Galvannealed steel sheet and member

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

Galvannealed steel sheets and components

[0001] This application claims priority to Japanese Patent Application No. 2024-062779, filed on April 9, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, further improvements in automobile fuel efficiency have been required in light of greenhouse gas emission regulations as part of global warming countermeasures. To reduce the weight of vehicle bodies and ensure collision safety, the use of high-strength steel sheets in automobile parts is becoming increasingly widespread. Since automobile parts also require rust resistance, galvannealed steel sheets are sometimes used to improve rust resistance.

[0003] For example, Patent Document 1 discloses a method for forming δ １ Phase alone or δ １ phase and Γ with a thickness of less than 1 μm １ The present invention discloses a galvannealed steel sheet characterized by having an Fe—Zn alloy coating consisting of a Fe—Zn alloy phase and having a surface layer crystal with an average aspect ratio of 3 or less.

[0004] Japanese Patent Application Publication No. 3-249162

[0005] The technology of Patent Document 1 is said to be capable of exhibiting excellent surface treatability and powdering resistance in addition to the sliding properties required for press forming. However, the technology of Patent Document 1 does not anticipate producing a high-strength galvannealed steel sheet having a tensile strength of 1180 MPa or more. Therefore, it can be said that the technology of Patent Document 1 has room for improvement in achieving both excellent workability (plating adhesion and sliding properties) and high strength. Furthermore, adjusting the components and manufacturing process to increase strength is conceivable, but depending on the conditions, this may result in poor appearance, such as the appearance of spots of metallic luster on the plating surface.

[0006] Therefore, an object of the present invention is to provide a galvannealed steel sheet that has high strength and excellent formability, and that is suppressed from causing defects in appearance, and a member that includes the galvannealed steel sheet.

[0007] In order to solve the above problems, the present invention proposes the following means. (1) A first aspect of the present invention is a steel sheet having a chemical composition, in mass%, of C: 0.10 to 0.35%, Si: 0.01 to 2.00%, Mn: 2.8 to 4.0%, P: 0 to 0.100%, S: 0 to 0.100%, N: 0 to 0.020%, Al: 0.001 to 1.500%, O: 0 to 0.010%, Cr: 0 to 0.80%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.1000%, Nb: 0 to 0.4000%, V: 0 to 0.50%, Ni: 0 to 1.0000%, Cu: 0 to 1.00%, REM: 0 to 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: 0-0.100%, 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 balance being Fe and impurities. The steel sheet has a Zn-containing plating layer on the surface of the steel material, wherein the aspect ratio of 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 galvannealed steel sheet according to (1) above, wherein the chemical composition of the steel material is, in mass %, Cr: 0.001 to 0.80%, Mo: 0.001 to 1.00%, B: 0.0003 to 0.0100%, Ti: 0.001 to 0.1000%, Nb: 0.001 to 0.4000%, V: 0.001 to 0.50%, Ni: 0.001 to 1.0000%, Cu: 0.001 to 1.00%, REM: 0.0003 to 0.0100%, As: 0.001 to 0.20 (3) A second aspect of the present invention is a member comprising the galvannealed steel sheet according to (1) or (2) above.

[0008] According to the above-described aspects of the present invention, it is possible to provide a galvannealed steel sheet that has high strength and excellent formability, and in which the occurrence of defective appearance is suppressed, and a member including the galvannealed steel sheet.

[0009] FIG. 1 is a schematic diagram showing a surface layer of a coating layer in which spot-like metallic luster portions are formed. FIG. 2 is a schematic diagram showing a cross section of a galvannealed steel sheet in the vicinity of a metallic luster portion formed in the coating layer. FIG. 3 is a cross-sectional view of a galvannealed steel sheet according to an embodiment. FIG. 4 is a first schematic diagram for explaining a method for evaluating coating adhesion (after bending and bending back) of a galvannealed steel sheet in an example. FIG. 5 is a second schematic diagram for explaining a method for evaluating coating adhesion (after bending and bending back) of a galvannealed steel sheet in an example. FIG. 6 is a third schematic diagram for explaining a method for evaluating coating adhesion (after bending and bending back) of a galvannealed steel sheet in an example. FIG. 7 is a first schematic diagram for explaining a method for evaluating coating adhesion (after ironing), i.e., slidability, of a galvannealed steel sheet in an example. FIG. 8 is a second schematic diagram for explaining a method for evaluating coating adhesion (after ironing), i.e., slidability, of a galvannealed steel sheet in an example. FIG. 9 is a graph showing the relationship between Γ phase thickness and adhesion (after bending and bending back) for the examples. 1 is a graph showing the relationship between the aspect ratio of surface crystals and adhesion (after ironing) for Examples.

[0010] The present inventors have conducted extensive research into a galvannealed steel sheet (and a member including the galvannealed steel sheet) that can suppress 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 is a structure mainly composed of a hard martensite phase. For this reason, an appropriate chemical composition is selected, and then the plating process and alloying process are controlled so that the austenite phase becomes the main phase, and the martensite phase is obtained by rapid cooling (quenching) after alloying heating.

