Resistance spot welding method

Abstract

The purpose of the present invention is to provide a resistance spot welding method. The present invention is a resistance spot welding method for two or more steel sheets including at least one surface-treated steel sheet having a metal plating layer, the method comprising a cooling step, a post-energization step, and a holding step after a main energization step for forming a welding spot, the cooling time Ct in the cooling step being 140 ms or more, and the post-energization step being less than the cooling time Ct in the holding step being less than the cooling time Ct in the post-energization step. The holding time Ht (ms) in the holding step satisfies the following formula (1) or formula (2). # imgabs 0 #, # imgabs 1 # imgabs 2 # in # imgabs 3 #, # imgabs 4 # in # imgabs 0 #, # imgabs 2 # in # imgabs 3 #, # imgabs 4 #

Description

Technical Field

[0001] The present invention relates to a method of resistance spot welding. Background Art

[0002] Generally, a resistance spot welding method, which is a kind of overlapping resistance welding method, is used for joining steel plates that overlap two or more steel plates. As Figure 1 shown, this welding method is as follows: taking two overlapping steel plates 1 and 2 as a plate group, clamping the upper and lower sides of this plate group by a pair of welding electrodes 3 and 4, applying pressure from the upper and lower sides of this plate group by the pair of welding electrodes 3 and 4, and simultaneously passing a welding current between the upper and lower electrodes to join the steel plates to each other. Using the resistance heat generated by flowing the welding current, a dot-shaped welded portion 5 is obtained. This dot-shaped welded portion 5 is called a weld nugget, and is a portion where the two steel plates 1 and 2 melt and solidify at the contact portion of the steel plates when the welding current flows through the overlapping steel plates, whereby the steel plates are joined to each other in a dot shape. In addition, in Figure 1 an example of a structure in which two steel plates are overlapped is shown.

[0003] However, in the resistance spot welding of a plate group in which multiple steel plates including surface-treated steel plates are overlapped, there is a problem that cracks sometimes occur in the welded portion. Here, the surface-treated steel plate refers to a steel plate having a metal coating such as galvanizing represented by electro-galvanizing or hot-dip galvanizing (this hot-dip galvanizing includes alloyed hot-dip galvanizing) or a galvanized alloy containing elements such as aluminum and magnesium in addition to zinc on the surface of a base material (also called a base steel plate). The melting point of galvanizing or galvanized alloy is lower than that of the base material, so there is a problem of cracks in the welded portion as follows.

[0004] Moreover, the cracks in the welded portion are considered to be cracks (LME cracks) caused by so-called liquid metal embrittlement in which the low-melting-point metal coating on the surface of the steel plate melts during the welding process, and when the compressive stress generated by the electrode and the tensile stress generated by the thermal expansion and contraction of the steel plate are applied to the welded portion, the molten low-melting-point metal invades the grain boundaries of the base material of the surface-treated steel plate and reduces the grain boundary strength, causing cracks.

[0005] As Figure 2 shown, the generation positions of such cracks are various, such as the surfaces of the steel plates 1 and 2 on the side in contact with the welding electrodes 3 and 4 and the surfaces of the steel plates 1 and 2 on the side where the steel plates are in contact with each other.

[0006] As a countermeasure against such a problem, for example, the technique described in Patent Document 1 can be cited. In Patent Document 1, the following method is proposed: in spot welding of high-strength plated steel sheets, generation of cracks is suppressed by adjusting the welding energization time and the holding time after welding energization to a prescribed time or more. However, when the technique described in Patent Document 1 is applied to high-strength steel sheets with a tensile strength of 980 MPa or more, there is a problem that the toughness is reduced due to rapid cooling of the welded portion during the long holding time, and thus the cross-tensile strength (CTS) of the joint is reduced.

[0007] As a countermeasure against such a problem of CTS reduction, for example, the technique described in Patent Document 2 can be cited. In Patent Document 2, the following method is proposed: in addition to the main energization for welding, pre-energization or post-energization is provided, and upper and lower limits are defined within an appropriate range of the holding time, thereby suppressing cracks during welding and ensuring CTS.

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-103377

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-47476

[0010] However, in the technique described in Patent Document 2, for joints with a relatively small total plate thickness, the appropriate range of the holding time is narrow, and there is a case where it is difficult to manage the welding conditions within an appropriate range during actual construction. Summary of the Invention

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide a resistance spot welding method capable of stably suppressing cracks during welding and ensuring CTS of a joint in a surface-treated steel sheet.

