Spot welded member, steel sheet for spot welded member, and method for manufacturing spot welded member

Abstract

The present invention provides: a spot welded member which has excellent LME cracking resistance; a steel sheet for a spot welded member; and a method for manufacturing a spot welded member. If region R1 is a 100 μm square region centered on a point 10 that is separated by 300 µm from an end part 9 of a nugget toward a base material part 3 side, and region R2 is the area having a Zn concentration of more than 0 mass% in the region R1, the ratio Sa / S1 of the length Sa of a grain boundary Ga in which the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V in the region R2 to the length S1 of a grain boundary G1 in the region R2 is 0.50 or more.

Description

Spot-welded component, steel plate for spot-welded component, and method for manufacturing spot-welded component

[0001] The present invention relates to a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component.

[0002] In recent years, due to the growing awareness of environmental issues, carbon dioxide (CO ２ Aiming to simultaneously reduce the weight of automotive materials to reduce CO₂ emissions and improve collision safety performance by increasing the strength of the vehicle body, efforts are being made to increase the strength of automotive steel sheets. For example, the use of high-strength steel sheets with a tensile strength of 980 MPa or more is required as the framework material for automobile cabins. When assembling automobiles using such high-strength steel sheets, from the perspective of efficiency and cost, press-formed steel sheets are often stacked and joined by resistance spot welding.

[0003] Recently, it has been confirmed that cracks occur in spot welds when zinc (Zn)-plated steel sheets are included in a sheet assembly (sheet assembly) during resistance spot welding. Here, the term "Zn-plated steel sheet" refers to a steel sheet having a Zn-based plating layer on the surface of a base steel sheet. The term "Zn-based plating layer" refers to an electrolytic Zn-plated layer, a hot-dip Zn-plated layer (including alloyed hot-dip Zn-plated layer), or a Zn-based alloy plating layer containing elements such as aluminum (Al) and magnesium (Mg) in addition to zinc.

[0004] Because the Zn-based plating layer has a lower melting point than the steel sheet, it melts during spot welding. As a result, when the welding electrode pressure during spot welding or tensile stress due to thermal expansion and contraction of the steel sheet is applied to the spot weld, it is thought that Zn constituting the molten Zn-based plating layer penetrates the grain boundaries of the steel sheet, reducing the grain boundary strength and causing cracks. Because this cracking is caused by the molten Zn-based plating layer, i.e., the Zn constituting the Zn-based plating layer in a liquid state, it is called cracking due to liquid metal embrittlement (LME) (hereinafter referred to as "LME cracking").

[0005] It is known that LME cracking during spot welding is likely to occur when excessive tensile stress is generated in the spot weld due to disturbances during spot welding, etc. In particular, it is known that LME cracking is likely to occur in a region of high local tensile stress on the mating surface side of the spot weld where the sheets come into contact with each other when the pair of welding electrodes are released after current and pressure application during spot welding has ended. LME cracking has become even more of a problem as the strength of steel sheets constituting spot welds has increased.

[0006] For example, Non-Patent Document 1 describes that silicon (Si) in steel affects LME cracking. Si in steel is an element contained in steel to increase the strength and ductility of steel sheets, and high-strength steel sheets with a tensile strength of 980 MPa or more have a high Si content. Therefore, when high-strength steel sheets are included in a sheet assembly that constitutes a spot weld, it is expected that the risk of LME cracking will increase.

[0007] Therefore, various studies have been conducted to prevent LME cracking in spot welds. For example, Patent Literature 1 proposes a spot welding method that includes a step of removing a plating layer before sandwiching and spot welding a plurality of overlapping steel sheets, including at least one steel sheet coated with a plating layer at a welding location on at least one surface, between opposing welding electrodes. Patent Literature 1 describes that this spot welding method can easily prevent the occurrence of liquid metal cracking during spot welding.

[0008] Furthermore, Patent Document 2 proposes a steel sheet having excellent resistance to molten metal embrittlement cracking, which steel sheet has an internal oxide layer in which at least a portion of the grain boundaries are covered with oxide from the surface of a steel sheet base material having a predetermined chemical composition to a depth of 5.0 μm or more, and in which the grain boundary coverage by oxide is 60% or more in a region from the surface of the base material to a depth of 5.0 μm.

[0009] International Publication No. WO 2016 / 159169 International Publication No. WO 2019 / 116531

[0010] D. Bhattacharya et al. Materials Science & Engineering A 823 (2021) 141569

[0011] However, the spot welding method described in Patent Document 1 requires a step of removing the plating layer before spot welding, which poses a problem in terms of workability.

[0012] Furthermore, the steel plate described in Patent Document 2 may have insufficient LME cracking resistance depending on the spot welding conditions, and there is room for improvement.

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component, which are excellent in LME cracking resistance.

[0014] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0015] That is, the gist and configuration of the present invention are as follows.

[0016] [1] A spot-welded component in which at least one set of two adjacent steel plates among a plurality of steel plates are arranged with a Zn-based plating layer present between them, and the plurality of steel plates are spot-welded together, wherein the spot-welded component comprises at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel plates with a Zn-based plating layer present between them, and the spot weld comprises a nugget and a heat-affected zone, and when a region R1 in the spot weld is defined as a region R2 where the Zn concentration exceeds 0 mass%, a ratio Sa / S1 of the length Sa of a grain boundary Ga in region R2 where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V is 0.1 mass% or more to the length S1 of a grain boundary G1 in region R2 is 0.50 or more. Here, the region R1 is a 100 μm square region in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget, and centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion.

[0017] [2] The spot-welded component according to [1], wherein the grain boundary Ga includes a prior austenite grain boundary and / or a phase interface with a different phase.

[0018] [3] A steel sheet for a spot-welded member, the steel sheet constituting one of two adjacent steel sheets in a spot-welded member formed by spot-welding at least one set of a plurality of steel sheets, the plurality of steel sheets being arranged with a Zn-based plating layer therebetween, wherein a surface layer portion from the surface of the steel sheet to a depth of 5 μm in the sheet thickness direction contains a solid solution region R3 in which one or more elements selected from the group consisting of Nb, Mo, and V are solid-solved, and / or precipitates R4 having a particle size of 20 nm or less and containing one or more elements selected from the group consisting of Nb, Mo, and V, and the sum of the sum of the average contents of Nb, Mo, and V contained in the solid solution region R3 and the average contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass% or more.

[0019] [4] The steel sheet for spot-welded components according to [3], further comprising a Zn-based plating layer on the surface.

[0020] [5] A method for manufacturing a spot-welded component by arranging at least one set of two adjacent steel plates among a plurality of steel plates with a Zn-based plating layer present between them, and spot-welding the plurality of steel plates, comprising: arranging, of at least the two adjacent steel plates, the steel plate for spot-welded components described in [3] on the steel plate side that is subjected to more stress during spot welding, so that the surface of the steel plate for spot-welded components described in [3] is in contact with the Zn-based plating layer arranged on the surface of the other steel plate of the two adjacent steel plates; and spot-welding the plurality of steel plates.

[0021] [6] A method for manufacturing a spot-welded component by arranging at least one set of two adjacent steel plates among a plurality of steel plates with a Zn-based plating layer present between them, and spot-welding the plurality of steel plates, comprising: arranging the steel plate for spot-welded components according to [4] on at least one of the two adjacent steel plates that is subjected to more stress during spot welding, such that the Zn-based plating layer of the steel plate for spot-welded components is in contact with the surface of the other of the two adjacent steel plates; and spot-welding the plurality of steel plates.

[0022] According to the present invention, it is possible to provide a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component, which are excellent in LME cracking resistance.

[0023] 1 is a schematic diagram of a cross section in the plate thickness direction passing through the center of a spot weld of a spot-welded member according to an embodiment of the present invention. FIG.

