Molten zinc–aluminum–magnesium-based plated steel sheet

Abstract

The purpose of the present invention is to provide a molten zinc-aluminum-magnesium-based plated steel sheet that demonstrates both high corrosion resistance and high scratch resistance.　In order to achieve the foregoing, the present invention is a molten zinc-aluminum-magnesium-based plated steel sheet comprising a plating film 20 formed from an interface alloy layer 22 and a main layer 21, the interface alloy layer 22 being present next to a base steel sheet 10 as an interface, and the main layer 21 being present on the interface alloy layer 22. The molten zinc-aluminum-magnesium-based plated steel sheet is characterized in that the plating film 20 contains 10-22 mass% Al, 0.01-2 mass% Si, and 3-10 mass% Mg, with the remainder being Zn and unavoidable impurities, and that the percentage of the area accounted for by MgZn2 present in the main layer 21 is 30% or more when observed in cross-section taken in the thickness direction of the plating film 20.

Description

Hot-dip Zn-Al-Mg coated steel sheet

[0001] The present invention relates to a hot-dip Zn-Al-Mg coated steel sheet having excellent corrosion resistance and scratch resistance.

[0002] Due to its excellent corrosion resistance, hot-dip galvanized steel sheets have been widely used as rust-resistant steel sheets in the fields of automobiles, electrical machinery, building materials, etc. In general, hot-dip galvanized coatings consist of an interfacial alloy layer present at the interface with a substrate steel sheet and a main layer present on the interfacial alloy layer, and exhibit superior corrosion resistance compared to cold-rolled steel sheets and hot-rolled steel sheets, mainly due to the sacrificial corrosion protection ability of the Zn present in the main layer against Fe. When a typical cold-rolled steel sheet or hot-rolled steel sheet is used as the substrate steel sheet, the above-mentioned interfacial alloy layer contains, as a constituent, an Fe-Al alloy or an Fe-Zn alloy formed by the reaction of Fe in the substrate steel sheet with Zn or Al in the coating bath.

[0003] In recent years, to meet market needs for high corrosion resistance, multi-element alloy-plated steel sheets have been developed, such as hot-dip Zn-Al-Mg-plated steel sheets, which contain Al, Mg, and Si in addition to Zn as plating components. For example, Patent Document 1 discloses a hot-dip Zn-Al-Mg-plated steel sheet having a plating film composition of 4.0 to 10 wt.% Al, 1.0 to 4.0 wt.% Mg, and the balance being Zn and unavoidable impurities. Patent Document 2 also discloses a hot-dip Zn-Al-Mg-plated steel sheet having a plating film composition of 2 to 19 wt.% Al, 1.0 to 10 wt.% Mg, 0.01 to 2 wt.% Si, and the balance being Zn and unavoidable impurities, with the total Al and Mg content being 20 mass% or less.

[0004] In the case of typical hot-dip Zn-Al-Mg-plated steel sheets such as those disclosed in Patent Documents 1 and 2, complex solidification reactions occur during the film formation process, resulting in a complex and non-uniform structure of the plated film. Although hot-dip Zn-Al-Mg-plated steel sheets tend to have superior corrosion resistance compared to conventional hot-dip Zn-plated steel sheets due to this non-uniform structure, there has been a demand for even more stable corrosion resistance. Furthermore, the plated films of typical hot-dip Zn-Al-Mg-plated steel sheets such as those disclosed in Patent Documents 1 and 2 have a problem in that the plated surface is more susceptible to scratches than conventional hot-dip Zn-plated steel sheets due to the presence of soft Al and Zn phases.

[0005] Japanese Patent Laid-Open No. 10-226865 Japanese Patent Laid-Open No. 2000-104154

[0006] In view of the above circumstances, an object of the present invention is to provide a hot-dip Zn-Al-Mg coated steel sheet that achieves both high levels of corrosion resistance and scratch resistance.

