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

Abstract

The present invention provides a spot welded member which has excellent LME cracking resistance. Disclosed is a spot welded member which satisfies two or more of the conditions (a), (b), and (c) described below, where region R1 is a 100 μm square region centered on a point 10 that is separated by 300 µm from an end part 9 of a nugget toward a base material part 3 side, region R2 is an area having a Zn concentration of 10 mass% or more in the region R1, and S2 is the area of the region R2. (a) If region Ra is an area having an Si concentration of less than 0.5 mass% in the region R2, and Sa is the area of the region Ra, the ratio Sa / S2 of Sa to S2 is 0.50 or more. (b) If region Rb is an area having a Zn concentration of 50 mass% or more in the region R2, and Sb is the area of the region Rb, the ratio Sb / S2 of Sb to S2 is 0.20 or less. (c) If region Rc is an area of the ferrite phase in the region R2, and Sc is the area of the region Rc, the ratio Sc / S2 of Sc to S2 is 0.30 or more.

Description

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0016] [1] A spot-welded component in which at least one set of two adjacent steel plates among a plurality of steel plates are arranged with a Zn-based plating layer present between them, and the plurality of steel plates are spot-welded together, wherein the spot-welded component comprises at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel plates with a Zn-based plating layer present between them, and the spot weld comprises a nugget and a heat-affected zone, and wherein, in a region R1 within the spot weld, a region R2 where the Zn concentration is 10 mass % or more is defined as a region R2, and when the area of ​​the region R2 is defined as S2, the region R2 satisfies two or more of the following conditions (a), (b), and (c): Here, the region R1 is a 100 μm square region in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget, and centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion. (a) When a region in the region R2 where the Si concentration is less than 0.5 mass% is defined as region Ra and the area of ​​the region Ra is defined as Sa, the ratio Sa / S2 of the area Sa to the area S2 is 0.50 or more; (b) When a region in the region R2 where the Zn concentration is 50 mass% or more is defined as region Rb and the area of ​​the region Rb is defined as Sb, the ratio Sb / S2 of the area Sb to the area S2 is 0.20 or less; (c) When a region in the region R2 where the ferrite phase is defined as region Rc and the area of ​​the region Rc is defined as Sc, the ratio Sc / S2 of the area Sc to the area S2 is 0.30 or more.

[0017] [2] A steel sheet for a spot-welded component, in which at least one set of two adjacent steel sheets among a plurality of steel sheets are arranged with a Zn-based plating layer present between them, and the plurality of steel sheets are spot-welded together, the steel sheet constituting one of the two adjacent steel sheets, comprises a first layer L1 having a solute Si concentration of less than 0.5 mass% and a second layer L2 having a C concentration of 0.02 mass% or less, the first layer L1 being in contact with a surface of the steel sheet for the spot-welded component, and the second layer L2 (i) being in contact with the surface of the steel sheet for the spot-welded component, (ii) not in contact with the surface of the steel sheet for the spot-welded component and overlapping with the first layer L1, or (iii) a surface of the second layer L2 on the surface side of the steel sheet for the spot-welded component being in contact with a surface of the first layer L1 opposite to the surface of the steel sheet for the spot-welded component, A steel sheet for spot-welded components, which satisfies any one of the following conditions (1), (2), and (3), where T1 is the thickness of the first layer L1 and T2 is the thickness of the second layer L2: (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-welded components according to [2], further comprising a Zn-based plating layer on the surface.

[0019] [4] A method for manufacturing a spot-welded component by arranging at least one set of two adjacent steel plates among a plurality of steel plates with a Zn-based plating layer present between them, and spot-welding the plurality of steel plates, comprising: arranging, of at least the two adjacent steel plates, the steel plate for spot-welded components described in [2] on the steel plate side that is subjected to more stress during spot welding, so that the surface of the steel plate for spot-welded components described in [2] is in contact with the Zn-based plating layer arranged on the surface of the other steel plate of the two adjacent steel plates; and spot-welding the plurality of steel plates.

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

[0021] According to the present invention, it is possible to provide a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component, which are excellent in LME cracking resistance.

[0022] 1 is a schematic diagram of a cross section in the plate thickness direction passing through the center of a spot weld of a spot-welded member according to an embodiment of the present invention. FIG.

[0023] (Spot-welded component) Hereinafter, an embodiment of the present invention will be described with reference to the drawings. A spot-welded component according to the present invention is a spot-welded component in which at least one set of two adjacent steel sheets among a plurality of steel sheets are arranged with a Zn-based plating layer present between them, and the plurality of steel sheets are spot-welded together, the spot-welded component comprising at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel sheets with the Zn-based plating layer present between them, the spot weld comprising a nugget and a heat-affected zone, and wherein, in a region R1 within the spot weld, a region R2 having a Zn concentration of 10 mass % or more is designated as a region R2, and the area of ​​the region R2 is designated as S2, the region R2 satisfies two or more of the following conditions (a), (b), and (c): Here, the region R1 is a 100 μm square region centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion, in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget. (a) When a region in the region R2 where the Si concentration is less than 0.5 mass% is defined as region Ra and the area of ​​the region Ra is defined as Sa, the ratio Sa / S2 of the area Sa to the area S2 is 0.50 or more; (b) When a region in the region R2 where the Zn concentration is 50 mass% or more is defined as region Rb and the area of ​​the region Rb is defined as Sb, the ratio Sb / S2 of the area Sb to the area S2 is 0.20 or less; (c) When a region in the region R2 where the ferrite phase is defined as region Rc and the area of ​​the region Rc is defined as Sc, the ratio Sc / S2 of the area Sc to the area S2 is 0.30 or more.

