Surface-treated steel sheet and method for manufacturing the same

Abstract

Provided is a surface-treated steel sheet having stable and excellent corrosion resistance and white rust resistance. [Solution] To achieve the above-mentioned object, the present invention provides a surface-treated steel sheet comprising a plating film and a chemical conversion film formed on the plating film, wherein the plating film contains 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the remainder consisting of Zn and unavoidable impurities, and the Ni content of the unavoidable impurities is 0.010 mass% or less relative to the total mass of the plating film, and the chemical conversion film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound.

Description

[Technical Field]

[0001] This invention relates to a surface-treated steel sheet having stable and excellent corrosion resistance and white rust resistance, and a method for manufacturing the same. [Background technology]

[0002] Hot-dip galvanized steel sheets, such as the 55% Al-Zn type, are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection of zinc with the high corrosion resistance of aluminum. For this reason, due to their excellent corrosion resistance, hot-dip galvanized steel sheets are mainly used in building materials such as roofs and walls that are exposed to the outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring and piping, and sound barriers. In particular, the demand for materials with superior corrosion resistance and maintenance-free materials in harsher operating environments, such as acid rain caused by air pollution, the application of de-icing agents to prevent road freezing in snowy areas, and coastal development, has been increasing in recent years.

[0003] The plating film of hot-dip Al-Zn plated steel sheets is characterized by a structure in which multiple α-Al phases are stacked in the direction of the plating film thickness, consisting of a portion where Al containing supersaturated Zn has solidified in a dendrite-like manner (α-Al phase) and a Zn-Al eutectic structure present in the interdendrite gaps. Due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to progress easily. As a result, hot-dip Al-Zn plated steel sheets are known to achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same plating film thickness.

[0004] Attempts have been made to further extend the lifespan of such hot-dip Al-Zn plated steel sheets, and hot-dip Al-Zn-Si-Mg plated steel sheets with added Mg have been put into practical use. As an example of such a hot-dip Al-Zn-Si-Mg plated steel sheet, Patent Document 1 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the plating film contains an Al-Zn-Si alloy containing Mg, the Al-Zn-Si alloy is an alloy containing 45-60% by weight of elemental aluminum, 37-46% by weight of elemental zinc, and 1.2-2.3% by weight of Si, and the concentration of Mg is 1-5% by weight. Furthermore, Patent Document 2 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that aims to improve corrosion resistance and enhance the protective effect after the underlying steel sheet is exposed by containing 2-10% Mg and 0.01-10% Ca in the plating film. Furthermore, Patent Document 3 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that improves the corrosion resistance of flat plates and end faces by forming a coating layer containing Mg: 1-15%, Si: 2-15%, and Zn: 11-25% by mass, with the remainder being Al and unavoidable impurities, and by making the size of intermetallic compounds such as Mg2Si phase and MgZn2 phase present in the plating film 10 μm or less.

[0005] As mentioned above, hot-dip molten Al-Zn plated steel sheets have a beautiful appearance with a white metallic spangle pattern, and are often used without painting. Therefore, there is a strong demand for a high-quality appearance. Consequently, technologies to improve the appearance of hot-dip molten Al-Zn plated steel sheets have been developed. For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film. Furthermore, Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film.

[0006] Furthermore, the aforementioned hot-dip Al-Zn plated steel sheets had a problem in that, when used in harsh corrosive environments, white rust would occur due to corrosion of the plating film. Since this white rust degrades the appearance of the steel sheet, development is underway to improve the resistance to white rust in plated steel sheets. For example, Patent Document 6 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the mass ratio of Mg in the Si-Mg phase to the total amount of Mg in the plating layer is optimized for the purpose of improving the white rust resistance of the processed part. Furthermore, Patent Document 7 discloses a technology that improves resistance to blackening and white rust by forming a chemical conversion film containing urethane resin on the plating film of a molten Al-Zn-Si-Mg plated steel sheet. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 5020228 [Patent Document 2] Patent No. 5000039 [Patent Document 3] Japanese Patent Publication No. 2002-12959 [Patent Document 4] Patent No. 3983932 [Patent Document 5] Special Publication No. 2011-514934 [Patent Document 6] Patent No. 5751093 [Patent Document 7] Japanese Patent Publication No. 2019-155872 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the technique of incorporating Mg into the plating film, as disclosed in Patent Documents 1 to 3, does not necessarily guarantee an improvement in corrosion resistance. In the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 1 to 3, corrosion resistance is improved solely by including Mg in the plating components. However, the influence of components other than the four elements mentioned above (Al, Zn, Si, Mg) and the characteristics of the metallic phase and intermetallic compound phase constituting the plating film are not considered, making it impossible to uniformly discuss the superiority or inferiority of corrosion resistance. Therefore, even when molten Al-Zn-Si-Mg plated steel sheets are manufactured using a plating bath composition with equivalent content of the four elements mentioned above, corrosion acceleration tests show variability in corrosion resistance, and there is a problem in that they are not necessarily superior to Al-Zn plated steel sheets without added Mg. Similarly, in improving the appearance of the plating, simply adding Sr to the plating film does not necessarily eliminate wrinkle-like uneven defects, and the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 4 and 5 sometimes failed to achieve both corrosion resistance and appearance. In addition, because Mg is an element that oxidizes easily, the Mg contained in the plating bath can generate oxides (top dross) near the bath surface, and in the case of molten plating, over time, Fe-Al compounds containing iron (bottom dross) may be generated, which are unevenly distributed in the bath or at the bottom of the plating bath. These drosses can adhere to the surface of the plating film, causing convex defects and potentially impairing the appearance of the plating film surface. Furthermore, it is known that when steel plates are plated using a molten Al-Zn-Si bath to which Mg has been added, in addition to the α-Al phase, Mg2Si phase, MgZn2 phase, and Si phase precipitate in the plating film. However, the influence of the amount and relative abundance of each phase on corrosion resistance had not been clarified. Furthermore, regarding white rust resistance, none of the technologies were able to achieve sufficient improvement. While Patent Document 6 describes improvements in white rust resistance in the processed and heated flat sections of the hot-dip molten Al-Zn-Si-Mg plated steel sheet, it does not consider white rust resistance in the unheated flat section, and achieving stable white rust resistance remains a challenge. Similarly, Patent Document 7 does not guarantee consistently excellent corrosion resistance or white rust resistance, and further improvements were desired.

[0009] In view of these circumstances, the present invention aims to provide a surface-treated steel sheet having stable and excellent corrosion resistance and white rust resistance, and a method for manufacturing the same. [Means for solving the problem]

[0010] As a result of their investigations to solve the above problems, the inventors focused on the fact that, in order to control the composition of the plating film of a molten Al-Zn-Si-Mg plated steel sheet, it is important not only to control the concentrations of Al, Zn, Si, and Mg, but also to control the concentration of elements included as impurities. They found that by appropriately controlling the Ni content, the deterioration of corrosion resistance can be effectively suppressed, and furthermore, by appropriately controlling the size and distribution of Ni-based compounds present as impurities in the plating film, the deterioration of corrosion resistance can be suppressed even more effectively. In addition, regarding the Mg2Si phase, MgZn2 phase, and Si phase formed in the plating film of the molten Al-Zn-Si-Mg plated steel sheet, the precipitation amount increases or decreases depending on the balance of each component in the plating film and the formation conditions of the plating film, and the existing ratio thereof changes. Depending on the balance of the composition, any one of the phases may not precipitate. It has been investigated that the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet changes depending on the existing ratio of these phases, and particularly when the MgZn2 phase is more than the Mg2Si phase and Si phase, the corrosion resistance is stably improved. However, regarding these Mg2Si phase, MgZn2 phase, and Si phase, it is known that it is very difficult to distinguish the difference in phases even by using a general method, for example, by utilizing a scanning electron microscope and observing the plating film from the surface or cross-section to obtain a secondary electron image or a reflected electron image. Although it is possible to obtain microscopic information by observing using a transmission electron microscope, it has not been possible to grasp the existing ratios of the Mg2Si, MgZn2, and Si phases that affect macroscopic information such as corrosion resistance and appearance. Therefore, as a result of further intensive research by the present inventors, focusing on the X-ray diffraction method, it is possible to quantitatively define the existing ratio of the phases by using the intensity ratio of specific diffraction peaks of the Mg2Si phase, MgZn2 phase, and Si phase. Furthermore, it has been found that when the Mg2Si phase and MgZn2 phase in the plating film satisfy a specific existing ratio, not only can stable and excellent corrosion resistance be achieved, but the generation of dross can be suppressed and good surface appearance can also be ensured. In addition, the present inventors have also found that by controlling the Sr concentration in the plating bath after controlling the Ni content and film structure in the above-described plating film, the generation of wrinkled concavo-convex defects can be surely suppressed, and a plated steel sheet with excellent surface appearance can be obtained.