[0007] As a result of investigations conducted by the present inventors to solve the above-mentioned problems, they found that it is important not only to control the concentrations of Zn, Al, Mg, and Si in the composition of the plating film of a hot-dip Zn-Al-Mg-plated steel sheet, but also to control the plating film structure. They particularly focused on the effectiveness of the hardness of MgZn2 formed in the plating film and its effect of stabilizing corrosion products, and found that it is possible to achieve high levels of both corrosion resistance and scratch resistance by controlling, within a specific range, the amount of MgZn2 present in the main layer when a cross section of the plating film in the thickness direction is observed, specifically its area ratio.

[0008] The present invention was made based on the above findings, and its gist is as follows: 1. A hot-dip Zn-Al-Mg-plated steel sheet having a plating film consisting of an interfacial alloy layer present at the interface with a base steel sheet and a main layer present on the interfacial alloy layer, wherein the plating film has a composition containing 10 to 22 mass% Al, 0.01 to 2 mass% Si, and 3 to 10 mass% Mg, with the balance consisting of Zn and unavoidable impurities, and wherein, when a cross-section of the plating film is observed in the thickness direction, the area ratio of MgZn2 present in the main layer is 30% or more. 2. The hot-dip Zn-Al-Mg-plated steel sheet according to item 1 above, wherein, when a cross-section of the plating film is observed in the thickness direction, the area ratio of MgZn2 having a portion exposed at the outermost coating surface is 30% or more. 3. 3. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, characterized in that, when a cross section of the plated film is observed in the thickness direction, the area ratio of the MgZn2 present within 50% of the thickness range from the surface of the main layer exceeds 50%. 4. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, characterized in that, when a cross section of the plated film is observed in the thickness direction, the area ratio of the Al phase present within 50% of the thickness range from the surface of the main layer is less than 50%. 5. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, characterized in that the plated film further contains 0.1 to 5 mass % in total of one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb and Bi.

[0009] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg coated steel sheet that achieves both high levels of corrosion resistance and scratch resistance.

[0010] Fig. 1 is an enlarged schematic view of a cross section of a hot-dip Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention. Fig. 2 is an enlarged schematic view of a cross section of a hot-dip Zn-Al-Mg-plated steel sheet according to another embodiment of the present invention. Fig. 3 is an enlarged schematic view of a cross section of a hot-dip Zn-Al-Mg-plated steel sheet according to another embodiment of the present invention.

[0011] (Hot-dip Zn-Al-Mg-plated steel sheet) As shown in Fig. 1, the hot-dip Zn-Al-Mg-plated steel sheet of the present invention comprises a plating film 20 on a substrate steel sheet 10, the plating film 20 comprising an interface alloy layer 22 present at the interface with the substrate steel sheet 10 and a main layer 21 present on the interface alloy layer. The plating film 20 has a composition containing 10 to 22 mass% Al, 0.01 to 2 mass% Si, and 3 to 10 mass% Mg, with the balance being Zn and unavoidable impurities.

[0012] Zn, the main component of the plating film, is an element necessary for imparting sacrificial corrosion protection to the plating film and achieving excellent corrosion resistance. When considering the Zn content in terms of atomic composition, the plating layer is composed of elements with low specific gravity, such as Al and Mg, so Zn must be the main component in terms of atomic composition. Therefore, the Zn content in the plating film must be 60% by mass or more, and preferably 70% by mass or more. The upper limit of the Zn content is the content remaining after elements other than Zn and impurities.

[0013] Al in the plating film is an essential element for forming an Al phase in the main layer and achieving excellent corrosion resistance. When the Al content of the plating film exceeds 5 mass%, an Al phase can be formed in the plating film, and the amount of Al phase formed increases with increasing Al content. To achieve more stable and excellent corrosion resistance, a certain amount of Al phase must be formed in the plating film, and the Al content in the plating film should be 10 mass% or more. Therefore, the lower limit of the Al concentration is set to 10 mass%. On the other hand, as the Al concentration in the plating film increases, sacrificial corrosion protection tends to deteriorate. Therefore, the upper limit of the Al concentration must be set to 22 mass% or less. From the same perspective, the Al content in the plating film is preferably 12 to 20 mass%, and more preferably 15 to 19 mass%.

