Steel sheet and plated steel sheet

Abstract

The present invention addresses the problem of providing a steel sheet and a plated steel sheet having high LME resistance. A steel sheet and a plated steel sheet according to the present invention are characterized by having a prescribed chemical composition, having a depth of 3 [mu] m or more at which the C concentration measured by GDS is 0.01% or less in the depth direction from the surface of the steel sheet, and having a thickness of 5 [mu] m or more at which the area ratio of cementite is 10% or less in the depth direction from the surface of the steel sheet. The surface roughness of the steel sheet is 3.0 [mu] m or less in terms of arithmetic average roughness Ra.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a plated steel sheet, and more particularly to a steel sheet and a plated steel sheet having high LME resistance. Background Art

[0002] In recent years, the strength of steel sheets used in various fields, including automobiles, home appliances, and building materials, has been increasing. For example, in the automotive field, the use of high-strength steel sheets is increasing to reduce vehicle body weight in order to improve fuel efficiency.

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

[0004] It should be noted that Patent Document 2 discloses a steel plate having improved weldability by suppressing LME cracking, wherein Si oxide particles having a particle size of 20 nm or more are present at a density of 3,000 to 6,000 particles / mm in the surface layer of the steel plate. 2 The number density of the particles is high and they exist in a suitable particle size distribution.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: International Publication No. 2019 / 116531

[0008] Patent Document 2: International Publication No. 2020 / 218575 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] In order to prevent LME cracking, it is effective to suppress the intrusion of Zn and the like contained in the coating into the steel sheet after austenite transformation. In this regard, there is room for improvement.

[0011] 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.

[0012] Means for solving problems

[0013] The present inventors conducted intensive research into means for solving the above-mentioned problems and found that by applying strain to a pre-annealed steel sheet using a projection material under appropriate conditions to achieve an appropriate surface condition and then subjecting it to high dew-point annealing, the surface layer of the steel sheet is decarburized, thereby forming a layer with a low cementite fraction, thereby suppressing LME.

[0014] The present invention has been further studied based on the above-mentioned knowledge, and the gist of the research is as follows.

[0015] (1) A steel plate having a tensile strength of 780 MPa or more, wherein the chemical composition comprises, in mass%, C: 0.08-0.40%, Si: 0.4-2.0%, Mn: 0.1-5.0%, sol. Al: 0-2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.0%, Ni: 0-2.00%, Cu: 0-2.0 000%, 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%, and the remainder is Fe and impurities. In the depth direction from the steel plate surface, the depth at which the C concentration is 0.01% or less as measured by GDS is 3 μm or more, and in the depth direction from the steel plate surface, the thickness of the layer having an area ratio of cementite of 10% or less is 5 μm or more. The surface roughness of the steel plate is 3.0 μm or less in terms of arithmetic mean roughness Ra.

[0016] (2) The steel sheet according to (1) above, wherein the total content of Si and sol.Al is less than 1.8%.

[0017] (3) The steel sheet according to (1) above is characterized in that the depth from the surface of the steel sheet where the C concentration measured by GDS is 0.01% or less is 5 μm or more.

[0018] (4) The steel sheet according to (1) above is characterized in that the depth from the surface of the steel sheet where the C concentration measured by GDS is 0.01% or less is 7 μm or more.

[0019] (5) The steel sheet according to (1) above is characterized in that the surface roughness of the steel sheet is 2.0 μm or less in terms of arithmetic mean roughness Ra.

[0020] (6) A plated steel sheet comprising a Zn-containing plating layer on at least a portion of the surface of the steel sheet according to any one of (1) to (5).

[0021] Effects of the Invention

[0022] According to the present invention, a steel sheet and a plated steel sheet having high LME resistance can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram showing layers formed on the surface layer of the steel plate of the present invention.

[0024] Figure 2 This is a diagram illustrating the LME resistance evaluation in Examples. DETAILED DESCRIPTION

[0025] The present invention will be described below. The present invention is not limited to the following embodiments. First, the structure for improving LME resistance in the steel sheet of the present invention will be briefly described.

[0026] When the plated steel sheet is spot welded, the coating melts, and the surface of the steel sheet is heated, causing the steel sheet structure to transform into austenite. At this time, the hot-dip coating invades the steel sheet structure along the grain boundaries of the austenite, causing the crystal grain boundaries to become brittle. Therefore, if stress is applied to the steel sheet, LME cracking is likely to occur at the crystal grain boundaries. It is believed that LME is likely to occur, especially during welding, because tensile stress is applied to the steel sheet. The steel sheet of the present invention improves LME resistance by forming a structure in the surface layer of the steel sheet. It should be noted that in this specification, the surface layer of the steel sheet is defined as the range from the outermost surface of the steel sheet to a depth of 100 μm.

[0027] If the steel sheet surface contains carbon, LME cracking is more likely to occur. Therefore, keeping the carbon concentration in the steel sheet surface low is effective in preventing LME cracking. Generally, when a steel sheet is heated, as in annealing, external oxidation occurs, forming oxides (scale) on the steel sheet surface, making decarburization difficult. Therefore, it is difficult to reduce the carbon concentration in the steel sheet surface. On the other hand, in the steel sheet of the present invention, the depth from the steel sheet surface at which the carbon concentration is 0.01% or less, as measured by GDS, is 3 μm or more. This means that the concentration of carbon, an element that is prone to LME, is low in the steel sheet surface.

[0028] However, even when the carbon concentration in the steel sheet surface layer is low, if carbon exists as carbides (cementite), the molten coating may penetrate into the steel sheet along the grain boundaries of the cementite, becoming the starting point of LME cracking. Therefore, it is believed that if carbon exists as cementite in the steel sheet surface layer, it may sometimes cause a decrease in LME resistance. Therefore, in the steel sheet of the present invention, the thickness of the layer with an area ratio of cementite of 10% or less in the depth direction from the steel sheet surface is 5 μm or more. In other words, the steel sheet of the present invention improves LME resistance by controlling the morphology of carbon in the surface layer.

[0029] The present inventors discovered that, in order to reduce the area ratio of cementite in the surface layer, in addition to applying strong strain to the surface layer of the steel sheet and performing annealing, it is important to control the dew point during annealing. In the present invention, by applying strong strain to the surface layer without increasing the surface roughness of the steel sheet, it is possible to promote the diffusion of oxygen into the interior of the steel sheet and reduce the carbon concentration in the surface layer of the steel sheet. Furthermore, the present inventors discovered that by setting the carbon concentration and the area ratio of cementite in the surface layer of the steel sheet as described above, LME resistance can be improved, thus completing the present invention.