[0011] Furthermore, the present inventors have also examined the chemical conversion film formed on the plating film, and by constituting the chemical conversion film from a specific resin and a specific metal compound, the affinity between the chemical conversion film and the plating film, the rust prevention effect, etc. are enhanced, and it has also been found that the stable improvement of the white rust resistance is improved.

[0012] This invention is based on the above findings, and its gist is as follows. 1. A surface-treated steel sheet comprising a plating film and a chemical conversion film formed on the plating film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The Ni content in the aforementioned unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. The surface-treated steel sheet is characterized in that the chemical conversion coating contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound.

[0013] 2. The surface-treated steel sheet according to claim 1, characterized in that the plating film contains a Ni-based compound, and the major axis of the Ni-based compound is 4.0 μm or less. 3. The surface treatment according to 1 or 2, characterized in that the plating film contains a Ni-based compound, and the number of Ni-based compounds present in a direction parallel to the surface of the underlying steel sheet is 5 or less per mm. Steel plate. 4. The surface-treated steel sheet according to claim 1, characterized in that the plating film does not contain a Ni-based compound.

[0014] 5. A surface-treated steel sheet according to any one of 1 to 4, characterized in that the diffraction intensity of Mg2Si and MgZn2 in the plating film, measured by X-ray diffraction, satisfies the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 (1) Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si. MgZn2(100): Diffraction intensity of the (100) plane (interplanar spacing d=0.4510nm) of MgZn2

[0015] 6. The surface-treated steel sheet according to any one of items 1 to 5, characterized in that the diffraction intensity of Si in the plating film by X-ray diffraction satisfies the following relationship (2). Si (111)=0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si

[0016] 7. The surface-treated steel sheet according to any one of claims 1 to 6, characterized in that the plating film further contains Sr: 0.01 to 1.0% by mass.

[0017] 8. The surface-treated steel sheet according to any one of 1 to 7, characterized in that the Al content in the plating film is 50 to 60% by mass.

[0018] 9. A surface-treated steel sheet according to any one of 1 to 8, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

[0019] 10. A surface-treated steel sheet according to any one of 1 to 9, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

[0020] 11. A method for manufacturing a surface-treated steel sheet comprising a plating film and a chemical conversion film formed on the plating film, The aforementioned chemical conversion coating contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. A method for manufacturing a surface-treated steel sheet, comprising a hot-dip plating step of immersing a base steel sheet in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, characterized in that the Ni content in the unavoidable impurities of the plating bath is controlled to 0.010% by mass or less relative to the total mass of the plating bath. 12. The method for manufacturing a surface-treated steel sheet according to 10, characterized in that the plating bath further contains Sr: 0.01 to 1.0% by mass. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a surface-treated steel sheet with consistently excellent corrosion resistance and white rust resistance, and a method for manufacturing the same. [Brief explanation of the drawing]

[0022] [Figure 1] This diagram illustrates the flow of the combined cycle test (JASO-CCT) according to Japanese automotive standards. [Modes for carrying out the invention]

[0023] <Surface-treated steel sheet> The surface-treated steel sheet of the present invention comprises a plating film on the surface of the steel sheet and a chemical conversion film formed on the plating film.

[0024] (Plating film) In the surface-treated steel sheet of the present invention, the plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities.

[0025] The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, considering the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45% by mass, Al dendrite solidification occurs, and a plating film structure mainly consisting of a dendrite solidified α-Al phase can be obtained. When this dendrite solidified structure is stacked in the direction of the thickness of the plating film, the corrosion progression path becomes complex, and the corrosion resistance of the plating film itself is improved. Furthermore, the more of these α-Al phase dendrites that are stacked, the more complex the corrosion progression path becomes, making it more difficult for corrosion to easily reach the underlying steel plate, thus improving corrosion resistance. For this reason, it is preferable to have an Al content of 50% by mass or more. On the other hand, if the Al content in the plating film exceeds 65% by mass, the structure changes to one in which most of the Zn is solid-dissolved in α-Al, the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg plating deteriorates. Therefore, the Al content in the plating film must be 65% by mass or less, preferably 60% by mass or less.

[0026] The Si in the aforementioned plating film is added primarily to suppress the growth of Fe-Al and / or Fe-Al-Si interfacial alloy layers that form at the interface with the underlying steel sheet, thereby preventing deterioration of the adhesion between the plating film and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn plating bath containing Si, Fe on the surface of the steel sheet reacts with Al and Si in the bath to form an alloy, and Fe-Al and / or Fe-Al-Si intermetallic compound layers are formed at the underlying steel sheet / plating film interface. However, since the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. For this reason, the Si content in the plating film must be 1.0% by mass or more. On the other hand, if the Si content in the plating film exceeds 4.0% by mass, not only does the aforementioned effect of suppressing the growth of the interfacial alloy layer saturate, but the presence of an excess Si phase in the plating film promotes corrosion, so the Si content should be 4.0% or less. Furthermore, the Si content in the plating film is preferably 3.0% or less, from the viewpoint of suppressing the presence of an excess Si phase. In addition, from the viewpoint of easily satisfying the relationship (1) described below in relation to the Mg content, it is preferable that the Si content be 1.0 to 3.0% by mass.

[0027] The aforementioned plating film contains 1.0 to 10.0% Mg. By including Mg in the plating film, the aforementioned Si can be present in the form of an intermetallic compound of the Mg2Si phase, thereby suppressing the acceleration of corrosion. Furthermore, when Mg is included in the plating film, an intermetallic compound, the MgZn2 phase, is also formed in the plating film, resulting in an improved corrosion resistance. If the Mg content in the plating film is less than 1.0% by mass, sufficient corrosion resistance cannot be ensured because Mg is used for solid solution into the α-Al phase, which is the main phase, rather than for the formation of the intermetallic compounds (Mg2Si, MgZn2). On the other hand, if the Mg content in the plating film is too high, the effect of improving corrosion resistance saturates, and the processability decreases due to the weakening of the α-Al phase; therefore, the content should be 10.0% or less. Moreover, from the viewpoint of suppressing dross generation during plating formation and facilitating plating bath management, the Mg content in the plating film is preferably 5.0% by mass or less. Furthermore, in relation to the Si content, from the viewpoint of easily satisfying the relationship (1) described below, it is preferable to set the Mg content to 3.0 mass%, and from the viewpoint of compatibility with dross suppression, it is more preferable to set the Mg content to 3.0 to 5.0 mass%.

[0028] Furthermore, the plating film contains Zn and unavoidable impurities. Of these, the unavoidable impurities include Fe. This Fe is inevitably included in the plating film as a result of the elution of steel plates and equipment in the plating bath, and as a result of diffusion from the underlying steel plate during the formation of the interfacial alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%. Other unavoidable impurities include Cr, Ni, and Cu. These components inevitably become present in the plating film when the underlying steel plate or stainless steel bath equipment dissolves into the plating bath, when they are present as impurities in the metal ingots that serve as the raw materials for the plating bath, and when the plated steel sheets are manufactured using pots and bath equipment that were used in the production of plated steel sheets that had these components intentionally added.