[0024] (Spot-welded member) Hereinafter, an embodiment of the present invention will be described with reference to the drawings. A spot-welded member according to the present invention is a spot-welded member in which at least one set of adjacent two steel sheets among a plurality of steel sheets are arranged with a Zn-based plating layer present between them, and the plurality of steel sheets are spot-welded together, the spot-welded member comprising at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel sheets with the Zn-based plating layer present between them, the spot weld including a nugget and a heat-affected zone, and wherein, when a region R1 in the spot weld is defined as a region R2 where the Zn concentration exceeds 0 mass %, a ratio Sa / S1 of a length Sa of a grain boundary Ga in region R2 where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V in region R2 is 0.1 mass % or more to a length S1 of a grain boundary G1 in region R2 is 0.50 or more. Here, the region R1 is a 100 μm square region in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget, and centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion.

[0025] First, the present inventors conducted extensive research into spot-welded components having excellent LME cracking resistance, a manufacturing method thereof, and a steel sheet for spot-welded components. The present inventors focused on the structure of spot-welded components obtained by spot-welding a zinc-based plated steel sheet and a steel sheet not having a zinc-based plated layer together, and conducted a detailed investigation into the relationship between the structure and LME cracking of spot-welded components.

[0026] LME cracking during spot welding is a phenomenon that occurs when the Zn-based plating layer at the spot weld melts due to the heat during spot welding, and the heat-affected zone shrinks during the cooling process after spot welding, or when tensile stress is generated by electrode pressure during spot welding, causing Zn in the melted Zn-based plating layer to penetrate and segregate into grain boundaries in the surface layer of the steel sheet.

[0027] In the course of their investigations, the inventors discovered that there are regions where LME cracking is likely to occur, that Zn penetrates into some of the grain boundaries in those regions, causing grain boundary embrittlement and resulting in LME cracking, and that in order to suppress grain boundary embrittlement, one or more elements selected from the group consisting of Nb, Mo, and V penetrate into the grain boundaries and segregate there, thereby suppressing grain boundary embrittlement and thereby suppressing LME cracking. After conducting various studies, they completed the present invention.

[0028] The present invention has been made based on the above findings. However, the present invention is not limited to the following embodiments. Furthermore, the components in the following embodiments include those that are easily replaceable by a person skilled in the art, or those that are substantially identical.

[0029] The spot-welded component of the present invention is a spot-welded component in which at least one set of two adjacent steel sheets among a plurality of steel sheets is arranged with a Zn-based plating layer present between them, and the plurality of steel sheets are spot-welded together.

[0030] 1 shows a schematic diagram of a cross section in the plate thickness direction passing through the center of a spot weld of a spot weld that constitutes a spot-welded member according to one embodiment of the present invention. The spot-welded member 1 includes at least one pair of a spot weld 2 that is the subject of the present invention and a base material portion 3. The spot weld 2 is also composed of a nugget 4 and a heat-affected zone 5.

[0031] 1 is an example of a spot-welded component 1 formed by spot-welding two adjacent steel sheets, in which the first steel sheet (lower sheet) is a zinc-based plated steel sheet with a zinc-based plating layer (not shown), and the second steel sheet (upper sheet) is a steel sheet (without a zinc-based plating layer), and a mating surface is formed where the Zn-based plating layer surface of the lower sheet meets the steel sheet surface (lower surface of the upper sheet), and the spot-welded component 1 includes at least one pair (one in the example of FIG. 1 ) of a spot weld 2 and a base material portion 3 obtained by spot welding. The spot weld 2 is composed of a nugget 4 and a heat-affected zone (HAZ) 5.

[0032] When the spot-welded member 1 of the present invention is formed from two or more steel plates, it is sufficient that the spot-welded member 1 includes at least one pair of a spot-welded portion 2 and a base material portion 3 formed by spot welding two adjacent steel plates, as shown in FIG. 1 , among the plurality of steel plates, with a Zn-based plating layer present between them.

[0033] For example, when n steel sheets (n≧2) are overlapped and spot-welded, a mating surface (1) where the surface of the first steel sheet meets the surface of the second steel sheet, a mating surface (2) where the surface of the second steel sheet meets the surface of the third steel sheet, ..., a mating surface (n−1) where the surface of the (n−1)th steel sheet meets the surface of the n-th steel sheet are formed by spot welding, resulting in n−1 spot welds. In this case, the spot welds formed by spot welding with a Zn-based plating layer present on the mating surfaces are the spot welds 2 that are the subject of the present invention. Note that the spot-welded member 1 of the present invention is required to have at least one spot weld 2 that is the subject of the present invention, and all n−1 spot welds may be the spot welds 2 that are the subject of the present invention.

[0034] The Zn-based plating layer may be provided as a Zn-based plated steel sheet having a Zn-based plating layer on the surface of the steel sheet, as at least one of the multiple (two or more) steel sheets that make up the spot-welded member 1. Of the steel sheets that make up the spot-welded portion 2 that is the subject of the present invention, one or both may be Zn-based plated steel sheets.

[0035] Furthermore, the spot-welded component of the present invention may be formed from two steel plates, or from three or more steel plates, and the upper limit of the number of steel plates (n) is not particularly limited, but may be five or less.

[0036] <Region R1> Region R1 is a 100 μm square region in a plate thickness direction cross section passing through the center 7 of the spot welded portion 2 (i.e., the nugget 4), with the edge 9 of the nugget 4 being the intersection of a line 8 passing through the center 7 of the nugget 4 and the tip 6 of the gap between the two steel plates sandwiching the nugget 4 with the boundary of the nugget 4. The intersection is a point 10 300 μm away from the edge 9 of the nugget 4 toward the base metal portion 3 along the line 8. Region R1 exists within the heat-affected zone 5. Of the four sides of region R1, two sides can be parallel to the plate thickness direction, and the remaining two sides can be perpendicular to the plate thickness direction (parallel to the steel plate surface).

[0037] <Region R2> While investigating the region where LME cracking occurs, the inventors discovered that region R1 shown in FIG. 1 is a region where tensile stress concentrates when the electrode is released during spot welding, and where LME cracking frequently occurs, and that LME cracking can be suppressed by segregating a predetermined element at the grain boundary in region R1.

[0038] The inventors have found that, in particular, when a region R2 in region R1 where the Zn concentration exceeds 0 mass % is designated as region R2, it is important to segregate a predetermined element at the grain boundary within region R2. Specifically, the ratio Sa / S1 of the length Sa of the grain boundary Ga in region R2, where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V in region R2 is 0.1 mass % or more, to the length S1 of the grain boundary G1 in region R2, should be set to 0.50 or more. When Sa / S1 is 0.50 or more, Zn is prevented from penetrating and segregating at the grain boundary, suppressing grain boundary embrittlement and thereby suppressing the occurrence of LME cracking. On the other hand, when Sa / S1 is less than 0.05, Zn is not completely prevented from penetrating the grain boundary, resulting in the occurrence of LME cracking. Here, the grain boundary includes the interface.

[0039] It is known that LME cracking occurs at prior austenite grain boundaries. Furthermore, analysis of grain boundaries at which LME cracking occurs has revealed that LME cracking occurs particularly when the grain boundary where Zn segregates is a phase boundary with a different phase. Therefore, if the grain boundary where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V is 0.1 mass % or more is a prior austenite grain boundary and / or a phase boundary with a different phase, LME cracking is expected to be further suppressed.

