Resistance spot welding member and resistance spot welding method thereof

Abstract

The purpose of the present invention is to provide a resistance spot welded member and a resistance spot welding method for the same. The present invention pertains to a resistance spot welded member in which two or more overlapping steel sheets are subjected to resistance spot welding. At least one of the two or more overlapping steel sheets is a Zn-based plated steel sheet. In a first sheet set that has the largest difference in tensile strength among the combinations in each of which the Zn-based plating is present between two steel sheets abutting in the vertical direction, the Fe concentration of a Fe-Zn alloy layer formed from a position that is 300 μm away from a nugget end to a position that is 700 μm away from the nugget end in the sheet width direction satisfies formulae (1) to (3). (1): 98≥CFe (2): CFe≥{25×SiHigh×(CE1-CE2)}+{θ+(gSheet / 1.2)}+[{(SZn×dGB)0.5} / 30]+{60+(C0Fe / 3)} (3): |m|≤0.10

Description

[Technical Field]

[0001] The present invention relates to a resistance spot welded member formed by resistance spot welding multiple steel plates, and more particularly to a resistance spot welded member suitable as a structural component for automobiles and the like, and to a resistance spot welding method thereof. [Background technology]

[0002] From the perspective of protecting the global environment, reducing CO2 emissions by improving fuel efficiency while maintaining the strength of the vehicle body is a crucial challenge for the automotive industry. Furthermore, maintaining the strength of the vehicle body while reducing its weight is also required from the perspective of collision safety and improved fuel efficiency. To achieve these goals, it is effective to thin the steel sheets used as materials for automotive parts by increasing their strength, and in recent years, the application of steel sheets with a tensile strength (TS) of 980 MPa or higher has been progressing.

[0003] In the automobile assembly process, when combining press-formed automobile parts, it is common to join overlapping steel plates by welding. From the standpoint of cost and manufacturing efficiency, resistance spot welding, a type of overlapping resistance welding, is often used to join automobile parts. In this welding method, two or more overlapping steel plates are sandwiched between a pair of welding electrodes from above and below, and while applying pressure from the pair of welding electrodes from above and below the steel plates, a high-current welding current is passed between the upper and lower welding electrodes for a short time to join the steel plates together. Figure 1 shows an example where two steel plates 1 and 2 are overlapped and sandwiched between welding electrodes 8 and 9. In this method, resistance heating generated by passing a high-current welding current between the welding electrodes is used to obtain a point-shaped weld 4. This elliptical-shaped weld 4 is called a nugget, and it is the part where the two steel plates 1 and 2 melt and solidify at the point of contact when current is passed through the overlapping steel plates. This joins the steel plates together in a point-like manner.

[0004] To ensure the crash safety of automobiles, it is necessary to improve not only the strength of the steel plates but also the strength of the welded joints. There are various test methods for evaluating the strength of resistance spot welds, and one common evaluation method is the tensile shear test specified in JIS Z3136. This is a test method that measures the tensile shear strength (hereinafter referred to as TSS) by applying a tensile load in the tensile shear direction to the welded joint.

[0005] Furthermore, in automotive steel sheets, from the viewpoint of corrosion resistance, steel sheets with rust-preventive properties such as zinc (Zn) plated steel sheets (so-called "surface-treated steel sheets") are used for parts at risk of corrosion. However, in resistance spot welding of plate assemblies that include surface-treated steel sheets, there is a problem that cracks may occur in the weld area. It is believed that these cracks in the weld area are caused by so-called liquid metal embrittlement (hereinafter referred to as "LME cracks"), where the low-melting-point metal plating layer on the surface of the surface-treated steel sheet melts during welding, and when tensile stress due to the pressure of the welding electrode, thermal expansion of the steel sheet, or contraction of the steel sheet is applied to the weld area, the molten low-melting-point metal penetrates the grain boundaries of the base material of the surface-treated steel sheet, reducing the grain boundary strength and causing cracks. The location of LME cracks varies, as shown in Figure 7, including the surfaces of steel sheets 1 and 2 that are in contact with the welding electrodes 8 and 9, and the surfaces of steel sheets 1 and 2 that are in contact with each other.

[0006] As countermeasures against such LME cracking, for example, there are technologies described in Patent Documents 1 to 4. Patent Document 1 proposes setting the component composition of the steel sheet in the sheet assembly to a specific range, specifically, in weight percent, C: 0.003 to 0.01%, Mn: 0.05 to 0.5%, P: 0.02% or less, sol.Al: 0.1% or less, Ti: 48 × (N / 14) to 48 × {(N / 14) + (S / 32)}%, Nb: 93 × (C / 12) to 0.1%, B: 0.0005 to 0.003%, N: 0.01% or less, Ni: 0.05% or less, with the remainder being Fe and unavoidable impurities.

[0007] Patent Document 2 proposes a spot welding method for high-strength plated steel sheets, in which spot welding is performed by setting the welding energization time and the holding time after welding energization so as to satisfy the following conditions (A) and (B). 0.25×(10×t+2) / 50≦WT≦0.50×(10×t+2) / 50 …(A) 300 - 500 × t + 250 × t 2 ≤HT …(B) However, in conditions (A) and (B), t: plate thickness (mm), WT: welding energization time (ms), and HT: holding time after welding energization (ms).

[0008] Furthermore, Patent Document 2 proposes performing resistance spot welding using high-strength galvanized steel sheets in which the amount of alloying elements in the steel sheet is below a certain level, while appropriately setting the energizing time and the holding time of the electrodes after energizing according to the thickness of the steel sheet.

[0009] Patent Document 3 proposes a resistance spot welding method in which the energizing pattern is multi-stage energizing with three or more stages, the energizing time and welding current are adjusted so that the appropriate current range (ΔI) is 1.0 kA or more, preferably 2.0 kA or more, and a cooling time is provided between each stage. The above-mentioned appropriate current range is the current range in which a nugget with a desired nugget diameter or greater and a molten residue thickness of 0.05 mm or more can be stably formed.

[0010] Patent Document 4 proposes a technique for preventing LME cracking by removing the plating layer from the area to be welded prior to resistance spot welding. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 10-195597 [Patent Document 2] Japanese Patent Publication No. 2003-103377 [Patent Document 3] Japanese Patent Publication No. 2003-236676 [Patent Document 4] International Publication No. 2016 / 159169 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, Patent Document 1 has drawbacks, such as the need to limit the amount of alloying elements in the steel sheet, which restricts the use of steel sheets that meet the required performance. In particular, given the recent trend towards higher alloy content in steel sheets due to increased strength, the application of the technology in Patent Document 1 is extremely limited.

[0013] Patent document 2 only proposes a method for suppressing LME cracking when an excessively high welding current is set that causes spatter, and does not mention LME cracking when spatter does not occur.

[0014] Patent Document 3 presents challenges, such as the need for significant man-hours to optimize welding conditions and its inability to be applied to steel plates and plate assemblies where securing an appropriate current range is difficult. In addition, Patent Documents 2 and 3 do not consider the influence of the welding electrode angle, and therefore may be insufficient as countermeasures when considering the actual work performed during automobile assembly.

[0015] Patent Document 4 requires a step to remove the plating layer beforehand, which increases manufacturing costs. Furthermore, because the plating layer is removed, the corrosion resistance of the welded area is likely to decrease.

[0016] The present invention has been made in view of the above circumstances, and aims to provide a resistance spot welding member and a resistance spot welding method thereof that can suppress LME cracking of resistance spot welds in plate assemblies using multiple steel plates, including surface-treated steel plates. [Means for solving the problem]

[0017] In order to achieve the above objective, the inventors conducted extensive research and obtained the following findings.

[0018] The effects of the present invention on cracks that occur during welding cannot be simply explained because various factors have a complex influence, but the inventors believe the following: LME cracks in resistance spot welds are likely to occur when excessive tensile residual stress is generated in the resistance spot weld due to welding disturbances or other factors. In particular, it is known that LME cracks are likely to occur in areas with locally high tensile stress when the welding electrode is released after the current and pressure of the resistance spot welding is completed, on the side of the steel plate joint where the steel plates meet. In addition, it is thought that tensile stress due to differences in transformation behavior during cooling is also likely to occur when there is a significant difference in strength between the overlapping steel plates.

[0019] As mentioned above, LME cracking occurs when tensile stress is applied while liquid metal such as Zn is in contact with the steel sheet. Therefore, the aim is to promote the interdiffusion of Fe and Zn between the overlapping steel sheets (i.e., the steel sheet joint surface) where the strength difference between the overlapping steel sheets is large, and to promote the alloying of Fe and Zn so that a Zn alloy layer with a certain or higher Fe concentration is formed near the nugget. This ensures that no liquid Zn is present between the sheets when the tensile stress is applied. The inventors conceived that LME cracking can be suppressed by this technical concept. They also found that there are appropriate welding conditions for controlling the Zn alloy layer to have a certain or higher Fe concentration.

