Resistance spot welding method and method for manufacturing welded parts

Abstract

The purpose of the present invention is to provide a resistance spot welding method and a method for manufacturing a welded member. The present invention is a resistance spot welding method in which a plate set obtained by overlapping a plurality of steel plates is sandwiched between a pair of welding electrodes and energized while being pressurized to join the steel plates together. The method comprises a holding step for holding the welding electrodes after the completion of energization. In this holding step, the pressing force holding time is controlled to fall within a predetermined range of time defined by a specific relational expression that uses the C content, the Si content, and the Mn content of the steel plates, as well as the decarburized layer thickness and the internal oxidation amount of the steel plate surface layer.

Description

[Technical field]

[0001] The present invention relates to a method for resistance spot welding of steel plates and a method for manufacturing welded components, and more particularly to a method for resistance spot welding and a method for manufacturing welded components that are suitable for welding components that are structural parts of automobiles and the like. [Background technology]

[0002] In recent years, CO2 emission regulations have become stricter due to growing environmental concerns, and in the automotive industry, reducing the weight of car bodies to improve fuel efficiency has become an issue. To this end, the use of high-strength steel sheets to reduce thickness in automotive parts is being promoted, and the use of steel sheets with a tensile strength (TS) of 980 MPa or more is being promoted. In addition, from the perspective of corrosion resistance, zinc-plated steel sheets with rust-preventing properties are used in areas exposed to rainwater.

[0003] In addition, in the assembly of automobiles, from the viewpoint of cost and manufacturing efficiency, automobile parts that have been press-formed by resistance spot welding are often combined. In general, resistance spot welding, which is a type of lap resistance welding, is used to join overlapping steel sheets. In this welding method, two or more overlapping steel sheets are sandwiched between a pair of welding electrodes from above and below, and a high welding current is passed between the upper and lower electrodes for a short period of time while applying pressure with the welding electrodes, to join them. FIG. 1 shows an example of resistance spot welding in which two overlapping steel sheets 1 and 2 are sandwiched between welding electrodes 4 and 5. According to this method, a point-like weld 6 is obtained by utilizing resistance heat generated by passing a high welding current. This point-like weld 6 is called a nugget, and is a portion where the two steel sheets 1 and 2 melt and solidify at the contact point of the steel sheets when a current is passed through the overlapping steel sheets. This joins the steel sheets in a point-like manner.

[0004] In resistance spot welding of a sheet assembly in which multiple steel sheets, including surface-treated steel sheets, are overlapped, there is a problem that cracks due to liquid metal embrittlement (hereinafter referred to as "LME cracks") may occur in the welded part. Here, the surface-treated steel sheet refers to a steel sheet having a metal plating layer on the surface of the base material (base steel sheet), such as zinc plating represented by electrolytic zinc plating and hot-dip galvanizing (hot-dip galvanizing includes alloyed hot-dip galvanizing), or zinc alloy plating containing elements such as aluminum and magnesium in addition to zinc. The melting points of zinc plating and zinc alloy plating are lower than the melting point of the base material of the surface-treated steel sheet, which causes the following problems.

[0005] That is, LME cracking in welds is thought to be caused by liquid metal embrittlement, in which the low-melting-point metal plating layer on the surface of the steel sheet melts during welding, and when the pressure of the welding electrodes and tensile stress due to the thermal expansion and contraction of the steel sheet are 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 cracking. As shown in Figure 1, LME cracking occurs in various locations, such as the surfaces of steel sheets 1 and 2 that come into contact with welding electrodes 4 and 5, and the surfaces of steel sheets 1 and 2 that come into contact with each other.

[0006] As countermeasures against such LME cracks, for example, the techniques of Patent Documents 1 to 4 can be mentioned. Patent Document 1 proposes that the composition of the steel sheets in the plate assembly should be within a specific range, specifically, by weight, a composition consisting of C: 0.003-0.01%, Mn: 0.05-0.5%, P: 0.02% or less, sol. Al: 0.1% or less, Ti: 48×(N / 14)-48×{(N / 14)+(S / 32)}%, Nb: 93×(C / 12)-0.1%, B: 0.0005-0.003%, N: 0.01% or less, Ni: 0.05% or less, and the balance: Fe and unavoidable impurities.

[0007] Patent Document 2 proposes a method for spot welding high-strength plated steel sheet, in which resistance 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 conditional formulas (A) and (B), t is plate thickness (mm), WT is welding current time (ms), and HT is holding time after welding current (ms).

[0008] Patent Document 3 also proposes that welding be performed 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-tensile galvanized steel sheet in which the amount of alloy elements in the steel sheet is below a certain level.

[0009] Patent Document 3 proposes a method in which a current pattern is three or more stages, the current flow time, welding current, and other welding conditions are adjusted so that the appropriate current range 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 refers to a current range in which a nugget having a desired nugget diameter (ΔI) or more and a molten residual thickness of 0.05 mm or more can be stably formed.

[0010] Patent Document 4 discloses a resistance spot welding method for welded members including zinc-plated steel sheets. Patent Document 4 proposes a method for suppressing LME cracking that occurs immediately outside the corona bond and at the nugget edge of the corona bond and for ensuring joint strength by setting the post-weld holding time Ht (seconds) from the end of welding current flow between the welding electrodes until the welding electrodes and the welded members are no longer in contact with each other within the range of the following conditional formula (C). 0.015t 2 +0.020≦Ht≦0.16t 2 -0.40t+0.70 …(C) In conditional formula (C), t is the total thickness of the steel plate (mm). [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 10-195597 [Patent Document 2] JP 2003-103377 A [Patent Document 3] JP 2003-236676 A [Patent Document 4] JP 2017-47475 A Summary of the Invention [Problem to be solved by the invention]

[0012] However, since it is necessary to limit the amount of alloy elements in the steel plate in Patent Document 1, there is a problem that the use of steel plates that satisfy the required performance is restricted. In particular, in the current situation where high alloying is being promoted along with the increase in strength of recent steel plates, the application of the technology in Patent Document 1 is extremely limited.

[0013] Patent Document 2 only proposes a method for suppressing cracking when an excessively large welding current is set such that expulsion occurs, and makes no mention of cracking in a state in which expulsion does not occur.

[0014] Patent Document 3 has the problem that many man-hours are required to optimize the welding conditions, and 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 effects of welding disturbances such as electrode strike angles, so they may be insufficient as a countermeasure when considering the actual work during automobile assembly.

[0015] Patent Document 4 proposes an electrode holding time that suppresses LME cracking on the steel sheet surface on the side where the steel sheets come into contact with each other. However, the effects of the steel sheet components and surface properties are not considered, and there is an issue that the range of electrode holding times that is effective in suppressing LME cracking is limited.

[0016] In addition, Patent Documents 1 to 4 do not consider cracks (hereinafter referred to as "low-temperature cracks") that occur on the surface of the steel plates in the weld heat-affected zone where the steel plates come into contact with each other after the weld heat-affected zone on the side where the steel plates come into contact undergoes martensitic transformation due to cooling after the end of current flow.

[0017] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a resistance spot welding method and a manufacturing method for welded components that are capable of simultaneously preventing LME cracking and cold cracking, particularly on the surface of the weld heat-affected zone where the steel sheets come into contact, by optimizing the electrode holding time in accordance with the composition and surface condition of the steel sheets, and welding procedure disturbances. [Means for solving the problem]

[0018] The present inventors have conducted extensive research in order to achieve the above object.

[0019] First, we will discuss LME cracking. The effect of the present invention on LME cracking that occurs on the surface of steel sheets where the steel sheets come into contact during welding (see Figure 1) cannot be explained simply because it is influenced by a variety of complex factors, but the basic mechanism is thought to be as follows.

[0020] One of the causes of cracking in welds is the generation of tensile stress, as described below, when the hot-temperature zinc-plated steel sheet is in contact with the base material (base steel sheet). This tensile stress has a region where it becomes locally large when the welding electrode separates from the steel sheet after welding is completed. Specifically, the expansion of the weld while current is flowing causes compressive deformation around the weld. Then, solidification shrinkage occurs in the weld as the weld cools after current is stopped, but while pressure is applied by the welding electrode, the stress remains in a compressive state due to the pressure applied, or even if the stress is in a tensile state, it is relaxed. However, when released from the restraint of the electrode pressure, a region where the tensile stress becomes locally large occurs, and it is thought that LME cracking occurs in this region.

