RESISTANCE SPOT WELDING METHOD AND WELD MEMBER PRODUCTION METHOD

Abstract

A resistance spot welding method is provided where the main current step includes two or more electrode force application steps, including a first electrode force application step and a second electrode force application step after the first electrode force application step, an electrode force F1 in the first electrode force application step and an electrode force F2 in the second electrode force application step in the main current step satisfy a ratio F1 < F2, and an electrode force switching point Tf from the first electrode force application step to the second electrode force application step in the main current step is set to satisfy predetermined ratio formulas.

Description

RESISTANCE SPOT WELDING METHOD AND WELD MEMBER PRODUCTION METHOD FIELD OF INVENTION This description relates to a resistance spot welding method. Specifically, it aims to ensure a stable desired nugget diameter regardless of spatter from alterations in a combination of three or more overlapping sheets with a high sheet thickness ratio. BACKGROUND OF THE INVENTION Overlapping metal sheets, especially overlapping steel sheets, are typically joined by resistance spot welding, which is a type of overlapping resistance welding. Resistance spot welding is a method of compressing two or more overlapping steel sheets using a pair of electrodes from above and below. While applying electrode force, a high welding current is passed between the upper and lower electrodes for a short time to join the steel sheets. With this welding method, the heat generated by the resistance to the flow of the high welding current is used to form a weld. The weld is called a nugget. The nugget results from the fusion of the overlapping steel sheets and solidification at the contact point when current flows through them. The steel sheets are joined by the nugget. The bond strength of resistance spot welding depends on the nugget diameter. Particularly in cases where high bond strength is required, such as in automotive parts and similar applications, it is important to ensure at least a certain nugget diameter. Typically, when electrode force and welding time (current flow time) are constant, the nugget diameter gradually increases as the welding current increases. However, when the welding current reaches a certain value or higher, spatter occurs. Spatter is a phenomenon in which molten metal splashes between the steel plates. Spatter causes variations in nugget diameter and joint tensile strength, resulting in unstable weld quality. Regarding automotive part structures, for example, a center pillar has a structure in which a reinforcement is sandwiched between an outer and an inner portion. In this structure, three or more metal sheets need to be overlapped and spot-welded, unlike the case of simply spot-welding two overlapping metal sheets. The recent demand for further improved crashworthiness in automobiles has encouraged increased strength and thickness of reinforcements and similar components. This often necessitates spot welding a combination of sheet metal layers, where an outer portion with a small sheet thickness is located on the outside, and an inner portion and a reinforcement, each with a large sheet thickness, are located on the inside. In sheet metal fabrication, a sheet of metal with a relatively small thickness is referred to as thin sheet, and a sheet of metal with a relatively large thickness as thick sheet. The same applies here and later. In the case where that combination of sheets of three or more overlapping sheets with a QRZfrnn / zznz / B / Ywi When a high sheet thickness ratio ((total thickness of the sheet metal combination) / (thickness of the thinnest sheet metal in the combination)) is subjected to conventional resistance spot welding using constant electrode force and welding current, a nugget of a required size hardens to form between the outermost thinner sheet (i.e., the sheet in contact with the electrode tip) and the thicker sheet. This tendency is particularly noticeable when the sheet thickness ratio of the sheet metal combination is greater than 3 and most noticeable when the sheet thickness ratio is 5 or more. This is thought to be because contact with the electrode tip prevents the temperature increase between the outer thin sheet and the thick sheet. In detail, the nugget typically forms due to the heat generated by the volumetric resistance, which varies according to the specific resistance of each steel sheet, starting near the center between the electrodes. However, contact with the electrode tip prevents the temperature from increasing between the outermost thin sheet and the thicker sheet. Consequently, the nugget grows significantly between the thicker sheets located near the center between the electrodes before it grows between the thin and thick sheets. As a result, the molten metal cannot be suppressed by the applied electrode force, and expulsion occurs. Since the outer portion is required to be formable, mild steel is frequently used as the thin plate for this portion. Meanwhile, the inner portion and reinforcement are strength-bearing members, and therefore high-tensile-strength steel plates are often used as the thick plates for these members. In this combination of thin and thick plates, the heat generation location tends to be closer to the side of the high-tensile-strength steel plate (thick plate) with its high specific strength. Furthermore, when the metal plates used are coated steel plates, the low-temperature coated layers cause the current path between the steel plates to expand, and the current density decreases. This further hinders nugget formation between the thin and thick plates. The following technique has been proposed as a resistance spot welding method used for that combination of three or more overlapping sheets with a high sheet thickness ratio: The main current path to form a nugget is divided into two or more electrode force application steps, and