[0012] The inventors of the present invention have repeatedly conducted careful studies in order to achieve the above object.

[0013] LME cracks can be suppressed by adjusting the holding time after welding energization to a prescribed time or more. However, an increase in the holding time results in a reduction in CTS, and from the viewpoint of ensuring CTS, it is preferable that the holding time be short. Due to these two problems, there is a problem that it is difficult to control an appropriate holding time during actual construction.

[0014] The inventors of the present invention have found that by providing a cooling process having a cooling time of a prescribed time or more after a main energization process for forming a solder joint, (i) it is possible to promote solidification of molten zinc and restore the toughness of the welded portion by providing a post-energization process after the cooling process, and (ii) it is possible to ensure a wider appropriate range of the holding time in the holding process after the post-energization process, and thus it is possible to stably suppress LME cracks and ensure CTS even during actual construction.

[0015] In this process, the following points can be cited as the effects of the cooling time.

[0016] · From the viewpoint of suppressing LME cracks: Promote the solidification of the molten plating layer and prevent the re-melting of zinc in the subsequent post-electrification process.

[0017] · From the viewpoint of ensuring CTS: Lower the temperature of the welded part and enhance the effect of toughness recovery brought about by tempering in the subsequent post-electrification process.

[0018] As described above, in order to stably suppress LME cracks while ensuring CTS, the control of the cooling process is particularly important. Specifically, it is important to set the cooling time in the cooling process to a specified time or more, and adjust the holding time in the holding process according to the set cooling time. In addition, the inventors of the present invention have found that by setting the cooling time to be longer, an appropriate range of the holding time can be ensured more widely.

[0019] The present invention has been completed based on the above insights, and its gist is as follows.

[0020] [1] A resistance spot welding method, in which two or more steel sheets including at least one surface-treated steel sheet having a metal plating layer are overlapped to form a sheet group, and the sheet group is clamped by a pair of welding electrodes and energized while applying pressure to perform joining, wherein,

[0021] After the main electrification process for forming a weld spot, there are a cooling process, a post-electrification process, and a holding process,

[0022] The cooling time Ct (ms) in the above cooling process is 140 ms or more,

[0023] The holding time Ht (ms) in the above holding process satisfies the following formula (1) or (2).

[0024] When Ct ≤ 2 × D × √TS - 80, D × √TS - 0.5 × Ct < Ht ≤ 2700 × D / √TS + Ct... (1)

[0025] When Ct > 2 × D × √TS - 80, 40 < Ht ≤ 2700 × D / √TS + Ct... (2)

[0026] Here, in the formulas (1) and (2), D is the total thickness (mm) of the above sheet group, and TS is the tensile strength (MPa) of the steel sheet having the maximum tensile strength in the above sheet group.

[0027] [2] The resistance spot welding method according to [1], wherein,

[0028] The current value Ip (kA) and the energization time Pt (ms) in the subsequent energization process described above satisfy the following formulas (3) and (4).

[0029] 0.5×Im ≤ Ip ≤ 1.1×Im…(3)

[0030] 150×Im / Ip ≤ Pt ≤ 700×Im / Ip…(4)

[0031] Here, Im shown in formulas (3) and (4) is the current value (kA) in the above-mentioned main energization process.

[0032] [3] According to the resistance spot welding method described in [1] or [2], wherein

[0033] Before applying pressure using the above-mentioned welding electrodes, one or more of the following states (a) to (e) are satisfied.

[0034] (a) A state where the welding electrode and the overlapping steel plates have a chamfer angle of 0.2 degrees or more

[0035] (b) A state where a pair of welding electrodes are eccentric by 0.1 mm or more

[0036] (c) A state where there is a gap of 0.5 mm or more between any one of the welding electrodes and the overlapping steel plates

[0037] (d) A state where there is a gap of 0.5 mm or more between at least one set of the overlapping steel plates

[0038] (e) A state where the shortest distance from the center of the welding dot to the end face of the overlapping steel plates is 10 mm or less

[0039] [4] According to the resistance spot welding method described in any one of [1] to [3], wherein

[0040] The tensile strength of at least one of the overlapping steel plates is 980 MPa or more.

