Welded joint

Abstract

The present invention addresses the problem of obtaining a welded joint having excellent LME resistance during production. This welded joint is characterized by being provided with a plurality of steel plates that overlap each other, a nugget that joins the plurality of steel plates together, a spot-welded section that has a pressure-welded section and a heat-affected section that are formed around the nugget, a non-heat-affected section that is a region outside the heat-affected section, and a separation section that is formed around the pressure-welded section, at least one steel sheet disposed on the outermost side among the plurality of steel sheets is a high-strength steel sheet having a Vickers hardness of 240 Hv or more at the center of the sheet thickness, the high-strength steel sheet having a predetermined chemical composition, and the thickness of the high-strength steel sheet being less than the thickness of the steel sheet at a position 50 [mu] m outward from the end of the pressure-welded section. A high ferrite layer having a ferrite phase area ratio of 90% or more is present at a thickness of 5 [mu] m or more in the thickness direction from the surface, and a B-concentrated portion having a B strength of two or more times the B strength at a depth of 50 [mu] m determined by TOF-SIMS measurement is present at a thickness of 1.0 [mu] m or more from the surface.

Description

TECHNICAL FIELD

[0001] The present application relates to a welded joint. BACKGROUND

[0002] In recent years, high strength of steel sheets used in various fields such as automobiles, home electric appliances, building materials and the like is being promoted. For example, in the automobile field, in order to improve fuel efficiency, the use of high-strength steel sheets for the purpose of weight reduction of the vehicle body is increasing.

[0003] Particularly in the automobile field, a welded joint obtained by spot welding a steel sheet on which zinc-based plating is performed is sometimes used. With respect to welding of a steel sheet on which zinc-based plating, particularly a high-strength steel sheet, is performed, a decrease in weldability caused by liquid metal embrittlement (LME) cracking is sometimes a problem, as described in Patent Literature 1. The LME cracking is considered to be caused by phase transformation of the surface layer of the steel sheet to austenite at the time of welding, embrittlement of the steel sheet by molten zinc invading the grain boundaries thereof, and further, application of tensile stress to the steel sheet at the time of welding.

[0004] In addition, in Patent Literature 2, as a steel sheet in which LME cracking is suppressed and weldability is improved, a steel sheet in which Si oxide particles having a particle diameter of 20 nm or more are present at a number density of 3000 to 6000 / mm 2 at the surface layer of the steel sheet at an appropriate particle diameter.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: International Publication No. 2019 / 116531 Patent Literature 2: International Publication No. 2020 / 218575 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION The present application relates to a welded joint.

[0007] MEANS FOR SOLVING THE PROBLEMS Various studies have been made on improvement of LME resistance at the time of manufacture of a welded joint. In relation thereto, in the present application, it has been found that it is preferable to concentrate B at the surface layer of a steel sheet constituting a welded joint. It has been found that, in order to concentrate B at the surface layer of a steel sheet, it is effective to perform internal oxidation toward the inside of the steel sheet at the surface layer of the steel sheet without performing external oxidation at the surface of the steel sheet in annealing. The gist of the present application is as described below.

[0008] [1] A welded joint characterized by comprising: a plurality of steel sheets that overlap, a nugget that joins the plurality of steel sheets, a spot weld portion having a press weld portion and a heat-affected portion formed around the nugget, a non-heat-affected portion that is an area outside the heat-affected portion, and a separation portion formed around the press weld portion, wherein at least one steel sheet disposed at the outermost side among the plurality of steel sheets is a high-strength steel sheet having a Vickers hardness of 240 Hv or more at the center of the thickness, the high-strength steel sheet contains, in mass%, C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0.0005 to 0.0050%, Ti: 0.0010 to 0.1000%, Nb: 0 to 0.2000%, V: 0 to 0.15%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and REM: 0 to 0.100%, and the remainder is Fe and impurities, a high-ferrite layer in which an area ratio of a ferrite phase present at a position 50 μm or more outward from an end of the press weld portion at a thickness of 5 μm or more from a surface of the high-strength steel sheet is 90% or more, and a B concentration portion present at a thickness of 1.0 μm or more from the surface of the high-strength steel sheet at the position 50 μm or more outward from the end of the press weld portion, a B intensity in the B concentration portion found by TOF-SIMS measurement being 2 times or more a B intensity at a position 50 μm in depth found by TOF-SIMS measurement.

[0009] [2] The welded joint according to the above [1], characterized in that the high-strength steel sheet has a Zn-containing plated layer formed on one or both faces of the high-strength steel sheet.

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

[0011] [4] The welded joint according to the above [3], characterized in that, in the non-heat affected portion, a maximum value Bmax of the luminescence intensity of B in a range of 5 μm or less in depth in GDS measurement from the surface of the steel sheet in the thickness direction, and a luminescence intensity B150 at a depth of 150 μm satisfy the following formula (2), Bmax / B150 ≥ 8 (2).

[0012] [5] The welded joint according to any one of the above [1] to [4], characterized in that the ferrite-austenite layer has a thickness of 10 μm or more.

[0013] Effects of Invention According to the present application, a welded joint having good LME resistance at the time of manufacture can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is an example of a microstructure photograph of a ferrite-austenite layer which is a surface layer portion of a steel sheet constituting the welded joint of the present application.

[0015] Figure 2 is an example of a TOF-SIMS measurement result of a B concentration portion which is a surface layer portion of a steel sheet constituting the welded joint of the present application.

[0016] Figure 3 is a drawing illustrating the position of a crack which is an object in the LME resistance evaluation of the example. DETAILED DESCRIPTION

[0017] Hereinafter, one embodiment of the present application will be described. The present application is not limited to the following embodiment. First, in the present embodiment, an outline of improving the LME resistance at the time of manufacture of a welded joint will be described.

[0018] When spot welding is performed on a plated steel sheet, the plating layer is melted, and the surface layer portion of the steel sheet is heated and the steel sheet microstructure is transformed into austenite. At this time, the molten plating layer intrudes into the steel sheet microstructure along the grain boundaries of the austenite, thereby grain boundary embrittlement. Therefore, if stress is applied to the steel sheet, LME cracks are easily generated at the grain boundaries. In particular, since tensile stress is applied to the steel sheet at the time of welding, it is considered that LME is easily generated. As a method of improving the LME resistance, the present inventors have conceived the idea of effectively utilizing B (hereinafter also referred to as "boron"). Specifically, the idea of suppressing the intrusion of molten zinc into the Fe grain boundaries (austenite grain boundaries or ferrite grain boundaries) by segregating B at the surface layer microstructure, and suppressing the occurrence of LME has been conceived.

[0019] Generally in B-containing steel, particularly when heated to the austenite temperature region, a phenomenon of boron depletion in which the amount of B near the surface decreases is known. As a result, generally in B-containing steel, the concentration of B in the surface layer of the steel decreases compared to the center portion of the steel sheet. In the present embodiment, when the steel sheet is annealed, internal oxidation that easily proceeds at the grain boundaries occurs in the surface layer of the steel sheet toward the inside of the steel sheet. Thereby, an internal oxidation layer is formed in the surface layer of the steel sheet, and thus, particularly, Si that deteriorates the LME resistance can be fixed as an oxide in the internal oxidation layer. Further, the oxide formed by the internal oxidation takes in B that diffuses from the steel sheet to the surface layer due to heating, and thus, the boron depletion phenomenon is suppressed. In this way, in contrast to the conventional B-containing steel, by forming a region in which B is concentrated in the surface layer of the steel, the LME resistance at the time of manufacturing the welded joint is improved by the effect of the complex thereof.

[0020] Generally, in the case where the steel sheet is heated such as annealing, external oxidation in which an oxide (scale) is formed is generated on the surface of the steel sheet. The present embodiment is completed based on the insight that by imparting a strain to the surface layer of the steel sheet in advance to promote the diffusion of oxygen to the inside of the material, and by performing annealing at an appropriate dew point, internal oxidation can be performed without performing external oxidation.

