Resistance spot welding method and manufacturing method of welded components

Abstract

To provide a resistance spot welding method capable of stably forming a nugget of a desired nugget diameter even if there is external disturbance.SOLUTION: This resistance spot welding method is for holding a welded member formed by stacking a plurality of metal plates 1, 2 between a pair of electrodes 4 and 5, and pressuring and energizing the member to join it. In a pressurizing step before welding, a voltage is applied between the electrodes 4 and 5 so as to supply current during contact of the metal plates, time T from when a pressurizing force applied from the metal plates 1, 2 to the electrodes 4 and 5 is detected to when energization is confirmed between the electrodes 4 and 5 is measured, an aimed accumulation heat generation amount is set on the basis of the measured time T, and the welded member to be joined is joined by controlling an energization amount in accordance with the aimed accumulation heat generation amount.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a resistance spot welding method and a method for manufacturing a welded component.

[0002] Resistance spot welding, a type of lap resistance welding, is commonly used to join overlapping metal sheets, such as steel sheets. This welding method involves sandwiching two or more overlapping metal sheets and applying pressure from above and below with a pair of electrodes while passing a high welding current between the upper and lower electrodes for a short period of time to join them. The high-current welding current generates resistance heat, resulting in a spot-like weld. This spot-like weld is called a "nugget," and is the area where the overlapping metal sheets melt and solidify at the point where they contact when a current is passed through them. This nugget joins the metal sheets together in a spot-like manner.

[0003] In order to obtain good weld quality, it is important to adjust the nugget diameter within an appropriate range. The nugget diameter is determined by welding conditions such as welding current, welding time, electrode shape, and welding pressure. Therefore, in order to obtain an appropriate nugget diameter, it is necessary to appropriately set the above welding conditions according to the conditions of the workpieces, such as the material, plate thickness, and number of overlapping sheets.

[0004] For example, when manufacturing an automobile, thousands of spot welds are applied to each vehicle, and it is necessary to weld the workpieces that flow in one after another. In this case, if the conditions of the workpieces at each welding point, such as the material, plate thickness, and number of overlapping sheets, are the same, the same nugget diameter can be obtained under the same welding conditions, such as the welding current, welding time, and pressure.

[0005] However, with continuous welding, the contact surface of the electrode with the workpiece gradually wears, and the contact area gradually becomes larger than the initial state. If the same welding current as in the initial state is applied when the electrode contact area becomes larger, the current density in the workpiece decreases, the temperature rise in the weld becomes smaller, and the nugget diameter becomes smaller. For this reason, the electrode is polished or replaced every few hundred to several thousand welding points to prevent the electrode tip diameter from becoming too large.

[0006] In addition, resistance welding devices have been used that have a function (stepper function) that increases the welding current value after a predetermined number of welding operations to compensate for the decrease in current density due to electrode wear. To use this stepper function, it is necessary to properly set the welding current change pattern described above in advance.

[0007] However, this requires a lot of time and money to derive welding current change patterns that correspond to a large number of welding conditions and weld material conditions through testing, etc. Furthermore, in actual welding, the progress of electrode wear varies, so it cannot be said that a predetermined welding current change pattern is always appropriate.

[0008] Furthermore, if there is a disturbance during welding, such as a previously welded point near the welding point, or if the surface of the workpiece is uneven and a contact point is present near the welding point, the current will be shunted to the previously welded point and the contact point. In such a situation, even if welding is performed under specified conditions, the current density at the desired welding position directly below the electrode will be low, and a nugget of the required diameter will not be obtained. To compensate for this lack of heat generation and obtain a nugget of the required diameter, a high welding current must be set in advance.

[0009] Furthermore, if the area around the welding point is tightly constrained due to surface irregularities or the shape of the components, or if a foreign object is trapped between the metal plates around the welding point, the gap between the metal plates (hereinafter also referred to as the "plate gap") will become larger, narrowing the contact diameter between the metal plates and making splashing more likely to occur.