[0030] Hereinafter, the present invention will be described in detail.

[0031] First, the chemical composition of the steel plate will be described. Hereinafter, "%" in chemical composition refers to "mass %." Furthermore, numerical ranges within chemical composition expressed using "to" refer to ranges that include the numerical values ​​before and after "to" as the lower and upper limits.

[0032] (C: 0.08-0.40%)

[0033] Carbon (C) is an element that ensures the strength of steel. To achieve a tensile strength of 780 MPa or higher, which is the target of the present invention, and to balance this with weldability, the C content is set to 0.08% to 0.40% to prevent excessive carbon concentration in the surface layer of the steel plate. If the C content is too high, the surface C concentration and cementite fraction will not decrease even through high dew point annealing, which will be described later. The C content can be 0.10% or higher, 0.12% or higher, or 0.15% or higher. The C content can be 0.35% or lower, 0.30% or lower, or 0.25% or lower.

[0034] (Si: 0.4-2.0%)

[0035] Si (silicon) is an element that promotes ferrite stabilization and decarburization. By containing Si, decarburization is carried out in the surface layer during the pretreatment and heat treatment described later, and the ferrite in the surface layer is stabilized, thereby improving LME resistance. To achieve this effect, the Si content is set to 0.4-2.0%. If the Si content is too high, external oxidation will occur even if high dew point annealing is performed, forming oxides (scale) on the surface of the steel plate, which in turn inhibits decarburization in the outermost layer and reduces the effect of improving LME resistance. The Si content can be 0.5% or more, 0.6% or more, 0.7% or more, or 0.8% or more. The Si content can be 1.8% or less, 1.6% or less, 1.4% or less, or 1.2% or less.

[0036] (sol.Al: 0-2.0%)

[0037] Al (aluminum) is an element that, like Si, promotes ferrite stabilization and decarburization by dissolving in steel. Sol-Al refers to acid-soluble Al that is not converted into oxides such as Al2O3 and is soluble in acid. It is determined by removing the insoluble residue on the filter paper produced during the Al analysis process. In the steel plate of the present invention, the effect of sol-Al can also be achieved by containing Si, so sol-Al is not essential. The lower limit of the sol-Al content is 0%. If the sol-Al content is too high, external oxidation will occur even when high dew point annealing is performed, forming oxides (scale) in the surface layer of the steel plate. This, in turn, inhibits decarburization at the outermost surface, reducing the effect of improving LME resistance. Therefore, the sol-Al content is set to 2.0% or less. The sol-Al content can be 0.1% or more, 0.2% or more, or 0.3% or more. The sol-Al content can be 1.5% or less, 1.2% or less, or 1.0% or less.

[0038] Note that Si and Al are also elements that reduce LME resistance, so the total content of Si and sol.Al is preferably less than 1.8%. The total content of Si and sol.Al can be less than 1.7% or less than 1.6%.

[0039] (Mn: 0.1-5.0%)

[0040] Mn (manganese) is an element effective in increasing the strength of steel by creating a hard structure. Considering the balance between steel strength and the reduction in workability caused by Mn segregation, the Mn content is set to 0.1-5.0%. The Mn content can be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content can be 4.5% or less, 4.0% or less, or 3.5% or less.

[0041] (P: 0.0300% or less)

[0042] Phosphorus (P) is generally contained in steel as an impurity. A P content exceeding 0.0300% may reduce weldability. Therefore, the P content is set to 0.0300% or less. The P content can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. P is preferably absent, and the lower limit of the P content is 0%. From the perspective of dephosphorization costs, the P content can be greater than 0%, 0.0001% or more, or 0.0005% or more.

[0043] (S: 0.0300% or less)

[0044] S (sulfur) is generally contained as an impurity in steel. If the S content exceeds 0.0300%, weldability may be reduced, and further, the amount of MnS precipitation may increase, reducing workability such as bendability. Therefore, the S content is set to 0.0300% or less. The S content can be 0.0100% or less, 0.0050% or less, or 0.0020% or less. S is preferably not contained, and the lower limit of the S content is 0%. From the perspective of desulfurization costs, the S content can be greater than 0%, 0.0001% or more, or 0.0005% or more.

[0045] (N: 0.0100% or less)

[0046] Nitrogen (N) is generally contained as an impurity in steel. A N content exceeding 0.0100% may reduce weldability. Therefore, the N content is set to 0.0100% or less. The N content can be 0.0080% or less, 0.0050% or less, or 0.0030% or less. N is preferably absent, and the lower limit of the N content is 0%. From the perspective of manufacturing costs, the N content can be greater than 0%, 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0047] (B: 0 to 0.0100%)

[0048] B (boron) is an element that improves hardenability and contributes to the improvement of strength, and further segregates at grain boundaries to strengthen grain boundaries and improve toughness, so it can also be contained as needed. Since it is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even if it is contained in a trace amount, but the B content in the case of inclusion is preferably 0.0001% or more. The B content can be 0.0002% or more or 0.0003% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.0100% or less. The B content can be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less or 0.0020% or less.

[0049] (Ti: 0 to 0.1500%)

[0050] Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to the improvement of strength, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Ti content is 0%. This effect can be achieved even with a trace amount of inclusion, but the Ti content when included is preferably 0.0001% or more. The Ti content can be 0.0002% or more or 0.0003% or more. On the other hand, if it is included in excess, coarse TiN may be generated and the toughness may be impaired, so the Ti content is set to 0.1500% or less. The Ti content can be 0.1350% or less, 0.1200% or less, 0.0900% or less, 0.0600% or less, 0.0450% or less, 0.0300% or less, 0.0150% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0051] (Nb: 0-0.150%)

[0052] Nb (niobium) is an element that contributes to the improvement of strength by improving hardenability, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be achieved even with a trace amount of Nb, but when it is contained, the Nb content is preferably 0.001% or more. The Nb content can be 0.002% or more, 0.004% or more, 0.006% or more, or 0.007% or more. On the other hand, from the perspective of ensuring sufficient toughness, the Nb content is set to 0.150% or less. The Nb content can be 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

[0053] (V: 0~0.150%)

[0054] V (vanadium) is an element that contributes to the improvement of strength by improving hardenability, so it can also 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 if it is contained in a trace amount, but the V content in the case of being contained is preferably 0.001% or more. The V content can be 0.003% or more, 0.005% or more, or 0.006% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. The V content can be 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.045% or less, 0.025% or less, or 0.020% or less.

