Steel sheet, plated steel sheet, and automobile component

Abstract

The present invention addresses the problem of providing a steel sheet and a plated steel sheet having high LME resistance. This steel sheet has a tensile strength of 780 MPa or more, has prescribed chemical components, has a surface roughness (Ra) of 3.0 [mu] m or less, has a depth of 3 [mu] m or more at which the C concentration measured by GDS is 0.02% or less in the thickness direction from the surface of the steel sheet, and has a depth (Ra) of 3 [mu] m or more at which the C concentration measured by GDS is 0.02% or less in the thickness direction from the surface of the steel sheet. The thickness of a layer having an area ratio of ferrite of 90% or more is 3 [mu] m or more, and when I (110) is the diffraction intensity corresponding to a (110) plane, I (200) is the diffraction intensity corresponding to a (200) plane, and I (211) is the diffraction intensity corresponding to a (211) plane, 0.45 < = I (110) / (I (110) + I (200) + I (211)) < = 0.90 is satisfied, where I (110) is the diffraction intensity corresponding to the (110) plane, I (200) is the diffraction intensity corresponding to the (200) plane, and I (211) is the diffraction intensity corresponding to the (211) plane.

Description

Technical Field

[0001] This invention relates to steel sheets and coated steel sheets. More specifically, this invention relates to steel sheets and coated steel sheets with high LME resistance. Background Technology

[0002] In recent years, the use of high-strength steel sheets in various fields such as automobiles, home appliances, and building materials has been trending towards higher strength. For example, in the automotive industry, the use of high-strength steel sheets is increasing to improve fuel efficiency and reduce vehicle weight.

[0003] In the welding of zinc-plated steel sheets, especially high-strength steel sheets, weldability is sometimes reduced due to liquid metal embrittlement (LME) cracks, as described in Patent Document 1, for example.

[0004] Patent document 2 discloses a method for improving weldability of steel plates by suppressing LME cracks, using 3000 to 6000 cracks per mm on the surface of the steel plate. 2 A steel plate containing Si oxide particles with a diameter of 20 nm or larger, with an appropriate number density and particle size distribution.

[0005] Existing technical documents Patent documents Patent Document 1: International Publication No. 2019 / 116531 Patent Document 2: International Publication No. 2020 / 218575 Summary of the Invention

[0006] The problem that the invention aims to solve To suppress the formation of LME cracks, it is effective, for example, during welding, to prevent the intrusion of elements such as Zn contained in the coating into the steel sheet where the metal structure has undergone austenitic phase transformation. There is room for improvement in this regard.

[0007] In view of such actual conditions, the present invention aims to provide steel sheets and coated steel sheets with high resistance to LME.

[0008] Methods for solving problems The inventors conducted in-depth research on methods for solving the aforementioned problems. As a result, they discovered that by applying strain to the steel sheet before annealing under appropriate conditions using a projected material to form a suitable surface state, and then performing high dew point annealing, the surface layer of the steel sheet is decarburized, thereby forming a layer with a high ferrite fraction and random ferrite orientation. As a result, LME (Low Metal Emission) can be suppressed.

[0009] This invention was completed based on the above insights and further research, and its main points are as follows.

[0010] (1) A steel plate, characterized in that it is a steel plate with a tensile strength of 780 MPa or higher, and its chemical composition, by mass%, contains: C: 0.05~0.40%, Si: 0.5~3.0%, Mn: 0.1~5.0%, sol.Al: 0~3.0%, P: less than 0.0300%, S: less than 0.0300%, N: less than 0.0100%, B: 0~0.0100%, Ti: 0~0.1500%, Nb: 0~0.1500%, 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 remainder is composed of Fe and impurities, the surface roughness Ra of the steel plate is less than 3.0μm, the depth of the C concentration less than 0.02% as measured by GDS in the thickness direction from the surface of the steel plate is more than 3μm, the thickness of the layer with an area ratio of ferrite of more than 90% in the thickness direction from the surface of the steel plate is more than 3μm, in oblique incident X-ray diffraction with an incident angle of 1° relative to the surface of the steel plate, when the diffraction intensity of the (110) plane is set as I(110), the diffraction intensity of the (200) plane is set as I(200), and the diffraction intensity of the (211) plane is set as I(211), the following condition is met: 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.90.

[0011] (2) The steel plate according to (1) above is characterized in that the depth of the C concentration of 0.02% or less in the thickness direction from the surface of the steel plate is 10 μm or more.

[0012] (3) The steel plate according to (1) or (2) above is characterized in that it satisfies 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.75.

[0013] (4) The steel plate according to any one of (1) to (3) above is characterized in that the thickness of the layer in the thickness direction from the surface of the steel plate having an area ratio of ferrite of 90% or more is 8 μm or more.

[0014] (5) The steel plate according to any one of (1) to (4) above, characterized in that the surface roughness Ra of the steel plate is less than 2.0 μm.

[0015] (6) A coated steel sheet, wherein at least a portion of the surface of the steel sheet of any one of (1) to (5) above has a coating containing Zn.

[0016] (7) An automotive component comprising a steel sheet of any one of (1) to (5) above or a plated steel sheet of (6) above.

[0017] Invention Effects According to the present invention, steel sheets and coated steel sheets with high LME resistance can be obtained. Attached Figure Description

[0018] Figure 1 This is an example of the results of oblique incidence X-ray diffraction measurements showing the random orientation and non-random orientation of the ferrite phase.

[0019] Figure 2 This is a diagram illustrating the location of the crack, which is the subject of the LME resistance evaluation in the embodiment. Detailed Implementation

[0020] The present invention will now be described. The present invention is not limited to the following solutions. First, a general overview of the improved LME resistance in the present invention will be provided.

[0021] LME cracking originates when, for example, during spot welding, the metal structure of the steel plate is heated and transforms into austenite. Molten zinc, generated from the melting of the coating, penetrates into the grain boundaries of the austenite in the surface layer of the steel plate. It can be considered that the molten zinc penetrating into the austenite grain boundaries embrittles the steel plate, thereby applying tensile stress during welding. As a method to improve LME resistance, the inventors have conceived of utilizing the metal structure of the surface layer of the steel plate. Specifically, the surface layer of the steel plate is made to have a metal structure dominated by a ferrite phase with low carbon concentration and low LME susceptibility, thereby randomly orienting the ferrite phase and suppressing the formation of LME. Here, in this specification, "LME resistance" refers to the characteristic of suppressing LME cracking in the steel plate, and "LME susceptibility" refers to the characteristic of easily forming LME cracks in the steel plate.