[0020] This invention is based on the above findings, and its gist is as follows. [1] A resistance spot welded member in which two or more overlapping steel plates are resistance spot welded, Of the two or more overlapping steel sheets mentioned above, at least one is a Zn-plated steel sheet. Among the combinations in which a Zn-based plating exists between two steel plates that are in contact in the vertical direction, the combination with the largest difference in tensile strength is called the first plate combination. The Fe concentration of the Fe-Zn alloy layer formed in the first region from position A, which is 300 μm in the sheet width direction from the end of the lug on the sheet mating surface in the first plate set, to position B, which is 700 μm in the sheet width direction from the end of the lug, is C Fe (mass %), and when the gradient of C Fe of the Fe-Zn alloy layer in the first region is m, the resistance spot welding member in which the Fe concentration of the Fe-Zn alloy layer in the first region satisfies all of the formulas (1) to (3). 98 ≥ C Fe …(1) C Fe ≥ {25 × Si High × (CE1 - CE2)} + {θ + (g Sheet / 1.2)} + [{(S Zn × d GB ) 0.5} / 30] + {60 + (C 0Fe / 3)} …(2) |m| ≤ 0.10 …(3) Here, in the formula (2), C 0Fe (mass %): the Fe concentration of the steel sheet having the highest average Fe concentration in the Zn-based plating among the Zn-based plated steel sheets in the first plate set, S Zn (g / m 2 ): the Zn-based plating adhesion amount of the steel sheet having the highest Zn-based plating adhesion amount among the Zn-based plated steel sheets in the first plate set, CE1 (%): the carbon equivalent of the steel sheet having a high carbon equivalent in the first plate set, CE2 (%): the carbon equivalent of the steel sheet having a low carbon equivalent in the first plate set, Si High (mass %): the Si concentration of the steel sheet having a large Si content in the first plate set, d GB (number / mm): the grain boundary density of the outermost surface layer of the base material of the steel sheet having a high tensile strength in the first plate set, θ (degree): the inclination of the lug obtained by the angle formed by the straight line connecting the shoulders of the outermost steel sheet and the tangent line of the outer peripheral edge of the lug of the outermost steel sheet in the two or more superimposed steel sheets, gSheet (mm): The sum of the gaps between the joining surfaces of the two or more overlapping steel plates. That is the case. [2] In the first plate assembly, the carbon equivalents of the two steel plates are CE1-CE2≦0.7, and the grain boundary density of the outermost layer of the base material is d GB A resistance spot welding member as described in [1], wherein the resistance is ≤ 700. [3] A resistance spot welding method for a resistance spot welding member described in [1] or [2], This process involves clamping two or more overlapping steel plates with a pair of welding electrodes, applying pressure, and then applying current to form a nugget. Let I1 (kA) be the average current value for the current-setting process, T1 (s) be the current-setting time for the current-setting process, p (kN) be the average pressure force of the welding electrode, and T be the hold time after the current-setting process is complete. Hold (s) When the number of steel plates is N, A resistance spot welding method for resistance spot welding members, wherein the welding conditions of the current application process satisfy equation (4). I1 2 ×T1+(T Hold (x400) 2 ≥1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB 0.5 ) + (CE1 - CE2) × 70} / N …(4) Here, in equation (4), c is the gap (mm) between the welding electrode and the outermost layer of the two or more overlapping steel plates, and g Axis is the misalignment (mm) of the pair of welding electrodes. [4] The process includes a post-energization step in which the formed nugget is subjected to post-heat treatment after the main energization step, Let I1 (kA) be the average current value of the main energizing process, T1 (s) be the energizing time of the main energizing process, I2 (kA) be the average current value of the subsequent energizing process, T2 (s) be the energizing time of the subsequent energizing process, and N be the number of energizing cycles in the subsequent energizing process. P(Times) The energizing interval between the main energizing process and the post-energizing process is T3 (s), the average pressure of the welding electrode is p (kN), and the holding time after the end of energizing is T Hold (s) When the number of steel plates is N, A resistance spot welding method for a resistance spot welding member according to [3], wherein, in place of equation (4), the welding conditions for the main energizing step and the post-energizing step satisfy equation (5). I1 2 ×T1+{3×I2 2 ×(5×T2-T3)×N P}+(T Hold (x400) 2 ≥1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB 0.5 ) + (CE1 - CE2) × 70} / N …(5) [5] The resistance spot welding method for a resistance spot welded member according to [3] or [4], wherein, in the current application step, at least one welding point satisfies one or more conditions selected from (a) to (d) immediately before applying pressure with the welding electrode. (a) The welding angle between the welding electrode and the two or more overlapping steel plates is 0.2 degrees or more. (b) In the case of two or more overlapping steel plates, there is a gap of 0.5 mm or more between at least one pair of steel plates. (c) A state in which there is a gap of 0.5 mm or more between any of the welding electrodes and the outermost layer of the two or more overlapping steel plates. (d) A state in which the misalignment of the pair of welding electrodes is 0.1 mm or more. [6] A resistance spot welding method for a resistance spot welding member according to [4] or [5], comprising a no-energy step between the main energizing step and the post-energizing step, wherein the energizing is suspended. [7] The resistance spot welding method for a resistance spot welding member according to [6], wherein the de-energization step and the post-energization step are repeated after the main energization step. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a resistance spot welding member that can suppress LME cracking of the resistance spot weld, regardless of the component composition of the steel plates or construction disturbances during welding, even in plate assemblies using multiple steel plates, particularly surface-treated steel plates. Furthermore, it is possible to provide a resistance spot welding method for a resistance spot welding member that enables the manufacture of a welded joint without having to remove the plating layer of the Zn-plated steel plate included in the plate assembly beforehand. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic cross-sectional view in the thickness direction showing an example of resistance spot welding. [Figure 2] Figure 2(A) is a cross-sectional view in the thickness direction illustrating the resistance spot weld portion and its surroundings, as well as the method for measuring CFe, in a resistance spot welded member according to one embodiment of the present invention, and Figure 2(B) is a diagram illustrating the gradient of CFe. [Figure 3] Figure 3(A) is a cross-sectional view in the thickness direction schematically illustrating the resistance spot weld portion and its surrounding area, as well as the method for measuring CFe, in a resistance spot welded member according to another embodiment of the present invention, and Figure 3(B) is a diagram illustrating the gradient of CFe. [Figure 4] Figure 4 shows the relationship between CFe in the resistance spot weld of the present invention, the right-hand side value of equation (2), and the evaluation of LME cracking. [Figure 5] Figure 5 is a cross-sectional view in the thickness direction illustrating a schematic method for measuring the nugget inclination (θ), which is a construction disturbance during resistance spot welding. [Figure 6] Figures 6(A) and 6(B) are cross-sectional views in the thickness direction illustrating a schematic method for measuring the gap between plates (gSheet), which is a construction disturbance during resistance spot welding. [Figure 7] Figure 7 is a schematic cross-sectional view in the thickness direction showing an example of crack occurrence during conventional resistance spot welding. [Modes for carrying out the invention]

[0023] The resistance spot welding member and the resistance spot welding method of the present invention will be described in detail below. However, the present invention is not limited to this embodiment.

[0024] [Resistance spot welding components] The resistance spot welding member (hereinafter referred to as "welded member") of the present invention will be described with reference to Figures 2 and 3. As an example, Figures 2(A) and 3(A) show an enlarged cross-sectional view in the thickness direction of the plate, showing a portion of the resistance spot weld and its surrounding area in the welded member. This enlarged area is the region enclosed by the rectangular frame shown in Figures 2(A) and 3(A).

[0025] The present invention relates to a welded member having a resistance spot welded portion formed by resistance spot welding two or more overlapping steel plates. That is, the welded member of the present invention comprises two or more steel plates and a resistance spot welded portion formed by welding the steel plates. As described later, at least one of the overlapping steel plates is a Zn-plated steel plate having a Zn-based plating on its surface as a plating layer. The number of overlapping steel plates is preferably three or more. There is no particular upper limit to the number, but it is preferably five or less.

[0026] Furthermore, as the number of overlapping steel plates increases, the heat input to the weld increases, making LME cracking more likely. Therefore, the effects of the present invention can be more effectively obtained when the number of steel plates is three or more.

[0027] The example shown in Figure 2(A) is a welded member 6 formed by welding together two overlapping steel plates 1 and 2. Both or either of the steel plate 2 (hereinafter sometimes referred to as the "lower plate") placed on the bottom and the steel plate 1 (hereinafter sometimes referred to as the "upper plate") placed on the top are Zn-plated steel plates. In the case of the welded member 6 in Figure 2(A), a resistance spot weld 4, which will be described below, is formed on the steel plate joining surface (i.e., the overlapping surface of the steel plates) 7 where the upper plate 1 and the lower plate 2 meet.

[0028] Furthermore, as an example where there are three or more steel plates, Figure 3(A) shows a welded member 6 formed by welding together three overlapping steel plates 1, 2, and 3. In the welded member 6 of Figure 3(A), all or at least one of the steel plates—the bottommost steel plate 2 (i.e., the lower plate), the topmost steel plate 1 (i.e., the upper plate), and the steel plate 3 (hereinafter sometimes referred to as the "middle plate") placed between them—are Zn-plated steel plates. In the case of the welded member 6 of Figure 3(A), the resistance spot weld 4 described below is formed so as to include the steel plate joining surfaces 7 (7a, 7b) where the bottom plate 2 and the middle plate 3, and the middle plate 3 and the upper plate 1, are in contact.

[0029] [Resistance spot weld] First, the technical concept of the present invention will be explained in detail with reference to Figure 4.

[0030] Figure 4 shows the Fe concentration (C) of the Fe-Zn alloy layer formed between plates in the resistance spot welded portion (hereinafter referred to as the "welded portion") of the welded member of the present invention. Fe The relationship between this and the right-hand side value of equation (2) described below, and the correspondence between these and LME crack evaluation are shown. Here, we use the welded members welded under the plate assembly and welding conditions described in the example below, and the evaluation criteria for LME crack evaluation. The "right-hand side value of equation (2)" above refers to "{25 × Si High ×(CE1-CE2)}+{θ+(g Sheet / 1.2)}+[{(S Zn ×d GB ) 0.5} / 30]+{60+(C 0Fe This is the value calculated using the formula ` / 3)`.

[0031] As described above, LME cracking occurs when tensile stress is applied while liquid metal such as Zn is in contact with a steel plate. In addition, if there is a significant difference in strength between stacked steel plates, tensile stress due to differences in transformation behavior during cooling is also likely to occur. Therefore, in this invention, it is important to promote the interdiffusion of Fe and Zn between plates in a specific region, to advance the alloying of Fe and Zn, and to control the Zn alloy layer to have a certain level of Fe concentration or higher. This makes it possible to prevent liquid Zn from being present between plates when tensile stress is applied.