the electrode force in the first electrode force application step is set so that it is less than the electrode force in the second electrode force application step, to narrow the contact diameter (current path) between thin and thick sheets and increase the current density (in other words, preferably promote heat generation between the thin and thick sheets) in the first electrode force application step to thereby form a nugget of a required size between the thin and thick sheets. For example, JP 2004-358500 A (PTL 1) proposes “a spot welding method of passing a welding current while compressing parts to be welded by means of a pair of electrodes, comprising: a first step of passing a welding current while applying a first electrode force to the parts to be welded; and a second step of passing a welding current again while applying a second force of QRZfrnn / zznz / Β / γΐΛΐ electrode greater than the first electrode force in a state in which the parts to be welded remain compressed.” APPOINTMENT LIST Patent Literature PTL 1: JP 2004-358500 A BRIEF DESCRIPTION OF THE INVENTION Technical Problem In cases where an alteration occurs during welding, such as when a previously welded spot (hereafter referred to as an "existing weld") is present near the current weld point, or when the parts to be welded have considerable surface roughness and a contact point between them is present near the current weld point, some of the current is diverted to the existing weld or contact point during welding. In this state, even though the current flow is established under a predetermined condition, the current density at the position to be welded, which is directly above and below the electrodes, decreases, and consequently, a weld nugget of the required diameter cannot be obtained. Furthermore, when the area surrounding the weld is severely restricted due to surface roughness, member shape, etc., the gap between the steel plates increases. This results in a smaller contact diameter between the steel plates, making it impossible to achieve the required nugget diameter or facilitating ejection. In particular, resistance spot welding of a combination of three or more overlapping sheets with a high sheet thickness ratio tends to be affected by that alteration. In actual operations such as vehicle manufacturing, parts to be welded are transported one after another and welded continuously. The state of alteration varies between welding positions or parts being welded due to working conditions, dimensional errors in the parts being welded, or similar factors. Therefore, it is difficult to accurately assess the state of alteration of the parts to be welded before welding actually begins. Thus, the technique described in PTL 1 has the problem that a desired nugget diameter cannot be ensured without ejection when a greater alteration than expected occurs. As mentioned previously, the state of alteration varies between welding positions or parts to be welded, due to working conditions, dimensional errors in the parts being welded, or similar factors. Consequently, even if the state of alteration of the parts to be welded can be detected before welding, it is necessary to establish, for each state of alteration, the optimal welding conditions based on that state, which is problematic in terms of efficiency and labor costs. Therefore, it would be useful to provide a resistance spot welding method that can reliably ensure the desired nugget diameter without ejection regardless of the effect of disturbance, particularly in a combination of three or more overlapping sheets with a high sheet thickness ratio. QRZfrnn / zznz / Β / γΐΛΐ It might also be useful to provide a method for producing welded joining members from a plurality of overlapping metal sheets by the resistance spot welding method. Solution to the Problem We conducted an intensive study to achieve the previously established goal and discovered the following: (1) In the case where the main current passage to form a nugget is divided into two or more electrode force application steps and the electrode force in the first electrode force application step is less than the electrode force in the second electrode force application step, if a desired current path is ensured, i.e., if a desired heat generation pattern is obtained, it can be determined from the time integration value of the resistance between the electrodes from the start of the main current passage until a predetermined time has elapsed, regardless of the effect of a disturbance. (2) Furthermore, the effect of the disturbance can be effectively mitigated by adjusting, depending on the time integration value of the resistance between the electrodes, the switching timing of the electrode force in the first electrode force application step to the electrode force in the second electrode force application step. (3) Therefore, it is possible to reliably ensure a desired nugget diameter without ejection by responding effectively to variations in the alteration state, even when continuously welding the parts to be welded that are transported one after the other in an actual operation such as in vehicle manufacturing (i.e., even when the alteration state varies between welding positions or parts to be welded). This description is based on those discoveries and additional studies. In this way we provide: 1. A resistance spot welding compression method, by means of a pair of electrodes, the parts to be welded, which are a plurality of overlapping metal sheets, and passing a current while applying an electrode force to join the parts to be welded,where the main current step includes two or more electrode force application steps including a first electrode force application step and a second electrode force application step after the first electrode force application step and an electrode force Fi in the first electrode force application step and an electrode force Fa in the second electrode force application step in the main current step satisfies a ratio Fi < F2 and an electrode force switching point Tf from the first electrode force application step to the second electrode force application step in the main current step is set to satisfy the following Formulas (1) to (3):, in the case where Ta < 0.8 χ To, TA <Tf<To ...