[0022] Random orientation of the ferrite phase refers to the overall averaging of the characteristics of ferrite grain boundaries. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is random. By randomly aligning the crystal orientation of ferrite particles, the bias of grain boundaries oriented in a specific direction can be suppressed, preventing continuous or discontinuous connections. LME cracks can be considered to be caused by the concentrated intrusion of Zn from the coating into grain boundaries where the grain boundary energy is locally lower. That is, if grain boundaries with locally lower grain boundary energy exist continuously, Zn from the coating will concentrate there, easily leading to LME cracks. By averaging the characteristics of the grain boundaries overall, grain boundaries with locally lower grain boundary energy will not connect continuously, suppressing the local concentration of Zn from the coating, and thus improving LME resistance.

[0023] In this invention, the random orientation of the ferrite phase is represented by the following conditional expression. That is, it means that the random orientation of the ferrite phase in a steel plate satisfies the following conditional expression.

[0024] (conditional expression) In oblique incident X-ray diffraction with an incident angle of 1° relative to the surface of the steel plate, when the diffraction intensity corresponding to the (110) plane is set as I(110), the diffraction intensity corresponding to the (200) plane is set as I(200), and the diffraction intensity corresponding to the (211) plane is set as I(211), the following condition is satisfied: 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.90.

[0025] To achieve the aforementioned structure in the steel sheet, in this invention, strain is applied to the cold-rolled steel sheet during manufacturing, followed by annealing at a high dew point. This promotes decarburization and facilitates the formation of a ferrite phase on the steel sheet surface. Furthermore, this invention was completed based on the discovery that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification begins. The invention will now be described in detail.

[0026] [tensile strength] The steel plate of this invention is a high-strength steel plate with a tensile strength of 780 MPa or higher. This invention suppresses low-molecular-weight elongation (LME) formation in high-strength steel plates. Specifically, the steel plate of this invention has a tensile strength of 780 MPa or higher. There is no particular upper limit to the tensile strength; from the viewpoint of ensuring toughness, it can be, for example, 2000 MPa or lower. The tensile strength is measured by collecting JIS 5 tensile test specimens with the length direction perpendicular to the rolling direction and the thickness direction, according to JIS Z 2241:2011. The tensile strength can be 980 MPa or higher, or 1180 MPa or higher. When the rolling direction in the steel plate cannot be determined, JIS 5 test specimens with any direction on the surface of the steel plate as the length direction can be collected when measuring the tensile strength.

[0027] [Chemical composition] The following explains the chemical composition of the steel plate. The "%" in chemical composition refers to "mass %". Furthermore, in the numerical range of chemical composition, the range indicated by "~" includes the values ​​before and after the "~" as the lower and upper limits.

[0028] (C: 0.05~0.40%) Carbon (C) is an element that ensures the strength of steel. To obtain a tensile strength of 780 MPa or higher, which is the subject of this invention, the C content is set to 0.05% or higher. To prevent the C concentration in the surface layer described later from becoming too high, and also considering weldability, the C content is set to 0.40% or lower. The C content can be 0.08% or higher, 0.10% or higher, or 0.15% or higher. The C content can be 0.37% or lower, 0.35% or lower, or 0.30% or lower.

[0029] (Si: 0.5~3.0%) Silicon (Si) is an element that promotes ferrite stabilization and decarburization. By including Si and utilizing the pretreatment and heat treatment described later, decarburization occurs in the surface layer, and the ferrite in the surface layer is stabilized, thereby improving LME resistance. To achieve this effect, the Si content is set to 0.5% or more. When the Si content is too high, even with high dew point annealing, external oxidation will occur, forming oxides (mill scale) on the surface of the steel sheet. Conversely, decarburization on the outermost surface is inhibited, and the effect of improving LME resistance is diminished. Considering this, the Si content is set to 3.0% or less. The Si content can be 0.6% or more, 0.7% or more, or 0.8% or more. The Si content can be 2.5% or less, 2.0% or less, or 1.5% or less.

[0030] (Mn: 0.1~5.0%) Manganese (Mn) is an effective element for increasing the strength of steel by obtaining a hard structure. Considering the strength of the steel, the Mn content is set to 0.1% or more. Furthermore, considering the reduction in workability caused by Mn segregation, the Mn content is set to 5.0% or less. 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.

[0031] (sol.Al: 0~3.0%) Al (aluminum) is an element that, like Si, promotes ferrite stabilization and decarburization by being dissolved in steel. sol.Al refers to acid-soluble Al that is soluble in acids and does not form oxides such as Al₂O₃. It is determined by subtracting the insoluble residue on filter paper generated during Al analysis. In the steel sheet of this invention, the effect of sol.Al can also be achieved by containing Si; therefore, sol.Al is not essential, and the lower limit of its content is 0%. Excessive sol.Al content leads to external oxidation even with high dew point annealing, forming oxides (scale) on the surface of the steel sheet. Conversely, surface decarburization is inhibited, and the effect of improving LME resistance diminishes. Considering this, the sol.Al content is set to 3.0% or less. The sol.Al content can be 0.1% or more, 0.3% or more, or 0.5% or more. The sol.Al content can be 2.0% or less, 1.5% or less, or 1.0% or less.

[0032] As mentioned above, Si and sol.Al are elements that reduce LME resistance when added in excess. Therefore, the total content of Si and sol.Al is preferably 1.8% or less. The total content of Si and sol.Al can be 1.7% or less, 1.6% or less, or 1.5% or less.

[0033] (P: below 0.0300%) Phosphorus (P) is typically an impurity found in steel. When the P content exceeds 0.0300%, weldability may decrease. Therefore, the P content is set to be below 0.0300%. The P content can be below 0.0200%, below 0.0100%, or below 0.0050%. Preferably, it contains no P, with a lower limit of 0%. From the perspective of dephosphorization cost, the P content can be above 0%, above 0.0001%, or above 0.0005%.

