Spot welding member, steel plate for spot welding member, and method for manufacturing spot welding member

Abstract

The present invention provides a spot-welded member with excellent resistance to LME cracking, a steel plate for spot-welded members, and a method for manufacturing spot-welded members. Region R1 is defined as a 100 μm square area centered on a point 10 300 μm away from the end 9 of the nugget towards the base material 3, and Region R2 is defined as a region where the Zn concentration exceeds 0 mass%, and the ratio Sa / S1 of the length Sa of the grain boundary Ga to the length S1 of the grain boundary G1 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, is 0.50 or more.

Description

Technical Field

[0001] The present invention relates to a spot welding member, a steel sheet for a spot welding member, and a method for manufacturing a spot welding member.

Background Art

[0002] In recent years, due to the increasing awareness of environmental issues, efforts have been made to reduce the weight of automotive materials to reduce carbon dioxide (CO2) emissions and improve the collision safety performance by increasing the strength of the vehicle body. As a result, the strength of automotive steel sheets has been increasing. For example, the application of high-strength steel sheets with a tensile strength of 980 MPa or more is required as the skeletal material of automotive cabins. In the assembly of automobiles using such high-strength steel sheets, it is often the case that press-formed steel sheets are overlapped and joined by resistance spot welding from the viewpoints of efficiency and cost.

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

[0004] Since 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's pressing force during spot welding and the tensile stress due to the thermal expansion and contraction of the steel sheet are applied to the spot weld, Zn that constitutes the melted Zn-based plating layer penetrates into the grain boundaries of the steel sheet, reducing the grain boundary strength and causing cracks. This crack is caused by Zn that constitutes the melted Zn-based plating layer, that is, the Zn-based plating layer in a liquid state, and is called a crack due to liquid metal embrittlement (LME).

[0005] LME cracks that occur during spot welding are known to be more likely to occur when excessive tensile stress is generated in the spot weld area due to construction disturbances during spot welding. In particular, after the current and pressure are applied during spot welding and the pair of welding electrodes are released, LME cracks are known to be more likely to occur in areas with locally high tensile stress on the mating surface where the plates of the spot weld meet. LME cracks are becoming an even greater problem due to the increasing strength of the steel plates that make up the spot welds.

[0006] For example, Non-Patent Document 1 states that silicon (Si) in steel affects LME cracking. Si in steel is an element included to increase the strength and improve the ductility of steel sheets, and high-strength steel sheets with a tensile strength of 980 MPa or higher have a high Si content. Therefore, if high-strength steel sheets are included in the plate assembly that makes up the spot weld, the risk of LME cracking is expected to increase.

[0007] Therefore, various studies have been conducted to prevent LME cracking in spot welds. For example, Patent Document 1 proposes a spot welding method that includes a step of removing the plating layer before spot welding multiple overlapping steel plates, each containing one or more steel plates with a plating layer coated on at least one surface of the weld area, between opposing welding electrodes. Patent Document 1 states that this spot welding method makes it possible to easily prevent the occurrence of liquid metal cracking in spot welding.

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

[0009] [Patent Document 1] International Publication No. 2016 / 159169 [Patent Document 2] International Publication No. 2019 / 116531 [Non-patent literature]

[0010] [Non-Patent Document 1] D. Bhattacharya et al. Materials Science & Engineering A 823 (2021) 141569 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, the spot welding method described in Patent Document 1 requires a step to remove the plating layer before spot welding, which presents challenges in terms of workability.

[0012] Furthermore, the steel plate described in Patent Document 2 sometimes exhibited insufficient LME crack resistance depending on the spot welding conditions, indicating room for improvement.

[0013] This invention has been made in view of the above circumstances, and aims to provide a spot-welded member with excellent resistance to LME cracking, a steel plate for spot-welded members, and a method for manufacturing spot-welded members. [Means for solving the problem]

[0014] As a result of diligent research, the inventors of this invention discovered that the above objective can be achieved by adopting the following configuration, and thus completed the present invention.

[0015] In other words, the gist of the present invention is as follows:

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

[0017] [2] The spot welded member 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 plate for a spot-welded member in which at least one pair of 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, the steel plate constituting one of the two adjacent steel plates in the spot-welded member, The surface layer of the steel plate, from the surface to a depth of 5 μm in the 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-dissolved, and / or precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V with a particle size of 20 nm or less. The sum of the average values of the contents of Nb, Mo, and V contained in the solid solution region R3 and the sum of the average values of the contents of Nb, Mo, and V contained in the precipitate R4 are 0.05% by mass or more, a steel sheet for a spot welding member.

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

[0020] [5] A method for manufacturing a spot welding member by arranging at least one set of two adjacent steel sheets among a plurality of steel sheets in a state where a Zn-based plating layer exists between them, and spot welding the plurality of steel sheets, Among at least the two adjacent steel sheets, on the side of the steel sheet that is more stressed during spot welding, the surface of the steel sheet for a spot welding member according to [3] is arranged to contact the Zn-based plating layer on the surface of the other steel sheet among the two adjacent steel sheets, and spot welding is performed on the plurality of steel sheets, a method for manufacturing a spot welding member.

[0021] [6] A method for manufacturing a spot welding member by arranging at least one set of two adjacent steel sheets among a plurality of steel sheets in a state where a Zn-based plating layer exists between them, and spot welding the plurality of steel sheets, Among at least the two adjacent steel sheets, on the side of the steel sheet that is more stressed during spot welding, the steel sheet for a spot welding member according to [4] is arranged such that the Zn-based plating layer of the steel sheet for a spot welding member contacts the surface of the other steel sheet among the two adjacent steel sheets, and spot welding is performed on the plurality of steel sheets, a method for manufacturing a spot welding member. [Effect of the Invention]

[0022] According to the present invention, it is possible to provide a spot welding member excellent in LME crack resistance, a steel sheet for a spot welding member, and a method for manufacturing a spot welding member. [Brief Description of the Drawings]

[0023] [Figure 1] This is a schematic diagram of a cross-section in the thickness direction passing through the center of the spot weld of a spot-welded member according to one embodiment of the present invention. [Modes for carrying out the invention]

