Zn-Al-Mg coated steel sheet

Abstract

The present invention provides a Zn-Al-Mg-plated steel sheet that exhibits excellent corrosion resistance in paint cut areas, appearance corrosion resistance, and corrosion resistance in flat joints, even in a corrosive environment where temperatures vary greatly from below freezing to high temperatures. The Zn-Al-Mg-plated steel sheet of the present invention comprises a substrate steel sheet and a coating layer formed on at least one side of the substrate steel sheet, the coating layer having a predetermined chemical composition, and in a backscattered electron image obtained by irradiating the surface of the coating layer with an electron beam using a scanning electron microscope, (I) a Zn-phase single-phase structure exists in the form of islands in a matrix of a Zn-Al-MgZn2 ternary eutectic structure, and (II) the Zn-phase single-phase structure has a circle-equivalent mean radius r ave (III) The average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn phase single phase structures ave is 4.0 μm or more and 6.0 μm or less.

Description

[Technical Field]

[0001] The present invention relates to a Zn-Al-Mg plated steel sheet. [Background technology]

[0002] Coated steel sheets, which have a zinc-based plating layer formed on the surface of a base steel sheet, are used in a wide range of applications, including automobiles, home appliances, and building materials. Coated steel sheets have excellent corrosion resistance because the zinc-based plating layer suppresses corrosion of the iron through sacrificial corrosion protection.

[0003] In the automotive industry, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, which are types of zinc-based coated steel sheets, have traditionally been used. However, due to the worsening of air pollution and corrosive environments that accompany industrial advancements, the adoption of Zn-Al-Mg coated steel sheets, which offer better corrosion resistance and a longer life than conventional zinc coatings, is being considered.

[0004] For example, Patent Document 1 discloses a Zn-Al-Mg-plated steel sheet having a coating layer containing 1.0-3.0% Al and 1.5-4.0% Mg and a Zn-Al-MgZn2 ternary eutectic structure in the base structure. It describes that the addition of aluminum and magnesium to zinc provides high corrosion resistance, and that the low aluminum and magnesium contents make it suitable for automotive applications. Patent Document 2 discloses a Zn-Al-Mg-plated steel sheet that has improved initial corrosion resistance compared to conventional Zn-Al-Mg-plated steel sheets by controlling the coating structure. Patent Document 3 discloses a Zn-Al-Mg-plated steel sheet that has reduced blistering from cut areas after painting by controlling the coating structure, thereby ensuring the corrosion resistance of damaged areas after painting, which is necessary for automotive body applications. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2014-501334 [Patent Document 2] Japanese Patent Application Publication No. 2023-74874 [Patent Document 3] Patent Publication No. 2021-195564 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the coated steel sheets described in the above documents had the problem that the Zn-Al-MgZn2 ternary eutectic structure in the coating structure corroded early in a corrosive environment where the temperature fluctuated greatly from below freezing to high temperatures (above 50°C).As a result, the corrosion resistance of damaged areas after painting and the corrosion resistance of flat-sheet joints in a corrosive environment with large temperature changes, which are necessary when assuming application to automobile bodies, could not be ensured.

[0007] In view of the above problems, an object of the present invention is to provide a Zn-Al-Mg-plated steel sheet that exhibits excellent blister corrosion resistance and appearance corrosion resistance in paint-cut areas and excellent corrosion resistance in joints between flat sheets, even in corrosive environments where temperatures vary greatly from below freezing to high temperatures (50°C or higher). [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have come to the following conclusions: On the surface of the coating layer of a Zn-Al-Mg-based coated steel sheet, a Zn-phase single-phase structure is present in the form of islands in a matrix of a Zn-Al-MgZn2 ternary eutectic structure, and the Zn-phase single-phase structure has an average circle-equivalent radius r ave , and the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures aveBy setting the temperature within a predetermined range, it is possible to improve the blister corrosion resistance and appearance corrosion resistance of the paint cut portion, as well as the corrosion resistance of the joints of flat plates, even in a severe corrosive environment with temperatures ranging from below freezing to high temperatures (50°C or higher). Furthermore, in the production of a plated steel sheet, a mixed gas consisting of H2 and N2 is sprayed onto the plated steel sheet during gas wiping, and the average cooling rate in the cooling process until the temperature of the plated steel sheet surface after gas wiping reaches 200°C is set within a predetermined range, thereby obtaining the above-mentioned structure of the plated layer.

[0009] That is, the gist and configuration of the present invention are as follows.

