Steel sheet and galvannealed steel sheet

Abstract

The present invention addresses the problem of providing a steel sheet and galvannealed steel sheet both having good liquid metal embrittlement (LME) resistance.　This steel sheet has a tensile strength of 780 MPa or more and has specified chemical components. A GDS measurement performed in the thickness direction of the steel sheet finds that the depth at which Bx / B150 ≥ 5.0 is satisfied, where Bx is the light emission intensity at a depth x (μm) and B150 is the light emission intensity at a depth of 150 μm, is 0.5 μm or more from the surface of the steel sheet. The thickness of an oxide formed on the surface of the steel sheet is 0.5 μm or less. An internal oxide layer having a thickness of 1.0 μm or more exists in the thickness direction of the steel sheet from the surface of the steel sheet. Also, this galvannealed steel sheet comprises a galvannealed layer on at least a part of the surface of the steel sheet and the thickness of the oxide formed on the surface of the galvannealed layer is 0.5 μm or less.

Description

Steel sheets and galvannealed steel sheets

[0001] The present invention relates to a steel sheet and a galvannealed steel sheet.

[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] In particular, in the automotive field, zinc-based plated steel sheets are often spot-welded for use. When welding zinc-based plated steel sheets, particularly high-strength steel sheets, the 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, and molten zinc penetrates the grain boundaries, embrittling the steel sheet, and further when tensile stress is applied to the steel sheet during welding.

[0004] In addition, Patent Document 2 discloses 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 rate 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] Various studies have been conducted on improving the LME resistance of steel sheets and galvannealed steel sheets. In response to these findings, the present invention has discovered that it is preferable to concentrate B in the surface layer of a steel sheet. Furthermore, it has been discovered that concentrating B in the surface layer of a steel sheet improves hydrogen desorption. This is thought to be because the surface diffusion rate of hydrogen atoms increases in the surface layer of a steel sheet where B is concentrated, thereby accelerating the hydrogen gasification reaction. It has been discovered that, in order to concentrate B in the surface layer of a steel sheet, it is effective to prevent external oxidation from progressing on the surface of the steel sheet during annealing, and instead promote internal oxidation, in which oxidation progresses toward the interior of the steel sheet in the surface layer of the steel sheet. The gist of the present invention is as follows.

[0010] [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 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0.0005 to 0.0050%, Ti: 0.0010 to 0.1000%, Nb: 0 to 0.2000%, V: 0 to 0.15%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and a depth of 0.5 μm or more from the surface of the steel sheet, where an emission intensity Bx at a depth x (μm) and an emission intensity B150 at a depth of 150 μm satisfy Bx / B150≧5.0 (1), the depth is 0.5 μm or more from the surface of the steel sheet, the thickness of an oxide formed on the surface of the steel sheet is 0.5 μm or less, and an internal oxide layer with a thickness of 1.0 μm or more is present from the surface of the steel sheet in the thickness direction of the steel sheet.

[0011] [2] The steel plate according to [1], characterized in that the depth at which the luminous intensity Bx and the luminous intensity B150 satisfy Bx / B150≧5.0 is 2.0 μm or more from the surface of the steel plate.

[0012] [3] The steel sheet according to [1], wherein in a GDS measurement in the thickness direction of the steel sheet, the maximum value Bmax of the B emission intensity in the range from the surface of the steel sheet to a depth of 5.0 μm satisfies Bmax / B150≧8...(2).

[0013] [4] A galvannealed steel sheet comprising a galvannealed layer on at least a part of a surface of the steel sheet according to any one of [1] to [3], wherein the thickness of an oxide formed on the surface of the galvannealed layer is 0.5 μm or less.

[0014] [5] The galvannealed steel sheet according to [4], wherein the galvannealed layer contains, by mass%, 0 to 1.5% Al and 3 to 20% Fe, with the balance being Zn and impurities.

[0015] According to the present invention, a steel sheet and a galvannealed steel sheet having good LME resistance can be obtained.

[0016] Fig. 1 is a diagram showing an example of the distribution of B in the surface layer portion of a steel sheet according to the present invention Fig. 2 is a diagram illustrating the positions of cracks targeted in the evaluation of LME resistance in Examples.

[0017] 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.

[0018] When plated steel sheets are spot welded, the coating melts and the surface layer of the steel sheet is heated, transforming the steel sheet structure to austenite. During this process, the hot-dip coating penetrates into the steel sheet structure along the austenite grain boundaries, embrittling the grain boundaries. Therefore, when stress is applied to the steel sheet, LME cracking is likely to occur at the grain boundaries. It is believed that LME is particularly likely to occur because tensile stress is applied to the steel sheet during welding. The inventors have come up with the idea of ​​utilizing B (hereinafter also referred to as "boron" or "boron") as a method for improving LME resistance. Specifically, they have come up with the idea of ​​suppressing the penetration of molten zinc into the Fe grain boundaries (austenite grain boundaries or ferrite grain boundaries) in the surface layer structure, thereby suppressing the occurrence of LME. In this specification, the "surface layer" refers to the range from the outermost surface of the steel sheet to approximately 100 μm in the thickness direction.

[0019] Generally, in boron-containing steel, a deboronization phenomenon is known in which the amount of boron near the surface decreases, particularly when heated to the austenite temperature range. As a result, in boron-containing steel, the boron concentration in the surface layer of the steel is generally lower than that in the center of the steel sheet. In this embodiment, when the steel sheet is annealed, internal oxidation, which tends to progress to grain boundaries in the surface layer of the steel sheet, is induced toward the interior of the steel sheet. This results in the formation of an internal oxidation layer in the surface layer of the steel sheet, which makes it possible to fix Si, which particularly deteriorates LME resistance, as an oxide. Furthermore, the oxide formed by internal oxidation captures boron that diffuses from the steel sheet to the surface layer upon heating, thereby suppressing the deboronization phenomenon. In this way, contrary to typical boron-containing steel, by forming a boron-enriched region in the surface layer of the steel, the combined effects of these factors improve LME resistance.

