WO2025121109A1 - Molten zinc–aluminum–magnesium-based plated steel sheet - Google Patents

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 plated steel sheet

The present invention relates to hot-dip Zn-Al-Mg plated steel sheets with excellent corrosion resistance and scratch resistance.

2. Description of the Related Art Hot-dip galvanized steel sheets have excellent corrosion resistance and have conventionally been widely used as rust-resistant steel sheets in the fields of automobiles, electrical machinery, building materials, and the like.
In general, a hot-dip Zn-based plating coating is composed of an interfacial alloy layer present at the interface with the base steel sheet and a main layer present on the interfacial alloy layer, and exhibits 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 base steel sheet, the above-mentioned interface alloy layer contains as its constituents an Fe-Al alloy or an Fe-Zn alloy formed by the reaction between the Fe in the base steel sheet and the Zn or Al in the coating bath.

In recent years, in order to meet the market need 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 remainder being Zn and unavoidable impurities.
Furthermore, Patent Document 2 discloses a hot-dip Zn-Al-Mg plated steel sheet having a plating film composition consisting of 2 to 19 wt % Al, 1.0 to 10 wt % Mg, 0.01 to 2 wt % Si, with the balance being Zn and unavoidable impurities, with the total content of Al and Mg being 20 mass % or less.

Here, in the case of a typical hot-dip Zn-Al-Mg-plated steel sheet as disclosed in Patent Document 1 or Patent Document 2, a complex solidification reaction occurs during the film formation process, resulting in a complex and non-uniform structure of the plated film. Due to this non-uniform structure, the hot-dip Zn-Al-Mg-plated steel sheet tends to have superior corrosion resistance compared to conventional hot-dip Zn-plated steel sheets, but there has been a demand for a more stable improvement in corrosion resistance.
Furthermore, the plating coating of a general hot-dip Zn-Al-Mg plated steel sheet as disclosed in Patent Document 1 and Patent Document 2 has a problem in that the plating surface is more susceptible to scratches than a conventional hot-dip Zn plated steel sheet due to the presence of soft Al phases and Zn phases.

Japanese Patent Application Publication No. 10-226865 JP 2000-104154 A

In view of these circumstances, the present invention aims to provide a hot-dip Zn-Al-Mg plated steel sheet that combines high levels of corrosion resistance and scratch resistance.

As a result of investigations to solve the above problems, the present inventors have 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, and in particular, that the hardness of _MgZn2 formed in the plating film and its effect of stabilizing corrosion products are effective. They have found that it is possible to achieve both high levels of corrosion resistance and scratch resistance by controlling the amount of _MgZn2 present in the main layer, specifically the area ratio, within a specific range when observing a cross section of the plating film in the thickness direction.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A hot-dip Zn-Al-Mg-plated steel sheet having a plating film consisting of an interfacial alloy layer existing at the interface with a base steel sheet and a main layer existing on the interfacial alloy layer,
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 remainder being Zn and unavoidable impurities;
The hot-dip Zn-Al-Mg plated steel sheet, characterized in that, when a cross section of the plating film is observed in a thickness direction, an 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 ₁ above, characterized in that, when a cross section of the plating film is observed in a thickness direction, an area ratio of MgZn ₂ having a portion exposed on the outermost plating surface among the observed MgZn 2 is 30% or more.
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 plating film is observed in a thickness direction, an area ratio of _MgZn2 present within a thickness range of up to 50% from the surface of the main layer exceeds 50% of the observed MgZn2.
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 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% of the observed Al phase.
5. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, 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.

The present invention provides hot-dip Zn-Al-Mg plated steel sheets that combine high levels of corrosion resistance and scratch resistance.

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. 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.

(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 has a plating film 20 on a base steel sheet 10, and the plating film 20 comprises an interfacial alloy layer 22 present at the interface with the base steel sheet 10 and a main layer 21 present on the interfacial alloy layer.
The plating film 20 has a composition containing 10 to 22 mass % Al, 0.01 to 2 mass % Si, 3 to 10 mass % Mg, with the remainder being Zn and unavoidable impurities.