[0010] To solve this welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in welding phenomena due to electrode wear and disturbances are directly measured or calculated as electrical signals in the form of changes in current, voltage, resistance, and heat generation during welding, and input parameters such as welding current and voltage are controlled based on these values.

[0011] Patent Document 1 describes a method for controlling welding conditions for a resistance welding machine that aims to perform good welding by detecting the welding current and the voltage between the tips, simulating the welded part through heat conduction calculations, and estimating the nugget formation state of the welded part during welding.

[0012] Patent Document 2 describes a resistance welding system that uses a welding system that calculates the cumulative heat generation amount per unit volume that will allow the workpieces to be welded well, based on the plate thickness and current flow time of the workpieces, and adjusts the welding current or voltage to generate the calculated amount of heat generation per unit volume and unit time, thereby achieving good welding regardless of the type of workpieces to be welded or the state of wear of the electrodes.

[0013] Patent Document 3 describes a resistance spot welding method in which a welding pattern is divided into two steps: a step for securing a current path directly below the electrode, and a subsequent step for forming a nugget of a specified diameter.The method stores as target values ​​the change in instantaneous heat generation per unit volume over time and the cumulative heat generation per unit volume, which are calculated from the electrical characteristics between the electrodes when a current is passed through using constant current control in a test welding to form an appropriate nugget.The method then controls the amount of current passed so that the cumulative heat generation in the actual welding matches the cumulative heat generation determined in advance in the test welding, thereby achieving a nugget diameter of a certain size or larger.

[0014] Patent Document 4 describes a resistance spot welding method in which, when storing the cumulative heat generation amount in test welding, a disturbance state is simulated and stored, and the amount of current is controlled so that the cumulative heat generation amount in the actual welding matches the cumulative heat generation amount previously determined in the test welding, thereby achieving a nugget diameter of a certain size or larger.

[0015] Patent Document 5 describes a resistance spot welding method that can stably obtain a desired nugget diameter by applying pressure until an initial initial pressure is reached before starting current application for actual welding, measuring the pressure in the same way during actual welding, and setting the pressure during current application using a pressure index parameter obtained from the start of current application until the initial pressure is reached. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Publication No. 10-94883 [Patent Document 2] Japanese Patent Application Publication No. 11-33743 [Patent Document 3] International Publication No. 2015 / 049998 [Patent Document 4] Japanese Patent Application Laid-Open No. 2019-34341 [Patent Document 5] International Publication No. 2020 / 095847 Summary of the Invention [Problem to be solved by the invention]

[0017] However, the technology described in Patent Document 1 requires complex calculations to estimate the nugget temperature based on a heat conduction model (heat conduction simulation), etc., which not only makes the configuration of the welding control device complex, but also makes the welding control device itself expensive.

[0018] Furthermore, the technology described in Patent Document 2 is believed to be able to perform good welding even if the electrode has worn down a certain amount by controlling the cumulative heat generation to a target value. However, if the set conditions for the workpieces differ significantly from the actual conditions for the workpieces, for example, if there is a large gap between the metal plates to be welded, even if the final cumulative heat generation can be matched to the target value, the heat generation pattern, i.e., the change over time in the temperature distribution of the weld, may deviate from the heat pattern that results in the desired good weld, and the required nugget diameter may not be obtained or expulsion may occur.

[0019] Furthermore, in the techniques described in Patent Documents 3 and 4, the cumulative heat generation amount is stored regardless of whether or not there is a disturbance, but there is a problem in that the weldability becomes unstable in response to an unknown disturbance.

[0020] Furthermore, the technology described in Patent Document 5 only deals with the increase in pressure application time due to the plate gap, but there was a problem in that it may not be the optimal solution for fluctuations caused by multiple disturbances, such as when the plate gap is combined with the impact angle.

