Zn-Al-Mg-BASED PLATED STEEL SHEET

Abstract

Provided is a Zn-Al-Mg-based plated steel sheet having exceptional resistance against swelling corrosion and appearance corrosion in a coating cut part, as well as exceptional corrosion resistance in a flat-plate joint part, even in a corrosive environment where the temperature varies greatly from below freezing to high temperatures. A Zn-Al-Mg-based plated steel sheet according to the present invention is characterized in having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer having a prescribed component composition and being such that, 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-phase single-phase structures are present in an island form in a base of a Zn-Al-MgZn2 ternary eutectic structure; (II) the equivalent circle average radius rave of the Zn-phase single-phase structures is 6.0-14.0 μm; and (III) the average width dave of the Zn-Al-MgZn2 ternary eutectic structure positioned between adjacent Zn-phase single-phase structures is 4.0-6.0 μm.

Description

Zn-Al-Mg plated steel sheet

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

[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 superior corrosion resistance and a longer life than conventional zinc coatings, is being considered.

[0004] For example, Patent Document 1 discloses a coating layer having a composition containing Al: 1.0 to 3.0% and Mg: 1.5 to 4.0%, which is called Zn-Al-MgZn. ２ A Zn-Al-Mg-plated steel sheet having a ternary eutectic structure in its base structure has been disclosed, and it is described 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 use in automobiles. 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 use in automobile bodies.

[0005] JP 2014-501334 A JP 2023-74874 A JP 2021-195564 A

[0006] However, in the plated steel sheet described in the above document, in a corrosive environment where the temperature changes greatly from below freezing to high temperatures (50°C or higher), the Zn-Al-MgZn in the plated structure ２The ternary eutectic structure had the problem of early corrosion, which meant that the corrosion resistance of damaged areas after painting and of flat joints in a corrosive environment with large temperature changes, which is 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 a painted cut area and excellent corrosion resistance in a joining area of ​​flat sheets, even in a corrosive environment where the temperature varies greatly from below freezing to high temperatures (50°C or higher).

[0008] As a result of intensive research to solve the above problems, the present inventors have found the following: ２ The Zn single-phase structure exists in the matrix of the ternary eutectic structure in the form of islands, and the average circle-equivalent radius of the Zn single-phase structure is r ａｖｅ , and Zn-Al-MgZn located between adjacent Zn phase single phase structures ２ Average width d of ternary eutectic structure ａｖｅ By setting the temperature within the predetermined range, it is possible to improve the blister corrosion resistance and appearance corrosion resistance of the coating cut portion, as well as the corrosion resistance of the joint portion of the flat plate, even in a severe corrosive environment where the temperature is from below freezing to high temperatures (50°C or higher). ２ and N ２ and a mixed gas consisting of the above is sprayed onto the plated steel sheet, and the average cooling rate in the cooling step until the temperature of the plated steel sheet surface after gas wiping reaches 200°C is set within a predetermined range, thereby making it possible to obtain 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 substrate steel sheet and a plating layer formed on at least one surface of the substrate steel sheet, wherein 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 consisting of Zn and inevitable impurities, and wherein a backscattered electron image obtained by irradiating an electron beam onto the surface of the plating layer using a scanning electron microscope shows that (I) Zn-Al-MgZn ２A 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.

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

[0012] According to the present invention, it is possible to provide a Zn-Al-Mg-plated steel sheet that exhibits excellent blister corrosion resistance and appearance corrosion resistance in a painted cut area, and excellent corrosion resistance in a joining area of ​​flat sheets, even in a corrosive environment where the temperature varies greatly from below freezing to high temperatures (50°C or higher).

[0013] 1 is a backscattered electron image at 800x magnification 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. It is a diagram showing an example of the circle-equivalent radius r of the Zn-phase single-phase structure in the backscattered electron image of example No. 1. It is a diagram showing the Zn-Al-MgZn interlayers located between adjacent Zn-phase single-phase structures in the backscattered electron image of example No. 1. ２ FIG. 1 is a diagram showing widths d1, d2, d3, and d4 of the ternary eutectic structure.

