Resistance spot welded member, and resistance spot welding method for same

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

Resistance spot welded member and resistance spot welding method

[0001] The present invention relates to a resistance spot-welded component formed by resistance spot welding a plurality of steel plates, and more particularly to a resistance spot-welded component suitable for use as a structural component for automobiles and the like, and a resistance spot welding method for the same.

[0002] From the perspective of global environmental conservation ２ In order to reduce emissions, improving automobile fuel efficiency by reducing the weight of automobile bodies while maintaining their strength has always been an important issue in the automotive industry. Furthermore, from the perspectives of collision safety and improved fuel efficiency, it is also necessary to reduce the weight of automobile bodies while maintaining their strength. To achieve these goals, it is effective to reduce the thickness of steel sheets used as materials for automotive parts by increasing their strength, and in recent years, the use of steel sheets with a tensile strength (TS) of 980 MPa or more has been promoted.

[0003] In the automobile assembly process, overlapping steel sheets are typically joined by welding when assembling press-formed automotive parts. From the perspectives of cost and manufacturing efficiency, resistance spot welding, a type of lap resistance welding, is often used to join automotive parts. This welding method involves sandwiching two or more overlapping steel sheets between a pair of welding electrodes from above and below, applying pressure to the steel sheets from above and below with the pair of welding electrodes, and passing a high welding current between the upper and lower welding electrodes for a short period of time to join the steel sheets. Note that FIG. 1 shows an example of two overlapping steel sheets 1 and 2 sandwiched between welding electrodes 8 and 9. This method utilizes resistance heating generated by passing a high welding current between the welding electrodes to obtain a point-like weld 4. This elliptical weld 4 is called a nugget and is the portion where the overlapping steel sheets 1 and 2 melt and solidify at the contact point when a current is passed through them. This results in a point-like weld.

[0004] To ensure the crashworthiness of automobiles, it is necessary to improve not only the strength of the steel sheet but also the strength of the welds. 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 test method measures the tensile shear strength (TSS) by applying a tensile load to the welded joint in the tensile shear direction.

[0005] Furthermore, in automotive steel sheets, rust-resistant steel sheets such as zinc (Zn)-plated steel sheets (so-called "surface-treated steel sheets") are used for parts at risk of corrosion from the viewpoint of corrosion resistance. However, resistance spot welding of overlapping steel sheets including surface-treated steel sheets poses a problem of cracks occurring in the weld. The cracks in the weld are thought to be caused by so-called liquid metal embrittlement (hereinafter referred to as "LME cracking"), in which a low-melting-point metal plating layer on the surface of the surface-treated steel sheet melts during welding. When tensile stress due to the welding electrode pressure or the thermal expansion or contraction of the steel sheet is applied to the weld, 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. LME cracking can occur in various locations, such as the surfaces of steel sheets 1 and 2 that contact welding electrodes 8 and 9, as shown in FIG. 7, or the surfaces of steel sheets 1 and 2 that contact each other.

[0006] Countermeasures against such LME cracking include, for example, the technologies disclosed in Patent Documents 1 to 4. Patent Document 1 proposes that the chemical composition of the steel sheets used in the sheet assembly be within a specific range, specifically, a chemical composition consisting of, 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 balance 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 current duration and the holding time after welding current duration so as to satisfy the following conditional expressions (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) In the conditional expressions (A) and (B), t is the plate thickness (mm), WT is the welding current application time (ms), and HT is the holding time after welding current application (ms).

[0008] Patent Document 2 also proposes performing resistance spot welding by appropriately setting the current application time and the electrode holding time after current application according to the thickness of the steel sheet, and by using a high-strength galvanized steel sheet in which the amount of alloy elements in the steel sheet is equal to or less than a certain level.

[0009] Patent Document 3 proposes a resistance spot welding method in which a current pattern is a multi-stage current pattern of three or more stages, welding conditions such as current application 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 appropriate current range is a current range that can stably form a nugget that is equal to or larger than the desired nugget diameter and has a molten residual thickness of 0.05 mm or more.

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

[0011] Japanese Patent Application Laid-Open No. 10-195597 Japanese Patent Application Laid-Open No. 2003-103377 Japanese Patent Application Laid-Open No. 2003-236676 International Publication No. 2016 / 159169

[0012] However, since Patent Document 1 requires that the amount of alloying elements in the steel sheet be limited, there are problems such as limitations on the use of steel sheets that satisfy required performance. In particular, under the circumstances where high alloying is being promoted in line with the recent trend toward higher strength steel sheets, the application of the technology of Patent Document 1 is extremely limited.

[0013] Patent Document 2 only proposes a method for suppressing LME cracking when an excessively large welding current is set such that expulsion occurs, and does not mention LME cracking in a state where expulsion does not occur.

[0014] Patent Document 3 has the problem that a large number of steps are required to optimize the welding conditions, and that it cannot be applied to steel plates and plate assemblies for which it is difficult to ensure an appropriate current range. In addition, Patent Documents 2 and 3 do not consider the influence of the welding electrode impact angle, and therefore may be insufficient as a countermeasure when considering the actual work during automobile assembly.

[0015] In Patent Document 4, a step of removing the plating layer in advance is required, which increases the manufacturing cost. Furthermore, since the plating layer is removed, it is thought that the corrosion resistance of the welded portion is reduced.

[0016] The present invention has been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a resistance spot-welded component and a resistance spot welding method thereof that can prevent LME cracking at resistance spot welds in sheet assemblies using multiple steel sheets, particularly surface-treated steel sheets.

[0017] The present inventors have conducted extensive research to achieve the above object and have come to the following findings.

[0018] The effect of the present invention on cracking that occurs during welding cannot be simply explained because various factors have a complex influence. However, the inventors believe as follows. LME cracking in resistance spot welds is likely to occur when excessive tensile residual stress is generated in the resistance spot weld due to construction disturbances during welding. It is known that LME cracking is particularly likely to occur on the mating surfaces of steel sheets where the steel sheets come into contact, in areas where localized tensile stress is high when the welding electrode is released after the current and pressure application of resistance spot welding is completed. Furthermore, when there is a significant difference in strength between overlapping steel sheets, tensile stress is also likely to occur due to differences in transformation behavior during cooling.

[0019] As described above, LME cracking occurs when tensile stress is applied to a liquid metal such as Zn in contact with a steel sheet. Therefore, interdiffusion of Fe and Zn is promoted between the sheets (i.e., the steel sheet mating surfaces) where the strength difference between the overlapping steel sheets is large, and alloying of Fe and Zn is promoted, thereby controlling the formation of a Zn alloy layer with a certain Fe concentration or higher near the nugget. This prevents liquid Zn from being present between the sheets when the tensile stress is applied. The inventors of the present invention came up with the idea that LME cracking can be prevented based on this technical concept. They also discovered that there are appropriate welding conditions for controlling the Zn alloy layer to have a certain Fe concentration or higher.