[0055] (Cr: 0-2.00%)

[0056] Cr (chromium) is effective in improving the hardenability of steel and thus the strength of steel, so it can be contained as needed. Since it is not an essential element, the lower limit of the Cr content is 0%. This effect can be achieved even with a trace amount of Cr, but when it is contained, the Cr content is preferably 0.001% or more. The Cr content can be 0.01% or more, 0.02% or more, 0.04% or more, 0.06% or more, or 0.07% or more. On the other hand, if it is contained in excess, a large amount of Cr carbides may be formed, which in turn impairs the hardenability, so the Cr content is set to 2.00% or less. The Cr content can be 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0057] (Ni: 0-2.00%)

[0058] Ni (nickel) is effective in improving the hardenability of steel and thus the strength of steel, so it can also 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 if it is contained in a trace amount, but the Ni content in the case of containing it is preferably 0.001% or more. The Ni content can be more than 0.01%, more than 0.03%, more than 0.04% or more than 0.05%. On the other hand, excessive addition of Ni will increase the cost, so the Ni content is set to less than 2.00%. The Ni content can be less than 1.80%, less than 1.60%, less than 1.25%, less than 0.85%, less than 0.65%, less than 0.40%, less than 0.25% or less than 0.15%.

[0059] (Cu: 0-2.0000%)

[0060] Cu (copper) is effective in improving the hardenability of steel and thus the strength of steel, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Cu content is 0%. This effect can be achieved even with a trace amount of Cu, but the Cu content is preferably 0.0001% or more. The Cu content can be 0.0002% or more or 0.0004% or more. On the other hand, from the viewpoint of suppressing the reduction of toughness, cracking of the slab after casting, and reduction of weldability, the Cu content is set to 2.0000% or less. The Cu content may be 1.8000% or less, 1.6000% 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.

[0061] (Mo: 0-1.00%)

[0062] Mo (molybdenum) is effective in improving the hardenability of steel and thus the strength of steel, so it can also 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 if it is contained in a trace amount, but the Mo content in the case of inclusion is preferably 0.001% or more. The Mo content can be 0.01% or more, 0.03% or more, 0.05% or more, or 0.06% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness, the Mo content is set to 1.00% or less. The Mo content can be 0.90% or less, 0.80% or less, 0.65% or less, 0.45% or less, 0.35% or less, 0.30% or less, or 0.20% or less.

[0063] (W: 0~1.000%)

[0064] W (tungsten) is effective in improving the hardenability of steel and thus the strength of steel, so it can also 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 if it is contained in a trace amount, but the W content in the case of being contained is preferably 0.001% or more. The W content can be 0.002% or more or 0.003% or more. On the other hand, from the viewpoint of suppressing the reduction of toughness, the W content is set to 1.000% or less. The W content can 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, 0.025% or less, 0.015% or less or 0.010% or less.

[0065] (Ca: 0 to 0.1000%)

[0066] Ca (calcium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Ca content is 0%. This effect can be achieved even with a trace amount of inclusion, but the Ca content is preferably 0.0001% or more. The Ca content can be 0.0002% or more or 0.0003% or more. On the other hand, if it is contained in excess, the deterioration of the surface properties sometimes becomes obvious, so the Ca content is set to 0.1000% or less. The Ca content can be 0.0900% or less, 0.0800% or less, 0.0600% 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.

[0067] (Mg: 0-0.100%)

[0068] Mg (magnesium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Mg content is 0%. This effect can be obtained even if it is contained in a trace amount, but the Mg content in the case of inclusion is preferably 0.0001% or more. The Mg content can be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if it is contained in excess, the deterioration of the surface properties sometimes becomes obvious, so the Mg content is set to 0.100% or less. The Mg content can 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.

[0069] (Zr: 0 to 0.100%)

[0070] Zr (zirconium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Zr content is 0%. This effect can be obtained even if it is contained in a trace amount, but the Zr content in the case of inclusion is preferably 0.001% or more. The Zr content can be 0.003% or more, 0.006% or more, 0.009% or more, or 0.010% or more. On the other hand, if it is contained in excess, the deterioration of the surface properties sometimes becomes obvious, so the Zr content is set to 0.100% or less. The Zr content can be 0.090% or less, 0.085% or less, 0.065% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0071] (Hf: 0~0.100%)

[0072] Hf (Hafnium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can also be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0%. This effect can be achieved even with a trace amount of inclusion, but the Hf content is preferably 0.0001% or more. The Hf content can be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if it is contained in excess, the deterioration of the surface properties may sometimes become obvious, so the Hf content is set to 0.100% or less. The Hf content can 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.

[0073] (REM: 0~0.1000%)

[0074] REM (rare earth elements) can be contained as needed because they contribute to inclusion control, especially fine dispersion of inclusions, and have the effect of improving toughness. Since they are not essential elements, the lower limit of the REM content is 0%. This effect can be achieved even with a trace amount of inclusion, but when included, the REM content is preferably 0.0001% or more. The REM content can be 0.0003% or more, 0.0004% or more, or 0.0005% or more. On the other hand, if excessively contained, the deterioration of surface properties may sometimes become apparent, so the REM content is set to 0.1000% or less. The REM content can be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0040% or less, 0.0025% or less, or 0.0015% or less. REM stands for Rare Earth Metal and refers to elements belonging to the lanthanide series. REM is usually added as a mixed rare earth alloy.

[0075] In the steel sheet according to the present invention, the remainder other than the above-mentioned chemical components includes Fe and impurities. Here, impurities refer to components that are mixed into the steel sheet during industrial production due to various factors in the production process, typified by raw materials such as ores and scraps, and are contained within a range that does not adversely affect the LME resistance of the steel sheet according to the present invention and can achieve the LME resistance required of the steel sheet according to the present invention.

[0076] The chemical composition of the steel plate can be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). However, C and S can be measured using the combustion-infrared absorption method, while N can be measured using the inert gas fusion-thermal conductivity method. These analyses can be performed on samples collected from the steel plate using methods in accordance with JIS G0417:1999.