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

Al in the plating film is an essential element for forming an Al phase in the main layer and obtaining 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 an increase in the Al content. In order to obtain more stable and excellent corrosion resistance, it is necessary to form a certain amount of Al phase in the plating film, and the Al content in the plating film may be 10 mass% or more. Therefore, the lower limit of the Al concentration is set to 10 mass%. On the other hand, when the Al concentration in the plating film increases, the sacrificial corrosion protection tends to deteriorate. Therefore, the upper limit of the Al concentration needs to be 22 mass% or less.
From the same viewpoint, the Al content in the plating film is preferably 12 to 20 mass %, and more preferably 15 to 19 mass %.

The Si in the plating film is used to suppress the abnormal growth of the Fe-Al interfacial alloy layer that is generated mainly at the interface with the base steel sheet, and to ensure the workability of the plating film. When the base steel sheet is immersed in a molten Zn-Al-Mg plating bath containing the Si, the Fe on the surface of the base steel sheet reacts with the Al and Si in the bath to form an Fe-Al and/or Fe-Al-Si intermetallic compound layer at the interface between the base steel sheet and the plating film. At this time, since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, the higher the ratio of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film must be 0.01 mass% or more. On the other hand, if the Si content in the plating film exceeds 2 mass%, not only will the effect of suppressing the growth of the interfacial alloy layer described above saturate, but the presence of excess Si in the plating film will also promote corrosion, so the Si content is set to 2 mass% or less.

Furthermore, Mg in the plating film has a function of stabilizing the corrosion products formed during corrosion, and is an essential element for obtaining excellent corrosion resistance. In order to obtain the effect of stabilizing the corrosion products, the Mg content in the plating film needs to be 3 mass% or more, and to obtain a more reliable effect, it is preferable that the Mg content is 5 mass% or more.
On the other hand, if the Mg content in the plating film exceeds 10 mass %, the plating film becomes hard and brittle, and the workability deteriorates, so the upper limit of the Mg content is set to 10 mass %.
From the same viewpoint, the Mg content in the plating film is preferably 5 to 8 mass %, and more preferably 6 to 8 mass %.

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 when the interfacial alloy layer is formed. The Fe content in the plating film is usually about 0.1 to 0.5 mass%.

In addition, the plating film preferably further contains, as necessary, 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 in a total amount of 0.1 to 5 mass %. These elements can improve the stability of the corrosion products when the plating film corrodes, slowing the progress of corrosion, and can stabilize the spangle size on the plating surface, improving the surface appearance.

As shown in FIGS. 1 to 3, the plating film is composed of an interface alloy layer 22 present at the interface with the base steel sheet 10 and a main layer 21 present on the interface alloy layer 22.
In addition, in Figures 1 to 3, the cross sections of the base steel plate 10, the main layer 21 and the interface alloy layer 22 are shown diagrammatically for the convenience of explanation, and the actual shapes, dimensions, etc. differ from those shown in Figures 1 to 3.

The interfacial alloy layer is formed in the plating process by the reaction of the base 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 has low wettability, is used as the base steel sheet, the base steel sheet may be pre-plated with Ni, Fe, or the like before the plating process in order to ensure wettability. In particular, when a steel sheet pre-plated with Ni is used as the base steel sheet, Ni-Al and/or Fe-Ni-Al intermetallic compounds containing Ni are formed as the interface alloy layer.

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. When the average thickness is less than 0.1 μm, the interfacial alloy layer may not be formed over the entire plating film, i.e., the base steel sheet and the plating bath may not react, which may result in a stable plating adhesion and film formation. On the other hand, when the average thickness exceeds 1 μm, the interfacial alloy layer may crack during processing, causing the plating to peel off. Therefore, it is preferable that the average thickness of the interfacial alloy layer is 0.1 to 1 μm.

In addition, as shown in Figures 1 to 3, the plating bath components that were not consumed in the formation of the interface alloy layer 22 are solidified in the main layer, and thus mainly an Al phase, a Zn phase, and _MgZn2 are formed.

The Al phase is a necessary structure 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.

