Steel sheet and plated steel sheet

Abstract

The present invention relates to a steel sheet and a plated steel sheet, each of which has high LME resistance. The steel sheet according to the present invention is characterized in that: the steel sheet has a tensile strength of 780 MPa or more; the steel sheet has a specific chemical component; the depth, at which the C concentration as measured by GDS is 0.05% or less, from the surface of the steel sheet in the thickness direction of the steel sheet is 8 μm or more; the thickness of a layer, in which the area ratio of the ferrite phase is 90% or more, in the thickness direction of the steel sheet from the surface of the steel sheet is 8 μm or more; and the solid solution amount of Si metal and Al metal in the ferrite phase is 0.3% or more in total at the position of 4 μm from the surface of the steel sheet in the thickness direction of the steel sheet.

Description

Steel sheets and plated steel sheets

[0001] The present invention relates to a steel sheet and a plated steel sheet, and more specifically to a steel sheet and a plated steel sheet having high LME resistance and hydrogen desorption properties.

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, building materials, etc. For example, in the automobile field, the use of high-strength steel sheets has been increasing with the aim of reducing the weight of vehicle bodies to improve fuel efficiency.

[0003] When welding zinc-based plated steel sheets, particularly high-strength steel sheets, deterioration of weldability due to liquid metal embrittlement (LME) cracking can become a problem, as described in Patent Document 1, for example. LME cracking is thought to occur when the surface layer of the steel sheet transforms to austenite during welding, molten zinc penetrates into the grain boundaries, embrittling the steel sheet, and further when tensile stress is applied to the steel sheet during welding.

[0004] Patent Document 2 describes a steel sheet that suppresses LME cracking and improves weldability, and describes a steel sheet having a surface layer containing silicon oxide particles with a particle size of 20 nm or more at a density of 3,000 to 6,000 particles / mm 2 The patent discloses a steel sheet having a suitable grain size distribution at a number density of 1000.degree.

[0005] International Publication No. WO 2019 / 116531 International Publication No. WO 2020 / 218575

[0006] In order to prevent LME cracking, it is effective to prevent Zn and other elements contained in the coating from penetrating into the austenitic steel sheet. In this respect, there is room for improvement.

[0007] It is also known that when high-strength steel plates are exposed to atmospheric corrosion environments with large fluctuations in temperature and humidity, hydrogen generated during the corrosion process penetrates into the steel. Hydrogen that penetrates into the steel segregates at the martensite grain boundaries of the steel structure, embrittling the grain boundaries and potentially causing cracks in the steel plate. The phenomenon of cracks caused by penetrated hydrogen is called hydrogen embrittlement cracking (delayed fracture) and is often a problem during steel plate processing. To prevent this, it is effective to promote hydrogen desorption from the steel plate into the atmosphere when hydrogen penetrates into the steel plate.

[0008] In view of the above circumstances, an object of the present invention is to provide a steel sheet and a plated steel sheet having high LME resistance and hydrogen desorption properties.

[0009] The present inventors have intensively investigated means for solving the above-mentioned problems. As a result, they have found that it is possible to suppress LME by forming a layer of a decarburized ferrite phase containing Si and Al in the surface layer of a steel sheet. Furthermore, they have found that covering the surface layer of a steel sheet with a ferrite phase having a low solid solution content of C promotes hydrogen desorption from the steel sheet into the atmosphere even when hydrogen penetrates into the steel sheet.

[0010] The present invention was made based on the above findings and through further investigation, and the gist of the present invention is as follows.

[0011] (1) A steel plate having a tensile strength of 780 MPa or more, the chemical composition of which is, in mass %, C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.5 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.1500%, V: 0 to 0.150%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.0000%, Mo: 0 to 1.00%, W: 0 to 1.000%, Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, REM: 0 to 0.1000%, and the balance being Fe and impurities, and Si and sol. a depth of 8 μm or more from the surface of the steel sheet in the thickness direction of the steel sheet at which a C concentration measured by GDS is 0.05% or less; a thickness of a layer in which an area ratio of ferrite phase is 90% or more from the surface of the steel sheet in the thickness direction of the steel sheet is 8 μm or more; and a total amount of solid solution of metallic Si and metallic Al in the ferrite phase at a position 4 μm from the surface of the steel sheet in the thickness direction of the steel sheet is 0.3% or more.

[0012] (2) The steel sheet according to (1), wherein the depth at which the C concentration measured by GDS is 0.05% or less is 15 μm or more from the surface of the steel sheet in the thickness direction of the steel sheet.

[0013] (3) The steel sheet according to (1), characterized in that a layer having an area ratio of ferrite of 90% or more has a thickness of 20 μm or more from the surface of the steel sheet in the thickness direction of the steel sheet, and the total amount of solid solution of metal Si and metal Al in the ferrite phase is 0.5% or more.

[0014] (4) A galvannealed steel sheet comprising any one of the steel sheets (1) to (3) above, and a galvannealed layer on a part of the surface thereof.

[0015] (5) The galvannealed steel sheet according to (4), wherein the galvannealed layer contains, by mass%, 0 to 1.5% Al and 3 to 15% Fe, with the balance being Zn and impurities.

[0016] According to the present invention, a steel sheet and a plated steel sheet having high LME resistance and hydrogen desorption properties can be obtained.

[0017] 1 is a schematic diagram illustrating a lamellar ferrite phase in a steel sheet according to the present invention, and FIG. 2 is a diagram illustrating the position of a crack targeted in an LME resistance evaluation in an example.

[0018] An embodiment of the present invention will be described below. The present invention is not limited to the following embodiment. First, an outline of improving LME resistance in this embodiment will be described.

[0019] LME cracking is believed to occur when the surface layer of a steel sheet is heated and transformed into austenite during spot welding, and molten zinc from the coating penetrates the austenite grain boundaries, embrittling the steel sheet. Furthermore, tensile stress is applied to the steel sheet during welding. The inventors have come up with the idea of ​​utilizing the microstructure of the surface layer of the steel sheet, which is located near the interface with the coating layer, as a method for improving LME resistance. Specifically, they have come up with the idea of ​​suppressing LME occurrence by making the microstructure of the surface layer of the steel sheet mainly ferrite with a low C concentration and low LME susceptibility, and further stabilizing the ferrite by incorporating metallic Si and metallic Al into the ferrite phase as a solid solution. Furthermore, they have come up with the idea of ​​covering the surface layer of the steel sheet with a ferrite phase with a low C solid solution content to promote hydrogen desorption from the steel sheet into the atmosphere.

