore, in the hot-dip galvanized Al-Zn steel sheet of this invention, the Mg content in the aforementioned galvanized film, relative to the total mass of the galvanized film, needs to be below 0.3% by mass. Since the Mg contained in the aforementioned galvanized film can deteriorate the processability and corrosion resistance of the processed parts of the hot-dip galvanized Al-Zn steel sheet, by appropriately controlling the Al, Zn, and Si contents in the aforementioned galvanized film, the deterioration of processability and corrosion resistance of the processed parts can be suppressed by inhibiting the Mg content. Based on the same viewpoint, the Mg content in the aforementioned galvanized film, relative to the total mass of the aforementioned galvanized film, is preferably made to be below 0.1% by mass. Furthermore, in the hot-dip galvanized Al-Zn steel sheet of the present invention, if the Mg content relative to the total mass of the aforementioned coating is 0.3% by mass or less, it can be included as an arbitrary component. However, from the viewpoint of more effectively suppressing the reduction of processability and corrosion resistance of the processed part of the hot-dip galvanized Al-Zn steel sheet, it is preferable not to include Mg as an arbitrary additive component of the aforementioned coating. That is, relative to the total mass of the aforementioned coating, the Mg content in the unavoidable impurities of the aforementioned coating is preferably 0.3% by mass or less, and preferably 0.1% by mass. Therefore, since the lower the Mg content in the aforementioned coating, the better the corrosion resistance of the hot-dip galvanized Al-Zn steel sheet of the present invention, a lower limit value is not particularly limited. However, since it is technically difficult to completely reduce the Mg content in the aforementioned coated film to 0.000% by mass, the lower limit of the Mg content in the aforementioned coated film is essentially around 0.001% by mass.

Furthermore, the hot-dip Al-Zn steel sheet of this invention is characterized in that the diffraction intensities of MgZn ₂ and Mg ₂ Zn ₁₁ in the aforementioned coated film obtained by X-ray diffraction satisfy the following relationships (1) and (2). MgZn ₂ (100)=0 ・・・(1) Mg ₂ Zn ₁₁ (321)=0 ・・・(2) MgZn ₂ (100): Diffraction intensity of the ( ₁₀₀ ) plane (plane spacing d=0.4510nm) of MgZn _{2, Mg 2} Zn ₁₁ (321): Diffraction intensity of the (321) plane (plane spacing d=0.2290 nm) of Mg ₂ Zn ₁₁ .

In the aforementioned coated film _, the presence of a small amount of Mg can lead to _the formation of _MgZn2 and _Mg2Zn11 intermetallic compounds. _MgZn2 and _Mg2Zn11 are generally hard and brittle. Therefore, the presence of these intermetallic compounds in the coated film is considered to be a starting point for cracking during harsh bending or stretching processes, resulting in deterioration of the processability and corrosion resistance of the hot-dip Al-Zn steel sheet. Therefore, by reducing the Mg content in the aforementioned coated film as described above, and by creating a coated film without _MgZn2 and _Mg2Zn11 as shown in equations (1) and ( ₂ ), processability and corrosion resistance of the processed area can be improved more reliably and stably.

Here, in the above relationships (1) and (2), MgZn ₂ (100) is the diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm), and Mg ₂ Zn ₁₁ (321) is the diffraction intensity of the (321) plane of Mg ₂ Zn ₁₁ (plane spacing d = 0.2290 nm). As for the aforementioned method of measuring MgZn ₂ (100) and Mg ₂ Zn ₁₁ (321) by X-ray diffraction, it can be calculated by mechanically cutting out a portion of the aforementioned coated film and performing X-ray diffraction in the powder state (powder X-ray diffraction measurement method). Regarding the measurement of diffraction intensity, the diffraction peak intensity of _MgZn2 at an interplanar spacing d=0.4510nm and the diffraction peak intensity _{of Mg2Zn11} at an interplanar spacing d=0.2290nm can be measured.