[0032] Here, we will explain the "Zn alloy layer having a certain or higher Fe concentration" mentioned above. As shown in the welded member 6 in Figures 2(A) and 3(A), a Zn alloy layer is formed on the outside of the nugget end and between the overlapping steel plates (i.e., on the steel plate joint surface 7 side). The higher the Fe concentration of the Zn alloy layer, the more liquid Zn is suppressed during alloying, which is effective in preventing LME cracking. Therefore, in this invention, the region of the Zn alloy layer formed between the plates with a high Fe concentration is defined as the "Fe-Zn alloy layer," and the Fe concentration of the Fe-Zn alloy layer is defined as "C Fe It is defined as "

[0033] Furthermore, the degree of Fe-Zn alloying can be controlled by controlling the current flow pattern during welding. In particular, for alloying, it is effective to maintain a high temperature range for a long period of time during current flow so that Fe-Zn alloying progresses. By controlling this current flow pattern, it is possible to appropriately control the Fe concentration of the Fe-Zn alloy layer in response to the weld plate assembly, the strength, component composition, surface structure, and plating layer of the steel plate used in the assembly, and the disturbances during welding.

[0034] Furthermore, the "specific area" mentioned above refers to C mentioned above. FeThis refers to the range in the plate width direction for measurement. Specifically, when the intersection point of the steel plate mating surface 7 and the outer edge of the nugget 4a is defined as the nugget end E, a position 300 μm from the nugget end E on the steel plate mating surface 7 in the plate width direction (i.e., the base material direction) is defined as A, and a position 700 μm from the nugget end E on the steel plate mating surface 7 in the plate width direction is defined as B, this refers to the region from A to B. In this invention, this region is referred to as the "first region". The first region is formed on both ends of the nugget 4a.

[0035] Note that in Figures 2(A) and 3(A), for explanatory purposes, the first region is shown only on one nugget end, and the first region on the other nugget end is omitted. This first region is located inside the weld heat-affected zone 4b formed on the outer circumference of the nugget 4a. The rectangular frames shown in Figures 2(A) and 3(A) are shown with lengths set appropriately in the thickness direction so that they include the Fe-Zn alloy layer 5 within the first region.

[0036] The first region is considered to be a region where LME cracking is likely to occur during welding. The reason for this is not clear, but it is thought to be as follows: Since the maximum temperature reached during welding is higher the closer to the nugget, it is thought that liquid Zn is more likely to be present. On the other hand, in the region that is more than 700 μm away from the nugget end E on the steel plate mating surface 7 in the plate width direction, the maximum temperature reached during welding may be below the melting point of the Zn plating, and it is thought that LME cracking is less likely to occur.

[0037] For the reasons stated above, as shown in Figure 4, the present invention focuses on the material properties between plates and determines the amount of C needed to suppress LME cracking based on the TS difference between steel plates, Si content, steel plate surface structure, Zn concentration in Zn-based plating, amount of Zn-based plating adhesion, and construction disturbances. Fe We found that it changes depending on the material properties between the plates. Furthermore, C in the first region described above changes depending on the material properties between the plates. Fe We found that LME cracking can be suppressed by appropriately controlling this.

[0038] Next, we will describe the welded joint of the present invention, which was completed based on this technical concept.

[0039] As described above, in the welded joint 4 of the present invention, it is important to appropriately control the Fe concentration of the Fe-Zn alloy layer 5 in the first region near the nugget 4a.

[0040] Specifically, as shown in Figures 2(A) and 3(A), among combinations of two or more overlapping steel plates where a Zn-based plating exists between two steel plates that are in contact in the vertical direction, the combination with the largest difference in tensile strength is called the first plate combination. In the example shown in Figure 2(A), the top plate 1 and the bottom plate 2 are the first plate combination, and in the example shown in Figure 3(A), the top plate 1 and the middle plate 3 are the first plate combination.

[0041] In this first plate assembly, the region from A to B on the steel plate joining surface 7 is referred to as the first region, and the Fe concentration of the Fe-Zn alloy layer formed within this first region is C Fe (This is expressed as mass %). Also, the C of the Fe-Zn alloy layer in the first region. Fe Let m be the gradient of the Fe-Zn alloy layer in this first region. Fe ) is controlled so that all of equations (1) to (3) are satisfied. 98≧C Fe …(1) C Fe ≥{25×Si} High ×(CE1-CE2)}+{θ+(g Sheet / 1.2)}+[{(S Zn ×d GB ) 0.5} / 30]+{60+(C 0Fe / 3)} …(2) |m|≦0.10 …(3) Here, in equation (2), C 0Fe (Mass %): Among the Zn-plated steel sheets in the first sheet assembly described above, the Fe concentration of the steel sheet with the highest average Fe concentration in the Zn-plated coating. S Zn (g / m 2 ): Among the Zn-plated steel sheets in the above first sheet assembly, the amount of Zn-plated coating on the steel sheet with the highest amount of Zn-plated coating, CE1(%): The carbon equivalent of the steel plate with a high carbon equivalent in the above first plate assembly. CE2(%):: The carbon equivalent of the steel plate with a low carbon equivalent in the above first plate assembly, Si High (Mass %): The Si concentration of the steel sheet with the highest Si content in the first plate assembly described above. d GB (pieces / mm): The grain boundary density of the outermost surface layer of the base material of the steel plate with high tensile strength in the above first plate assembly. θ (degrees): The angle between the line connecting the shoulders of the outermost steel plate and the tangent to the outer edge of the nugget of the outermost steel plate in the two or more superimposed steel plates described above (see Figure 5). g Sheet (mm): The sum of the gaps between the two or more overlapping steel plates at the joint surfaces 7 of each steel plate (see Figure 6). That is the case.

[0042] Note that CE1 and CE2 in equation (2) are carbon equivalents (CE), and CE can be calculated using the following equation (6).

[0043] Carbon equivalent CE=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) …(6) In equation (6) above, the [element symbol %] represents the content (mass %) of each element, and elements that are not present are represented as 0.

[0044] The reasons for defining equations (1) and (2) in this invention are as follows:

[0045] While various factors influence LME cracking, it is believed that an increase in Si content in the steel sheet composition increases the risk of LME cracking. The effect of Si in steel sheets on LME cracking is complex and cannot be simply explained, but Si is known to be an element that inhibits the Fe-Zn alloying reaction. Therefore, as Si increases, liquid Zn is more likely to exist between the sheets, and as the Si content of the steel sheet increases, C FeThe lower limit of the Si concentration (Si) of steel plates with a high Si content in the first plate assembly rises. High ) shall be taken into consideration.

[0046] Furthermore, the difference in carbon equivalents (CE1-CE2) is such that the larger the value of CE1-CE2, the more C Fe The lower limit of C increases. The reason for this is not clear, but it is thought to be as follows: When the difference in carbon equivalent is large, the difference in transformation point and material properties is also thought to be large. When the transformation points are different, the amount of volume expansion and contraction due to phase transformation during welding will differ between plates, which may cause an increase in local tensile stress. Also, when the difference in material properties is large, it is thought that tensile stress will be generated on the surface of the plate with a larger carbon equivalent due to plastic deformation of the plate with a smaller carbon equivalent and friction between the plates. From this, as the difference in carbon equivalent increases C Fe The lower limit is rising.

[0047] Furthermore, the inclination (θ) of the nugget is thought to be caused by the welding angle. Due to the moment generated during pressurization caused by the welding angle, tensile stress is generated in a part of the steel plate. From this, C increases in proportion to the increase in the inclination of the nugget. Fe The lower limit is rising.

[0048] Also, the total gap between the steel plates at the joint surface (g Sheet ) Also, similar to the inclination of the nugget, tensile stress is generated in a part of the steel plate when pressurized, and C increases in proportion to the increase in the total gap between the plates at the joint surface of the steel plates. Fe The lower limit is rising.

[0049] Furthermore, among the Zn-plated steel sheets of the first sheet assembly, the Fe concentration (C) of the steel sheet with the highest average Fe concentration in the Zn-plated steel sheet was determined. 0Fe ) is C 0Fe The smaller C Fe The lower limit of C rises. The reason for this is not clear, but it is thought that if the average Fe concentration in the Zn-based plating before welding is high, the Fe concentration in the Fe-Zn alloy layer of the alloy layer after welding will be high. From this, C rises in accordance with the decrease in the average Fe concentration. Fe The lower limit is rising. In addition, in the "Fe concentration (C 0Fe ) of the steel sheet having the highest average Fe concentration in the Zn-based plating", the "Fe concentration" refers to the Fe concentration in the Zn-based plating of the steel sheet.

[0050] Also, among the Zn-based plated steel sheets of the first plate set, the Zn-based plating adhesion amount (S Zn ) of the steel sheet having the highest Zn-based plating adhesion amount is such that the larger S Zn is, the lower limit value of C Fe increases. The reason for this is not clear, but it is considered as follows. When S Zn is large, the plating layer becomes thick and the diffusion distance becomes long, so it is considered that the alloying of the Fe-Zn alloy layer becomes difficult to progress. Therefore, the lower limit value of C Fe increases in accordance with the increase in this adhesion amount.

[0051] Also, the grain boundary density (d GB ) of the outermost surface layer of the base material in the steel sheet having a large tensile strength among the first plate set is such that the larger d GB is, the lower limit value of C Fe increases. The reason for this is not clear, but it is considered as follows. Since LME cracking occurs due to embrittlement of grain boundaries, the contact points between the liquid Zn and the grain boundaries of the base material surface layer become the starting points of cracking. When d GB is large, the contact points between the liquid Zn and the grain boundaries of the base material surface layer increase, and as a result, it is considered that cracking is likely to occur. Therefore, the lower limit value of C Fe increases in accordance with the increase in this grain boundary density.

[0052] And as described above, when the strength difference between the overlapped steel sheets is remarkable, it is considered that tensile stress due to the difference in transformation behavior during cooling is also likely to occur.