[0011] Regarding workability, it is important to improve the sliding properties and plating adhesion on the surface of the plating layer. Excellent sliding properties can be obtained by suppressing the formation of a phase called the ζ phase, which easily adheres to soft molds, on the surface of the plating layer. To suppress the formation of the ζ phase, alloying heating can be performed under conditions that result in 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 is thicker than a certain level, the plating will peel off during press forming. To suppress the formation of the ζ phase while suppressing excessive formation of the Γ phase, heating to the alloying temperature can be performed under conditions that result in rapid temperature rise. Therefore, excellent workability can be obtained by controlling the chemical composition and manufacturing conditions to suppress the formation of the ζ phase and the growth of the Γ phase.

[0012] However, when rapid heating is performed to ensure workability, a defective appearance as shown in Figure 1 may occur on the surface of the plating layer. Figure 1 is a schematic diagram showing the surface of a plating layer on which spot-like metallic luster portions a appear. As shown in Figure 1, each spot-like metallic luster portion a has a circle-equivalent diameter of about 2 to 5 mm based on area measurement, and appears in a row. The inventors analyzed the defective appearance accompanied by a large number of localized metallic luster portions a that occurs during rapid alloying heating.

[0013] Fig. 2 is a schematic diagram showing a cross section of the vicinity of a metallic luster portion a that appeared in the coating layer of an alloyed hot-dip galvanized steel sheet 1001. As shown in Fig. 2, it was found that the alloying reaction had progressed sufficiently outside the metallic luster portion a, and an Fe-Zn alloy layer b (healthy portion) had been formed, whereas the alloying reaction had not progressed locally within the metallic luster portion a, and an initial alloy layer a1 composed of Fe and Al remained, with an extremely thin Fe-Zn alloy layer a2 (defective portion).

[0014] The inventors speculate that the reason for the above phenomenon is as follows. The steel material 1011 used as the base material for plating contains microscopic variations in structure and composition. In particular, when a C-deficient layer (decarburized layer) or a layer deficient in alloying elements such as Si and Mn is present in the surface layer of the steel sheet, crystal grains in these deficient layers grow significantly when the steel sheet is heated to a high temperature during an annealing process or other process. Furthermore, when the temperature is increased rapidly and instantaneously, the alloying reaction is less likely to occur during the heating process, and the alloying reaction progresses rapidly at higher temperatures. Therefore, when a steel material with variations in structure and composition is used, if the alloying reaction progresses rapidly, the alloying reaction will proceed sufficiently in relatively reactive areas, but will be delayed in less reactive areas. In this case, Zn, a plating component, is consumed in the surrounding alloying reaction, resulting in an extremely thin plating in the less reactive areas, resulting in a poor metallic appearance.

[0015] In fact, the inventors have confirmed that when the heating rate is slowed, no significant appearance defects occur. This is thought to be because, if the heating rate is slow, the alloying reaction proceeds slowly even during heating, and even if there are some areas where the alloying reaction proceeds slowly due to some microscopic variation in reactivity, alloying begins before the alloying reaction in surrounding areas where the alloying reaction proceeds quickly has progressed sufficiently.

[0016] From the above investigations, the inventors have found that improving the alloying rate is an effective heating method that does not result in poor appearance, even when rapid heating is performed. They have also found that it is not necessary to selectively improve the alloying rate only in poorly reactive areas with a diameter of several millimeters that would result in poor appearance, and that improving the average alloying rate throughout the entire coating layer is sufficient to achieve the desired effect.

[0017] The present invention has been completed based on the above findings. Hereinafter, a galvannealed steel sheet according to an embodiment of the present invention will be described. In this specification, "excellent workability" means "excellent coating adhesion and sliding properties." Furthermore, % used for components means "mass %" unless otherwise specified.

[0018] The galvannealed steel sheet 1 according to this embodiment is a steel sheet used as a component. Fig. 3 is a cross-sectional view of the galvannealed steel sheet 1 according to this embodiment. The galvannealed steel sheet 1 has a Zn-containing coating layer 13 on the surface of a steel material 11 having a predetermined chemical composition. The chemical composition of the steel material will be described below.

[0019] (C: 0.10 to 0.35%) 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 C content exceeds 0.35%, the hydrogen embrittlement resistance and weldability of the steel material decrease, so the C content is 0.35% or less, preferably 0.33% or less, or 0.30% or less.

[0020] (Si: 0.01 to 2.00%) If the Si content is low, Si-containing inner oxides cannot be formed in the cold rolling process, and the crystal grains in the surface layer tend to become coarse. Therefore, the Si content is 0.01% or more, preferably 0.02% or more, or 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 an excellent 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 to 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%, quenching will be insufficient and the desired tensile strength will tend to be difficult to obtain, so the Mn content is 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 interfere with the alloying reaction will form on the steel sheet surface, and the distribution of Mn oxides will likely cause variation in the alloying rate and deteriorate the appearance quality. Therefore, the Mn content is 4.0% or less, preferably 3.9% or less, 3.8% or less, or 3.5% or less.

