Hot-dip Al-Zn coated steel sheet and its manufacturing method

Abstract

To provide a hot-dip Al-Zn alloy-plated steel sheet excellent in bending workability, non-aging properties of yield elongation, and moldability.SOLUTION: A hot-dip Al-Zn alloy-plated steel sheet comprises a plated layer with Al content of 40 to 70% by mass, Si content of 0.5 to 3.0% by mass and remaining content being Zn and inevitable impurities. Given that bending workability is represented by a minimum amount of an inner distance nt (where t is a thickness of plated steel sheet and n is the number of steel sheets) of a test piece which does not cause the test piece to have cracks when, after an aging acceleration test, the same is bent at 180 degrees and an external surface of bent section thereof is visually observed with a loupe having a 10 time magnification lens in a bending test conforming to a plate adhesion test specified in JIS G 3321 (2019), has the bending workability of 6t or less. The hot-dip Al-Zn alloy-plated steel sheet also has yield elongation (YEL) after the aging acceleration test of 10% or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a hot-dip Al-Zn coated steel sheet that has excellent bending workability and non-aging of yield point elongation, and also has good formability, and a method for producing the same. [Background technology]

[0002] Hot-dip Al-Zn coated steel sheets (hereinafter sometimes referred to as "55% Al-Zn coated steel sheets"), which contain 20 to 95 mass% Al in the coating layer, such as 55% Al-Zn coated steel sheets, have shown excellent corrosion resistance and have seen an increase in demand in recent years, mainly in the field of building materials.

[0003] However, because the coating layer of 55% Al-Zn coated steel sheets is harder than that of pure zinc coatings, etc., there is a problem in that cracks are easily generated in the processed part of the coating layer when the coated steel sheet is bent. Such cracks in the coating layer during bending may cause a deterioration in the corrosion resistance of the processed part, so improvement is desired. For the purpose of improving such workability and corrosion resistance of worked portions, a technique is known in which a steel sheet after the formation of a plating layer is subjected to a heat treatment to soften the plating layer (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-322573 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-213395 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-70326 Summary of the Invention [Problem to be solved by the invention]

[0005] The techniques of subjecting a steel sheet to heat treatment after the formation of a coating layer as disclosed in Patent Documents 1 to 3 can improve the workability of a hot-dip Al-Zn coated steel sheet to some extent, but have had a problem in that when low-carbon steel, which is widely used as a general building material, is used as a base steel sheet, the heat treatment makes the steel sheet more susceptible to yield point elongation.

[0006] This is thought to be because when steel sheets are heat-treated to improve formability after the formation of the coating layer, large amounts of Fe3C precipitate at dislocations introduced by processes such as the skin pass of a continuous hot-dip galvanizing line (CGL). This precipitated Fe3C then re-dissolves in the subsequent paint baking process, resulting in an increase in the amount of solute C in the steel near the dislocations. Figure 2 shows the relationship between cooling time and temperature after reheating a steel sheet with a coating layer, as well as the state of the FeC nose. When examining the FeC precipitation nose of low-carbon steel sheet, the cooling rate after coating on a typical continuous hot-dip galvanizing line (CGL) is typically sufficient to avoid this precipitation nose, resulting in almost no FeC precipitation. However, when heat treatments such as those described in Patent Documents 1 to 3 are performed to improve the bending workability of the coating, the thermal history of the steel sheet during the heat treatment process, particularly during cooling after heating, can affect the precipitation nose. In this case, FeC precipitates at dislocations introduced during the skin-pass process of the CGL. In the production of coated steel sheet, the precipitated FeC redissolves in the subsequent paint baking process, resulting in an increase in solute C in the steel near the dislocations compared to the bulk. This solute C then attaches to the dislocations during aging, potentially resulting in yield elongation. Furthermore, since the yield elongation that occurs can cause wrinkles (stretcher strain) during processing, there is a need to develop technology that can suppress this.

[0007] Furthermore, when the plated steel sheet disclosed in the present invention is used as a building material, it is often used as a painted steel sheet. In this case, in order to suppress the above-mentioned yield point elongation, it is known to impart strain to the steel sheet using a skin pass or a tension leveler in the final process of the coating line (CCL). However, this method lowers the yield point (YP) of the steel sheet, which causes the steel sheet to become more prone to dents (deteriorating oil-can resistance). Therefore, another means was needed to suppress the yield point elongation of low-carbon steel sheets while maintaining good oil-can resistance.

[0008] Furthermore, the bending workability and yield point elongation of the above-mentioned hot-dip Al-Zn coated steel sheet also tend to deteriorate after a certain period of time, and there has been a problem in improving physical properties such as bending workability and yield point elongation after aging (improving non-aging properties).