[0053] For the above reasons, in the present invention, the Fe concentration (C Fe ) of the Fe-Zn alloy layer in the first region on the mating surface side of the steel sheets in the first plate set is made to satisfy the formula (2).

[0054] In addition, since the higher C Fe is, the melting point of the Zn alloy layer rises and it is effective in suppressing LME cracking, CFe The higher the value, the better. However, C Fe To make it excessively high requires a very long period of power supply, which can lead to an increase in cycle time. Also, C Fe If the ratio is excessively high, the boundary between the Fe-Zn alloy layer and the base material may become unclear, making identification difficult. Therefore, C Fe The upper limit of is to satisfy equation (1). C in equation (1) Fe The upper limit is preferably 95% (mass%) or less.

[0055] Furthermore, from the viewpoint of more effectively obtaining the effects of equations (1) and (2) above, in addition to the provisions of each equation, the difference in carbon equivalents above is CE1-CE2≦0.7, and the grain boundary density of the outermost layer of the base material is d GB It is preferable that the value is ≤700. Also, Si High It is preferable that the amount be 2.5% or less, and more preferably 2.0% or less.

[0056] Furthermore, the reason for defining formula (3) in this invention is as follows:

[0057] In equation (3), the C of the Fe-Zn alloy layer in the first region Fe It defines the gradient. C Fe The gradient is C at a position 300 μm from the nugget end (position A shown in Figure 2(A), etc.) and at a position 700 μm from the nugget end (position B shown in Figure 2(A), etc.). Fe This can be determined from the following. As shown in the examples in Figure 2(B) and Figure 3(B), the absolute value of the slope of the line connecting A and B is determined. If the slope is large, C is in the regions 300 μm and 700 μm from the nugget edge, where the risk of LME cracking is high. Fe Because the change is large, locally C Fe There may be areas with low C, where LME may occur. Therefore, locally C Fe To prevent LME from occurring at low values, equation (3) is defined such that the absolute value of the slope (|m|) is 0.10 or less.

[0058] In this invention, the Fe concentration (C) of the Fe-Zn alloy layer Fe The first region that satisfies all of equations (1) to (3) is formed on both ends of the nugget. For example, as shown in Figures 2(A) and 3(A), if the nugget 4a is not tilted with respect to the steel plate mating surface 7, the first region formed on one side of either end of the nugget is to be measured. Also, for example, as shown in Figure 5, if the nugget 4a is tilted with respect to the steel plate mating surface 7, the first region formed on the opposite side of the tilt is to be measured. Specifically, if the nugget 4a is tilted in the direction shown in Figure 5 (i.e., the nugget 4a is tilted so that the left side of the paper is lower with respect to the steel plate mating surface 7), the first region on the right side of the nugget is to be measured. Furthermore, if there are three or more overlapping steel plates and Fe-Zn alloy layers are formed on multiple steel plate mating surfaces, the first region formed on the steel plate mating surface with the largest difference in TS between the contacting steel plates (i.e., the steel plate mating surface of the first plate assembly) is to be measured.

[0059] In this invention, the Si content of the steel sheet, the amount of plating, and the Fe concentration (C) are specified. Fe , C 0Fe The grain boundary density of the outermost layer of the base material can be measured by the method described in the examples below. Also, θ, g Sheet This can be measured using the method described later with reference to Figures 5 and 6.

[0060] [Multiple overlapping steel plates] [Zn-plated steel sheet] As described above, at least one of the two or more overlapping steel sheets is a zinc-plated steel sheet. This is because LME cracking is a phenomenon that occurs when at least one zinc-plated steel sheet is used. The multiple overlapping steel sheets (i.e., the sheet assembly) may consist entirely of zinc-plated steel sheets, or they may consist of zinc-plated steel sheets and steel sheets without a metal plating layer (so-called "cold-rolled steel sheets"). In either case, the effects of the present invention can be obtained.

[0061] In this invention, "Zn-plated steel sheet" refers to a steel sheet having a Zn-plated layer on the surface of a base steel sheet, such as zinc plating represented by electro-zinc plating (EG), hot-dip galvanizing (GI), and alloyed hot-dip galvanizing (GA), or zinc alloy plating containing elements such as aluminum and magnesium in addition to zinc.

[0062] While the composition of the plating layer is not particularly limited, it is preferable that the Fe concentration in the plating layer be 5% by mass or higher, as a higher Fe concentration facilitates the formation of an alloying layer during welding. Furthermore, from the viewpoint of preventing a decrease in the powdering properties of the steel sheet, it is preferable that the Fe concentration in the plating layer be 30% by mass or lower.

[0063] In order to more effectively obtain the above-mentioned effects in the present invention, the steel plates used in the plate assembly may have the following configurations as needed.

[0064] [Si content of steel plate] In the present invention, it is preferable that the steel plates used in the plate assembly include at least one steel plate with a Si content of 0.5% by mass or more.

[0065] When the Si content is less than 0.5 mass%, LME cracking may not occur regardless of disturbances or other welding conditions. The reason for this is not clear, but it is thought to be as follows: Si is thought to have the effect of expanding the liquid-solid coexistence region in the phase diagram, and it is thought that the addition of Si delays the solidification of Zn during the cooling process during welding. As a result, the state in which liquid Zn exists is prolonged, and it is thought that LME cracking is more likely to occur due to interaction with local tensile stress that occurs during holding or when the electrode is released. When the Si content is low, interaction between liquid Zn and tensile stress is less likely to occur, and depending on the welding conditions, LME cracking may not occur even if a method other than the welding conditions of the present invention is used. Therefore, in the present invention, LME cracking when the Si content is 0.5 mass% or more is considered. Accordingly, it is preferable that at least one of the steel plates used in the plate assembly is within the range of steel plate composition described later and has a Si content of 0.5 mass% or more.

[0066] Furthermore, in order to promote the alloying of Zn present between all overlapping steel plates, the Si content of all steel plates used in the plate assembly may be 0.5% by mass or more.

[0067] [Strength difference between steel plates] As described above, when there is a significant difference in strength between multiple overlapping steel plates, LME cracking is likely to occur due to tensile stress resulting from differences in transformation behavior during cooling in welding. Therefore, in this invention, among the two or more overlapping steel plates described above, the TS difference (hereinafter referred to as "ΔTS") is the largest difference in tensile strength between two steel plates that are in contact in the vertical direction. Max It is preferable that the pressure (referred to as ")" be 200 MPa or higher. Max This is because, at pressures below 200 MPa, tensile stress due to differences in transformation behavior during cooling is less likely to occur, and therefore, depending on the welding conditions, LME cracking may not occur even when using welding methods other than those of the present invention.

[0068] Therefore, in the plate assembly, ΔTS Max It is preferable that there is at least one combination of steel plates where the pressure is 200 MPa or higher. In this case, LME cracking can be suppressed even in plate assemblies that are prone to LME cracking, improving the degree of freedom in the structural design of automobiles, and thus the effects of the present invention can be obtained more effectively.

[0069] ΔTS Max The larger ΔTS is, the more effectively the effects of the present invention can be obtained. Max While no upper limit is specifically specified, from the viewpoint of practical strength levels for automotive steel sheets, 2000 MPa or less is preferable. ΔTS Max The pressure is more preferably 250 MPa or higher, and more preferably 1800 MPa or lower.

[0070] [Tensile strength of steel plate] Furthermore, LME cracking is more likely to occur in high-strength steel plates. Therefore, in this invention, it is preferable that at least one of the two or more overlapping steel plates has a tensile strength of 980 MPa or higher. In this case, LME cracking can be suppressed even if the steel plate is made stronger, so effects such as improved impact characteristics of the welded member can be expected, and the effects of this invention can be obtained more effectively. Preferably, the tensile strength of the steel plate is 3000 MPa or less.

[0071] [Composition of steel sheets] The component composition of the high-strength steel sheet used in the present invention is not particularly limited as long as it has the above-described welded joint configuration. From the viewpoint of applying the present invention to structural parts of automobiles, the component composition shown below is preferable. In the following description, the "%" notation for component composition means "mass%" unless otherwise specified.

[0072] C: 0.01~0.40% Carbon (C) is an element that contributes to increasing the strength of steel plates. Therefore, it is preferable that the C content be 0.01% or more. More preferably, the C content is 0.02% or more. On the other hand, if C is added in excess, the welded joint will harden excessively, causing a decrease in the toughness of the welded joint. Therefore, it is preferable that the C content be 0.40% or less. More preferably, the C content is 0.38% or less.

[0073] Si: 0.02~2.50% Si is an effective element for improving the strength and elongation of steel sheets. Therefore, it is preferable that the Si content be 0.02% or more. More preferably, the Si content is 0.10% or more. On the other hand, excessive addition of Si can lead to a decrease in LME resistance and plating properties. Therefore, it is preferable that the Si content be 2.50% or less. More preferably, the Si content is 2.00% or less.

[0074] Mn: 1.0~5.0% Mn is an element that contributes to increasing the strength of steel sheets. Therefore, it is preferable that the Mn content be 1.0% or more. More preferably, the Mn content is 1.2% or more. On the other hand, excessive addition of Mn promotes solidification segregation of alloying elements in the nugget, causing a decrease in the toughness of the weld. Therefore, it is preferable that the Mn content be 5.0% or less. More preferably, the Mn content is less than 3.5%.

[0075] P:0.050% or less If added in excess, phosphorus (P) can cause a decrease in the toughness of the weld due to solidification segregation of the nugget. Therefore, it is preferable that the P content be 0.050% or less. More preferably, the P content should be 0.020% or less. There is no specific lower limit for the P content, but extremely low P content increases steelmaking costs. Therefore, it is preferable that the P content be 0.005% or more.

