Alloyed hot-dip galvanized steel sheets and components

Abstract

A galvannealed steel sheet according to the present invention has a Zn-containing plating layer on a surface of a steel material having a specific component. The aspect ratio of crystal grains on the surface of the plating layer is 4.0 or less. In the plating layer, the thickness of the Γphase is 1.0 μm or less and the tensile strength is 1180 MPa or more.

Description

[Technical Field]

[0001] This invention relates to alloyed hot-dip galvanized steel sheets and components. This application claims priority based on Japanese Patent Application No. 2024-062779, filed in Japan on April 9, 2024, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a growing demand for further improvements in automobile fuel efficiency, driven by regulations on greenhouse gas emissions to combat global warming. To achieve this, and to reduce vehicle weight while ensuring collision safety, the application of high-strength steel sheets in automotive components is expanding rapidly. Since automotive parts also require corrosion resistance, alloyed hot-dip galvanized steel sheets are sometimes used to improve corrosion resistance.

[0003] For example, Patent Document 1 discloses an alloyed hot-dip galvanized steel sheet characterized by having an Fe-Zn alloy coating on the surface of the cold-rolled steel sheet, which consists of a δ1 phase alone or a δ1 phase and a Γ1 phase with a thickness of 1 μm or less, and in which the average aspect ratio of the surface crystals is 3 or less. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 3-249162 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] According to the technology described in Patent Document 1, it is possible to achieve excellent surface treatment properties and powdering resistance in addition to the sliding properties required for press forming. However, the technology in Patent Document 1 does not envision producing high-strength alloyed hot-dip galvanized steel sheets with a tensile strength of 1180 MPa or more. Therefore, it can be said that there is room for improvement in the technology of Patent Document 1 in terms of achieving both excellent workability (plating adhesion and sliding properties) and high strength. Furthermore, while it is possible to adjust the components and manufacturing process to increase strength, under certain conditions, this could result in appearance defects such as the appearance of spot-like metallic luster areas on the plated surface.

[0006] Therefore, the present invention aims to provide an alloyed hot-dip galvanized steel sheet that is high in strength, has excellent workability, and suppresses the occurrence of appearance defects, as well as a component containing the alloyed hot-dip galvanized steel sheet. [Means for solving the problem]

[0007] To solve the above problems, the present invention proposes the following means. (1) A first aspect of the present invention is a chemical composition in mass% of: C:0.10~0.35%, Si:0.01~2.00%, Mn:2.8~4.0%, P:0~0.100%, S:0~0.100%, N:0~0.020%, Al:0.001~1.500%, O:0~0.010%, Cr:0~0.80%, Mo:0~1.00%, B:0~0.0100%, Ti:0~0.1000%, Nb:0~0.4000%, V:0~0.50%, Ni:0~1.0000%, Cu:0~1.00%, REM:0~0.0100%, As:0~0.200%, Sb:0~0.200 This alloyed hot-dip galvanized steel sheet has a Zn-containing plating layer on its surface, comprising a steel material containing %, Sn: 0~0.20%, W: 0~0.100%, Co: 0~2.0%, Zn: 0~0.100%, Zr: 0~0.0500%, Mg: 0~0.050%, Ca: 0~0.050%, Ta: 0~0.100%, Bi: 0~0.050%, and Te: 0~0.050%, with the remainder being Fe and impurities, the aspect ratio of the crystal grains on the surface of the plating layer is 4.0 or less, the thickness of the Γ phase in the plating layer is 1.0 μm or less, and the tensile strength is 1180 MPa or more. (2) The alloyed hot-dip galvanized steel sheet described in (1) above has a chemical composition of the steel material in mass percent of: Cr: 0.001~0.80%, Mo: 0.001~1.00%, B: 0.0003~0.0100%, Ti: 0.001~0.1000%, Nb: 0.001~0.4000%, V: 0.001~0.50%, Ni: 0.001~1.0000%, Cu: 0.001~1.00%, REM: 0.0003~0.0100%, As: 0.001~0.20% It may contain one or more elements selected from the group consisting of 0%, Sb: 0.001-0.200%, Sn: 0.001-0.20%, W: 0.001-0.100%, Co: 0.01-2.0%, Zn: 0.0005-0.100%, Zr: 0.0010-0.0500%, Mg: 0.001-0.050%, Ca: 0.001-0.050%, Ta: 0.001-0.100%, Bi: 0.001-0.050%, and Te: 0.001-0.050%. (3) A second aspect of the present invention is a member comprising the alloyed hot-dip galvanized steel sheet described in (1) or (2) above. [Effects of the Invention]

[0008] According to the above aspects of the present invention, it is possible to provide an alloyed hot-dip galvanized steel sheet that is highly strong, has excellent workability, and suppresses the occurrence of appearance defects, as well as a component including the alloyed hot-dip galvanized steel sheet. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram showing the surface of a plated layer where spot-like metallic luster areas have appeared. [Figure 2] This is a schematic diagram showing a cross-section near the metallic luster area that has appeared in the plating layer of an alloyed hot-dip galvanized steel sheet. [Figure 3] This is a cross-sectional view of the alloyed hot-dip galvanized steel sheet according to this embodiment. [Figure 4A] This is the first schematic diagram illustrating the method for evaluating the adhesion of the plating (after bending and unbending) of alloyed hot-dip galvanized steel sheets in the examples. [Figure 4B] This is a second schematic diagram illustrating the method for evaluating the adhesion of the plating (after bending and unbending) of alloyed hot-dip galvanized steel sheets in the examples. [Figure 4C] This is a third schematic diagram illustrating the method for evaluating the adhesion of the plating (after bending and unbending) of alloyed hot-dip galvanized steel sheets in the examples. [Figure 5A] This is the first schematic diagram illustrating the method for evaluating the adhesion (after ironing), i.e., sliding properties, of the alloyed hot-dip galvanized steel sheet in the example. [Figure 5B] This is a second schematic diagram illustrating the method for evaluating the adhesion (after ironing), i.e., sliding properties, of the alloyed hot-dip galvanized steel sheet in the example. [Figure 6] This graph shows the relationship between the Γ phase thickness and adhesion (after bending and unbending) for the example. [Figure 7] This graph shows the relationship between the aspect ratio of the surface crystal and adhesion (after ironing) in the example.

Best Mode for Carrying Out the Invention

[0010] The inventors have intensively studied an alloyed hot-dip galvanized steel sheet (and a member including the alloyed hot-dip galvanized steel sheet) capable of suppressing the occurrence of appearance defects while achieving both strength and workability. First, in order to obtain a high strength of 1180 MPa or more, it is important that the metal structure constituting the steel material has a structure mainly composed of a hard martensite phase. For this reason, after setting an appropriate component composition, in the plating process and the alloying process, the austenite phase is controlled to be the main phase, and after alloying heating, rapid cooling (quenching) is performed to obtain a martensite phase.

