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 coating adhesion. To achieve the above objectives, the present invention provides a hot-dip Zn-Al-Mg based steel sheet, which is a hot-dip Zn-Al-Mg based 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), needle-shaped inorganic compounds (23) with a major diameter of 1 μm or more and an aspect ratio (minor diameter / major diameter) of 0.2 or less are formed on the interface alloy layer (22).

Description

Technical Field

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

[0002] Hot-dip Zn-coated steel sheets have excellent corrosion resistance and have therefore been widely used as rust-proof steel sheets in the fields of automobiles, motors, and building materials. Generally speaking, hot-dip Zn-plated coatings consist of an interface alloy layer at the interface with the base steel sheet and a main layer located above the interface alloy layer. The Zn, primarily present in the main layer, provides superior corrosion resistance to Fe, resulting in superior corrosion resistance compared to cold-rolled 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 an Fe-Zn alloy formed by the reaction of Fe in the base steel sheet with Zn or Al in the plating bath as a component.

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

[0004] However, as disclosed in Patent Document 1 or Patent Document 2, in the case of a typical hot-dip Zn-Al-Mg-based steel sheet, 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 uneven structure, hot-dip Zn-Al-Mg steel sheets have a problem of unstable and low adhesion between the base steel sheet and the coating film (hereinafter referred to as "coating adhesion"), especially the adhesion between the interface alloy layer and the main layer, compared to conventional hot-dip Zn-coated steel sheets. Prior Art Literature Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 10-226865 Patent Document 2: Japanese Patent Application Laid-Open 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-based steel sheet that achieves both high corrosion resistance and high plating adhesion. Means of solving problems

[0007] The present inventors conducted intensive research to solve the above-mentioned problems and found that it is important to control not only the concentrations of Zn, Al, Mg, and Si but also the structure of the coating film in the composition of hot-dip Zn-Al-Mg steel sheets. They found that when observing a cross-section of the coating film in the thickness direction, the adhesion between the main layer of the coating film and the interface alloy layer can be improved by forming needle-shaped inorganic compounds on the interface alloy layer existing at the interface between the coating film and the base steel sheet. Therefore, in addition to corrosion resistance, the coating adhesion can also be improved.

[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 coated steel sheet having a coating film, wherein the coating film comprises an interface alloy layer at the interface with a base steel sheet and a main layer on the interface alloy layer, and wherein: The plating film has the following composition: Al: 10-22 mass %, Si: 0.01-2 mass % and Mg: 3-10 mass %, the remainder being Zn and inevitable impurities. When observing a cross section of the plated film in the thickness direction, a needle-shaped inorganic compound having a long diameter of 1 μm or more and an aspect ratio (short diameter / long diameter) of 0.2 or less is formed on the interface alloy layer. 2. The hot-dip Zn-Al-Mg plated steel sheet as described in 1 above, wherein, when observing a cross section of the plated film in the thickness direction, the needle-shaped inorganic compound extends from the surface of the interface alloy layer into the main layer. 3. The hot-dip Zn-Al-Mg-coated steel sheet according to 1 or 2 above, wherein the needle-shaped inorganic compound contains Si. 4. The hot-dip Zn-Al-Mg-based steel sheet according to 3 above, wherein the needle-shaped inorganic compound further contains Ni. 5. The hot-dip Zn-Al-Mg-coated steel sheet according to 1 or 2 above, wherein the interface alloy layer contains Ni. 6. The hot-dip Zn-Al-Mg-coated steel sheet according to item 1 or 2 above, wherein the coating further contains 0.1 to 5% by 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. Effects of the Invention

[0009] According to the present invention, a hot-dip Zn-Al-Mg-based steel sheet can be provided that achieves both high levels of corrosion resistance and plating adhesion. Simple diagram description

[0010] FIG1 is an enlarged schematic diagram showing a cross section of a hot-dip Zn-Al-Mg coated steel sheet according to the present embodiment. Implementation Method

[0011] Modes for carrying out the invention (Hot-dip Zn-Al-Mg steel sheet) The hot-dip Zn-Al-Mg coated steel sheet of the present invention is shown in FIG1 , and comprises a coating film 20 on a base steel sheet 10. 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. Furthermore, the plated film 20 has the following composition: Al: 10-22 mass %, Si: 0.01-2 mass % and Mg: 3-10 mass %, with the remainder being Zn and inevitable impurities. 1 is an enlarged, schematic diagram showing a cross-section of a hot-dip Zn-Al-Mg coated steel sheet according to this embodiment. However, the sizes and shapes of the various components are shown schematically for the sake of convenience of explanation and are different from the actual ones.