[0024] The present inventors first conducted extensive research into spot-welded components with excellent LME cracking resistance, their manufacturing methods, and steel sheets for spot-welded components. The inventors focused on the microstructure of spot-welded components obtained by spot-welding a Zn-based plated steel sheet and a steel sheet without a Zn-based plated layer together, and conducted detailed research into the relationship between LME cracking and the microstructure of spot-welded components. As a result, they found that there are regions prone to LME cracking, and that when Zn penetrates into some of the grain boundaries in those regions, causing intergranular embrittlement, tensile stress during spot welding concentrates at the locations where intergranular embrittlement occurs, becoming the initiation points for LME cracking. In other words, they believed that controlling the microstructure and composition of the regions that serve as initiation points for LME cracking is important for suppressing LME cracking, and they conducted various studies to complete the present invention.

[0025] The present invention has been made based on the above findings. However, the present invention is not limited to the following embodiments. Furthermore, the components in the following embodiments include those that are easily replaceable by a person skilled in the art, or those that are substantially identical.

[0026] The spot-welded component of the present invention is a spot-welded component in which at least one set of two adjacent steel sheets among a plurality of steel sheets are arranged with a Zn-based plating layer present between them, and the plurality of steel sheets are spot-welded together.

[0027] 1 shows a schematic diagram of a cross section in the plate thickness direction passing through the center of a spot weld of a spot weld that constitutes a spot-welded member according to one embodiment of the present invention. The spot-welded member 1 includes at least one pair of a spot weld 2 that is the subject of the present invention and a base material portion 3. The spot weld 2 is also composed of a nugget 4 and a heat-affected zone 5.

[0028] 1 is an example of a spot-welded component 1 formed by spot-welding two adjacent steel sheets, in which the first steel sheet (lower sheet) is a zinc-based plated steel sheet with a zinc-based plating layer (not shown), and the second steel sheet (upper sheet) is a steel sheet (without a zinc-based plating layer), and a mating surface is formed where the Zn-based plating layer surface of the lower sheet meets the steel sheet surface (lower surface of the upper sheet), and the spot-welded component 1 includes at least one pair (one in the example of FIG. 1 ) of a spot weld 2 and a base material portion 3 obtained by spot welding. The spot weld 2 is composed of a nugget 4 and a heat-affected zone (HAZ) 5.

[0029] When the spot-welded member 1 of the present invention is formed from two or more steel plates, it is sufficient that the spot-welded member 1 includes at least one pair of a spot-welded portion 2 and a base material portion 3 formed by spot welding two adjacent steel plates, as shown in FIG. 1 , among the plurality of steel plates, with a Zn-based plating layer present between them.

[0030] For example, when n steel sheets (n≧2) are overlapped and spot-welded, a mating surface (1) where the surface of the first steel sheet meets the surface of the second steel sheet, a mating surface (2) where the surface of the second steel sheet meets the surface of the third steel sheet, ..., a mating surface (n−1) where the surface of the (n−1)th steel sheet meets the surface of the n-th steel sheet are formed by spot welding, resulting in n−1 spot welds. In this case, the spot welds formed by spot welding with a Zn-based plating layer present on the mating surfaces are the spot welds 2 that are the subject of the present invention. Note that the spot-welded member 1 of the present invention is required to have at least one spot weld 2 that is the subject of the present invention, and all n−1 spot welds may be the spot welds 2 that are the subject of the present invention.

[0031] The Zn-based plating layer may be provided as a Zn-based plated steel sheet having a Zn-based plating layer on the surface of the steel sheet, as at least one of the multiple (two or more) steel sheets that make up the spot-welded member 1. Of the steel sheets that make up the spot-welded portion 2 that is the subject of the present invention, one or both may be Zn-based plated steel sheets.

[0032] Furthermore, the spot-welded component of the present invention may be formed from two steel plates, or from three or more steel plates, and the upper limit of the number of steel plates (n) is not particularly limited, but may be five or less.

[0033] <Region R1> Region R1 is a 100 μm square region in a plate thickness direction cross section passing through the center 7 of the spot welded portion 2 (i.e., the nugget 4), with the edge 9 of the nugget 4 being the intersection of a line 8 passing through the center 7 of the nugget 4 and the tip 6 of the gap between the two steel plates sandwiching the nugget 4 with the boundary of the nugget 4. The intersection is a point 10 300 μm away from the edge 9 of the nugget 4 toward the base metal portion 3 along the line 8. Region R1 exists within the heat-affected zone 5. Of the four sides of region R1, two sides can be parallel to the plate thickness direction, and the remaining two sides can be perpendicular to the plate thickness direction (parallel to the steel plate surface).

[0034] <Region R2> The inventors have discovered that when the region within region R1 where the Zn concentration is 10 mass% or more is designated region R2, it is important to control the composition and structure of region R2. This is based on the following findings: (i) a Zn concentration of a certain value or more is required for LME cracking to occur; (ii) the coexistence of Zn and Si when the Zn concentration is a certain value or more 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. Region R2 can suppress LME cracking by satisfying two or more of the following conditions (a), (b), and (c): (a) When the region within region R2 where the Si concentration is less than 0.5 mass% is designated region Ra and the area of ​​region Ra is designated region Sa, the ratio Sa / S2 of the area Sa to the area S2 of region R2 is 0.50 or more. (b) When a region in region R2 where the Zn concentration is 50 mass% or more is defined as region Rb and the area of ​​region Rb is defined as Sb, the ratio Sb / S2 of the area Sb to the area S2 of region R2 is 0.20 or less. (c) When a region in region R2 where the ferrite phase is defined as region Rc and the area of ​​region Rc is defined as Sc, the ratio Sc / S2 of the area Sc to the area S2 of region R2 is 0.30 or more.