[0020] Normally, when a steel sheet is heated, such as during annealing, external oxidation occurs, in which oxides (scale) are formed on the surface of the steel sheet. This embodiment realizes the above-described structure of the surface layer of the steel sheet, based on the finding that imparting strain to the surface layer of the steel sheet in advance promotes oxygen diffusion into the material, and that annealing at an appropriate dew point can promote internal oxidation without promoting external oxidation.

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

[0022] <Steel Plate> The steel plate of this embodiment will be described in detail below.

[0023] [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.

[0024] [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.

[0025] (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 that is the target of this embodiment, the C content is set to 0.05% or more. In consideration of weldability, 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.

[0026] (Si: 0.7 to 3.0%) Si (silicon) is an element that suppresses the deboronization phenomenon through internal oxidation. It is also an element that improves the corrosion resistance of steel. This forms a B distribution, described below, in the surface layer of the steel sheet. To achieve this effect, the Si content is set to 0.7% or more. If the Si content is too high, Si is an element that generally reduces LME resistance, and therefore the effect of the B distribution, described below, is hindered, and the effect of improving LME resistance is reduced. In consideration of this, 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 2.8% or less, 2.5% or less, or 2.0% or less.

[0027] It has been known that adding Si to steel reduces LME resistance, but as a result of investigations by the present inventors, it has been found that, contrary to conventional knowledge, adding a large amount of Si improves LME resistance. This is thought to be because the inclusion of B and the manufacturing method described below cause B to concentrate in the surface layer and segregate at the Fe grain boundaries.

[0028] (Mn: 0.1 to 5.0%) Mn (manganese) is an element that is effective in improving the strength of steel by obtaining a hard structure. Also, like Si, it is an element that suppresses the deboronization phenomenon through internal oxidation. In order to obtain these effects, the lower limit of the Mn content is set to 0.1%. Furthermore, taking into consideration 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.

[0029] (Sol. Al: 0 to 2.0%) Aluminum (aluminum) is an element that dissolves in steel and promotes ferrite stabilization and decarburization, thereby improving LME resistance, and may be added as needed. Sol. Al refers to acid-soluble Al that is not in the form of an oxide such as Al2O3 and is soluble in acid. The amount of Al measured is determined by subtracting the insoluble residue on the filter paper that occurs during the Al analysis process. The inclusion of sol. Al is not essential, and the lower limit of the sol. Al content is 0.0%. To obtain the desired effect, the sol. Al content may be 0.1% or more, 0.2% or more, or 0.3% or more. If the sol. Al content is too high, external oxidation progresses, even when high-dew-point annealing is performed, forming an oxide (scale) on the surface of the steel sheet, thereby reducing LME resistance. In consideration of this, the sol. Al content is set to 2.0% or less. The Al content may be 1.5% or less, 1.2% or less, or 1.0% or less.

[0030] (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% or 0.0001% or more.

[0031] (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% or 0.0001% or more.

[0032] (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 manufacturing costs, the N content may be more than 0% or 0.0010% or more.

[0033] (B: 0.0005 to 0.0050%) 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. Furthermore, in the steel sheet of this embodiment, boron is concentrated in the surface layer of the steel sheet and segregates at the Fe grain boundaries. In order to achieve this effect, the B content is set to 0.0005% or more. From the viewpoint of toughness and weldability, the B content is set to 0.0050% or less. The B content may be 0.0006% or more, 0.0008% or more, or 0.0010% or more. The B content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0034] Generally, the B concentration in the surface layer of steel is lower than that in the center of the steel sheet due to the deboronization phenomenon. In this embodiment, the manufacturing method described below promotes internal oxidation toward the inside of the steel sheet during the annealing process, thereby capturing B in the oxides, suppressing the deboronization phenomenon and forming the concentration distribution described below in the surface layer of the steel. It is believed that B segregating at the Fe grain boundaries suppresses LME.

[0035] (Ti: 0.0010 to 0.1000%) Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to improving strength. To achieve this effect, the Ti content is set to 0.0010% or more. Excessive Ti content may result in the formation of coarse TiN, which may impair toughness, so the Ti content is set to 0.1000% or less. The Ti content may be 0.0020% or more, 0.0030% or more, 0.0040% or more, 0.0080% or more, 0.0110% or more, or 0.0130% or more. The Ti content may be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, or 0.0400% or less.

[0036] (Nb: 0 to 0.2000%) 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. This effect can be obtained even with a small amount of Nb contained, but when Nb is contained, the Nb content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0010% or more. From the viewpoint of ensuring toughness, the Nb content is set to 0.2000% or less. The Nb content may be 0.1500% or less, 0.1000% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0037] (V: 0 to 0.15%) 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. This effect can be obtained even with a small amount of V contained, but when V is contained, the V content may be 0.001% or more, 0.01% or more, 0.03% or more, 0.05% or more, or 0.06% or more. From the viewpoint of ensuring toughness, the V content is set to 0.15% or less. The V content may be 0.14% or less, 0.13% or less, 0.12% or less, 0.11% or less, or 0.10% or less.

[0038] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength, and may 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 a small amount of Cr, but when Cr is contained, the Cr content may be 0.001% or more, 0.01% or more, 0.05% or more, 0.07% or more, or 0.10% or more. Since excessive Cr content may form a large amount of Cr carbide, which may adversely impair hardenability, the Cr content is set to 2.00% or less. 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.

[0039] (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 may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. Since excessive addition of Ni increases costs, the Ni content is set to 2.00% or less. 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.