[0021] Therefore, the inventors performed resistance spot welding under various welding conditions, as described in the technical concept below, and obtained the following findings regarding LME cracking. (i) It was found that each alloying element in the steel sheet, such as C, Si, and Mn, has an effect on LME cracking. (ii) In addition, it was found that since LME cracking occurs in the surface layer of the steel sheet in contact with the molten low-melting point metal, not only the average alloying element content in the steel sheet but also the concentration of the alloying element solid-solubilized in the surface layer of the steel sheet is important. It was found that the concentration of the alloying element solid-solubilized in the surface layer of the steel sheet is particularly affected by the thickness of the decarburized layer in the surface layer of the steel sheet and the amount of internal oxidation. (iii) In addition, as a result of evaluating LME cracking under various disturbances, it was also found that LME cracking is more likely to occur when there is a welding disturbance such as a striking angle.

[0022] Next, we will discuss cold cracking. As with LME cracking, the effect of the present invention on cold cracking that occurs during welding is not easily explained because it is influenced by a variety of complex factors, but the basic mechanism is thought to be as follows.

[0023] The heat-affected zone is heated to a temperature range where it becomes austenite single phase by the temperature rise during resistance spot welding. The heat-affected zone is then transformed into martensite by cooling, and then cooled to room temperature while undergoing auto-tempering. If the pressure holding time of the welding electrode after the end of current flow is long, the cooling rate after the completion of martensite transformation becomes very large due to the heat transfer from the welding electrode to the steel plate. Then, auto-tempering does not occur sufficiently, and the heat-affected zone becomes a very brittle martensite structure. Therefore, it is considered that, after being released from the restraint by the electrode pressure, if the area where the tensile stress is locally increased on the surface of the steel plate on the side where the steel plates contact each other is a brittle martensite structure, brittle fracture occurs within the grains or at the grain boundaries of the martensite structure, causing cold cracking. In particular, hydrogen contained in rust-preventive oil or contaminants attached to the surface of the steel plate diffuses from the surface of the steel plate on the side where the steel plates contact each other to the inside of the steel plate, promoting cold cracking.

[0024] Therefore, as described in the technical idea to be described later, the present inventors performed resistance spot welding under various welding conditions in the same manner as the above-described LME cracking, and obtained the following findings regarding low-temperature cracking. (iv) Each alloy element such as C, Si, and Mn in the steel sheet affects low-temperature cracking. (v) In addition, since low-temperature cracking occurs starting from the surface layer of the steel sheet where the steel sheets are in contact with each other, not only the average alloy element content in the steel sheet but also the concentration of the alloy element dissolved in the surface layer of the steel sheet, particularly the decarburized layer thickness and the amount of internal oxidation of the surface layer of the steel sheet, are greatly affected. (vi) Further, as a result of evaluating low-temperature cracking in the presence of various disturbances, it was also found that low-temperature cracking is likely to occur when there are welding construction disturbances such as corner blows.

[0025] From the above, it was found that appropriately controlling the holding time of the applied pressure after the energization is effective in reducing LME cracking and low-temperature cracking according to the content of each alloy element contained in the steel sheet and the surface state of the steel sheet. Further, it was found that it is more effective to consider construction disturbances during welding for reducing these cracks.

[0026] The present invention is based on the above findings, and the gist is as follows. [1] A resistance spot welding method for joining steel sheets by sandwiching a stack of a plurality of steel sheets between a pair of welding electrodes and energizing while applying pressure, having a holding step of holding the welding electrodes after the energization ends, in the holding step, when the holding time of the applied pressure is H (ms), the C content, Si content, and Mn content of the steel sheet are C (mass %), Si (mass %), and Mn (mass %), respectively, the decarburized layer thickness per side of the surface layer of the steel sheet is tc (mm), and the amount of internal oxidation per side of the surface layer of the steel sheet is Os (g / m 2 ), and when the calculated value of X1 of formula (1) for each steel sheet is X1max, which is the maximum value, and the calculated value of X2 of formula (2) for each steel sheet is X2min, which is the minimum value, the resistance spot welding method in which the holding time of the applied pressure satisfies X1max < H < X2min. 100 × {(C - tc) + (Si - 2×Os) / 5 + Mn / 40} = X1 …(1) 2000 - 600 × {(C - tc) + (Si - 2×Os) / 20 + Mn / 40} = X2 …(2) [2] The holding step further Taking the holding time of the pressing force as H (ms), the C content, Si content, and Mn content of the steel plate as C (% by mass), Si (% by mass), and Mn (% by mass) respectively, the decarburized layer thickness per side of the surface layer of the steel plate as tc (mm), the internal oxidation amount per side of the surface layer of the steel plate as Os (g / m 2 ), taking the total thickness of the stacked plurality of steel plates as t all (mm), taking the calculated value of the formula (3) for each steel plate as Y1, and taking the calculated value of the steel plate with the maximum value among them as Y1max, and taking the calculated value of the formula (4) for each steel plate as Y2, and taking the calculated value of the steel plate with the minimum value among them as Y2min, The holding time of the pressing force satisfies Y1max < H < Y2min, and the resistance spot welding method according to [1]. (1 + 0.1×t all ) × [100 × {(C - tc) + (Si - 2×Os) / 5 + Mn / 40}] = Y1 …(3) (1 - 0.8 / t all ) × [2000 - 600 × {(C - tc) + (Si - 2×Os) / 20 + Mn / 40}] = Y2 …(4) [3] During the energization, The shortest distance Le from the center of the welding spot to the end face of the steel plate is more than 3.0 mm, And when there is one or more existing welded spots around the welding spot, the resistance spot welding is performed with the center - to - center distance Lp between the welding spot and the nearest existing welded spot being 6.0 mm or more, and the resistance spot welding method according to [1] or [2]. [4] A first step of obtaining a plate stack by overlapping a plurality of steel plates, A second step of resistance spot - welding the obtained plate stack by the resistance spot - welding method according to any one of [1] to [3], And a method for manufacturing a welded member having the above steps.

Advantages of the Invention

[0027] According to the present invention, the electrode holding time after the end of current flow can be optimized according to the components and surface layer conditions of the overlapping steel sheets, regardless of the strength of the steel sheets, sheet combination, etc. As a result, it is possible to provide a resistance spot welding method and a manufacturing method for welded components that reduce LME cracking and low temperature cracking that occur on the steel sheet surfaces on the side where the steel sheets come into contact with each other in the weld heat affected zone. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a cross-sectional view that illustrates an example of how cracks occur during resistance spot welding. [Diagram 2] FIG. 2 is a graph showing the relationship in the present invention between the composition (C content) and surface layer condition of the steel sheet, the pressure holding time after the end of current flow, and the LME crack depth of the welded member. [Diagram 3] FIG. 3 is a graph showing the relationship between welding disturbance, pressure holding time after termination of current flow, and LME crack depth in welded members in the present invention. [Figure 4] FIG. 4 is a graph showing the relationship between the composition (C content) and surface layer condition of the steel sheet, the pressure holding time after the end of current flow, and the cold crack depth of the welded member in the present invention. [Diagram 5] FIG. 5 is a graph showing the relationship between welding disturbance, pressure holding time after termination of current flow, and cold crack depth of welded members in the present invention. [Figure 6] FIG. 6 is a graph showing the relationship in the present invention between the composition (Si content) and surface layer condition of the steel sheet, the pressure holding time after the end of current flow, and the LME crack depth of the welded member. [Figure 7] FIG. 7 is a cross-sectional view in the plate thickness direction illustrating the state of the surface layer of an example of a steel plate used in the present invention. [Figure 8] FIG. 8 is a diagram for explaining Le, which is the shortest distance from the center of a weld point to the end face of a steel sheet in the present invention. [Figure 9]FIG. 9 is a diagram for explaining Lp, which is the distance between the centers of adjacent welds and an existing weld in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] A resistance spot welding method and a method for manufacturing a welded member according to an embodiment of the present invention will be described below. Note that the present invention is not limited to this embodiment.

[0030] First, the technical concept of the present invention will be described in detail with reference to FIGS.

[0031] As described above, the cracks targeted by the present invention are LME cracks and low-temperature cracks that occur on the surface of the steel sheet on the side where the steel sheets come into contact with each other in the heat-affected zone of welding (see reference numeral 7 in FIG. 1). The inventors have found that in order to reduce such cracks, it is effective to control the time during which the pressure is held after the end of current flow to a certain value or less depending on the thickness of the decarburized layer on the surface of the steel sheet and the amount of internal oxidation, in addition to the disturbance during welding and the content of each alloy element contained in the steel sheet. Representative findings are shown below.

