RESISTANCE SPOT WELDING METHOD AND METHOD FOR PRODUCING A RESISTANCE SPOT WELDING JOINT

Abstract

An object of this invention is to provide a resistance spot welding method and a method for producing a resistance spot weld joint. The present invention provides a resistance spot welding method for joining two or more steel sheets, including at least one steel sheet with a tensile strength of 980 MPa or higher. The resistance spot welding method consists of stacking the steel sheets one on top of the other to form a steel sheet assembly to be welded, clamping the steel sheet assembly with a pair of electrodes, and passing a current through the steel sheets while applying pressure to join them. The resistance spot welding method includes an initial welding step in which a current I1 (kA) is passed, satisfying 2 x vF1 < l1 = 10 x vF1, while a welding force F1 (kN) is applied, satisfying 0.2 x vt1 < F1 = 4 x vt1, and a main welding pass where a nugget with a predetermined nugget diameter is formed. Spatter occurs in the initial welding pass.

Description

RESISTANCE SPOT WELDING METHOD AND METHOD FOR PRODUCING A RESISTANCE SPOT WELDING JOINT Technical field The present invention relates to a resistance spot welding method and to a method for producing a resistance spot weld joint. Background of the Technique Resistance spot welding is widely used in vehicle body assembly, such as for automobiles. Assembling a single body requires resistance spot welding at thousands of points. Resistance spot welding involves passing a current through two or more steel sheets placed one on top of the other and held together by a pair of welding electrodes positioned on the top and bottom of the steel sheets, respectively, while applying pressure. This forms a predetermined-sized nugget at the interface of the steel sheets, bonding them together and producing a welded joint. In recent years, from an environmental protection standpoint, there has been a demand to reduce CO2 emissions from automobiles. In an effort to achieve lighter weight and thus improve fuel efficiency, high-strength steel sheets have been used to reduce the thickness of car body walls. The increased strength of high-strength steel sheets is generally achieved by adding various alloying elements, as well as a large amount of carbon, which results in greater susceptibility to hydrogen embrittlement.In resistance spot welding, for example, rust-preventive oil, moisture, and a coated layer on the surface of the steel sheet enter the weld metal (molten portion) during the melting and solidification process in the weld, and remain, after cooling, as a source of hydrogen that causes delayed fracture. Therefore, when high-strength steel sheets are resistance spot welded, the weld of the resulting joint may suffer delayed fracture due to the ingress of hydrogen, during welding, into the weld metal which is highly sensitive to hydrogen embrittlement. The following patent literature discloses methods for preventing delayed fracture in welds. For example, patent literature 1 discloses a technique in which, immediately after the welding current is applied (main weld), the welding force is increased and the current is decreased to control the residual stress in the welds and thus prevent delayed fracture. Also, for example, patent literature 2 discloses a technique in which the welding force is increased immediately after the welding current is applied (main weld), and the current is applied after a cooling period during which no current is applied, such that the microstructure and hardness of the welds are controlled, thus preventing delayed fracture. List of references Patent Literature PTL 1: Publication of unexamined Japanese patent application no. 2015-93282 PTL 2: International Publication No. 2014 / 171495 Brief description of the invention Technical problem As previously described, resistance spot welding of high-strength steel sheets presents the problem of hydrogen entering the weld metal. Therefore, in resistance spot welding of high-strength steel sheets, it is important not only to improve the strength of the weld joint but also to reduce the amount of residual hydrogen in the weld to prevent delayed fracture. However, the techniques disclosed in patent literature 1 and 2 do not involve reducing the amount of hydrogen in the weld to prevent delayed fracture. Furthermore, these techniques have drawbacks in that if the welding force is excessively increased when the nugget is molten immediately after the welding current is applied, the thickness of the sheet in the weld tends to decrease, which can reduce the strength of the resulting weld joint or affect its appearance. The problem of delayed fracture caused by the ingress of hydrogen during welding in weld metal that is highly sensitive