Steel sheet and alloyed hot-dip galvanized steel sheet

Abstract

The present invention addresses the problem of providing a steel sheet having excellent LME resistance and an alloyed hot-dip galvanized steel sheet. This steel sheet has a tensile strength of 780 MPa or more, has a prescribed chemical composition, has a depth of 0.5 [mu] m or more from the surface of the steel sheet at which the luminous intensity Bx at a depth x ([mu] m) and the luminous intensity B150 at a depth 150 [mu] m satisfy Bx / B150 > = 5.0 in a GDS measurement in the thickness direction of the steel sheet, and has a thickness of 0.5 [mu] m or less for an oxide formed on the surface of the steel sheet. An internal oxide layer having a thickness of 1.0 [mu] m or more is present in the thickness direction of the steel sheet from the surface of the steel sheet. In addition, this alloyed hot-dip galvanized steel sheet is provided with an alloyed hot-dip galvanized layer on at least a portion of the surface of the steel sheet, and the thickness of an oxide formed on the surface of the alloyed hot-dip galvanized layer is 0.5 [mu] m or less.

Description

Technical Field

[0001] This invention relates to steel plates and alloyed hot-dip galvanized steel plates. Background Technology

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

[0003] Especially in the automotive industry, zinc-plated steel sheets are sometimes used for spot welding. Regarding the welding of zinc-plated steel sheets, particularly high-strength steel sheets, a decrease in weldability due to liquid metal embrittlement (LME) cracking can become a problem, as described in Patent Document 1, for example. LME cracking is believed to occur because the surface layer of the steel sheet transforms into austenite during welding, molten zinc infiltrates at the grain boundaries, causing embrittlement of the steel sheet, and subsequently applying tensile stress to the steel sheet during welding.

[0004] It should be noted that Patent Document 2 discloses a steel plate that improves weldability by suppressing LME cracking, wherein the surface layer of the steel plate contains Si oxide particles with a particle size of 20 nm or larger at a density of 3000 to 6000 particles / mm. 2 The number density exists with a suitable 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 prevent LME cracking, it is effective to suppress the intrusion of Zn and other substances contained in the coating into the steel sheet after the austenitic phase transformation. There is room for improvement in this regard.

[0007] Furthermore, it is known that if high-strength steel sheets are exposed to atmospheric corrosion environments with significant temperature or humidity fluctuations, hydrogen generated during the corrosion process can penetrate the steel. This hydrogen segregates at the martensite grain boundaries in the steel structure, causing grain boundary embrittlement and thus contributing to cracking. This cracking due to hydrogen intrusion is called hydrogen embrittlement (delayed fracture), and it often becomes a problem during the processing of the steel sheet. To prevent this problem, promoting the removal of hydrogen from the steel sheet to the atmosphere is effective when hydrogen has intruded into the steel.

[0008] In view of the actual situation, the present invention aims to provide steel sheets and coated steel sheets with high resistance to LME and hydrogen detachment.

[0009] Methods for solving problems Various studies have been conducted on improving the LME resistance of steel plates and alloyed hot-dip galvanized steel plates. In this invention, it has been found that it is preferable to concentrate boron (B) in the surface layer of the steel plate. Furthermore, it has been found that by concentrating B in the surface layer of the steel plate, hydrogen desorption is improved. This is believed to be because the surface diffusion rate of hydrogen atoms is faster in the B-concentrated steel plate surface layer, promoting the hydrogenation reaction. It has been found that, in order to concentrate B in the surface layer of the steel plate, internal oxidation is performed in the surface layer of the steel plate, rather than external oxidation, and the oxidation proceeds towards the interior of the steel plate during annealing; this is effective. The main points of this invention 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, wherein the chemical composition of the steel plate, by mass%, contains C: 0.05–0.40%, Si: 0.7–3.0%, Mn: 0.1–5.0%, sol.Al: 0–2.0%, P: less than 0.0300%, S: less than 0.0300%, N: less than 0.0100%, B: 0.0005–0.0050%, Ti: 0.0010–0.1000%, Nb: 0–0.2000%, V: 0–0.15%. , Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.00%, W: 0~1.00%, Ca: 0~0.1000%, Mg: 0~0.100%, Zr: 0~0.100%, Hf: 0~0.100% and REM: 0~0.100%, the remainder is Fe and impurities. In the GDS measurement in the thickness direction of the above steel plate, the luminous intensity Bx at depth x (μm) and the luminous intensity B150 at depth 150μm satisfy Bx / B150≥5.0 (1) at a depth of 0.5μm or more from the surface of the steel plate, the oxide thickness formed on the surface of the above steel plate is less than 0.5μm, and there is an internal oxide layer with a thickness of more than 1.0μm in the thickness direction of the above steel plate at a distance from the surface of the above steel plate.

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

[0012] [3] According to the steel plate of [1] above, the characteristic is that, in the GDS measurement in the thickness direction of the steel plate, the maximum value of the luminous intensity Bmax of B in the range from the surface of the steel plate to a depth of 5.0 μm satisfies Bmax / B150≥8 (2).

[0013] [4] An alloyed hot-dip galvanized steel sheet, characterized in that at least a portion of the surface of the steel sheet of any one of [1] to [3] above has an alloyed hot-dip galvanized layer, and the thickness of the oxide formed on the surface of the alloyed hot-dip galvanized layer is 0.5 μm or less.

[0014] [5] According to the alloyed hot-dip galvanized steel sheet of [4] above, the alloyed hot-dip galvanized layer contains 0 to 1.5% Al and 3 to 20% Fe by mass, and the remainder is Zn and impurities.

[0015] Invention Effects According to the present invention, it is possible to obtain steel sheets with good LME resistance and alloyed hot-dip galvanized steel sheets. Attached Figure Description

[0016] Figure 1 This is a diagram illustrating an example of the distribution of B in the surface layer of the steel plate of the present invention.

[0017] Figure 2 This is a diagram illustrating the location of cracks in the LME resistance evaluation of the embodiments. Detailed Implementation

[0018] The following describes one embodiment of the present invention. The present invention is not limited to this embodiment. First, a general overview of improving LME resistance in this embodiment will be described.

[0019] When a coated steel sheet is spot-welded, the coating melts, and the surface portion of the steel sheet is heated, causing the steel sheet microstructure to transform into austenite. At this time, the molten coating penetrates the steel sheet microstructure along the austenite grain boundaries, leading to grain boundary embrittlement. Therefore, if stress is applied to the steel sheet, LME cracking is likely to occur at the grain boundaries. It is believed that LME is particularly prone to occur during welding due to the tensile stress applied to the steel sheet. As a method to improve LME resistance, the inventors of this invention conceived of utilizing boron (hereinafter sometimes also referred to as "boron"). Specifically, it is conceived of suppressing the intrusion of molten zinc into the Fe grain boundaries by causing boron to segregate at the Fe grain boundaries (austenite or ferrite grain boundaries) in the surface microstructure, thereby suppressing the formation of LME. It should be noted that in this specification, the term "surface layer" is defined as the area from the outermost surface of the steel sheet to approximately 100 μm in the thickness direction.

[0020] Generally, in boron-containing steels, especially when heated to the austenite temperature range, deboronization, a reduction in boron content near the surface, is known. As a result, the concentration of boron in the surface layer of boron-containing steel is generally lower than that in the center of the steel sheet. In this embodiment, during annealing of the steel sheet, internal oxidation is induced in the surface layer towards the interior of the steel sheet, readily reaching the grain boundaries. This forms an internal oxide layer within the surface layer of the steel sheet, thus enabling the fixation of Si, which particularly degrades LME resistance, as oxides. Furthermore, the oxides formed by internal oxidation absorb boron, which diffuses from the steel sheet to the surface layer upon heating, thereby suppressing deboronization. Thus, unlike conventional boron-containing steels, by forming regions of boron concentration in the surface layer of the steel, the combined effect of these regions improves LME resistance.

