Welded joints

Abstract

The present invention addresses the problem of obtaining a welded joint having good LME resistance in production.　A welded joint according to the present invention comprises: a plurality of steel sheets which are stacked; a nugget for joining the plurality of steel sheets together; a spot welded part which includes a pressure welded part and a heat-affected part formed around the nugget; a non-heat-affected part which is a region on the outer side of the heat-affected part; and a separation part which is formed around the pressure welded part. Said welded joint is characterized in that: among the plurality of steel sheets, at least one steel sheet which is disposed on the outermost side is a high-strength steel sheet having a Vickers hardness of not less than 240 Hv at the sheet thickness center; the high-strength steel sheet has a prescribed chemical component; at a position apart from the edge of the pressure welded part by 50 μm to the outer side, a highly ferritic layer having an area ratio of the ferrite phase of not less than 90% exists with a thickness of not less than 5 μm from the surface in the thickness direction; and a B-concentrated part having a B strength which is obtained by TOF-SIMS measurement and is two or more times as large as that at a depth position of 50 μm exists with a thickness of not less than 1.0 μm from the surface.

Description

[Technical Field]

[0001] The present invention relates to welded joints. [Background technology]

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

[0003] In particular, in the automotive field, welded joints are often used in which zinc-based plated steel sheets are spot-welded. When welding zinc-based 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 thought to occur when the surface layer of the steel sheet transforms to austenite during welding, and molten zinc penetrates the grain boundaries, embrittling the steel sheet, and when tensile stress is applied to the steel sheet during welding.

[0004] In addition, Patent Document 2 describes a steel sheet that suppresses LME cracking and improves weldability, and describes a steel sheet having a surface layer containing Si oxide particles with a particle size of 20 nm or more at a rate of 3000 to 6000 particles / mm 2 With a population density of , discloses a steel sheet in which the grains are present in a suitable grain size distribution. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 116531 [Patent Document 2] International Publication No. 2020 / 218575 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a welded joint that has good LME resistance during manufacturing. [Means for solving the problem]

[0007] Various studies have been conducted to improve LME resistance during the production of welded joints. In response to this, the present invention has discovered that it is preferable to concentrate B in the surface layer of the steel sheet that constitutes the welded joint. It has been discovered that in order to concentrate B in the surface layer of the steel sheet, it is effective to prevent external oxidation from progressing on the surface of the steel sheet during annealing, and to promote internal oxidation in which oxidation progresses toward the inside of the steel sheet in the surface layer of the steel sheet. The gist of the present invention is as follows.

[0008] [1] A welded joint comprising a plurality of overlapping steel plates, a nugget joining the plurality of steel plates, a spot weld having a pressure-welded portion and a heat-affected portion formed around the nugget, a non-heat-affected portion which is an area outside the heat-affected portion, and a separation portion formed around the pressure-welded portion, wherein at least one steel plate arranged on the outermost side of the plurality of steel plates has a Vickers hardness at the center of the plate thickness. a high-strength steel plate having a hardness of 240Hv or more, and a chemical composition of the high-strength steel plate, in mass%, of C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol.Al: 0 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0.0005 to 0.0050%, Ti: 0.0010 to 0.1000%, Nb: 0 to 0.2000%, V : 0-0.15%, 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%, with the balance being Fe and impurities, and ferrite is present at a position 50 μm outward from the end of the pressure-welded portion. a high-ferrite layer having an area ratio of 90% or more of the ferrite phase is present in a thickness of 5 μm or more from the surface of the high-strength steel plate in the thickness direction of the high-strength steel plate, and a B-enriched portion having a B intensity at least twice the B intensity at a depth of 50 μm determined by TOF-SIMS measurement is present in a thickness of 1.0 μm or more from the surface of the high-strength steel plate at a position 50 μm outward from the end of the pressure weld.

[0009] [2] The welded joint according to [1], wherein the high-strength steel plate has a Zn-containing plating layer formed on one or both sides of the high-strength steel plate.

[0010] [3] The welded joint according to [1] or [2], characterized in that in the non-heat-affected zone, the depth at which the emission intensity Bx at a depth x (μm) in a GDS measurement from the steel sheet surface in the thickness direction and the emission intensity B150 at a depth of 150 μm satisfy Bx / B150≧5...(1) is 1.5 μm or more.

[0011] [4] The welded joint according to [3], characterized in that in the non-heat-affected zone, in GDS measurement from the steel plate surface in the thickness direction, the maximum B emission intensity Bmax within a depth of 5 μm or less and the emission intensity B150 at a depth of 150 μm satisfy Bmax / B150≧8...(2).

[0012] [5] The welded joint according to any one of [1] to [4], wherein the high ferrite layer has a thickness of 10 μm or more. [Effects of the Invention]

[0013] According to the present invention, a welded joint having good LME resistance during manufacturing can be obtained. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an example of a photograph of the structure of a high ferrite layer in a surface layer portion of a steel plate that constitutes a welded joint of the present invention. [Figure 2] 1 is an example of the results of TOF-SIMS measurement of a B-enriched portion in a surface layer portion of a steel plate that constitutes a welded joint of the present invention. [Figure 3] FIG. 1 is a diagram illustrating the positions of cracks targeted in the LME resistance evaluation in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, one embodiment of the present invention will be described. The present invention is not limited to the following embodiment. First, an outline of the present embodiment for improving LME resistance during production of a welded joint will be described.

[0016] When plated steel sheets are spot welded, the coating melts and the surface layer of the steel sheet is heated, transforming the steel sheet structure to austenite. During this process, the hot-dip coating penetrates into the steel sheet structure along the austenite grain boundaries, embrittling the grain boundaries. Therefore, when stress is applied to the steel sheet, LME cracking is likely to occur at the grain boundaries. LME is particularly likely to occur because tensile stress is applied to the steel sheet during welding. The inventors came up with the idea of ​​utilizing B (hereinafter sometimes referred to as "boron" or "boron") as a method for improving LME resistance. Specifically, they came up with the idea of ​​suppressing the penetration of molten zinc into the Fe grain boundaries (austenite grain boundaries or ferrite grain boundaries) in the surface layer structure, thereby suppressing the occurrence of LME.

[0017] Generally, in boron-containing steel, a deboronization phenomenon is known in which the amount of boron near the surface decreases, especially when heated to the austenite temperature range. As a result, in boron-containing steel, the boron concentration in the surface layer of the steel is generally lower than that in the center of the steel sheet. In this embodiment, when the steel sheet is annealed, internal oxidation, which tends to progress to grain boundaries in the surface layer of the steel sheet, is induced toward the interior of the steel sheet. This forms an internal oxidation layer in the surface layer of the steel sheet, making it possible to fix Si, which particularly deteriorates LME resistance, as an oxide in the internal oxidation layer. Furthermore, the oxide formed by internal oxidation captures boron that diffuses from the steel sheet to the surface layer upon heating, thereby suppressing the deboronization phenomenon. In this way, contrary to conventional boron-containing steel, by forming a boron-enriched region in the surface layer of the steel, the combined effects of these factors improve LME resistance during weld joint production.