In the present invention, 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.
Since MgZn ₂ present in the main layer 21 has the function of preferentially dissolving in the initial stage when the plating film corrodes and stabilizing the formed corrosion product, the preferential dissolution of MgZn ₂ in the initial stage of corrosion of the plating film 20 allows stable corrosion products to be formed early, and the corrosion rate of the plating film can be reduced. In addition, since the MgZn ₂ is a hard intermetallic compound, the presence of the MgZn 2 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 of the effects of corrosion resistance and scratch resistance can be stably obtained. From the same viewpoint, the area ratio of _MgZn2 present in the main layer 21 when observing a cross section of the plating film 20 in the thickness direction is preferably 40% or more, and more preferably 50% or more.

In addition, as shown in FIG. 2, when the cross section of the plating film 20 in the thickness direction is observed, the area ratio of the MgZn ₂ having a portion exposed on the outermost plating surface among the observed MgZn ₂ is preferably 30% or more, more preferably 40% or more. In order to efficiently perform preferential dissolution by MgZn ₂ at the early stage of corrosion, it is effective that there is a lot _{of MgZn 2 exposed} on the surface of the plating film 20. In addition, the larger the ratio of MgZn ₂ having a portion exposed on the surface of the plating film 20, the more the surface scratches can be suppressed. Therefore, by making the area ratio of MgZn ₂ having a portion exposed on the plating surface among the observed MgZn ₂ 30% or more, it is possible to achieve both corrosion resistance and scratch resistance at a higher level.

Furthermore, as shown in FIG. 3, when the cross section of the plating film 20 is observed in the thickness direction, the area ratio of the MgZn ₂ present in the main layer ₂₁ within 50% of the thickness range from the surface of the main layer ₂₁ is preferably more than 50%, and more preferably 60% or more. As described above, in order to efficiently perform preferential dissolution by MgZn ₂ at the beginning of corrosion, it is effective that a large amount of MgZn ₂ is present on the surface side of the plating film 20. In addition, the presence of a large amount of MgZn ₂ on the surface side of the plating film 20 can further suppress surface scratches. Therefore, when the cross section of the plating film 20 is observed in the thickness direction, the area ratio of the MgZn ₂ present in the thickness range from 50% of the surface of the main layer ₂₁ is more than 50%, so that both corrosion resistance and scratch resistance can be achieved at a higher level.

Furthermore, the Al phase formed in the main layer is a structure necessary for obtaining stable excellent corrosion resistance, and when a cross section of the plating film is observed in the thickness direction, the area ratio occupied by the Al phase is preferably 30% or more, and more preferably 40% or more.
Furthermore, in order to obtain the effect of improving corrosion resistance brought about by the Al phase, it is sufficient that the Al phase occupies a certain proportion in the main layer 21, and there is no limitation on the distribution location in the main layer. Therefore, from the viewpoint of more efficiently achieving both corrosion resistance and scratch resistance without impeding the distribution of the above-mentioned MgZn ₂ toward the surface layer side of the main layer 21, it is preferable that, when observing a cross section of the plating film 20 in the 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 21 is less than 50%, as shown in Fig. 3.

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 _MgZn2 and Al phases in the main layer 21. For example, the cross section can be observed and measured by SEM-EDX (energy dispersive X-ray analysis using a scanning electron microscope).

From the viewpoint of satisfying various characteristics, the coating weight of the plating film is preferably 30 to 300 g/ ^m2 per side. When the coating weight of the plating film is 30 g/ ^m2 or more, sufficient corrosion resistance is obtained for applications requiring long-term corrosion resistance, such as building materials, while when the coating weight of the plating film is 300 g/m2 or ^less , excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the coating weight of the plating film is more preferably 50 to 150 g/ ^m2 .

The amount of plating adhesion can be calculated, for example, by dissolving and peeling off a specific area of the plating film in a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the difference in the weight of the steel sheet before and after peeling. To determine the amount of plating adhesion per side using this method, the non-target side is sealed with tape so that the plated surface is not exposed, and then the dissolution described above is carried out.