[0014] The Si content in the plating film is primarily used to suppress the abnormal growth of an Fe-Al interfacial alloy layer that forms at the interface with the substrate steel sheet, thereby ensuring the workability of the plating film. When a substrate steel sheet is immersed in a molten Zn-Al-Mg plating bath containing Si, an alloying reaction occurs between the Fe on the substrate steel sheet surface and the Al and Si in the bath, resulting in the formation of an Fe-Al and / or Fe-Al-Si intermetallic compound layer at the interface between the substrate steel sheet and the plating film. Because 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 plating film must be 0.01% by mass or more. On the other hand, if the Si content in the plating film exceeds 2% by mass, not only does the effect of suppressing the growth of the interfacial alloy layer saturate, but the presence of excess Si in the plating film also promotes corrosion. Therefore, the Si content is limited to 2% by mass or less.

[0015] Furthermore, Mg in the plating film stabilizes the corrosion products formed during corrosion and is an essential element for achieving excellent corrosion resistance. To achieve this corrosion product stabilization effect, the Mg content in the plating film must be 3% by mass or more, and a more reliable effect is preferably 5% by mass or more. On the other hand, if the Mg content in the plating film exceeds 10% by mass, the plating film becomes hard and brittle, resulting in poor workability. Therefore, the upper limit of the Mg content is set to 10% by mass. From the same perspective, the Mg content in the plating film is preferably 5 to 8% by mass, and more preferably 6 to 8% by mass.

[0016] The plating film contains unavoidable impurities. Among these, the unavoidable impurities include Fe. This Fe is inevitably contained in the plating film as a result of dissolution of the steel sheet or bath-immersed equipment into the plating bath, and as a result of being supplied by diffusion from the base steel sheet during the formation of the interfacial alloy layer. The Fe content in the plating film is typically about 0.1 to 0.5 mass%.

[0017] Furthermore, the plating film preferably further contains, as necessary, 0.1 to 5 mass % in total of one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi. These elements have the effect of improving the stability of corrosion products when the plating film corrodes, thereby delaying the progression of corrosion, and the effect of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

[0018] 1 to 3, the plating film is composed of an interface alloy layer 22 present at the interface with the substrate steel sheet 10 and a main layer 21 present on the interface alloy layer 22. In FIGS. 1 to 3, cross sections of the substrate steel sheet 10, the main layer 21, and the interface alloy layer 22 are shown schematically for the sake of convenience, and the actual shapes, dimensions, etc. may differ from those shown in FIGS. 1 to 3.

[0019] The interfacial alloy layer is formed in the plating process by the reaction of the substrate steel sheet with bath components such as Zn, Al, Mg, and Si in the plating bath, and is generally an Fe-Al and / or Fe-Al-Si intermetallic compound. Furthermore, when a hot-rolled steel sheet or a high-tensile steel sheet, which have low wettability, is used as the substrate steel sheet, the substrate steel sheet may be pre-plated with Ni, Fe, or the like before the plating process to ensure wettability. In particular, when a Ni-preplated steel sheet is used as the substrate steel sheet, Ni-Al and / or Fe-Ni-Al intermetallic compounds containing Ni are formed as the interfacial alloy layer.

[0020] When the interfacial alloy layer exists with an average thickness of 0.1 to 1 μm, a stable main layer can be formed on the interfacial alloy layer. If the average thickness is less than 0.1 μm, the interfacial alloy layer may not form over the entire plating film, i.e., the substrate steel sheet and the plating bath may not react, which may result in insufficient plating adhesion and film formation. On the other hand, if the average thickness exceeds 1 μm, the interfacial alloy layer may crack during processing, causing plating peeling. Therefore, the average thickness of the interfacial alloy layer is preferably 0.1 to 1 μm.

[0021] Furthermore, as shown in FIGS. 1 to 3, the plating bath components that were not consumed in forming the interface alloy layer 22 solidify, and as a result, an Al phase, a Zn phase, and MgZn2 are mainly formed in the main layer.

[0022] The Al phase is a structure necessary for obtaining stable and excellent corrosion resistance, and when observing a cross section of the plating film in the thickness direction, the area ratio occupied by the Al phase is preferably 30% or more, and more preferably 40% or more.