[0034] (S: below 0.0300%) Sulfur (S) is typically an impurity found in steel. When the S content exceeds 0.0300%, weldability decreases, and consequently, the precipitation of MnS increases, potentially reducing workability such as flexibility. Therefore, the S content is set to be 0.0300% or less. The S content can be 0.0100% or less, 0.0050% or less, or 0.0020% or less. Preferably, it contains no S, with a lower limit of 0% for the S content. From the perspective of desulfurization cost, the S content can be greater than 0%, greater than 0.0001%, or greater than 0.0005%.

[0035] (N: below 0.0100%) Nitrogen (N) is typically an impurity found in steel. When the N content exceeds 0.0100%, weldability may decrease. Therefore, the N content is set to 0.0100% or less. The N content can also be 0.0080% or less, 0.0050% or less, or 0.0030% or less. Preferably, it contains no N, with a lower limit of 0% for the N content. From a manufacturing cost perspective, the N content can be greater than 0%, greater than 0.0005%, or greater than 0.0010%.

[0036] (O: below 0.0030%) Oxygen (O) is an element that forms oxides and reduces the workability of steel sheets. If the O content is too high, excessive oxide formation can easily reduce the workability of the steel sheet. Therefore, the O content is set to 0.0030% or less. The O content can be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. Preferably, it contains no O, and the lower limit of the O content is 0%. From a manufacturing cost perspective, the O content can be more than 0%, more than 0.0005%, or more than 0.0010%.

[0037] (B: 0~0.0100%) Boron (B) is an element that improves hardenability, thus contributing to increased strength, and also strengthens grain boundaries through segregation, thereby improving toughness. Therefore, it can be included as needed. Since it is not an essential element, the lower limit for B content is 0%. This effect can be achieved even in trace amounts, but the B content when present is 0.0001% or more. Furthermore, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.0100% or less. The B content can be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content can be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0038] (Ti: 0~0.1500%) Titanium (Ti) is an element that contributes to strength improvement by precipitating TiC during the cooling of steel, and therefore can be included as needed. Since it is not an essential element, the lower limit for Ti content is 0%. This effect can be achieved even in trace amounts, but the Ti content when present is 0.0001% or more. The Ti content can be 0.0003% or more, or 0.0005% or more. On the other hand, if it is present in excess, coarse TiN will form, potentially impairing toughness; therefore, the Ti content is set to 0.1500% or less. The Ti content can be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.

[0039] (Nb: 0~0.1500%) Niobium (Nb) is an element that contributes to increased strength by improving hardenability, and therefore can be included as needed. Since it is not an essential element, the minimum Nb content is 0%. This effect can be achieved even in trace amounts, but the Nb content when present is 0.0001% or more. The Nb content can be 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.1500% or less. The Nb content can be 0.1000% or less, 0.0600% or less, or 0.0200% or less.

[0040] (V: 0~0.150%) Vanadium (V) is an element that contributes to increased strength by improving hardenability, and therefore can be included as needed. Since it is not an essential element, the lower limit for V content is 0%. This effect can be achieved even in trace amounts, but the V content when present is 0.001% or more. The V content can be 0.003% or more, 0.005% or more, or 0.008% 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 0.100% or less, 0.060% or less, or 0.020% or less.

[0041] (Cr: 0~2.00%) Chromium (Cr) is effective in improving the hardenability and thus strength of steel, and can therefore be included as needed. Since it is not an essential element, the lower limit for Cr content is 0%. Even trace amounts can achieve this effect, but the Cr content is preferably 0.001% or more. The Cr content can be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if it is present in excess, a large amount of Cr carbides will form, which may conversely impair hardenability; therefore, the Cr content is set to 2.00% or less. The Cr content can be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0042] (Ni: 0~2.00%) Nickel (Ni) is effective in improving the hardenability and thus strength of steel, and can therefore be included as needed. Since it is not an essential element, the minimum Ni content is 0%. This effect can be achieved even in trace amounts, but the preferred Ni content is 0.001% or more. The Ni content can be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive Ni addition increases costs, so the Ni content is set to 2.00% or less. The Ni content can be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0043] (Cu: 0~2.0000%) Copper (Cu) is effective in improving the hardenability and thus strength of steel, and can therefore be included as needed. Since it is not an essential element, the lower limit for Cu content is 0%. Even trace amounts can achieve this effect, but the Cu content is preferably 0.0001% or more. The Cu content can be 0.0002% or more, or 0.0005% or more. On the other hand, from the viewpoint of suppressing reduced toughness or cracking of the slab after casting, the Cu content is set to 2.0000% or less. The Cu content can be 1.8000% or less, 1.5000% or less, 0.0050% or less, or 0.0020% or less.

[0044] (Mo: 0~1.00%) Mo (Mo) is effective in improving the hardenability of steel and thus its strength, and therefore can be included as needed. Since it is not an essential element, the lower limit for Mo content is 0%. Even trace amounts can achieve this effect, but the Mo content when present is 0.001% or more. The Mo content can be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content can be 0.80% or less, 0.60% or less, or 0.20% or less.

[0045] (W: 0~1.000%) Tungsten (W) is effective in improving the hardenability and thus strength of steel, and can therefore be included as needed. Since it is not an essential element, the lower limit for W content is 0%. This effect can be achieved even in trace amounts, but the preferred W content is 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 a decrease in toughness, the W content is set to 1.000% or less. The W content can be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.

[0046] (Ca: 0~0.1000%) Ca (calcium) is an element that helps control inclusions, particularly in the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit for Ca content is 0%. This effect can be achieved even in trace amounts, but the Ca content when present is 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 present in excess, the deterioration of surface properties may become significant; therefore, the Ca content is set to 0.1000% or less. The Ca content can be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.

[0047] (Mg: 0~0.100%) Magnesium (Mg) is an element that helps control inclusions, particularly in the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit for Mg content is 0%. This effect can be achieved even in trace amounts, but the Mg content when present is 0.0001% or more. The Mg content can be 0.0005% or more, or 0.0008% or more. On the other hand, if it is present in excess, the deterioration of surface properties may become significant; therefore, the Mg content is set to 0.100% or less. The Mg content can be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, or 0.002% or less.