[0021] Hereinafter, the present embodiment will be described in detail.

[0022] Welded joint The welded joint of the present embodiment has a plurality of steel sheets that are overlapped, a nugget that joins the plurality of steel sheets, a spot weld portion that has a press weld portion formed around the nugget and a heat affected portion, a non-heat affected portion that is a region outside the heat affected portion, and a separation portion formed around the press weld portion. The so-called "heat affected portion" herein is a portion of the steel sheet that has changed in structure, metallurgical property, mechanical property, and the like due to welding heat, and the "non-heat affected portion" is a portion other than the heat affected portion.

[0023] [High-strength steel sheet] At least one steel sheet disposed at the outermost side among the plurality of steel sheets is a high-strength steel sheet. The present invention aims to suppress LME cracks generated when a steel sheet having high strength is spot-welded. LME cracks are generated in a high-strength steel sheet. The welded joint of the present embodiment has an effect of suppressing LME cracks at the time of manufacturing the welded joint, and is particularly effective for suppressing cracks in the shoulder portion of the welded portion. Here, the so-called high-strength steel sheet refers to a steel sheet in which the Vickers hardness at a position that is the non-heat affected portion of the welded joint, a 1 / 2 depth position, is 240 Hv or more. In addition, the "shoulder portion" refers to a slanted portion of the edge of a depression formed by an electrode at the time of spot welding.

[0024] The hardness of the steel sheet was measured at a position of 1 / 2 depth in a non-heat affected portion of the steel sheet constituting the welded joint. The hardness measurement was performed in accordance with JIS Z 2244:2009. The measurement load was set to 200 gf. The Vickers hardness at the center of the plate thickness of the steel sheet constituting the welded joint can be 260 Hv or greater, 280 Hv or greater, or 300 Hv or greater.

[0025] In the welded joint of the present embodiment, as long as at least one steel sheet disposed at the outermost side among the steel sheets constituting the welded joint is the above-described high-strength steel sheet, the other steel sheets can also be general commercially available steel sheets. Of course, all the steel sheets can be the above-described high-strength steel sheet. In manufacturing the welded joint, at least one of the faces at which the welding electrode is brought into contact in spot welding is the high-strength steel sheet described below.

[0026] [Chemical composition of high-strength steel sheet] Hereinafter, the chemical composition of the high-strength steel sheet will be described. Hereinafter, "%" with respect to the chemical composition of the high-strength steel sheet means "mass %". In addition, in the numerical range of the chemical composition, the numerical range indicated using "~" means a range including the numerical values recited before and after the "~" as lower limit values and upper limit values.

[0027] (C: 0.05 to 0.40%) C (carbon) is an element that ensures the strength of steel. In order to obtain the hardness of 240 Hv or greater, which is the object of the present embodiment, the content of C is 0.05% or greater. In consideration of weldability, the content of C is set to 0.40% or less. The content of C can be 0.08% or greater, 0.10% or greater, or 0.15% or greater. The content of C can be 0.37% or less, 0.35% or less, or 0.30% or less.

[0028] (Si: 0.7 to 3.0%) Si (silicon) is an element that promotes decarburization and ferrite stabilization in the annealing process at the time of manufacturing the steel sheet, and is also an element that suppresses the boron loss phenomenon through internal oxidation. In addition, it is also an element that improves the corrosion resistance of steel. As a result, the high-ferrite layer and the B concentration portion described later are formed in the surface layer of the steel sheet. In order to obtain this effect, the content of Si is set to 0.7% or greater. When the content of Si is too high, Si is generally an element that reduces the LME resistance at the time of manufacturing the welded joint, and thus the effect of the B distribution described later is hindered, and the effect of improving the LME resistance at the time of manufacturing the welded joint becomes small. In consideration of this, the content of Si is set to 3.0% or less. The content of Si can be 0.8% or greater, 0.9% or greater, or 1.0% or greater. The content of Si can be 2.5% or less, 2.0% or less, or 1.5% or less.

[0029] In the past, it has been known that the addition of Si to steel decreases the LME resistance at the time of manufacturing a welded joint, but the inventors' research has found that, contrary to the past understanding, the LME resistance at the time of manufacturing a welded joint is improved by containing a large amount of Si. It is believed that this is because, by the following-described B content and manufacturing method, the surface layer portion becomes a structure with ferrite at the center, and in addition, B is concentrated in the surface layer portion and exists in a segregated manner at the Fe grain boundaries.

[0030] (Mn: 0.1 to 5.0%) Mn (manganese) is an element effective for improving the strength of steel by obtaining a hard structure. In addition, like Si, it is an element that suppresses the boron loss phenomenon by internal oxidation. In order to obtain these effects, the lower limit of the content of Mn is set to 0.1%. In addition, in consideration of the decrease in workability due to Mn segregation, the content of Mn is set to 5.0% or less. The content of Mn can be 0.5% or more, 1.0% or more, or 1.5% or more. The content of Mn can be 4.5% or less, 4.0% or less, or 3.5% or less.

[0031] (sol. Al: 0 to 2.0%) Al (aluminum) is an element that, when solid-solved in steel, promotes ferrite stabilization and decarburization, and thus can improve the LME resistance at the time of manufacturing a welded joint, and thus can be contained as needed. sol. Al refers to acid-soluble Al that does not become an oxide such as Al2O3 and is soluble in acid, and is obtained as Al measured by deducting insoluble residues on filter paper generated during the analysis process of Al. The content of sol. Al is not necessary, and the lower limit of the content of sol. Al is 0. In order to obtain the effect of the content, the content of sol. Al can be 0.1% or more, 0.2% or more, or 0.3% or more. When the content of sol. Al is too much, even if high dew point annealing is performed, external oxidation will proceed, and an oxide (scale) will be formed on the surface layer of the steel sheet, and the LME resistance at the time of manufacturing a welded joint will decrease. In consideration of this, the content of sol. Al is set to 2.0% or less. The content of sol. Al can be 1.5% or less, 1.2% or less, or 1.0% or less.

[0032] (P: 0.0300% or less) P (phosphorus) is generally an impurity contained in steel. When the content of P exceeds 0.0300%, there is a possibility that the weldability will decrease. Therefore, the content of P is set to 0.0300% or less. The content of P can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that P not be contained, and the lower limit of the content of P is 0. From the viewpoint of the cost of dephosphorization, the content of P can exceed 0% or be 0.0001% or more.

[0033] (S: 0.0300% or less) S (sulfur) is generally an impurity contained in steel. When the content of S exceeds 0.0300%, the weldability decreases, and furthermore, the amount of MnS precipitates increases, and the workability such as bendability can decrease. Therefore, the content of S is set to 0.0300% or less. The content of S can be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that S is not contained, and the lower limit of the content of S is 0. From the viewpoint of desulfurization cost, the content of S can exceed 0% or be 0.0001% or more.

[0034] (N: 0.0100% or less) N (nitrogen) is generally an impurity contained in steel. When the content of N exceeds 0.0100%, the weldability can decrease. Therefore, the content of N is set to 0.0100% or less. The content of N can 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 content of N is 0. From the viewpoint of manufacturing cost, the content of N can exceed 0% or be 0.0010% or more.

[0035] (B: 0.0005 to 0.0050%) B (boron) is an element that contributes to an increase in strength by improving hardenability, and also improves toughness by segregating at grain boundaries and strengthening the grain boundaries. Furthermore, in the steel sheet of the present embodiment, B is concentrated in the surface layer of the steel sheet and exists by segregating at Fe grain boundaries. In order to obtain this effect, the content of B is set to 0.0005% or more. From the viewpoints of toughness and weldability, the content of B is set to 0.0050% or less. The content of B can be 0.0006% or more, 0.0008% or more, or 0.0010% or more. The content of B can be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0036] Generally, in the surface layer of steel, the concentration of B decreases compared to the center portion of the steel sheet due to the phenomenon of boron loss. In the present embodiment, by the manufacturing method described later, internal oxidation toward the inside of the steel sheet is performed in the annealing step, thereby taking in B by oxides, suppressing the phenomenon of boron loss, and forming the concentration distribution described later in the surface layer of the steel. It is considered that this B segregating at Fe grain boundaries suppresses LME at the time of manufacturing a welded joint.

