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 spot-welded member that satisfies two or more of the following conditions (a), (b), and (c), where region R1 is a 100 μm square area centered on a point 10 300 μm away from the end 9 of the nugget toward the base material 3, region R2 is an area where the Zn concentration is 10% by mass or more, and the area of ​​region R2 is S2. (a) When the region R2 has a Si concentration of less than 0.5 mass%, and the area of ​​region Ra is denoted as Sa, the ratio of Sa to S2, Sa / S2, is 0.50 or more. (b) In region R2, when the region with a Zn concentration of 50% by mass or more is called region Rb, and the area of ​​region Rb is called Sb, the ratio of Sb to S2, Sb / S2, is 0.20 or less. (c) In region R2, when the ferrite phase region is denoted as region Rc and the area of ​​region Rc as Sc, the ratio of Sc to S2, Sc / S2, is 0.30 or greater.

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 simultaneously 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 steel sheets for automobiles has been increased. For example, as a skeletal material for an automobile cabin, the application of high-strength steel sheets with a tensile strength of 980 MPa or more is required. In the assembly of automobiles using such high-strength steel sheets, from the viewpoints of efficiency and cost, it is often the case that press-formed steel sheets are overlapped and joined by resistance spot welding.

[0003] Recently, it has been confirmed that cracks occur in the spot weld when a zinc (Zn)-based plated steel sheet is included in the combination of plates (plate set) during resistance spot welding. Here, the Zn-based plated steel sheet refers to a steel sheet provided with 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 pressure applied by the welding electrode 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 said to be 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. A spot-welded member in which, within region R1 of the spot-welded area, a region R2 is defined as a region where the Zn concentration is 10% by mass or more, and the area of ​​region R2 is defined as S2, such that region R2 satisfies two or more of the following conditions (a), (b), and (c). 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. (a) When the region R2 is defined as region Ra in which the Si concentration is less than 0.5 mass%, and the area of ​​region Ra is defined as Sa, the ratio of the area Sa to the area S2, Sa / S2, is 0.50 or more. (b) When the region R2 is defined as region Rb in which the Zn concentration is 50% by mass or more, and the area of ​​region Rb is defined as Sb, the ratio of the area Sb to the area S2, Sb / S2, is 0.20 or less. (c) When the region R2 is defined as region Rc, and the area of ​​region Rc is defined as Sc, the ratio of the area Sc to the area S2, Sc / S2, is 0.30 or greater.

[0017] [2] 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, A first layer L1 with a solid solution Si concentration of less than 0.5% by mass and a second layer L2 with a C concentration of 0.02% by mass or less are provided. The first layer L1 is in contact with the surface of the steel sheet for spot welding members. The second layer L2 is (i) in contact with the surface of the steel sheet for spot welding members, (ii) not in contact with the surface of the steel sheet for spot welding members and overlapping with the first layer L1, or (iii) the surface of the second layer L2 on the surface side of the steel sheet for spot welding members is in contact with the surface of the first layer L1 on the side opposite to the surface of the steel sheet for spot welding members. A steel sheet for spot welding members that satisfies any one of the following conditions (1), (2), and (3) when the thickness of the first layer L1 is T1 and the thickness of the second layer L2 is T2. (1) T1 ≥ 3.0 μm (2) 15T1 + T2 ≥ 50 μm and 2.0 μm ≤ T1 < 3.0 μm (3) 15T1 + T2 ≥ 50 μm and 1.0 μm ≤ T1 < 2.0 μm

[0018] [3] The steel sheet for spot welding members according to [2], further comprising a Zn-based plating layer on the surface.

[0019] [4] A method for manufacturing a spot welding member by arranging at least one set of two adjacent steel sheets out of a plurality of steel sheets in a state where a Zn-based plating layer exists between them and performing spot welding on the plurality of steel sheets. Among at least the two adjacent steel sheets, the surface of the steel sheet for spot welding members according to [2] is arranged to contact the Zn-based plating layer disposed on the surface of the other steel sheet among the two adjacent steel sheets on the side where more stress is applied during spot welding, and spot welding is performed on the plurality of steel sheets. A method for manufacturing a spot welding member.