[0048] (Zr: 0~0.100%) Zirconium (Zr) is an element that helps control inclusions, particularly by promoting their fine dispersion and improving toughness; therefore, it may be included as needed. Since it is not an essential element, the minimum Zr content is 0%. This effect can be achieved even in trace amounts, but the Zr content is preferably 0.001% or more. The Zr content can be 0.005% or more, or 0.010% or more. On the other hand, excessive Zr content can sometimes lead to significant deterioration of surface properties; therefore, the Zr content is set to 0.100% or less. The Zr content can be 0.050% or less, or 0.030% or less.

[0049] (Hf: 0~0.100%) Hafnium (Hf) is an element that helps control inclusions, particularly in the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit for Hf content is 0%. This effect can be achieved even in trace amounts, but the Hf content when included is preferably 0.0001% or more. The Hf content can be 0.0003% or more, or 0.0005% or more. On the other hand, excessive inclusion can sometimes lead to significant deterioration of surface properties; therefore, the Hf content is set to 0.100% or less. The Hf content can be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0050] (REM: 0~0.1000%) Rare earth elements (REMs) are elements that help control inclusions, particularly by promoting the fine dispersion of inclusions and improving toughness; therefore, they can be included as needed. Since they are not essential, the minimum REM content is 0%. Even trace amounts can achieve this effect, but the REM content is preferably 0.0001% or more. The REM content can be 0.0003% or more, or 0.0005% or more. On the other hand, excessive REM content can sometimes lead to significant deterioration of surface properties; therefore, the REM content is set to 0.1000% or less. The REM content can be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. Furthermore, REM is short for Rare Earth Metal, referring to elements belonging to the lanthanide series. REMs are typically added as a mixed rare earth metal.

[0051] In the steel plate of the present invention, the remaining portion other than the aforementioned chemical composition consists of Fe and impurities. In the steel plate of the present invention, the remaining portion may be Fe and impurities; that is, the remaining portion may consist only of Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial manufacturing of steel plates due to various reasons in the manufacturing process, represented by raw materials such as ores and scrap iron. These components are present within a range that does not adversely affect the LME resistance of the steel plate of the present invention, i.e., within which the LME resistance required by the steel plate of the present invention can be obtained.

[0052] For the analysis of the chemical composition of the steel plate, elemental analysis methods known to those skilled in the art can be used, such as inductively coupled plasma mass analysis (ICP-MS). Specifically, C and S can be determined using combustion-infrared absorption, N can be determined using inert gas melting-thermal conductivity, and O can be determined using inert gas melting-infrared absorption. These analyses can be performed using samples collected from the steel plate according to JIS G0417:1999.

[0053] Next, the surface layer of the steel plate will be explained. Here, the surface layer of the steel plate refers to a layered region having a depth extending from the surface of the steel plate (the surface of the steel plate) along the thickness direction of the steel plate to a specified distance. In the case of galvanized steel plate, the surface of the steel plate (the surface of the steel plate) refers to the surface of the steel plate (the surface of the steel plate) without the coating. It should be noted that the specified distance in the thickness direction can be set as the longer of the following: "the depth where the C concentration of the surface layer is 0.02% or less" or "the depth of the layer where the area fraction of the ferrite phase is 90% or more".

[0054] [Surface Roughness Ra] The surface roughness Ra of the steel sheet of the present invention is 3.0 μm or less. The surface roughness Ra is the arithmetic mean roughness Ra defined by JIS B0601:2013. In the case of plated steel sheet, the surface roughness Ra is set as the surface roughness of the interface between the steel sheet and the plated layer, excluding the plated layer.

[0055] In this invention, when measuring the surface roughness Ra, according to JIS B 0601:2013, 10 measurement points are randomly selected on the surface of the steel plate, with each measurement point spaced at least 1 mm apart. At each measurement point, the surface profile is measured using a laser microscope (e.g., Keyence VK-X3000). Specifically, using a laser microscope, images are taken at 20x magnification. In the captured images, a reference length of 2000 μm is set, and the arithmetic mean roughness (Ra) of each measurement point is calculated. The arithmetic mean of the 10 points of roughness (Ra) calculated at each measurement point is taken as the "surface roughness Ra".

[0056] [Depth where the surface C concentration is below 0.02%] In the steel plate of the present invention, the depth of the C concentration of less than 0.02% as measured by GDS (glow discharge spectroscopy) in the thickness direction from the surface of the steel plate is 3 μm or more.

[0057] If the carbon concentration decreases, the LME sensitivity decreases; therefore, reducing the carbon concentration in the surface layer improves LME resistance. Furthermore, carbon is an austenite stabilizing element; therefore, a lower carbon concentration in the surface layer results in a more stable layer with lower LME sensitivity.

[0058] Such a surface structure (the metallic structure of the surface portion of the steel plate) can be obtained as a decarburized layer generated by setting the chemical composition of the steel plate as described above and performing the pretreatment and annealing processes described later.

[0059] If the depth at which the C concentration is 0.02% or less is 3 μm or more, it helps to improve LME resistance; therefore, there is no particular upper limit to the depth at which the C concentration is 0.02% or less. For example, the depth at which the C concentration is 0.02% or less can be 50 μm or less, 40 μm or less, or 30 μm or less. Preferably, the depth at which the C concentration is 0.02% or less is 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more, 15 μm or more, or 20 μm or more.

[0060] GDS measurement was performed at 5 measurement points along the thickness direction of the plate. At each measurement point, the arithmetic mean of the depth of the area where the C concentration was 0.02% or less was taken as the depth of the surface layer where the C concentration was 0.02% or less. The 5 measurement points were randomly determined such that each measurement point on the steel plate surface was spaced at least 5 mm apart from the others. The measurement conditions are as described below. Of course, measurement results can be obtained even without following the conditions, for example, with the measuring device, but in the case where the measurement results differ, the steel plate of the present invention is determined based on the measurement results under the conditions described below.

[0061] Apparatus: High-frequency glow discharge emission spectrometer (manufactured by LECO Japan, model "GDS850A") Ar gas pressure: 0.3 MPa Anode diameter: 4mmφ RF output: 30W Measurement time: 200~1500 seconds [Thickness of the ferrite layer (a layer with a ferrite area ratio of over 90%)] In the steel plate of the present invention, the thickness of the layer in which the area fraction of the ferrite phase is 90% or more (hereinafter referred to as the "high ferrite layer") in the thickness direction from the surface of the steel plate is 3 μm or more.