[0037] (Ti: 0.0010 to 0.1000%) Ti (titanium) is an element that precipitates as TiC during cooling of the steel and contributes to an increase in strength. In order to obtain this effect, the content of Ti is set to 0.0010% or more. If it is contained in excess, coarse TiN is generated, and the toughness can be impaired, so the content of Ti is set to 0.1000% or less. The content of Ti 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 content of Ti can be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, or 0.0400% or less.

[0038] (Nb: 0 to 0.2000%) Nb (niobium) is an element that contributes to an increase in strength through an increase in hardenability, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Nb is 0. This effect can be obtained even with a trace amount, and the content of Nb when contained 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 securing toughness, the content of Nb is set to 0.2000% or less. The content of Nb 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.

[0039] (V: 0 to 0.15%) V (vanadium) is an element that contributes to an increase in strength through an increase in hardenability, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of V is 0. This effect can be obtained even with a trace amount, and the content of V when contained can 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 securing toughness, the content of V is set to 0.15% or less. The content of V can 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.

[0040] (Cr: 0 to 2.00%) Cr (chromium) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Cr is 0. The effect can be obtained even with a small amount, and the content of Cr when contained can be 0.001% or more, 0.01% or more, 0.05% or more, 0.07% or more, or 0.10% or more. If Cr is contained in excess, Cr carbides are formed in large amounts, and conversely, the hardenability can be impaired, and thus the content of Cr is set to 2.00% or less. The content of Cr 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, 0.30% or less, or 0.20% or less.

[0041] (Ni: 0 to 2.00%) Ni (nickel) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Ni is 0. The effect can be obtained even with a small amount, and the content of Ni when contained 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 addition of Ni increases the cost, and thus the content of Ni is set to 2.00% or less. The content of Ni 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.

[0042] (Cu: 0 to 2.00%) Cu (copper) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Cu is 0. The effect can be obtained even with a small amount, and the content of Cu when contained 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 a decrease in toughness, cracking of a slab after casting, and a decrease in weldability, the content of Cu is set to 2.00% or less. The content of Cu 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, 0.20% or less, or 0.15% or less.

[0043] (Mo: 0 to 1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing the strength of steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Mo is 0. The effect can be obtained even with a small amount, and the content of Mo when contained 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 content of Mo is set to 1.00% or less. The content of Mo 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.

[0044] (W: 0 to 1.00%) W (tungsten) is effective in improving the hardenability of steel and increasing the strength of steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of W is 0. The effect can be obtained even with a small amount, and the content of W when contained 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 content of W is set to 1.00% or less. The content of W 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.

[0045] (Ca: 0 to 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly contributes to the fine dispersion of inclusions, and has an effect of increasing toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Ca is 0. The effect can be obtained even with a small amount, and the content of Ca when contained 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. If Ca is contained in excess, the deterioration of surface properties can sometimes be significantly increased, and thus the content of Ca is set to 0.1000% or less. The content of Ca 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.

[0046] (Mg: 0 to 0.100%) Mg (magnesium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Mg is 0. The effect can be obtained even with a small amount, and when contained, the content of Mg can 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. If Mg is contained in excess, the deterioration of the surface properties can sometimes be noticeable, and thus the content of Mg is set to 0.100% or less. The content of Mg can 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.

[0047] (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 improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Zr is 0. The effect can be obtained even with a small amount, and when contained, the content of Zr can be 0.001% or more, 0.03% or more, 0.005% or more, 0.08% or more, or 0.010% or more. If Zr is contained in excess, the deterioration of the surface properties can sometimes be noticeable, and thus the content of Zr is set to 0.100% or less. The content of Zr 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.

[0048] (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 improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Hf is 0. The effect can be obtained even with a small amount, and when contained, the content of Hf can be 0.001% or more, 0.002% or more, or 0.005% or more. If Hf is contained in excess, the deterioration of the surface properties can sometimes be noticeable, and thus the content of Hf 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.

[0049] (REM: 0 to 0.100%) REM (rare earth element) is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and has an effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of REM is 0. The effect can be obtained even with a small amount, and the content of REM when contained can be 0.0001% or more, 0.005% or more, or 0.001% or more. If REM is contained in excess, the deterioration of surface properties can sometimes be noticeable, and thus the content of REM is set to 0.100% or less. The content of REM can 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. In addition, REM is an abbreviation of Rare Earth Metal, and refers to an element belonging to the lanthanide series. REM is generally added as a mixed rare earth metal.

[0050] (residual portion) In the steel sheet that constitutes the welded joint of the present embodiment, the residual portion other than the above-described chemical components is Fe and impurities. Here, the so-called impurities refer to components that are mixed due to various reasons in the manufacturing process, with raw materials such as ores and scrap iron and the like as representatives, at the time of industrially manufacturing a steel sheet, and are components that can be contained within a range that does not adversely affect the LME resistance at the time of manufacturing the welded joint of the present embodiment, i.e., a range in which the LME resistance required for the steel sheet of the present embodiment can be obtained. As specific elements, for example, O (oxygen) can be cited. The content of O contained as impurities 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 content of O can be 0.00001% or more, 0.00005% or more, or 0.0001% or more.

[0051] (analytical method of chemical components) For the analysis of the chemical components of the steel sheet, an elemental analysis method known to those skilled in the art can be used, for example, by an inductively coupled plasma mass spectrometry (ICP-MS method). Among these, for C and S, a combustion-infrared absorption method can be used for measurement, and for N, a non-active gas melting-thermal conductivity method can be used for measurement. For O, a non-active gas melting-infrared absorption method is used. These analyses can be performed using a sample collected from the steel sheet by a method according to JIS G0417: 1999.

[0052] [high ferrite layer] In the high-strength steel sheet constituting the welded joint of the present embodiment, a high ferrite layer exists at a position 50 μm outward from the end of the pressure welding portion, with a thickness of 5 μm or more in the thickness direction of the high-strength steel sheet from the surface of the high-strength steel sheet. Here, the high ferrite layer refers to a structure in which the area ratio of ferrite phase is 90% or more. Also, the position 50 μm outward from the end of the pressure welding portion can be regarded as a "heat-affected portion". The welded joint of the present embodiment is characterized in that the heat-affected portion affected by welding heat at the time of manufacture of the welded joint also has a high ferrite layer formed at the time of manufacture of the steel sheet.

[0053] Figure 1 An example of an SEM-based microstructure photograph of the surface layer vicinity at a position 50 μm outward from the end of the pressure welding portion of the high-strength steel sheet constituting the welded joint of the present embodiment is shown. Figure 1 is a cross section in the thickness direction of the steel sheet, with the upper side being the surface of the steel sheet. As shown in Figure 1 , in the welded joint of the present embodiment, at the heat-affected portion resulting from welding, a high ferrite layer 1 in which the area ratio of ferrite phase is 90% or more exists on the surface side of the steel sheet. By the presence of the B concentration portion described later in this layer, the LME resistance at the time of manufacture of the welded joint is improved.

[0054] If the thickness of the high ferrite layer is 5 μm or more, an effect of improved LME resistance can be obtained, and thus the upper limit of the thickness is not particularly limited. For example, it can be 100 μm or less, 80 μm or less, 60 μm or less, 40 μm or less. From the viewpoint of LME resistance, the thickness of the high ferrite layer is preferably relatively thick, 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, 25 μm or more.