(1) en el caso donde 0.8 χ To < Ta < To o 0.9 χ Ro < Ra < Ro, 0.9 χ To < Tf < 1.1 χ To ...(2) in the case where Ra < 0.9 χ Ro, QRZfrnn / zznz / B / YiAi To < Tf < To + 2 x (Ro - Ra) / Ro x Tm ...(3) where To is a reference electrode force switching point from the first electrode force application step to the second electrode force application step, Tm is a total welding time in the main current step, Ra is a time integration value of a resistance between the electrodes from the start of the current step from the main current step to the reference electrode force switching point To, Ro is a time integration value of a resistance between the electrodes from the start of the current step to the reference electrode force switching point To in the case where the current step is carried out under the same condition as the main current step when the parts to be welded have no alteration and Ta is a time at which the time integration value of a resistance between the electrodes in the main current step reaches Ro. 2. The resistance spot welding method according to 1., where the reference electrode force switching point To satisfies the following formula: 0.1 X Tm < To < 0.8 X Tm. 3. The resistance spot welding method according to 1 or 2, comprising: performing the test weld;and performing the actual welding, including the main current pass, after the test welding, wherein in a main current pass in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume, calculated from an electrical property between the electrodes in the formation of an appropriate nugget, performing the current pass by constant current control, are stored, and in the main current pass in the actual welding, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume, stored in the main current pass in the test welding, are established as a target, and an amount of current pass is controlled according to the target. 4. A method of producing welded members comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to any of 1 to 3 Advantageous Effect In this way it is possible to reliably ensure a desired nugget diameter without ejection regardless of the effect of an alteration, even in a combination of sheets of three or more overlapping sheets with a high sheet thickness ratio. It is also possible to reliably ensure a desired nugget diameter by effectively responding to variations in the state of alteration, even when continuously welding parts that are transported one after the other in a real-world operation such as vehicle manufacturing (i.e., even when the state of alteration varies between welding positions or parts being welded). This is highly advantageous for improving operational efficiency and throughput. BRIEF DESCRIPTION OF THE FIGURES In the accompanying figures: Figure 1 is a diagram that illustrates the relationship between electrode force and time in the main current passage and the relationship between the time integration value of the resistance QRZfrnn / zznz / B / YiAi between the electrodes and the time in the main current passage when Formula (1) is satisfied (in the case where Ta < 0.8 x To) in a resistance spot welding method according to one of the described modalities; Figure 2 is a diagram that illustrates the relationship between the electrode force and the time in the main current passage and the relationship between the time integration value of the resistance between the electrodes and the time in the main current passage when Formula (2) is satisfied (in the case where 0.8 χ To < Ta < To or 0.9 χ Ro < Ra < Ro) in the resistance spot welding method according to one of the described modalities; Figure 3 is a diagram that illustrates the relationship between the electrode force and the time in the main current passage and the relationship between the time integration value of the resistance between the electrodes and the time in the main current passage when Formula (3) is satisfied (in the case where Ra < 0.9 χ Ro) in the resistance spot welding method according to one of the described modalities; Figure 4A is a diagram that schematically illustrates an example of carrying out welding in an undisturbed state; Figure 4B is a diagram that schematically illustrates an example of carrying out welding in an undisturbed state; Figure 5A is a diagram that schematically illustrates an example of performing welding on a combination of sheets that has a sheet gap; Figure 5B is a diagram that schematically illustrates an example of performing welding on a combination of sheets that has a sheet gap; Figure 6A is a diagram that schematically illustrates an example of how to perform welding on a combination of sheets that have existing welds; and Figure 6B is a diagram that schematically illustrates an example of performing welding on a combination of sheets that have existing welds. DETAILED DESCRIPTION OF THE INVENTION One of the described modalities will be described below. One of the described methods is a resistance spot welding compression method, using a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets, and passing a current while applying an electrode force to join the parts to be welded, where the main current passage includes two or more electrode force application steps, including a first electrode force application step and a second electrode force application step after the first electrode force application step.an electrode force Fi in the first electrode force application step and an electrode force Fg in the second electrode force application step in the main current step satisfy a relation Fi < F2 and an electrode force switching point Tf from the first electrode force application step to the second electrode force application step in the main current step (hereafter also referred to as “electrode