Hot-dip galvanized Zn-Al-Mg series steel sheet

Abstract

The purpose is to provide a hot-dip Zn-Al-Mg coated steel sheet that achieves a high level of both corrosion resistance and damage resistance. To achieve the above objective, the present invention is a hot-dip Zn-Al-Mg coated steel sheet, which is a hot-dip Zn-Al-Mg coated steel sheet having a coating film (20). The coating film (20) is composed of an interface alloy layer (22) existing at the interface with the base steel sheet (10) and a main layer (21) existing on the interface alloy layer (22). The coating film (20) is characterized by having the following composition: containing Al: 10-22% by mass, Si: 0.01-2% by mass and Mg: 3-10% by mass, with the remainder consisting of Zn and unavoidable impurities. When viewed in cross-section in the thickness direction of the coating film (20), the area ratio of MgZn2 present in the main layer (21) is more than 30%.

Description

Technical Field

[0001] The present invention relates to a hot-dip Zn-Al-Mg-based steel sheet having excellent corrosion resistance and damage resistance. Prior Art

[0002] Hot-dip Zn-plated steel sheets have excellent corrosion resistance and have been widely used as rust-proof steel sheets in the fields of automobiles, motors, and building materials. Generally speaking, hot-dip Zn-based coatings are composed of an interface alloy layer present at the interface with the base steel sheet and a main layer present on the interface alloy layer. Zn, which mainly exists in the main layer, has a sacrificial corrosion resistance to Fe, and thus exhibits superior corrosion resistance compared to cold-rolled steel sheets or hot-rolled steel sheets. Furthermore, when a general cold-rolled steel sheet or hot-rolled steel sheet is used as the base steel sheet, the interface alloy layer includes an Fe-Al alloy or Fe-Zn alloy formed by the reaction of Fe of the base steel sheet and Zn or Al of the plating bath components as a constituent component.

[0003] In recent years, in order to meet the market's demand for high corrosion resistance, multi-element alloy plated steel sheets such as hot-dip Zn-Al-Mg steel sheets have been developed that have Al, Mg, and Si added to the Zn as plating components. For example, Patent Document 1 discloses a hot-dip Zn-Al-Mg steel plate, wherein the composition of the coating film is Al: 4.0~10 wt%, Mg: 1.0~4.0 wt%, and the remainder is composed of Zn and unavoidable impurities. In addition, Patent Document 2 discloses a hot-dip Zn-Al-Mg steel plate, wherein the composition of the plated film is Al: 2~19% by weight, Mg: 1.0~10% by weight, Si: 0.01~2% by weight, and the remainder is composed of Zn and unavoidable impurities, and the content of Al and Mg is set to a total of less than 20% by weight.

[0004] Here, as disclosed in Patent Document 1 or Patent Document 2, in the case of a general hot-dip Zn-Al-Mg steel sheet, a complex solidification reaction occurs during the film formation process, so the plating film has a complex and non-uniform structure. Moreover, due to this non-uniform structure, the hot-dip Zn-Al-Mg steel sheet tends to have excellent corrosion resistance compared to the conventional hot-dip Zn steel sheet, but further improvement of stable corrosion resistance is desired. Furthermore, as disclosed in Patent Document 1 or Patent Document 2, the coating film of a general hot-dip Zn-Al-Mg steel sheet has a problem that the coating surface is easily damaged compared to conventional hot-dip Zn-coated steel sheets due to the presence of soft Al phase or Zn phase. [Prior Technical Literature] [] [Patent Document] [] []

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 10-226865 Patent Document 2: Japanese Patent Application Publication No. 2000-104154 Summary of the invention

[0006] [Problems to be solved by the invention] The present invention is based on the above circumstances and aims to provide a hot-dip Zn-Al-Mg steel plate having both high corrosion resistance and damage resistance. [Methods to solve the problem] [] []