[0009] In view of the above circumstances, an object of the present invention is to provide a hot-dip Al-Zn coated steel sheet that is excellent in bending workability and non-aging of yield point elongation, and also has good formability, and a method for producing the same. [Means for solving the problem]

[0010] The inventors conducted research to solve the above-mentioned problems with respect to hot-dip Al-Zn plated steel sheets, in which the plating layer has a composition containing 40 to 70 mass% Al and 0.5 to 3.0 mass% Si, with the remainder consisting of Zn and unavoidable impurities. As a result, they found that by optimizing the heat treatment conditions applied to improve the bending workability of the plating after an accelerated aging test and improving the yield point elongation (YEL) after the accelerated aging test to be low at 10% or less, it is possible to maintain good oil can resistance and further suppress wrinkles during processing, thereby achieving excellent bending workability and non-aging of yield point elongation, as well as good formability.

[0011] The present invention has been made based on the above findings, and the gist of the present invention is as follows. 1. A hot-dip Al-Zn coated steel sheet, the coating layer of which contains 40 to 70 mass% Al and 0.5 to 3.0 mass% Si, with the remainder being Zn and unavoidable impurities, A hot-dip Al-Zn coated steel sheet characterized in that, in a bending test conforming to the coating adhesion test specified in JIS G 3321 (2019), after an accelerated aging test, the test piece is bent 180° at an inner interval nt (where t is the thickness of the coated steel sheet and n is the number of coated steel sheets), and when the outer surface of the bent section is observed with a 10x magnifying glass, the bending workability, indicated by the smallest nt at which no cracks are observed, is 6t or less, and the yield elongation (YEL) after the accelerated aging test is 10% or less.

[0012] 2. The Vickers hardness of the primary Al crystals in the dendritic phase of the plating layer is 120 HV 0.01 2. The hot-dip Al-Zn plated steel sheet according to 1 above, characterized in that:

[0013] 3. The hot-dip Al-Zn plated steel sheet according to 1 or 2 above, characterized in that the plated layer has an interfacial alloy layer at the interface with the base steel sheet, and the average thickness of the interfacial alloy layer is 2 μm or less.

[0014] 4. Forming a coating layer on a substrate steel sheet using a coating bath containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, and the remainder consisting of Zn and unavoidable impurities; The cooling rate after the steel sheet leaves the coating bath is 12°C / s or more, A method for producing a hot-dip Al-Zn coated steel sheet, characterized in that the following formulas (1) and (2) are satisfied, where T (°C) is the maximum temperature that the steel sheet reaches when reheating the steel sheet on which the coating layer has been formed, and x (hr) is the cooling time from T°C to 150°C. 150≦T≦300 (1) 0.5≦x≦1000 / (T+273) ···(2) [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a hot-dip Al-Zn coated steel sheet that is excellent in bending workability and non-aging of yield point elongation, and also has good formability, and a method for producing the same. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram schematically illustrating a state in which a hot-dip Al-Zn-plated steel sheet has been bent 180°. [Figure 2] FIG. 1 is a diagram showing the relationship between cooling time and temperature after reheating a steel sheet on which a coating layer has been formed, as well as the state of an Fe3C nose. [Figure 3] 10 is a photograph for explaining the distance between dendrite arms. [Figure 4] FIG. 2 is a diagram schematically illustrating the shape formed for evaluating formability in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0017] <Hot-dip Al-Zn coated steel sheet> The hot-dip Al-Zn plated steel sheet of the present invention is a hot-dip Al-Zn plated steel sheet having a coating layer containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, and the remainder consisting of Zn and unavoidable impurities.

[0018] (Composition of plating layer) The coating layer has a composition containing 40 to 70 mass % Al and 0.5 to 3.0 mass % Si, with the balance being Zn and unavoidable impurities. When the coating layer of the hot-dip Al-Zn coated steel sheet has the above-mentioned composition, it is possible to form a dendrite phase and an interdendritic phase surrounding the dendrite phase in a network shape in the coating layer, thereby improving corrosion resistance.

[0019] The Al content in the coating layer is 40 to 70% by mass, preferably 50 to 60% by mass, to balance corrosion resistance and operability. When the Al content in the main layer of the coating layer is at least 40% by mass, sufficient dendritic solidification of Al occurs. As a result, the main layer mainly contains Zn in a supersaturated state, and is composed of dendritic solidification of Al (α-Al phase dendritic portions) and the remaining interdendritic portions (interdendritic portions), resulting in a structure with excellent corrosion resistance in which the dendritic portions are stacked in the thickness direction of the coating layer. Furthermore, the more α-Al phase dendritic portions are stacked, the more complex the corrosion progression path becomes, making it more difficult for corrosion to reach the substrate steel sheet, thereby improving corrosion resistance. On the other hand, if the Al content in the coating layer exceeds 70% by mass, the content of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, resulting in a deterioration of corrosion resistance. For this reason, the Al content in the coating layer is set to 70% by mass or less. Furthermore, if the Al content in the coating layer is 60% by mass or less, the coating weight is reduced and even if the substrate steel sheet is easily exposed, the coating layer has a sacrificial corrosion protection effect against Fe, and sufficient corrosion resistance is obtained. Therefore, the Al content in the main coating layer is preferably 70% by mass or less.