[0077] Next, the surface layer portion of the steel plate will be described.

[0078] [C concentration]

[0079] In the steel sheet of the present invention, the depth at which the C concentration measured by GDS (glow discharge spectrometry) is 0.01% or less is 3 μm or more in the depth direction from the steel sheet surface.

[0080] Since LME sensitivity decreases as the C concentration decreases, LME resistance is improved by reducing the C concentration in the surface layer. In addition, since C is an austenite stabilizing element, a low C concentration stabilizes the layer with low LME sensitivity described later.

[0081] Such a surface structure can be obtained by setting the chemical composition of the steel plate as described above and performing the pretreatment and heat treatment described later.

[0082] If the depth at which the C concentration is 0.01% or less is 3 μm or more, LME resistance can be improved, so the upper limit of this depth is not particularly limited. For example, the depth at which the C concentration is 0.01% or less can be 50 μm or less, 40 μm or less, or 30 μm or less. For example, the depth at which the C concentration is 0.01% or less can be 5 μm or more, 7 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more.

[0083] GDS measurements were performed five times along the thickness direction of the plate, and the average value was used as the C concentration. The measurement conditions were set as follows. The starting point of "depth" was the surface of the steel plate for unplated steel plates and the interface between the steel plate and the plated layer for plated steel plates. The interface between the steel plate and the plated layer was defined as the point where the Fe concentration measured by GDS reached 93% of the Fe concentration at a depth of 150 μm.

[0084] Equipment: High-frequency glow discharge luminescence analyzer (manufactured by LECO JAPAN Co., Ltd., model "GDS850A"

[0085] Ar gas pressure: 0.3MPa

[0086] Anode diameter: 4mmφ

[0087] RF output power: 30W

[0088] Measuring time: 200 to 1500 seconds

[0089] [Low cementite layer]

[0090] In the steel sheet of the present invention, the thickness of a layer having an area ratio of cementite of 10% or less (hereinafter referred to as "low cementite layer") in the depth direction from the steel sheet surface is 5 μm or more. Figure 1 1 shows an example of a microstructure photograph of the vicinity of the surface layer of the steel plate of the present invention taken with a SEM at a magnification of 1000 times. Figure 1 It is a cross section parallel to the thickness direction of the steel plate, and the upper side of the figure is the surface of the steel plate. In the surface layer of the steel plate, there is a layer with a low C concentration and an area ratio of cementite with ferrite as the main body of less than 10%, namely a low cementite layer 11. Compared with the low cementite layer 11, there is a hard structure 12 containing a relatively large amount of cementite on the inner side of the steel plate (the lower side of the figure). Ferrite and cementite can be distinguished by the difference in brightness in the SEM image. Figure 1When the steel plate portion is classified into a relatively bright (bright) portion and a relatively dark (dark) portion, the relatively bright portion can be determined to be cementite, and the relatively dark portion can be determined to be ferrite.

[0091] Since cementite tends to segregate at grain boundaries, if the amount of cementite increases, the molten coating will intrude into the grains, making LME more likely to occur. Therefore, by having a thick low-cementite layer, LME is less likely to occur even when the coating melts, improving LME resistance. This surface structure can be achieved by setting the chemical composition of the steel plate as described above and performing the pretreatment and heat treatment described below.

[0092] If the thickness of the low cementite layer is 5 μm or greater, the LME resistance can be improved, so the upper limit of the thickness is not particularly limited. The thickness of the low cementite layer can be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. The thickness of the low cementite layer can be 10 μm or more or 20 μm or more.

[0093] The structure other than cementite in the low cementite layer is not limited. For example, it can be set to any one or more of martensite, bainite, and ferrite. Ferrite has low LME sensitivity, so a structure mainly composed of ferrite is preferred from the perspective of improving LME resistance.

[0094] The thickness of the low cementite layer is obtained by etching the C section of the steel plate (the plate thickness section parallel to the rolling direction (L direction)) with nitric alcohol and observing the 50μm×50μm field of view containing the surface layer of the steel plate at a magnification of 1000 times by SEM. The organizational morphology on the SEM image obtained by SEM observation can distinguish between hard structures such as martensite and bainite containing relatively more cementite and ferrite. The thickness of the low cementite layer is measured within a measurement range of 500μm in the L direction, and the measurement range is set to 5 ranges with an interval of 1000μm in the L direction. It is set to the average value of the thickness of the low cementite layer in the plate thickness direction within the 5 measurement ranges. Here, the area ratio of cementite refers to the area ratio obtained by observation in the C section. When observing the L section (the plate thickness section perpendicular to the rolling direction) locally in the thickness direction, for example, even if there is a part where the area ratio of cementite exceeds 10%, there is no problem as long as the area ratio of cementite in the C section up to a depth of 5μm is less than 10%.

[0095] The steel sheet of the present invention may have a coating as described below. In the case of a coating, the starting point of the thickness of the layer having a C concentration of 0.01% or less and a cementite area ratio of 10% or less in GDS measurement is the interface between the steel sheet and the coating.

[0096] [Surface roughness]

[0097] The surface roughness of the steel sheet of the present invention is 3.0 μm or less, as measured by the arithmetic mean height Ra defined in JIS B0601:2013. Increased roughness increases the likelihood of cracking due to stress concentration, thereby reducing LME resistance. When a coating (described below) is present, this roughness refers to the roughness of the interface between the steel sheet and the coating.

[0098] [tensile strength]

[0099] The present invention is an invention for suppressing the formation of LME in high-strength steel sheets. Specifically, high-strength steel sheets are steel sheets having a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the perspective of ensuring toughness, it can be, for example, 2000 MPa or less. The tensile strength can be measured by collecting JIS No. 5 tensile test pieces with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction and performing the measurement in accordance with JIS Z 2241:2011. The tensile strength can be 880 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The tensile strength can be 1900 MPa or less, or 1800 MPa or less.