[0055] The structure other than ferrite in the high ferrite layer is not limited. For example, it can be any one or more of martensite, bainite, cementite.

[0056] The thickness of the high ferrite layer is obtained by observing the cross section of the plate thickness of the steel sheet by nitric acid ethanol etching at a magnification of 1000 times by SEM, and distinguishing between hard structures containing much cementite such as martensite and bainite, and ferrite, based on the microstructure. Regarding the thickness of the high ferrite layer, a range of 500 μm in a direction perpendicular to the plate thickness direction is taken as the measurement range, and measurement is performed at intervals of 1000 μm in the direction perpendicular to the plate thickness direction in 5 measurement ranges, and the average value is taken. Here, the area ratio of ferrite refers to the area ratio obtained by observing the cross section of the plate thickness as described above. In the case of observing a local portion in the middle of the thickness direction, for example, even if there is a portion in which the area ratio of ferrite is less than 90%, it is not a problem as long as the area ratio of ferrite is 90% or more in the cross section of the plate thickness from the surface to a depth of 5 μm.

[0057] The ferrite area ratio is found by cutting a cross section of the steel sheet in a direction orthogonal to the rolling direction, mirror-polishing it, developing the steel structure using a nitric acid ethanol solution, and taking a secondary electron image using a field emission scanning electron microscope. The observation position is set to a range of 500 μm from the surface of the sheet thickness or from the interface of the plated layer and the steel sheet, and 5 fields of view are observed at equal intervals. For the obtained structure photograph, the fraction of each structure is calculated by a point counting method. More specifically, first, an equidistant grid is drawn on the structure photograph. Next, it is determined which of tempered martensite, pearlite, ferrite, primary martensite or residual austenite, or bainite the structure of each grid point corresponds to. By finding the number of grid points corresponding to each structure and dividing by the total number of grid points, the fraction of each structure can be determined. The more the total number of grid points, the more accurately the area ratio can be found. In the present embodiment, the grid interval is set to 2 μm x 2 μm, and the total number of grid points is set to 1500 points.

[0058] A criterion for determining tempered martensite, pearlite, ferrite, primary martensite or residual austenite, or bainite is shown. A region in which the lower structure (lath boundary, block boundary) is present within the grain and the carbide is precipitated in a plurality of variants is determined to be tempered martensite. In addition, a region in which cementite is precipitated in a layered form is determined to be pearlite. A region in which the brightness is small and no lower structure is confirmed is determined to be ferrite. A region in which the brightness is large and no lower structure is developed by etching is determined to be primary martensite or residual austenite. A region which does not correspond to any of the above is determined to be bainite. In short, if ferrite and structures other than ferrite are distinguished, the area ratio of the ferrite phase can be found.

[0059] Ferrite is a structure having low LME sensitivity compared to austenite, and by providing a high ferrite layer in the surface layer of the steel sheet, the LME resistance at the time of manufacturing the welded joint is improved. In the present embodiment, the LME resistance is further improved by the presence of the B concentration portion described later.

[0060] [B concentration portion] In the high-strength steel sheet constituting the welded joint of the present embodiment, a B concentration portion exists at a position 50 μm outward from the end of the pressure welding portion, with a thickness of 1.0 μm or more in the thickness direction of the high-strength steel sheet from the surface of the high-strength steel sheet. Here, the B concentration portion refers to a portion in which the B intensity found in TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) measurement is 2 times or more the B intensity at a position 50 μm in depth found in TOF-SIMS measurement. Figure 2 An example of the measurement result based on TOF-SIMS is shown. Figure 2The results of measurement of the C section of the portion where the two steel sheets overlap in the welded joint are shown in FIG. 1. The up-and-down direction is the thickness direction of the steel sheet. Figure 2 The brighter the results of measurement of the B concentration, the higher the B concentration. The dark portion near the center is the gap between the two steel sheets. From the results of measurement of the B concentration, Figure 2 From the results of measurement of the B concentration, it is found that the B concentration becomes high near the surface of the steel sheet.

[0061] The analysis based on TOF-SIMS was performed using a TOF-SIMS (manufactured by ION-TOF) as the device, under the conditions of primary ions: Bi3 2+ , acceleration voltage: 25 kV, measurement area: 50 μm square. The measurement was performed in a manner that the B concentration portion on the surface of the high-strength steel sheet and the position at a depth of 50 μm from the surface were measured, and the field of view was moved in the thickness direction of the high-strength steel sheet.

[0062] From the viewpoint of the resistance to LME at the time of manufacture of the welded joint, the thicker the B concentration portion, the better, and the thickness is preferably 2.0 μm or more, more preferably 3.0 μm or more, 4.0 μm or more, 5.0 μm or more.

[0063] In the welded joint of the present embodiment, it is considered that the B concentrated in the surface layer of the steel sheet constituting the welded joint is segregated at the Fe grain boundary, the invasion of Zn is suppressed, and thus the LME at the time of manufacture of the welded joint is suppressed. By manufacturing the steel sheet from molten steel having the above-described chemical composition using the manufacturing method described later, the B distribution of the surface layer can be obtained.

[0064] [Plating layer] The high-strength steel sheet constituting the welded joint of the present embodiment can have a plating layer containing Zn on the surface. The plating layer can be formed on only one side of the high-strength steel sheet, or can be formed on both sides, and in addition, the plating layer can be formed on only a part of the side.

[0065] The plating layer is not particularly limited as long as it contains Zn. As an example, Zn-0.2% Al (GI), Zn-(0.3-1.5)% Al, Zn-4.5% Al, Zn-0.09% Al-10% Fe (GA), Zn-1.5% Al-1.5% Mg, Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, Zn-15% Mg, Zn-20% Al-7% Mg, Zn-30% Al-10% Mg can be listed.

[0066] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which a corrosion inhibitor for suppressing corrosion of the steel sheet is added, and measuring the obtained solution by ICP (high-frequency inductively coupled plasma) emission spectrometry.

[0067] The thickness of the plated layer can be, for example, 3 to 50 μm. In addition, the amount of adhesion of the plated layer is not particularly limited, and can be, for example, 10 to 170 g / m 2 In the present application, the amount of adhesion of the plated layer is determined by dissolving the plated layer in an acid solution to which an inhibitor for suppressing corrosion of the steel sheet is added, and from the change in weight before and after pickling and stripping of the plated layer. As the acid solution to which the inhibitor is added, for example, a 10% hydrochloric acid solution to which 0.06 mass% of the inhibitor (IBIT 710K manufactured by Asahi Denka Kogyo Co., Ltd.) is added can be used. The base steel sheet after removal of the plated layer is washed with water and dried.

[0068] The thickness of the plated layer can be 5 μm or more, 7 μm or more, or 10 μm or more. The thickness of the plated layer can be 45 μm or less, 40 μm or less, 35 μm or less, or 30 μm or less. The amount of adhesion of the plated layer can be 15 g / m 2 or more, 20 g / m 2 or more, 25 g / m 2 or more, 30 g / m 2 or more. The amount of adhesion of the plated layer can be 160 g / m 2 or less, 140 g / m 2 or less, 120 g / m 2 or less, 100 g / m 2 or less.

[0069] In the case where the welded joint includes a general steel sheet or the like that is not the high-strength steel sheet described above, the steel sheet that is not the high-strength steel sheet can also have the plated layer described above.

[0070] Note that, for the welded joint of the present embodiment, even in the case where the steel sheet that constitutes the welded joint, particularly the high-strength steel sheet disposed at the outermost side, does not have a plated layer, the effect of improving the resistance to LME at the time of manufacture is exhibited. In the absence of molten zinc, an LME crack is not generated, but, for example, in the case where a steel sheet that has not been plated is welded using a welding electrode obtained by spot welding a steel sheet that has a zinc plated layer, the plated layer adhering to the welding electrode melts and is transferred to the steel sheet that has not been plated, and thus an LME crack is sometimes generated. According to the welded joint of the present embodiment, even in such a case, the intrusion of molten zinc into the Fe grain boundaries is suppressed by the B concentrated in the surface layer, and thus the effect of suppressing LME cracks at the time of manufacture of the welded joint is exhibited. Therefore, in the welded joint of the present embodiment, plating is not an essential constituent element.