[0021] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a resistance spot welding method that can stably form a nugget with a desired nugget diameter even when external disturbances are present. [Means for solving the problem]

[0022] Now, the inventors have conducted extensive experimental studies to find a method of setting conditions that will solve the above-mentioned problems and enable stable formation of a nugget with a desired nugget diameter even when external disturbances are present, and have obtained the following findings.

[0023] (1) When the gap is large, the contact area between the metal plates (hereinafter simply referred to as "metal plates") that make up the welded materials is small at the start of current flow, resulting in a high detected resistance and increased likelihood of expulsion. Furthermore, if excessive pressure is applied to the metal plates when the gap is large, the metal plates will warp significantly. As a result, the contact area between the metal plates and the electrode increases excessively, reducing the contact resistance in that area. This significantly changes the electrical characteristics of the plate assembly and also promotes heat transfer to the electrode, resulting in a smaller nugget diameter and thickness. (2) In order to mitigate the influence of such a gap, it is effective to set the pressing force during current application according to the size of the gap and to apply a cumulative heat amount according to the contact state between the metal plates during current application, especially at the start of current application. (3) The effect of the gap is reflected in a parameter that serves as an index of the electrode position from the start of pressure application until the specified set pressure (initial pressure) is reached. For example, if pressure is applied to metal plates under the same conditions when there is no gap between them and when there is a gap between them, the change in pressure will be the same in both cases after the plates come into contact. (4) If there is a gap between the plates, the pressure force is not transmitted directly to the metal plate, and the rate at which the pressure force increases will be slower than the set rate until the gap disappears. (5) However, when relying solely on the pressure parameter, there are cases where the fluctuation in the gap cannot be accurately captured due to factors such as vibration of the metal plate.

[0024] Based on the above findings, the inventors have further investigated a method for setting the cumulative heat generation amount during energization depending on the size of the plate gap, and have come to the following findings. (6) To accurately determine the gap state, it is necessary to use at least one parameter in addition to the applied pressure. High accuracy can be achieved by using the electrical parameter between the electrodes as this parameter. (7) This mechanism is based on the fact that when metal plates are in contact with each other, there is a significant difference in the resistance value compared to when they are not in contact, and it is considered reasonable to capture this difference. (8) Therefore, contact between the metal plates and the electrodes is detected using the pressure parameter, and as an electrical parameter, a voltage is applied between the electrodes so that a weak current flows when multiple metal plates come into contact, from the perspective of capturing resistance fluctuations. Then, by measuring the time T from when the measured pressure is first detected until current flow is confirmed, it is possible to detect the gap with high accuracy, which was previously difficult. Then, by setting the target heat quantity based on this detection, it becomes possible to stably obtain a nugget of the desired diameter without the influence of the gap and without the occurrence of expulsion.

[0025] Specifically, in a resistance spot welding method, a workpiece consisting of multiple overlapping metal plates is sandwiched between a pair of electrodes and joined by applying pressure and passing an electric current through it. In the pressure application step prior to welding, a voltage is applied between the pair of electrodes so that current flows when the multiple metal plates come into contact. The time T is measured from the time when the pressure applied from the metal plates to the electrodes is detected to the time when current is confirmed between the pair of electrodes. A target cumulative heat generation amount is set based on the measured time T, and the amount of current passed is controlled in accordance with the target cumulative heat generation amount. This discovery led to the completion of the present invention, which has made it possible to stably form a nugget of the desired diameter even when external disturbances are present.

[0026] That is, the present invention that solves the above problems is as follows. [1] A resistance spot welding method in which a workpiece made of overlapping metal plates is sandwiched between a pair of electrodes and joined by applying current while applying pressure, In a pressure application step before welding, a voltage is applied between the pair of electrodes so that a current flows when the plurality of metal plates contact each other, measuring a time T from the time when the pressure force from the metal plate to the electrode is detected to the time when current conduction between the pair of electrodes is confirmed; a resistance spot welding method for joining the workpieces to be joined, the resistance spot welding method comprising: setting a target cumulative heat generation amount based on the measured time T; and controlling the amount of current flowing in accordance with the target cumulative heat generation amount.