[0040] The heat-affected zone 5 is a high-strength steel plate (e.g., a low-alloy carbon steel having a tensile strength of 980 MPa or more), and the area around the nugget 4 within the heat-affected zone 5 (e.g., a region extending from the edge 9 of the nugget 4 to the region R1, i.e., a region within approximately 500 μm square) is a martensite phase. If Zn is present in the surface layer of the steel plate, the Zn diffuses from the steel plate surface, causing the steel plate surface to transform into ferrite, thereby generating a ferrite phase. Therefore, the structure of region R2 contains ferrite and martensite in a total area ratio of 80% or more. The remaining structure may be bainite with an area ratio of less than 20% and / or a Zn-rich phase with a Zn concentration of 50 mass% or more. However, if the Zn-rich phase exceeds 20% in area ratio, the effect of suppressing LME cracking may not be sufficient, which is undesirable.

[0041] Here, the extraction of region R2 within region R1, the length S1 of grain boundary G1 within region R2, the length Sa of grain boundary Ga where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V within region R2 is 0.1 mass% or more, Sa / S1, the area ratios of the ferrite phase, martensite phase, and Zn-rich phase within region R2, and the segregation amounts of Nb, Mo, and V at grain boundary G1 within region R2 are measured as follows.

[0042] First, a sample is cut out so that the observation surface is a cross section in the plate thickness direction passing through the center 7 of the spot weld 2 (i.e., the nugget 4) that constitutes the spot-welded component 1. Next, the observation surface of the sample is roughly polished using waterproof abrasive paper, and then mirror-polished using diamond paste. If water is used during polishing, the Zn-rich phase (here, the region with a Zn concentration of 50 mass% or more) will react with the water and disappear. Therefore, polishing is performed using alcohol or the like without using water (water-free). Furthermore, the obtained polished surface may be subjected to ion milling using Ar ions.

[0043] Region R2 is extracted as a region where the Zn concentration exceeds 0 mass % from the obtained Zn concentration map obtained by using an energy dispersive X-ray spectroscopy (EDS) method (SEM-EDS) using a scanning electron microscope (SEM) so as to include region R1.

[0044] The area ratio of the ferrite phase and the area ratio of the martensite phase in region R2 are calculated by identifying the ferrite phase and the martensite phase from the SEM image of region R2, calculating their areas, and dividing the calculated values ​​by the area of ​​region R2. Here, the area of ​​region R2, the area of ​​the ferrite phase in region R2, and the area of ​​the martensite phase in region R2 are calculated using image processing software such as Adobe Photoshop from Adobe Systems Incorporated.

[0045] The area ratio of the Zn-rich phase in region R2 is calculated by extracting a region where the Zn concentration is 50 mass% or more from the Zn concentration map, calculating the area using the image processing software, and dividing the area by the area of ​​region R2.

[0046] The length S1 of the grain boundary G1 in the region R2, and the lengths Sa and Sa / S1 of the grain boundary Ga in the region R2 where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V is 0.1 mass % or more are calculated by the following procedure.

[0047] The length S1 of the grain boundary G1 in the region R2 is determined by cutting out an analytical sample (e.g., size: approximately 10 μm square) from the region R2 using a focused ion beam (FIB) method, and then performing electron backscattered diffraction (EBSD) measurement on the obtained analytical sample using an SEM. The grain boundary is extracted using data analysis software attached to the EBSD device. The obtained EBSD image data is loaded into the image processing software, and the total length of the grain boundary is calculated.

[0048] The prior austenite grain boundaries are extracted by reconstructing the prior austenite grains using a technique for reconstructing prior austenite grains from the crystal orientation of martensite, as described in, for example, C. Ranger et al., "Austenite Reconstruction Elucidates Prior Grain Size Dependence of Toughness in a Low Alloy Steel," Metall. Mater. Trans. A, Vol. 49A, 4521 (2018), pp. 4521-4535. Specifically, the prior austenite grains can be reconstructed using EBSD data acquired from the martensite structure by using the matrix reconstruction function of the EBSD data analysis software "OIM Analysis ver. 8.6." Grain boundaries were extracted from the obtained reconstructed structure, and the obtained grain boundaries were designated as prior austenite grain boundaries.

[0049] The length Sa of the grain boundary Ga in region R2 where one or more elements selected from the group consisting of Nb, Mo, and V are segregated in a total amount of 0.1 mass% or more is calculated by the following method. First, elemental analysis is performed on the grain boundaries of the analysis sample using energy dispersive X-ray spectroscopy (EDS) (STEM-EDS) using a scanning transmission electron microscope (STEM), and the segregation amounts (concentrations) of Nb, Mo, and V segregated at each grain boundary are analyzed. The total segregation amount of Nb, Mo, and V is calculated, and grain boundaries with a total segregation amount of 0.1 mass% or more are extracted. Then, the length Sa is calculated using the image analysis software. Here, elemental analysis of the grain boundaries within region R2 is performed by tilting the analysis sample so that the grain boundaries to be analyzed are parallel to the direction of electron beam incidence. This is because if the grain boundaries are not parallel to the direction of electron beam incidence, the electron beam will penetrate both the grain boundaries and the parent phase, resulting in measured values ​​of lower element concentrations than the actual concentrations. If the shape of the grain boundaries makes it impossible to tilt the sample parallel to the direction of electron beam incidence, it is preferable to remove the analysis sample and reset it on the sample holder so that it can be tilted parallel. Sa / S1 is calculated from S1 and Sa obtained by the above method.

[0050] <Zn-based plating layer> The Zn-based plating layer is a Zn-based plating layer disposed on the surface of a steel sheet, and a Zn-based plated steel sheet may be provided as at least one of the multiple steel sheets constituting the spot-welded member 1. LME cracking occurs when the spot-welded member 1 is manufactured to include at least one spot weld 2 formed by spot welding two Zn-based plated steel sheets together or by spot welding two overlapping Zn-based plated steel sheets together. Therefore, the spot-welded member 1 of the present invention is intended to be a spot-welded member including at least one pair of a spot weld and a base material portion formed by spot welding two adjacent steel sheets among the multiple steel sheets with a Zn-based plating layer present between them.

[0051] The Zn-based plating layer is not particularly limited as long as it is a plating layer containing Zn. Examples include an electrolytic Zn-based plating layer and a hot-dip Zn-based plating layer. The Zn-based plating layer may contain elements such as aluminum (Al) and magnesium (Mg) in addition to Zn.

[0052] Examples of Zn-based plated steel sheets include electrolytic Zn-based plated steel sheets and hot-dip Zn-based plated steel sheets. In particular, in the automotive field, where LME cracking is a problem, hot-dip Zn-based plated steel sheets are used from the viewpoints of corrosion resistance, manufacturability, and the like. Specific examples include hot-dip Zn-plated steel sheets (GI steel sheets) having a hot-dip Zn-plated layer on the steel sheet surface, and alloyed hot-dip Zn-plated steel sheets (GA steel sheets) having a hot-dip Zn-plated layer on the steel sheet surface. Among these, GA steel sheets are often used in the automotive field, taking into consideration pressability and continuous spot weldability when manufacturing automotive parts, and therefore it is preferable to manufacture spot-welded members using at least one GA steel sheet.

[0053] Furthermore, the risk of LME cracking tends to increase as the strength of the steel sheet used for the spot-welded member 1 increases. Therefore, in the present invention, the tensile strength of the Zn-based plated steel sheet is preferably 980 MPa or more. This allows the effects of the present invention to be more effectively exhibited.

[0054] (Steel Plate for Spot-Welded Member) Next, a steel plate for spot-welded members according to the present invention will be described. The steel plate for spot-welded members according to the present invention is a steel plate for spot-welded members, which is a steel plate for constituting one of the two adjacent steel plates in a spot-welded member formed by spot-welding at least one set of a plurality of steel plates, the plurality of steel plates being arranged with a Zn-based plating layer therebetween, wherein a surface layer portion from the surface of the steel plate to a depth of 5 μm in the plate thickness direction contains a solid solution region R3 in which one or more elements selected from the group consisting of Nb, Mo, and V are solid-solved, and / or precipitates R4 having a particle size of 20 nm or less and containing one or more elements selected from the group consisting of Nb, Mo, and V, and the sum of the average contents of Nb, Mo, and V contained in the solid solution region R3 and the average contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass % or more.