Furthermore, from the viewpoint of accurately measuring MgZn ₂ (100) and Mg ₂ Zn ₁₁ (321), the amount of coating film required for powder X-ray diffraction measurement (the amount of coating film removed) only needs to be 0.1 g or more, preferably 0.3 g or more. Also, when removing the aforementioned coating film, there may be cases where the powder contains steel plate components other than the coating film. These intermetallic compound phases are only contained in the coating film and will not affect the aforementioned peak intensity. Furthermore, the reason for making the aforementioned coated film into powder for X-ray diffraction is that when X-ray diffraction is performed on the coated film formed on the coated steel plate, the orientation of the solidified structure of the coated film will be affected, making it difficult to obtain the peak intensity corresponding to the amount of substance present.

Here, the method used to satisfy the above relationship (1) or relationship (2) is not particularly limited. For example, by controlling and reducing the content of Mg in the aforementioned coated film, the ratio of Mg content to Zn content can be reduced (for example, Mg/Zn can be reduced to 0.008 or less, preferably 0.006 or less), thereby controlling and reducing the amount of MgZn ₂ and Mg ₂ Zn ₁₁ (the diffraction intensity of MgZn ₂ (100) and Mg ₂ Zn ₁₁ (321)).

In addition to controlling the Mg content in the aforementioned coated film, the above relationships (1) and (2) can also be satisfied by controlling the Mg content in the aforementioned coated film to a specific value and then adjusting the conditions during the formation of the coated film (e.g., the cooling conditions after coating).

Furthermore, the aforementioned coating film may also contain one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi, at a total weight percentage of 0.01 to 3.0%. These elements can enhance the stability of corrosion products during corrosion, thereby slowing down the corrosion process, or stabilize the size of the zinc spangle on the coated surface, resulting in a better surface appearance.

Furthermore, from the perspective of satisfying various characteristics, the adhesion amount of the aforementioned coating is preferably 45~120 g/ ^m² on each side. This is because when the adhesion amount of the aforementioned coating is 45 g/ ^m² or higher, sufficient corrosion resistance can be achieved for applications such as building materials that require long-term corrosion resistance. Conversely, when the adhesion amount of the aforementioned coating is below 120 g/ ^m² , coating damage during processing can be suppressed while simultaneously achieving excellent corrosion resistance. Based on the same viewpoint, the adhesion amount of the aforementioned coating is preferably 45~100 g/ ^m² .

Regarding the aforementioned coating adhesion amount, it can be derived using, for example, the method described in JIS H0401:2013. This method involves dissolving and peeling off a specific area of the coating using a mixture of hydrochloric acid and hexamethylenetetramine, and calculating the amount from the weight difference of the steel sheet before and after peeling. The coating adhesion amount on each side can be determined by sealing the non-target coating surface with tape and then performing the aforementioned dissolution. Furthermore, the composition of the aforementioned coating can be determined by immersing the coating in a hydrochloric acid solution or similar solution to dissolve it, similar to the method used for coating adhesion, and then confirming the solution using ICP emission spectroscopy or atomic absorption spectrometry. This method is merely one example; any method that can accurately quantify the composition of the coating film is acceptable and is not particularly limited to any particular method.

Furthermore, the coating film obtained by this invention for hot-dip Al-Zn steel sheets is almost identical in composition to the coating bath. Therefore, the composition of the aforementioned coating film can be precisely controlled by controlling the composition of the coating bath.

Furthermore, there are no particular limitations on the base steel plate used in the hot-dip galvanized Al-Zn steel sheet of this invention. Cold-rolled steel sheet or hot-rolled steel sheet can be used depending on the required performance or specifications. Furthermore, there are no particular limitations on the method for obtaining the aforementioned base steel plate. For example, in the case of the aforementioned hot-rolled steel sheet, a hot-rolling and pickling process can be used; in the case of the aforementioned cold-rolled steel sheet, a cold-rolling process can also be applied. Moreover, to obtain the characteristics of the steel sheet, a recrystallization annealing process can be performed before the hot-dip galvanizing process.