[0071] [B distribution of surface layer] In the welded joint of the present embodiment, in the non-heat affected portion of the steel sheet, particularly the high-strength steel sheet, which constitutes the welded joint, the depth of the following formula (1) is preferably 1.5 μm or more from the surface of the steel sheet in the GDS (high-frequency glow discharge emission spectroscopy) measurement in the thickness direction of the steel sheet.

[0072] Bx / B150 ≥ 5 (1) In the above formula (1), Bx represents the emission intensity at a point in the thickness direction of the steel sheet at a depth x (μm) from the interface between the steel sheet and the plating layer. In addition, B150 represents the emission intensity at a depth of 150 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer. Note that the "thickness direction" in the present embodiment refers to the direction perpendicular to the interface between the steel sheet and the plating layer. The point in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer refers to the point in the direction toward the center of the thickness of the steel sheet.

[0073] The left side of the above formula (1) represents the ratio of the B concentration at the depth x to the B concentration at the depth of 150 μm. That is, Bx / B150 ≥ 5 means that the B concentration at the depth x is 5 times or more the B concentration at the depth of 150 μm. The B concentration at the depth of 150 μm can be regarded as the B concentration at the center of the thickness of the steel sheet, and Bx / B150 ≥ 5 means that the B is concentrated at the depth x. The depth of 1.5 μm or more from the surface of the steel sheet at which the formula (1) is satisfied means that the formula (1) is satisfied in the range from the surface of the steel sheet to the depth of 1.5 μm or more in the thickness direction of the steel sheet, and means that the B is concentrated in the range to the depth of 1.5 μm or more in the thickness direction of the steel sheet.

[0074] The surface of the steel sheet and the interface between the steel sheet and the plating layer in the present embodiment are determined as follows. First, the Fe content in the thickness direction of the plated steel sheet is measured by GDS measurement. The value of the highest Fe content is taken as the Fe content of the steel sheet. The point at which the Fe content is 5% of the Fe content of the steel sheet is defined as the "surface of the steel sheet". In the case where the plating layer is formed on the surface of the steel sheet, the interface between the steel sheet and the plating layer is regarded as the surface of the steel sheet, and is taken as the starting point of the depth for the GDS measurement. The "interface between the steel sheet and the plating layer" is defined as the position at which the Fe content measured by GDS measurement is 93% of the Fe content of the steel sheet.

[0075] As described above, by the concentration of B in the surface layer of the steel sheet, the B segregates at the Fe grain boundaries of the surface layer structure. Thereby, the penetration of molten zinc into the Fe grain boundaries at the time of spot welding can be suppressed, and the occurrence of LME at the time of manufacture of the welded joint can be further suppressed.

[0076] (Measurement method of GDS) Bx, B150 were measured using the following method: by GDS, the surface of the steel sheet as the object was made to be in an Ar atmosphere, in a state where a voltage was applied to generate glow plasma, and the surface of the steel sheet was sputtered while being analyzed in the depth direction. Then, based on the wavelength of the light emission spectrum specific to the element, which is emitted when an atom is excited in the glow plasma, the element contained in the material was identified, and the light emission intensity of the identified element was estimated.

[0077] The data in the depth direction can be estimated from the sputtering time. Specifically, by previously using a standard sample to find the relationship between the sputtering time and the sputtering depth, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The sputtering time was set in such a manner that at least the sputtering depth exceeds 150 μm.

[0078] The GDS was measured in the sheet thickness direction 5 times, and the average thereof was taken as the B concentration. The measurement conditions are described below. The B concentrations at depths of x (μm) and 150 μm were Bx, B150, respectively.

[0079] Apparatus: High-frequency glow discharge emission spectrometer (LECO Japan Co., Ltd., Model "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds In order to obtain the effect of suppressing the occurrence of LME, the depth satisfying Bx / B150 ≥ 5.0 is preferably 1.5 μm or more. From the viewpoint of LME resistance, the larger the Bx / B150 is, the more preferable it is, and it can 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. Bx / B150 is the ratio of the B concentration at a depth of x to the B concentration at a depth of 150 μm, and therefore the depth satisfying Bx / B150 ≥ 5.0 is less than 150 μm. Even if the depth satisfying Bx / B150 ≥ 5.0 is deep, the LME resistance does not decrease, but the depth satisfying Bx / B150 ≥ 5.0 can 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 the present embodiment, the B concentrated in the surface layer of the steel sheet constituting the welded joint is segregated to the Fe grain boundary. It is considered that thereby the molten zinc is suppressed from invading the Fe grain boundary at the time of spot welding, and the LME at the time of manufacturing the welded joint is suppressed. By manufacturing the steel sheet from molten steel having the above-described chemical composition using the manufacturing method described later, such a B distribution in the surface layer can be obtained.

[0081] B concentration in the range from the surface of the steel sheet to a depth of 5.0 μm In the steel sheet constituting the welded joint of the present embodiment, in the above-described GDS measurement, it is preferable to further satisfy the following formula (2).

[0082] Bmax / B150≥ 8 (2) In the above-described formula (2), Bmax represents the maximum value of the emission intensity of B in the range from the surface of the steel sheet to a depth of 5.0 μm.

[0083] The left side of formula (2) represents the ratio of the maximum value of the emission intensity of B in the range from the surface of the steel sheet to a depth of 5.0 μm to the B concentration at a depth of 150 μm. That is, Bmax / B150≥ 8 means that the B concentration at the position where B is most concentrated in the range from the surface of the steel sheet to a depth of 5.0 μm is 8 times or more the B concentration at a depth of 150 μm. The B concentration at a depth of 150 μm can be regarded as the B plate-thickness center concentration of the steel sheet, and Bmax / B150≥ 8 means that the concentration of B is large in the range from the surface of the steel sheet to a depth of 5.0 μm, that is, in the vicinity of the surface.

[0084] From the viewpoint of LME resistance, Bmax / B150 is preferably large, and is preferably 10 or more, more preferably 12 or more, 14 or more, or 16 or more.

[0085] It is not necessary for the steel sheet of the present embodiment to satisfy formula (2), and even if formula (2) is not satisfied, as long as formula (1) is satisfied, good LME resistance can be obtained. By providing the surface layer so as to satisfy formula (2), B that is more concentrated in the surface layer of the steel segregates at the Fe grain boundaries, and the invasion of molten zinc can be suppressed, and thus the effect of suppressing LME can be more greatly obtained.

[0086] [Plate thickness] The plate thickness of the steel sheet constituting the welded joint of the present application 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, 2.5 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0087] Method for manufacturing steel sheet Next, a method for manufacturing the steel sheet constituting the welded joint of the present embodiment will be described.

[0088] The steel sheet constituting the welded joint of the present embodiment can be obtained by a manufacturing method including: a casting step of casting molten steel in which a chemical composition is adjusted to form a steel bloom; a hot rolling step of hot-rolling the steel bloom to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet; an acid pickling step of acid-pickling the hot-rolled steel sheet after coiling; a cold rolling step of cold-rolling the hot-rolled steel sheet after acid pickling to obtain a cold-rolled steel sheet; and an annealing step of annealing the cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet can be acid-pickled without being coiled after the hot rolling step and directly cold-rolled.

[0089] [Casting Step] The conditions of the casting step are not particularly limited. For example, after melting by a blast furnace, an electric furnace, or the like, various secondary smelting is performed, and then casting is performed by a method such as a general continuous casting, a casting by an ingot casting method, or the like.