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

[0008] The present invention has been completed based on the above knowledge and insights, and its gist is as follows. 1. A hot-dip Zn-Al-Mg steel sheet, which is a hot-dip Zn-Al-Mg steel sheet having a coating film, wherein the coating film is composed of an interface alloy layer existing at the interface with a base steel sheet and a main layer existing on the interface alloy layer, and is characterized by: The above-mentioned plated film has the following composition: containing Al: 10-22 mass %, Si: 0.01-2 mass % and Mg: 3-10 mass %, and the remainder is composed of Zn and inevitable impurities. When observing a cross section in the thickness direction of the plated film, the area ratio of MgZn2 present in the main layer is 30% or more. 2. The hot-dip Zn-Al-Mg-coated steel sheet as described in 1 above, wherein when observing a cross section in the thickness direction of the coating film, the area ratio of the MgZn 2 having a portion exposed on the coating outermost surface among the observed MgZn 2 is 30% or more. 3. The hot-dip Zn-Al-Mg-coated steel sheet as described in 1 or 2 above, wherein when observing a cross section in the thickness direction of the coating film, the area ratio of MgZn2 existing in the thickness range from the surface of the main layer to 50% among the observed MgZn2 exceeds 50%. 4. The hot-dip Zn-Al-Mg-coated steel sheet as described in 1 or 2 above, wherein when observing a cross section in the thickness direction of the coating film, among the Al phases observed, the area ratio of the Al phase existing in the thickness range from the surface of the main layer to 50% is less than 50%. 5. The hot-dip Zn-Al-Mg-plated steel sheet according to 1 or 2 above, wherein the plating film further contains 0.1 to 5 mass % in total of one or more selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb and Bi. [Effects of the Invention] [] []

[0009] According to the present invention, there can be provided a hot-dip Zn-Al-Mg-based steel sheet which achieves both corrosion resistance and scratch resistance at a high level. Simple diagram description

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

[0011] [Modes for carrying out the invention] (Hot-dip Zn-Al-Mg steel sheet) The hot-dip Zn-Al-Mg steel plate of the present invention is shown in FIG1 , and has a plated film 20 on a base steel plate 10, wherein the plated film 20 is composed of an interface alloy layer 22 existing at the interface with the base steel plate 10 and a main layer 21 existing on the interface alloy layer. Furthermore, the plated film 20 has the following composition: Al: 10-22 mass %, Si: 0.01-2 mass % and Mg: 3-10 mass %, and the remainder is composed of Zn and inevitable impurities.

[0012] The Zn as the main component of the above-mentioned plating film is an element necessary to impart the above-mentioned plating film with a sacrificial corrosion resistance and obtain excellent corrosion resistance. When the content of the above-mentioned Zn is considered in terms of atomic composition ratio, since the plating layer is composed of elements with low specific gravity such as Al or Mg, the atomic composition ratio must be mainly Zn. Therefore, the Zn content in the above-mentioned plated film must be set to 60 mass % or more, preferably 70 mass % or more. Moreover, the upper limit of the Zn content is the content of the remaining part excluding elements other than Zn and impurities.

[0013] The Al in the aforementioned plated film is an element necessary to form an Al phase in the aforementioned main layer and obtain excellent corrosion resistance. If the Al content of the aforementioned plated film exceeds 5% by mass, an Al phase can be formed in the plated film, and the amount of Al phase formed increases as the Al content increases. In order to obtain more stable and excellent corrosion resistance, a certain Al phase must be formed in the aforementioned plated film, and the Al content in the aforementioned plated film can be set to 10% by mass or more. Therefore, the lower limit of the Al concentration is set to 10% by mass. On the other hand, if the Al concentration in the aforementioned plated film increases, there is a tendency to sacrifice corrosion resistance. Therefore, the upper limit of the Al concentration must be set to 22% by mass or less. Based on the same viewpoint, the Al content in the aforementioned plated film is preferably 12-20 mass %, more preferably 15-19 mass %.