[0020] In this embodiment, when a steel sheet is annealed, decarburization occurs in the surface layer of the steel sheet and internal oxidation from the surface layer of the steel sheet toward the inside of the steel sheet occurs in the first half of the annealing, and in the second half of the annealing, only decarburization occurs, and Si and Al are diffused from the inside of the steel sheet toward the surface side, thereby dissolving them in the ferrite phase of the surface layer of the steel sheet. This embodiment was made based on the finding that imparting strain to the surface layer of the steel sheet and annealing at an appropriate dew point can promote decarburization and internal oxidation as described above. In this specification, the "surface layer" refers to a range from the surface of the steel sheet to a depth of about 100 μm.

[0021] This embodiment will be described in detail below.

[0022] <Steel Plate> [Tensile Strength] The steel plate of this embodiment has a tensile strength of 780 MPa or more. Since this embodiment suppresses LME that occurs in high-strength steel plates, the steel plate of this embodiment has high strength. Specifically, it has a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness. The tensile strength is measured in accordance with JIS Z 2241:2011 using a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction. The tensile strength may be 980 MPa or more, or 1180 MPa or more.

[0023] [Chemical Composition of Steel Sheet] The chemical composition of the steel sheet of this embodiment will be described below. Hereinafter, "%" regarding the chemical composition of the steel sheet means "mass %." Furthermore, in the numerical range of the chemical composition, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0024] (C: 0.05 to 0.40%) C (carbon) is an element that ensures the strength of steel. In order to obtain a tensile strength of 780 MPa or more, which is the target of this embodiment, the C content is set to 0.05% or more. In order to prevent the C concentration in the surface layer, which will be described later, from becoming too high, and taking weldability into consideration, the C content is set to 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0025] (Si: 0.7 to 3.0%) Si (silicon) is an element that promotes ferrite stabilization and decarburization when added in combination with Al (aluminum). It has been known that adding Si to steel reduces LME resistance. However, as a result of research by the present inventors, it has been found that, contrary to conventional knowledge, adding large amounts of both Si and Al improves LME resistance. This is thought to be because the heat treatment described below promotes decarburization in the surface layer and stabilizes ferrite in the surface layer, thereby resulting in a greater effect of improving LME resistance due to decarburization and ferrite stabilization than the decrease in LME resistance caused by Si. To achieve this effect, the Si content is set to 0.7% or more. If the Si content is too high, external oxidation will progress, even when high-dew-point annealing is performed, and oxides (scale) will form in the surface layer of the steel sheet. Conversely, decarburization at the outermost surface will be suppressed, reducing the effect of improving LME resistance. In consideration of this point, the Si content is set to 3.0% or less. The Si content may be 0.8% or more, 0.9% or more, or 1.0% or more. The Si content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0026] (Mn: 0.1 to 5.0%) Mn (manganese) is an element effective in improving the strength of steel by obtaining a hard structure. In consideration of the strength of the steel, the Mn content is set to 0.1% or more. In addition, in consideration of the deterioration of workability due to Mn segregation, the Mn content is set to 5.0% or less. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

[0027] (Sol. Al: 0.5 to 2.0%) Aluminum (Al) is an element that dissolves in steel when added in combination with silicon (Si) and, like silicon, promotes ferrite stabilization and decarburization. Sol. Al refers to acid-soluble Al that is soluble in acid and does not form oxides such as Al2O3. The amount of Al measured is determined by subtracting the insoluble residue on the filter paper generated during the Al analysis process. To achieve the above-mentioned effect, the sol. Al content is set to 0.5% or more. If the sol. Al content is too high, even with high dew-point annealing, external oxidation progresses, forming oxides (scale) on the surface of the steel sheet. Conversely, decarburization on the outermost surface is suppressed, reducing the effect of improving LME resistance. Taking this into consideration, the sol. Al content is set to 3.0% or less. The sol. Al content may be 0.6% or more, 0.7% or more, 0.8% or more, or 1.0% or more. The Al content may be 1.8% or less, 1.6% or less, or 1.5% or less.

[0028] (Si + sol. Al: 1.3% or more) In order to obtain the effect of improving LME resistance as described above, the total content of Si and sol. Al is set to 1.3% or more. In order to increase the contents of metal Si and metal Al dissolved in the ferrite phase, which will be described later, the total content of Si and sol. Al is set to 1.3% or more. The total content of Si and sol. Al may be 1.4% or more, 1.5% or more, 1.6% or more, 1.8% or more, or 2.0% or more. The upper limit of the content of Si and sol. Al may be the sum of the respective upper limits of the content of Si and the content of sol. Al described above.

[0029] (P: 0.0300% or less) P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, weldability may be reduced. Therefore, the P content is set to 0.0300% or less. The P content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that no P is contained, and the lower limit of the P content is 0. From the viewpoint of dephosphorization costs, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.

[0030] (S: 0.0300% or less) S (sulfur) is an impurity generally contained in steel. If the S content exceeds 0.0300%, weldability will decrease, and further, the amount of MnS precipitation will increase, which may decrease workability such as bendability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that no S is contained, and the lower limit of the S content is 0. From the viewpoint of desulfurization costs, the S content may be more than 0%, 0.0001% or more, or 0.0005% or more.

[0031] (N: 0.0100% or less) N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may be reduced. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N is not contained, and the lower limit of the N content is 0. From the viewpoint of production costs, the N content may be more than 0%, 0.0005% or more, or 0.0010% or more.

[0032] (B: 0 to 0.0100%) B (boron) is an element that improves hardenability and contributes to improving strength, and also segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as needed. Since B is not an essential element, the lower limit of the B content is 0. Although this effect can be obtained even with a small amount of B, if B is contained, the B content is preferably 0.0001% or more. Furthermore, from the viewpoint of ensuring toughness, the B content is set to 0.0100% or less. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0020% or less.

[0033] (Ti: 0 to 0.1500%) Ti (titanium) is an element that precipitates as TiC during cooling of the steel and contributes to improving strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ti content is 0. This effect can be obtained even with a small amount of Ti, but when Ti is contained, the Ti content is preferably 0.0001% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, which may impair toughness, so the Ti content may be 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0005% or more. The Ti content is 0.1500% or less. The Ti content may be 0.1200% or less, 0.1000% or less, 0.0500% or less, 0.0300% or less, 0.0150% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0034] (Nb: 0 to 0.150%) Nb (niobium) is an element that contributes to improving strength by improving hardenability, so it may be contained as needed. Since it is not an essential element, the lower limit of the Nb content is 0. Although this effect can be obtained even with a small amount of Nb, the Nb content, when contained, is preferably 0.001% or more. On the other hand, from the viewpoint of ensuring toughness, the Nb content is set to 0.150% or less. The Nb content may be 0.002% or more, 0.003% or more, 0.005% or more, 0.007% or more, or 0.010% or more. The Nb content may be 0.120% or less, 0.100% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