[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 will be described later in the measurement method, regions Ra, Rb, and Rc are determined so as not to overlap with each other.

[0036] First, regarding condition (a), during spot welding, the Zn-based coating layer melts due to the heat generated during spot welding, and Zn diffuses into the interior of the upper and lower steel sheets in contact with the Zn-based coating layer, forming region R2 within region R1. If Sa / S2 is less than 0.50 within region R2, the effect of Si, which promotes LME cracking, becomes significant; that is, grain boundary embrittlement due to Zn is promoted, making LME cracking more likely to occur. Therefore, Sa / S2 is set to be 0.50 or more.

[0037] Regarding condition (b), region R2 is present in the heat-affected zone 5, and if Sb / S2 exceeds 0.20 in region R2, grain boundary embrittlement due to Zn progresses in region R2, making LME cracking more likely to occur. Therefore, the Sb / S2 ratio is set to 0.20 or less.

[0038] Furthermore, regarding condition (c), if Sc / S2 is less than 0.30 in region R2, the amount of ferrite phase present in region R2 will be reduced, resulting in excessive tensile residual stress due to the tensile stress generated during spot welding, making LME cracking more likely to occur. This is because the ferrite phase has a low concentration of solute carbon, which has the effect of lowering the yield stress (YS) at high temperatures during welding and reducing the tensile residual stress. Therefore, the Sc / S2 is set to be 0.30 or more.

[0039] To suppress LME cracking in the spot-welded component 1, it is sufficient for the region R2 in the spot weld 2 constituting the spot-welded component 1 to satisfy at least two of the three conditions (a) to (c), and it is preferable for all three conditions to be satisfied. For example, even if the region R1 satisfies the conditions (a) and (b) but not the condition (c), satisfying the conditions (a) and (b) suppresses Zn-induced grain boundary embrittlement in the region R2, which is the Zn penetration region. This makes it possible to suppress LME cracking even when excessive tensile residual stress is present. Furthermore, even if the condition (a) (condition (b)) is not satisfied, satisfying the condition (b) (condition (a)) suppresses Zn-induced grain boundary embrittlement, and satisfying the condition (c) enables the tensile residual stress to be reduced, thereby suppressing LME cracking.

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

[0041] The composition of region R2 within region R1 and the area ratios of regions Ra, Rb, and Rc within region R2 are measured as follows. First, a sample is cut out so that the observation surface is a cross section in the plate thickness direction passing through the center 7 of the spot weld 2 (i.e., nugget 4) constituting the spot-welded component 1. Next, the observation surface of the sample is roughly polished using waterproof abrasive paper, and then mirror-polished using diamond paste. If water is used during polishing, the Zn-rich phase, i.e., region Rb with a Zn concentration of 50% by mass or more, will react with the water and disappear. Therefore, the polishing is performed using alcohol or the like without water (water-free). Furthermore, the obtained polished surface may be subjected to ion milling using Ar ions. Next, using a field emission electron probe microanalyzer (FE-EPMA), the 100 μm × 100 μm area of ​​the region R1 is analyzed with an electron beam diameter of 1 μm to measure the Si concentration and Zn concentration. From the obtained Zn concentration map and Si concentration map, a region R2 where the Zn concentration is 10 mass% or more is extracted, and the area S2 of the region R2 is calculated. Furthermore, a region Rb where the Zn concentration is 50 mass% or more within the region R2 is extracted, and the area Sb of the region Rb is calculated. Next, the region R2 is observed using an SEM (Scanning Electron Microscope) at an acceleration voltage of 15 kV, and the ferrite phase region Rc is extracted from the obtained structural image, and the area Sc of the region Rc is calculated. Finally, within region R2, which does not belong to either region Rb or region Rc, a region Ra having a Si concentration of less than 0.5 mass % is extracted, and the area Sa of region Ra is calculated. Sa / S2, Sb / S2, and Sc / S2 are calculated from the values ​​of the areas Sa, Sb, Sc, and S2 obtained above.

[0042] <Zn-based plating layer> The Zn-based plating layer is a Zn-based plating layer disposed on the surface of a steel sheet, and a Zn-based plated steel sheet may be provided as at least one of the multiple steel sheets constituting the spot-welded member 1. LME cracking occurs when the spot-welded member 1 is manufactured to include at least one spot weld 2 formed by spot welding two Zn-based plated steel sheets together or by spot welding two overlapping Zn-based plated steel sheets together. Therefore, the spot-welded member 1 of the present invention is intended to be a spot-welded member including at least one pair of a spot weld and a base material portion formed by spot welding two adjacent steel sheets among the multiple steel sheets with a Zn-based plating layer present between them.

[0043] The Zn-based plating layer is not particularly limited as long as it is a plating layer containing Zn. Examples include an electrolytic Zn-based plating layer and a hot-dip Zn-based plating layer. The Zn-based plating layer may contain elements such as aluminum (Al) and magnesium (Mg) in addition to Zn.

