Galvanized steel sheet and method for manufacturing the same

Abstract

This galvanized steel sheet comprises a base steel sheet having a specified chemical composition, an Fe-Al alloy layer formed on at least a portion of the base steel sheet, and a zinc plating layer formed on the surface of the base steel sheet or the Fe-Al alloy layer, in which the base steel sheet has an internal oxide layer lying between the surface of the base steel sheet and a depth of 0.2 μm or more when observed in the thickness direction, the Fe-Al alloy layer has an average thickness of 1 nm or more and less than 100 nm, the tensile strength is 980 MPa to 2000 MPa inclusive, and, in a cross section in the thickness direction, the ratio of coating of a grain boundary with an oxide is 60% or more in the internal oxide layer and the ratio of coating of the aforementioned surface of the base steel sheet with the Fe-Al alloy layer is 40% or more.

Description

[Technical Field]

[0001] This invention relates to galvanized steel sheets and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2022-027919, filed in Japan on February 25, 2022, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] High-strength steel sheets are used as automotive steel sheets to lighten vehicles, improve fuel efficiency, reduce carbon dioxide emissions, and ensure passenger safety. In recent years, in order to ensure sufficient corrosion resistance of vehicle bodies and parts, high-strength alloyed hot-dip galvanized steel sheets are also used as automotive steel sheets in addition to high-strength hot-dip galvanized steel sheets (see, for example, Patent Document 1).

[0003] Furthermore, high-strength steel sheets used for automotive parts require not only strength but also properties necessary for part formation, such as uniform elongation (formability, elongation, and bending resistance). While there is a trade-off relationship between strength and formability, methods to achieve both include TRIP (Transformation Induced Plasticity) steel sheets, which utilize the transformation-induced plasticity of retained austenite, and DP steel sheets, which have a soft phase and a hard phase.

[0004] However, when spot welding galvanized steel sheets (hot-dip galvanized steel sheets, electro-galvanized steel sheets, or alloyed hot-dip galvanized steel sheets) to each other, or when spot welding cold-rolled steel sheets to galvanized steel sheets, cracks called liquid metal embrittlement (LME) cracks may occur at the spot weld. LME cracks occur when the heat generated during spot welding melts the zinc in the galvanized layer, the molten zinc penetrates the grain boundaries of the steel sheet structure at the weld, and tensile stress acts on this state. This LME cracking is more likely to occur in high-alloy steel sheets such as TRIP steel sheets and DP steel sheets. It is particularly noticeable when spot welding high-strength TRIP steel sheets (transformation-induced plastic steel sheets). High-strength TRIP steel sheets are steel sheets that have higher concentrations of C, Si, and Mn than ordinary high-strength steel sheets and contain retained austenite, resulting in excellent energy absorption capacity and press formability. Therefore, galvanized steel sheets intended for use in automotive parts are required to have high resistance to LME (Luminous Metal Emission Control).

[0005] Furthermore, by applying high-strength steel sheets to automotive parts, the thickness of the steel sheets can be reduced, making it possible to lighten the vehicle while maintaining collision resistance. However, a challenge arises in that the bending fatigue strength decreases as the thickness of the steel sheets decreases.

[0006] To address these challenges, for example, Patent Document 2 discloses a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet that have excellent resistance to molten metal embrittlement cracking, having an internal oxide layer in which at least a portion of the grain boundaries are covered with oxide to a depth of 5.0 μm or more from the surface of the base material, and having a grain boundary coverage rate of 60% or more of the oxide in the region up to a depth of 5.0 μm from the surface of the base material. Patent Document 2 discloses that LME generation is suppressed by ensuring that a layer in which internal oxidation occurs exists to a predetermined depth, and by increasing the coverage of the grain boundaries by oxides.

[0007] However, Patent Document 2 does not consider bending fatigue strength or bending resistance. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2018 / 043453 [Patent Document 2] Japanese Patent No. 6388099 [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, conventionally, galvanized steel sheets that possess high strength, sufficient bending resistance, and excellent LME resistance and bending fatigue strength have not been provided. Therefore, the object of the present invention is to provide a galvanized steel sheet that is high strength and has excellent LME resistance and bending fatigue strength without reducing bending resistance. [Means for solving the problem]

[0010] In view of the above problems, the present inventors investigated methods to improve LME resistance and bending fatigue strength in high-strength galvanized steel sheets. As a result of our investigations, we found that in galvanized steel sheets, by providing an Fe-Al alloy layer of a predetermined thickness between the base steel sheet and the galvanized layer, it is possible to improve LME resistance without reducing bending resistance. Furthermore, it was found that forming a predetermined internal oxide layer on the surface of the base steel plate improves the bending fatigue strength. Furthermore, it was found that controlling the annealing and plating processes is effective in forming such Fe-Al alloy layers and internal oxide layers.