[0024] (Spot-welded components) Embodiments of the present invention will be described below with reference to the drawings. The spot-welded member according to the present invention is a spot-welded member in which at least one pair of 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, wherein the spot-welded member comprises at least one set of a spot-welded portion and a base material portion formed by spot-welding the two adjacent steel plates with a Zn-based plating layer between them, the spot-welded portion comprises a nugget and a heat-affected zone, and in a region R1 within the spot-welded portion, when the region where the Zn concentration is greater than 0 mass% is defined as region R2, the ratio Sa / S1 of the length Sa of the grain boundary Ga in region R2 to the length S1 of the grain boundary G1 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, is 0.50 or more. Here, region R1 is a cross-section in the thickness direction passing through the center of the nugget, The end of the nugget is defined as the point where a straight line passing through the center of the nugget and the leading edge of the gap between the two adjacent steel plates intersects with the boundary of the nugget, and the area is a 100 μm square region centered on a point 300 μm away from the end of the nugget along the straight line toward the base material.

[0025] First, the inventors diligently studied spot-welded members with excellent resistance to LME cracking, their manufacturing method, and steel sheets for spot-welded members. The inventors focused on the microstructure of spot-welded members obtained by overlapping a Zn-plated steel sheet and a steel sheet without a Zn-plated layer and performing spot welding, and investigated in detail the relationship between the spot-welded member and LME cracking.

[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, the heat-affected zone shrinks during the cooling process after spot welding, and tensile stress generated by electrode pressure during spot welding causes the Zn in the molten Zn-based plating layer to penetrate the grain boundaries of the steel sheet surface and segregate.

[0027] During their investigation, the inventors discovered that there are regions where LME cracking is likely to occur, that Zn penetrates into a portion of the grain boundaries in those regions, causing grain boundary embrittlement and leading to LME cracking, and that grain boundary embrittlement can be suppressed by the penetration and segregation of one or more elements selected from the group consisting of Nb, Mo, and V into the grain boundaries, thereby suppressing grain boundary embrittlement and LME cracking. After conducting various studies, the inventors completed the present invention.

[0028] The present invention is 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 substituted or substantially identical to those that are easily substituted by those skilled in the art.

[0029] The present invention relates to a spot-welded member in which at least one pair of 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.

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

[0031] The spot-welded member 1 shown in Figure 1 is an example of a spot-welded member in which two adjacent steel plates are spot-welded. The first steel plate (lower plate) is a Zn-plated steel plate with a Zn-plated layer (not shown), and the second steel plate (upper plate) is a steel plate (without a Zn-plated layer). A joint surface is formed where the surface of the Zn-plated layer of the lower plate and the surface of the upper steel plate (the lower surface of the steel plate) are in contact, and the spot-welded member 1 comprises at least one set (one in the example of Figure 1) of a spot-welded portion 2 and a base material portion 3 obtained by spot welding. The spot-welded portion 2 consists 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 at least one set of a spot-welded portion 2 and a base material portion 3 is included, which is formed by spot-welding two adjacent steel plates, as shown in Figure 1, with a Zn-based plating layer between them.

[0033] For example, when n steel plates (n≧2) are stacked and spot-welded, a joint surface (1) is formed where the surface of the first steel plate meets the surface of the second steel plate, a joint surface (2) is formed where the surface of the second steel plate meets the surface of the third steel plate, ..., a joint surface (n-1) is formed where the surface of the (n-1) steel plate meets the surface of the n steel plate, and by spot welding, n-1 spot welds are formed. In this case, the spot welds formed by spot welding while a Zn-based plating layer is present at the joint surface become the spot welds 2 targeted by the present invention. Note that the spot-welded member 1 of the present invention only needs to have at least one spot weld 2 targeted by the present invention, and all n-1 spot welds may be spot welds 2 targeted by the present invention.

[0034] The Zn-based plating layer can be provided by providing a Zn-based plated steel sheet having a Zn-based plating layer on its surface as at least one of the multiple (two or more) steel sheets constituting the spot-welded member 1. In the case of the spot-welded portion 2 targeted by the present invention, one of the steel sheets may be a Zn-based plated steel sheet, or both may be Zn-based plated steel sheets.

[0035] Furthermore, the spot-welded member of the present invention may be formed from two steel plates, or from three or more steel plates, and there is no particular upper limit to the number of steel plates (n), but it may be five or less.

[0036] <Area R1> Region R1 is a 100 μm square region centered at a point 10 located 300 μm away from the end 9 of the nugget 4 towards the base material 3, along the line 8, in a cross-section in the thickness direction passing through the center 7 of the spot weld 2 (i.e., the nugget 4). The intersection of the 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 is defined as the end 9 of the nugget 4. Region R1 is located within the heat-affected zone 5. Of the four sides of region R1, two sides can be parallel to the thickness direction, and the remaining two sides can be perpendicular to the thickness direction (parallel to the steel plate surface).

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

[0038] In particular, the inventors have found that when region R2 is defined as a region within region R1 where the Zn concentration exceeds 0 mass%, it is important to segregate a predetermined element at the grain boundaries within region R2. Specifically, the ratio Sa / S1 of 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, to the length S1 of grain boundary G1 within region R2 should be 0.50 or more. When Sa / S1 is 0.50 or more, the penetration and segregation of Zn into the grain boundaries is inhibited, and grain boundary embrittlement is suppressed, thereby suppressing the occurrence of LME cracking. On the other hand, when Sa / S1 is less than 0.05, the penetration of Zn into the grain boundaries cannot be sufficiently inhibited, and LME cracking occurs. Here, grain boundaries include interfaces.

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

[0040] In the heat-affected zone 5, the steel sheet is a high-strength steel sheet (for example, a low-alloy carbon steel with a tensile strength of 980 MPa or more), and the area around the nugget 4 within the heat-affected zone 5 (for example, the area where region R1 extends from the edge 9 of the nugget 4, i.e., an area within approximately 500 μm square) is a martensite phase. At this time, if Zn is present on the surface of the steel sheet, Zn diffuses from the surface of the steel sheet, and the surface of the steel sheet undergoes a ferrite transformation, thus generating a ferrite phase. Therefore, the structure of region R2 will contain ferrite and martensite in a total area ratio of 80% or more. The remaining structure may consist of bainite in 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 LME crack suppression effect may not be sufficiently obtained, which is undesirable.