[0076] S: 0.100% or less If the sulfur content is high, solidification segregation of the nugget can cause a decrease in the toughness of the weld. Therefore, it is preferable to keep the sulfur content at 0.100% or less. More preferably, the sulfur content should be 0.010% or less, and even more preferably 0.005% or less. There is no specific lower limit for the sulfur content, but extremely low sulfur content increases steelmaking costs. Therefore, it is preferable to keep the sulfur content at 0.001% or more.

[0077] Al: 0.010~1.000% Al is an element necessary for deoxidation, and it is desirable to contain 0.010% or more Al to obtain this effect. On the other hand, excessive addition of Al increases the number of inclusions in the steel sheet, reduces local deformability, and decreases the ductility of the steel sheet. Therefore, it is preferable to set the upper limit of the Al content to 1.000%. More preferably, the Al content is 0.800% or less.

[0078] N: 0.0100% or less Since nitrogen (N) forms coarse nitrides, reducing local deformability and decreasing the ductility of the steel sheet, it is desirable to keep the N content low. This tendency becomes particularly pronounced when N exceeds 0.0100%, so it is preferable to keep the N content below 0.0100%. More preferably, the N content is below 0.0075%. There is no specific lower limit for the N content, but extremely low N levels increase steelmaking costs. Therefore, it is preferable to keep the N content above 0.0001%.

[0079] The above is the basic component composition, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Co, Sn, and Zn, and the permissible content ranges for these are Co: 0.05% or less, Sn: 0.01% or less, and Zn: 0.01% or less.

[0080] In this invention, in addition to the basic component composition described above, one or more components selected from the following may be included as needed. Note that Ti, B, Nb, Cr, Ni, Mo, Cu, Sb, V, Ca, and REM can be included as needed, and their presence may be 0%. In other words, these components are preferably present in amounts of 0% or more.

[0081] Ti: 0.1% or less Ti is effective in precipitation hardening of steel sheets by forming fine carbonitrides. When Ti is included to obtain this effect, it is preferable to include 0.005% or more Ti. On the other hand, adding a large amount of Ti significantly reduces elongation, so it is preferable to keep the Ti content at 0.1% or less. More preferably, the Ti content is 0.065% or less.

[0082] B: 0.010% or less B is an element that improves the hardenability of steel plates and contributes to increasing their strength. When B is included to achieve this effect, it is preferable to include 0.0002% or more of B. On the other hand, the effect of B saturates when the content exceeds 0.010%, so it is preferable to keep the B content at 0.010% or less. The B content is preferably 0.008% or less.

[0083] Nb: 0.1% or less Nb is effective in precipitation hardening of steel sheets by forming fine carbonitrides. When Nb is included to obtain this effect, it is preferable to include 0.005% or more Nb. On the other hand, if a large amount of Nb is added, not only will the elongation decrease significantly, but slab cracking will occur after continuous casting, so it is preferable to keep the Nb content at 0.1% or less. The Nb content is more preferably 0.07% or less, and even more preferably 0.055% or less.

[0084] Cr:1.0% or less Cr is an element that contributes to increasing shear tensile strength because it readily promotes the formation of martensite in resistance welds. When Cr is included to achieve this effect, it is preferable to include 0.05% or more of Cr. On the other hand, if the Cr content exceeds 1.0%, surface defects are more likely to occur, so it is preferable to keep the Cr content at 1.0% or less. The Cr content is preferably 0.8% or less.

[0085] Ni: 0.50% or less Ni is an element that contributes to increasing the strength of steel sheets through solid solution strengthening and transformation strengthening. When Ni is included to exert these effects, it is preferable to include Ni at a concentration of 0.005% or more. Furthermore, when Ni is added simultaneously with Cu, it has the effect of suppressing surface defects caused by Cu, making it effective when Cu is added. On the other hand, the effect of Ni saturates even if the concentration exceeds 0.50%, so it is preferable to keep the Ni content at 0.50% or less.

[0086] Mo: 0.5% or less Like Cr, Mo (Mo) readily induces martensite formation in resistance welds, thus contributing to increased shear and tensile strength. To achieve these effects, it is desirable to include Mo at a concentration of 0.01% or more. Preferably, the Mo content is 0.02% or more. However, including more than 0.5% Mo will saturate the aforementioned effects, only increasing costs. Therefore, it is preferable to keep the Mo content at 0.5% or less. More preferably, the Mo content is 0.42% or less.

[0087] Cu: 1.0% or less Cu is an element that contributes to the solid solution strengthening of steel sheets. When Cu is included to achieve this effect, it is preferable to include Cu at a concentration of 0.005% or more. On the other hand, if the Cu concentration exceeds 1.0%, the effect saturates, and surface defects caused by Cu become more likely to occur. Therefore, it is preferable to keep the Cu content at 1.0% or less.

[0088] Sb: 0.20% or less Since Sb has the effect of suppressing the decarburization layer that forms on the surface of the steel sheet, it can suppress the reduction of martensite on the surface of the steel sheet. When Sb is included to achieve this effect, it is preferable that the Sb content be 0.001% or more. On the other hand, if the Sb content exceeds 0.20%, the rolling load increases, which reduces productivity, so it is preferable that the Sb content be 0.20% or less.

[0089] V:0.05% or less V is effective in precipitation hardening of steel sheets by forming fine carbonitrides. When V is included to obtain this effect, it is preferable to include V at a concentration of 0.005% or more. On the other hand, adding a large amount of V results in only a small increase in strength beyond 0.05%, and also leads to an increase in alloy costs. Therefore, it is preferable to keep the V content at 0.05% or less.

[0090] Ca: 0.05% or less, REM: 0.05% or less Ca and REM (rare earth metals) are elements that contribute to improving delayed fracture resistance by spheroidizing the shape of sulfides, and can be added as needed. When included to exert these effects, it is preferable to include at least 0.0005% of Ca and REM each. On the other hand, since the effects of Ca and REM saturate when included at concentrations exceeding 0.05% each, it is preferable to keep the content of Ca and REM at 0.05% or less each.

[0091] [Resistance spot welding method] An embodiment of a resistance spot welding method for manufacturing a welded component of the present invention will be described.

[0092] The welded member of the present invention is manufactured by resistance spot welding, in which a plate assembly made by overlapping two or more steel plates, each including at least one of the Zn-plated steel plates, is clamped between a pair of welding electrodes and joined by applying current while pressurizing.

[0093] For example, as shown in Figure 1, two steel plates 1 and 2 are stacked to form a plate assembly. Then, the plate assembly is clamped between a pair of welding electrodes 8 and 9 positioned on the lower and upper sides of the plate assembly, and current is applied while applying pressure with the welding electrodes, controlling the process to achieve predetermined welding conditions. This forms the aforementioned welded joint 4 between the steel plates that form the steel plate joining surface 7 of steel plates 1 and 2, thereby joining the steel plates together. Note that the plate assembly may also be made using steel plates with a Zn-based plating layer (e.g., GI steel plates, GA steel plates, EG steel plates) and steel plates without a plating layer (e.g., cold-rolled steel plates). In this case, the plates are stacked so that the side with the Zn-based plating layer is in contact with the cold-rolled steel plate.

[0094] As a welding apparatus usable with the resistance spot welding method of the present invention, a welding apparatus equipped with a pair of upper and lower welding electrodes, and capable of arbitrarily controlling the applied pressure and welding current during welding, can be used. The pressurizing mechanism, type, welding electrode shape, etc., of the welding apparatus are not particularly limited. Examples of pressurizing mechanisms include air cylinders and servo motors, and examples of types include stationary type and robotic gun. Examples of welding electrode tip types include DR type (dome radius type), R diameter (radius type), D type (dome type), etc., as described in JIS C 9304:1999. The tip diameter of the welding electrode is, for example, 4 mm to 16 mm. The radius of curvature of the welding electrode tip is, for example, 10 mm to 400 mm. Furthermore, the present invention can be applied to both DC and AC welding power sources. In the case of AC, "current" means "effective current".

[0095] Next, the welding conditions for the present invention will be described.

[0096] To promote Fe-Zn alloying between plates near the nugget, it is effective to promote interdiffusion of Fe and Zn by appropriately controlling the current flow pattern during current application, as described above.

[0097] In the resistance spot welding of the present invention, the current flow pattern during welding can be controlled by the main current flow process alone, or by the main current flow process and the post-current flow process. In particular, by combining the main current flow process and the post-current flow process, it is possible to maintain the Fe-Zn alloying process at a high temperature for a longer period of time more stably compared to using only the main current flow process. Furthermore, in this invention, the "hold time after the end of energization (T Hold )" refers to the "hold time after the end of the main energizing process" in the case of the first energizing pattern described below, and to the "hold time after the end of the subsequent energizing process" in the case of the second energizing pattern described below.

[0098] In the present invention, the current energizing step is the energizing step for forming the nugget, and the post-energizing step is the post-energizing step for performing post-heat treatment after the nugget has been formed.

[0099] [First energization pattern] Here, the case in which the current supply pattern of the present invention has only this current supply step will be referred to as the "first current supply pattern." An embodiment of the first current supply pattern will be described below.

[0100] In the first energizing pattern, the average current value of the energizing process is I1 (kA), the energizing time of the energizing process is T1 (s: seconds), the average pressure applied to the welding electrode during energizing is p (kN), and the hold time after the energizing process is T Hold (s) Let N be the number of steel plates. The welding conditions for this energizing process and the holding time after the energizing process are controlled so that the relationship in equation (4) is satisfied. I1 2 ×T1+(T Hold (x400) 2 ≥1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB 0.5 ) + (CE1 - CE2) × 70} / N …(4) Here, in equation (4), c is the gap (mm) between the welding electrode and the outermost layer of two or more overlapping steel plates, and g Axis This represents the misalignment (mm) of a pair of welding electrodes.