[0011] Regarding workability, it is important to improve the slidability and plating adhesion on the surface of the plating layer. Excellent slidability can be obtained by suppressing the formation of a soft phase called the ζ phase that easily adheres to the die on the surface of the plating layer. In order to suppress the formation of the ζ phase, the alloying heating may be carried out under conditions of high temperature and rapid temperature rise. Excellent plating adhesion can be obtained by suppressing the formation of a hard and brittle Fe alloyed phase called the Γ phase in the plating layer and reducing the thickness of the Γ phase. This is because if the Γ phase has a thickness of a certain amount or more, the plating will peel off during press forming. In order to suppress the excessive formation of the Γ phase and suppress the formation of the ζ phase, the heating to the alloying temperature may be carried out under conditions of rapid temperature rise. Therefore, by controlling the chemical composition and manufacturing conditions to suppress the formation of the ζ phase and the growth of the Γ phase, excellent workability can be obtained.

[0012] However, when rapid heating is carried out to ensure workability, surface defects as shown in FIG. 1 may occur on the surface layer of the plating layer. FIG. 1 is a schematic diagram showing the surface of a plating layer in which spot-like metallic luster portions a are developed. As shown in FIG. 1, each of the spot-like metallic luster portions a has a circle-equivalent diameter of about 2 to 5 mm based on area measurement and is developed in a row. The inventors analyzed the appearance defect accompanied by a large number of local metallic luster portions a generated during rapid alloying heating.

[0013] FIG. 2 is a schematic diagram showing a cross section in the vicinity of the metallic luster portion a developed in the plating layer for the alloying hot-dip galvanized steel sheet 1001. As shown in FIG. 2, outside the metallic luster portion a, the alloying reaction proceeds sufficiently and an Fe-Zn alloy layer b (healthy portion) is formed, whereas in the metallic luster portion a, the alloying reaction does not proceed locally, and an initial alloy layer a1 composed of Fe and Al remains, and it was found that the Fe-Zn alloy layer a2 (defective portion) is extremely thin.

[0014] The inventors speculated on the reason for the above events as follows. The steel material 1011 serving as the plating base material contains microscopic variations in structure, composition, etc. In particular, when a C-deficient layer (decarburized layer) or alloy element-deficient layers such as Si and Mn occur on the surface layer of the steel sheet, the crystal grains in these deficient layers grow significantly when heated to a high temperature in an annealing process or the like. Further, when heating is instantaneously performed by rapid heating, the alloying reaction is unlikely to occur during the heating process, and the alloying reaction proceeds rapidly at a higher temperature. Therefore, when a steel material with variations in structure, composition, etc. is used, when the alloying reaction proceeds rapidly, the alloying reaction proceeds sufficiently at relatively reactive portions, but the progress of the alloying reaction is delayed at less reactive portions. At that time, since Zn of the plating component is consumed by the surrounding alloying reaction, the plating becomes extremely thin at the less reactive portion, resulting in an appearance defect as a metallic luster portion.

[0015] In fact, the inventors confirmed that no significant appearance defects occur when the heating rate is low. This is thought to be because, at a low heating rate, the alloying reaction proceeds slowly even during heating, so even if there are some microscopic variations in reactivity and some areas where the alloying reaction proceeds slowly, alloying begins before the surrounding areas where the alloying reaction proceeds quickly have fully progressed.

[0016] From the above studies, the inventors have found that improving the alloying rate is an effective heating method that prevents surface defects even when rapid heating is performed. Furthermore, they have found that it is not necessary to selectively improve the alloying rate only in the poorly reactive areas with a diameter of several mm where surface defects occur, but that improving the average alloying rate across the entire plating layer is sufficient to achieve the desired effect.

[0017] The present invention was completed based on the above findings. Hereinafter, an alloyed hot-dip galvanized steel sheet according to an embodiment of the present invention will be described. In this specification, "excellent workability" means "excellent plating adhesion and sliding properties." Furthermore, unless otherwise specified, the percentages used for components refer to "mass percent."

[0018] The alloyed hot-dip galvanized steel sheet 1 according to this embodiment is a steel sheet used as a component. Figure 3 is a cross-sectional view of the alloyed hot-dip galvanized steel sheet 1 according to this embodiment. The alloyed hot-dip galvanized steel sheet 1 has a plating layer 13 containing Zn on the surface of a steel material 11 having a predetermined chemical composition. The following describes the chemical composition of steel materials.

[0019] (C: 0.10~0.35%) Carbon (C) is an essential element for obtaining the desired tensile strength. If the C content is less than 0.10%, the desired tensile strength cannot be obtained, so the C content is 0.10% or more, preferably 0.13% or more, 0.15% or more, or 0.20% or more. On the other hand, if the carbon content exceeds 0.35%, the hydrogen embrittlement resistance and weldability of the steel material will decrease. Therefore, the carbon content should be 0.35% or less, preferably 0.33% or less, or 0.30% or less.

[0020] (Si: 0.01~2.00%) If the Si content is low, Si-containing internal oxides cannot be formed during the cold rolling process, and the surface grains tend to coarseen. Therefore, the Si content is 0.01% or more, preferably 0.02% or more, and 0.03% or more. On the other hand, if the Si content exceeds 2.00%, a film-like oxide containing Si covers the surface of the steel material, delaying the progress of the alloying reaction. As a result, it becomes difficult to obtain a good appearance. Therefore, the Si content is 2.00% or less, preferably 1.90% or less, 1.50% or less, and more preferably 1.00% or less.

[0021] (Mn: 2.8~4.0%) Mn is an austenite-stabilizing element and is effective in improving the hardenability of steel materials. If the Mn content is less than 2.8%, hardening tends to be insufficient and the desired tensile strength cannot be obtained. Therefore, the Mn content should be 2.8% or more, preferably 2.9% or more, 3.0% or more, and more preferably 3.2% or more. If the Mn content exceeds 4.0%, Mn-containing oxides that hinder the alloying reaction are formed on the surface of the steel sheet, and variations in the alloying rate due to the distribution of Mn oxides tend to occur, resulting in a decrease in appearance quality. Therefore, the Mn content should be 4.0% or less, preferably 3.9% or less, 3.8% or less, or 3.5% or less.

[0022] (P: 0~0.100%) P is a solid solution strengthening element and is effective in increasing the strength of steel materials. However, if the P content exceeds 0.100%, the weldability and toughness of the steel material will decrease, so it should be 0.100% or less, preferably 0.050% or less, 0.040% or less, or 0.030% or less. The P content may be 0%, 0.0001% or more, or 0.001% or more.