[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 sheet] There is no particular limitation on the type of base steel sheet used in the present invention, 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 limitation on 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 content of Al in the plating layer is less than 0.10 mass%, a sufficient area ratio (%) of Zn-Al-MgZn ２ A ternary eutectic structure is not obtained, and most of the surface of the coating layer is occupied by a Zn-phase single-phase structure. As a result, the Zn-Al-MgZn structure located between adjacent Zn-phase single-phase structures is ２ Average width d of ternary eutectic structure ａｖｅ The thickness is less than 4.0 μm, and sufficient blister corrosion resistance and appearance corrosion resistance of the paint cut portion, as well as corrosion resistance of the flat plate joints, are not 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%, blister corrosion resistance of the paint cut portion is not obtained. Furthermore, if the Al content is 5.00 mass% or more, the primary crystal becomes an Al phase containing Zn as a solid solution, and a Zn single-phase structure cannot 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 plating layer is less than 0.50 mass%, a sufficient area ratio (%) of Zn—Al—MgZn ２ A ternary eutectic structure is not obtained, and most of the surface of the coating layer is occupied by a Zn-phase single-phase structure. As a result, the Zn-Al-MgZn structure located between adjacent Zn-phase single-phase structures is ２ Average width d of ternary eutectic structure ａｖｅ The thickness of the coating layer is less than 4.0 μm, and the effect of improving the blister corrosion resistance and appearance corrosion resistance of the coating cut portion, and the corrosion resistance of the joining portion of the flat plate, becomes insufficient. 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, when the Mg content in the coating layer exceeds 3.00 mass%, the MgZn ２ As a result, the area percentage of the ternary eutectic structure increases dramatically. ２ Average width d of ternary eutectic structure ａｖｅFurthermore, when the Mg content in the plating layer is 5.00 mass % or more, the MgZn aggregates ２ 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 a decrease in 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 composition of the plating layer 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. Figure 1 shows an example of a backscattered electron image obtained by irradiating the surface of the coating layer with an electron beam using a scanning electron microscope in a Zn-Al-Mg-based coated steel sheet according to one embodiment of the present invention. As shown in Figure 1, the backscattered electron image shows a Zn-phase single-phase structure 10 and a Zn-Al-MgZn single-phase structure 11, which have different composition contrasts. ２ A ternary eutectic structure 20 is observed, and Zn—Al—MgZn ２ The Zn-phase single-phase structure 10 exists in the form of islands in the matrix of the ternary eutectic structure 20. The Zn-Al-Mg-based coating layer has a Zn-phase single-phase structure and a Zn-Al-MgZn ２ If one or both of the ternary eutectic structures are not present, the desired corrosion resistance cannot be obtained. ２ It is preferably made of a ternary eutectic structure.

[0022] Zn phase single phase structure and Zn-Al-MgZn ２The presence or absence of a 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 plating 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 are performed. As shown in Figure 1, a region with a smooth and white appearance is observed in the backscattered electron image. If only Zn is detected in the compositional analysis using EDS and an η-Zn phase is confirmed in the analysis using XRD, 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, three elements, Zn, Al, and Mg, are detected in the compositional analysis using EDS, and a Zn phase, an Al phase, and an MgZn phase are confirmed in the analysis using XRD. ２ If an alloy phase is confirmed, the region is classified as Zn-Al-MgZn. ２ It is determined to be a ternary eutectic structure.

[0023] The Zn-Al-Mg plating layer is Zn-Al-MgZn in a corrosive environment. ２ Mg is dissolved from the ternary eutectic structure. The dissolved Mg buffers the surrounding pH, thereby suppressing the increase in pH that accompanies the progression of corrosion and stabilizing the basic zinc chloride, a highly protective Zn-based corrosion product produced by the corrosion reaction, thereby improving corrosion resistance. In a corrosive environment, Zn-Al-MgZn ２ If magnesium dissolution from the ternary eutectic structure occurs early, it becomes difficult to stabilize zinc chloride for a long period of time, and the effect of improving long-term corrosion resistance cannot be sufficiently obtained. ２ If Mg elution from the ternary eutectic structure occurs long after the start of corrosion, the effect of improving corrosion resistance in the early stages of corrosion cannot be fully achieved. ２ It is important that Mg elution from the ternary eutectic structure has a sustained release property.