to hydrogen embrittlement exists not only in the case of resistance spot welding of high-strength automotive steel sheets, but also in the case of resistance spot welding of other types of steel sheets. The present invention has been made in view of the problems described above. An object of the present invention is to provide a resistance spot welding method and a method for producing a resistance spot weld joint that allows for the stable formation of a large-diameter nugget, as well as suppressing delayed fracture in the weld. Solution to the problem To suppress delayed fracture in a weld joint obtained by resistance spot welding of high-tensile-strength steel sheets, the present inventors studied the behavior of hydrogen entering the weld metal during the process, that is, the behavior that would cause delayed fracture. The following results were obtained. As described above, hydrogen first enters the weld during welding. Since hydrogen diffuses more slowly at low temperatures, the rapid cooling after welding allows a large proportion of hydrogen to remain in the nugget without diffusing. Then, over time, hydrogen accumulates in areas where high tensile stress is concentrated, such as a notched end portion of the nugget. This results in delayed fracture. One effective way to suppress delayed fracture is to release more hydrogen from the nugget during welding and reduce the amount of residual hydrogen in the nugget. Accordingly, the present inventors sought to identify preferred conditions for resistance spot welding under which the amount of residual hydrogen in the weld can be reduced. The results are presented below. In a welding process, by first producing spatter at the interface of the steel sheets, a source of hydrogen present at the interface can be discharged with the spatter. This has been shown to reduce the hydrogen content in the weld nugget in subsequent welding steps and improve the weld joint's resistance to delayed fracture. However, if spatter occurs in later welding steps, it is difficult to reduce the hydrogen content in the nugget before spattering occurs. This can hinder delayed fracture suppression, affect nugget growth, and make it difficult to obtain a large nugget diameter. Consequently, the welding process was divided into two steps: a first welding step (or initial welding stage described below) intended to produce spatter, and a second welding step (or main welding stage described below) intended to form a nugget after the first welding step. This allowed spatter to be produced in the initial step of the welding process and made it possible to suppress spatter in subsequent welding steps. Furthermore, the contact resistance at the interface of the steel sheets in the initial phase of the welding process was used to selectively melt only a portion near the surface of the steel sheet where a hydrogen source was present, in order to produce spatter at the minimum required level. This allowed for efficient hydrogen release. It was then found that, to achieve this, it was important to properly control the welding force and current value in the first welding pass (initial welding stage). By performing the first welding step (initial welding step) described above, the moisture and oil content present at the interface of the steel sheets, or any adhering substances such as dirt, were discharged along with the spatter. As a result, it was possible to keep the interface of the steel sheets clean and moderately soften the steel sheets with heat by passing the current through them before nugget formation. This ensured good contact between the steel sheets and was found to improve resistance to delayed fracture. Furthermore, it was discovered that in the second welding step (main welding stage), the stable formation of a large-diameter nugget could also be achieved. The present invention has been made based on the results described above and can be summarized as follows. [1] The present invention provides a resistance spot welding method for joining two or more steel sheets, including at least one steel sheet with a tensile strength of 980 MPa or higher. The resistance spot welding method consists of placing the steel sheets one on top of the other to form a stack of steel sheets to be welded, clamping the stack of steel sheets with a pair of electrodes, and passing a current through the steel sheets while applying pressure to join them. The resistance spot welding method includes an initial welding step of passing a current h (kA) that satisfies relation (2) while applying a welding force Fi (kN) that satisfies relation (1): 0.2 x Vti < Fi < 4 x ^ti (1) x VFi < h < 10 x VFi (2) where ti is the total sheet thickness (mm) of the steel sheets to be welded; and a main nugget forming weld pass having a predetermined