[0055] The steel sheet for spot-welded components used in producing spot-welded components is not particularly limited as long as it is a steel sheet from which the above-mentioned spot-welded components can be produced.

[0056] On the other hand, LME cracking is thought to occur when tensile stress due to the welding electrode pressure during spot welding or thermal expansion and contraction of the steel sheet is applied to the spot weld 2, causing Zn in the Zn-based plating layer to penetrate into the grain boundaries of the steel sheet, resulting in grain boundary embrittlement caused by the Zn. In other words, the Zn reduces the grain boundary strength and causes cracking. Therefore, it is preferable to use the steel sheet for spot-welded components according to the present invention for the steel sheet on the side that is subjected to tensile stress during spot welding, with the side that comes into contact with the Zn-based plating layer as the steel sheet surface. This allows the above-mentioned spot-welded component 1 to be manufactured under typical spot welding conditions.

[0057] Nb, Mo, and V are elements that improve LME cracking resistance. The inventors have found that when these elements are present in a solid solution state in the surface layer of the steel sheet at the time of welding in a content equal to or greater than a predetermined amount, these elements segregate at the grain boundary G1 in region R2, suppressing the penetration of Zn into the grain boundary G1 and making it possible to suppress LME cracking.

[0058] Here, in order to achieve the desired Sa / S1 at the grain boundaries in region R2, it is important to control the structure and composition of the surface layer portion from the steel sheet surface to a depth of 5 μm in the sheet thickness direction. The steel sheet surface layer portion needs to contain one or more elements selected from the group consisting of Nb, Mo, and V in a solid solution state during welding. To achieve this, a solid solution region R3 in which the above elements exist in a solid solution state may be formed in the steel sheet surface layer portion in the steel sheet state. Alternatively, precipitates R4 containing the above elements may be formed in the steel sheet surface layer portion. However, because the precipitates R4 must dissolve during welding, the size of the precipitates R4 must be 20 nm or less in equivalent circle diameter. The equivalent circle diameter of the precipitates is preferably 10 nm or less. The steel sheet surface layer portion may include both the solid solution region R3 and precipitates R4. There is no preferred lower limit for the size of precipitates R4, and it is most preferable that precipitates R4 are in a solid solution state.

[0059] However, the sum of the average values ​​of the Nb, Mo, and V contents contained in the solid solution region R3 and the average values ​​of the Nb, Mo, and V contents contained in the precipitates R4 is set to 0.05 mass% or more. Specifically, (average Nb content contained in the solid solution region R3 + average Mo content + average V content) + (average Nb content contained in the precipitates R4 + average Mo content + average V content) ≥ 0.05 mass% (1). This allows the solid solution region R3 and the precipitates R4 to dissolve during welding, and Nb, Mo, and V penetrate and segregate into the grain boundaries in the steel sheet surface layer, thereby preventing Zn, which causes LME cracking, from penetrating the grain boundaries. Meanwhile, the upper limit of the left side of the above formula (1) is not particularly limited, but from the viewpoint of manufacturing costs, it is preferably set to 0.5 mass% or less.

[0060] The structure and composition of the steel sheet other than the surface layer portion are not particularly limited, and the structure of the region other than the surface layer portion may be the same as the structure of the surface layer portion.

[0061] Regarding the chemical composition of steel sheets, in order to improve various properties such as mechanical properties, for example, solid solution strengthening by adding interstitial solid solution elements such as C and N and substitutional solid solution elements such as Si, Mn, P and Cr, precipitation strengthening by carbonitrides such as Ti, Nb, V and Al, chemical composition modification such as adding strengthening elements such as W, Zr, Hf, Co, B, Cu and rare earth elements, strengthening by recovery annealing at a temperature where recrystallization does not occur or partial recrystallization strengthening where unrecrystallized regions are left without complete recrystallization, strengthening by transformation structure such as single phase bainite or martensite or a composite structure of ferrite and these transformed structures, and strengthening by the Hall-Petch formula: σ = σ where d is the ferrite grain size. ０ +kd-1 / 2 (σ: stress, σ ０ , k: material constant), and processing strengthening by rolling or the like.

[0062] Examples of the composition of the steel sheet include one having 0.1 to 0.4 mass% C, 0.5 to 2.5 mass% Si, 1 to 3 mass% Mn, 0 to 0.05 mass% P, 0 to 0.005 mass% S, and the balance being Fe and unavoidable impurities, and one to which one or more of Cu, Ti, V, Al, Cr, etc. are further added.

[0063] The steel sheet for spot-welded members may further include a Zn-based plating layer on the surface thereof, and the Zn-based plating steel sheet may be supplied as the steel sheet for spot-welded members. In this case, the Zn-based plating layer is provided on the surface layer portion.

[0064] (Method for manufacturing a spot-welded member) Next, a method for manufacturing a spot-welded member according to the present invention will be described. The method for manufacturing a spot-welded member according to the present invention is a method for manufacturing a spot-welded member by arranging at least one set of two adjacent steel sheets among a plurality of steel sheets with a Zn-based plating layer present between them, and spot-welding the plurality of steel sheets, in which the steel sheet for a spot-welded member according to the present invention described above is arranged on the steel sheet that is subjected to more stress during spot welding, so that its surface comes into contact with the Zn-based plating layer arranged on the surface of the other of the two adjacent steel sheets, and spot welding is performed on the plurality of steel sheets.

[0065] Here, the steel sheet that is more subjected to stress during spot welding refers to the steel sheet on the side where LME cracking occurs during spot welding. If it is not known in advance which steel sheet will cause LME cracking, it is preferable to perform a spot welding test in advance to identify which of the two steel sheets to be welded will be more subjected to stress.

[0066] The other of the two steel sheets is a steel sheet having a Zn-based plating layer on its surface. The other steel sheet may or may not be the steel sheet for spot-welded components according to the present invention.

[0067] The steel sheet for spot-welded components of the present invention may be supplied as a zinc-based plated steel sheet having a surface coated with zinc-based plating as the steel sheet to be placed on the side where more stress is applied during spot welding. In this case, the zinc-based plated steel sheet is placed so that the zinc-based plating layer of the zinc-based plated steel sheet is in contact with the surface of the other of the two steel sheets, and spot welding is performed on the multiple steel sheets.

[0068] The other of the two adjacent steel sheets is not particularly limited and may or may not be a steel sheet for spot-welded components according to the present invention. Furthermore, the other steel sheet may or may not have a Zn-based plating layer on its surface. When the other steel sheet has a Zn-based plating layer on its surface, the two adjacent steel sheets are positioned so that the Zn-based plating layers of both steel sheets are in contact with each other, and then spot-welded.

[0069] When both of the two adjacent steel sheets are steel sheets for spot-welded components according to the present invention, as long as a Zn-based plating layer is provided on at least one surface, the spot-welded component according to the present invention can be manufactured without having to specify a steel sheet to which more stress is applied during spot welding. Note that all of the steel sheets constituting the spot-welded component 1 may be steel sheets for spot-welded components according to the present invention.

[0070] By manufacturing a spot-welded component using the above-described method, it is possible to manufacture a spot-welded component having a spot-welded portion with a desired composition and structure, even when spot welding is performed under known spot welding conditions.