[0023] (S: 0~0.100%) S is an impurity element, and a lower amount is preferable. However, if the S content exceeds 0.100%, it forms MnS in the steel material, degrading toughness and hole-expanding properties. Therefore, the S content should be 0.100% or less, preferably 0.090% or less, 0.080% or less, 0.055% or less, 0.030% or less, or 0.020% or less. The S content may be 0%, 0.0001% or more, or 0.001% or more.

[0024] (N: 0~0.020%) N is an impurity element, and the less it is present, the better. If the N content exceeds 0.020%, coarse nitrides are formed in the steel material, reducing its hole-expanding properties. Therefore, the N content should be 0.020% or less, preferably 0.015% or less, 0.012% or less, or 0.010% or less. The N content may be 0%, 0.0001% or more, or 0.001% or more.

[0025] (Al: 0.001~1.500%) Al is an element added for deoxidation. To obtain the desired effect, the Al content should be 0.001% or higher. Preferably, the Al content is 0.003% or higher, 0.005% or higher, 0.008% or higher, or 0.010% or higher. On the other hand, sufficient effects can be obtained even with an Al content of 1,500% or less, so an increase in cost can be avoided by keeping the Al content at 1,500% or less. Furthermore, by keeping the Al content at 1,500% or less, it is possible to avoid an increase in the load during hot rolling due to an increase in the transformation temperature of the steel material. Therefore, the Al content should be 1,500% or less, preferably 1,000% or less, 0,500% or less, 0,300% or less, and more preferably 0,200% or less.

[0026] (0:0~0.010%) If the O content exceeds 0.010%, various oxides may be formed, which can adversely affect the mechanical properties of the steel sheet. Therefore, the upper limit of the O content is 0.010%, preferably 0.008% or less, 0.006% or less, or 0.005% or less. The O content may be 0%, 0.0001% or more, or 0.001% or more.

[0027] Cr, Mo, B, Ti, Nb, V, Ni, Cu, and REM are optional elements, and their respective lower limits are 0%. The preferred lower and upper limits for each element when included are explained below.

[0028] (Cr: 0.001~0.80%) When the Cr content is 0.001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Cr content may also be 0.01% or higher, 0.05% or higher, or 0.10% or higher. By keeping the Cr content below 0.80%, the inhibition of the plating alloying reaction is suppressed. Therefore, the upper limit of the Cr content is preferably 0.80%, but it may also be 0.70% or less, 0.60% or less, or 0.50% or less.

[0029] (Mo: 0.001~1.00%) When the Mo content is 0.001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Mo content may also be 0.01% or higher, 0.05% or higher, or 0.10% or higher. By keeping the Mo content below 1.00%, the inhibition of the plating alloying reaction is suppressed. Therefore, the upper limit of the Mo content is preferably 1.00%, but it may also be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0030] (B: 0.0001~0.0100%) When the B content is 0.0001% or more, various properties of the steel material such as strength, hole-expandability, or ductility can be improved. The B content may also be 0.0003% or more, 0.0005% or more, or 0.0010% or more. Since sufficient effects can be obtained even with a B content of 0.0100% or less, the upper limit of the B content is preferably 0.0100%, but it may also be 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0031] (Ti: 0.0001~0.1000%) When the Ti content is 0.001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Ti content may also be 0.0003% or higher, 0.0005% or higher, 0.0010% or higher, or 0.0020% or higher. By setting the Ti content to 0.1000% or less, it is possible to suppress the increase in recrystallization temperature, which leads to non-uniformity of the metal structure of the cold-rolled steel sheet and impairs its bendability. Therefore, the upper limit is preferably 0.1000%, but it may also be 0.0800% or less, 0.0600% or less, or 0.0500% or less.

[0032] (Nb: 0.0001~0.4000%) When the Nb content is 0.0001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Nb content may be 0.001% or higher, or 0.005% or higher. By limiting the Nb content to 0.4000% or less, it is possible to suppress the increase in recrystallization temperature, which leads to non-uniformity of the metal structure of the cold-rolled steel sheet and impairs its bendability. Therefore, the upper limit is preferably 0.4000%, but it may also be 0.2000% or less, 0.1000% or less, or 0.0500% or less.

[0033] (V: 0.0001~0.50%) When the V content is 0.0001% or more, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The V content may also be 0.001% or more, 0.005% or more, or 0.010% or more. By keeping the V content below 0.50%, it is possible to suppress the rise in recrystallization temperature, which can lead to non-uniformity of the metal structure of the cold-rolled steel sheet and impair its bendability. Therefore, the upper limit is preferably 0.50%, but it may also be 0.30% or less, 0.20% or less, or 0.10% or less.

[0034] (Ni: 0.0001~1.0000%) When the Ni content is 0.0001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Ni content may also be 0.001% or higher, 0.005% or higher, 0.010% or higher, 0.050% or higher, or 0.100% or higher. Since sufficient effects can be obtained even with a Ni content of 1.0000% or less, the upper limit of the Ni content is preferably 1.0000%, but it may also be 0.8000% or less, 0.5000% or less, or 0.3000% or less.

[0035] (Cu: 0.001~1.00%) When the Cu content is 0.001% or higher, various properties of the steel material, such as strength, hole-expandability, or ductility, can be improved. The Cu content may also be 0.005% or higher, 0.010% or higher, 0.050% or higher, or 0.10% or higher. By limiting the Cu content to 1.00% or less, it is possible to suppress the increase in recrystallization temperature, which can lead to non-uniformity of the metal structure of the cold-rolled steel sheet and impair its bendability. Therefore, the upper limit is preferably 1.00%, but it may also be 0.80% or less, 0.50% or less, or 0.30% or less.

[0036] (REM: 0.0003~0.0100%) If the REM content is 0.0003% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. The REM content may also be 0.0005% or higher, or 0.0010% or higher. Since sufficient effects can be obtained even with a REM content of 0.0100% or less, the upper limit of the REM content is preferably 0.0100%, but it may also be 0.0090% or less, 0.0080% or less, 0.0050% or less, or 0.0030% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and REM content refers to the total content of these elements.

[0037] Furthermore, other optional elements such as As, Sb, Sn, W, Co, Zn, Zr, Mg, Ca, Ta, Bi, and Te may also be included. The lower limit for each is 0%. The preferred lower and upper limits for each element when included will be explained below.

[0038] (As: 0.001~0.200%) As is an element that can be introduced when scrap is used as a raw material. Because it can strongly segregate at grain boundaries, potentially leading to grain boundary embrittlement, reduced ductility, and decreased cold formability, the upper limit is preferably 0.200%. However, excessive removal would increase the number of processes and steps, so the content may be 0.001% or more.