Furthermore, the hot-dip Al-Zn based steel sheet of this invention, as shown in Figure 1, has a coating film 20 formed on a base steel sheet 10. However, if necessary, an intermediate layer or coating film can also be formed on this coating film. There are no particular limitations on the type of coating film or the method of forming the coating film; it can be appropriately selected according to the required performance. Examples of methods include roller coating, curtain coating, and spray coating. After applying a coating containing organic resin, the coating film can be formed by heating and drying using methods such as hot air drying, infrared heating, or induction heating. Furthermore, regarding the aforementioned intermediate layer, there is no particular limitation as long as it is a layer formed between the plating film on the hot-dip Al-Zn based steel sheet and the aforementioned coating film. Examples include chemically treated films or primer layers such as adhesive layers. The aforementioned chemically treated films can be formed by, for example, applying a chromate treatment solution or a chromate-free chemical treatment solution, without water washing, and drying the steel sheet at a temperature of 80-300°C through a chromate treatment or chromate-free chemical treatment. These chemically treated films can be a single layer or multiple layers; in the case of multiple layers, multiple chemical treatments can be performed sequentially.

(Manufacturing Method of Hot-Dip Galvanized Al-Zn Steel Sheet) The present invention provides a manufacturing method for hot-dip galvanized Al-Zn steel sheets having a coated film.

Furthermore, the manufacturing method of the hot-dip Al-Zn steel sheet of the present invention includes the step of forming the aforementioned coating film on the substrate steel sheet using a coating bath, wherein the coating bath contains Al: 45~65% by mass and Si: 1.0~3.0% by mass, and the remainder is composed of Zn and unavoidable impurities, and the Mg content is controlled to be below 0.3% by mass relative to the total mass of the aforementioned coating bath.

Furthermore, the steps for forming the aforementioned coating film are not particularly limited, except for the coating bath conditions described later. For example, it can be manufactured by using a continuous hot-dip galvanizing machine to clean, heat, and immerse the aforementioned substrate steel sheet in the coating bath. During the heating step of the steel sheet, recrystallization annealing or similar processes are applied to control the microstructure of the substrate steel sheet itself, while simultaneously preventing oxidation of the steel sheet and reducing any trace oxide film present on the surface. Heating in a reducing environment such as a nitrogen-hydrogen environment is effective.

The plating bath used in the step of forming the aforementioned coated film has the following composition: containing Al: 45~65% by mass and Si: 1.0~3.0% by mass, with the remainder consisting of Zn and unavoidable impurities. As mentioned above, the composition of the aforementioned coated film is almost identical to that of the plating bath.

Furthermore, the manufacturing method of the hot-dip galvanized Al-Zn steel sheet of this invention is characterized by controlling the Mg content to below 0.3% by mass relative to the total mass of the aforementioned plating bath. As described above, since the Mg contained in the aforementioned plating film can degrade the processability and corrosion resistance of the processed parts of the hot-dip galvanized Al-Zn steel sheet, by appropriately controlling the contents of Al, Zn, and Si in the plating bath and then suppressing the Mg content, the degradation of processability and corrosion resistance of the processed parts can be suppressed.

Furthermore, relative to the total mass of the aforementioned coating bath, the Mg content in the aforementioned coating bath needs to be controlled below 0.3% by mass, preferably below 0.1% by mass. If the Mg content in the aforementioned coating bath is below 0.3% by mass, the manufactured hot-dip galvanized Al-Zn steel sheet can have sufficiently excellent workability and corrosion resistance of the processed parts; if it is below 0.1% by mass, even better workability and corrosion resistance of the processed parts can be achieved. Therefore, since the lower the Mg content in the coating bath, the better the corrosion resistance of the hot-dip galvanized Al-Zn steel sheet, there is no particular limit to the lower limit of the Mg content. However, since it is technically difficult to make the Mg content in the aforementioned plating bath completely 0.000% by mass, the lower limit of the Mg content in the aforementioned plating is actually around 0.001% by mass.