[0021] Typically, when steel sheets are heated, such as during annealing, external oxidation occurs on the surface of the steel sheet, forming oxides (scale). This embodiment achieves the aforementioned surface structure of the steel sheet based on the understanding that by pre-stressing the surface of the steel sheet to promote oxygen diffusion into the material, and by performing annealing at a suitable dew point, internal oxidation can be achieved without external oxidation.

[0022] The following is a detailed description of this embodiment.

[0023] Steel Plate The steel plate of this embodiment will be described in detail below.

[0024] [tensile strength] The steel plate of this embodiment has a tensile strength of 780 MPa or higher. Since this embodiment aims to suppress LME formation in high-strength steel plates, the steel plate of this embodiment is of high strength. Specifically, it has a tensile strength of 780 MPa or higher. There is no particular upper limit to the tensile strength, but 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, according to JIS Z 2241:2011. The tensile strength can also be 980 MPa or higher, or 1180 MPa or higher.

[0025] [Chemical composition of steel plate] The chemical composition of the steel plate according to this embodiment will be described below. Hereinafter, "%" in the chemical composition of the steel plate refers to "mass %". Furthermore, in the numerical range of the chemical composition, the range indicated by "~" refers to the range including the values ​​listed before and after "~" as the lower and upper limits.

[0026] (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 more, as desired in this embodiment, the C content is set to 0.05% or more. Considering weldability, the C content is set to 0.40% or less. The C content can be 0.08% or more, 0.10% or more, or 0.15% or more. The C content can be 0.37% or less, 0.35% or less, or 0.30% or less.

[0027] (Si: 0.7-3.0%) Silicon (Si) is an element that suppresses deboronization through internal oxidation. It is also an element that improves the corrosion resistance of steel. This results in the formation of the boron distribution described later in the surface layer of the steel sheet. To achieve this effect, the Si content is set to 0.7% or more. If the Si content is too high, Si, which generally reduces LME resistance, will hinder the effect of the boron distribution described later, thus diminishing the improvement in LME resistance. Considering this, the Si content is set to 3.0% or less. The Si content can be 0.8% or more, 0.9% or more, or 1.0% or more. The Si content can be 2.8% or less, 2.5% or less, or 2.0% or less.

[0028] Previously, it was known that adding Si to steel would reduce LME resistance, but the inventors of this invention have found that, contrary to previous understanding, LME resistance is improved by including a large amount of Si. This is believed to be because, through the inclusion of B and the manufacturing method described later, B is concentrated in the surface layer, segregated, and present at the Fe grain boundaries.

[0029] (Mn: 0.1~5.0%) Manganese (Mn) is an effective element for improving the strength of steel by obtaining a hard microstructure. Furthermore, like silicon (Si), it is an element that suppresses deboronization through internal oxidation. To achieve these effects, the lower limit for Mn content is set at 0.1%. Additionally, considering the reduction in workability caused by Mn segregation, the Mn content is set at 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.

[0030] (sol.Al: 0~2.0%) Al (aluminum) is an element dissolved in steel to promote ferrite stabilization and decarburization, thereby improving LME resistance. Therefore, it can be included as needed. Sol.Al refers to acid-soluble Al that has not formed oxides such as Al₂O₃ and is soluble in acids. It is determined by subtracting insoluble residues on filter paper during Al analysis. The presence of sol.Al is not mandatory, and the lower limit for its content is 0. To achieve the desired effect, the sol.Al content can be 0.1% or more, 0.2% or more, or 0.3% or more. If the sol.Al content is too high, even with high dew point annealing, external oxidation will progress, forming oxides (scale) on the surface of the steel plate, reducing LME resistance. Considering this, the sol.Al content is set to 2.0% or less. The sol.Al content can be 1.5% or less, 1.2% or less, or 1.0% or less.

[0031] (P: below 0.0300%) Phosphorus (P) is a common impurity found in steel. When the P content exceeds 0.0300%, weldability may decrease. Therefore, the P content is set to be 0.0300% or less. The P content can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. Ideally, P should not be present, and the lower limit for P content is 0. From the perspective of dephosphorization cost, the P content can be greater than 0% or greater than 0.0001%.

[0032] (S: below 0.0300%) Sulfur (S) is an impurity commonly found in steel. When the S content exceeds 0.0300%, weldability may decrease, leading to increased MnS precipitation and reduced 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. Ideally, S should be absent, with a lower limit of 0%. From the perspective of desulfurization cost, the S content can be greater than 0% or greater than 0.0001%.

[0033] (N: below 0.0100%) Nitrogen (N) is an impurity commonly 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 be 0.0080% or less, 0.0050% or less, or 0.0030% or less. Ideally, N should not be present, and the lower limit for the N content is 0. From a manufacturing cost perspective, the N content can be more than 0% or more than 0.0010%.

[0034] (B: 0.0005~0.0050%) Boron (B) is an element that improves hardenability, thus contributing to increased strength, and also enhances toughness by segregating at grain boundaries to strengthen them. Therefore, in the steel sheet of this embodiment, it is concentrated in the surface layer of the steel sheet, segregated, and present at the Fe grain boundaries. To achieve this effect, the B content is set to 0.0005% or more. From the viewpoint of toughness and weldability, the B content is set to 0.0050% or less. The B content can be 0.0006% or more, 0.0008% or more, or 0.0010% or more. The B content can be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0035] Generally, in the surface layer of steel, the concentration of boron (B) is lower compared to the center of the steel plate due to deboration. In this embodiment, by means of a manufacturing method described later, internal oxidation is performed towards the interior of the steel plate during the annealing process, thereby absorbing B through oxides, suppressing deboration, and forming the concentration distribution described later in the surface layer of the steel. It is believed that the B segregated at the Fe grain boundaries suppresses LME (lower metal esterification).

[0036] (Ti: 0.0010~0.1000%) Titanium (Ti) is an element that contributes to strength improvement by precipitating TiC during the cooling of steel. To achieve this effect, the Ti content is set at 0.0010% or more. Excessive content may lead to the formation of coarse TiN, impairing toughness; therefore, the Ti content is set at 0.1000% or less. The Ti content can be 0.0020% or more, 0.0030% or more, 0.0040% or more, 0.0080% or more, 0.0110% or more, or 0.0130% or more. The Ti content can be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, or 0.0400% or less.

[0037] (Nb: 0~0.2000%) 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 lower limit for Nb content is 0. This effect can be achieved even in trace amounts, but the Nb content can be 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0010% or more. From the viewpoint of ensuring toughness, the Nb content is set to 0.2000% or less. The Nb content can be 0.1500% or less, 0.1000% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0038] (V: 0~0.15%) 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 can be 0.001% or more, 0.01% or more, 0.03% or more, 0.05% or more, or 0.06% or more. From the viewpoint of ensuring toughness, the V content is set to 0.15% or less. The V content can be 0.14% or less, 0.13% or less, 0.12% or less, 0.11% or less, or 0.10% or less.