[0101] (4) When current is applied under conditions that do not satisfy equation, the temperature and time required for interdiffusion of Fe·Zn cannot be maintained during current application, and as a result, Fe-Zn alloying between plates becomes insufficient, C Fe It becomes impossible to increase the Si content and transformation point of the steel plate. Also, as mentioned above, LME cracking is more likely to occur when the difference in Si content and transformation point of the steel plate increases, so the left-hand side of equation (4) (i.e., "I1 2 ×T1+(T Hold (x400) 2 The lower limit of the value of '' is Si High It is effective to increase them according to (CE1-CE2).

[0102] For the reasons stated above, in the first energizing pattern, the welding conditions of the main energizing process are controlled to satisfy the relationship in equation (4). The left-hand side value of equation (4) is preferably 1.05 or more (the right-hand side value of equation (4)). Note that the "right-hand side value of equation (4)" above refers to 1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB 0.5 This is the value of ) + (CE1 - CE2) × 70 / N.

[0103] Furthermore, the upper limit of equation (4) is not particularly limited. In order to prevent significant heat dissipation due to excessive heat input and excessive increases in the cycle time of the automobile manufacturing process, in addition to controlling according to equation (4), it is preferable that I1 be 15.0 kA or less and T1 be 2.0 s or less. More preferably, I1 be 3.0 kA or more and T1 be 0.2 s or more.

[0104] [Second energization pattern] Here, the present invention's energizing pattern, which includes both an initial energizing step and a subsequent energizing step, is referred to as the "second energizing pattern." An embodiment of the second energizing pattern will be described below.

[0105] In the second energizing pattern, the average current value of the main energizing process is I1 (kA), and the energizing time of the main energizing process is T1 (s). Furthermore, the average current value of the subsequent energizing process is I2 (kA), the energizing time of the subsequent energizing process is T2 (s), and the number of energizing cycles in the subsequent energizing process is N. P Let (times). Also, the energizing interval (non-energizing time) between the main energizing process and the subsequent energizing process be T3 (s), and the average pressure applied to the welding electrode during energizing be p (kN). Also, the holding time after the end of energizing is T Hold Let (s) be the number of steel plates, and let N be the number of plates. The welding conditions for the main energizing process and the post-energizing process, as well as the holding time after the end of energizing, are controlled so that the relationship in equation (5) is satisfied. I1 2 ×T1+{3×I2 2 ×(5×T2-T3)×N P}+(T Hold (x400) 2 ≥1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB 0.5 ) + (CE1 - CE2) × 70} / N …(5) Here, in equation (5), g Axis is the misalignment (mm) of the pair of welding electrodes. c is the gap (mm) between the welding electrode and the outermost layer of two or more overlapping steel plates.

[0106] (5) When current is applied under welding conditions that do not satisfy equation (5), it is not possible to maintain a temperature and time sufficient for interdiffusion of Fe·Zn, resulting in insufficient Fe-Zn alloy formation between plates during the post-current application process, and consequently, C Fe It becomes impossible to increase this. Also, as mentioned above, LME cracking becomes more likely when the difference in Si content and carbon equivalent content of the steel sheet increases. In addition, the surface structure of the steel sheet and Zn-based plating also affect the susceptibility to LME cracking. Therefore, the lower limit of the left-hand side value of equation (5) is Si High (CE1-CE2) and C 0Fe Ya S Zn ya d GB The inventors conceived the idea that increasing it accordingly would be effective.

[0107] For the reasons stated above, in the second energizing pattern, the welding conditions for the main energizing process and the post-en energizing process are controlled to satisfy the relationship in equation (5). The left-hand side value of equation (5) is preferably 1.05 or more (the right-hand side value of equation (5)). Note that the "right-hand side value of equation (5)" above refers to "1.5 × C 0Fe ×Si High ×p{(θ 1.8 +g Sheet +c+g Axis ) + ((0.05 × S Zn / 3)+d GB0.5 This is the value of ) + (CE1 - CE2) × 70 / N.

[0108] Furthermore, the upper limit of equation (5) is not particularly limited. In order to prevent significant spatter due to excessive heat input and excessive increase in the cycle time of the automobile manufacturing process, in addition to controlling the welding conditions of the main energizing process and the post-energizing process to equation (5), it is preferable that I1 and I2 each be 15.0 kA or less, and T1 and T2 total 2.0 s or less. More preferably, I1 and I2 each be 3.0 kA or more, and T1 and T2 total 0.2 s or more.

[0109] [Electrode holding time] After the main energizing process in the first energizing pattern or the post-energing process in the second energizing pattern described above is completed, the electrodes are held for a predetermined time (i.e., a hold time). This hold time is set to 0.02 to 1.00 s. Holding the welding electrodes with a constant pressure after the energizing is complete suppresses the occurrence of blowholes in the nugget and an excessive increase in cycle time. The hold time is preferably 0.05 s or more, and preferably 0.50 s or less.

[0110] [Non-energized process] In the case of the second energizing pattern described above, there may be a de-energizing step between the main energizing step and the post-energizing step in which the energizing is suspended. If there is a de-energizing step, it is preferable to repeat the de-energizing step and the post-energizing step after the main energizing step. This is because the effects of the present invention can be obtained more effectively.

[0111] In equation (5) above, "T3(s)" represents the time during which the power is not supplied during the power-off process. Therefore, if there is no power-off process, T3(s) in equation (5) above should be set to 0. Also, in equation (5) above, "N P "(times)" indicates the number of repetitions.

[0112] In the post-energization process, the scattering due to the remelting of the nugget is suppressed while the mutual diffusion of Fe and Zn is promoted, and C FeTo increase the effect, it is effective to maintain the vicinity of the nugget within a certain temperature range after the completion of the main energizing process. Therefore, in the present invention, it is preferable to provide a period of no energizing as the energizing interval between the main energizing process and the subsequent energizing process.

[0113] When a post-energization process is performed with a constant current value, if the current value in the post-energization process is large, the temperature near the nugget will gradually rise, and conversely, if the current value in the post-energization process is small, the temperature near the nugget will gradually decrease. Even in such a state, it is thought that the desired effect can be obtained by appropriately setting the current value in the post-energization process, but the amount of work required to derive the optimal conditions may increase.

[0114] Furthermore, maintaining a constant temperature range requires strict condition control, and this becomes even more difficult during welding disturbances, as will be discussed later. On the other hand, by repeating the no-energy and post-energy processes, the temperature near the nugget can be kept within a relatively constant range, even if the total energizing time in the post-energy process increases.

[0115] For these reasons, the second energizing pattern of the present invention may include a de-energizing step after the main energizing step, and the de-energizing step and the post-energizing step may be repeated. This allows the vicinity of the nugget to be easily maintained within a constant temperature range, and the appropriate current range in the post-energizing step is broadened, which is thought to improve robustness against welding disturbances. However, if the de-energizing time is too short, there is a concern that spatter will increase due to the re-melting of the nugget, while if the de-energizing time is too long, the desired heat treatment effect may not be obtained. Therefore, the duration of the de-energizing step is preferably 0.01 s or more, and preferably 0.20 s or less.

[0116] The no-energy process and the post-energy process only need to be performed at least once. If the no-energy process and the post-energy process are repeated, the number of repetitions should more preferably be two or more, and even more preferably three or more. There is no specific upper limit on the number of repetitions, but generally there is an upper limit to the number of repetitions that can be set in a welding device, and setting a number of repetitions exceeding this limit requires modification of the welding device. For this reason, due to the increase in equipment costs in the automobile manufacturing process, the number of repetitions should preferably be 20 or less, and even more preferably 10 or less.

[0117] [Welding disturbances] In addition to the welding conditions in each of the above steps, the present invention may also have the following welding conditions.

[0118] As described above, LME cracking is more likely to occur in the presence of construction disturbances during welding. Therefore, in this energizing process, it is preferable that at least one welding point satisfies one or more of the following conditions (a) to (d) immediately before applying pressure with the welding electrode. This makes it possible to obtain the effects of the present invention more effectively. (a) A condition in which the welding angle between the welding electrode and two or more overlapping steel plates is 0.2 degrees or more. (b) In a condition where there is a gap of 0.5 mm or more between at least one pair of overlapping steel plates. (c) A condition in which there is a gap of 0.5 mm or more between any welding electrode and the outermost layer of two or more overlapping steel plates. (d) A state in which the misalignment of a pair of welding electrodes is 0.1 mm or more. These welding disturbances all locally increase the temperature and / or tensile stress of the weld when the electrodes are released, making it prone to LME cracking. However, according to the present invention C Fe By optimizing these factors, LME cracking can be suppressed even when these welding disturbances are present, improving the margin for managing construction disturbances during component manufacturing. The details of each construction disturbance are described below.

[0119] (a) A state in which the welding angle between the welding electrode and the two or more overlapping steel plates is 0.2 degrees or more. The welding angle refers to the angle at which the electrode is tilted relative to the steel plate, that is, "the angle between the direction of electrode pressure and the thickness direction of the steel plate." If the welding angle is large, bending stress is applied to the weld, causing large localized compressive plastic deformation, which increases the tensile stress after cooling.

[0120] Furthermore, when welding is performed with a beading angle, the resulting nugget will also be tilted. Therefore, when determining the beading angle from the cross-section after welding, the tilt of the nugget was used as the beading angle. In other words, this "beading angle" is one of the construction disturbances during welding, but for the reasons mentioned above, the "nugget tilt" in the member obtained after welding can be used as a substitute for this "beading angle".

[0121] The angle θ, which indicates the "nugget inclination," is determined by the method shown in Figure 5. As shown in Figure 5, a straight line connecting the outermost parts of the left and right shoulders of the upper plate 1 side (i.e., the outermost steel plate side of the plate assembly) (i.e., the boundary where deformation due to the pressure of the welding electrode is no longer observed) is used as the reference. The angle θ is defined as the angle between the reference line (hereinafter referred to as the "reference line") and the line passing through the intersections of two perpendicular lines drawn from positions 500 μm to the left and right of the center of this reference line and the outermost edge of the nugget (i.e., the outer edge of the nugget 4a). The "two perpendicular lines" mentioned above are lines parallel to the median line drawn from the center of the reference line. The effects of the present invention can be effectively obtained when the welding angle is 0.2 degrees or more. If the welding angle is too large, nugget formation becomes unstable and causes scattering, so it is preferable to set the welding angle to 10.0 degrees or less. More preferably, the welding angle is 1.0 degrees or more, and even more preferably 8.0 degrees or less.

