Hot-dip zn-al-mg-based plated steel sheet

Abstract

The purpose of the present invention is to provide a hot-dip Zn-Al-Mg-based plated steel sheet which has both corrosion resistance and plating adhesion at a high level.　To achieve the purpose, the present invention is a hot-dip Zn-Al-Mg-based plated steel sheet comprising a plated film 20 which is composed of: an interfacial alloy layer 22 present at an interface with a base steel sheet 10; and a main layer 21 present on the interfacial alloy layer 22, the hot-dip Zn-Al-Mg-based plated steel sheet being characterized in that the plated film 20 has a composition containing 10-22 mass% of Al, 0.01-2 mass% of Si, and 3-10 mass% of Mg, with the remainder consisting of Zn and inevitable impurities, and when a cross section of the plated film 20 is observed in the thickness direction, a needle-like inorganic compound 23 having a major diameter of at least 1 μm and an aspect ratio (minor diameter / major diameter) of at most 0.2 is formed on the interfacial alloy layer 22.

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 coating adhesion.

[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] However, 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. Due to this non-uniform structure, hot-dip Zn-Al-Mg-plated steel sheets have a problem in that the adhesion between the substrate steel sheet and the plated film (hereinafter referred to as "plating adhesion"), in particular the adhesion between the interface alloy layer and the main layer, is unstable and low compared to conventional hot-dip Zn-plated steel sheets.

[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 coating adhesion.

[0007] As a result of investigations aimed at solving the above-mentioned problems, the present inventors have focused on the fact that it is important not only to control the concentrations of Zn, Al, Mg, and Si when it comes to 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 have found that by forming needle-shaped inorganic compounds on the interfacial alloy layer that exists at the interface between the plating film and the substrate steel sheet when a cross section of the plating film in the thickness direction is observed, it is possible to increase the adhesion between the main layer of the plating film and the interfacial alloy layer, thereby making it possible to improve plating adhesion in addition to corrosion resistance.

[0008] The present invention was made based on the above findings and is summarized 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 substrate 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, needle-shaped inorganic compounds having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less are formed on the interfacial alloy layer. 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 needle-shaped inorganic compounds extend from the surface of the interfacial alloy layer into the main layer. 3. 3. The hot-dip Zn-Al-Mg plated steel sheet according to the above 1 or 2, characterized in that the needle-shaped inorganic compounds contain Si. 4. The hot-dip Zn-Al-Mg plated steel sheet according to the above 3, characterized in that the needle-shaped inorganic compounds further contain Ni. 5. The hot-dip Zn-Al-Mg plated steel sheet according to the above 1 or 2, characterized in that the interfacial alloy layer contains Ni. 6. The hot-dip Zn-Al-Mg plated steel sheet according to the above 1 or 2, characterized in that the plated coating 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 coating adhesion.

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

[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 has a plating film 20 on a substrate steel sheet 10, and the plating film 20 consists of an interfacial alloy layer 22 present at the interface with the substrate 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, and 3 to 10 mass% Mg, with the balance being Zn and unavoidable impurities. Note that Fig. 1 shows an enlarged cross section of the hot-dip Zn-Al-Mg-plated steel sheet of this embodiment, but the dimensions and shapes of the components are shown schematically for ease of explanation and may differ from the actual dimensions and shapes.

[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%, 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] As shown in Fig. 1, 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. Note that Fig. 1 shows a schematic cross section of the substrate steel sheet 10, the main layer 21, and the interface alloy layer 22 for ease of explanation, and the actual shapes, dimensions, etc. differ from those shown in Fig. 1.

[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 FIG. 1, 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] Furthermore, the MgZn2 dissolves preferentially in the early stages of corrosion of the plating film, stabilizing the resulting corrosion products. Furthermore, since MgZn2 is a hard intermetallic compound, its presence in the plating main layer can improve the scratch resistance of the plating film. To consistently achieve both effects, the area ratio of the MgZn2 in the main layer is preferably 10% or more, more preferably 30% or more, when observing a cross section of the plating film in the thickness direction. Furthermore, the Zn phase, together with the MgZn2, exerts sacrificial corrosion protection against Fe, improving the corrosion resistance of the end surface where Fe is exposed. To consistently achieve these effects, the total area ratio of the Zn phase and the MgZn2 in the main layer is preferably 30% or more when observing a cross section of the plating film in the thickness direction.

[0024] The hot-dip Zn-Al-Mg-plated steel sheet of the present invention is characterized in that, when a cross section of the plating film is observed in the thickness direction, needle-shaped inorganic compounds having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less are formed on the interfacial alloy layer. As shown in Fig. 1, when a cross section of the plating film 20 is observed in the thickness direction, needle-shaped inorganic compounds 23 are formed on the interfacial alloy layer 22, which can exert an anchoring effect between the interfacial alloy layer 22 and the main layer 21, thereby achieving excellent corrosion resistance as a hot-dip Zn-Al-Mg-plated steel sheet and also high levels of plating adhesion.