[0071] Known spot welding conditions include, for example, a resistance spot welding method in which, when n steel sheets are overlapped, n steel sheets are clamped between a pair of welding electrodes positioned on the surface opposite the mating surface (1) of the first steel sheet and the surface opposite the mating surface (n-1) of the nth steel sheet, and current is applied while applying pressure and controlling it to achieve predetermined welding conditions. For example, resistance spot welding is performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance spot welder, using an electrode (DR type, tip diameter 6 mm) of the resistance spot welder under conditions of a pressure of 3.5 kN, a hold time of 0.1 second, and a welding current and welding time that result in a nugget diameter of 4.5√t mm, where t is the maximum sheet thickness of the n steel sheets.

[0072] A welding device that can be used for the spot welding of the present invention may be a resistance spot welding device equipped with a pair of upper and lower welding electrodes, each capable of freely controlling the welding pressure and welding current during welding. The welding device's pressure mechanism (e.g., air cylinder, servo motor, etc.), type (e.g., stationary, robot gun, etc.), and electrode shape are not particularly limited. Examples of electrode tip types include DR type (dome radius type), R type (radius type), and D type (dome type), as described in JIS C 9304:1999. The tip diameter of a DR type electrode may be, for example, 4 mm to 16 mm.

[0073] (Method for manufacturing steel sheet for spot-welded member) Next, an example of a method for manufacturing a steel sheet for spot-welded member will be described. For example, a clad steel sheet may be manufactured by preparing a clad material and a steel sheet to be laminated together. Specifically, a steel sheet having a thickness of 5 μm or more and containing a solid solution region R3 containing one or more elements selected from the group consisting of Nb, Mo, and V, and / or precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V and having a circle-equivalent diameter of 20 nm or less, wherein the sum of the average contents of Nb, Mo, and V contained in the solid solution region R3 and the average contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass% or more is prepared as the clad material.

[0074] Next, steel sheets to be laminated to the laminated material are prepared. Here, the steel sheets to be laminated are not particularly limited, but it is preferable to prepare steel sheets that are prone to LME cracking. Examples of steel sheets that are prone to LME cracking include steel sheets having a composition with a Si concentration of 0.5 mass% or more. Examples of the steel sheets include cold-rolled steel sheets. The cold-rolled steel sheets may be manufactured by a known manufacturing method. For example, a steel slab having the above-described composition may be hot-rolled to form a hot-rolled steel sheet, and the hot-rolled steel sheet may then be pickled and cold-rolled to form a cold-rolled steel sheet.

[0075] Examples of methods for producing a laminate include the following. First, molten steel having a total content of Nb, Mo, and V of 0.05% by mass or more is produced by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and then the resulting molten steel is solidified to produce a steel material. The method for producing the steel material from the molten steel is not particularly limited, and a continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used. In order to prevent macrosegregation, it is preferable to use a steel slab produced by a continuous casting method as the steel material.

[0076] The obtained steel slab is preferably heated at a heating temperature of 1100°C or higher for a heating time of 1 hour or longer. By setting the heating temperature to 1100°C or higher and the heating time to 1 hour or longer, the Nb, Mo, and / or V contained in the steel slab can be solid-dissolved in the steel slab. On the other hand, if the heating temperature is lower than 1100°C or the heating time is shorter than 1 hour, the solid solution of Nb, Mo, and / or V may be insufficient. The heated steel slab is hot-rolled to obtain a hot-rolled steel sheet. If the finish rolling start temperature is lower than 1000°C, precipitates containing the dissolved Nb, Mo, and / or V may precipitate, resulting in an increase in the particle size of the precipitates. On the other hand, if the finish rolling start temperature exceeds 1200°C, scale loss in the steel slab increases and cracking may occur in the steel slab. Therefore, it is preferable to set the finish rolling start temperature to 1200°C or lower.

[0077] The finish rolling temperature of the hot rolling is preferably 800°C or higher. If the finish rolling temperature is lower than 800°C, Nb, Mo, and / or V dissolved during heating of the steel slab may precipitate as precipitates, such as NbC, VC, and MoC. As a result, when subsequent cold rolling is performed, the rolling load during cold rolling increases, which impedes the cold rolling. Therefore, the finish rolling temperature of the hot rolling is preferably 800°C or higher. The finish rolling temperature of the hot rolling is more preferably 850°C or higher, and even more preferably 870°C or higher. Note that the upper limit of the finish rolling temperature is not particularly limited, but if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases rapidly, causing the interface between the base steel and the oxide to become rough. As a result, the surface quality of the resulting hot-rolled steel sheet tends to deteriorate after pickling or cold rolling. The finish rolling temperature of the hot rolling is more preferably 930°C or less, and further preferably 900°C or less.

[0078] When the obtained hot-rolled steel sheet is subjected to cold rolling, an intermediate heat treatment of 600°C or less may be performed as needed to prevent an increase in load during cold rolling. Furthermore, the obtained hot-rolled steel sheet may be subjected to treatment such as pickling as needed. The pickling method for the hot-rolled steel sheet may be a conventional method. Furthermore, skin-pass rolling may be performed to correct the shape of the hot-rolled steel sheet and improve its pickling properties.

[0079] After hot rolling and / or intermediate heat treatment and / or pickling, the heat treatment may be carried out directly, or after cold rolling, the heat treatment may be carried out.

[0080] Cold rolling may be performed after hot rolling and / or intermediate heat treatment and / or pickling. When cold rolling is performed, the cold reduction is preferably 25% or more, more preferably 30% or more. On the other hand, excessive reduction increases the rolling load and leads to an increase in the load on the cold rolling mill, so the upper limit is preferably 75% or less, more preferably 70% or less.

[0081] After cold rolling, a heat treatment may be performed. When annealing, which is one type of heat treatment, is performed, it is desirable to perform annealing at an average heating rate of 2.0°C / s or more in a temperature range of 600°C or higher. If the average heating rate in a temperature range of 600°C or higher is less than 2.0°C / s, the steel will remain in a temperature range where precipitation and growth of precipitates containing Nb, Mo, and / or V may occur for a long period of time, resulting in the formation of a large amount of precipitates containing Nb, Mo, and / or V with a grain size of more than 20 nm in the final structure.

[0082] That is, the solid solution region R3 in which one or more elements selected from the group consisting of Nb, Mo, and V are dissolved, and / or the precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V and having a circle equivalent diameter of 20 nm or less are reduced, and the average total content of Nb, Mo, and V contained in the solid solution region R3 and the precipitates R4 no longer satisfies the requirement of 0.05 mass% or more, which may result in a deterioration of LME cracking resistance. Therefore, the average heating rate in the temperature range of 600°C or higher is preferably 2.0°C / s or higher. Note that there is no particular upper limit to the average heating rate in the temperature range of 600°C or higher, but in consideration of industrially feasible heating rates, it is preferable to set it to 100°C / s or lower.

[0083] The annealing temperature is preferably 800° C. or higher and 950° C. or lower. If the annealing temperature is lower than 800° C. or higher than 950° C., the growth of precipitates containing Nb, Mo, and / or V is excessively promoted. Therefore, the annealing temperature is set to 800° C. or higher and 950° C. or lower.

[0084] The holding time at the annealing temperature is set to 20 seconds or more and 60 seconds or less. By setting the holding time to 20 seconds or more, the phase transformation to austenite at the annealing temperature can be promoted. On the other hand, if the holding time exceeds 60 seconds, the growth of precipitates containing Nb, Mo, and / or V is also promoted. Therefore, the holding time is preferably 60 seconds or less.

[0085] The steel sheets thus obtained may be used as a laminated material. The resulting laminated material may also be subjected to annealing, pickling, polishing, and the like, as necessary. The above-mentioned laminated materials are laminated so that the surfaces (bonding surfaces) of the steel sheets are vacuum-sealed, and the four peripheries of the bonding surfaces are sealed by welding to assemble. The joining method is not particularly limited, but for example, the laminated material and the steel sheets are joined by laminating them together and then performing electron beam welding (EBW), arc welding, or laser beam welding on the four peripheries of the edge of the laminated material.