[0041] Hereinafter, the extraction of region R2 within region R1, the length S1 of grain boundary G1 within region R2, the length Sa, Sa / S1 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, the area ratio of ferrite phase, martensite phase, and Zn-rich phase within region R2, and the method for measuring the segregation amounts of Nb, Mo, and V at grain boundary G1 within region R2 are as follows.

[0042] First, the sample is cut out so that the cross-section in the thickness direction passing through the center 7 of the spot weld 2 (i.e., nugget 4) that constitutes the spot welded member 1 becomes the observation surface. 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, in this case the region with a Zn concentration of 50% by mass or more, will react with the water during polishing and disappear, so polishing is carried out using alcohol or the like (water-restricted) without using water. Furthermore, ion milling using Ar ions may be performed on the obtained polished surface.

[0043] Region R2 is extracted from the obtained Zn concentration map by obtaining a Zn concentration map using energy dispersive X-ray spectroscopy (EDS) (SEM-EDS) with a scanning electron microscope (SEM) that includes region R1, and then identifying the region where the Zn concentration exceeds 0 mass%.

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

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

[0046] The length S1 of the grain boundary G1 within region R2, the length Sa of the grain boundary Ga within 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, and Sa / S1 are calculated using the following procedure.

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

[0048] For example, the prior austenite grain boundaries are extracted by reconstructing the prior austenite grains using a technique that reconstructs prior austenite grains from the crystal orientation of martensite, as described in the reference (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 obtained from the martensite structure by using the matrix reconstruction function of the EBSD data analysis software "OIM Analysis ver8.6". Grain boundaries are extracted from the resulting reconstructed structure, and these obtained grain boundaries are considered to be the prior austenite grain boundaries.

[0049] The length Sa of grain boundary Ga in which one or more elements selected from the group consisting of Nb, Mo, and V in region R2 are segregated in a total amount of 0.1% by mass or more is calculated by the following method. First, for the grain boundaries of the above analysis sample, elemental analysis using energy dispersive X-ray spectroscopy (EDS) method (STEM-EDS) using a scanning transmission electron microscope (STEM: Scanning Transmission Electron Microscope) is performed to analyze the segregation amount (concentration) of each of Nb, Mo, and V segregated at each grain boundary, calculate the total segregation amount of Nb, Mo, and V, and extract the grain boundaries with a total segregation amount of 0.1% by mass or more. Then, the length Sa is calculated using the above image analysis software. Here, the elemental analysis of the grain boundaries in region R2 is performed by tilting the analysis sample so that the grain boundaries to be analyzed are parallel to the electron beam incident direction. This is because when the grain boundaries are not parallel to the electron beam incident direction, the electron beam penetrates both the grain boundaries and the matrix phase, resulting in a measured value with a lower concentration than the actual elemental concentration. If the grain boundaries cannot be tilted parallel to the electron beam incident direction from their shape, it is preferable to take out the analysis sample once and re-set it in the sample holder so that it can be tilted parallel. Calculate Sa / S1 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 the steel sheet. As at least one of the plurality of steel sheets constituting the spot welding member 1, a Zn-based plated steel sheet may be supplied. When the spot welding member 1 is manufactured to include at least one spot welding portion 2 formed by overlapping Zn-based plated steel sheets with each other or by overlapping a Zn-based plated steel sheet and a steel sheet and performing spot welding, LME cracking occurs. Therefore, the spot welding member 1 of the present invention is directed to a spot welding member including at least one set of a spot welding portion and a base material portion formed by spot welding two adjacent steel sheets among the plurality of steel sheets in a state where a Zn-based plating layer exists between them.

[0051] The Zn-based plating layer is not particularly limited as long as it contains Zn. Examples include electroplated Zn-based plating layers and hot-dip Zn-based plating layers. The Zn-based plating layer may also contain elements other than Zn, such as aluminum (Al) or magnesium (Mg).

[0052] Examples of zinc-plated steel sheets include electroplated zinc-plated steel sheets and hot-dip zinc-plated steel sheets. In particular, in the automotive sector where LME cracking is a problem, hot-dip zinc-plated steel sheets are used from the viewpoint of corrosion resistance and manufacturability. Specifically, examples include hot-dip zinc-plated steel sheets (GI steel sheets) which have a hot-dip zinc plating layer on the steel sheet surface, and alloyed hot-dip zinc-plated steel sheets (GA steel sheets) which have an alloyed hot-dip zinc plating layer on the steel sheet surface. Among these, considering pressability and continuous spot welding properties when manufacturing automotive parts, GA steel sheets are often used in the automotive sector, so 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 with increasing strength of the steel sheet used in the spot-welded member 1. Therefore, in this invention, it is preferable that the tensile strength of the Zn-plated steel sheet be 980 MPa or higher. This allows the effects of the present invention to be more fully realized.

[0054] (Steel plate for spot welding components) Next, the steel sheet for spot welding members according to the present invention will be described. The steel sheet for spot welding members according to the present invention is a steel sheet for spot welding members which constitutes one of the two adjacent steel sheets in a spot welding member in which at least one pair of adjacent steel sheets from a plurality of steel sheets are arranged with a Zn-based plating layer between them, and the plurality of steel sheets are spot-welded together, wherein the surface layer from the surface of the steel sheet to a depth of 5 μm in the 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-dissolved and / or precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V with a particle size of 20 nm or less, and the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the solid solution region R3 and the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the precipitates R4 is 0.05 mass% or more.

[0055] The steel plate used when manufacturing spot-welded components is not particularly limited, as long as it is a steel plate capable of manufacturing the above-mentioned spot-welded components.