[0039] (Sb: 0.001~0.200%) Sb is an element that can be introduced when scrap is used as a raw material. Because it can strongly segregate at grain boundaries, potentially leading to grain boundary embrittlement, reduced ductility, and decreased cold formability, the upper limit is preferably 0.200%. However, excessive removal would increase the number of processes and steps, so the content may be 0.001% or more.

[0040] (Sn: 0.001~0.20%) Sn is an element that can be introduced when scrap is used as a raw material. Because it can strongly segregate at grain boundaries, potentially leading to grain boundary embrittlement, reduced ductility, and decreased cold formability, the upper limit is preferably 0.20%. However, excessive removal would increase the number of processes and steps, so the content may be 0.001% or more.

[0041] (W: 0.001~0.100%) If the W content is 0.001% or higher, the strength of the steel material can be improved. If the W content exceeds 0.100%, it leads to a decrease in ductility and reduces the cold workability of the steel sheet; therefore, the upper limit is preferably 0.100%.

[0042] (Co: 0.01~2.0%) If the Co content is 0.01% or higher, the strength of the steel material can be improved. If the Co content exceeds 2.0%, it leads to a decrease in ductility and reduces the cold workability of the steel sheet; therefore, the upper limit is preferably 2.0%.

[0043] (Zn: 0.0005~0.100%) If the Zn content exceeds 0.100%, coarse Zn oxides are formed, reducing cold formability; therefore, the upper limit is preferably 0.100%. The lower limit of the Zn content may be substantially 0%, or it may be 0.0005%.

[0044] (Zr: 0.0010~0.0500%) When the Zr content is 0.0010% or higher, it can contribute to improving the bendability of steel sheets. If the Zr content exceeds 0.0500%, coarse Zr oxides are formed, reducing cold formability; therefore, the upper limit is preferably 0.0500%.

[0045] (Mg: 0.001~0.050%) If the Mg content is 0.001% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. Since sufficient effects can be obtained even with an Mg content of 0.050% or less, it is preferable that the upper limit of the Mg content be 0.050%.

[0046] (Ca: 0.001~0.050%) If the Ca content is 0.001% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. Since sufficient effects can be obtained even with a Ca content of 0.050% or less, the upper limit of the Ca content is preferably 0.050%.

[0047] (Ta: 0.001~0.100%) If the Ta content is 0.001% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. Since sufficient effects can be obtained even with a Ta content of 0.100% or less, it is preferable that the upper limit of the Ta content be 0.100%.

[0048] (Bi: 0.001~0.050%) If the Bi content is 0.001% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. Since sufficient effects can be obtained even with a Bi content of 0.050% or less, it is preferable that the upper limit of the Bi content be 0.050%.

[0049] (Te: 0.001~0.050%) If the Te content is 0.001% or higher, the strength and flexibility can be improved by adjusting the shape of the inclusions. Since sufficient effects can be obtained even with a Te content of 0.050% or less, the upper limit of the Te content is preferably 0.050%.

[0050] (The remainder is Fe and impurities) The remainder consists of Fe and impurities. The impurities are elements that are introduced from the steel raw materials and / or during the steelmaking process and whose presence is permissible as long as they do not impair the properties of the steel sheet according to the embodiment of the present invention.

[0051] The chemical composition of steel materials can be measured using general analytical methods. For example, the chemical composition of the steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on the chips in accordance with JIS G 1201:2014. Specifically, for example, a 35mm square test piece can be obtained from approximately the 1 / 4 thickness point of the steel material, and the specific location can be identified by measuring it using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, should be measured using combustion-infrared absorption spectroscopy, N using inert gas fusion-thermal conductivity spectroscopy, and O using inert gas fusion-nondispersive infrared absorption spectroscopy. The plating layer on the surface of the steel material can be removed by mechanical grinding or other means before the chemical composition can be analyzed.

[0052] Furthermore, the thickness of the steel material is not particularly limited and can be between 0.6 mm and 5.0 mm, preferably between 1.0 mm and 3.0 mm or between 1.2 mm and 2.4 mm.

[0053] The plating layer, for example, contains Fe: 7.0-15.0% and Al: 0.1-1.0%, with the remainder being Zn and impurities. Furthermore, impurities refer to components contained in the raw materials or components introduced during the manufacturing process, and not components that were intentionally included. For example, trace amounts of components other than Fe may be mixed into the plating layer as impurities due to atomic diffusion between the base steel material and the plating bath. Also, the above content in the plating layer refers to the content in the plating layer as a whole, and not the content in localized parts such as parts corresponding to specific phases within the plating layer.

[0054] In this embodiment, the plating layer is an alloyed hot-dip galvanized layer. In the plating layer, the remainder other than Fe, Al, and impurities is Zn. Preferably, the impurity content in the plating layer is 3.0% or less.

[0055] When the Fe content of the plating layer is 7.0% or more, the formation of a soft ζ phase is suppressed, and adhesion between the mold and the plating layer during press molding can be suppressed. Preferably, the Fe content in the plating layer is 8.0% or more, and more preferably 9.0% or more. When the Fe content of the plating layer is 15.0% or less, the growth of the Γ phase can be suppressed, and the adhesion of the plating can be ensured. Preferably, the Fe content in the plating layer is 13.0% or less, and more preferably 12.0% or less.

[0056] When the Al content of the plating layer is 0.1% or more, the growth of the Γ phase can be suppressed, and a plating layer with excellent adhesion can be obtained. The Al content in the plating layer is preferably 0.15% or more, and more preferably 0.2% or more. When the Al content of the plating layer is 1.0% or less, the temperature required for alloying heating can be reduced, improving the appearance quality after plating. Preferably, the Al content in the plating layer is 0.6% or less, and more preferably 0.5% or less.

[0057] The chemical composition of the plating layer can be measured using general analytical methods. For example, the plating layer of a sample is dissolved in dilute hydrochloric acid containing a commercially available inhibitor (e.g., Asahi Chemical's "Ibit 710K," concentration 0.04%), and the amount of Fe contained in the dilute hydrochloric acid after the plating layer has been dissolved is measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, the amount of plating deposited (g / m²) can be determined from the mass difference of the sample before and after the plating layer has been dissolved. 2 The amount of Fe in the plating is determined by calculating the ratio of Fe content to plating adhesion. Other elements in the plating layer can also be measured using the same procedure.

[0058] (Γ phase thickness 1.00 μm or less) The Γ phase is an Fe alloying phase that forms near the interface with the steel material in the plating layer. Because the Γ phase is hard and brittle, if it exceeds a certain thickness, the plating may peel off during press forming. If the thickness of the Γ phase is 1.00 μm or less, plating peeling during press molding is suppressed, and excellent plating adhesion is achieved. The thickness of the Γ phase is preferably 0.90 μm or less, and more preferably 0.80 μm or less.