[0020] The Si in the coating layer is added to the coating bath to improve corrosion resistance and workability and to suppress the growth of an interfacial alloy layer formed at the interface with the substrate steel sheet, and is therefore inevitably contained in the coating layer. In the case of the hot-dip Al-Zn-coated steel sheet used in the hot-dip Al-Zn-coated steel sheet of the present invention, when hot-dip coating is performed with Si in the coating bath, Fe on the steel sheet surface reacts with Al and Si in the bath to form an alloy consisting of Fe-Al and / or Fe-Al-Si compounds as soon as the substrate steel sheet is immersed in the coating bath. The formation of this Fe-Al-Si interfacial alloy layer suppresses the growth of the interfacial alloy layer. Furthermore, when the Si content in the coating layer is 0.5% by mass or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content in the coating layer exceeds 3.0% by mass, the coating layer deteriorates in workability and is prone to the precipitation of Si phases that serve as cathode sites. For this reason, the Si content in the coating layer is set to 0.5 to 3.0% by mass.

[0021] The plating layer contains Zn as a main component of the plating layer. By including Zn in the plating layer, a sacrificial anticorrosion effect can be obtained, and it becomes possible to improve corrosion resistance. When the Zn content is 80 mass % or less, the Al content can be ensured, and the corrosion resistance due to the dendrite phase and interdendrite phase can be realized, which is preferable.

[0022] Furthermore, the plating layer may contain 5 mass % or less of optional additive elements in addition to the above-mentioned Al, Si, and Zn. The optional additives can be selected appropriately depending on the performance required of the plating layer, and examples thereof include alkaline earth metals such as Ca and Mg, and additives such as Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B. Although these optional addition elements can provide effects such as further improving corrosion resistance, they may also reduce the workability of the coating layer and deteriorate the ultimate elongation of the hot-dip Al-Zn coated steel sheet. Therefore, the content of these optional addition elements is preferably 5 mass% or less.

[0023] The plating layer may contain Mg and / or Ca. When the plating layer corrodes, the corrosion products contain Mg and / or Ca, improving the stability of the corrosion products and slowing the progression of corrosion, resulting in improved corrosion resistance. The total content of Ca and / or Mg is not particularly limited as long as it is 5% by mass or less, but is preferably 0.01 to 5% by mass. A content of 0.01% by mass or more provides a sufficient corrosion retardation effect, while a content of 5% by mass or less prevents the effect from saturating, suppresses increases in production costs, and facilitates control of the plating bath composition. Furthermore, the plating layer preferably contains at least Mg. This is because the inclusion of Mg in the plating layer allows the formation of Mg2Si together with Si, thereby achieving a corrosion retardation effect. Here, the Mg content in the plating layer is preferably 0.01 to 5 mass%, more preferably 2 to 4.9 mass%.

[0024] Furthermore, similar to the alkaline earth metals Ca and Mg, the plating layer may further contain one or more elements selected from Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B in a total amount of 5 mass % or less, preferably 0.01 to 5 mass %, since these elements have the effect of improving the stability of corrosion products and delaying the progression of corrosion.

[0025] The coating layer contains components of the base steel sheet that are incorporated into the coating due to the reaction between the coating bath and the base steel sheet during the coating process, unavoidable impurities contained in the ingot used to prepare the coating bath, and unavoidable impurities that are slightly eluted from the equipment in the bath. The base steel sheet components that are incorporated into the coating may contain several percent of Fe. Examples of unavoidable impurities in the coating bath include, for example, Fe, Mn, P, S, C, Nb, Ti, and B as base steel sheet components. Furthermore, impurities in the ingot include Fe, Pb, Sb, Cd, As, Ga, and V. Furthermore, impurities from the equipment in the bath include Cr, Ni, W, and Co. It is not possible to quantify the Fe in the coating layer separately, distinguishing between that incorporated from the base steel sheet and that present in the coating bath. The total content of the unavoidable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, the total amount of unavoidable impurities excluding Fe is preferably 1 mass % or less.

[0026] The means for forming the coating layer on the base steel sheet is not particularly limited, and a conventional continuous hot-dip galvanizing facility can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere, and while annealing, rolling oil and the like adhering to the steel sheet surface are removed and an oxide film is reduced and removed. After that, the steel sheet is immersed in a hot-dip galvanizing bath containing predetermined concentrations of Al and Zn through a snout whose lower end is immersed in the coating bath. The steel sheet immersed in the coating bath is then pulled up above the coating bath via a sink roll, and the coating weight is adjusted by spraying pressurized gas toward the surface of the steel sheet from a gas wiping nozzle arranged above the coating bath. The steel sheet is then cooled by a cooling device, thereby forming a coating layer.

[0027] The cooling rate after plating (after the steel sheet is removed from the plating bath) is not particularly limited, and can be set to a normal condition (for example, 12°C / s or more). The reasons for cooling under normal conditions are thought to be as follows: The softening of the coating layer due to heat treatment occurs when Zn crystallizes from the Zn-supersaturated primary Al crystals into the primary A crystals during heat treatment. In this case, if the cooling rate after coating is slow, a certain amount of Zn will crystallize from the primary A crystals during cooling, reducing the degree of supersaturation of the primary Al crystals before heat treatment and reducing the effect of the subsequent heat treatment. Although the crystallization of Zn from the primary Al crystals during the cooling process after plating slightly softens the plating layer, it does not have the effect of significantly improving bending workability. To improve workability, it is necessary to efficiently crystallize Zn from the primary Al crystals, so it is important to increase the degree of supersaturation of Zn in the primary Al crystals by ensuring a sufficient cooling rate in the temperature range from leaving the plating bath to 450°C.