force switching point Tf”) is set to satisfy a predetermined relation. The electrode force switching point Tf (and the reference electrode force switching point described below To) is the time at which the operation begins QRZfrnn / zznz / Β / γΐΛΐ electrode force switching. The electrode force switching point Tt (and the reference electrode force switching point To described below) is expressed relative to the start of current flow in the main current flow (i.e., expressed as the elapsed time from the start of current flow in the main current flow). The same applies to Ta (the time at which the time integration value of the resistance between the electrodes in the main current flow reaches Ro) described below, etc. The resistance spot welding method, according to one of the described modalities, is particularly suitable for plate combinations where the plate thickness ratio (the total thickness of the plate combination / the thickness of the thinnest plate in the combination) is greater than 3, and even more suitable for plate combinations where the plate thickness ratio is 5 or more, for which it has been difficult to obtain a nugget of the required size between the thin and thick plates without ejection, regardless of alteration. The resistance spot welding method is also effective for plate combinations of two overlapping plates. The term “thin sheet” refers to a sheet of metal with a relatively small thickness, and the term “thick sheet” refers to a sheet of metal with a relatively large thickness, of the steel sheets used in the sheet metal assembly. The thickness of a thin sheet is no more than 3 / 4 that of the thickest sheet of metal. Any welding device that includes a pair of upper and lower electrodes and is capable of freely controlling the electrode force and welding current during welding can be used in the resistance spot welding method according to one of the described modalities. The type (stationary, robotic gun, etc.), electrode shape, and the like are not limited. The resistance spot welding method according to one of the described modalities will be described below. (A) The main current pass (also referred to as “main current pass in actual welding” to distinguish it from the main current pass in test welding (described later here). The term “main current pass” when used alone denotes the main current pass in actual welding and not the main current pass in test welding. Here, “main current pass” denotes the current pass to form a nugget. “Actual welding” denotes a process of actually welding the parts to be welded, which should be distinguished from test welding.) For a combination of three or more overlapping sheets with a high sheet thickness ratio, dividing the main current step for nugget formation into two or more electrode force application steps and satisfying the following relationship, i.e., adjusting the electrode force Fi in the first electrode force application step (hereafter also referred to simply as “Fi”) so that it is less than the electrode force Fz in the second electrode force application step (hereafter also simply referred to as “Fz”), heat generation between the thin and thick sheets is preferably promoted in the first electrode force application step: QRZfrnn / zznz / B / YiAi Fi < F2. Preferably, 1.1 x Fi < F2. More preferably, 1.2 x Fi < F2. Most preferably, 1.5 x Fi < F2. Fi and F2 can be set as appropriate depending on the materials, thicknesses, etc. of the metal sheets as the parts to be welded, as long as the above relationship is satisfied. For example, when using a combination of three or more overlapping plates with a high plate thickness ratio (e.g., a combination of three overlapping plates consisting of two thick plates (mild steel or zinc-coated steel or zinc alloy plates of grade 490 MPa to 2000 MPa or uncoated steel plates of 0.8 mm to 3.0 mm thickness) and one thin plate (zinc-coated steel or zinc alloy plate or uncoated steel plate (mild steel) of 0.5 mm to 2.0 mm thickness), it is preferable that Fi be from 1.0 kN to 6.0 kN and F2 be from 2.0 kN to 10.0 kN. In the case of using a typical combination of two overlapping plates, it is preferable that Fi be from 1.0 kN to 5.0 kN and F2 be from 2.0 kN to 7.0 kN. In the resistance spot welding method according to one of the described modalities, it is important to establish the synchronization of the switching from Fi to F2, that is, the electrode force switching point Tt, to satisfy the following Formulas (1) to (3), depending on the time integration value of the resistance between the electrodes from the start of the main current flow until a predetermined time elapses: - in the case where Ta < 0.8 χ To, Ta <T(<T0...(1) - in the case where 0.8 χ Το < Τα < Το or 0.9 χ Ro <Ra< Ro, 0.9 χ Το < Tf< 1.1 χ Το ... (2) - in the case where Ra < 0.9 χ Ro, To < Tf < To + 2 x (Ro - Ra) / Ro x Tm ...(3) where To is the reference electrode force switching point from the first electrode force application step to the second electrode force application step, T is the total welding time in the main current step, Ra is the time integration value of the resistance between the electrodes from the start of the current step from the main current step to the reference electrode force switching point To, Ro is the time integration value of the resistance between the electrodes from the start of the current step to the reference electrode force switching point To in the case where the current step is carried out under the same condition as the main current step when the parts to be welded have no alteration and Ta is the time at which the time integration value of the resistance between the electrodes in the main current step reaches Ro. In the case where Ta < 0.8 χ To, that is, in the case where Ra is expected to be greater than Ro by a certain amount (see Figure 1), in the first step of electrode force application, the contact diameter between the metal sheets is smaller than in an undisturbed state due to sheet separation. In other words, the desired heat generation