[0086] Here, the method of evacuation is not particularly limited, but for example, when laser beam welding the four peripheries of the end of a laminated slab, a vacuum valve is provided between the clad material and the steel plate at the end (before the four peripheries of the end are completely joined), and a vacuum is drawn between the clad material and the steel plate by connecting a vacuum pump to the valve. By using the clad steel plate obtained in this way as a steel plate for spot-welded components, the effects of the present invention can be more reliably obtained.

[0087] As another method for producing a steel sheet for spot-welded components according to the present invention, an Fe-based plating layer may be formed on a surface of the steel sheet, the Fe-based plating layer including a solid solution region R3 in which one or more elements selected from the group consisting of Nb, Mo, and V are dissolved, and / or precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V and having an equivalent circle diameter of 20 nm or less, wherein the sum of the average contents of Nb, Mo, and V contained in the solid solution region R3 and the average contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass % or more.

[0088] Specifically, an Fe-based plating layer is formed on the surface of the steel sheet. The Fe-based plating layer is preferably an Fe-based electroplated layer. The Fe-based electroplated layer preferably contains a solid solution region R3 and / or precipitates R4, with the balance being Fe and unavoidable impurities.

[0089] When the Fe-based plating layer is formed by an electroplating method, for example, the following method can be mentioned. The Fe-based electroplating method is not particularly limited, and may be carried out by a known method. As the Fe-based electroplating bath, for example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both may be used. The Fe ion content in the Fe-based electroplating bath before the start of current application is Fe ２＋ The Fe ion content in the Fe-based electroplating bath is preferably 0.5 mol / L or more. ２＋A sufficient Fe deposition amount can be obtained if the Fe-based electroplating bath has a concentration of 0.5 mol / L or more. Furthermore, the Fe-based electroplating bath must contain at least one element selected from the group consisting of Nb, Mo, and V. These elements may be contained as metal ions. Furthermore, the Fe-based electroplating bath may contain a conductivity aid, a chelating agent, and / or a pH buffer, as needed. For example, in the case of an iron sulfate plating bath, a conductivity aid such as sodium sulfate or potassium sulfate may be contained.

[0090] There is no particular upper limit to the amount of the Fe-based electroplated layer deposited on one side. However, from the viewpoint of cost, the amount of the Fe-based electroplated layer deposited on one side is set to 60 g / m. ２ On the other hand, the lower limit is 40.0 g / m or less, since the thickness of the surface layer of the steel sheet needs to be 5 μm or more. ２ It is preferable to exceed this limit.

[0091] The methods for measuring the contents of Nb, Mo, and V in the solid solution region R3 in the surface layer portion, the contents of Nb, Mo, and V in the precipitates R4, and the size of the precipitates R4 are as follows.

[0092] The measurement method for precipitates R4 with a particle size of 20 nm or less present in the surface layer will be described. The specific procedure is as follows. First, a plurality of test pieces with a size of 20 × 50 mm are cut out from the prepared steel sheet for spot-welded components or the steel sheet for spot-welded components that has been subjected to a Zn-based plating treatment. However, if a Zn-based plating layer is formed on the surface of the steel sheet for spot-welded components, the test pieces are immersed in hydrochloric acid containing an inhibitor to remove the Zn-based plating layer from the surface.

[0093] The obtained test piece was masked except for the surface portion with insulating tape, and two or more electrolytic test pieces were prepared so that electrolytic extraction could be performed only on the surface portion. The amount of dissolution of the test piece was calculated from the relationship between the current and electrolysis time of the electrolytic treatment, and this was divided by the area to convert it into the distance from the surface of the test piece, and electrolysis of the electrolytic test piece was performed so that this distance was 5 μm.

[0094] The electrolytic solution is not particularly limited as long as it dissolves Fe, which is a matrix component of the steel sheet, and allows the precipitate to be extracted, and an electrolytic solution having a conventionally known composition can be used as appropriate, such as 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol.

[0095] First, the first electrolytic test piece (1) was electrolyzed, and the electrolytic test piece (1) obtained after electrolysis was immersed in methanol. Residues adhering to the electrolytic test piece (1) after electrolysis were dispersed in methanol using ultrasonic vibration to obtain a dispersion a (1). Subsequently, the dispersion a (1) and the electrolytic electrolysis solution (1) were filtered using a filter with a pore size of 20 nm, thereby capturing residues with a particle size exceeding 20 nm on the filter. Here, the resulting residues are aggregates of inclusions and precipitates with a particle size exceeding 20 nm contained in the surface layer of the test piece (1) (aggregate (1)). Since aggregate (1) exists as an aggregate, it also contains inclusions and precipitates with a particle size of 20 nm or less. In other words, aggregate (1) can be said to represent all precipitates and inclusions present in the surface layer. The obtained aggregate (1) was subjected to acid decomposition, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. The mass Wa(1) of the electrolysis test piece (1) before electrolysis and the mass Wb(1) of the electrolysis test piece (1) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference between them was defined as the amount of electrolysis W(1). The obtained absolute amounts of Nb, Mo, and V were divided by the amount of electrolysis W(1) to obtain the average values ​​of the contents of Nb, V, and Mo contained in the aggregates (1) present in the surface layer of the electrolysis test piece, i.e., all inclusions and precipitates (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)). It is preferable to perform the above analysis on two or more pieces and use the average values.

[0096] Next, the second electrolysis test piece (2) was subjected to the same electrolysis treatment to obtain a dispersion a (2). The obtained dispersion a (2) and the electrolytic solution (2) after electrolysis were filtered using a filter with a pore size of 20 nm, thereby obtaining an aggregate (aggregate (2)) of inclusions and precipitates having a particle size greater than 20 nm. The obtained aggregate (2) was immersed in hexametaphosphoric acid, and ultrasonic vibration was used to obtain a dispersion b (2) in which the aggregated inclusions and precipitates were dispersed in hexametaphosphoric acid. The obtained dispersion b (2) was filtered using a new filter with a pore size of 20 nm, thereby capturing precipitates and inclusions having a particle size greater than 20 nm (precipitate (2)) on the filter. The obtained precipitate (2) was subjected to acid decomposition, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. The mass Wa(2) of the electrolysis test piece (2) before electrolysis and the mass Wb(2) of the electrolysis test piece (2) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference was designated as the amount of electrolysis W(2). The obtained absolute amounts of Nb, Mo, and V were divided by the amount of electrolysis W(2) to obtain the average values ​​of the contents of Nb, V, and Mo contained in precipitates with a particle size of more than 20 nm contained in the surface layer portion of the electrolysis test piece (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)). It is preferable to perform the above analysis on two or more pieces and calculate the average values.

[0097] Next, the average values ​​of the Nb, V, and Mo contents contained in precipitates having a particle size of more than 20 nm (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)) are calculated from the average values ​​of the Nb, V, and Mo contents contained in all the obtained inclusions and precipitates (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)), to obtain the average values ​​of the Nb, V, and Mo contents contained in precipitates having a particle size of 20 nm or less, i.e., precipitate R4.

[0098] Furthermore, for the third test piece, the average values ​​of all the contents of Nb, V, and Mo contained in the steel sheet (Nb content 3 (mass%), Mo content 3 (mass%), V content 3 (mass%)) were determined by wet chemical analysis. It is preferable to perform the above analysis on two or more sheets and use the average values. Subsequently, the average values ​​of the contents of Nb, V, and Mo contained in the solid solution region R3 of the surface layer portion can be obtained by determining the respective differences of Nb content 1 (mass%), Mo content 1 (mass%), and V content 1 (mass%) from the obtained Nb content 3 (mass%), Mo content 3 (mass%), and V content 3 (mass%).