[0122] (b) In a condition where there is a gap of 0.5 mm or more between at least one pair of overlapping steel plates. Because steel plates deform during welding, the gap between steel plates may change before and after welding. However, if a gap exists between steel plates in the cross-section after welding, it is highly likely that a gap also existed before welding. Therefore, when determining the gap between steel plates from the cross-section after welding, the gap between steel plates after welding should be considered as the gap between steel plates before welding. In other words, the "gap between steel plates" described in (b) above is one of the construction disturbances during welding, but for the reasons stated above, the "gap between steel plates after welding" in the member obtained after welding may be substituted for this "gap between steel plates".

[0123] Note that this "g" indicates the "gap between steel plates after welding". Sheet The value of the nuggets is determined by the method shown in Figures 6(A) and 6(B). As shown in Figure 6(A), in the case of continuous dotting, the difference between the maximum thickness of steel plates 1 and 2 between two adjacent nuggets 4a and the total thickness of the overlapping steel plates 1 and 2 (i.e., the plate gap 10) is calculated, and g Sheet In other cases (for example, as shown in Figure 6(B)), the difference between the maximum thickness of steel plates 1 and 2 between nugget 4a and the end of the plate assembly and the total thickness of the overlapping steel plates 1 and 2 is calculated, and g Sheet Let's assume that.

[0124] The effects of the present invention can be effectively obtained when this gap is 0.5 mm or more. If this gap is too large, nugget formation becomes unstable and causes scattering, so it is preferable that this gap be 4.0 mm or less. More preferably, the gap is 1.0 mm or more, and even more preferably 3.0 mm or less.

[0125] (c) A condition in which there is a gap of 0.5 mm or more between any welding electrode and the outermost layer of two or more overlapping steel plates. Similar to (b) above, if there is a gap between either welding electrode and the steel plate immediately before pressurization begins, the steel plate will undergo bending deformation, and bending stress will be applied to the weld, making LME cracking more likely. If there is a gap between either welding electrode and the steel plate immediately before pressurization begins, for example, if one electrode is movable (hereinafter referred to as the "movable electrode") and the other electrode is fixed (hereinafter referred to as the "fixed electrode"), and there is a gap between the fixed electrode and the steel plate, when pressurization by the movable electrode begins, the steel plate will undergo bending deformation, and bending stress will be applied to the weld. This makes LME cracking more likely. Let the amount of this gap between the welding electrode and the steel plate be c. The effects of the present invention can be effectively obtained when this gap amount is 0.5 mm or more. If this gap amount is excessive, nugget formation becomes unstable and causes spatter, so it is preferable that this gap amount be 5.0 mm or less. The gap amount is more preferably 1.0 mm or more, and even more preferably 3.0 mm or less.

[0126] Furthermore, if the values ​​of the welding disturbances in equation (4) are within the preferred ranges described above and below, and welding is performed under conditions that satisfy equation (4), then the influence of the welding disturbances described in (c) on LME cracking is considered to be small. Also, in equation (1), the influence of the welding disturbances described in (c) is considered to be small.

[0127] (d) A state in which the misalignment of a pair of welding electrodes is 0.1 mm or more. Misalignment refers to a condition where the central axes of a pair of welding electrodes are not aligned. The amount of misalignment of this pair of welding electrodes is g. Axis As with the impact angle mentioned above, if the misalignment is large, bending stress is applied to the weld, making LME cracking more likely. The effects of the present invention can be effectively obtained when the misalignment is 0.1 mm or more. If the misalignment is excessive, nugget formation becomes unstable and causes spatter, so it is preferable to keep the misalignment at 5.0 mm or less. The misalignment is more preferably 0.2 mm or more, and even more preferably 3.0 mm or less.

[0128] Furthermore, if each value of the welding disturbance in equation (4) is within the above-mentioned preferred range and welding is performed that satisfies equation (4), then the influence of the welding disturbance described in (d) on LME cracking is considered to be small. Therefore, in equation (1), it is possible to obtain the effects of the present invention even if the welding disturbance described in (d) is ignored.

[0129] In this invention, the pressurization conditions in each step are not particularly limited. From the viewpoint of automotive applications, it is preferable to adjust the pressurization conditions to a range of 2.0 to 8.0 kN. [Examples]

[0130] 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.

[0131] A plate assembly was constructed using the steel plates shown in Table 1, and this assembly was welded under the welding conditions shown in Tables 2 and 3 to produce a welded joint (welded member). The plate assembly was arranged and stacked in the order of bottom plate, top plate, or bottom plate, middle plate, top plate, as shown in Table 2. A single-phase AC (50Hz in this case) resistance welding machine with a servo motor pressure attached to a welding gun was used as the welding apparatus. The pair of electrode tips used were DR type electrodes made of chromium copper with a tip radius of curvature R40mm and a tip diameter of 6mm.

[0132] In Table 1, "GI" in the "Plating" column refers to steel sheets with a hot-dip galvanized layer (i.e., hot-dip galvanized steel sheets), "GA" refers to steel sheets with an alloyed hot-dip galvanized layer (i.e., alloyed hot-dip galvanized steel sheets), "EG" refers to steel sheets with an electro-galvanized layer (i.e., electro-galvanized steel sheets), and "-" refers to steel sheets without a plating layer (in this case, cold-rolled steel sheets).

[0133] Furthermore, the component composition shown in Table 1 was measured by inductively coupled plasma (ICP) emission spectroscopy.

[0134] Furthermore, the "Tensile Strength" column in Table 1 shows the tensile strength (TS) (unit: MPa) measured by taking JIS No. 5 tensile test specimens in the rolling direction from each steel plate and conducting tensile tests in accordance with JIS Z 2241.

[0135] Furthermore, the "Grain Boundary Density" column in Table 1 shows the grain boundary density of the outermost layer of the base material for each steel sheet. The grain boundary density was measured using EBSD (Electron Backscatter Diffraction). Large-angle grain boundaries with a crystal orientation difference of 15° or more were defined as grain boundaries, and an IQ Map displaying the grain boundaries was created. Next, the number of grain boundaries in the outermost layer of the base material was counted in the created IQ Map, and the grain boundary density for five fields of view with a size of 50 μm × 40 μm or larger was measured, and then the average value of the measurements for all fields of view was calculated as d GB Table 2, "d GB In the first plate assembly, the grain boundary density (unit: grains / mm) of the outermost layer of the base material on the steel plate side with the greater tensile strength is listed.

[0136] Furthermore, the "Fe concentration" shown in the "Plating" column of Table 1 refers to the C described later. Fe The concentration was measured using the same method. The "amount of adhesion" was measured using GDS (Glow Discharge Spectroscopy). See Table 2 for "S Zn In the section, the amount of plating adhesion on the steel sheet with the highest amount of plating adhesion among the Zn-plated steel sheets in the stacked steel sheets is indicated.

[0137] The symbols shown in the "Construction Disturbances" column of Table 3 correspond to (a) to (d) shown above for welding construction disturbances.

[0138] Also, the "N" shown in the "Welding Conditions" column of Table 3. P " indicates the number of repetitions of the no-energy process and the post-energy process. For example, if it is "main energy application process only", then N P :0(times), and for example, in the case of "main power-on process - no power-on process - post-power-on process", N P :1 (times), and for example, in the case of "Main power-on process - Non-power-on process (1) - Post-power-on process (1) - Non-power-on process (2) - Post-power-on process (2)", NP It was assumed that this would be 2 (times). Thus, the number of times the power is not applied is N P This will be the same number.

[0139] Using the obtained welded member, the Fe concentration (C) of the Fe-Zn alloy layer in the Zn alloy layer is determined by the method shown below. Fe Measurements of the weld and evaluation of LME cracking in the welded area were performed.

[0140] <C Fe Measurement > C Fe The measurement method is not particularly limited as long as the Fe concentration can be quantitatively measured. For example, it can be measured by EPMA or SEM-EDS. Here, as an example, it was measured by SEM-EDS. The conditions for SEM-EDS were as follows. EDS conditions: Ka source used as the X-ray source for irradiation, magnification: 1500x or higher, acceleration voltage: 15kV, scan time: 1 min or higher However, gaps are excluded. Specifically, the thickness of the gaps is subtracted from the total thickness including the gaps. If the thickness of the gaps is difficult to determine, the measurement is taken from the nearest adjacent area without gaps.

[0141] As shown in Figures 2(A) and 3(A), in the steel plate joint surface 7 where the Fe-Zn alloy layer is formed in the welded joint, the region from A to B (i.e., the first region) was designated as the observation region.

[0142] Here, the observation position was defined as starting at position A, 300 μm away from the nugget end E in the plate width direction (i.e., one side of the first region perpendicular to the steel plate mating surface 7, located on the left side of the paper in each figure), and ending at position B, 700 μm away from the nugget end E in the plate width direction (i.e., one side of the first region perpendicular to the steel plate mating surface 7, located on the right side of the paper in each figure). The region from the starting point to the ending point was observed at equal intervals, and the Zn-enriched area between the plates in each field of view was identified as the Fe-Zn alloy layer, and the Fe concentration of the Fe-Zn alloy layer was measured. The "Zn-enriched area" was determined from the contrast in the composition image (COMPO image). As shown in each figure, the Fe concentration was measured at the 1 / 4 position in the thickness direction of the Fe-Zn alloy layer. The average value of the obtained Fe concentrations, excluding the maximum and minimum values, was defined as the Fe concentration (C) of the Fe-Zn alloy layer. Fe )

[0143] The phrase "observe at equal intervals" above refers to observing at least 10 fields of view at equal intervals. These fields of view may be set appropriately from, for example, 11 to 100 locations. Since accurate values ​​may not be obtained if the measurement interval is less than 10 locations, 10 fields of view were used in this embodiment. Furthermore, the following analytical methods were used to measure the "Fe concentration" and "Zn concentration" mentioned above. Note that Figures 2(A) and 3(A) are schematic diagrams for illustrative purposes, and only six locations are shown for clarity.