[0027] [2]Prior to this welding, under conditions without disturbance, a test weldment in which a plurality of metal plates are stacked is sandwiched between a pair of electrodes, and from the time when the pressing force from the metal plate to the electrode is detected until the time when energization between the pair of electrodes is confirmed, the time Ts is measured, and based on the time T and Ts, the target cumulative heat generation amount is set, the resistance spot welding method according to [1].

[0028] [3]Taking the cumulative heat generation amount Es that forms a nugget desired when there is no disturbance, when T / Ts > 1, the target cumulative heat generation amount E at the time of this welding is set so that E < Es is satisfied, the resistance spot welding method according to [2].

[0029] [4]Prior to this welding, test welding is performed, In the test welding, the cumulative heat generation amount per unit volume calculated from the electrical characteristics between the pair of electrodes when energizing under constant current control to form a nugget with the predetermined nugget diameter is obtained, In the energization of this welding, the cumulative heat generation amount per unit volume obtained in the test welding is set as the target cumulative heat generation amount, and the energization amount is controlled according to the target cumulative heat generation amount, the resistance spot welding method according to any one of [1] to [3].

[0030] [5]By the resistance spot welding method according to any one of [1] to [4], a weldment in which a plurality of metal plates are stacked is joined to obtain a welded member, a method for manufacturing a welded member.

Advantages of the Invention

[0031] According to the present invention, even when there is disturbance, a nugget with a desired nugget diameter can be stably formed.

Brief Description of the Drawings

[0032] [Figure 1] It is a diagram for explaining the outline of the resistance spot welding method. [Figure 2]10A and 10B are diagrams showing the distance L between the electrodes from when pressure starts to reach the initial pressure setting, where (a) is for when there is no gap between the metal plates and (b) is for when there is a gap between the metal plates. [Figure 3] 10A is a diagram showing the relationship between time and measured current, and FIG. 10B is a diagram showing the relationship between time and measured applied pressure. [Figure 4] 10A and 10B are diagrams illustrating a mechanism for detecting contact between metal plates. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. As shown in Fig. 1, a resistance spot welding method according to the present invention is a resistance spot welding method in which a workpiece 3 consisting of multiple overlapping metal sheets 1 and 2 is sandwiched between a pair of electrodes 4 and 5 and joined by applying pressure and passing current through them. Resistance heat generated by the current passing through the workpiece 3 forms a spot-like weld 6. This spot-like weld 6 is called a "nugget," and is formed when a welding current is passed through the overlapping metal sheets 1 and 2, melting and solidifying the metal sheets 1 and 2 at their contact points, thereby joining the metal sheets 1 and 2 together at a spot.

[0034] In the resistance spot welding method according to the present invention, in the pressure application step prior to welding, a voltage is applied between a pair of electrodes so that current flows when multiple metal plates contact each other. The time T is measured from the time when the pressure applied from the metal plates to the electrodes is detected to the time when current is confirmed between the pair of electrodes. A target cumulative heat generation amount is set based on the measured time T, and the amount of current flow is controlled in accordance with the target cumulative heat generation amount to join the workpieces to be joined.

[0035] As mentioned above, in order to mitigate the effects of disturbances such as plate gaps, it is effective to set a target cumulative heat generation amount according to the magnitude of the disturbance. Furthermore, the effects of disturbances are reflected in a parameter that serves as an index of the electrode position (electrode movement amount) from the start of pressure application to the end. Therefore, when applying pressure to the workpiece to reach the initial pressurized pressure before the start of current application for the actual welding, it is important to set the cumulative heat generation amount during current application for the actual welding using a parameter that serves as an index of the electrode position obtained from the start of pressure application before the start of current application for the actual welding until the initial pressurized pressure is reached.

[0036] For example, when pressure is applied to metal plates 1 and 2 under the same conditions in a case where there is no gap between them as shown in Figure 2(a) and a case where there is a gap between them as shown in Figure 2(b), the amount of movement of electrodes 4 and 5 (i.e., the distance between the electrodes) L from the start of pressure application until the initial pressure is reached will be different for each case.