[0032] Figure 2 is a graph evaluating the relationship between the pressure holding time (ms) after the end of current flow and the LME crack depth (μm) when resistance spot welding was performed in the presence of welding disturbances using steel plates with C contents of 0.1 mass%, 0.2 mass%, and 0.3 mass%. In Figure 2, the square plots show the evaluation of steel plates with C content: 0.1 mass%, the triangular plots show the evaluation of steel plates with C content: 0.2 mass%, and the circular and diamond plots show the evaluation of steel plates with C content: 0.3 mass%.

[0033] Here, a plate set is prepared by overlapping two steel plates of the same type, and welding is performed under various welding conditions. The welding conditions are appropriately set within the ranges of pressure: 2.0 to 10.0 kN, welding current: 4.0 to 15.0 kA, and current flow time: 100 to 2000 ms. The presence of the welding disturbance mentioned above refers to a state in which one or more of the following are present: impact angle, misalignment between welding electrodes, gap between welding electrodes and steel plates, and gap between steel plates. Using the welded welded members, the LME crack depth of each welded member is measured by the method described in the Examples below.

[0034] As shown in Figure 2, in all steel sheets, LME cracking did not occur when the pressure holding time was 200 ms or more, but the pressure holding time required to suppress LME cracking tended to increase as the C content of the steel sheet increased. This is thought to be because an increase in C content increases the high-temperature strength due to solid solution strengthening in the weld heat-affected zone, and the tensile stress applied to the steel sheet surface layer in the weld heat-affected zone when the electrode is released increases, making LME cracking more likely to occur.

[0035] In addition, even if the C content is 0.3 mass%, the thickness of the decarburized layer (tc) per one side of the steel sheet surface layer described later is 0.1 mm, and the amount of internal oxidation (Os) per one side of the steel sheet surface layer is 0.2 g / m 2 In the case of the steel plate with a C content of 0.3 mass% (see the diamond plots in Fig. 2), which has no decarburized layer or internal oxidation, the pressure holding time required to suppress LME cracking was reduced compared to the steel plate with a C content of 0.3 mass% (see the circular plots in Fig. 2).

[0036] This is believed to be due to the fact that the decarburized layer has a low C content, which reduces the high-temperature strength of the steel sheet surface layer in the welded heat-affected zone, and the tensile stress applied to the steel sheet surface layer in the welded heat-affected zone when the electrode is released is reduced, making LME cracking less likely to occur. In addition, when the amount of internal oxidation is large, the amount of solute Si in the steel sheet surface layer is reduced. In the Fe-Zn phase diagram, Si lowers the solidus temperature at the same Zn concentration and shifts the solidus concentration to the Fe side. Therefore, when the zinc-based coating on the steel sheet surface melts during resistance spot welding and Zn diffuses into the steel sheet, in steel sheets with a high Si content, the steel sheet and liquid Zn come into contact for a long time at a lower temperature and in a low Zn concentration region, making LME cracking more likely to occur. Therefore, when the amount of internal oxidation is large and the amount of solute Si in the steel sheet surface layer is reduced, the temperature and time at which the steel sheet and liquid Zn come into contact become higher and shorter. As a result, it is believed that LME cracking becomes less likely to occur.

[0037] Furthermore, as shown in Figure 3, the evaluation of LME cracking under various disturbances revealed that LME cracking is more likely to occur when welding disturbances such as impact angles are present. This is thought to be because welding disturbances cause large localized compressive plastic deformation during current flow, which causes the tensile stress after the electrode is released to become very large. As mentioned above, when tensile stress is applied to the weld, the molten low-melting point metal penetrates the grain boundaries of the base material, reducing the grain boundary strength and causing LME cracking. Therefore, if the temperature of the weld when large tensile stress is generated can be appropriately reduced to suppress the penetration of the low-melting point metal into the grain boundaries of the steel plate, the occurrence of LME cracking can be more effectively reduced.

[0038] In FIG. 3, steel plates with the same C amount were used, and the "strike angle" and "gap between the welding electrode and the steel plate" were used as compound disturbances, and the strike angle was changed from 5 to 15 degrees.

[0039] Fig. 4 is a graph showing the relationship between the pressure holding time after the end of current flow and the cold crack depth when resistance spot welding is performed using the same steel plates as in Fig. 2. Here, welding is performed using the same plate combinations, welding conditions, and welding disturbances as in Fig. 2. Then, using the welded components, the cold crack depth of each welded component is measured by the method described in the Examples below.

[0040] As shown in Figure 4, no cold cracking occurred in any of the steel sheets when the pressure holding time was 1000 ms or less, but the pressure holding time required to suppress cold cracking tended to decrease as the C content of the steel sheet increased. This is thought to be because an increase in C content makes the weld heat-affected zone more likely to turn into hard, brittle martensite, and because the tensile stress applied to the steel sheet surface layer in the weld heat-affected zone when the electrode is released increases, making cold cracking more likely to occur.

[0041] In addition, even if the C content is 0.3 mass%, the decarburized layer thickness (tc) described later is 0.1 mm and the internal oxidation amount (Os) is 0.2 g / m 2 In the case of the steel plate with 0.3 mass% C (see the diamond plot in Figure 4), the time required to hold the applied pressure to prevent cold cracking was increased compared to the steel plate with 0.3 mass% C (see the circular plot in Figure 4) that does not have a decarburized layer or internal oxidation. This is because the decarburized layer has a low C content, which suppresses the embrittlement of martensite in the steel plate surface layer of the welded heat-affected zone, and the tensile stress applied to the steel plate surface layer of the welded heat-affected zone when the electrode is released is reduced. As a result, cold cracking is less likely to occur. In addition, when the amount of internal oxidation is large, the amount of solid solution Si in the steel plate surface layer decreases, and the martensitic transformation start temperature increases. Then, the temperature and time range in which autotempering occurs after the start of martensitic transformation increases, which suppresses the embrittlement of the steel plate surface layer of the welded heat-affected zone, and as a result, cold cracking is less likely to occur.

[0042] In addition, when the pressure holding time after the end of current flow was the minimum value of 1000 ms or less, no cold cracking occurred under any of the conditions.

[0043] Furthermore, as shown in Figure 5, the evaluation of cold cracking under various disturbances revealed that cold cracking, like LME cracking, is likely to occur when welding disturbances such as impact angles are present. This is believed to be due to the fact that welding disturbances cause large localized compressive plastic deformation during current flow, which causes the tensile stress after the electrode is released to become very large. As mentioned above, when tensile stress is applied to the weld, if the area where the tensile stress is locally large on the steel sheet surface on the side where the steel sheets come into contact with each other is a brittle martensite structure, cold cracking occurs. Therefore, if the pressure holding time after the end of current flow is prevented from becoming excessive, the cooling rate after martensite transformation is reduced, promoting auto-tempering and preventing the martensite structure near the steel sheet surface on the side where the steel sheets come into contact from becoming brittle. As a result, it is believed that the occurrence of cold cracking can be more effectively reduced.

[0044] In FIG. 5, steel plates with the same C amount were used, and the "strike angle" and "gap between the welding electrode and the steel plate" were used as compound disturbances, and the strike angle was changed from 5 to 15 degrees.

[0045] Fig. 6 is a graph showing the relationship between the applied pressure holding time after the end of current flow and the LME crack depth when resistance spot welding is performed under the presence of welding disturbances using steel plates with Si contents of 0.1 mass%, 1.1 mass%, and 1.7 mass%. In Fig. 6, the square plots are the evaluations of steel plates with Si content: 0.1 mass%, the triangular plots are the evaluations of steel plates with Si content: 1.1 mass%, and the circular and diamond plots are the evaluations of steel plates with Si content: 1.7 mass%. Here, as in Fig. 2, a plate set is prepared by overlapping two identical steel plates, and welding is performed under various welding conditions. The LME crack depth is then measured using the welded components.

[0046] As shown in Fig. 6, LME cracking did not occur in any of the steel sheets when the pressure holding time was 200 ms or more, but the pressure holding time required to suppress LME cracking tended to increase as the Si content of the steel sheet increased. In the Fe-Zn phase diagram, Si lowers the solidus temperature at the same Zn concentration and shifts the solidus concentration toward the Fe side. Therefore, when the zinc-based coating on the steel sheet surface melts and the Zn diffuses into the steel sheet during resistance spot welding, in steel sheets with a high Si content, the steel sheet and liquid Zn are in contact for a long time at a lower temperature and in a low Zn concentration region, which is thought to make LME cracking more likely to occur.