[0010] [1] A steel sheet having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer has a component composition containing, in mass%, Al: 0.10 to 3.00%, Mg: 0.50 to 3.00%, and Fe: 0.30% or less, with the balance being Zn and unavoidable impurities; In a backscattered electron image obtained by irradiating the surface of the coating layer with an electron beam using a scanning electron microscope, (I) a Zn-phase single-phase structure is present in the form of islands in a matrix of a Zn-Al-MgZn2 ternary eutectic structure, and (II) the circle-equivalent average radius r of the Zn-phase single-phase structure is ave (III) the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn-phase single phase structures ave Zn-Al-Mg-plated steel sheet characterized in that the grain size is 4.0 μm or more and 6.0 μm or less.

[0011] [2] The Zn-Al-Mg-plated steel sheet according to [1], wherein, in the backscattered electron image, the Zn-Al-MgZn2 ternary eutectic structure accounts for 30 area % or more, and the Zn-phase single-phase structure accounts for 70 area % or less. [Effects of the Invention]

[0012] The present invention can provide a Zn-Al-Mg-plated steel sheet that exhibits excellent blister corrosion resistance and appearance corrosion resistance in paint-cut areas, as well as excellent corrosion resistance in flat sheet joints, even in corrosive environments where temperatures vary greatly from below freezing to high temperatures (50°C or higher). [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a backscattered electron image at a magnification of 800 times obtained by irradiating the surface of the plating layer with an electron beam at an accelerating voltage of 15.0 kV in an example (No. 1) according to one embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing an example of the circle-equivalent radius r of a Zn single-phase structure in a backscattered electron image of Invention Example No. 1. [Figure 3] FIG. 1 is a diagram showing widths d1, d2, d3, and d4 of Zn-Al-MgZn2 ternary eutectic structures located between adjacent Zn single-phase structures in a backscattered electron image of Invention Example No. 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the Zn-Al-Mg-plated steel sheet according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

[0015] [Base steel plate] The type of the base steel sheet used in the present invention is not particularly limited, and for example, a hot-rolled steel sheet or hot-rolled steel strip that has been pickled and descaled, or a cold-rolled steel sheet or cold-rolled steel strip obtained by cold rolling such a hot-rolled steel sheet or hot-rolled steel strip, can be used. There is also no particular limit to the thickness of the base steel sheet, but it is preferably 0.7 to 2.0 mm.

[0016] [Plating layer] Next, the composition of the Zn-Al-Mg-based plating layer in the present invention will be described.

[0017] When the Al content in the coating layer is less than 0.10 mass%, a sufficient area percentage (%) of the Zn-Al-MgZn2 ternary eutectic structure cannot be obtained, and the Zn phase single phase structure occupies most of the surface of the coating layer. As a result, the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn phase single phase structures becomes ave If the thickness is less than 4.0 μm, sufficient blister corrosion resistance and appearance corrosion resistance of the paint cut area, as well as sufficient corrosion resistance of the joints of the flat plates, will be obtained. Therefore, the Al content in the coating layer is set to 0.10 mass% or more, and preferably 0.50 mass% or more. On the other hand, if the Al content in the coating layer exceeds 3.00 mass%, sufficient blister corrosion resistance of the paint cut area will not be obtained. Furthermore, if the Al content is 5.00 mass% or more, the primary crystal will be an Al phase containing Zn as a solid solution, and a single Zn phase structure will not be obtained. Therefore, the Al content in the coating layer is set to 3.00 mass% or less, and preferably 2.00 mass% or less.

[0018] When the Mg content in the coating layer is less than 0.50 mass%, a sufficient area percentage (%) of the Zn-Al-MgZn2 ternary eutectic structure cannot be obtained, and the Zn phase single phase structure occupies most of the surface of the coating layer. As a result, the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn phase single phase structures becomes ave The average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn-phase single phase structures is increased, resulting in an insufficient effect of improving the blister corrosion resistance and appearance corrosion resistance of the coating cut area, as well as the corrosion resistance of the joints of the flat plates. Therefore, the Mg content in the coating layer is set to 0.50 mass% or more, and preferably 1.00 mass% or more. On the other hand, if the Mg content in the coating layer exceeds 3.00 mass%, the MgZn2 phase increases, and the area % of the ternary eutectic structure increases drastically. As a result, the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn-phase single phase structures is increased. ave If the thickness exceeds 6.0 μm, the plate joints will not be able to withstand corrosion. Furthermore, if the Mg content in the coating layer exceeds 5.00 mass%, the coating layer will be dominated by a massive MgZn2 phase, making it impossible to obtain a single Zn phase structure. Therefore, the Mg content in the coating layer is set to 3.00 mass% or less, and preferably 2.50 mass% or less.