[0023] The present invention is characterized in that, as shown in FIG. 1 , when observing a cross section of the plating film 20 in the thickness direction, the area ratio of MgZn2 present in the main layer 21 is 30% or more. The MgZn2 present in the main layer 21 preferentially dissolves in the early stages of corrosion of the plating film and stabilizes the corrosion products that form. Therefore, preferential dissolution of MgZn2 in the early stages of corrosion of the plating film 20 allows stable corrosion products to form early, thereby slowing the corrosion rate of the plating film. Furthermore, since MgZn2 is a hard intermetallic compound, its presence in the plating main layer can improve the scratch resistance of the plating film. Therefore, by setting the area ratio of MgZn2 present in the main layer 21 to 30% or more, both corrosion resistance and scratch resistance can be stably achieved. From the same viewpoint, when observing a cross section of the plating film 20 in the thickness direction, the area ratio occupied by MgZn2 present in the main layer 21 is preferably 40% or more, and more preferably 50% or more.

[0024] Furthermore, as shown in FIG. 2 , when a cross section of the plating film 20 is observed in the thickness direction, the area ratio of the MgZn2 present in the main layer 21 that has a portion exposed on the outermost plating surface is preferably 30% or more, and more preferably 40% or more. To efficiently achieve preferential dissolution by MgZn2 in the early stages of corrosion, it is effective for a large amount of MgZn2 to be exposed on the surface of the plating film 20. Additionally, a larger proportion of MgZn2 that has a portion exposed on the surface of the plating film 20 can better suppress surface scratches. Therefore, by ensuring that the area ratio of the MgZn2 that has a portion exposed on the plating surface is 30% or more, both corrosion resistance and scratch resistance can be achieved at a higher level.

[0025] Furthermore, as shown in FIG. 3 , when observing a cross section of the plating film 20 in the thickness direction, the area ratio of the MgZn2 present in the main layer 21 within the first 50% of the thickness range from the surface of the main layer 21 is preferably greater than 50%, and more preferably greater than 60%. As described above, the presence of a large amount of MgZn2 on the surface side of the plating film 20 is effective for efficiently achieving preferential dissolution by MgZn2 in the early stages of corrosion. Additionally, the presence of a large amount of MgZn2 on the surface side of the plating film 20 can further suppress surface scratches. Therefore, when observing a cross section of the plating film 20 in the thickness direction, by ensuring that the area ratio of the MgZn2 present in the first 50% of the thickness range from the surface of the main layer 21 exceeds 50%, both corrosion resistance and scratch resistance can be achieved at a higher level.

[0026] The Al phase formed in the main layer is a structure necessary for consistently achieving excellent corrosion resistance. When observing a cross section of the plating film in the thickness direction, the area ratio of the Al phase is preferably 30% or more, and more preferably 40% or more. Furthermore, to obtain the improved corrosion resistance brought about by the Al phase, a certain proportion of the Al phase in the main layer 21 is sufficient, and the location of its distribution in the main layer is not limited. Therefore, from the viewpoint of not inhibiting the distribution of the above-described MgZn2 toward the surface side of the main layer 21 and more efficiently achieving both corrosion resistance and scratch resistance, when observing a cross section of the plating film 20 in the thickness direction, it is preferable that the area ratio of the Al phase present within the first 50% of the thickness of the main layer 21 be less than 50%, as shown in FIG. 3 .

[0027] The method for observing the cross section of the plating film 20 in the thickness direction is not particularly limited as long as it is a method that can observe the distribution state of the MgZn and Al phases in the main layer 21. For example, observation and measurement can be performed by SEM-EDX (energy dispersive X-ray analysis using a scanning electron microscope).