[0022] (P: 0 to 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 the P content is 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 to 0.100%) S is an impurity element, and the lower the content, the better. However, if the S content exceeds 0.100%, MnS is formed in the steel material, deteriorating toughness and hole expandability, so the S content is 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 to 0.020%) N is an impurity element, and the lower the content, the better. If the N content exceeds 0.020%, coarse nitrides are formed in the steel material, reducing hole expandability, so the N content is 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 to 1.500%) Al is an element added for deoxidation. To obtain the added effect, the Al content is set to 0.001% or more. The Al content is preferably 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, since a sufficient effect can be obtained even with an Al content of 1.500% or less, an increase in cost can be avoided by setting the Al content to 1.500% or less. Furthermore, by setting the Al content to 1.500% or less, an increase in load during hot rolling due to an increase in the transformation temperature of the steel material can be avoided. Therefore, the Al content is set to 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] (O: 0 to 0.010%) If the O content exceeds 0.010%, various oxides are formed, which may adversely affect the mechanical properties of the steel sheet, so 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 the lower limit of each is 0%. Preferred lower and upper limits of each element when included will be described below.

[0028] (Cr: 0.001 to 0.80%) When the Cr content is 0.001% or more, various properties such as strength, hole expandability, and elongation of the steel material can be improved. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. By setting the Cr content to 0.80% or less, inhibition of the plating alloying reaction is suppressed, so the upper limit of the Cr content is preferably 0.80%, and may be 0.70% or less, 0.60% or less, or 0.50% or less.

[0029] (Mo: 0.001 to 1.00%) When the Mo content is 0.001% or more, various properties such as strength, hole expandability, and elongation of the steel material can be improved. The Mo content may be 0.01% or more, 0.05% or more, or 0.10% or more. By setting the Mo content to 1.00% or less, inhibition of the plating alloying reaction is suppressed, so the upper limit of the Mo content is preferably 1.00%, and may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

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

[0031] (Ti: 0.0001 to 0.1000%) When the Ti content is 0.001% or more, various properties such as the strength, hole expandability, and elongation of the steel material can be improved. The Ti content may be 0.0003% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more. By setting the Ti content to 0.1000% or less, the recrystallization temperature increases, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability can be prevented from being impaired. Therefore, the upper limit is preferably 0.1000%, and may be 0.0800% or less, 0.0600% or less, or 0.0500% or less.

[0032] (Nb: 0.0001 to 0.4000%) When the Nb content is 0.0001% or more, various properties such as the strength, hole expandability, and elongation of the steel material can be improved. The Nb content may be 0.001% or more, or 0.005% or more. By setting the Nb content to 0.4000% or less, the recrystallization temperature increases, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability can be prevented from being impaired. Therefore, the upper limit is preferably 0.4000%, and may be 0.2000% or less, 0.1000% or less, or 0.0500% or less.

[0033] (V: 0.0001 to 0.50%) When the V content is 0.0001% or more, various properties such as the strength, hole expandability, and elongation of the steel material can be improved. The V content may be 0.001% or more, 0.005% or more, or 0.010% or more. By setting the V content to 0.50% or less, the recrystallization temperature increases, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability can be prevented from being impaired. Therefore, the upper limit is preferably 0.50%, and may be 0.30% or less, 0.20% or less, or 0.10% or less.

[0034] (Ni: 0.0001 to 1.0000%) When the Ni content is 0.0001% or more, various properties such as the strength, hole expandability, and elongation of the steel material can be improved. The Ni content may be 0.001% or more, 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more. Since a sufficient effect can be obtained even with a Ni content of 1.0000% or less, the upper limit of the Ni content is preferably 1.0000%, and may be 0.8000% or less, 0.5000% or less, or 0.3000% or less.

[0035] (Cu: 0.001 to 1.00%) When the Cu content is 0.001% or more, various properties such as the strength, hole expandability, and elongation of the steel material can be improved. The Cu content may be 0.005% or more, 0.010% or more, 0.050% or more, or 0.10% or more. By setting the Cu content to 1.00% or less, the recrystallization temperature increases, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability can be prevented from being impaired. Therefore, the upper limit is preferably 1.00%, and may be 0.80% or less, 0.50% or less, or 0.30% or less.

[0036] (REM: 0.0003 to 0.0100%) When the REM content is 0.0003% or more, strength and bendability can be improved by adjusting the shape of inclusions. The REM content may be 0.0005% or more, or 0.0010% or more. Since a sufficient effect can be obtained even with a REM content of 0.0100% or less, the upper limit of the REM content is preferably 0.0100%, and may 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 lanthanoids, and the 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 be contained. The lower limit of each element is 0%. The preferable lower and upper limits of each element when contained will be described below.

[0038] (As: 0.001 to 0.200%) As is an element that is mixed in when scrap is used as the raw material. It may strongly segregate at grain boundaries, causing embrittlement of the grain boundaries, reducing ductility, and reducing cold formability, so the upper limit is preferably 0.200%. However, excessive removal increases the number of steps and man-hours, so the content may be 0.001% or more.

[0039] (Sb: 0.001 to 0.200%) Sb is an element that is mixed in when scrap is used as the raw material. It may strongly segregate at grain boundaries, causing embrittlement of the grain boundaries, reducing ductility, and reducing cold formability, so the upper limit is preferably 0.200%. However, excessive removal increases the number of steps and man-hours, so the content may be 0.001% or more.

[0040] (Sn: 0.001 to 0.20%) Sn is an element that is mixed in when scrap is used as the raw material. It may strongly segregate at grain boundaries, leading to embrittlement of the grain boundaries, a decrease in ductility, and a decrease in cold formability, so the upper limit is preferably 0.20%. However, since excessive removal leads to an increase in the number of steps and man-hours, the content may be set to 0.001% or more.