[0025] As shown in FIG. 1 , the major axis of the acicular inorganic compounds 23 is determined by observing a cross section of the plating film 20 in the thickness direction in a range of 2 mm or more parallel to the surface of the base steel sheet, measuring the major axis L of at least 10 randomly selected acicular inorganic compounds 23, and averaging the measured values. The minor axis of the acicular inorganic compounds 23 is determined by measuring the minor axis D of the acicular inorganic compounds 23 over the same measurement range and number of measurements as the major axis. If the major axis of the acicular inorganic compounds 23 is less than 1 μm, a sufficient anchoring effect may not be achieved, and the desired plating adhesion may not be achieved. On the other hand, if the major axis of the acicular inorganic compounds 23 is too large, the acicular inorganic compounds 23 may penetrate deeply into the main layer 21, potentially deteriorating workability and corrosion resistance of the processed portion. Therefore, the major axis of the acicular inorganic compounds 23 is preferably 10 μm or less, and more preferably 5 μm or less. The method for observing the cross section of the plating film 20 in the thickness direction is not particularly limited as long as it allows for observing the presence or absence of the acicular inorganic compounds 23 and the major axis L and minor axis D. For example, observation and measurement can be performed using SEM-EDX (energy dispersive X-ray analysis using a scanning electron microscope).

[0026] As shown in FIG. 1 , the aspect ratio of the needle-shaped inorganic compound 23 is the ratio of the minor axis to the major axis of the needle-shaped inorganic compound 23 (minor axis D / major axis L). If the aspect ratio of the needle-shaped inorganic compound 23 exceeds 0.2, a sufficient anchoring effect may not be obtained, and the desired plating adhesion may not be achieved. On the other hand, if the aspect ratio of the needle-shaped inorganic compound 23 is less than 0.05, the needle-shaped inorganic compound 23 may penetrate deeply into the main layer 21, potentially deteriorating workability and corrosion resistance of the processed portion. From the same perspective, the aspect ratio of the needle-shaped inorganic compound 23 is preferably 0.05 to 0.2, and more preferably 0.10 to 0.15. Furthermore, the average aspect ratio (average aspect ratio) of the aspect ratios of 10 or more randomly selected acicular inorganic compounds 23 is preferably 0.2 or less when a cross section of the plating film 20 in the thickness direction is observed over a range of 2 mm or more in a direction parallel to the surface of the base steel sheet. When the average aspect ratio of the acicular inorganic compounds 23 is 0.2 or less, stable and excellent adhesion can be achieved. On the other hand, when the average aspect ratio of the acicular inorganic compounds 23 is less than 0.05, the acicular inorganic compounds 23 penetrate deeply into the main layer 21, potentially resulting in poor workability and corrosion resistance in the processed area. From the same perspective, the average aspect ratio of the acicular inorganic compounds 23 is preferably 0.05 to 0.2, and more preferably 0.10 to 0.15.

[0027] 1 , when a cross section of the plating film 22 is observed in the thickness direction, the acicular inorganic compounds 23 preferably extend from the surface of the interface alloy layer 22 into the main layer 21. This is because the acicular inorganic compounds 23 further increase the adhesive strength between the interface alloy layer 22 and the main layer 21, thereby achieving better plating adhesion. Note that the present invention specifies that it is preferable to be able to confirm that the acicular inorganic compounds 23 extend from the surface of the interface alloy layer 22 into the main layer 21 when a cross section of the plating film 22 is observed in the thickness direction, but in reality, it is believed that most of the acicular inorganic compounds 23 extend from the surface of the interface alloy layer 22.

[0028] The constituent components of the acicular inorganic compound are not particularly limited as long as they are inorganic compounds having the above-mentioned shape, but more specifically, Si-based compounds containing Si are preferred. Since Si is present in large amounts near the interfacial alloy layer, it becomes a constituent component of the acicular inorganic compound, and it tends to have a shape with a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less. This can more reliably improve plating adhesion.

[0029] Furthermore, the constituent components of the acicular inorganic compound preferably further contain Ni in addition to the above-mentioned Si. When the Ni is contained in the interface alloy layer, the acicular inorganic compound extending from the interface alloy layer will contain Ni. By containing Ni in the acicular inorganic compound, integration with the interface alloy layer is promoted, and better plating adhesion can be obtained.

[0030] 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:

[0031] 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.

[0032] 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.

[0033] (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 22 mass% Al, 0.01 to 2 mass% Si, and 3 to 10 mass% Mg, with the balance being Zn and unavoidable impurities.

[0034] 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.

[0035] 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.

[0036] The method for forming the acicular inorganic compound on the interfacial alloy layer is not particularly limited. For example, the acicular inorganic compound can be formed by adding the acicular inorganic compound to a plating bath and performing a hot-dip plating process. In this case, the acicular inorganic compound added preferably has a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less.

[0037] Furthermore, 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, in the case of the hot-rolled steel sheet, a steel sheet 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, a steel sheet can be produced by further adding a cold-rolling process. Furthermore, in order to obtain the desired steel sheet properties, a recrystallization annealing process or the like can also be performed before the hot-dip plating process.