[0020] [5] A method for manufacturing a spot welding member by arranging at least one set of two adjacent steel sheets out of a plurality of steel sheets in a state where a Zn-based plating layer exists between them and performing spot welding on the plurality of steel sheets. A method for manufacturing a spot-welded member, comprising: placing the spot-welded member steel plate described in [3] 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. [Effects of the Invention]

[0021] 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. [Brief explanation of the drawing]

[0022] [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]

[0023] (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, a region R2 is defined as a region where the Zn concentration is 10% by mass or more, and the area of ​​the region R2 is defined as S2, and the region R2 satisfies two or more of the following conditions (a), (b), and (c). Here, region R1 is a 100 μm square region centered on a point 300 μm away from the end of the nugget towards the base material side along the straight line, in a cross-section in the thickness direction passing through the center of the nugget, where the intersection of the straight line passing through the center of the nugget and the leading edge of the gap between the two adjacent steel plates and the boundary of the nugget is defined as the end of the nugget. (a) When the region R2 is defined as region Ra in which the Si concentration is less than 0.5 mass%, and the area of ​​region Ra is defined as Sa, the ratio of the area Sa to the area S2, Sa / S2, is 0.50 or more. (b) When the region R2 is defined as region Rb in which the Zn concentration is 50% by mass or more, and the area of ​​region Rb is defined as Sb, the ratio of the area Sb to the area S2, Sb / S2, is 0.20 or less. (c) When the region R2 is defined as region Rc, and the area of ​​region Rc is defined as Sc, the ratio of the area Sc to the area S2, Sc / S2, is 0.30 or greater.

[0024] 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. As a result, they found that there are regions where LME cracking is likely to occur, and that when Zn penetrates into a part of the grain boundary in that region, causing grain boundary embrittlement, the tensile stress during spot welding concentrates at the location where grain boundary embrittlement occurs, becoming the starting point for LME cracking. In other words, they considered that controlling the microstructure and composition of the region that becomes the starting point for LME cracking is important for suppressing LME cracking, and after various studies, completed the present invention.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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 present between them.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] <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).

[0034] <Area R2> The inventors have found that when region R2 is defined as a region within region R1 where the Zn concentration is 10% by mass or more, it is important to control the composition and microstructure of region R2. This is based on the finding that (i) a certain level of Zn concentration is necessary for LME cracking to occur, (ii) the coexistence of Zn and Si under conditions where the Zn concentration is above a certain level promotes LME cracking, and (iii) the presence of a certain proportion or more of ferrite, which has the effect of reducing tensile residual stress at high temperatures, suppresses LME cracking. LME cracking can be suppressed in region R2 by satisfying two or more of the following conditions (a), (b), and (c). (a) In region R2, the region where the Si concentration is less than 0.5 mass% is defined as region Ra, and the area of ​​region Ra is defined as Sa. The ratio of the area Sa to the area S2 of region R2, Sa / S2, is 0.50 or greater. (b) In region R2, the region where the Zn concentration is 50% by mass or more is defined as region Rb, and the area of ​​region Rb is defined as Sb. In this case, the ratio of the area of ​​Sb to the area of ​​region R2, Sb / S2, is 0.20 or less. (c) In region R2, when the region of the ferrite phase is defined as region Rc and the area of ​​region Rc is defined as Sc, the ratio of the area Sc to the area S2 of region R2, Sc / S2, is 0.30 or greater.

[0035] Note that region Ra in region R2 exists as a single region. In contrast, regions Rb and Rc are scattered within region R2. As shown in the measurement method described later, regions Ra, Rb, and Rc are determined so that they do not overlap.

[0036] First, regarding condition (a), during spot welding, the heat generated during spot welding melts the Zn-based plating layer, and the Zn diffuses into the interior of the upper and lower steel plates in contact with the Zn-based plating layer, forming region R2 within region R1. Within region R2, if Sa / S2 is less than 0.50, the influence of Si, which promotes LME cracking, becomes significantly apparent; that is, grain boundary embrittlement by Zn is accelerated, making LME cracking more likely. Therefore, Sa / S2 should be 0.50 or higher.

[0037] Furthermore, regarding condition (b), region R2 is located in the heat-affected zone 5. If Sb / S2 exceeds 0.20 within region R2, grain boundary embrittlement by Zn progresses in region R2, making LME cracking more likely. Therefore, the above Sb / S2 should be 0.20 or less.