[0040] (Cu: 0 to 2.00%) Cu (copper) is effective in improving the hardenability of steel and increasing its strength, 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 contained, but when contained, the Cu content may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.07% or more. 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 2.00% or less. The Cu content may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0041] (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 may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. From the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0042] (W: 0 to 1.00%) W (tungsten) 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 W content is 0. This effect can be obtained even with a small amount of W contained, but when W is contained, the W content may be 0.001% or more, 0.01% or more, 0.02% or more, or 0.03% or more. From the viewpoint of suppressing a decrease in toughness, the W content is set to 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, 0.15% or less, or 0.10% or less.

[0043] (Ca: 0 to 0.1000%) Ca (calcium) 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 Ca content is 0. This effect can be obtained even with a trace amount of Ca, but when Ca is contained, the Ca content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, 0.0020% or more, 0.0040% or more, 0.0060% or more, or 0.0070% or more. If Ca 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 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[0044] (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, the Mg content, when contained, may be 0.0001% or more, 0.0005% or more, or 0.001% or more. Since excessive Mg content may cause deterioration of surface properties to become apparent, the Mg content may be 0.100% or less, 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.

[0045] (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, and therefore 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 trace amount, the Zr content, when contained, may be 0.001% or more, 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. Since excessive Zr content may cause deterioration of surface properties to become apparent, the Zr content is set to 0.100% or less. 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.

[0046] (Hf: 0 to 0.10%) Hf (hafnium) 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 Hf content is 0. This effect can be obtained even with a trace amount, but when contained, the Hf content may be 0.001% or more, 0.002% or more, or 0.005% or more. If Hf is contained in excess, deterioration of surface properties may become apparent, so the Hf content may be 0.10% or less, 0.08% or less, 0.06% or less, 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less.

[0047] (REM: 0 to 0.100%) 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. Although this effect can be obtained even with a small amount, the REM content, when contained, may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.005% or more. Since excessive REM content may cause deterioration of surface properties to become apparent, the REM content is set to 0.100% or less. The REM content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% 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.

[0048] (Balance) In the steel sheet according to this embodiment, the balance other than the above chemical components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scrap, when industrially manufacturing a steel sheet. The impurities do not adversely affect the LME resistance and hydrogen desorption properties of the steel sheet according to this embodiment, that is, they are contained in a range that allows the LME resistance and hydrogen desorption properties required for the steel sheet according to this embodiment to be obtained. 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 content of O may be 0.00001% or more, 0.00005% or more, or 0.0001% or more.

[0049] (Method of Analyzing Chemical Components) The chemical components of steel sheets 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 sheets according to JIS G0417:1999.

[0050] [B Distribution in Surface Layer] In the steel sheet of this embodiment, in a GDS (high frequency glow discharge optical emission spectroscopy) measurement in the thickness direction of the steel sheet, the depth that satisfies the following formula (1) is 0.5 μm or more from the surface of the steel sheet.

[0051] Bx / B150≧5.0…(1)

[0052] In the above formula (1), Bx represents the luminescence intensity at a point at a depth x (μm) away from the interface between the steel sheet and the plating layer in the thickness direction of the steel sheet. Furthermore, B150 represents the luminescence intensity at a depth of 150 μm away from the interface between the steel sheet and the plating layer in the thickness direction of the steel sheet. Note that the "thickness direction" in this embodiment refers to a direction perpendicular to the interface between the steel sheet and the plating layer. A point away from the interface between the steel sheet and the plating layer in the thickness direction of the steel sheet refers to a point away from the interface between the steel sheet and the plating layer in the thickness direction of the steel sheet.

[0053] The left side of the above formula (1) represents the ratio of the B concentration at depth x to the B concentration at a depth of 150 μm. That is, Bx / B150≧5.0 means that the B concentration at depth x is 5.0 times or more the B concentration at a depth of 150 μm. The B concentration at a depth of 150 μm may be considered to be the B concentration at the center of the steel sheet thickness, and Bx / B150≧5.0 means that B is enriched at depth x. The expression "a depth of 0.5 μm or more from the surface of the steel sheet that satisfies formula (1)" means that formula (1) is satisfied in a range from the surface of the steel sheet to a depth of 0.5 μm or more in the thickness direction of the steel sheet, and that B is enriched in a range to a depth of 0.5 μm or more in the thickness direction of the steel sheet.

[0054] In this embodiment, the surface of the steel sheet is defined as the point where the Fe content determined by GDS measurement is 5% of the Fe content at a depth of 150 μm. When a galvannealed layer (described later) is formed on the surface of the steel sheet, the interface between the steel sheet and the galvannealed layer is regarded as the surface of the steel sheet and is used as the starting point of the depth for GDS measurement. The interface between the steel sheet and the galvannealed layer is defined as follows. First, the Fe content in the thickness direction of the plated steel sheet is measured by GDS measurement. The value with the highest Fe content is defined as the Fe content of the steel sheet. The point where the Fe content is 93% of the Fe content of the steel sheet is defined as the "interface between the steel sheet and the galvannealed layer."

[0055] As described above, the concentration of B in the surface layer of the steel sheet causes B to segregate at the Fe grain boundaries in the surface layer structure. This prevents molten zinc from penetrating the Fe grain boundaries during spot welding, thereby suppressing the occurrence of LME. In addition, the hydrogen desorption property of the steel sheet can be improved.

[0056] (GDS measurement method) For Bx and B150, a method is used in which the surface of the target steel sheet is placed in an Ar atmosphere by GDS, a voltage is applied to generate glow plasma, and the steel sheet surface is sputtered while being analyzed in the depth direction. Then, elements contained in the material are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated.

[0057] The data in the depth direction can be estimated from the sputtering time. Specifically, by determining the relationship between the sputtering time and the sputtering depth in advance using a standard sample, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The sputtering time is set so that the sputtering depth exceeds at least 150 μm.

[0058] The GDS measurement is performed five times in the thickness direction, and the average value is taken as the B concentration. The measurement conditions are as follows: Bx and B150 are the B concentrations corresponding to a depth of x (μm) and a depth of 150 μm, respectively.