[0018] Normally, when a steel sheet is heated, such as during annealing, external oxidation occurs, in which oxides (scale) are formed on the surface of the steel sheet. This embodiment is based on the finding that imparting strain to the surface layer of the steel sheet in advance promotes oxygen diffusion into the material, and that annealing at an appropriate dew point can promote internal oxidation without promoting external oxidation.

[0019] This embodiment will be described in detail below.

[0020] Welded joints The welded joint of this embodiment comprises a plurality of overlapping steel plates, a nugget joining the plurality of steel plates, a spot weld having a pressure-welded portion and a heat-affected portion formed around the nugget, a non-heat-affected portion which is the region outside the heat-affected portion, and a separation portion formed around the pressure-welded portion. Here, the "heat-affected portion" refers to a portion of the steel plate that has not melted and whose structure, metallurgical properties, mechanical properties, etc. have been changed by welding heat, and the "non-heat-affected portion" refers to a portion other than the heat-affected portion.

[0021] [High strength steel plate] Among the plurality of steel plates, at least one steel plate arranged on the outermost side is a high-strength steel plate. The present invention aims to suppress LME cracking that occurs when high-strength steel plates are spot-welded. LME cracking occurs in high-strength steel plates. The welded joint of this embodiment is effective in suppressing LME cracking during the manufacture of a welded joint, and is particularly effective in suppressing cracking in the shoulder of a weld. Here, a high-strength steel plate refers to a steel plate having a Vickers hardness of 240 Hv or more at a position that is the non-heat-affected zone of a welded joint and at a half-depth position. Furthermore, the "shoulder" refers to the inclined portion of the edge of a depression formed by an electrode during spot welding.

[0022] The hardness of the steel plate is measured at a position in the non-heat-affected zone of the steel plate that constitutes the weld joint, at a depth of 1 / 2. The hardness measurement is performed in accordance with JIS Z 2244:2009. The measurement load is 200 gf. The Vickers hardness at the center of the plate thickness of the steel plate that constitutes the weld joint may be 260 Hv or more, 280 Hv or more, or 300 Hv or more.

[0023] In the welded joint of this embodiment, as long as at least one of the steel plates constituting the welded joint, the outermost one, is the high-strength steel plate described above, the other steel plates may be ordinary commercially available steel plates. Of course, all of the steel plates may be the high-strength steel plates described above. When manufacturing a welded joint, at least one of the surfaces that comes into contact with a welding electrode during spot welding is a high-strength steel plate described below.

[0024] [Chemical composition of high-strength steel plate] The chemical composition of the high-strength steel plate will be described below. Hereinafter, "%" regarding the chemical composition of the high-strength steel plate means "mass %." Furthermore, in the numerical range of the chemical composition, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0025] (C: 0.05 to 0.40%) C (carbon) is an element that ensures the strength of steel. To obtain a hardness of 240 Hv or more, which is the target of this embodiment, the C content is set to 0.05% or more. In consideration of weldability, the C content is set to 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0026] (Si: 0.7 to 3.0%) Silicon (Si) is an element that promotes decarburization and ferrite stabilization during the annealing process in steel sheet production. It also prevents boron removal through internal oxidation. It also improves the corrosion resistance of steel. This results in the formation of a high-ferrite layer and a B-enriched region (described below) on the surface of the steel sheet. To achieve this effect, the Si content is set to 0.7% or more. Since Si generally reduces LME resistance during the production of welded joints, too much Si inhibits the effect of the B distribution (described below) and reduces the effect of improving LME resistance during the production of welded joints. Taking this into consideration, the Si content is set to 3.0% or less. The Si content may be 0.8% or more, 0.9% or more, or 1.0% or more. The Si content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0027] It has been known that adding Si to steel reduces LME resistance during the production of welded joints, but as a result of investigations by the present inventors, it has been found that, contrary to conventional knowledge, adding a large amount of Si improves LME resistance during the production of welded joints. This is thought to be because the inclusion of B and the production method described below result in a surface layer structure centered on ferrite, and B is concentrated in the surface layer and segregates at the Fe grain boundaries.

[0028] (Mn: 0.1 to 5.0%) Manganese (Mn) is an element that effectively improves the strength of steel by providing a hard structure. Similarly to Si, it also inhibits the deboronization phenomenon through internal oxidation. To achieve these effects, the lower limit of the Mn content is set to 0.1%. Furthermore, taking into consideration the deterioration of workability due to Mn segregation, the Mn content is set to 5.0% or less. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

[0029] (sol.Al:0-2.0%) Aluminum (Al) is an element that dissolves in steel and promotes ferrite stabilization and decarburization, thereby improving LME resistance during the production of welded joints. Therefore, it may be added as needed. Sol. Al refers to acid-soluble Al that is soluble in acid and does not form oxides such as Al2O3. It is calculated as the Al content measured after subtracting the insoluble residue on the filter paper generated during the Al analysis process. The inclusion of sol. Al is not essential, and the lower limit of the sol. Al content is 0.01%, 0.2%, or 0.3%. To obtain the desired effect, the sol. Al content may be 0.1%, 0.2%, or 0.3% or more. If the sol. Al content is too high, external oxidation will progress, even with high-dew-point annealing, resulting in the formation of oxides (scale) on the surface of the steel sheet, reducing LME resistance during the production of welded joints. Taking this into consideration, the sol. Al content is limited to 2.0% or less. The sol. Al content may be 1.5%, 1.2%, or 1.0% or less.

[0030] (P:0.0300% or less) P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, weldability may be reduced. Therefore, the P content is set to 0.0300% or less. The P content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that no P is contained, and the lower limit of the P content is 0. From the viewpoint of dephosphorization costs, the P content may be more than 0% or 0.0001% or more.

[0031] (S:0.0300% or less) S (sulfur) is an impurity generally contained in steel. If the S content exceeds 0.0300%, weldability will decrease, and further, the amount of MnS precipitation will increase, which may decrease workability such as bendability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that no S is contained, and the lower limit of the S content is 0. From the viewpoint of desulfurization costs, the S content may be more than 0% or 0.0001% or more.

[0032] (N:0.0100% or less) N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may be reduced. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N is not contained, and the lower limit of the N content is 0. From the viewpoint of manufacturing costs, the N content may be more than 0% or 0.0010% or more.

[0033] (B: 0.0005 to 0.0050%) B (boron) is an element that improves hardenability and contributes to improving strength, and also segregates at grain boundaries to strengthen the grain boundaries and improve toughness. Furthermore, in the steel sheet of this embodiment, B is concentrated in the surface layer of the steel sheet and segregates 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 may be 0.0006% or more, 0.0008% or more, or 0.0010% or more. The B content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

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

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

[0036] (Nb: 0 to 0.2000%) Niobium (Nb) is an element that contributes to improving strength by improving hardenability, so it may be added as needed. Since it is not an essential element, the lower limit of the Nb content is 0. This effect can be achieved even with trace amounts, but when added, the Nb content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0010% or more. To ensure toughness, the Nb content is set to 0.2000% or less. The Nb content may be 0.1500% or less, 0.1000% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0037] (V:0~0.15%) V (vanadium) is an element that contributes to improving strength by improving hardenability, so it may be added as needed. Since it is not an essential element, the lower limit of the V content is 0. This effect can be obtained even with a small amount of vanadium. However, when vanadium is added, the V content may be 0.001% or more, 0.002% or more, 0.003% or more, 0.005% or more, 0.006% or more, 0.008% or more, or 0.01% or more. From the viewpoint of ensuring toughness, the V content is set to 0.15% or less. The V content may be 0.14% or less, 0.13% or less, 0.12% or less, 0.10% or less, 0.08% or less, 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less.