[0038] Furthermore, regarding condition (c), if Sc / S2 is less than 0.30 within region R2, the amount of ferrite phase present in region R2 decreases, resulting in excessive tensile residual stress due to the tensile stress generated during spot welding, making LME cracking more likely. This is because the ferrite phase has a low solid solution carbon concentration, which lowers the yield stress (YS) at high temperatures during welding and reduces tensile residual stress. Therefore, Sc / S2 should be 0.30 or higher.

[0039] Here, in order to suppress LME cracking of the spot welded member 1, in the region R2 within the spot weld 2 constituting the spot welded member 1, it is sufficient that at least two of the three conditions (a) to (c) above be satisfied, and it is preferable that all three conditions be satisfied. For example, even if conditions (a) and (b) are satisfied but condition (c) is not, by satisfying conditions (a) and (b), grain boundary embrittlement by Zn in region R2, which is the Zn penetration region, is suppressed. This makes it possible to suppress LME cracking even if there is an excessive tensile residual stress. Furthermore, even if conditions (a) and (condition (b)) are not satisfied, by satisfying conditions (b) and (condition (a)), grain boundary embrittlement by Zn is suppressed, and by satisfying condition (c), it is possible to reduce the tensile residual stress, thereby suppressing LME cracking.

[0040] Furthermore, the structure other than Ra, Rb, and Rc is not limited and may include, for example, martensite containing 0.5% by mass or more of Si.

[0041] Here, the method for measuring the composition of the region R2 within the above region R1 and the area ratios of the regions Ra, Rb, and Rc within the region R2 is as follows. First, a sample is cut out such that the cross-section in the plate thickness direction passing through the center 7 of the spot weld portion 2 (i.e., the nugget 4) constituting the spot weld member 1 serves as the observation surface. Next, after coarsely polishing the observation surface of the sample using waterproof abrasive paper, it is mirror-polished using diamond paste. When water is used during polishing, the Zn-rich phase, i.e., the region Rb with a Zn concentration of 50 mass% or more, reacts with the water during polishing and disappears, so it is carried out using alcohol etc. without using water (water prohibited). Further, ion milling using Ar ions may be performed on the obtained polished surface. Next, using a field emission electron probe microanalyzer (FE-EPMA), a 100 μm × 100 μm range, which is the above region R1, is analyzed with an electron beam diameter of 1 μm, and the Si concentration and Zn concentration are measured. From the obtained Zn concentration map and Si concentration map, the region R2 with a Zn concentration of 10 mass% or more is extracted, and the area S2 of the region R2 is calculated. Further, the region Rb with a Zn concentration of 50 mass% or more within the region R2 is extracted, and the area Sb of the region Rb is calculated. Subsequently, under the condition that the acceleration voltage is 15 kV, using a SEM (Scanning Electron Microscope), the region R2 is observed, the ferrite phase region Rc is extracted from the obtained tissue image, and the area Sc of the region Rc is calculated. Finally, in the region within the region R2 that does not belong to either of the above regions Rb and Rc, the region Ra with a Si concentration of less than 0.5 mass% is extracted, and the area Sa of the region Ra is calculated. From the values of each area Sa, Sb, Sc, S2 obtained above, Sa / S2, Sb / S2, Sc / S2 are calculated.

[0042] <Zn-based plating layer> The Zn-based plating layer is a Zn-based plating layer placed on the surface of a steel sheet, and a Zn-based plated steel sheet should be supplied 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-welded portion 2 formed by spot-welding two Zn-based plated steel sheets together, or by overlapping a Zn-based plated steel sheet and another steel sheet. Therefore, the spot-welded member 1 of the present invention is a spot-welded member comprising at least one set of a spot-welded portion and a base metal portion, formed by spot-welding two adjacent steel sheets among a plurality of steel sheets with a Zn-based plating layer between them.

[0043] 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).

[0044] 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.

[0045] 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 base steel sheet of the Zn-plated steel sheet be 980 MPa or higher. This allows the effects of the present invention to be more fully realized.