Furthermore, there are no particular limitations on the means of reducing the Mg content in the aforementioned plating bath. For example, effective methods include intentionally not adding Mg to the plating bath, or not reusing the tanks or bath equipment used for manufacturing plating steel sheets with intentionally added Mg, such as Zn-Al-Mg or Al-Zn-Si-Mg plating steel sheets, in the manufacture of hot-dip galvanized Al-Zn steel sheets. This is because it inhibits the dissolution of Mg-containing metal blocks adhering to the aforementioned tanks or bath equipment, preventing their contamination into the plating bath. Furthermore, as another means of reducing the Mg content in the aforementioned plating bath, it is preferable to use metal blocks with a low Mg content among impurities as the raw material for the plating bath.

The bath temperature for the aforementioned plating process is not particularly limited, but a range of (melting point + 20°C) to 650°C is preferred. Setting the lower limit of the bath temperature to melting point + 20°C is necessary for hot-dip plating, which requires the bath temperature to be above the solidification point. Setting it to melting point + 20°C prevents solidification caused by localized temperature drops in the plating bath. On the other hand, setting the upper limit of the bath temperature to 650°C is due to concerns that above 650°C, the plating film becomes difficult to cool rapidly, and the interfacial alloy layer formed between the plating film and the steel sheet thickens.

Furthermore, there are no particular limitations on the temperature of the base steel plate immersed in the plating bath (immersion temperature). However, from the perspective of ensuring the plating characteristics in the aforementioned continuous hot-dip plating operation or preventing changes in the bath temperature, it is preferable to control the temperature to be within ±20°C relative to the aforementioned plating bath temperature.

Furthermore, the immersion time of the aforementioned substrate steel plate in the aforementioned plating bath is preferably 0.5 seconds or more. This is because if the immersion time is less than 0.5 seconds, there is a concern that a sufficient plating film may not be formed on the surface of the aforementioned substrate steel plate. There is no particular upper limit to the immersion time, but since there is a concern that the interfacial alloy layer formed between the plating film and the steel plate may become thicker if the immersion time is too long, it is preferable to make it within 8 seconds.

Furthermore, the manufacturing of the hot-dip galvanized Al-Zn steel sheet of this invention is characterized by setting the maximum reheating temperature T of the steel sheet with the aforementioned coating to 130~300°C. After reheating, the average cooling rate C (°C/hr) from the aforementioned reheating temperature T (°C) to 100°C satisfies the following relationship (3): C≦(T-100)/2　・・・(3) This is because by applying heat treatment that satisfies the above conditions, the coating softens and its ductility is improved, thus giving the hot-dip galvanized Al-Zn steel sheet excellent workability.

Generally, the solidification of the coating in the manufacturing of hot-dip Al-Zn steel sheets occurs under non-equilibrium conditions with a high cooling rate. Therefore, because the α-Al phase in the coating becomes supersaturated with Zn, the resulting hot-dip Al-Zn coating is hard and has low ductility. By applying the aforementioned heat treatment (heating) to this state of the hot-dip Al-Zn coating, the distortion accumulated in the coating during solidification is released, and solid diffusion occurs within the coating, thereby achieving the separation of the Al and Zn two phases. In other words, hot-dip galvanized Al-Zn steel sheets subjected to heat treatment exhibit high ductility due to the reduced Zn concentration in the α-Al phase, resulting in a softened coating and superior workability and corrosion resistance of the machined areas.

In the aforementioned heating process, the heating temperature T is 130~300℃. When the heating temperature T is below 130℃, the solid diffusion rate is low, so the two-phase separation will not occur sufficiently, thus failing to adequately soften the coated film. On the other hand, when the heating temperature T exceeds 300℃, the growth of alloy phases at the interface between the substrate steel plate and the coated film will be promoted, which will adversely affect processability. Therefore, the range of the aforementioned heating temperature T is set to 130~300℃. Furthermore, based on the same viewpoint, the aforementioned heating temperature T is preferably 130~200℃.