[0035] (V: 0 to 0.150%) V (vanadium) is an element that contributes to improving strength by improving hardenability, so it may be contained as needed. Since it is not an essential element, the lower limit of the V content is 0. Although this effect can be obtained even with a small amount of V, when V is contained, the V content is preferably 0.001% or more. On the other hand, from the viewpoint of ensuring toughness, the V content is set to 0.150% or less, and the V content may be 0.002% or more, 0.003% or more, 0.005% or more, or 0.008% or more. The V content may be 0.120% or less, 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

[0036] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength, and may therefore be contained as needed. Since Cr is not an essential element, the lower limit of the Cr content is 0. This effect can be obtained even with trace amounts, but if Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, excessive inclusion of Cr can result in the formation of a large amount of Cr carbide, which may in turn impair hardenability. Therefore, the Cr content is set to 2.00% or less. The Cr content may be 0.02% or more, 0.04% or more, 0.05% or more, 0.07% or more, or 0.10% or more. The Cr content may be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0037] (Ni: 0 to 2.00%) Ni (nickel) is effective in improving the hardenability of steel and increasing its strength, and may be contained as needed. Since it is not an essential element, the lower limit of the Ni content is 0. This effect can be obtained even with a small amount of Ni contained, but when Ni is contained, the Ni content is preferably 0.001% or more. On the other hand, since excessive addition of Ni increases costs, the Ni content is set to 2.00% or less. The Ni content may be 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. The Ni content may be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.20% or less, or 0.15% or less.

[0038] (Cu: 0 to 2.0000%) Cu (copper) is effective in improving the hardenability of steel and increasing the strength of steel, so it may be contained as needed. Since it is not an essential element, the lower limit of the Cu content is 0. This effect can be obtained even with a small amount of Cu, but when Cu is contained, the Cu content is preferably 0.0001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, cracking of the slab after casting, and a decrease in weldability, the Cu content is set to 2.0000% or less. The Cu content may be 0.0002% or more, 0.0004% or more, or 0.0005% or more. The Cu content may be 1.8000% or less, 1.5000% or less, 1.2000% or less, 0.8000% or less, 0.6000% or less, 0.4000% or less, 0.2000% or less, 0.1000% or less, 0.0070% or less, 0.0050% or less, 0.0035% or less, 0.0020% or less, or 0.0015% or less.

[0039] (Mo: 0 to 1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing its strength, and may therefore be contained as needed. Since it is not an essential element, the lower limit of the Mo content is 0. This effect can be obtained even with a small amount of Mo contained, but when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. The Mo content may be 0.90% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, or 0.20% or less.

[0040] (W: 0 to 1.000%) W (tungsten) is effective in improving the hardenability of steel and increasing its strength, and may be contained as needed. Since it is not an essential element, the lower limit of the W content is 0. This effect can be obtained even with a small amount of W, but when W is contained, the W content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. The W content may be 0.002% or more, or 0.003% or more. The W content may be 0.900% or less, 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0041] (Ca: 0 to 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the Ca content is 0. This effect can be obtained even with a small amount of Ca contained, but when Ca is contained, the Ca content is preferably 0.0001% or more. On the other hand, if it is contained in excess, deterioration of surface properties may become apparent, so the Ca content is set to 0.1000% or less. The Ca content may be 0.0002% or more, or 0.0003% or more. The Ca content may be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0025% or less, 0.0015% or less, or 0.0010% or less.

[0042] (Mg: 0 to 0.100%) Mg (magnesium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the Mg content is 0. Although this effect can be obtained even with a small amount of Mg contained, the Mg content, if contained, is preferably 0.0001% or more. On the other hand, if contained in excess, deterioration of surface properties may become apparent, so the Mg content is set to 0.100% or less. The Mg content may be 0.0003% or more, 0.0005% or more, or 0.0008% or more. The Mg content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

[0043] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Zr content is 0. Although this effect can be obtained even with a small amount of Zr, the Zr content, if contained, is preferably 0.001% or more. On the other hand, excessive Zr content may cause deterioration of surface properties to become apparent, so the Zr content is set to 0.100% or less. The Zr content may be 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. The Zr content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0044] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0. This effect can be obtained even with a small amount of Hf contained, but when contained, the Hf content is preferably 0.0001% or more. On the other hand, if contained in excess, deterioration of surface properties may become apparent, so the Hf content is set to 0.100% or less. The Hf content may be 0.0002% or less, 0.0003% or more, 0.0005% or more, or 0.0008% or more. The Hf content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

[0045] (REM: 0 to 0.1000%) REM (rare earth element) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the REM content is 0. This effect can be obtained even with a small amount of REM contained, but when REM is contained, the REM content is preferably 0.0001% or more. On the other hand, if it is contained in excess, deterioration of surface properties may become apparent, so the REM content is set to 0.1000% or less. The REM content may be 0.0003% or more, 0.0004% or more, or 0.0005% or more. The REM content may be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. REM is an abbreviation for Rare Earth Metal and refers to elements belonging to the lanthanide series. REM is usually added as misch metal.

[0046] In the steel sheet according to this embodiment, the balance other than the above chemical components consists of Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap, and that do not adversely affect the LME resistance and hydrogen desorption properties of the steel sheet according to this embodiment, i.e., are contained in a range that allows the steel sheet according to this embodiment to achieve the LME resistance and hydrogen desorption properties required for the steel sheet according to this embodiment. Specific examples of elements include O (oxygen). The content of O contained as an impurity may be, for example, 0.0500% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less. However, from the viewpoint of manufacturing costs, the lower limit of the O content may be 0.00001%, 0.00005%, or 0.0001%.

[0047] The chemical composition of the steel sheet may be analyzed using an elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. O may be measured using an inert gas fusion-infrared absorption method. These analyses may be performed on samples collected from the steel sheet according to JIS G0417:1999.

[0048] [Surface Layer Portion] Next, the surface layer portion of the steel sheet will be described.

[0049] (C concentration in surface layer) In the steel sheet of this embodiment, the depth from the surface of the steel sheet in the thickness direction of the steel sheet at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.05% or less is 8 μm or more.

[0050] Since the susceptibility to LME decreases as the carbon concentration decreases, a low carbon concentration in the surface layer improves LME resistance. Furthermore, carbon is an austenite-stabilizing element, so a low carbon concentration stabilizes the layer with low LME susceptibility, as described below. Furthermore, a low carbon content in the surface layer facilitates the escape of hydrogen that has penetrated into the steel, improving hydrogen desorption. This is presumably because the low presence of carbon, an interstitial element, in the ferrite phase makes it easier for hydrogen to pass through.

[0051] Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and subjecting it to the pretreatment and heat treatment described below.

[0052] Since the effect of improving LME resistance can be obtained as long as the depth at which the C concentration is 0.05% or less is 8 μm or more, there is no particular limitation on the upper limit of the depth at which the C concentration is 0.05% or less. The depth at which the C concentration is 0.05% or less may be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. The depth at which the C concentration is 0.02% or less is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more, or 20 μm or more.