[0046] (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, comprising a first layer L1 having a solid solution Si concentration of less than 0.5 mass%, and a second layer L2 having a C concentration of 0.02 mass% or less, wherein the first layer L1 is in contact with the surface of the steel sheet for spot welding members, and Layer L2 of layer 2 satisfies any one of the following conditions (1), (2), and (3), where (i) it is in contact with the surface of the steel sheet for spot welding members, (ii) it is not in contact with the surface of the steel sheet for spot welding members and overlaps with the first layer L1, or (iii) the surface of the second layer L2 on the side facing the surface of the steel sheet for spot welding members is in contact with the surface of the first layer L1 on the side opposite to the surface of the steel sheet for spot welding members, and when the thickness of the first layer L1 is T1 and the thickness of the second layer L2 is T2. (1) T1 ≥ 3.0 μm (2) 15T1 + T2 ≥ 50 μm, and 2.0 μm ≤ T1 < 3.0 μm (3) 15T1 + T2 ≥ 50 μm, and 1.0 μm ≤ T1 < 2.0 μm

[0047] 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.

[0048] 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 as the steel plate on the side where tensile stress is applied during spot welding, and to arrange the first layer L1 and the second layer L2 on the Zn-based plating layer side. This allows the spot welding member 1 to be manufactured under general spot welding conditions.

[0049] Here, the first layer L1 is a layer in which the solid solution Si concentration is less than 0.5 mass% in the thickness direction from the surface on the Zn-based plating layer side, and the second layer L2 is a layer in which the C concentration is 0.02 mass% or less. The first layer L1 is in contact with the surface of the steel sheet for spot welding (i.e., the upper surface of the first layer L1 is exposed). The second layer L2 can be (i) in contact with the above surface of the steel sheet for spot welding (i.e., the upper surface of the second layer L2 is exposed), (ii) not in contact with the above surface of the steel sheet for spot welding and overlapping with the first layer L1, or (iii) the surface of the second layer L2 on the side of the surface of the steel sheet for spot welding is in contact with the surface of the first layer L1 on the opposite side of the surface of the steel sheet for spot welding (i.e., the upper surface of the second layer L2 and the lower surface of the first layer L1 are in continuous contact). Furthermore, when the thickness of the first layer L1 is T1 and the thickness of the second layer L2 is T2, one or more of the following conditions (1), (2), and (3) are satisfied. (1) T1 ≥ 3.0 μm (2) 15T1 + T2 ≥ 50 μm, and 2.0 μm ≤ T1 < 3.0 μm (3) 15T1 + T2 ≥ 50 μm, and 1.0 μm ≤ T1 < 2.0 μm

[0050] By satisfying any one of the above conditions (1), (2), and (3) of the steel plate for spot welding members, the region R2 within region R1 in the spot welded portion 2 constituting the spot welding member 1 will satisfy any two or more of the above conditions (a), (b), and (c), making it possible to suppress the occurrence of LME cracks.

[0051] Specifically, in a steel sheet for spot-welded members, if the thickness T1 of the first layer L1 in condition (1) is 3.0 μm or more, even if Si diffusion occurs from the inside of the steel sheet due to the heat during spot welding, a first layer L1 of sufficient thickness remains. As a result, Si does not diffuse into region R2 of the spot-welded member 1, the area Sa of region Ra increases, and the area Sb of region Rb decreases. Therefore, conditions (a) and (b) can be satisfied. If condition (1) is satisfied, LME crack occurrence can be suppressed regardless of the thickness of T2, but it is preferable if T2 is 20 μm or more. Here, since the heat-affected time during welding is short, the diffusion of C from the inside of the steel sheet to the surface of the steel sheet does not need to be considered. Therefore, when T2 satisfies 20 μm or more, the second layer L2 can maintain a low amount of C, an austenite-stabilizing element, making austenite transformation less likely to occur even at high temperatures during welding, and allowing the ferrite phase to exist stably. Thus, in addition to conditions (a) and (b), condition (c) can also be satisfied.

[0052] When condition (2) is satisfied, that is, when T1 is present with a thickness of 2.0 μm or more and 15T1 + T2 ≥ 50 μm is satisfied, the amount of ferrite increases in region R2 of the spot-welded member. Since ferrite has a high solid solubility limit for Zn and dissolves the surrounding Zn, the area Sb of region Rb decreases and the area Sa of region Ra increases. Therefore, conditions (a) and (b) can be satisfied.