[0041] According to the present invention, it is possible to stably suppress the generation of cracks during welding in the surface-treated steel plate while ensuring CTS, and thus it has a remarkable effect in the industry. Description of the Drawings

[0042] Figure 1 is a cross-sectional view schematically showing an example of the resistance spot welding of the present invention.

[0043] Figure 2 is a cross-sectional view schematically showing the cracks generated in the resistance spot welding part.

[0044] Figure 3It is a diagram schematically showing the state in which the welding electrode in the present invention has a fillet with the overlapping steel plates.

[0045] Figure 4 It is a diagram schematically showing the state in which a pair of welding electrodes in the present invention are eccentric.

[0046] Figure 5 It is a diagram schematically showing the state in which there is a gap between the welding electrode and the overlapping steel plates in the present invention.

[0047] Figure 6 It is a diagram schematically showing the state in which there is a gap between the overlapping steel plates in the present invention.

[0048] Figure 7 It is a diagram for explaining the distance from the center of the welding spot to the end face of the steel plate in the present invention. Detailed Embodiments

[0049] Hereinafter, the resistance spot welding method of the present invention will be described with reference to the respective drawings. In addition, the present invention is not limited to this embodiment.

[0050] The present invention is a resistance spot welding method in which two or more steel plates are overlapped to form a plate group, and the plate group is clamped by a pair of welding electrodes and energized under the welding conditions described below while applying pressure to form a welding spot and join the steel plates to each other. The above plate group includes at least one surface-treated steel plate having a metal coating. The number of overlapped steel plates is preferably 4 or less.

[0051] The present invention is applicable to a resistance spot welding method for a plate group, wherein at least one of the plurality of steel plates constituting the plate group is a surface-treated steel plate having a metal coating. In addition, it is preferably applied to a steel plate in which the melting point of the metal coating is lower than the melting point of the base material of the surface-treated steel plate.

[0052] At least one of the plurality of steel plates constituting the plate group is preferably a high-strength steel plate having a tensile strength of 980 MPa or more. This is because high-strength steel plates are highly sensitive to cracks, and crack generation during welding is likely to become a problem. Therefore, the effects of the present invention can be further exerted. The tensile strength is more preferably 1180 MPa or more. In addition, the upper limit of the tensile strength of the steel plate is not particularly specified, but the tensile strength is preferably 2500 MPa or less.

[0053] In the present invention, the plate thickness of the steel plate for which resistance spot welding is performed is not particularly limited. For example, it is preferably in the range of 0.5 mm or more and 3.0 mm or less. Steel plates with a plate thickness in this range can be appropriately used as automotive components.

[0054] For example, as Figure 1As shown, in the case of performing resistance spot welding on a stack of two steel plates, the overlapping steel plates (upper steel plate 1 and lower steel plate 2) are clamped by a pair of welding electrodes, namely, an electrode disposed on the lower side of the stack of plates (hereinafter referred to as the lower electrode) 4 and an electrode disposed on the upper side of the stack of plates (hereinafter referred to as the upper electrode) 3, and current is applied while applying pressure.

[0055] The device for implementing the resistance spot welding method of the present invention only needs to be a structure in which the lower electrode and the upper electrode apply pressure and control the pressing force thereof, and this structure is not particularly limited. For example, devices such as cylinders and servo motors can be used. In addition, the structure for supplying current and controlling the current value during power-on is not particularly limited, and both direct current and alternating current can apply the present invention. Further, in the case of alternating current, "current" means "effective current".

[0056] In addition, the form of the front ends of the lower electrode and the upper electrode is not particularly limited. For example, the DR shape (dome arc shape), R shape (arc shape), D shape (dome shape), etc. described in JIS C 9304:1999 can be cited. The front end diameter of each electrode is, for example, 4 mm to 16 mm. The radius of curvature is, for example, 10 mm to 400 mm, and a Flat type electrode with a flat front end can also be used.

[0057] In this way, while applying pressure and applying current in a state where the overlapping steel plates (stack of plates) are clamped by a pair of welding electrodes, a solder joint of a required size is formed by resistance heating, and the overlapping steel plates are joined, thereby obtaining a welded joint. Moreover, in the present invention, the current application is performed in a specific mode described later.

[0058] In addition, the two or more steel plates on which resistance spot welding is performed may be steel plates of the same type and the same shape, or steel plates of different types and different shapes. Further, a surface-treated steel plate having a metal coating may be overlapped with a steel plate without a metal coating.