[0020] The present invention was made based on the above findings, and its gist is as follows: [1] A resistance spot welded component in which two or more overlapping steel sheets are resistance spot welded, wherein at least one of the two or more overlapping steel sheets is a Zn-based plated steel sheet, and among the combinations of two steel sheets abutting in the vertical direction and having a Zn-based plating between them, the combination in which the difference in tensile strength is greatest is called a first sheet combination, and the Fe concentration of an Fe-Zn alloy layer formed in a first region on the mating surfaces of the steel sheets in the first sheet combination from position A, which is 300 μm from the nugget edge in the sheet width direction, to position B, which is 700 μm from the nugget edge in the sheet width direction, is defined as C Ｆｅ (mass %), and C of the Fe—Zn alloy layer in the first region Ｆｅ The resistance spot welded component has an Fe concentration in the Fe—Zn alloy layer of the first region that satisfies all of the following formulas (1) to (3): 98≧C Ｆｅ …(1) C Ｆｅ ≧{25×Si Ｈｉｇｈ × (CE １ -CE ２ )}+{θ+(g Ｓｈｅｅｔ / 1.2)}+[{(S Ｚｎ ×d ＧＢ ) ０．５} / 30]+{60+(C ０Ｆｅ / 3)} ... (2) |m| ≦ 0.10 ... (3) Here, in formula (2), C ０Ｆｅ(mass%): 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 sheet set, S Ｚｎ (g / m ２ CE): The Zn-based coating weight of the steel sheet having the highest Zn-based coating weight among the Zn-based coated steel sheets in the first sheet set. １ (%): carbon equivalent of the steel plate with a high carbon equivalent in the first plate pair, CE ２ (%): carbon equivalent of the steel plate with a low carbon equivalent in the first plate set, Si Ｈｉｇｈ (mass%): Si concentration of the steel plate having a large Si content in the first plate set, d ＧＢ (number / mm): grain boundary density of the outermost layer of the base material of the steel plate having the large tensile strength in the first sheet pair, θ (degrees): inclination of the nugget obtained by the angle between the line connecting the shoulders of the steel plate of the outermost layer in the two or more overlapping steel plates and the tangent to the outer peripheral edge of the nugget of the steel plate of the outermost layer, g Ｓｈｅｅｔ (mm): the total gap between the contact surfaces of the two or more overlapping steel plates. [2] The carbon equivalent of the two steel plates in the first plate set is CE １ -CE ２ ≦0.7, and the grain boundary density of the outermost layer of the base material is d ＧＢ [3] A resistance spot welding method for a resistance spot welded member according to [1] or [2], comprising a main current application step of clamping two or more overlapping steel sheets between a pair of welding electrodes and applying current while applying pressure to form a nugget, wherein the average current value of the main current application step is I １ (kA), the energization time of the main energization step is T １ (s), the average pressure of the welding electrode is p (kN), and the hold time after the end of current application is T Ｈｏｌｄ (s), when the number of steel plates is N (pieces), the welding conditions of the main current application step satisfy formula (4). １ ２ ×T １ + (T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ１．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２ ) × 70} / N (4) where, in formula (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) between the pair of welding electrodes. [4] A post-current application process is performed after the main current application process to perform a post-heat treatment on the formed nugget, and the average current value of the main current application process is I １ (kA), the energization time of the main energization step is T １ (s), the average current value of the post-current application step is I ２ (kA), the energization time of the post-energization step is T ２ (s), the number of energizations in the post-energization step is N Ｐ (times), and the energization interval between the main energization step and the post-energization step is T ３ (s), the average pressure of the welding electrode is p (kN), and the hold time after the end of the current is T Ｈｏｌｄ (s) and the number of steel plates is N (plates), the welding conditions for the main current application step and the post-current application step satisfy formula (5) by replacing formula (4). １ ２ ×T １ + {3 × I ２ ２ × (5 × T ２ -T ３ ) x N Ｐ}+(T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２)×70} / N ... (5) [5] The resistance spot welding method of resistance spot-welded members according to [3] or [4], wherein, in the main current application step, for at least one weld spot, one or more of the following conditions are satisfied immediately before applying pressure with the welding electrode: (a) a state in which the impact angle between the welding electrode and the two or more overlapping steel sheets is 0.2 degrees or more, (b) a state in which a gap between at least one pair of steel sheets among the two or more overlapping steel sheets is 0.5 mm or more, (c) a state in which a gap between one of the welding electrodes and the outermost layer of the two or more overlapping steel sheets is 0.5 mm or more, and (d) a misalignment amount between the pair of welding electrodes is 0.1 mm or more. [6] The resistance spot welding method of resistance spot-welded members according to [4] or [5], further comprising a non-current application step in which current application is suspended between the main current application step and the post-current application step. [7] The resistance spot welding method for resistance spot-welded members according to [6], wherein the non-energizing step and the post-energizing step are repeated after the main energizing step.

[0021] The present invention provides a resistance spot-welded component that can prevent LME cracking at resistance spot welds, even in sheet assemblies using multiple steel sheets, particularly surface-treated steel sheets, regardless of the steel sheet composition or welding disturbances. It also provides a resistance spot welding method for resistance spot-welded components that can produce welded joints without prior removal of the plating layer of the Zn-based plated steel sheets included in the sheet assembly.

[0022] 1 is a cross-sectional view in the thickness direction showing an example of resistance spot welding. FIG. 2(A) shows a resistance spot weld and its periphery in a resistance spot welded member according to an embodiment of the present invention, and FIG. Ｆｅ 2(A) is a cross-sectional view in the thickness direction that schematically explains the measurement method of Ｆｅ 3A is a diagram illustrating the gradient of the resistance spot weld and its periphery in a resistance spot welded member according to another embodiment of the present invention, and Ｆｅ 3(A) is a cross-sectional view in the thickness direction that schematically explains the measurement method of Ｆｅ 4 is a diagram illustrating the gradient of C in the resistance spot weld of the present invention. Ｆｅ5 is a diagram showing the relationship between the value of the right side of equation (2) and the evaluation of LME cracking. Fig. 5 is a cross-sectional view in the thickness direction for schematically explaining a method for measuring the nugget inclination (θ), which is a processing disturbance during resistance spot welding. Figs. 6(A) and 6(B) are diagrams showing the relationship between the gap between sheets (g Ｓｈｅｅｔ 7 is a cross-sectional view in the thickness direction that schematically illustrates a method for measuring the cracking strength during conventional resistance spot welding.

[0023] The resistance spot welded member and the resistance spot welding method of the present invention will be specifically described below, but the present invention is not limited to the following embodiments.

[0024] [Resistance Spot Welded Component] A resistance spot welded component (hereinafter referred to as "welded component") of the present invention will be described with reference to Figures 2 and 3. Figures 2(A) and 3(A) show enlarged cross-sectional views in the thickness direction of a resistance spot weld and a portion of its surrounding area in the welded component as an example. The enlarged area is the area enclosed by a rectangular frame in Figures 2(A) and 3(A).

[0025] The present invention relates to a welded component having a resistance spot weld formed by resistance spot welding two or more overlapping steel sheets. That is, the welded component of the present invention has two or more steel sheets and a resistance spot weld formed by welding the steel sheets. As described below, at least one of the overlapping steel sheets is a zinc-based plated steel sheet having a zinc-based plating on the surface of the steel sheet as a plating layer. The number of overlapping steel sheets is preferably three or more. There is no particular upper limit to the number of sheets, but it is preferably five or less.

[0026] In addition, since the increase in the number of overlapping steel plates increases the heat input to the welded portion, making LME cracking more likely to occur, the effects of the present invention can be more effectively obtained when the number of the above steel plates is three or more.

[0027] The example shown in Figure 2(A) is a welded member 6 formed by welding two overlapping steel sheets 1 and 2, in which both or one of the lower steel sheet 2 (hereinafter sometimes referred to as the "lower sheet") and the upper steel sheet 1 (hereinafter sometimes referred to as the "upper sheet") is a zinc-based plated steel sheet. In the case of the welded member 6 shown in Figure 2(A), a resistance spot weld 4, which will be described below, is formed at the steel sheet mating surface 7 where the upper sheet 1 and the lower sheet 2 come into contact (i.e., the overlapping surface of the steel sheets).

[0028] As an example of a welded member 6 using three or more steel plates, Fig. 3(A) shows a welded member 6 formed by welding three overlapping steel plates 1, 2, and 3. In the welded member 6 shown in Fig. 3(A), all or at least one of the steel plate 2 (i.e., the lower plate), the steel plate 1 (i.e., the upper plate), and the steel plate 3 (hereinafter sometimes referred to as the "middle plate") disposed therebetween are zinc-based plated steel plates. In the welded member 6 shown in Fig. 3(A), resistance spot welds 4, which will be described below, are formed to include steel plate mating surfaces 7 (7a, 7b) where the lower plate 2 and the middle plate 3 and the middle plate 3 and the upper plate 1 meet.

[0029] [Resistance Spot Welds] First, the technical concept of the present invention will be described in detail with reference to FIG.

[0030] FIG. 4 shows the Fe concentration (C Ｆｅ ) and the right-hand side value of equation (2) described later, and the correspondence between these and LME cracking evaluation. Here, the evaluation criteria for LME cracking evaluation of welded components welded under the plate assembly and welding conditions described in the examples described later are used. The "right-hand side value of equation (2)" above refers to "{25 × Si Ｈｉｇｈ × (CE １ -CE ２ )}+{θ+(g Ｓｈｅｅｔ / 1.2)}+[{(S Ｚｎ ×d ＧＢ ) ０．５} / 30]+{60+(C ０Ｆｅ / 3))" is the value calculated.