[0053] The GDS measurement is performed five times in the sheet thickness direction, and the average value is taken as the C concentration. The measurement conditions are as follows:

[0054] Apparatus: High-frequency glow discharge optical emission analyzer (manufactured by LECO Japan LLC, model number "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds

[0055] (High Ferrite Layer) In the steel sheet of this embodiment, a layer having an area ratio of ferrite phase of 90% or more (hereinafter referred to as "high ferrite layer") has a thickness of 8 μm or more from the surface of the steel sheet in the thickness direction of the steel sheet. FIG. 1 shows a schematic diagram of the vicinity of the surface layer of the steel sheet of this embodiment. FIG. 1 is a cross section of the steel sheet in the thickness direction, with the upper side being the steel sheet surface. In FIG. 1, the high ferrite layer 12 is located in the surface layer portion of the steel sheet 11 and has a thickness of 8 μm or more. The inner side 13 of the steel sheet has a structure mainly composed of martensite and containing ferrite. As will be described later, when a galvannealed layer is formed on the surface of the steel sheet to produce a galvannealed steel sheet, the galvannealed layer is formed on the surface of the high ferrite layer 12 (the side opposite the inner side 13 of the steel sheet).

[0056] Since the effect of improving LME resistance can be obtained as long as the thickness of the high ferrite layer is 8 μm or more, the upper limit of the thickness is not particularly limited. For example, it may be 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. The thickness of the high ferrite layer is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more, 20 μm or more, or 25 μm or more.

[0057] The structure of the high ferrite layer other than ferrite is not limited, and may be, for example, one or more of martensite, bainite, and cementite.

[0058] The thickness of the high ferrite layer is determined by nital etching the thickness cross section of the steel sheet and observing it with an SEM at 1000x magnification, and distinguishing between martensite, bainite, and hard structures containing relatively large amounts of cementite, and ferrite, based on the structural morphology. The thickness of the high ferrite layer is determined by measuring a 500 μm range in the direction perpendicular to the thickness direction, and measuring five measurement ranges at 1000 μm intervals in the direction perpendicular to the thickness direction, and averaging the measurements. Here, the ferrite area ratio refers to the area ratio determined by observing the above-mentioned thickness cross section. Even if there are localized locations in the thickness direction where the ferrite area ratio is less than 90%, for example, there is no problem as long as the ferrite area ratio is 90% or more in the thickness cross section from the surface to a depth of 8 μm.

[0059] The ferrite area ratio is determined by cutting a cross section of the steel sheet in the thickness direction perpendicular to the rolling direction, mirror-polishing it, revealing the steel structure with nital solution, and capturing a secondary electron image using a field-emission scanning electron microscope. The observation position is within 500 μm from the surface of the sheet thickness or the interface between the coating layer and the steel sheet, and five equally spaced fields are observed. The fraction of each structure is calculated using the point-counting method for the obtained structure photograph. More specifically, a grid is first drawn with equal intervals on the structure photograph. Next, it is determined whether the structure at each grid point corresponds to tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite. The fraction of each structure can be measured by determining the number of grid points corresponding to each structure and dividing by the total number of grid points. The greater the total number of grid points, the more accurately the volume fraction can be determined. In this embodiment, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0060] The criteria for determining whether a material is tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite are as follows: A region that has a substructure (lath boundaries, block boundaries) within the grains and where carbides are precipitated with multiple variants is determined to be tempered martensite; A region where cementite is precipitated in a lamellar form is determined to be pearlite; A region that has low brightness and no substructure is visible is determined to be ferrite; A region that has high brightness and where the substructure is not revealed by etching is determined to be fresh martensite or retained austenite; A region that does not fall into any of the above categories is determined to be bainite. Simply put, the area fraction of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[0061] (Metal Si and Metal Al in Ferrite Phase) The ferrite phase in the high ferrite layer contains a total of 0.3% or more of metal Si and metal Al in solid solution at a position 4 μm from the surface of the steel sheet in the thickness direction of the steel sheet. This stabilizes the ferrite phase and makes it less likely to transform into the austenite phase, which is highly susceptible to LME, even with heat from spot welding. Since ferrite has low LME susceptibility, suppressing its transformation into austenite during welding is preferable from the perspective of improving LME resistance. Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and performing the pretreatment and heat treatment described below. The content of metal Si and metal Al dissolved in the ferrite phase may be 0.4% or more, 0.5% or more, or 0.6% or more.

[0062] The amount of metal Si dissolved in the ferrite phase was measured by measuring the Si (SiO x The Al content is calculated by determining the ratio of the peak area intensity to the peak area intensity of Fe-Si (Si-Si, Fe-Si) + Si (Si-Si, Fe-Si), and multiplying this by the Si content in the steel at a position 4 μm from the interface between the steel sheet and the coating layer in the thickness direction of the steel sheet. The Si content can be measured using the GDS described above. The amount of Al dissolved in the ferrite phase is determined as Al measured after deducting the insoluble residue on the filter paper that is generated during the Al analysis process.

[0063] Preferably, the amounts of metallic Si and metallic Al dissolved in the ferrite phase are measured by TEM-EDS as follows.

[0064] First, a thin film sample is prepared using the FIB (Focused Ion Beam) method for TEM observation, with the observation surface being a plate thickness cross section perpendicular to the L direction (rolling direction). A Hitachi High-Technologies NB5000 is used to prepare the thin film sample, with an acceleration voltage of 5 to 40 kV during processing. Note that 10 kV or less is used for finish processing. C vapor deposition is performed on the outermost surface of the sample for protection. A Mo mesh is used.

[0065] Next, the prepared sample is observed with a transmission electron microscope (TEM), and line analysis is performed using TEM-EDS (Energy Dispersive X-ray Spectroscopy) along a position 4 μm from the steel sheet surface, and quantitative analysis is performed at any 10 points. The elements to be quantitatively analyzed are Fe, Si, Al, Mn, and O. That is, the proportions of metal Si and metal Al in the ferrite phase in the present invention refer to the proportions of Si and Al in the total concentration of Fe, Si, Al, Mn, and O in the ferrite phase, respectively. The concentration is the average value of the measurement results at 10 points. Here, if the O concentration is 5% or more, it is determined that the structure at the analysis position was an internal oxide and is not included in the measurement points, and analysis is performed at 10 points where the O concentration is less than 5%.

[0066] The TEM used is a JEM-2100F manufactured by JEOL Ltd., with an accelerating voltage of 200 kV for observation. The EDS used is a JED-2300T manufactured by JEOL Ltd., with an accelerating voltage of 200 kV for analysis.