[0047] In addition, even if the Si content is 1.7 mass%, the decarburized layer thickness (tc) described later is 0.1 mm and the internal oxidation amount (Os) is 0.2 g / m 2 In the case of the steel sheet with 1.7 mass% Si (see the diamond plots in Fig. 6), the pressure holding time required to suppress LME cracking was shorter than that of the steel sheet with 1.7 mass% Si (see the circular plots in Fig. 6), which has no decarburized layer or internal oxidation. As mentioned above, when the amount of internal oxidation is large and the amount of solute Si in the steel sheet surface layer is reduced, the temperature and time at which the steel sheet comes into contact with liquid Zn become higher and shorter.

[0048] Regarding the evaluation of LME cracking performed under various disturbances, the trends were similar to those shown in FIG. 3 above, so the explanation will be omitted.

[0049] As explained above, LME cracking is likely to occur when the electrode holding time after the end of current is short, while cold cracking is likely to occur when the electrode holding time after the end of current is long. In addition, for both types of cracking, the presence or absence of cracking varies depending on the composition and surface condition of the overlapping steel sheets, and welding disturbances, even if the electrode holding time is the same. In addition, the degree of influence of the composition and surface condition of the steel sheets, and welding disturbances on the presence or absence of cracking differs between LME cracking and cold cracking. Therefore, in the present invention, it has been found that, as a method for suppressing these cracks that occur on the steel sheet surface in the weld heat affected zone, it is important to optimize the electrode holding time after the end of current depending on the composition and surface condition of the overlapping steel sheets, and welding disturbances.

[0050] Next, the conditions for optimizing the electrode holding time after the end of current flow will be described.

[0051] [Resistance spot welding method] First, an embodiment of the resistance spot welding method of the present invention will be described.

[0052] The present invention relates to a resistance spot welding method in which a sheet set of multiple overlapping steel sheets is sandwiched between a pair of welding electrodes, and current is passed through the welding electrodes while applying pressure to the steel sheets to join them. After the current is passed through (i.e., the current passing step described below), the method includes a step of holding the welding electrodes with a constant pressure (i.e., the holding step described below).

[0053] The number of the "plurality of steel plates" may be two or more, and there is no particular upper limit on the number of overlapping steel plates. The number of the steel plates is preferably five or less. The steel plates used in the present invention will be described later, and therefore will not be described here.

[0054] The welding device that can be used in the resistance spot welding method of the present invention is equipped with a pair of upper and lower electrodes, and can arbitrarily control the pressure and welding current during welding. The present invention can be applied to both DC and AC welding power sources. In the case of a single-phase AC type, "current" means "effective current". The pressure mechanism, type, electrode shape, etc. of the welding device are not particularly limited. Examples of the pressure mechanism include an air cylinder and a servo motor, and examples of the type include a stationary type and a robot gun. Examples of the type of the electrode tip include the DR type (dome radius type), R diameter (radius type), and D type (dome type) described in JIS C 9304:1999. The tip diameter of the electrode is, for example, 4 mm to 16 mm.

[0055] Next, each step in the resistance spot welding method of the present invention will be described.

[0056] [Electrification process] A current application process is performed to melt the overlapping surfaces of overlapping steel sheets to form a weld. First, multiple steel sheets are overlapped to form a sheet assembly. Next, the sheet assembly is sandwiched between a pair of welding electrodes arranged on the lower and upper sides of the sheet assembly, and current is applied while applying pressure with the welding electrodes and controlling to achieve predetermined welding conditions. This forms a weld on the overlapping surfaces of the overlapping steel sheets, joining the steel sheets together.

[0057] The welding conditions in the current application process are not particularly specified. From the viewpoint of application to automobile parts, the following welding conditions in the current application process are preferable. Specifically, the current value, current application time, and pressure during current application may be constant, or may not need to be constant (i.e., may be variable).

[0058] In the former case, i.e., when welding is performed by one-stage current application, the preferred ranges of the welding current value, current application time, and pressure during current application are as follows: The welding current value during current application is preferably 10 kA or less, and more than 2 kA, for example. The current application time is preferably 100 ms to 1000 ms, for example. The pressure during current application is preferably 2.0 kN to 10.0 kN, for example.

[0059] In the latter case, that is, when welding is performed by multi-stage energization, the welding current value and the pressing force may be changed in two or more stages, or a cooling time may be provided between each stage, or parameters such as the resistance value and voltage value during welding may be monitored, and there is no problem at all in using a control method of changing the welding current value and the energization time during energization according to the fluctuations.

[0060] For example, in the case of multi-stage energization with short-time pre-energization and post-energization before and after the main energization, the upper limit of the preferable welding current value during energization extends from the above-mentioned "10 kA" to 15 kA. Also, the upper limit of the preferable energization time extends from the above-mentioned "1000 ms" to 2000 ms. However, the energization time in the case of multi-stage energization is the sum of the energization times of each stage. That is, the welding current value during energization is preferably 15 kA or less and preferably exceeds 2 kA. Also, the energization time is preferably 100 ms to 2000 ms. Also, the pressing force during energization is preferably, for example, 2.0 kN to 10.0 kN.

[0061] 〔Holding step〕 After the energization step is completed, a holding step of holding the welding electrode with a constant pressing force is performed.

[0062] In the holding step of the present invention, the pressing force holding time after the energization is completed is set as H (ms), the C content, Si content, and Mn content of the steel sheet are set as C (mass %), Si (mass %), and Mn (mass %), respectively, the decarburized layer thickness per one side of the surface layer of the steel sheet is set as tc (mm), and the internal oxidation amount per one side of the surface layer of the steel sheet is set as Os (g / m 2 )). Among the calculated values X1 of formula (1) for each steel sheet, the calculated value of the steel sheet having the maximum value is set as X1max, and among the calculated values X2 of formula (2) for each steel sheet, the calculated value of the steel sheet having the minimum value is set as X2min. The welding electrode is held so that this pressing force holding time (H) satisfies X1max < H < X2min. 100×{(C - tc)+(Si - 2×Os) / 5+Mn / 40}=X1 …(1) 2000 - 600×{(C - tc)+(Si - 2×Os) / 20+Mn / 40}=X2 …(2)

[0063] The reason for defining formula (1) will be explained below.

[0064] As described above in (i) to (iii), LME cracking is more likely to occur with an increase in the alloying elements C, Si, and Mn, so increasing the pressure holding time after the end of current flow in accordance with the increase in the C, Si, and Mn concentrations in the steel sheet is effective in reducing LME cracking. In addition, the tensile stress that acts as the driving force for LME cracking on the steel sheet surface where the steel sheets come into contact with each other increases due to welding disturbances. Therefore, increasing the pressure holding time in the holding process after the end of current flow in accordance with the magnitude of the disturbance parameter D shown in equation (7) described later is even more effective in reducing LME cracking.

[0065] The effect of each alloying element on LME cracking cannot be explained simply because various factors have a complex influence, but the basic mechanism is thought to be as follows.

[0066] C is an element that promotes solid solution strengthening of steel sheets. Therefore, if the C content increases, the tensile stress after electrode release increases in the area where compressive plastic deformation occurs during current flow, making LME cracking more likely to occur.

[0067] In the Fe-Zn phase diagram, Si lowers the solidus temperature at the same Zn concentration and shifts the solidus concentration toward the Fe side. Therefore, when the zinc-based coating on the steel sheet surface melts and the Zn diffuses into the steel sheet during resistance spot welding, in steel sheets with a high Si content, the steel sheet and liquid Zn are in contact for a long time at a lower temperature and in a low Zn concentration region, making LME cracking more likely to occur.

[0068] Like C, Mn is an element that promotes solid solution strengthening of steel sheets, so an increase in the Mn content increases the tensile stress after the electrode is released, making LME cracking more likely to occur. However, in the high temperature range where LME cracking occurs, the degree of solid solution strengthening by Mn is smaller than that of C, so the effect on tensile stress after the electrode is released is also smaller than that of C.

[0069] Since LME cracking occurs in the surface layer of the steel sheet in contact with the molten low melting point metal, it is also affected by the concentration of alloy elements dissolved in the surface layer of the steel sheet. When a decarburized layer is formed in the surface layer of the steel sheet, the C concentration in the surface layer of the steel sheet decreases. Therefore, an increase in the thickness of the decarburized layer in the surface layer of the steel sheet has the same effect on reducing LME cracking as a decrease in the C content of the steel sheet. Therefore, as the thickness of the decarburized layer increases, the lower limit of the pressure holding time after the end of current application, as specified by formula (1), decreases. Furthermore, when internal oxidation occurs in the surface layer of the steel sheet, Si near the surface layer of the steel sheet is fixed as Si oxide, and the amount of solid-solubilized Si decreases. Therefore, an increase in the amount of internal oxidation in the surface layer of the steel sheet has the same effect on reducing LME cracking as a decrease in the Si content of the steel sheet. Therefore, as the amount of internal oxidation increases, the lower limit of the pressure holding time after the end of current application, as specified by formula (1), decreases.

