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 having a resistance spot welded part 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 plated steel sheet. In a first sheet set that has the largest difference in tensile strength between two steel sheets abutting in the vertical direction, the average thickness of a Fe-Zn alloy layer that has a Fe concentration of 70-95 mass% and a Zn concentration of 5-30 mass% and that is formed in a region located within 300-500 μm from a nugget end in the sheet width direction satisfies formula (1). (1): tFe-Zn≥SiHigh×(Ms1-Ms2)1 / 2×0.5+(θ+8×gSheet) / 5

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 set 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 becomes large, promoting alloying of Fe and Zn and controlling the thickness 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 conceived the idea that this technical concept could prevent LME cracking. Furthermore, the inventors also discovered appropriate welding conditions for controlling the thickness of the Zn alloy layer with 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 having a resistance spot weld formed by resistance spot welding two or more overlapping steel sheets, wherein at least one of the two or more overlapping steel sheets is a Zn-plated steel sheet, and the combination of the two or more overlapping steel sheets that have the largest difference in tensile strength between the two steel sheets that are in contact with each other in the vertical direction is called a first sheet combination, and the average thickness of an Fe-Zn alloy layer having an Fe concentration of 70 to 95 mass % and a Zn concentration of 5 to 30 mass % formed in a region 300 to 500 μm from the nugget edge on the steel sheet mating surfaces in the first sheet combination is called t Ｆｅ－Ｚｎ (μm), the average thickness of the Fe—Zn alloy layer satisfies formula (1). Ｆｅ－Ｚｎ ≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×0.5+(θ+8×g Ｓｈｅｅｔ ) / 5 ... (1) where, Ms shown in formula (1) １ (°C): Ms point of the steel plate with a higher Ms point in the first plate pair, ２ (°C): Ms point of the steel plate with a lower Ms point in the first plate pair; Si Ｈｉｇｈ(mass %): Si concentration of the steel plate with the largest Si content in the first sheet pair, θ (degrees): inclination of the nugget, which is obtained by the angle between a line connecting the shoulders of the outermost steel plate side and a tangent to the outer peripheral edge of the nugget on the outermost steel plate side in the overlapping plurality of steel plates, g Ｓｈｅｅｔ (mm): the total gap between the mating surfaces of each steel sheet. [2] The resistance spot welded component according to [1], wherein at least one of the two or more overlapping steel sheets has a Si content of 0.5 mass % or more. [3] The resistance spot welded component according to [1] or [2], wherein the difference in tensile strength between the two steel sheets in the first sheet pair is 200 MPa or more. [4] The resistance spot welding method for the resistance spot welded component according to any one of [1] to [3], comprising a main current application step of clamping the two or more overlapping steel sheets with a pair of welding electrodes and applying current while applying pressure to form a nugget, wherein the average current value in 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 the steel plates is N (pieces), the welding conditions of the main current application step satisfy formula (2). １ ２ ×T １ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N (2) Here, Si shown in formula (2) Ｈｉｇｈ is the Si concentration (mass%) of the steel plate in the first sheet pair having the largest Si content, and Ms １ is the Ms point (°C) of the steel plate side of the first sheet pair having a higher Ms point, and Ms ２ is the Ms point of the steel sheet of the first sheet pair having a lower Ms point. [5] A post-current application process is performed after the main current application process to perform a post-heat treatment on the formed nugget, and an average current value of 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 the steel plates is N (plates), the welding conditions for the main current application step and the post-current application step satisfy the relationship of formula (3) by replacing formula (2). １ ２ ×T １ +I ２ ２ ×T ２ ×N Ｐ / T ３ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N ... (3) [6] The resistance spot welding method of resistance spot-welded members according to [4] or [5], wherein, in the main current application step, for at least one weld spot, immediately before applying pressure with the welding electrode, one or more of the following conditions are satisfied: (a) a hit angle between the welding electrode and the two or more overlapping steel sheets is 0.2 degrees or more, (b) a misalignment between a pair of the welding electrodes is 0.1 mm or more, (c) a gap between any of the welding electrodes and the outermost layers of the two or more overlapping steel sheets is 0.5 mm or more, and (d) a gap between at least one pair of steel sheets among the two or more overlapping steel sheets is 0.5 mm or more. [7] The resistance spot welding method of resistance spot-welded members according to [5] or [6], 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. [8] The resistance spot welding method for resistance spot-welded members according to [7], 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-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 is a cross-sectional view in the thickness direction showing a resistance spot weld and its periphery in a resistance spot welded member according to an embodiment of the present invention. Ｆｅ－Ｚｎ 3 is a cross-sectional view in the plate thickness direction illustrating a resistance spot weld and its periphery, and a method for measuring t Ｆｅ－Ｚｎ 4 is a cross-sectional view in the thickness direction illustrating the measurement method of t in the resistance spot weld of the present invention. Ｆｅ－Ｚｎ 5 is a diagram showing the relationship between the value of the right side of Equation (1) and the evaluation of LME cracking. Fig. 5 is a cross-sectional view in the plate thickness direction that schematically explains 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 plates (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 and 3 show, as an example, enlarged cross-sectional views in the thickness direction of a resistance spot weld in the welded component and a portion of its surrounding area. The enlarged area is the area enclosed by a rectangular frame in Figures 2 and 3.