[0053] When condition (3) is satisfied, that is, when T1 is present with a thickness of 1.0 μm or more and less than 2.0 μm, and 15T1 + T2 ≥ 50 μm is satisfied, the amount of ferrite in region R2 increases significantly. Ferrite has a high solid solubility limit for Zn, and it solid dissolves the surrounding Zn, so the area Sb of region Rb decreases. Therefore, condition (b) can be satisfied. Since the thickness of T1 is thinner compared to conditions (1) and (2), condition (a) cannot be satisfied, but when 15T1 + T2 ≥ 50 μm is satisfied, a state in which the amount of C, an austenite-stabilizing element, is low can be maintained, austenite transformation is less likely to occur even at high temperatures during welding, and the ferrite phase can exist stably, so conditions (b) and (c) can be satisfied.

[0054] In the present invention, when a spot-welded member is manufactured using a steel sheet with a Si concentration of 0.5% by mass or more in its component composition, LME cracking becomes significant. Therefore, it is preferable to use a steel sheet for spot-welded members that has the above-mentioned component composition and whose surface is manufactured to satisfy any one of the conditions (1), (2), and (3) of the present invention.

[0055] As for the composition of the steel sheet mentioned above, in order to improve various properties such as mechanical properties, various structural or microstructural modifications can be carried out individually or in combination, 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; 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 at which recrystallization does not occur or partial recrystallization strengthening that leaves an unrecrystallized region without complete recrystallization; strengthening by transformation microstructure such as single-phase formation of bainite or martensite or composite microstructure formation of ferrite and these transformation microstructures; fine-grain strengthening represented by the Hall-Petch formula: 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.

[0056] Examples of the component 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.

[0057] Here, the method for measuring the solid-solution Si concentration and its thickness T1 in the first layer L1 is as follows. First, a sample is cut out using focused ion beam (FIB) spectroscopy from the surface of the steel sheet or from the steel sheet surface directly beneath the Zn-based plating layer of a Zn-based plated steel sheet, so that the cross-section in the thickness direction becomes the observation surface. Next, the solid-solution Si concentration is measured using energy-dispersive X-ray spectroscopy (EDS) with a scanning transmission electron microscope (STEM). Specifically, first, an EDS line profile is obtained at intervals of 0.01 μm from the surface layer of the observation surface obtained from FIB using STEM-EDS. Next, the Si concentration distribution in the region excluding precipitates and oxides is determined from the obtained line profile, and the layer containing the region with a solid-solution Si concentration of less than 0.5 mass% from the surface of the Zn-based plating layer on the steel sheet is defined as L1, and T1 is obtained by measuring its thickness. Note that precipitates and oxides can be removed by STEM observation. Furthermore, the thickness of layer L1 can be determined, for example, by fitting a plot of Si concentration with an appropriate function and basing the result on the obtained fitting curve. It is preferable to measure the solid solution Si concentration and the thickness of layer L1 at two or more locations, and if measurements are taken at two or more locations, the average value can be used as the thickness of the first layer L1.

[0058] Furthermore, the carbon concentration and thickness T2 of the second layer L2 can be measured using a glow discharge spectrometry (GDS). From the carbon concentration profile in the thickness direction obtained from the GDS, the region where the carbon concentration is 0.02 mass% or less is defined as the second layer L2, and its thickness is defined as T2. It is preferable to perform measurements at two or more locations, and if measurements are taken at two or more locations, the average value can be used as the thickness of the second layer L2.

[0059] Furthermore, the steel sheet for spot welding can 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. In this case, the Zn-based plating layer is provided on the first layer L1.

[0060] (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 on the side of the two adjacent steel plates where the stress is greater during spot welding, such that the surface of the steel plate for spot-welded member according to the present invention (i.e., the surface of the first layer L1) 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 is performed on the plurality of steel plates.

[0061] 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.

[0062] 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.