[0019] If the Fe content in the coating layer exceeds 0.30 mass%, the growth of an alloy layer formed at the interface between the base steel sheet and the coating layer increases, resulting in reduced workability. Therefore, the Fe content in the coating layer is set to 0.30 mass% or less, and preferably 0.20 mass% or less. On the other hand, the lower limit of the Fe content in the coating layer is not particularly limited, and may be 0.00 mass%.

[0020] The remainder of the plating layer composition other than those mentioned above consists of Zn and unavoidable impurities.

[0021] Next, the structure of the Zn-Al-Mg-based coating layer will be described. FIG. 1 shows an example of a backscattered electron image obtained by irradiating the surface of a coating layer of a Zn-Al-Mg-based coated steel sheet according to one embodiment of the present invention with an electron beam using a scanning electron microscope. As shown in FIG. 1, a Zn-phase single-phase structure 10 and a Zn-Al-MgZn2 ternary eutectic structure 20, which have different compositional contrasts, are observed in the backscattered electron image, and the Zn-phase single-phase structure 10 is present in an island-like form in the matrix of the Zn-Al-MgZn2 ternary eutectic structure 20. If the Zn-Al-Mg-based coating layer does not have one or both of the Zn-phase single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure, desired corrosion resistance cannot be obtained. The structure of the coating layer preferably consists of a Zn-phase single-phase structure and a Zn-Al-MgZn2 ternary eutectic structure.

[0022] The presence or absence of a Zn-phase single-phase structure and a Zn-Al-MgZn2 ternary eutectic structure can be confirmed by combining analyses using a scanning electron microscope (SEM), an attached energy dispersive X-ray analyzer (EDS), and an X-ray diffractometer (XRD) as follows. A sample of Zn-Al-Mg-plated steel sheet is sheared to a size of 10 × 20 mm. The coating layer of the sheared sample is observed from the surface direction of the plated steel sheet using an SEM, and compositional analysis using EDS and XRD is performed. As shown in Figure 1, a smooth, white region is observed in the backscattered electron image. If only Zn is detected in the EDS composition analysis in this region and the η-Zn phase is confirmed in the XRD analysis, the region is determined to have a Zn-phase single-phase structure. Furthermore, as shown in Figure 1, a region with a lamellar structure is observed in the backscattered electron image. In this region, if the three elements Zn, Al, and Mg are detected during composition analysis by EDS, and if Zn phase, Al phase, and MgZn2 alloy phase are confirmed by analysis by XRD, the region is determined to have a Zn-Al-MgZn2 ternary eutectic structure.

[0023] In a corrosive environment, magnesium dissolves from the Zn-Al-MgZn2 ternary eutectic structure of a Zn-Al-Mg-Zn2 coating layer. The dissolved magnesium buffers the surrounding pH, thereby suppressing the pH increase associated with corrosion progression and stabilizing the highly protective zinc-based corrosion product, basic zinc chloride, produced during the corrosion reaction, thereby improving corrosion resistance. If magnesium dissolution from the Zn-Al-MgZn2 ternary eutectic structure occurs too early in a corrosive environment, it becomes difficult to stabilize the zinc chloride over the long term, resulting in insufficient long-term corrosion resistance. On the other hand, if magnesium dissolution from the Zn-Al-MgZn2 ternary eutectic structure occurs long after corrosion has begun, the corrosion resistance improvement effect in the early stages of corrosion is insufficient. Therefore, to further improve the corrosion resistance of a Zn-Al-Mg-based coating layer, it is important to ensure that magnesium dissolution from the Zn-Al-MgZn2 ternary eutectic structure is sustained.

[0024] To achieve sustained magnesium release from the Zn-Al-MgZn2 ternary eutectic structure, it is important to control the size of the Zn-phase single-phase structure on the coating surface and the distance between the Zn-phase single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure. The Zn-phase single-phase structure is slightly more resistant to corrosion than the Zn-Al-MgZn2 ternary eutectic structure. Therefore, the corrosion rate of the Zn-Al-MgZn2 ternary eutectic structure is slightly reduced in areas where the Zn-phase single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure are close to each other. Therefore, increasing or decreasing the contact area between the Zn-phase single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure changes the rate and amount of magnesium release. Excessively large contact areas significantly suppress corrosion of the Zn-Al-MgZn2 ternary eutectic structure, significantly slowing magnesium release and resulting in insufficient corrosion resistance in the early stages of corrosion. On the other hand, if the contact area is too small, the Zn-Al-MgZn2 ternary eutectic structure is preferentially corroded, Mg dissolution occurs very quickly, and the effect of improving corrosion resistance over the long term is not sufficiently achieved.