[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. As described below, at least one of the overlapping steel sheets is a Zn-plated steel sheet having a Zn-based plating layer on its surface. There is no particular upper limit to the number of overlapping steel 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 Fig. 2 is a welded component 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 galvanized steel sheet. In the case of the welded component 6 shown in Fig. 2, 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 component 6 using three or more steel plates, Fig. 3 shows a welded component 6 formed by welding three overlapping steel plates 1, 2, and 3. In the welded component 6 shown in Fig. 3, 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 galvanized steel plates. In the case of the welded component 6 shown in Fig. 3, 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 average thickness (t Ｆｅ－Ｚｎ) and the right-hand side value of the formula (1) described later, and the correspondence between these and the LME cracking evaluation. Here, the evaluation criteria for the LME cracking evaluation and the welded components welded under the plate assembly and welding conditions described in the examples described later are used. The unit of the thickness is μm. The "right-hand side value of the formula (1)" above refers to "Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×0.5+(θ+8×g Ｓｈｅｅｔ ) / 5".

[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. It is also believed that when there is a significant difference in strength between overlapping steel sheets, tensile stress is more likely to occur due to differences in transformation behavior during cooling. Therefore, in the present invention, it is important to promote interdiffusion of Fe and Zn between the sheets in specific regions, promote alloying of Fe and Zn, and control the thickness of the Zn alloy layer with 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 Figures 2 and 3, a Zn alloy layer is formed outside the nugget 4a and between the sheets of 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 as a result of alloying, and the more effective it is in preventing LME cracking. Therefore, in the present invention, the region where the Fe concentration is 70 to 95 mass % and the Zn concentration is 5 to 30 mass % is defined as the "Fe-Zn alloy layer", and the average thickness of the Fe-Zn alloy layer is defined as "t Ｆｅ－Ｚｎ " is defined as

[0033] The reasons for setting the Fe concentration to 70 to 95 mass% and the Zn concentration to 5 to 30 mass% are as follows: The Fe concentration and the Zn concentration are indicators of the degree of Fe-Zn alloying, and as alloying progresses, the Fe concentration increases and the Zn concentration decreases. If the Fe concentration is less than 70 mass% and the Zn concentration exceeds 30 mass%, alloying will be insufficient and the effects of the present invention may not be obtained. Therefore, the Fe concentration is set to 70 mass% or more and the Zn concentration is set to 30 mass% or less. On the other hand, if the Fe concentration exceeds 95% and the Zn concentration is less than 5% in order to progress alloying, the current application time and number of current applications will significantly increase, which may lead to a decrease in construction efficiency. Therefore, the Fe concentration is set to 95 mass% or less and the Zn concentration to 5 mass% or more.

[0034] 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 average thickness of the Fe-Zn alloy layer can be appropriately controlled depending on the welded sheet pair, the strength, composition, and structure of the steel sheets used in the sheet pair, and disturbances during welding.

[0035] Furthermore, when the intersection point between the steel sheet mating surface 7 and the outer peripheral edge of the nugget 4a is defined as the nugget edge E, the "specific region" refers to a region located 300 to 500 μm away from the nugget edge E in the sheet width direction on the steel sheet mating surface 7. In the present invention, this region is referred to as the "first region." This first region is a region present inside the welding heat-affected zone 4b formed on the outer periphery of the nugget 4a, and is formed on both end sides of the nugget 4a. Note that for the sake of explanation, in FIGS. 2 and 3, the first region is illustrated only on one nugget edge side, and the first region on the other nugget edge side is omitted. The rectangular frame shown in the figures is illustrated with its length in the sheet thickness direction appropriately set so as to include the Fe—Zn alloy layer 5 in the first region.