[0070] Next, the reason for defining the formula (2) will be explained.

[0071] As described in (iv) to (vi) above, similar to the case of LME cracking, cold cracking is more likely to occur with an increase in the alloying elements C, Si, and Mn, so reducing the time for which pressure is held after the end of current flow in accordance with the increase in the C, Si, and Mn concentrations in the steel sheet is effective in reducing cold cracking. In addition, the tensile stress that acts as the driving force for cold cracking on the steel sheet surface on the side where the steel sheets come into contact with each other increases due to welding disturbances. Therefore, reducing the time for which pressure is held after the end of current flow in accordance with the magnitude of the disturbance parameter D shown in equation (7) described below is even more effective in reducing cold cracking.

[0072] The effect of each alloying element on cold cracking cannot be explained simply because various factors have a complex influence, but the basic mechanism is thought to be as follows.

[0073] C is an element that greatly affects the hardness and toughness of martensite. When C increases, hard and brittle martensite is more likely to form near the surface where the steel sheets come into contact after the current is turned off. In addition, it increases the tensile stress after the electrode is released, making low-temperature cracking more likely to occur.

[0074] Silicon inhibits the precipitation of carbides after the completion of martensitic transformation. Therefore, if the amount of silicon increases, hard and brittle martensite is more likely to form near the surface where the steel sheets come into contact with each other after the passage of current is stopped.

[0075] Mn improves the hardenability of steel sheets, so if the Mn content increases, hard and brittle martensite tends to form near the surface where the steel sheets come into contact with each other after the passage of current is terminated.

[0076] Since cold cracking occurs from the surface layer of the steel sheet where the steel sheets contact each other, it is also affected by the concentration of alloy elements dissolved in the surface layer of the steel sheet. When a decarburized layer is formed in the surface layer of the steel sheet, the C concentration in the surface layer of the steel sheet decreases. Therefore, an increase in the thickness of the decarburized layer in the surface layer of the steel sheet has the same effect on reducing cold cracking as a decrease in the C content of the steel sheet. Therefore, as the thickness of the decarburized layer increases, the upper limit of the pressure holding time after the end of current application, which is specified by formula (2), increases. Furthermore, when internal oxidation occurs in the surface layer of the steel sheet, Si near the surface layer of the steel sheet is fixed as Si oxide, and the amount of solid-solubilized Si decreases. Therefore, an increase in the amount of internal oxidation in the surface layer of the steel sheet has the same effect on reducing cold cracking as a decrease in the Si content of the steel sheet. Therefore, as the amount of internal oxidation increases, the upper limit of the pressure holding time after the end of current application, which is specified by formula (2), increases.

[0077] The thickness of the decarburized layer and the amount of internal oxidation per one side of the steel sheet surface layer used in each formula can be measured by the method described in the examples below.

[0078] Although an example of a steel sheet having a decarburized layer and an internal oxidation layer on one side of the steel sheet is shown in FIG. 7 described below, the steel sheet used in the present invention may have a decarburized layer and an internal oxidation layer on both sides of the steel sheet. For example, in the case of welding in which two steel sheets are overlapped and the decarburized layer thickness and the amount of internal oxidation are different on both sides of the steel sheet, the decarburized layer thickness and the amount of internal oxidation of the surface layer of the steel sheet on the side where the steel sheets are in contact with each other are measured, and the calculations of formulas (1) and (2) are performed. Further, for example, in the case of welding in which three or more steel sheets are overlapped and both sides of one steel sheet are in contact with other steel sheets, the decarburized layer thickness and the amount of internal oxidation on both sides of the steel sheet in the one steel sheet are measured, and the calculations of formulas (1) and (2) are performed using the respective maximum values.

[0079] When the components of the plurality of steel sheets used for welding and the decarburized layer thickness and the amount of internal oxidation per side of the surface layer of the steel sheet are different, it is necessary to satisfy the relationships of formulas (1) and (2) with the components of each steel sheet and the decarburized layer thickness and the amount of internal oxidation per side of the surface layer of the steel sheet, respectively.

[0080] Therefore, in the present invention, in order to optimize the electrode holding time in the holding step after the energization according to the components and the surface state of the steel sheets to be overlapped, formulas (1) and (2) are defined as conditional expressions. The electrode holding time (H) in the holding step shall satisfy the range obtained from these conditional expressions (that is, X1max < H < X2min). If the electrode holding time (H) is not controlled within this numerical range, the above-described effects cannot be obtained, and thus the LME cracks and the low-temperature cracks generated on the surface of the welded heat-affected zone on the side where the steel sheets are in contact with each other cannot be suppressed.

[0081] In order to more effectively obtain the above-described effects in the present invention, in the holding step, in addition to the provisions of formulas (1) and (2), the following conditional expressions can be defined as necessary.

[0082] Specifically, in the holding step, the pressure holding time is further defined as H (ms), the C content, Si content, and Mn content of the steel sheet are defined as C (mass %), Si (mass %), and Mn (mass %), respectively, the decarburized layer thickness per side of the surface layer of the steel sheet is defined as tc (mm), the internal oxidation amount per side of the surface layer of the steel sheet is defined as Os (g / m 2 ), the total thickness of the stacked plurality of steel sheets is defined as t all (mm), the calculated value of the steel sheet with the maximum value among the calculated values Y1 of formula (3) for each steel sheet is defined as Y1max, and the calculated value of the steel sheet with the minimum value among the calculated values Y2 of formula (4) for each steel sheet is defined as Y2min. It is preferable that this pressure holding time (H) satisfies Y1max < H < Y2min. (1 + 0.1×t all )×[100×{(C - tc)+(Si - 2×Os) / 5 + Mn / 40}] = Y1 …(3) (1 - 0.8 / t all )×[2000 - 600×{(C - tc)+(Si - 2×Os) / 20 + Mn / 40}] = Y2 …(4)

[0083] The reasons for defining formulas (3) and (4) are explained below.

[0084] Regarding LME cracks and low-temperature cracks, the temperature at the time of electrode liberation affects them respectively, but the temperature at the time of electrode liberation depends on the total thickness of the steel sheets. That is, as the total thickness of the steel sheets increases, it becomes difficult to extract heat from the electrodes, so the temperature at the time of electrode liberation becomes higher. Then, since liquid Zn is likely to exist at the time of electrode liberation, LME cracks are likely to occur. On the other hand, since the auto-tempering after martensite transformation is promoted by the decrease in the cooling rate after electrode liberation, low-temperature cracks are less likely to occur. Therefore, as the total thickness t all (mm) of the stacked plurality of steel sheets increases, it is effective to increase the lower limit value of the pressure holding time after the end of energization required to suppress LME cracks defined by formula (3), and also to increase the upper limit value of the pressure holding time after the end of energization required to suppress low-temperature cracks defined by formula (4).

[0085] Therefore, in addition to making the electrode holding time (H) in the holding step of the present invention fall within the range that satisfies the conditional expressions (1) and (2) of the above conditional expression, it is preferable that the electrode holding time (H) satisfies the range obtained from the conditional expressions (3) and (4) (that is, Y1max < H < Y2min).

[0086] <Welding construction disturbance> In the present invention, in order to further optimize the electrode holding time (H), in addition to the components and surface states of the steel plates to be overlapped, it is preferable to consider the welding construction disturbance. Note that the following conditional expressions (5), (6), (8), and (9) are conditional expressions in which the welding construction disturbance is considered in the above-mentioned conditional expressions (1) to (4). Since the reasons for defining each conditional expression have been described above, the description is omitted.

[0087] Specifically, in the holding step, the pressure holding time after the energization is terminated is H (ms), the C content, Si content, and Mn content of the steel plate are C (mass %), Si (mass %), and Mn (mass %), respectively, the decarburized layer thickness per side of the surface layer of the steel plate is tc (mm), and the internal oxidation amount per side of the surface layer of the steel plate is Os (g / m 2 ). Let the calculated value of the steel plate that becomes the maximum value among the calculated values X1' of the conditional expression (5) for each steel plate be X1'max, and the calculated value of the steel plate that becomes the minimum value among the calculated values X2' of the conditional expression (6) for each steel plate be X2'min. The welding electrode is held so that this pressure holding time (H) satisfies X1'max < H < X2'min. 100×{(C - tc)+(Si - 2×Os) / 5+Mn / 40}+D = X1' …(5) 2000 - 600×{(C - tc)+(Si - 2×Os) / 20+Mn / 40}-5×D = X2' …(6) Here, D shown in the conditional expressions (5) and (6) is a disturbance parameter obtained by the following conditional expression (7), and A, M, CL, and G shown in the conditional expression (7) are, respectively, immediately before the pressure is applied by the welding electrode, A: The included angle (degrees) between the welding electrode and the plurality of steel plates to be overlapped, M: The amount of core deviation (mm) between the welding electrodes, CL: The maximum value (mm) of the gap between any welding electrode and the plurality of steel plates to be overlapped, G: The maximum value (mm) of the gaps between at least one set of steel plates among the overlapped multiple steel plates is as follows. D = 5×A + 5×M + 5×CL + 2×G …(7) In addition, when these welding construction disturbances do not occur, the value of the disturbance parameter D is 0.