Furthermore, the average cooling rate C (°C/hr) from the aforementioned heating temperature T (°C) to 100°C after the heating is performed satisfies the aforementioned relationship (3) in order to promote the separation of the two phases of Al and Zn. Since the solid diffusion of Al and Zn continues even during the cooling process, obtaining sufficient cooling time in a manner that satisfies relationship (3) can effectively promote the separation of the two phases of Al and Zn. On the other hand, when the aforementioned average cooling rate C (°C/hr) is greater than (T-100)/2, the separation of the two phases of Al and Zn becomes insufficient, and the softening of the coated film cannot be sufficiently induced, thus failing to meet the requirements for processability and corrosion resistance of the processed part.

Furthermore, the heat treatment or subsequent heat preservation applied to the aforementioned heating system can be performed using heating or heat preservation devices installed within or outside the continuous hot-dip galvanizing equipment. Heating mechanisms (e.g., induction heaters, hot air furnaces, etc.) can be installed within the aforementioned continuous hot-dip galvanizing equipment for continuous heating on the production line. Alternatively, heating can be performed in batches by winding the material into coils and then removing it from the coil. Continuous heating can also be performed in continuous processing equipment outside the plating production line using heating mechanisms (e.g., induction heaters, hot air furnaces, etc.). Furthermore, the continuously heated coated steel sheet can be coiled into a coil within the coating production line or the aforementioned continuous processing equipment, and then appropriately insulated or kept heated. Additionally, during the cooling process of the hot-dip coated metal after solidification, a heat preservation device can be installed to keep the aforementioned coated film warm and allow for slow cooling. However, there are no particular restrictions on the method, shape, or scale of the heating or heat preservation device; the important thing is that it can provide the aforementioned thermal trace to the coated film.

Furthermore, in the manufacturing method of hot-dip Al-Zn steel sheet of the present invention, in addition to the above-mentioned steps of forming the coating film and the heating and cooling steps after the formation of the coating film, the steps commonly used for coating steel sheet can also be implemented.

[Example] [Samples 1-37] Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by conventional methods as the base steel sheet, and annealing and plating treatment were performed using a hot-dip galvanizing simulator manufactured by Rhesca, samples 1-37 of hot-dip galvanized steel sheets under the conditions shown in Table 1 were produced. Furthermore, regarding the composition of the plating bath used in manufacturing the hot-dip galvanized steel sheets, the composition of the plating bath was such that the plating film composition of each sample shown in Table 1 was as follows: Al: 0.2-70% by mass, Si: 0.0-3.2% by mass, B: 0.00-0.02% by mass, Ca: 0.0-1.0% by mass, Ti: 0.0-0.1% by mass, V: 0.1-0.1% by mass. Various variations are made within the range of Cr: 0.0~0.2% by mass, Mn: 0.0~0.1% by mass, Sr: 0.0~0.1% by mass, Mo: 0.0~0.1% by mass, In: 0.0~0.5% by mass, Sn: 0.0~0.1% by mass, Sb: 0.0~0.1% by mass, Ce: 0.0~1.0% by mass, and Bi: 0.00~0.05% by mass. Furthermore, the plating bath temperature is set to 460℃ when Al is 0.2~5% by mass, 600℃ when Al is 35~55% by mass, and 660℃ when Al exceeds 60% by mass. The immersion temperature of the base steel plate is controlled to be the same as the plating bath temperature. Furthermore, the plating treatment was performed at a temperature range of 520-500°C with a mass percentage of Al of 35-70%, and cooled for 3 seconds. The amount of plating film adhered was controlled as follows: 85±5 g/ ^m² for each side of samples 1-34, 50±5 g/ ^m² for each side of sample 35, 100±5 g/ ^m² for each side of sample 36, and 125±5 g/ ^m² for each side of sample 37. For samples 1-37, except for sample 16, after the above plating treatment, a specified heat cycle (heating and cooling) was applied in a heating furnace. The heat cycle conditions are shown in Table 1.

[Evaluation] The following evaluation was conducted on each of the obtained hot-dip galvanized steel sheet samples. The evaluation results are shown in Table 1.