[0025] From the above, it is important to control the contact area between the Zn single phase structure and the Zn-Al-MgZn2 ternary eutectic structure to ensure that Mg elution from the Zn-Al-MgZn2 ternary eutectic structure is sustained, and for this purpose, it is useful to control the arrangement of the Zn single phase structure and the Zn-Al-MgZn2 ternary eutectic structure. As a result of extensive research, the present inventors have found that the circle-equivalent average radius r of the Zn single phase structure ave The average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn phase single phase structures is 6.0 μm or more and 14.0 μm or less. ave It has been found that it is important to control the metal structure arrangement so that the grain size is 4.0 μm or more and 6.0 μm or less.

[0026] In the backscattered electron image obtained by irradiating the surface of the plating layer with an electron beam using a scanning electron microscope, the average circle-equivalent radius r of the Zn single-phase structure was aveIf the radius is 6.0 μm or more, the blister corrosion resistance and appearance corrosion resistance of the painted cut part, as well as the corrosion resistance of the joint part of the flat plate, can be improved even in a corrosive environment where the temperature changes greatly from below freezing to high temperatures (above 50°C). ave is 6.0 μm or more, and preferably 9.0 μm or more. Similarly, the circle-equivalent average radius r of the Zn single-phase structure ave If the radius is 14.0 μm or less, the blister corrosion resistance and appearance corrosion resistance of the painted cut part, as well as the corrosion resistance of the joint part of the flat plate, can be improved even in a severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (above 50°C). ave is 14.0 μm or less, and preferably 12.0 μm or less.

[0027] Zn phase single-phase structure circle equivalent average radius r ave can be measured as follows. Using a scanning electron microscope, three randomly selected locations on the surface of the coating layer are observed at an accelerating voltage of 15.0 kV and a magnification of 800x. Here, the circle-equivalent radius r of the Zn-phase single-phase structure is the radius r of a circle having the same area as the Zn-phase single-phase structure, as exemplified in Figure 2. Using general-purpose image processing software (e.g., Image-J), the obtained backscattered electron image is binarized, and the area S of the Zn-phase single-phase structure is measured for each Zn-phase single-phase structure that is completely contained in the observed field of view. Using the obtained area S, the circle-equivalent radius r of the Zn-phase single-phase structure is calculated using the following formula (1). The average value of r in the three observed fields of view is taken as the circle-equivalent average radius r of the Zn-phase single-phase structure. ave Let's say. r=(S / π) 1 / 2 ···(1)

[0028] In a backscattered electron image obtained by irradiating the surface of the coating layer with an electron beam using a scanning electron microscope, the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures is aveIf the average width d of the Zn-Al-MgZn2 ternary eutectic structure is 4.0 μm or more, the blister corrosion resistance and appearance corrosion resistance of the painted cut area, as well as the corrosion resistance of the joints of the flat plates, can be improved even in a severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (above 50°C). ave is 4.0 μm or more, and preferably 4.5 μm or more. Similarly, the average width d of the Zn-Al-MgZn2 ternary eutectic structure ave If the average width d of the Zn-Al-MgZn2 ternary eutectic structure is 6.0 μm or less, the blister corrosion resistance and appearance corrosion resistance of the painted cut area, as well as the corrosion resistance of the joints of the flat plates, can be improved even in a severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (above 50°C). ave is 6.0 μm or less, and preferably 5.5 μm or less.