[0024] Zn-Al-MgZn ２In order for Mg to be released gradually from the ternary eutectic structure, the size of the Zn single phase structure on the plating surface and the relationship between the Zn single phase structure and Zn-Al-MgZn must be considered. ２ It is important to control the distance to the ternary eutectic structure. The Zn-phase single phase structure is Zn-Al-MgZn ２ It is slightly less corroded than the ternary eutectic structure. Therefore, the Zn phase single phase structure and the Zn-Al-MgZn ２ In the area where the ternary eutectic structure is close, Zn-Al-MgZn ２ The corrosion rate of the ternary eutectic structure is slightly reduced. Therefore, the Zn-phase single phase structure and the Zn-Al-MgZn ２ The rate and amount of Mg dissolved can be changed by increasing or decreasing the contact area with the ternary eutectic structure. ２ Corrosion of the ternary eutectic structure is greatly suppressed, Mg dissolution becomes very slow, and the effect of improving corrosion resistance in the early stage of corrosion cannot be sufficiently obtained. ２ The ternary eutectic structure is preferentially corroded, Mg dissolution occurs very quickly, and the effect of improving corrosion resistance over the long term cannot be sufficiently obtained.

[0025] From the above, it is clear that the Zn-phase single-phase structure and the Zn-Al-MgZn ２ By controlling the contact area with the ternary eutectic structure, Zn-Al-MgZn ２ It is important that Mg dissolution from the ternary eutectic structure is slow-release. To achieve this, it is necessary to use a Zn-phase single-phase structure and a Zn-Al-MgZn structure. ２ As a result of extensive investigations, the present inventors have found that it is useful to control the arrangement of the ternary eutectic structure by controlling the average circle-equivalent radius r of the Zn-phase single-phase structure. ａｖｅ is 6.0 μm or more and 14.0 μm or less, and the Zn-Al-MgZn located between adjacent Zn phase single phase structures ２ Average width d of ternary eutectic structure ａｖｅ It has been found that it is important to control the metal structure arrangement so that the thickness is 4.0 μm or more and 6.0 μm or less.

[0026] In a 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-phase single-phase structure is ａｖｅIf the mean radius r is 6.0 μm or more, the blister corrosion resistance and appearance corrosion resistance of the coating cut portion, as well as the corrosion resistance of the joint portion of the flat plate, can be improved even in a corrosive environment where the temperature changes greatly from below freezing to high temperatures (50°C or higher). ａｖｅ is 6.0 μm or more, and preferably 9.0 μm or more. Similarly, the circle-equivalent average radius r of the Zn-phase single-phase structure ａｖｅ If the mean radius r is 14.0 μm or less, the blister corrosion resistance and appearance corrosion resistance of the coating cut portion, as well as the corrosion resistance of the joint portion of the flat plate, can be improved even in a severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (50°C or higher). 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 plating layer are observed at an acceleration 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 that is completely contained in the observed field of view is measured. 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 r = (S / π) １／２ ...(1)

[0028] In a backscattered electron image obtained by irradiating the surface of the plating layer with an electron beam using a scanning electron microscope, the Zn-Al-MgZn phases located between adjacent Zn single-phase structures are ２ Average width d of ternary eutectic structure ａｖｅ If the thickness is 4.0 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 severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (50°C or higher). ２Average width d of ternary eutectic structure ａｖｅ is 4.0 μm or more, and preferably 4.5 μm or more. ２ Average width d of ternary eutectic structure ａｖｅ If the thickness is 6.0 μm or less, the blister corrosion resistance and appearance corrosion resistance of the coating cut portion, as well as the corrosion resistance of the joint portion of the flat plate, can be improved even in a severe corrosive environment where the temperature changes greatly from below freezing to high temperatures (50°C or higher). ２ Average width d of ternary eutectic structure ａｖｅ is 6.0 μm or less, and preferably 5.5 μm or less.