[0067] <<Galvannealed Steel Sheet>> The steel sheet according to this embodiment may have a galvannealed layer formed on the surface thereof to form a galvannealed steel sheet. The galvannealed layer may be formed on one side or both sides of the steel sheet. Furthermore, the galvannealed layer may be formed on only a portion of the surface of the steel sheet. The galvannealed steel sheet according to this embodiment is primarily used in the automotive field, and the galvannealed coating is not particularly limited as long as it is a coating containing Zn. Elements other than Zn may include, for example, elements commonly contained in Zn coating, such as Fe, Al, Mg, Si, Ni, and Sn. Furthermore, elements contained in the steel sheet may be diffused and contained in the coating. The Zn content may be 50% or more, 55% or more, or 60% or more.

[0068] [Chemical Composition of Coating Layer] Hereinafter, an example of the chemical composition of the coating suitable for the galvannealed steel sheet of this embodiment will be described. Unless otherwise specified, "%" regarding the content of an element means "mass %." In the numerical range of the chemical composition of the coating layer, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits, unless otherwise specified.

[0069] (Al: 0 to 1.5%) Al is an element that improves the corrosion resistance of the coating layer when contained together with Zn or alloyed with Zn, and therefore may be contained as needed. The Al content may be 0%. To form a coating layer containing Zn and Al, the Al content is preferably 0.01% or more. The Al content may be 0.1% or more. When the Al content in the coating layer is in the range of 0.3 to 1.5%, the effect of Al significantly reduces the rate at which Zn penetrates into the steel grain boundaries, making it possible to improve LME resistance. Therefore, from the viewpoint of improving LME resistance, the Al content in the coating layer is desirably 0.3 to 1.5%. The Al content may be 0.4% or more, 0.5% or more, or 0.6% or more. The Al content may be 1.4% or more, 1.3% or more, or 1.2% or more.

[0070] (Fe: 3 to 15%) Fe is contained in the plating layer by diffusing from the steel sheet when the plated steel sheet is heat-treated after forming a plating layer containing Zn on the steel sheet. The Fe content is preferably 3.0% or more. The Fe content may be 4.0% or more or 5.0% or more. The Fe content is preferably 15.0% or less. The Fe content may be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0071] In one example of the chemical composition of the coating suitable for the galvannealed steel sheet of this embodiment, the remainder other than the above components consists of Zn and impurities. Impurities in the coating layer refer to components that are mixed in due to various factors in the manufacturing process, including raw materials, when producing the coating layer, and are not components that are intentionally added to the coating layer. For example, elements other than Zn, Fe, and Al contained in the steel sheet that are diffused and included in the coating layer are considered impurities. In one example of the chemical composition of the coating suitable for the plated steel sheet of this embodiment, elements other than the basic components and optional additional components described above may be included as impurities in trace amounts within a range that does not interfere with the effects of this embodiment.

[0072] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution by inductively coupled plasma (ICP) emission spectroscopy. The acid solution containing the inhibitor may be, for example, a 10 mass % hydrochloric acid solution containing 0.06 mass % inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.).

[0073] The thickness of the plating layer may be, for example, 3 to 50 μm. The coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side. 2 In this embodiment, the coating weight of the plating layer is determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and determining the weight change before and after the plating layer is removed by pickling. After the plating layer is removed, the steel sheet is washed with water and dried. The thickness of the plating layer may be 5 μm or more, 7 μm or more, or 10 μm or more. The thickness of the plating layer may be 45 μm or less, 40 μm or less, 35 μm or less, or 30 μm or less. The coating weight of the plating layer is 15 g / m per side. 2 Above, 20g / m 2 Above, 25g / m 2 Above, 30g / m 2 The coating weight of the plating layer may be 160 g / m or more per side. 2 Below, 140g / m 2 Below, 120g / m 2 Below, 100g / m 2It may be the following:

[0074] The roughness of the interface between the plating layer and the steel sheet is the surface roughness of the steel sheet. Considering the adhesion of the plating, the interface roughness is preferably 3.0 μm or less, and more preferably 2.0 μm or less, in terms of Ra. The interface roughness is the surface roughness of the steel sheet measured after removing the plating.

[0075] The steel sheet of this embodiment exhibits improved LME resistance even when it does not have a galvannealed layer. LME cracking does not occur when ungalvanized steel sheets are welded together. However, even when one galvanized steel sheet and the other ungalvanized steel sheet are welded, molten galvanization occurs on the overlapping surface of the steel sheets during welding. Therefore, the molten galvanization may come into contact with the surface of the ungalvanized steel sheet, potentially causing LME cracking. Furthermore, when an ungalvanized steel sheet is welded using a welding electrode that has been used to spot-weld a galvanized steel sheet, the galvanization attached to the welding electrode may melt and come into contact with the steel sheet surface, potentially causing LME cracking. When the steel sheet of this embodiment is used as an ungalvanized steel sheet, even in such a case, LME cracking can be suppressed due to the low C concentration in the surface layer and the high ferrite content of the surface layer, even during the welding process.

[0076] Furthermore, when the steel sheet has a hot-dip galvanized layer, the depth at which the C concentration measured by GDS is 0.05% or less, and the starting point of the thickness of the high ferrite layer, is the interface between the steel sheet and the galvanized layer. In this embodiment, the interface between the steel sheet and the galvanized layer is defined as the point at which the Fe content determined by GDS measurement is 93% of the Fe content at a depth of 150 μm.

[0077] [Thickness] The thickness of the steel plate of this embodiment is not particularly limited. For example, it may be 0.6 to 3.2 mm. The thickness may be 0.8 mm or more, or 1.0 mm or more. The thickness may be 3.0 mm or less, 2.6 mm or less, 2.5 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0078] <<Manufacturing Method>> Next, a method for manufacturing the steel sheet according to this embodiment will be described.

[0079] The steel sheet according to this embodiment can be obtained by a manufacturing method including, for example, a casting process in which molten steel having adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot rolled to obtain a hot rolled steel sheet, a coiling process in which the hot rolled steel sheet is coiled, a cold rolling process in which the coiled hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, a pretreatment process in which the cold rolled steel sheet is brush ground, and an annealing process in which the pretreated cold rolled steel sheet is annealed. Alternatively, the hot rolled steel sheet may be pickled after the hot rolling process and then cold rolled directly without being coiled.

[0080] <Casting Step> The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.

[0081] <Hot Rolling Process> A hot-rolled steel plate can be obtained by hot-rolling the steel slab obtained by casting. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction rate of each rolling step may be changed as appropriate depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050°C, and the reduction rate of finish rolling may be 10 to 50%.

[0082] <Coiling step> The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by recoiling before or after coiling. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step without being coiled, and then cold-rolled as described below.

[0083] <Cold Rolling Step> After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0084] <Pretreatment Step> In order to obtain the above-described structure of the surface layer of the steel sheet, it is necessary to carry out a predetermined pretreatment and then anneal the steel sheet.