[0062] If the thickness of the ferrite layer is 3 μm or more, it helps to improve LME resistance; therefore, there is no particular upper limit to its thickness. The thickness of the ferrite layer can be, for example, less than 100 μm, less than 80 μm, less than 60 μm, or less than 40 μm. Preferably, the thickness of the ferrite layer is 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more or 20 μm or more.

[0063] There are no restrictions on the microstructure other than ferrite in the ferrite layer. For example, it can be any one or more of martensite, bainite, and cementite.

[0064] The thickness of the high-ferrite body layer was determined by mirror finishing of the steel plate section through mechanical grinding, followed by SEM observation of the secondary electron images of the section after nitric acid-ethanol etching. A field emission scanning electron microscope (e.g., JSM 7000F, NEC Corporation, accelerating voltage: 15kV) was used for SEM observation. The observation field of view was defined as a depth of 500 μm along the thickness direction (longitudinal direction of the observation section) and a width of 600 μm along the direction orthogonal to the thickness direction (transverse direction of the observation section). Within this observation section, five observation fields were observed at intervals of at least 1000 μm in the transverse direction (orthogonal direction of the thickness direction) to obtain secondary electron images. The observation resolution was set to 1280 × 960 pixels. It should be noted that, in the case of coated steel sheets, the surface of the steel sheet (plate surface) is the surface of the steel sheet with the coating removed.

[0065] For the five secondary electron images obtained, the ferrite fraction was calculated using a point counting method. More specifically, firstly, equally spaced grids were drawn on the secondary electron images. Next, the number of ferrite-containing grid points in each grid point was counted, and this number was divided by the total number of grid points to determine the ferrite fraction. The more grid points there are, the more accurately the area fraction can be calculated. In this embodiment, the grid spacing was set to 2 μm × 2 μm, and the total number of grid points was set to 1500.

[0066] In secondary electron microscopy (SEEM) images, regions with relatively low brightness and where the underlying microstructure is not clearly identified can be identified as ferrite. Here, the underlying microstructure refers to the phase transformation structure formed within the original austenitic phase, such as laths or lath blocks. In SEEM images, ferrite has relatively low brightness and is observed as an extended region with a monotonous brightness and hue. In this invention, no special distinction is needed for metallic structures other than ferrite. The following represents the criteria for identifying tempered martensite, pearlite, ferrite, primary martensite or retained austenite, or bainite in SEEM images. Regions with underlying microstructure (lath boundaries, lath block boundaries) within the grains and where carbides precipitate in various forms are identified as tempered martensite. Additionally, regions where cementite precipitates in layers are identified as pearlite. Regions with high brightness and where the underlying microstructure is not revealed by etching are identified as primary martensite or retained austenite. Regions that do not correspond to any of the above are identified as bainite. Simply put, if we can distinguish ferrite from other structures, we can determine the area fraction of the ferrite phase.

[0067] Ferrite exhibits low LME susceptibility. From the viewpoint of improving LME resistance, a ferrite-dominated surface microstructure is preferred for the steel sheet. Such a surface microstructure can be obtained by setting the chemical composition of the steel sheet as described above and performing the pretreatment and annealing processes described later.

[0068] [Diffraction intensity ratio of ferrite phase based on oblique incidence X-ray diffraction (XRD)] In oblique incident X-ray diffraction with an incident angle of 1° relative to the surface of the steel plate of the present invention, when the diffraction intensity corresponding to the (110) plane is set as I(110), the diffraction intensity corresponding to the (200) plane is set as I(200), and the diffraction intensity corresponding to the (211) plane is set as I(211), the condition 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.90 is satisfied.

[0069] The value of the middle side "I(110) / (I(110)+I(200)+I(211))" in the conditional expression is preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. The conditional expression refers to the random orientation of the ferrite phase. In the case of completely random orientation of the ferrite phase, the value of the middle side is 0.67.

[0070] Here, oblique incidence X-ray diffraction (also known as oblique incidence XRD, low-angle incidence XRD, or tilted XRD) is a measurement method in which the incident angle of the X-rays is set to a small value, and the detector is scanned (the detection angle is changed) while maintaining this incident angle. This allows for the efficient detection of grain orientation information down to a depth of approximately a few μm in the surface layer of the sample. In this invention, the incident angle of the X-rays is fixed at 1°, and the orientation of ferrite in the surface layer of the steel plate is detected. Furthermore, the incident angle is the angle between the surface of the sample (steel plate) and the incident direction of the X-rays.

[0071] Figure 1 Examples of oblique incidence XRD analysis results for the case of randomized ferrite phase (b) and the case of non-randomized ferrite phase (a) are shown. (a) is the result of oblique incidence XRD analysis of a conventional steel sheet, which shows an orientation towards the (110) direction. Therefore, the value of the side "I(110) / (I(110)+I(200)+I(211))" in the conditional expression increases relatively to 0.91. (b) is the result of oblique incidence XRD analysis of the steel sheet of the present invention, which shows a smaller orientation towards the (110) direction compared to (a). Therefore, the value of the side "I(110) / (I(110)+I(200)+I(211))" in the conditional expression decreases relatively to 0.58.

[0072] [Coating] The steel sheet of the present invention can have a coating as described later. When a coating is present, the starting point of the thickness of the layer where the C concentration is less than 0.02% in GDS measurements and the ferrite phase area fraction is 90% or more is defined as the interface between the steel sheet and the coating. The interface between the steel sheet and the coating in the present invention is determined as follows: First, the Fe content in the thickness direction of the coated steel sheet is determined by GDS measurements. The highest value of this Fe content is taken as the Fe content of the steel sheet. The location 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 coating".

[0073] <Coated steel sheet> The coated steel sheet of the present invention is a coated steel sheet having a Zn-containing coating on the steel sheet described above. This coating may be formed on one side of the steel sheet or on both sides. Alternatively, it may be formed only on a portion of the surface. The coating may be an alloyed coating.

[0074] [Chemical composition of the coating] There are no restrictions on the chemical composition of the coating as long as it contains Zn. For example, coatings containing Zn can be 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, etc.