[0037] That is, if there is no gap between the metal sheets 1 and 2, after the electrodes 4 and 5 start to apply pressure, the initial pressure is reached with a relatively small amount of movement of the electrodes 4 and 5. On the other hand, if there is a gap between the metal sheets 1 and 2, the metal sheets 1 and 2 will be deformed in the initial stage of pressure application, causing them to come into contact with each other. As a result, the rate at which the pressure increases differs before and after contact, and the electrodes 4 and 5 will move a greater distance before the initial pressure is reached.

[0038] However, it can be difficult to accurately detect the distance between the electrodes (the amount of movement of the electrodes). In particular, when the metal plates 1 and 2 come into contact with each other, an impact occurs, which can cause errors in the measurement of the applied pressure. Plastic deformation of the metal plates 1 and 2 can also cause errors in the measurement position. In contrast, the contact between the electrodes 4 and 5 and the metal plates 1 and 2 is easier to detect than the contact between the metal plates 1 and 2, because it is the timing when the applied pressure to the electrodes 4 and 5 changes from zero to a significant value.

[0039] Figure 3 shows the relationship between time and the measured current between electrodes 4 and 5 and the measured applied pressure of electrodes 4 and 5, with (a) showing the relationship between time and the measured current between electrodes 4 and 5, and (b) showing the relationship between time and the measured applied pressure of electrodes 4 and 5. As shown in Figure 3(a), by applying a voltage between electrodes 4 and 5, current flows when contact occurs between metal plates 1 and 2, and the current value increases from zero to a predetermined value. On the other hand, as shown in Figure 3(b), when electrodes 4 and 5 come into contact with metal plates 1 and 2, the applied pressure gradually increases from zero and reaches a constant value when the gap between metal plates 1 and 2 disappears.

[0040] Therefore, in the present invention, as shown in Figure 4, the welding pressure transmitted from the metal sheets 1 and 2 to the electrodes 4 and 5 and the current between the electrodes 4 and 5 can be measured during welding, and in the pressure application step before welding, a voltage is applied between the electrodes 4 and 5 so that a current flows when the metal sheets 1 and 2 come into contact. Then, by measuring the time T from the time when the pressure from the metal sheets 1 and 2 to the electrodes 4 and 5 is detected to the time when current is confirmed between the electrodes 4 and 5, the gap distance can be accurately measured. In addition, if there is a difference between the time when the pressure force from metal plate 1 to electrode 4 is detected and the time when the pressure force from metal plate 2 to electrode 5 is detected, the time when the pressure force is detected first will be considered to be the ``time when the pressure force from the metal plate to the electrode is detected.''

[0041] The voltage to be applied between the electrodes 4 and 5 is not particularly limited as long as a current sufficient to detect contact between the metal plates 1 and 2 flows between the electrodes 4 and 5. However, it is preferable to apply a voltage such that the current flowing between the electrodes 4 and 5 is 200 A or more and 3000 A or less.

[0042] The parameters that serve as indicators of the electrode positions are: Electrode displacement, electrode displacement speed, and pressure are measured directly by an external sensor. Electrode displacement and pressure are indirectly measured by the distortion of the gun or housing. In the case of a servo pressure mechanism, the reaction force detected by the mechanism, the torque value of the servo motor, the rotation speed, and the number of revolutions -The above variable times Examples include:

[0043] In addition, the parameters that serve as indicators of current and voltage are: Current and voltage output by the welding timer used for welding Current and voltage output by external testers and monitoring devices It is desirable to use these parameters to detect the gap.

[0044] In the present invention, it is necessary to determine the electrode contact time points, but a combination of methods may be used to calculate this. For example, the electrode movement amount may be determined by measuring the electrode position with a sensor, and the contact time points of each electrode with the metal plate may be measured using the fluctuation in the pressure.