[0039] (Cr: 0~2.00%) Chromium (Cr) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Since it is not an essential element, the lower limit for Cr content is 0. This effect can be achieved even in trace amounts, but the Cr content can be 0.001% or more, 0.01% or more, 0.05% or more, 0.07% or more, or 0.10% or more. Excessive Cr content may lead to the formation of large amounts of Cr carbides, which can impair hardenability; therefore, the Cr content is set below 2.00%. Cr content can be below 1.80%, 1.50%, 1.20%, 0.80%, 0.60%, 0.50%, 0.30%, or 0.20%.

[0040] (Ni: 0~2.00%) Nickel (Ni) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Since it is not an essential element, the lower limit for Ni content is 0. This effect can be achieved even with trace amounts, but the Ni content can be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. Excessive Ni addition increases costs, so the Ni content is set to 2.00% or less. Ni content can be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.20% or less, or 0.15% or less.

[0041] (Cu: 0~2.00%) Cu (copper) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Since it is not an essential element, the lower limit for Cu content is 0. This effect can be achieved even in trace amounts, but the Cu content can be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.07% or more. From the viewpoint of suppressing reduced toughness, cracking of the cast slab, and reduced weldability, the Cu content is set to 2.00% or less. The Cu content can be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0042] (Mo: 0~1.00%) Mo (Mo) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Since it is not an essential element, the lower limit for Mo content is 0. This effect can be achieved even in trace amounts, but the Mo content can be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. From the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content can be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0043] (W: 0~1.00%) Tungsten (W) is effective in improving the hardenability of steel, thereby increasing its strength, 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 W content can be 0.001% or more, 0.01% or more, 0.02% or more, or 0.03% or more. From the viewpoint of suppressing a decrease in toughness, the W content is set to 1.00% or less. The W content can be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, 0.15% or less, or 0.10% or less.

[0044] (Ca: 0~0.1000%) Ca (calcium) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can 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 with trace amounts, but the Ca content can be 0.0001% or more, 0.0005% or more, 0.0010% or more, 0.0020% or more, 0.0040% or more, 0.0060% or more, or 0.0070% or more. Excessive Ca content may lead to significant deterioration of surface properties; therefore, the Ca content is set to 0.1000% or less. Ca content can be 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[0045] (Mg: 0~0.100%) Magnesium (Mg) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can 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 with trace amounts, but the Mg content can be 0.0001% or more, 0.0005% or more, or 0.001% or more. Excessive Mg content may lead to significant deterioration of surface properties; therefore, the Mg content can be 0.100% or less, 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

[0046] (Zr: 0~0.100%) Zirconium (Zr) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Since it is not an essential element, the lower limit for Zr content is 0. This effect can be achieved even with trace amounts, but the Zr content can be 0.001% or more, 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. Excessive Zr content may lead to significant deterioration of surface properties; therefore, the Zr content is set to 0.100% or less. Zr content can be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0047] (Hf: 0~0.10%) Hafnium (Hf) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can 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 with trace amounts, but the Hf content can be 0.001% or more, 0.002% or more, or 0.005% or more. Excessive Hf content may lead to significant deterioration of surface properties; therefore, the Hf content can be 0.10% or less, 0.08% or less, 0.06% or less, 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less.

[0048] (REM: 0~0.100%) Rare earth elements (REMs) are elements that help control inclusions, especially the fine dispersion of inclusions, and improve toughness; therefore, they can be included as needed. Since they are not essential elements, the lower limit for REM content is 0. This effect can be achieved even in trace amounts, but the REM content can be above 0.001%, 0.002%, 0.003%, or 0.005%. Excessive REM content may significantly degrade surface properties; therefore, the REM content is set below 0.100%. REM content can be below 0.090%, 0.080%, 0.060%, 0.050%, 0.040%, 0.030%, 0.020%, 0.015%, or 0.010%. It should be noted that REM is an abbreviation for Rare Earth Metal, referring to elements belonging to the lanthanide series. REM is typically added in the form of mixed rare earth alloys.

[0049] (The rest of the text) In the steel sheet of this embodiment, the remaining components other than the aforementioned chemical composition are Fe and impurities. Here, impurities refer to components introduced during the industrial manufacturing of steel sheets due to various factors in the manufacturing process, such as raw materials like ores and waste. These components are those that do not adversely affect the LME resistance and hydrogen detachment resistance of the steel sheet of this embodiment, and are present within the range required for the LME resistance and hydrogen detachment resistance of the steel sheet of this embodiment. Specific elements include, for example, O (oxygen). The content of O as an impurity can be, for example, 0.0500% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less. However, from the viewpoint of manufacturing cost, the O content can be 0.00001% or more, 0.00005% or more, or 0.0001% or more.

[0050] (Analytical methods for chemical composition) 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, it is preferable to use combustion-infrared absorption for C and S, and inert gas melting-thermal conductivity for N. For O, inert gas melting-infrared absorption is used. These analyses can be performed on samples collected from the steel plate according to the method in JIS G0417:1999.

[0051] [Surface B distribution] In the steel plate of this embodiment, in the GDS (high-frequency glow discharge luminescence analysis) measurement in the thickness direction of the steel plate, the depth satisfying the following formula (1) is 0.5 μm or more from the surface of the steel plate.

[0052] Bx / B150≥5.0 (1) In equation (1) above, Bx represents the luminous intensity at a point at a depth x (μm) from the interface between the steel plate and the coating in the thickness direction of the steel plate. Furthermore, B150 represents the luminous intensity at a depth of 150 μm from the interface between the steel plate and the coating in the thickness direction of the steel plate. It should be noted that in this embodiment, "thickness direction" refers to the direction perpendicular to the interface between the steel plate and the coating. The point at the interface between the steel plate and the coating in the thickness direction of the steel plate refers to the point at a distance from the center of the steel plate thickness.

[0053] The left side of equation (1) above represents the ratio of the B concentration at depth x to the B concentration at depth 150 μm. That is, Bx / B150≥5.0 means that the B concentration at depth x is more than 5.0 times that at depth 150 μm. The B concentration at depth 150 μm can be regarded as the B concentration at the center of the steel plate thickness, and Bx / B150≥5.0 means that B concentration occurs at depth x. The depth of 0.5 μm or more from the surface of the steel plate satisfies equation (1) means that the meaning of equation (1) is satisfied within the range from the surface of the steel plate to a depth of 0.5 μm or more in the thickness direction of the steel plate, which means that B concentration occurs within the range up to a depth of 0.5 μm or more in the thickness direction of the steel plate.

[0054] In this embodiment, the surface of the steel sheet is defined as the location where the Fe content, as determined by GDS measurement, is 5% of the Fe content at a depth of 150 μm. It should be noted that when an alloyed hot-dip galvanized layer (described later) is formed on the surface of the steel sheet, the interface between the steel sheet and the alloyed hot-dip galvanized layer is considered the surface of the steel sheet and serves as the starting point for the depth measurement of GDS. The interface between the steel sheet and the alloyed hot-dip galvanized layer is defined as follows: First, the Fe content in the thickness direction of the coated steel sheet is measured by GDS measurement. The highest value of this Fe content is taken as the Fe content of the steel sheet. The location where the Fe content reaches 93% of the Fe content of the steel sheet is defined as the "interface between the steel sheet and the alloyed hot-dip galvanized layer".

[0055] By concentrating boron (B) in the surface layer of the steel sheet as described above, B segregates at the Fe grain boundaries in the surface microstructure. This suppresses the intrusion of molten zinc into the Fe grain boundaries during spot welding, thus inhibiting the formation of low-electromagnetic metal (LME). Furthermore, it improves the hydrogen desorption properties of the steel sheet.