[0012] Zn, the main component of the aforementioned plating film, is an element necessary to impart sacrificial corrosion protection to the plating film, resulting in excellent corrosion resistance. When considering the Zn content in terms of atomic composition, since the plating layer is composed of low-density elements such as Al and Mg, the atomic composition must be primarily Zn. Therefore, the Zn content in the plated film must be 60% by mass or more, preferably 70% by mass or more. Furthermore, the upper limit of the Zn content is the content of the remainder excluding elements other than Zn and impurities.

[0013] The Al in the plated film is an essential element for forming an Al phase in the main layer and achieving excellent corrosion resistance. If the Al content in the plated film exceeds 5% by mass, an Al phase may form in the film, and the amount of Al phase formed increases with increasing Al content. To achieve more stable and excellent corrosion resistance, a certain Al phase must be formed in the plated film. Setting the Al content in the plated film to 10% by mass or higher suffices. Therefore, the lower limit of the Al concentration is set to 10% by mass. On the other hand, increasing the Al concentration in the plated film tends to compromise corrosion resistance. Therefore, the upper limit of the Al concentration must be set to 22% by mass or lower. Based on the same viewpoint, the Al content in the aforementioned plating film is preferably 12-20 mass %, more preferably 15-19 mass %.

[0014] Furthermore, the Si in the aforementioned coating film is primarily used to suppress the abnormal growth of the Fe-Al interfacial alloy layer formed at the interface with the base steel sheet, thereby ensuring the workability of the coating film. When the base steel sheet is immersed in the aforementioned Si-containing hot-dip Zn-Al-Mg bath, the Fe on the base steel sheet surface alloys with the Al or Si in the bath, forming Fe-Al and / or Fe-Al-Si intermetallic compound layers at the base steel sheet / coating film interface. Since the Fe-Al-Si alloy grows more slowly than 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 aforementioned coating film must be set to 0.01% by mass or greater. On the other hand, if the Si content in the aforementioned coating film exceeds 2% by mass, not only will the effect of suppressing the growth of the interfacial alloy layer saturate, but the presence of excess Si phase in the coating film will promote corrosion. Therefore, the Si content is set to 2% by mass or less.

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

[0016] Furthermore, the aforementioned plating film contains unavoidable impurities. These unavoidable impurities include Fe. This Fe is inevitably contained in the plating film due to dissolution from the steel sheet or bath equipment into the plating bath, and is also inevitably contained in the plating film as a result of diffusion from the base steel sheet during formation of the interface alloy layer. The Fe content in the plating film is typically approximately 0.1 to 0.5 mass%.

[0017] The plated film preferably further contains, if necessary, 0.1 to 5% by mass 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 can improve the stability of corrosion products during corrosion of the plated film, thereby delaying the progression of corrosion, or stabilize the size of spangles on the plated surface, thereby improving the surface appearance.

[0018] Furthermore, the aforementioned plating film 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, as shown in FIG. 1 , for convenience of explanation, schematically shows a cross-section of the base steel plate 10, the main layer 21, and the interface alloy layer 22, and the actual shape or size may be different from that shown in FIG. 1 .

[0019] The aforementioned interface alloy layer is formed during the plating process by the reaction of the base steel sheet with the 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 hot-rolled steel sheets or high-tensile strength steel sheets with low plating wettability are used as base steel sheets, pre-plating with Ni, Fe, etc. may be applied to the base steel sheet prior to the plating process to ensure wettability. In particular, when the base steel sheet is pre-plated with Ni, Ni-Al and / or Fe-Ni-Al intermetallic compounds containing Ni are formed as the interface alloy layer.

[0020] Furthermore, when the interface alloy layer has an average thickness of 0.1 to 1 μm, a stable main layer can be formed on the interface alloy layer. If the average thickness is less than 0.1 μm, the interface alloy layer will not form throughout the entire coating, meaning that the base steel sheet and the coating bath may not react, leading to unstable coating adhesion and coating formation. On the other hand, if the average thickness exceeds 1 μm, there is a risk of cracking the interface alloy layer during processing, causing coating delamination. Therefore, the average thickness of the interface alloy layer is preferably 0.1 to 1 μm.