[0041] (W: 0.001 to 0.100%) When the W content is 0.001% or more, the strength of the steel material can be improved. When the W content exceeds 0.100%, the ductility is reduced, and the cold workability of the steel sheet is reduced. Therefore, the upper limit is preferably 0.100%.

[0042] (Co: 0.01 to 2.0%) When the Co content is 0.01% or more, the strength of the steel material can be improved. When the Co content exceeds 2.0%, the ductility is reduced and the cold workability of the steel sheet is reduced, so the upper limit is preferably 2.0%.

[0043] (Zn: 0.0005 to 0.100%) If the Zn content exceeds 0.100%, coarse Zn oxides are generated and cold formability is reduced, so the upper limit is preferably 0.100%. The lower limit of the Zn content may be substantially 0%, or may be 0.0005%.

[0044] (Zr: 0.0010 to 0.0500%) When the Zr content is 0.0010% or more, it can contribute to improving the bending formability of the steel sheet. When the Zr content exceeds 0.0500%, coarse Zr oxides are generated and the cold formability is reduced, so the upper limit is preferably 0.0500%.

[0045] (Mg: 0.001 to 0.050%) When the Mg content is 0.001% or more, strength and bendability can be improved by adjusting the shape of inclusions. Since a sufficient effect can be obtained even with a Mg content of 0.050% or less, the upper limit of the Mg content is preferably 0.050%.

[0046] (Ca: 0.001 to 0.050%) When the Ca content is 0.001% or more, strength and bendability can be improved by adjusting the shape of inclusions. Since a sufficient effect 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 to 0.100%) When the Ta content is 0.001% or more, strength and bendability can be improved by adjusting the shape of inclusions. Since a sufficient effect can be obtained even with a Ta content of 0.100% or less, the upper limit of the Ta content is preferably 0.100%.

[0048] (Bi: 0.001 to 0.050%) When the Bi content is 0.001% or more, strength and bendability can be improved by adjusting the shape of inclusions. Since a sufficient effect can be obtained even with a Bi content of 0.050% or less, the upper limit of the Bi content is preferably 0.050%.

[0049] (Te: 0.001 to 0.050%) When the Te content is 0.001% or more, strength and bendability can be improved by adjusting the shape of inclusions. Since a sufficient effect 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] (Balance is Fe and Impurities) The balance consists of Fe and impurities. The impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the embodiment of the present invention.

[0051] The chemical composition of the steel material may be measured using a common analytical method. For example, the chemical composition of the steel plate may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the steel material at approximately 1 / 4 of the plate thickness and measured using a Shimadzu ICPS-8100 (measuring device) or similar under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. The plating layer on the surface of the steel material may be removed by mechanical grinding or the like before analyzing the chemical composition.

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

[0053] The coating layer contains, for example, 7.0 to 15.0% Fe and 0.1 to 1.0% Al, with the remainder consisting of Zn and impurities. Note that impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, trace amounts of components other than Fe may be mixed into the coating layer as impurities due to mutual atomic diffusion between the base steel and the coating bath. Furthermore, the above-mentioned content in the coating layer refers to the content of the coating layer as a whole, and not the content of localized portions, such as portions corresponding to specific phases in the coating layer.

[0054] In this embodiment, the plating layer is a galvannealed layer. The remainder of the plating layer other than Fe, Al, and impurities is Zn. The content of impurities in the plating layer is preferably 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 plating layer and the mold during press forming can be suppressed. The Fe content in the plating layer is preferably 8.0% or more, and more preferably 9.0% or more. When the Fe content in 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. The Fe content in the plating layer is preferably 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, and the appearance quality after plating can be improved. The Al content in the plating layer is preferably 0.6% or less, and more preferably 0.5% or less.

[0057] The chemical composition of the plating layer can be measured by a general analytical method. For example, the plating layer of a sample is dissolved in dilute hydrochloric acid containing a commercially available inhibitor (for example, "Ivit 710K" manufactured by Asahi Chemical Industry, concentration 0.04%), and the amount of Fe contained in the dilute hydrochloric acid after dissolving the plating layer is measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, the plating coating weight (g / m) can be determined from the difference in mass of the sample before and after dissolving the plating layer. ２ The amount of Fe in the plating is calculated from the ratio of the amount of Fe to the amount of plating coverage. Elements other than Fe 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-alloyed phase that forms in the plating layer near the interface with the steel material. Because the Γ phase is hard and brittle, if it has a thickness greater than a certain level, the plating may peel off during press forming. If the Γ phase thickness is 1.00 μm or less, plating peeling during press forming is suppressed and excellent plating adhesion is exhibited. 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 by an electrochemical method. ４ A sample of a galvannealed steel sheet, in which the surface of the coating layer is masked with tape except for a measurement area of ​​20 mm in diameter, is immersed in the aqueous Cl solution, and a potential of −0.940 V is applied to the sample. When the current stops flowing, the sample is removed from the NH ４ Remove the sample from the Cl solution and rub the surface of the sample measurement area with absorbent cotton to remove any residue. ４ Immersed in a Cl aqueous solution, 4 mA / cm ２ A voltage is applied to the sample so that the current density is 0.55 V. The measurement is terminated when the potential becomes constant at -0.55 V. The time t from the start of dissolution until the potential becomes constant at -0.55 V is １ (s) is measured, and the thickness d (μm) of the Γ phase is calculated using the following formula (1): d = t １ × 4 × 4.448 … (1)