[0144] Concentration measurement can be performed, for example, by line analysis or point analysis. When performing concentration measurement using these analyses, concentration measurements are taken at least 5 lines for each field of view. After measuring the concentration at each line and calculating the average concentration per field of view, the average value obtained by excluding the maximum and minimum values ​​from all measured values ​​across the entire field of view is calculated as C Fe This was decided upon. Furthermore, in the case of point analysis, it is preferable to measure at a pitch of 1 μm or less per location.

[0145] As mentioned above, C Fe The area where this occurs is formed on both ends of the nugget. If the nugget is not tilted, the C on one side will be formed. FeMeasured, and when the nugget was tilted, C on the side opposite to the tilted side was measured. Fe was measured.

[0146] <Evaluation of LME cracks> After cutting the center of the welded part of the obtained welded member with a micro cutter, the cross-section of the welded part was observed to evaluate the presence or absence of LME cracks. Specifically, five welded members were fabricated under each welding condition shown in Tables 2 and 3, and the presence or absence of LME cracks on the plate-plate interface (i.e., the side of the steel plate mating surface) was confirmed. The results evaluated according to the following evaluation criteria are shown in Table 3. Here, when the evaluation result was an A judgment or a B judgment, it was evaluated as "qualified". <Evaluation criteria> A judgment: 0 / 5 (i.e., the number of cracked specimens among 5 specimens is 0) B judgment: 1 / 5 (i.e., the number of cracked specimens among 5 specimens is 1) C judgment: 2 / 5 to 4 / 5 (i.e., the number of cracked specimens among 5 specimens is 2 to 4)

[0147]

Table 1

[0148]

Table 2

[0149]

Table 3

[0150] As shown in Tables 2 and 3, it was found that for the welded members of the invention examples, all the evaluation results of LEM cracks were qualified, indicating that LME cracks were suppressed.

Explanation of reference numerals

[0151] 1, 2, 3 Steel plates 4 Welded part 4a Nugget 4b Weld heat-affected zone 5 Fe-Zn alloy layer 6. Resistance spot welding components 7 Steel plate joining surface 8, 9 Welding electrodes 10 Gap

Claims

1. A resistance spot welded member in which two or more overlapping steel plates are resistance spot welded, Of the two or more overlapping steel plates mentioned above, at least one is a Zn-plated steel plate. Among the combinations in which a Zn-based plating exists between two steel plates that are in contact in the vertical direction, the combination with the largest difference in tensile strength is called the first plate combination. In the first plate assembly, the Fe concentration of the Fe-Zn alloy layer formed in the first region from position A, 300 μm in the plate width direction from the nugget end on the steel plate mating surface, to position B, 700 μm in the plate width direction from the nugget end, is C Ｆｅ (As mass%), C in the Fe-Zn alloy layer of the first region Ｆｅ When the gradient is m, A resistance spot welding member wherein the Fe concentration of the Fe-Zn alloy layer in the first region satisfies all of equations (1) to (3). 98≧C Ｆｅ …(1) C Ｆｅ ≧{25×Yes Ｈｉｇｈ ×(CE １ -CE ２ )}+{θ+(' Ｓｈｅｅｔ / 1.2)}+[{(S Ｚｎ ×d ＧＢ ) ０．５ } / 30]+{60+(C ０Ｆｅ / 3)} …(2) |m|≦0.10…(3) Here, in equation (2), C ０Ｆｅ (Mass %): Among the Zn-plated steel sheets in the first plate assembly, the Fe concentration of the steel sheet with the highest average Fe concentration in the Zn-plated plating, S Ｚｎ (g / m ２ ): Among the Zn-plated steel sheets in the first plate assembly, the amount of Zn-plated coating on the steel sheet with the highest amount of Zn-plated coating, CE １ (%): The carbon equivalent of the steel plate with a high carbon equivalent in the first plate assembly, CE ２ (%): The carbon equivalent of the steel plate with a low carbon equivalent in the first plate assembly, Si Ｈｉｇｈ (Mass %): Si concentration of the steel plate with a high Si content in the first plate assembly, d ＧＢ (pieces / mm): The grain boundary density of the outermost surface layer of the base material of the steel plate with high tensile strength in the first plate assembly. θ (degrees): The inclination of the nugget, which is determined by the angle between the straight line connecting the shoulders of the outermost steel plate and the tangent to the outer edge of the nugget of the outermost steel plate in the two or more superimposed steel plates. g Ｓｈｅｅｔ (mm): The sum of the gaps between the joining surfaces of the two or more overlapping steel plates. That is the case.

2. In the first plate assembly, the carbon equivalent of the two steel plates is CE １ -CE ２ ≤ 0.7, and the grain boundary density of the outermost layer of the base material is d ＧＢ A resistance spot welding member according to claim 1, wherein the coefficient of resistance is ≤ 700.

3. A resistance spot welding method for a resistance spot welding member according to claim 1 or 2, This process involves clamping two or more overlapping steel plates with a pair of welding electrodes, applying pressure, and then applying current to form a nugget. The average current value of the above-mentioned main energization process is I １ (kA), the energizing time of the above energizing process is T １ (s), the average pressure applied to the welding electrode is p (kN), and the holding time after the energization is finished is T. Ｈｏｌｄ (s) When the number of steel plates is N (sheets), A resistance spot welding method for resistance spot welding members, wherein the welding conditions of the current application process satisfy equation (4). I １ ２ ×T １ +(T Ｈｏｌｄ ×400) ２ ≧1.5×C ０Ｆｅ ×Yes Ｈｉｇｈ ×{(θ) １．８ +' Ｓｈｅｅｔ +c+g Ａｘｉｓ )+((0.05×S Ｚｎ / 3)+$ ＧＢ ０．５ )+(FE １ -CE ２ )×70} / N …(4) Here, in equation (4), c is the gap (mm) between the welding electrode and the outermost layer of the two or more overlapping steel plates, and g Ａｘｉｓ is the misalignment (mm) of the pair of welding electrodes.

4. A resistance spot welding method for a resistance spot welding member according to claim 1 or 2, The process includes a main energizing step in which two or more overlapping steel plates are clamped between a pair of welding electrodes and energized while pressurized to form a nugget, and a post-energizing step in which the formed nugget is subjected to post-heat treatment after the main energizing step. The average current value of the above-mentioned main energization process is I １ (kA), the energizing time of the above energizing process is T １ (s), the average current value of the post-energization process is I ２ (kA), the energizing time of the subsequent energizing process is T ２ (s), the number of times the energy is applied in the subsequent energizing process is N Ｐ (times), the energizing interval between the main energizing process and the subsequent energizing process is set to T ３ (s), let p (kN) be the average pressure applied to the welding electrode, and T be the holding time after the energization is finished. Ｈｏｌｄ (s) When the number of steel plates is N (sheets), A resistance spot welding method for a resistance spot welding member, wherein the welding conditions for the main energizing step and the post-energizing step satisfy equation (5). I １ ２ ×T １ +{3×I ２ ２ ×(5×T ２ -T ３ )×N Ｐ }+(T Ｈｏｌｄ ×400) ２ ≧1.5×C ０Ｆｅ ×Yes Ｈｉｇｈ ×{(θ) １．８ +' Ｓｈｅｅｔ +c+g Ａｘｉｓ )+((0.05×S Ｚｎ / 3)+$ ＧＢ ０．５ )+(FE １ -CE ２ )×70} / N …(5)

5. The resistance spot welding method for a resistance spot welded member according to claim 3, wherein, in the current application step, at least one welding point satisfies one or more conditions selected from (a) to (d) immediately before applying pressure with the welding electrode. (a) A state in which the welding angle between the welding electrode and the two or more overlapping steel plates is 0.2 degrees or more. (b) A state in which there is a gap of 0.5 mm or more between at least one pair of steel plates among the two or more overlapping steel plates. (c) A state in which there is a gap of 0.5 mm or more between any of the welding electrodes and the outermost layer of the two or more overlapping steel plates. (d) A state in which the misalignment of the pair of welding electrodes is 0.1 mm or more.

6. The resistance spot welding method for a resistance spot welded member according to claim 4, wherein, in the current application step, at least one welding point satisfies one or more conditions selected from (a) to (d) immediately before applying pressure with the welding electrode. (a) A state in which the welding angle between the welding electrode and the two or more overlapping steel plates is 0.2 degrees or more. (b) A state in which there is a gap of 0.5 mm or more between at least one pair of steel plates among the two or more overlapping steel plates. (c) A state in which there is a gap of 0.5 mm or more between any of the welding electrodes and the outermost layer of the two or more overlapping steel plates. (d) A state in which the misalignment of the pair of welding electrodes is 0.1 mm or more.

7. The resistance spot welding method for a resistance spot welding member according to claim 4, wherein between the main energizing step and the post-energizing step, there is a no-energy step in which the energizing is suspended.

8. The resistance spot welding method for a resistance spot welding member according to claim 6, wherein between the main energizing step and the post-energizing step, there is a no-energy step in which the energizing is suspended.

9. A resistance spot welding method for a resistance spot welding member according to claim 7, wherein the de-energization step and the post-energization step are repeated after the main energization step.

10. A resistance spot welding method for a resistance spot welding member according to claim 8, wherein the de-energization step and the post-energization step are repeated after the main energization step.

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