[0038] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability and strength of steel, so it may be added as needed. Since Cr is not an essential element, the lower limit of the Cr content is 0. This effect can be achieved even with trace amounts, but if added, the Cr content may be 0.001% or more, 0.01% or more, 0.05% or more, 0.07% or more, or 0.10% or more. Excessive Cr content can result in the formation of large amounts of Cr carbides, which may adversely affect hardenability. Therefore, the Cr content is set to 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0039] (Ni: 0 to 2.00%) Ni (nickel) is effective in improving the hardenability and strength of steel, so it may be added as needed. Since it is not an essential element, the lower limit of the Ni content is 0. This effect can be achieved even with trace amounts, but if Ni is added, the Ni content may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. Because excessive Ni addition increases costs, the Ni content is set to 2.00% or less. The Ni content may be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.20% or less, or 0.15% or less.

[0040] (Cu: 0-2.00%) Copper (Cu) is effective in improving the hardenability and strength of steel, and may be added as needed. Since it is not an essential element, the lower limit of the Cu content is 0. This effect can be achieved even with trace amounts, but when added, the Cu content may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.07% or more. To prevent a decrease in toughness, cracking of the slab after casting, or a decrease in weldability, the Cu content is set to 2.00% or less. The Cu content may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0041] (Mo: 0-1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing its strength, so it may be added as needed. Since it is not an essential element, the lower limit of the Mo content is 0. This effect can be achieved even with trace amounts, but when added, the Mo content may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. To prevent a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0042] (W:0~1.00%) W (tungsten) is effective in improving the hardenability of steel and increasing its strength, so it may be added as needed. Since it is not an essential element, the lower limit of the W content is 0. This effect can be achieved even with trace amounts, but when W is added, the W content may be 0.001% or more, 0.01% or more, 0.02% or more, or 0.03% or more. To prevent a decrease in toughness, the W content is set to 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, 0.15% or less, or 0.10% or less.

[0043] (Ca: 0 to 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be added as needed. Since it is not an essential element, the lower limit of the Ca content is 0. Although this effect can be obtained even with trace amounts, the Ca content, if added, may be 0.0001% or more, 0.0005% or more, 0.0010% or more, 0.0020% or more, 0.0040% or more, 0.0060% or more, or 0.0070% or more. Since excessive Ca content may cause noticeable deterioration of surface properties, the Ca content is set to 0.1000% or less. The Ca content may be 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[0044] (Mg: 0 to 0.100%) Magnesium (Mg) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and enhances toughness, and may be added as needed. Since it is not an essential element, the lower limit of the Mg content is 0. While this effect can be achieved even with trace amounts, the Mg content, if added, may be 0.0001% or more, 0.0005% or more, 0.001% or more, 0.002% or more, 0.004% or more, 0.008% or more, 0.010% or more, or 0.015% or more. Since excessive Mg content can significantly deteriorate surface properties, the Mg content is set to 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0045] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be added as needed. Since it is not an essential element, the lower limit of the Zr content is 0. Although this effect can be obtained even with trace amounts, the Zr content, if added, may be 0.001% or more, 0.03% or more, 0.005% or more, 0.08% or more, or 0.010% or more. Since excessive Zr content may cause noticeable deterioration in surface quality, the Zr content is set to 0.100% or less. The Zr content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0046] (Hf: 0 to 0.10%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0. Although this effect can be obtained even with a small amount of Hf, when contained, the Hf content may be 0.001% or more, 0.002% or more, or 0.005% or more. Since excessive Hf content may cause noticeable deterioration of surface properties, the Hf content may be 0.10% or less, 0.08% or less, 0.06% or less, 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less.

[0047] (REM: 0 to 0.100%) REM (rare earth elements) contribute to inclusion control, particularly to the fine dispersion of inclusions, and thus enhance toughness. Therefore, they may be added as needed. Since they are not essential elements, the lower limit of REM content is 0. This effect can be achieved even with trace amounts, but if added, the REM content may be 0.0001% or more, 0.005% or more, or 0.001% or more. Excessive REM content can significantly deteriorate surface properties, so the REM content should be 0.100% or less. The REM content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, or 0.005% or less. REM stands for Rare Earth Metal and refers to elements belonging to the lanthanide series. REM is usually added as mischmetal.

[0048] (Remainder) The remainder of the steel plate constituting the welded joint according to this embodiment, other than the above chemical components, is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial production of steel plate due to various factors in the manufacturing process, including raw materials such as ores and scrap, and that do not adversely affect the LME resistance of the welded joint according to this embodiment during production, i.e., are contained in a range that achieves the LME resistance required for the steel plate according to this embodiment. Specific elements include, for example, O (oxygen). The content of O contained as an impurity may be, for example, 0.0500% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less. However, from the viewpoint of manufacturing costs, the content of O may be 0.00001% or more, 0.00005% or more, or 0.0001% or more.

[0049] (Method of analyzing chemical components) The chemical composition of steel sheets can be analyzed by any elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. O can be measured using the inert gas fusion-infrared absorption method. These analyses are carried out by measuring the chemical composition of steel sheets. This can be done using samples taken from the plate in accordance with JIS G0417:1999.

[0050] [High ferrite layer] In the high-strength steel plate constituting the welded joint of this embodiment, a high-ferrite layer is present 50 μm outward from the end of the pressure-welded joint, with a thickness of 5 μm or more from the surface of the high-strength steel plate in the thickness direction of the high-strength steel plate. Here, the high-ferrite layer refers to a structure in which the area ratio of ferrite phase is 90% or more. Furthermore, the position 50 μm outward from the end of the pressure-welded joint may be considered the "heat-affected zone." The welded joint of this embodiment is characterized in that the high-ferrite layer formed during steel plate manufacturing is present even in the heat-affected zone that was affected by the welding heat during the production of the welded joint.

[0051] Figure 1 shows an example of an SEM micrograph of the surface layer of a high-strength steel plate constituting a welded joint of this embodiment, located 50 µm outward from the end of the pressure weld. Figure 1 shows a cross section of the steel plate in the thickness direction, with the steel plate surface on the upper side. As shown in Figure 1, in the welded joint of this embodiment, a high-ferrite layer 1, in which the area ratio of the ferrite phase is 90% or more, is present on the surface side of the steel plate in the heat-affected zone caused by welding. The presence of a B-enriched portion, described below, in this layer improves LME resistance during the production of the welded joint.

[0052] Since the effect of improving LME resistance can be obtained when the thickness of the high ferrite layer is 5 μm or more, there is no particular upper limit to the thickness. For example, it may be 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. From the viewpoint of LME resistance, the thickness of the high ferrite layer is preferably thicker, and is preferably 8 μm or more, more preferably 10 μm or more, 12 μm or more, 15 μm or more, 20 μm or more, or 25 μm or more.