[0059] Next, the current application mode in the resistance spot welding method of the present invention will be described.

[0060] The current application mode in the present invention has a cooling process, a post-current application process, and a holding process after the main current application process. These processes are controlled to satisfy the welding conditions described below.

[0061] <Main current application process>

[0062] The main energization process is a process aimed at forming solder joints (molten parts). The energization conditions and pressing conditions for forming solder joints are not particularly limited. Conditions that can ensure the solder joint diameter of the required size can be appropriately selected. In addition, the solder joint diameter of the required size varies depending on the steel type. For example, it is preferable that the current in the main energization process is 1.0 kA or more and 15 kA or less, and the pressing force is 2.0 kN or more and 7.0 kN or less. Additionally, the energization time in the main energization process is 100 ms or more and 1000 ms or less.

[0063] <Cooling process>

[0064] After the main energization process, a cooling process is performed. As described above, in the present invention, in order to stably suppress LME cracks while ensuring CTS, the control of the cooling process is particularly important.

[0065] In the present invention, the cooling time Ct (ms) in the cooling process is set to 140 ms or more.

[0066] When the cooling time is less than 140 ms, the cooling is insufficient. As a result, the effect of solidifying the molten plating cannot be obtained sufficiently. That is, the effect (specifically, preventing the re-melting of zinc in the subsequent post-energization process) from the viewpoint of suppressing the above-mentioned LME cracks cannot be obtained. In addition, as a result of insufficient cooling, the effect of improving the toughness recovery brought about by tempering in the subsequent post-energization process (that is, the effect from the viewpoint of ensuring the above-mentioned CTS) cannot be obtained sufficiently.

[0067] The upper limit of the cooling time is not particularly limited, and it is preferably 2000 ms or less. This is because if the cooling time exceeds 2000 ms, the total time of the entire welding process becomes long and the productivity decreases. The cooling time is more preferably 1000 ms or less, and further preferably 800 ms or less.

[0068] In addition, in order to more significantly exert the effect of promoting the solidification of the molten plating and improve the effect of toughness recovery brought about by tempering in the post-energization process, the cooling time is preferably 240 ms or more.

[0069] <Post-energization process>

[0070] After the cooling process, a post-energization process is performed. As described above, in the present invention, as long as the cooling process and the subsequent holding process are adjusted to appropriate conditions, the effects of the present invention can be obtained. In order to more significantly exert the effects from the viewpoints of suppressing LME cracks and ensuring CTS, in addition to the cooling process and the holding process, it is also effective to adjust the conditions of the post-energization process as follows.

[0071] In the post-energization process after the cooling process, the current value Ip (kA) and the energization time Pt (ms) in the post-energization process preferably satisfy the following formulas (3) and (4).

[0072] 0.5×Im ≤ Ip ≤ 1.1×Im…(3)

[0073] 150×Im / Ip ≤ Pt ≤ 700×Im / Ip…(4)

[0074] Here, Im shown in formulas (3) and (4) is the current value (kA) in the main energization process.

[0075] In the case where Ip < 0.5×Im or Pt < 150×Im / Ip, the heat input is insufficient and the toughness recovery effect brought by tempering cannot be obtained. As a result, there is a possibility of CTS reduction.

[0076] In the case where Ip > 1.1×Im or Pt > 700×Im / Ip, the heat input is too large and exceeds the appropriate temperature range for obtaining the tempering effect, and the toughness recovery effect cannot be obtained. As a result, there is a possibility of CTS reduction. In addition, since it becomes high temperature and the coating melts, there is a possibility of not obtaining the effect of suppressing LME cracks.

[0077] <Holding process>

[0078] After the post-energization process, a holding process of maintaining the pressing force without energization is performed. As described above, in the present invention, it is important to set the cooling time in the cooling process to a prescribed time or more and adjust the holding time in the subsequent holding process according to the set cooling time.

[0079] In the holding process after the post-energization process, the holding time Ht (ms) in the holding process is set to satisfy the following formula (1) or (2).

[0080] In the case where Ct ≤ 2×D×√TS - 80, D×√TS - 0.5×Ct < Ht ≤ 2700×D / √TS + Ct…(1)

[0081] In the case where Ct > 2×D×√TS - 80, 40 < Ht ≤ 2700×D / √TS + Ct…(2)

[0082] Here, in formulas (1) and (2), D is the total plate thickness (mm) of the plate group, TS is the tensile strength (MPa) of the steel plate with the maximum tensile strength in the plate group. In addition, Ct is the cooling time (ms) in the cooling process.