[0085] The pretreatment involves grinding the surface of the cold-rolled steel sheet with a grinding brush (brush grinding treatment). An example of a grinding brush that can be used is the M-33 manufactured by Hotani Co., Ltd. This allows strain to be introduced into the surface layer of the steel. During grinding, a 1.0 to 5.0% NaOH aqueous solution is preferably applied to the steel sheet surface. The brush reduction is preferably 0.5 to 10.0 mm, and the rotation speed is preferably 100 to 1000 rpm. By performing the brush grinding treatment while controlling the coating solution conditions, brush reduction, and rotation speed, decarburization is promoted in the annealing process described below, and a stable ferrite structure can be formed in the surface layer of the steel sheet.

[0086] In addition, a smaller roughness of the steel sheet after the pretreatment process is preferable because it reduces stress concentration and improves LME resistance. The unevenness of the steel sheet surface is determined in accordance with JIS B 0601:2013 by randomly selecting 10 locations on the surface of the surface layer side, measuring the surface profile at each location with a contact surface roughness meter, and arithmetically averaging the surface roughnesses at those locations to obtain the arithmetic mean roughness Ra. The smaller the Ra, the better, with a value less than 3.0 μm being preferred, and 2.5 μm or less being more preferred, with 2.0 μm or less being more preferred.

[0087] <Annealing Process> The pretreated cold-rolled steel sheet is annealed. In this embodiment, the steel sheet that has been strained by pretreatment is held at a high dew point. Specifically, the holding temperature in the annealing process is 760 to 900°C to promote decarburization and internal oxidation. The holding temperature may be 770 to 870°C. If the holding temperature is low, decarburization and internal oxidation will not proceed, resulting in a shallower depth where the C concentration is 0.05% or less and a thinner ferrite-rich layer. The holding time at the holding temperature in the annealing process is 20 to 300 seconds. The holding time may be 50 to 200 seconds. A non-oxidizing atmosphere is preferred, and examples include N2-1 to 10 vol% H2 and N2-2 to 4 vol% H2. The heating rate to the holding temperature is not particularly limited, but may be 1 to 10°C / second.

[0088] In the annealing process of the steel sheet manufacturing method of this embodiment, the dew point of the atmosphere is changed between the first and second halves of the holding time. The first half of the holding time refers to the period from the start of holding to 40±1% of the total holding time, and the dew point of the annealing atmosphere is set to −20 to 20°C during the first half of the holding time. This allows both decarburization of the surface layer of the steel sheet and internal oxidation to proceed. As a result, Si and Al in the surface layer become oxides, and the amount of solute Si and Al decreases. If the dew point during the first half of the holding time is low, decarburization and internal oxidation do not proceed, resulting in a shallower depth at which the C concentration is 0.05% or less and a thinner high ferrite layer. If the dew point during the first half of the holding time is high, external oxidation proceeds, suppressing decarburization, resulting in a shallower depth at which the C concentration is 0.05% or less and a thinner high ferrite layer. Furthermore, if the proportion of the first half of the holding time to the total holding time is large, Si and Al do not sufficiently diffuse from the interior of the steel sheet to the surface, and the content of solute Si and solute Al in the ferrite phase decreases.

[0089] Thereafter, in the second half of the holding time, after 40±1% of the holding time has elapsed, the dew point of the annealing atmosphere is set to -40 to -20°C. Furthermore, the dew point in the second half is set to be 10°C or more lower than the dew point in the first half. As a result, only decarburization proceeds, and Si and Al diffuse from the interior of the steel sheet to the surface side of the steel sheet. This increases the amount of Si and Al dissolved in the ferrite phase in the high ferrite layer. If the dew point in the second half of the holding time is low, decarburization does not proceed, so the depth at which the C concentration is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin. If the dew point in the second half of the holding time is high, external oxidation proceeds, suppressing decarburization, and the depth at which the C concentration is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin. Furthermore, if the ratio of the second half of the holding time to the total holding time is small, decarburization does not proceed in the surface layer, so the depth at which the C concentration is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin. If the proportion of the first half of the holding time to the entire holding time is small, the proportion of the second half of the holding time to the entire holding time becomes relatively large, C diffuses from the inside of the steel sheet to the surface during the second half of the holding time, the layer decarburized at the surface during the first half of the holding time disappears, the depth at which the C concentration is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin.

[0090] The dew point during the first half of the holding time may be -15°C or higher, -10°C or higher. Also, the dew point during the first half of the holding time may be 15°C or lower, 10°C or lower. The dew point during the second half of the holding time may be -37°C or higher, -35°C or higher. Also, the dew point during the second half of the holding time may be -22°C or lower, -25°C or lower.

[0091] By setting the annealing conditions within the above ranges, it is possible to promote decarburization, reduce the C concentration in the surface layer, and increase the fraction of the ferrite phase in which Si and Al are dissolved.

[0092] Annealing is performed under a tension of, for example, 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, and promotes decarburization of the surface layer.

[0093] <<Method for Manufacturing Galvannealed Steel Sheet>> Using the steel sheet according to the present embodiment manufactured as described above, a galvannealed steel sheet can be manufactured by a manufacturing method including a plating treatment step and an alloying treatment step.

[0094] The plating treatment may be carried out according to a method known to those skilled in the art. The plating treatment may be carried out, for example, by hot-dip plating, electroplating, vapor deposition plating, thermal spraying, or cold spraying. Preferably, the plating treatment is carried out by hot-dip plating. After the plating treatment step, a known alloying treatment is carried out to obtain alloyed plating. The conditions for the plating treatment step and the alloying treatment step may be appropriately set taking into consideration the chemical composition, thickness, and coating weight of the desired plating layer, etc.

[0095] The steel sheet and plated steel sheet according to this embodiment have high strength and high LME resistance and hydrogen desorption properties, and therefore can be suitably used in a wide range of fields, such as automobiles, home appliances, and building materials. They are particularly preferably used in the automobile field. Steel sheets and plated steel sheets used for automobiles are often spot welded, in which case LME cracking can become a significant problem. Therefore, when the steel sheet and plated steel sheet according to this embodiment are used as automotive steel sheets, the effect of this embodiment, namely high LME resistance, is suitably exhibited.

[0096] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0097] Example No. 1 Molten steel was produced in a blast furnace and cast by continuous casting to obtain a slab having the chemical composition listed in No. 1 of Table 1. The obtained slab was heated to 1200°C and hot-rolled to a finish rolling temperature of 950°C and a finish rolling reduction of 30%, thereby obtaining a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold-rolled at a reduction of 50%, thereby obtaining a cold-rolled steel sheet. The cold-rolled steel sheet had a thickness of 1.6 mm.

[0098] Next, the cold-rolled steel sheet was pretreated by applying a 2.0% aqueous solution of NaOH and brush grinding it using a Hotani M-33 grinding brush at a brush reduction of 2.0 mm and a rotation speed of 600 rpm (condition "A" in Table 2).