[0044] Examples of Zn-based plated steel sheets include electrolytic Zn-based plated steel sheets and hot-dip Zn-based plated steel sheets. In particular, in the automotive field, where LME cracking is a problem, hot-dip Zn-based plated steel sheets are used from the viewpoints of corrosion resistance, manufacturability, and the like. Specific examples include hot-dip Zn-plated steel sheets (GI steel sheets) having a hot-dip Zn-plated layer on the steel sheet surface, and alloyed hot-dip Zn-plated steel sheets (GA steel sheets) having a hot-dip Zn-plated layer on the steel sheet surface. Among these, GA steel sheets are often used in the automotive field, taking into consideration pressability and continuous spot weldability when manufacturing automotive parts, and therefore it is preferable to manufacture spot-welded members using at least one GA steel sheet.

[0045] Furthermore, the risk of LME cracking tends to increase as the strength of the steel sheet used for the spot-welded member 1 increases. Therefore, in the present invention, the tensile strength of the steel sheet of the base material of the Zn-based plated steel sheet is preferably 980 MPa or more. This allows the effects of the present invention to be more effectively exhibited.

[0046] (Steel Plate for Spot-Welded Members) Next, a steel plate for spot-welded members according to the present invention will be described. The steel plate for spot-welded members according to the present invention is a steel plate for spot-welded members, which is a steel plate for constituting one of the two adjacent steel plates in a spot-welded member formed by spot-welding a plurality of steel plates, at least one set of two adjacent steel plates being arranged with a Zn-based plating layer therebetween, and which comprises a first layer L1 having a solute Si concentration of less than 0.5 mass % and a second layer L2 having a C concentration of 0.02 mass % or less, the first layer L1 being in contact with the surface of the steel plate for spot-welded members, The second layer L2 (i) is in contact with the surface of the steel plate for spot-welded components, (ii) is not in contact with the surface of the steel plate for spot-welded components and overlaps with the first layer L1, or (iii) the surface of the second layer L2 facing the surface of the steel plate for spot-welded components is in contact with the surface of the first layer L1 opposite to the surface of the steel plate for spot-welded components, and satisfies any one of the following conditions (1), (2) and (3): (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 sheet for spot-welded components used in producing spot-welded components is not particularly limited as long as it is a steel sheet from which the above-mentioned spot-welded components can be produced.

[0048] On the other hand, LME cracking is thought to occur when tensile stress due to the welding electrode pressure during spot welding or thermal expansion and contraction of the steel sheet is applied to the spot weld 2, causing Zn in the Zn-based plating layer to penetrate into the grain boundaries of the steel sheet, resulting in grain boundary embrittlement. In other words, Zn reduces the grain boundary strength and causes cracking. Therefore, it is preferable to use the steel sheet for spot-welded components according to the present invention as the steel sheet on the side on which 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 above-mentioned spot-welded component 1 to be manufactured under general spot welding conditions.

[0049] Here, the first layer (L1) is a layer in a region where the solute Si concentration is less than 0.5 mass % from the surface on the Zn-based coating layer side toward the sheet thickness direction, and the second layer (L2) is a layer in a region where the C concentration is 0.02 mass % or less. The first layer (L1) is in contact with the surface of the steel sheet for a spot-welded component (i.e., the upper surface of the first layer (L1) is exposed). The second layer (L2) (i) is in contact with the surface of the steel sheet for a spot-welded component (i.e., the upper surface of the second layer (L2) is exposed), (ii) is not in contact with the surface of the steel sheet for a spot-welded component and overlaps with the first layer (L1), or (iii) the surface of the second layer (L2) facing the surface of the steel sheet for a spot-welded component is in contact with the surface of the first layer (L1) opposite to the surface of the steel sheet for a spot-welded component (i.e., the upper surface of the second layer (L2) is continuously in contact with the lower surface of the first layer (L1)). 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] When the steel plate for spot-welded components satisfies any one of the above conditions (1), (2) and (3), the region R2 within the region R1 in the spot weld 2 constituting the spot-welded component 1 satisfies any two or more of the above conditions (a), (b) and (c), making it possible to suppress the occurrence of LME cracking.

[0051] Specifically, in a steel sheet for spot-welded components, if the thickness T1 of the first layer L1 in condition (1) is 3.0 μm or more, even if Si diffuses from the interior of the steel sheet due to the heat generated during spot welding, the first layer L1 remains thick enough to prevent Si from diffusing into region R2 of the spot-welded component 1, increasing the area Sa of region Ra and decreasing the area Sb of region Rb. Therefore, conditions (a) and (b) are satisfied. If condition (1) is satisfied, LME cracking can be suppressed regardless of the thickness of T2, but it is preferable for T2 to be 20 μm or more. Here, because the heat-affected time during welding is short, the diffusion of C from the interior of the steel sheet to the surface layer of the steel sheet need not be considered. Therefore, when T2 satisfies 20 μm or more, the second layer L2 can maintain a low amount of C, which is an austenite stabilizing element, making it difficult for austenite transformation to occur even at high temperatures during welding, and allowing the ferrite phase to exist stably, thereby satisfying condition (c) in addition to condition (a) and condition (b).

[0052] When condition (2) is satisfied, i.e., when T1 is 2.0 μm or more in thickness and 15T1 + T2 ≧ 50 μm is satisfied, ferrite increases in region R2 of the spot-welded component. Ferrite has a high solubility limit for Zn, and dissolves the surrounding Zn. This reduces the area Sb of region Rb and increases the area Sa of region Ra. Therefore, conditions (a) and (b) are satisfied.