[0014] Furthermore, Si in the aforementioned plated film is mainly used to suppress the abnormal growth of the Fe-Al-based interface alloy layer generated at the interface with the base steel sheet, thereby ensuring the workability of the plated film. If the base steel sheet is immersed in the aforementioned hot-dip Zn-Al-Mg-based bath containing Si, the Fe on the surface of the base steel sheet reacts with the Al or Si in the bath to form an Fe-Al-based and / or Fe-Al-Si-based intermetallic compound layer at the interface of the base steel sheet / plated film. At this time, since the growth rate of the Fe-Al-Si-based alloy is slower than that of the Fe-Al-based alloy, the higher the ratio of the Fe-Al-Si-based alloy, the more the growth of the entire interface alloy layer can be suppressed. Therefore, the Si content in the aforementioned plated film must be set to 0.01% by mass or more. On the other hand, if the Si content in the aforementioned plated film exceeds 2% by mass, not only will the growth suppression effect of the aforementioned interface alloy layer be saturated, but also the corrosion will be promoted due to the presence of excess Si phase in the plated film, so the Si content is set to 2% by mass or less.

[0015] Furthermore, Mg in the aforementioned plating film has the function of stabilizing the corrosion products formed during corrosion, and is an essential element for obtaining excellent corrosion resistance. To obtain the effect of stabilizing the corrosion products, the Mg content in the aforementioned plating film needs to be 3% by mass or more, and in order to obtain a more reliable effect, it is preferably set to 5% by mass or more. On the other hand, if the Mg content in the above-mentioned plating film exceeds 10% by mass, the plating film becomes harder and more brittle, and the workability deteriorates. Therefore, the upper limit of the Mg content is set to 10% by mass. Based on the same viewpoint, the Mg content in the above-mentioned plated film is preferably 5 to 8% by mass, more preferably 6 to 8% by mass.

[0016] Furthermore, the aforementioned plating film contains inevitable impurities. Among them, the aforementioned inevitable impurities contain Fe. The Fe is inevitably contained in the plating bath due to the dissolution of the steel plate or the equipment in the bath, and is inevitably contained in the aforementioned plating film as a result of diffusion from the base steel plate when the interface alloy layer is formed. The Fe content in the aforementioned plating film is generally about 0.1 to 0.5 mass %.

[0017] In addition, the aforementioned plated film may further contain 0.1 to 5 mass % of one or more selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb and Bi as needed. These elements can improve the stability of corrosion products when the plated film corrodes, delay the progress of corrosion, or stabilize the size of spangles on the plated surface, thereby improving the surface appearance.

[0018] Moreover, the aforementioned plating film is composed of an interface alloy layer 22 existing at the interface with the base steel plate 10 and a main layer 21 existing on the interface alloy layer 22, as shown in FIGS. 1 to 3 . 1 to 3 , for the sake of convenience, schematically show the cross-sections of the aforementioned base steel plate 10, the aforementioned main layer 21 and the aforementioned interface alloy layer 22, and the actual shapes or sizes are different from those shown in FIGS. 1 to 3 .

[0019] The aforementioned interface alloy layer is formed during the plating process by the reaction of the base steel plate with bath components such as Zn, Al, Mg, Si, etc. in the plating bath, and is generally an intermetallic compound of the Fe-Al system and / or the Fe-Al-Si system. Furthermore, when a hot-rolled steel sheet or a high-tensile strength steel sheet with low plating wettability is used as a base steel sheet, in order to ensure wettability, a pre-plating of Ni or Fe is applied to the base steel sheet before the plating process. In particular, when the base steel sheet is a pre-plated Ni sheet, a Ni-Al-based and / or Fe-Ni-Al-based intermetallic compound containing Ni is formed as an interface alloy layer.

[0020] Moreover, when the interface alloy layer exists in a state of an average film thickness of 0.1 to 1 μm, a stable main layer can be formed on the interface alloy layer. When the average film thickness is less than 0.1 μm, the interface alloy layer is not formed in the entire plated film, that is, there is a situation where the base steel plate and the plating bath do not react, and there is a risk of unstable plating adhesion and film formation. On the other hand, if the average film thickness exceeds 1 μm, there is a risk that the interface alloy layer will break during processing and cause the plating layer to peel off. Therefore, the average film thickness of the interface alloy layer is preferably 0.1 to 1 μm.