[0056] On the other hand, LME cracking is thought to occur when tensile stress due to the pressure applied by the welding electrode during spot welding, or due to thermal expansion and contraction of the steel plate, is applied to the spot weld 2. This causes Zn, which constitutes the Zn-based plating layer, to penetrate the grain boundaries of the steel plate, resulting in grain boundary embrittlement by Zn. In other words, Zn reduces the grain boundary strength and causes cracking. Therefore, it is preferable to use the steel plate for spot welding members according to the present invention, with the side in contact with the Zn-based plating layer as the steel plate surface, for the steel plate on the side where tensile stress is applied during spot welding. This allows the above spot welding member 1 to be manufactured under general spot welding conditions.

[0057] Nb, Mo, and V are elements that improve resistance to LME cracking. The inventors have found that when these elements are present in a solid solution state in a predetermined amount or more in the surface layer of the steel sheet during welding, these elements segregate at the grain boundary G1 within region R2, suppressing the penetration of Zn into the grain boundary G1 and thereby suppressing LME cracking.

[0058] Here, in order to achieve the desired Sa / S1 ratio at the grain boundary within region R2, it is important to control the structure and composition of the surface layer from the steel sheet surface to a depth of 5 μm in the thickness direction. The surface layer of the steel sheet 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 containing the above elements in a solid solution state should be formed in the surface layer of the steel sheet. Alternatively, precipitates R4 containing the above elements should be formed in the surface layer of the steel sheet. However, since the precipitates R4 need to dissolve during welding, the size of the precipitates R4 should be 20 nm or less in equivalent circular diameter. Preferably, the equivalent circular diameter of the precipitates is 10 nm or less. The surface layer of the steel sheet may contain both the solid solution region R3 and the precipitates R4. Furthermore, there is no preferred lower limit for the size of the precipitates R4, and it is most preferable that the precipitates R4 are in a solid solution state.

[0059] However, the sum of the average values ​​of the Nb, Mo, and V content in the solid solution region R3 and the sum of the average values ​​of the Nb, Mo, and V content in the precipitate R4 shall be 0.05% by mass or more. Specifically, (Average Nb content in solid solution region R3 + Average Mo content + Average V content) + (Average Nb content in precipitate R4 + Average Mo content + Average V content) ≥ 0.05 mass% (1) This allows the solid solution region R3 and precipitate R4 to dissolve during welding, and Nb, Mo, and V to penetrate and segregate into the grain boundaries of the surface layer of the steel sheet, thereby preventing Zn, which is a cause of LME cracking, from penetrating into the grain boundaries. On the other hand, the upper limit of the left side of the above formula (1) is not particularly limited, but from the viewpoint of manufacturing cost, it is preferable to set it to 0.5 mass% or less.

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

[0061] In order to improve various properties such as mechanical properties, the compositional composition of steel sheets can be modified individually or in combination by chemical compositional modifications such as 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; and strengthening by adding strengthening elements such as W, Zr, Hf, Co, B, Cu, and rare earth elements; strengthening by recovery annealing at a temperature at which recrystallization does not occur, or partial recrystallization strengthening that leaves an unrecrystallized region without complete recrystallization; strengthening by transformation structures such as single-phase formation of bainite or martensite, or composite formation of ferrite and these transformation structures; fine-graining strengthening represented by the Hall-Petch equation: σ = σ0 + kd - 1 / 2 (where σ is stress, σ0 and k are material constants) when the ferrite grain size is d; and work strengthening by rolling.

[0062] Examples of the composition of the above-mentioned steel sheet include, for example, a composition in which C: 0.1-0.4 mass%, Si: 0.5-2.5 mass%, Mn: 1-3 mass%, P: 0-0.05 mass%, S: 0-0.005 mass%, with the remainder being Fe and unavoidable impurities, and further, a composition in which one or more elements such as Cu, Ti, V, Al, and Cr are added.

[0063] Furthermore, the steel sheet for spot welding components described above may be further provided with a Zn-based plating layer on its surface, and the Zn-based plated steel sheet can be supplied as a steel sheet for spot welding components. In this case, the Zn-based plating layer is provided on top of the surface layer.

[0064] (Method of manufacturing spot-welded components) 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 spot welding the plurality of steel plates, wherein at least one pair of 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 arranged with a Zn-based plating layer arranged on the surface of the other of the two adjacent steel plates, and spot welding is performed on the plurality of steel plates.

[0065] Here, the steel plate that experiences more stress during spot welding is the steel plate on which LME cracking occurs during spot welding. If the steel plate on which LME cracking occurs is not known in advance, it is preferable to perform a spot welding test beforehand to determine which of the two steel plates to be welded will experience more stress.

[0066] The other of the two steel plates is a steel plate having a Zn-based plating layer on its surface. The other steel plate may or may not be the steel plate for spot welding members according to the present invention.

[0067] As the steel plate to be placed on the side where more stress is applied during spot welding, the steel plate for spot welding members of the present invention may be supplied as a Zn-plated steel plate with a Zn-based plating applied to its surface. In this case, the Zn-plated layer of the Zn-plated steel plate is placed in contact with the surface of the other of the two steel plates, and spot welding is performed on multiple steel plates.

[0068] The other of the two adjacent steel plates described above is not particularly limited and may or may not be a steel plate for spot welding members according to the present invention. Furthermore, the other steel plate may or may not have a Zn-based plating layer on its surface. If the other steel plate has a Zn-based plating layer on its surface, the two adjacent steel plates are arranged so that the Zn-based plating layers of both plates are in contact, and spot welding is performed.

[0069] If both of the two adjacent steel plates described above are steel plates for spot welding according to the present invention, then as long as a Zn-based plating layer is provided on at least one of their surfaces, it is possible to manufacture the spot welding member according to the present invention without having to specify which steel plate experiences more stress during spot welding. Furthermore, all the steel plates constituting the spot welding member 1 may be steel plates for spot welding according to the present invention.

[0070] By manufacturing spot-welded members using the method described above, it becomes possible to manufacture spot-welded members having spot welds 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 plates are stacked, the n steel plates are clamped between a pair of welding electrodes positioned on the opposite side of the mating surface (1) of the first steel plate and the opposite side of the mating surface (n-1) of the nth steel plate, and current is applied while applying pressure and controlling it to achieve predetermined welding conditions. For example, resistance spot welding is performed using a servo motor-driven single-phase AC (50Hz) resistance spot welding machine, with the electrodes (DR type, tip diameter 6mm) of the resistance spot welding machine set to a pressure of 3.5kN, a hold time of 0.1 seconds, and a welding current and welding time such that the nugget diameter is 4.5√tmm, where t is the maximum thickness of the n steel plates.