[0045] As described above, the present invention is characterized by the method for setting the target cumulative heat generation amount when performing main welding, and other configurations are not particularly limited.

[0046] A welding device that can be used in the resistance spot welding method of the present invention is not particularly limited as long as it has a pair of upper and lower electrodes and can arbitrarily control the welding force and welding current during welding, and the type (stationary type, robot gun, etc.), electrode shape, etc.

[0047] The multiple metal plates constituting the workpieces to be welded can be, for example, steel plates. The steel plates may be untreated steel plates. Alternatively, the steel plates may be surface-treated steel plates that have been subjected to a surface treatment such as plating, or two or more of the multiple metal plates may be steel plates with a plating layer. Alternatively, all of the multiple steel plates may be steel plates with a plating layer.

[0048] Examples of the steel sheet having the plating layer include zinc-based plated steel sheets. The zinc-based plated steel sheet is a steel sheet having a zinc-based plating coated on the steel sheet by various methods such as a hot-dip plating method, an electroplating method, a vapor deposition plating method, a thermal spraying method, etc. Examples of the zinc-based plating include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, electro-zinc plating, electro-zinc-nickel alloy plating, etc., but are not limited thereto, and all known zinc-based platings containing zinc are applicable. Examples of such zinc-based plated steel sheets include a hot-dip zinc plated steel sheet (GI) without an alloying treatment and an alloyed hot-dip zinc plated steel sheet (GA) with an alloying treatment.

[0049] In the present invention, prior to this welding, test pressurization is performed, and under conditions without disturbances such as plate gaps, a test welded material obtained by overlapping a plurality of metal plates is sandwiched between a pair of electrodes, and from the time when the pressing force from the metal plate to the electrode is detected, the time Ts from the time when energization between the pair of electrodes is confirmed is measured, and based on the time T and Ts, it is preferable to set the target cumulative heat generation amount. Thereby, welding can be stabilized even when the measurement time fluctuates due to various disturbances.

[0050] And, when the cumulative heat generation amount for forming a nugget desired in the case of no disturbance is defined as Es, when T / Ts > 1, it is preferable to set the target cumulative heat generation amount E at the time of this welding so that E < Es (that is, E / Es < 1) is satisfied. Thereby, a nugget with an appropriate nugget diameter can be formed without applying a heat generation amount that becomes unnecessary due to the presence of a plate gap.

[0051] Also, in the present invention, prior to this welding, test welding is performed under the same conditions as the above test pressurization. In the test welding, the cumulative heat generation amount per unit volume calculated from the electrical characteristics between a pair of electrodes when forming a nugget with a predetermined nugget diameter by energization under constant current control is obtained Mel which is preferable. Thereby, control can be performed based on the heat generation phenomenon, and welding can be stabilized.

[0052] Here, the method for calculating the calorific value is not particularly limited, but an example is disclosed in Patent Document 2, and this method can also be adopted in the present invention. The calculation procedure for the calorific value q per unit volume / unit time and the cumulative calorific value Q per unit volume according to the method described in Patent Document 2 is as follows.

[0053] That is, let t be the total thickness of the workpieces, such as steel plates, r be the electrical resistivity of the workpieces, V be the voltage between the electrodes, I be the welding current, and S be the area of ​​contact between the electrode and workpieces. In this case, the welding current I passes through a columnar portion with a cross-sectional area of ​​S and a thickness of t, generating resistance heat. The amount of heat q generated per unit volume and unit time in this columnar portion can be calculated using the following equation (1): q=(V·I) / (S·t) (1)

[0054] The electrical resistance R of the columnar portion is calculated by the following formula (2). R=(r·t) / S (2)

[0055] By solving equation (2) for S and substituting it into equation (1), the heat generation amount q is given by the following equation (3). q=(V·I·R) / (r·t 2 )=(V 2 ) / (r·t 2 ) (3)