[0011] This invention was made in view of the above findings. The gist of this invention is as follows. [1] A zinc-plated steel sheet according to one aspect of the present invention comprises a base steel sheet, an Fe-Al alloy layer formed on at least a part of the surface of the base steel sheet, and a zinc plating layer formed on the surface of the base steel sheet or the Fe-Al alloy layer, wherein the base steel sheet has a composition of C: 0.10~0.40%, Si: 0.10~3.00%, Mn: 1.00~5.00%, sol. Al: 0.001~1.500%, P: 0.0010~0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0~0.200 %, B:0~0.0100%, Cr:0~1.000%, Mo:0~1.000%, Ni:0~1.000%, Cu:0~1.000%, Sn:0~0.500%, Nb:0~0 The Fe-Al alloy has a chemical composition consisting of 0.200%, V:0~0.500%, W:0~0.500%, Ca:0~0.0100%, Mg:0~0.0100%, Bi:0~0.0100%, Sb:0~0.1000%, Zr:0~0.0100%, REM:0~0.1000%, and the remainder being Fe and impurities, the base steel sheet has an internal oxide layer of 0.2 μm or more in the thickness direction from the surface of the base steel sheet, the average thickness of the Fe-Al alloy layer is 1 nm or more and less than 100 nm, in the cross-section in the thickness direction, the grain boundary coverage rate by oxides in the internal oxide layer is 60% or more, and the coverage rate of the surface of the base steel sheet by the Fe-Al alloy layer is 40% or more, and the tensile strength is 980 MPa or more and 2000 MPa or less. The zinc-plated steel sheet described in [2][1] is a base steel sheet whose chemical composition is, in mass%, Ti: 0.005~0.200%, B: 0.0005~0.0100%, Cr: 0.001~1.000%, Mo: 0.001~1.000%, Ni: 0.001~1.000%, Cu: 0.001~1.000%, Sn: 0.001~0.500%, Nb: 0.001~0.200% It may contain one or more substances selected from the group consisting of %, V: 0.001~0.500%, W: 0.001~0.500%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Bi: 0.0001~0.0100%, Sb: 0.0001~0.1000%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.1000%. [3] A method for manufacturing a galvanized steel sheet according to another aspect of the present invention is: A method for manufacturing a galvanized steel sheet as described in [1], In mass percent, C: 0.10~0.40%, Si: 0.10~3.00%, Mn: 1.00~5.00%, sol.Al: 0.001~1.500%, P: 0.0010~0.0300%, S: ≤0.0200%, N: ≤0.0100%, O: ≤0.0100%, Ti: 0~0.200%, B: 0~0.0100%, Cr: 0~1.000%, Mo: 0~1.000%, Ni: 0~1.000%, Cu: 0~1. Annealing a steel sheet having a chemical composition consisting of 000%, Sn: 0~0.500%, Nb: 0~0.200%, V: 0~0.500%, W: 0~0.500%, Ca: 0~0.0100%, Mg: 0~0.0100%, Bi: 0~0.0100%, Sb: 0~0.1000%, Zr: 0~0.0100%, REM: 0~0.1000%, and the remainder: Fe and impurities, by holding it at an annealing temperature of 700~1000℃ for 1 second or more. The process comprises a step and a plating step of forming a zinc plating layer on the surface of the steel sheet after the annealing step, wherein in the heating process up to the annealing temperature, the average heating rate in the first temperature range of 400 to 650°C is 2.0°C / second or more, the average heating rate in the second temperature range from 650°C to the annealing temperature is 0.5 to 5.0°C / second, and in the second temperature range, the (P(H2O) / P(H2)) of the atmosphere is 0.05 to 2.00. In the plating process, the steel plate is cooled to 440-550°C at an average cooling rate of 0.5°C / second or more, immersed in a plating bath mainly composed of Zn with an effective Al content of 0.050-0.250% by mass, removed from the plating bath, cooled so that it takes 10 seconds or less to reach 400°C, and then cooled to 350°C or below, with an average cooling rate of 1.0°C / second or more and 5.0°C / second or less between 400°C and 350°C. [Effects of the Invention]

[0012] According to the above aspects of the present invention, it is possible to provide a galvanized steel sheet that has high strength, sufficient bending resistance, and excellent LME resistance and bending fatigue strength. [Brief explanation of the drawing]

[0013] [Figure 1] It is a schematic diagram showing an example of the cross section of the steel sheet according to this embodiment.

Mode for Carrying Out the Invention

[0014] A galvanized steel sheet according to an embodiment of the present invention (sometimes simply referred to as the steel sheet according to this embodiment) and a method for manufacturing the same will be described. FIG. 1 is a schematic diagram showing an example of the cross section of the steel sheet according to this embodiment. The steel sheet 1 according to this embodiment includes a base steel sheet 10 having a predetermined chemical composition, an Fe—Al alloy layer 20 formed on at least a part of the surface of the base steel sheet, and a galvanized layer 30 formed on the surface of the base steel sheet 10 or the Fe—Al alloy layer 20. Further, the base steel sheet 10 has an internal oxide layer 11 in the surface layer portion on the interface side with the Fe—Al alloy layer 20 or the galvanized layer 30. When the Fe—Al alloy layer 20 is formed only on a part of the base steel sheet 10, on the portion where the Fe—Al alloy layer 20 is formed on the surface of the base steel sheet 10, the galvanized layer 30 is formed on the Fe—Al alloy layer, and on the portion where the Fe—Al alloy layer 20 is not formed, the galvanized layer 30 is formed on the base steel sheet 10. Although the Fe—Al alloy layer and the galvanized layer are formed only on one side in FIG. 1, they may be formed on the other side as well. Hereinafter, each component of the steel sheet according to this embodiment will be described. In the description, the range indicated with “~” is, in principle, included in the range with the values at both ends as the lower limit value and the upper limit value. However, the numerical values indicated with “exceeding” and “less than” are not included in the range.

[0015] "Base steel sheet" First, the base steel sheet 10 included in the steel sheet 1 according to this embodiment will be described.

[0016] <Chemical composition> The base steel sheet 10 included in the steel sheet 1 according to this embodiment contains the following elements. In this embodiment, % of the content of each element means mass %.

[0017] C: 0.10~0.40% Carbon (C) is an essential element for increasing the strength of steel sheets. Sufficient tensile strength cannot be obtained if the C content is less than 0.10%. Therefore, the C content should be 0.10% or higher. Preferably, the C content is 0.12% or higher. Furthermore, C is an element that contributes to the formation of retained austenite. Retained austenite contributes to improved elongation through the TRIP effect. To obtain this effect, a C content of 0.16% or higher is preferable. On the other hand, if the carbon content exceeds 0.40%, the weldability deteriorates significantly. Therefore, the carbon content should be 0.40% or less. From the viewpoint of suppressing deterioration of press formability and weldability, the carbon content is preferably 0.30% or less.

[0018] Si: 0.10~3.00% Silicon (Si) is a solid solution strengthening element and is effective in increasing the strength of steel sheets. Furthermore, Si contributes to the formation of retained austenite. To obtain these effects, the Si content should be 0.10% or higher. Preferably, the Si content is 0.30% or higher. On the other hand, excessive Si content significantly deteriorates the chemical conversion treatment properties of the steel sheet and its wettability with hot-dip galvanizing. Therefore, the Si content should be 3.00% or less. Preferably, the Si content is 2.00% or less.

[0019] Mn: 1.00~5.00% Manganese (Mn) is a powerful austenite-stabilizing element and is effective in improving the hardenability of steel sheets. To achieve this effect, the Mn content should be 1.00% or higher. Preferably, the Mn content is 1.50% or higher. On the other hand, excessive Mn content deteriorates weldability and low-temperature toughness. Therefore, the Mn content should be 5.00% or less. From the viewpoint of suppressing deterioration of weldability and low-temperature toughness, the Mn content is preferably 3.20% or less.

[0020] sol.Al: 0.001~1.500% Aluminum (Al) is an element that has a deoxidizing effect on steel. Furthermore, Al is also an element that contributes to the formation of retained austenite. To obtain these effects, the sol.Al content should be 0.001% or more. Preferably, the sol.Al content is 0.005% or more. On the other hand, excessive Al content not only leads to saturation of the effect and increased costs, but also raises the transformation temperature of the steel, increasing the load during hot rolling. Therefore, the sol.Al content should be 1.500% or less. Preferably, the sol.Al content is 1.000% or less.