[0053] The structure other than ferrite in the high ferrite layer is not limited, and may be, for example, one or more of martensite, bainite, and cementite.

[0054] The thickness of the high ferrite layer is determined by nital etching the thickness cross section of the steel plate and observing it with an SEM at 1000x magnification, distinguishing between ferrite and hard structures containing relatively large amounts of cementite, such as martensite and bainite, based on the microstructural morphology. The thickness of the high ferrite layer is determined by measuring five measurement ranges, each 500 μm apart in the direction perpendicular to the thickness direction, at intervals of 1000 μm, and averaging the measurements. Here, the ferrite area ratio refers to the area ratio determined by observing the above-mentioned thickness cross section. Even if there are localized areas in the thickness direction where the ferrite area ratio is less than 90%, this is not a problem as long as the ferrite area ratio is 90% or more in the thickness cross section from the surface to a depth of 5 μm.

[0055] The ferrite area ratio is determined by cutting a cross section of the steel sheet in the thickness direction perpendicular to the rolling direction, mirror-polishing it, revealing the steel structure with nital solution, and capturing a secondary electron image using a field-emission scanning electron microscope. The observation position is within 500 μm from the surface of the sheet thickness or the interface between the coating layer and the steel sheet, and five equally spaced fields are observed. The fraction of each structure is calculated using the point-counting method for the obtained structure photograph. More specifically, a grid is first drawn with equal intervals on the structure photograph. Next, it is determined whether the structure at each grid point corresponds to tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite. The fraction of each structure can be measured by determining the number of grid points corresponding to each structure and dividing by the total number of grid points. The greater the total number of grid points, the more accurately the area ratio can be determined. In this embodiment, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0056] The criteria for determining whether a material is tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite are as follows: A region that has a substructure (lath boundaries, block boundaries) within the grains and where carbides are precipitated with multiple variants is determined to be tempered martensite. A region where cementite is precipitated in a lamellar form is determined to be pearlite. A region with low brightness and no visible substructure is determined to be ferrite. A region with high brightness and where the substructure is not revealed by etching is determined to be fresh martensite or retained austenite. A region that does not fall into any of the above categories is determined to be bainite. Simply put, the area fraction of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[0057] Ferrite is a structure that is less susceptible to LME than austenite, and the presence of a ferrite-rich layer in the surface layer of a steel sheet improves LME resistance during the production of a welded joint. In this embodiment, the presence of a B-enriched portion, which will be described later, further improves LME resistance.

[0058] [B concentrated part] In the high-strength steel plate constituting the welded joint of this embodiment, a B-enriched portion exists at a position 50 μm outward from the end of the pressure welded portion, with a thickness of 1.0 μm or more from the surface of the high-strength steel plate in the thickness direction of the high-strength steel plate. Here, the B-enriched portion is a portion of the high-strength steel plate that is observed by TOF-SIMS (Time-of-Flight This refers to a location where the B intensity determined by TOF-SIMS (Time of Flight-Secondary Ion Mass Spectrometry) measurement is at least twice the B intensity at a depth of 50 μm determined by TOF-SIMS measurement. Figure 2 shows an example of the measurement results using TOF-SIMS. Figure 2 shows the measurement results for cross section C of the overlapping part of two steel plates in a welded joint, with the top and bottom direction being the thickness direction of the steel plate. The measurement results in Figure 2 indicate that the brighter the area, the higher the B concentration. The dark area near the center is the gap between the two steel plates. The measurement results in Figure 2 show that the B concentration is high near the surface of the steel plate.

[0059] The TOF-SIMS analysis was performed using a TOF-SIMS (manufactured by ION-TOF) as the device, and the primary ions were Bi3 2+ , Applied voltage: 25 kV, Measurement area: 50 μm square The measurement is performed by moving the field of view in the thickness direction of the high-strength steel plate so that the B-enriched area on the surface of the high-strength steel plate and a position 50 μm deep from the surface are measured.

[0060] The thicker the B-enriched portion, the better from the viewpoint of LME resistance during the production of a welded joint. The thickness is preferably 2.0 μm or more, more preferably 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more.

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

[0062] [Plating layer] The high-strength steel sheet constituting the welded joint of this embodiment may have a Zn-containing plating layer on its surface. The plating layer may be formed on only one side of the high-strength steel sheet, or on both sides, or may be formed on only a part of the surface.

[0063] The plating layer is not particularly limited as long as it contains Zn. Examples include Zn-0.2%Al (GI), Zn-(0.3 to 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, Zn-15%Mg, Zn-20%Al-7%Mg, and Zn-30%Al-10%Mg.

[0064] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and measuring the resulting solution using ICP (inductively coupled plasma) atomic emission spectroscopy.

[0065] The thickness of the plating layer may be, for example, 3 to 50 μm. The coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side. 2 This embodiment may be In this method, the coating weight of the coating layer is determined by dissolving the coating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the change in weight before and after the coating layer is removed by pickling. The acid solution containing the inhibitor may be, for example, a 10% hydrochloric acid solution containing 0.06 mass% inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.). After the coating layer is removed, the base steel sheet is rinsed with water and dried.

[0066] The thickness of the plating layer may be 5 μm or more, 7 μm or more, or 10 μm or more. The thickness of the plating layer may be 45 μm or less, 40 μm or less, 35 μm or less, or 30 μm or less. The coating weight of the plating layer is 15 g / m per side. 2 More than 20g / m 2 More than 25g / m 2 That's all, 30g / m 2 The coating weight of the plating layer may be 160 g / m per side. 2 Below 140g / m 2 Below 120g / m 2 Below 100g / m 2 It may be the following:

[0067] When a welded joint includes a general steel plate or the like that is not the high-strength steel plate described above, the steel plate that is not the high-strength steel plate may have the above-described plating layer.

[0068] The welded joint of this embodiment effectively improves LME resistance during manufacturing even when the steel sheets constituting the welded joint, particularly the outermost high-strength steel sheet, are not plated. LME cracking does not occur in the absence of molten zinc. However, for example, when a welding electrode spot-welded to a zinc-plated steel sheet is used to weld an unplated steel sheet, the plating adhering to the welding electrode melts and transfers to the unplated steel sheet, potentially causing LME cracking. Even in such cases, the welded joint of this embodiment effectively suppresses LME cracking during manufacturing because the B concentrated in the surface layer prevents the molten zinc from penetrating the Fe grain boundaries. Therefore, plating is not an essential component of the welded joint of this embodiment.

[0069] [B distribution in the surface layer] In the welded joint of this embodiment, in the non-heat-affected zone of the steel plate constituting the welded joint, particularly the high-strength steel plate, it is preferable that the depth satisfying the following formula (1) in a GDS (high-frequency glow discharge optical emission spectroscopy) measurement in the thickness direction of the steel plate is 1.5 μm or more from the surface of the steel plate.

[0070] Bx / B150≧5 …(1)

[0071] In the above formula (1), Bx represents the luminescence intensity at a point a depth x (μm) away from the interface between the steel sheet and the coating layer in the thickness direction of the steel sheet. This represents the luminescence intensity at a depth of 150 μm in the thickness direction of the steel sheet. In this embodiment, the "thickness direction" refers to the direction perpendicular to the interface between the steel sheet and the plating layer. A point away from the interface between the steel sheet and the plating layer in the thickness direction of the steel sheet refers to a point away from the interface toward the center of the steel sheet thickness.