[0021] 1 to 3, the plating bath components that are not consumed in the formation of the interface alloy layer 22 solidify and mainly form Al phase, Zn phase, and MgZn 2.

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

[0023] Furthermore, in the present invention, as shown in FIG. 1 , the area ratio of MgZn 2 present in the main layer 21 is 30% or more when the cross section of the plated film 20 in the thickness direction is observed. The MgZn2 in the main layer 21 has the function of dissolving preferentially in the early stage when the plated film corrodes, and stabilizing the formed corrosion products. Therefore, in the early stage of corrosion of the plated film 20, MgZn2 dissolves preferentially, and stable corrosion products are formed early, which can reduce the corrosion rate of the plated film. In addition, since the MgZn2 is a hard intermetallic compound, it can improve the damage resistance of the plated film by existing in the main layer of the plated film. Therefore, by setting the area ratio of MgZn2 in the main layer 21 to 30% or more, both the corrosion resistance and the scratch resistance can be stably obtained. Based on the same viewpoint, when observing the cross section in the thickness direction of the plated film 20, the area ratio of MgZn2 in the main layer 21 is preferably 40% or more, and more preferably 50% or more.

[0024] Furthermore, as shown in FIG. 2, when observing the cross section in the thickness direction of the plated film 20, the area ratio of the MgZn 2 having the portion exposed to the uppermost surface of the plated film among the observed MgZn 2 is preferably 30% or more, and more preferably 40% or more. In order to efficiently carry out the preferential dissolution of MgZn 2 at the initial stage of corrosion, it is effective to have more MgZn 2 exposed to the surface of the plated film 20. In addition, the larger the ratio of the MgZn 2 having the portion exposed to the surface of the plated film 20, the more the surface damage can be suppressed. Therefore, by setting the area ratio of the MgZn 2 having the portion exposed to the plated surface among the observed MgZn 2 to 30% or more, it is possible to achieve both corrosion resistance and damage resistance at a higher level.

[0025] Furthermore, as shown in FIG. 3, the MgZn 2 present in the main layer 21 is preferably present in the thickness range from the surface of the main layer 21 to 50% when the cross section of the plated film 20 is observed, the area ratio of the MgZn 2 present in the thickness range from the surface of the main layer 21 to 50% is preferably more than 50%, and more preferably more than 60%. As described above, in order to efficiently carry out the preferential dissolution of MgZn 2 at the initial stage of corrosion, it is effective to have more MgZn 2 present on the surface side of the plated film 20. In addition, the surface damage can be further suppressed when MgZn 2 is present on the surface side of the plated film 20. Therefore, when the cross section of the plated film 20 is observed in the thickness direction, by setting the area ratio of the MgZn 2 present in the thickness range from the surface of the main layer 21 to 50% to more than 50%, it is possible to achieve both corrosion resistance and damage resistance at a higher level.

[0026] Furthermore, the Al phase formed in the main layer is a necessary structure for obtaining stable and excellent corrosion resistance, and when observing the cross section in the thickness direction of the plated film, the area ratio of the Al phase is preferably 30% or more, more preferably 40% or more. Furthermore, in order to obtain the corrosion resistance improvement effect brought by the Al phase, the distribution location in the main layer is not limited as long as the proportion of the Al phase in the main layer 21 is a certain degree. Therefore, from the perspective of not hindering the distribution of the MgZn2 to the surface side of the main layer 21 and more efficiently taking into account both corrosion resistance and damage resistance, as shown in FIG3, when observing the cross-section in the thickness direction of the plated film 20, among the observed Al phases, the area ratio of the Al phase existing in the thickness range from the surface of the main layer 21 to 50% is preferably less than 50%.

[0027] Furthermore, the method for observing the cross section of the aforementioned plated 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 or Al phase in the aforementioned main layer 21, and for example, it can be observed and measured by SEM-EDX (energy dispersive X-ray analysis of a scanning electron microscope).

[0028] Furthermore, the coating weight of the aforementioned plating film is preferably 30 to 300 g / m 2 per single side from the viewpoint of satisfying various characteristics. This is because when the coating weight of the aforementioned plating film is 30 g / m 2 or more, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance such as building materials, and when the coating weight of the aforementioned plating film is 300 g / m 2 or less, excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. Based on the same viewpoint, the coating weight of the aforementioned plating film is more preferably 50 to 150 g / m 2.