nugget diameter. Spatter occurs in the initial weld pass. [2] In the resistance spot welding method of [1], the welding time in the initial step is 10 ms or more and within 200 ms. [3] The resistance spot welding method of [1] or [2] further includes a cooling step between the initial welding step and the main welding step. The cooling step is a welding step at a current le (kA) that satisfies relation (3): < le < h (3) where le is a current (kA) in the cooling step eh is a current (kA) in the initial welding step. [4] In the resistance spot welding method of any one of [1] to [3], a welding voltage Vs (V) at the time of spattering satisfies relation (4): Vs > 0.7 x Va (4) where Va is a welding voltage (V) 5 ms before the appearance of spatter, and Vs is a welding voltage (V) at the time of the appearance of spatter. [5] The present invention provides a method for producing a resistance spot welded joint using the resistance spot welding method of any of [1] to [4], Advantageous effects of the invention The present invention has great industrial advantages, since it allows the stable formation of a large diameter nugget, as well as the suppression of delayed fracture in welding. Brief description of the figures Figure 1 shows a cross-sectional view that schematically illustrates resistance spot welding in accordance with an embodiment of the present invention. Figure 2A and Figure 2B show diagrams illustrating an example weld joint used in the examples of the present invention; Figure 2A shows a plan view of the weld joint, and Figure 2B shows a side view of the weld joint. Description of the modalities A resistance spot welding method and a method for producing a resistance spot weld joint according to the present invention will now be described with reference to the drawings. The present invention is not limited to the embodiments described herein. First, a resistance spot welding method according to the present invention will be described with reference to Figure 1. The present invention provides a technique for joining two or more steel sheets by resistance spot welding. Figure 1 schematically illustrates an example of resistance spot welding. Figure 1 illustrates an example where resistance spot welding is performed on two steel sheets. First, two or more steel sheets are placed one on top of the other. In the example illustrated in Figure 1, two steel sheets are placed, including a steel sheet (hereafter referred to as lower steel sheet 1) on the bottom side and a steel sheet (hereafter referred to as upper steel sheet 2) on the top side, one on top of the other to form a set of steel sheets to be welded. Next, the steel sheets (the lower steel sheet 1 and the upper steel sheet 2), stacked one on top of the other, are held together by a pair of welding electrodes (electrodes) 4 and 5 positioned on the lower and upper sides of the steel sheets, respectively. A current is then passed through the lower steel sheet 1 and the upper steel sheet 2 in a predetermined pattern (described later) while pressure is applied. In the example illustrated in Figure 1, the electrode on the lower side of the steel sheets is called the lower electrode 4, and the electrode on the upper side is called the upper electrode 5. With the overlapping steel sheets held together by the pair of welding electrodes 4 and 5, a nugget 3 of a required size is formed by resistance heating by passing a current through the overlapping steel sheets while applying pressure to them, and the overlapping steel sheets are joined to form a welded joint. Although not shown in the present invention, three or more steel sheets placed one on top of the other can be resistance-spot welded. In this case, a welded joint can also be obtained by the same welding method as described above. An apparatus used to implement the resistance spot welding method according to the present invention is not limited to a particular type. The apparatus may have any configuration provided it is capable of applying pressure through the lower electrode 4 and the upper electrode 5 and controlling the welding force applied by the lower electrode 4 and the upper electrode 5. For example, a conventionally known device, such as an air cylinder or a servomotor, may be used. The configuration of the current supply during welding and the control of the current value are also not particularly limited, and a conventionally known device may be used. The present invention is applicable to both direct current and alternating current. In the case of alternating current, the term "current" refers to the effective current. The shape of the tip of the lower electrode 4 and the upper electrode 5 is not limited to one particular shape. Examples of electrode tip shapes include a radial or dome shape (DR), a radial