[0072] As a welding apparatus usable in spot welding according to the present invention, any resistance spot welding apparatus equipped with a pair of upper and lower welding electrodes, capable of arbitrarily controlling the applied pressure and welding current during welding, may be used. The pressurizing mechanism (air cylinder, servo motor, etc.), type (stationary, robotic gun, etc.), electrode shape, etc. of the welding apparatus are not particularly limited. Examples of electrode tip types include DR type (dome radius type), R type (radius type), D type (dome type), etc., as described in JIS C 9304:1999. Furthermore, the tip diameter of the DR type electrode may be, for example, 4 mm to 16 mm.

[0073] (Method of manufacturing steel plates for spot welding components) Next, an example of a method for manufacturing steel sheets for spot welding components will be described. For example, a clad steel sheet can be manufactured by preparing a cladding material and a steel sheet to be bonded. Specifically, a steel sheet is prepared as the cladding material, having a thickness of 5 μm or more, containing a solid solution region R3 containing one or more elements selected from the group consisting of Nb, Mo, and V, and / or precipitate R4 with an equivalent circle diameter of 20 nm or less containing one or more elements selected from the group consisting of Nb, Mo, and V, and the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the solid solution region R3 and the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the precipitate R4 is 0.05 mass% or more.

[0074] Next, the steel sheets to be bonded together are prepared. While the steel sheets to be bonded are not particularly limited, it is preferable to prepare steel sheets that are prone to LME cracking. Examples of steel sheets prone to LME cracking include those with a Si concentration of 0.5% by mass or more. Examples of such steel sheets include cold-rolled steel sheets. Cold-rolled steel sheets can be manufactured using known manufacturing methods. For example, a steel slab having the above-mentioned composition can be hot-rolled to produce a hot-rolled steel sheet, and then the hot-rolled steel sheet can be pickled and then cold-rolled to produce a cold-rolled steel sheet.

[0075] The following methods can be used to manufacture the composite material. First, molten steel is prepared using a known method such as a converter, electric furnace, or vacuum melting furnace, with a total content of Nb, Mo, and V of 0.05% by mass or more. Then, the obtained molten steel is solidified to produce the steel material. The method for producing the steel material from molten steel is not particularly limited, and continuous casting, ingot casting, or thin slab casting methods can be used. To prevent macrosegregation, it is preferable to use steel slabs produced by continuous casting as the steel material.

[0076] It is preferable to heat the obtained steel slab at a heating temperature of 1100°C or higher and for a heating time of 1 hour or more. By heating at a temperature of 1100°C or higher and for a heating time of 1 hour or more, the Nb, Mo, and / or V contained in the steel slab can be dissolved in the steel slab. On the other hand, if the heating temperature is less than 1100°C or the heating time is less than 1 hour, the solid solution of Nb, Mo, and / or V may be insufficient. The heated steel slab is hot-rolled to make a hot-rolled steel sheet. At this time, if the finishing rolling start temperature is less than 1000°C, precipitates containing the dissolved Nb, Mo, and / or V may precipitate, and the particle size of the precipitates may become large. On the other hand, if the finishing rolling start temperature exceeds 1200°C, the scale loss of the steel slab will increase and it may cause cracking of the steel slab, so it is preferable to keep it at 1200°C or lower.

[0077] The finishing temperature for hot rolling is preferably 800°C or higher. If the finishing temperature is below 800°C, Nb, Mo, and / or V dissolved in the steel slab during heating may precipitate as deposits such as NbC, VC, MoC, etc. As a result, when cold rolling is performed afterward, the rolling load during cold rolling increases, the rolling load becomes larger, and this hinders cold rolling. Therefore, the finishing temperature for hot rolling is preferably 800°C or higher. More preferably, the finishing temperature for hot rolling is 850°C or higher, and even more preferably 870°C or higher. There is no particular upper limit to the finishing temperature, but if the finishing temperature exceeds 950°C, the amount of oxide (scale) generated increases rapidly, and the interface between the base metal and the oxide becomes rough. As a result, the surface quality of the steel sheet after pickling or cold rolling tends to deteriorate. The finishing rolling temperature for hot rolling is more preferably 930°C or lower, and even more preferably 900°C or lower.

[0078] When the obtained hot-rolled steel sheet is subjected to cold rolling, an intermediate heat treatment at 600°C or lower 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 pickling or other treatments as needed. The pickling method for the hot-rolled steel sheet should follow conventional methods. Additionally, 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 material may be immediately subjected to heat treatment, or it may be cold-rolled and then subjected to heat treatment.

[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 ratio is preferably 25% or more, and more preferably 30% or more. On the other hand, excessive reduction leads to excessive rolling load and increases the load on the cold rolling mill, so the upper limit is preferably 75% or less, and more preferably 70% or less.

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

[0082] In other words, 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 precipitate R4 containing one or more elements selected from the group consisting of Nb, Mo, and V with an equivalent circle diameter of 20 nm or less, decreases, and the average total content of Nb, Mo, and V contained in the solid solution region R3 and precipitate R4 no longer satisfies the requirement of 0.05 mass% or more, which may lead to a deterioration in LME crack resistance. For this reason, it is preferable to set the average heating rate in the temperature range of 600°C or higher to 2.0°C / s or higher. There is no particular upper limit to the average heating rate in the temperature range of 600°C or higher, but considering the heating rate that can be industrially achieved, it is preferable to set it to 100°C / s or lower.

[0083] Furthermore, the annealing temperature is preferably between 800°C and 950°C. If the annealing temperature is below 800°C or above 950°C, the growth of precipitates containing Nb, Mo, and / or V will be excessively promoted. Therefore, the annealing temperature should be between 800°C and 950°C.