[0029] Furthermore, the surface-treated steel sheet of the present invention is characterized in that the Ni content in the unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. Since Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet, the degradation of corrosion resistance can be suppressed by appropriately controlling the content of Al, Zn, Si, and Mg in the plating film as described above, and further suppressing the Ni content as an unavoidable impurity. From a similar viewpoint, it is preferable that the Ni content in the unavoidable impurities be 0.005% by mass or less relative to the total mass of the plating film.

[0030] Furthermore, if Ni is included in the aforementioned unavoidable impurities, Ni-based compounds may be present as impurities in the plating film of the molten Al-Zn-Si-Mg plated steel sheet. Here, the Ni-based compounds mainly refer to binary intermetallic compounds such as Ni-Al compounds, and ternary intermetallic compounds such as Ni-Al-Fe compounds. Examples of Ni-Al compounds include intermetallic compounds such as NiAl3, and examples of Ni-Al-Fe compounds include intermetallic compounds such as (Ni,Fe)Al3 in which some of the Ni in NiAl3 is replaced with Fe, but the compound is not limited to these. The presence of Ni-based compounds in the plating film can be confirmed, for example, by using a scanning electron microscope to observe the plating film from the surface or cross-section using secondary electron images or backscattered electron images, and then analyzing it by energy-dispersive X-ray spectroscopy (EDS). For example, by selecting 5 to 10 locations on a 100 μm cross-section of the plating, observing and performing elemental mapping analysis on each location at an accelerating voltage of 5 kV or less, and then performing point analysis on the areas where Ni is detected, the composition of the Ni-based content can be confirmed. This method is merely one example, and any method that can confirm the presence of Ni-based compounds is acceptable and is not particularly limited.

[0031] Furthermore, if the plating film contains a Ni-based compound, it is preferable that the major axis of the Ni-based compound is 4.0 μm or less. Ni-based compounds present in the aforementioned plating film can function as cathodes in corrosive environments, forming local galvanic cells with the surrounding solidified tissue, which can lead to a deterioration in corrosion resistance. In particular, the presence of coarse Ni-based compounds in the plating film can significantly reduce the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet. Therefore, in order to obtain a molten Al-Zn-Si-Mg plated steel sheet with superior corrosion resistance, it is effective to control the size of the Ni-based compounds contained as impurities in the plating film. Specifically, it is preferable to have a major axis of Ni-based compounds of 4.0 μm or less, more preferably 3.0 μm or less, and even more preferably 2.0 μm or less. The major axis of the Ni-based compound can be measured, for example, by using a scanning electron microscope to observe the plating film in cross-section using a backscattered electron image, confirming that it is a Ni-based compound using EDS, and then observing a magnified backscattered electron image of the observation field containing the Ni-based compound. The major axis of the Ni-based compound is defined as the maximum major axis of the Ni-based compound confirmed within the observation field of the plating film.

[0032] Furthermore, when the plating film contains a Ni-based compound, it is also effective to reduce the amount of the Ni-based compound present, which acts as the starting point for corrosion, from the viewpoint of obtaining more stable and high corrosion resistance. Specifically, it is preferable to have 5 or fewer Ni-based compound particles in the plating film in the direction parallel to the surface of the underlying steel sheet, more preferably 2 or fewer, and most preferably 0 or none (absent). Therefore, by suppressing the amount of Ni-containing compounds present in the plating film, the deterioration of the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet can be more reliably suppressed. In order to obtain such a film structure (a film structure that does not contain Ni-based compounds), it is important to reduce the Ni content in the unavoidable impurities, specifically, to set the Ni content to 0.005% by mass or less relative to the total mass of the plating film. Furthermore, the number of Ni-based compound particles can be determined, for example, by using a scanning electron microscope to continuously observe a cross-section parallel to the surface of the underlying steel sheet with a plating film using backscattered electron images for a length of 1 mm or more, and then dividing the number of Ni-based compounds confirmed by EDS by the measured length (mm) to calculate the number of Ni-based compounds present within a 1 mm length range.

[0033] Furthermore, while there are no particular limitations on the total content of unavoidable impurities in the plating film, it is preferable to keep the total content at 5.0% by mass or less, as excessive impurities may affect various properties of the plated steel sheet.

[0034] In the surface-treated steel sheet of the present invention, from the viewpoint of being able to more stably improve corrosion resistance while controlling the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above, it is preferable that the diffraction intensity of Mg2Si and MgZn2 in the plating film, measured by X-ray diffraction, satisfies the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 (1) Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si, MgZn2(100): Diffraction intensity of the (100) plane (interplanar spacing d=0.4510 nm) of MgZn2

[0035] As described above, in this invention, it is important to control the relative proportions of intermetallic compounds such as Mg2Si and MgZn2 that are formed in the plating film due to the inclusion of Mg to a specific ratio. The effects of these on corrosion resistance are currently under investigation and many aspects remain unclear, but the following mechanism is hypothesized.

[0036] When surface-treated steel sheets are exposed to a corrosive environment, the intermetallic compounds described above dissolve preferentially over the α-Al phase, resulting in a Mg-rich environment near the formed corrosion products. In such a Mg-rich environment, the formed corrosion products are less likely to decompose, and as a result, the protective effect of the plating film is presumed to be enhanced. Furthermore, since the protective effect of MgZn2 is greater than that of Mg2Si, it is considered effective to increase the proportion of MgZn2 in the intermetallic compounds present in the plating film.

[0037] Furthermore, it is preferable that the ratio of Mg2Si to MgZn2 in the plating film satisfies the relationship (1): Mg2Si (111) / MgZn2(100) ≤ 2.0, using the diffraction peak intensity obtained by X-ray diffraction. If the ratio of Mg2Si and MgZn2 in the plating film does not satisfy relationship (1), that is, if Mg2Si (111) / MgZn2(100) > 2.0, then there is a large amount of Mg2Si in the intermetallic compounds present in the plating film, making it difficult to obtain the aforementioned Mg-rich environment near the corrosion products, and thus the protective effect of the plating film may not be improved.

[0038] Here, in relation (1) above, Mg2Si (111) is the diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si, and MgZn2 (100) is the diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) of MgZn2. As a method for measuring Mg2Si (111) and MgZn2 (100) by the aforementioned X-ray diffraction, a portion of the plating film is mechanically scraped off, and X-ray diffraction is performed on the powdered material (powder X-ray diffraction measurement method). For the measurement of diffraction intensity, the diffraction peak intensity of Mg2Si corresponding to the interplanar spacing d = 0.3668 nm and the diffraction peak intensity of MgZn2 corresponding to the interplanar spacing d = 0.4510 nm are measured, and the ratio of these can be calculated to obtain Mg2Si (111) / MgZn2 (100). Furthermore, the amount of plating film required for powder X-ray diffraction measurement (the amount of plating film to be removed) is 0.1g or more, and preferably 0.3g or more, from the viewpoint of accurately measuring Mg2Si (111) and MgZn2 (100). In addition, when removing the plating film, steel sheet components other than the plating film may be included in the powder, but these intermetallic compound phases are contained only in the plating film and do not affect the peak intensity mentioned above. Moreover, the reason for performing X-ray diffraction on the plating film in powder form is that if X-ray diffraction is performed on the plating film formed on the plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the surface orientation of the solidification structure of the plating film.

[0039] Furthermore, in the surface-treated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film, measured by X-ray diffraction, satisfies the following relationship (2), since the corrosion resistance can be improved more stably by controlling the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above. Si (111)=0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si Generally, in the dissolution reaction of Al alloys in aqueous solutions, the presence of the Si phase as a cathode site is known to promote the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is effective in suppressing the dissolution of the α-Al phase, and among these methods, creating a film without any Si phase (making the diffraction peak intensity of Si(111) zero), as shown in relation (2), is the best for stabilizing corrosion resistance. Furthermore, the method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction can be the same as the method for measuring Mg2Si (111) and MgZn2 (100) described above.