between the thin sheets QRZfrnn / zznz / Β / γΐΛΐ and thick will probably be reached in a shorter time. In that case, it is effective to advance the timing of the Fi to Fz switching. Specifically, it is effective to set the electrode strength switching point Tf to satisfy Formula (1) above. In the case where Ra < 0.9 χ Ro, that is, where Ra is less than Ro by a certain amount (see Figure 3), heat generation between the thin and thick plates is likely to be insufficient due to current shunt or similar factors. In that case, it is effective to delay the timing of the Fi to Fz switching. Specifically, it is effective to set the electrode force switching point Tf to satisfy Formula (3) above, to further promote heat generation between the thin and thick plates. In the case where 0.8 χ To < Ta < To or 0.9 χ Ro < Ra < Ro (see Figure 2), that is, in the case where Ra is (or is expected to be) approximately equal to Ro, the effect of a disturbance is unlikely to be significant. In that case, the electrode strength switching point Tf is set so as to satisfy Formula (2) above. Thus, in the resistance spot welding method according to one of the described modalities, it is important to establish the electrode force switching point Tf to satisfy Formulas (1) to (3) depending on, for example, the time integration value of the resistance between the electrodes from the start of the main current flow until the predetermined time elapses. As with Formulas (1) to (3), it is preferable to satisfy the following Formulas (1)' to (3)' respectively: - in the case where Ta < 0.8 χ To, Ta < Tf < 0.95 x To ...(1)' - in the case where 0.8 χ To < Ta < To or 0.9 χ Ro <Ra< Ro, 0.95 χ To < Tf < 1.05 χ To ...(2)' - in the case where Ra < 0.9 χ Ro, .05 χ To < Tf < To + 2 χ (Ro - Ra) / Ro χ Tm... (3)'. For example, the time integration value Rodé the resistance between the electrodes from the start of the current flow to the reference electrode force switching point To in the case where the current flow is carried out under the same condition as the main current flow when the parts to be welded have no alteration can be obtained by separately preparing the parts to be welded composed of metal sheets of the same thicknesses of sheet materials as the main current flow and having no alterations and carrying out a preliminary welding test of the parts to be welded under the same condition as the main current flow. In the case of performing the test welding described below, the time integration value of the resistance between the electrodes from the start of the current flow to the reference electrode force switching point To in the main current flow in the test welding can be Ro. The reference electrode force switching point To (ms) from the first electrode force application step to the second electrode force application step can be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets as the parts to be welded, but preferably it is set using QRZfrnn / zznz / Β / γΐΛΐ the total welding time Tm(ms) in the main current step to satisfy the following formula: 0.1 x Tm h To h 0.8 x Tm. If To is less than 0.1 x Tm, there is a possibility that the effect of a disturbance cannot be effectively mitigated by controlling the electrode force switching time. If To is greater than 0.8 x Tm, there is also a possibility that the effect of a disturbance cannot be effectively mitigated by controlling the electrode force switching time. To is therefore preferably 0.1 x Tm plus and 0.8 x Tm or less. To is more preferably 0.15 χ Tm or more, and even more preferably 0.2 χ Tm or more. To is more preferably 0.7 χ Tmo less, and even more preferably 0.5 χ Tmo less. The total welding time Tm(ms) in the main current pass can be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets as the parts to be welded. For example, when using a combination of three or more overlapping sheets with a high sheet thickness ratio, as mentioned above, Tm is preferably between 120 ms and 1000 ms. When using a typical combination of two overlapping sheets, Tm is preferably between 80 ms and 800 ms. In the case where the main current step is divided into two or more current step steps and a cooling time is provided between the current step steps, the total welding time in the main current step includes the cooling time between the current step steps. The main current flow can be controlled by constant current control. Alternatively, after performing the test weld described below, adaptive control welding can be performed to control the amount of current flow between the target assembly and the test weld. In the case of constant current control, the welding current can be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets being welded. The main current pass can be divided into two or more current passes, and a cooling time can be provided between these passes. The current step division time can be the same as, or different from, the electrode force application division time. The switching point of the current value from the first current step to the second current step (i.e., the current step division time) in the main current step does not need to be modified according to the change in the electrode force switching point in the main current step. The same applies to adaptive control welding described below. For example, in the case of welding a typical combination of two sheets overlapped by a current passage, the current value is preferably from 4.0 kA to 12.0 kA. In the case of performing welding by two or more current steps obtained by dividing the current step, it is preferable that the current value and welding time in the first current step be from 4.0 kA to 14.0 kA and from 20 ms to 400 ms respectively and the QRZfrnn / zznz / B / YiAi The current value and welding time in the second current step should be from 3.0 kA to 12.0 kA and from 40 ms to 800 ms, respectively. Particularly in the case of welding a combination of three or more overlapping sheets with a high sheet thickness ratio, as mentioned above, it is