[0031] As mentioned above, LME cracking occurs when tensile stress is applied to a liquid metal such as Zn in contact with a steel sheet. Furthermore, when there is a significant difference in strength between overlapping steel sheets, it is thought that tensile stress is also more likely to occur due to differences in transformation behavior during cooling. Therefore, in the present invention, it is important to promote the interdiffusion of Fe and Zn between the sheets in a specific region, promote the alloying of Fe and Zn, and control the Zn alloy layer to have a certain Fe concentration or higher. This is because it is possible to prevent liquid Zn from being present between the sheets when tensile stress is applied.

[0032] Here, the above-mentioned "Zn alloy layer having a certain or higher Fe concentration" will be explained. As shown in the welded member 6 in Fig. 2(A) and Fig. 3(A), a Zn alloy layer is formed outside the nugget end and between the overlapping steel sheets (i.e., on the steel sheet mating surface 7 side). The higher the Fe concentration of the Zn alloy layer, the more liquid Zn is suppressed with alloying, which is effective in suppressing LME cracking. Therefore, in the present invention, the region of the Zn alloy layer formed between the sheets where the Fe concentration is high is defined as an "Fe-Zn alloy layer", and the Fe concentration of the Fe-Zn alloy layer is defined as "C Ｆｅ " is defined as

[0033] The degree of Fe-Zn alloying can be controlled by controlling the current flow pattern during welding. In particular, maintaining the temperature at a high temperature for a long period of time so that the Fe-Zn alloying progresses during current flow is effective for alloying. By controlling the current flow pattern, the Fe concentration in the Fe-Zn alloy layer can be appropriately controlled depending on the welded sheet pair, the strength, chemical composition, surface structure, and plating layer of the steel sheets used in the sheet pair, and disturbances during welding.

[0034] The "specific region" refers to the C ＦｅSpecifically, when the intersection point between the steel sheet mating surface 7 and the outer peripheral edge of the nugget 4a is defined as nugget edge E, a position 300 μm from the nugget edge E on the steel sheet mating surface 7 in the sheet width direction (i.e., toward the base material) is defined as A, and a position 700 μm from the nugget edge E on the steel sheet mating surface 7 in the sheet width direction is defined as B, this refers to the region ranging from A to B. In the present invention, this region is referred to as the "first region." The first region is formed on both end sides of the nugget 4a.

[0035] 2(A) and 3(A), for the sake of explanation, the first region is illustrated only on one nugget end side, and the first region on the other nugget end side is omitted. This first region is a region present inside the welding heat-affected zone 4b formed on the outer periphery of the nugget 4a. The rectangular frame shown in FIGS. 2(A) and 3(A) is illustrated with its length in the plate thickness direction appropriately set so as to include the Fe—Zn alloy layer 5 in 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: The maximum temperature reached during welding increases the closer to the nugget, so it is thought that liquid Zn is more likely to exist. On the other hand, in a region that is more than 700 μm away from the nugget edge E on the steel sheet mating surface 7 in the sheet width direction, the maximum temperature reached during welding may be below the melting point of the Zn coating, and it is thought that LME cracking is less likely to occur.

[0037] For the above reasons, as shown in Figure 4, in the present invention, attention is paid to the material properties between sheets, and the C required to suppress LME cracking is determined based on the TS difference between steel sheets, the Si content, the steel sheet surface structure, the Zn concentration in the Zn-based coating, the coating weight of the Zn-based coating, and construction disturbances. Ｆｅ Furthermore, it was found that the C of the first region changes depending on the material properties between the plates. Ｆｅ It was found that LME cracking can be suppressed by appropriately controlling

[0038] Next, the welded portion of the present invention, which has been completed based on this technical concept, will be described.

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

[0040] Specifically, as shown in Figures 2(A) and 3(A), among combinations of two or more overlapping steel sheets in which Zn-based plating is present between two steel sheets that are abutting in the vertical direction, the combination that has the largest difference in tensile strength is referred to as the first sheet pair. In the example shown in Figure 2(A), the upper sheet 1 and the lower sheet 2 constitute the first sheet pair, and in the example shown in Figure 3(A), the upper sheet 1 and the middle sheet 3 constitute the first sheet pair.

[0041] In this first sheet pair, the region from A to B on the steel sheet mating surface 7 is referred to as the first region, and the Fe concentration of the Fe—Zn alloy layer formed in this first region is C Ｆｅ (mass %). Ｆｅ The gradient of the Fe concentration (C Ｆｅ ) is controlled so as to satisfy all of the formulas (1) to (3). Ｆｅ …(1) C Ｆｅ ≧{25×Si Ｈｉｇｈ × (CE １ -CE ２ )}+{θ+(g Ｓｈｅｅｔ / 1.2)}+[{(S Ｚｎ ×d ＧＢ ) ０．５} / 30]+{60+(C ０Ｆｅ / 3)} ... (2) |m| ≦ 0.10 ... (3) Here, in formula (2), C ０Ｆｅ (mass%): 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 sheet set, S Ｚｎ (g / m ２ CE): The Zn-based coating weight of the steel sheet having the highest Zn-based coating weight among the Zn-based coated steel sheets in the first sheet set. １ (%): carbon equivalent of the steel plate with a high carbon equivalent in the first plate pair, CE ２ (%): Carbon equivalent of the steel plate with a low carbon equivalent in the first plate set, Si Ｈｉｇｈ(mass%): Si concentration of the steel plate with the largest Si content in the first plate set, d ＧＢ (number / mm): grain boundary density of the outermost layer of the base material of the steel plate with the large tensile strength in the first sheet pair, θ (degrees): inclination of the nugget, which is obtained by the angle between the line connecting the shoulders of the outermost steel plate in the two or more overlapping steel plates and the tangent to the outer peripheral edge of the nugget of the outermost steel plate (see FIG. 5), g Ｓｈｅｅｔ (mm): the total gap between the plates at the mating surfaces 7 of the two or more overlapping steel plates (see FIG. 6 ).

[0042] In addition, CE in equation (2) １ and C.E. ２ stands for carbon equivalent (CE), and CE can be calculated by the following formula (6):

[0043] Carbon equivalent CE=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) (6) The element symbol % in the above formula (6) represents the content (mass%) of each element, and elements that are not contained are set to 0.

[0044] The reason why the formulas (1) and (2) are defined in the present invention is as follows.

[0045] There are various factors that affect LME cracking, but it is thought that an increase in Si in the steel sheet's composition increases the risk of LME cracking. The effect of Si in steel sheets on LME cracking is complex and cannot be explained simply, but Si is known as an element that inhibits the alloying reaction of Fe-Zn. Therefore, as the Si content increases, liquid Zn becomes more likely to exist between sheets, and as the Si content of steel sheets increases, the C Ｆｅ Therefore, the lower limit of the Si concentration (Si Ｈｉｇｈ ) shall be taken into consideration.

[0046] Also, the difference in carbon equivalent (CE １ -CE ２ ) is C.E. １ -CE ２ The larger the value of C ＦｅThe lower limit of 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 differences in transformation point and material property values ​​are also thought to increase. When the transformation point is different, the amount of volume expansion and contraction that accompanies phase transformation during welding varies between plates, which can cause an increase in local tensile stress. Also, when the difference in material property values ​​is large, it is thought that tensile stress occurs in the surface layer of the plate with a large carbon equivalent due to plastic deformation of the plate with a small carbon equivalent and friction between the plates. From this, it is thought that the C Ｆｅ The lower limit of increases.

[0047] The inclination of the nugget (θ) is thought to be caused by the impact angle during welding. The moment generated by the impact angle when pressure is applied generates tensile stress in a part of the steel plate. From this, it can be seen that the C increases with the increase in the inclination of the nugget. Ｆｅ The lower limit of increases.

[0048] In addition, the total gap between the steel plates (g Ｓｈｅｅｔ ) also generates tensile stress in part of the steel plate when pressure is applied, similar to the inclination of the nugget. Therefore, C Ｆｅ The lower limit of increases.

[0049] In addition, the Fe concentration (C ０Ｆｅ ) is C ０Ｆｅ The smaller is C Ｆｅ The reason for this is unclear, but it is thought to be as follows: 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 is thought to be high. From this, it is thought that the lower limit of C increases as the average Fe concentration decreases. Ｆｅ The lower limit of the Fe concentration (C ０Ｆｅ The "Fe concentration" in (2) refers to the Fe concentration in the Zn-based plating of the steel sheet.