[0029] The average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures avecan be measured as follows. Using a scanning electron microscope, three randomly selected locations on the surface of the coating layer are observed at an accelerating voltage of 15.0 kV and a magnification of 800x. As shown in Figure 3, using general-purpose image processing software (e.g., Image-J), a Zn-phase single-phase structure that is completely contained within the observation field is selected, and lines are drawn in the horizontal and vertical directions in the observation field, with the center of gravity of the Zn-phase single-phase structure as the intersection point. On the line extending vertically upward from the center of gravity, the distance between the grain boundary of the Zn-phase single-phase structure and the grain boundary of the other Zn-phase single-phase structure closest to the Zn-phase single-phase structure, i.e., the width of the Zn-Al-MgZn2 ternary eutectic structure surrounding the Zn-phase single-phase structure, is defined as d1. Similarly, the width of the Zn-Al-MgZn2 ternary eutectic structure on a line extending horizontally to the left from the center of gravity is defined as d2, the width of the Zn-Al-MgZn2 ternary eutectic structure on a line extending vertically downward from the center of gravity is defined as d3, and the width of the Zn-Al-MgZn2 ternary eutectic structure on a line extending horizontally to the right from the center of gravity is defined as d4. Distances d1 to d4 are measured and averaged. If one or more of d1 to d4 cannot be measured because the Zn-phase single-phase structure is located at the edge of the observation field, for example, the horizontally and vertically drawn lines are rotated clockwise, and if all of d1 to d4 can be measured on the rotated lines, the measurements are repeated in the same manner. If one or more of d1 to d4 cannot be measured even after rotating the horizontally and vertically drawn lines, the Zn-phase single-phase structure is excluded from the measurement. Similar measurements are performed on all Zn-phase single-phase structures that are completely contained within the observation field of view and are included in the measurement target. The average value of the measurement results in the three photographed fields was calculated as the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn phase single phase structures. ave Let's say.

[0030] In a backscattered electron image obtained by irradiating the surface of a coating layer with an electron beam using a scanning electron microscope, if the Zn-Al-MgZn2 ternary eutectic structure is 30 area % or more and the Zn-phase single-phase structure is 70 area % or less, red rust formation from the paint cut can be significantly suppressed, further improving the appearance and corrosion resistance of the paint cut. If the area proportions of the Zn-Al-MgZn2 ternary eutectic structure and the Zn-phase single-phase structure are within the above ranges, sufficient amounts of basic zinc chloride and sufficient magnesium elution to stabilize the basic zinc chloride are generated during corrosion. More preferably, the Zn-Al-MgZn2 ternary eutectic structure is 40 area % or more and the Zn-phase single-phase structure is 60 area % or less. It is preferable that the Zn-Al-MgZn2 ternary eutectic structure is 50 area % or less. It is also preferable that the Zn-phase single-phase structure is 40 area % or more.

[0031] The area proportions of the Zn single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure can be measured as follows. Using a scanning electron microscope, three randomly selected locations on the surface of the coating layer are observed at an accelerating voltage of 15.0 kV and a magnification of 800x. Using general-purpose image processing software (e.g., Image-J), the obtained backscattered electron image is binarized to clearly separate the Zn single-phase structure, and the area Z of the Zn single-phase structure is measured. The total area T of the backscattered electron image is calculated, and the area proportion A (%) of the Zn single-phase structure is calculated using the following formula (2). The area proportion B (%) of the Zn-Al-MgZn2 ternary eutectic structure is calculated using the following formula (3). The average values ​​of A and B for the three observed fields are calculated, and these are defined as the area proportions of the Zn single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure, respectively. A = (Z / T) × 100 (2) B=100-A (3)

[0032] [Manufacturing method for Zn-Al-Mg-based coated steel sheets] Next, a method for producing a Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention will be described. One example of the method for producing a Zn-Al-Mg-plated steel sheet includes the steps of preparing a substrate steel sheet, optionally annealing the substrate steel sheet, immersing the substrate steel sheet in a coating bath containing, by mass, 0.10 to 3.00% Al, 0.50 to 3.00% Mg, and 0.30% or less Fe, with the balance consisting of Zn and unavoidable impurities, thereby subjecting the substrate steel sheet to a coating treatment to obtain a coated steel sheet, gas wiping the coated steel sheet and adjusting the coating weight to obtain a conditioned coated steel sheet, and cooling the conditioned coated steel sheet. The gas wiping step is characterized in that a mixed gas consisting of H2 and N2 is sprayed onto the coated steel sheet, and the cooling step, after gas wiping, is performed until the surface temperature of the coated steel sheet reaches 200°C, at an average cooling rate of 3.0°C / s or more and 8.0°C / s or less.

[0033] The type and thickness of the base steel sheet are as described above. When the base steel sheet is subjected to annealing heat treatment, the furnace atmosphere during annealing needs to be a reducing atmosphere, and preferably contains H2, with the remainder being N2 from the viewpoint of production costs. When the H2 concentration in the furnace atmosphere during annealing is 1% by volume or more, oxides on the steel sheet surface can be suitably reduced. Therefore, the H2 concentration in the furnace atmosphere during annealing is preferably 1% by volume or more. On the other hand, when the H2 concentration in the furnace atmosphere during annealing is 10% by volume or less, production costs can be suitably reduced. Therefore, the H2 concentration in the furnace atmosphere during annealing is preferably 10% by volume or less. The temperature of the annealing heat treatment is not particularly limited, but is preferably 600 to 850°C. The time of the annealing heat treatment is also not particularly limited, but is preferably 10 to 60 seconds.