preferable that the current value in the first current step be greater than the current value in the second current step. In the case where a cooling time is provided between the first and second current steps, the cooling time is preferably from 10 ms to 400 ms. In adaptive control welding, the welding process is carried out according to the target (the time-vary curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated) obtained from the test weld described below. If the time-vary curve of the instantaneous amount of heat generated per unit volume follows the target curve, the welding continues unchanged and is completed. If the time-vary curve of the instantaneous amount of heat generated per unit volume differs from the target curve, the current flow rate is adjusted to compensate for the difference during the remaining welding time so that the cumulative amount of heat generated per unit volume in the actual weld matches the target cumulative amount of heat generated per unit volume. In the case of adaptive control welding, the main current path can also be divided into two or more current path steps, and adaptive control welding can be performed for each current path step. In detail, the main current step in the actual welding and the main current step in the test welding are each divided into two or more current step steps to correspond to each other. The welding is then performed according to the target (the time-variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated) for each current step obtained as a result of the test weld. If the time-variation curve of the instantaneous amount of heat generated per unit volume differs from the time-variation curve at any current step, the current step is adjusted to compensate for the difference within the remaining welding time at that current step so that the cumulative amount of heat generated per unit volume at that current step matches the cumulative amount of heat generated per unit volume at that current step during the test weld. The method for calculating the amount of heat generated is not limited. JP H11-33743 A describes an example of the method, which may be used herein. The following is the procedure for calculating the amount q of heat generated per unit volume and per unit time, and the cumulative amount Q of heat generated per unit volume, according to this method. Let t be the total thickness of the parts to be welded, let r be the electrical resistivity of the parts to be welded, let V be the voltage between the electrodes, let I be the welding current, and let S be the contact area of ​​the electrodes and the parts to be welded. In this case, the welding current passes through a columnar portion whose cross-sectional area is S and thickness is t, to generate heat by resistance. The amount q of heat generated per unit of QRZfrnn / zznz / Β / γΐΛΐ volume and per unit of time in the columnar portion is provided by the following Formula (4): q = (Vl) / (St) ...(4). The electrical resistance R of the columnar portion is provided by the following Formula (5): R = (rt) / S ...(5). Solving Formula (5) for S and substituting the solution into Formula (4) yields the amount q of heat generated according to the following Formula (6): q = (VlR) / (r-t2) = (V2) / (r-t2) ... (6). As is clear from Formula (6), the amount q of heat generated per unit volume and per unit time can be calculated from the voltage V between the electrodes, the total thickness t of the parts to be welded, and the electrical resistivity ρ of the parts to be welded, and is not affected by the contact area S of the electrodes and the parts to be welded. Although the amount of heat generated is calculated from the voltage V between the electrodes in Formula (6), the amount q of heat generated can also be calculated from the current between electrodes I. The contact area S of the electrodes and the parts to be welded need not be used in this case. By accumulating the amount q of heat generated per unit volume and per unit time for the welding time, the cumulative amount Q of heat generated per unit volume for the weld is obtained.As is clear from Formula (6), the cumulative amount Q of heat generated per unit volume can also be calculated without using the contact area S of the electrodes and the parts to be welded. Although the above describes the case of calculating the cumulative amount Q of heat generated by the method described in JP H11-33743 A, the cumulative amount Q may be calculated by any other method. (B) Test welding In the case of performing the main current flow in the actual welding process using adaptive current control, the test weld is performed before the actual welding. During the main current flow in the test weld, the time-variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume, calculated from the electrical properties between the electrodes during the formation of the appropriate nugget by performing the current flow using constant current control, is stored. In detail, in the test welding, a preliminary welding test is carried out with the same types and thicknesses of steel as the parts that are to be welded in the actual welding under various conditions by controlling constant current in a state without sheet separation or current deviation to an existing weld, in order to find an optimal condition in the test welding. The current flow then occurs under this condition, and the time-variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume, calculated from the electrical property between the electrodes during the current flow, are stored as the target for the actual welding. Here, "electrical property between the electrodes" means the resistance between the electrodes or the voltage between them. QRZfrnn / zznz / e / YiAi the electrodes. The main current step in test welding can be divided into two or more current step steps, and adaptive control welding can be performed for each current step in the actual welding, as mentioned above. In the case of welding a combination of sheets