[0063] 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 two adjacent steel plates, and spot welding is performed on multiple steel plates.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] (Method of manufacturing steel plates for spot welding components) Next, an example of a method for manufacturing steel sheets for spot welding members will be described. For example, a material (cladding material) adjusted to a composition and thickness that satisfies any one of conditions (1), (2), and (3) can be prepared, and a clad steel sheet can be manufactured by bonding the cladding material to the surface of a steel sheet, and this can be used as a steel sheet for spot welding members. Here, the steel sheet to which the cladding material is bonded is not particularly limited, but it is preferable to apply it to a steel sheet that is prone to LME cracking. Examples of steel sheets prone to LME cracking include steel sheets having a composition with a Si concentration of 0.5 mass% or more. Examples of the above steel sheets include cold-rolled steel sheets. Cold-rolled steel sheets can be manufactured by known manufacturing methods. For example, a steel slab having the above-mentioned composition can be hot-rolled to make a hot-rolled sheet, and then the hot-rolled sheet can be pickled and then cold-rolled to make a cold-rolled steel sheet.

[0070] The following methods can be used to manufacture the composite material. First, molten low-alloy steel is prepared using known methods such as a converter, electric furnace, or vacuum melting furnace, and then the obtained molten steel is solidified to produce a steel slab. The method for producing the steel slab from the 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 manufacture the steel slab by continuous casting. The obtained steel slab is hot-rolled under commonly used conditions to form the composite material.

[0071] The composite material obtained as described above may be subjected to annealing, pickling, polishing, etc., as necessary. The composite material is stacked so that the surface (compression surface) of the steel plate is in a vacuum, and the four sides of the compression surface are sealed by welding to assemble it. The joining method is not particularly limited, but for example, the composite material and the steel plate are stacked, and the composite material and the steel plate are joined by performing electron beam welding (EBW), arc welding, or laser beam welding on the four sides of the ends of the stacked material.

[0072] 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.

[0073] Alternatively, when manufacturing steel sheets, a first layer L1 may be formed on the surface of the steel sheet by forming an internal Si oxide layer, and a second layer L2 may be formed by forming a decarburized layer.

[0074] Here, we will describe an example of a method for forming an internal Si oxide layer and a decarburized layer on the surface of a steel sheet. For example, the dew point during annealing can be increased to form the internal Si oxide layer and the decarburized layer on the surface of the steel sheet. Specifically, annealing should be performed with a dew point of -15°C or higher, an annealing temperature of 740°C to 900°C, and a holding time of 20 seconds or more. Here, the internal Si oxide layer refers specifically to a region in which Si oxide is formed within the crystal grains and / or at part of the grain boundaries.

[0075] The steel sheet for spot welding components described above may have a Zn-based plating layer formed on its surface using the method described above. For example, a molten zinc plating treatment can be used to form the Zn-based plating layer. The molten zinc plating treatment can be performed by applying the molten zinc plating treatment to the steel sheet surface as described above; the conditions for the molten zinc plating treatment are not particularly limited and can be carried out using conventional methods. The amount of molten zinc plating layer applied is 20 g / m² per side of the steel sheet surface. 2 More than 80g / m 2 The following is preferable. Alternatively, an alloying treatment may be performed after the hot-dip Zn plating treatment described above to form an alloyed hot-dip Zn plating layer. The conditions for the alloying treatment are not particularly limited and can be carried out by conventional methods. [Examples]

[0076] 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.

[0077] <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.

[0078] [Table 1] TIFF0007831714000002.tif233119

[0079] 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.

[0080] The spot welding conditions were set to incorporate one of the following welding disturbances (I) to (V) in order to reproduce conditions that are prone to LME cracking. All of these conditions allow for a localized increase in the temperature and / or tensile stress of the weld when the electrode is released, thus reproducing conditions that are prone to LME cracking. Table 1 shows the above welding disturbances (I) to (V) incorporated during spot welding in the "Pressure Start State" column of Table 1.

[0081] (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.

[0082] (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.

[0083] (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 greater of the two values.

[0084] (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.

[0085] (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.

[0086] 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.

[0087] <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.

[0088] 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 and Si concentrations, and Zn concentration maps and Si concentration maps were created. From the obtained Zn concentration map, region R2, in which the Zn concentration was 10 mass% or higher, was extracted, and the area S2 of region R2 was calculated.