[0050] In addition, the Zn-based coating weight (S Ｚｎ ) is S Ｚｎ The larger the CＦｅ The reason for this is unclear, but is thought to be as follows: Ｚｎ It is considered that when the coating weight is large, the plating layer becomes thicker and the diffusion distance becomes longer, making it difficult for the alloying of the Fe—Zn alloy layer to proceed. Ｆｅ The lower limit of increases.

[0051] In addition, the grain boundary density (d ＧＢ ) is d ＧＢ The larger the C Ｆｅ The lower limit of the value of d increases. The reason for this is not clear, but it is thought to be as follows: LME cracking occurs due to the embrittlement of grain boundaries, so the point of contact between the liquid Zn and the grain boundaries in the surface layer of the base material becomes the starting point of the crack. ＧＢ It is considered that when the grain boundary density is large, the number of contact points between the liquid Zn and the grain boundary of the base material surface increases, and as a result, cracks tend to occur. Ｆｅ The lower limit of increases.

[0052] As described above, when there is a significant difference in strength between overlapping steel sheets, it is believed that tensile stress is also likely to occur due to differences in transformation behavior during cooling.

[0053] For the above reasons, in the present invention, the Fe concentration (C Ｆｅ ) satisfies equation (2).

[0054] In addition, C Ｆｅ The larger the value, the higher the melting point of the Zn alloy layer, which is effective in suppressing LME cracking. Ｆｅ However, the higher the C Ｆｅ In order to make C excessively high, it is necessary to energize for a very long time, which may result in an increase in tact time. Ｆｅ If C is too high, the boundary between the Fe-Zn alloy layer and the base material may become unclear, making it difficult to distinguish. Ｆｅ The upper limit of C in equation (1) must satisfy the following: Ｆｅ The upper limit is preferably 95 (mass %) or less.

[0055] In order to more effectively obtain the effects of the above formulas (1) and (2), in addition to the definitions of each formula, the difference in carbon equivalents is １ -CE ２ ≦0.7, and the grain boundary density of the outermost layer of the base material is d ＧＢ It is preferable that Si is ≦700. Ｈｉｇｈ is preferably 2.5% or less, and more preferably 2.0% or less.

[0056] The reason why the formula (3) is defined in the present invention is as follows.

[0057] In formula (3), C of the Fe—Zn alloy layer in the first region Ｆｅ The gradient of C is specified. Ｆｅ The gradient of C is at a position 300 μm from the nugget edge (position A shown in FIG. 2(A) etc.) and at a position 700 μm from the nugget edge (position B shown in FIG. 2(A) etc.). Ｆｅ As shown in the examples of Figures 2(B) and 3(B), the absolute value of the slope of the line connecting A and B is calculated. If the slope is large, the C Ｆｅ Because the change in C is large, Ｆｅ There may be areas with low C, which may cause LME. Ｆｅ To prevent LME from occurring at low values, the formula (3) is defined so that the absolute value of the slope (|m|) is 0.10 or less.

[0058] In the present invention, the Fe concentration (C Ｆｅ) satisfy all of the formulas (1) to (3), are formed on both end sides of the nugget. For example, when the nugget 4a is not tilted with respect to the steel sheet mating surface 7 as shown in FIGS. 2(A) and 3(A), the first region formed on either side of the nugget end is the measurement target. Also, when the nugget 4a is tilted with respect to the steel sheet mating surface 7 as shown in FIG. 5, the first region formed on the opposite side to the tilted side is the measurement target. Specifically, when the nugget 4a is tilted in the direction shown in FIG. 5 (i.e., when the nugget 4a is tilted so that the left side of the drawing surface is lowered with respect to the steel sheet mating surface 7), the first region on the right side of the nugget is the measurement target. Note that when three or more steel sheets are overlapped and Fe—Zn alloy layers are formed on multiple steel sheet mating surfaces, the measurement target is the first region formed on the steel sheet mating surface where the difference in TS between the abutting steel sheets is largest (i.e., the steel sheet mating surface of the first sheet pair).

[0059] In the present invention, the Si content, coating weight, and Fe concentration (C Ｆｅ , C ０Ｆｅ ) and the grain boundary density of the outermost layer of the base material can be measured by the method described in the examples below. Ｓｈｅｅｔ can be measured by the method described later with reference to FIGS. 5 and 6.

[0060] [Multiple Superimposed Steel Sheets] [Zn-based Plated Steel Sheet] As described above, at least one of two or more superimposed steel sheets is a Zn-based plated steel sheet. This is because LME cracking is a phenomenon that occurs when at least one Zn-based plated steel sheet is used. Note that the superimposed multiple steel sheets (i.e., sheet set) may have all Zn-based plated steel sheets, or may have a Zn-based plated steel sheet superimposed on a steel sheet not having a metal plating layer (so-called "cold-rolled steel sheet"). In either case, the effects of the present invention can be obtained.

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

[0062] Although the composition of the plating layer is not particularly limited, it is believed that the higher the Fe concentration, the easier it is to form an alloyed layer during welding, so the Fe concentration in the plating layer is preferably 5% by mass or more. Furthermore, from the viewpoint of preventing a decrease in the powdering properties of the steel sheet, the Fe concentration in the plating layer is preferably 30% by mass or less.

[0063] In order to more effectively obtain the above-described effects of the present invention, the steel sheets used in the sheet assembly may have the following configurations as necessary.

[0064] [Si Content of Steel Sheets] In the present invention, it is preferable that at least one steel sheet among the plurality of steel sheets used in the sheet assembly has an Si content of 0.5 mass % or more.

[0065] When the Si content is less than 0.5% by mass, LME cracking may not occur regardless of disturbances or other welding conditions. The reason for this is unclear, 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 the addition of Si is thought to delay the solidification of Zn during the cooling process during welding. This prolongs the period in which liquid Zn exists, and LME cracking is likely to occur due to interaction with local tensile stresses that occur during holding or electrode release. 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 when using methods other than the welding conditions of the present invention. Therefore, in the present invention, LME cracking is considered when the Si content is 0.5% by mass or more. Therefore, it is preferable that at least one steel sheet used in the sheet assembly be within the steel sheet composition range described below and have a Si content of 0.5% by mass or more.

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

[0067] [Strength Difference Between Steel Plates] As described above, when there is a significant difference in strength between the steel plates in a plurality of overlapping steel plates, LME cracking is likely to occur due to tensile stress caused by a difference in transformation behavior during cooling during welding. Therefore, in the present invention, the TS difference (hereinafter referred to as "ΔTS"), which is the largest difference in tensile strength between two steel plates that are in contact with each other in the vertical direction among the two or more overlapping steel plates, is used. Ｍａｘ ") is preferably 200 MPa or more. Ｍａｘ If the stress is less than 200 MPa, tensile stress due to differences in transformation behavior during cooling is unlikely to occur, and depending on the welding conditions, LME cracking may not occur even when a method other than the welding conditions of the present invention is used.

[0068] Therefore, in the plate assembly, ΔTS Ｍａｘ It is preferable that there is at least one combination of steel sheets in which the strength of the steel sheets is 200 MPa or more. In this case, LME cracking can be suppressed even in a sheet combination in which LME cracking is likely to occur, and the degree of freedom in structural design of automobiles is improved, so that the effects of the present invention can be more effectively obtained.

[0069] ΔTS Ｍａｘ The larger ΔTS is, the more effectively the effect of the present invention can be obtained. Ｍａｘ Although there is no particular upper limit for ΔTS, it is preferably 2000 MPa or less from the viewpoint of the practical strength level of steel sheets for automobiles. Ｍａｘ is more preferably 250 MPa or more, and more preferably 1800 MPa or less.

[0070] [Tensile strength of steel plate] Furthermore, LME cracking is likely to occur in high-strength steel plates. Therefore, in the present invention, it is preferable that at least one of the two or more overlapping steel plates has a tensile strength of 980 MPa or more. In this case, since LME cracking can be suppressed even when the steel plate is strengthened, effects such as improved crashworthiness of the welded member can be expected, and the effects of the present invention can be more effectively obtained. The tensile strength of the steel plate is preferably 3000 MPa or less.

[0071] [Composition of Steel Sheet] The composition of the high-strength steel sheet used in the present invention is not particularly limited as long as it can provide the above-described weld configuration. From the viewpoint of applying the present invention to structural parts of automobiles, the composition shown below is preferred. In the following description, the "%" designation for the composition means "mass %" unless otherwise specified.