[0040] Here, the method for satisfying the above-mentioned relationships (1) and (2) is not particularly limited. For example, in order to satisfy relationships (1) and (2), the relative abundance of Mg2Si, MgZn2, and Si (diffraction intensities of Mg2Si (111), MgZn2 (100), and Si (111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. The balance of the Si content, Mg content, and Al content in the plating film does not necessarily satisfy relationships (1) and (2) by setting the content ratio to a constant ratio; for example, it may be necessary to change the content ratio of Mg and Al depending on the Si content (mass%). In addition to adjusting the balance of Si, Mg, and Al content in the plating film, the diffraction intensities of Mg2Si(111), MgZn2(100), and Si(111) can be controlled to satisfy relationships (1) and (2) by adjusting the conditions during plating film formation (for example, cooling conditions after plating).

[0041] Furthermore, in the surface-treated steel sheet of the present invention, it is preferable that the plating film contains 0.01 to 1.0% by mass of Sr. By including Sr in the plating film, the occurrence of surface defects such as wrinkle-like unevenness defects can be more reliably suppressed, and a good surface appearance can be achieved. The aforementioned wrinkle-like defects are defects that appear as wrinkle-like irregularities on the surface of the plating film, and are observed as whitish streaks on the surface of the plating film. Such wrinkle-like defects are more likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip galvanized steel sheet, by including Sr in the plating film, Sr is preferentially oxidized over Mg on the surface layer of the plating film, thereby suppressing the oxidation reaction of Mg and making it possible to suppress the occurrence of the aforementioned wrinkle-like defects.

[0042] Furthermore, in the surface-treated steel sheet of the present invention, it is preferable that the ratio of Mg2Si and MgZn2 in the plating film satisfies relationship (1) and that the plating film contains 0.01 to 1.0 mass% of Sr. This is because the effect of improving the surface appearance due to Sr described above can be enjoyed to a greater extent. Although the reason for this is not clear, it is presumed that if the amount of Mg2Si in the plating film is high, oxidation of the plating surface layer is not easily suppressed in the first place, which affects the effect of improving the appearance when Sr is added. If the Sr content in the plating film is less than 0.01 mass%, it is difficult to obtain the effect of suppressing the occurrence of the wrinkle-like defects described above, and if the Sr content in the plating film exceeds 1.0 mass%, Sr may be excessively incorporated into the interface alloy layer, which may affect the plating adhesion and other aspects beyond the effect of improving the appearance. Therefore, it is preferable that the Sr content in the plating film be 0.01 to 1.0 mass%.

[0043] Furthermore, the plating film, like the Mg mentioned above, can improve the stability of corrosion products and slow down the progression of corrosion. Therefore, it is preferable that it further contains one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Co, Sb, and B in a total amount of 0.01 to 10% by mass. The reason for setting the total content of the above-mentioned components to 0.01 to 10% by mass is that a sufficient corrosion delaying effect can be obtained without the effect becoming saturated.

[0044] Furthermore, the amount of the plating film applied is 45 to 120 g / m² per side, from the viewpoint of satisfying various characteristics. 2 Preferably, the amount of the plating film is 45 g / m². 2 In the above case, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance, such as building materials, and the amount of the plating film attached is 120 g / m². 2 In the following cases, excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From a similar viewpoint, the amount of plating film attached is 45-100 g / m². 2 It is preferable that it be so.

[0045] The amount of plating film attached can be determined, for example, by dissolving and peeling the plating film from a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the amount from the difference in steel plate weight before and after peeling. To determine the amount of plating attached to one side using this method, the plated surface of the non-target side can be sealed with tape to prevent exposure, and then the aforementioned dissolution process can be carried out.

[0046] Furthermore, the component composition of the plating film can be determined in the same way as the Ni content described above. This can be done by immersing the plating film in hydrochloric acid or the like to dissolve it, and then examining the solution using ICP emission spectrometry or atomic absorption spectrometry. This method is merely one example, and any method that can accurately quantify the component composition of the plating film is acceptable and is not particularly limited.

[0047] Furthermore, the plating film of the surface-treated steel sheet obtained by this invention has a composition that is approximately the same as that of the plating bath. Therefore, the composition of the plating film can be controlled with high precision by controlling the composition of the plating bath.

[0048] Furthermore, the base steel sheet constituting the surface-treated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheets, hot-rolled steel sheets, etc., can be used as appropriate depending on the required performance and specifications.

[0049] Furthermore, there are no particular limitations on the method for obtaining the base steel sheet. For example, in the case of hot-rolled steel sheets, those that have undergone a hot-rolling process and a pickling process can be used, and in the case of cold-rolled steel sheets, a cold-rolling process can be added to the manufacturing process. In addition, it is possible to go through a recrystallization annealing process or the like before the hot-dip galvanizing process in order to obtain the properties of the steel sheet.

[0050] Furthermore, the conditions for forming the plating film when manufacturing the surface-treated steel sheet of the present invention are not particularly limited to a hot-dip galvanizing process in which the base steel sheet is immersed in a plating bath. The hot-dip galvanizing process may, for example, involve cleaning the base steel sheet in a continuous hot-dip galvanizing facility. This can be carried out by heating and immersion in a plating bath. In the heating process of the steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing the trace oxide film present on the surface.

[0051] Furthermore, as mentioned above, since the composition of the plated film is approximately the same as that of the plating bath, a plating bath containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, can be used for the plating bath used in the molten plating process.

[0052] Furthermore, in the hot-dip plating process, the Ni content in the unavoidable impurities of the plating bath must be controlled to 0.010% by mass or less relative to the total mass of the plating bath. As mentioned above, Ni contained in the plating film may degrade the corrosion resistance of the hot-dip Al-Zn-Si-Mg plated steel sheet. Therefore, by appropriately controlling the content of Al, Zn, Si, and Mg in the plating bath, and further suppressing the Ni content as an unavoidable impurity, the degradation of corrosion resistance can be suppressed. Furthermore, the Ni content in the plating bath as an unavoidable impurity must be controlled to 0.010% by mass or less relative to the total mass of the plating bath, and preferably to 0.005% by mass or less. If the Ni content in the plating bath exceeds 0.005% by mass, the corrosion resistance of the manufactured molten Al-Zn-Si-Mg plated steel sheet may deteriorate, and if it exceeds 0.010%, a significant deterioration in corrosion resistance may occur. There is no lower limit for the Ni content that adversely affects corrosion resistance.

[0053] The means for reducing the Ni content in the plating bath are not particularly limited. For example, since it is effective to suppress the elution of stainless steel bath equipment into the plating bath, it is preferable to treat the surface of the bath equipment with a thermal spray coating or the like. This is because the formation of the thermal spray coating or the like can impart corrosion resistance to the plating bath to the bath equipment, thereby suppressing the elution of the bath equipment into the plating bath. The type of thermal spray coating is not particularly limited, but a coating with heat resistance and corrosion resistance such as WC-based or MoB-based coatings can be selected. Furthermore, it is more effective to use bath equipment made of heat-resistant material that does not contain Ni. In this case, even if the bath equipment elutes, an increase in Ni content can be prevented.

[0054] Furthermore, as another means of reducing the Ni content in the plating bath, it is preferable to use a metal ingot with a low Ni content among impurities as the raw material for the plating bath. Furthermore, it is also effective to avoid using pots and bath equipment used in the production of plated steel sheets with intentionally added Ni for the production of molten Al-Zn-Si-Mg plated steel sheets. This is because it prevents Ni-containing metal lumps adhering to the pots and bath equipment from dissolving and mixing into the plating bath.

[0055] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the temperature range of (melting point + 20°C) to 650°C. The reason the lower limit of the bath temperature is set to the melting point + 20°C is that in order to perform the molten plating process, it is necessary to raise the bath temperature above the solidification point, and setting it to the melting point + 20°C prevents solidification due to a localized drop in the bath temperature of the plating bath. On the other hand, the reason the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet will become thicker.