[0036] This first region is considered to be a region where LME cracking is likely to occur. The reason for this is not clear, but it is thought to be as follows: Because the maximum temperature reached during welding is higher closer to the nugget, Fe-Zn alloying due to the interdiffusion of Fe and Zn is promoted closer to the nugget. From the perspective of Fe-Zn alloying, it is considered that LME cracking is less likely to occur in this region close to the nugget. However, the higher the maximum temperature reached, the higher the temperature when the welding electrode is released, which is thought to make it more likely that liquid Zn will exist. As a result, it is considered that LME cracking is likely to occur in the first region, which is a region located 300 to 500 μm away from the nugget edge E on the steel sheet mating surface 7 in the sheet width direction.

[0037] In the region from the nugget edge E to less than 300 μm on the steel sheet mating surface 7 in the sheet width direction, the t Ｆｅ－Ｚｎ In many cases, this is obtained, and the risk of LME cracking is considered to be low. Furthermore, in the region beyond 500 μm in the sheet width direction from the nugget edge E on the steel sheet mating surface 7, local tensile stress due to disturbances is unlikely to occur, and the maximum temperature reached during welding is also low, so the risk of LME cracking is considered to be low. Therefore, in the present invention, attention is focused on the region located 300 to 500 μm away from the nugget edge E on the steel sheet mating surface 7 in the sheet width direction.

[0038] Therefore, as shown in FIG. 4, in the present invention, attention is paid to the material properties between plates, and the t required to suppress LME cracking is calculated based on the transformation point taking into account the TS difference between steel plates, the Si content, and the structure fraction, as well as on the processing disturbance. Ｆｅ－Ｚｎ Furthermore, it was found that the t of the first region changes depending on the material properties between the plates. Ｆｅ－Ｚｎ It was found that LME cracking can be suppressed by appropriately controlling

[0039] Next, the welded portion of the present invention, which has been completed based on this technical concept, will be described.

[0040] As described above, in the welded portion 4 of the present invention, it is important to appropriately control the thickness of the Fe—Zn alloy layer in the first region near the nugget.

[0041] Specifically, as shown in Figures 2 and 3, among the multiple overlapping steel plates, the combination in which the difference in tensile strength between two steel plates abutting in the vertical direction is the largest is referred to as the first plate pair. In the example shown in Figure 2, the upper plate 1 and the lower plate 2 constitute the first plate pair, and in the example shown in Figure 3, the upper plate 1 and the middle plate 3 constitute the first plate pair. Note that when the number of overlapping steel plates is three or more, and the difference in tensile strength between two steel plates abutting in the vertical direction is the same in all combinations, any one of the combinations may be designated as the first plate pair.

[0042] In this first sheet pair, the Fe—Zn alloy layer formed in a region 300 to 500 μm from the nugget end E on the steel sheet mating surface 7 in the sheet width direction and having an Fe concentration of 70 to 95 mass % and a Zn concentration of 5 to 30 mass % is referred to as the first region, and the average thickness of the Fe—Zn alloy layer in this first region is referred to as t Ｆｅ－Ｚｎ In addition, the Ms point on the steel plate side with the higher Ms point in the first sheet pair is defined as Ms １ (°C), and the Ms point of the steel plate with the lower Ms point is Ms ２ (°C), and the Si concentration on the steel sheet side with the highest Si content is Si Ｈｉｇｈ In addition, the inclination of the nugget, which is calculated by the angle between the line connecting the shoulders of the outermost steel sheets and the tangent to the outer periphery of the nugget on the outermost steel sheet side, is defined as θ (degrees) (see FIG. 5 ), and the total gap between the sheets at the mating surfaces 7 of the steel sheets is defined as g Ｓｈｅｅｔ (mm) (see Figure 6). Ｆｅ－Ｚｎ However, the above Si Ｈｉｇｈ , Ms. １ , Ms. ２ , θ and g Ｓｈｅｅｔ The relationship between t and t satisfies equation (1). Ｆｅ－Ｚｎ ≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×0.5+(θ+8×g Ｓｈｅｅｔ ) / 5 ... (1) where Ms in equation (1) １ and Ms. ２ is the martensitic transformation start temperature (Ms point), and the Ms point can be calculated by the following formula (4).