[0072] C: 0.01 to 0.40% C is an element that contributes to increasing the strength of steel plate. Therefore, the C content is preferably 0.01% or more. The C content is more preferably 0.02% or more. On the other hand, if C is added in excess, the welded portion will become excessively hardened, causing a decrease in the toughness of the welded portion. Therefore, the C content is preferably 0.40% or less. The C content is more preferably 0.38% or less.

[0073] Si: 0.02 to 2.50% Si is an element effective in improving the strength and elongation of steel sheet. Therefore, the Si content is preferably 0.02% or more. The Si content is more preferably 0.10% or more. On the other hand, excessive addition of Si causes a decrease in LME resistance and platability. Therefore, the Si content is preferably 2.50% or less. The Si content is more preferably 2.00% or less.

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

[0075] P: 0.050% or less If added in excess, P causes solidification segregation in the nugget, reducing the toughness of the weld. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.020% or less. There is no particular lower limit for the P content, but extremely low P content increases steelmaking costs. Therefore, the P content is preferably 0.005% or more.

[0076] S: 0.100% or less If the S content is high, solidification segregation of the nugget causes a decrease in the toughness of the weld. Therefore, the S content is preferably 0.100% or less. The S content is more preferably 0.010% or less, and even more preferably 0.005% or less. There is no particular lower limit for the S content, but extremely low S content increases steelmaking costs. Therefore, the S content is preferably 0.001% or more.

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

[0078] N: 0.0100% or less N forms coarse nitrides, which reduces local deformability and reduces the ductility of the steel sheet, so it is desirable to limit its content. This tendency becomes more pronounced when the N content exceeds 0.0100%, so it is preferable to set the N content to 0.0100% or less. The N content is more preferably 0.0075% or less. There is no particular lower limit for the N content, but extremely low N content increases steelmaking costs. Therefore, it is preferable to set the N content to 0.0001% or more.

[0079] The above is the basic composition, with the balance being Fe and unavoidable impurities, such as Co, Sn, and Zn, with the allowable ranges for their contents being Co: 0.05% or less, Sn: 0.01% or less, and Zn: 0.01% or less.

[0080] In the present invention, in addition to the basic component composition described above, one or more components selected from the following may be contained as needed. Note that, since each of Ti, B, Nb, Cr, Ni, Mo, Cu, Sb, V, Ca, and REM components can be contained as needed, the content of these components may be 0%. In other words, the content of these components is preferably 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 contained to obtain this effect, it is preferable that the Ti content be 0.005% or more. On the other hand, if a large amount of Ti is added, elongation is significantly reduced, so the Ti content is preferably 0.1% or less. The Ti content is more preferably 0.065% or less.

[0082] B: 0.010% or less B is an element that improves the hardenability of steel sheets and contributes to high strength. When B is contained to exert this effect, it is preferable to contain 0.0002% or more of B. On the other hand, even if B is contained in an amount exceeding 0.010%, the effect saturates, so the B content is preferably 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 contained to obtain this effect, it is preferable to contain 0.005% or more of Nb. On the other hand, if a large amount of Nb is added, not only does it significantly reduce elongation but also causes slab cracking after continuous casting, so the Nb content is preferably 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 easily generates martensite in resistance welds. When Cr is contained to exert this effect, it is preferable to contain 0.05% or more of Cr. On the other hand, if Cr is contained in excess of 1.0%, surface defects are likely to occur, so the Cr content is preferably 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 contained to exert these effects, it is preferable to contain 0.005% or more of Ni. Furthermore, when Ni is added simultaneously with Cu, it is effective in suppressing surface defects caused by Cu, so it is effective when Cu is added. On the other hand, even if Ni is contained in an amount exceeding 0.50%, the effect saturates, so it is preferable to set the Ni content to 0.50% or less.

[0086] Mo: 0.5% or less Like Cr, Mo also easily forms martensite in resistance welds, and is therefore an element that contributes to increasing shear tensile strength. When Mo is contained to exert these effects, it is desirable to contain Mo at 0.01% or more. The Mo content is preferably 0.02% or more. Furthermore, even if Mo is contained at more than 0.5%, the above-mentioned effects saturate, and the cost simply increases. Therefore, the Mo content is preferably 0.5% or less. The Mo content is more preferably 0.42% or less.

[0087] Cu: 1.0% or less Cu is an element that contributes to solid solution strengthening of steel sheets. When Cu is contained to exert this effect, it is preferable to contain Cu in an amount of 0.005% or more. On the other hand, even if Cu is contained in an amount exceeding 1.0%, the effect saturates and surface defects caused by Cu tend to occur. Therefore, it is preferable that the Cu content be 1.0% or less.

[0088] Sb: 0.20% or less Sb has the effect of suppressing the formation of a decarburized layer in the surface layer of the steel sheet, and can therefore suppress the reduction of martensite on the steel sheet surface. When Sb is contained to exert such an effect, the Sb content is preferably 0.001% or more. On the other hand, if Sb is contained in an amount exceeding 0.20%, the rolling load increases, thereby reducing productivity. Therefore, the Sb content is preferably 0.20% or less.

[0089] V: 0.05% or less V forms fine carbonitrides, which is effective in precipitation hardening of steel sheets. When V is added to obtain this effect, it is preferable that the V content be 0.005% or more. On the other hand, even if a large amount of V is added, the effect of increasing strength by more than 0.05% is small, and in addition, it also leads to an increase in alloy costs. Therefore, it is preferable that the V content be 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 making the shape of sulfides spheroidal, and can be added as needed. When contained to exert these effects, it is preferable to contain 0.0005% or more of Ca and REM, respectively. On the other hand, even if Ca and REM are contained in excess of 0.05%, the effects saturate, so it is preferable to limit the content of Ca and REM to 0.05% or less, respectively.

[0091] [Resistance Spot Welding Method] An embodiment of a resistance spot welding method for producing a welded member of the present invention will be described.

[0092] The welded member of the present invention is produced by resistance spot welding in which a sheet set consisting of two or more overlapping steel sheets, including at least one of the above-described zinc-based plated steel sheets, is clamped between a pair of welding electrodes and joined by passing an electric current through the electrodes while applying pressure.

[0093] For example, as shown in FIG. 1 , two steel sheets 1 and 2 are overlapped to form a sheet assembly. Next, the sheet assembly is clamped between a pair of welding electrodes 8 and 9 arranged on the lower and upper sides of the sheet assembly, and current is applied while applying pressure with the welding electrodes under controlled conditions to achieve predetermined welding conditions. This forms the above-mentioned weld 4 between the steel sheets that form the steel sheet mating surfaces 7 of the steel sheets 1 and 2, thereby joining the steel sheets together. Note that the sheet assembly may also be formed using a steel sheet having a Zn-based plating layer (e.g., a GI steel sheet, a GA steel sheet, or an EG steel sheet) and a steel sheet not having a plating layer (e.g., a cold-rolled steel sheet). In this case, the steel sheets are overlapped so that the surface having the Zn-based plating layer contacts the cold-rolled steel sheet.

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

[0095] Next, the welding conditions of the present invention will be explained.

[0096] In order to promote the alloying of Fe—Zn between the plates in the vicinity of the nugget, it is effective to promote the interdiffusion of Fe and Zn by appropriately controlling the current pattern during current application, as described above.

[0097] In the resistance spot welding of the present invention, the current pattern during welding can be controlled by only the main current process, or by both the main current process and the post-current process. In particular, by combining the main current process and the post-current process, it becomes possible to more stably maintain the high temperature range where the alloying of Fe—Zn progresses for a long period of time compared to current application in the main current process alone. Note that the "hold time after the end of current application (T Ｈｏｌｄ ) is the "hold time after the end of current application in the main current application process" in the case of the first current application pattern described below, and is the "hold time after the end of current application in the post-current application process" in the case of the second current application pattern described below.

[0098] In the present invention, the main current application step refers to current application for forming a nugget, and the post-current application step refers to post-current application for performing post-heat treatment after the nugget is formed.

[0099] [First Current-Applying Pattern] Here, the current-applying pattern of the present invention that includes only this current-applying step is referred to as a “first current-applying pattern.” One embodiment of the first current-applying pattern will be described below.