[0099] Thereafter, annealing was performed in a furnace with an oxygen concentration of 20 ppm or less in an N2-4 vol% H2 gas atmosphere at a holding temperature of 800°C for 100 seconds, to prepare steel sheet samples. The temperature rise rate during annealing was 6.0°C / sec up to 500°C, and 2.0°C / sec from 500°C to the holding temperature. The dew point of the annealing atmosphere was 0°C for the first 40 seconds of the holding time and -20°C for the last 60 seconds. The annealing was performed under a tension of 15 MPa.

[0100] <Examples Nos. 2 to 37> Steel sheets or plated steel sheets were produced under the same conditions as in Example 1, except that the chemical compositions were as shown in Table 1, and the brush grinding conditions, annealing conditions, and plating conditions were as shown in Table 2. In Table 2, "Present" in the "Galvannealed Layer" column indicates plated steel sheets with a galvannealed layer, while "Absent" indicates steel sheets without a plating layer. In addition, in Example No. 36, a Hotani D-100 grinding brush was used for brush grinding (condition "B" in Table 2). The D-100 is a brush with higher grinding power than the M-33.

[0101] In the example where a plated steel sheet was produced, the annealed steel sheet was immersed in a 450°C hot-dip galvanizing bath (Zn-0.2% Al) for 3 seconds, then pulled out at 100 mm / sec, and the coating weight was reduced to 50 g / m using N2 wiping gas. 2 The temperature was controlled to 520°C for 30 seconds, and alloying treatment was carried out.

[0102] (Surface roughness after brush grinding) When producing a steel sheet, the surface roughness of the hot-rolled steel sheet after brush grinding was measured. The surface roughness was measured in accordance with JIS B 0601:2013 by randomly selecting 10 locations on the surface of the surface layer side, measuring the surface profile at each location using a contact surface roughness meter, and calculating the arithmetic average of the surface roughnesses at those locations to obtain the arithmetic average roughness Ra. The surface roughness after brush grinding is shown in Table 2 according to the following evaluation.

[0103] Evaluation AA: Less than 2.0 μm Evaluation A: 2.0 μm or more and less than 3.0 μm Evaluation B: 3.0 μm or more

[0104]

[0105]

[0106] (Tensile strength evaluation) For each steel plate, a JIS No. 5 tensile test piece was taken with the direction perpendicular to the rolling direction as the longitudinal direction, and a tensile test was performed in accordance with JIS Z 2241:2011 to determine the tensile strength, which was evaluated as follows.

[0107] Grade AAA: 1180 MPa or more Grade AA: 980 MPa or more, less than 1180 MPa Grade A: 780 MPa or more, less than 980 MPa

[0108] (Surface Layer Structure) A sample cut to 30 mm × 30 mm was collected, and GDS measurement was performed five times in the thickness direction using the method described above. The depth at which the C concentration was 0.05% or less was determined and shown in Table 3 under "C≦0.05% depth."

[0109] In addition, samples cut to 25 mm x 15 mm were collected and subjected to nital etching. The T-section of each sample was observed with an SEM to measure the thickness of the layer with a ferrite area ratio of 90% or more. The thickness was measured at five equally spaced points within a range of 500 μm in the T-direction, and the average value was calculated. Here, when the steel sheet is provided with a galvannealed coating, the starting point of the "depth" is the interface between the coating layer and the steel sheet.

[0110] In addition, the amount of dissolved Si in the ferrite phase of the layer (high ferrite layer) with a ferrite area ratio of 90% or more was measured using an X-ray photoelectron spectrometer, and Si(SiO x The ratio of the peak area intensity to the peak area intensity of Fe-Si (Si-Si, Fe-Si) + Si (Si-Si, Fe-Si) was calculated and multiplied by the Si content in the steel at a position 4 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the coating layer. The Si content was measured using the GDS described above. Furthermore, the amount of solute Al was calculated as Al measured after excluding the insoluble residue on the filter paper that occurs during the Al analysis process.

[0111] (LME Resistance) Two samples were taken from each plated steel sheet by cutting them into a size of 50 mm × 100 mm, and these two samples were spot-welded using a dome radius welding electrode having a tip diameter of 8 mm at an impact angle of 2°, a pressure of 5.0 kN, a current flow time of 1.2 seconds, and a current flow of 12 kA to produce welded joints.

[0112] The evaluation of LME resistance will be described with reference to Figure 2. LME resistance was evaluated based on the length of an LME crack (shoulder crack 23) that occurred in the shoulder of a weld 22 formed by spot welding two overlapping steel sheets 21. The shoulder refers to the inclined portion of the edge of a depression created by spot welding. The evaluation was performed as follows based on the length of the shoulder crack 23. Note that when evaluating steel sheets not provided with a galvannealed coating layer, the welding electrode used was the same as that used immediately before in spot welding a steel sheet provided with a galvannealed coating layer.

[0113] Rating AAA: 0 μm Rating AA: More than 0 μm, less than 50 μm Rating A: 50 μm or more, less than 160 μm Rating B: 160 μm or more

[0114] (Hydrogen desorption test) A test piece measuring 80 mm x 80 mm was cut out from each steel plate and electrochemically charged with hydrogen. The test piece was placed in a 3% NaCl + 3 g / L NH4SCN aqueous solution under constant current control (cathode current density 1 mA / cm2). 2 ) for 48 hours. After hydrogen charging, the steel sheet or plated steel sheet was left to stand in a constant temperature and humidity chamber at 30°C and a relative humidity of 28% in the air for 48 hours, and after the specified time had passed, the amount of diffusible hydrogen contained in the steel sheet or plated steel sheet was measured using a thermal desorption method and evaluated as follows. In the thermal desorption method, the temperature was raised to 200°C at a rate of 100°C / h, and the total amount of hydrogen released from room temperature to 200°C was taken as the amount of diffusible hydrogen.

[0115] Grade AAA: 5% or less of the initial hydrogen amount Grade AA: 15% or less of the initial hydrogen amount Grade A: Less than 40% of the initial hydrogen amount Grade B: 40% or more of the initial hydrogen amount

[0116] The results of each evaluation are shown in Table 3. In Table 3, "C≦0.05% depth" means the depth where the C concentration is 0.05% or less, "high α layer thickness" means the thickness of the layer where the ferrite area ratio is 90% or more, and "Si+Al in α phase" means the total amount of dissolved Si and dissolved Al in the ferrite phase.

[0117]

[0118] Nos. 1 to 22 are examples of the present invention and had high LME resistance.

[0119] No. 23 steel sheet had a high C content. Therefore, the C content was high even near the surface layer, and the C content in the surface layer did not decrease even by decarburization, so the depth where the C concentration measured by GDS was 0.05% or less was shallow, and the thickness of the high ferrite layer was thin. As a result, the LME resistance and hydrogen desorption performance were poor.