[0072] The left side of the above formula (1) represents the ratio of the B concentration at depth x to the B concentration at a depth of 150 μm. That is, Bx / B150≧5 means that the B concentration at depth x is five times or more the B concentration at a depth of 150 μm. The B concentration at a depth of 150 μm can be considered to be the B concentration at the center of the steel sheet thickness, and Bx / B150≧5 means that B is enriched at depth x. The expression "a depth of 1.5 μm or more from the surface of the steel sheet that satisfies formula (1)" means that formula (1) is satisfied in a range from the surface of the steel sheet to a depth of 1.5 μm or more in the thickness direction of the steel sheet, and that B is enriched in a range to a depth of 1.5 μm or more in the thickness direction of the steel sheet.

[0073] In this embodiment, the surface of the steel sheet and the interface between the steel sheet and the coating layer are defined as follows. First, the Fe content in the thickness direction of the coated steel sheet is measured by GDS measurement. The highest Fe content is defined as the Fe content of the steel sheet. The point at which the Fe content is 5% of the Fe content of this steel sheet is defined as the "surface of the steel sheet." If a coating layer is formed on the surface of the steel sheet, the interface between the steel sheet and the coating layer is considered to be the surface of the steel sheet and is used as the starting point for the depth of the GDS measurement. The "interface between the steel sheet and the coating layer" is defined as the point at which the Fe content determined by GDS measurement is 93% of the Fe content of the steel sheet.

[0074] As described above, the concentration of B in the surface layer of the steel sheet causes B to segregate at the Fe grain boundaries in the surface structure, which prevents molten zinc from penetrating the Fe grain boundaries during spot welding and further suppresses the occurrence of LME during the production of welded joints.

[0075] (GDS measurement method) For Bx and B150, the surface of the target steel plate is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma, and the steel plate surface is sputtered while undergoing depth analysis using GDS. The elements contained in the material are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated.

[0076] Depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The sputtering time is set so that the sputtering depth exceeds at least 150 μm.

[0077] The GDS measurement was performed five times in the thickness direction, and the average value was taken as the B concentration. The measurement conditions were as follows: Bx and B150 are the B concentrations corresponding to a depth of x (μm) and a depth of 150μm, respectively.

[0078] Equipment: High-frequency glow discharge optical emission spectrometer (LECO Japan, model number GDS850A) Ar gas pressure: 0.3MPa Anode diameter: 4mmφ RF output: 30W Measurement time: 200 to 1500 seconds

[0079] To obtain the effect of suppressing the occurrence of LME, the depth at which Bx / B150≧5.0 is satisfied is preferably 1.5 μm or more. From the viewpoint of LME resistance, a larger Bx / B150 is more preferable, and it may be 1.6 μm or more, 1.8 μm or more, 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more. Since Bx / B150 is the ratio of the B concentration at depth x to the B concentration at a depth of 150 μm, the depth at which Bx / B150≧5.0 is satisfied is less than 150 μm. Even if the depth at which Bx / B150≧5.0 is greater, LME resistance is not reduced, but the depth at which Bx / B150≧5.0 is satisfied may be 100.0 μm or less, 50.0 μm or less, 30.0 μm or less, 20.0 μm or less, or 10.0 μm or less.

[0080] In the welded joint of this embodiment, B concentrated in the surface layer of the steel sheet constituting the welded joint segregates at the Fe grain boundaries. This is thought to suppress the penetration of molten zinc into the Fe grain boundaries during spot welding, thereby suppressing LME during the production of the welded joint. Such a B distribution in the surface layer can be obtained by producing a steel sheet from molten steel having the above-mentioned chemical composition using the production method described below.

[0081] <B enrichment in the range up to a depth of 5.0 μm> In the steel plate that constitutes the welded joint of this embodiment, it is preferable that the following formula (2) be further satisfied in the above-mentioned GDS measurement.

[0082] Bmax / B150≧8 …(2)

[0083] In the above formula (2), Bmax represents the maximum value of the luminescence intensity of B in the range from the surface of the steel sheet to a depth of 5.0 μm.

[0084] The left side of equation (2) represents the ratio of the maximum B emission intensity in the range from the surface of the steel sheet to a depth of 5.0 μm to the B concentration at a depth of 150 μm. In other words, Bmax / B150≧8 means that the B concentration at the position where B is most concentrated in the range from the surface of the steel sheet to a depth of 5.0 μm is 8 times or more the B concentration at a depth of 150 μm. The B concentration at a depth of 150 μm can be considered to be the B concentration at the center of the steel sheet thickness, and Bmax / B150≧8 means that B is highly concentrated in the range from the surface of the steel sheet to a depth of 5.0 μm, i.e., in the vicinity of the surface.

[0085] From the viewpoint of LME resistance, the larger Bmax / B150 is the more preferable, and it is preferably 10 or more, more preferably 12 or more, 14 or more, or even more preferably 16 or more.

[0086] It is not essential for the steel sheet of this embodiment to satisfy formula (2), and good LME resistance can be obtained as long as formula (1) is satisfied, even if formula (2) is not satisfied. By making the B distribution in the surface layer such that formula (2) is satisfied, B that is more highly concentrated in the surface layer of the steel segregates at the Fe grain boundaries, preventing the penetration of molten zinc, and therefore, a greater effect of suppressing LME can be obtained.

[0087] Plate Thickness The thickness of the steel plate constituting the welded joint of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The thickness may be 0.8 mm or more, or 1.0 mm or more. The thickness may be 3.0 mm or less, 2.8 mm or less, 2.6 mm or less, 2.5 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0088] <Steel plate manufacturing method> Next, a method for manufacturing the steel plates that form the welded joint of this embodiment will be described.

[0089] The steel sheet that constitutes the welded joint according to this embodiment can be obtained by a manufacturing method including a casting step of casting molten steel with adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a coiling step of coiling the hot-rolled steel sheet, a pickling step of pickling the coiled hot-rolled steel sheet, a cold-rolling step of cold-rolling the pickled hot-rolled steel sheet to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet may be pickled and then cold-rolled directly without being coiled after the hot rolling step.

[0090] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0091] [Hot rolling process] A hot-rolled steel plate can be obtained by hot-rolling a steel slab obtained by casting. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction rate of each rolling step may be appropriately changed depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050°C, and the reduction rate of finish rolling may be 10 to 50%.

[0092] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step without being coiled, and then cold-rolled as described below.

[0093] [Pickling process] The hot-rolled steel sheet is subjected to pickling. In the method for manufacturing a steel sheet constituting a welded joint according to this embodiment, the unevenness of the steel sheet surface after pickling is controlled in order to concentrate B in the surface layer of the steel sheet in the subsequent annealing process. Specifically, the unevenness is set to 1.5 μm or more in terms of Ra, which is the arithmetic mean height defined in JIS B0601:2013. This condition means that a certain degree of unevenness exists on the steel sheet surface. If the unevenness is small, the strain imparted to the surface layer of the steel sheet is small, so that internal oxidation does not proceed even when annealing at a high dew point, as described below, and the concentration of B in the surface layer of the steel sheet does not proceed.