[0084] Furthermore, the holding time at the annealing temperature should be between 20 and 60 seconds. Holding for 20 seconds or more can promote the phase transformation to austenite at the annealing temperature. On the other hand, if the holding time exceeds 60 seconds, the growth of precipitates containing Nb, Mo, and / or V will also be promoted. Therefore, it is preferable that the holding time be 60 seconds or less.

[0085] The steel plates obtained in this way can be used as the bonding material. The resulting bonding material may also be subjected to annealing, pickling, polishing, etc., as needed. The bonding materials are stacked so that the surface (compression surface) of the steel plates is under vacuum, and the four sides of the compression surface are sealed by welding to assemble the parts. The joining method is not particularly limited, but for example, the bonding material and the steel plates are stacked, and the bonding material and the steel plates are joined by applying electron beam welding (EBW), arc welding, or laser beam welding to the four sides of the ends of the stacked material.

[0086] Here, the method of vacuuming is not particularly limited, but for example, when laser beam welding the edges of a laminated slab, a vacuum valve is installed between the cladding material and the steel plate at the end (before the edges are completely joined), and a vacuum pump is connected to it to create a vacuum between the cladding material and the steel plate. By using the clad steel plate obtained in this way as a steel plate for spot welding members, the effects of the present invention can be obtained more reliably.

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

[0088] Specifically, an Fe-based plating layer is formed on the surface of the steel sheet. Preferably, the Fe-based plating layer is an Fe-based electroplating layer. Furthermore, it is preferable that the Fe-based electroplating layer has a component composition that includes a solid solution region R3 and / or precipitates R4, with the remainder consisting of Fe and unavoidable impurities.

[0089] When forming an Fe-based plating layer using an electroplating method, for example, the following method can be used. The Fe-based electroplating method is not particularly limited and can 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 can be used. The Fe ion content in the Fe-based electroplating bath before the start of current application is Fe 2+ It is preferable that the Fe ion content in the Fe-based electroplating bath be 0.5 mol / L or higher. 2+A sufficient amount of Fe can be obtained if the concentration is 0.5 mol / L or higher. In addition, the Fe-based electroplating bath must contain at least one element from Nb, Mo, and V. These elements can be included as metal ions. Furthermore, the Fe-based electroplating bath may contain conductivity boosters, chelating agents, and / or pH buffers as needed. For example, in the case of an iron sulfate plating bath, conductivity boosters such as sodium sulfate and potassium sulfate may be included.

[0090] There is no particular upper limit to the amount of Fe-based electroplating layer deposited on one side, but from a cost perspective, the amount of Fe-based electroplating layer deposited on one side is 60 g / m². 2 The following is preferable. On the other hand, the lower limit is 40.0 g / m², since the thickness of the surface layer of the steel plate must be 5 μm or more. 2 It is preferable to exceed this value.

[0091] The methods for measuring the Nb, Mo, and V content in the solid solution region R3 at the surface, the Nb, Mo, and V content in the precipitate R4, and the size of the precipitate R4 are as follows.

[0092] This section describes a measurement method for precipitates R4 with a particle size of 20 nm or less present in the surface layer. The specific procedure is as follows: First, cut out several 20 x 50 mm test pieces from the prepared spot-welded steel sheet or the spot-welded steel sheet with Zn-based plating. However, if the Zn-based plating layer is formed on the surface of the spot-welded steel sheet, immerse the test piece in hydrochloric acid with an inhibitor added to remove the Zn-based plating layer from the surface.

[0093] Two or more electrolytic test specimens were prepared by masking all but the surface of the obtained test specimen with insulating tape, allowing electrolytic extraction only from the surface. Furthermore, the amount of material dissolved in the test specimen was determined from the relationship between the electrolytic current and the electrolysis time, and this was converted to a distance from the surface of the test specimen by dividing it by the area. Electrolysis was then performed on the electrolytic test specimens so that this distance was 5 μm.

[0094] The electrolyte is not particularly limited as long as it is an electrolyte that dissolves Fe, which is a matrix component of the steel plate, and from which precipitates can be extracted. Conventionally known electrolytes can be used as appropriate. For example, 10% by volume acetylacetone - 1% by mass tetramethylammonium chloride - methanol can be used.

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

[0096] Next, the same electrolytic treatment was performed on the second electrolytic test piece (2) to obtain dispersion a(2). By filtering the obtained dispersion a(2) and the electrolytic solution (2) after electrolysis using a filter with a pore size of 20 nm, aggregates of inclusions and precipitates with particle sizes exceeding 20 nm (aggregates (2)) were obtained. The obtained aggregates (2) were immersed in hexametaphosphoric acid, and by using ultrasonic vibration, dispersion b(2) was obtained in which the aggregated inclusions and precipitates were dispersed in the hexametaphosphoric acid. By filtering the obtained dispersion b(2) using a new filter with a pore size of 20 nm, precipitates and inclusions with particle sizes exceeding 20 nm (precipitates (2)) could be captured on the filter. The obtained precipitates (2) were acid-decomposed, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. Furthermore, the mass Wa(2) of the electrolytic test specimen (2) before electrolysis and the mass Wb(2) of the electrolytic test specimen (2) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference was defined as the electrolysis amount W(2). By dividing the obtained absolute amounts of Nb, Mo, and V by the electrolysis amount W(2), the average values ​​of Nb, V, and Mo contained in precipitates with particle sizes exceeding 20 nm contained in the surface layer of the electrolytic test specimen (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)) are obtained. It is preferable to perform the above analysis on two or more specimens and calculate the average value.

[0097] Next, by calculating the difference between the average values ​​of Nb, V, and Mo content contained in all obtained inclusions and precipitates (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)) and the average values ​​of Nb, V, and Mo content contained in precipitates with a particle size greater than 20 nm (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)), the average values ​​of Nb, V, and Mo content contained in precipitates with a particle size of 20 nm or less, i.e., precipitate R4, can be obtained.