In addition, in the hot-dip Zn-Al-Mg plated steel sheet of the present invention, as shown in FIG. 1 , a plating film 20 is formed on a base steel sheet 10, but an intermediate layer or a coating film can also be further formed on the plating film, if necessary.
The type of the coating film and the method for forming the coating film are not particularly limited and can be appropriately selected depending on the required performance. For example, the coating film can be formed by a method such as roll coater coating, curtain flow coating, spray coating, etc. After coating the coating material containing an organic resin, the coating film can be formed by heating and drying the coating film by means of hot air drying, infrared heating, induction heating, etc.
The intermediate layer is not particularly limited as long as it is a layer 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 a chromium-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion coating liquid is applied, and then dried at a steel sheet temperature of 80 to 300°C without rinsing with water. These chemical conversion coating films may be single-layered or multi-layered, and in the case of multi-layered films, multiple chemical conversion treatments may be performed sequentially.

(Manufacturing method of hot-dip Zn-Al-Mg coated 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 plating film of the hot-dip Zn-Al-Mg plated steel sheet obtained by the present invention has a composition generally equivalent to that of the plating bath. Therefore, the present invention includes a step of forming the plating film on the base steel sheet using a plating bath whose composition is controlled to contain 10-25 mass% Al, 0.01-2 mass% Si, 3-10 mass% Mg, with the remainder being Zn and unavoidable impurities.

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 coating bath in a continuous hot-dip galvanizing facility. In the steel sheet heating process, 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 a small amount of oxide film present on the surface.

The temperature of the plating 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 the melting point + 20° C. is that the bath temperature needs to be equal to or higher than the solidification point in order to perform hot-dip plating, and by setting the temperature to the melting point + 20° C., solidification due to a local drop in the bath temperature of the plating bath is prevented. 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., 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 becomes thick.

The method for controlling the area ratio of _MgZn2 in the main layer to 30% or more is not particularly limited. For example, it can be formed by adding _MgZn2 to a plating bath and performing hot-dip plating, or by spraying _MgZn2 powder on the surface of the steel sheet where the plating has not solidified immediately after the hot-dip plating process.

In addition, there is no particular limitation on the method for controlling the area ratio of _MgZn2 having a portion 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 on the surface of a steel sheet where the coating has not yet solidified immediately after hot-dip coating.
Furthermore, there is no particular limitation on the method for controlling the area _ratio of _MgZn2 existing within a thickness range of 50% from the surface of the main layer to exceed 50%. For example, it can be formed by a method of manufacturing a Zn-Al-Mg-based plated steel sheet by a normal hot-dip plating process, and then subjecting the steel sheet to a second hot-dip plating process in a hot-dip Zn-Al-Mg plating bath containing _MgZn2 , or by a method of spraying _MgZn2 powder on the surface of a steel sheet where the plating has not yet solidified immediately after the hot-dip plating process.

The base steel sheet constituting the Zn-Al-Mg-plated steel sheet of the present invention is not particularly limited, and a cold-rolled steel sheet, a hot-rolled steel sheet, or the like 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 not particularly limited. For example, in the case of the hot-rolled steel sheet, one that has been subjected to a hot rolling process and a pickling process can be used, and in the case of the cold-rolled steel sheet, it can be manufactured by further adding a cold rolling process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to pass through a recrystallization annealing process or the like before the hot-dip plating 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 immersed in a sulfate bath or a chloride bath containing metal ions of various pre-plating components to perform electrolytic treatment. In the displacement plating method, the base steel sheet may be immersed in an aqueous solution containing metal ions of various pre-plating components and having a pH adjusted with sulfuric acid to cause displacement precipitation of metal. A representative example of a pre-plated steel sheet is a Ni pre-plated steel sheet.

In addition, in the manufacturing method of the hot-dip Zn-Al-Mg plated steel sheet of the present invention, in addition to the above-mentioned plating film formation process and the heating/cooling process after plating film formation, it is possible to appropriately carry out processes that are used in normal plated steel sheets.

[Samples 1-2]
(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 Corporation to produce hot-dip Zn-Al-Mg plated steel sheet samples 1 and 2 under the conditions shown in Table 1.
Table 1 shows the composition and bath temperature of the plating bath used in the production of the hot-dip Zn-Al-Mg plated steel sheets, and the composition and coating weight of the plating film of each sample.