[0090] [Hot Rolling Step] The steel bloom obtained by casting can be hot-rolled to obtain a hot-rolled steel sheet. The hot rolling step is performed by hot-rolling the steel bloom after casting, directly or after temporary cooling, and then reheating. In the case of reheating, the heating temperature of the steel bloom can be, for example, 1100 to 1250°C. In the hot rolling step, rough rolling and finish rolling are generally performed. The temperature and reduction rate of each rolling can be appropriately changed depending on the desired metal structure and sheet thickness. For example, the finish rolling can be performed at a finish rolling end temperature of 900 to 1050°C, and the reduction rate of the finish rolling can be 10 to 50%.

[0091] [Coiling Step] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately changed depending on the desired metal structure or the like, and can be, for example, 500 to 800°C. A predetermined heat treatment can be applied to the hot-rolled steel sheet before or after coiling. Alternatively, the hot-rolled steel sheet can be acid-pickled without the coiling step after the hot rolling step and cold-rolled as described later.

[0092] [Acid Pickling Step] The steel sheet after hot rolling is subjected to acid pickling. In the manufacturing method of the steel sheet constituting the welded joint of the present embodiment, the surface of the steel sheet after acid pickling is controlled with the aim of concentrating B in the surface layer of the steel sheet in the annealing step described later. Specifically, the surface of the steel sheet after acid pickling has a concave-convex shape with an arithmetic average height Ra of 1.5 μm or more as defined in JIS B0601:2013. This condition means that there is a certain degree of concave-convex shape on the surface of the steel sheet. If the concave-convex shape is small, the strain imparted to the surface layer of the steel sheet is small, and thus even if annealing at a high dew point is performed as described later, internal oxidation does not proceed, and the concentration of B in the surface layer of the steel sheet does not proceed.

[0093] The larger the Ra of the concave-convex is, the more preferable it is, and it is preferably 2.0 μm or more, more preferably 2.5 μm or more, 3.0 μm or more, 3.5 μm or more.

[0094] The concave-convex of the surface of the steel sheet after pickling is found as follows: 10 places are randomly selected on the surface on the side of the surface layer portion in accordance with JIS B 0601:2013, the surface profile is measured at each place using a contact type surface roughness meter, and the surface roughness at these places is arithmetically averaged to find the arithmetic average roughness Ra. The concave-convex of the surface of the steel sheet after pickling does not need to be measured all the time, and the measurement of the concave-convex can be omitted after the pickling conditions to form the desired concave-convex are determined.

[0095] The roughness of the surface of the steel sheet varies depending on the pickling conditions, and thus it is appropriately adjusted in a manner to obtain the above-described concave-convex. 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 less than 30 to 200 seconds.

[0096] The surface of the steel sheet after pickling has such a concave-convex, and the concave-convex is rolled in the subsequent cold rolling step to impart strain to the surface layer of the steel, thereby promoting the concentration of B in the surface layer of the steel sheet in the subsequent annealing step.

[0097] [Cold rolling step] The hot-rolled steel sheet can be subjected to pickling or the like and then cold-rolled to obtain a cold-rolled steel sheet. In the cold rolling step, the concave-convex imparted in the above-described pickling step is flattened by rolling to impart strain to the surface layer of the steel sheet. Therefore, the roll used for cold rolling is preferably a roll having a small surface roughness, and the surface roughness of the roll is preferably 1.0 μm or less in terms of Ra. The surface roughness of the roll can be 0.8 μm or less, 0.6 μm or less, or 0.5 μm or less in terms of Ra. The reduction rate of cold rolling can be appropriately changed depending on the desired metal structure and sheet thickness, and can be, for example, 20 to 80%. After the cold rolling step, air cooling to room temperature can be performed, for example.

[0098] Strain is imparted to the surface layer of the steel sheet by rolling the concave-convex of the surface of the hot-rolled steel sheet in the cold rolling step, and thereby the concentration of B in the surface layer of the steel sheet is promoted in the subsequent annealing step.

[0099] [Annealing step] After the cold rolling step, the obtained cold-rolled steel sheet is subjected to the following high dew point annealing. In the manufacturing method of the steel sheet and the galvannealed steel sheet of the present embodiment, in the annealing step, external oxidation is not performed on the surface of the steel sheet, and internal oxidation toward the inside of the steel sheet is performed on the surface layer of the steel sheet.

[0100] In the annealing step of the present embodiment, the steel sheet to which the strain is imparted to the surface layer by the above-described steps is maintained at a high dew point. Specifically, in order to perform internal oxidation and promote the concentration of B, the holding temperature of the annealing step is set to 760 to 900°C, and the holding time at the holding temperature is set to 0 to 360 seconds. The holding temperature can be 770°C or higher, 780°C or higher, or 790°C or higher. The holding temperature can be 890°C or lower, 880°C or lower, or 870°C or lower. The holding time can be 10 seconds or more, 30 seconds or more, 50 seconds or more, or 60 seconds or more. The holding time can be 330 seconds or less, 300 seconds or less, 270 seconds or less, 240 seconds or less, or 200 seconds or less.

[0101] The atmosphere in the annealing is preferably a non-oxidizing atmosphere, and can be set to N2-2 to 4 vol% H2, for example. The oxygen concentration of the atmosphere is preferably 50 ppm or less, and can be 30 ppm or less, 20 ppm or less, or 10 ppm or less. By setting to such conditions, it is possible to perform internal oxidation while suppressing oxidation of the surface of the plated layer. The temperature increase rate until the holding temperature is not particularly limited. The temperature increase rate can be 1 to 10°C / second, for example. If the temperature increase rate is less than 1°C / second, it takes too much time to increase the temperature to the control temperature, and the oxide on the surface of the plated layer can become thick. On the other hand, if the temperature increase rate exceeds 10°C / second, internal oxidation cannot be sufficiently performed, the strain imparted to the surface layer of the steel sheet cannot be sufficiently released, and the concentration of B can not be sufficient. From these viewpoints, the temperature increase rate can be 2°C / second or more, 3°C / second or more, or 4°C / second or more. The temperature increase rate can be 9°C / second or less, 8°C / second or less, or 7°C / second or less.

[0102] In the annealing step in the method for manufacturing the steel sheet that constitutes the welded joint of the present embodiment, the dew point of the atmosphere is changed in the first half and the second half of the temperature increase. Specifically, the dew point from room temperature to the control temperature is made different from the dew point from the control temperature to the holding temperature.

[0103] The control temperature is the changed temperature of the dew point, and is set to 450 to 550°C. During the temperature increase 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 control temperature to the holding temperature, the dew point of the annealing atmosphere is set to more than -20°C and 20°C or lower.

[0104] If the control temperature is lower than 450°C, the dew point rises at a low temperature, and thus internal oxidation is performed at a low temperature, the strain imparted to the surface layer of the steel sheet is released, and sufficient internal oxidation for suppressing the boron loss phenomenon does not occur during the control temperature to the holding temperature. If the control temperature exceeds 550°C, the strain imparted to the surface layer of the steel sheet is released before internal oxidation is performed at a high temperature, and sufficient internal oxidation for suppressing the boron loss phenomenon does not occur.

[0105] When the dew point during the period from the room temperature to the control temperature is below -40°C, Si and Mn are externally oxidized, and internal oxidation during the period from the control temperature to the holding temperature can not be performed. When the dew point during the period from the room temperature to the control temperature exceeds -20°C, internal oxidation is performed at a low temperature, and thus strain imparted to the surface layer of the steel sheet is released. As a result, internal oxidation for suppressing the boron loss phenomenon can not occur during the period from the control temperature to the holding temperature.

[0106] Likewise, when the dew point during the period from the control temperature to the holding temperature is -20°C or less, internal oxidation for suppressing the boron loss phenomenon can not be sufficiently performed. When the dew point during the period from the control temperature to the holding temperature exceeds 20°C, external oxidation is performed, and internal oxidation for suppressing the boron loss phenomenon can not be sufficiently performed.