[0075] The chemical composition of the coating can be determined by dissolving the coating in an acidic solution containing an inhibitor that inhibits corrosion of the steel sheet, and then measuring the resulting solution using ICP (inductively coupled plasma) emission spectroscopy. For example, a 10% hydrochloric acid solution containing 0.06% by mass of the inhibitor (manufactured by Asahi Chemical Industry Co., Ltd., IBIT 710K) can be used as the acidic solution containing the inhibitor.

[0076] The coating thickness can be, for example, 3~50μm. Furthermore, there is no particular limitation on the coating amount; for example, it can be 10~170g / m² per side. 2 In this invention, the amount of coating adhesion is determined by dissolving the coating in an acid solution containing an inhibitor that suppresses steel corrosion, and by the weight change before and after acid pickling and stripping of the coating.

[0077] The surface roughness (Ra) of the interface between the steel sheet and the coating is equivalent to the surface roughness of the steel sheet described above, and the surface roughness Ra is 3.0 μm or less. Considering the adhesion of the coating, the surface roughness Ra is preferably 2.0 μm or less. The surface roughness (Ra) of the interface can be the surface roughness of the steel sheet measured after removing the coating. The coating is removed by pickling in an acid solution containing an inhibitor.

[0078] Furthermore, the steel sheet of the present invention can achieve improved resistance to LME even without a zinc coating. Normally, when spot-welding uncoated steel sheets to each other, LME cracks will not occur as long as there is no contact between the spot weld and molten zinc. However, when spot-welding a zinc-coated steel sheet and an uncoated steel sheet, molten zinc is generated at the overlapping surface of the steel sheets during welding. Therefore, contact between the molten zinc and the surface of the uncoated steel sheet may cause LME cracks.

[0079] The thickness of the steel sheet and galvanized steel sheet of the present invention is not particularly limited. For example, it can be set to 0.1 to 3.2 mm. The sheet thickness can be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The sheet thickness can be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0080] Manufacturing Methods Next, the manufacturing method of the steel sheet of the present invention will be described. The steel sheet of the present invention can be obtained, for example, by a manufacturing method comprising the following steps: 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 cold rolling step of cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a pretreatment step of pretreatment (shot peening) of the cold-rolled steel sheet; and an annealing step of annealing the pretreated cold-rolled steel sheet. Alternatively, after the hot rolling step, the hot-rolled steel sheet may be pickled and then directly cold-rolled without coiling.

[0081] <Casting Process> There are no particular restrictions on the conditions for the casting process. For example, after smelting in a blast furnace or electric furnace, various secondary smelting processes can be carried out, followed by casting using conventional continuous casting or ingot casting methods.

[0082] <Hot Rolling Process> Hot-rolled steel sheets can be obtained by hot rolling cast steel billets. The hot rolling process involves directly or temporarily cooling the cast steel billet before reheating it. When reheating is involved, the billet heating temperature can be, for example, 1100~1250℃. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate for each rolling pass can be appropriately adjusted according to the desired microstructure and plate thickness. For example, the finishing rolling end temperature can be 900~1050℃, and the finishing rolling reduction rate can be 10~50%.

[0083] <Winding Process> Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be adjusted appropriately according to the desired metal structure, for example, 500~800℃. Alternatively, the hot-rolled steel sheet can be uncoiled before or after coiling and subjected to a specified heat treatment. Alternatively, the coiling process can be omitted, and the hot-rolled steel sheet can be pickled and then subjected to the cold rolling process described later after the hot rolling process.

[0084] <Cold rolling process> After pickling, hot-rolled steel sheets can be cold-rolled to obtain cold-rolled steel sheets. The reduction rate during cold rolling can be adjusted appropriately according to the desired metal structure and sheet thickness, for example, 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0085] <Pre-treatment process> To obtain the surface microstructure of the steel plate as described above, a prescribed pretreatment is required, followed by annealing.

[0086] Pretreatment includes shot peening of the surface of the cold-rolled steel sheet using spherical abrasive materials. There are no particular limitations on the abrasive materials that can be used; for example, steel balls (shots) with a center diameter of 40-450 μm can be used. Examples of such steel balls (shots) include WINOA IKK JAPAN and TSH30. The shot velocity can be 5-400 kg / m³. 2 Therefore, strain can be introduced into the surface layer of the steel plate without increasing the surface roughness Ra. By performing this shot peening treatment, decarburization can be promoted during the annealing process described later, effectively forming a ferrite-stabilized structure on the surface of the steel plate. The greater the amount of shot peening, the greater the improvement in LME resistance; however, if a certain amount of shot peening is exceeded, for example, 400 kg / m², the resistance may decrease. 2 The effect will be saturated. Additionally, the projection amount is 400 kg / m². 2 The level is 4.0 × 10⁻⁶ per unit time per unit area. -4 kg / (mm 2 •min).

[0087] Annealing process Annealing is performed on cold-rolled steel sheets that have undergone pretreatment processes.

[0088] In this invention, an annealing process is performed, which involves holding a steel sheet strained by shot peening at a specified holding temperature and a high dew point. The heating rate up to the specified holding temperature is not particularly limited and can be 1 to 10°C / second. Dew point control is set to humidification control above 300°C, preferably 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or higher and less than 600°C, preferably within the range of 450 to 550°C. The dew point when not controlled (humidified) is typically below -30°C. To promote decarburization, the dew point (high dew point) during annealing is set to -30 to 20°C. The dew point (high dew point) during annealing is preferably -10°C or higher. Furthermore, the dew point during annealing is preferably below 5°C. To promote decarburization, the specified holding temperature (maximum heating temperature) in the annealing process is 750 to 900°C, preferably 770 to 870°C. The holding time at the holding temperature (maximum heating temperature) during the annealing process is set to 20-300 seconds, preferably 50-200 seconds. Furthermore, the atmosphere is preferably a non-oxidizing atmosphere, such as N2-1~10 vol% H2 or N2-2~4 vol% H2.

[0089] By maintaining the dew point, holding temperature, and holding time within the aforementioned ranges, decarburization can be promoted, the C concentration on the surface can be reduced, and the ferrite phase fraction can be appropriately controlled. Furthermore, by setting the dew point (humidification) control start temperature within the aforementioned range, decarburization of the surface layer of the steel plate can be promoted. Simultaneously, the internal oxidation of Si and Mn proceeds rapidly, and internal oxides are formed rapidly. These formed internal oxides function as nucleation sites, resulting in randomization of the ferrite phase orientation. If the dew point (humidification) control start temperature is too low, external oxidation occurs, and the internal oxidation of Si and Mn ceases, thus making it difficult to randomize the ferrite phase orientation.