[0021] 1, in the main layer, the plating bath components that are not consumed in the formation of the interface alloy layer 22 solidify to 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 of the plating film in the thickness direction, the area ratio occupied by the Al phase is preferably 30% or more, more preferably 40% or more.

[0023] Furthermore, the MgZn2 has the function of preferentially dissolving in the early stages of corrosion of the plated film, stabilizing the corrosion products formed. Furthermore, since MgZn2 is a hard intermetallic compound, its presence in the main plating layer improves the scratch resistance of the plated film. To stably achieve both of these effects, the area ratio of the MgZn2 in the main layer, when viewed in a cross-section through the thickness of the plated film, is preferably 10% or greater, and more preferably 30% or greater. Furthermore, the Zn phase primarily works together with the MgZn2 to provide sacrificial corrosion protection against Fe, improving the corrosion resistance of the end faces where Fe is exposed. To stably achieve this effect, the combined area ratio of the Zn phase and the MgZn2 in the main layer, when viewed in a cross-section along the thickness direction of the plated film, is preferably 30% or more.

[0024] Furthermore, the hot-dip Zn-Al-Mg-coated steel sheet of the present invention is characterized in that, when observing a cross section in the thickness direction of the aforementioned coating film, a needle-shaped inorganic compound having a long diameter of 1 μm or more and an aspect ratio (short diameter / long diameter) of 0.2 or less is formed on the aforementioned interface alloy layer. As shown in FIG1 , when observing a cross-section of the plated film 20 in the thickness direction, the needle-shaped inorganic compound 23 is formed on the interface alloy layer 22, thereby exhibiting an anchoring effect between the interface alloy layer 22 and the main layer 21. This allows for the excellent corrosion resistance of a hot-dip Zn-Al-Mg-coated steel sheet while also achieving a high level of plating adhesion.

[0025] The major diameter of the needle-shaped inorganic compound 23 is determined by observing a cross section of the coating film 20 in the thickness direction within a range of 2 mm or more parallel to the surface of the base steel sheet, as shown in FIG1 . The major diameters L of ten or more randomly selected needle-shaped inorganic compounds 23 are measured and averaged. Furthermore, the minor diameters D of the needle-shaped inorganic compound 23 are determined by measuring the minor diameters D of the compounds within the same measurement range and number of measurements as for the major diameters, and averaging the values. If the major diameter of the needle-shaped inorganic compound 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 diameter of the needle-shaped inorganic compound 23 is too large, the needle-shaped inorganic compound 23 may penetrate deeply into the main layer 21, potentially deteriorating workability and corrosion resistance of the processed portion. Therefore, the major diameter of the needle-shaped inorganic compound 23 is preferably 10 μm or less, and more preferably 5 μm or less. Furthermore, the cross-sectional observation of the aforementioned plated film 20 in the thickness direction is not particularly limited as long as the method can observe the presence or absence of the aforementioned needle-shaped inorganic compound 23 or the major axis L and minor axis D. For example, it can be observed and measured by SEM-EDX (energy dispersive X-ray analysis using a scanning electron microscope).

[0026] The aspect ratio of the needle-shaped inorganic compound 23 is the ratio of the minor diameter to the major diameter (minor diameter D / major diameter L) of the needle-shaped inorganic compound 23, as shown in Figure 1. If the aspect ratio of the needle-shaped inorganic compound 23 exceeds 0.2, a sufficient anchoring effect may not be achieved, 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 or corrosion resistance of the processed portion. Based on the same considerations, 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, when observing a cross-section of the coating film 20 in the thickness direction within a range of 2 mm or more parallel to the surface of the base steel sheet, the average aspect ratio of ten or more randomly selected acicular inorganic compounds 23 is preferably 0.2 or less (average aspect ratio). When the average aspect ratio of the acicular inorganic compounds 23 is 0.2 or less, stable and excellent adhesion is 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 deteriorating workability or corrosion resistance of the worked portion. Based on the same viewpoint, 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] Furthermore, as shown in FIG1 , the needle-shaped inorganic compound 23 preferably extends from the surface of the interface alloy layer 22 into the main layer 21 when viewed in a cross-section along the thickness direction of the plated film 22. This is because the needle-shaped inorganic compound 23 further enhances the adhesion between the interface alloy layer 22 and the main layer 21, thereby achieving even better plating adhesion. Furthermore, the present invention stipulates that when observing the cross-section of the aforementioned plated film 22 in the thickness direction, it is preferably confirmed that the aforementioned needle-shaped inorganic compound 23 extends from the surface of the aforementioned interface alloy layer 22 into the aforementioned main layer 21, but in reality, it is believed that most of the aforementioned needle-shaped inorganic compound 23 extends from the surface of the aforementioned interface alloy layer 22.