[0021] P: 0.0010~0.0300% Phosphorus (P) is a solid solution strengthening element and is effective in increasing the strength of steel sheets. To achieve this effect, the P content should be 0.0010% or more. Preferably, the P content is 0.0050% or more. On the other hand, if the P content exceeds 0.0300%, the steel sheet becomes brittle due to P segregation at the grain boundaries. Furthermore, weldability and toughness deteriorate. Therefore, the P content should be 0.0300% or less. Preferably, the P content is 0.0200% or less.

[0022] S: 0.0200% or less S (sulfur) is an element that causes hot brittleness and also inhibits weldability and corrosion resistance. If the sulfur content exceeds 0.0200%, hot workability, weldability, and corrosion resistance will be significantly reduced, so the sulfur content should be kept below 0.0200%. Preferably, the sulfur content is below 0.0100%. A low sulfur (S) content is preferable, and it may even be 0%, but reducing the S content to less than 0.0001% significantly increases manufacturing costs. Therefore, the S content may be 0.0001% or higher. The S content may also be 0.0010% or higher.

[0023] N: 0.0100% or less Nitrogen (N) is an element that forms coarse nitrides in steel, degrading its bending resistance and hole-expanding properties. When the N content exceeds 0.0100%, the above degradation becomes significant, so the N content should be kept below 0.0100%. Preferably, the N content is below 0.0050%. A low N content is preferable, and it may even be 0%, but since extremely low N content would increase the cost of removing N, from an economic standpoint, the N content may be set at 0.0005% or more.

[0024] O: 0.0100% or less Oxygen (O) is an element that forms coarse oxides in steel, degrading its bending resistance and hole-expanding properties. When the O content exceeds 0.0100%, the above-mentioned property degradation becomes significant, so the O content should be kept below 0.0100%. Preferably, the O content is below 0.0070%. A low oxygen content is preferable, and it may be 0%, but from the perspective of manufacturing costs, the oxygen content may be 0.0001% or more. The oxygen content may also be 0.0010% or more.

[0025] The steel sheet according to this embodiment may contain the above elements, with the remainder being Fe and impurities. However, for the purpose of improving various properties, it may further contain one or more elements (arbitrary elements) selected from the following: Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM. Since the arbitrary elements are not required to be included, the lower limit is 0%.

[0026] Ti: 0~0.200% Titanium (Ti) is an element that suppresses the formation of BN, which is a factor that reduces hardenability, by fixing N as TiN in steel. Furthermore, Ti is an element that refines the austenite grain size during heating, thereby improving toughness. To obtain this effect, a Ti content of 0.005% or more is preferable. A Ti content of 0.010% or more is more preferable. On the other hand, excessive Ti content reduces the ductility of the steel sheet. Therefore, when Ti is included, the Ti content should be 0.200% or less. Preferably, the Ti content should be 0.050% or less.

[0027] B: 0~0.0100% Boron (B) is an element that, during welding, segregates at austenite grain boundaries, strengthening them and contributing to improved resistance to molten metal embrittlement cracking (LME resistance). To achieve this effect, it is preferable to have a B content of 0.0005% or more. It is more preferable to have a B content of 0.0008% or more. On the other hand, if the B content exceeds 0.0100%, carbides and nitrides are formed, the above effects become saturated, and the hot workability decreases. Therefore, the B content should be 0.0100% or less. Preferably, the B content is 0.0050% or less.

[0028] Cr: 0~1.000% Mo: 0~1.000% Ni: 0~1.000% Cu: 0~1.000% Sn: 0~0.500% Cr (chromium), Mo (molybdenum), Ni (nickel), Cu (copper), and Sn (tin) are all elements that are effective in increasing the strength of steel sheets. To obtain the above effects, it is preferable to include one or more elements selected from Cr, Mo, Ni, Cu, and Sn in an amount of 0.001% or more, more preferably 0.010% or more, and even more preferably 0.050% or more. On the other hand, excessive amounts of these elements lead to saturation of the effect and increased costs. Therefore, when including them, the content of Cr, Mo, Ni, and Cu should all be 1.000% or less, and the Sn content should be 0.500% or less. Preferably, the content of Cr, Mo, Ni, and Cu should all be 0.600% or less, and the Sn content should be 0.300% or less.

[0029] Nb: 0~0.200% V: 0~0.500% W: 0~0.500% Nb (niobium), V (vanadium), and W (tungsten) are carbide-forming elements and are effective in increasing the strength of steel sheets. To obtain the above effects, it is preferable to include one or more elements selected from Nb, V, and W in an amount of 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, excessive amounts of these elements lead to saturation of the effect and increased costs. Therefore, when including them, the Nb content should be 0.200% or less, and the V and W content should both be 0.500% or less. Preferably, the Nb content should be 0.100% or less, and the V and W content should both be 0.300% or less.

[0030] Ca: 0~0.0100% Mg: 0~0.0100% Bi: 0~0.0100% Sb: 0~0.1000% Zr: 0~0.0100% REM: 0~0.1000% Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) are elements that contribute to the fine dispersion of inclusions in steel, while Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Each of these elements contributes to improving the bending resistance of steel sheets. Therefore, they may be included as needed. To obtain the above effects, it is preferable to include at least 0.0001% of one or more elements selected from Ca, Mg, Bi, Sb, Zr, and REM, and more preferably at least 0.0010%. On the other hand, excessive amounts of these elements degrade ductility. Therefore, the Ca content, Mg content, Bi content, and Zr content should all be 0.0100% or less. In addition, the Sb content and REM content should be 0.1000% or less. The Ca content, Mg content, Bi content, and Zr content should all be preferably 0.0080% or less, and more preferably 0.0060% or less. The Sb content and REM content should be preferably 0.0800% or less, more preferably 0.0600% or less, and even more preferably 0.0200% or less. Here, REM refers to the 17 elements totaling Sc, Y, and lanthanides, and REM content means the total content of these elements. Industrially, lanthanides are added in the form of mischmetals.

[0031] The chemical composition of the base steel sheet of the steel sheet according to this embodiment can be determined by the following method. The chemical composition of the base steel sheet can be measured using general chemical composition methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol.Al can be measured by ICP-AES using the filtrate obtained after heating and decomposing the sample with acid. In addition, C and S can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy. If the steel sheet has a plating layer on its surface, the plating layer should be removed by mechanical grinding before the chemical composition analysis can be performed.

[0032] As described above, the chemical composition of the base steel sheet of the steel sheet according to this embodiment is either containing C, Si, Mn, sol.Al, P, S, O, N, with the remainder being Fe and impurities, or containing C, Si, Mn, sol.Al, P, S, O, N, and further containing one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, with the remainder being Fe and impurities. Impurities are elements that are mixed in during the raw material or manufacturing process. The total amount of impurities is preferably 0.5% or less, and more preferably 0.1% or less.