[0056] (Methods for determining GDS) For Bx and B150, the following method is used: Using GDS, the surface of the steel plate to be analyzed is set to an Ar atmosphere. While sputtering the steel plate surface to generate glow plasma under applied voltage, analysis is performed along the depth direction. Then, the elements contained in the material are identified by the characteristic emission spectrum wavelengths of the elements emitted by atoms excited in the glow plasma, and the emission intensity of the identified elements is estimated.

[0057] Data in the depth direction can be estimated from the sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, the sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from sputtering time can be defined as the depth from the material surface. The sputtering time is set in such a way that the sputtering depth is at least 150 μm.

[0058] GDS measurements were performed five times along the thickness of the plate, and the average value was set as the B concentration. The measurement conditions were set as follows. The B concentrations at depths of x (μm) and 150 μm are Bx and B150, respectively.

[0059] Apparatus: High-frequency glow discharge luminescence analyzer (manufactured by LECO JAPAN, model "GDS850A") Ar gas pressure: 0.3 MPa Anode diameter: 4mmφ RF output power: 30W Measurement time: 200–1500 seconds To effectively suppress LME formation, the depth satisfying Bx / B150 ≥ 5.0 is set to 0.5 μm or more. From the viewpoint of LME resistance, a larger Bx / B150 is preferred, which can be 1.0 μm or more, 1.2 μm or more, 1.4 μm or more, 1.6 μm or more, 1.8 μm or more, or 2.0 μm or more. Since Bx / B150 is the ratio of B concentration at depth x to B concentration at depth 150 μm, the depth satisfying Bx / B150 ≥ 5.0 is less than 150 μm. Even at a deeper depth satisfying Bx / B150 ≥ 5.0, LME resistance will not decrease, but the depth satisfying Bx / B150 ≥ 5.0 can also be less than 100.0 μm, 50.0 μm or less, 30.0 μm or less, 20.0 μm or less, or 10.0 μm or less.

[0060] It is believed that in the steel sheet of this embodiment, the segregation of B, which is concentrated in the surface layer of the steel, at the Fe grain boundaries suppresses the intrusion of Zn, thereby suppressing LME. Such a surface B distribution can be obtained by manufacturing a steel sheet from molten steel having the above-described chemical composition using the manufacturing method described later.

[0061] Figure 1 An example of the distribution of B in the surface layer of the steel plate according to this embodiment, as determined by GDS, is shown. (Refer to...) Figure 1 It was confirmed that the B concentration increased sharply from a position approximately 2 μm from the surface to the surface (at a depth of 0 μm).

[0062] [Oxide Thickness] In the steel sheet of this embodiment, the oxide thickness formed on the surface of the steel sheet is 0.5 μm or less. In the case of the alloyed hot-dip galvanized layer described later, the oxide thickness formed on the surface of the alloyed hot-dip galvanized layer is also 0.5 μm or less. If a considerably thick oxide layer exceeding 0.5 μm is formed on the surface of the coating, the diffusion rate of hydrogen in the oxide decreases, thus reducing hydrogen removal efficiency. To ensure that the oxide thickness on the surface of the coating is set to 0.5 μm or less, heat treatment using an atmospheric furnace or similar equipment should be avoided after the manufacture of the steel sheet or alloyed hot-dip galvanized steel sheet.

[0063] The oxide thickness on the coating surface was measured using cross-sectional SEM observation. Specifically, areas containing more than 20% O in the EDS were defined as oxides, and their thickness was measured. The measurement field of view was set to 80 μm horizontally × 50 μm vertically, and the oxide thickness of the thickest portion within the field of view was measured. The same measurement was performed on five fields of view, and the average of the maximum oxide thicknesses in each field of view was taken as the surface oxide thickness. A thinner oxide thickness on the coating surface is preferred, and can be less than 0.4 μm, 0.3 μm, 0.2 μm, or 0.1 μm.

[0064] [Internal oxide layer thickness] In the coated steel sheet of this embodiment, an internal oxide layer with a thickness of 1.0 μm or more exists in the thickness direction of the steel sheet, away from the surface of the steel sheet. When the alloyed hot-dip galvanized layer described later is present, the internal oxide layer with a thickness of 1.0 μm or more exists in the thickness direction of the steel sheet, away from the interface between the steel sheet and the alloyed hot-dip galvanized layer. By precipitating Si and Mn as internal oxides, the concentration of Si and Mn dissolved in the surface layer of the steel sheet is reduced. In particular, Si is an element that promotes LME and reduces LME resistance; therefore, if the concentration of dissolved Si in the surface layer of the steel sheet is reduced through internal oxidation, the LME resistance is improved. The thickness of the internal oxide layer can be 2.0 μm or more, 3.0 μm or more, 4.0 μm or more, 5.0 μm or more, 6.0 μm or more, or 7.0 μm or more. The thickness of the internal oxide layer can be 100.0 μm or less, 50.0 μm or less, 40.0 μm or less, 30.0 μm or less, or 25.0 μm or less.

[0065] The thickness of the internal oxide layer was determined by cross-sectional SEM observation. Specifically, in the steel plate surface layer, and in the case of an alloyed hot-dip galvanized layer (described later), the internal oxide layer was defined as the precipitate containing more than 20% O in the EDS. The thickness of the oxide layer was measured. The measurement field of view was set to 80 μm horizontally × 50 μm vertically, and the thickness of the thickest internal oxide layer in the field of view was measured. The same measurement was performed for five fields of view, and the average of the maximum internal oxide layer thickness in each field of view was taken as the internal oxide layer thickness.

[0066] <B concentration up to a depth of 5.0 μm> In the steel plate of this embodiment, in the above-mentioned GDS measurement, it is preferable to satisfy the following formula (2).

[0067] Bmax / B150≥8 (2) In the above formula (2), Bmax represents the maximum value of the luminous intensity of B in the range from the surface of the steel plate to a depth of 5.0 μm.

[0068] The left side of equation (2) represents the ratio of the maximum luminescence intensity of B from the surface of the steel plate to a depth of 5.0 μm to the B concentration at a depth of 150 μm. That is, Bmax / B150≥8 means that the B concentration at the location where B is most concentrated from the surface of the steel plate to a depth of 5.0 μm is more than 8 times that at a depth of 150 μm. The B concentration at a depth of 150 μm can be regarded as the B concentration at the center of the steel plate thickness. Bmax / B150≥8 means that the concentration of B is large from the surface of the steel plate to a depth of 5.0 μm, that is, near the surface.

[0069] From the perspective of LME resistance, a larger Bmax / B150 is preferred, preferably 10 or more, and more preferably 12 or more, 14 or more, or 16 or more.

[0070] Satisfying equation (2) is not necessary in the steel plate of this embodiment. Even if equation (2) is not satisfied, good resistance to LME can be obtained as long as equation (1) is satisfied. By setting the surface B distribution to satisfy equation (2), the B that is more significantly concentrated in the surface of the steel segregates at the Fe grain boundaries, which can suppress the intrusion of molten zinc, and thus the effect of suppressing LME can be obtained more significantly.

[0071] Alloyed hot-dip galvanized steel sheet In this embodiment, an alloyed hot-dip galvanized layer is formed on the surface of the steel sheet, resulting in an alloyed hot-dip galvanized steel sheet. The alloyed hot-dip galvanized layer can be formed on one side or both sides of the steel sheet. Alternatively, it can be formed in a portion of one side. The alloyed hot-dip galvanized steel sheet of this embodiment is primarily used in the automotive industry. The alloyed hot-dip galvanized layer is not particularly limited as long as it contains Zn. Elements other than Zn can include, for example, Fe, Al, Mg, Si, Ni, Sn, and other elements commonly found in Zn coatings. Furthermore, elements present in the steel sheet can be diffused into the coating. The Zn content can be set to 50% or more, or 55% or more, or 60% or more.