[0029] Zn-Al-MgZn located between adjacent Zn single phase structures ２ Average width d of ternary eutectic structure 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 acceleration 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 another Zn-phase single-phase structure that is closest to the Zn-phase single-phase structure, i.e., the distance between the grain boundary of the Zn-Al-MgZn single-phase structure surrounding the Zn-phase single-phase structure, is calculated. ２ The width of the ternary eutectic structure is d1. Similarly, the Zn-Al-MgZn structure on the horizontal line to the left from the center of gravity ２ The width of the ternary eutectic structure is d2, and the Zn-Al-MgZn on the vertical line downward from the center of gravity ２ The width of the ternary eutectic structure is d3, and the Zn-Al-MgZn on the horizontal line to the right from the center of gravity ２The width of the ternary eutectic structure is defined as d4. Distances d1 to d4 are measured and the average value is calculated. If it is not possible to measure one or more of d1 to d4 because the Zn-phase single-phase structure is located at the edge of the observation field, for example, a line drawn in the horizontal and vertical directions is rotated clockwise, and if all of d1 to d4 can be measured on the rotated line, the measurement is carried out in the same manner. If it is not possible to measure one or more of d1 to d4 even when the line drawn in the horizontal and vertical directions is rotated, the Zn-phase single-phase structure is excluded from the measurement target. The same measurement is carried out for all Zn-phase single-phase structures that are completely included in the observation field and are the measurement target. The average value of the measurement results in the three photographed fields is calculated as the Zn-Al-MgZn ... ２ Average width d of ternary eutectic structure ａｖｅ Let's say.

[0030] In a backscattered electron image obtained by irradiating the surface of the plating layer with an electron beam using a scanning electron microscope, Zn—Al—MgZn ２ When the ternary eutectic structure is 30% by area or more and the Zn-phase single phase structure is 70% by area or less, the occurrence of red rust from the cut portion of the paint can be significantly suppressed, and the appearance and corrosion resistance of the cut portion of the paint can be further improved. ２ If the area ratio of the ternary eutectic structure and the Zn-phase single phase structure is within the above range, a sufficient amount of basic zinc chloride and sufficient Mg elution to stabilize the basic zinc chloride are generated during corrosion. ２ It is more preferable that the ternary eutectic structure is 40% or more by area and the Zn-phase single phase structure is 60% or less by area. ２ The ternary eutectic structure preferably accounts for 50% or less by area, and the Zn-phase single phase structure preferably accounts for 40% or more by area.

[0031] Zn phase single phase structure and Zn-Al-MgZn ２The area proportion of the 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 acceleration 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-phase single-phase structure, and the area Z of the Zn-phase single-phase structure is measured. The total area T of the backscattered electron image is determined, and the area proportion A (%) of the Zn-phase single-phase structure is calculated using the following formula (2). The area proportion A (%) of the Zn-Al-MgZn single-phase structure is calculated using the following formula (3). ２ The area ratio B (%) of the ternary eutectic structure is calculated. The average value of the three observed fields is calculated for A and B, and the area ratio B is calculated as the Zn-phase single phase structure and the Zn-Al-MgZn single phase structure, respectively. ２ The area ratio of the ternary eutectic structure is: A = (Z / T) × 100 (2) B = 100 - A (3)

[0032] [Method for manufacturing Zn-Al-Mg-plated steel sheet] Next, a method for manufacturing a Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention will be described. One example of the method for manufacturing a Zn-Al-Mg-plated steel sheet includes the steps of preparing a substrate steel sheet, an optional step of annealing the substrate steel sheet, a step of immersing the substrate steel sheet in a coating bath having a composition, by mass%, of 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, a step of gas wiping the coated steel sheet and adjusting the coating weight to obtain an adjusted coated steel sheet, and a step of cooling the adjusted coated steel sheet. Furthermore, in the gas wiping, the coated steel sheet may be coated with H ２ and N ２ and a cooling step of blowing a mixed gas consisting of the above components onto the plated steel sheet until the temperature of the surface of the plated steel sheet reaches 200°C after gas wiping, wherein the average cooling rate is 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 atmosphere in the furnace during annealing must be a reducing atmosphere. ２ The remainder is N from the viewpoint of manufacturing cost.２ It is preferable that the atmosphere in the furnace is H ２ When the concentration is 1% by volume or more, oxides on the steel sheet surface can be suitably reduced. ２ The concentration is preferably 1% by volume or more. ２ When the concentration is 10% by volume or less, the manufacturing cost can be suitably reduced. ２ The concentration 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 sheet 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 inevitable impurities. The explanation of each component in the coating bath is the same as the explanation of each component of the coating layer described above. The component composition of the coating bath and the component composition of the coating layer are equivalent.