[0100] When the rolling direction of the steel plate is unclear, the following method is used as a method for determining the rolling direction of the steel plate. After the plate thickness section of the steel plate is finished by mirror polishing, the S concentration is measured using an electron probe micro analyzer (EPMA). The measurement conditions are to set the acceleration voltage to 15kV, set the measurement interval to 1μm, and measure the distribution image of a 500μm square range in the center of the plate thickness. At this time, the extended area with a high S concentration is determined to be an inclusion such as MnS. During observation, it is also possible to observe in multiple fields of view. Then, using the plate thickness section initially observed by the above method as a reference, a cross-sectional observation is performed on the surface that becomes parallel to the surface rotated by 5° in the range of 0° to 180° around the axis by the above method. In each cross section, the average value of the length of the major axis of the multiple inclusions in each cross section is calculated, and the cross section where the average value of the length of the major axis of the inclusion becomes the largest is determined. The direction in the cross section parallel to the major axis direction of the inclusion is determined as the rolling direction.

[0101] It should be noted that when it is impossible to collect a test piece for tensile strength measurement from the steel plates constituting the weld joint, the hardness (Vickers hardness) of the steel plate in the non-heat-affected zone (NAZ) at a distance of 5 mm or more from the outer end of the nugget of the spot weld can be measured as an alternative, and the tensile strength value can be estimated using the following correlation formula (Correlation between Static Strength Parameters, Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials," Vol. 39, No. 442, pp. 859-863). The "heat-affected zone" refers to the portion of the steel plate that has not melted due to changes in its structure, metallurgical properties, mechanical properties, etc., caused by welding heat, and the "non-heat-affected zone" refers to the portion other than the heat-affected zone. The portion at a distance of 5 mm or more from the outer end of the nugget of the spot weld can be determined as the NZZ.

[0102] Hv=0.301×TS+5.701

[0103] (Where Hv is Vickers hardness, TS is tensile strength (unit: MPa).)

[0104] That is, if the hardness is approximately 240 Hv or higher, it can be considered that the tensile strength is 780 MPa or higher.

[0105] <Plated Steel Sheet>

[0106] The plated steel sheet according to the present invention comprises a Zn-containing plating layer on the steel sheet according to the present invention. The plating layer is formed on at least a portion of the surface of the steel sheet and may be formed on one or both surfaces of the steel sheet. The plating layer may also be a plating layer subjected to an alloying treatment.

[0107] [Chemical composition of the coating]

[0108] The chemical composition of the plating layer is not limited as long as it contains Zn. Examples of Zn-containing plating layers include Zn-0.2% Al (GI), Zn-(0.3-1.5)% Al, Zn-4.5% Al, Zn-0.09% Al-10% Fe (GA), Zn-1.5% Al-1.5% Mg, Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, or Zn-15% Mg.

[0109] The chemical composition of the coating can be determined by dissolving the coating in an acid solution containing an inhibitor for inhibiting corrosion of the steel sheet and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. The acid solution containing the inhibitor can be, for example, a 10% by mass hydrochloric acid solution containing 0.06% by mass of an inhibitor (manufactured by Asahi Chemical Industries, Ltd., IBIT).

[0110] The thickness of the plating layer can be, for example, 3 to 50 μm. The amount of the plating layer applied is not particularly limited, but can be, for example, 10 to 170 g / m per side.2 In the present invention, the coating adhesion amount is determined by dissolving the coating in an acid solution containing an inhibitor for inhibiting corrosion of the steel plate, and determining the weight change of the coating before and after pickling and stripping. The coating thickness may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The coating thickness may be 40 μm or less, or 30 μm or less. The coating adhesion amount may be 20 g / m per side. 2 Above, 30g / m 2 Above, 40g / m 2 Above or 50g / m 2 The coating adhesion can be 150g / m2 per side. 2 Below, 130g / m 2 Below, 120g / m 2 Below or 100g / m 2 the following.

[0111] The roughness of the interface between the steel sheet and the coating becomes the surface roughness of the steel sheet described above, so the arithmetic mean roughness Ra is 3.0 μm or less. Considering the adhesion of the coating, the Ra can be 2.5 μm or less, or 2.0 μm or less. The roughness of the interface between the steel sheet and the coating can be set as the surface roughness of the steel sheet measured after dissolving and removing the coating.

[0112] It should be noted that the steel sheet of the present invention can achieve improved LME resistance even without a galvanized layer. Generally speaking, when spot welding ungalvanized steel sheets together, LME cracking will not occur unless molten zinc comes into contact near the spot weld. However, when spot welding one galvanized steel sheet to the other ungalvanized steel sheet, molten zinc forms on the overlapping surface of the steel sheets during welding. Therefore, the molten zinc may come into contact with the surface of the ungalvanized steel sheet, causing LME cracking. Furthermore, when spot welding galvanized steel sheets using a welding electrode with a molten coating, when spot welding another ungalvanized steel sheet, the galvanized coating on the welding electrode may melt and come into contact with the surface of the other steel sheet, causing LME cracking. When the steel sheet of the present invention is used as an unplated steel sheet, even when molten zinc can come into contact during spot welding as described above, LME cracking can be suppressed because the surface layer has a low C concentration and is a low cementite layer.

[0113] When creating a welded joint made by spot welding multiple overlapping steel plates, the use of the steel plate of the present invention as the outermost steel plate can suppress the occurrence of LME cracking in the outermost layer of the welded joint. LME cracking in the outermost layer of the welded joint can occur when the outermost steel plate is a high-strength steel plate with a high carbon concentration and has a galvanized layer on the surface side, or when a molten galvanized layer adheres to the welding electrode. Examples of LME cracking in the outermost layer of the welded joint include cracking in the indentation of the welding electrode (cracks directly below the welding electrode) and cracking in the inclined portion (shoulder) around the edge of the indentation (cracks in the weld shoulder). These can be suppressed by using the steel plate of the present invention as the outermost steel plate.

[0114] Furthermore, in the welded joint described above, even when one steel plate is galvanized and the other is a high-strength steel plate without a galvanized layer, LME cracking can be suppressed by using the steel plate of the present invention as the high-strength steel plate. Examples of LME cracking in such an overlapping surface include cracking in the vicinity of the outer side of the portion where the steel plates are press-bonded by spot welding (cracks outside the press-bonded portion), and these can be suppressed by using the steel plate of the present invention as the high-strength steel plate.

[0115] The thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it can be set to 0.6 to 3.2 mm. The thickness can be 0.8 mm or more, or 1.0 mm or more. The thickness can be 3.0 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0116] Next, the method for producing the steel sheet according to the present invention will be described.