[0059] The thickness of the Γ phase can be measured using electrochemical methods. First, a sample of a hot-dip galvanized steel sheet, with all areas except the φ20 mm diameter measurement area on the surface of the plating layer masked with tape, is immersed in a 150 g / L NH4Cl aqueous solution, and a potential of -0.940 V is applied to the sample. Once the current stops flowing, remove the sample from the NH4Cl aqueous solution and rub the surface of the sample's measurement area vigorously with cotton wool to remove any residue. Subsequently, the sample was immersed again in NH4Cl aqueous solution and tested at 4 mA / cm². 2 Apply a voltage to the sample to achieve the specified current density. The measurement is terminated when the potential stabilizes at -0.55V. The time t1 (s) from the start of dissolution until the potential becomes constant at -0.55V is measured, and the thickness d (μm) of the Γ phase is determined by the following equation (1). d = t1 × 4 × 4.448 …(1)

[0060] (Aspect ratio of crystal grains observed from the surface of the plating layer is 4.0 or less) The ζ phase is a soft Fe-Zn alloy phase that forms in the plating layer. Because the ζ phase on the surface of the plating layer is soft, it can adhere to the mold during press forming, causing the plating to peel off. In some cases, insufficient sliding properties between the mold and the surface of the plating layer during press forming can cause the steel material to crack along with the plating. By suppressing the formation of the ζ phase on the surface of the plating layer, sliding properties during press forming can be improved.

[0061] In this embodiment, the presence of the ζ phase on the surface of the plating layer is determined by the morphology of the crystal grains. The ζ phase is a columnar crystal with a large aspect ratio. If the aspect ratio of the crystal grains on the surface of the plating layer is 4.0 or less, it can be said that the formation of the ζ phase is sufficiently suppressed. By controlling the aspect ratio of the crystal grains on the surface of the plating layer to 4.0 or less, the sliding properties of the alloyed hot-dip galvanized steel sheet can be improved. The aspect ratio of the crystal grains on the surface of the plating layer is preferably 3.5 or less, 2.0 or less, or 1.5 or less, and more preferably 1.0 or less.

[0062] The aspect ratio of crystal grains on the surface of the plating layer can be measured as follows. The surface of the plating layer of an alloyed hot-dip galvanized steel sheet was ultrasonically cleaned with acetone, and then an SEM (JEOL JSM-7001F, accelerating voltage 20kV) was used to obtain an SEM image at a magnification of 1000x. In the SEM images, more than 20 crystal grains that are judged to have a larger aspect ratio by visual inspection are selected, and the aspect ratio of each selected crystal grain is measured. The aspect ratio measurement function of the image analysis software "Image J 1.54f" is used to measure the aspect ratio. If more than 20 crystal grains cannot be observed in the captured 1000x SEM image, change the scanning range and take additional SEM images under the same conditions for observation. If necessary, take additional SEM images until more than 20 crystal grains can be observed. For the measured aspect ratios, calculate the average of the aspect ratios of the top 20 points in descending order of aspect ratio, and define this value as the "aspect ratio of crystal grains on the surface of the plating layer."

[0063] (Tensile strength of 1180 MPa or more) The alloyed hot-dip galvanized steel sheet according to this embodiment has a tensile strength of 1180 MPa (HV: 370) or higher. Preferably, the tensile strength is 1300 MPa (HV: 410) or higher, and more preferably, the tensile strength is 1450 MPa (HV: 460) or higher. There is no particular upper limit to the tensile strength of the alloyed hot-dip galvanized steel sheet according to this embodiment, but the practical upper limit is 2000 MPa (HV: 650). The Vickers hardness (HV) was measured at a position 1 / 4 of the thickness from the surface of the steel sheet in the cross-section of the test specimen, under a load of 0.490 N.

[0064] To obtain an alloyed hot-dip galvanized steel sheet with a strength of 1180 MPa or higher, which is the subject of the present invention, it is preferable that the steel material contains hard martensite as the main phase. The steel material may contain trace amounts of bainite, ferrite, and austenite, but since these can reduce strength, it is preferable that at least 90% by volume, preferably 95% by volume or more, be martensite. The martensite fraction (by volume) in the steel material is a value estimated from the tensile strength of the alloyed hot-dip galvanized steel sheet. If the tensile strength of the alloyed hot-dip galvanized steel sheet is 1180 MPa or higher, it can be estimated that the martensite fraction in the steel material is 90% or higher.

[0065] The alloyed hot-dip galvanized steel sheet according to this embodiment has a superior appearance because the occurrence of metallic luster on the surface, which tends to occur when the heating rate during alloying is high, is suppressed. The metallic luster is due to the coarsening of the crystal grains in the surface layer of the steel material. In the alloyed hot-dip galvanized steel sheet according to this embodiment, the average crystal grain size in the surface layer of the steel material is preferably 15 μm or less in terms of equivalent circle diameter, and more preferably 12 μm or less, 10 μm or less, or 9 μm or less. There is no particular lower limit to the average grain size in the surface layer of the steel material, but the average grain size is in terms of the equivalent diameter of a circle, and may be, for example, 1 μm or more, or 2 μm or more.

[0066] The average grain size of the surface layer of a steel material can be measured as follows: The plating layer of the sample is removed with dilute hydrochloric acid containing a commercially available inhibitor (e.g., "Ibit 710K" from Asahi Chemical, concentration 0.04%), and the microstructure of the outermost surface of the steel material is analyzed by analyzing this sample without polishing using EBSD (Electron Backscatter Diffraction). Using an FE-SEM (JEOL JSM-7001F, accelerating voltage 20kV) equipped with an EBSD detector (TSL OIM), the crystal orientation is analyzed at 80x magnification for a 1000μm × 250μm region with a measurement pitch of 1μm. Using the EBSD's accompanying software, OIM ANALYSIS, the boundaries of regions with a crystal orientation difference of 5° or more are considered grain boundaries, and the equivalent diameter of a circle is measured based on the area of ​​each crystal grain when the region enclosed by the grain boundaries is considered a crystal grain. The particle size distribution of the measured equivalent circle diameter is evaluated as an area fraction. Measurement and analysis, including the measurement of the equivalent circle diameter and the analysis of the particle size distribution, can be performed using the "OIM ANALYSIS" software included with the EBSD detector (TSL OIM).

[0067] An example of a method for manufacturing alloyed hot-dip galvanized steel sheet according to this embodiment will be described below. An example of the manufacturing process includes (A) hot rolling, (B) pickling and cold rolling, (C) annealing, (D) plating, (E) alloying, and (F) quenching and tempering. Each process is described below.