[0033] <Metallic structure (microstructure)> The base steel sheet of this embodiment is not limited in terms of its microstructure, but to obtain a tensile strength of 980 MPa or more, it is preferable that the total volume ratio of fresh martensite and tempered martensite in the microstructure at the 1 / 4 thickness position, centered at the 1 / 4 thickness position from the surface of the base steel sheet and ranging from 1 / 8 to 3 / 8 of the thickness from the surface, be 40% or more. More preferably, it is greater than 50%, and even more preferably 55% or more. If an even higher tensile strength is to be obtained, it is preferable that the total volume ratio of fresh martensite and tempered martensite be 80% or more. Aside from fresh martensite and tempered martensite, other materials include one or more of the following: ferrite, bainite, pearlite, cementite, and retained austenite.

[0034] The volume fractions of ferrite, bainite, martensite (tempered martensite and fresh martensite), pearlite, cementite, and retained austenite contained in the metal structure at the 1 / 4 thickness position can be measured using the method described below. A sample is taken from a cross-section of the steel plate parallel to the rolling direction and thickness direction, which is used as the observation surface. The observation surface is then polished and etched with Nital. Next, when observing the tissue at the 1 / 4 thickness position, the field of view should be 250 μm, centered on the 1 / 4 thickness position from the surface, in the range of 1 / 8 to 3 / 8 thickness, at a magnification of 5000x. 2In summary, a total of five fields of view are observed using a field emission scanning electron microscope (FE-SEM). The area fractions of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite are then measured and considered as their volume fractions. Here, regarding the identification of each phase, regions that have a substructure within the grain and where carbides precipitate with multiple variants are identified as tempered martensite. Regions where cementite precipitates in a lamellar pattern are identified as pearlite or cementite. Regions with low brightness and no visible substructure are identified as ferrite. Regions with high brightness and where the substructure has not been revealed by etching are identified as fresh martensite or retained austenite. The remainder is identified as bainite. The volume fraction of each structure is calculated using the point counting method. The volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite, as determined by the EBSD method described later, from the volume fraction of fresh martensite or retained austenite.

[0035] In the steel sheet according to this embodiment, the volume fraction of retained austenite at the 1 / 4 thickness position is evaluated by performing high-resolution crystal structure analysis using the EBSD method (electron beam backscatter diffraction). Specifically, a sample is taken from a cross section of the steel sheet parallel to the rolling direction and thickness direction, and the observation surface is polished to a mirror finish. Furthermore, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed surface layer. Next, at a position where the steel plate is 1 / 4 thickness, the magnification was 5000x, and the size of one field of view was 250 μm. 2 Based on the above, crystal structure analysis will be performed on the five fields of view using the EBSD method. The step distance between evaluation points will be set to 0.01 to 0.20 μm. The data obtained by the EBSD method will be analyzed using TSL's "OIM Analysys 6.0". Based on the observation results at each location, regions identified as FCC iron will be identified as retained austenite, and the volume fraction of each retained austenite at the 1 / 4 thickness position will be calculated.

[0036] <Internal Oxidation Layer> The base steel sheet 10 of the steel sheet 1 according to this embodiment has an internal oxide layer of 0.2 μm or more from the surface in the thickness direction (the interface with the Fe-Al alloy layer 20, or, in the case of portions where the Fe-Al alloy layer 20 is not formed, the interface with the zinc plating layer 30) (the thickness of the internal oxide layer is 0.2 μm or more). In this embodiment, the internal oxide layer is a layer in which at least a part of the grain boundaries of the base material is covered with an oxide of an easily oxidizable element such as Si or Mn (an oxide can be observed on the grain boundaries when observing the cross-section). Furthermore, in the base steel sheet 10 of the steel sheet 1 according to this embodiment, the grain boundary coverage rate of oxides in the internal oxide layer 11 is 60% or more. The grain boundary coverage rate is the ratio (%) of the length of the grain boundaries covered by oxides to the total length of the grain boundaries in the internal oxide layer 11. When grain boundaries are covered with oxides, dislocation movement is hindered, improving fatigue strength. If the thickness of the internal oxide layer 11 is less than 0.2 μm, or if the grain boundary coverage rate is less than 60%, the fatigue strength improvement effect cannot be sufficiently obtained. While there is no particular upper limit to the thickness of the internal oxide layer 11, if it exceeds 3.0 μm, the effect of improving fatigue strength saturates, and the deformability and bendability may decrease. Therefore, it is preferable that the thickness of the internal oxide layer 11 be 3.0 μm or less.

[0037] Furthermore, the presence of easily oxidizable elements such as Si and Mn as oxides at the grain boundaries suppresses the concentration of oxides on the surface of the base material. Oxides formed on the surface of the base material reduce the wettability of the hot-dip galvanized metal and can cause non-plating. Therefore, by forming an internal oxide layer, the occurrence of non-plating can be prevented and the plating performance can be improved. In the steel sheet according to this embodiment, since oxides are mainly formed on the grain boundaries in the internal oxide layer 11, the oxides often exist in a network-like structure.

[0038] The thickness (depth) of the internal oxide layer and the grain boundary coverage rate are determined by the following method. A sample for microstructural observation is taken from the steel plate so that the structure of the cross-section in the thickness direction can be observed. In the collected samples, wet polishing with emery paper is performed on surfaces parallel to the rolling direction and the thickness direction, and then buff polishing with diamond abrasive grains with an average diameter of 1 μm is performed to finish the observation surface to a mirror surface. Next, in order to remove the distortion introduced to the polished surface by the aforementioned mechanical polishing, colloidal silica polishing is performed using a suspension with alcohol as the solvent. In colloidal silica polishing, increasing the load during polishing can introduce further strain; therefore, the load should be kept low during polishing. For this reason, for example, in colloidal silica polishing, an automated polishing process may be performed for one hour using a BUEHLER Vibromet 2 at a power setting of 40%. However, if electrolytic polishing or chemical etching is applied during the process of removing the distortion introduced by mechanical polishing, the oxides will dissolve, making it impossible to observe the actual state of the oxides present on the grain boundaries. The same precautions are necessary when performing polishing using water as a solvent. Water-soluble oxides dissolve during polishing with water as a solvent, making it impossible to observe the internal oxides on the grain boundaries. For this reason, the finishing process of polishing should not include the above steps.