shape (R), and a dome shape (D), as described in JIS C 9304: 11999. The diameter of the tip of the lower electrode 4 and the upper electrode 5 is, for example, from 4 mm to 16 mm. Resistance spot welding is performed while the electrodes are continuously cooled with water. In the present invention, the type of steel used for the resistance-spot welded steel sheets is not limited to a particular type. At least one of the steel sheets to be stacked on top of the other is a high-strength steel sheet having a tensile strength of 980 MPa or higher. This is because delayed fracture in resistance-spot welds tends to be a problem, especially in high-strength steel sheets with a tensile strength of 980 MPa or higher. This means that the advantageous effects of the present invention are particularly beneficial for a use case involving such high-strength steel sheets. The thickness of the steel sheet to be resistance spot welded is not particularly limited. For example, the sheet thickness is preferably 0.5 mm or more and 3.0 mm or less. This is because steel sheets with a thickness within this range can be used appropriately as automotive components. Steel sheets to be resistance-spot welded may be rolled to have a coating on their surface. Examples of coatings that may be used in the present invention include zinc coatings and aluminum coatings. Examples of zinc coatings include hot-dip galvanizing (Gl), Zn-Ni coating, and Zn-Al coating. Examples of aluminum coatings include Al-Si coating (e.g., Al-Si coating containing 10% to 20% Si by mass). The hot-dip coating may be an alloyed hot-dip coating. Examples of an alloyed hot-dip coating include a galvanized (GA) coating. The two or more steel sheets to be resistance-spot welded can be the same or different. That is, the steel sheets can be of the same type and shape, or they can be of different types and shapes. A surface-treated steel sheet with a coating and an uncoated steel sheet can be placed one on top of the other. The following will describe a current flow pattern in the resistance spot welding method according to the present invention. The present invention provides a resistance spot welding method for joining two or more steel sheets, including at least one steel sheet with a tensile strength of 980 MPa or higher. The resistance spot welding method consists of stacking the steel sheets one on top of the other to form the stack of steel sheets to be welded, holding the stack of steel sheets to be welded with a pair of electrodes, and passing a current through the steel sheets while applying pressure to them to form a nugget and join the stacked steel sheets (i.e., the steel sheets to be welded). In the example illustrated in Figure 1, the steel sheets 1 and 2, held by the lower electrode 4 and the upper electrode 5, are welded by passing a current through them in a particular pattern while applying pressure.The welding of the present invention includes an initial welding step and a main welding step intended to form a nugget having a predetermined nugget diameter. First, the initial welding pass produces spatter by controlling the current h (kA) that satisfies relation (2) (described later) while applying a welding force Fi (kN) that satisfies relation (1) (described later). That is, in the initial welding pass, a source of hydrogen present at the interface of the steel sheets is discharged along with the spatter, and good contact between the steel sheets is maintained. In the present invention, it is important to produce spatter during the initial welding step. If spatter occurs during any step (e.g., a cooling step or a main welding step described below) following the initial welding step, a large amount of hydrogen is mixed into the weld nugget before spatter occurs. Since this hinders the achievement of the hydrogen reduction effect expected from spatter formation, a delayed crack suppression effect cannot be achieved. An effective way to improve the hydrogen reduction effect is to reduce the welding time preceding spatter formation and minimize hydrogen input. In the present invention, it is preferable to produce spatter within 200 ms of the start of welding in the initial welding pass. It is more preferable to produce spatter within 100 ms of the start of welding in the initial welding pass. The minimum time from the start of welding to the appearance of spatter is not limited to a specific length, but it is preferable that it be 20 ms or more. To reliably form a large-diameter weld nugget in the main welding pass described below, it is preferable that the spatter produced in the initial welding pass be low-level spatter (hereafter referred to as minor spatter). Since spatter reduces the electrode resistance, a voltage drop appears as a measured value when measuring the electrode