[0083] That is, in the relationship between the cooling time (Ct) in the cooling process, the total plate thickness (D) of the plate group, and the above-mentioned tensile strength (TS), when Ct ≤ 2×D×√TS - 80, the holding time (Ht) is set in a manner that satisfies formula (1). On the other hand, when Ct > 2×D×√TS - 80, the holding time (Ht) is set in a manner that satisfies formula (2).

[0084] When Ct ≤ 2×D×√TS - 80 and Ht ≤ D×√TS - 0.5×Ct, and when Ct > 2D×√TS - 80 and Ht ≤ 40, when the welding electrodes are opened after the holding process, it becomes a form where tensile stress is applied before the plating is fully solidified, and LME cracks are likely to occur.

[0085] When Ct ≤ 2×D×√TS - 80 and Ht > 2700×D / √TS + Ct, and when Ct > 2×D×√TS - 80 and Ht > 2700×D / √TS + Ct, the holding time is too long and the welded part is rapidly cooled. As a result, there is a concern that CTS may decrease due to reduced toughness.

[0086] By welding in the energization mode described above, it is possible to stably suppress crack generation during welding in the surface-treated steel sheet while ensuring the CTS of the obtained welded joint.

[0087] In addition, the pressing force is the same pressing force in all processes.

[0088] <Welding construction interference>

[0089] LME cracks are likely to occur in the presence of welding construction interference during welding. Therefore, in the present invention, in addition to adjusting to the above-mentioned energization mode, for at least one welding dot, it is preferable to satisfy one or more of the following (a) to (e) states immediately before pressing with the welding electrodes. Thereby, the effects of the present invention can be obtained more effectively.

[0090] (a) A state where the welding electrode and the overlapping steel plates have a welding angle of 0.2 degrees or more

[0091] (b) A state where a pair of welding electrodes are eccentric by 0.1 mm or more

[0092] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0093] (d) A state where there is a gap of 0.5 mm or more between at least one set of the overlapping steel plates

[0094] (e) The state where the shortest distance from the center of the welding dot to the end face of the overlapping steel plates is 10 mm or less

[0095] These welding construction interferences all cause local increases in the temperature and / or tensile stress of the welded part when the electrodes are open, thus becoming a state prone to LME cracks. However, by rationalizing the cooling time and holding time using the present invention, even in the presence of these welding construction interferences, LME cracks can be suppressed, and the margin for construction interference management during component manufacturing is increased.

[0096] Hereinafter, Figures 3 to 7 the details of each construction interference will be described. Figures 3 to 7 A view showing the welding electrodes and the plate group in side view. In Figures 3 to 7 the upper electrode 3 represents the movable-side electrode, and the lower electrode 4 represents the fixed-side electrode.

[0097] (a) The state where the welding electrodes and the overlapping steel plates have a welding angle of 0.2 degrees or more

[0098] As Figure 3 shown, the welding angle means the angle at which the welding electrodes 3, 4 are inclined with respect to the steel plates 1, 2, that is, "the angle formed by the electrode pressing direction and the steel plate thickness direction". If the welding angle is large, bending stress is applied to the welded part, and large compressive plastic deformation is locally generated, resulting in an increase in the tensile stress after the cooling process. When the welding angle is 0.2 degrees or more, the effects of the present invention can be effectively obtained. When the welding angle is too large, the formation of the solder joint becomes unstable and becomes a cause of splash generation. Therefore, the welding angle is preferably 10 degrees or less.

[0099] (b) The state where a pair of welding electrodes is eccentric by 0.1 mm or more

[0100] As Figure 4 shown, eccentricity means the state where the central axes of a pair of welding electrodes 3, 4 do not coincide. Similar to the above-mentioned welding angle, if the eccentricity is large, bending stress is applied to the welded part, and LME cracks are likely to occur. When the eccentricity is 0.1 mm or more, the effects of the present invention can be effectively obtained. When the eccentricity is too large, the formation of the solder joint becomes unstable and becomes a cause of splash generation. Therefore, the eccentricity is preferably 5 mm or less.