[0028] Furthermore, the constituent components of the needle-shaped 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 mostly present near the interface alloy layer, it becomes a component of the needle-shaped inorganic compound, easily achieving a shape with a long diameter of 1 μm or greater and an aspect ratio (short diameter / long diameter) of 0.2 or less. This more reliably improves plating adhesion.

[0029] Furthermore, regarding the constituent components of the needle-shaped inorganic compound, it is preferred that it further contain Ni in addition to the above-mentioned Si. When Ni is contained in the interface alloy layer, the needle-shaped inorganic compound extending from the interface alloy layer also contains Ni. Since the needle-shaped inorganic compound contains Ni, it is integrated with the interface alloy layer, thereby achieving better plating adhesion.

[0030] Furthermore, from the perspective of satisfying various properties, the coating weight is preferably 30 to 300 g / m² per side. This is because a coating weight of 30 g / m² or greater provides sufficient corrosion resistance even for applications requiring long-term corrosion resistance, such as building materials. Furthermore, a coating weight of 300 g / m² or less achieves excellent corrosion resistance while suppressing plating cracking during processing. Based on the same perspective, a coating weight of 50 to 150 g / m² is more preferably applied.

[0031] Regarding the adhesion amount of the aforementioned plating film, for example, This method, as specified in JIS H 0401:2013, involves dissolving and peeling a specific area of the plated film in a mixture of hydrochloric acid and hexamethylenetetramine, and calculating the weight difference between the steel sheet before and after the peeling. In this method, the amount of plating applied per side is determined by sealing the non-plated surface with tape, then performing the dissolution.

[0032] 1, the hot-dip Zn-Al-Mg coated steel sheet of the present invention has a coating film 20 formed on a base steel sheet 10. However, an intermediate layer or coating film may be further formed on the coating film as needed. The type of coating or the method for forming the coating is not particularly limited and can be appropriately selected based on the desired performance. Examples include roll coating, curtain coating, and spray coating. After applying the coating containing an organic resin, the coating film can be formed by heating and drying using methods 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 hot-dip Zn-Al-Mg plated steel sheet and the coating film. Examples include a chemical conversion coating and a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by applying a chromate treatment solution or a chromium-free chemical conversion solution, drying the steel sheet at a temperature of 80-300°C without rinsing, and then performing a chromate treatment or a chromium-free chemical conversion treatment. These chemical conversion coatings can be single or multiple layers. In the case of multiple layers, multiple chemical conversion treatments can be performed sequentially.

[0033] (Method for manufacturing hot-dip Zn-Al-Mg coated steel sheet) There is no particular limitation on the method for producing the hot-dip Zn-Al-Mg coated steel sheet of the present invention. However, the coating film of the hot-dip Zn-Al-Mg-coated steel sheet obtained according to the present invention has a substantially identical overall composition to the coating bath. Therefore, the present invention includes a step of forming the coating film on a base steel sheet using a coating bath, wherein the coating bath composition is controlled to contain 10-22% by mass of Al, 0.01-2% by mass of Si, and 3-10% by mass of Mg, with the remainder being Zn and unavoidable impurities.

[0034] Furthermore, the steps for forming the aforementioned plating film are not particularly limited except for the composition of the aforementioned plating bath. For example, continuous hot-dip coating equipment can be used to clean, heat, and then immerse the base steel sheet in a coating bath. During the heating step, recrystallization annealing is performed to control the microstructure of the base steel sheet. Heating in a reducing environment, such as a nitrogen-hydrogen atmosphere, is effective to prevent oxidation and reduce trace oxide films on the surface.