[0059] 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

[0060] In order to obtain the effect of suppressing the occurrence of LME, the depth at which Bx / B150≧5.0 is satisfied is 0.5 μm or more. From the viewpoint of LME resistance, a larger Bx / B150 is preferable, and it may be 1.0 μm or more, 1.2 μm or more, 1.4 μm or more, 1.6 μm or more, 1.8 μm or more, or 2.0 μm or more. Since Bx / B150 is the ratio of the B concentration at depth x to the B concentration at a depth of 150 μm, the depth at which Bx / B150≧5.0 is satisfied is less than 150 μm. Even if the depth at which Bx / B150≧5.0 is deep, LME resistance is not reduced, but the depth at which Bx / B150≧5.0 is satisfied may be 100.0 μm or less, 50.0 μm or less, 30.0 μm or less, 20.0 μm or less, or 10.0 μm or less.

[0061] In the steel sheet of this embodiment, it is believed that B concentrated in the surface layer of the steel segregates at Fe grain boundaries and inhibits the penetration of Zn, thereby suppressing LME. Such a B distribution in the surface layer can be obtained by producing a steel sheet from molten steel having the above-mentioned chemical composition by the production method described below.

[0062] An example of the distribution of B obtained by GDS measurement in the surface layer of the steel sheet of this embodiment is shown in Figure 1. Referring to Figure 1, it can be seen that the B concentration increases sharply from a position at a depth of about 2 µm from the surface to the surface (position at a depth of 0 µm).

[0063] [Oxide Thickness] In the steel sheet of this embodiment, the oxide thickness formed on the surface of the steel sheet, or on the surface of the galvannealed layer when a galvannealed layer described later is provided, is 0.5 μm or less. If a thick oxide exceeding 0.5 μm is formed on the surface of the coating layer, the diffusion rate of hydrogen in the oxide decreases, and therefore hydrogen desorption performance decreases. In order to make the oxide thickness on the surface of the coating layer 0.5 μm or less, it is necessary to avoid heat treatment in an atmospheric furnace or the like after production of the steel sheet or galvannealed steel sheet.

[0064] The oxide thickness on the surface of the plating layer is measured by cross-sectional SEM observation. Specifically, the oxide is defined as a location containing 20% ​​or more O by EDS, and the oxide thickness is measured. The measurement field is 80 μm wide x 50 μm long, and the oxide thickness is measured at the thickest part in the field. Similar measurements are performed on five fields, and the average of the maximum oxide thicknesses in each field is taken as the surface oxide thickness. The thinner the oxide thickness on the surface of the plating layer, the better, and it may be 0.4 μm or less, 0.3 μm or less, 0.2 μm or less, or 0.1 μm or less.

[0065] [Thickness of Internal Oxide Layer] In the plated steel sheet of this embodiment, when a galvannealed coating layer described below is provided, an internal oxide layer having a thickness of 1.0 μm or more is present from the surface of the steel sheet to the interface between the steel sheet and the galvannealed coating layer in the thickness direction of the steel sheet. Precipitation of Si and Mn as internal oxides reduces the concentrations of Si and Mn dissolved in the surface layer of the steel sheet. In particular, since Si is an element that promotes LME and reduces LME resistance, a reduction in the solute Si concentration in the surface layer of the steel sheet due to internal oxidation improves LME resistance. The thickness of the internal oxide layer may be 2.0 μm or more, 3.0 μm or more, 4.0 μm or more, 5.0 μm or more, 6.0 μm or more, or 7.0 μm or more. The thickness of the internal oxide layer may be 100.0 μm or less, 50.0 μm or less, 40.0 μm or less, 30.0 μm or less, or 25.0 μm or less.

[0066] The thickness of the internal oxide layer is measured by cross-sectional SEM observation. Specifically, in the steel sheet surface layer, or in the steel sheet surface layer directly below the galvannealed hot-dip coating layer (if the steel sheet has a galvannealed hot-dip coating layer described below), the thickness of the oxide layer is measured, with the internal oxide being defined as a precipitate containing 20% ​​or more of O as determined by EDS. The measurement field size is 80 μm wide x 50 μm long, and the thickness of the thickest internal oxide layer in the field is measured. Similar measurements are performed on five fields, and the average of the maximum internal oxide layer thicknesses in each field is taken as the internal oxide layer thickness.

[0067] <B Concentration in a Range up to a Depth of 5.0 μm> In the steel sheet of this embodiment, it is preferable that the following formula (2) is satisfied in the above-described GDS measurement.

[0068] Bmax / B150≧8…(2)

[0069] In the above formula (2), Bmax represents the maximum value of the emission intensity of B in the range from the surface of the steel sheet to a depth of 5.0 μm.

[0070] The left side of equation (2) represents the ratio of the maximum value of B emission intensity in the range from the surface of the steel sheet to a depth of 5.0 μm to the B concentration at a depth of 150 μm. That is, Bmax / B150≧8 means that the B concentration at the position where B is most concentrated in the range from the surface of the steel sheet to a depth of 5.0 μm is 8 times or more the B concentration at a depth of 150 μm. The B concentration at a depth of 150 μm can be regarded as the B concentration at the center of the steel sheet thickness, and Bmax / B150≧8 means that B is highly concentrated in the range from the surface of the steel sheet to a depth of 5.0 μm, i.e., in the vicinity of the surface.

[0071] From the viewpoint of LME resistance, the larger Bmax / B150 is the more preferable, and it is preferably 10 or more, more preferably 12 or more, 14 or more, or even more preferably 16 or more.

[0072] It is not essential for the steel sheet of this embodiment to satisfy formula (2), and good LME resistance can be obtained as long as formula (1) is satisfied, even if formula (2) is not satisfied. By making the B distribution in the surface layer such that formula (2) is satisfied, B that is more highly concentrated in the surface layer of the steel segregates at the Fe grain boundaries, preventing the penetration of molten zinc, and therefore, a greater effect of suppressing LME can be obtained.