of three or more overlapping sheets with a high sheet thickness ratio as mentioned above, it is preferable that the current value in the first current step be greater than the current value in the second current step in the test weld as well. (C) Other modifications The preliminary current pass to stabilize the contact diameter can be performed before the main current pass (the main current pass in the actual weld and / or the test weld) for nugget formation, and the subsequent current pass can be performed for the subsequent heat treatment. The preliminary and subsequent current passes can be performed by constant current control or with an ascending or descending current pattern. A cooling time may be provided between the preliminary current pass and the main current pass, and between the main current pass and the subsequent current pass. The parts to be welded are not limited. The resistance spot welding method can be used to weld steel sheets and coated steel sheets with varying strengths, from mild steel to ultra-high tensile strength steel, and lightweight metal sheets of aluminum alloys and similar materials. The resistance spot welding method can also be used for combinations of three or more overlapping steel sheets. By joining a plurality of overlapping metal sheets using the resistance spot welding method described above, various welded members, particularly welded members of automotive and similar parts, can be produced while stably ensuring a desired nugget diameter and responding effectively to variations in the state of alteration. EXAMPLES The techniques described herein are further described below as examples. The conditions in the examples are illustrative of conditions used to determine the operability and effects of the techniques described herein, and this description is not limited to these exemplary conditions. Various conditions may be used in this description as long as the objective of this description is met, without deviating from its scope. The actual welding (main current flow) was performed for each combination of sheet metal sheets listed in Table 1 under the conditions listed in Tables 3 and 4 in the states illustrated in Figures 4A to 6B, to produce weld joints. In Figures 5A and 5B, spacers 15 were inserted between metal sheets, and a combination of sheets was clamped from above and below (not polished) to create a sheet separation of any of the different thicknesses. The distance between the spacers was 60 mm in each case. QRzi?nn / zznz / e / YiAi In Figures 6A and 6B, two existing welds 16 and the welding position (the center between the electrodes) was set to be the midpoint between the existing welds (i.e., the distance L from each existing weld was equal). The nugget diameter of each existing weld was 4 / t' (mm) (where t' is the sheet thickness (mm) of the thinnest sheet metal in the sheet combination). In some examples, prior to the actual welding, a test weld was performed under the conditions listed in Table 2 in an unaltered state, as illustrated in Figures 4A and 4B. The time-variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume during the main current pass in the test weld were recorded. Additionally, the time integration value of the resistance between the electrodes from the start of the current pass to the reference electrode force switching point To during the main current pass in the test weld was measured and taken as Ro. In each example in which the current passage was carried out by constant current control, the parts to be welded, composed of metal sheets of the same thicknesses and sheet materials as in the main current passage and without alterations, were prepared separately and a preliminary welding test was carried out on the parts to be welded under the same condition as the actual welding to obtain Ro. The Ro thus obtained is listed in Table 3. For each weld joint produced, the weld was cut and etched in section and then examined under an optical microscope to evaluate the weld joint according to three levels A, B, and F, based on nugget diameter and whether ejection occurred. For a combination of three overlapping sheets, the evaluation was performed using the diameter of a nugget formed between the thinnest sheet metal 11 and the outer side of the sheet metal 12 adjacent to sheet metal 11. The evaluation results are listed in Table 4. A: The nugget diameter was 4.5 / t' (mm) or more (where t' is the sheet thickness (mm) of the thinnest sheet metal in the sheet combination) and no ejection occurred regardless of disturbance. B: the nugget diameter was 4 µt' (mm) or more and no expulsion occurred regardless of the alteration (excluding case A). F: The kernel diameter was less than 4Vt' (mm) and / or expulsion occurred depending on the alteration. QRZfrnn / zznz / e / Ywi Qfizbnn / zznzE / YiAi Table 1 4.5Ú' (mm) 3.76 4.02 5.32 (UJLU) 3.35 3.58 4.73 Metal sheet of reference sign 13 in the Figures GA steel sheet grade 1470 MPa (sheet thickness: 1.6 mm) GA steel sheet grade 1470 MPa (sheet thickness: 1.4 mm) Metal sheet of reference sign 12 in the Figures GA steel sheet grade 1470 MPa (sheet thickness: 1.6 mm) Cold rolled steel sheet grade 1470 MPa (sheet thickness: 1.6 mm) GA steel sheet grade 1470 MPa (sheet thickness: 1.4 mm) Metal sheet of reference sign 11 in the Figures GA steel sheet grade 270 MPa (sheet thickness: 0.7 mm) GA grade 270 MPa steel sheet (sheet thickness: 0.8 mm) GA grade 1470 MPa steel sheet (sheet thickness: 1.4 mm) ID of the sheet combination < m O Table 2 Observations Example Example Example Example Comparative Ί: Electrode force in the first step of electrode force application in the main current step in the test weld *2: Electrode force in the second step of electrode force application in the main current step in the test weld Test weld conditions Current step condition Welding time (ms) 300 3.0 5.0 100 6.5 100 - 6.5 180 Second step with Current value (kA) 6.5 Cooling time (ms) Welding time (ms) with First step with Current value (kA) 8.0 Electrode