[0034] The coating bath used in the production of Zn-Al-Mg coated steel sheets has a composition containing, by mass%, 0.10 to 3.00% Al, 0.50 to 3.00% Mg, and 0.30% or less Fe, with the balance consisting of Zn and unavoidable impurities. The explanation of each component in the coating bath is the same as the explanation of each component in the coating layer described above. The component composition of the coating bath and the component composition of the coating layer are the same.

[0035] When the temperature of the plating bath during plating is 430°C or higher, the fluidity of the plating bath can be favorably obtained, and the coating weight can be favorably uniform. Therefore, the temperature of the plating bath is preferably 430°C or higher, and more preferably 450°C or higher. On the other hand, by setting the temperature of the plating bath to 520°C or lower, it is possible to favorably suppress the increase in oxides on the surface of the plating bath due to oxidation of Mg in the plating bath, and the erosion of the refractory material in the plating bath by Al and Mg. Therefore, the temperature of the plating bath is preferably 520°C or lower, more preferably 500°C or lower, and even more preferably 490°C or lower.

[0036] In a method for producing a Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention, the gas composition during gas wiping when adjusting the coating weight of the plated steel sheet obtained after immersion in a plating bath and the conditions for the subsequent cooling step are important. The atmosphere during gas wiping can be air. The cooling method for the plated steel sheet after adjustment is not particularly limited, and various methods such as air cooling, air-water cooling, and water cooling can be used.

[0037] In the present invention, it is important that the wiping gas used during gas wiping has reducing properties. Therefore, gas wiping is performed by spraying a mixed gas consisting of H2 and N2 onto the steel sheet. Conventionally, air or N2 gas has been used as the wiping gas in the production of hot-dip galvanized steel sheets from an economical standpoint. However, in the present invention, it is important to sufficiently form a ternary eutectic structure on the surface of the coating layer using small amounts of Al and Mg present in the coating bath composition. Mg is an easily oxidizable element, and if the wiping gas contains O2, Mg is oxidized during gas wiping, resulting in an insufficient Mg content during the subsequent cooling process, resulting in insufficient formation of the Zn-Al-MgZn2 ternary eutectic structure. If the Zn-Al-MgZn2 ternary eutectic structure is insufficient on the surface of the coating layer, a Zn-phase single-phase structure will instead occupy most of the surface, and the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn-phase single-phase structures will be reduced. ave As a result, the contact area between the Zn single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure decreases, causing preferential corrosion of the Zn-Al-MgZn2 ternary eutectic structure to occur more rapidly, resulting in extremely rapid Mg dissolution and making it impossible to obtain the long-term effect of improving corrosion resistance.

[0038] As a result of extensive research into improving the corrosion resistance of Zn-Al-Mg-plated steel sheets, the inventors have discovered that the higher the H concentration in the wiping gas, the more effectively Mg oxidation during gas wiping can be suppressed. This ensures a sufficient gap between the Zn single-phase structure and the Zn-Al-MgZn2 ternary eutectic structure, thereby enabling corrosion of the Zn-Al-MgZn2 ternary eutectic structure and sustained Mg release.

[0039] When the H2 concentration of the wiping gas is 1% by volume or more, oxidation of molten Mg can be suitably suppressed, and the effect of contributing to improved corrosion resistance can be sufficiently obtained. Therefore, the H2 concentration of the wiping gas is set to 1% by volume or more. The higher the H2 concentration of the wiping gas, the more effectively it can suppress oxidation of molten Mg. However, from the viewpoint of manufacturing cost and economic efficiency, the H2 concentration of the wiping gas is preferably 5% by volume or less, and more preferably 4% by volume or less. The remainder of the wiping gas is preferably N2.