[0035] 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 lower limit of the bath temperature is set at the melting point + 20°C because hot-dip coating requires the bath temperature to be above the solidification point. By setting the bath temperature at the melting point + 20°C, solidification due to a localized drop in the bath temperature of the coating bath can be prevented. On the other hand, the upper limit of the bath temperature is set at 550°C because temperatures exceeding 550°C make it difficult to rapidly cool the coating film, potentially increasing the thickness of the interface alloy layer formed between the coating film and the steel sheet.

[0036] Furthermore, the method for forming the needle-shaped inorganic compound on the interface alloy layer is not particularly limited. For example, the needle-shaped inorganic compound can be added to a plating bath and then hot-dip plated. In this case, the needle-shaped inorganic compound preferably has a major diameter of 1 μm or greater and an aspect ratio (minor diameter / major diameter) of 0.2 or less.

[0037] The base steel sheet constituting the Zn-Al-Mg-based plated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheet or hot-rolled steel sheet can be used as appropriate depending on the required performance or specifications. Furthermore, 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, a steel sheet that has undergone a hot rolling step and a pickling step can be used, while in the case of the cold-rolled steel sheet, a cold rolling step can be added. Furthermore, to enhance the properties of the steel sheet, a recrystallization annealing step or the like may be performed before the hot-dip galvanizing step.

[0038] Furthermore, a pre-plated steel sheet can be used as the base steel sheet. The pre-plated steel sheet is plated, for example, by electrolytic treatment or displacement plating. In the electrolytic treatment method, the base steel sheet is immersed in a sulfuric acid bath or chloride bath containing various metal ions of the pre-plating components to perform the electrolytic treatment. In the displacement plating method, the base steel sheet is immersed in an aqueous solution containing various metal ions of the pre-plating components and having its pH adjusted with sulfuric acid to displace the deposited metal. A representative example of a pre-plated steel sheet is a Ni-plated steel sheet. Furthermore, when a pre-Ni-plated steel sheet is hot-dip plated in a bath containing the aforementioned needle-shaped inorganic compound, the needle-shaped inorganic compound easily extends from the surface of the interface alloy layer into the main plating layer, and the resulting hot-dip Zn-Al-Mg-based steel sheet has improved adhesion, making it preferable.

[0039] Furthermore, in the method for manufacturing a hot-dip Zn-Al-Mg-based steel sheet of the present invention, in addition to the above-mentioned steps of forming the coating film and the steps of heating and cooling after the coating film is formed, steps commonly used for coating steel sheets can also be appropriately implemented. Example

[0040] [Samples 1~3] (Manufacturing Method A:) Using a conventionally manufactured cold-rolled steel sheet having a thickness of 0.8 mm as the base steel sheet, annealing and plating were performed using a hot-dip plating simulator manufactured by RHESCA Co., Ltd. to produce samples 1 to 3 of hot-dip Zn-Al-Mg steel sheets under the conditions shown in Table 1. Furthermore, 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] [Samples 4~9] (Manufacturing Method B:) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as a base steel sheet, annealing treatment and hot-dip plating treatment were performed using a hot-dip plating simulator manufactured by RHESCA Co., Ltd. to produce samples 4 to 9 of hot-dip Zn-Al-Mg steel sheets under the conditions shown in Table 1. Furthermore, 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. In addition, for samples 4 to 9, the needle-shaped inorganic compound was added to the plating bath so that the amount of each needle-shaped inorganic compound was 0.1% relative to the total weight of the plating bath. Table 1 shows the type, average major diameter, and aspect ratio of the needle-shaped inorganic compound.

[0042] [Samples 10~14] (Manufacturing Method C:) Using a cold-rolled steel sheet with a thickness of 0.8 mm produced by a conventional method and pre-plated with Ni as the base steel sheet, annealing treatment and hot-dip plating treatment were performed using a hot-dip plating simulator manufactured by RHESCA Co., Ltd. to produce samples 10 to 14 of hot-dip Zn-Al-Mg steel sheets under the conditions shown in Table 1. Furthermore, the pre-Ni plating treatment of cold-rolled steel sheets uses a plating bath with a concentration of 300 g / L NiSO₄·6H₂O, 40 g / L H₃BO₃, 100 g / L Na₂SO₄, and a pH of 2.7. The bath temperature is 60°C, and the current density is 50 A / dm₂, with the Ni deposition weight controlled to 1 g / m₂. 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. Furthermore, for samples 10 to 14, the needle-shaped inorganic compound was added to the plating bath so that the concentration of each was 0.1% relative to the total weight of the plating bath. Table 1 shows the type, average major diameter, and aspect ratio of the needle-shaped inorganic compound.