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

[0061] In this embodiment, the presence of the ζ phase on the surface of the coating layer is determined by the shape 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 coating layer is 4.0 or less, it can be said that the generation of the ζ phase is sufficiently suppressed. By controlling the aspect ratio of the crystal grains on the surface of the coating layer to 4.0 or less, the sliding properties of the galvannealed steel sheet can be improved. The aspect ratio of the crystal grains on the surface of the coating 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 coating layer can be measured as follows. After ultrasonically cleaning the surface of the coating layer of a galvannealed steel sheet with acetone, an SEM image is taken at 1000x magnification using an SEM (JEOL JSM-7001F, accelerating voltage 20 kV). In the SEM image, 20 or more crystal grains that are visually judged to have a large aspect ratio are selected, and the aspect ratio of each selected crystal grain is measured. The aspect ratio is measured using the aspect ratio measurement function of the image analysis software "Image J 1.54f." If 20 or more crystal grains cannot be observed in the 1000x SEM image, the imaging range is changed, and another SEM image is taken under the same conditions and observed. If necessary, additional SEM images are taken until 20 or more crystal grains are observable. The measured aspect ratios are averaged for the top 20 aspect ratios in descending order, and this value is defined as the "aspect ratio of the crystal grains on the surface of the plating layer."

[0063] (Tensile strength of 1180 MPa or more) The galvannealed steel sheet according to this embodiment has a tensile strength of 1180 MPa (HV: 370) or more. Preferably, the tensile strength is 1300 MPa (HV: 410) or more, and more preferably, the tensile strength is 1450 MPa (HV: 460) or more. The upper limit of the tensile strength of the galvannealed steel sheet according to this embodiment is not particularly limited, but the substantial upper limit is 2000 MPa (HV: 650). The Vickers hardness (HV) was measured at a position ¼ of the sheet thickness from the surface of the steel sheet on the sheet thickness cross section of the test specimen, under a load of 0.490 N.

[0064] To obtain a galvannealed steel sheet having a strength of 1180 MPa or more, 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 this can cause a decrease in strength, it is preferable that at least 90% by volume, preferably 95% by volume, of the steel material be martensite. The martensite fraction (volume %) in the steel material is a value estimated from the tensile strength of the galvannealed steel sheet. When the tensile strength of the galvannealed steel sheet is 1180 MPa or more, it can be estimated that the martensite fraction in the steel material is 90% or more.

[0065] The galvannealed steel sheet according to this embodiment has an excellent appearance because the occurrence of metallic luster portions on the surface, which is likely to occur when the heating rate during alloying is high, is suppressed. The metallic luster portions are caused by coarsening of crystal grains in the surface layer portion of the steel material. In the galvannealed steel sheet according to this embodiment, the average crystal grain size in the surface layer portion of the steel material is preferably 15 μm or less, more preferably 12 μm or less, 10 μm or less, or 9 μm or less, in equivalent circle diameter. There is no particular restriction on the lower limit of the average crystal grain size in the surface layer portion of the steel material, but the average crystal grain size may be, for example, 1 μm or more, or 2 μm or more, in equivalent circle diameter.

[0066] The average crystal grain size in the surface layer of a steel material can be measured as follows. The coating layer of a sample is removed with dilute hydrochloric acid containing a commercially available inhibitor (e.g., Asahi Chemical's "Ivit 710K," concentration 0.04%), and the sample is analyzed as is using EBSD (Electron Backscatter Diffraction) without being polished, to analyze the structure of the outermost surface of the steel material. Using an FE-SEM (JEOL JSM-7001F, accelerating voltage 20 kV) equipped with an EBSD detector (TSL OIM), the crystal orientation is analyzed at a magnification of 80x over a 1000 μm × 250 μm area with a measurement pitch of 1 μm. Using the software OIM ANALYSIS attached to the EBSD, the boundary of an area where the crystal orientation difference is 5° or more is regarded as a grain boundary, and the area surrounded by the grain boundary is regarded as a crystal grain, and the circle-equivalent diameter is measured based on the area of ​​each crystal grain. The grain size distribution of the measured circle-equivalent diameter is evaluated as an area fraction. Measurement and analysis, including measurement of the circle-equivalent diameter and analysis of the grain size distribution, can be performed using the software "OIM ANALYSIS" attached to the EBSD detector (OIM manufactured by TSL).

[0067] An example of a method for producing a galvannealed steel sheet according to this embodiment will be described below. The example of the production method includes (A) a hot rolling step, (B) a pickling and cold rolling step, (C) an annealing step, (D) a plating step, (E) an alloying step, and (F) a quenching and tempering step. Each step will be described below.