[0117] The steel plate according to the present invention can be obtained, for example, by a manufacturing method comprising the following steps: a casting step in which molten steel with adjusted chemical composition is cast to form a steel slab; a hot rolling step in which the steel slab is hot-rolled to obtain a hot-rolled steel plate; a coiling step in which the hot-rolled steel plate is coiled; a cold rolling step in which the coiled hot-rolled steel plate is cold-rolled to obtain a cold-rolled steel plate; a pretreatment step in which the cold-rolled steel plate is shot-peened; and an annealing step in which the pretreated cold-rolled steel plate is annealed. Alternatively, after the hot rolling step, the hot-rolled steel plate may be pickled without being coiled and then directly cold-rolled.

[0118] [Casting process]

[0119] The conditions of the casting process are not particularly limited. For example, the smelting process may be followed by various secondary refining steps using a blast furnace or electric furnace, and then casting may be performed by conventional continuous casting or ingot casting.

[0120] [Hot rolling process]

[0121] Hot-rolled steel sheets can be obtained by hot rolling the steel slabs obtained by casting. The hot rolling process is carried out by hot rolling the cast steel slabs directly or temporarily cooling them and then reheating them. In the case of reheating, the heating temperature of the steel slab can 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 process can be appropriately changed according to the desired metal structure and plate thickness. For example, the end temperature of the finish rolling can be 900 to 1050°C, and the reduction ratio of the finish rolling can be 10 to 50%.

[0122] [Coiling process]

[0123] Hot-rolled steel sheets can be coiled at a predetermined temperature. The coiling temperature can be adjusted appropriately based on the desired metallurgical structure, for example, between 500°C and 800°C. The hot-rolled steel sheets can also be uncoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheets can be pickled after the hot rolling process and then cold-rolled as described below, without coiling.

[0124] [Cold rolling process]

[0125] After pickling, the hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet. The cold-rolling reduction can be adjusted appropriately based on the desired metallographic structure and sheet thickness, and can be, for example, 20% to 80%. After the cold-rolling step, the sheet can be cooled to room temperature, for example, by air cooling.

[0126] [Pre-treatment process]

[0127] In order to obtain the above-mentioned structure of the surface layer of the steel plate, it is necessary to perform a predetermined pretreatment and then perform annealing.

[0128] The pretreatment includes shot peening the surface of the cold-rolled steel sheet using a spherical abrasive. The abrasive that can be used is not particularly limited, but for example, steel balls with a center particle size of 40 to 450 μm, preferably 120 to 420 μm, and more preferably 180 to 350 μm can be used. An example of such abrasive is TSH30 from WINOA IKK JAPAN. The shot peening rate can be 5 to 400 kg / m 2 . Thus, strain can be introduced without increasing the surface roughness. By performing such shot peening, decarburization can be promoted in the annealing process described later, and a structure with reduced cementite can be effectively formed in the surface layer of the steel plate. It should be noted that the shot amount is 400kg / m 2 The level of per unit time per unit area is 4.0×10 -4 kg / (mm 2·min). The surface roughness of the steel sheet after pretreatment is preferably 3.0 μm or less in terms of arithmetic mean roughness Ra. The roughness in terms of Ra may be 2.5 μm or less or 2.0 μm or less. It should be noted that the surface roughness of the steel sheet after pretreatment can also be maintained in the steel sheet and plated steel sheet according to the present invention after the annealing step and the plating step (including the alloying step) described below.

[0129] [Annealing process]

[0130] After the pretreatment process, the cold-rolled steel sheet is annealed. Annealing is performed under a tension of 1 to 20 MPa. Applying tension during annealing effectively introduces strain into the steel sheet, promoting decarburization of the surface layer.

[0131] The holding temperature in the annealing step is set to 750-900°C. Alternatively, the holding temperature can be 770-870°C. This range promotes decarburization, reduces the carbon concentration in the surface layer, and reduces cementite. The heating rate to the holding temperature is not particularly limited; it can be 1-10°C / second.

[0132] The holding time at the holding temperature in the annealing process is set to 40 to 300 seconds. The holding time can also be 50 to 250 seconds. By setting it within this range, decarburization can be promoted, the carbon concentration in the surface layer can be reduced, and cementite can be reduced.

[0133] The dew point of the annealing atmosphere is set to -30 to 20°C. Alternatively, the dew point can be -10 to 5°C. For example, the atmosphere can be N2-1 to 10 vol% H2 or N2-2 to 4 vol% H2. Setting this range promotes decarburization, reduces the carbon concentration in the surface layer, and reduces cementite. Furthermore, if the dew point is too high or too low, a phase containing oxides such as Si, Mn, and Al forms on the exterior of the steel sheet, failing to promote decarburization. Furthermore, this can hinder the interdiffusion of plating components and steel components, resulting in insufficient plating properties.

[0134] According to the production method including the above-mentioned steps, decarburization can be promoted in the surface layer of the steel sheet, and a steel sheet with reduced cementite can be obtained.

[0135] <Method for Manufacturing Plated Steel Sheet>

[0136] The plated steel sheet according to the present invention can be obtained by a production method including a plating treatment step of forming a plated layer on the steel sheet produced as described above.

[0137] The plating treatment process can be carried out according to the hot dip method known to those skilled in the art. The conditions of the plating treatment process can be appropriately set in consideration of the chemical composition, thickness and adhesion amount of the desired coating. For example, it can be immersed in a hot dip galvanizing bath of 420 to 480°C with adjusted chemical composition for 1 to 10 seconds. After dipping, it can be scooped up at 20 to 200 mm / second and the coating adhesion amount can be controlled by N2 wiping gas. After the plating treatment process, a well-known alloying treatment process can also be provided to produce an alloyed coating. The alloying treatment can be carried out, for example, at 500 to 550°C for 10 to 60 seconds.

[0138] The steel sheets and plated steel sheets of the present invention are high-strength and have high LME resistance, making them suitable for use in a wide range of fields, including automobiles, home appliances, and building materials. They are particularly preferred for use in the automotive field. Steel sheets and plated steel sheets used for automobiles are often spot welded, in which case LME cracking can be a significant problem. Therefore, when the steel sheets and plated steel sheets of the present invention are used as automotive steel sheets, the advantages of the present invention, such as their high LME resistance, can be advantageously demonstrated.