[0029] The amount of the plating film can be derived, for example, by dissolving and peeling off a specific area of the plating film in a mixed solution of hydrochloric acid and hexamethylenetetramine, and calculating from the weight difference of the steel sheet before and after peeling, as shown in JIS H 0401:2013. In this method, in order to obtain the amount of plating per single side, the plating surface of the non-target side is sealed with tape so that the plating surface is not exposed, and then the dissolution is performed to obtain the amount of plating.

[0030] In addition, the hot-dip Zn-Al-Mg steel sheet of the present invention is shown in FIG. 1 , in which a plating film 20 is formed on a base steel sheet 10, but an intermediate layer or a coating film may be further formed on the plating film as needed. There is no particular limitation on the type of coating or the method for forming the coating, and it can be appropriately selected according to the required performance. For example, the forming method may be roller coating, curtain coating, spray coating, etc. After the coating containing organic resin is applied, it can be heated and dried by means of hot air drying, infrared heating, induction heating, etc. to form a coating. In addition, the intermediate layer is not particularly limited as long as it is a layer formed between the plated film of the hot-dip Zn-Al-Mg steel sheet and the coating film. For example, a primer such as a chemical conversion film or an adhesive layer can be cited. The chemical conversion film can be formed by, for example, applying a chromate treatment solution or a chromium-free chemical conversion solution, and performing a chromate treatment or a chromium-free chemical conversion treatment at a temperature of 80 to 300° C. without water washing. The chemical conversion films can be single-layer or multi-layer. In the case of multi-layer, multiple chemical conversion treatments can be performed in sequence.

[0031] (Method for producing hot-dip Zn-Al-Mg steel sheet) There is no particular limitation on the method for producing the hot-dip Zn-Al-Mg-based steel sheet of the present invention. However, the coating film of the hot-dip Zn-Al-Mg steel sheet obtained by the present invention is substantially the same as the composition of the coating bath. Therefore, there is a step of forming the coating film on the base steel sheet using a coating bath, wherein the coating bath composition is controlled to contain Al: 10-25 mass %, Si: 0.01-2 mass % and Mg: 3-10 mass %, and the remainder is composed of Zn and inevitable impurities.

[0032] Furthermore, the step of forming the aforementioned plating film is not particularly limited except for the composition of the aforementioned plating bath. For example, the base steel sheet can be cleaned, heated, and immersed in a plating bath by a continuous hot-dip plating device. In the heating step of the steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself. In order to prevent oxidation of the steel sheet and reduce the trace oxide film on the surface, heating in a reducing environment such as a nitrogen-hydrogen environment is effective.

[0033] The bath temperature of the plating bath is not particularly limited, but is preferably set to a temperature 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 must be set to above the solidification point in order to perform the hot dip treatment, and by setting it to the melting point + 20°C, solidification due to a local decrease in the bath temperature of the plating bath can be prevented. On the other hand, the reason why the upper limit of the bath temperature is set to 550°C is that if it exceeds 550°C, the plating film becomes difficult to cool rapidly, and there is a possibility that the interface alloy layer formed between the plating film and the steel sheet becomes thicker.

[0034] Furthermore, there is no particular limitation on the method for controlling the area ratio of MgZn2 in the main layer to be 30% or more. For example, it can be formed by hot-dip plating MgZn2 in a plating bath, or by spraying MgZn2 powder on the surface of the steel sheet where the plating has not solidified immediately after the hot-dip plating.