[0053] When condition (3) is satisfied, i.e., when T1 has a thickness of 1.0 μm or more but less than 2.0 μm and 15T1+T2≧50 μm is satisfied, ferrite increases significantly in region R2. Ferrite has a high solubility limit for Zn, and dissolves surrounding Zn, reducing the area Sb of region Rb. Therefore, condition (b) can be satisfied. Because the thickness of T1 is thinner than conditions (1) and (2), condition (a) cannot be satisfied. However, when 15T1+T2≧50 μm is satisfied, the amount of C, an austenite stabilizing element, can be maintained low, making austenite transformation less likely to occur even at high temperatures during welding, and allowing the ferrite phase to exist stably, thereby satisfying conditions (b) and (c).

[0054] In the present invention, when a spot-welded component is manufactured using a steel plate having a chemical composition in which the Si concentration is 0.5 mass% or more, LME cracking becomes significant. Therefore, it is preferable to use a steel plate for spot-welded components that is manufactured so that the surface of the steel plate having the above chemical composition satisfies any one of the conditions (1), (2) and (3) of the present invention.

[0055] The component composition of the steel sheet may be such that, in order to improve various properties such as mechanical properties, for example, solid solution strengthening is achieved 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 in which an unrecrystallized region is left without complete recrystallization; strengthening by transformed structure such as bainite or martensite single phase or a composite structure of ferrite and these transformed structures; and strengthening by the Hall-Petch formula: σ=σ where d is the ferrite grain size. ０ +kd-1 / 2 (σ: stress, σ ０ , k: material constant), and processing strengthening by rolling or the like.

[0056] Examples of the composition of the steel sheet include C: 0.1 to 0.4 mass%, Si: 0.5 to 2.5 mass%, Mn: 1 to 3 mass%, P: 0 to 0.05 mass%, S: 0 to 0.005 mass%, and the balance being Fe and unavoidable impurities, and further includes those to which one or more of Cu, Ti, V, Al, Cr, etc. are added.

[0057] The solute Si concentration and thickness T1 of the first layer L1 are measured as follows. First, a sample is cut out from the surface of the steel sheet or the surface side of the steel sheet directly below the Zn-based plating layer of a Zn-based plated steel sheet using a focused ion beam (FIB) method so that the cross section in the sheet thickness direction serves as the observation surface. Next, the solute Si concentration is measured using energy dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM). Specifically, EDS line profiles are first obtained at 0.01 μm intervals from the surface of the observation surface obtained by 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 L1 including the region where the solute Si concentration is less than 0.5 mass % from the surface of the steel sheet facing the Zn-based coating layer is designated as L1, and its thickness is measured to obtain T1. The precipitates and oxides can be removed by STEM observation. The thickness of layer L1 can be determined, for example, by fitting the plot of Si concentration with an appropriate function and then based on the obtained fitting curve. The solute Si concentration and the thickness of layer L1 are preferably measured at two or more locations. When measurements are performed at two or more locations, the average value can be used as the thickness of the first layer L1.

[0058] The carbon concentration of the second layer L2 and its thickness T2 can be measured using a glow discharge optical emission spectrometer (GDS). From the carbon concentration profile in the sheet thickness direction obtained by GDS, a 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. Measurement is preferably performed at two or more locations. When measurements are performed at two or more locations, the average value can be used as the thickness of the second layer L2.

[0059] The steel sheet for spot-welded members may further include a Zn-based plating layer on the surface thereof, and the Zn-based plating steel sheet may be supplied as the steel sheet for spot-welded members. In this case, the Zn-based plating layer is provided on the first layer L1.

[0060]

[0023] (Method for manufacturing a spot-welded member) Next, a method for manufacturing a spot-welded member according to the present invention will be described. The method for manufacturing a spot-welded member according to the present invention is a method for manufacturing a spot-welded member by arranging at least one set of two adjacent steel sheets among a plurality of steel sheets with a Zn-based plating layer present between them, and spot-welding the plurality of steel sheets, in which one of the two adjacent steel sheets that is subjected to more stress during spot welding is arranged so that the surface of the steel sheet for a spot-welded member according to the present invention described above (i.e., the surface of the first layer L1) comes into contact with the Zn-based plating layer arranged on the surface of the other of the two adjacent steel sheets, and spot welding is performed on the plurality of steel sheets.

[0061] Here, the steel sheet that is more subjected to stress during spot welding refers to the steel sheet on the side where LME cracking occurs during spot welding. If it is not known in advance which steel sheet will cause LME cracking, it is preferable to perform a spot welding test in advance to identify which of the two steel sheets to be welded will be more subjected to stress.

[0062] The other of the two steel sheets is a steel sheet having a Zn-based plating layer on its surface. The other steel sheet may or may not be the steel sheet for spot-welded components according to the present invention.

[0063] The steel sheet for spot-welded components of the present invention may be supplied as a zinc-based plated steel sheet having a surface coated with zinc-based plating as the steel sheet to be placed on the side where more stress is applied during spot welding. In this case, the zinc-based plated steel sheet is placed so that the zinc-based plating layer of the zinc-based plated steel sheet is in contact with the surface of the other of the two adjacent steel sheets, and spot welding is performed on the plurality of steel sheets.