[0101] (c) The state where there is a gap of 0.5 mm or more between any one of the welding electrodes and the overlapping steel plates

[0102] In the state where there is a gap between any one of the welding electrodes and the steel plate just before starting pressurization, for example, when one electrode is movable (hereinafter, the movable-side electrode) and the other electrode is fixed (hereinafter, the fixed-side electrode), as Figure 5As shown, when the movable-side electrode 3 starts to apply pressure while there is a gap between the fixed-side electrode 4 and the steel plate 2, the steel plate undergoes bending deformation. As a result, bending stress is applied to the welded portion, making it prone to LME cracks. When the gap amount (i.e., electrode-plate gap) is 0.5 mm or more, the effects of the present invention can be effectively obtained. When the gap amount is too large, the formation of the solder joint becomes unstable, causing splashing. Therefore, the gap amount is preferably 5 mm or less.

[0103] (d) In a state where there is a gap of 0.5 mm or more between at least one set of overlapping steel plates

[0104] Similar to the above (c), as Figure 6 shown, before starting to apply pressure, in a state where there is a gap between any two steel plates (in the example shown in Figure 6 the upper steel plate 1 and the lower steel plate 2), the steel plate undergoes bending deformation, so bending stress is applied to the welded portion, making it prone to LME cracks. When the gap amount (i.e., plate-plate gap) is 0.5 mm or more, the effects of the present invention can be effectively obtained. When the gap amount is too large, the formation of the solder joint becomes unstable, causing splashing. Therefore, the gap amount is preferably 4 mm or less.

[0105] (e) In a state where the shortest distance from the center of the welding dot to the end face of the overlapping steel plates is 10 mm or less

[0106] If the shortest distance from the center of the welding dot shown in Figure 7 to the end face of the steel plate (i.e., the end of the steel plate) is small, heat conduction from the welded portion is blocked at the end face of the steel plate, so there may be a situation where the cooling rate of the welded portion is excessively reduced. Therefore, since the temperature increases when the electrode is open, it is prone to LME cracks. When the shortest distance from the center of the welding dot to the end face of the steel plate is 10 mm or less, the effects of the present invention can be effectively obtained. In addition, when this distance is less than 3 mm, splashing during welding becomes significant, and the solder joint diameter is prone to deviation, making the strength of the welded portion unstable. Therefore, this distance is preferably 3 mm or more.

[0107] In the above description, the case of performing resistance spot welding on two overlapping steel plates has been mainly described, but the present invention can also be applied to the case of welding three or more overlapping steel plates, and the above effects can also be obtained.

[0108] Examples

[0109] Hereinafter, the functions and effects of the present invention will be described using examples. In addition, the present invention is not limited to the following examples.

[0110] In this embodiment, two steel sheets including a galvanized steel sheet as a surface-treated steel sheet were used, and the steel sheets were overlapped to form a sheet group (see Table 1). Here, as the upper steel sheet and the lower steel sheet, an alloyed hot-dip galvanized (GA) steel sheet and a cold-rolled steel sheet whose tensile strength and sheet thickness of the steel sheet satisfied the conditions shown in Table 1 were used.

[0111] The sheet group was spot-welded under the conditions shown in Table 2-1 and Table 2-2 to produce welded joints. In addition, the symbols shown in the "Construction interference during LME evaluation" column of Table 2-1 correspond to (a) to (e) shown in the above-mentioned welding construction interference.

[0112] The spot welding was carried out at room temperature and in a state where the welding electrodes (lower electrode, upper electrode) were always water-cooled. Both the lower electrode and the upper electrode were DR-shaped electrodes made of chromium-copper with a front-end diameter (i.e., the front-end diameter) of 6 mm and a curvature radius of 40 mm. In addition, the pressing force was controlled by driving the upper electrode with a servo motor, and a DC power supply was supplied during energization.

[0113] Based on the following judgment criteria, the presence or absence of LME cracks and CTS of the obtained welded joints were evaluated.

[0114] 〔Evaluation of cracks〕

[0115] Regarding the evaluation of cracks (i.e., LME cracks), the LME cracks between the plates were confirmed based on the production of 10 welded components. Specifically, welding was carried out in a state where one or more of the welding construction interferences satisfying the above (a) to (e) were present, and the center of the welded part of the obtained welded joint was cut with a micro-cutting machine, and then the cross-section of the welded part was observed. Based on the observation results, the presence or absence of LME crack generation was evaluated according to the following criteria.