[0120] In No. 24, the Si content of the steel plate was low. Therefore, decarburization did not progress in the surface layer, the depth at which the C concentration measured by GDS was 0.05% or less was shallow, and the contents of solute Si and solute Al in the ferrite phase were low. As a result, the LME resistance and hydrogen desorption performance were poor.

[0121] No. 25 steel sheet had a high Si content. As a result, external oxidation progressed, decarburization on the surface was suppressed, and the depth at which the C concentration measured by GDS was 0.05% or less became shallow. As a result, the steel sheet had poor LME resistance and hydrogen desorption performance.

[0122] In No. 26, the amount of sol. Al in the steel sheet was low. As a result, decarburization did not progress in the surface layer, the depth at which the C concentration measured by GDS was 0.05% or less was shallow, the thickness of the high ferrite layer was thin, and the contents of solute Si and solute Al in the ferrite phase were low. As a result, the LME resistance and hydrogen desorption performance were poor.

[0123] No. 27 steel sheet had a high Al content. As a result, external oxidation progressed, decarburization on the surface was suppressed, and the depth at which the C concentration measured by GDS was 0.05% or less became shallow. As a result, the LME resistance and hydrogen desorption performance were poor.

[0124] In No. 28, the total amount of Si and Al in the steel sheet was low. Therefore, decarburization did not progress in the surface layer, the depth where the C concentration measured by GDS was 0.05% or less was shallow, the thickness of the high ferrite layer was thin, and the contents of solute Si and solute Al in the ferrite phase were low. As a result, the LME resistance and hydrogen desorption performance were poor.

[0125] No. 29 had a low dew point in the first half of the annealing period. Therefore, decarburization and internal oxidation of the surface layer did not progress, and the depth at which the C concentration measured by GDS was 0.05% or less and the thickness of the high ferrite layer were both zero. Therefore, the contents of solute Si and solute Al in the ferrite phase could not be measured. As a result, the LME resistance and hydrogen desorption performance were poor.

[0126] No. 30 had a high dew point during the first half of annealing. This led to increased external oxidation and suppressed surface decarburization, resulting in a shallow depth where the C concentration measured by GDS was 0.05% or less, and a thin high-ferrite layer. As a result, the specimen had poor LME resistance and hydrogen desorption properties.

[0127] In No. 31, the dew point during the latter half of annealing was low. As a result, decarburization of the surface layer did not progress, and the depth at which the C concentration measured by GDS was 0.05% or less was shallow, resulting in a thin high-ferrite layer. As a result, the LME resistance and hydrogen desorption performance were poor.

[0128] No. 32 had a high dew point during the latter half of annealing. External oxidation progressed, surface decarburization was suppressed, and the depth at which the C concentration measured by GDS was 0.05% or less was shallow, resulting in a thin high-ferrite layer. As a result, the specimen had poor LME resistance and hydrogen desorption properties.

[0129] In No. 33, the holding temperature during annealing was low. As a result, decarburization and internal oxidation of the surface layer did not progress, and the depth at which the C concentration measured by GDS was 0.05% or less was shallow, resulting in a thin high-ferrite layer. As a result, the specimen was inferior in LME resistance and hydrogen desorption performance.

[0130] In No. 34, the holding time during annealing was 30 seconds, which was within the range described above. However, because the dew point was changed after 20 seconds, the proportion of the holding time in the first half was large and the proportion of the holding time in the second half was relatively small. As a result, Si and Al did not sufficiently diffuse from the interior of the steel sheet to the surface side, and the contents of solute Si and solute Al in the ferrite phase were low. As a result, the steel sheet had poor LME resistance and hydrogen desorption properties.

[0131] For No. 35, the holding time during annealing was 34 seconds, which was within the range described above. However, the dew point was changed after 4 seconds, so the proportion of the holding time in the first half was small. As a result, decarburization did not progress in the surface layer, and the depth where the C concentration measured by GDS was 0.05% or less was shallow, resulting in a thin high-ferrite layer. As a result, the specimen had poor LME resistance and hydrogen desorption properties.

[0132] In No. 36, the brush used in the brush grinding had a large grinding force. As a result, the amount of grinding in the surface layer of the steel sheet increased, and the strain entering the surface layer was small. As a result, decarburization did not progress in the surface layer, and Si and Al did not sufficiently diffuse from the interior of the steel sheet to the surface side. As a result, the depth where the C concentration measured by GDS was 0.05% or less was shallow, the thickness of the high ferrite layer was thin, and the contents of solute Si and solute Al in the ferrite phase were low. As a result, the LME resistance and hydrogen desorption performance were poor.

[0133] In No. 37, the dew point was kept constant between the first and second halves of annealing. As a result, Si and Al did not sufficiently diffuse from the interior of the steel sheet to the surface, resulting in low solute Si and solute Al contents in the ferrite phase. As a result, the LME resistance was poor.

[0134] According to the present invention, it is possible to provide a high-strength steel sheet and a plated steel sheet having high LME resistance and hydrogen desorption property, and the steel sheet and the plated steel sheet can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles. Therefore, the present invention has extremely high industrial applicability.

[0135] REFERENCE SIGNS LIST 11 steel plate 12 high ferrite layer 13 inner side of steel plate 21 steel plate 22 welded portion 23 shoulder crack

Claims

1. A steel plate having a tensile strength of 780 MPa or more, the chemical components of the steel plate being, in mass %, C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.5 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.1500%, V: 0 to 0.150%, Cr: 0 to 2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, REM: 0~0.1000% with the balance being Fe and impurities, and the total content of Si and sol.Al is 1.3% or more, a depth from the surface of the steel sheet in the thickness direction of the steel sheet where the C concentration measured by GDS is 0.05% or less is 8 μm or more, a thickness of a layer where the area ratio of the ferrite phase is 90% or more is 8 μm or more in the thickness direction of the steel sheet from the surface of the steel sheet, and a total amount of solid solution of metallic Si and metallic Al in the ferrite phase is 0.3% or more at a position 4 μm from the surface of the steel sheet in the thickness direction of the steel sheet.

2. The steel plate according to claim 1, characterized in that the depth at which the C concentration measured by GDS is 0.05% or less is 15 μm or more in the thickness direction of the steel plate from the surface of the steel plate.

3. A steel plate as described in claim 1, characterized in that the thickness of a layer having an area ratio of ferrite of 90% or more from the surface of the steel plate in the thickness direction of the steel plate is 20 μm or more, and the amount of solid solution of metallic Si and metallic Al in the ferrite phase is 0.5% or more in total.

4. A galvannealed steel sheet according to any one of claims 1 to 3, comprising a galvannealed layer on a portion of the surface of the steel sheet.

5. The galvannealed steel sheet according to claim 4, characterized in that the galvannealed layer contains, by mass%, 0 to 1.5% Al, 3 to 15% Fe, and the balance being Zn and impurities.

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