[0028] The coating weight of the plating film is set to 30 to 300 g / m per side from the viewpoint of satisfying various properties. 2 It is preferable that the coating weight of the plating film is 30 g / m 2 In the above cases, sufficient corrosion resistance can be obtained for applications requiring long-term corrosion resistance, such as building materials, and the coating weight of the plating film is 300 g / m 2 This is because, in the following cases, it is possible to achieve excellent corrosion resistance while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the coating weight of the plating film is 50 to 150 g / m 2 It is more preferable that:

[0029] The coating weight of the plating film can be determined, for example, by a method in which a specific area of ​​the plating film is dissolved and stripped in a mixed solution of hydrochloric acid and hexamethylenetetramine, and the coating weight is calculated from the difference in weight of the steel sheet before and after stripping, as specified in JIS H 0401: 2013. To determine the coating weight per side using this method, the non-target side can be sealed with tape so that the plated surface is not exposed, and then the dissolution described above is carried out.

[0030] Furthermore, as shown in FIG. 1 , the hot-dip Zn-Al-Mg-plated steel sheet of the present invention has a plating film 20 formed on a base steel sheet 10. If necessary, an intermediate layer or a coating film can be further formed on the plating film. The type of coating film and the method for forming the coating film are not particularly limited and can be appropriately selected depending on the required performance. Examples of methods include roll coater coating, curtain flow coating, and spray coating. A coating film can be formed by applying a paint containing an organic resin and then heating and drying it using means such as hot air drying, infrared heating, and induction heating. The intermediate layer is not particularly limited as long as it is formed between the plating film of the hot-dip Zn-Al-Mg-plated steel sheet and the coating film. Examples include a chemical conversion coating film and a primer such as an adhesive layer. The chemical conversion coating film can be formed, for example, by a chromate treatment or chromium-free chemical conversion treatment, in which a chromate treatment solution or a chromium-free chemical conversion treatment solution is applied, followed by drying at a steel sheet temperature of 80 to 300°C without rinsing with water. These chemical conversion coatings may be single-layer or multi-layer, and in the case of multi-layer, multiple chemical conversion treatments may be carried out in sequence.

[0031] (Method for producing hot-dip Zn-Al-Mg-plated steel sheet) The method for producing the hot-dip Zn-Al-Mg-plated steel sheet of the present invention is not particularly limited. However, the coating film on the hot-dip Zn-Al-Mg-plated steel sheet obtained by the present invention will have a composition that is generally equivalent to that of the coating bath. Therefore, the method includes a step of forming the coating film on a substrate steel sheet using a coating bath whose composition is controlled to contain 10 to 25 mass% Al, 0.01 to 2 mass% Si, and 3 to 10 mass% Mg, with the balance being Zn and unavoidable impurities.

[0032] The step of forming the plating film is not particularly limited except for the composition of the plating bath described above. For example, the steel sheet can be produced by cleaning, heating, and immersing the base steel sheet in a plating bath using continuous hot-dip galvanizing equipment. In the steel sheet heating step, recrystallization annealing or the like 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 small amount of oxide film present on the surface.

[0033] The bath temperature of the coating bath is not particularly limited, but is preferably in the range of (melting point + 20°C) to 550°C. The reason why the lower limit of the bath temperature is set to melting point + 20°C is that the bath temperature needs to be at or above the solidification point in order to perform hot-dip coating, and setting the temperature to melting point + 20°C prevents solidification due to a local drop in the bath temperature of the coating bath. On the other hand, the reason why the upper limit of the bath temperature is set to 550°C is that if the bath temperature exceeds 550°C, it becomes difficult to rapidly cool the coating film, and there is a risk that the interfacial alloy layer formed between the coating film and the steel sheet will become thick.

[0034] The method for controlling the area ratio of MgZn2 present in the main layer to 30% or more is not particularly limited, and can be, for example, by adding MgZn2 to a coating bath and performing hot-dip coating, or by spraying MgZn2 powder onto the surface of the steel sheet immediately after hot-dip coating treatment where the coating has not yet solidified.