[0068] (A) Hot Rolling Process First, a slab having the chemical composition described above for the steel material is heated. The slab is not particularly limited as long as it has the chemical composition described above. The slab may be one 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 is preferably 1300°C or lower in terms of the capacity and productivity of the heating equipment. The heated slab is subjected to rough rolling and finish rolling as appropriate, and then cooled to obtain a hot-rolled steel sheet of a predetermined thickness. The temperature at which the finish rolling is completed is preferably within the range of 860 to 960°C. After the finish rolling is completed, the slab is cooled to a temperature range of 450 to 700°C, and the hot-rolled steel sheet is coiled. The thickness of the sheet after finish rolling is preferably within the range of 2.0 to 4.0 mm.

[0069] (Parameter f(t, [Si]), consisting of the residence time t (s) of the hot-rolled steel sheet at 500°C or higher after coiling and the Si content [Si] (%) in the steel, is 1.30 or more and 4.80 or less.) After coiling of the hot-rolled steel sheet, oxides of easily oxidizable elements are formed in the surface layer of the steel sheet, and the solid solution concentration of the easily oxidizable elements decreases. When the solid solution concentration of Si, an easily oxidizable element, decreases in the surface layer of the steel sheet, a Si-depleted layer is formed in the surface layer of the steel sheet. If the Si-depleted layer thus formed is thick, it is difficult for internal oxides containing Si to be formed in the surface layer of the steel sheet in the subsequent annealing step, and crystal grains in the surface layer of the steel sheet are likely to coarsen. The Si-depleted layer in the surface layer of the steel sheet is generated by holding the hot-rolled steel sheet at a relatively high temperature (500°C or higher) for a long time after coiling. Furthermore, if the Si concentration in the steel sheet is low, it is difficult for internal oxides containing Si to be formed in the annealing step (C) described below.

[0070] Therefore, by setting the parameter f(t, [Si]), which is the residence time t (s) of the hot-rolled steel sheet at 500°C or higher after coiling and the Si content [Si] (%) in the steel, as shown in formula (2), to 4.80 or less, it is possible to suppress coarsening of crystal grains in the surface layer of the steel sheet in the subsequent annealing process. That is, by shortening the residence time t (s) and controlling the balance between [Si] (%), it is possible to suppress coarsening of crystal grains in the surface layer of the steel sheet. By suppressing coarsening of crystal grains in the surface layer of the steel sheet, even in the case of rapid heating, variation in the progress of the alloying reaction is less likely to occur, and it is possible to suppress the occurrence of poor appearance in the galvannealed steel sheet. f(t, [Si]) = Log １０ (t+10) / [Si] ０．０９５ ...(2) The parameter f(t, [Si]) is preferably 4.70 or less, more preferably 4.60 or less, 4.50 or less, 4.40 or less, 4.30 or less, 4.10 or less, and further preferably 3.50 or less.

[0071] In order to more effectively suppress the occurrence of poor appearance, it is preferable that f(t, [Si]) is smaller. However, to make f(t, [Si]) too small, for example, strong cooling such as immersion in water after coiling is necessary. In that case, large-scale equipment is required, which is not practical, and there is also the possibility that the shape of the steel sheet may be deteriorated and the subsequent sheet threadability may be reduced. Furthermore, by setting the coiling temperature to 500°C or less, the value of the residence time t at 500°C or higher can be reduced, but the coiling temperature may be limited by the amount of Si in the steel. The lower limit of f(t, [Si]) is preferably 1.30.

[0072] The residence time t at 500°C or higher is adjusted by controlling the coiling temperature in hot rolling, controlling the coil unit weight, or a combination of these. In addition, the residence time t at 500°C or higher may also be adjusted, as necessary, by cooling the coil by spraying water mist onto it or by keeping the coil warm by covering it with a cover made of a heat insulating material.

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

[0074] (C) Annealing Step 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 lower than 800°C, the ferrite phase remains at the annealing temperature, and an austenite fraction of 90% or higher cannot be obtained. Therefore, it becomes difficult to obtain a martensite-based structure in the steel material, and the desired tensile strength cannot be achieved. If the annealing temperature is higher than 880°C, excessive energy is consumed and the furnace body may be damaged.

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

[0076] The dew point of the annealing atmosphere is -30°C or higher and 15°C or lower. The dew point of the annealing atmosphere is preferably -25°C or higher, and preferably 0°C or lower. At a dew point of -30°C or lower, internal oxides are not formed in the cold-rolled steel sheet, and the crystal grains in the surface layer of the cold-rolled steel sheet become coarse. 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. The cooling conditions after annealing are not particularly limited.

[0077] (D) Galvanizing Step: The cold-rolled steel sheet that has undergone the annealing step is plated with zinc to obtain a hot-dip galvanized steel sheet. The method for plating is not particularly limited, but the cold-rolled steel sheet can be plated by immersing it 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 0.145% or less. A low Al concentration in the plating bath increases the amount of bottom dross formed in the plating bath, which can cause poor appearance. On the other hand, a high Al concentration in the plating bath requires high-temperature alloying heating in the alloying step described below, which is likely to result in poor appearance. The plating bath may contain additional elements such as Fe, Mg, Si, Ti, Sb, Sn, Pb, and Ca, as well as other impurities.