[0028] The structure of the plating layer contains a dendritic phase and an interdendritic phase, and the Vickers hardness of the Al primary crystal of the dendritic phase is 120 HV. 0.01 Preferably, the Vickers hardness of the Al primary crystals in the dendritic phase is 120 HV or less. 0.01By reducing the Vickers hardness to 110 HV or less, the bending workability of the plated steel sheet can be further improved. 0.01 It is more preferable that it is less than 100HV 0.01 On the other hand, from the viewpoint of improving the scratch resistance of the plating layer surface, it is more preferable that the Vickers hardness of the Al primary crystals of the dendrite phase is 10 HV or less. 0.01 It is preferable that the above is set. The Vickers hardness is measured under a 10g indentation load (HV 0.01 ) is conducting the test.

[0029] The plating layer preferably has dendrites mainly composed of Al primary crystals and dendritic gaps containing Al-Zn eutectic, the Al primary crystals including an α-Al phase matrix and Zn precipitates, and the Zn content in the matrix is ​​preferably 30 mass% or less. In the present invention, the coating layer is reheated to soften it and thereby improve its workability. This softening occurs due to the crystallization of Zn from the Al primary crystals, as described above. In this case, the Zn concentration in the Al primary crystal matrix decreases compared to before the heat treatment due to the crystallization of Zn; specifically, it is preferable that the Zn concentration be 30 mass% or less.

[0030] Furthermore, the coating weight of the plating layer is not particularly limited, but from the viewpoint of achieving both corrosion resistance and bending workability, the coating weight on one side is preferably 30 to 90 g / m 2 It is preferable that the density is 40 to 80 g / m 2 It is more preferable that:

[0031] Here, Fig. 3 is a photograph showing a portion of the coating layer observed at 200x magnification using an SEM to explain dendrite arms and dendrite arm distance. Because the coating layer is an Al-Zn-based coating, the cross-sectional structure of the coating is such that the Al primary crystals are surrounded by a Zn-rich phase. The dendrite arm distance (dendrite arm spacing) is an index related to the number of layers of these Al primary crystals. The dendrite arm distance refers to the center-to-center distance between adjacent dendrite arms, and can be observed by enlarging the surface of the main layer of the coating layer using a scanning electron microscope (SEM) or the like, as shown in Fig. 3. As mentioned above, the smaller the dendrite arm distance, the greater the number of layers, which leads to a complex corrosion path and a longer time until the steel sheet corrodes, resulting in high corrosion resistance. Therefore, from the viewpoint of obtaining good corrosion resistance, the distance between dendrite arms in the plating layer is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 16 μm or less. This value is determined by the cooling rate to 450°C after plating, and if the cooling rate is constant (12°C / s or more), a distance between dendrite arms of 20 μm or less can be achieved. can. The distance between the dendrite arms can be measured, for example, by observing the distance between the secondary dendrite arms branching from the primary arm extending from the center of the spangle using an SEM at 200x magnification in a randomly selected field of view. An aligned portion is selected (in Figure 3, three arms between A and B are selected), and the distance (distance L in Figure 3) is measured along the direction in which the arms are aligned. The measured distance is then divided by the number of dendrite arms (L / 3 in Figure 3) to calculate the inter-dendrite arm distance. The inter-dendrite arm distance can be measured at three or more locations within one field of view, and the average of the inter-dendrite arm distances obtained at each location can be calculated to determine the average inter-dendrite arm distance.

[0032] The plating layer has an interfacial alloy layer present at the interface with the base steel sheet. The interfacial alloy layer is an Fe-Al and / or Fe-Al-Si compound that is inevitably formed by an alloying reaction between Fe on the steel sheet surface and Al or Si in the bath. Because this interfacial alloy layer is hard and brittle, if it grows too thick it can become the starting point for cracks during processing. Therefore, it is preferable to make it as thin as possible. Therefore, the average thickness of the interfacial alloy layer is preferably 2 μm or less, and more preferably 1 μm or less. As mentioned above, this interfacial alloy layer is primarily formed in the coating bath. After the coating layer is formed, it further grows due to a reaction between the steel sheet and Al in the coating layer while the coating layer is in a high temperature state. Therefore, an interfacial alloy layer thickness of 2 μm or less can be achieved by setting the cooling rate after coating within a certain range (12°C / s or more). The average thickness of the interfacial alloy layer can be obtained, for example, by observing a cross section of the plating layer near the interface with the base steel sheet at a magnification of 5000 times using, for example, a scanning electron microscope (SEM), measuring the cross sections of the interfacial alloy layer at five randomly selected locations, and calculating the average value.