[0035] Furthermore, there is no particular limitation on the method for controlling the area ratio of the MgZn2 having the portion exposed on the topmost surface of the coating to be 30% or more among the aforementioned MgZn2 observed. For example, it can be formed by spraying MgZn2 powder on the surface of the coated unsolidified steel sheet immediately after the hot dip coating treatment. Furthermore, there is no particular limitation on the method for controlling the area ratio of MgZn2 existing in the thickness range from the surface of the main layer to 50% to more than 50% in the observed MgZn2. For example, after manufacturing a Zn-Al-Mg plated steel sheet by a conventional hot dip treatment, the steel sheet is subjected to a second hot dip treatment in a hot dip Zn-Al-Mg bath to which MgZn2 is added, or a method of spraying MgZn2 powder on the surface of the plated steel sheet that has not solidified immediately after the hot dip treatment.

[0036] Furthermore, there is no particular limitation on the base steel sheet constituting the Zn-Al-Mg based plated steel sheet of the present invention, and a cold rolled steel sheet or a hot rolled steel sheet can be appropriately used according to the required performance or specification. In addition, there is no particular limitation on the base steel sheet. Furthermore, there is no particular limitation on the method for obtaining the base steel sheet. For example, in the case of the hot-rolled steel sheet, a steel sheet that has been subjected to a hot rolling step and a pickling step may be used, and in the case of the cold-rolled steel sheet, a cold rolling step may be added for production. Furthermore, in order to obtain the characteristics of the steel sheet, a recrystallization annealing step may be performed before the hot-dip galvanizing step. Furthermore, a pre-plated steel sheet may 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 is immersed in a sulfuric acid bath or a chloride bath containing metal ions of various pre-plating components to perform an electrolytic treatment. Furthermore, in the displacement plating method, the base steel sheet is immersed in an aqueous solution containing metal ions of various pre-plating components and the pH of which is adjusted with sulfuric acid to displace the precipitated metal. As a representative example of the pre-plated steel sheet, a Ni pre-plated steel sheet can be cited.

[0037] Furthermore, in the manufacturing method of the hot-dip Zn-Al-Mg-based steel sheet of the present invention, in addition to the above-mentioned steps of forming the plating film and the steps of heating and cooling after the plating film is formed, the steps used in conventional plated steel sheets can also be appropriately implemented. [Example] [] []

[0038] [Sample 1~2] (Manufacturing method A:) A cold-rolled steel sheet with 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 samples 1 to 2 of hot-dip Zn-Al-Mg steel sheets under the conditions 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-based steel sheet, and the composition and adhesion amount of the coating film of each sample.

[0039] [Samples 3~9] (Preparation method B:) A cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method was used as a base steel sheet. After annealing and hot-dip plating were performed using a hot-dip plating simulator manufactured by RHESCA Co., Ltd., powdered MgZn2 (average particle size: less than 2 μm) was sprayed on the plating surface before the plating solidified, thereby preparing samples 3 to 9 of hot-dip Zn-Al-Mg steel sheets under the conditions shown in Table 1. Furthermore, the adhesion amount of hot-dip Zn-Al-Mg system and the adhesion amount of sprayed MgZn2 were appropriately adjusted to obtain the plating film with the composition 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-based steel sheet, and the composition and adhesion amount of the coating film of each sample.

[0040] [Sample 10] (Preparation method C:) A cold-rolled steel sheet with a thickness of 0.8 mm produced by a conventional method and pre-plated with Ni was used as a base steel sheet. After annealing and hot-dip plating were performed using a hot-dip plating simulator manufactured by RHESCA Co., Ltd., powdered MgZn2 (average particle size: less than 2 μm) was sprayed on the plated surface before the plating solidified, thereby producing sample 10 of a hot-dip Zn-Al-Mg steel sheet under the conditions shown in Table 1. In addition, the pre-Ni plating treatment was carried out using a plating bath with a concentration of 300 g / L NiSO 4･6H 2O, 40 g / L H 3BO 3, 100 g / L Na 2SO 4, and pH 2.7, at a bath temperature of 60°C and a current density of 50 A / dm 2, and the Ni deposition amount was controlled to be 1 g / m 2. In addition, the deposition amount of the hot-dip Zn-Al-Mg system and the deposition amount of the sprayed MgZn 2 were appropriately adjusted to obtain a plating film with the composition 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-based steel sheet, and the composition and adhesion amount of the coating film of each sample.