[0100] In the first current pattern, the average current value of the current flowing step is I １ (kA), the energization time of this energization step is T １ (s: seconds), the average pressure of the welding electrode during current application is p (kN), and the hold time after current application is finished is T Ｈｏｌｄ The welding conditions of the main current application process and the hold time after the end of current application are controlled so that the relationship of equation (4) is satisfied. １ ２ ×T １ + (T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２) × 70} / N (4) where, in formula (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) between the pair of welding electrodes.

[0101] If current is applied under conditions that do not satisfy formula (4), the temperature and time required for the mutual diffusion of Fe and Zn cannot be maintained during current application, resulting in insufficient Fe-Zn alloying between the plates, resulting in C Ｆｅ Furthermore, as mentioned above, when the difference in the Si content and transformation point of the steel sheet increases, LME cracking becomes more likely to occur. Therefore, the left side value of the formula (4) (i.e., "I １ ２ ×T １ + (T Ｈｏｌｄ x 400) ２ The lower limit of the value of "Si Ｈｉｇｈ Ya (CE １ -CE ２ ) is effective.

[0102] For the above reasons, in the first current pattern, the welding conditions of the main current application process are controlled so as to satisfy the relationship of formula (4). The value of the left side of formula (4) is preferably set to be equal to or greater than "the value of the right side of formula (4) x 1.05". Note that the "value of the right side of formula (4)" above is "1.5 x C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２ ) × 70} / N".

[0103] The upper limit of the formula (4) is not particularly limited. In order to prevent significant splashing due to excessive heat input and excessive increase in takt time in the automobile manufacturing process, in addition to controlling the formula (4), １ is 15.0 kA or less, and T １ It is preferable that I is 2.0 s or less. １ is 3.0 kA or more, and T １ is 0.2 seconds or more.

[0104] [Second Current-Applying Pattern] Here, the current-applying pattern of the present invention that includes the main current-applying step and the post-current-applying step is referred to as a “second current-applying pattern.” One embodiment of the second current-applying pattern will be described below.

[0105] In the second current pattern, the average current value of the current flowing step is I １ (kA), the energization time of this energization step is T １ The average current value of the post-current application step is I ２ (kA), the energization time of the post-energization step is T ２ (s), the number of energizations in the post-energization process is N Ｐ The energization interval (non-energization time) between the main energization process and the post-energization process is T ３ (s), and the average pressure of the welding electrode during current application is p (kN). Also, the hold time after current application is terminated is T Ｈｏｌｄ The welding conditions for the main current process and the post-current process and the hold time after the end of current application are controlled so that the relationship in equation (5) is satisfied. １ ２ ×T １ + {3 × I ２ ２ × (5 × T ２ -T ３ ) x N Ｐ}+(T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２ ) × 70} / N (5) where g shown in formula (5) Ａｘｉｓ is the misalignment (mm) between the pair of welding electrodes, and c is the gap (mm) between the welding electrode and the outermost layer of the two or more overlapping steel plates.

[0106] If current is applied under welding conditions that do not satisfy the formula (5), the temperature and time required for the mutual diffusion of Fe and Zn cannot be maintained, and the Fe-Zn alloying between the plates in the post-current application process becomes insufficient. As a result, C Ｆｅ Furthermore, as mentioned above, when the difference between the Si content and carbon equivalent of the steel sheet increases, LME cracking becomes more likely to occur. Furthermore, the steel sheet surface structure and Zn-based coating also affect the susceptibility to LME cracking. Therefore, the lower limit of the left side of equation (5) is Si Ｈｉｇｈ Ya (CE １ -CE ２ ) and C ０Ｆｅ and S Ｚｎ and ＧＢ The present inventors came up with the idea that it is effective to increase the concentration of the hydroxyl group in accordance with the amount of the hydroxyl group.

[0107] For the above reasons, in the second current pattern, the welding conditions for the main current application process and the post-current application process are controlled so as to satisfy the relationship in equation (5). The value of the left side of equation (5) is preferably set to be equal to or greater than "the value of the right side of equation (5) x 1.05". Note that the "right side of equation (5)" above means "1.5 x C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ -CE ２ ) × 70} / N".

[0108] The upper limit of the formula (5) is not particularly limited. In order to prevent significant expulsion due to excessive heat input and excessive increase in takt time in the automobile manufacturing process, in addition to controlling the welding conditions of the main current process and the post-current process to the formula (5), １ and I ２ are each 15.0 kA or less, and T １ and T ２ It is preferable that the total of I is 2.0 s or less. １ and I ２ are each 3.0 kA or more, and T １ and T ２ The total time must be 0.2 seconds or more.

[0109] [Electrode Holding Time] After the main current application process using the first current application pattern or the post-current application process using the second current application pattern is completed, the electrode is held for a predetermined time (i.e., hold time). This hold time is 0.02 to 1.00 seconds. By holding the welding electrode with a constant pressure after current application is complete, the occurrence of blowholes in the nugget and an excessive increase in takt time are suppressed. The hold time is preferably 0.05 seconds or more and preferably 0.50 seconds or less.

[0110] [No-current-passing step] In the case of the second current-passing pattern described above, a no-current-passing step in which current is suspended may be included between the main current-passing step and the post-current-passing step. When the no-current-passing step is included, it is preferable to repeat the no-current-passing step and the post-current-passing step after the main current-passing step. This is because the effects of the present invention can be obtained more effectively.

[0111] In addition, "T" in the above formula (5) ３ Therefore, when there is no non-energizing step, T in the above formula (5) ３ (s) is set to 0. Also, "N Ｐ "(times)" indicates the number of repetitions.

[0112] In the post-heat treatment process, the mutual diffusion of Fe and Zn is promoted while suppressing the expulsion due to the remelting of the nugget. Ｆｅ In order to increase the temperature, it is effective to maintain the vicinity of the nugget at a certain temperature range after the completion of the main current application process. Therefore, in the present invention, it is preferable to provide a no-current application process as an interval between the main current application process and the post-current application process.

[0113] When the post-heat process is performed at a constant current value, if the current value of the post-heat process is large, the temperature near the nugget gradually rises, and conversely, if the current value of the post-heat process is small, the temperature near the nugget gradually drops. Even in such a state, it is thought that the desired effect can be obtained by appropriately setting the current value of the post-heat process, but this may increase the number of steps required to derive the optimal conditions.

[0114] Furthermore, maintaining the temperature within a certain range not only requires strict control of conditions, but also becomes even more difficult when disturbances in the welding process occur, as described below. On the other hand, by repeating the no-energization step and the post-energization step, the temperature near the nugget can be maintained within a relatively constant range, even if the total energization time in the post-energization step increases.

[0115] For these reasons, the second current application pattern of the present invention may include a de-energization step after the main current application step, and the de-energization step and the post-current application 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 for the post-current application step is widened, which is thought to improve robustness against welding disturbances. Note that if the de-energization time is too short, there is a concern that the nugget will re-melt and cause increased expulsion, while if the de-energization time is too long, the desired heat treatment effect may not be achieved. Therefore, the time for the de-energization step is preferably 0.01 s or more and 0.20 s or less.

[0116] The de-energization step and the post-energization step need only be performed at least once, and if the de-energization step and the post-energization step are repeated, the number of repetitions is preferably two or more, and even more preferably three or more. There is no particular upper limit to 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 repetition number exceeding the upper limit requires modification of the welding device. Therefore, because this would increase the equipment costs in the automobile manufacturing process, the number of repetitions is preferably 20 or less, and even more preferably 10 or less.

[0117] [Welding Procedure Disturbance] In addition to the welding conditions in the above-described steps, the present invention may have the following welding conditions.

[0118] As described above, LME cracking is likely to occur in the presence of welding disturbances. Therefore, in this current application process, it is preferable that at least one welding point satisfy one or more of the following conditions (a) to (d) immediately before applying pressure with the welding electrode. This makes it possible to more effectively achieve the effects of the present invention. (a) The impact angle between the welding electrode and two or more overlapping steel sheets is 0.2 degrees or more. (b) Of the two or more overlapping steel sheets, there is a gap of 0.5 mm or more between at least one pair of steel sheets. (c) The gap between any welding electrode and the outermost layer of the two or more overlapping steel sheets is 0.5 mm or more. (d) The misalignment between the pair of welding electrodes is 0.1 mm or more. All of these welding disturbances locally increase the temperature and / or tensile stress of the weld when the electrodes are released, making the weld more susceptible to LME cracking. However, according to the present invention, C Ｆｅ By optimizing the above, it is possible to suppress LME cracking even when these welding disturbances are present, and the margin for managing the welding disturbances during component manufacturing is improved.