[0056] Furthermore, while there are no particular limitations on the temperature of the base steel plate that enters the plating bath (entry plate temperature), it is preferable to control it to within ±20°C of the temperature of the plating bath from the viewpoint of ensuring plating characteristics and preventing changes in the bath temperature during continuous hot-dip galvanizing operations.

[0057] Furthermore, the immersion time of the base steel plate in the plating bath is preferably 0.5 seconds or longer. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer formed between the plating film and the steel plate, it is more preferable to keep it within 8 seconds.

[0058] (Chemical conversion coating) The surface-treated steel sheet of the present invention has a chemical conversion coating formed on the aforementioned plating film. Furthermore, the chemical conversion coating only needs to be formed on at least one side of the surface-treated steel sheet, and may also be formed on both sides of the surface-treated steel sheet depending on the application and required performance.

[0059] Furthermore, in the surface-treated steel sheet of the present invention, the chemical conversion coating is characterized by containing at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. By forming the aforementioned chemical conversion film on the plated film, the affinity with the plated film is increased, enabling the uniform formation of the chemical conversion film on the plated film. In addition, the rust-preventive and barrier effects of the chemical conversion film can be enhanced. As a result, stable corrosion resistance and white rust resistance of the surface-treated steel sheet of the present invention can be achieved.

[0060] Here, the resin constituting the chemical conversion coating is selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, from the viewpoint of improving corrosion resistance. From a similar viewpoint, it is preferable that the resin contains at least one of urethane resin and acrylic resin. The resin constituting the chemical conversion coating also includes addition polymers of the above-mentioned resins.

[0061] For example, the epoxy resin can be a glycidyl ether of bisphenol A type, bisphenol F type, novolac type epoxy resin, a glycidyl ether of bisphenol A type epoxy resin with propylene oxide, ethylene oxide, or polyalkylene glycol added, aliphatic epoxy resin, alicyclic epoxy resin, polyether epoxy resin, etc.

[0062] For example, the urethane resin can be an oil-modified polyurethane resin, an alkyd polyurethane resin, a polyester polyurethane resin, a polyether polyurethane resin, a polycarbonate polyurethane resin, or the like.

[0063] Examples of the acrylic resin include polyacrylic acid and its copolymers, polyacrylic acid esters and their copolymers, polymethacrylic acid and its copolymers, polymethacrylic acid esters and their copolymers, urethane-acrylic acid copolymers (or urethane-modified acrylic resins), styrene-acrylic acid copolymers, and moreover, resins modified with other alkyd resins, epoxy resins, phenolic resins, etc. can be used.

[0064] Examples of the aforementioned acrylic silicone resin include a resin having hydrolyzable alkoxysilyl groups in the side chains or terminals of an acrylic copolymer as the main component, to which a curing agent is added. Furthermore, when using acrylic silicone resin, in addition to corrosion resistance, excellent weather resistance is also provided. Sex is possible.

[0065] Examples of the alkyd resins mentioned above include oil-modified alkyd resins, rosin-modified alkyd resins, phenol-modified alkyd resins, styrene-modified alkyd resins, silicon-modified alkyd resins, acrylic-modified alkyd resins, oil-free alkyd resins, and high molecular weight oil-free alkyd resins.

[0066] The polyester resin is a polycondensate synthesized by dehydrating and condensing a polycarboxylic acid and a polyalcohol to form an ester bond. Examples of polycarboxylic acids include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of polyalcohols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, and 1,4-cyclohexanedimethanol. Specifically, examples of polyesters include polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate. Acrylic-modified versions of these polyester resins can also be used.

[0067] Examples of the polyalkylene resins include ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, carboxyl-modified polyolefin resins, ethylene-unsaturated carboxylic acid copolymers, ethylene-ionomers, and the like. Furthermore, these resins can be modified with other alkyd resins, epoxy resins, phenolic resins, and the like.

[0068] The amino resin mentioned above is a thermosetting resin produced by the reaction of an amine or amide compound with an aldehyde, and examples include melamine resin, guanamine resin, and thiourea resin. However, from the viewpoint of corrosion resistance, weather resistance, and adhesion, it is preferable to use melamine resin. There are no particular limitations on the melamine resin, but examples include butylated melamine resin, methylated melamine resin, and aqueous melamine resin.

[0069] Examples of the aforementioned fluororesins include fluoroolefin polymers and copolymers of fluoroolefins with alkyl vinyl ethers, synchronous alkyl vinyl ethers, carboxylic acid-modified vinyl esters, hydroxyalkyl allyl ethers, tetrafluoropropyl vinyl ethers, etc. When these fluororesins are used, not only corrosion resistance but also excellent weather resistance and excellent hydrophobicity can be expected.

[0070] Furthermore, the resin constituting the chemical conversion film preferably uses a curing agent in particular, with the aim of improving corrosion resistance and processability. As curing agents, urea resins (such as butylated urea resin), melamine resins (such as butylated melamine resin, butylated ether melamine resin, etc.), butylated urea / melamine resin, amino resins such as benzoguanamine resin, blocked isocyanates, oxazoline compounds, phenolic resins, etc. can be used as appropriate.

[0071] Furthermore, the metal compound constituting the chemical conversion film is 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. From a similar viewpoint, it is preferable that the metal compound contains at least one of P compounds, Si compounds, and V compounds.

[0072] Here, the P compound, when included in the chemical conversion coating, can improve corrosion resistance and sweat resistance. The P compound is a compound containing P, and may contain one or more selected from, for example, inorganic phosphoric acid, organic phosphoric acid, and salts thereof.

[0073] The inorganic phosphoric acid, organic phosphoric acid, and salts thereof can be any compound without particular limitations. For example, it is preferable to use one or more selected from phosphoric acid, monophosphate, dicaphosphate, tertiary phosphate, pyrophosphate, pyrophosphate, tripolyphosphate, tripolyphosphate, phosphorous acid, phosphate, hypophosphorous acid, and hypophosphorous acid as the inorganic phosphoric acid. It is also preferable to use phosphonic acid (phosphonic acid compound) as the organic phosphoric acid. Furthermore, it is preferable to use one or more selected from nitrilotrismethylenephosphonic acid, phosphobutanetricarboxylic acid, methyldiphosphonic acid, methylenephosphonic acid, and ethylidenediphosphonic acid as the phosphonic acid. Furthermore, if the P compound is a salt, it is preferable that the salt is a salt of an element from Group 1 to Group 13 of the periodic table, more preferably a metal salt, and more preferably one or more selected from alkali metal salts and alkaline earth metal salts.

[0074] When the chemical conversion treatment solution containing the above-mentioned P compound is applied to a molten Al-Zn-Si-Mg plated steel sheet, the P compound etches the surface of the plated film, and a concentrated layer incorporating the constituent elements of the plated film, Al, Zn, Si, and Mg, is formed on the plated film side of the chemical conversion film. The formation of this concentrated layer strengthens the bond between the chemical conversion film and the plated film surface, improving the adhesion of the chemical conversion film. The concentration of the P compound in the chemical conversion treatment solution is not particularly limited, but can be between 0.25% by mass and 5% by mass. If the concentration of the P compound is less than 0.25% by mass, the etching effect will be insufficient, reducing adhesion to the plating interface and potentially decreasing corrosion resistance of flat surfaces. Furthermore, the corrosion resistance and sweat resistance of damaged areas of the plating and film caused by defects, cut edges, and processing may also decrease. From a similar viewpoint, the concentration of the P compound is preferably 0.35% by mass or more, more preferably 0.50% by mass or more. On the other hand, if the concentration of the P compound exceeds 5% by mass, not only will the lifespan of the chemical conversion treatment solution be shortened, but the appearance of the formed film may become uneven, and the amount of P leached from the chemical conversion film may increase, potentially reducing resistance to blackening. From a similar viewpoint, the concentration of the P compound is preferably 3.5% by mass or less, more preferably 2.5% by mass or less. Regarding the content of the P compound in the aforementioned chemical conversion coating, for example, by applying and drying a chemical conversion treatment solution with a P compound concentration of 0.25% to 5% by mass, the amount of P adhering to the chemical conversion coating after drying is 5 to 100 mg / m². 2 It can be done this way.