force application condition Switching point from Fi' to F2' (ms) with F2'*2 (kN) 5.0 Fi'*1 (kN) 3.0 ID of the sheet combination <c m O < No. 1-1 1-2 1-3 1-4 2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4 4-1 4-2 4-3 4-4 -- CXJ co QAZbnn / zznzE / YiAi QR7bnn / 77n7E / YIAI Table 3 Observations Example Example Example Example Comparative Actual Welding Condition Appropriate Ti Range According to Formulas (1) to (3) Formula (2) Formula (1) Formula (1) Formula (3) Formula (2) Formula (1) Formula (1) Formula (3) Formula (2) Formula (1) Formula (1) Formula (3) Formula (2) Formula (1) Formula (1) Formula (3) More than 72 to less than 88 From 50 to less than 80 From 34 to less than 80 More than 80 to 260 More than 54 to less than 66 From 46 to less than 60 From 38 to less than 60 More than 60 to 236 More than 90 to less than 110 From 74 to less than 100 From 62 to less than 100 More than 100 to 245 More than 72 to less than 88 From 56 to less than 80 From 42 to less than 80 More than 80 to 200 Ra / Ro A CD 04 5 OO cq A cq g A OJ co cq Ra (Ω-ms) 0.015 0.018 0.022 0.010 0.014 0.017 0.008 0.029 0.035 0.035 080'0 0.015 ¿LOO 0.020 0.012 Ro (Ω-ms) 0.014 0.010 0.027 0.014 (SLU) V1 oo so co LO oo co σο £ £ CO g CO lo OE tz 5 g en θ θ s CM Electrode Force Application Condition θ «L 1- ES sg co E oo s θ s S θ oo fe co θ co 81 81 8I O 5 S s O LO OO Electrode Force Switching Timing Change Control Applied Applied Applied Not Applied Alteration State None Sheet Gap (tg=0.5 mm) Sheet Gap (tg=1 mm) Existing Weld (L=10 mm) None Sheet Gap (tg=0.5 mm) Sheet Gap (tg=1 mm) Existing Weld (L=10 mm) None Sheet Gap (tg=0.5 mm) Sheet Gap (tg=1 mm) Existing Weld (L=10 mm) None Sheet separation (tg=0.5 mm) Sheet separation (tg=1 mm) Existing weld (L=10 mm) ID of the sheet combination m O 04 Si CO CQ qj -4- 2 3 2 T- C\J CO. Observations Example Example Example Example Comparative Evaluation Results Evaluation <c Ll_ Expulsión No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió No ocurrió Ocurrió Ocurrió No ocurrió Diámetro de pepita (mm) 3.9 4.0 3.9 4.2 4.3 5 co 5 5.5 LO 5.6 5.5 3.9 co cxi 3.2 Condición de soldadura real Condición de paso de corriente Segundo paso de paso de corriente Tiempo de soldadura (ms) 300 320 180 300 Valor de corriente (kA) 6.0 6.5 Tiempo de enfriamiento (ms) g S g Primer paso de paso de corriente Tiempo de soldadura (ms) co s 100 co Valor de corriente (kA) 7.5 8.0 Método de paso de corriente Control adaptable Control de corriente constante Control adaptable Control de corriente constante ID de la combinación de chapas CQ O <c No. CM c? ^i- cU 2-2 2-3 2-4 có 3-2 ε-ε cxj ετ CXJ co In each example, a sufficient kernel diameter was obtained without expulsion regardless of the alteration. In each Comparative Example, a sufficient kernel diameter was not obtained and / or expulsion occurred depending on the alteration. QRZfrnn / zznz / e / γΐΛΐ LIST OF REFERENCE SIGNS 11, 12, 13 sheet metal electrode separator 16 existing weld

Claims

1. A resistance spot welding compression method, by means of a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets and passing a current while applying an electrode force to join the parts to be welded,characterized in that the main current step includes two or more electrode force application steps, including a first electrode force application step and a second electrode force application step after the first electrode force application step, and an electrode force Fi in the first electrode force application step and an electrode force Fz in the second electrode force application step in the main current step satisfies a ratio Fi < Fz, and an electrode force switching point Tr from the first electrode force application step to the second electrode force application step in the main current step is established to satisfy the following Formulas (1) to (3): in the case where Ta < 0.8 χ To, Ta <T(<To ...(1) en el caso donde 0.8 χ To < Ta < To o 0.9 χ Ro < Ra < Ro, 0.9 xTo<Tf < 1.1 χ To ... (2) en el caso donde Ra < 0.9 χ Ro,To < Tt < To + 2 χ (Ro - Ra) / Ro x Tm ... (3) where To is a reference electrode force switching point from the first electrode force application step to the second electrode force application step, Tm is a total welding time in the main current step, Ra is a time integration value of a resistance between the electrodes from the start of the main current step to the reference electrode force switching point To,Ro is a time integration value of a resistance between the electrodes from the start of the current flow to the reference electrode force switching point To in the case where the current flow is carried out under the same conditions as the main current flow when the parts to be welded have no alteration and Ta is a time at which the time integration value of a resistance between the electrodes in the main current flow reaches Ro.

2. The resistance spot welding method according to claim 1, further characterized in that the reference electrode force switching point To satisfies the following formula: 0.1 X Tm < To < 0.8 X Tm.

3. The resistance spot welding method according to claim 1 or 2, further characterized in that it comprises: performing the test weld;and performing the actual welding, including the main current passage, after the test welding, wherein in the main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property QRZfrnn / zznz / e / YiAi between the electrodes to form an appropriate nugget by performing the current passage by means of constant current control is stored, and in the main current passage in the actual welding, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that is stored in the main current passage in the test welding set as a target and an amount of current passage is controlled according to the target.

4. A method of producing welded members characterized in that it comprises joining a plurality of overlapping metal sheets by the resistance spot welding method 10 as claimed in any of claims 1 to 3.

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