[0088] In the present invention, in order to more effectively obtain the above-described operational effects, in the holding process, in addition to defining Formula (5) and Formula (6), the following conditional expressions can be defined as necessary.

[0089] Specifically, in the holding process, further, the holding time of the pressing force is H (ms), the C content, Si content, and Mn content of the steel plate are C (mass %), Si (mass %), and Mn (mass %), respectively, the decarburized layer thickness per one side of the surface layer of the steel plate is tc (mm), the internal oxidation amount per one side of the surface layer of the steel plate is Os (g / m 2 ), the total thickness of the overlapped multiple steel plates is t all (mm), the calculated value of the steel plate with the maximum value among the calculated values Y1´ of Formula (8) for each steel plate is Y1´max, and the calculated value of the steel plate with the minimum value among the calculated values Y2´ of Formula (9) for each steel plate is Y2´min. It is preferable that this holding time of the pressing force (H) satisfies Y1´max < H < Y2´min. (1 + 0.1×t all )×[100×{(C - tc)+(Si - 2×Os) / 5 + Mn / 40}+D]=Y1´ …(8) (1 - 0.8 / t all )×[2000 - 600×{(C - tc)+(Si - 2×Os) / 20 + Mn / 40}-5×D]=Y2´ …(9)

[0090] Below, the details of each welding construction disturbance (A, M, CL, G) in the disturbance parameter shown in Formula (7) will be described.

[0091] (a) A: The included angle (degrees) between the welding electrode and the overlapped multiple steel plates The impact angle means the angle at which the electrode is inclined with respect to the steel sheet, that is, the angle between the electrode pressure direction and the thickness direction of the steel sheet. If the impact angle is large, bending stress is applied to the welded part, and large compressive plastic deformation occurs locally, which increases the tensile stress after the electrode is released, making LME cracking and low temperature cracking more likely to occur. The effect of the present invention can be effectively obtained when the impact angle is 0.2 degrees or more. If the impact angle is too large, the nugget formation becomes unstable and causes splashing, so it is preferable to set the impact angle to 10 degrees or less. The impact angle is more preferably 1 degree or more, and even more preferably 8 degrees or less.

[0092] (b) M: Displacement between welding electrodes (mm) Misalignment means a state in which the central axes of a 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 and cold cracking more likely to occur. When the misalignment amount is 0.1 mm or more, the effect of the present invention can be effectively obtained. If the misalignment amount is excessive, the nugget formation becomes unstable and causes expulsion, so it is preferable to set the misalignment amount to 5 mm or less. The misalignment amount is more preferably 0.2 mm or more, and even more preferably 3 mm or less.

[0093] (c) CL: Maximum gap between any welding electrode and multiple overlapping steel plates (mm) In a state where there is a gap between one of the welding electrodes and the steel sheet immediately before the start of pressure application, for example, when one welding electrode is movable (hereinafter, movable electrode) and the other welding electrode is fixed (hereinafter, fixed electrode), pressure application by the movable electrode is started in a state where there is a gap between the fixed electrode and the steel sheet. As a result, bending deformation occurs in the steel sheet, and bending stress is applied to the welded portion, which makes LME cracking and low temperature cracking more likely to occur. When this gap amount (maximum gap amount) is 0.5 mm or more, the effect of the present invention can be effectively obtained. If this gap amount is excessive, nugget formation becomes unstable and causes expulsion, so this gap amount is preferably 5 mm or less. The gap amount is more preferably 1 mm or more, and more preferably 3 mm or less.

[0094] (d) G: The maximum gap (mm) between at least one pair of overlapping steel plates As in the above (c), when there is a gap between any of the steel plates immediately before the start of pressurization, the steel plates undergo bending deformation, and bending stress is applied to the welded portion, which makes LME cracking and low-temperature cracking more likely to occur. When the maximum gap amount is 0.5 mm or more, the effect of the present invention can be effectively obtained. If the gap amount is excessively large, the nugget formation becomes unstable and causes splashing, so it is preferable that the maximum gap amount is 4 mm or less. The maximum gap amount is more preferably 1 mm or more, and more preferably 3 mm or less. The above "gap between one or more pairs of steel plates" means that when two or more steel plates are stacked together and two steel plates arranged vertically are considered as one pair, there is a gap between one or more pairs of steel plates.

[0095] For the above reasons, increasing the pressure holding time after the end of current application depending on the magnitude of the disturbance parameter D is effective in reducing LME cracking and cold cracking.

[0096] <Welding point> In the present invention, in addition to the above-mentioned conditions, it is desirable to control the welding points in the current application process. Therefore, the welding points will be described with reference to Figs. 8 and 9.

[0097] As shown in Figures 8 and 9, when current is applied, when the distance from the center of the weld point to the end face of the steel plate is Le, it is preferable that Le exceeds 3.0 mm, and when there are one or more previously welded points around the weld point, when the distance between the centers of the weld point and the previously welded point closest to the weld point is Lp, it is preferable that Lp exceeds 6.0 mm.

[0098] The above-mentioned "distance (Le) from the center of the welding point to the end face of the steel plate" refers to the distance from the center of the welding point 8 to the end face of the steel plate that is closest to the center, as shown in Fig. 8. Also, the above-mentioned "distance (Lp) between the centers of the welding points at the welding point and the existing weld point closest to the welding point" refers to the distance between the centers of adjacent welding points (here, welding point 8 and existing weld point 9) as shown in Fig. 9.

[0099] 8 and 9 are top views showing the periphery of the welded portion of welded member 10 (that is, views of the welded member as seen from above).

[0100] If the distance (Le) from the center of the weld point to the edge of the steel plate is 3.0 mm or less, the occurrence of expulsion becomes significant and LME cracking becomes more likely. There is no upper limit for the distance (Le). If the distance (Le) is large, welding itself becomes difficult with a small welding gun, and a large welding gun is required, which creates equipment constraints, so the distance (Le) is preferably 200 mm or less.

[0101] If the distance (Lp) between the centers of a weld point and the previous weld point closest to the weld point is less than 6.0 mm, the weld point is restrained by the previous weld point, increasing residual stress and making LME cracking and low-temperature cracking more likely to occur. There is no upper limit for the distance (Lp). In order to ensure the strength and rigidity of the welded component, it is preferable that the distance (Lp) be 200 mm or less.

[0102] <Steel plates used in plate assembly> Examples of the steel sheet used in the present invention include cold-rolled steel sheets and surface-treated steel sheets having a metal plating layer on the surface of the steel sheet. For example, all the steel sheets included in the sheet set may be cold-rolled steel sheets, or all the steel sheets included in the sheet set may be surface-treated steel sheets, or the steel sheets included in the sheet set may be cold-rolled steel sheets and surface-treated steel sheets. The steel sheets will be described below.

[0103] [Thickness of steel plate] The thickness of the steel sheets included in the sheet assembly of the present invention is not particularly limited, but from the viewpoint of application to automobile parts, it is preferably, for example, 0.5 mm to 6.0 mm.

[0104] [Tensile strength of steel plate] The tensile strength of the steel plates included in the sheet combination of the present invention is not particularly limited, and is, for example, 250 MPa to 2500 MPa. Since cracks are more likely to occur when the tensile strength of the sheet combination is high, the present invention can be applied to a sheet combination in which at least one of the multiple steel plates has a tensile strength of 590 MPa or more to more effectively suppress cracks. In particular, a greater effect can be obtained when at least one of the steel plates included in the sheet combination has a tensile strength of 780 MPa or more.

[0105] [Steel plate composition] The composition of the steel plates included in the sheet assembly of the present invention is not particularly limited. From the viewpoint of using steel plates having the above tensile strength range, for example, it is preferable that at least one of the steel plates included in the sheet assembly has the composition shown below. In this case, the effects of the present invention can be obtained more effectively. In the following description, the "%" designation of the composition means "mass %" unless otherwise specified.