[0075] The Si compound is a component that forms the backbone of the chemical conversion film together with the resin, and enhances its affinity with the plating film, enabling the uniform formation of the chemical conversion film. The Si compound is a compound containing Si, and preferably contains one or more selected from, for example, silica, trialkoxysilane, tetraalkoxysilane, and silane coupling agents.

[0076] The silica is not particularly limited and any type can be used. For example, at least one of wet silica and dry silica can be used. As a type of wet silica, colloidal silica, for example, Snowtex O, C, N, S, 20, OS, OXS, NS, etc. manufactured by Nissan Chemical Corporation can be suitably used. As a type of dry silica, for example, AEROSIL 50, 130, 200, 300, 380, etc. manufactured by Nippon Aerosil Co., Ltd. can be suitably used.

[0077] The trialkoxysilane mentioned above is not particularly limited and any trialkoxysilane can be used. For example, it is preferable to use a trialkoxysilane represented by the general formula: R1Si(OR2)3 (wherein R1 is hydrogen or an alkyl group having 1 to 5 carbon atoms, and R2 is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such trialkoxysilanes include trimethoxysilane, triethoxysilane, and methyltriethoxysilane. .

[0078] The tetraalkoxysilane mentioned above is not particularly limited and any type can be used. For example, it is preferable to use a tetraalkoxysilane represented by the general formula: Si(OR)4 (wherein R is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane.

[0079] The silane coupling agent can be any agent without particular limitations. Examples include γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-mercaptopropylmethyldimethoxysilane, and γ-mercaptopropyltrimethoxysilane, vinyltriethoxysilane, γ-isocyanatetopropyltriethoxysilane, and the like.

[0080] Furthermore, by incorporating the Si compound into the chemical conversion coating, the Si compound undergoes dehydration condensation to form an amorphous chemical conversion coating having siloxane bonds that provide a high barrier effect against corrosion factors. In addition, by bonding with the aforementioned resin, a chemical conversion coating with even higher barrier properties is formed. Moreover, in corrosive environments, dense and stable corrosion products are formed in defective areas and damaged areas of the plating or coating caused by processing, and the combined effect with the plating film also has the effect of suppressing corrosion of the underlying steel plate. From the viewpoint of having a high effect in forming stable corrosion products, it is preferable to use at least one of colloidal silica and dry silica as the Si compound.

[0081] The concentration of the Si compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is set to 0.2% by mass to 9.5% by mass. If the concentration of the Si compound in the chemical conversion treatment solution is 0.2% by mass or higher, a barrier effect due to siloxane bonding can be obtained, resulting in improved corrosion resistance not only on flat surfaces but also on defective areas, cut areas, and damaged areas caused by processing, as well as improved sweat resistance. Furthermore, if the concentration of the Si compound is 9.5% by mass or lower, the lifespan of the chemical conversion treatment solution can be extended. By applying and drying a chemical conversion treatment solution with a Si compound concentration of 0.2% by mass to 9.5% by mass, the amount of Si adhering to the chemical conversion film after drying is 2 to 95 mg / m². 2 It can be done this way.

[0082] The Co compound and the Ni compound, when included in the chemical conversion coating, can improve resistance to blackening. This is thought to be because Co and Ni have the effect of delaying the elution of water-soluble components from the coating in a corrosive environment. Furthermore, Co and Ni are elements that are less susceptible to oxidation than Al, Zn, Si, and Mg. Therefore, by concentrating at least one of the Co compound and the Ni compound at the interface between the chemical conversion coating and the plating film (forming a concentrated layer), the concentrated layer acts as a barrier against corrosion, thereby improving resistance to blackening.

[0083] By using a chemical conversion treatment solution containing the aforementioned Co compound, Co can be incorporated into the chemical conversion coating and incorporated into the concentrated layer. It is preferable to use a cobalt salt as the Co compound. More preferably, one or more cobalt salts selected from cobalt sulfate, cobalt carbonate, and cobalt chloride are used. Furthermore, by using a chemical conversion treatment solution containing the Ni compound, Ni can be incorporated into the chemical conversion coating and into the concentrated layer. It is preferable to use a nickel salt as the Ni compound. It is even more preferable to use one or more nickel salts selected from nickel sulfate, nickel carbonate, and nickel chloride.

[0084] The concentration of the Co compound and / or Ni compound in the chemical conversion treatment solution is not particularly limited, but can be 0.25% to 5% by mass in total. If the concentration of the Co compound and / or Ni compound is less than 0.25% by mass, the interfacial enrichment layer will be uneven, which may reduce the corrosion resistance of flat surfaces as well as the corrosion resistance of defective areas, cut edges, and areas with plating or film damage caused by processing. From a similar viewpoint, it is preferably 0.5% by mass or more, and more preferably 0.75% by mass or more. On the other hand, if the concentration of the Co compound and / or Ni compound exceeds 5% by mass, the appearance of the film when it is formed tends to be uneven, which may reduce corrosion resistance. From a similar viewpoint, it is preferably 4.0% by mass or less, and more preferably 3.0% by mass or less. By applying and drying a chemical conversion treatment solution in which the total concentration of the Co compound and / or Ni compound is 0.25% to 5% by mass, the total amount of Co and Ni deposited on the chemical conversion film after drying can be 5 to 100 mg / m². 2 It can be done this way.

[0085] By including the Al compound, Zn compound, and Mg compound in the chemical conversion treatment solution, a concentrated layer containing at least one of Al, Zn, and Mg can be formed on the plating side of the chemical conversion coating. The formed concentrated layer can improve corrosion resistance. The Al compound, the Zn compound, and the Mg compound are not particularly limited as long as they are compounds containing Al, Zn, and Mg, respectively, but they are preferably inorganic compounds, and preferably salts, chlorides, oxides, or hydroxides.

[0086] Examples of the Al compound include one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, and aluminum hydroxide. Examples of the Zn compound include one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide, and zinc hydroxide. Examples of the Mg compound include one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium hydroxide.

[0087] The concentrations of the Al compound, Zn compound, and / or Mg compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film are preferably 0.25% by mass to 5% by mass in total. If the total concentration is 0.25% by mass or more, the concentrated layer can be formed more effectively, and as a result, corrosion resistance can be further improved. On the other hand, if the total concentration is 5% by mass or less, the appearance of the chemical conversion film becomes more uniform, and the corrosion resistance of flat surfaces, defective areas, and damaged areas of the plating or film caused by processing is further improved.

[0088] The V compound, when included in the chemical conversion coating, dissolves appropriately in a corrosive environment and combines with zinc ions and other plating components that also dissolve in a corrosive environment, forming a dense protective film. The formed protective film further enhances corrosion resistance not only to the flat surfaces of the steel plate, but also to defects, damaged areas of the plating film caused by processing, and corrosion progressing from the cut edges to the flat surfaces.

[0089] The V compound is a compound containing V, and examples include one or more selected from sodium metavanadate, vanadyl sulfate, and vanadium acetylacetonate.

[0090] The concentration of the V compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.05% to 4% by mass. If the concentration of the V compound is 0.05% by mass or higher, it will dissolve in a corrosive environment and form a protective film more easily, improving the corrosion resistance of defective areas, cut edges, and damaged areas of the plating film caused by processing. On the other hand, if the concentration of the V compound exceeds 4% by mass, the chemical conversion film will not form. The resulting appearance tends to be uneven, and resistance to blackening is also reduced.