[0064] The other of the two adjacent steel sheets is not particularly limited and may or may not be a steel sheet for spot-welded components according to the present invention. Furthermore, the other steel sheet may or may not have a Zn-based plating layer on its surface. When the other steel sheet has a Zn-based plating layer on its surface, the two adjacent steel sheets are positioned so that the Zn-based plating layers of both steel sheets are in contact with each other, and then spot-welded.

[0065] When both of the two adjacent steel sheets are steel sheets for spot-welded components according to the present invention, as long as a Zn-based plating layer is provided on at least one surface, the spot-welded component according to the present invention can be manufactured without having to specify a steel sheet to which more stress is applied during spot welding. Note that all of the steel sheets constituting the spot-welded component 1 may be steel sheets for spot-welded components according to the present invention.

[0066] By manufacturing a spot-welded component using the above-described method, it is possible to manufacture a spot-welded component having a spot-welded portion with a desired composition and structure, even when spot welding is performed under known spot welding conditions.

[0067] Known spot welding conditions include, for example, a resistance spot welding method in which, when n steel sheets are overlapped, n steel sheets are clamped between a pair of welding electrodes positioned on the surface opposite the mating surface (1) of the first steel sheet and the surface opposite the mating surface (n-1) of the nth steel sheet, and current is applied while applying pressure and controlling it to achieve predetermined welding conditions. For example, resistance spot welding is performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance spot welder, using an electrode (DR type, tip diameter 6 mm) of the resistance spot welder under conditions of a pressure of 3.5 kN, a hold time of 0.1 second, and a welding current and welding time that result in a nugget diameter of 4.5√t mm, where t is the maximum sheet thickness of the n steel sheets.

[0068] A welding device that can be used for the spot welding of the present invention may be a resistance spot welding device equipped with a pair of upper and lower welding electrodes, each capable of freely controlling the welding pressure and welding current during welding. The welding device's pressure mechanism (e.g., air cylinder, servo motor, etc.), type (e.g., stationary, robot gun, etc.), and electrode shape are not particularly limited. Examples of electrode tip types include DR type (dome radius type), R type (radius type), and D type (dome type), as described in JIS C 9304:1999. The tip diameter of a DR type electrode may be, for example, 4 mm to 16 mm.

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

[0070] Examples of methods for producing a laminated material include the following. First, molten low-alloy steel is produced by a known method such as a converter, electric furnace, or vacuum melting furnace, and then the resulting molten steel is solidified to produce a steel slab. The method for producing a steel slab from molten steel is not particularly limited, and a continuous casting method, an ingot casting method, or a thin slab casting method can be used. In order to prevent macrosegregation, it is preferable to produce the steel slab by the continuous casting method. The resulting steel slab is hot-rolled under commonly used conditions to produce a laminated material.

[0071] The laminated material obtained as described above may be subjected to annealing, pickling, polishing, etc. as necessary. The laminated material is assembled by stacking the steel sheets so that the surfaces (bonding surfaces) are vacuum-vacuumed, and sealing the four peripheries of the bonding surfaces by welding. The joining method is not particularly limited, but for example, the laminated material and the steel sheets are joined by stacking the laminated material and performing electron beam welding (EBW), arc welding, or laser beam welding on the four peripheries of the end of the laminated material.

[0072] Here, the method of evacuation is not particularly limited, but for example, when laser beam welding the four peripheries of the end of a laminated slab, a vacuum valve is provided between the clad material and the steel plate at the end (before the four peripheries of the end are completely joined), and a vacuum is drawn between the clad material and the steel plate by connecting a vacuum pump to the valve. By using the clad steel plate obtained in this way as a steel plate for spot welding components, the effects of the present invention can be more reliably obtained.

[0073] Alternatively, when manufacturing a steel sheet, a steel sheet may be manufactured in which an Si internal oxidation layer is formed on the surface layer of the steel sheet to form a first layer L1 and a decarburized layer is formed on the surface layer of the steel sheet to form a second layer L2.

[0074] Here, an example of a method for forming an Si internal oxidation layer and a decarburized layer in the surface layer of a steel sheet will be described. For example, the dew point during annealing is increased to form an Si internal oxidation layer and a decarburized layer in the surface layer of the steel sheet. Specifically, annealing may be performed at 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 longer. Here, the Si internal oxidation layer specifically refers to a region where Si oxide is formed within crystal grains and / or in part of crystal grain boundaries.

[0075] The steel sheet for spot-welded components may have a Zn-based plating layer formed on the surface of the steel sheet obtained by the method described above. A method for forming the Zn-based plating layer is, for example, a hot-dip Zn plating process. The hot-dip Zn plating process may be performed by subjecting the surface of the steel sheet described above to hot-dip Zn plating. The conditions for the hot-dip Zn plating process are not particularly limited, and the hot-dip Zn plating process may be performed in a conventional manner. The coating weight of the hot-dip Zn plating layer is 20 g / m per one surface of the steel sheet. ２ 80g / m or more ２ The following is preferable. After the above-mentioned hot-dip Zn plating treatment, an alloying treatment may be carried out to form an alloyed hot-dip Zn plating layer. The conditions for the alloying treatment are not particularly limited, and the treatment may be carried out in a conventional manner.