[0043] Ms (°C) = 539 - 423 x {[C%] x 100 / (100 - [α area %])} - 30 x [Mn%] - 12 x [Cr%] - 18 x [Ni%] - 8 x [Mo%] (4) The [element symbol %] in the above formula (4) represents the content (mass %) of each element, and elements that are not contained are set to 0. In addition, [α area %] is the ferrite area ratio (%) of the steel sheet (base material) after annealing.

[0044] The reason why the formula (1) is 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 the Si content of steel sheet components increases the risk of LME cracking. The effect of Si in steel sheet on LME cracking is complex and cannot be explained simply, but Si is known to be 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 the t Ｆｅ－Ｚｎ The lower limit of increases.

[0046] Furthermore, the martensitic transformation start temperature (Ms point) of a steel sheet is a factor that affects the martensite fraction, i.e., the strength of the steel sheet. If the difference in Ms point is large, the difference in strength between the steel sheets also becomes large, and residual stress generated in the steel sheets during welding may be concentrated on one side. For this reason, as the difference in Ms point increases, t Ｆｅ－Ｚｎ In the present invention, the lower limit of (Ms １ -Ms. ２ )>0, the effect of the present invention is large, and (Ms １ -Ms. ２ The effect of the present invention is further enhanced when t ) ≧ 10. The overlapped steel sheets in the present invention are preferably different types of steel sheets. Ｆｅ－Ｚｎ The Ms point is influenced not only by the steel plate's composition but also by the steel plate's structural fraction. The ferrite phase fraction, which does not dissolve C, has a particularly large effect. Based on this, by using the Ms point taking the ferrite phase fraction into account, the t Ｆｅ－Ｚｎ We have discovered a technology to prevent LME cracking by appropriately controlling the

[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. Therefore, as the inclination of the nugget increases, the t Ｆｅ－Ｚｎ 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 sheet when pressure is applied, as with the inclination of the nugget, and as the total gap between the steel sheets increases, t Ｆｅ－Ｚｎ The lower limit of increases.

[0049] 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.

[0050] For the above reasons, in the present invention, the average thickness (t Ｆｅ－Ｚｎ ) satisfies equation (1).

[0051] In addition, t Ｆｅ－Ｚｎ The larger t is, the higher the melting point of the Zn alloy layer becomes, which is effective in suppressing LME cracking. Therefore, the upper limit of the formula (1) is not particularly specified. Ｆｅ－Ｚｎ In order to make t excessively large, it is necessary to energize for a very long time, which may lead to an increase in the tact time. Ｆｅ－Ｚｎ The upper limit is preferably 20 (μm) or less, more preferably 15 (μm) or less, and even more preferably 14.0 (μm) or less.

[0052] In the present invention, t satisfies the formula (1). Ｆｅ－ＺｎThe first regions, which correspond to the first region, are formed on both end sides of the nugget. For example, when the nugget is not tilted with respect to the steel sheet mating surface 7 as shown in FIGS. 2 and 3, the first region formed on either side of the nugget end is the measurement target. Furthermore, when the nugget 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 first region formed on the steel sheet mating surface where the difference in TS between the abutting steel sheets is largest is the measurement target.

[0053] In the present invention, the above-mentioned Si Ｈｉｇｈ Concentration of Ms １ , Ms. ２ , θ, g Ｓｈｅｅｔ , t Ｆｅ－Ｚｎ can be measured by the method described in the Examples below.

[0054] [Multiple Superimposed Steel Sheets] [Zinc-Plated Steel Sheet] As described above, at least one of two or more superimposed steel sheets is a zinc-plated steel sheet. LME cracking is a phenomenon that occurs when at least one zinc-plated steel sheet is used. Note that the superimposed multiple steel sheets (i.e., sheet set) may have all steel sheets that are zinc-plated steel sheets, or may have a zinc-plated steel sheet superimposed on a steel sheet that does not have a metal plating layer (so-called "cold-rolled steel sheet"). In either case, the effects of the present invention can be obtained.

[0055] The term "Zn-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.

[0056] The composition of the plating layer is not particularly limited, but Ｆｅ－ＺｎIn order to increase the Fe content, the Fe concentration in the coating layer is preferably 5% by mass or more. In addition, from the viewpoint of preventing a decrease in the powdering properties of the steel sheet, the Fe concentration in the coating layer is preferably 30% by mass or less.

[0057] 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.

[0058] [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.

[0059] 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.

[0060] 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.