[0072] [Chemical composition of alloyed hot-dip galvanized coating] Hereinafter, an example of the chemical composition of the preferred coating of the alloyed hot-dip galvanized steel sheet of this embodiment will be described. Unless otherwise specified, the "%" for element content refers to "mass %". In the numerical range of the chemical composition of the coating, the range indicated by "~" unless otherwise specified refers to the range including the values ​​listed before and after "~" as the lower and upper limits.

[0073] (Al: 0-1.5%) Al is an element that improves the corrosion resistance of the coating by being included together with Zn or alloyed, and therefore can be included as needed. The Al content can also be 0%. To form a coating containing Zn and Al, the Al content is preferably set to 0.01% or more. The Al content can also be 0.1% or more. When the Al in the coating is in the range of 0.3% to 1.5%, the rate at which Zn invades the steel grain boundaries is significantly reduced due to the effect of Al, thus improving LME resistance. Therefore, from the viewpoint of improving LME resistance, the Al in the coating is preferably 0.3% to 1.5%. The Al content can also be 0.4% or more, 0.5% or more, or 0.6% or more. The Al content can also be 1.4% or more, 1.3% or more, or 1.2% or more.

[0074] (Fe: 3-20%) When a Zn-containing coating is formed on a steel sheet and the coated steel sheet is then heat-treated and alloyed, Fe is incorporated into the coating by diffusion from the steel sheet. The Fe content is preferably set to 3% or more. The Fe content can be 4% or more, 5% or more. Furthermore, the Fe content is preferably 20% or less. The Fe content can be 15% or less, 12% or less, 10% or less, or 8% or less.

[0075] In one example of the chemical composition of the preferred coating of the alloyed hot-dip galvanized steel sheet of this embodiment, the remainder other than the aforementioned components consists of Zn and impurities. Impurities in the coating refer to components introduced during the manufacturing process, such as the raw materials, and are not intentionally added to the coating. For example, elements other than Zn, Fe, and Al contained in the steel sheet diffuse into the coating and are thus included, and are considered impurities. In one example of the chemical composition of the preferred coating of the coated steel sheet of this embodiment, elements other than the basic components described above and optional additives may be included in trace amounts, without impairing the effects of the present invention.

[0076] The chemical composition of the coating can be determined by dissolving the coating in an acid solution containing an inhibitor that inhibits steel corrosion, and then measuring the resulting solution by ICP (inductively coupled plasma) luminescence spectrophotometry.

[0077] [Coating thickness] The thickness of the coating can be, for example, 3–50 μm. Furthermore, the coating amount is not particularly limited, but can be, for example, 10–170 g / m² per single side. 2In this embodiment, the amount of coating adhered is determined by the weight change of the coating before and after pickling and peeling, after dissolving the coating in an acid solution containing an inhibitor that inhibits steel corrosion. For example, a 10% hydrochloric acid solution containing 0.06% by mass of the inhibitor (Asahi Chemical Industry Co., Ltd., IBIT710K) can be used as the acid solution containing the inhibitor. The base steel sheet after removing the coating is washed with water and dried.

[0078] The coating thickness can be 5μm or more, 7μm or more, or 10μm or more. The coating thickness can also be less than 45μm, 40μm or less, 35μm or less, or 30μm or less. The coating adhesion can be 15g / m² per single side. 2 Above, 20g / m 2 Above, 25g / m 2 Above, 30g / m 2 The coating adhesion can be 160g / m² per single side. 2 Below, 140g / m 2 Below, 120g / m 2 Below, 100g / m 2 the following.

[0079] It should be noted that the steel sheet of the present invention exhibits improved resistance to LME even when it lacks an alloyed hot-dip galvanized coating. No LME cracking occurs when ungalvanized steel sheets are welded together. However, even when welding one galvanized steel sheet to another ungalvanized steel sheet, molten zinc coating may form on the overlapping surface of the steel sheets during welding. Therefore, it is possible for the molten zinc coating to come into contact with the surface of the ungalvanized steel sheet, causing LME cracking. Furthermore, when welding an ungalvanized steel sheet using a welding electrode that has been used to spot-weld a galvanized steel sheet, the zinc coating adhering to the welding electrode may melt and come into contact with the steel sheet surface, causing LME cracking. If the steel sheet of the present invention is used as an ungalvanized steel sheet, even in the aforementioned situations, the intrusion of molten zinc into the Fe grain boundaries can be suppressed by the concentration of boron in the surface layer, thus suppressing LME cracking.

[0080] [Plate thickness] The thickness of the steel plate and alloyed hot-dip galvanized steel plate in this embodiment is not particularly limited. For example, it can be set to 0.6 to 3.2 mm. The plate thickness can be 0.8 mm or more or 1.0 mm or more. The plate thickness can be 3.0 mm or less, 2.8 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, or 2.0 mm or less.

[0081] Manufacturing Methods of Steel Plates Next, the manufacturing method of the steel plate according to this embodiment will be described.

[0082] The steel sheet of this embodiment can be obtained by a manufacturing method comprising the following steps: a casting process in which molten steel with adjusted chemical composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled; a pickling process in which the coiled hot-rolled steel sheet is pickled; a cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and an annealing process in which the cold-rolled steel sheet is annealed. Alternatively, the hot-rolled steel sheet may not be coiled after the hot rolling process, but may be pickled and then directly cold-rolled.

[0083] [Casting Process] There are no particular restrictions on the conditions for the casting process. For example, as long as the smelting is carried out in a blast furnace, electric furnace, etc., followed by various secondary refining processes, and then casting is carried out by conventional continuous casting, casting using ingot casting, or other methods, it is acceptable.

[0084] [Hot rolling process] Hot-rolled steel sheets can be obtained by hot rolling steel billets obtained through casting. The hot rolling process involves directly hot rolling the cast steel billet or by temporarily cooling and then reheating it. In the case of reheating, the billet heating temperature can be, for example, 1100–1250°C. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate of each rolling pass can be appropriately varied according to the desired microstructure and plate thickness. For example, the finishing rolling end temperature can be 900–1050°C, and the finishing rolling reduction rate can be 10–50%.

[0085] [Winding process] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be appropriately varied according to the desired metal structure, for example, 500–800°C. Uncoiling can be performed before or after coiling, and the hot-rolled steel sheet can be subjected to a specified heat treatment. Alternatively, coiling can be omitted, and the hot-rolled steel sheet can be pickled and then cold-rolled as described later after the hot-rolling process.

[0086] [Pickling process] The hot-rolled steel sheet is pickled. In the steel sheet manufacturing method of this embodiment, the surface roughness of the pickled steel sheet is controlled to concentrate boron (B) in the surface layer of the steel sheet during the subsequent annealing process. Specifically, the roughness is set to 1.5 μm or more based on the arithmetic mean height (Ra) as defined by JIS B0601:2013. This condition means that a certain degree of roughness exists on the surface of the steel sheet. If the roughness is small, the strain imparted to the surface layer of the steel sheet is small, and therefore, even with annealing at a high dew point as described later, the concentration of B in the surface layer of the steel sheet does not occur.

[0087] The larger the Ra value of the unevenness, the better, preferably 2.0 μm or more, and more preferably 2.5 μm or more, 3.0 μm or more, or 3.5 μm or more.