(1) Coated Film (Composition, Adhesion Amount, X-ray Diffraction Intensities of _{MgZn2 and Mg2Zn11} ) For each sample of the prepared coated steel sheet, a 100mm diameter punch was made. After sealing the non-measurement surfaces with tape, the coating was dissolved and peeled off using a mixture of hydrochloric acid and hexamethylenetetramine as shown in JIS H 0401:2013. The adhesion amount of the coated film was calculated from the mass difference of the sample before and after peeling. The calculated results and the adhesion amount of the obtained coated film are shown in Table 1. Subsequently, the peeling liquid was filtered, and the composition of the filtrate and solids were analyzed separately. Specifically, the components other than insoluble Si were quantified by ICP emission spectroscopy analysis of the filtrate. Furthermore, the solid composition was determined by drying and ashing in a furnace at 650°C, followed by the addition of sodium carbonate and sodium tetraborate to dissolve the solids. The melt was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the coated film was calculated by adding the soluble Si concentration obtained from filtrate analysis to the insoluble Si concentration obtained from solid composition analysis. The calculated results, along with the composition of the obtained coated film, are shown in Table 1. Furthermore, for each sample, after being cut into 100mm×100mm sizes, the coating film on the evaluation symmetry surface was mechanically cut until the base steel plate appeared. After well mixing the obtained powder, 0.3g was taken out and qualitative analysis of the powder was performed using an X-ray diffraction apparatus (RIGAKU Inc. "SmartLab"). The X-rays were Cu-Kα (wavelength = 1.54178Å), Kβ line removal was performed using a Ni filter, tube voltage was 40kV, tube current was 30mA, scanning speed was 4°/min, sampling interval was 0.020°, divergence slit was 2/3°, Soller slit was 5°, and the detector was a high-speed one-dimensional detector (D/teX Ultra). The intensity obtained by subtracting the base strength from each peak intensity was used as the diffraction intensity (kcps). The diffraction intensity of _the (100) plane (interface spacing d = 0.4510 nm) of MgZn2 and the diffraction intensity of the ( ₃₂₁ ) plane (interface spacing d = 0.2290 nm) of _Mg2Zn11 were measured. The measurement results are shown in Table 1.

(2) Processability Evaluation Each hot-dip galvanized steel sheet sample was cut to a size of 70mm × 150mm. Sheets of the same thickness were then subjected to a 180° bend (nT bend) by clamping n sheets (n=4, 5, 6, 7, 8) together on the inside, ensuring a 150mm apex. Cellotape (registered trademark) was then firmly affixed to the outer surface of the bend and peeled off. The crack formation morphology was confirmed by observing the outer surface (apex) of the bend using a scanning electron microscope (Carl Zeiss ULTRA55) under an accelerating voltage of 5kV. Processability was evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: No large cracks with an opening width exceeding 20μm were found. ×: Larger cracks with an opening width exceeding 20μm were found.

(3) Corrosion Resistance Evaluation of the Processed Part Each hot-dip galvanized steel sheet sample was cut to a size of 70mm × 150mm. Each end face was sealed with tape. Six or eight sheets of the same thickness were sandwiched inside each sample, ensuring a 150mm apex, and 6T and 8T bending tests were applied respectively. For each sample prepared as described above, the Japanese Automobile Standard Combined Cyclic Test (JASO-CCT) was performed. The corrosion acceleration test started with wet conditions and continued for 60 cycles. The appearance of the outer surface (apex) of the bend of each sample was visually inspected, and the evaluation was performed according to the following criteria. The evaluation results are shown in Table 1. ○: No red or white rust was found in the samples subjected to 8T bending. ×: Red or white rust was found in the samples subjected to 8T bending.

As shown in Table 1, compared with the comparative examples, the processability and corrosion resistance of the processed parts of the samples of this invention are well-balanced and excellent. [Industrial Applicability]

According to the present invention, a hot-dip galvanized Al-Zn steel sheet with reliable and stable excellent workability and corrosion resistance of the processed parts and a method thereof can be provided.