[0078] The temperature of the coating bath is 440°C or higher and 480°C or lower. The temperature of the coating bath is preferably 450°C or higher and 470°C or lower. If the temperature of the coating bath is lower than 440°C, the low-temperature portion near the bath surface solidifies, which can lead to poor appearance. On the other hand, if the temperature of the coating bath is higher than 480°C, zinc tends to evaporate, resulting in loss of coating raw materials. The temperature of the cold-rolled steel sheet when entering the coating bath is 400°C or higher and 490°C or lower. The temperature is preferably 440°C or higher and 470°C or lower. If the temperature of the cold-rolled steel sheet when entering the coating bath is 400°C or lower, the amount of Fe eluted from the cold-rolled steel sheet decreases, which can result in a coating defect in which some coating is not formed, known as uncoated. If the temperature of the sheet is 490°C or higher, a very dense Fe-Zn alloy is formed in the coating bath, which can inhibit the reaction between Fe and molten Zn, thereby reducing the alloying processability in the alloying step described below.

[0079] Plating weight is 30 to 80 g / m ２ and preferably 40 to 60 g / m ２ The basis weight is 30 g / m ２ On the other hand, if the weight per unit area is less than 80 g / m, it is difficult to obtain sufficient corrosion resistance. ２ Anything above this will result in an excessive plating thickness, which will increase costs.

[0080] (E) Alloying step The hot-dip galvanized steel sheet obtained in the plating step is subjected to an alloying treatment to obtain a hot-dip galvanized steel sheet. In the alloying treatment, the heating rate when heating to the alloying temperature is set to 50°C / s or more and 400°C / s or less. If the heating rate is less than 50°C / s, it is difficult to suppress the formation of both the Γ phase and the ζ phase. Achieving a heating rate of more than 400°C / s requires an increase in the sheet passing speed and a smaller, higher-output 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. If the alloying temperature is 500°C or lower, the ζ phase crystallizes, making it difficult to control the aspect ratio of the crystal grains on the surface of the coating layer to 4.0 or lower. If the alloying temperature is 630°C or higher, the alloying reaction proceeds rapidly in advance in part of the coating layer, which is likely to cause poor appearance and reduce coating adhesion during press forming. Any method, such as electrical heating or induction heating, can be used to heat the hot-dip galvanized steel sheet at the alloying temperature.

[0082] (F) Quenching and Tempering Process After the alloying process, the galvannealed steel sheet may be quenched for quenching, and then heated for tempering, if necessary. This allows for a martensite-based galvannealed steel sheet with a desired strength-ductility balance to be obtained. The cooling end temperature of quenching 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, for example, 200°C or higher and 350°C or lower. Furthermore, before or after the above-mentioned quenching or tempering, temper rolling may be performed to adjust the surface roughness and strength of the galvannealed steel sheet. The elongation of temper rolling can be set, for example, in the range of 0.1 to 5.0%.

[0083] (Examples) Steel types A to Y having the chemical compositions shown in Tables 1 and 2 were used to evaluate the coating structure and properties as galvannealed hot-dip galvannealed steel sheets when alloyed and heated under the production conditions shown in Table 3. In Tables 1 to 3, values ​​outside the ranges of the present invention are underlined.

[0084]

[0085]

[0086] Each galvannealed steel sheet was manufactured under the following conditions.

[0087] Slabs having the chemical compositions shown in Tables 1 and 2 were cast, heated to 1,150°C or higher, rough-rolled, and then finish-rolled at a total reduction of 50% and a finish-rolling completion temperature of 950°C. The hot-rolled steel sheets were then coiled. The coiled hot-rolled steel sheets were cooled or kept at a temperature of 500°C or higher for the residence time t shown in Table 3. The hot-rolled steel sheets were then 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. The cold-rolled steel sheets were then immersed in a hot-dip galvanizing bath with an Al concentration of 0.135% and a bath temperature of 460°C to obtain hot-dip galvanized steel sheets. The coating weight 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 production conditions shown in Table 3 to obtain galvannealed steel sheets. The thickness of the obtained galvannealed steel sheets was 1.6 mm. The tensile strength (MPa) and the grain size (μm) in the surface layer portion of the steel material were measured for the obtained galvannealed steel sheets by the methods described above.

[0089] The coiling temperatures in Example Nos. 3, 4, 10, 11, 12, and 42 were 480, 450, 480, 480, 480, and 450, respectively, which were lower than 500°C, and therefore the residence time t at 500°C or higher after coiling was 0 (zero) seconds. In the other Examples, the coiling temperature was 500°C or higher, and the residence time t (s) at 500°C or higher after coiling was as shown in Table 3. The appearance, adhesion, and sliding properties were evaluated as follows.