[0040] Next, we will explain the cooling process after adjusting the coating weight by gas wiping. Immediately after gas wiping, the temperature of the surface of the conditioned plated steel sheet is approximately 450°C, and the plated steel sheet is cooled at an average cooling rate of 3.0°C / s to 8.0°C / s until the temperature of the surface of the conditioned plated steel sheet reaches 200°C. If the average cooling rate is less than 3.0°C / s, the Zn single-phase structure becomes coarse, and the average circle-equivalent radius r of the Zn single-phase structure becomes smaller. ave and the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures. ave Therefore, in the cooling step, the average cooling rate until the temperature of the surface of the conditioned plated steel sheet reaches 200°C is set to 3.0°C / s or more. If the average cooling rate is greater than 8.0°C / s, the Zn single-phase structure becomes smaller, and the circle-equivalent average radius r of the Zn single-phase structure becomes smaller. ave and the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures. ave Therefore, in the cooling step, the average cooling rate until the temperature of the surface of the conditioned plated steel sheet reaches 200°C is set to 8.0°C / s or less.

[0041] For steps and conditions not described in the present invention, conventional methods can be used. [Example]

[0042] All Zn-Al-Mg plated steel sheets used as samples were manufactured using hot-dip galvanizing equipment, using an ultra-low carbon cold-rolled steel sheet with a thickness of 0.8 mm as the base steel sheet. The coating bath composition was changed in various ways, and the base steel sheet was subjected to hot-dip galvanizing for 1 second to obtain coated steel sheets. The temperature of the coating bath was set to 460°C, which is 20°C higher than the solidification start temperature estimated from the Zn-Al-Mg equilibrium phase diagram. In each example, the substrate steel sheet before coating was heated in an N2 + H2 (5% by volume) atmosphere to ensure that the temperature of the substrate steel sheet when immersed in the coating bath was the same as the coating bath temperature. The coated steel sheet was then gas wiped, and the thickness of the coating layer was reduced to 7 μm (deposition weight 50 g / m) by adjusting the flow rate of the wiping gas. 2 The gas wiping was carried out under atmospheric conditions. After gas wiping, the plated steel sheet was cooled while controlling the average cooling rate until the temperature of the plated steel sheet surface after adjustment reached 200°C. Table 1 shows the plating bath composition, wiping gas composition, plating layer composition, and average cooling rate for each example.

[0043] Using the method described above, the presence or absence of a Zn single-phase structure and a Zn-Al-MgZn2 ternary eutectic structure in each sample was confirmed. In each example, if the respective structure was confirmed, it is recorded as "present," and if it was not confirmed, it is recorded as "absent" in Table 1. Furthermore, using the method described above, the circle-equivalent average radius r of the Zn single-phase structure on the coating surface of each sample was measured. ave , the average width d of the Zn-Al-MgZn2 ternary eutectic structure located between adjacent Zn single-phase structures ave The area ratios of the Zn-Al-MgZn2 ternary eutectic structure and the Zn single phase structure were measured, and the results are shown in Table 1.

[0044] [Corrosion resistance of painted cut areas] Next, to evaluate the blister corrosion resistance and appearance corrosion resistance of the resulting Zn-Al-Mg-plated steel sheets, each sample was sheared to a size of 70 x 80 mm. The samples were then phosphate-treated in a bath containing PB-SX35 as the base material at pH 4 and 35°C, followed by electrodeposition coating with a 15 μm thick epoxy-based water-based paint. After coating, the sample surface was cut with a blade, creating cross-shaped cuts. The blade depth reached the surface of the base steel sheet. To evaluate corrosion resistance after coating, the N-VDA corrosion test (VDA 233-102) was performed. The N-VDA corrosion test is a cyclic dry-wet test simulating atmospheric corrosion in the temperature range of -15°C to 50°C. Compared to conventional cyclic dry-wet tests (e.g., SAE J2334 tests, which involve repeated dry-wet cycles from room temperature to approximately 60°C), this corrosion test involves significantly larger temperature changes, from below freezing to high temperatures (50°C). In the N-VDA corrosion test, one cycle is defined as one week of a specified temperature and humidity control pattern according to the standard number. After 18 cycles of the N-VDA corrosion test (VDA 233-102), the blister corrosion resistance and appearance corrosion resistance of the painted cut area were evaluated.

[0045] The blister corrosion resistance of the cut area of ​​the paint was evaluated by measuring the blister width from the cross-cut scratch on the paint surface and determining the maximum blister width. Table 1 lists the results as follows: if the maximum blister width was less than 3 mm, it was marked "◎", if it was 3 mm or more but less than 4 mm, it was marked "○", if it was 4 mm or more but less than 5 mm, it was marked "△", and if it was 5 mm or more, it was marked "×". Evaluation results of "◎" or "○" were considered pass, and "△" or "×" were considered fail.