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

[0120] Furthermore, when welding is performed with a strike angle, the nugget formed is also tilted, so when determining the strike angle from the cross section after welding, the tilt of the nugget is used as the strike angle. In other words, this "strike angle" is one of the construction disturbances during welding, but for the reasons mentioned above, the "slope of the nugget" in the component obtained after welding can be used instead as this "strike angle."

[0121] The "θ" representing the "nugget inclination" is determined by the method shown in Figure 5. As shown in Figure 5, a line connecting the outermost portions (i.e., the boundary portion where deformation due to the pressure of the welding electrode is no longer observed) of the left and right shoulders on the upper plate 1 side (i.e., the steel plate side of the outermost layer of the sheet assembly) is used as a reference. Two perpendicular lines are drawn from positions 500 μm away on the left and right from the center of this reference line (hereinafter referred to as the "reference line"), and the angle θ is defined as the line passing through the intersection of the two lines with the outermost edge of the nugget (i.e., the outer peripheral edge of the nugget 4a). The "two perpendicular lines" are lines parallel to the median line drawn from the center of the reference line. The effects of the present invention can be effectively achieved when the impact angle is 0.2 degrees or more. An excessive impact angle can cause unstable nugget formation and expulsion, so the impact angle is preferably 10.0 degrees or less. The impact angle is more preferably 1.0 degrees or more, and even more preferably 8.0 degrees or less.

[0122] (b) A state in which, 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. Because steel plates deform during welding, the gap between the steel plates may change before and after welding. However, if there is a gap between the steel plates in the cross section after welding, there is a high possibility that there was a gap between the steel plates before welding. Therefore, when determining the gap between steel plates from the cross section after welding, the gap between the steel plates after welding is taken as the gap between the 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 mentioned above, the "gap between steel plates after welding" in the member obtained after welding may be substituted for this "gap between steel plates."

[0123] The gap between the steel plates after welding is called "g" Ｓｈｅｅｔ " is obtained by the method shown in Figures 6(A) and 6(B). As shown in Figure 6(A), in the case of continuous nuggets, the difference between the maximum thickness of the steel sheets 1 and 2 between two adjacent nuggets 4a and the total thickness of the overlapping steel sheets 1 and 2 (i.e., the sheet gap 10) is calculated, and g Ｓｈｅｅｔ In other cases (for example, the case shown in FIG. 6(B)), the difference between the maximum thickness of the steel sheets 1 and 2 from the nugget 4a to the end of the sheet assembly and the total thickness of the overlapped steel sheets 1 and 2 is calculated, and gＳｈｅｅｔ Let's say.

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

[0125] (c) A state in which a gap of 0.5 mm or more exists between one of the welding electrodes and the outermost layer of two or more overlapping steel sheets. Similar to (b) above, when a gap exists between one of the welding electrodes and the steel sheet immediately before the start of pressure application, the steel sheet undergoes bending deformation, which applies bending stress to the weld, making LME cracking more likely to occur. When a gap exists between one of the welding electrodes and the steel sheet immediately before the start of pressure application, for example, when one electrode is movable (hereinafter referred to as the "movable electrode") and the other electrode is fixed (hereinafter referred to as the "fixed electrode"), if a gap exists between the fixed electrode and the steel sheet, when pressure application by the movable electrode begins, the steel sheet bends and deforms, applying bending stress to the weld. This makes LME cracking more likely to occur. The gap between the welding electrode and the steel sheet is designated c. The effects of the present invention can be effectively achieved when this gap is 0.5 mm or more. An excessively large gap results in unstable nugget formation, which can lead to expulsion. Therefore, it is preferable to set this gap to 5.0 mm or less. The gap is more preferably 1.0 mm or more, and even more preferably 3.0 mm or less.

[0126] If the values ​​of the welding disturbances in formula (4) are within the preferred ranges described above and below, and welding is performed under conditions that satisfy formula (4), it is believed that the influence of the welding disturbances described in (c) on LME cracking will be small. Also, in formula (1), it is believed that the influence of the welding disturbances described in (c) on LME cracking will be small.

[0127] (d) A state in which the misalignment amount of the pair of welding electrodes is 0.1 mm or more. Misalignment means a state in which the central axes of the pair of welding electrodes are not aligned. ＡｘｉｓAs with the impact angle described above, if the misalignment is large, bending stress is applied to the weld, making LME cracking more likely to occur. The effects of the present invention can be effectively obtained when the misalignment amount is 0.1 mm or more. If the misalignment amount is excessive, nugget formation becomes unstable, which can cause expulsion, so the misalignment amount is preferably 5.0 mm or less. The misalignment amount is more preferably 0.2 mm or more, and even more preferably 3.0 mm or less.

[0128] It is considered that, if each value of the welding disturbance in formula (4) is within the above-mentioned preferable range and welding is performed to satisfy formula (4), the influence of the welding disturbance described in (d) on LME cracking is small. Therefore, in formula (1), the effect of the present invention can be obtained even if the welding disturbance described in (d) is ignored.

[0129] In the present invention, the pressure conditions in each step are not particularly limited. From the viewpoint of automotive applications, the pressure conditions are preferably adjusted to the range of 2.0 to 8.0 kN.

[0130] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0131] The steel plates shown in Table 1 were used to form a plate assembly, which was then welded under the welding conditions shown in Tables 2 and 3 to produce a welded joint (welded member). The plate assembly was overlapped with the bottom plate and top plate in this order, or the bottom plate, middle plate, and top plate in this order, as shown in Table 2. A single-phase AC (50 Hz in this case) resistance welding machine with a servo motor pressure system attached to a welding gun was used as the welding device. The pair of electrode tips used were chromium copper DR-type electrodes with a tip curvature radius R of 40 mm and a tip diameter of 6 mm.

[0132] In the "Plating" column of Table 1, "GI" indicates a steel sheet having a hot-dip galvanized layer (i.e., a hot-dip galvanized steel sheet), "GA" indicates a steel sheet having a galvannealed layer (i.e., a galvannealed steel sheet), "EG" indicates a steel sheet having an electrogalvanized layer (i.e., an electrogalvanized steel sheet), and "-" indicates a steel sheet having no plating layer (here, a cold-rolled steel sheet).

[0133] The component compositions shown in Table 1 were measured by inductively coupled plasma (ICP) emission spectrometry.

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

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

[0136] In addition, the "Fe concentration" shown in the "Plating" column of Table 1 is the same as that of C described later. Ｆｅ The "adhesion amount" was measured using Glow Discharge Spectroscopy (GDS). Ｚｎ " indicates the coating weight of the Zn-based plated steel sheet with the highest coating weight among the stacked steel sheets.

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

[0138] In addition, the "N" shown in the "Welding conditions" column of Table 3 Ｐ " indicates the number of times the non-energizing step and the post-energizing step are repeated. For example, in the case of "only the main energizing step", N Ｐ : 0 (times), and for example, in the case of "main energization process - non-energization process - post-energization process", N Ｐ: 1 (times), and for example, in the case of "main energization process - non-energization process (1) - post-energization process (1) - non-energization process (2) - post-energization process (2)", N Ｐ : 2 (times). In this way, the number of non-energizing steps is N Ｐ It will be the same number as.

[0139] The obtained welded member was used to measure the Fe concentration (C Ｆｅ ) and LME cracking of the welds were evaluated.

[0140] <C Ｆｅ Measurement of C Ｆｅ The measurement method is not particularly limited as long as it can quantitatively measure the Fe concentration. 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: A Ka ray source was used as the source of X-rays to be irradiated, magnification: 1500 times or more, acceleration voltage: 15 kV, scan time: 1 minute or more, excluding gaps. Specifically, the thickness of the gaps is subtracted from the thickness including the gaps. If it is difficult to determine the thickness of the gaps, the measurement is made from the nearest adjacent part where there are no gaps.

[0141] As shown in FIGS. 2A and 3A, the region from A to B (i.e., first region) was observed on the steel sheet mating surface 7 where the Fe—Zn alloy layer was formed in the weld.