[0139] Example

[0140] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[0141] (Preparation of Steel Plate Samples)

[0142] <Example 1>

[0143] Molten steel adjusted to the chemical composition listed in Table 1, No. 1, was melted in a blast furnace and cast by continuous casting to produce a steel slab. The resulting steel slab was heated to 1200°C and hot rolled with a finishing temperature of 950°C and a finishing reduction of 30% to produce a hot-rolled steel sheet. The resulting hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold rolled at a reduction of 50% to produce a cold-rolled steel sheet. The cold-rolled steel sheet had a thickness of 1.6 mm.

[0144] Next, the cold rolled steel sheet was projected with WINOA IKK JAPAN’s TSH30 at a rate of 5 kg / m 2 Perform shot peening.

[0145] After the pretreatment step and before the annealing step, the surface roughness of the steel sheet was measured in accordance with JIS B 0601:2013. Specifically, 10 locations on the surface layer side were randomly selected, and the surface profile was measured at each location using a contact-type surface roughness meter. The surface roughness values ​​at these locations were arithmetic averaged, and the arithmetic average roughness Ra was evaluated as follows.

[0146] Evaluation AA: 2.0μm or less

[0147] Evaluation A: more than 2.0 μm and 3.0 μm or less

[0148] Evaluation B: more than 3.0 μm

[0149] Afterwards, annealing treatment was performed in a furnace with an oxygen concentration of 20 ppm or less in an N₂-4% H₂ atmosphere at a dew point of 0°C, a holding temperature of 800°C, and a holding time of 40 seconds to produce steel plate samples. The heating rate during annealing was set at 6.0°C / second up to 500°C and 2.0°C / second from 500°C to the holding temperature. Annealing was performed under a tension of 5.0 MPa.

[0150] Following annealing, the plated steel sheet was plated. The plated steel sheet was immersed in a hot-dip galvanizing bath (Zn-0.14% Al) at 450°C for 3 seconds. After immersion, the sheet was pulled up at a speed of 100 mm / s and the coating weight was controlled to 50 g / m² by wiping with N2 gas. 2 Following the plating treatment, alloying treatment was performed at 520°C for 30 seconds.

[0151] <Examples 2 to 24, Comparative Examples 25 to 37>

[0152] Welded joints were produced in the same manner as in Example 1, except that the chemical composition was set to that listed in Table 1, the pretreatment and annealing conditions were set to those listed in Table 2, and the plating type was set to that listed in Table 3. The LME resistance during production was evaluated. Note that shot peening was omitted for No. 32. For No. 33, a steel plate with increased surface roughness by skin-pass rolling was used. For No. 37, the surface was treated by brush grinding instead of shot peening.

[0153] In Table 3, "a" of the coating types indicates Zn-0.14%Al alloyed hot-dip galvannealing, "b" indicates Zn-0.14%Al hot-dip galvannealing without alloying treatment, and "c" indicates Zn-1.5%Al-1.5%Mg.

[0154] Furthermore, “non-plated” means that plating is not performed.

[0155]

[0156] [Table 2]

[0157]

[0158] ※Underlined elements are outside the range of preferred manufacturing conditions.

[0159] The obtained steel sheets were evaluated for surface structure, roughness of the steel sheet surface or the steel sheet / plating interface, tensile strength, and LME resistance.

[0160] (Surface tissue evaluation)

[0161] A sample cut into 30 mm x 30 mm was collected and GDS was measured five times in the plate thickness direction to determine the depth at which the C concentration was 0.01% or less. The depth is shown in Table 3 as "C ≤ 0.01% depth".

[0162] In addition, samples were cut into 25 mm x 15 mm plates and nital-etched. The C-section (a cross-section parallel to the rolling direction (L direction)) of each sample was observed using a SEM to measure the thickness of the low-cementite layer, which is shown in the "Thickness of Cementite 10% or Less" column in Table 3. The starting point of "depth" is the surface of the steel plate for unplated steel plates and the interface between the steel plate and the plating layer for plated steel plates. The interface between the steel plate and the plating layer is defined as the position where the Fe concentration measured by GDS is 93% of the Fe concentration at a depth of 150 μm.

[0163] In addition, the surface of the steel sheet was removed using a 10 mass % hydrochloric acid solution containing 0.06 mass % of an inhibitor (manufactured by Asahi Chemical Industries, Ltd., IBIT) in the case of the unplated steel sheet, and the plating layer was removed using a 10 mass % hydrochloric acid solution containing 0.06 mass % of an inhibitor (manufactured by Asahi Chemical Industries, Ltd., IBIT). The roughness of the exposed surface of the steel sheet was measured by the same method as before annealing, and the results are shown in the "Steel Sheet Surface or Steel Sheet / Plating Interface Roughness" section of Table 3.

[0164] (Tensile Strength Evaluation)

[0165] For each steel plate, a JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction was collected and subjected to a tensile test in accordance with JIS Z 2241:2011 to determine the tensile strength and evaluate it as follows.

[0166] Rating AAA: 1180MPa or above

[0167] Rating AA: 980MPa or higher and lower than 1180MPa

[0168] Evaluation A: 780 MPa or higher and lower than 980 MPa

[0169] Evaluation B: less than 780MPa

[0170] (LME resistance evaluation)

[0171] Two samples, cut into 50 mm x 100 mm dimensions, were collected from each steel plate. These two samples were spot welded using a dome-radius-type welding electrode with an 8 mm tip diameter at a 2° angle, a 4.0 kN force, a 0.5 second current, and a 12 kA current to produce welded joints. When using "unplated" steel plates for welded joint production as shown in Table 3, spot welding was performed ten or more times on steel plates previously plated with zinc. Spot welding was then performed using a zinc-coated welding electrode, with the electrode in contact with the "unplated" steel plate.

[0172] Reference Figure 2 The evaluation of LME resistance is now described. LME resistance is evaluated based on the length of LME cracks (cracks 23 on the shoulder and outside the shoulder) that form at the weld 22 formed by spot welding two overlapping steel plates 21. The shoulder refers to the deep, inclined portion of the depression created by spot welding, while the outside shoulder refers to the outer portion of the shoulder. Based on the length of the shoulder crack 23, the evaluation is performed as follows. In this example, a rating of A or higher indicates excellent LME resistance, thus addressing the present invention's objectives.

[0173] Rating AAA: 0μm

[0174] Evaluation AA: more than 0μm and less than 60μm

[0175] Evaluation A: 60 μm or more and less than 120 μm

[0176] Evaluation B: 120 μm or more

[0177] Table 3 shows the results of each evaluation.