[0068] (A) Hot rolling process First, a slab of steel material having the chemical composition described above is heated. The slab is not particularly limited as long as it has the chemical composition described above. The slab may be cast by a continuous casting method. The heating temperature of the slab is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower. After heating the slab, rough rolling and finish rolling are performed as appropriate, followed by cooling to obtain a hot-rolled steel sheet of a predetermined thickness. The temperature at which the finish rolling is completed is preferably in the range of 860 to 960°C. After the finish rolling is completed, the hot-rolled steel sheet is cooled to a temperature range of 450 to 700°C and then wound up. The thickness of the sheet after finish rolling is preferably in the range of 2.0 to 4.0 mm.

[0069] (The parameter f(t,[Si]), consisting of the residence time t(s) at 500°C or higher after winding of the hot-rolled steel sheet and the Si content [Si](%) in the steel, is between 1.30 and 4.80.) After hot-rolled steel sheets are wound, oxides of easily oxidizable elements form on the surface of the steel sheet, reducing the solid solution concentration of these easily oxidizable elements. When the solid solution concentration of Si, an easily oxidizable element, decreases on the surface of the steel sheet, a Si-deficient layer is formed on the surface. If this Si-deficient layer is thick, it becomes difficult for internal oxides containing Si to form on the surface of the steel sheet during the subsequent annealing process, and the crystal grains on the surface of the steel sheet tend to coarseen. The Si-deficient layer on the surface of the steel sheet is formed when the hot-rolled steel sheet is held at a relatively high temperature (500°C or higher) for a long period of time after being rolled. Furthermore, if the Si concentration in the steel sheet is low, internal oxides containing Si are less likely to form in the annealing process (C) described later.

[0070] Therefore, by setting the parameter f(t,[Si]), which consists of the residence time t(s) at 500°C or higher after winding of the hot-rolled steel sheet as shown in equation (2) and the amount of Si in the steel [Si](%), to 4.80 or less, the coarsening of crystal grains on the surface of the steel sheet can be suppressed in the subsequent annealing process. In other words, by shortening the residence time t(s) and controlling the balance with [Si](%), the coarsening of crystal grains on the surface of the steel sheet can be suppressed. By suppressing the coarsening of crystal grains on the surface of the steel sheet, even in the case of rapid heating, variations in the progress of the alloying reaction are less likely to occur, and the occurrence of appearance defects in alloyed hot-dip galvanized steel sheets can be suppressed. f(t,[Si])=Log 10 (t+10) / [Si] 0.095 …(2) The parameter f(t,[Si]) is preferably 4.70 or less, and preferably 4.60 or less. It is more preferable that the value is 4.50 or less, 4.40 or less, 4.30 or less, or 4.10 or less, and even more preferable that it is 3.50 or less.

[0071] To more effectively suppress the occurrence of appearance defects, it is preferable that f(t,[Si]) be smaller. However, making it excessively small would require strong cooling, such as immersion in water after winding. In that case, large-scale equipment would be required, which is not practical, and it may also worsen the shape of the steel sheet and reduce its subsequent passability. Furthermore, by setting the winding temperature to 500°C or lower, the value of the residence time t at temperatures above 500°C can be reduced, but the winding temperature may be limited by the amount of Si in the steel. It is preferable that the lower limit of f(t,[Si]) be 1.30.

[0072] The residence time t at temperatures above 500°C can be adjusted by controlling the winding temperature during hot rolling, controlling the coil weight, or a combination thereof. In addition, the residence time t at temperatures above 500°C may be adjusted as needed by cooling the coil by spraying water mist onto it or by keeping the coil warm by covering it with an insulating cover.

[0073] (B) Pickling and cold rolling process The hot-rolled steel sheet obtained through the hot-rolling process is pickled to remove oxides and other contaminants, and then cold-rolled to the desired thickness to obtain a cold-rolled steel sheet. Note that heat treatment may be performed either before or after pickling prior to cold rolling.

[0074] (C) Annealing process Next, the cold-rolled steel sheet is annealed under the following annealing conditions. The lower limit of the annealing temperature is 800°C or higher, preferably 820°C or higher. If the annealing temperature is below 800°C, the ferrite phase remains at the annealing temperature, and an austenite fraction of 90% or more cannot be obtained. Therefore, it becomes difficult to obtain a martensite-dominant structure in the steel material, and the desired tensile strength cannot be achieved. If the annealing temperature exceeds 880°C, excessive energy will be consumed, which may damage the furnace body.

[0075] The annealing atmosphere is a nitrogen atmosphere containing 2% to 30% hydrogen. If the hydrogen concentration in the annealing atmosphere is too low, the oxide film on the surface of the cold-rolled steel sheet will not be reduced, making it difficult for the plating to adhere in the plating process described later. On the other hand, there is no need to raise the hydrogen concentration in the annealing atmosphere more than necessary, and it would also increase costs, so the upper limit is set at 30%.

[0076] The dew point of the annealing atmosphere should be between -30°C and 15°C. Preferably, the dew point of the annealing atmosphere should be above -25°C, and preferably below 0°C. At a dew point of -30°C or lower, internal oxides are not formed in the cold-rolled steel sheet, and the crystal grains on the surface of the cold-rolled steel sheet become coarser. At a dew point of 15°C or higher, the oxide film on the surface of the cold-rolled steel sheet is not sufficiently reduced, or an excessively thick decarburized layer is formed, resulting in a decrease in the strength of the steel material. There are no particular restrictions on the cooling conditions after annealing.

[0077] (D) Plating process A cold-rolled steel sheet that has undergone an annealing process is subjected to zinc plating to obtain a hot-dip galvanized steel sheet. The method of plating is not particularly limited, but plating can be applied by immersing the cold-rolled steel sheet in a plating bath. The plating bath is a Zn bath containing 0.12% to 0.15% Al. The Al concentration in the plating bath is preferably 0.135% or more, and preferably 0.145% or less. If the Al concentration in the plating bath is low, the bottom dross that forms in the plating bath will increase, which can lead to a poor appearance. On the other hand, if the Al concentration in the plating bath is high, high-temperature heating is required in the alloying process described later, which can easily lead to defects in appearance. The plating bath may contain additive elements such as Fe, Mg, Si, Ti, Sb, Sn, Pb, Ca, and other impurities.