[0033] (Bending workability after accelerated aging test) Here, in the hot-dip Al-Zn-plated steel sheet of the present invention, in a bending test conforming to the coating adhesion test described in JIS G 3321 (2019), after an accelerated aging test, the test piece is bent 180° with an inner spacing nt, and the outer surface of the bent portion is observed with a 10x magnification magnifier. The bending workability, indicated by the minimum nt at which no cracks are observed, is 6t or less. A bending workability of 6t or less can achieve excellent bending workability and, ultimately, corrosion resistance after bending. Furthermore, because the bending workability after accelerated aging is specified, there is no deterioration in bending workability due to subsequent aging, and the steel sheet also has excellent non-aging properties. From the same viewpoint, the bendability of the hot dip Al-Zn plated steel sheet of the present invention after an accelerated aging test is preferably 5t or less, and more preferably 4t or less. In the present invention, the accelerated aging test is carried out by holding the specimen at 100° C. for 3600 seconds.

[0034] The bending test for determining the bending workability after the accelerated aging test was a bending test conforming to the plating adhesion test described in JIS G 3321 (2019). As shown in Figure 1, in the inner spacing nt, t is the thickness of the plated steel sheet, and n is the number of plated steel sheets. For example, when two steel sheets are sandwiched, the spacing is 2t, and when three steel sheets are sandwiched, the spacing is 3t. The entire surface of the bent portion of the test piece is observed with a 10x magnifying glass to check for the presence or absence of cracks. For example, if there are no cracks up to 3t in a bending test, but cracks are confirmed when the test is performed at 2t, the bending workability of the steel plate is 3t.

[0035] (YEL, YP after accelerated aging test) The hot-dip Al-Zn coated steel sheet of the present invention satisfies the bending workability (6t or less) after the accelerated aging test described above, and has a yield elongation (YEL) of 10% or less after the accelerated aging test. It is characterized by: By keeping the YEL after accelerated aging test at 10% or less, it is possible to maintain good oil can resistance while suppressing wrinkles during processing, thereby achieving excellent formability. Furthermore, since the YEL after accelerated aging is specified, there is no deterioration of the YEL due to subsequent aging, and the non-aging properties are also excellent. From the same perspective, the YEL after the accelerated aging test is preferably 9.0% or less, more preferably 8.5% or less, and even more preferably 8.0% or less.

[0036] Here, the YEL can be measured by a tensile test. In the present invention, a No. 5 test piece of JIS Z 2201 is taken from a hot-dip Al-Zn-plated steel sheet, and is subjected to a strain rate of 10 -3 Conduct a tensile test under the conditions of / s. In the present invention, the accelerated aging test is carried out by holding the specimen at 100° C. for 3600 seconds, as in the bending workability test described above. That is, the YEL after the accelerated aging test can be measured by taking a test piece from the hot-dip Al-Zn-plated steel sheet, subjecting the test piece to accelerated aging treatment by holding it at 100°C for 3600 s, and then conducting a tensile test.

[0037] Regarding the hot-dip Al-Zn-plated steel sheet of the present invention, there are no particular limitations on the method for suppressing the YEL after an accelerated aging test to 10% or less and the method for maintaining the bending workability after the above-mentioned accelerated aging test to 6T or less. Examples include methods of controlling the composition of the coating layer, the temperature conditions during formation of the coating layer, the temperature conditions during reheating after formation of the coating layer, etc. For example, in the present invention, as will be described later, the YEL after an accelerated aging test and the bending workability after an accelerated aging test are controlled by controlling the temperature conditions during reheating after the formation of the plating layer.

[0038] Furthermore, the hot-dip Al-Zn coated steel sheet of the present invention has a yield point (YP) of 400 N / mm 2 It is preferable that the resistance is 450N / mm or more. 2 It is more preferable that the YP is 400 N / mm or more. 2 If the content is less than 100%, even if the YEL is good, the oil can property may be reduced, which may lead to deterioration of moldability.

[0039] In addition, in the hot dip Al-Zn plated steel sheet of the present invention, a coating film can be formed directly or via an intermediate layer on the plated layer depending on the required performance.

[0040] The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. Examples include roll coater coating, curtain flow coating, and spray coating. After applying a coating material containing an organic resin, the coating film can be formed by heating and drying the coating material using means such as hot air drying, infrared heating, and induction heating. The intermediate layer is not particularly limited as long as it is a layer formed between the coating layer of the hot-dip coated steel sheet and the coating film.

[0041] <Method of manufacturing hot-dip Al-Zn coated steel sheet> The method for producing a hot-dip Al-Zn coated steel sheet of the present invention (hereinafter sometimes referred to as "the production method of the present invention") comprises forming a coating layer on a substrate steel sheet using a coating bath having a composition containing 40 to 70 mass % Al, 0.5 to 3.0 mass % Si, and the balance consisting of Zn and unavoidable impurities, and then The present invention is characterized in that the following formulas (1) to (3) are satisfied when the maximum temperature that the steel sheet on which the coating layer is formed is T (°C), the average cooling rate of the steel sheet from T to 150°C is CL1 (°C / hr), and the average cooling rate of the steel sheet from 150°C to 80°C is CL2 (°C / hr). 151≦T≦300 (1) CL1≧T-150 (2) CL2≦100 (3)