[0099] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0100] <Preparation of Spot-Welded Members> For the steel sheets for the spot-welded members, galvannealed steel sheets (GA) were used, which were prepared by forming hot-dip galvanized layers on both sides of steel sheet 1 shown in Table 1 and then subjecting the steel sheets to heat treatment. However, for some steel sheets, hot-dip galvanized steel sheets (GI) were used, which were not subject to heat treatment. The spot-welded members (welded joints) were prepared by placing steel sheet 2 on steel sheet 1 shown in Table 1 when two steel sheets were overlapped, or by placing steel sheet 1 in the middle, steel sheet 2 on the upper side of steel sheet 1, and steel sheet 3 on the lower side of steel sheet 1, and then performing resistance spot welding.

[0101]

[0102] The welding equipment used was a servomotor-driven, single-phase AC (50 Hz) resistance spot welder equipped with a welding gun. A pair of chromium-copper DR-type electrodes was used as the electrode tips. The radius of curvature of the tip of the DR-type electrode was 40 mm, and the tip diameter was 6 mm.

[0103] The spot welding conditions were set to incorporate one of the following welding disturbances (I) to (V) in order to reproduce the conditions that are likely to cause LME cracking. All of these conditions can locally increase the temperature and / or tensile stress of the weld when the electrode is open, and therefore can reproduce the conditions that are likely to cause LME cracking. The welding disturbances (I) to (V) that were incorporated during spot welding are shown in the "Pressure Start Condition" column of Table 1.

[0104] (I) A state in which the welding electrode and the overlapping sheet combination have an impact angle of 0.2 degrees or more. Here, the impact angle is defined as the angle at which the electrode is inclined relative to the steel sheet, i.e., the angle formed between the direction of the electrode pressure and the direction of the steel sheet thickness.

[0105] (II) A state in which the misalignment amount of a pair of welding electrodes is 0.1 mm or more. Here, misalignment means a state in which the central axes of a pair of welding electrodes are not aligned, and the amount of misalignment is defined as the distance between the central axis of the upper electrode and the central axis of the lower electrode.

[0106] (III) A state in which there is a gap of 0.5 mm or more between either electrode and the steel sheet (gap between the electrode and the steel sheet). Here, the gap between the electrode and the steel sheet is defined as the larger distance between either electrode and the steel sheet when the steel sheet and the pair of electrodes are arranged before the start of pressing.

[0107] (IV) A state in which, among two or more overlapping steel plates, there is a gap (distance between the steel plates) of 0.5 mm or more between at least one pair of steel plates. Here, the gap between steel plates is defined as the maximum distance between the steel plates when two steel plates arranged in the vertical direction are considered as one pair.

[0108] (V) A state in which the shortest distance from the center of the welding point to the end face of the steel sheet is 10 mm or less. Here, the distance from the center of the welding point to the end face of the steel sheet is defined as the shortest distance from the center of the welding point to the end face of any of the steel sheets in two or more overlapping steel sheets.

[0109] The spot welding conditions were as follows: Specifically, resistance spot welding was performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance spot welder, with a pressure of 3.5 kN applied to an electrode (DR type, tip diameter 6 mm) of the resistance spot welder, a hold time of 0.1 seconds, and a welding current and welding time that resulted in a nugget diameter of 4.5√t mm, where t is the maximum thickness of the overlapping steel sheets.

[0110] <Evaluation of Spot-Welded Members> First, samples were cut out so that the observation surface was a cross section in the sheet thickness direction passing through the center of the spot weld constituting the obtained spot-welded member. Specifically, when two steel sheets were overlapped, the spot weld formed between steel sheet 1 and steel sheet 2 was cut out, and when three steel sheets were overlapped, the spot weld formed between steel sheet 1 and steel sheet 2 and the spot weld formed between steel sheet 1 and steel sheet 3 were cut out. Here, all spot welds formed in the spot-welded member were spot welds formed by spot welding two adjacent steel sheets with a Zn-based plating layer present between them, and are the spot welds targeted by the present invention.

[0111] Next, the observation surface of the sample was roughly polished using waterproof abrasive paper, and then mirror-polished with alcohol using diamond paste. Furthermore, the resulting polished surface was subjected to ion milling using Ar ions. Using a field emission electron probe microanalyzer (FE-EPMA), a 100 μm × 100 μm area (region R1) was analyzed with an electron beam diameter of 1 μm to measure the Zn concentration, and a Zn concentration map was created. From the resulting Zn concentration map, regions with a Zn concentration exceeding 0% by mass were extracted.

[0112] Next, the length S1 of the grain boundary G1 in region R2 and the length Sa of the grain boundary Ga in region R2 where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V was 0.1 mass% or more were measured, and Sa / S1 was calculated. Specifically, the length S1 of the grain boundary G1 in region R2 was measured by cutting out an analysis sample from region R2 using a focused ion beam (FIB) method, and performing electron backscattered diffraction (EBSD) measurement on the obtained analysis sample using an SEM. In addition, the grain boundary was extracted using data analysis software attached to the EBSD device. The obtained EBSD image data was loaded into the image processing software, and the total length S1 of the grain boundary was calculated.

[0113] Then, the length Sa of the grain boundary Ga in which one or more elements selected from the group consisting of Nb, Mo, and V in region R2 segregated in a total of 0.1 mass% or more was calculated by the following method. First, the grain boundaries of the above analysis sample were subjected to elemental analysis using an energy dispersive X-ray spectroscopy (EDS: Energy Dispersive X-ray Spectroscopy) method (STEM-EDS) using a scanning transmission electron microscope (STEM). The segregation amounts (concentrations) of Nb, Mo, and V segregated at each grain boundary were analyzed, the total segregation amount of Nb, Mo, and V was calculated, and grain boundaries with a total segregation amount of 0.1 mass% or more were extracted. Then, the length Sa was calculated using the image analysis software. Here, elemental analysis of the grain boundaries in region R2 was performed by tilting the analysis sample so that the grain boundaries to be analyzed were parallel to the electron beam incident direction. This was because if the grain boundaries were not parallel to the electron beam incident direction, the electron beam would penetrate both the grain boundaries and the parent phase, resulting in measured values ​​of lower element concentrations than the actual concentrations. If the shape of the grain boundaries made it impossible to tilt the sample parallel to the electron beam incident direction, the analysis sample was removed and re-mounted on the sample holder so that it could be tilted parallel. Sa / S1 was calculated from the obtained S1 and Sa. When there were three sheets in the plate set, the spot welds between the plate sets of steel plates 1 and 2 and steel plates 1 and 3 were observed.

[0114] <Evaluation of Steel Plate for Spot-Welded Member> The solid solution region R3 and precipitates R4 present in the surface layer portion of the steel plate for spot-welded members (steel plate 1) from the surface to a depth of 5 μm in the plate thickness direction were analyzed for Nb content, Mo content, and V content. First, three test pieces measuring 20 × 50 mm were cut out from the prepared steel plate for spot-welded members.

[0115] Next, the Zn-based plating layer was immersed in hydrochloric acid containing an inhibitor to remove the Zn-based plating layer from the surface of the test piece. The resulting test piece was masked with insulating tape except for the surface, and two electrolytic test pieces were prepared to enable electrolytic extraction only from the surface. The amount of dissolution of the test piece was calculated from the relationship between the current and electrolysis time of the electrolytic treatment, and this was divided by the area to convert it into the distance from the surface of the test piece. The electrolysis conditions under which this distance became 5 μm were determined in advance, and the electrolytic test piece was electrolyzed. The electrolyte used was 10% by volume of acetylacetone-1% by mass of tetramethylammonium chloride-methanol.