[0078] The temperature of the plating bath shall be 440°C or higher and 480°C or lower. The temperature of the plating bath is preferably 450°C or higher and 470°C or lower. If the temperature of the plating bath is lower than 440°C, the low-temperature part near the bath surface will solidify, which is likely to cause poor appearance. On the other hand, if the temperature of the plating bath is higher than 480°C, zinc is likely to evaporate and the plating raw material will be depleted. The plate temperature when the cold-rolled steel sheet enters the plating bath shall be 400°C or higher and 490°C or lower. The plate temperature is preferably 440°C or higher and 470°C or lower. If the plate temperature when the cold-rolled steel sheet enters the plating bath is 400°C or lower, the elution amount of Fe from the cold-rolled steel sheet will decrease, and plating defects called non-plating, where partial plating does not form, may occur. If the plate temperature is 490°C or higher, a very dense Fe-Zn alloy is formed in the plating bath, which may inhibit the reaction between Fe and molten Zn, and the alloying treatment property will decrease in the alloying process described later.

[0079] The coating weight of the plating is 30~80 g / m 2 and preferably 40~60 g / m 2 . If the coating weight is 30 g / m 2 or less, it is difficult to obtain sufficient corrosion resistance. On the other hand, if the coating weight is 80 g / m 2 or more, the plating thickness will be excessive and the cost will increase.

[0080] (E) Alloying process The galvanized steel sheet obtained by the plating process is subjected to an alloying treatment to obtain an alloyed galvanized steel sheet. In the alloying treatment, the heating rate when heating to the alloying temperature shall be 50°C / s or higher and 400°C / s or lower. If the heating rate is less than 50°C / s, it is difficult to suppress both the formation of the Γ phase and the formation of the ζ phase. To achieve a heating rate exceeding 400°C / s, it is necessary to increase the passing speed of the steel sheet or miniaturize and increase the output of the heating device, which is industrially difficult.

[0081] The alloying temperature is 500°C or higher and 630°C or lower. The alloying temperature is preferably 530°C or higher and 600°C or lower. Below an alloying temperature of 500°C, the ζ phase crystallizes, making it difficult to control the aspect ratio of the crystal grains on the surface of the plating layer to 4.0 or less. Above an alloying temperature of 630°C, the alloying reaction proceeds rapidly in some parts of the plating layer, leading to appearance defects and reduced plating adhesion during press forming. Any method can be used to heat the hot-dip galvanized steel sheet at the alloying temperature, such as electric heating or induction heating.

[0082] (F) Hardening and tempering process After the alloying process, the alloyed hot-dip galvanized steel sheet may be rapidly cooled for quenching, and then heated for tempering as needed. This makes it possible to obtain a martensite-based alloyed hot-dip galvanized steel sheet with a desired strength-ductility balance. The quenching end temperature can be set arbitrarily, but may be set to, for example, 80°C or lower. The tempering temperature can also be set arbitrarily, but may be set to, for example, 200°C to 350°C. Furthermore, before or after the above-mentioned quenching or tempering, temper rolling may be performed to adjust the surface roughness and strength of the alloyed hot-dip galvanized steel sheet. The elongation of the temper rolling can be set in the range of, for example, 0.1 to 5.0%.

[0083] (Examples) For steel grades A to Y having the chemical compositions shown in Tables 1 and 2, the plating structure and properties as alloyed hot-dip galvanized steel sheets were evaluated when alloying heating was performed under the manufacturing conditions shown in Table 3. In Tables 1 to 3, values ​​outside the scope of the present invention are underlined.

[0084] [Table 1]

[0085] [Table 2]

[0086] Each alloyed hot-dip galvanized steel sheet was manufactured under the following conditions.

[0087] Slabs having the chemical compositions shown in Table 1 and Table 2 were cast, heated to 1150°C or higher, rough rolled, and then finish rolled under the conditions of a total reduction ratio of 50% and a finish rolling completion temperature of 950°C, after which the hot-rolled steel sheets were coiled. The hot-rolled steel sheets after coiling were cooled or kept warm so that the residence time in the temperature range of 500°C or higher became the residence time t shown in Table 3. Thereafter, the hot-rolled steel sheets were cooled, pickled, and cold rolled. The cold-rolled steel sheets were annealed in a nitrogen atmosphere with a hydrogen concentration of 5%, a dew point of -5°C, and an annealing temperature of 850°C. Then, the cold-rolled steel sheets were immersed in a hot-dip galvanizing bath with an Al concentration of 0.135% and a bath temperature of 460°C at a sheet temperature of 460°C to apply hot-dip galvanizing, thereby obtaining hot-dip galvanized steel sheets. The coating weight of the plating was 55 g / m 2 It was.

[0088] The hot-dip galvanized steel sheets obtained as described above were subjected to an alloying treatment under the manufacturing conditions shown in Table 3 to obtain alloyed hot-dip galvanized steel sheets. The sheet thickness of the obtained alloyed hot-dip galvanized steel sheets was 1.6 mm. For the obtained alloyed hot-dip galvanized steel sheets, the tensile strength (MPa) and the crystal grain size (μm) in the surface layer portion of the steel material were measured by the above method.

[0089]

Table 3

[0090] <Appearance> The appearance of each alloyed hot-dip galvanized steel sheet was observed on the surface of each alloyed hot-dip galvanized steel sheet (the surface of the plating layer) and evaluated in 7 grades according to the evaluation criteria of 1 to 7 shown below. An evaluation of 3 or higher was considered a pass. 7: Those without visible unevenness in appearance 6: Products exhibiting pinpoint irregularities in appearance without metallic luster, where the diameter of each pinpoint is less than 1 mm. 5: Items exhibiting pinpoint irregularities in appearance without metallic luster, where the diameter of each pinpoint is 1 mm or more. 4: Metallic luster is present in a dotted, uneven distribution on only a portion of the surface of the plating layer, with each dot having a diameter of less than 1 mm. 3: A metallic luster is observed as a series of dots on only a portion of the surface of the plating layer, with each dot having a diameter of 1 mm or more but less than 3 mm, or A plated layer exhibits pinpoint irregularities with metallic luster across its entire surface, with each pinpoint having a diameter of less than 1 mm. 2: Metallic luster is observed as a series of dots on only a portion of the surface of the plating layer, with each dot having a diameter of 3 mm or more but less than 5 mm. Or, A plated layer exhibits pinpoint irregularities with metallic luster across its entire surface, with each pinpoint having a diameter of 1 mm or more but less than 3 mm. 1: Items exhibiting metallic luster and pinpoint unevenness in appearance, with each pinpoint having a diameter of 5 mm or more, or A plated layer exhibits pinpoint irregularities with metallic luster across its entire surface, with each pinpoint having a diameter of 3 mm or more but less than 5 mm. In the above evaluation criteria, "appearance irregularities" refer to areas on the surface of the plating layer that have a stronger metallic luster than other areas. "Point-like appearance irregularities with metallic luster" correspond to the aforementioned metallic luster areas.