[0073] <<Galvannealed Steel Sheet>> A galvannealed steel sheet can be obtained by forming a galvannealed layer on the surface of the steel sheet according to this embodiment. The galvannealed layer may be formed on one side or both sides of the steel sheet. It may also be formed on a portion of the surface. 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 coatings, 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.

[0074] [Chemical Composition of Galvannealed Layer] An example of the chemical composition of the coating suitable for the galvannealed steel sheet according to this embodiment will be described below. 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.

[0075] (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 necessary. 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 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.

[0076] (Fe: 3 to 20%) When a Zn-containing plating layer is formed on a steel sheet and then the plated steel sheet is heat-treated and alloyed, Fe diffuses from the steel sheet and is contained in the plating layer. The Fe content is preferably 3% or more. The Fe content may be 4% or more, or 5% or more. The Fe content is preferably 20% or less. The Fe content may be 15% or less, 12% or less, 10% or less, or 8% or less.

[0077] In one example of the chemical composition of the coating suitable for the galvannealed steel sheet of this embodiment, the balance other than the above-mentioned 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 impair the effects of the present invention.

[0078] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy.

[0079] [Thickness of plating layer] 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 measuring the change in weight before and after the plating layer is removed by pickling. As the acid solution containing an inhibitor, for example, a 10% hydrochloric acid solution containing 0.06 mass % of an inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) may be used. After the plating layer is removed, the base steel sheet is washed with water and dried.

[0080] 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 2 It may be the following:

[0081] The steel sheet of the present invention exhibits improved LME resistance even when the steel sheet is not provided with a galvannealed hot-dip galvanizing coating. 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 at the overlapping surface of the steel sheets during welding. This may result in LME cracking when the molten galvanization comes into contact with the surface of the ungalvanized steel sheet. 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 resulting in LME cracking. Even in such cases, when the steel sheet of the present invention is used as an ungalvanized steel sheet, the B concentrated in the surface layer can prevent the penetration of molten zinc into Fe grain boundaries, thereby suppressing LME cracking.

[0082] [Thickness] The thickness of the steel sheet and galvannealed steel sheet of this embodiment is not particularly limited. For example, it can 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, 2.8 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, or 2.0 mm or less.

[0083] <Method for Manufacturing Steel Sheet> Next, a method for manufacturing the steel sheet according to this embodiment will be described.

[0084] The steel sheet according to this embodiment can be obtained by a manufacturing method including a casting step of casting molten steel with adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a coiling step of coiling the hot-rolled steel sheet, a pickling step of pickling the coiled hot-rolled steel sheet, a cold-rolling step of cold-rolling the pickled hot-rolled steel sheet to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet may be pickled and then cold-rolled directly without being coiled after the hot rolling step.

[0085] [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 conventional method such as continuous casting or ingot casting.

[0086] [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 ratio 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 ratio of finish rolling may be 10 to 50%.

[0087] [Coiling process] 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 process without being coiled, and then cold-rolled as described below.

[0088] [Pickling Process] The steel sheet after hot rolling is subjected to pickling. In the steel sheet manufacturing method of this embodiment, the unevenness of the steel sheet surface after pickling is controlled for the purpose of concentrating B in the surface layer of the steel sheet in the subsequent annealing process. Specifically, the unevenness is set to 1.5 μm or more in terms of Ra, which is the arithmetic mean height defined in JIS B0601:2013. This condition means that a certain degree of unevenness exists on the steel sheet surface. If the unevenness is small, the strain imparted to the surface layer of the steel sheet becomes small, and therefore, even by annealing at a high dew point as described below, the concentration of B in the surface layer of the steel sheet does not proceed.

[0089] The larger the Ra of the unevenness, the better, and it is preferably 2.0 μm or more, more preferably 2.5 μm or more, 3.0 μm or more, and even more preferably 3.5 μm or more.

[0090] The unevenness of the steel sheet surface after pickling is determined as an arithmetic mean roughness Ra 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, in accordance with JIS B 0601:2013. It is not necessary to constantly measure the unevenness of the steel sheet surface after pickling, and once the pickling conditions under which the desired unevenness is formed have been determined, measurement of the unevenness may be omitted.

[0091] The surface roughness of the steel sheet varies depending on the pickling conditions, and therefore, the conditions may be adjusted appropriately to obtain the above-mentioned unevenness. For example, pickling may be performed using a 1 to 10 mass % hydrochloric acid solution at a temperature of 20 to 95°C for a pickling time of 30 to less than 200 seconds.

[0092] The surface of the steel sheet after pickling has such irregularities, and by rolling out these irregularities in the subsequent cold rolling process to impart strain to the surface layer of the steel, the concentration of B in the surface layer of the steel sheet is promoted in the subsequent annealing process.

[0093] [Cold Rolling Process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet. In the cold rolling process, the unevenness imparted in the above-mentioned pickling process is rolled and crushed, thereby imparting strain to the surface layer of the steel sheet. Therefore, the roll used for cold rolling preferably has a small surface roughness, and the surface roughness of the roll is preferably 1.0 μm or less in Ra. The surface roughness of the roll may be 0.8 μm or less, 0.6 μm or less, or 0.5 μm or less in Ra. 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 process, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0094] In the cold rolling process, the unevenness on the surface of the hot-rolled steel sheet is rolled out, imparting strain to the surface layer of the steel sheet, which promotes the concentration of B in the surface layer of the steel sheet in the subsequent annealing process.

[0095] [Annealing Step] After the cold rolling step, the obtained cold-rolled steel sheet is subjected to the following high dew-point annealing. In the method for producing a steel sheet and a galvannealed steel sheet according to the present embodiment, the annealing step prevents external oxidation from progressing on the surface of the steel sheet, but promotes internal oxidation in the surface layer of the steel sheet, i.e., oxidation progresses toward the inside of the steel sheet.