[0088] The surface roughness of the pickled steel sheet is determined according to JIS B 0601:2013. Ten random locations are selected on the surface of the pickled portion, and the surface profile at each location is measured using a contact surface roughness tester. The arithmetic mean roughness Ra is obtained by averaging the surface roughness of these locations. It is not necessary to measure the surface roughness of the pickled steel sheet frequently; once the pickling conditions for achieving the desired roughness are clear, the measurement can be omitted.

[0089] The surface roughness of the steel plate varies depending on the pickling conditions, so it can be adjusted appropriately to achieve the aforementioned unevenness. For example, pickling can be performed using a 1-10% by mass hydrochloric acid solution at a temperature of 20-95°C for a pickling time of 30 seconds or more but less than 200 seconds.

[0090] The surface of the steel sheet after pickling has such unevenness, and the unevenness is rolled in the subsequent cold rolling process to introduce strain into the surface of the steel, thereby promoting the concentration of boron on the surface of the steel sheet in the subsequent annealing process.

[0091] [Cold rolling process] After pickling, hot-rolled steel sheets can be cold-rolled to obtain cold-rolled steel sheets. In the cold rolling process, the unevenness and roughness imparted in the pickling process are rolled to break down the steel sheet, thereby inducing strain on the surface. Therefore, the rolls used for cold rolling preferably have low surface roughness, preferably 1.0 μm or less in Ra. The surface roughness of the rolls in Ra can be 0.8 μm or less, 0.6 μm or less, or 0.5 μm or less. The reduction rate of cold rolling can be appropriately varied according to the desired metal structure and sheet thickness, for example, from 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0092] By rolling the unevenness of the hot-rolled steel sheet surface during the cold rolling process, strain is imparted to the surface layer of the steel sheet, thereby promoting the concentration of boron (B) on the surface layer of the steel sheet during the subsequent annealing process.

[0093] [Annealing process] After the cold rolling process, the obtained cold-rolled steel sheet is subjected to the following high dew point annealing. In the manufacturing method of steel sheet and alloyed hot-dip galvanized steel sheet of this embodiment, in the annealing process, external oxidation is not performed on the surface of the steel sheet, but internal oxidation is performed on the surface layer of the steel sheet, oxidizing towards the interior of the steel sheet.

[0094] In the annealing process of this embodiment, the steel sheet to which strain has been applied to the surface through the above-described process is held at a high dew point. Specifically, in the annealing process of the steel sheet and alloyed hot-dip galvanized steel sheet manufacturing method of this embodiment, the dew point of the atmosphere is changed during the first and second halves of the heating process up to the holding temperature. Specifically, the dew point from room temperature to the control temperature and the dew point from the control temperature to the holding time are set to different values. Here, the "control temperature" is set to the temperature at which the dew point is changed. The heating rate up to the holding temperature is not particularly limited. The heating rate can be, for example, 1 to 10 °C / second. If the heating rate is less than 1 °C / second, it takes too long to reach the control temperature, and the oxide layer on the coating surface may thicken. On the other hand, if the heating rate exceeds 10 °C / second, internal oxidation may not be sufficient, the strain applied to the steel sheet surface may not be fully released, and boron concentration may be insufficient. From these viewpoints, the heating rate can be 2 °C / second or more, 3 °C / second or more, or 4 °C / second or more. The heating rate can be below 9℃ / second, below 8℃ / second, or below 7℃ / second.

[0095] The control temperature is set to 450–550°C. During the period from room temperature to the control temperature, the dew point of the annealing atmosphere is set to -40°C to -20°C. During the period from the control temperature to the holding temperature, the dew point of the annealing atmosphere is set to be above -20°C but below 20°C.

[0096] If the controlled temperature is below 450°C, the dew point rises at low temperatures, thus internal oxidation occurs at low temperatures, releasing the strain imposed on the steel plate surface and preventing the accumulation of boron during the period from the controlled temperature to the holding temperature. If the controlled temperature exceeds 550°C, the strain imposed on the steel plate surface before internal oxidation occurs at high temperatures is released, preventing the accumulation of boron.

[0097] If the dew point is below -40°C during the period from room temperature to the control temperature, Si and Mn may undergo external oxidation, and internal oxidation may not occur during the period from the control temperature to the holding temperature. If the dew point is above -20°C during the period from room temperature to the control temperature, internal oxidation occurs at low temperature, and the strain imposed on the surface of the steel plate is released. As a result, it may become less likely to promote the concentration of B during the period from the control temperature to the holding temperature.

[0098] Similarly, if the dew point is below -20°C during the period from the controlled temperature to the holding temperature, the internal oxidation used to suppress deboronization may become insufficient. If the dew point exceeds 20°C during the period from the controlled temperature to the holding temperature, external oxidation may occur, and the internal oxidation used to suppress deboronization may become insufficient.

[0099] Furthermore, the dew point during the period from the controlled temperature to the maintained temperature is more than 10°C higher than the dew point during the period from room temperature to the controlled temperature. This allows for internal oxidation, promoting the concentration of boron (B).

[0100] From the viewpoint of suitable B concentration, the control temperature can be above 460°C, above 470°C, or above 480°C. The control temperature can be below 540°C, below 530°C, or below 520°C. From the viewpoint of suitable internal oxidation and B concentration, the dew point during the period from room temperature to the control temperature can be above -38°C, above -37°C, or above -35°C. The dew point during the period from room temperature to the control temperature can be below 18°C, below 17°C, or below 15°C. The dew point from the control temperature to the holding temperature can be above -18°C, above -17°C, or above -15°C. The dew point from the control temperature to the holding temperature can be below 18°C, below 17°C, or below 15°C.

[0101] To facilitate internal oxidation and promote B concentration, the holding temperature is set to 760–900°C. Furthermore, the holding time at this temperature is set to 0–360 seconds. Holding temperatures can be above 770°C, 780°C, or 790°C. Holding temperatures can be below 890°C, 880°C, or 870°C. Holding times can be 10 seconds or more, 30 seconds or more, 50 seconds or more, or 60 seconds or more. Holding times can be below 330 seconds, 300 seconds or less, 270 seconds or less, 240 seconds or less, or 200 seconds or less.

[0102] The atmosphere used in annealing is preferably a non-oxidizing atmosphere, such as N2-1 to 10 vol% H2 or N2-2 to 4 vol% H2. The oxygen concentration of the atmosphere is preferably below 50 ppm, and can be below 30 ppm, 20 ppm, or 10 ppm. By setting these conditions, oxidation of the coating surface can be suppressed, and internal oxidation can be carried out.

[0103] By applying strain to the surface of the steel plate using the method described above, and raising the dew point above the controlled temperature during the annealing process, the internal oxidation of the steel plate surface proceeds rapidly. B is absorbed by the oxides formed inside the steel plate, thereby concentrating B in the surface of the steel plate. B segregates at the Fe grain boundaries, resulting in the B concentration distribution of the surface layer as described above.

[0104] Annealing is carried out under a tension of 1–20 MPa. Applying tension during annealing allows for a more significant introduction of strain into the steel sheet, promoting the concentration of boron on the surface.

[0105] Manufacturing Method of Alloyed Hot-Dip Galvanized Steel Sheet Using the steel sheet of the present invention manufactured as described above, alloyed hot-dip galvanized steel sheet can be manufactured by a manufacturing method that includes a plating process and an alloying process.

[0106] [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, electroplating, vapor deposition, spraying, or cold spraying. Hot-dip plating is preferred.

[0107] [Alloying process] After the plating process, a known alloying treatment is performed to produce an alloyed coating.