[0061] [Difference in Strength Between Steel Plates] As described above, when there is a significant difference in strength between the steel plates in the overlapped steel plates, LME cracking is likely to occur due to the tensile stress caused by the difference in transformation behavior during cooling in 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 in the vertical direction among the above-mentioned overlapped 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.

[0062] Therefore, in the plate assembly, ΔTS Ｍａｘ It is preferable that there is at least one combination of steel sheets in which the ΔTS is 200 MPa or more. Ｍａｘ It is believed that in the case of a combination of steel sheets with a modulus of elasticity of 200 MPa or more, tensile stress due to differences in transformation behavior during cooling is likely to occur, making LME cracking more likely to occur. In this case, the LME cracking suppression effect achieved by promoting the Fe-Zn alloying of the present invention can be more effectively obtained, thereby improving the degree of freedom in the structural design of automobiles.

[0063] 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 further preferably 350 MPa or more. Ｍａｘ is more preferably 1800 MPa or less, and further preferably 1700 MPa or less.

[0064] [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 above-mentioned multiple 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 made high-strength, effects such as improved impact resistance 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.

[0065] [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.

[0066] 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.

[0067] 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.

[0068] 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%.

[0069] 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, so the P content is preferably 0.005% or more.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] In the present invention, in addition to the above basic component composition, one or more components selected from the following may be contained as needed. Note that each of Ti, B, Nb, Cr, Ni, Mo, Cu, Sb, V, Ca, and REM may be contained as needed, so these components may be 0%.

[0075] 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.

[0076] B: 0.010% or less B is an element that improves the hardenability of steel sheets and contributes to increasing 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] [Resistance Spot Welding Method] An embodiment of a resistance spot welding method for producing a welded member of the present invention will be described.

[0086] The welded member of the present invention is produced by resistance spot welding, in which a sheet set consisting of a plurality of overlapping steel sheets, including at least one of the above-described Zn-plated steel sheets, is clamped between a pair of welding electrodes and joined by passing an electric current through the electrodes while applying pressure.

[0087] 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 and controlling 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 (GI steel sheet, GA steel sheet, EG steel sheet) and a steel sheet not having a plating layer (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.

[0088] 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."

[0089] Next, the welding conditions of the present invention will be explained.

[0090] In order to promote the alloying of Fe-Zn between the plates in the vicinity of the nugget, it is effective to maintain the temperature within the range in which the alloying progresses during current flow, as described above. For this purpose, in the present invention, the current flow pattern during welding is appropriately controlled to promote the interdiffusion of Fe and Zn.

[0091] In the resistance spot welding of the present invention, the welding current pattern 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 at which Fe—Zn alloying proceeds for a long period of time compared to when only the main current process is used.

[0092] [First current application pattern] As described above, the current application pattern of the present invention may include only the main current application step, or may include both the main current application step and the post-current application step. Here, the case where only the main current application step is included is referred to as the "first current application pattern," and one embodiment of the first current application pattern will be described below.

[0093] 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), 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 this current application process and the hold time after the end of current application are controlled so as to satisfy the relationship of formula (2). １ ２ ×T １ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N (2) Here, Si shown in formula (2) Ｈｉｇｈ is the Si concentration (mass%) of the steel plate with the largest Si content in the first plate pair, and Ms １ is the Ms point (°C) of the steel plate side having a higher Ms point of the first plate pair, and Ms ２ is the Ms point of the steel plate side of the first plate pair having a lower Ms point.

[0094] If formula (2) is not satisfied, the temperature range where alloying progresses during current application is not maintained sufficiently. As a result, the Fe—Zn alloying between the plates becomes insufficient, and the average thickness (t Ｆｅ－Ｚｎ) cannot be increased. As described 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 (2) (i.e., "I １ ２ ×T １ + (T Ｈｏｌｄ x 1000) ２ The lower limit of the value of " / 50" is Si Ｈｉｇｈ Ya (Ms １ -Ms. ２ ) is effective.

[0095] 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 (2). The value of the left side of formula (2) is preferably set to be equal to or greater than the value of the right side of formula (2) multiplied by 1.05. The "value of the right side of formula (2)" refers to "(Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N).

[0096] The upper limit of the formula (2) 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 (2), １ is 15.0 kA or less, and T １ It is preferable that I is 2.0 s or less. １ is 4.0 kA or more, and T １ is 0.2 seconds or more.

[0097] [Second Current-Filling Pattern] As described above, the case including the main current-fill step and the post-current-fill step is referred to as a "second current-filling pattern," and one embodiment of the second current-filling pattern will be described below.