voltage in resistance spot welding. In the present invention, the spatter level can be controlled by controlling the amount of voltage drop at spatter formation. Specifically, the current and welding force in the initial welding pass are preferably set so that the electrode voltage (welding voltage) Vs (V) at the time of spatter formation satisfies the following relationship (4): Vs > 0.7 x Va (4) where Va is an electrode-to-electrode voltage (welding voltage) (V) 5 ms before spatter occurs, and Vs is an electrode-to-electrode voltage (welding voltage) (V) at the moment spatter occurs. The spatter produced in the weld that satisfies the relationship refers to the minor spatter in the present invention. If the electrode voltage Vs (V) at the moment of spattering is less than 0.7 x Va, the spatter level is too high to maintain good welding conditions in the main welding pass. In this case, a large-diameter nugget (hereafter referred to as the diameter) cannot be stably formed. Therefore, it is preferable for the electrode voltage Vs (V) at the moment of spattering to be 0.7 x Va or higher. Minimizing the spatter level is an effective way to maintain good contact between the steel sheets and improve the stable deformation effect of a large-diameter nugget in the main welding pass. Therefore, it is preferable for the electrode voltage Vs (V) at the moment of spattering to be 0.8 x Va or higher.As described above, the electrode spatter voltage typically decreases when spatter occurs during spot welding. That is, spatter does not increase the electrode spatter voltage. Therefore, the electrode spatter voltage Vs (V) is unlikely to be (1.0 × Va) or higher in relation (4). Therefore, it is preferable that the interelectrode voltage Vs (V) be less than (1.0 χ Va). The initial welding pass, described above, is followed by the main welding pass, which aims to form a nugget with a predetermined diameter. The current flow conditions, such as current value and welding time, and the pressure applied to form this nugget in the main welding pass are not particularly restricted. Conventional welding conditions can be used in this case. For example, to form a nugget of a suitable diameter, the current value in the main welding pass is preferably 1.0 kA or more and 15.0 kA or less, and the welding force in the main welding pass is preferably 1.0 kN or more and 9.0 kN or less. The welding time in the main welding pass is preferably 100 ms or more and 1000 ms or less. The main welding pass can be a single step where the current and welding force values ​​are varied in multiple steps. To cool a molten nugget, the main welding step may be followed by a holding step, which involves maintaining pressure without passing current. Although the duration of the holding step is not specified here, it is preferable that the duration of the holding step be in the range of 20 ms to 1000 ms, as in a typical resistance spot welding process. In the present invention, a nugget having a predetermined nugget diameter is preferably a nugget having a nugget diameter of 3 / ta 6t (t: sheet thickness) (mm). When steel sheets of different thicknesses are placed on top of each other and welded, the thickness of the thinner sheet of the two adjacent steel sheets to be joined is represented by the letter t described above. frjbrnn / zznz / e / YiAi In the present invention, a cooling step (described below) can be added between the initial welding step and the main welding step. The welding conditions in the initial step for implementing the resistance spot welding method of the present invention will now be described in detail. In the initial welding step, the welding force Fi (kN) and the current h (kA) are set to satisfy relation (1) and relation (2): 0.2 x Yti < Fi < 4 x Yti (1) xa / Fi < h < 10 xa / Fi · (2) where ti is the total thickness of the sheet (mm) of the steel sheets to be welded. The conditions described above are necessary in the initial welding step to discharge, in the form of splashes, a molten part due to contact resistance near the interface of the steel sheets. If the welding force Fi (kN) is (0.2 x -Vti) or less, the welding force is too small to control the heat of fusion generated by the current. This results in extremely high spatter levels and hinders the stable formation of a large nugget in the subsequent main welding pass. If the welding force Fi (kN) exceeds (4 x Yti), it is difficult to discharge molten material as spatter due to the contact resistance. This means that the delayed crack suppression effect cannot be achieved. Although applying a high current can produce spatter, the level of spatter is extremely high in this case. This hinders the stable formation of a large nugget in the subsequent main welding pass. If the current h (kA) is (2 x