[0106] The composition of the steel sheet preferably contains, in mass %, C: 0.04 to 0.50%, Si: 0.01 to 2.50%, Mn: 1.00 to 5.00%, P: 0.05% or less, S: 0.01% or less, and Al: 0.01 to 0.30%.

[0107] C: 0.04 to 0.50% C is an element that contributes to increasing the strength of steel plates. The C content is preferably 0.04% or more. On the other hand, if C is added in excess, the welded portion becomes excessively hard, causing a decrease in the toughness of the welded portion. The C content is preferably 0.50% or less.

[0108] Silicon: 0.01 to 2.50% Silicon is an effective element for improving the strength and elongation of steel sheets. The silicon content is preferably 0.01% or more. On the other hand, excessive addition of silicon causes a decrease in LME resistance and plating ability. The silicon content is preferably 2.50% or less.

[0109] Mn: 1.00-5.00% Mn is an element that contributes to increasing the strength of steel plate. The Mn content is preferably 1.00% 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. The Mn content is preferably 5.00% or less. The Mn content is more preferably less than 3.00%.

[0110] P:0.05% or less When P is added in excess, it causes solidification segregation in the nugget, reducing the toughness of the weld. The P content is preferably 0.05% 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.

[0111] S: 0.01% or less S causes a decrease in the toughness of the weld due to solidification segregation in the nugget. The S content is preferably 0.01% or less. Although there is no particular lower limit for the S content, extremely low S content increases the steelmaking cost, so the S content is preferably 0.0002% or more.

[0112] Al: 0.01 to 0.30% Al is an element necessary for deoxidization, and in order to obtain this effect, the Al content is preferably 0.01% or more. On the other hand, excessive addition of Al increases the number of inclusions in the steel sheet, reduces the local deformability, and reduces the ductility of the steel sheet. The Al content is preferably 0.30% or less.

[0113] In addition to the above components, if necessary, one or more optional elements selected from Ti, B, Cr, Ni, Mo, W, V, Nb, Cu, Sb, Ca, REM (rare earth elements), N, and O may be contained in a total amount of 10% or less. Since these elements can be contained as necessary, each of these components may be 0%.

[0114] If the total content of the above optional elements exceeds 10%, the cost of alloy addition becomes excessive, and it causes various problems such as embrittlement of the steel sheet, deterioration of weldability, etc. When these optional elements are contained, it is more preferable that the total content is 0.1% or more.

[0115] The balance other than the above elements is Fe and unavoidable impurities.

[0116] In addition, there is no problem even if hot-pressed steel sheets or hot-rolled steel sheets are used as the steel sheets for the sheet set used in the present invention.

[0117] [Steel plate surface layer] The steel sheets included in the sheet combination of the present invention may have a decarburized layer and an Si internal oxidation layer in the surface layer of the steel sheet, or may not have a decarburized layer and an Si internal oxidation layer.

[0118] It is known that LME cracking is more likely to occur when the amount of C in steel increases. For this reason, it is preferable to use a steel sheet having a decarburized layer 13 on the surface of the steel sheet, and containing an Si internal oxidation layer 14 in the decarburized layer 13, as shown in Fig. 7. This is because the effect of reducing LME cracking can be more effectively obtained when using a steel sheet having this decarburized layer and Si internal oxidation layer.

[0119] In the case of a steel sheet having this decarburized layer, the decarburized layer can be formed on the outermost surface of the steel sheet by increasing the dew point during annealing in the steel sheet production. There is no particular limit to the thickness of the decarburized layer in the steel sheet having this decarburized layer. If the decarburized layer becomes too thick, the tensile strength of the steel sheet decreases, so from the viewpoint of keeping the tensile strength within the above-mentioned range, the decarburized layer thickness (tc) per side of the steel sheet is preferably 0.20 mm or less. The decarburized layer thickness (tc) is preferably 0.01 mm or more.

[0120] It is also known that LME cracking is more likely to occur when the amount of Si in steel increases. Therefore, as shown in FIG. 7, it is preferable to use a steel sheet having a Si internal oxidation layer 14 in the decarburized layer 13. This is because the effect of reducing LME cracking can be more effectively obtained. In the steel sheet having a Si internal oxidation layer in the decarburized layer, the Si internal oxidation layer can be formed in the outermost layer of the steel sheet by increasing the dew point during annealing in the steel sheet production. In addition, the effect of reducing low-temperature cracking can be more effectively obtained as described above.

[0121] The above-mentioned "Si internal oxidation layer" specifically refers to a region in which Si oxide is formed within a crystal grain and / or in part of a crystal grain boundary.

[0122] There is no particular limit to the amount of internal oxidation. If the amount of internal oxidation is excessive, the bendability of the steel sheet will deteriorate, so the amount of internal oxidation (Os) per one side of the steel sheet should be 1.0 g / m 2 The internal oxidation amount (Os) is preferably 0.1 g / m 2 The above is preferable.

[0123] [Metal plating layer on steel sheet surface] When a surface-treated steel sheet having a metal plating layer is used as the steel sheet included in the sheet assembly of the present invention, the composition of the metal plating layer is also arbitrary.

[0124] Since LME cracking is caused in part by melting of a metal plating layer with a low melting point, the effect of suppressing cracking in the weld is greater when the metal plating layer has a lower melting point than the base material. From the viewpoint of application to automobile parts, it is preferable that the melting point of the base material (base steel sheet) in the present invention is, for example, 1400 to 1570°C, and the melting point of the metal plating layer is, for example, 300 to 1200°C.

[0125] Examples of the metal plating layer having the melting point include a Zn-based plating layer and an Al-based plating layer. For welding members that require corrosion resistance, a Zn-based plating is superior to an Al-based plating. This is because the sacrificial anticorrosive action of zinc Zn can reduce the corrosion rate of the base steel sheet. Examples of Zn-based plating include hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), electrogalvanizing (EG), Zn-Ni-based plating (e.g., Zn-Ni-based plating containing 10 to 25 mass% Ni), Zn-Al-based plating, Zn-Mg-based plating, and Zn-Al-Mg-based plating. Examples of Al-based plating include Al-Si-based plating (e.g., Al-Si-based plating containing 10 to 20 mass% Si).

[0126] The amount of metal plating applied can be freely selected, but from the viewpoint of weldability, the amount of metal plating applied per side is 120 g / m 2 The lower limit of the coating weight of the metal plating layer per side is not particularly limited, but from the viewpoint of ensuring rust prevention, the coating weight per side is preferably 20 g / m 2 It is preferable that the amount is equal to or more than that.

[0127] As shown in FIG. 7, the steel sheet having a metal plating layer may have an Fe-based pre-plating layer 12 between the metal plating layer 11 and the base steel of the steel sheet. In this case, the pre-plating layer is preferably an Fe-based electroplating layer. As the Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be used. The composition of the Fe-based electroplating layer is not particularly limited. In the present invention, it is preferable that the composition contains one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in total of 10 mass% or less, with the balance being Fe and unavoidable impurities. By making the amount of elements other than Fe 10 mass% or less in total, it is possible to prevent a decrease in electrolysis efficiency and form an Fe-based electroplating layer at low cost.

[0128] In the present invention, the sheet combination is not particularly limited. For example, the sheet combination may be a combination of a plurality of the same steel sheets, or a combination of a plurality of different steel sheets. There is no problem even if the thicknesses of the steel sheets are different. The sheet combination may also be a combination of a steel sheet having a metal plating layer with a lower melting point than the steel sheet and a steel sheet having no metal plating layer. It is preferable that at least one of the sheets in the sheet combination is a galvannealed steel sheet.

[0129] Unlike LME cracking, cold cracking occurs even when surface-treated steel sheets having a metal plating layer are not used in the sheet assembly. In other words, even if all the steel sheets in the sheet assembly do not have a metal plating layer, the effects of the present invention can be effectively obtained as long as the above-mentioned conditions are met.

[0130] Next, an embodiment of a method for manufacturing a welded component according to the present invention will be described. Here, a method for manufacturing a welded component in which a plurality of steel plates are resistance spot welded together using the above-mentioned resistance spot welding method according to the present invention will be described.

[0131] [Method of manufacturing welded components] In the manufacturing method of the welded component of the present invention, the welded component can be manufactured through a manufacturing process including a first step of overlapping a plurality of steel plates to obtain a plate assembly, and a second step of resistance spot welding the obtained plate assembly by the above-mentioned resistance spot welding method.

[0132] <1st process> In this process, two or more steel plates are prepared and stacked vertically to form a plate assembly. Note that the steel plates included in the plate assembly have already been described, so their description will be omitted. Next, the second process is carried out.