[0094] The larger the Ra of the unevenness, the better, and it is preferably 2.0 μm or more, more preferably 2.5 μm or more, 3.0 μm or more, and even more preferably 3.5 μm or more.

[0095] The unevenness of the steel sheet surface after pickling is determined as the arithmetic mean roughness Ra by randomly selecting 10 locations on the surface of the surface layer side, measuring the surface profile at each location with a contact surface roughness meter, and arithmetically averaging the surface roughness at those locations, in accordance with JIS B 0601:2013. It is not necessary to constantly measure the unevenness of the steel sheet surface after pickling, and once the pickling conditions that form the desired unevenness have been determined, measurement of the unevenness may be omitted.

[0096] The surface roughness of the steel sheet varies depending on the pickling conditions, so these conditions can be adjusted appropriately to obtain the above-mentioned unevenness. For example, pickling can be performed using a 1 to 10 mass % hydrochloric acid solution at a temperature of 20 to 95°C for a pickling time of 30 to less than 200 seconds.

[0097] The surface of the steel sheet after pickling has such irregularities, and by rolling out these irregularities in the subsequent cold rolling process and imparting strain to the surface layer of the steel, the concentration of B in the surface layer of the steel sheet is promoted in the subsequent annealing process.

[0098] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet. In the cold-rolling step, the unevenness imparted in the above-mentioned pickling step is rolled and crushed, thereby imparting strain to the surface layer of the steel sheet. For this reason, rolls used in cold rolling preferably have a small surface roughness, and the roll surface roughness Ra is preferably 1.0 μm or less. The roll surface roughness Ra may be 0.8 μm or less, 0.6 μm or less, or 0.5 μm or less. The reduction ratio in cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the steel sheet may be cooled to room temperature, for example, by air-cooling.

[0099] In the cold rolling process, the unevenness on the surface of the hot-rolled steel sheet is rolled out, imparting strain to the surface layer of the steel sheet, which promotes the concentration of B in the surface layer of the steel sheet in the subsequent annealing process.

[0100] [Annealing process] After the cold rolling step, the obtained cold-rolled steel sheet is subjected to the following high dew-point annealing. In the method for producing a steel sheet and a galvannealed steel sheet according to the present embodiment, the annealing step prevents external oxidation from progressing on the surface of the steel sheet, but promotes internal oxidation in the surface layer of the steel sheet, i.e., oxidation progresses toward the inside of the steel sheet.

[0101] In the annealing step of this embodiment, the steel sheet having the surface layer strained by the above-described step is held at a high dew point. Specifically, the holding temperature in the annealing step is 760 to 900°C to promote internal oxidation and B concentration, and the holding time at the holding temperature is 0 to 360 seconds. The holding temperature may be 770°C or higher, 780°C or higher, or 790°C or higher. The holding temperature may be 890°C or lower, 880°C or lower, or 870°C or lower. The holding time may be 10 seconds or longer, 30 seconds or longer, 50 seconds or longer, or 60 seconds or longer. The holding time may be 330 seconds or shorter, 300 seconds or shorter, 270 seconds or shorter, 240 seconds or shorter, or 200 seconds or shorter.

[0102] The annealing atmosphere is preferably a non-oxidizing atmosphere, and can be, for example, N2-2 to 4 vol% H2. The oxygen concentration of the atmosphere is preferably 50 ppm or less, and may be 30 ppm or less, 20 ppm or less, or 10 ppm or less. By adopting such conditions, internal oxidation can proceed while suppressing oxidation of the coating surface. The rate of temperature rise to the holding temperature is not particularly limited. The rate of temperature rise may be, for example, 1 to 10°C / sec. If the rate of temperature rise is less than 1°C / sec, it may take too long to raise the temperature to the control temperature, which may result in thick oxide on the surface of the coating layer. On the other hand, if the rate of temperature rise is more than 10°C / sec, internal oxidation may not proceed sufficiently, and the strain imparted to the steel sheet surface layer may not be sufficiently released, resulting in insufficient boron concentration. From these viewpoints, the rate of temperature rise may be 2°C / sec or more, 3°C / sec or more, or 4°C / sec or more. The temperature ramp rate may be 9°C / sec or less, 8°C / sec or less, or 7°C / sec or less.

[0103] In the annealing process of the method for manufacturing the steel sheet that constitutes the weld joint of this embodiment, the dew point of the atmosphere is changed in the first half and the second half of the temperature rise. Specifically, the dew point from room temperature to the controlled temperature is different from the dew point from the controlled temperature to the holding time.

[0104] The control temperature is the temperature at which the dew point is changed, and is set to 450 to 550°C. During the temperature rise from room temperature to the control temperature, the dew point of the annealing atmosphere is set to -40°C or higher and -20°C or lower. During the temperature rise from the control temperature to the holding temperature, the dew point of the annealing atmosphere is set to -20°C or higher and 20°C or lower.

[0105] If the control temperature is less than 450°C, the dew point rises at low temperatures, causing internal oxidation to proceed at low temperatures, releasing the strain imparted to the steel sheet surface, and preventing sufficient internal oxidation to suppress deboronization from occurring between the control temperature and the holding temperature.If the control temperature is more than 550°C, the strain imparted to the steel sheet surface is released before internal oxidation progresses at high temperatures, preventing sufficient internal oxidation to suppress deboronization.

[0106] If the dew point during the temperature rise from room temperature to the control temperature is less than -40°C, external oxidation of Si and Mn may occur, preventing internal oxidation from progressing between the control temperature and the holding temperature. If the dew point during the temperature rise from room temperature to the control temperature is more than -20°C, internal oxidation will progress at a low temperature, releasing the strain imparted to the surface layer of the steel sheet. As a result, internal oxidation to suppress boron removal may not occur between the control temperature and the holding temperature.

[0107] Similarly, if the dew point between the control temperature and the holding temperature is -20°C or lower, internal oxidation to suppress the deboronization may not proceed sufficiently. If the dew point between the control temperature and the holding temperature is higher than 20°C, external oxidation may proceed, and internal oxidation to suppress the deboronization may not proceed sufficiently.

[0108] Furthermore, the dew point during the temperature rise from the control temperature to the holding temperature is set to be at least 10°C higher than the dew point during the temperature rise from room temperature to the control temperature. This allows internal oxidation to proceed and promotes the concentration of B.

[0109] From the viewpoint of suitably concentrating B, the control temperature may be 460°C or higher, 470°C or higher, or 480°C or higher. The control temperature may be 540°C or lower, 530°C or lower, or 520°C or lower. From the viewpoint of suitably concentrating internal oxidation and B, the dew point during the temperature rise from room temperature to the control temperature may be -38°C or higher, -37°C or higher, or -35°C or higher. The dew point during the temperature rise from room temperature to the control temperature may be 18°C ​​or lower, 17°C or lower, or 15°C or lower. The dew point during the temperature rise from the control temperature to the holding temperature may be -18°C or higher, -17°C or higher, or -15°C or higher. The dew point during the temperature rise from the control temperature to the holding temperature may be 18°C ​​or lower, 17°C or lower, or 15°C or lower.