[0039] Observe the surface layer of the sample prepared according to the above procedure using SEM and SEM-EBSD. Select a magnification from 1000 to 9000x that contains 10 or more ferrite crystal grains in the microstructure, for example, 3000x. First, the oxide present at the grain boundaries is identified using backscattered electron imaging with a scanning electron microscope (SEM). Since the color of the backscattered electron image changes depending on the atomic number (or mass), the oxide and the steel structure can be easily distinguished. In backscattered electron microscopy, if, for example, a setting is configured to display elements with a small atomic number (or mass) as a "blackish color," oxides with a small mass relative to iron will appear as blackish colors in the observed image. Under these observation conditions, the microstructure of the surface layer of a steel sheet is photographed in five fields of view to confirm the presence of internal oxides. The maximum depth of the observed internal oxide layer is defined as the thickness of the internal oxide layer.

[0040] Next, crystal orientation data of BCC-iron is acquired by SEM-EBSD at the same position as the field of view observed by the SEM-backscattered electron image described above. The measurement magnification can be selected from 1000 to 9000 times, for example, the same magnification as the observation of the SEM-backscattered electron image described above. The measurement interval (STEP) is set to 0.01 to 0.1 μm, and 0.05 μm may be selected. In the BCC-iron crystal orientation MAP data obtained under these measurement conditions, grain boundaries are defined as those where the crystal orientation difference is 15° or greater, excluding regions where the confidence value (CI value) is less than 0.1. The CI value is a numerical indicator of the reliability of crystal orientation determination shown in the analysis software, and generally, a value of less than 0.1 is considered unreliable. When oxides are present at the grain boundaries of ferrite, BCC-iron crystal orientation data cannot be obtained, resulting in many regions with a CI value of less than 0.1 between adjacent grains. In this case, although the grain boundaries cannot be clearly identified, at boundaries where the orientation difference between adjacent ferrite grains is 15° or more, the grain boundaries are drawn on the MAP so as to pass through the center of the region with a CI value of less than 0.1.

[0041] In the ferrite grain boundary map obtained by the above procedure, the length of the grain boundaries covered by oxide (hereinafter referred to as "oxide-covered length") from the surface to the maximum depth of the internal oxide layer obtained above is measured. Next, the length of the grain boundaries not covered by oxide (hereinafter referred to as "uncovered length") is measured. Then, the grain boundary coverage rate (%) is calculated by dividing the obtained oxide-covered length by the total length of all grain boundaries.

[0042] "Fe-Al alloy layer" In the steel sheet 1 according to this embodiment, an Fe-Al alloy layer 20 with an average thickness of 1 nm or more is formed on the surface of the base steel sheet 10 with a coverage rate of 40% or more. The Fe-Al alloy layer 20 is formed between the base steel sheet 10 and the zinc plating layer 30. LME cracking occurs when molten zinc penetrates the grain boundaries during welding. Therefore, the presence of an Fe-Al alloy layer at the interface between the plating layer and the base material acts as a barrier against zinc penetration, improving LME resistance. When viewed in cross-section in the thickness direction (the thickness direction of the base steel plate), if the average thickness of the Fe-Al alloy layer is less than 1 nm or the coverage rate is less than 40%, sufficient barrier properties cannot be ensured, and the LME resistance cannot be sufficiently improved. Therefore, the average thickness of the Fe-Al alloy layer should be 1 nm or more, and the coverage rate should be 40% or more. The average thickness of the Fe-Al alloy layer is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more. Furthermore, the coverage rate of the Fe-Al alloy layer is preferably 50% or more, and more preferably 60% or more. On the other hand, if the average thickness of the Fe-Al alloy layer exceeds 100 nm, the bending resistance decreases. Therefore, the average thickness of the Fe-Al alloy layer should be less than 100 nm. Furthermore, while there is no need to limit the upper limit of the coverage rate, achieving 100% coverage would significantly increase costs, so the coverage rate may be set to less than 100%, or 98% or less. Here, the coverage ratio is the ratio (%) of the length of the interface between the Fe-Al alloy layer and the base steel sheet to the length of the surface of the base steel sheet when viewed in cross-section in the thickness direction.

[0043] The average thickness and coverage of the Fe-Al alloy layer are determined by the following method. A sample is taken so that the cross-section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. This sample is then examined using FE-SEM at a magnification of 50,000x near the surface of the base steel plate, at a thickness of 1.5 μm. 2 The above / 1 field of view will be photographed. The Fe-Al alloy layer is observed as black in backscattered electron images at the interface between the matrix phase and the plating layer. Therefore, the Fe-Al alloy layer is identified visually, and its thickness is measured. Photography is performed at 5 locations per field of view for 5 fields of view, and the average thickness of the Fe-Al alloy layer in the 5 fields of view (25 locations) is taken as the average thickness of the Fe-Al alloy layer in the steel plate according to this embodiment. Furthermore, the length of the interface between the base steel sheet and the Fe-Al alloy layer is measured along the length of the surface of the base steel sheet within the observation field of view to determine the coverage rate. Measurements are performed for five fields of view, and the average of the coverage rates for each field of view is taken as the coverage rate of the steel sheet according to this embodiment.

[0044] "Plating layer" The steel sheet 1 according to this embodiment has a zinc plating layer 30 on the surface of the base steel sheet 10 (the portion without the Fe-Al alloy layer) and / or the Fe-Al alloy layer 20 (the portion on the base steel sheet where the Fe-Al alloy layer is present). The zinc plating layer is, for example, a hot-dip galvanized layer. In this embodiment, the zinc plating layer means a plating layer containing 80% by mass or more of Zn. The presence of a hot-dip galvanized layer on the surface improves corrosion resistance. There are no specific restrictions on the amount of zinc plating layer that can be applied. However, if the amount is too high, the amount of molten zinc used during welding will increase. Therefore, to more effectively suppress the occurrence of LME (Long Metal Emission Mass), the amount of zinc applied should be 100 g / m². 2 The following is preferable: 80g / m 2 The following is preferable. On the other hand, in terms of improving corrosion resistance, the amount of adhesion is 10 g / m 2 It is preferable that the above conditions are met.

[0045] The chemical composition of the zinc plating layer is not limited, but preferably, for example, it contains Al: 0.1 to 2.0% and Fe: 5.0% or less by mass, with the remainder being Zn and impurities. Preferably, the total amount of impurities is 0.1% by mass or less.

[0046] The amount and chemical composition of the zinc plating layer are determined by the following method. The plating layer is melted using hydrochloric acid containing an inhibitor, and the amount of plating deposited is determined by comparing the weight before and after melting. Furthermore, the chemical composition of the plating layer is measured by quantitatively analyzing the resulting solution using ICP.