[0089] Next, region Rb, where the Zn concentration is 50 mass% or higher within region R2, was extracted, and the area Sb of region Rb was calculated. Furthermore, region R2 was observed using a Scanning Electron Microscope (SEM) under the condition of an acceleration voltage of 15 kV, and the ferrite phase region Rc was extracted from the obtained tissue image, and the area Sc of region Rc was calculated. In addition, region Ra, where the Si concentration is less than 0.5 mass% within the region R2 excluding regions Rb and Rc, was extracted, and the area Sa of region Ra was calculated.

[0090] Subsequently, Sa / S2, Sb / S2, and Sc / S2 were calculated.

[0091] In this case, when there were three plates in the assembly, the spot welds between the plates 1 and 2, and between 1 and 3 were observed.

[0092] <Evaluation of steel plates for spot welding components> The solid solution Si concentration and its thickness T1 in the first layer L1 of a spot-welded steel sheet (steel sheet 1) were determined by the following method. First, a sample was cut out using focused ion beam (FIB) spectroscopy so that the cross-section in the thickness direction from the surface of the steel sheet (the surface of the steel sheet directly beneath the Zn-based plating layer of the Zn-based plated steel sheet) served as the observation surface. The sample was then measured using energy-dispersive X-ray spectroscopy (EDS) with a scanning transmission electron microscope (STEM).

[0093] Specifically, for the observation surface obtained from FIB, EDS-line profiles were acquired at 0.01 μm intervals using STEM-EDS, moving from the surface side of the steel plate towards the interior in the thickness direction. The Si concentration distribution was determined in the region excluding precipitates, and the first layer containing a region with a solid solution Si concentration of less than 0.5 mass% was defined as L1, and its thickness T1 was measured.

[0094] Furthermore, the carbon concentration and thickness T2 of the second layer L2 were measured using a glow discharge spectrometry (GDS). Specifically, the region where the carbon concentration was 0.02 mass% or less was defined as the second layer L2 from the carbon profile obtained from the steel plate surface toward the interior in the thickness direction obtained from the GDS, and its thickness T2 was determined.

[0095] <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.

[0096] 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.

[0097] 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]

[0098] 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]

[0099] 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 10 A point 300 μm away from the end 9 of the nugget, along the straight line 8, towards the base material 3.

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. A spot-welded member in which, within a region R1 in the spot-welded area, a region R2 is defined as a region where the Zn concentration is 10% by mass or more, and the area of ​​region R2 is defined as S2, such that region R2 satisfies two or more of the following conditions (a), (b), and (c). 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. (a) When the region R2 in which the Si concentration is less than 0.5 mass% is defined as region Ra, and the area of ​​region Ra is defined as Sa, the ratio of the area Sa to the area S2, Sa / S2, is 0.50 or more. (b) When the region R2 has a Zn concentration of 50% by mass or more, and the area of ​​the region Rb is defined as Sb, the ratio of the area Sb to the area S2, Sb / S2, is 0.20 or less. (c) When the region R2 is defined as region Rc, and the area of ​​region Rc is defined as Sc, the ratio of the area Sc to the area S2, Sc / S2, is 0.30 or greater.

2. 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, It comprises a first layer L1 having a solid solution Si concentration of less than 0.5% by mass, and a second layer L2 having a C concentration of 0.02% by mass or less. The first layer L1 is in contact with the surface of the steel plate for the spot welding member, The second layer L2 is (i) in contact with the surface of the spot welding member steel plate, (ii) not in contact with the surface of the spot welding member steel plate and overlapping with the first layer L1, or (iii) the surface of the second layer L2 on the side facing the surface of the spot welding member steel plate is in contact with the surface of the first layer L1 opposite to the surface of the spot welding member steel plate. A steel sheet for spot welding members that satisfies any one of the following conditions (1), (2), and (3), when the thickness of the first layer L1 is T1 and the thickness of the second layer L2 is T2. (1) T1≧3.0μm (2) 15T1 + T2 ≥ 50 μm, and 2.0 μm ≤ T1 < 3.0 μm (3) 15T1 + T2 ≥ 50 μm, and 1.0 μm ≤ T1 < 2.0 μm

3. The steel sheet for spot welding member according to claim 2, further comprising a Zn-based plating layer on the surface.

4. 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, among at least 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 2 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.

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: placing the spot-welded member steel plate according to claim 3 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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