[0096] In the annealing step of this embodiment, the steel sheet having strain imparted to the surface layer by the above-described steps is held at a high dew point. In particular, in the annealing step in the method for producing the steel sheet and galvannealed steel sheet of this embodiment, the dew point of the atmosphere is changed in the first and second halves of the temperature rise to the holding temperature. Specifically, the dew point from room temperature to the control temperature is different from the dew point from the control temperature to the holding time. Here, the "control temperature" refers to the temperature at which the dew point is changed. The rate of temperature rise to the holding temperature is not particularly limited. The rate of temperature rise may be, for example, 1 to 10°C / sec. If the rate of temperature rise is less than 1°C / sec, it may take too long to raise the temperature to the control temperature, which may result in thick oxides on the surface of the coating layer. On the other hand, if the rate of temperature rise is more than 10°C / sec, internal oxidation may not proceed sufficiently, and the strain imparted to the steel sheet surface may not be fully released, resulting in insufficient boron enrichment. From these perspectives, the rate of temperature rise may be 2°C / sec or more, 3°C / sec or more, or 4°C / sec or more. The temperature rise rate may be 9°C / sec or less, 8°C / sec or less, or 7°C / sec or less.

[0097] The control temperature is 450 to 550° C. During the temperature rise from room temperature to the control temperature, the dew point of the annealing atmosphere is −40° C. or higher and −20° C. or lower. During the temperature rise from the control temperature to the holding temperature, the dew point of the annealing atmosphere is −20° C. or higher and 20° C. or lower.

[0098] If the control temperature is less than 450°C, the dew point increases at low temperatures, so that internal oxidation progresses at low temperatures, the strain imparted to the steel sheet surface layer is released, and the concentration of B is not promoted between the control temperature and the holding temperature.If the control temperature is more than 550°C, the strain imparted to the steel sheet surface layer is released before internal oxidation progresses at high temperatures, and the concentration of B is not promoted.

[0099] If the dew point during the temperature rise from room temperature to the control temperature is less than −40° C., external oxidation of Si and Mn may occur, and internal oxidation may not proceed between the control temperature and the holding temperature. If the dew point during the temperature rise from room temperature to the control temperature is more than −20° C., internal oxidation may proceed at a low temperature, releasing the strain imparted to the surface layer of the steel sheet, and as a result, the concentration of B may not be promoted between the control temperature and the holding temperature.

[0100] Similarly, if the dew point between the control temperature and the holding temperature is −20° C. or lower, internal oxidation to suppress the deboronization may not proceed sufficiently. If the dew point between the control temperature and the holding temperature is higher than 20° C., external oxidation may proceed, and internal oxidation to suppress the deboronization may not proceed sufficiently.

[0101] Furthermore, the dew point during the temperature rise from the control temperature to the holding temperature is set to be 10° C. or more higher than the dew point during the temperature rise from room temperature to the control temperature. This allows internal oxidation to proceed, and the concentration of B to be promoted.

[0102] From the viewpoint of suitably concentrating B, the control temperature may be 460°C or higher, 470°C or higher, or 480°C or higher. The control temperature may be 540°C or lower, 530°C or lower, or 520°C or lower. From the viewpoint of suitably concentrating internal oxidation and B, the dew point during the temperature rise from room temperature to the control temperature may be -38°C or higher, -37°C or higher, or -35°C or higher. The dew point during the temperature rise from room temperature to the control temperature may be 18°C ​​or lower, 17°C or lower, or 15°C or lower. The dew point during the temperature rise from the control temperature to the holding temperature may be -18°C or higher, -17°C or higher, or -15°C or higher. The dew point during the temperature rise from the control temperature to the holding temperature may be 18°C ​​or lower, 17°C or lower, or 15°C or lower.

[0103] The holding temperature is set to 760 to 900°C to promote internal oxidation and the concentration of B. The holding time at the holding temperature is set to 0 to 360 seconds. The holding temperature may be 770°C or higher, 780°C or higher, or 790°C or higher. The holding temperature may be 890°C or lower, 880°C or lower, or 870°C or lower. The holding time may be 10 seconds or higher, 30 seconds or higher, 50 seconds or higher, or 60 seconds or higher. The holding time may be 330 seconds or lower, 300 seconds or lower, 270 seconds or lower, 240 seconds or lower, or 200 seconds or lower.

[0104] The annealing atmosphere is preferably a non-oxidizing atmosphere, such as N-1 to 10 vol% H or N-2 to 4 vol% H. The oxygen concentration of the atmosphere is preferably 50 ppm or less, and may be 30 ppm or less, 20 ppm or less, or 10 ppm or less. By using these conditions, internal oxidation can be promoted while suppressing oxidation of the plating surface.

[0105] In a state in which strain is imparted to the surface layer of the steel sheet by the above-described method, by raising the dew point at a temperature equal to or higher than the control temperature in the annealing step, internal oxidation rapidly progresses in the surface layer of the steel sheet, and B is incorporated into the oxides formed inside the steel sheet, thereby concentrating B in the surface layer of the steel sheet and segregating B at the Fe grain boundaries, thereby obtaining the above-described B concentration distribution in the surface layer.

[0106] Annealing is performed under a tension of 1 to 20 MPa. Applying tension during annealing allows for greater strain to be introduced into the steel sheet, which promotes B enrichment in the surface layer.

[0107] <<Method for Producing Galvannealed Steel Sheet>> Using the steel sheet according to the present invention produced as described above, a galvannealed steel sheet can be produced by a production method including a plating treatment step and an alloying treatment step.