VFi) or less, spatter is unlikely to occur, and the delayed crack suppression effect cannot be achieved. If the current h (kA) is greater than (10 x YFi), the resulting extremely large spatter hinders the stable formation of a large-diameter nugget in the subsequent main welding pass. To improve the delayed crack suppression effect and the stable formation of a large-diameter nugget in the subsequent main welding pass, it is preferable to set the welding force Fi (kN) and the current h (kA) to satisfy ratios (5) and (6): 0.5 x Vti < Fi < 2 x λ / h (5) x YFi < h < 8 x YFi (6) The welding time in the initial welding step is preferably set at 10 ms or more and within 200 ms. If the desoldering time is less than 10 ms, the welding time is too short to reliably produce spatter, and the delayed crack suppression effect cannot be consistently achieved. Heat generation from contact resistance generally occurs in the initial phase of welding. In the present invention, spatter occurs during the contact resistance heat generation phase. In this case, a prolonged welding process after spattering occurs not only leads to an unnecessary increase in welding time but also results in a high level of spatter.For these reasons, the welding time is preferably within 200 ms, more preferably 20 ms or more and within 140 ms, and even more preferably 20 ms or more and within 100 ms. In the present invention, a cooling step can be added between the initial step current and the main welding step. The cooling step consists of welding at a current le (kA) that satisfies relation (3) to stabilize the contact between the steel sheets: 0 < le < h (3) where le is a current (kA) in the cooling step and eh is a current (kA) in the initial welding step. By adding the cooling step, the contact state between the steel sheets, temporarily disrupted by spatter, can be stabilized again. This ensures more stable nugget formation in the subsequent main welding step. If the current le (kA) in the cooling step exceeds the current h (kA) in the initial welding step, the likelihood of spatter in the cooling step increases, and the effect of maintaining contact between the steel sheets may not be achieved. The purpose of the cooling step is to stabilize the contact state between the steel sheets without spatter occurring during the cooling step.If the current le in the cooling step simply satisfies relation (3), the step current pattern in the cooling step is not limited to a particular one, and can be a no-step current step that does not involve current flow, a multi-step step current step, or a falling step current step. The current le (kA) in the cooling step is most preferably (0.5 xh) kA or less. The duration of the cooling step is preferably 500 ms or less. If the current flows for more than 500 ms during the cooling step, the resulting increase in the total welding process time may lead to reduced productivity. The duration of the cooling step is more preferably 300 ms or less, and even more preferably 20 ms or more. The following will describe a method for producing a resistance spot weld joint. The present invention provides a method for producing a resistance spot weld using the resistance spot welding method described above. In the method for producing a resistance spot weld joint according to the present invention, for example, two or more steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or higher, are placed one on top of the other, held together by a pair of welding electrodes, and resistance spot welded to form a nugget of a required size by passing a current through the steel sheets while applying pressure to them, under the welding conditions for each of the steps described above, and a resistance spot weld joint is produced. The steel sheets, welding conditions, etc., will not be described herein, as they are the same as those described above. As described above, the present invention allows for the suppression of delayed fracture in welds. Furthermore, since low-level spatter is produced in the initial welding step, satisfying the electrode voltage condition described above, a large-diameter nugget can be stably formed in the subsequent main welding step. Furthermore, the present invention effectively suppresses the ingress of hydrogen into the weld metal, which is highly susceptible to hydrogen embrittlement. Therefore, the advantageous effects described above are achieved not only in resistance spot welding of high-strength automotive steel sheets, but also in resistance spot welding of other types of steel sheets. Examples The operations and effects of the present invention will now be described by means of Examples. Note that the present invention is not limited to the Examples described below. In the examples of the present invention, as described