[0046] The corrosion resistance of the cut area was evaluated by measuring the area of ​​red rust from a photograph of the sample after the test. 2 If it is less than 50mm, mark it as "◎" 2 More than 150mm 2 If it is less than 150mm, mark it as "○" 2 More than 300mm 2 If it is less than 300mm, it will be marked "△" 2 In the above cases, an "X" is recorded in Table 1. An evaluation result of "◎" or "〇" was considered to be a pass, and "△" or "×" was considered to be a fail.

[0047] [Corrosion resistance of plate joints] To evaluate the corrosion resistance of flat Zn-Al-Mg-plated steel sheets, each sample was sheared into a large sheet measuring 80 × 70 mm and a small sheet measuring 60 × 40 mm. The large and small sheets, each with the same plating type, were joined together with their long and short edges aligned and then spot-welded at two points to prepare samples for the lamination corrosion test. The samples were then phosphate-treated in a PB-SX35-based bath at pH 4 and 35°C, followed by electrodeposition coating with a 15 μm layer of epoxy-based water-based paint. The edges of the large sheets, the backside opposite the mating surfaces of the large sheets, and the backside opposite the mating surfaces of the small sheets were covered with tape sealant to prevent corrosion progression outside the mating surfaces. The samples were then subjected to the N-VDA corrosion test (VDA 233-102). After 24 cycles of corrosion testing, the spot welds were disassembled, the electrochemical deposition on the large plate side was removed, and the corrosion products on the mating surfaces were peeled off with hydrochloric acid. The maximum corrosion depth of the samples after the test was then measured using a laser device.

[0048] If the maximum corrosion depth was less than 0.3 mm, it was marked with "◎", if it was 0.3 mm or more but less than 0.4 mm, it was marked with "○", if it was 0.4 mm or more but less than 0.5 mm, it was marked with "△", and if it was 0.5 mm or more, it was marked with "×" in Table 1. Evaluation results of "◎" or "○" were considered to be pass, and "△" or "×" were considered to be fail.

[0049] [Table 1] TIFF0007736226000002.tif233111

[0050] As shown in Table 1, examples that satisfied the requirements of the present invention were excellent in blister corrosion resistance and appearance corrosion resistance of the paint cut area, as well as in corrosion resistance of the joints of the flat plates. Furthermore, examples in which the area ratio of the Zn-Al-MgZn2 ternary eutectic structure was 30 area % or more and the area ratio of the Zn-phase single phase structure was 70 area % or less had even better appearance corrosion resistance of the paint cut area. In contrast, examples that did not satisfy the requirements of the present invention were inferior in either the blister corrosion resistance of the paint cut area, the appearance corrosion resistance of the paint cut area, or the corrosion resistance of the joints of the flat plates. [Industrial Applicability]

[0051] The present invention can provide a Zn-Al-Mg-plated steel sheet that exhibits excellent blister corrosion resistance and appearance corrosion resistance in paint-cut areas, as well as excellent corrosion resistance in flat sheet joints, even in corrosive environments where temperatures vary greatly from below freezing to high temperatures (50°C or higher). [Explanation of symbols]

[0052] 10 Zn phase single phase structure 20 Zn-Al-MgZn2 ternary eutectic structure r Equivalent circle radius of Zn single-phase structure d1 Width of Zn-Al-MgZn2 ternary eutectic structure d2 Width of Zn-Al-MgZn2 ternary eutectic structure d3 Width of Zn-Al-MgZn2 ternary eutectic structure d4 Width of Zn-Al-MgZn2 ternary eutectic structure

Claims

1. A steel sheet having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer has a component composition containing, in mass%, Al: 0.10 to 3.00%, Mg: 0.50 to 3.00%, and Fe: 0.30% or less, with the balance being Zn and inevitable impurities; In a backscattered electron image obtained by irradiating the surface of the plating layer with an electron beam using a scanning electron microscope, (I) Zn—Al—MgZn ２ A Zn-phase single-phase structure exists in the form of islands in a matrix of a ternary eutectic structure, and (II) the Zn-phase single-phase structure has an average circle-equivalent radius r ａｖｅ (III) the Zn-Al-MgZn phases located between adjacent Zn-phase single phase structures ２ Average width d of ternary eutectic structure ａｖｅ The Zn-Al-Mg plated steel sheet is characterized in that the thickness of the surface roughness is 4.0 μm or more and 6.0 μm or less.

2. In the backscattered electron image, the Zn—Al—MgZn ２ 2. The Zn-Al-Mg plated steel sheet according to claim 1, wherein the ternary eutectic structure accounts for 30% by area or more, and the Zn-phase single phase structure accounts for 70% by area or less.

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