[0098] Furthermore, for the third test specimen, the average values ​​of all Nb, V, and Mo content contained in the steel plate were determined by wet chemical analysis (Nb content 3 (mass%), Mo content 3 (mass%), V content 3 (mass%)). It is preferable to perform the above analysis on two or more specimens and use the average value. Next, by calculating the difference between the obtained Nb content of 3 (mass%), Mo content of 3 (mass%), and V content of 3 (mass%), and the Nb content of 1 (mass%), and the Mo content of 1 (mass%), the average values ​​of Nb, V, and Mo contained in the solid solution region R3 of the surface layer can be obtained. [Examples]

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

[0100] <Fabrication of spot-welded components> For the spot-welded components, alloyed hot-dip Zn plated steel sheets (GA) were used, which were formed by applying a hot-dip Zn plating layer to both sides of steel sheet 1 as shown in Table 1 and then heat-treating them. However, for some steel sheets, hot-dip Zn plated steel sheets (GI) that were not heat-treated were used. When overlapping two steel sheets, the spot-welded components (welded joints) were fabricated by placing steel sheet 2 on top of steel sheet 1 as shown in Table 1, and when overlapping three steel sheets, steel sheet 1 was placed in the middle, steel sheet 2 was placed above steel sheet 1, and steel sheet 3 was placed below steel sheet 1, and resistance spot welding was performed.

[0101] [Table 1] TIFF0007831715000002.tif233166

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

[0103] The spot welding conditions were designed to replicate conditions prone to LME cracking by incorporating one of the following welding disturbances (I) to (V). Each of these conditions allows for a localized increase in the temperature and / or tensile stress of the weld area when the electrode is released, thus replicating conditions conducive to LME cracking. The welding disturbances (I) to (V) incorporated during spot welding are shown in the "Pressure Start State" column of Table 1.

[0104] (I) A state in which the plate assembly superimposed on the welding electrode has a striking angle of 0.2 degrees or more. Here, the striking angle is defined as the angle at which the electrode is inclined with respect to the steel plate, that is, the angle between the direction of electrode pressure and the direction of steel plate thickness.

[0105] (II) A condition in which the misalignment of a pair of welding electrodes is 0.1 mm or more. Here, misalignment means a condition 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 plate (gap between electrode and steel plate). Here, the gap between electrode and steel plate is defined as the distance between either electrode and the steel plate when the steel plate and the pair of electrodes are placed before pressurization begins, and is defined as the larger of the two values.

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

[0108] (V) The shortest distance from the center of the weld point to the edge of the steel plate is 10 mm or less. Here, the distance from the center of the weld point to the edge of the steel plate is defined as the shortest distance from the center of the weld point to the edge of any of the two or more overlapping steel plates.

[0109] The spot welding conditions were as follows: Specifically, a servo motor-driven resistance spot welding machine with single-phase AC (50Hz) was used. The electrode (DR type, tip diameter 6mm) of the resistance spot welding machine was subjected to a pressure of 3.5kN, a hold time of 0.1 seconds, and a welding current and welding time that resulted in a nugget diameter of 4.5√tmm, where t was the maximum thickness of the overlapping steel plates.

[0110] <Evaluation of spot-welded components> First, a sample was cut out so that the cross-section in the thickness direction passing through the center of the spot weld constituting the obtained spot-welded member would be the observation surface. Specifically, when two steel plates were stacked, the spot weld formed between steel plate 1 and steel plate 2 was cut out, and when three steel plates were stacked, the spot weld formed between steel plate 1 and steel plate 2, and the spot weld formed between steel plate 1 and steel plate 3 were cut out. Here, all the spot welds formed in the spot-welded member are spot welds formed by spot welding two adjacent steel plates with a Zn-based plating layer between them, and these 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 diamond paste and alcohol. Furthermore, ion milling using Ar ions was performed on the resulting polished surface. Using a field emission electron probe microanalyzer (FE-EPMA), a 100 μm × 100 μm area, which is region R1, was analyzed with an electron beam diameter of 1 μm to measure the Zn concentration and create a Zn concentration map. From the obtained Zn concentration map, regions where the Zn concentration was greater than 0 mass% were extracted.

[0112] Next, the length S1 of grain boundary G1 within region R2 and the length Sa of grain boundary Ga within 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, were measured, and Sa / S1 was calculated. Specifically, the length S1 of grain boundary G1 within region R2 was determined by cutting an analytical sample from region R2 using the focused ion beam (FIB) method, and performing electron backscattered diffraction (EBSD) measurements on the obtained analytical sample using a scanning electron microscope (SEM). The grain boundaries were extracted using data analysis software attached to the EBSD instrument. The obtained EBSD image data was loaded into the image processing software, and the total length S1 of the grain boundaries was calculated.

[0113] Subsequently, the length Sa of the grain boundary Ga where one or more elements selected from the group consisting of Nb, Mo, and V within region R2 were segregated in a total of 0.1 mass% or more was calculated by the following method. First, elemental analysis was performed on the grain boundaries of the analytical sample using energy dispersive X-ray spectroscopy (EDS) (STEM-EDS) with 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 described above. Here, the elemental analysis of the grain boundaries within region R2 was performed by tilting the analytical sample so that the grain boundary to be analyzed was parallel to the direction of electron beam incidence. This is because if the grain boundary is not parallel to the direction of electron beam incidence, the electron beam will penetrate both the grain boundary and the matrix phase, resulting in a measurement value lower than the actual elemental concentration. If the grain boundary shape did not allow for tilting to be parallel to the direction of electron beam incidence, the sample for analysis was removed and repositioned in the sample holder so that it could be tilted parallel. Sa / S1 was calculated from the obtained S1 and Sa. In this case, when there were three plates in the assembly, the spot welds between steel plate 1 and steel plate 2, and between steel plate 1 and steel plate 3 were observed.

[0114] <Evaluation of steel plates for spot welding components> The Nb, Mo, and V content of the solid solution region R3 and precipitate R4 present in the surface layer of a steel plate for spot welding (steel plate 1) up to a depth of 5 μm in the thickness direction was analyzed. First, three 20 × 50 mm test pieces were cut from the prepared steel plate for spot welding.