[0042] Conventionally, known methods for suppressing the YEL of hot-dip Al-Zn coated steel sheets have been to apply distortion to the steel sheets on the painting line using a skin pass or tension leveler. However, these methods lower the yield point (YP) of the steel sheet, which causes the steel sheet to have poor oil canvas resistance. Therefore, in the manufacturing method of the present invention, the temperature conditions for reheating the steel sheet on which the coating layer has been formed are determined by specifying the temperature range of the maximum temperature T of the steel sheet, while specifying the average cooling rate CL1 from the maximum temperature T of the steel sheet to 150°C and the average cooling rate CL2 from 150°C to 80°C. In other words, by satisfying formulas (1) to (3), excellent bending workability after an accelerated bending test can be achieved, and the YEL of the hot-dip Al-Zn coated steel sheet after an accelerated aging test can be suppressed while maintaining good formability such as oil can resistance. Furthermore, in the manufacturing method of the present invention, there is no need to apply distortion to the steel sheet on which the plating layer is formed using a skin pass or tension leveler on the painting line, so there is no deterioration in formability such as oil can resistance due to a decrease in YP.

[0043] In the method for producing a hot-dip Al-Zn coated steel sheet of the present invention, although not particularly limited, a continuous hot-dip coating facility is usually adopted from the viewpoint of production efficiency and quality stability. The type of base steel sheet used in the manufacturing method of the present invention is not particularly limited. For example, a hot-rolled steel sheet or strip that has been pickled and descaled, or a cold-rolled steel sheet or strip obtained by cold rolling such a hot-rolled steel sheet or strip, can be used. However, from the viewpoint of obtaining a more significant effect of suppressing YEL after an accelerated aging test according to the present invention, it is preferable to use low-carbon steel.

[0044] Furthermore, the present invention does not particularly limit the steel components, but for example, a steel containing 0.01 to 0.10 mass% C can be used. However, steel sheets containing less than 0.01% C are not excluded by the present invention. Furthermore, steel sheets containing trace amounts of N, S, O, B, V, Nb, Ti, Cu, Mo, Cr, Co, Ni, Ca, Sr, In, Sn, Sb, etc. in addition to the constituent elements C, Al, Si, Mn, and P are also within the scope of the present invention.

[0045] In the production method of the present invention, the plating bath has a composition containing 40 to 70 mass % of Al, 0.5 to 3.0 mass % of Si, and the remainder consisting of Zn and unavoidable impurities. This allows a hot-dip Al-Zn coated steel sheet having a desired composition to be obtained. The types, contents, and functions of the elements contained in the coating bath are explained in the description of the hot-dip Al-Zn coated steel sheet of the present invention described above.

[0046] The hot-dip Al-Zn coated steel sheet obtained by the production method of the present invention has a composition that is almost the same as that of the coating bath as a whole, and therefore the composition of the coating layer of the hot-dip Al-Zn coated steel sheet can be controlled by adjusting the composition of the coating bath.

[0047] In the manufacturing method of the present invention, when the steel sheet on which the plating layer is formed is reheated, the maximum temperature T reached satisfies the following formula (1). 151≦T≦300 (1) Formula (1) defines the range of the maximum temperature T that can be reached when reheating a steel sheet on which the coating layer has been formed. The reason why the maximum temperature T is set to 151°C or higher is that if the coating layer is not sufficiently softened below this temperature, the bending workability of the hot-dip Al-Zn-coated steel sheet and the bending workability after an accelerated aging test will deteriorate. From the same perspective, the maximum temperature T is preferably set to 160°C or higher. On the other hand, the reason why the maximum temperature T is set to 300°C or lower is that if the temperature is higher than this temperature, the thickness of the interfacial alloy layer formed at the interface between the coating and the steel sheet will increase, resulting in a deterioration in bending workability. From the same perspective, the maximum temperature T is preferably set to 280°C or lower.

[0048] Furthermore, in the production method of the present invention, when the steel sheet on which the coating layer has been formed is reheated, the average cooling rate from T°C to 150°C satisfies the following formula (2). CL1≧T-150 (2) By specifying CL1 (°C / hr), which is the average cooling rate of the steel sheet from T to 150°C, as in equation (2), it is possible to avoid the formation of an Fe3C precipitation nose (a condition that makes Fe3C more likely to precipitate) during the cooling process after reheating, and by suppressing the precipitation of Fe3C at dislocations, it is possible to reduce the amount of solute C near the dislocations during paint baking, thereby making it possible to reduce the yield point elongation after accelerated aging tests. Note that if the average cooling rate CL1 is less than (T-150), there is a possibility that the steel sheet will pass through the Fe3C precipitation nose during cooling, making it impossible to reduce the amount of solute C near the dislocations and therefore impossible to reduce the yield point elongation. Furthermore, in the manufacturing method of the present invention, when the steel sheet on which the plating layer is formed is reheated, the following formula (3) is satisfied. CL2≦100 (3) By specifying CL2 (°C / hr), which is the average cooling rate of the steel sheet from 150°C to 80°C, as shown in formula (3), it is possible to avoid the formation of an Fe3C precipitation nose (a condition that makes Fe3C more likely to precipitate) during the cooling process after reheating, while also suppressing hardening of the coating layer. The reason why the cooling rate CL2 is set to 100°C / hr or less is that if it is not this rate or less, the coating layer will harden by aging after heat treatment, and sufficient bending workability will not be obtained.