[0178] [Table 3]

[0179]

[0180] ※ Underlined elements are outside the scope of the present invention or do not provide the desired properties.

[0181] No. 25 is a comparative example in which the steel sheet contains a high carbon content. It is believed that due to the high carbon content in the steel sheet, the carbon concentration in the surface layer of the steel sheet did not decrease even after high dew-point annealing. Consequently, the depth at which the carbon concentration is 0.01% or less and the thickness of the layer with a cementite area fraction of 10% or less are reduced. Consequently, LME resistance is poor.

[0182] Steel No. 26 is a comparative example in which the Si content of the steel plate is low. It is believed that due to the low Si content in the steel plate, decarburization did not proceed in the surface layer even after high dew-point annealing. Consequently, the depth at which the C concentration is 0.01% or less and the thickness of the layer at which the cementite area fraction is 10% or less are reduced. Consequently, LME resistance is poor.

[0183] No. 27 is a comparative example in which the Si content of the steel plate is high. It is believed that due to the high Si content in the steel plate, external oxidation proceeds even after high dew-point annealing, forming oxides (scale) in the surface layer of the steel plate, which inhibits decarburization at the outermost surface. Consequently, the depth at which the C concentration is 0.01% or less and the thickness of the layer at which the cementite area fraction is 10% or less are reduced. Consequently, LME resistance is poor.

[0184] No. 28 is a comparative example in which the sol. Al content of the steel plate is high. This high sol. Al content is believed to cause external oxidation to occur even after high dew-point annealing, forming oxides (scale) in the surface layer of the steel plate, thereby suppressing decarburization at the outermost surface. Consequently, the thickness of the layer with a cementite area fraction of 10% or less decreases. Consequently, LME resistance is poor.

[0185] In No. 29, the holding temperature during annealing was low, so decarburization was not sufficiently promoted during annealing. As a result, the thickness of the layer with a cementite area ratio of 10% or less was reduced, resulting in poor LME resistance.

[0186] In No. 30, the holding temperature during annealing was high, so decarburization was not sufficiently promoted during annealing. Consequently, the depth at which the carbon concentration was 0.01% or less was reduced, resulting in poor LME resistance.

[0187] In No. 31, the short holding time during annealing presumably prevented sufficient decarburization. Consequently, the depth at which the carbon concentration was 0.01% or less and the thickness of the layer at which the cementite area ratio was 10% or less were reduced. Consequently, LME resistance was poor.

[0188] Since steel No. 32 was not shot peened during the pretreatment process, it is believed that no strain was introduced into the steel plate surface, and decarburization did not occur during annealing. Consequently, the depth at which the carbon concentration was 0.01% or less and the thickness of the layer at which the cementite area ratio was 10% or less were reduced. Consequently, LME resistance was poor.

[0189] Since No. 33 used a steel sheet with a large surface roughness, the roughness of the steel sheet / plating interface after annealing also became large, which easily caused stress concentration. As a result, the LME resistance was poor.

[0190] In No. 34, it is considered that the shot peening amount was large, so decarburization proceeded excessively, and the tensile strength of the steel plate decreased.

[0191] In Steel No. 35, due to the low dew point during annealing, a phase containing oxides such as Si, Mn, and Al formed on the outer surface of the steel plate, failing to promote decarburization. Consequently, the depth at which the carbon concentration was 0.01% or less and the thickness of the layer at which the cementite area ratio was 10% or less were reduced. Consequently, LME resistance was poor.

[0192] In Steel No. 36, due to the high dew point during annealing, a phase containing oxides such as Si, Mn, and Al formed on the outer surface of the steel plate, failing to promote decarburization. Consequently, the depth at which the carbon concentration was 0.01% or less and the thickness of the layer at which the cementite area ratio was 10% or less were reduced. Consequently, LME resistance was poor.

[0193] Since steel No. 37 was surface-treated using brush grinding instead of shot peening, it is believed that insufficient strain was introduced into the steel plate surface, and decarburization was not performed during annealing. Consequently, the depth at which the carbon concentration was 0.01% or less and the thickness of the layer at which the cementite area ratio was 10% or less were reduced. As a result, LME resistance was poor.

[0194] On the other hand, Nos. 1 to 24 are examples of the present invention and have high LME resistance. Examples with a large depth of a C concentration of 0.01% or less and a cementite area ratio of 10% or less have particularly excellent LME resistance.

[0195] Industrial applicability

[0196] According to the present invention, a high-strength steel sheet and a plated steel sheet having high LME resistance can be provided, which can be suitably used in applications such as automobiles, home appliances, and building materials, particularly automobiles. Therefore, the present invention has extremely high industrial applicability.

[0197] Explanation of symbols

[0198] 11 Low carburized layer

[0199] 12 Hard tissue

[0200] 21 Steel Plate

[0201] 22 Welding Department

[0202] 23 Cracking of the shoulder and outside the shoulder

Claims

1. A steel plate, characterized in that: It is a steel plate with a tensile strength of 780 MPa or more. The chemical composition is expressed in mass %: C：0.08～0.40％、 Si: 0.4-2.0%, Mn: 0.1-5.0%, sol.Al: 0~2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B：0～0.0100％、 Ti: 0~0.1500%, Nb: 0-0.150%, V：0～0.150％、 Cr：0～2.00％、 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%, The rest is Fe and impurities. In the depth direction from the steel sheet surface, the depth at which the C concentration measured by GDS is 0.01% or less is 3 μm or more. The thickness of the layer having an area ratio of cementite of 10% or less is 5 μm or more in the depth direction from the steel plate surface. The surface roughness of the steel plate is 3.0 μm or less in terms of arithmetic mean roughness Ra.

2. The steel plate according to claim 1, wherein The total content of Si and sol.Al is less than 1.8%.

3. The steel plate according to claim 1, wherein: In the depth direction from the surface of the steel sheet, the depth at which the C concentration measured by GDS is 0.01% or less is 5 μm or more.

4. The steel plate according to claim 1, wherein In the depth direction from the surface of the steel sheet, the depth at which the C concentration measured by GDS is 0.01% or less is 7 μm or more.

5. The steel plate according to claim 1, wherein The surface roughness of the steel sheet is 2.0 μm or less in terms of arithmetic mean roughness Ra.

6. A plated steel sheet, wherein: The steel sheet according to any one of claims 1 to 5 comprises a plating layer containing Zn on at least a portion of the surface thereof.

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