[0091] The Mo compound, when included in the chemical conversion coating, can enhance the resistance of the surface-treated steel sheet to blackening. The Mo compound is a compound containing Mo and can be obtained by adding one or both of molybdic acid and / or molybdate to the chemical conversion treatment solution. Examples of the molybdate salt include one or more selected from sodium molybdenate, potassium molybdenate, magnesium molybdenate, and zinc molybdenate.

[0092] The concentration of the Mo compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.01% to 3% by mass. If the concentration of the Mo compound is 0.01% by mass or higher, the formation of oxygen-deficient zinc oxide is further suppressed, and resistance to blackening can be further improved. On the other hand, if the concentration of the Mo compound is 3% by mass or lower, the lifespan of the chemical conversion treatment solution is further extended, and corrosion resistance can be further improved.

[0093] The inclusion of the Zr compound and the Ti compound in the chemical conversion coating prevents the coating from becoming porous and densifies the coating. As a result, corrosion factors become less likely to penetrate the chemical conversion coating, thereby improving corrosion resistance.

[0094] The Zr compound is a Zr-containing compound, and for example, one or more selected from zirconyl acetate, zirconyl sulfate, potassium zirconyl carbonate, sodium zirconyl carbonate, and ammonium zirconyl carbonate can be used. Among these, organotitanium chelate compounds are preferred because they densify the film when the chemical treatment solution is dried to form a film, resulting in superior corrosion resistance.

[0095] The Ti compound is a Ti-containing compound, and for example, one or more selected from titanium sulfate, titanium chloride, titanium hydroxide, titanium acetylacetonate, titanium octylene glycolate, and titanium ethylacetoacetate can be used.

[0096] The concentration of the Zr compound and / or Ti compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.2% by amount to 20% by mass in total. If the total concentration of the Zr compound and / or Ti compound is 0.2% by mass or more, the effect of suppressing the penetration of corrosive factors is enhanced, and the corrosion resistance of not only flat surfaces but also defective areas, cut edges, and areas of plating film damage caused by processing can be further improved. On the other hand, if the total concentration of the Zr compound and / or Ti compound is 20% by mass or less, the life of the chemical conversion treatment solution can be further extended.

[0097] The aforementioned Ca compound, when included in the chemical conversion coating, can exert the effect of reducing the corrosion rate.

[0098] The Ca compound is a compound containing Ca, such as Ca oxides, Ca nitrates, Ca sulfates, and Ca-containing intermetallic compounds. More specifically, the Ca compound can be CaO, CaCO3, Ca(OH)2, Ca(NO3)2·4H2O, CaSO4·2H2O, etc. The content of the Ca compound in the chemical conversion film is not particularly limited.

[0099] In addition, the chemical conversion film can contain various known components commonly used in the paint field as needed. For example, various surface modifiers such as leveling agents and defoaming agents, dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, various additives such as silane coupling agents and titanate coupling agents, coloring pigments, extender pigments, various pigments such as brighteners, curing catalysts, organic solvents, lubricants, etc. can be mentioned.

[0100] In the surface-treated steel sheet of the present invention, it is preferable that the chemical conversion film does not contain harmful components such as hexavalent chromium, trivalent chromium, and fluorine. This is because these harmful components are not contained in the chemical conversion treatment liquid for forming the chemical conversion film, so the safety is high and the impact on the environment is small.

[0101] Also, the adhesion amount of the chemical conversion film is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance and preventing peeling of the chemical conversion film, it is preferable that the adhesion amount of the chemical conversion film is 0.1 to 3.0 g / m 2 and more preferably 0.5 to 2.5 g / m 2 By setting the adhesion amount of the chemical conversion film to 0.1 g / m 2 or more, corrosion resistance can be more reliably ensured, and by setting the adhesion amount of the chemical conversion film to 3.0 g / m 2 or less, cracking and peeling of the chemical conversion film can be prevented. The adhesion amount of the chemical conversion film may be determined by appropriately selecting a method from existing methods, such as a method of measuring the abundance of elements whose content in the film is known in advance by fluorescence X-ray analysis of the film.

[0102] The method for forming the chemical conversion coating is not particularly limited and can be appropriately selected according to the required performance and manufacturing equipment. For example, the coating can be formed by continuously applying a chemical conversion solution to the plating film using a roll coater or the like, and then drying it at a peak metal temperature (PMT) of about 60 to 200°C using hot air or induction heating. In addition to a roll coater, known methods such as airless spraying, electrostatic spraying, and curtain flow coating can be appropriately used for applying the chemical conversion solution. Furthermore, the chemical conversion coating may be a single layer or a multi-layer film, as long as it contains the resin and the metal compound, and is not particularly limited.

[0103] Furthermore, the surface-treated steel sheet of the present invention may also have a coating film formed on the chemical conversion film as needed.

[0104] <Manufacturing method for surface-treated steel sheets> The present invention relates to a method for manufacturing a surface-treated steel sheet, comprising a plating film and a chemical conversion film formed on the plating film. Furthermore, in the manufacturing method of the present invention, the chemical conversion film contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. The formation of the aforementioned plating film comprises a hot-dip plating process in which a base steel plate is immersed in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, and the Ni content in the unavoidable impurities of the plating bath is controlled to 0.010% by mass or less.

[0105] As mentioned above, the Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet. Therefore, by appropriately controlling the content of Al, Zn, Si, and Mg in the plating bath, and further reducing the Ni content as an unavoidable impurity, the degradation of corrosion resistance can be suppressed.

[0106] The conditions for the hot-dip galvanizing process are the same as those described in the section on the galvanized film of the surface-treated steel sheet of the present invention. Furthermore, the structure of the chemical conversion coating is the same as that described for the chemical conversion coating of the surface-treated steel sheet of the present invention. [Examples]

[0107] (Samples 1-148) (1) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as the base steel sheet, samples of hot-dip galvanized steel sheets with the plating film conditions shown in Tables 2 and 3 were prepared by performing annealing and plating treatments using a hot-dip galvanizing simulator manufactured by Resca Co., Ltd. The composition of the plating bath used in the production of the hot-dip galvanized steel sheets was varied within the range of Al: 5-75 mass%, Si: 0.0-4.5 mass%, Mg: 0-10 mass%, and Ni: 0.000-0.025 mass%, so that the composition of the plating film for each sample shown in Tables 2 and 3 would match. The bath temperature of the plating bath was set to 450°C for Al: 5 mass%, 480°C for Al: 15 mass%, 590°C for Al: 30-60 mass%, and 630°C for Al: over 60 mass%, so that the plate temperature of the base steel sheet upon immersion in the plating was the same as the plating bath temperature. Furthermore, for Al: 30-60 mass%, the plating process was carried out under conditions where the plate temperature was cooled to the 520-500°C range in 3 seconds. Furthermore, the amount of plating film adhering to each side was 85±5 g / m² for samples 1-118 and 131-148. 2 For samples 119-120, the g / m² was 50±5 g / m² per side. 2 For samples 121-122, the density was 100±5 g / m² per side. 2 For samples 123-124, the density was 125 g / m² per side. 2±5g / m 2 , sa For sample sizes 125-130, the weight per side is approximately 70±5g / m². 2 It was controlled to be such that...

[0108] (2) Subsequently, a chemical conversion solution was applied to the plating film of each sample of the prepared hot-dip galvanized steel sheet using a bar coater, and a chemical conversion film was formed by drying in a hot air oven (heating rate: 60°C / s, PMT: 120°C), thereby producing the surface-treated steel sheet samples shown in Tables 2 and 3. The chemical treatment solutions were prepared by dissolving each component in water as a solvent, creating surface treatment solutions A to F. The types of components (resins, metal compounds) contained in the surface treatment solutions are as follows. (resin) Urethane resin: Superflex 130, Superflex 126 (Daiichi Kogyo Seiyaku Co., Ltd.) Acrylic resin: Boncoat EC-740EF (DIC Corporation) (metal compound) P compound: Aluminum dihydrogen tripolyphosphate Si compounds: Silica Compound V: Sodium metavanadate Mo compound: Molybdic acid Zr compound: Potassium zirconyl carbonate Table 1 shows the composition of the prepared chemical conversion treatment solutions A to F and the amount of the resulting chemical conversion film attached. Note that the concentrations of each component in Table 1 of this specification represent the solid content concentration (mass%).