with reference to Figure 1, the lower steel sheet (1) and the upper steel sheet (2) were placed one on top of the other and resistance spot welded. The resistance spot welding was performed at room temperature, with the lower electrode (4) and the upper electrode (5) continuously water-cooled. The lower electrode (4) and the upper electrode (5) used herein were DR chromium copper electrodes, both with a diameter (tip diameter) of 6 mm and a tip bend radius of 40 mm. The lower electrode (4) and the upper electrode (5) were driven by a servomotor to control the welding force, and an AC power supply with a frequency of 50 Hz was used for welding. The steel sheets to be welded were of three types of steel, as indicated below: [Type I Steel] An unrolled steel sheet with a tensile strength of 1470 MPa, 100 mm long on the longer side, 30 mm long on the shorter side, and 1.0 mm thick; [Type II Steel] A steel sheet with a tensile strength of 1470 MPa, 100 mm long on the longer side, 30 mm long on the shorter side, 1.6 mm thick, and rolled (hot-dip galvanized (Gl) with a coating weight of 50 g / m2 on one side); and [Type III Steel] Steel sheet with a tensile strength of 1320 MPa, 100 mm long on the longer side, 30 mm long on the shorter side, 2.0 mm sheet thickness, and rolled (hot-dip galvanized (Gl) with a coating weight of 50 g / m2 on one side). A welded joint used in a test will be described with reference to Figure 2A and Figure 2B. Figure 2A shows a plan view of a welded joint, and Figure 2B shows a side view of the welded joint. In the resistance spot welding, as illustrated in Figure 2A and Figure 2B, the two steel sheets 1 and 2 (100 mm long on the longer side and 30 mm long on the shorter side) of the steel types described above were spot welded together, with spacers (6) interposed between them on both sides. The spacers (6) are 1.6 mm thick and 30 mm long on all four sides. The two steel sheets, stacked one on top of the other to form a sheet assembly, were then center welded under the conditions specified in Table 1. As illustrated in Figure 2A, the sheet assembly was spot welded (8) at both ends and spot welded (7) in the center. The welding was performed by adjusting the current value so that the nugget diameter was approximately 3.5¥t (t: sheet thickness (mm)) in all examples and comparative examples. Welding steel sheets 1.6 mm thick yields a nugget diameter of 3.5¥t = 4.43 mm. In the case of welding steel sheets of different thicknesses, the current value was adjusted so that the nugget diameter was 3.5¥t, based on the thickness of the thinnest steel sheet. The delayed fracture characteristics and nugget stability were evaluated using the method described below. The characteristics of the delayed fracture were evaluated as follows. In the delayed fracture test, each weld joint was left to stand in the atmosphere at room temperature (20°C) for 24 hours. The delayed fracture of the weld was then checked. Welding was performed with n = 3 in all examples and comparative examples. In Table 2, welds that do not exhibit delayed fracture after standing for 24 hours are indicated with the symbol “O”, while welds that exhibit delayed fracture are indicated with the symbol “x”. Regarding the determination of delayed fracture characteristics, if the splitting of a weld nugget (or the splitting of a nugget into two at the interface) was observed visually after welding, it was determined that the welded joint had delayed fracture. The final determination of delayed fracture characteristics is shown in Table 2. As shown, a set of conditions where none of the three weld joints (n = 3) had delayed fracture is indicated as “A (excellent)”, while a set of conditions where at least one of the three weld joints (n = 3) had delayed fracture is indicated as “B (failure)”. Using the same specimens described above, the stability of the nuggets was evaluated as follows. Each weld joint was cut at its center and etched by applying an aqueous solution of picric acid to the cross-section. The length of the etched nugget microstructure was then measured to calculate the nugget diameter. The nugget diameter was calculated with n = 3 under all conditions. In Table 2, welded joints with a nugget diameter equal to or greater than 3.5t are indicated by the symbol “O”, while welded joints with a nugget diameter less than 3.5t are indicated by the symbol “x”. The determination of nugget stability is shown in Table 2. As shown, a set of conditions where all three weld joints (n = 3) had a nugget diameter of 3.5Vt or more is indicated as “A (excellent)”, while a set of conditions where at least one of the three weld joints (n = 3) had a nugget diameter of less than 3.5Vt is indicated as “B (failure)”. toJwaniTZfwiYW Tabla 1 N. 0 Top sheet Bottom sheet Welding