[0038] A pre-plated steel sheet may also be used as the base steel sheet. The pre-plated steel sheet is plated by, for example, 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 chloride bath containing metal ions of various pre-plating components for 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, the pH of which is adjusted with sulfuric acid, for displacement deposition of the metal. A typical example of a pre-plated steel sheet is a Ni pre-plated steel sheet. Note that, when a Ni pre-plated steel sheet is subjected to a hot-dip plating treatment in a bath containing the aforementioned acicular inorganic compound, the acicular inorganic compound is likely to extend from the surface of the interface alloy layer into the main plating layer, thereby improving the adhesion of the resulting hot-dip Zn-Al-Mg-plated steel sheet, which is particularly preferred.

[0039] 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.

[0040] [Samples 1 to 3] (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 to 3 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.

[0041] [Samples 4 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 galvanizing were performed using a hot-dip galvanizing simulator manufactured by Rhesca Corporation to produce hot-dip Zn-Al-Mg-plated steel sheet Samples 4 to 9 under the conditions shown in Table 1. The composition and bath temperature of the plating bath used in manufacturing 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. For Samples 4 to 9, an acicular inorganic compound was added to the plating bath in an amount of 0.1% relative to the total weight of the plating bath. The type, average major axis, and aspect ratio of the acicular inorganic compound are shown in Table 1.

[0042] [Samples 10 to 14] (Manufacturing method C:) A 0.8 mm thick cold-rolled steel sheet manufactured by a conventional method was subjected to Ni pre-plating and used as a base steel sheet. This was then annealed and hot-dip plated using a hot-dip plating simulator manufactured by Rhesca Corporation, to produce hot-dip Zn-Al-Mg plated steel sheet samples 10 to 14 under the conditions shown in Table 1. The Ni pre-plating of the cold-rolled steel sheet 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, and 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 / m 2 The composition and bath temperature of the plating bath used in producing 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. Furthermore, for Samples 10 to 14, an acicular inorganic compound was added to the plating bath in an amount of 0.1% relative to the total weight of the plating bath. The type, average major axis, and aspect ratio of the acicular inorganic compound are shown in Table 1.

[0043] <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.

[0044] (1) Acicular inorganic compounds For each sample of hot-dip Zn-Al-Mg-plated steel sheet prepared, a cross section was observed and analyzed at a random location using a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). For each sample, the presence or absence of acicular inorganic compounds observed in the cross section in the thickness direction of the plating film, the components contained in the acicular inorganic compounds, the average size (major axis, aspect ratio) of the acicular inorganic compounds, and the presence or absence of acicular inorganic compounds extending from the interfacial alloy layer were measured and calculated. The results are shown in Table 1.

[0045] (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, and a 10 mm area from each edge of the surface to be evaluated, as well as the sample edge and the non-evaluation surface, were sealed with tape, leaving a 50 mm x 100 mm area of ​​the surface to be evaluated. The evaluation samples were subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). The accelerated corrosion test began with wetting and continued for 90 cycles. The corrosion weight loss of each sample was then measured using the methods specified in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: Corrosion weight loss of 60 g / m 2 Below: ○: Corrosion loss is 80g / m 2 Less than ×: Corrosion loss is 80g / m 2 Overcoming

[0046] (3) Coating Adhesion Each sample of the obtained hot-dip Zn-Al-Mg-plated steel sheet was sheared to a size of 70 mm × 100 mm and then subjected to 180° close bending (0T bending) to obtain a 100 mm apex. Cellotape (registered trademark) was then firmly attached to the outer surface of the bent portion after bending and then peeled off. The outer surface (apex) of the bent portion was then observed with an optical microscope (OLYMPUS DSX1000) at 30x magnification to confirm the damage state of the plating film, and the plating adhesion was evaluated according to the following criteria. The evaluation results are shown in Table 1. ⊚: No peeling of the plating film (no cracks or only cracks); ○: Slight peeling of the plating film (total diameter of peeled portion less than 5 mm); ×: Obvious peeling of the plating film (total diameter of peeled portion 5 mm or more).

[0047]

[0048] The results in Table 1 show that the samples of the invention have a good balance of corrosion resistance and plating adhesion compared to the samples of the comparative examples.

[0049] 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 coating adhesion.

[0050] REFERENCE SIGNS LIST 10: Base steel sheet 20: Plating film 21: Main layer 22: Interface alloy layer 23: Acicular inorganic compound L: Long diameter of acicular inorganic compound D: Short diameter of acicular inorganic compound

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, needle-shaped inorganic compounds having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less are formed on the interfacial alloy layer.

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 the thickness direction, the needle-like inorganic compounds extend from the surface of the interface alloy layer into the main layer.

3. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the needle-shaped inorganic compound contains Si.

4. The hot-dip Zn-Al-Mg plated steel sheet according to claim 3, characterized in that the needle-shaped inorganic compounds further contain Ni.

5. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the interface alloy layer contains Ni.

6. 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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