[0116] First, the first electrolytic test piece (1) was electrolyzed, and the electrolytic test piece (1) obtained after electrolysis was immersed in methanol. Residues adhering to the electrolytic test piece (1) after electrolysis were dispersed in methanol using ultrasonic vibration to obtain a dispersion a (1). Subsequently, the dispersion a (1) and the electrolytic solution (1) after electrolysis were filtered using an aluminum filter with a pore size of 20 nm, and residues (aggregates (1)) with particle sizes exceeding 20 nm were captured on the aluminum filter. The obtained aggregates (1) were subjected to acid decomposition, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. In addition, the mass Wa (1) of the electrolytic test piece (1) before electrolysis and the mass Wb (1) of the electrolytic test piece (1) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference was taken as the amount of electrolysis W (1). The obtained absolute amounts of Nb, Mo, and V were divided by the amount of electrolysis W(1) to calculate the average values ​​of the contents of Nb, V, and Mo contained in all inclusions and precipitates present in the surface layer portion of the electrolytic test piece (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)).

[0117] Next, the second electrolysis test piece (2) was subjected to the same electrolysis treatment to obtain a dispersion a (2). The obtained dispersion a (2) and the electrolytic solution (2) after electrolysis were filtered using an aluminum filter with a pore size of 20 nm to obtain an aggregate of inclusions and precipitates having a particle size greater than 20 nm (aggregate (2)). The obtained aggregate (2) was immersed in hexametaphosphoric acid, and ultrasonic vibration was used to obtain a dispersion b (2) in which the aggregated inclusions and precipitates were dispersed in hexametaphosphoric acid. The obtained dispersion b (2) was filtered using a new aluminum filter with a pore size of 20 nm, and precipitates and inclusions having a particle size greater than 20 nm (precipitate (2)) were captured on the aluminum filter. The obtained precipitate (2) was subjected to acid decomposition, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. The mass Wa(2) of the electrolysis test piece (2) before electrolysis and the mass Wb(2) of the electrolysis test piece (2) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference between them was designated as the amount of electrolysis W(2). The obtained absolute amounts of Nb, Mo, and V were divided by the amount of electrolysis W(2) to obtain the average values ​​of the contents of Nb, V, and Mo contained in precipitates having a particle size of more than 20 nm contained in the surface layer portion of the electrolysis test piece (Nb content 2 (mass%), Mo content 2 (mass%), and V content 2 (mass%)).

[0118] Next, the average values ​​of the Nb, V, and Mo contents contained in precipitates having a particle size of 20 nm or more (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)) were calculated from the average values ​​of the Nb, V, and Mo contents contained in all the obtained inclusions and precipitates (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)), to obtain the average values ​​of the Nb, V, and Mo contents contained in precipitates having a particle size of 20 nm or less, i.e., precipitate R4.

[0119] Furthermore, for the third test specimen, the average values ​​of the contents of all of Nb, V, and Mo contained in the steel plate (Nb content 3 (mass%), Mo content 3 (mass%), V content 3 (mass%)) were determined by wet chemical analysis.

[0120] Thereafter, the average values ​​of the contents of Nb, V, and Mo contained in the solid solution region R3 of the surface layer portion were obtained by calculating the differences between the obtained Nb content 3 (mass%), Mo content 3 (mass%), and V content 3 (mass%) and the obtained Nb content 1 (mass%), Mo content 1 (mass%), and V content 1 (mass%).

[0121] <Evaluation of LME Cracking Resistance> First, welding was performed under five levels of welding conditions that satisfied one or more of the welding disturbances (I) to (V) described above, to produce welded joints. The centers of the welds of the obtained welded joints were then cut with a microcutter, and the cross sections of the welds were observed to evaluate the presence or absence of cracks and their depths. Here, the occurrence of cracks 5 μm or more deep was considered to be cracks, and LME cracking resistance was evaluated according to the following criteria.

[0122] Rating 4: 5 welded joints without cracks Rating 3: 4 welded joints without cracks, 1 welded joint with cracks Rating 2: 3 welded joints without cracks, 1 welded joint with cracks Rating 1: 2 or less welded joints without cracks, 3 or more welded joints with cracks Here, ratings of 4 to 2 were evaluated as passing.

[0123] As is clear from Table 1, the spot-welded components of the present invention have excellent LME cracking resistance. Furthermore, if the manufacturing method for spot-welded components using the steel plate for spot-welded components of the present invention is used, spot-welded components with excellent LME cracking resistance can be manufactured even in the presence of the above-mentioned welding disturbances. Therefore, the tolerance for managing the welding disturbances during manufacturing of spot-welded components is improved.

[0124] According to the present invention, it is possible to provide a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component, which are excellent in LME cracking resistance.

[0125] REFERENCE SIGNS LIST 1 Spot welded member 2 Spot welded portion 3 Base material portion 4 Nugget 5 Heat affected zone (HAZ) 6 Tip of gap between two steel plates 7 Center of spot welded portion (nugget) 8 Straight line 9 End of nugget

Claims

1. A spot-welded component in which at least one set of two adjacent steel plates from a plurality of steel plates are arranged with a Zn-based plating layer between them, and the plurality of steel plates are spot-welded together, wherein the spot-welded component comprises at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel plates with a Zn-based plating layer between them, and the spot weld comprises a nugget and a heat-affected zone, and when a region R1 in the spot weld is defined as a region R2 where the Zn concentration exceeds 0 mass%, the ratio Sa / S1 of the length Sa of grain boundaries Ga in region R2 where the total segregation amount of one or more elements selected from the group consisting of Nb, Mo, and V is 0.1 mass% or more to the length S1 of grain boundaries G1 in region R2 is 0.50 or more. Here, the region R1 is a 100 μm square region in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget, and centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion.

2. The spot-welded component according to claim 1, wherein the grain boundary Ga includes a prior austenite grain boundary and / or a phase interface with a different phase.

3. A steel sheet for a spot-welded component, which constitutes one of two adjacent steel sheets in a spot-welded component formed by spot-welding a plurality of steel sheets, at least one set of two adjacent steel sheets being arranged with a Zn-based plating layer between them, wherein a surface layer portion from the surface of the steel sheet to a depth of 5 μm in the sheet thickness direction contains a solid solution region R3 in which one or more elements selected from the group consisting of Nb, Mo, and V are solid-solubilized, and / or precipitates R4 having a particle size of 20 nm or less and containing one or more elements selected from the group consisting of Nb, Mo, and V, and wherein the sum of the average contents of Nb, Mo, and V contained in the solid solution region R3 and the average contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass% or more.

4. The steel sheet for spot-welded components according to claim 3, further comprising a Zn-based plating layer on the surface.

5. A method for manufacturing spot-welded components, comprising arranging at least one set of two adjacent steel plates from a plurality of steel plates with a Zn-based plating layer between them, and spot-welding the plurality of steel plates, wherein at least one of the two adjacent steel plates that is subjected to greater stress during spot welding is arranged so that the surface of the steel plate for spot-welded components described in claim 3 comes into contact with the Zn-based plating layer arranged on the surface of the other of the two adjacent steel plates, and spot-welding the plurality of steel plates.

6. A method for manufacturing spot-welded components by arranging at least one set of two adjacent steel plates from a plurality of steel plates with a Zn-based plating layer between them, and spot-welding the plurality of steel plates, comprising: arranging a steel plate for spot-welded components according to claim 4 on at least the steel plate that is subjected to greater stress during spot welding, of the two adjacent steel plates, so that the Zn-based plating layer of the steel plate for spot-welded components contacts the surface of the other of the two adjacent steel plates; and spot-welding the plurality of steel plates.

Images