[0110] In a state in which strain is imparted to the surface layer of a steel sheet by the above-described method, the dew point is raised to a temperature equal to or higher than the control temperature in the annealing process, whereby internal oxidation rapidly progresses in the surface layer of the steel sheet, and B is incorporated into the oxides formed inside the steel sheet, thereby concentrating B in the surface layer of the steel sheet and segregating B at the Fe grain boundaries, thereby obtaining the above-described B concentration distribution in the surface layer.

[0111] Annealing is performed under tension of 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, which promotes B concentration in the surface layer.

[0112] [Plating process] A plated steel sheet may be produced by a production method including a plating step using the steel sheet produced as described above. The plating step may be carried out according to a method known to those skilled in the art. The plating step may be carried out by, for example, a hot-dip plating method, an electroplating method, a vapor deposition plating method, a thermal spraying method, or a cold spraying method. Preferably, the plating step is carried out by a hot-dip plating method. The conditions for the plating step may be appropriately set taking into consideration the chemical composition, thickness, and coating weight of the desired plating layer, etc.

[0113] [Alloying process] After the plating step, a known alloying treatment may be carried out to form alloyed plating. The conditions for the alloying treatment step may be appropriately set in consideration of the chemical composition, thickness, coating weight, etc. of the desired plating layer.

[0114] [Spot welding process] A plurality of the above-described steel plates are stacked and spot-welded to obtain a welded joint. The steel plates may be of the same type or may have different chemical compositions. As long as at least one of the outermost steel plates is the above-described high-strength steel plate, the other steel plates may be general commercially available steel plates.

[0115] The conditions for spot welding are not particularly limited. For example, spot welding can be performed using a dome radius type welding electrode with a tip diameter of 8 mm, with a pressure of 5.0 kN, a welding time of 1.2 seconds, and a current of 12 kA.

[0116] The welded joint according to the present invention is suppressed from LME cracking during manufacturing, and therefore can be suitably used in a wide range of fields such as automobiles, home appliances, building materials, etc. It is particularly preferably used in the automobile field. [Example]

[0117] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. First, a method for manufacturing steel plates used to manufacture the welded joints in each example will be described.

[0118] <Example No. 1> Molten steel was melted in a blast furnace and cast by continuous casting to obtain a slab having the chemical composition shown in No. 1 of Table 1. The obtained slab was heated to 1200°C and hot rolled with a finish rolling ending temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C.

[0119] The coiled steel sheet was pickled for 40 seconds using a 5 mass % hydrochloric acid solution at 40°C. After pickling, the steel sheet was cold rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel sheet was 1.6 mm.

[0120] Thereafter, annealing was performed in a furnace with an oxygen concentration of 20 ppm or less in an N2-4 vol% H2 gas atmosphere at a holding temperature of 800°C and a holding time of 0 seconds to prepare steel sheet samples. The temperature increase rate during annealing was 5.0°C / second. The dew point of the annealing atmosphere was controlled at 500°C, and was -20°C from room temperature to the controlled temperature and -10°C from the controlled temperature to the holding temperature. The annealing was performed under a tension of 15 MPa. Note that a holding time of 0 seconds means that the temperature was increased to 800°C and then immediately decreased.

[0121] <Examples Nos. 2 to 32> Steel sheets were produced under the same conditions as in Example 1, except that the chemical compositions were as shown in Table 1, the annealing conditions were as shown in Table 2, and the coating type was as shown in Table 2. Regarding the coating type in Table 2, "a" indicates that the steel sheet was immersed in a 450°C hot-dip galvanizing bath (Zn-0.14%Al) for 3 seconds, then pulled out at 100 mm / s, and then wiped with N2 wiping gas to a coating weight of 50 g / m 2 "b" refers to hot-dip galvanizing, in which the alloying treatment in "a" was omitted; and "c" refers to hot-dip galvanizing, in which the plating bath in "a" was Zn-1.5%Al-1.5%Mg, in which the alloying treatment was omitted.

[0122] (Unevenness after pickling) When manufacturing the steel sheets, the surface roughness of the hot-rolled steel sheets after pickling was measured. According to JIS B 0601:2013, 10 locations were randomly selected on the surface of the surface layer side, and the surface profile at each location was measured using a contact surface roughness meter. The surface roughness at these locations was arithmetically averaged to obtain the arithmetic mean roughness Ra. The surface roughness of the hot-rolled steel sheets is shown in Table 2.

[0123] [Table 1]

[0124] [Table 2]

[0125] Two steel plates obtained in each example were stacked together, and spot welding was performed using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 5°, a pressure of 5.0 kN, a welding time of 1.2 seconds, and a current of 12 kA to produce a welded joint.The microstructures of the heat-affected zone and non-heat-affected zone, as well as LME resistance during production, were evaluated.

[0126] (Hardness of steel plate) The hardness of the steel plate was measured in the non-heat-affected zone, located at a distance of 5 mm or more from the outer edge of the spot weld of the welded joint. The hardness was measured at a position halfway down the steel plate in accordance with JIS Z 2244:2009. The measurement load was 200 gf. The hardness was evaluated as follows: A rating of A or higher was considered to be good hardness.

[0127] Rating AAA: 380Hv or higher Rating AA: 300Hv or more, less than 380Hv Rating A: 240Hv or more, less than 300Hv Rating B: Less than 240Hv

[0128] (high ferrite layer) Samples were cut to 25mm x 15mm from a position 50µm outward from the edge of the pressure welded joint, etched with nital, and the T-section of each sample was observed using an SEM to measure the thickness of the high-ferrite layer, which had a ferrite area ratio of 90% or more. Thickness was measured at five equally spaced points within a 500µm range in the T-direction, and the average value was used. Here, the starting point of "thickness" was the surface of the steel sheet for unplated steel sheets, and the starting point was the interface between the plating layer and the steel sheet for plated steel sheets, and was determined from the SEM image.

[0129] (B concentrated part) A 30mm x 30mm cut sample was taken from a position 50µm outward from the edge of the pressure welded joint, and the thickness of the B-enriched area was measured by TOF-SIMS. The B-enriched area was defined as a region where the B intensity measured by TOF-SIMS was more than twice the B intensity measured at a depth of 50µm. The TOF-SIMS analysis was performed using a TOF-SIMS (manufactured by ION-TOF) with primary ions: Bi3 2+ The measurement was carried out under the conditions of a voltage of 25 kV and a measurement area of ​​50 μm square. The field of view was moved in the thickness direction of the high-strength steel plate so that the B-enriched portion on the surface of the high-strength steel plate and a position 50 μm deep from the surface were measured.

[0130] (B distribution in the steel plate surface layer in the non-heat-affected zone) The B distribution in the surface layer of the steel plate in the non-heat-affected zone was evaluated as follows.

[0131] GDS was performed on samples cut to a size of 50 mm x 50 mm from the non-heat-affected zone of each weld joint. GDS measurements were performed five times in the plate thickness direction, and the average value was taken as the B concentration. The measurement conditions were as follows: Bx and B150 are the B concentrations corresponding to a depth of x (μm) and a depth of 150 μm, respectively.