[0043] <Evaluation> The following evaluations were performed on each sample of the obtained hot-dip Zn-Al-Mg-based steel sheet. The evaluation results are shown in Table 1.

[0044] (1) Needle-shaped inorganic compounds For each sample of the hot-dip Zn-Al-Mg-coated steel sheet, a cross-section was observed and analyzed at any location using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX). Table 1 shows the measured and calculated results for each sample regarding the presence of needle-shaped inorganic compounds in a cross-section along the thickness direction of the plated film, the composition of the needle-shaped inorganic compounds, the average size (length, aspect ratio) of the needle-shaped inorganic compounds, and the presence or absence of needle-shaped inorganic compounds extending from the interface alloy layer.

[0045] (2) Corrosion resistance evaluation For each sample of hot-dip Zn-Al-Mg steel plate, cut it into a size of 70 mm × 120 mm. Then, seal the area 10 mm from each edge of the evaluation target surface, the end surface of the sample, and the non-evaluation target surface with tape, leaving the evaluation target surface exposed in a size of 50 mm × 100 mm as the evaluation sample. Evaluation samples were subjected to the Japanese Automotive Standard Combined Cycle Test (JASO-CCT). The test consisted of a corrosion acceleration test starting with wetting and continuing for 90 cycles. The corrosion loss of each sample was then measured using the methods described in JIS Z 2383 and ISO 8407, and evaluated using the following criteria. Table 1 shows the evaluation results. ◎: Corrosion loss is less than 60g / m2 ○: Corrosion loss is 80g / m2 or less ×: Corrosion loss exceeds 80g / m2

[0046] (3) Plating adhesion Each sample of the resulting hot-dip Zn-Al-Mg-coated steel sheet was cut into 70 mm x 100 mm dimensions and then subjected to a 180° adhesive bend (0T bend) to obtain a 100 mm apex. Cellotape (registered trademark) was then firmly applied to the outer surface of the bent section and then removed. The outer surface of the bent section (apex) was then observed using an optical microscope (DSX1000, manufactured by Olympus) at 30x magnification to determine the morphology of damage to the plating film. The plating adhesion was evaluated using the following criteria. The evaluation results are shown in Table 1. ◎: No peeling of the plating film (no cracks, or only cracks) ○: The plating film is slightly peeled off (the total diameter of the peeled part is less than 5mm) ×: The plating film is obviously peeled off (the total diameter of the peeled part is more than 5mm)

[0047]

[0048] As can be seen from the results in Table 1, compared with the samples of the comparative examples, the samples of the inventive examples have a better balance between corrosion resistance and plating adhesion. Industrial Application Possibilities

[0049] According to the present invention, a hot-dip Zn-Al-Mg-based steel sheet can be provided that achieves both high levels of corrosion resistance and plating adhesion.

[0050] 10: Base steel plate 20: Coated film 21: Main Floor 22: Interface alloy layer 23: Needle-shaped inorganic compounds L: long diameter of needle-shaped inorganic compound D: short diameter of needle-shaped inorganic compound

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 existing at the interface with a base steel sheet and a main layer existing 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, Mg: 3-10% by mass and Fe: 0.1-0.5% 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, needle-shaped inorganic compounds with a major diameter of 1 μm or more and an aspect ratio (minor diameter / major diameter) of 0.2 or less are formed on the aforementioned interface alloy layer, and the aforementioned needle-shaped inorganic compounds further contain Si and Ni.

2. The hot-dip Zn-Al-Mg steel sheet of claim 1, wherein when viewed in cross-section along the thickness direction of the aforementioned coated film, the aforementioned needle-like inorganic compound extends from the surface of the aforementioned interface alloy layer into the aforementioned main layer.

3. Hot-dip Zn-Al-Mg series steel sheet as requested in item 1 or 2, wherein the aforementioned interface alloy layer contains Ni.

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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