[0116] <Judgment criteria>

[0117] A: None of the 10 have cracks

[0118] B: The number of welded joints with cracks is 2 or less, and the maximum value of the crack depth is less than 100 μm

[0119] F: The number of welded joints with cracks is 3 or more, or the maximum value of the crack depth is 100 μm or more

[0120] Here, when the evaluation results are A and B, the evaluation is qualified.

[0121] 〔Evaluation of CTS〕

[0122] The evaluation of the cross-tensile strength (CTS) was carried out based on the cross-tensile test method (JIS Z3137). In the cross-tensile test, cross-tensile test pieces were cut out from each steel plate shown in Table 1, and welded joints (cross-tensile joints) were produced by performing resistance spot welding under the plate groups shown in Table 1 and the welding conditions shown in Tables 2-1 and 2-2. The CTS was measured based on the production of 3 cross-tensile joints, and the average value of the 3 CTSs was obtained. The obtained average value was compared with the CTS target value of each plate group shown in Table 1, and the evaluation was carried out according to the following criteria.

[0123] <Judgment Criteria>

[0124] A: The average value of CTS exceeds 120% of the CTS target value

[0125] B: The average value of CTS is in the range of 100 - 120% of the CTS target value

[0126] F: The average value of CTS is less than the CTS target value

[0127] In addition, the evaluation of CTS was carried out without applying welding construction interference.

[0128] [Judgment]

[0129] Regarding the judgment, the condition where the evaluation results of both the LME crack and the CTS are A or B was evaluated as "〇 (qualified)", and the condition where any one or both of the evaluation results are F was evaluated as "× (unqualified)". The evaluation results are shown in Table 2-2.

[0130] [Table 1]

[0131]

[0132] [Table 2-1]

[0133]

[0134] [Table 2-2]

[0135]

[0136] According to Table 2-2, all the evaluation results of the examples of the present invention are "〇". That is, in the examples of the present invention, welded joints that stably suppress crack generation during welding and ensure CTS were obtained in the surface-treated steel plates.

[0137] Explanation of Reference Numerals

[0138] 1... upper steel plate; 2... lower steel plate; 3... upper electrode; 4... lower electrode; 5... welding point.

Claims

1. A resistance spot welding method, wherein two or more steel sheets including at least one surface-treated steel sheet with a metal coating are overlapped to form a sheet group, and the sheet group is clamped by a pair of welding electrodes and energized while applying pressure to perform bonding. The resistance spot welding method is characterized in that after the main energization process for forming a weld spot, there are a cooling process, a post-energization process, and a holding process. The cooling time Ct (ms) in the cooling process is 140 ms or more. The holding time Ht (ms) in the holding process satisfies the following formula (1) or (2). When Ct ≤ 2×D×√TS - 80, D×√TS - 0.5×Ct < Ht ≤ 2700×D / √TS + Ct...(1) When Ct > 2×D×√TS - 80, 40 < Ht ≤ 2700×D / √TS + Ct...(2) Here, in formulas (1) and (2), D is the total thickness (mm) of the sheet group, and TS is the tensile strength (MPa) of the steel sheet with the maximum tensile strength in the sheet group.

2. The resistance spot welding method according to claim 1, characterized in that the current value Ip (kA) and the energization time Pt (ms) in the post-energization process satisfy the following formulas (3) and (4). 0.5×Im ≤ Ip ≤ 1.1×Im...(3) 150×Im / Ip ≤ Pt ≤ 700×Im / Ip...(4) Here, Im in formulas (3) and (4) is the current value (kA) of the main energization process.

3. The resistance spot welding method according to claim 1 or 2, characterized in that before applying pressure using the welding electrodes, one or more of the following states (a) to (e) are satisfied. (a) A state where the welding electrodes and the overlapped steel sheets have an included angle of 0.2 degrees or more. (b) A state where a pair of welding electrodes are eccentric by 0.1 mm or more. (c) A state where there is a gap of 0.5 mm or more between any one of the welding electrodes and the overlapped steel sheets. (d) A state where there is a gap of 0.5 mm or more between at least one set of the overlapped steel sheets. (e) A state where the shortest distance from the weld spot center to the end face of the overlapped steel sheets is 10 mm or less.

4. The resistance spot welding method according to claim 1 or 2, characterized in that the tensile strength of at least one of the overlapped steel sheets is 980 MPa or more.

5. The resistance spot welding method according to claim 3, characterized in that the tensile strength of at least one of the overlapped steel sheets is 980 MPa or more.

Images