[0041] [evaluate] The following evaluations were performed on each sample of the hot-dip Zn-Al-Mg-based steel sheet obtained. Table 1 shows the evaluation results.

[0042] (1) Existence state of MgZn2 and Al phase in the main layer For each sample of the produced hot-dip Zn-Al-Mg-based steel sheet, a cross section was observed at an arbitrary location using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX). In addition, for each sample, the area ratio of MgZn2 observed in the cross section in the thickness direction of the plated film, the area ratio of MgZn2 having a portion exposed on the outermost surface of the plated film, the area ratio of MgZn2 existing in the thickness range from the surface of the main layer to 50%, and the area ratio of the Al phase existing in the thickness range from the surface of the main layer to 50% were measured or calculated, and are shown in Table 1.

[0043] (2) Corrosion resistance evaluation For each sample of hot-dip Zn-Al-Mg steel plate, cut it into a size of 70mm×120mm, seal the area 10mm away from each edge of the evaluation target surface and the end surface of the sample with the evaluation non-target surface with tape, and expose the evaluation target surface with a size of 50mm×100mm as the evaluation sample. The three evaluation samples prepared as described above were subjected to the Japanese Automotive Standards Combined Cycle Test (JASO-CCT). The test was to start the corrosion acceleration test from wetting, visually confirm the appearance of the surface of each sample, measure the number of cycles until red rust occurs, and evaluate according to the following criteria. Table 1 shows the evaluation results. ◎: The number of cycles for red rust to occur is ≧160 cycles ○: 120 cycles ≦ the number of cycles for red rust to occur < 160 cycles ×: The number of cycles for red rust to occur is <120 cycles (3) Damage resistance For each sample of the obtained hot-dip galvanized steel sheet, a scratch hardness test according to JIS K 6902 (2008) was conducted by pushing a diamond scratching needle with a 45° tip onto the steel sheet surface at a specific load to scratch it, and then visually checking for damage. The minimum load at which damage occurs was measured, and evaluation was performed according to the following criteria. The evaluation results are shown in Table 1. 0: Minimum load ≧0.5N ×: Minimum load <0.5N

[0044]

[0045] From the results in Table 1, it can be seen that, compared with the samples of the comparative examples, the samples of the examples of the present invention have excellent balance between corrosion resistance and scratch resistance. [Industrial Application Possibility] [] []

[0046] According to the present invention, there can be provided a hot-dip Zn-Al-Mg-based steel sheet which achieves both corrosion resistance and scratch resistance at a high level.

[0047] 10: Base steel plate 20: Coated film 21: Main Floor 22: Interface alloy layer

Claims

1. A hot-dip Zn-Al-Mg based steel sheet, which is a hot-dip Zn-Al-Mg based steel sheet having a coating film, the coating film being composed of an interface alloy layer present at the interface with a base steel sheet and a main layer present on the interface alloy layer, characterized in that: the aforementioned coating film has the following composition: containing Al: 10-22% by mass, Si: 0.01-2% by mass and Mg: 3-10% by mass, the remainder being composed of Zn and unavoidable impurities; when viewed in cross-section along the thickness direction of the aforementioned coating film, the area proportion of MgZn2 present in the aforementioned main layer is more than 40%; when viewed in cross-section along the thickness direction of the aforementioned coating film, among the observed MgZn2, the area proportion of MgZn2 present from the surface of the aforementioned main layer to 50% of the thickness range exceeds 50%.

2. For the hot-dip Zn-Al-Mg series steel sheet of claim 1, when viewed in cross-section along the thickness direction of the aforementioned coating film, the area proportion of MgZn2 with the portion exposed on the outermost surface of the coating is 30% or more.

3. For hot-dip Zn-Al-Mg steel sheets as requested in item 1 or 2, when viewed in cross-section along the thickness direction of the aforementioned coated film, the area proportion of the Al phase present in the observed Al phase from the surface of the aforementioned main layer to 50% of the thickness does not reach 50%.

4. The hot-dip Zn-Al-Mg series steel sheet as claimed in claim 1 or 2, wherein the aforementioned coating further contains a total of 0.1 to 5% by mass of one or more 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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