conditions Pass where spatter occurred Spatter behavior Observations Initial welding pass Cooling pass Main welding pass Holding pass Time from start of welding until spatter appears (ms) Voltage drop Vs / Va Welding force Fi (kN) Current h (kA) Welding time (ms) Current L (kA) Cooling time (ms) Welding force (kN) Current I2 (kA) Welding time (ms) Holding time (ms) 1 II 1.0 6.0 80 0.0 60 3.0 6.8 300 20 Initial welding pass 40 0.9 Example of the invention 2 II 1.0 6.0 80 0.0 60 3.0 6.8 300 100 Initial welding pass 40 0.9 Example of the invention 3 II 1.0 12.0 80 0.0 60 3.0 6.8 300 20 Initial welding step 40 0.6 Comparative example 4 II 1.0 6.0 80 Downhill 100 3.0 6.8 300 20 Initial welding step 40 0.9 Example of the Invention 5 II 3.0 9.0 80 - - 3.0 6.8 300 20 Initial welding step 40 0.9 Example of the invention 6 II 3.0 31 80 - - 3.0 6.8 300 20 - No spatter - Comparative example 7 II 0.6 91 100 - - 3.0 6.8 300 20 Initial welding step 40 0.6 Comparative example 8 I II 1.5 7.0 80 1.0 60 3.0 7.2 300 20 Initial welding step 40 0.9 Example of the invention 9 I II 1.5 7.0 80 1.0 60 3.0 7.2 300 200 Initial welding step 40 0.9 Example of the invention 1 0 I II 3.0 10.0 60 - - 3.0 7.3 300 20 Initial welding step 40 0.8 Example of the Invention 1 1 I II 71 15.0 60 - - 3.0 7.3 300 20 Initial welding step 40 0.6 Comparative example. > shchhccc 4 * 4 5 1 2 II II 2.0 7.0 80 0.0 60 4.0 7.5 300 20 Initial welding step 20 0.9 Example of the invention 1 3 II II 3.0 10.0 60 - - 3.0 7.5 300 20 Initial welding step 40 0.9 Example of the invention 1 4 II II 3.0 10.0 60 0.0 60 3.0 7.5 300 20 Initial welding step 40 0.9 Example of the invention 10 1 5 III III 3.5 10.0 60 0.0 100 5.0 7.5 300 20 Initial welding step 40 0.8 Example of the invention > S hchhccc 4 * 4 Table 2 No. Test Result Observations Delayed Fracture Test Delayed Fracture Characteristics (*1) Nugget Diameter Nugget Stability (*1) 1st time 2nd time 3rd time 1st time 2nd time 3rd time 1 OO or AOOOA Example of the invention 2 OO or AOO or A Example of the invention 3 OO or AOXXB Comparative example 4 or O or AO oo A Example of the invention 5 or O or A ooo A Example of the invention 6 XXXB ooo A Comparative example 7 or XBXX or B Comparative example 8 oroo A ooo A Example of the invention 9 oroo A ooo A Example of the invention 10 oroo A ooo A Example of the invention 11 oroo A or XXB Comparative example 12 oroo A ooo A Example of the invention 13 oroo A ooo A Example of the invention 14 oroo A ooo A Example of the invention 15 ooo A ooo A Example of the invention 1. A: Excellent, B: Failure In the examples of the Invention, as can be seen in Table 2, the effect of stably forming a nugget was achieved while suppressing the occurrence of delayed fracture in a welded joint. List of reference signs 1: lower steel sheet 2: top steel sheet 3: nugget 4: lower electrode 5: upper electrode 6: separator 7: welding point 8: spot welding

Claims

1. A resistance spot welding method for joining two or more steel sheets, including at least one steel sheet with a tensile strength of 980 MPa or higher, the method comprising placing the steel sheets one on top of the other to form a steel sheet assembly to be welded, holding the steel sheet assembly with a pair of electrodes, and passing a current through the steel sheets while applying pressure to join the steel sheets, the resistance spot welding method comprising: a first welding step by passing a current h (kA) satisfying relation (2) while applying a welding force Fi (kN) satisfying relation (1), 0.2 x Vti < Fi < 4 x ^ti (1) 2x^Fi <h<10x^Fi (2) donde ti es el grosor total de la lámina (mm) de las láminas de acero a soldar; y un paso de soldadura principal de formación de una pepita que tiene un diámetro predeterminado déla pepita, en donde las salpicaduras se producen en el paso inicial de la soldadura.

2. The resistance spot welding method according to claim 1, wherein a welding time in the initial welding step is 10 ms or more and within 200 ms.

3. The resistance spot welding method according to claim 1 or 2 further comprises a cooling step between the initial welding step and the main welding step, the cooling step being a welding step at a current le (kA) satisfying relation (3): 0 < le < h (3) where le is a current (kA) in the cooling step and h is a current (kA) in the initial welding step.

4. The resistance spot welding method according to any one of claims 1 to 3, wherein a welding voltage Vs (V) at the time of spattering satisfies relation (4): Vs > 0.7 x Va · (4) where Va is a welding voltage (V) 5 ms before spattering occurs, and Vs is a welding voltage (V) at the time of spattering occurs.

5. A method for producing a resistance spot welded joint using the resistance spot welding method in accordance with any one of claims 1 to 4.

Images