[0109] [Table 1]

[0110] (evaluation) The following evaluations were performed on each sample of hot-dip galvanized steel sheet and surface-treated steel sheet obtained as described above. The evaluation results are shown in Tables 2 and 3.

[0111] (1) Plating film (composition, amount of coating, Ni-based compound, X-ray diffraction intensity) For each sample of hot-dip galvanized steel sheet, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amounts of plating film attached are shown in Tables 2 and 3. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material 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 plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Tables 2 and 3. Furthermore, after shearing each sample to a size of 15 mm × 15 mm and embedding it in conductive resin so that the cross-section of the steel plate could be observed, mechanical polishing was performed. Then, using a scanning electron microscope (Carl Zeiss ULTRA55), continuous backscattered electron images were taken at a width of 100 μm under an acceleration voltage of 3 kV for a continuous cross-section of an arbitrarily selected plating film with a length of 2 mm or more in the direction parallel to the surface of the underlying steel plate. Furthermore, elemental mapping analysis (Al, Zn, Si, Mg, Fe, Sr, and Ni) of each cross-section was performed using an energy-dispersive X-ray spectrometer (Oxford Instruments Ultim Extreme) under an acceleration voltage of 3 kV within the same apparatus. For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The major axis was measured for all Ni-based compounds confirmed in the observation field, and the largest major axis was determined. Furthermore, the number of Ni-based compound particles present in the observed continuous cross-section was counted and divided by the observed cross-sectional length (mm) to calculate the number of Ni-based compound particles per mm parallel to the surface of the underlying steel plate (particles / mm). For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Tables 2 and 3. In addition, for each sample, after shearing it to a size of 100 mm x 100 mm, the plating film on the evaluation symmetric surface was mechanically scraped off until the underlying steel plate was exposed. After thoroughly mixing the resulting powder, 0.3 g was taken out and qualitative analysis of the powder was performed using an X-ray diffractometer (Rigaku Corporation's "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity. The diffraction intensities of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si, the (100) plane (interplanar spacing d=0.4510 nm) of MgZn2, and the (111) plane (interplanar spacing d=0.3135 nm) of Si were measured. The measurement results are shown in Tables 2 and 3.

[0112] (2) Corrosion resistance evaluation For each sample of hot-dip galvanized steel sheet and surface-treated steel sheet, after shearing to a size of 120 mm x 120 mm, the area within 10 mm of each edge of the surface to be evaluated, as well as the end face of the sample and the surface not to be evaluated, were sealed with tape, leaving the surface to be evaluated exposed at a size of 100 mm x 100 mm. Three identical evaluation samples were prepared. The three evaluation samples prepared as described above were subjected to accelerated corrosion testing using the cycle shown in Figure 1. The accelerated corrosion testing started from a wet state and continued for 300 cycles. Afterward, the corrosion loss of each sample was measured according to the methods described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. The evaluation results are shown in Tables 2 and 3. ◎: The corrosion loss for all three samples was 30g / m². 2 below ○: The corrosion loss for all three samples was 75g / m². 2 below ×: Corrosion loss of 75g / m² for one or more samples. 2 cross

[0113] (3) White rust resistance For each sample of hot-dip galvanized steel sheet and surface-treated steel sheet, after shearing to a size of 120 mm x 120 mm, the area within 10 mm from each edge of the surface to be evaluated, as well as the end face of the sample and the surface not to be evaluated, were sealed with tape, and the surface to be evaluated was exposed to a size of 100 mm x 100 mm, which was used as the evaluation sample. Using the above evaluation samples, a salt spray test as described in JIS Z 2371 was conducted for 90 hours and evaluated according to the following criteria. The evaluation results are shown in Tables 2 and 3. ◎: No white rust on the flat surface. ○: White rust area on flat surface is less than 10% ×: White rust area on flat surface exceeds 10%

[0114] (4) Surface appearance For each sample of hot-dip galvanized steel sheet, the surface of the galvanized film was observed visually. The observation results were then evaluated according to the following criteria. The evaluation results are shown in Tables 2 and 3. ◎: No wrinkle-like defects were observed at all. ○: Wrinkle-like defects were observed only within a 50mm radius from the edge. ×: Wrinkle-like defects were observed outside the 50mm range from the edge.

[0115] (5) Processability Each sample of hot-dip galvanized steel sheet was sheared to a size of 70 mm x 150 mm, and then bent 180° (8T bend) with eight sheets of the same thickness sandwiched inside. After bending, cellophane tape (registered trademark) was firmly attached to the outer surface of the bent section and then peeled off. The surface condition of the galvanized film on the outer surface of the bent section and the presence or absence of adhesion (peeling) of the galvanized film on the surface of the tape used were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Tables 2 and 3. ○: No cracks or peeling are observed in the plating film. △: Cracks are present in the plating film, but no peeling is observed. ×: Both cracks and peeling are observed in the plating film.

[0116] (5) Bath stability During the hot-dip plating process, the condition of the plating bath surface was visually inspected and compared with the plating bath surface used when manufacturing hot-dip Al-Zn plated steel sheets (a bath surface free of Mg-containing oxides). The evaluation was performed according to the following criteria, and the evaluation results are shown in Tables 2 and 3. ○: Similar to a molten Al-Zn plating bath (55% by mass Al - remainder Zn - 1.6% by mass bath) △: Contains more white oxides compared to molten Al-Zn plating baths (55% by mass Al - remainder Zn - 1.6% by mass bath). ×: Formation of black oxide is observed during plating.

[0117] [Table 2]

[0118] [Table 3]

[0119] The results in Tables 2 and 3 show that each sample of the present invention is superior to each sample of the comparative examples in terms of a good balance of corrosion resistance, white rust resistance, surface appearance, processability, and bath stability. Furthermore, the results in Table 3 show that each sample subjected to chemical treatment A to D exhibits particularly excellent resistance to white rust. [Industrial applicability]

[0120] According to the present invention, it is possible to provide a surface-treated steel sheet with consistently excellent corrosion resistance and white rust resistance, and a method for manufacturing the same.

Claims

1. A surface-treated steel sheet comprising a plating film and a chemical conversion film formed on the plating film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The Ni content in the aforementioned unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. The plating film contains a Ni-based compound, and the number of Ni-based compounds present in a direction parallel to the surface of the underlying steel sheet is 5 or less per mm. Mg in the aforementioned plating film 2 Si and MgZn 2 The diffraction intensity obtained by X-ray diffraction satisfies the following relationship (1): Mg 2 Si(111) / MgZn 2 (100)≦2.0 ・・・(1) Mg 2 Si (111): Mg 2 Diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Si, MgZn 2 (100): MgZn 2 Diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) The surface-treated steel sheet is characterized in that the chemical conversion coating contains at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound.

2. The surface-treated steel sheet according to claim 1, characterized in that the major axis of the Ni-based compound is 4.0 μm or less.

3. The surface-treated steel sheet according to claim 1 or 2, characterized in that the diffraction intensity of Si in the plating film by X-ray diffraction satisfies the following relationship (2). Si (111)=0...(2) Si (111): Diffraction intensity of the (111) plane of Si (interplanar spacing d = 0.3135 nm)

4. The surface-treated steel sheet according to any one of claims 1 to 3, characterized in that the plating film further contains Sr: 0.01 to 1.0% by mass.

5. The surface-treated steel sheet according to any one of claims 1 to 4, characterized in that the Al content in the plating film is 50 to 60% by mass.

6. The surface-treated steel sheet according to any one of claims 1 to 5, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

7. The surface-treated steel sheet according to any one of claims 1 to 6, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

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