[0142] Here, the observation start point was set to position A, which was 300 μm away from the nugget edge E in the sheet width direction (i.e., one side of the first region perpendicular to the steel sheet mating surface 7, located on the left side of the paper in each figure), and the observation end point was set to position B, which was 700 μm away from the nugget edge E in the sheet width direction (i.e., one side of the first region perpendicular to the steel sheet mating surface 7, located on the right side of the paper in each figure). The region from the start point to the end point was observed at equal intervals, and the Zn-enriched portion between the sheets in each field of view was identified as an Fe—Zn alloy layer, and the Fe concentration of the Fe—Zn alloy layer was measured. The "Zn-enriched portion" was determined from the contrast in the composition image (COMPO image). As shown in each figure, the Fe concentration was measured at a quarter 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 determined as the Fe concentration (C Ｆｅ )

[0143] Note that the above phrase "observed at equal intervals" refers to observing at least 10 visual fields at equal intervals. The visual fields may be appropriately set within a range of, for example, 11 to 100. Since accurate values ​​may not be obtained if the measurement interval is less than 10, in this example, the visual fields were set to 10. The following analytical method was used to measure the above "Fe concentration" and "Zn concentration." Note that Figures 2(A) and 3(A) are schematic diagrams for explanation, and only six locations are shown for ease of viewing.

[0144] Density measurement can be performed by, for example, line analysis or point analysis. When density measurement is performed by these analyses, density is measured at at least five or more lines per visual field. After measuring the density at each line and calculating the average density per visual field, the average value excluding the maximum and minimum values ​​from the measured values ​​for the entire visual field is calculated as C. Ｆｅ In the case of point analysis, it is preferable to measure at a pitch of 1 μm or less per point.

[0145] As mentioned above, C Ｆｅ When the nugget is not tilted, the C on one side is formed on both end sides of the nugget. Ｆｅ If the nugget is tilted, measure the C on the opposite side to the tilted side.Ｆｅ was measured.

[0146] <Evaluation of LME cracking> The center of the welded portion of the obtained welded component was cut with a microcutter, and then the cross section of the welded portion was observed to evaluate the presence or absence of LME cracking. Specifically, five welded components were produced under each of the welding conditions shown in Tables 2 and 3, and the presence or absence of LME cracking between the plates (i.e., on the steel plate mating surface side) was checked. The results of evaluation using the following evaluation criteria are shown in Table 3. Here, evaluation results of A and B were evaluated as "pass". <Evaluation criteria> A: 0 / 5 (i.e., 0 out of 5 components had cracks confirmed) B: 1 / 5 (i.e., 1 out of 5 components had cracks confirmed) C: 2 / 5 to 4 / 5 (i.e., 2 to 4 out of 5 components had cracks confirmed)

[0147]

[0148]

[0149]

[0150] As shown in Tables 2 and 3, the welded components of the invention passed all of the evaluation results for LEM cracking, indicating that LME cracking was suppressed.

[0151] REFERENCE SIGNS LIST 1, 2, 3 steel plate 4 welded portion 4a nugget 4b weld heat affected zone 5 Fe—Zn alloy layer 6 resistance spot welded member 7 steel plate mating surface 8, 9 welding electrode 10 plate gap

Claims

1. A resistance spot welded component in which two or more overlapping steel sheets are resistance spot welded, at least one of the two or more overlapping steel sheets is a Zn-based plated steel sheet, and among the combinations in which Zn-based plating is present between the two steel sheets abutting in the vertical direction, the combination with the largest difference in tensile strength is called the first sheet combination, and the Fe concentration of the Fe-Zn alloy layer formed in the first region in the first sheet combination, from position A, 300 μm in the sheet width direction from the nugget end on the steel sheet mating surface, to position B, 700 μm in the sheet width direction from the nugget end, is defined as C. Ｆｅ (mass%), and C of the Fe—Zn alloy layer in the first region Ｆｅ A resistance spot welded component, wherein the Fe concentration of the Fe—Zn alloy layer in the first region satisfies all of formulas (1) to (3), where m is a gradient of the Fe concentration in the Fe—Zn alloy layer in the first region. Ｆｅ …(1) C Ｆｅ ≧{25×Si Ｈｉｇｈ × (CE １ -C.E. ２ )}+{θ+(g Ｓｈｅｅｔ / 1.2)) + [{(S Ｚｎ ×d ＧＢ ) ０．５ } / 30]+{60+(C ０Ｆｅ / 3)} ... (2) |m| ≤ 0.10 ... (3) Here, in formula (2), ０Ｆｅ (mass%): 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 sheet set; S Ｚｎ (g / m ２ CE): The Zn-based coating weight of the steel sheet having the highest Zn-based coating weight among the Zn-based coated steel sheets in the first sheet set; １ (%): carbon equivalent of the steel plate having a high carbon equivalent in the first plate pair, CE ２ (%): carbon equivalent of the steel plate having a low carbon equivalent in the first plate set, Si Ｈｉｇｈ (mass%): Si concentration of the steel plate having a large Si content in the first plate set, d ＧＢ (pieces / mm): grain boundary density of the outermost layer of the base material of the steel plate having the large tensile strength in the first plate set; θ (degrees): inclination of the nugget obtained by the angle between a straight line connecting the shoulders of the steel plate of the outermost layer in the two or more overlapping steel plates and a tangent to the outer peripheral edge of the nugget of the steel plate of the outermost layer; g Ｓｈｅｅｔ (mm): the sum of the sheet gaps between the mating surfaces of the two or more overlapping steel sheets.

2. The carbon equivalent of the two steel plates in the first plate set is CE １ - C.E. ２ ≦0.7, and the grain boundary density of the outermost layer of the base material is d ＧＢ 2. The resistance spot welded component of claim 1, wherein the resistance is ≦700.

3. A method for resistance spot welding of resistance spot welded components according to claim 1 or 2, comprising a main current application process in which two or more overlapping steel sheets are clamped between a pair of welding electrodes and current is applied while applying pressure to form a nugget, and the average current value of the main current application process is set to I １ (kA), the energization time of the main energization step is T １ (s), the average pressure of the welding electrode is p (kN), and the hold time after the end of current application is T Ｈｏｌｄ (s) and the number of the steel plates is N (pieces), the welding conditions of the main current passing step satisfy formula (4). １ ２ ×T １ + (T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ - C.E. ２ ) × 70} / N ... (4) In the formula (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 amount (mm) of the pair of welding electrodes.

4. A post-current process is provided for performing a post-heat treatment on the nugget formed after the main current process, and the average current value of the main current process is I １ (kA), the energization time of the main energization step is T １ (s), the average current value of the post-current application step is I ２ (kA), the energization time of the post-energization step is T ２ (s), the number of energization times in the post-energization step is N Ｐ (times), and the energization interval between the main energization step and the post-energization step is T ３ (s), the average pressure of the welding electrode is p (kN), and the hold time after the end of the current is T Ｈｏｌｄ The method for resistance spot welding a resistance spot welded member according to claim 3, wherein, when the number of steel plates is N (pieces), the welding conditions for the main current application step and the post-current application step satisfy the following formula (5) instead of the formula (4). １ ２ ×T １ + {3 × I ２ ２ × (5 × T ２ -T ３ ) x N Ｐ }+(T Ｈｏｌｄ x 400) ２ ≧1.5×C ０Ｆｅ ×Si Ｈｉｇｈ ×p{(θ １．８ +g Ｓｈｅｅｔ +c+g Ａｘｉｓ ) + ((0.05 x S Ｚｎ / 3) + d ＧＢ ０．５ ) + (CE １ - C.E. ２ )×70} / N…(5) 5. The method for resistance spot welding of resistance spot welded members according to claim 3 or 4, wherein in the main current application step, for at least one welding spot, immediately before applying pressure with the welding electrode, one or more of the following conditions are satisfied: (a) a state in which the impact angle between the welding electrode and the two or more overlapping steel plates is 0.2 degrees or more, (b) a state in which a gap between at least one pair of the two or more overlapping steel plates is 0.5 mm or more, (c) a state in which a gap between any one of the welding electrodes and the outermost layer of the two or more overlapping steel plates is 0.5 mm or more, and (d) a state in which the amount of misalignment between a pair of the welding electrodes is 0.1 mm or more.

6. The method for resistance spot welding of resistance spot welded components as set forth in claim 4 or 5, further comprising a non-energizing step for halting the energization between the main energizing step and the post-energizing step.

7. The method for resistance spot welding of resistance spot welded members according to claim 6, wherein the non-energizing step and the post-energizing step are repeated after the main energizing step.

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