[0049] Figure 2 shows the relationship between cooling time and temperature and the state of the Fe3C precipitation nose when the steel sheet bearing the coating layer is reheated. Figure 2 shows that when the steel sheet bearing the coating layer is reheated and then cooled in the temperature range from the maximum temperature T to 150°C, rapid cooling avoids the Fe3C precipitation nose, suppresses Fe3C precipitation, and reduces the amount of solute C near dislocations even after paint baking. On the other hand, slow cooling promotes Fe3C precipitation because the steel passes through the Fe3C precipitation nose, increasing the amount of solute C near dislocations even after paint baking. Regarding the relationship between cooling rate and Fe3C precipitation, the higher the maximum temperature T, the greater the average cooling rate (CL1) from T to 150°C must be to avoid the Fe3C precipitation nose. Therefore, CL1 must satisfy Equation (2). Furthermore, with regard to the residence time after reheating of the steel sheet on which the coating layer has been formed, from the viewpoint of suppressing hardening due to aging of the coating layer and improving bending workability, it is preferable to make the cooling rate CL2 at a temperature of 150°C or less slow, and in accordance with the present invention, the rate CL2 needs to satisfy formula (3). Therefore, in the manufacturing method of the present invention, by specifying the composition of the coating bath as described above, and then satisfying the conditions of formulas (1) and (3) that take into account the bending workability of the coating layer, and further satisfying formula (2) so as to avoid FeC precipitation noses, it is possible to provide a coated steel sheet that has excellent formability and excellent bending workability of the coating and is resistant to aging. Note that "CL1" and "CL2" shown in Figure 2 each show one example of the average cooling rate. The cooling curve in the figure is merely an example, and the present invention does not specify this cooling rate.

[0050] Furthermore, in the manufacturing method of the present invention, from the viewpoint of ensuring good material properties of the steel sheet, the heat pattern after the maximum temperature is reached must satisfy the above formulas (1) and (2) in order to avoid the above-mentioned FeC precipitation nose. However, with regard to the heating rate, there is a preferred range as shown below from the viewpoint of improving workability. In the manufacturing method of the present invention, although not particularly limited, the average heating rate from room temperature to the maximum temperature T°C when reheating the steel sheet is preferably 3°C / hr or more, more preferably 4°C / hr, and even more preferably 5°C / hr. This is to prevent the residence time in the high temperature range for improving workability from becoming excessively long. Furthermore, when the steel sheet is reheated, When heating from room temperature to the maximum temperature T°C, the average heating rate is preferably 10°C / hr or less, more preferably 15°C / hr or less, and even more preferably 20°C / hr or less, in order to ensure the minimum residence time in the high temperature range required to improve processability.

[0051] In the production method of the present invention, as described above, it is sufficient to satisfy the composition of the coating bath and the conditions for reheating the steel sheet on which the coating layer has been formed, and other conditions are not particularly limited and can be the same as those for known hot-dip Al-Zn coated steel sheets. [Example]

[0052] The present invention will be described below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.

[0053] <Samples 1-12 of hot-dip Al-Zn coated steel sheets> Samples 1 to 12 shown in Table 1 were prepared as follows. A cold-rolled steel sheet having a thickness of 0.40 mm and manufactured by a conventional method was used as a base steel sheet (steel sheet consisting of 0.075% by mass of C, 0.015% by mass of Si, 0.5% by mass of Mn, 0.025% by mass of Al, 0.013% by mass of P, 0.015% by mass of S, 0.002% by mass of N, the balance being Fe and unavoidable impurities), and was subjected to annealing, plating, and skin-pass treatment in a continuous hot-dip plating facility. Note that for all samples, the bath temperature of the plating bath was 590°C, the entry sheet temperature was 600°C, and the skin-pass reduction was 0.5%. Next, this coil was subjected to heat treatment in a batch-type heating furnace under the conditions shown in Table 1. The temperature of the steel plate was measured using a thermocouple attached to the steel plate. Furthermore, this coil was subjected to chemical conversion treatment on a continuous painting line, and then painted with a primer and top coat. In addition, for some samples, after painting, a tension leveler was used to impart an elongation of 0.1% to the steel plate.

[0054] The thickness of the interface alloy layer of each sample was measured by observing the cross section of the plating layer with an SEM (X5000), and the average value was calculated.

[0055] The Vickers hardness of the dendrite portion of the plating layer of each sample was measured by embedding each sample in room-temperature drying resin, polishing it, selecting the dendrite phase of the plating layer from the cross section, and measuring the Vickers hardness of the selected dendrite phase using a microhardness tester (Shimadzu Corporation, Shimadzu Microhardness Tester HMV-G21). The measurement method was in accordance with JIS Z 2244, and the indentation load was 10 gf.