[0107] Further, the dew point during the period from the control temperature to the holding temperature is higher than the dew point during the period from the room temperature to the control temperature by 10°C or more. Thus, internal oxidation is performed, and concentration of B is promoted.

[0108] From the viewpoint of appropriately performing concentration of B, the control temperature can be 460°C or higher, 470°C or higher, or 480°C or higher. The control temperature can be 540°C or lower, 530°C or lower, or 520°C or lower. From the viewpoint of appropriately performing internal oxidation and concentration of B, the dew point during the period from the room temperature to the control temperature can be -38°C or higher, -37°C or higher, or -35°C or higher. The dew point during the period from the room temperature to the control temperature can be 18°C or lower, 17°C or lower, or 15°C or lower. The dew point during the period from the control temperature to the holding temperature can be -18°C or higher, -17°C or higher, or -15°C or higher. The dew point during the period from the control temperature to the holding temperature can be 18°C or lower, 17°C or lower, or 15°C or lower.

[0109] In the state where strain is imparted to the surface layer of the steel sheet by the above-described method, the dew point is increased above the control temperature in the annealing step, and thus internal oxidation is sharply performed in the surface layer of the steel sheet, B is taken into oxides formed in the inside of the steel sheet, and thus B is concentrated in the surface layer of the steel sheet, and B is segregated at the Fe grain boundaries, and the above-described concentration distribution of B in the surface layer is obtained.

[0110] The annealing is performed in a state where a tension of 1 to 20 MPa is applied. When a tension is applied at the time of annealing, strain can be more effectively introduced into the steel sheet, and concentration of B in the surface layer is promoted.

[0111] [Plating treatment step] A plated steel sheet can also be manufactured from the steel sheet manufactured as described above by a manufacturing method provided with a plating treatment step. The plating treatment can be performed according to a method known to those skilled in the art. The plating treatment can be performed, for example, by a hot-dip plating method, or by an electroplating method, a vapor deposition method, a sputtering method, a cold spray method. The plating treatment is preferably performed by a hot-dip plating method. The conditions of the plating treatment can be appropriately set taking into account the chemical composition, thickness, and adhesion amount of the plated layer, and the like, of the desired plated layer.

[0112] [Alloying treatment step] After the plating treatment step, a known alloying treatment can be performed as an alloyed plating. The conditions of the alloying treatment step can be appropriately set taking into account the chemical composition, thickness, and adhesion amount of the plated layer, and the like, of the desired plated layer.

[0113] [Spot welding step] The above steel sheet is overlapped by a plurality of pieces, and spot welding is performed to obtain a welded joint. The steel sheets can be the same steel sheet, or can be different steel sheets in terms of chemical composition and the like. As long as at least the steel sheet disposed at the outermost side is the above high-strength steel sheet, the other steel sheets can also be a general commercially available steel sheet.

[0114] The conditions of the spot welding are not particularly limited. For example, a welding electrode having a tip diameter of 8 mm of a dome radius type can be used, and the spot welding can be performed at a pressing force of 5.0 kN, a current-on time of 1.2 seconds, and a current of 12 kA.

[0115] For the welded joint of the present application, since the LME cracking at the time of manufacturing is suppressed, it can be suitably used in a wide range of fields such as automobiles, home electric appliances, building materials, and the like. It is particularly preferable to be used in the field of automobiles.

[0116] Examples Hereinafter, the present application will be described in more detail by examples. The present application is not limited to these examples. First, the manufacturing method of the steel sheet used in the manufacturing of the welded joint of each example will be described.

[0117] [Example No. 1] A molten steel was smelted by a blast furnace, and casting was performed by continuous casting to obtain a steel slab having the chemical composition described in No. 1 of Table 1. The obtained steel slab was heated to 1200°C, the finish temperature of the finish rolling was set to 950°C, and the reduction rate of the finish rolling was set to 30%, and hot rolling was performed to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C.

[0118] For the steel sheet after coiling, pickling was performed using a 5 mass% hydrochloric acid solution at 40°C for 40 seconds. After the pickling was performed, cold rolling was performed at a reduction rate of 50% to obtain a cold-rolled steel sheet. The sheet thickness of the cold-rolled steel sheet was set to 1.6 mm.

[0119] Then, a steel sheet sample was produced by performing an annealing treatment in a furnace having an oxygen concentration of 20 ppm or less under an N2-4 vol% H2 gas atmosphere at a holding temperature of 800°C and a holding time of 0 seconds. The temperature increasing rate at the time of annealing was set to 5.0°C / sec. 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. In addition, the annealing treatment was performed in a state where a tension of 15 MPa was applied. Note that the holding time of 0 seconds means that the temperature was immediately decreased after being increased to 800°C.

[0120] <Example Nos. 2 to 32> A steel sheet was produced under the same conditions as in Example 1 except that the chemical components were those described in Table 1, the annealing treatment conditions, and the plating type were those described in Table 2. In Table 2, the plating type, "a" means that after being immersed in a hot-dip galvanizing bath (Zn-0.14% Al) at 450°C for 3 seconds, the plating layer was attached at an amount of 50 g / m 2 Thereafter, an alloying treatment was performed at 520°C for 30 seconds, "b" means hot-dip galvanizing, and the alloying treatment in "a" was omitted, and "c" means hot-dip galvanizing, the plating bath in "a" was changed to Zn-1.5% Al-1.5% Mg, and the alloying treatment was omitted.

[0121] (Roughness after pickling) When the steel sheet was produced, the roughness of the surface of the hot-rolled steel sheet after pickling was measured. Regarding the roughness of the surface, 10 places were randomly selected on the surface on the side of the surface layer portion, and the surface profile was measured at each place using a contact type surface roughness meter, and the arithmetic average roughness Ra obtained by performing an arithmetic average of the surface roughness of these places was calculated. The roughness of the surface of the hot-rolled steel sheet is shown in Table 2.

[0122] The steel sheet obtained in each example was overlapped by two sheets, and a welded joint was produced by spot welding using a welding electrode of a dome radius type with a tip diameter of 8 mm at an angle of 5°, a pressing force of 5.0 kN, an electric current time of 1.2 seconds, and an electric current of 12 kA, and the microstructure of the heat-affected portion and the non-heat-affected portion and the LME resistance at the time of production were evaluated.

[0123] (Hardness of steel sheet) The hardness of the steel sheet was measured in the non-heat affected portion at a distance of 5 mm or more from the outside end of the spot weld portion of the welded joint. The hardness of the steel sheet was measured at a position of 1 / 2 depth of the steel sheet in accordance with JIS Z 2244:2009. The measurement load was set to 200 gf. The hardness was evaluated as follows. If it was A or more, it was determined that the hardness was good.

[0124] Evaluation AAA: 380 Hv or more Evaluation AA: 300 Hv or more and less than 380 Hv Evaluation A: 240 Hv or more and less than 300 Hv Evaluation B: less than 240 Hv (High ferrite layer) A sample cut to 25 mm x 15 mm from the position 50 μm outside the end of the pressure weld portion of the welded joint was collected, nitric acid ethanol etching was performed, and the thickness of the high ferrite layer in which the ferrite area ratio was 90% or more was measured by observing the T cross section of each sample with an SEM. The thickness was measured at 5 points at equal intervals in a range of 500 μm in the T direction, and the average value was taken. In this case, the starting point of the "thickness" was the surface of the steel sheet in the steel sheet on which plating was not performed, and was the interface between the plated layer and the steel sheet in the steel sheet on which plating was performed, and was determined by the SEM image.