[0047] "Mechanical properties" When the steel sheet according to this embodiment is used as an automobile steel sheet, it is desirable that it has high strength. Considering its contribution to reducing the weight of automobiles, the tensile strength should be 980 MPa or higher. Preferably, the tensile strength should be 1050 MPa or higher, and more preferably 1100 MPa or higher. On the other hand, when the tensile strength exceeds 2000 MPa, the residual stress during welding increases, causing the internal oxide layer on the grain boundaries to crack, and the effect of suppressing LME cracking decreases significantly. For this reason, the tensile strength should be kept below 2000 MPa.

[0048] "Manufacturing method" The steel sheet according to this embodiment will achieve the above-described effects regardless of the manufacturing method, as long as it possesses the above-described characteristics. However, the manufacturing conditions described below are preferable because they allow for stable production.

[0049] In other words, the steel sheet according to this embodiment can be manufactured by annealing and plating a base steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) under predetermined conditions. There are no restrictions on the manufacturing conditions of the steel sheet used in the annealing process. For example, a hot-rolled steel sheet can be produced by casting molten steel having the above-mentioned chemical composition under normal conditions to form a steel billet, and then hot-rolling it under normal conditions. Alternatively, a cold-rolled steel sheet can be produced by cold-rolling the above-mentioned hot-rolled steel sheet under normal conditions.

[0050] <Annealing process> The annealing process includes a heating process in which a steel sheet having a predetermined chemical composition (the same chemical composition as the steel sheet according to this embodiment to be obtained) is heated to an annealing temperature (maximum heating temperature) of 700 to 1000°C, and a holding process in which the heated steel sheet is held at the annealing temperature for 1 second or more. From a productivity standpoint, it is preferable to perform annealing by passing the steel plates through a continuous annealing line. If the annealing temperature is below 700°C, the amount of austenite will be insufficient, and a sufficient amount of hard structure cannot be secured during the subsequent phase transformation in cooling, resulting in insufficient tensile strength. Therefore, the annealing temperature should be 700°C or higher. Preferably, the annealing temperature is 720°C or higher. On the other hand, if the annealing temperature exceeds 1000°C, the austenite grain size becomes coarser, making it difficult for the transformation to proceed during cooling, and making it difficult to obtain a sufficiently soft structure that contributes to improved formability. For this reason, the annealing temperature should be 1000°C or lower. Preferably, the annealing temperature is 900°C or lower.

[0051] (Heating process) During the heating process, the steel plate is heated to its annealing temperature (maximum heating temperature: 700-1000°C). In the heating process to the annealing temperature during the annealing process, the average heating rate in the first temperature range of 400 to 650°C is set to 2.0°C / second or higher, the average heating rate in the second temperature range of 650°C to the annealing temperature is set to 0.5 to 5.0°C / second, and the (P(H2O) / P(H2)) of the atmosphere in the second temperature range is set to 0.05 to 2.00.

[0052] In the first temperature range of 400-650°C, dislocation recovery primarily occurs during heating. By maintaining an average heating rate of 2.0°C / second or higher in this temperature range, dislocation recovery can be suppressed, leaving more dislocations that serve as nucleation sites for recrystallization. In this case, recrystallization, which typically occurs in the temperature range above 650°C, can be induced in many locations. While there is no upper limit to the average heating rate in the first temperature range, from a cost perspective, an average heating rate of 20.0°C / second or less is preferable.

[0053] The second temperature range, from 650°C to the annealing temperature, is the temperature range where recrystallization occurs, and where an internal oxide layer is formed by controlling the atmosphere. If the average heating rate in this temperature range exceeds 5.0°C / second, recrystallization of the steel sheet proceeds before oxides can form on the surface, resulting in the generation of coarse ferrite grains. In this case, an internal oxide layer, where the grain boundaries are covered with oxides, is not formed. On the other hand, if the average heating rate is less than 0.5°C / second, the decarburization reaction may proceed excessively, potentially reducing the tensile strength of the steel sheet. Furthermore, when heating in the second temperature range, the ratio of the partial pressure of water vapor P(H2O) to the partial pressure of hydrogen P(H2) in the furnace atmosphere, (P(H2O) / P(H2)), must be such that if (P(H2O) / P(H2)) is less than 0.05, a sufficient internal oxidation layer cannot be secured. Therefore, (P(H2O) / P(H2)) should be 0.05 or higher. (P(H2O) / P(H2)) is preferably 0.07 or higher, and more preferably 0.10 or higher. On the other hand, if (P(H2O) / P(H2)) exceeds 2.00, decarburization proceeds excessively, the thickness of the decarburized layer increases, and the tensile strength of the steel sheet decreases. Therefore, (P(H2O) / P(H2)) should be 2.00 or less. (P(H2O) / P(H2)) is preferably 1.50 or less, more preferably 1.20 or less. By controlling the average heating rate and atmosphere in the second temperature range, recrystallization is initiated from many recrystallization nuclei, and at the same time, internal oxides are generated at the grain boundaries, thereby inhibiting grain growth and simultaneously achieving a grain boundary coverage of 60% or more by oxide.

[0054] Furthermore, in order to control dislocation recovery, recrystallization, and grain growth, and to efficiently generate internal oxides at grain boundaries, it is preferable that the average heating rate in the first temperature range is faster than the average heating rate in the second temperature range, and more preferably that the average heating rate in the first temperature range is 2.0°C / second or more faster than the average heating rate in the second temperature range.

[0055] (retention process) After heating to the annealing temperature as described above, the steel plate is held at the predetermined maximum heating temperature for at least one second. If the holding time is less than one second, austenitization will not be sufficient. In this case, a sufficient amount of hard structure cannot be secured during the subsequent phase transformation while cooling, and sufficient tensile strength cannot be obtained. There is no specific upper limit on the holding time. However, since excessively long holding times can impair the manufacturability of the steel sheet, it is preferable to set the upper limit of the holding time at 1000 seconds.

[0056] <Plating Process> In the plating process, the steel sheet after the annealing process is cooled from the annealing temperature to 440-550°C at an average cooling rate of 0.5°C / second or more. The steel sheet is then immersed in a plating bath mainly composed of Zn with an effective Al content of 0.050-0.250% by mass. After being removed from the plating bath, it is cooled so that it reaches 400°C in 10 seconds or less, and then cooled to 350°C or below, with an average cooling rate of 1.0°C / second or more and 5.0°C / second or less between 400°C and 350°C. This forms a zinc plating layer on the surface of the steel sheet and also forms an Fe-Al alloy layer at least on a portion of the interface between the steel sheet and the plating layer.

[0057] If the average cooling rate from 440 to 550°C is less than 0.5°C / second, a hard structure will not form in the base steel sheet, resulting in a decrease in strength. Furthermore, if the cooling stop temperature (the temperature of the steel plate when immersed in the plating bath) is below 440°C, it becomes necessary to supply a large amount of heat to the plating bath in order to maintain the plating temperature, which increases manufacturing costs. On the other hand, if the temperature of the steel plate when it is immersed in the plating bath exceeds 550°C, equipment is required to remove a large amount of heat from the plating bath in order to maintain the temperature of the plating bath, which increases manufacturing costs.