[0098] 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 ＰThe 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 (3) is satisfied. １ ２ ×T １ +I ２ ２ ×T ２ ×N Ｐ / T ３ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N (3) Here, Si shown in formula (3) Ｈｉｇｈ is the Si concentration (mass%) of the steel plate with the largest Si content in the first plate pair, and Ms １ is the Ms point (°C) of the steel plate side having a higher Ms point of the first plate pair, and Ms ２ is the Ms point of the steel plate side of the first plate pair having a lower Ms point.

[0099] In the case of the second current supply pattern, the formula (3) is used instead of the formula (2) used in the above-described first current supply pattern.

[0100] If welding is performed under conditions that do not satisfy the formula (3), the temperature and time required for the mutual diffusion of Fe and Zn cannot be maintained, and the Fe-Zn alloying between the plates by the post-current work is insufficient. As a result, the average thickness of the Fe-Zn alloy layer (t Ｆｅ－Ｚｎ ) cannot be increased. In addition, 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 (3) (i.e., "I １ ２ ×T １ +I ２ ２ ×T ２ ×N Ｐ / T ３ + (TＨｏｌｄ x 1000) ２ The lower limit of the value of " / 50" is Si Ｈｉｇｈ Ya (Ms １ -Ms. ２ The present inventors came up with the idea that it is effective to increase the amount of the ion exchange medium in accordance with the amount of the ion exchange medium.

[0101] For the above reasons, in the second current pattern, the welding conditions for the main current process and the post-current process are controlled so as to satisfy the relationship in equation (3). The value of the left side of equation (3) is preferably set to be equal to or greater than the value of the right side of equation (3) multiplied by 1.05. The "value of the right side of equation (3)" refers to "(Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N).

[0102] The upper limit of the formula (3) 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 (3), 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.

[0103] [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 (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.

[0104] [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.

[0105] In addition, "T" in the above formula (3) ３ Therefore, when there is no non-energizing step, T in the above formula (3) ３ (s) is set to 0. Also, "N Ｐ "(times)" indicates the number of repetitions.

[0106] 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－Ｚｎ 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.

[0107] 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.

[0108] Furthermore, maintaining a constant temperature range not only requires strict control of conditions, but also makes condition control 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.

[0109] 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.

[0110] 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 increases the equipment costs in the automobile manufacturing process, the number of repetitions is preferably 20 or less, and even more preferably 10 or less.

[0111] [Welding Procedure Disturbance] In addition to the welding conditions in the above-described steps, the present invention may have the following welding conditions.

[0112] As mentioned above, LME cracking is likely to occur in the presence of welding disturbances. Therefore, in this current application step, 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 more effectively obtain the effects of the present invention. (a) The impact angle between the welding electrode and the two or more overlapping steel sheets is 0.2 degrees or more. (b) The misalignment between the pair of welding electrodes is 0.1 mm or more. (c) The gap between any one of the welding electrodes and the outermost layer of the two or more overlapping steel sheets is 0.5 mm or more. (d) The gap between at least one pair of the two or more overlapping steel sheets is 0.5 mm or more.

[0113] These welding disturbances all locally increase the temperature and / or tensile stress of the weld when the electrodes are released, making it more susceptible to LME cracking. Ｆｅ－Ｚｎ 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.

[0114] (a) A state in which the impact angle between the welding electrode and two or more overlapping steel sheets is 0.2 degrees or more. The impact angle refers to the angle at which the electrode is tilted 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.

[0115] 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."

[0116] 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.

[0117] (b) A state in which the misalignment amount of the pair of welding electrodes is 0.1 mm or more. Misalignment refers to a state in which the central axes of the pair of welding electrodes are not aligned. As 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. When the misalignment amount is 0.1 mm or more, the effects of the present invention can be effectively obtained. If the misalignment amount is excessive, nugget formation becomes unstable and may cause expulsion, so it is preferable that the misalignment amount be 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.

[0118] (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. When a gap exists between one of the welding electrodes and the steel sheet immediately before pressure application begins, 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"), the steel sheet undergoes bending deformation when pressure application by the movable electrode begins, resulting in bending stress being applied to the weld. This makes LME cracking more likely to occur. The effects of the present invention can be effectively achieved when the gap between the welding electrode and the steel sheet is 0.5 mm or more. An excessive gap size can cause unstable nugget formation and expulsion, so the gap size is preferably 5.0 mm or less. The gap size is more preferably 1.0 mm or more, and even more preferably 3.0 mm or less.