[0125] (B concentration portion) A sample cut to 30 mm x 30 mm from the position 50 μm outside the end of the pressure weld portion of the welded joint was collected, and the thickness of the B concentration portion was measured in TOF-SIMS measurement. The B concentration portion was a portion in which the B intensity found in TOF-SIMS measurement was 2 times or more the B intensity of the position 50 μm in depth found in TOF-SIMS measurement. The analysis based on TOF-SIMS used a TOF-SIMS (manufactured by ION-TOF) as the device, and measurement was performed under the conditions of primary ions: Bi3 2+ , acceleration voltage: 25 kV, measurement area: 50 μm square. Measurement was performed by moving the field of view in the thickness direction of the high-strength steel sheet in a manner that the B concentration portion on the surface of the high-strength steel sheet and the position 50 μm in depth from the surface could be measured.

[0126] (B distribution of the steel sheet surface layer in the non-heat affected portion) The B distribution of the steel sheet surface layer in the non-heat affected portion was evaluated as follows.

[0127] GDS was performed using a sample cut to a size of 50 mm x 50 mm from the non-heat affected portion of each welded joint. GDS measurement was performed 5 times in the sheet thickness direction, and the average value thereof was taken as the B concentration. The measurement conditions were as follows. The B concentrations corresponding to the x (μm) depth and the 150 μm depth were Bx and B150, respectively.

[0128] Apparatus: High frequency glow discharge emission spectrometer (LECO Japan Co., Ltd., Model "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds From the Bx thus obtained, the depth satisfying formula (1): Bx / B150≥5.0 and the value of the left side of formula (2): Bmax / B150 were obtained.

[0129] (Resistance to LME at the time of welded joint production) Two samples cut into a size of 50 mm x 100 mm were collected from each steel sheet and alloyed hot-dip galvanized steel sheet, and for the two samples, spot welding was performed at an angle of 5°, a pressing force of 5.0 kN, a current time of 1.2 seconds, and a current of 12 kA using a round tip radius type welding electrode having a front end diameter of 8 mm to produce a welded joint. Note that, when a steel sheet on which plating was not performed was used to produce a welded joint, a welding electrode obtained by performing spot welding using a steel sheet on which plating containing zinc was performed 10 or more times before was used.

[0130] Reference Figure 3 Evaluation of the resistance to LME at the time of welded joint production will be described. The resistance to LME is evaluated by the length of LME cracking (shoulder crack 21) of the shoulder portion 14 of the welded portion 12 produced by overlapping two steel sheets 11 and performing spot welding. The shoulder portion refers to the inclined portion of the edge of the depression 13 produced by spot welding. From the length of the shoulder crack 21, the evaluation is as follows. In the present embodiment, if A or more is evaluated, it is judged that the resistance to LME at the time of welded joint production is excellent.

[0131] Evaluation AAA: 0 μm Evaluation AA: More than 0 μm and less than 50 μm Evaluation A: 50 μm or more and less than 160 μm Evaluation B: 160 μm or more The results of each evaluation are shown in Table 3.

[0132] Nos. 1 to 22 are inventive examples in which the resistance to LME at the time of welded joint production was confirmed to be excellent.

[0133] In No. 23, the content of Si of the steel sheet is low. Therefore, in the annealing step at the time of manufacturing the steel sheet, decarburization and internal oxidation do not sufficiently proceed, the ferrite layer is thinned, and in addition, the phenomenon of boron loss cannot be suppressed, and a B concentration portion is not formed. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0134] In No. 24, the content of Si of the steel sheet is high. Therefore, even if a ferrite layer and a B concentration portion are formed, LME cannot be suppressed. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0135] In No. 25, the dew point is low in the first half in the annealing step at the time of manufacturing the steel sheet. Therefore, external oxidation of Si and Mn occurs, and internal oxidation does not proceed in the latter half of the annealing step, the ferrite layer is thinned, and in addition, a B concentration portion is not formed. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0136] In No. 26, the dew point is high in the first half in the annealing step at the time of manufacturing the steel sheet. Therefore, internal oxidation proceeds at a low temperature in the first half of the annealing step, the strain imparted to the surface layer of the steel sheet is released, internal oxidation does not occur during the period from the control temperature to the holding temperature, the phenomenon of boron loss cannot be suppressed, and the ferrite layer and the B concentration portion are thinned. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0137] In No. 27, the dew point is low in the latter half in the annealing step at the time of manufacturing the steel sheet. Therefore, internal oxidation for suppressing the phenomenon of boron loss does not sufficiently proceed, and the ferrite layer and the B concentration portion are thinned. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0138] In No. 28, the dew point is high in the latter half in the annealing step at the time of manufacturing the steel sheet. Therefore, external oxidation proceeds, internal oxidation for suppressing the phenomenon of boron loss does not sufficiently proceed, the ferrite layer is thinned, and a B concentration portion is not formed. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0139] In No. 29, the control temperature is low. Therefore, the dew point rises at a low temperature, internal oxidation proceeds at a low temperature, the strain imparted to the surface layer of the steel sheet is released, and sufficient internal oxidation for suppressing the phenomenon of boron loss does not occur during the period from the control temperature to the holding temperature, and the ferrite layer and the B concentration portion are thinned. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0140] In No. 30, the control temperature is high. Therefore, the strain imparted to the surface layer of the steel sheet is released before internal oxidation proceeds at a high temperature, and sufficient internal oxidation for suppressing the phenomenon of boron loss does not occur, and the B concentration portion is thinned. As a result, the LME resistance at the time of manufacturing the welded joint is deteriorated.

[0141] In No. 31, the annealing temperature is low. Therefore, internal oxidation and B concentration are not sufficiently performed, the high ferrite layer is thin, and in addition, a B concentration portion is not formed. As a result, the LME resistance at the time of manufacturing the welded joint is poor.

[0142] In No. 32, the unevenness of the steel sheet surface after pickling at the time of manufacturing the steel sheet is small. Therefore, sufficient strain cannot be imparted to the surface layer of the steel sheet, B does not concentrate in the surface layer of the steel sheet even by high dew point annealing, and the B concentration portion is thin. As a result, the LME resistance at the time of manufacturing the welded joint is poor.

[0143] BRIEF DESCRIPTION OF DRAWINGS 1 high ferrite layer 11 steel sheet 12 welded portion 13 depression 14 shoulder 21 crack of shoulder

Claims

1. A welded joint, characterized in that, It possesses: Multiple overlapping steel plates The fusion core that joins the multiple steel plates A spot weld having a pressure weld portion and a heat-affected portion formed around the weld nugget, The non-heat-affected portion, which is the region outside the heat-affected portion, and Separation portion formed around the pressure weld portion, Among these, at least one of the outermost steel plates is a high-strength steel plate with a Vickers hardness of 240 Hv or higher at the center of the plate thickness. The chemical composition of the high-strength 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. At a position 50 μm outward from the end of the pressure welded portion, there is a high ferrite layer with a ferrite phase area ratio of 90% or more and 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, there exists a B-concentrated portion with a thickness of 1.0 μm or more from the surface of the high-strength steel plate. The B intensity determined in the TOF-SIMS measurement of the B-concentrated portion is more than twice the B intensity at a depth of 50 μm determined in the TOF-SIMS measurement.

2. The welded joint according to claim 1, characterized in that, The high-strength steel plate has a Zn-containing coating formed on one or both sides of the high-strength steel plate.

3. The welded joint according to claim 1 or 2, characterized in that, In the non-heat-affected zone, the luminous intensity Bx at depth x (μm) and the luminous intensity B150 at depth 150 μm, measured by GDS from the steel plate surface toward the thickness direction, satisfy the following formula (1) at a depth of 1.5 μm or more. Bx / B150≥5 (1)。 4. The welded joint according to claim 3, characterized in that, In the non-heat-affected zone, the maximum value Bmax of the luminescence intensity of B in the range of depth 5μm and below in the GDS measurement from the surface of the steel plate to the thickness direction, and the luminescence intensity B150 at a depth of 150μm satisfy the following formula (2). Bmax / B150≥8 (2).

5. The welded joint according to claim 1 or 2, characterized in that, The thickness of the high-ferrite body layer is 10 μm or more.

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