[0108] The conditions for the plating and alloying processes can be appropriately set by considering the desired chemical composition, thickness, and adhesion amount of the coating.

[0109] The steel sheet and alloyed hot-dip galvanized steel sheet of this embodiment are suitable for use in a wide range of fields, including automobiles, home appliances, and building materials, due to their high strength, high resistance to LME, and hydrogen detachment. They are particularly preferred for use in the automotive industry. Most coated steel sheets used in automobiles are spot-welded, in which case LME cracking can become a significant problem. Therefore, when using the steel sheet and alloyed hot-dip galvanized steel sheet of this embodiment as automotive steel sheets, the high LME resistance of this embodiment can be appropriately utilized.

[0110] Furthermore, the steel sheet and alloyed hot-dip galvanized steel sheet of this embodiment also have excellent hydrogen removal properties due to the formation of a B-concentrated layer in the surface, and are therefore suitable for the automotive field.

[0111] Example The present invention will now be described in more detail through embodiments. However, the present invention is not limited to these embodiments.

[0112] <Example No. 1> Molten steel is smelted in a blast furnace and cast by continuous casting to obtain a steel billet with the chemical composition described in Table 1, No. 1. The obtained steel billet is heated to 1200°C, and hot rolling is performed with the finishing rolling end temperature set at 950°C and the finishing rolling reduction rate set at 30% to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet is coiled at a coiling temperature of 650°C.

[0113] The coiled steel sheet is pickled for 40 seconds using a 5% by mass hydrochloric acid solution at 40°C. After pickling, it is cold-rolled with a reduction rate of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel sheet is set to 1.6 mm.

[0114] Subsequently, steel plate samples were prepared by annealing in a furnace with an oxygen concentration of less than 20 ppm and a holding temperature of 800°C and a holding time of 0 seconds, under a N2-4 vol% H2 gas atmosphere. The heating rate during annealing was set to 5.0°C / second. Regarding the dew point of the annealing atmosphere, the control temperature was set to 500°C, the period from room temperature to the control temperature was set to -20°C, and the period from the control temperature to the holding temperature was set to -10°C. Furthermore, the annealing process was performed under a tension of 15 MPa. It should be noted that a holding time of 0 seconds means that cooling began immediately after the temperature was raised to 800°C.

[0115] <Examples No. 2-22, Comparative Examples No. 23-33> The chemical composition was set as described in Table 1, and the annealing conditions were set as described in Table 2. Otherwise, the steel sheet was manufactured under the same conditions as in Example 1. It should be noted that, in the example where the presence or absence of the alloyed hot-dip galvanized layer in Table 2 is marked "yes," the annealed steel sheet was immersed in a 450°C hot-dip galvanizing bath (Zn-0.2%Al) for 3 seconds, then removed at a speed of 100 mm / s. The coating adhesion was controlled to 50 g / m² using N2 wiping gas. 2 Then, alloying treatment is carried out at 520 degrees Celsius for 30 seconds to obtain alloyed hot-dip galvanized steel sheet with alloyed hot-dip galvanized layers on both sides. In the case of No. 33, after the formation of the alloyed hot-dip galvanized layer, a heat treatment of 800 degrees Celsius for 10 seconds is carried out in an atmospheric furnace.

[0116] (The unevenness after pickling) During the manufacturing of steel sheets and alloyed hot-dip galvanized steel sheets, the surface roughness of the pickled hot-rolled steel sheets is measured. The surface roughness is determined according to JIS B 0601:2013, by randomly selecting 10 locations on the surface of the outermost layer and measuring the surface profile at each location using a contact surface roughness tester. The arithmetic mean roughness Ra is obtained by averaging the surface roughness of these locations. The surface roughness of the hot-rolled steel sheets is shown in Table 2.

[0117] (B distribution on the steel surface) The B distribution on the surface of the steel plate is evaluated as follows.

[0118] GDS was performed using samples cut from various steel plates and alloyed hot-dip galvanized steel plates into 50mm × 50mm sizes. The GDS measurement was performed five times along the thickness direction, and the average value was taken as the B concentration. The measurement conditions were set as follows. The B concentrations at depths of x (μm) and 150 μm are Bx and B150, respectively.

[0119] Apparatus: High-frequency glow discharge luminescence analyzer (manufactured by LECO JAPAN, model "GDS850A") Ar gas pressure: 0.3 MPa Anode diameter: 4mmφ RF output power: 30W Measurement time: 200–1500 seconds The depth that satisfies equation (1): Bx / B150≥5.0 and the left side of equation (2): Bmax / B150 are obtained from the calculated Bx.

[0120] (Surface oxide thickness) By observing the cross-sectional area using SEM, regions containing more than 20% oxygen (O) in the EDS (electrode slurry) were defined as oxides. The thickness of the oxides on the surface of the steel plate or the alloyed hot-dip galvanized layer was measured. The measurement field of view was set to 80 μm horizontally and 50 μm vertically, and the oxide thickness of the thickest part in the field of view was measured. The same measurement was performed on five fields of view, and the average of the maximum oxide thickness in each field of view was taken as the surface oxide thickness, as shown in Table 3. In Table 3, "<0.5" indicates that the oxide thickness is thinner than 0.5 μm.

[0121] (Thickness of internal oxide layer) By observing the cross-sectional SEM, precipitates containing more than 20% O in EDS within the steel plate surface layer or directly beneath the alloyed hot-dip galvanized layer were defined as internal oxides, and the thickness of the oxide layer was measured. The measurement field of view was set to 80 μm horizontally × 50 μm vertically, and the thickness of the thickest internal oxide layer in the field of view was measured. The same measurement was performed on five fields of view, and the average of the maximum internal oxide layer thickness in each field of view was taken as the internal oxide layer thickness, as shown in Table 3.

[0122] (tensile strength) For all steel plates and alloyed hot-dip galvanized steel plates, JIS No. 5 tensile test specimens were collected with the length direction perpendicular to the rolling direction as the test length. Tensile tests were performed according to JIS Z 2241:2011 to determine the tensile strength, and the results were evaluated as follows. In this embodiment, if the evaluation is A or above, it is judged to have excellent LME resistance.

[0123] AAA rating: Above 1180MPa Rating AA: Above 980MPa and below 1180MPa Rating A: Above 780MPa and below 980MPa Rating B: Below 780 MPa (LME resistance) Two samples, each cut from a steel plate and an alloyed hot-dip galvanized steel plate, were collected and sized to 50mm × 100mm. For these two samples, a dome-shaped welding electrode with a front diameter of 8mm was used to perform spot welding at an angle of 5°, a pressure of 5.0kN, an energizing time of 1.2 seconds, and an energizing current of 12kA to create a welded joint.

[0124] Reference Figure 2 The evaluation of LME resistance is explained below. LME resistance is evaluated by the length of LME cracks (shoulder cracks 11) that occur at the shoulder of the welded portion 2 formed by overlapping and spot welding two steel plates 1. The shoulder refers to the sloping portion of the recessed edge caused by spot welding. The evaluation is set as follows based on the length of the shoulder cracks 11. In this embodiment, if the evaluation is A or above, it is judged to have excellent LME resistance.