[0133] <Second process> In the second step, the plate assembly prepared in the first step is joined. In this step, the plate assembly is clamped between a pair of welding electrodes arranged on the lower and upper sides of the plate assembly, and while applying pressure, a current application step and a holding step based on the above-mentioned resistance spot welding method are performed to form a weld between the steel plates, thereby joining the steel plates.

[0134] When welding is performed using the resistance spot welding method of the present invention, the occurrence of cracks in the welded portion can be suppressed. Therefore, according to the manufacturing method of the present invention, a welded component with reduced cracks in the welded portion can be manufactured. EXAMPLES

[0135] The operation and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0136] Using the sheet combination shown in Table 1, a welded joint (welded member) was produced under the welding conditions shown in Table 2. The sheet combination was arranged in the order of steel plate 1, steel plate 2, and steel plate 3 shown in Table 1, from the top, and overlapped. "None" in the "Type of plating" column in Table 1 refers to a steel plate (cold-rolled steel plate) that does not have the above-mentioned metal plating layer. The symbols in the "State immediately before pressure application" column in Table 2 correspond to (a) to (d) shown in the above-mentioned welding construction disturbance. In addition, a resistance welding machine consisting of a servo motor pressure type welding gun and a direct current inverter type welding power source was used as the welding device. The pair of electrode tips used was a chromium copper DR type electrode with a tip curvature radius R40 mm and tip diameter 6 mm.

[0137] The "tensile strength" column in Table 1 shows the tensile strength (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.

[0138] Furthermore, the column "Thickness of decarburized layer on steel plate surface" in Table 1 shows the value of the decarburized layer thickness (tc) (mm) per one side of the steel plate surface, which was determined by cutting the cross section of the steel plate with a microcutter, measuring the carbon content by electron beam microanalysis as described in JIS G 0558. From the viewpoint of measurement accuracy, when tc was less than 0.010 mm, tc was recorded as 0 mm in Table 1, and this value was used in the calculations of formulas (1) to (4).

[0139] In addition, the column "Amount of internal oxidation in the steel sheet surface layer" in Table 1 lists the amount of oxygen (Os) (g / m2) per unit area in the steel sheet surface layer on one side of the steel sheet, which was determined by the following method.2 ) was shown. For steel sheets without a metal plating layer (cold-rolled steel sheets), pickling was carried out in 5 mass% HCl to remove the outer oxide on the steel sheet surface. For steel sheets with a metal plating layer (GI steel sheets, GA steel sheets, EG steel sheets, etc.), the zinc plating layer was dissolved in a 20 mass% NaOH aqueous solution containing triethanolamine. After that, Os was measured as the amount of oxygen per unit area on one side of the steel sheet using the fusion infrared absorption method. From the viewpoint of measurement accuracy, it was determined that Os was less than 0.001 g / m 2 If it is less than 0g / m, set Os to 0g / m 2 These values ​​were used in the calculations of formulas (1) to (4).

[0140] In the "Total Plate Thickness" column of Table 2, the plate thickness of the steel plate before welding is measured, and the sum of the thicknesses is the total plate thickness of the plate assembly (t all ) (mm).

[0141] The "shortest distance Le" column in Table 2 shows the measurement results of the shortest distance Le (mm) from the center 8 of the weld point to the end face of the steel plate, as shown in FIG.

[0142] In some examples, as shown in FIG. 9, welding was performed in a state where there was one previously welded point 9 around the weld point 8. In this example, the "Center distance Lp between the weld point and the previously welded point" column in Table 2 shows the measurement results of the center distance Lp (mm) between the weld point 8 and the previously welded point 9. The welding conditions for the evaluation welding (welding point 8) and the previously welded point (previously welded point 9) were the same. When there was no previously welded point around the weld point to be evaluated, "none" was written in the "Center distance Lp between the weld point and the previously welded point" column in Table 2. This "when there is no previously welded point around the weld point to be evaluated" includes cases where there is no effect of welding because the distance between adjacent welding points is far.

[0143] The presence or absence of LME cracking and low temperature cracking was evaluated by the method described below.

[0144] The center of the welded portion of the obtained welded joint was cut with a microcutter, and then the cross-section of the welded portion was observed. The observation results were evaluated for the presence or absence of cracks according to the following criteria. Specifically, 10 welded joints were produced under each of the welding conditions shown in Table 2, and cross-sectional observations were made of the weld heat-affected zone on the mating surface side between the steel plates to confirm LME cracking and low-temperature cracking. <Evaluation criteria> A: All 10 pieces are crack-free B: The number of cracks is two or less, and the maximum crack depth is less than 100 μm. C: Three joints had cracks, and the maximum crack depth was less than 100 μm. D: Four joints had cracks, and the maximum crack depth was less than 100 μm. F: 5 or more joints have cracks, or the maximum crack depth is 100 μm or more Here, if the evaluation result was any of A to D, it was evaluated as passing.

[0145] The obtained values ​​and evaluation results are shown in Table 2. As is clear from Table 2, the evaluation results of all of the inventive examples were either A to D. It was found that according to the present invention, it is possible to achieve both reduction in LME cracking and reduction in low temperature cracking on the mating surface side of the steel sheet surface in the heat-affected zone.

[0146] [Table 1]

[0147] [Table 2] [Explanation of symbols]

[0148] 1, 2 steel plate 4, 5 Welding electrodes 6 Welding 7 Crack 8 Welding points 9 Already welded points 10 Welding materials 11 Metal plating layer 12 Pre-plating layer 13 Decarburized layer 14 Internal oxide layer

Claims

1. A resistance spot welding method in which a sheet set in which a plurality of steel sheets are overlapped is sandwiched between a pair of welding electrodes, and electric current is passed through the sheet set while applying pressure to join the steel sheets, comprising the steps of: a holding step of holding the welding electrode after the energization is terminated, In the holding step, The pressure holding time is defined as H (ms), the C content, Si content and Mn content of the steel sheet are defined as C (mass%), Si (mass%) and Mn (mass%), respectively, the thickness of the decarburized layer per one side of the surface layer of the steel sheet is defined as tc (mm), the amount of internal oxidation per one side of the surface layer of the steel sheet is defined as Os (g / m ２ ), the calculated value X1 of the steel plate having the maximum value among the calculated values ​​X1 of the formula (1) for each steel plate is defined as X1max, and the calculated value X2 of the steel plate having the minimum value among the calculated values ​​X2 of the formula (2) for each steel plate is defined as X2min. a pressure holding time satisfying X1max<H<X2min. 100×{(C-tc)+(Si-2×Os) / 5+Mn / 40}=X1...(1) 2000-600×{(C-tc)+(Si-2×Os) / 20+Mn / 40}=X2…(2)

2. The holding step further comprises: The pressure holding time is defined as H (ms), the C content, Si content and Mn content of the steel sheet are defined as C (mass%), Si (mass%) and Mn (mass%), respectively, the thickness of the decarburized layer per one side of the surface layer of the steel sheet is defined as tc (mm), the amount of internal oxidation per one side of the surface layer of the steel sheet is defined as Os (g / m ２ ) and the total thickness of the overlapped steel plates is t all (mm), the calculated value Y1 of the steel plate having the maximum value among the calculated values ​​Y1 of the formula (3) for each steel plate is Y1max, and the calculated value Y2 of the steel plate having the minimum value among the calculated values ​​Y2 of the formula (4) for each steel plate is Y2min, The resistance spot welding method according to claim 1 , wherein the pressure holding time satisfies Y1max<H<Y2min. (1+0.1×t) all )×[100×{(C-Mc)+(Si-2×Os) / 5+Mn / 40}]=Y1 …(3) (1-0.8 / t all )×[2000-600×{(C-tc)+(Si-2×Os) / 20+Mn / 40}]=Y2 …(4)

3. During the energization, The shortest distance Le from the center of the welding point to the end face of the steel plate is more than 3.0 mm, 2. The resistance spot welding method according to claim 1, further comprising the steps of: when there is one or more previously welded points around a welding point, resistance spot welding is performed with a center-to-center distance Lp between the welding point and the previously welded point closest to the welding point being set to 6.0 mm or more.

4. During the energization, The shortest distance Le from the center of the welding point to the end face of the steel plate is more than 3.0 mm, 3. The resistance spot welding method according to claim 2, further comprising the steps of: when there is one or more previously welded points around a welding point, resistance spot welding is performed with a center-to-center distance Lp between the welding point and the previously welded point closest to the welding point being set to 6.0 mm or more.

5. A first step of stacking a plurality of steel plates to obtain a plate assembly; A second step of resistance spot welding the obtained plate assembly by the resistance spot welding method according to any one of claims 1 to 4; The method for manufacturing a welded member includes the steps of:

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