[0119] (d) A state in which, among two or more overlapping steel plates, there is a gap of 0.5 mm or more between at least one pair of steel plates. As in (c), when there is a gap between any of the steel plates immediately before the start of pressure application, bending deformation occurs in the steel plates, and bending stress is applied to the weld, making LME cracking more likely to occur. Because the 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 the steel plates" described in (c) above is one of the construction disturbances during welding, but for the reasons mentioned above, the "gap between the steel plates after welding" in the component obtained after welding may be substituted for this "gap between the steel plates."

[0120] 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 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 g Ｓｈｅｅｔ Let's say.

[0121] 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.

[0122] 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 a range of 2.0 to 8.0 kN.

[0123] 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.

[0124] 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) 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.

[0125] In the "Plating" column of Table 1, "GI" indicates a steel sheet having a hot-dip galvanized layer (hot-dip galvanized steel sheet), "GA" indicates a steel sheet having a galvannealed layer (galvannealed hot-dip galvanized steel sheet), "EG" indicates a steel sheet having an electrogalvanized layer (electrogalvanized steel sheet), and "-" indicates a steel sheet having no plating layer (cold-rolled steel sheet).

[0126] The component compositions shown in Table 1 were measured by inductively coupled plasma (ICP) emission spectrometry.

[0127] 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.

[0128] The "structure fraction" column in Table 1 shows the area fraction (%) of ferrite (α) in the area observed by a scanning electron microscope (SEM) for each steel sheet. The obtained ferrite area fraction was then used to calculate the martensitic transformation start temperature (Ms) for each steel sheet using the above-mentioned formula, and the Ms was shown in the "transformation point" column in Table 1. The area fraction was measured as follows: A cross section of the steel sheet cut parallel to the rolling direction was polished and then corroded with 3% by volume of nital. Images were taken of three fields of view at a 1 / 4 position of the sheet thickness at 1500x magnification using an SEM, and the area fraction was determined from the obtained image data using Image-Pro manufactured by Media Cybernetics. The average value of the area fractions of the three fields of view is defined as the area fraction in the present invention. Ferrite can be distinguished as black in the image data.

[0129] The 10 kg-HV hardness (average value of the median 3 points / 5 points) at the 1 / 4 position of the plate thickness is multiplied by 0.32 to give TS, and the maximum strength between the steel plates (△TS ＭＡＸ ) was identified. １ and Ms. ２ The values ​​were calculated using the above formula (4) and are shown in Table 2.

[0130] The symbols shown in the "Construction disturbance" column of Table 3 correspond to (a) to (d) shown in the welding construction disturbances described above. Ａｘｉｓ " represents the "amount of misalignment" shown in (b) above, and "c" represents the "amount of gap between the welding electrode and the steel plate" shown in (c) above.

[0131] 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).

[0132] The obtained welded components were used to measure the average thickness (t Ｆｅ－Ｚｎ ) and LME cracking of the welds were evaluated.

[0133] [t Ｆｅ－Ｚｎ Measurement of t Ｆｅ－Ｚｎ 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.

[0134] As shown in Figures 2 and 3, the observation area was a region (i.e., a first region) 300 to 500 µm away from the end E of the nugget 4a in the sheet width direction on the steel sheet mating surface 7 where the Fe-Zn alloy layer was formed in the weld.

[0135] Here, the start point of the observation position was a position 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 end point of the observation position was a position 500 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 of the Zn-enriched portions between the sheets in each field of view, a region where the Fe concentration was 70 to 95 mass % and the Zn concentration was 5 to 30 mass % was identified as an Fe—Zn alloy layer, and the thickness of the Fe—Zn alloy layer was measured. The average value of the values ​​excluding the maximum and minimum values ​​of the thicknesses of the obtained Fe—Zn alloy layers was determined as the average thickness (t Ｆｅ－Ｚｎ )

[0136] 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, the visual fields were set to 10 in this example. The following analytical methods were used to measure the "Fe concentration," "Zn concentration," and "Fe—Zn alloy layer thickness."

[0137] The concentration measurement can be performed by, for example, line analysis or point analysis. When performing concentration measurement by these analyses, concentration measurements are performed at at least five or more lines per field of view, and the Fe concentration and Zn concentration for each measurement line are recorded. In the case of point analysis, it is preferable to measure at a pitch of 1 μm or less per point.