[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 an increase in oxides on the surface of the plating bath due to oxidation of Mg in the plating bath, and 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 to adjust 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 in gas wiping is a gas having reducing properties. ２ and N ２ Conventionally, in the production of hot-dip galvanized steel sheets, air or N has been used as the wiping gas from the viewpoint of economy. ２ However, in the present invention, it is important to sufficiently form a ternary eutectic structure on the surface of the plating layer by using a small amount of Al and Mg present in the plating bath composition. Mg is an easily oxidizable element, and the wiping gas is O. ２ When the alloy contains Zn—Al—MgZn, Mg is oxidized during gas wiping, and the amount of Mg becomes insufficient during the subsequent cooling process. ２ The ternary eutectic structure is not sufficiently formed on the surface of the coating layer. ２ If the formation of the ternary eutectic structure is insufficient, the Zn-phase single-phase structure will occupy most of the surface instead, and the Zn-Al-MgZn structure located between adjacent Zn-phase single-phase structures will ２ Average width d of ternary eutectic structure ａｖｅ As a result, the Zn-phase single phase structure and the Zn-Al-MgZn ２ The contact area with the ternary eutectic structure is reduced, and the Zn-Al-MgZn ２ Preferential corrosion of the ternary eutectic structure occurs significantly, Mg dissolution becomes very rapid, and the effect of improving corrosion resistance over the long term cannot be obtained.

[0038] The present inventors have conducted extensive research to improve the corrosion resistance of Zn-Al-Mg plated steel sheets, and have found that the H ２It was found that the higher the concentration, the more the oxidation of Mg can be suppressed during gas wiping. ２ A sufficient gap can be secured between the ternary eutectic structure and the Zn-Al-MgZn ２ It is possible to exert sustained release properties against corrosion of the ternary eutectic structure and Mg dissolution.

[0039] Wiping gas H ２ When the concentration is 1% by volume or more, oxidation of the molten Mg can be suitably suppressed, and the effect of contributing to the improvement of corrosion resistance can be sufficiently obtained. ２ The concentration is 1% by volume or more. ２ The higher the concentration, the more the oxidation of molten Mg can be suppressed. However, from the viewpoint of production cost and economic efficiency, the H ２ The concentration is preferably 5% by volume or less, more preferably 4% by volume or less. The remainder of the wiping gas is N ２ is preferred.

[0040] Next, the cooling step after adjusting the coating weight by gas wiping will be described. Immediately after gas wiping, the temperature of the surface of the regulated 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 regulated plated steel sheet reaches 200°C. If the average cooling rate is less than 3.0°C / s, the Zn-phase single-phase structure becomes coarse, and the circle-equivalent average radius r of the Zn-phase single-phase structure becomes smaller. ａｖｅ and Zn-Al-MgZn located between adjacent Zn phase single phase structures. ２ Average width d of ternary eutectic structure ａｖｅ Therefore, in the cooling step, the average cooling rate until the temperature of the surface of the regulated 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 small, and the circle-equivalent average radius r of the Zn single-phase structure becomes smaller. ａｖｅ and Zn-Al-MgZn located between adjacent Zn phase single phase structures. ２ Average width d of ternary eutectic structure ａｖｅTherefore, in the cooling step, the average cooling rate until the temperature of the surface of the plated steel sheet after adjustment 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.

[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 composition of the coating bath was changed in various ways, and the base steel sheet was subjected to hot-dip galvanizing for 1 second to obtain plated 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 base steel sheet before coating was heated to 460°C, which is 20°C higher than the solidification start temperature estimated from the Zn-Al-Mg equilibrium phase diagram. ２ +H ２ By heating in a 5% by volume atmosphere, the temperature of the base steel sheet when immersed in the coating bath was made the same as the temperature of the coating bath. Thereafter, the coated steel sheet was subjected to gas wiping, and the thickness of the coating layer was reduced to 7 μm (deposition amount 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] By the method described above, the Zn phase single phase structure and the Zn-Al-MgZn phase in each sample were ２ The presence or absence of a ternary eutectic structure 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, by the above-mentioned method, the circle-equivalent average radius r of the Zn-phase single-phase structure on the plated surface of each sample was measured. ａｖｅ Zn-Al-MgZn located between adjacent Zn single phase structures ２ Average width d of ternary eutectic structure ａｖｅ , and Zn—Al—MgZn ２ The area ratios of the ternary eutectic structure and the Zn single phase structure were measured, and the results are shown in Table 1.