[0058] The composition of the plating bath into which the steel plate is immersed is mainly Zn (for example, 80% by mass or more), with an effective Al content (total A in the plating bath). l If the value obtained by subtracting the total amount of Fe from the total amount is between 0.050 and 0.250 mass%, it is not limited and may contain one or more other elements as needed, such as Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, S, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM. If the amount of effective Al in the plating bath is less than 0.050 mass%, the formation of the Fe-Al alloy layer will be insufficient. Furthermore, excessive penetration of Fe into the plating layer may occur, potentially reducing the adhesion of the plating. The amount of effective Al in the plating bath is preferably 0.065 mass% or more. On the other hand, if the amount of effective Al in the plating bath exceeds 0.250% by mass, Al-based oxides that inhibit the movement of Fe and Zn atoms may be formed at the boundary between the steel sheet and the plating layer, potentially reducing the adhesion of the plating. The amount of effective Al in the plating bath is preferably 0.180% by mass or less. The plating bath temperature is not limited, but is preferably 450 to 490°C. If the plating bath temperature is below 450°C, the viscosity of the plating bath will increase excessively, making it difficult to control the thickness of the plating layer, which may impair the appearance of the hot-dip galvanized steel sheet. The plating bath temperature is preferably 455°C or higher. On the other hand, if the plating bath temperature exceeds 490°C, a large amount of fumes may be generated, making safe plating operations difficult. The plating bath temperature is preferably 480°C or lower.

[0059] After removing the steel plate from the plating bath, the amount of coating is adjusted by wiping with N2 gas or the like, and the plate is cooled so that it reaches 400°C in 10 seconds or less. Then, it is cooled to 350°C or below so that the average cooling rate between 400°C and 350°C is 1.0°C / second or more and 5.0°C / second or less. The plating layer primarily undergoes alloying with the matrix phase at temperatures above 400°C. Therefore, by cooling the plate so that the time it takes to reach 400°C after being removed from the plating bath is within 10 seconds (reaching 400°C within 10 seconds), excessive alloying of the plating layer can be prevented. If the time to reach 400°C exceeds 10 seconds, an Fe-Zn alloy layer is formed instead of an Fe-Al alloy layer, and the desired Fe-Al alloy layer cannot be obtained. Subsequently, by setting the average cooling rate at 400-350°C to 5.0°C / second or less so that the time spent at 400-350°C is 10.0 seconds or more, an Fe-Al alloy layer can be formed at the interface between the plating layer and the matrix phase, and its coverage can be made 40% or more. On the other hand, if the time spent at 400-350°C exceeds 50.0 seconds, the thickness of the Fe-Al alloy layer will exceed 100 nm. Therefore, the average cooling rate at 400-350°C should be 1.0°C / second or higher. Furthermore, it is preferable that the plating bath temperature is set to over 450°C, the time to reach 400°C is within 10 seconds (i.e., the average cooling rate to 400°C is over 5.0°C / second), and then the average cooling rate from 400°C to 350°C is 5.0°C / second or less, so that the average cooling rate to 400°C is greater than the average cooling rate from 400°C to 350°C. By performing cooling in this two-stage process, an appropriate Fe-Al alloy layer is formed at the interface between the zinc plating layer and the base steel sheet. The Fe-Al alloy layer formed at the interface between the zinc plating layer and the base steel sheet incorporates and integrates the internal oxides at the grain boundaries in the surface layer of the steel sheet. As a result, the Fe-Al alloy layer is less likely to peel off from the base steel sheet, and a higher bending fatigue strength can be obtained. It is more preferable that the average cooling rate to 400°C is 6.0°C / second or higher.

[0060] <Skin pass rolling process> In the steel sheet manufacturing method according to this embodiment, skin pass rolling may be performed on the steel sheet after the plating process for the purpose of shape adjustment, etc. When skin pass rolling is performed, it is preferable to set the rolling ratio to 0.5% or less. [Examples]

[0061] Slabs having the chemical compositions listed in Tables 1-1 to 1-4 were hot-rolled and coiled to obtain hot-rolled steel sheets with a thickness of 3.0 mm. These hot-rolled steel sheets were then cold-rolled to obtain cold-rolled steel sheets with a thickness of 1.6 mm. This cold-rolled steel sheet was heated to the annealing temperature and held under the conditions shown in Tables 2-1 and 2-2. After holding, the material was cooled to 440-550°C as shown in Tables 2-1 and 2-2, and then immersed in a plating bath to form a zinc plating layer. After immersion in the plating bath, the material was cooled to 400°C, with the cooling time and average cooling rate being as shown in Tables 2-1 and 2-2. Subsequently, the cooling process was switched, and the material was cooled to below 350°C, with the average cooling rate between 400°C and 350°C being as shown in Tables 2-1 and 2-2. Subsequently, skin pass rolling was performed under the conditions shown in Tables 2-1 and 2-2.

[0062] [Table 1-1]

[0063] [Table 1-2]

[0064] [Table 1-3]

[0065] [Table 1-4]

[0066] [Table 2-1]

[0067] [Table 2-2]

[0068] The obtained galvanized steel sheets (hot-dip galvanized steel sheets) were observed using the method described above, and the thickness of the internal oxide layer, the grain boundary coverage rate of the internal oxide layer, and the microstructure at the 1 / 4 thickness position were observed. The results are shown in Tables 3-1 and 3-2. Although not shown in the table, the amount of zinc plating layer attached is 10-80 g / m². 2 That was the case.

[0069] Furthermore, the thickness of the Fe-Al alloy layer and the coverage rate of the Fe-Al alloy layer were measured on the obtained galvanized steel sheet using the method described above. The results are shown in Tables 3-1 and 3-2.

[0070] Furthermore, the tensile strength (TS), LME resistance, bending fatigue strength, and bending resistance of the obtained galvanized steel sheets were evaluated according to the following procedure.

[0071] <Tensile strength> Tensile test specimens of JIS No. 5 were taken from the galvanized steel sheet in a direction perpendicular to the rolling direction and thickness direction (the rolling direction and thickness direction of the base steel sheet) (width direction), and tensile tests were performed in accordance with JIS Z 2241:2011 to measure the tensile strength (TS). We determined that a tensile strength of 980 MPa or higher constituted high strength.