[0132] Equipment: High-frequency glow discharge optical emission spectrometer (LECO Japan, model number GDS850A) Ar gas pressure: 0.3MPa Anode diameter: 4mmφ RF output: 30W Measurement time: 200 to 1500 seconds

[0133] From the determined Bx, the depth that satisfies the formula (1): Bx / B150≧5.0 and the value of the left side of the formula (2): Bmax / B150 were determined.

[0134] (LME resistance during welded joint manufacturing) Two samples were cut to a size of 50 mm x 100 mm from each steel plate and galvannealed steel plate, and welded joints were produced by spot welding these two samples using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 5°, a pressure of 5.0 kN, a welding time of 1.2 seconds, and a current of 12 kA. When producing welded joints using unplated steel plate, a welding electrode that had been previously used in spot welding of zinc-plated steel plate at least 10 times was used.

[0135] With reference to Figure 3, the evaluation of LME resistance during the production of welded joints will be described. LME resistance was evaluated by the length of an LME crack (shoulder crack 21) that occurred in the shoulder 14 of a weld 12 formed by spot welding two overlapping steel plates 11. The shoulder refers to the inclined portion of the edge of a depression 13 created by spot welding. The evaluation was performed as follows based on the length of the shoulder crack 21. In this example, a rating of A or higher was determined to indicate excellent LME resistance during the production of a welded joint.

[0136] Rating AAA: 0μm Rating AA: Over 0 μm, less than 50 μm Rating A: 50 μm or more, less than 160 μm Rating B: 160 μm or more

[0137] The results of each evaluation are shown in Table 3.

[0138] [Table 3]

[0139] Nos. 1 to 22 are inventive examples, and it was confirmed that they had excellent resistance to LME during the production of welded joints.

[0140] No. 23 steel sheet had a low Si content. As a result, decarburization and internal oxidation did not progress sufficiently during the annealing process during steel sheet production, resulting in a thin ferrite layer. In addition, the boron removal phenomenon could not be suppressed, and B-enriched areas were not formed. As a result, the LME resistance during weld joint production was poor.

[0141] No. 24 steel sheet had a high Si content. Therefore, even though a high ferrite layer and a B-enriched area were formed, LME could not be suppressed. As a result, the LME resistance during weld joint production was poor.

[0142] No. 25 had a low dew point in the first half of the annealing process during steel plate production. As a result, Si and Mn were externally oxidized, and internal oxidation did not progress in the second half of the annealing process. This resulted in a thin ferrite layer and no B-enriched areas. As a result, the LME resistance was poor during weld joint production.

[0143] For No. 26, the dew point was high in the first half of the annealing process during steel plate manufacturing. As a result, internal oxidation progressed at the low temperature in the first half of the annealing process, releasing the strain applied to the surface layer of the steel plate. This prevented internal oxidation from occurring between the controlled temperature and the holding temperature, making it impossible to prevent boron removal, resulting in thinning of the high ferrite layer and the B-enriched area. As a result, the LME resistance during weld joint manufacturing was poor.

[0144] No. 27 had a low dew point in the latter half of the annealing process during steel plate manufacturing. As a result, internal oxidation, which is necessary to suppress the deboronization phenomenon, did not progress sufficiently, resulting in thin high-ferrite layers and B-enriched areas. As a result, the LME resistance during weld joint manufacturing was poor.

[0145] No. 28 had a high dew point in the latter half of the annealing process during steel plate production. As a result, external oxidation progressed and internal oxidation, which is necessary to suppress the deboronization phenomenon, did not progress sufficiently. As a result, the ferrite-rich layer became thin and no B-enriched area was formed. As a result, the LME resistance during weld joint production was poor.

[0146] No. 29 had a low control temperature. As a result, the dew point rose at low temperatures, internal oxidation progressed at low temperatures, and the strain applied to the steel surface was released. As a result, sufficient internal oxidation to suppress the deboronization phenomenon did not occur between the control temperature and the holding temperature, and the high ferrite layer and B-enriched area became thinner. As a result, the LME resistance of the welded joint was poor when it was manufactured.

[0147] No. 30 had a high control temperature. As a result, the strain applied to the steel surface was released before internal oxidation progressed at high temperatures. This prevented sufficient internal oxidation from occurring to suppress the deboronization phenomenon, resulting in a thinner boron-enriched area. As a result, the LME resistance of the welded joint was poor.

[0148] No. 31 had a low annealing temperature. As a result, internal oxidation and B enrichment did not progress sufficiently, the ferrite layer became thin, and B enrichment did not form. As a result, the LME resistance during weld joint production was poor.

[0149] No. 32 had small surface irregularities after pickling during steel plate production. As a result, sufficient strain was not imparted to the surface layer of the steel plate, and even after high-dew-point annealing, B did not concentrate in the surface layer of the steel plate, resulting in a thin B-enriched area. As a result, LME resistance was poor during weld joint production. [Explanation of symbols]

[0150] 1. High ferrite layer 11 Steel plate 12 Welded parts 13. Hollow 14 Shoulder 21 Crack in shoulder

Claims

1. Multiple overlapping steel plates, a nugget joining the plurality of steel plates; a spot weld having a pressure welded portion and a heat affected zone formed around the nugget; a non-heat-affected zone, which is a region outside the heat-affected zone; and A separation portion formed around the pressure-welded portion A welded joint comprising: Among the plurality of steel plates, at least one steel plate arranged on the outermost side is a high-strength steel plate having a Vickers hardness of 240 Hv or more at the center of the plate thickness, The chemical composition of the high-strength steel plate is, in mass%, C: 0.05-0.40%, Si: 0.7-3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0.0005 to 0.0050%, Ti: 0.0010 to 0.1000%, Nb: 0 to 0.2000%, V: 0 to 0.15%, Cr: 0-2.00%, 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 to 0.100%, Hf: 0 to 0.100%, and REM: 0~0.100% and the balance being Fe and impurities, a high ferrite layer having an area ratio of ferrite phase of 90% or more is present at a position 50 μm outward from an end of the pressure-welded portion, with a thickness of 5 μm or more from the surface of the high-strength steel plate in the thickness direction of the high-strength steel plate; At a position 50 μm outward from the end of the pressure-welded portion, a B-enriched portion having a B intensity at least twice the B intensity at a position 50 μm deep determined by TOF-SIMS measurement is present to a thickness of 1.0 μm or more from the surface of the high-strength steel plate. A welded joint characterized by:

2. The welded joint according to claim 1, wherein the high-strength steel plate has a Zn-containing plating layer formed on one or both sides of the high-strength steel plate.

3. In the non-heat-affected zone, the emission intensity Bx at a depth x (μm) in the GDS measurement from the steel sheet surface in the thickness direction and the emission intensity B150 at a depth of 150 μm are Bx / B150≧5…(1) The depth that satisfies the above condition is 1.5 μm or more.

3. The welded joint according to claim 1 or 2.

4. In the non-heat-affected zone, in the GDS measurement from the steel sheet surface to the thickness direction, the maximum value Bmax of the B emission intensity in the range of a depth of 5 μm or less, and the emission intensity B150 at a depth of 150 μm are Bmax / B150≧8…(2) 4. The welded joint according to claim 3, wherein the following is satisfied:

5. 3. The welded joint according to claim 1, wherein the high ferrite layer has a thickness of 10 μm or more.

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