[0090] Annealing is performed under tension of 1-20 MPa. Applying tension during annealing allows for more effective strain introduction into the steel sheet, promoting surface decarburization.

[0091] By performing the above-mentioned processes, decarburization is promoted in the surface layer of the steel plate, resulting in a steel plate with a surface layer mainly composed of randomly oriented ferrite phase.

[0092] Manufacturing Method of Coated Steel Sheets The plated steel sheet of the present invention can be obtained by performing a plating process to form a coating on the surface of a steel sheet manufactured as described above.

[0093] <Plating Process> The plating process can be performed according to methods known to those skilled in the art. For example, plating can be performed by hot-dip plating or electroplating. Hot-dip plating is preferred. The plating conditions can be appropriately set considering the desired chemical composition, thickness, and adhesion amount of the coating. After the plating process, a known alloying process can be performed as alloying plating.

[0094] The steel sheet and clad steel sheet of the present invention are high-strength and have high resistance to LME (Low Metal Electrode Lamination), making them suitable for a wide range of applications, including automobiles, home appliances, and building materials. They are particularly suitable for use in the automotive industry. Most steel sheets and clad steel sheets used in automobiles are spot-welded, which easily leads to LME cracking. Therefore, in automotive components where the steel sheet and clad steel sheet of the present invention are used as automotive steel sheets, the high LME resistance of the present invention can be appropriately utilized.

[0095] Example The present invention will now be described in more detail through examples. However, the present invention is not limited to these examples.

[0096] <Experiment No. 1> Steel with the chemical composition adjusted to that recorded in Test No. 1 of Table 1 was smelted in a blast furnace and cast into billets through continuous casting. The billets were heated to 1200°C, the finishing rolling temperature was set to 950°C, and the finishing rolling reduction rate was set to 30%, resulting in hot-rolled steel sheets. The hot-rolled steel sheets were then coiled at a coiling temperature of 650°C, pickled, and then cold-rolled with a reduction rate of 50% to obtain cold-rolled steel sheets. The thickness of the cold-rolled steel sheets was 1.6 mm.

[0097] Next, using TSH 30 manufactured by WINOA IKK JAPAN as the projection material, the surface of the obtained cold-rolled steel sheet was treated with a projection amount of 5 kg / m. 2 Shot peening was performed. The surface roughness Ra of the cold-rolled steel sheet after shot peening was 2.9 μm.

[0098] Next, for the cold-rolled steel sheet that had undergone shot peening, an annealing process was performed in a furnace with an oxygen concentration of less than 20 ppm, under a N2-4% H2 atmosphere, with the temperature increased to 500°C at a rate of 6.0°C / second, and then increased to 800°C at a rate of 2.0°C / second, and held for 40 seconds. At this point, dew point control was initiated by reducing the dew point from 300°C to 0°C. The annealing process was carried out under a tension of 5.0 MPa on the steel sheet.

[0099] Then, the annealed steel sheet is immersed in a 450℃ hot-dip galvanizing bath (Zn-0.14%Al) for 3 seconds, then lifted at 100mm / second, and the coating adhesion is controlled to 50g / m using N2 wiping gas. 2 Then, alloying treatment is carried out at 520℃ for 30 seconds to obtain alloyed hot-dip galvanized steel sheet.

[0100] <Experiments No. 2~55> Except that the chemical composition of the steel sheet is as described in Table 1 or Table 2, the conditions of the pretreatment process and the annealing process are as described in Table 3, and the type of plating is as described in Table 4, the steel sheet or plated steel sheet is produced under the same conditions as in Example 1. Furthermore, shot peening was omitted in Test No. 32, and in Test No. 35, surface treatment based on brush grinding was performed instead of shot peening. In Table 4, "a" is alloyed hot-dip galvanizing, "b" is hot-dip galvanizing with the alloying treatment in Test No. 1 omitted, "c" is using a Zn-1.5%Al-1.5%Mg bath with the alloying treatment omitted, and "non-plated" refers to cold-rolled steel sheet that has not undergone plating treatment.

[0101] (Surface roughness after pretreatment) The "surface roughness after pretreatment" recorded in Table 3 is obtained by measuring the surface roughness Ra of the steel plate according to JISB 0601:2013 after the pretreatment process and before the annealing process.

[0102] Evaluation AA: Below 2.0μm Rating A: Greater than 2.0μm and less than 3.0μm Rating B: Over 3.0μm The following evaluations were conducted on the annealed steel sheets and galvanized steel sheets.

[0103] (Surface roughness Ra) After annealing, or after both annealing and plating, the surface roughness Ra of the steel sheet without plating is measured; for the steel sheet with plating, the surface roughness Ra of the exposed steel sheet after removing the plating is measured. The plating is removed by dissolving it in a 10% hydrochloric acid solution containing 0.06% by mass of an inhibitor (Asahi Chemical Industry Co., Ltd., IBIT 710K) to inhibit corrosion of the base steel sheet.

[0104] (Steel surface structure) Samples were collected from steel plates and cut into 30mm × 30mm pieces. GDS, ferrite layer thickness, and oblique incidence XRD analysis were performed using the methods described above. The results are shown in Table 4 under the columns "C≤0.02% depth", "ferrite layer thickness", and "oblique incidence XRD (values ​​of the sides in the conditional expression)".

[0105] (tensile strength) For each steel plate, JIS No. 5 tensile test specimens were collected, and tensile tests were conducted according to JIS Z 2241:2011 to determine the tensile strength. Based on the obtained tensile strength values, the following evaluations were performed.