[0115] Next, the Zn-based plating layer was immersed in hydrochloric acid with an inhibitor added to remove the Zn-based plating layer from the surface of the test specimen. Two electrolytic test specimens were prepared by masking all but the surface area with insulating tape, allowing electrolytic extraction only from the surface. Furthermore, the amount of material dissolved in the test specimen was determined from the relationship between the electrolytic current and electrolysis time, and this was converted to a distance from the surface of the test specimen by dividing it by the area. Electrolytic conditions that resulted in a distance of 5 μm were determined in advance, and electrolysis was performed on the electrolytic test specimens. The electrolyte used was 10 vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol.

[0116] First, the first electrolytic test piece (1) was electrolyzed, and the resulting electrolytic test piece (1) was immersed in methanol. The residue adhering to the electrolytic test piece (1) after electrolysis was dispersed in methanol using ultrasonic vibration to obtain dispersion a(1). Subsequently, dispersion a(1) and the electrolytic solution (1) after electrolysis were filtered using an aluminum filter with a pore size of 20 nm, capturing residue (aggregates (1)) with a particle size exceeding 20 nm on the aluminum filter. The obtained aggregates (1) were acid-decomposed, 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 defined as the electrolysis amount W(1). The average values ​​of Nb, V, and Mo contained in all inclusions and precipitates present on the surface of the electrolytic test specimen (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)) were calculated by dividing the absolute amounts of Nb, Mo, and V obtained by the electrolysis amount W(1).

[0117] Next, the second electrolytic test piece (2) was subjected to the same electrolytic treatment to obtain 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 aggregates of inclusions and precipitates with particle sizes exceeding 20 nm (aggregates (2)). The obtained aggregates (2) were immersed in hexametaphosphoric acid, and dispersion b(2) was obtained by dispersing the aggregated inclusions and precipitates in the hexametaphosphoric acid using ultrasonic vibration. The obtained dispersion b(2) was filtered using a new aluminum filter with a pore size of 20 nm to capture precipitates and inclusions with particle sizes exceeding 20 nm (precipitates (2)) on the aluminum filter. The obtained precipitates (2) were acid-decomposed, and the absolute amounts of Nb, Mo, and V were determined using ICP emission spectrometry. Furthermore, the mass Wa(2) of the electrolytic test specimen (2) before electrolysis and the mass Wb(2) of the electrolytic test specimen (2) after electrolysis, ultrasonic cleaning, and drying were measured, and the difference was defined as the electrolysis amount W(2). By dividing the obtained absolute amounts of Nb, Mo, and V by the electrolysis amount W(2), the average values ​​of Nb, V, and Mo contained in precipitates with particle sizes exceeding 20 nm contained in the surface layer of the electrolytic test specimen were obtained (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%).

[0118] Next, by calculating the difference between the average values ​​of Nb, V, and Mo content in all obtained inclusions and precipitates (Nb content 1 (mass%), Mo content 1 (mass%), V content 1 (mass%)) and the average values ​​of Nb, V, and Mo content in precipitates with a particle size of 20 nm or larger (Nb content 2 (mass%), Mo content 2 (mass%), V content 2 (mass%)), the average values ​​of Nb, V, and Mo content in precipitates with a particle size of 20 nm or less, i.e., precipitate R4, were obtained.

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

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

[0121] <Evaluation of LME crack resistance> First, welds were fabricated using five welding conditions that satisfied one or more of the welding disturbances described in (I) to (V) above. Then, the center of the welded joint was cut with a microcutter, and the cross-section of the weld was observed to evaluate the presence and depth of cracks. Here, cracks with a depth of 5 μm or more were considered present, and LME crack resistance was evaluated according to the following criteria.

[0122] Rating 4: Five welded joints without cracks. Rating 3: 4 welded joints without cracks, 1 welded joint with cracks. Rating 2: Three welded joints were free of cracks, and one welded joint had cracks. Score 1: Two or fewer welded joints without cracks, and three or more welded joints with cracks. Here, a score of 4 to 2 was considered a passing grade.

[0123] As is clear from Table 1, the spot-welded member of the present invention exhibits excellent resistance to LME cracking. Furthermore, the method for manufacturing a spot-welded member using the steel plate for spot-welded members of the present invention makes it possible to manufacture a spot-welded member with excellent resistance to LME cracking even under the above-mentioned welding work disturbances. Therefore, the margin for managing work disturbances during the manufacturing of spot-welded members is improved. [Industrial applicability]

[0124] According to the present invention, it is possible to provide a spot-welded member with excellent resistance to LME cracking, a steel plate for spot-welded members, and a method for manufacturing spot-welded members. [Explanation of Symbols]

[0125] 1 Spot welding component 2 Spot welds 3 Base metal part 4 nuggets 5 Heat affected zone (HAZ) 6. The tip of the gap between the two steel plates 7. Center of the spot weld (nugget) 8 straight line 9. The end of the nugget

Claims

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

2. The spot welded member 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 plate for a spot-welded member in which at least one pair of 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 steel plate constitutes one of the two adjacent steel plates in the spot-welded member, The surface layer of the steel plate, from the surface to a depth of 5 μm in the 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-dissolved, and / or precipitates R4 containing one or more elements selected from the group consisting of Nb, Mo, and V with a particle size of 20 nm or less. A steel sheet for spot welding members, wherein the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the solid solution region R3 and the sum of the average values ​​of the respective contents of Nb, Mo, and V contained in the precipitate R4 is 0.05% by mass or more.

4. The steel plate for spot welding member according to claim 3, further comprising a Zn-based plating layer on the surface.

5. A method for manufacturing a spot-welded member, comprising arranging at least one pair of 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, A method for manufacturing a spot-welded member, comprising arranging, at least one of the two adjacent steel plates, on the side of the steel plate that experiences more stress during spot welding, such that the surface of the spot-welded member steel plate according to claim 3 is in contact with the Zn-based plating layer disposed on the surface of the other of the two adjacent steel plates, and performing spot welding on the plurality of steel plates.

6. A method for manufacturing a spot-welded member, comprising arranging at least one pair of 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, A method for manufacturing a spot-welded member, comprising: placing the spot-welded member steel plate according to claim 4 on the side of at least two adjacent steel plates where more stress is applied during spot welding, such that the Zn-based plating layer of the spot-welded member steel plate is in contact with the surface of the other of the two adjacent steel plates; and performing spot welding on the plurality of steel plates.

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