10: Base steel plate 20: Coated film 21:Main floor 22: Interface alloy layer 211:Dendrite 212:Dendritic intergranular

[Figure 1] A schematic diagram showing an enlarged cross-section of the hot-dip galvanized Al-Zn steel sheet of this embodiment.

10: Base steel plate 20: Coated film 21:Main floor 22: Interface alloy layer 211:Dendrite 212:Dendritic intergranular

Claims (8)

A hot-dip galvanized Al-Zn steel sheet, characterized by a galvanized coating comprising: 45-65% by mass of Al and 1.0-3.0% by mass of Si, and a total of 0.01-3.0% by mass of one or more elements selected from B, Ca, Ti, Sr, Mo, In, Sn, Sb, Ce, and Bi, with the remainder consisting of Zn and unavoidable impurities. The galvanized coating substantially does not contain Cr or Mn, and the Mg content relative to the total mass of the galvanized coating is less than 0.3% by mass. The _MgZn₂ and _Mg₂Zn₂ components in the galvanized coating are... The diffraction intensity obtained by X-ray diffraction of ₁₁ satisfies the following relationships (1) and (2), the interface alloy layer is formed at the interface between the aforementioned coated film and the base steel plate, the interface alloy layer has Fe-Al system and Fe-Al-Si system intermetallic compounds; MgZn ₂ (100)=0 ・・・(1) Mg ₂ Zn ₁₁ (321)=0 ・・・(2) MgZn ₂ (100): diffraction intensity of the (100) plane of MgZn ₂ (interface spacing d=0.4510nm), Mg ₂ Zn ₁₁ (321): diffraction intensity of the (321) plane of Mg ₂ Zn ₁₁ (interface spacing d=0.2290 nm). For example, in the hot-dip galvanized Al-Zn steel sheet of claim 1, the unavoidable impurity content in the aforementioned coating is less than 5.0% by mass. For example, in the hot-dip galvanized Al-Zn steel sheet of claim 1 or 2, the Mg content in the unavoidable impurities of the aforementioned galvanized film is less than 0.3% by mass relative to the total mass of the aforementioned galvanized film. For example, in the hot-dip galvanized Al-Zn steel sheet of claim 2, the Mg content in the unavoidable impurities of the aforementioned galvanized film is less than 0.3% by mass relative to the total mass of the aforementioned galvanized film. A method for manufacturing hot-dip galvanized Al-Zn steel sheet, comprising a coated film, characterized in that: the aforementioned coated film is formed on a base steel sheet using a coating bath, the coating bath comprising: 45-65% by mass of Al and 1.0-3.0% by mass of Si, and a total of 0.01-3.0% by mass of one or more of B, Ca, Ti, Sr, Mo, In, Sn, Sb, Ce, and Bi, with the remainder consisting of Zn and unavoidable impurities, and substantially free of Cr and Mn; and the Mg content relative to the total mass of the aforementioned coating bath is controlled to be below 0.3% by mass. The heating temperature T at which the steel sheet with the aforementioned coating is reheated is set to the highest temperature reached is 130~300°C. After heating, the average cooling rate C (°C/hr) from the aforementioned heating temperature T (°C) to 100°C satisfies the following relationship (3): An interface alloy layer is formed at the interface between the aforementioned coating and the base steel plate. The interface alloy layer has intermetallic compounds of Fe-Al system and Fe-Al-Si system; C≦(T-100)/2・・・(3). For example, in the method for manufacturing hot-dip galvanized Al-Zn steel sheets as described in claim 5, the content of unavoidable impurities in the aforementioned plating bath is less than 5.0% by mass. For example, in the method of manufacturing hot-dip galvanized Al-Zn steel sheet as claimed in claim 5 or 6, the Mg content in the unavoidable impurities in the aforementioned galvanizing bath is less than 0.3% by mass relative to the total mass of the aforementioned galvanizing bath. For example, in the method for manufacturing hot-dip galvanized Al-Zn steel sheet as claimed in claim 6, the Mg content in the unavoidable impurities in the aforementioned galvanizing bath is less than 0.3% by mass relative to the total mass of the aforementioned galvanizing bath.