[0138] The thickness of the Fe—Zn alloy layer in the first region was measured as follows. First, the first region to be measured was identified as described above. Next, the thickness of the Fe—Zn alloy layer in the first region to be measured was measured. The thickness was measured using the measurement method described above at the location where the concentration measurement was performed, and the Fe—Zn alloy layer thickness in each line was measured. Of the Fe—Zn alloy layer thicknesses in all lines measured within the field of view, the average value was calculated, excluding the maximum and minimum values.

[0139] Using the average values ​​obtained for each line, graphs of length vs. Fe concentration and length vs. Zn concentration for the measurement line were created (see Figures 2 and 3). Then, the thickness (μm) of the region where both the Fe and Zn concentration ranges satisfied the ranges of the present invention was determined, and t Ｆｅ－Ｚｎ That is, the width of the double-headed arrows shown in FIGS. 2 and 3 is the "average thickness (t Ｆｅ－Ｚｎ )"

[0140] [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 Table 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)

[0141]

[0142]

[0143]

[0144] 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.

[0145] 1, 2, 3 Steel plate 4 Welded portion 4a Nugget 4b Welding heat affected zone 5 Fe—Zn alloy layer 6 Resistance spot welded member 7 Steel plate mating surface 8, 9 Welding electrode

Claims

1. A resistance spot welded component having a resistance spot welded portion formed by resistance spot welding two or more overlapping steel sheets, wherein at least one of the two or more overlapping steel sheets is a Zn-plated steel sheet, and among the two or more overlapping steel sheets, a combination having the largest difference in tensile strength between two steel sheets abutting in the vertical direction is called a first sheet combination, and the average thickness of an Fe-Zn alloy layer having an Fe concentration of 70 to 95 mass% and a Zn concentration of 5 to 30 mass% formed in an area 300 to 500 μm from the nugget end on the steel sheet mating surface in the first sheet combination is called t Ｆｅ－Ｚｎ (μm), the average thickness of the Fe—Zn alloy layer satisfies formula (1). Ｆｅ－Ｚｎ ≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×0.5+(θ+8×g Ｓｈｅｅｔ ) / 5 ... (1) where Ms １ (° C.): Ms point of the steel plate with a higher Ms point in the first plate pair; ２ (° C.): Ms point of the steel plate having a lower Ms point in the first plate pair; Si Ｈｉｇｈ (mass%): Si concentration of the steel plate side having the largest Si content 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 side of the outermost layer in the overlapped multiple steel plates and a tangent to the outer peripheral edge of the nugget on the steel plate side of the outermost layer, g Ｓｈｅｅｔ (mm): the sum of the gaps between the plates of each steel plate mating surface.

2. The resistance spot welded component according to claim 1, wherein at least one of the two or more overlapping steel plates has a Si content of 0.5 mass % or more.

3. A resistance spot welded component according to claim 1 or 2, wherein the difference in tensile strength between the two steel plates in the first plate set is 200 MPa or more.

4. A resistance spot welding method for resistance spot welding components according to any one of claims 1 to 3, 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) when the number of the steel plates is N (pieces), the welding conditions of the main current passing step satisfy formula (2). １ ２ ×T １ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N (2) Here, Si shown in formula (2) Ｈｉｇｈ is the Si concentration (mass%) of the steel plate side of the first plate set having the largest Si content, and Ms １ is the Ms point (°C) of the steel plate side having a higher Ms point of the first plate pair, and Ms ２ is the Ms point on the steel plate side having the lower Ms point of the first plate pair.

5. 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 4, wherein, when the number of steel plates is N (pieces), the welding conditions in the main current application step and the post-current application step satisfy the relationship in the following formula (3) instead of the formula (2). １ ２ ×T １ +I ２ ２ ×T ２ ×N Ｐ / T ３ + (T Ｈｏｌｄ x 1000) ２ / 50≧Si Ｈｉｇｈ × (Ms １ -Ms. ２ ) １／２ ×p×30 / N…(3) 6. The method for resistance spot welding of resistance spot-welded components according to claim 4 or 5, wherein in the main current application step, for at least one welding point, 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 sheets is 0.2 degrees or more, (b) a state in which the misalignment between a pair of the welding electrodes is 0.1 mm or more, (c) a state in which the gap between any 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 state in which the gap between at least one pair of the two or more overlapping steel sheets is 0.5 mm or more.

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

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

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