[0035] Furthermore, there is no particular limitation on the method for controlling the area ratio of the MgZn2 that is exposed on the outermost coating surface to 30% or more among the observed MgZn2. For example, it can be formed by a method of spraying MgZn2 powder onto the surface of a steel sheet immediately after hot-dip galvanizing treatment where the coating has not yet solidified. Furthermore, there is no particular limitation on the method for controlling the area ratio of the MgZn2 that is present within 50% of the thickness range from the surface of the main layer to exceed 50% among the observed MgZn2. For example, it can be formed by a method of producing a Zn-Al-Mg-based coated steel sheet by a conventional hot-dip galvanizing treatment, and then subjecting the steel sheet to a second hot-dip galvanizing treatment in a hot-dip Zn-Al-Mg coating bath containing MgZn2, or by a method of spraying MgZn2 powder onto the surface of a steel sheet immediately after hot-dip galvanizing treatment where the coating has not yet solidified.

[0036] The base steel sheet constituting the Zn-Al-Mg-plated 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. The base steel sheet is also not particularly limited. Furthermore, the method for obtaining the base steel sheet is also not particularly limited. For example, the hot-rolled steel sheet can be one that has undergone a hot-rolling process and a pickling process, while the cold-rolled steel sheet can be produced by adding a cold-rolling process. Furthermore, to obtain the desired steel sheet properties, a recrystallization annealing process or the like can be performed before the hot-dip galvanizing process. A pre-plated steel sheet may also be used as the base steel sheet. The pre-plated steel sheet is plated, for example, by an electrolytic treatment method or a displacement plating method. In the electrolytic treatment method, the base steel sheet may be electrolytically treated by immersing it in a sulfate bath or a chloride bath containing metal ions of various pre-plating components. In the displacement plating method, the base steel sheet is immersed in an aqueous solution containing metal ions of various pre-plating components and whose pH is adjusted with sulfuric acid, thereby causing displacement deposition of the metal. A typical example of a pre-plated steel sheet is a Ni pre-plated steel sheet.

[0037] In the method for producing a hot-dip Zn-Al-Mg-plated steel sheet of the present invention, in addition to the above-mentioned plating film formation step and the heating / cooling step after plating film formation, it is possible to appropriately carry out steps that are employed in the production of ordinary plated steel sheets.

[0038] [Samples 1 and 2] (Manufacturing method A:) A cold-rolled steel sheet having a thickness of 0.8 mm, manufactured by a conventional method, was used as a base steel sheet, and annealing and plating were performed using a hot-dip plating simulator manufactured by Rhesca Co., Ltd., to produce hot-dip Zn-Al-Mg plated steel sheet Samples 1 and 2 under the conditions shown in Table 1. Table 1 also shows the composition and bath temperature of the plating bath used in the manufacture of the hot-dip Zn-Al-Mg plated steel sheets, as well as the composition and coating weight of the plating film of each sample.

[0039] [Samples 3 to 9] (Manufacturing method B:) A cold-rolled steel sheet having a thickness of 0.8 mm manufactured by a conventional method was used as the base steel sheet. Annealing and hot-dip plating were performed using a hot-dip plating simulator manufactured by Rhesca Corporation. After that, powdered MgZn2 (average particle size: 2 μm or less) was sprayed onto the plated surface before the coating solidified, thereby producing hot-dip Zn-Al-Mg-plated steel sheet Samples 3 to 9 under the conditions shown in Table 1. By appropriately adjusting the coating weight of the hot-dip Zn-Al-Mg coating and the coating weight of the sprayed MgZn2, plating films with the compositions shown in Table 1 were obtained. The composition and bath temperature of the coating bath used in the production of the hot-dip Zn-Al-Mg-plated steel sheets, as well as the composition and coating weight of the plating film for each sample, are shown in Table 1.

[0040] [Sample 10] (Manufacturing Method C:) A 0.8 mm thick cold-rolled steel sheet manufactured by a conventional method and subjected to Ni pre-plating was used as a base steel sheet. This steel sheet was annealed and hot-dip plated in a hot-dip plating simulator manufactured by Rhesca Corporation, and then powdered MgZn2 (average particle size: 2 μm or less) was sprayed onto the plated surface before the plating solidified, thereby producing Sample 10 of hot-dip Zn-Al-Mg plated steel sheet under the conditions shown in Table 1. The Ni pre-plating was carried out using a plating bath containing NiSO4·6H2O at a concentration of 300 g / L, H3BO3 at a concentration of 40 g / L, Na2SO4 at a pH of 2.7, at a bath temperature of 60°C, and a current density of 50 A / dm 2 Under the condition, Ni deposition amount is 1g / m2 The coating weight of the hot-dip Zn-Al-Mg coating and the coating weight of the sprayed MgZn2 were appropriately adjusted to obtain coating films with the compositions shown in Table 1. Table 1 shows the composition and bath temperature of the coating bath used in the production of the hot-dip Zn-Al-Mg coated steel sheets, as well as the composition and coating weight of the coating of each sample.