[0072] <LME resistance> A 30mm x 30mm sample was taken from a galvanized steel sheet, and two of these samples were stacked together. A spot welding test was then performed by applying current under the following conditions. Electrode:φ8,R40 Pressing force: 4.5kN Electrode tilt angle: 4° Upslope: 0.02 seconds Power-on time: 0.4 seconds Holding time after energization: 0.3 seconds The current was set so that the nugget diameter would be 5 mm. The cross-section of the evaluation material after spot welding was observed using a scanning electron microscope (SEM), and the crack length was evaluated as follows. Crack length of 80 μm or less: OK (Excellent resistance to LME) Crack length exceeding 80 μm: NG

[0073] <Bending resistance> Bending tests were conducted according to VDA238-100, and the bending angle (°) was evaluated as follows. The test specimens were taken in a direction parallel to the rolling direction, with the bending ridge parallel to the rolling direction. Bending angle > 110-TS × 0.03: Ex (exceptionally good bending resistance) Bending angle > 110-TS × 0.04: OK (Excellent bending resistance) Bending angle ≤ 110 - TS × 0.04: NG

[0074] <Bending fatigue strength> A planar bending fatigue test was conducted in accordance with JIS Z 2275:1978. The test specimen was a No. 1 specimen with a width of 30 mm and a radius of R 40 mm. The test results showed that the number of repetitions was 10. 6The fatigue strength of the cycles was evaluated as follows. 10 6 Fatigue strength of cycles > 0.35 × TS: Ex (particularly excellent in bending fatigue strength) 10 6 Fatigue strength of cycles > 0.30 × TS: OK (excellent in bending fatigue strength) 10 6 Fatigue strength of cycles ≤ 0.30 × TS: NG

[0075]

Table 3-1

[0076]

Table 3-2

[0077] As can be seen from Tables 1-1 to 3-2, in Nos. 1, 8, 11, 12, 17 to 46 where the chemical composition and manufacturing method are within the scope of the present invention, favorable Fe-Al alloy layers and internal oxidation layers were formed, and all of the tensile strength, LME resistance, bending fatigue strength, and bending resistance were excellent. On the other hand, in Nos. 47 to 56 where the chemical composition was outside the scope of the present invention, at least one of the tensile strength, LME resistance, bending fatigue strength, and bending resistance was inferior. Also, even when the chemical composition was within the scope of the present invention, in Nos. 2 to 7, 9, 10, 13 to 16 where the manufacturing method was outside the scope of the present invention, the tensile strength was low, or the internal oxidation layer and Fe-Al alloy layer were not formed in a favorable state, resulting in inferiority in any one or more of the LME resistance, bending fatigue strength, and bending resistance.

Explanation of Signs

[0078] 1 Steel sheet (galvanized steel sheet) 10 Base metal steel sheet 11 Internal oxidation layer 20 Fe-Al alloy layer 30 Galvanized layer

Industrial Applicability

[0079] According to the present invention, it is possible to provide a galvanized steel sheet that is high in strength, has sufficient bending resistance, and exhibits excellent LME resistance and bending fatigue strength. Such a steel sheet is useful as a high-strength steel sheet for automobiles.

Claims

1. Base material steel plate, An Fe-Al alloy layer formed on at least a portion of the surface of the base steel sheet, A zinc plating layer formed on the surface of the base steel plate or the Fe-Al alloy layer, Equipped with, The aforementioned base steel plate is, by mass%, C: 0.10-0.40%, Si: 0.10-3.00%, Mn: 1.00-5.00%, Sol. Al: 0.001–1.500%, P: 0.0010-0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0-0.200%, B: 0 to 0.0100%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-1.000%, Cu: 0 to 1.000%, Sn: 0-0.500%, Nb: 0 to 0.200%, V: 0 to 0.500%, W: 0-0.500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Bi: 0 to 0.0100%, Sb: 0 to 0.1000%, Zr: 0 to 0.0100%, REM: 0 to 0.1000%, and Remainder: Fe and impurities, It has a chemical composition consisting of, The base steel sheet has an internal oxide layer of 0.2 μm or more in the thickness direction from the surface of the base steel sheet. The average thickness of the Fe-Al alloy layer is 1 nm or more and less than 100 nm. In the cross-section in the thickness direction, In the aforementioned internal oxide layer, the grain boundary coverage rate by oxides is 60% or more, The coverage rate of the surface of the base steel sheet by the Fe-Al alloy layer is 40% or more. The tensile strength is between 980 MPa and 2000 MPa. Galvanized steel sheet.

2. The chemical composition of the base steel sheet is, in mass%, Ti: 0.005-0.200%, B: 0.0005-0.0100%, Cr: 0.001-1.000%, Mo: 0.001 to 1.000%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Sn: 0.001 to 0.500%, Nb: 0.001-0.200%, V: 0.001-0.500%, W: 0.001-0.500%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Bi: 0.0001-0.0100%, Sb: 0.0001 to 0.1000%, Zr: 0.0001 to 0.0100%, and REM: 0.0001-0.1000%, It contains one or more selected from the group consisting of, The galvanized steel sheet according to claim 1.

3. A method for manufacturing a galvanized steel sheet according to Claim 1, In mass percent, C: 0.10-0.40%, Si: 0.10-3.00%, Mn: 1.00-5.00%, sol. Al: 0.001-1.500%, P: 0.0010-0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0-0.200%, B: 0-0.0100%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-1.000%, Cu: 0-1.000%, Sn: 0-0.500%, Nb: 0-0.200% An annealing step is performed by holding a steel sheet having a chemical composition consisting of %, V: 0-0.500%, W: 0-0.500%, Ca: 0-0.0100%, Mg: 0-0.0100%, Bi: 0-0.0100%, Sb: 0-0.1000%, Zr: 0-0.0100%, REM: 0-0.1000%, and the remainder: Fe and impurities, at an annealing temperature of 700-1000°C for 1 second or more. A plating step in which a zinc plating layer is formed on the surface of the steel sheet after the annealing step, It has, In the aforementioned annealing process, In the heating process up to the annealing temperature, the average heating rate in the first temperature range of 400 to 650°C is set to 2.0°C / second or more, the average heating rate in the second temperature range of 650°C to the annealing temperature is set to 0.5 to 5.0°C / second, and in the second temperature range, the (P(H2O) / P(H2)) of the atmosphere is set to 0.05 to 2.

00. In the aforementioned plating process, the steel plate is cooled to 440-550°C at an average cooling rate of 0.5°C / second or more, immersed in a plating bath mainly composed of Zn with an effective Al content of 0.050-0.250% by mass, removed from the plating bath, cooled so that it takes 10 seconds or less to reach 400°C, and then cooled to 350°C or below, with an average cooling rate of 1.0°C / second or more and 5.0°C / second or less between 400°C and 350°C. A method for manufacturing galvanized steel sheets.

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