[0090] <Appearance> The appearance of each galvannealed steel sheet was evaluated by observing the surface (surface of the coating layer) of each galvannealed steel sheet and rating it on a 7-point scale according to the following evaluation criteria of 1 to 7. A rating of 3 or higher was considered to be acceptable. 7: No visible unevenness in appearance 6: Spot-like unevenness in appearance without metallic luster is observed, and the diameter of each spot is less than 1 mm 5: Spot-like unevenness in appearance without metallic luster is observed, and the diameter of each spot is 1 mm or more 4: Spot-like unevenness in appearance with metallic luster is observed distributed in a dotted row only on a part of the surface of the plating layer, and the diameter of each spot is less than 1 mm 3: Spot-like unevenness in appearance with metallic luster is observed distributed in a dotted row only on a part of the surface of the plating layer, and the diameter of each spot is 1 mm or more and less than 3 mm, or Spot-like unevenness in appearance with metallic luster is observed over the entire surface of the plating layer, and the diameter of each spot is less than 1 mm 2: Spot-like unevenness in appearance with metallic luster is observed distributed in a dotted row only on a part of the surface of the plating layer, and the diameter of each spot is 3 mm or more and less than 5 mm, or Spot-like unevenness in appearance with metallic luster is observed over the entire surface of the plating layer, and the diameter of each spot is 1 mm or more and less than 3 mm 1: Dot-like irregularities in appearance with a metallic luster are observed, and each dot has a diameter of 5 mm or more, or Dot-like irregularities in appearance with a metallic luster are observed over the entire surface of the plating layer, and each dot has a diameter of 3 mm or more and less than 5 mm. In the above evaluation criteria, "uneven appearance" refers to a part on the surface of the plating layer that has a stronger metallic luster than other parts. "Dot-like irregularities in appearance with a metallic luster" corresponds to the metallic luster part mentioned above.

[0091] <Adhesion (After Bending and Unbending)> The method for evaluating the adhesion of the coating of each galvannealed steel sheet (after bending and unbending) will be described with reference to Figures 4A to 4C. (1) As shown in Figure 4A, a disk-shaped sample 100a with a diameter of 70 mm obtained by punching from a galvannealed steel sheet was bent 90° and then unbent to obtain sample 100b. (2) As shown in Figure 4B, transparent cellophane tape ("CT405AP-24" manufactured by Nichiban Co., Ltd.) was applied to the inside of the bend (inside of the bend, the valley fold side) of sample 100b and then peeled off, thereby obtaining measurement cellophane tape 200 with coating 201 peeled off linearly along the inside of the bend from sample 100b. (3) As shown in FIG. 4C , the cellophane tape 200 for measurement was attached to the whiteboard 300, and two locations with the largest width W (width perpendicular to the line) of the line-like plating 201 were selected. The larger of the measured values ​​was used to measure the release tape reflectance (%) for that sample. The larger the release tape reflectance (%), the smaller the release tape reflectance (%). In other words, the smaller the release tape peeled off, the larger the release tape reflectance (%), which is preferable. As an indicator of adhesion (after bending and unbending), a release tape reflectance of 40% or more was considered acceptable.

[0092] <Adhesion (after ironing)> The method for evaluating the adhesion of the coating of each galvannealed steel sheet (after ironing) will be described with reference to Figures 5A and 5B. (1) As shown in Figure 5A, a galvannealed steel sheet was pressed down on both sides by beads formed by meshing a convex portion 510U of an upper die 500U with a concave portion 510L of a lower die 500L, and ironed using a punch 500C to obtain an ironed sample 600. The pressing load was 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.) having a width of 24 mm and a length of 100 mm was attached to the lower end of the side wall portion 610 of the ironed sample 600 and then peeled off, thereby obtaining a cellophane tape 700 for visual evaluation with plating 601 peeled off in a line from the ironed sample 600. (3) This cellophane tape 700 for visual evaluation was attached to a whiteboard, and the adhesion (after ironing) was evaluated visually on a three-point scale of A to C based on the amount of plating adhered to the cellophane tape 700 for visual evaluation. A rating of A or B was deemed acceptable. A: No plating was observed on the cellophane tape for visual evaluation. B: A slight amount of plating was adhered to the cellophane tape for visual evaluation (less than 80% of the plating adhesion area). C: A large amount of plating was adhered to the cellophane tape for visual evaluation (80% or more of the plating adhesion area).

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

[0094]

[0095] Fig. 6 is a graph showing the relationship between the Γ phase thickness and adhesion (after bending and unbending). Fig. 7 is a graph showing the relationship between the aspect ratio of the surface crystals and the sliding properties. From the Examples, which are examples of the invention, it was confirmed that by satisfying the ranges specified in the present application, a galvannealed steel sheet having high strength and excellent formability and suppressing the occurrence of appearance defects can be obtained. On the other hand, from the Comparative Examples, which do not satisfy the ranges specified in the present application, it was confirmed that a galvannealed steel sheet having high strength and excellent formability and suppressing the occurrence of appearance defects cannot be obtained.

[0096] According to the present disclosure, it is possible to provide a galvannealed steel sheet having high strength and excellent formability, and suppressing the occurrence of appearance defects, and a member including the galvannealed steel sheet. Such a member can be used as an automobile member. Examples of the automobile member include structural members (framework members) of an automobile.

[0097] REFERENCE SIGNS LIST 1 steel plate 11 steel material 13 plating layer 1001 steel plate 1011 steel material a metallic luster portion a1 initial alloy layer a2 Fe—Zn alloy layer

Claims

1. Chemical composition, in mass%, is: 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%, 1. A galvannealed steel sheet comprising: a steel material containing Sb: 0-0.200%, 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 balance being Fe and impurities; and a Zn-containing plating layer formed on the surface of the steel material; wherein the aspect ratio of crystal grains on the surface of the plating layer is 4.0 or less; the thickness of a Γ phase in the plating layer is 1.0 μm or less; and the tensile strength is 1180 MPa or more.

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

3. A member comprising the galvannealed steel sheet according to claim 1 or 2.