[0056] Furthermore, the average inter-dendrite arm distance for each sample was determined by magnifying and observing the surface of the main plating layer using a scanning electron microscope (SEM) or the like (observation was performed at 200x magnification in the examples), measuring the spacing between secondary dendrite arms in randomly selected visual fields at three or more locations within each visual field, and calculating the average of the obtained inter-dendrite arm distances.

[0057] For each sample of hot-dip Al-Zn coated steel sheet, the cross section of the coating layer was observed using an ultra-low acceleration SEM and analyzed using energy dispersive X-ray spectroscopy (hereinafter referred to as "EDX"). The Zn concentration in the primary Al crystals of the above-mentioned coating layer was determined by point analysis of predetermined locations in the matrix, excluding spherical Zn precipitates, when observed at an accelerating voltage of 3 kV and a magnification of 20,000 times using a Zeiss ULTRA55 (ultra-low acceleration SEM) and an Oxford Instruments Ultim Extreme (EDX).

[0058] <Evaluation> The hot-dip Al-Zn coated steel sheets of each sample obtained as described above were evaluated as follows. The evaluation results are shown in Table 1.

[0059] (1) Bending resistance after accelerated aging test Each sample of hot-dip Al-Zn-plated steel sheet was subjected to a bending test in accordance with the coating adhesion test specified in JIS G 3321 (2019). Between the time each test piece was collected and the time of the bending test, an accelerated aging test was conducted in which the specimen was held at 100°C for 3,600 seconds. A 50 mm wide specimen was bent 180 degrees, and five cross sections of the bent section within a 30 mm width, excluding 10 mm at each end, were observed with a 10x magnifying glass to measure the minimum nt (where t is the thickness of the plated steel sheet and n is the number of plated steel sheets) at which no cracks were observed. The obtained nts were then evaluated according to the following criteria. 〇: 6t or less ×: 7t or more

[0060] (2) YEL after accelerated aging test JIS Z 2201 No. 5 test pieces were taken from each sample of hot-dip Al-Zn coated steel sheet and subjected to strain rate of 10 in accordance with JIS Z 2241. -3 Tensile tests were conducted under the conditions of / s. Between the time of collection of each test piece and the time of the tensile test, an accelerated aging test was conducted in which the test piece was held at 100°C for 3600 seconds, and then a tensile test in accordance with JIS Z 2241 was conducted to measure the yield point elongation. The tensile test was carried out three times (N=3) in each of the L and C directions of the steel sheet, and the average value was calculated.

[0061] (3) Formability (wrinkle resistance, oil resistance) Each sample of hot-dip Al-Zn plated steel sheet was formed into the shape shown in FIG. 4, and then the flat edge was visually inspected for the presence or absence of wrinkles and dents. The evaluation was made as follows: no wrinkles or dents were observed, and x was observed.

[0062] [Table 1]

[0063] The results in Table 1 show that the samples of the invention are well-balanced and superior in bending workability, YEL after accelerated aging, and formability compared to the samples of the comparative examples. [Industrial Applicability]

[0064] According to the present invention, it is possible to provide a hot-dip Al-Zn coated steel sheet that is excellent in bending workability and non-aging of yield point elongation and also has good formability, and a method for producing the same.

Claims

1. A hot-dip Al-Zn coated steel sheet, wherein the coating layer has a composition containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, 0.01 to 5 mass% Mg, and the balance consisting of Zn and unavoidable impurities, In a bending test conforming to the plating adhesion test described in JIS G 3321 (2019), after an accelerated aging test in which the specimen is held at 100°C for 3600 seconds, the specimen is bent 180° at an inner interval nt (where t is the thickness of the plated steel sheet and n is the number of plated steel sheets), and when the outer surface of the bent portion is observed with a 10x magnifying glass, the bending workability indicated by the smallest nt at which no cracks are observed is 6t or less, and the yield elongation (YEL) after an accelerated aging test in which the specimen is held at 100°C for 3600 seconds is 10% or less, The hot-dip Al-Zn plated steel sheet is characterized in that the plated layer has an interfacial alloy layer at the interface with the base steel sheet, and the average thickness of the interfacial alloy layer is 2 μm or less.

2. The Vickers hardness of the Al primary crystals of the dendritic phase in the plating layer is 120 HV 0.01 The hot-dip Al-Zn coated steel sheet according to claim 1, characterized in that:

3. A method for producing the hot-dip Al-Zn coated steel sheet according to claim 1 or 2, comprising: forming a coating layer on a substrate steel sheet using a coating bath having a composition containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, and 0.01 to 5 mass% Mg, with the balance being Zn and unavoidable impurities; The cooling rate after the steel sheet leaves the coating bath is 12°C / s or more, a method for producing a hot-dip Al-Zn coated steel sheet, characterized in that the following formulas (1) to (3) are satisfied, when T (°C) is the maximum temperature that the steel sheet will reach when reheating the steel sheet on which the coating layer has been formed, CL1 (°C / hr) is the average cooling rate of the steel sheet from T to 150°C, and CL2 (°C / hr) is the average cooling rate of the steel sheet from 150°C to 80°C. 151≦T≦300 ... (1) CL1≧T−150 (2) CL2≦100 (3)

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