[0076] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0077] <Preparation of Spot-Welded Members> For the steel sheets for the spot-welded members, galvannealed steel sheets (GA) were used, which were prepared by forming hot-dip galvanized layers on both sides of steel sheet 1 shown in Table 1 and then subjecting the steel sheets to heat treatment. However, for some steel sheets, hot-dip galvanized steel sheets (GI) were used, which were not subject to heat treatment. The spot-welded members (welded joints) were prepared by placing steel sheet 2 on steel sheet 1 shown in Table 1 when two steel sheets were overlapped, or by placing steel sheet 1 in the middle, steel sheet 2 on the upper side of steel sheet 1, and steel sheet 3 on the lower side of steel sheet 1, and then performing resistance spot welding.

[0078]

[0079] The welding equipment used was a servomotor-driven, single-phase AC (50 Hz) resistance spot welder equipped with a welding gun. A pair of chromium-copper DR-type electrodes was used as the electrode tips. The radius of curvature of the tip of the DR-type electrode was 40 mm, and the tip diameter was 6 mm.

[0080] The spot welding conditions were set to incorporate one of the following welding disturbances (I) to (V) in order to reproduce a state in which LME cracking is likely to occur. All of these conditions can locally increase the temperature and / or tensile stress of the weld when the electrode is open, making it possible to reproduce a state in which LME cracking is likely to occur. Table 1 shows the welding disturbances (I) to (V) that were incorporated during spot welding in the "Pressure Start Condition" column.

[0081] (I) A state in which the welding electrode and the overlapping sheet combination have an impact angle of 0.2 degrees or more. Here, the impact angle is defined as the angle at which the electrode is inclined relative to the steel sheet, i.e., the angle formed between the direction of the electrode pressure and the direction of the steel sheet thickness.

[0082] (II) A state in which the misalignment amount of a pair of welding electrodes is 0.1 mm or more. Here, misalignment means a state in which the central axes of a pair of welding electrodes are not aligned, and the amount of misalignment is defined as the distance between the central axis of the upper electrode and the central axis of the lower electrode.

[0083] (III) A state in which there is a gap of 0.5 mm or more between either electrode and the steel sheet (gap between the electrode and the steel sheet). Here, the gap between the electrode and the steel sheet is defined as the larger distance between either electrode and the steel sheet when the steel sheet and the pair of electrodes are arranged before the start of pressurization.

[0084] (IV) A state in which, among two or more overlapping steel plates, there is a gap (distance between the steel plates) of 0.5 mm or more between at least one pair of steel plates. Here, the gap between steel plates is defined as the maximum distance between the steel plates when two steel plates arranged in the vertical direction are considered as one pair.

[0085] (V) A state in which the shortest distance from the center of the welding point to the end face of the steel sheet is 10 mm or less. Here, the distance from the center of the welding point to the end face of the steel sheet is defined as the shortest distance from the center of the welding point to the end face of any of the steel sheets in two or more overlapping steel sheets.

[0086] The spot welding conditions were as follows: Specifically, resistance spot welding was performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance spot welder, with a pressure of 3.5 kN applied to an electrode (DR type, tip diameter 6 mm) of the resistance spot welder, a hold time of 0.1 seconds, and a welding current and welding time that resulted in a nugget diameter of 4.5√t mm, where t is the maximum thickness of the overlapping steel sheets.

[0087] <Evaluation of Spot-Welded Members> First, samples were cut out so that the observation surface was a cross section in the sheet thickness direction passing through the center of the spot weld constituting the obtained spot-welded member. Specifically, when two steel sheets were overlapped, the spot weld formed between steel sheet 1 and steel sheet 2 was cut out, and when three steel sheets were overlapped, the spot weld formed between steel sheet 1 and steel sheet 2 and the spot weld formed between steel sheet 1 and steel sheet 3 were cut out. Here, all spot welds formed in the spot-welded member were spot welds formed by spot welding two adjacent steel sheets with a Zn-based plating layer present between them, and are the spot welds targeted by the present invention.

[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, the resulting polished surface was subjected to ion milling using Ar ions. Using a field emission electron probe microanalyzer (FE-EPMA), a 100 μm × 100 μm area (region R1) was analyzed with an electron beam diameter of 1 μm, and the Zn and Si concentrations were measured, resulting in the creation of Zn and Si concentration maps. From the resulting Zn concentration map, region R2, where the Zn concentration was 10% by mass or more, was extracted, and the area S2 of region R2 was calculated.

[0089] Next, a region Rb in region R2 where the Zn concentration was 50 mass% or more was extracted, and the area Sb of region Rb was calculated. Region R2 was observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV, and a ferrite phase region Rc was extracted from the obtained structural image, and the area Sc of region Rc was calculated. Furthermore, a region Ra in region R2 excluding regions Rb and Rc where the Si concentration was less than 0.5 mass% was extracted, and the area Sa of region Ra was calculated.

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

[0091] At this time, when the plate pair consisted of three plates, the spot welds between the plate pairs of steel plate 1 and steel plate 2, and steel plate 1 and steel plate 3 were observed.

[0092] <Evaluation of Steel Sheet for Spot-Welded Member> The solute Si concentration and thickness T1 of the first layer L1 of the steel sheet for spot-welded members (steel sheet 1) were determined by the following method. First, a sample was cut out using a focused ion beam (FIB) method so that the cross section in the sheet thickness direction from the surface of the steel sheet (the steel sheet surface directly below the Zn-based plating layer of the Zn-based plated steel sheet) served as the observation surface, and measurement was performed using energy dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM).