[0044] [Corrosion Resistance of Cut Portions of Painted Surface] Next, to evaluate the blister corrosion resistance and appearance corrosion resistance of the cut portions of the painted surfaces of the obtained 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 main agent at pH 4 and 35°C, and then electrocoated with a 15 μm thick epoxy-based water-based paint. After coating, a blade was used to create cross-cut scratches on the sample surface. The blade was deep enough to reach the surface of the base steel sheet. The N-VDA corrosion test (VDA 233-102) was performed to evaluate the corrosion resistance after coating. The N-VDA corrosion test is a cyclic dry-wet test that simulates atmospheric corrosion in a temperature range of -15°C to 50°C. Compared to a typical cyclic dry-wet test (for example, the SAE J2334 test involves repeated dry-wet cycles in a temperature range of room temperature to approximately 60°C), this corrosion test involves much larger temperature changes, from below freezing to a high temperature (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 cut paint area were evaluated.

[0045] The blister corrosion resistance of the cut 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, "○" if it was 3 mm or more but less than 4 mm, "△" if it was 4 mm or more but less than 5 mm, and "×" if it was 5 mm or more. Evaluation results of "◎" or "○" were considered pass, and "△" or "×" were considered fail.

[0046] The appearance corrosion resistance of the cut part of the coating was evaluated by measuring the area of ​​red rust from a photograph of the appearance of the sample after the test. ２ If it is less than 50mm, mark it as "◎" ２ 150mm or more ２ If it is less than 150mm, mark it as "○" ２ More than 300 mm ２ If it is less than 300mm, it is marked as "△" ２ In the above cases, "×" is recorded in Table 1. The evaluation result of "◎" or "◯" was judged as passing, and "△" or "×" was judged as failing.

[0047] [Corrosion Resistance of Flat Plate Joints] To evaluate the corrosion resistance of flat plate joints of Zn-Al-Mg-plated steel sheets, each sample was sheared into a large plate size of 80 x 70 mm and a small plate size of 60 x 40 mm. Large and small plates with the same plating type were joined together so that their long and short edges were aligned, and then two-point spot welded to prepare samples for the joint corrosion test. The samples were then phosphate-treated in a bath containing PB-SX35 as the main agent at pH 4 and 35°C, followed by electrodeposition coating with a 15 μm coating of an epoxy-based water-based paint. The end faces of the large plate, the back surface opposite the mating surface of the large plate, and the back surface opposite the mating surface of the small plate were covered with tape seal 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 the corrosion test, the spot welds were disassembled, the chemical electrodeposition on the large plate side was removed, and the corrosion products on the mating surfaces were peeled off with hydrochloric acid. After that, the maximum corrosion depth of the sample after the test was measured using a laser device.

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

[0049]

[0050] As shown in Table 1, the examples that satisfied the requirements of the present invention were excellent in blister corrosion resistance and appearance corrosion resistance of the cut part of the coating, and in corrosion resistance of the joint part of the flat plate. ２ In the examples in which the area ratio of the ternary eutectic structure was 30 area % or more and the area ratio of the Zn-phase single phase structure was 70 area % or less, the appearance corrosion resistance of the paint cut portion was superior.In contrast, the examples that did not satisfy the requirements of the present invention were inferior in either the blister corrosion resistance of the paint cut portion, the appearance corrosion resistance of the paint cut portion, or the corrosion resistance of the joint of the flat plates.

[0051] According to the present invention, it is possible to provide a Zn-Al-Mg-plated steel sheet that exhibits excellent blister corrosion resistance and appearance corrosion resistance in a painted cut area, and excellent corrosion resistance in a joining area of ​​flat sheets, even in a corrosive environment where the temperature varies greatly from below freezing to high temperatures (50°C or higher).

[0052] 10 Zn phase single phase structure 20 Zn-Al-MgZn ２ Ternary eutectic structure r Equivalent circle radius of Zn phase single phase structure d1 Zn-Al-MgZn ２ Width of ternary eutectic structure d2 Zn-Al-MgZn ２ Width of ternary eutectic structure d3 Zn-Al-MgZn ２ Width of ternary eutectic structure d4 Zn-Al-MgZn ２ Ternary eutectic structure width

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, wherein the plating layer has a composition, in mass%, of Al: 0.10 to 3.00%, Mg: 0.50 to 3.00%, and Fe: 0.30% or less, with the balance consisting of Zn and inevitable impurities, and wherein a backscattered electron image obtained by irradiating the surface of the plating layer with an electron beam using a scanning electron microscope shows that (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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