[Samples 3 to 9]
(Manufacturing method B:) A cold-rolled steel sheet with a thickness of 0.8 mm produced by conventional methods was used as the base steel sheet, and annealing and hot-dip plating were performed using a hot-dip plating simulator manufactured by Rhesca Co., Ltd. After that, powdered _MgZn2 (average particle size: 2 μm or less) was sprayed onto the plated surface before the plating solidified, to produce samples 3 to 9 of hot-dip Zn-Al-Mg plated steel sheets according to the conditions shown in Table 1.
By appropriately adjusting the amount of the hot-dip Zn-Al-Mg coating and the amount of _MgZn2 sprayed, a coating film having the composition shown in Table 1 was obtained.
Table 1 shows the composition and bath temperature of the plating 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 of each sample.

[Sample 10]
(Manufacturing method C:) A 0.8 mm thick cold-rolled steel sheet manufactured by conventional methods was pre-plated with Ni and used as the base steel sheet. This 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 to produce sample 10 of hot-dip Zn-Al-Mg plated steel sheet with the conditions shown in Table 1.
The Ni pre-plating treatment was performed using a plating bath with a concentration _{of NiSO4.6H2O} of 300g/L, _a concentration of _H3BO3 of 40g/L, a concentration _{of Na2SO4} of 100g/L, and a pH of 2.7, under conditions of a bath temperature of 60°C and a current density of 50A/ ^dm2 , with the Ni deposition weight controlled to be 1g/ ^m2 . In addition, by appropriately adjusting the deposition weight of the hot-dip Zn-Al-Mg-based plating and the deposition weight of the sprayed _MgZn2 , a plating film with the composition shown in Table 1 was obtained.
Table 1 shows the composition and bath temperature of the plating 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 of each sample.

[evaluation]
The following evaluations were carried out on each sample of the obtained hot-dip Zn-Al-Mg plated steel sheet. The evaluation results are shown in Table 1.

(1) State of _MgZn2 and Al phase in main layer For each sample of hot-dip Zn-Al-Mg-plated steel sheet prepared, 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 observed in the cross section in the thickness direction of the plating film, the area ratio of _MgZn2 having a portion exposed on the outermost plating surface, the area ratio _{of MgZn2} present within 50% of the thickness range from the surface of the main layer, and the area ratio of the Al phase present within 50% of the thickness range from the surface of the main layer were measured or calculated, and are shown in Table 1.

(2) Evaluation of corrosion resistance Each sample of hot-dip Zn-Al-Mg coated steel sheet was sheared to a size of 70 mm × 120 mm, and then a range of 10 mm from each edge of the surface to be evaluated, as well as the end faces and non-evaluation surfaces of the sample were sealed with tape to expose the surface to be evaluated in a size of 50 mm × 100 mm, and these were used as samples for evaluation.
The three evaluation samples prepared as described above were subjected to the Japanese Automotive Standard Combined Cyclic Test (JASO-CCT). The accelerated corrosion test started with wetting, and the appearance of the surface of each sample was visually confirmed. The number of cycles until red rust appeared was measured, and the samples were evaluated according to the following criteria. The evaluation results are shown in Table 1.
◎: Number of cycles for red rust generation ≧160 cycles ○: 120 cycles ≦ Number of cycles for red rust generation <160 cycles ×: Number of cycles for red rust generation <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 with a specific load in accordance with the scratch hardness test of JIS K 6902 (2008), and then visually confirmed 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.
〇：Minimum load ≧0.5N
×：Minimum load＜0.5N

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

The present invention provides hot-dip Zn-Al-Mg plated steel sheets that combine high levels of corrosion resistance and scratch resistance.

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

Claims (5)

A hot-dip Zn-Al-Mg-plated steel sheet having a plating film composed of an interfacial alloy layer existing at the interface with a base steel sheet and a main layer existing on the interfacial alloy layer,
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 remainder being Zn and unavoidable impurities;
The hot-dip Zn-Al-Mg plated steel sheet, characterized in that, when a cross section of the plating film is observed in a thickness direction, an 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, an area ratio of the MgZn ₂ having a portion exposed on an outermost plating surface to the observed MgZn 2 is 30% or more. 3. 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, an area ratio of the _MgZn2 present within a thickness range of up to 50% from a surface of the main layer exceeds 50% of the observed MgZn2. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that, when a cross-section of the plated coating is observed in the thickness direction, the area ratio of the Al phase present within a thickness range of 50% from the surface of the main layer is less than 50%. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, 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.

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