[0125] Rating AAA: 0μm Evaluation AA: Greater than 0 μm and less than 50 μm Rating A: Above 50μm and below 160μm Rating B: Above 160μm (Hydrogen removal test) Test pieces measuring 80mm × 50mm were cut from various steel plates and alloyed hot-dip galvanized steel plates, and hydrogen was charged electrochemically. The hydrogen was controlled by a constant current (cathode current density 1 mA / cm²) in a 3% NaCl + 3g / L NH₄SCN aqueous solution. 2 The steel plates were energized for 48 hours. After hydrogen charging, the steel plates and alloyed hot-dip galvanized steel plates were left to stand in an atmospheric atmosphere at room temperature for 48 hours. After the specified time, the amount of diffusible hydrogen in the steel plates and alloyed hot-dip galvanized steel plates was measured using the temperature rise removal method, and evaluated as follows. It should be noted that in the temperature rise removal method, the temperature was raised to 400°C at a rate of 100°C / h, and the total amount of hydrogen released from room temperature to 200°C was taken as the diffusible hydrogen content.

[0126] AAA rating: Less than 5% of the initial hydrogen content Evaluation AA: Less than 15% of the initial hydrogen content Evaluation A: Less than 40% of the initial hydrogen content. Rating B: Initial hydrogen content above 40% The results of each evaluation are shown in Table 3.

[0127] Examples No. 1 to 22 are invention examples, which have been confirmed to have excellent LME resistance and hydrogen desorption properties.

[0128] The low Si content in No. 23 prevents sufficient internal oxidation during the annealing process, thus failing to suppress boron desorption. Consequently, the internal oxide layer is thin, and boron does not concentrate on the surface of the steel plate, failing to meet the requirement of Bx / B150 ≥ 5.0. As a result, it exhibits poor resistance to LME and hydrogen desorption.

[0129] No. 24 has a high Si content. Therefore, although B is concentrated in the surface layer, the reduction in LME resistance caused by Si is greater. As a result, both LME resistance and hydrogen desorption are poor.

[0130] In the annealing process of No. 25, the dew point is low during the first half of the heating phase, allowing Si and Mn to undergo external oxidation. However, internal oxidation is impossible during the second half of the heating phase. Consequently, the internal oxide layer becomes thinner, and boron (B) does not concentrate in the surface layer of the steel plate, failing to meet the requirement of Bx / B150 ≥ 5.0. As a result, both LME resistance and hydrogen desorption resistance are poor.

[0131] In the annealing process, No. 26 has a high dew point during the first half of the heating phase. Internal oxidation occurs at a low temperature, releasing the strain imposed on the steel surface. However, during the second half of the heating phase, there is no promotion of boron (B) concentration. Therefore, B does not concentrate in the surface layer of the steel, failing to satisfy Bx / B150 ≥ 5.0. As a result, it exhibits poor resistance to LME and hydrogen detachment.

[0132] In the annealing process, No. 27 has a low dew point during the latter half of the heating phase, resulting in insufficient internal oxidation to suppress deboronization. Consequently, the internal oxide layer is thinner, and boron does not concentrate in the surface layer of the steel plate, failing to meet the requirement of Bx / B150 ≥ 5.0. As a result, its resistance to LME and hydrogen desorption are both poor.

[0133] In the annealing process, the dew point of No. 28 is high during the latter half of the heating phase, resulting in external oxidation. Therefore, sufficient internal oxidation to suppress boron desorption occurs. Consequently, the internal oxide layer is thin, and boron (B) does not concentrate in the surface layer of the steel plate, failing to meet the requirement of Bx / B150 ≥ 5.0. As a result, both LME resistance and hydrogen desorption resistance are poor.

[0134] No. 29 Due to the low controlled temperature during the annealing process, the dew point rises at low temperatures, resulting in internal oxidation at low temperatures. This releases the strain imposed on the steel plate surface, and in the latter half of the heating process, there is no promotion of boron (B) concentration. Therefore, B does not concentrate in the surface layer of the steel plate, failing to satisfy Bx / B150 ≥ 5.0. Consequently, both LME resistance and hydrogen detachment properties are poor.

[0135] The high temperature control during the annealing process of No. 30 releases the strain applied to the steel surface before internal oxidation at high temperatures, failing to promote boron concentration. Therefore, boron does not concentrate in the surface of the steel, failing to satisfy Bx / B150 ≥ 5.0. Consequently, it exhibits poor resistance to LME and hydrogen detachment.

[0136] The holding temperature during the annealing process of No. 31 was low, and no internal oxidation was performed. Therefore, the internal oxide layer was thin, and boron (B) did not concentrate in the surface layer of the steel plate, failing to meet the requirement of Bx / B150 ≥ 5.0. As a result, its resistance to LME and hydrogen detachment are both poor.

[0137] No. 32 has a small surface roughness after pickling. Therefore, the surface layer is not adequately strained, and even with high dew point annealing, boron does not concentrate in the surface layer of the steel plate, failing to satisfy Bx / B150≥5.0. As a result, although hydrogen desorption is excellent, LME resistance is poor.

[0138] After the formation of the alloyed hot-dip galvanized layer, No. 33 underwent a heat treatment at 800°C for 10 seconds in an atmospheric furnace. As a result, a thick oxide layer was formed on the surface of the alloyed hot-dip galvanized layer, exhibiting poor hydrogen removal properties.

[0139] Industrial availability According to the present invention, steel sheets and alloyed hot-dip galvanized steel sheets with high resistance to LME and hydrogen detachment can be provided, which are suitable for use in automobiles, home appliances, building materials, etc., especially in automobiles. Therefore, the present invention is an invention with extremely high industrial applicability.

[0140] Explanation of symbols 1. Steel plate 2 Welding section 11. Cracks in the shoulder area

Claims

1. A steel plate, characterized in that, It is a steel plate with a tensile strength of 780MPa or higher. The chemical composition of the steel plate, expressed as a percentage by mass, contains: C：0.05～0.40%、 Si: 0.7–3.0% Mn: 0.1–5.0%, sol.Al: 0~2.0%, P: Below 0.0300% S: Below 0.0300% N: below 0.0100% B：0.0005～0.0050%、 Ti: 0.0010~0.1000% Nb: 0~0.2000%, V：0～0.15%、 Cr：0～2.00%、 Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.00%, W：0～1.00%、 Ca: 0~0.1000% Mg: 0~0.100%, Zr：0～0.100%、 Hf: 0~0.100%, and REM: 0~0.100%, The remainder consists of Fe and impurities. In the GDS measurement along the thickness direction of the steel plate, the luminous intensity Bx at depth x and the luminous intensity B150 at depth 150 μm satisfy the following equation (1) at a depth of 0.5 μm or more from the surface of the steel plate, where the unit of depth x is μm. Bx / B150≥5.0 (1) The thickness of the oxide formed on the surface of the steel plate is less than 0.5 μm. An internal oxide layer with a thickness of more than 1.0 μm exists on the surface of the steel plate in the thickness direction of the steel plate.

2. The steel plate according to claim 1, characterized in that, The depth at which the luminous intensity Bx and the luminous intensity B150 satisfy Bx / B150≥5.0 is greater than 2.0μm from the surface of the steel plate.

3. The steel plate according to claim 1, characterized in that, In GDS measurements along the thickness direction of the steel plate, the maximum value Bmax of the luminous intensity of B in the range from the surface of the steel plate to a depth of 5.0 μm satisfies: Bmax / B150≥8 (2).

4. An alloyed hot-dip galvanized steel sheet, characterized in that, The steel plate according to any one of claims 1 to 3 has an alloyed hot-dip galvanized layer in at least a portion of its surface, wherein the thickness of the oxide formed on the surface of the alloyed hot-dip galvanized layer is less than 0.5 μm.

5. The alloyed hot-dip galvanized steel sheet according to claim 4, characterized in that, The alloyed hot-dip galvanized layer contains 0-1.5% Al and 3-20% Fe by mass, with the remainder being Zn and impurities.

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