[0108] [Plating step] The plating process may be carried out according to a method known to those skilled in the art. For example, the plating process may be carried out by hot-dip plating, electroplating, vapor deposition plating, thermal spraying, or cold spraying. Preferably, the plating process is carried out by hot-dip plating.

[0109] [Alloying Treatment Step] After the plating treatment step, a known alloying treatment is carried out to form alloy plating.

[0110] The conditions for the plating process and alloying process may be appropriately set in consideration of the chemical composition, thickness, coating weight, etc. of the desired plating layer.

[0111] The steel sheet and galvannealed steel sheet according to the present 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 automotive field. Galvanized 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 galvannealed steel sheet according to the present embodiment are used as automotive steel sheets, the effect of the present embodiment, namely high LME resistance, is suitably exhibited.

[0112] Furthermore, the steel sheet and galvannealed steel sheet of the present embodiment have excellent hydrogen desorption properties due to the formation of a layer in which B is concentrated on the surface, and are therefore suitable for the automotive field in this respect as well.

[0113] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0114] Example No. 1 Molten steel was produced in a blast furnace and cast by continuous casting to obtain a slab having the chemical composition shown in No. 1 of Table 1. The obtained slab was heated to 1200°C and hot-rolled with a finish rolling ending temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C.

[0115] The coiled steel sheet was pickled for 40 seconds using a 5 mass % hydrochloric acid solution at 40° C. After pickling, the steel sheet was cold rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel sheet was 1.6 mm.

[0116] 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 and a holding time of 0 seconds to prepare steel sheet samples. The temperature rise rate during annealing was 5.0°C / second. The dew point of the annealing atmosphere was set to a controlled temperature of 500°C, -20°C from room temperature to the controlled temperature, and -10°C from the controlled temperature to the holding temperature. The annealing was performed under a tension of 15 MPa. A holding time of 0 seconds means that the temperature was raised to 800°C and then immediately started to drop.

[0117] <Examples Nos. 2 to 22, Comparative Examples Nos. 23 to 33> 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 annealing conditions were as shown in Table 2. In the examples in Table 2 where the presence or absence of an alloyed hot-dip galvanized layer is "present," the annealed steel sheet was immersed in a hot-dip galvanized bath (Zn-0.2% Al) at 450°C 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 steel sheets were then subjected to alloying treatment at 520°C for 30 seconds to obtain galvannealed steel sheets having galvannealed layers on both sides. In the case of No. 33, after the formation of the galvannealed coating, heat treatment was performed in an atmospheric furnace at 800°C for 10 seconds.

[0118] (Unevenness after pickling) When producing steel sheets and galvannealed steel sheets, the surface unevenness of the hot-rolled steel sheets after pickling was measured. According to JIS B 0601:2013, 10 locations were randomly selected on the surface of the surface layer side, and the surface profile at each location was measured using a contact surface roughness meter. The surface roughness at these locations was arithmetically averaged to obtain the arithmetic mean roughness Ra. The surface unevenness of the hot-rolled steel sheets is shown in Table 2.

[0119]

[0120]

[0121] (B Distribution in Steel Surface Layer) The B distribution in the steel surface layer of the steel plate was evaluated as follows.

[0122] GDS was performed using samples cut to a size of 50 mm x 50 mm from each steel sheet and galvannealed steel sheet. GDS measurement was performed five times in the sheet thickness direction, and the average value of these measurements was taken as the B concentration. The measurement conditions were as follows. The B concentrations corresponding to a depth of x (μm) and a depth of 150 μm are Bx and B150, respectively.

[0123] 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

[0124] From the obtained Bx, the depth satisfying the formula (1): Bx / B150≧5.0 and the value of the left side of the formula (2): Bmax / B150 were obtained.

[0125] (Surface oxide thickness) By cross-sectional SEM observation, the thickness of the oxide on the steel sheet surface or the galvannealed layer surface was measured, with the oxide being defined as a location containing 20% ​​or more of O by EDS. The measurement field was 80 μm wide x 50 μm long, and the oxide thickness was measured at the thickest part in the field. Similar measurements were performed on five fields, and the average of the maximum oxide thicknesses in each field was taken as the surface oxide thickness, which is shown in Table 3. In Table 3, "<0.5" indicates that the oxide thickness was thinner than 0.5 μm.

[0126] (Thickness of Internal Oxide Layer) By cross-sectional SEM observation, the thickness of the oxide layer was measured in the surface layer of the steel sheet or the surface layer of the steel sheet directly below the galvannealed layer, with precipitates containing 20% ​​or more of O as determined by EDS being defined as internal oxides. The measurement field size was 80 μm wide x 50 μm long, and the thickness of the thickest internal oxide layer in the field was measured. Similar measurements were performed on five fields, and the average of the maximum internal oxide layer thicknesses in each field was taken as the internal oxide layer thickness, which is shown in Table 3.

[0127] (Tensile strength) For each steel sheet and galvannealed steel sheet, a JIS No. 5 tensile test piece was taken with the longitudinal direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241:2011 to determine the tensile strength, which was evaluated as follows: In the present examples, a rating of A or higher was determined to be excellent in LME resistance.

[0128] Rating AAA: 1180 MPa or more Rating AA: 980 MPa or more, less than 1180 MPa Rating A: 780 MPa or more, less than 980 MPa Rating B: Less than 780 MPa

[0129] (LME Resistance) Two samples were cut to a size of 50 mm × 100 mm from each steel sheet and galvannealed steel sheet, and these two samples were spot-welded using a dome radius type welding electrode having a tip diameter of 8 mm at an impact angle of 5°, 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.

[0130] 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 11) that occurred in the shoulder of a weld 2 formed by overlapping two steel sheets 1 and spot welding them together. The shoulder refers to the inclined portion of the edge of a depression created by spot welding. The evaluation was performed based on the length of the shoulder crack 11 as follows: In this example, a rating of A or higher was determined to be excellent in LME resistance.