[0041] [Evaluation] Each sample of the obtained hot-dip Zn-Al-Mg coated steel sheet was evaluated as follows. The evaluation results are shown in Table 1.

[0042] (1) State of MgZn2 and Al phase in main layer For each sample of the prepared hot-dip Zn-Al-Mg coated steel sheet, a cross section was observed at one random location using a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). Then, for each sample, the area ratio of MgZn2, the area ratio of MgZn2 having a portion exposed at the outermost coating surface, the area ratio of MgZn2 present within 50% of the thickness of the main layer, and the area ratio of the Al phase present within 50% of the thickness of the main layer, all observed in the cross section in the thickness direction of the coating, were measured or calculated, and the results are shown in Table 1.

[0043] (2) Corrosion Resistance Evaluation: Each hot-dip Zn-Al-Mg-plated steel sheet sample was sheared to a size of 70 mm x 120 mm. A 10 mm area from each edge of the surface to be evaluated, as well as the sample edges and the non-evaluation surface, were sealed with tape, leaving a 50 mm x 100 mm area of ​​the surface to be evaluated. The three evaluation samples prepared as described above were subjected to the Japanese Automotive Standards Combined Cyclic Test (JASO-CCT). The accelerated corrosion test began with wetting, and the surface appearance of each sample was visually inspected. The number of cycles required for the development of red rust was measured and evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: Number of cycles for red rust occurrence ≧160 cycles ○: Number of cycles for red rust occurrence ≦120 cycles <160 cycles ×: Number of cycles for red rust occurrence <120 cycles (3) Scratch Resistance Each sample of the obtained hot-dip galvanized steel sheet was scratched by pressing a diamond scratch needle with a 45° tip against the steel sheet surface at a specific load in accordance with the scratch hardness test of JIS K 6902 (2008), and then visually inspected for the presence or absence of scratches. The minimum load at which scratches occurred was measured and evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: Minimum load ≧0.5N ×: Minimum load <0.5N

[0044]

[0045] The results in Table 1 show that the samples of the invention have a good balance of corrosion resistance and scratch resistance compared to the samples of the comparative examples.

[0046] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg coated steel sheet that achieves both high levels of corrosion resistance and scratch resistance.

[0047] 10: Base steel sheet 20: Plating film 21: Main layer 22: Interface alloy layer

Claims

1. A hot-dip Zn-Al-Mg plated steel sheet having a plating film consisting of an interfacial alloy layer present at the interface with a base steel sheet and a main layer present on the interfacial alloy layer, wherein the plating film has a composition containing 10-22 mass% Al, 0.01-2 mass% Si, 3-10 mass% Mg, with the balance being Zn and unavoidable impurities, and wherein, when a cross-section of the plating film is observed in the thickness direction, the area ratio of MgZn2 present in the main layer is 30% or more.

2. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1, characterized in that, when a cross section of the plating film is observed in a thickness direction, the area ratio of the MgZn2 having a portion exposed on the outermost plating surface to the observed MgZn2 is 30% or more.

3. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that, upon cross-sectional observation of the plating film in a thickness direction, the area ratio of MgZn2 present within a thickness range of up to 50% from the surface of the main layer among the observed MgZn2 exceeds 50%.

4. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that, when a cross section of the plating film is observed in a thickness direction, the area ratio of the Al phase present within a thickness range of up to 50% from the surface of the main layer is less than 50%.

5. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the plating film further contains 0.1 to 5 mass% in total of one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb and Bi.

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