[0093] Specifically, for the observation surface obtained by FIB, EDS-line profiles were obtained at intervals of 0.01 μm using STEM-EDS from the surface side of the steel plate toward the inside in the plate thickness direction, and the distribution of Si concentration was determined for the region excluding precipitates. A first layer including a region where the solid solution Si concentration was less than 0.5 mass% was designated as L1, and its thickness T1 was measured.

[0094] The C concentration of the second layer L2 and its thickness T2 were measured using a glow discharge optical emission spectrometer (GDS). Specifically, a region where the C concentration was 0.02 mass% or less was determined from a C profile obtained by GDS from the surface of the steel sheet toward the inside in the sheet thickness direction, and the thickness T2 of the second layer L2 was calculated.

[0095] <Evaluation of LME Cracking Resistance> First, welding was performed under five levels of welding conditions that satisfied one or more of the welding disturbances (I) to (V) described above, to produce welded joints. The centers of the welds of the obtained welded joints were then cut with a microcutter, and the cross sections of the welds were observed to evaluate the presence or absence of cracks and their depths. Here, the occurrence of cracks 5 μm or more deep was considered to be cracks, and LME cracking resistance was evaluated according to the following criteria.

[0096] Rating 4: 5 welded joints without cracks Rating 3: 4 welded joints without cracks, 1 welded joint with cracks Rating 2: 3 welded joints without cracks, 1 welded joint with cracks Rating 1: 2 or less welded joints without cracks, 3 or more welded joints with cracks Here, ratings of 4 to 2 were evaluated as passing.

[0097] As is clear from Table 1, the spot-welded components of the present invention have excellent LME cracking resistance. Furthermore, if the manufacturing method for spot-welded components using the steel plate for spot-welded components of the present invention is used, spot-welded components with excellent LME cracking resistance can be manufactured even in the presence of the above-mentioned welding disturbances. Therefore, the tolerance for managing the welding disturbances during manufacturing of spot-welded components is improved.

[0098] According to the present invention, it is possible to provide a spot-welded component, a steel plate for a spot-welded component, and a method for manufacturing a spot-welded component, which are excellent in LME cracking resistance.

[0099] REFERENCE SIGNS LIST 1 Spot-welded member 2 Spot welded portion 3 Base material portion 4 Nugget 5 Heat-affected zone (HAZ) 6 Tip of gap between two steel plates 7 Center of spot welded portion (nugget) 8 Straight line 9 End of nugget 10 Point 300 μm away from the end 9 of the nugget along the straight line 8 toward the base material portion 3

Claims

1. A spot-welded component in which at least one set of two adjacent steel plates from a plurality of steel plates are arranged with a Zn-based plating layer between them, and the plurality of steel plates are spot-welded together, wherein the spot-welded component comprises at least one set of a spot weld and a base material portion formed by spot welding the two adjacent steel plates with a Zn-based plating layer between them, and the spot weld comprises a nugget and a heat-affected zone, and wherein, within a region R1 in the spot weld, a region R2 is defined as a region where the Zn concentration is 10 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, the region R1 is a 100 μm square region in a thickness direction cross section passing through the center of the nugget, with the intersection of the boundary of the nugget with a straight line passing through the center of the nugget and the tip of the gap between the two adjacent steel plates as the end of the nugget, and centered at a point 300 μm away from the end of the nugget along the straight line toward the base material portion. (a) When a region in the region R2 where the Si concentration is less than 0.5 mass% is defined as region Ra and the area of ​​the region Ra is defined as Sa, the ratio Sa / S2 of the area Sa to the area S2 is 0.50 or more; (b) When a region in the region R2 where the Zn concentration is 50 mass% or more is defined as region Rb and the area of ​​the region Rb is defined as Sb, the ratio Sb / S2 of the area Sb to the area S2 is 0.20 or less; (c) When a region in the region R2 where the ferrite phase is defined as region Rc and the area of ​​the region Rc is defined as Sc, the ratio Sc / S2 of the area Sc to the area S2 is 0.30 or more.

2. A steel plate for a spot-welded component, in which at least one set of two adjacent steel plates among a plurality of steel plates are arranged with a Zn-based plating layer between them, and the plurality of steel plates are spot-welded together, the steel plate constituting one of the two adjacent steel plates, comprising a first layer L1 having a solute Si concentration of less than 0.5 mass% and a second layer L2 having a C concentration of 0.02 mass% or less, the first layer L1 being in contact with the surface of the steel plate for the spot-welded component, and the second layer L2 (i) being in contact with the surface of the steel plate for the spot-welded component, (ii) not in contact with the surface of the steel plate for the spot-welded component and overlapping with the first layer L1, or (iii) the surface of the second layer L2 on the surface side of the steel plate for the spot-welded component being in contact with the surface of the first layer L1 opposite to the surface of the steel plate for the spot-welded component, A steel sheet for spot-welded components, which satisfies any one of the following conditions (1), (2), and (3), where T1 is the thickness of the first layer L1 and T2 is the thickness of the second layer L2: (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 3. The steel sheet for spot-welded components according to claim 2, further comprising a Zn-based plating layer on the surface.

4. A method for manufacturing spot-welded components, comprising arranging at least one set of two adjacent steel plates from a plurality of steel plates with a Zn-based plating layer between them, and spot-welding the plurality of steel plates, wherein at least one of the two adjacent steel plates is arranged on the steel plate that is subjected to greater stress during spot welding, so that the surface of the steel plate for spot-welded components described in claim 2 comes into contact with the Zn-based plating layer arranged on the surface of the other of the two adjacent steel plates, and spot-welding the plurality of steel plates.

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

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