[0131] 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

[0132] (Hydrogen desorption test) Test specimens measuring 80 mm x 50 mm were cut out from each steel sheet and galvannealed steel sheet, and electrochemically charged with hydrogen. The specimens were 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 and the galvannealed steel sheet were left to stand in an air atmosphere at room temperature for 48 hours, and after the predetermined time had passed, the amount of diffusible hydrogen contained in the steel sheet and the galvannealed steel sheet was measured using a thermal desorption method and evaluated as follows. In the thermal desorption method, the temperature was raised to 400°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.

[0133] 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

[0134] The results of each evaluation are shown in Table 3.

[0135]

[0136] Nos. 1 to 22 are inventive examples, and it was confirmed that they had excellent LME resistance and hydrogen desorption properties.

[0137] In No. 23, the Si content was low, and internal oxidation could not be sufficiently generated during the annealing process, and the deboronization phenomenon could not be suppressed. As a result, the internal oxidation layer became thin, B was not concentrated in the surface layer of the steel sheet, and the Bx / B150≧5.0 was not satisfied. As a result, both the LME resistance and hydrogen desorption performance were poor.

[0138] No. 24 had a high Si content. Therefore, although B was concentrated in the surface layer, the effect of Si on the deterioration of LME resistance was large. As a result, both LME resistance and hydrogen desorption performance were poor.

[0139] In No. 25, the dew point was low in the first half of the temperature rise in the annealing process, causing external oxidation of Si and Mn, and preventing internal oxidation in the second half of the temperature rise. As a result, the internal oxidation layer became thin, B did not concentrate in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both LME resistance and hydrogen desorption performance were poor.

[0140] In No. 26, the dew point was high in the first half of the temperature rise in the annealing process, and internal oxidation progressed at a low temperature, causing the strain imparted to the steel sheet surface layer to be released, preventing the enrichment of B in the second half of the temperature rise. As a result, B did not enrich in the surface layer of the steel sheet, and the relationship Bx / B150≧5.0 was not satisfied. As a result, both the LME resistance and hydrogen desorption performance were poor.

[0141] In No. 27, the dew point in the latter half of the temperature rise in the annealing process was low, and sufficient internal oxidation did not occur to suppress the deboronization phenomenon. As a result, the internal oxidation layer became thin, B did not concentrate in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both the LME resistance and hydrogen desorption performance were poor.

[0142] In No. 28, the dew point was high in the latter half of the temperature rise in the annealing process, and external oxidation progressed, so that sufficient internal oxidation did not occur to suppress the deboronization phenomenon. As a result, the internal oxidation layer became thin, B did not concentrate in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both LME resistance and hydrogen desorption performance were poor.

[0143] In No. 29, the controlled temperature in the annealing process was low, and the dew point increased at low temperatures, so internal oxidation progressed at low temperatures, the strain imparted to the steel sheet surface layer was released, and the enrichment of B was not promoted in the latter half of the temperature rise. Therefore, B did not enrich in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both the LME resistance and hydrogen desorption performance were poor.

[0144] In No. 30, the controlled temperature in the annealing process was high, and the strain imparted to the surface layer of the steel sheet was released before internal oxidation progressed at high temperatures, preventing the enrichment of B. Therefore, B was not enriched in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both the LME resistance and hydrogen desorption performance were poor.

[0145] In No. 31, the holding temperature in the annealing process was low, and internal oxidation did not progress. As a result, the internal oxidation layer became thin, B did not concentrate in the surface layer of the steel sheet, and Bx / B150≧5.0 was not satisfied. As a result, both LME resistance and hydrogen desorption performance were poor.

[0146] In No. 32, the unevenness of the steel sheet surface after pickling was small. Therefore, sufficient strain was not imparted to the surface layer, and B did not concentrate in the surface layer of the steel sheet even after high dew-point annealing, so that Bx / B150≧5.0 was not satisfied. As a result, although the hydrogen desorption property was excellent, the LME resistance was poor.

[0147] In No. 33, after the formation of the galvannealed layer, heat treatment was performed in an atmospheric furnace at 800°C for 10 seconds, resulting in the formation of a thick oxide on the surface of the galvannealed layer and poor hydrogen desorption performance.

[0148] According to the present invention, it is possible to provide a steel sheet and a galvannealed steel sheet having high LME resistance and hydrogen desorption property, and the steel sheet and the galvannealed steel sheet can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles. Therefore, the present invention is an invention with extremely high industrial applicability.

[0149] 1 Steel plate 2 Welded part 11 Crack in shoulder part

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: 0-2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0.0005-0.0050%, Ti: 0.0010-0.1000%, Nb: 0-0.2000%, V: 0-0.15%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and REM: 0-0.100%. and the balance being Fe and impurities; in a GDS measurement of the steel sheet in the thickness direction, a depth at which an emission intensity Bx at a depth x (μm) and an emission intensity B150 at a depth of 150 μm satisfy Bx / B150≧5.0 ... (1) is 0.5 μm or more from the surface of the steel sheet; a thickness of an oxide formed on the surface of the steel sheet is 0.5 μm or less; and an internal oxide layer with a thickness of 1.0 μm or more is present 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 luminous intensity Bx and the luminous intensity B150 satisfy the relationship Bx / B150≧5.0 is 2.0 μm or more from the surface of the steel plate.

3. The steel plate according to claim 1, characterized in that, in GDS measurement of the steel plate in the thickness direction, the maximum B emission intensity Bmax in the range from the surface of the steel plate to a depth of 5.0 μm satisfies the following relationship: Bmax / B150≧8 ... (2).

4. A galvannealed steel sheet according to any one of claims 1 to 3, comprising a galvannealed layer on at least a portion of the surface thereof, and wherein the thickness of the oxide formed on the surface of the galvannealed layer is 0.5 μm or less.

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 20% Fe, and the balance being Zn and impurities.

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