[0091] <Adhesion (after bending and unbending)> The evaluation method for the adhesion of the plating (after bending and unbending) of each alloyed hot-dip galvanized steel sheet will be explained with reference to Figures 4A to 4C. (1) As shown in Figure 4A, a disc-shaped sample 100a with a diameter of 70 mm is obtained from an alloyed hot-dip galvanized steel sheet by punching, then bent at a 90° angle, and then the sample 100a is bent back to obtain sample 100b. (2) As shown in Figure 4B, transparent cellophane tape (Nichiban Co., Ltd. "CT405AP-24") is applied to the inner side of the bend (inner side of the bend, valley fold side) of sample 100b, and then peeled off to obtain a measuring cellophane tape 200 with the plating 201 that has been peeled off linearly along the inner side of the bend from sample 100b attached. (3) As shown in Figure 4C, the measuring cellophane tape 200 is attached to the whiteboard 300, and two locations with the largest width W (width in the direction perpendicular to the linear direction) of the plating 201 that is attached in a linear fashion are selected, and the reflectance (%) of the release tape is measured using a reflectance meter 400 (Tokyo Denshoku Co., Ltd. "TC-6MC-D"). Of the measurements obtained in this way, the larger value was taken as the reflectance (%) of the release tape for the sample. The reflectivity (%) of the release tape decreases as more plating is removed. In other words, a lower percentage of the release tape reflectivity (%) is preferable as less plating is removed. As an indicator of adhesion (after bending and unbending), a release tape reflectivity of 40% or higher was considered acceptable.

[0092] <Adhesion (after wiping process)> The evaluation method for the adhesion of the plating (after ironing) of each alloyed hot-dip galvanized steel sheet will be explained with reference to Figures 5A and 5B. (1) As shown in Figure 5A, a hot-dip galvanized alloy steel sheet, held down on both sides by a bead formed by interlocking the convex portion 510U of the upper die 500U and the concave portion 510L of the lower die 500L, was pressed with a punch 500C to obtain a processed sample 600. The pressing load was set to 1200 kg, the stroke of the punch 500C was 65 mm, and the radius of curvature of both corners of the bottom surface of the convex portion 510U was 1 mm. (2) A transparent cellophane tape (CT405AP-24, manufactured by Nichiban Co., Ltd.) measuring 24 mm in width and 100 mm in length was attached to the lower end of the side wall portion 610 of the ironing sample 600, and then peeled off to obtain a cellophane tape 700 for visual evaluation on which the linearly peeled plating 601 from the ironing sample 600 was attached. (3) The cellophane tape 700 for visual evaluation was attached to a whiteboard, and the adhesion (after wiping) was evaluated visually based on the amount of plating adhering to the cellophane tape 700, in three stages from A to C. An evaluation of A or B was considered acceptable. A: No plating was observed on the cellophane tape used for visual evaluation. B: A small amount of plating was found on the cellophane tape used for visual evaluation (less than 80% of the surface area was covered by the plating). C: A large amount of plating adhered to the cellophane tape used for visual evaluation (plating adhesion area of ​​80% or more).

[0093] The measurement results are shown in Table 4. In Table 4, values ​​outside the scope of the present invention, or evaluation results that do not meet the acceptance criteria, are underlined.

[0094] [Table 4]

[0095] Figure 6 is a graph showing the relationship between Γ phase thickness and adhesion (after bending and unbending). Figure 7 is a graph showing the relationship between the aspect ratio of the surface crystal and sliding properties. From the examples of the invention, it was confirmed that by satisfying the scope defined in this application, an alloyed hot-dip galvanized steel sheet can be obtained that is high in strength, has excellent workability, and suppresses the occurrence of appearance defects. On the other hand, comparative examples that did not meet the scope defined in this application were found to be insufficient to obtain alloyed hot-dip galvanized steel sheets that were high in strength, had excellent workability, and suppressed the occurrence of appearance defects. [Industrial applicability]

[0096] According to this disclosure, it is possible to provide alloyed hot-dip galvanized steel sheets and components including such alloyed hot-dip galvanized steel sheets that are high in strength, have excellent workability, and suppress the occurrence of appearance defects. Such components can be used as automotive components. Examples of automotive components include structural components (skeleton members) of automobiles. [Explanation of symbols]

[0097] 1 steel plate 11 Steel material 13 Plating layer 1001 steel plate 1011 Steel material a. Metallic luster a1 Initial alloy layer a2 Fe-Zn alloy layer

Claims

1. The chemical composition is expressed in mass percent. C: 0.10-0.35%, Si: 0.01-2.00%, Mn: 2.8-4.0%, P: 0-0.100%, S: 0-0.100%, N: 0 to 0.020%, Al: 0.001-1.500%, O: 0 to 0.010%, Cr: 0-0.80%, Mo: 0-1.00%, B: 0 to 0.0100%, Ti: 0 to 0.1000%, Nb: 0 to 0.4000%, V: 0-0.50%, Ni: 0 to 1.0000%, Cu: 0 to 1.00%, REM: 0-0.0100%, As: 0 to 0.200%, Sb: 0 to 0.200%, Sn: 0-0.20%, W: 0-0.100%, Co: 0-2.0%, Zn: 0 to 0.100%, Zr: 0 to 0.0500%, Mg: 0 to 0.050%, Ca: 0-0.050%, Ta: 0-0.100%, Bi: 0-0.050%, and Te: 0-0.050%, including The steel material, whose remainder consists of Fe and impurities, has a plating layer containing Zn on its surface. The average grain size in the surface layer of the steel material is 15 μm or less in terms of the equivalent diameter of a circle. The aspect ratio of the crystal grains on the surface of the plating layer is 4.0 or less. The thickness of the Γ phase in the aforementioned plating layer is 1.0 μm or less. The tensile strength is 1180 MPa or higher. A hot-dip galvanized steel sheet characterized by the following features.

2. The chemical composition of the aforementioned steel material is, in mass%, Cr: 0.001-0.80%, Mo: 0.001-1.00%, B: 0.0003 to 0.0100%, Ti: 0.001 to 0.1000%, Nb: 0.001 to 0.4000%, V: 0.001-0.50%, Ni: 0.001 to 1.0000%, Cu: 0.001 to 1.00%, REM: 0.0003-0.0100%, As: 0.001-0.200%, Sb: 0.001-0.200%, Sn: 0.001-0.20%, W: 0.001-0.100%, Co: 0.01-2.0%, Zn: 0.0005-0.100%, Zr: 0.0010-0.0500%, Mg: 0.001-0.050%, Ca: 0.001-0.050%, Ta: 0.001 to 0.100%, Bi: 0.001 to 0.050%, and Te: 0.001~0.050% The alloyed hot-dip galvanized steel sheet according to claim 1, characterized by containing one or more selected from the group consisting of the above.

3. A member comprising an alloyed hot-dip galvanized steel sheet according to claim 1 or 2.

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