[0106] Rating AAA: Above 1180MPa Rating AA: Above 980MPa and below 1180MPa Rating A: Above 780MPa and below 980MPa (LME resistance) Samples were taken from each steel plate and cut into 50mm × 100mm dimensions. Then, steel plates of the same size were prepared as the target material. For these two samples, spot welding was performed using a dome-radius type welding electrode with a front diameter of 8mm, at a 5° angle, a pressure of 4.0kN, an energizing time of 1.6 seconds, and a current of 13kA to create a welded joint. The target material steel plates used were those listed in the "Target Material" column of Table 4. "Same type" indicates that a steel plate of the same type as the steel plate in this test number (Test No.) was used as the target material steel plate. Additionally, "Non-plated same type" indicates that a steel plate of the same type as the steel plate in this test number (Test No.) was used as the target material steel plate, but without plating. "GA same type" indicates that a steel plate of the same type as the steel plate in this test number (Test No.) was used with alloyed zinc plating. In addition, "GI270IF" indicates that commercially available hot-dip galvanized steel sheet with a tensile strength of 270 MPa is used as the material, and "GA590" indicates that commercially available alloyed hot-dip galvanized steel sheet with a tensile strength of 590 MPa is used as the material.

[0107] Reference Figure 2 The evaluation method for LME resistance is explained. LME resistance is evaluated by overlapping and spot-welding two steel plates 1, and assessing the length of an LME crack (crack 11) that occurs immediately outside the welded portion 2. The two steel plates 1 refer to the steel plates with the respective test numbers (Test No.) and the steel plates of the target material. The immediately outside the welded portion refers to the portion outside the welded portion 3, formed by spot welding on the overlapping surface of the two steel plates, specifically the area near the welded portion 3 (approximately 1 mm outward from the end of the welded portion 3). For crack 11 immediately outside the welded portion, the crack length is evaluated. Furthermore, the spot welding test is performed three times, and the longest crack 11 immediately outside the welded portion is evaluated. The evaluation criteria are as follows. In this embodiment, if the evaluation is A or higher (i.e., evaluations A, AA, AAA), it is considered to have excellent LME resistance.

[0108] Rating AAA: 0μm Evaluation AA: Greater than 0 μm and less than 60 μm Evaluation A: 60μm or larger and less than 120μm Rating B: Above 120μm The results of each evaluation are shown in Table 4.

[0109] <Experiments No. 5-1~5-7> Except for setting the dew point as shown in Table 5, the holding temperature to 860°C, and the material to be tested to be "the same type", tests No. 5-1 to 5-7 were conducted under the same conditions as test No. 5 shown in Tables 1, 3, and 4. The results are shown in Table 5.

[0110] <Experiments No. 3-1~3-7> Except for the dew point control start temperature and the holding temperature set to 860°C in the annealing process shown in Table 6, tests No. 3-1 to 3-7 were conducted under the same conditions as test No. 3 shown in Tables 1, 3, and 4. The results are shown in Table 6.

[0111] The steel plates tested in tests No. 1-25, 37-55, and Nos. 3-2-3-6 and 5-1-5-7 are examples of the present invention and exhibit high LME resistance. On the other hand, the steel plates tested in tests Nos. 26-36, 3-1, and 3-7, which are comparative examples whose chemical composition or manufacturing conditions deviate from the conditions specified in the present invention, did not meet the acceptable standard for LME resistance.

[0112] For Test No. 32, no shot peening was performed, therefore no strain was introduced into the surface layer of the steel plate. Consequently, it is believed that decarburization was not promoted, and the depth of C concentration below 0.02% in the GDS measurement was shallow. Furthermore, it is believed that internal oxidation of Si and Mn did not occur, and the orientation of the ferrite phase was not randomized. As a result, LME resistance was poor.

[0113] For Experiment No. 35, due to the pretreatment using brush grinding instead of shot peening, insufficient strain was introduced into the surface layer. Therefore, it is believed that internal oxidation of Si and Mn did not occur during the annealing process, and the orientation of the ferrite phase was not randomized. As a result, poor resistance to LME was observed.

[0114] For Experiment No. 36, it was believed that due to the low dew point control start temperature in the annealing process, external oxidation occurred but decarburization did not, resulting in a shallower depth for areas with C concentrations below 0.02% in the GDS measurement. Furthermore, it was believed that internal oxidation of Si and Mn did not occur, and the orientation of the ferrite phase was not randomized. Consequently, LME resistance was poor.

[0115] Industrial availability According to the present invention, high-strength steel sheets and coated steel sheets with high resistance to LME can be provided, which are suitable for use in automobiles, home appliances, building materials, and especially in automotive applications. Therefore, the present invention is an invention with extremely high industrial applicability.

[0116] Explanation of reference numerals in the attached figures 1. Steel plate 2 Welding section 3. Pressure welding section 11. Cracks immediately adjacent to the outer side of the welded section

Claims

1. A steel plate, characterized in that, It is a steel plate with a tensile strength of 780MPa or higher. Its chemical composition, expressed as a percentage by mass, contains: C：0.05~0.40%、 Si: 0.5~3.0% Mn: 0.1~5.0%, sol.Al: 0~3.0% P: Below 0.0300% S: Below 0.0300% N: below 0.0100% B：0~0.0100%、 Ti: 0~0.1500%, Nb: 0~0.1500%, 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 remaining portion consists of Fe and impurities. The surface roughness Ra of the steel plate is below 3.0 μm. In the thickness direction from the surface of the steel plate, the depth where the C concentration is below 0.02% is 3 μm or more. In the thickness direction from the surface of the steel plate, the thickness of the layer with a ferrite area ratio of 90% or more is 3 μm or more. In oblique incident X-ray diffraction with an incident angle of 1° relative to the surface of the steel plate, when the diffraction intensity corresponding to the (110) plane is set as I(110), the diffraction intensity corresponding to the (200) plane is set as I(200), and the diffraction intensity corresponding to the (211) plane is set as I(211), the following condition is satisfied: 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.

90.

2. The steel plate according to claim 1, characterized in that, In the thickness direction from the surface of the steel plate, the depth where the C concentration is less than 0.02% is 10 μm or more.

3. The steel plate according to claim 1, characterized in that, Satisfies 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.

75.

4. The steel plate according to claim 1, characterized in that, In the thickness direction from the surface of the steel plate, the thickness of the layer in which the ferrite area ratio is 90% or more is 8 μm or more.

5. The steel plate according to claim 1, characterized in that, The surface roughness Ra of the steel plate is below 2.0 μm.

6. A coated steel sheet, wherein at least a portion of the surface of the steel sheet according to any one of claims 1 to 5 has a coating containing Zn.

7. An automotive component comprising the steel sheet according to any one of claims 1 to 5.

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