[0056] As is clear from the above equation (3), the amount of heat generated per unit volume and unit time, q, can be calculated from the interelectrode voltage, V, the total thickness of the metal plates, t, and the electrical resistivity, r, of the metal plates, and is not affected by the contact area, S, between the electrode and the metal plate. While equation (3) calculates the amount of heat generated, q, from the interelectrode voltage, V, it can also be calculated from the interelectrode current, I, without needing to use the contact area, S, between the electrode and the metal plate. Furthermore, by accumulating the amount of heat generated, q, per unit volume and unit time over the current application period, the cumulative amount of heat generated per unit volume, Q, during welding can be obtained. As is clear from equation (3), this cumulative amount of heat generated per unit volume, Q, can also be calculated without using the contact area, S, between the electrode and the metal plate.

[0057] The pressure P set in the present invention may be set appropriately depending on the material and thickness of the metal plates 1 and 2 constituting the workpiece 3. For example, when the workpieces are a sheet set consisting of two 1.4 mm thick, 270 MPa to 2000 MPa grade steel plates each having a Zn-plated surface, the initial pressure is preferably set to 1.0 kN to 7.0 kN. When the workpieces are a sheet set consisting of three 1.4 mm thick, 270 MPa to 2000 MPa grade steel plates each having a Zn-plated surface, the initial pressure is preferably set to 2.0 kN to 10.0 kN.

[0058] The present invention performs adaptive control welding, in which welding is performed using the target value for the amount of heat generated as a reference, and if the time change in the instantaneous amount of heat generated per unit volume is along the reference time change curve, the welding is continued as is and the welding is terminated. However, if the time change in the instantaneous amount of heat generated per unit volume deviates from the reference time change curve, the amount of current supplied is controlled so that the difference in heat generation amount per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume set as the target value in order to compensate for the difference within the remaining current supply time.

[0059] The desired nugget diameter of the nugget formed by the resistance spot welding method of the present invention may be any value greater than or equal to that required for production, but it is desirable to aim for a nugget diameter between the plates that is greater than or equal to 4√t (mm), where t (mm) is the thickness of the thinner plate between the two plates.

[0060] (Method of manufacturing welded components) A method for manufacturing a welded component according to the present invention is characterized in that a plurality of overlapping workpieces are joined by the resistance spot welding method according to the present invention described above.

[0061] As described above, in the resistance spot welding method according to the present invention, a voltage is applied between the pair of electrodes during the pressure application step prior to welding so that current flows when the multiple metal plates contact each other. The time T is measured from the time when the pressure applied from the metal plates to the electrodes is detected to the time when current flow between the pair of electrodes is confirmed. A target cumulative heat generation amount is set based on the measured time T, and the current flow amount is controlled according to the target cumulative heat generation amount. This allows a nugget of the desired diameter to be formed even in the presence of external disturbances. Therefore, by joining multiple overlapping metal plates to be welded using the resistance spot welding method according to the present invention, a welded component with a nugget of the desired diameter and high weld strength can be produced. [Example]

[0062] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0063] For sheet pairs Nos. 1 to 22, each consisting of the metal sheets shown in Table 1, welded joints were produced by welding under the conditions shown in Table 1, either with or without a sheet gap. For sheet pairs Nos. 11 and 12, each consisting of three metal sheets, a sheet gap was provided only between metal sheets 1 and 2. For each welded joint, the weld was cut, the cross section etched, and then observed under an optical microscope to measure the nugget diameter. The measured nugget diameter and the deviation of the measured nugget diameter from the target nugget diameter were evaluated as follows. The target nugget diameter was based on the case without a sheet gap, and sheet gaps were set in the range of 0.1 mm to 3.0 mm, and the deviations in these cases were compared. The evaluation results are shown in Table 1.

[0064] [Table 1]

[0065] Diameter determination: ○ (Pass): The nugget diameter is larger than the target nugget diameter. × (Fail): The nugget diameter is smaller than the target nugget diameter.

[0066] Diameter Variation: ○ (Pass): The nugget diameter varies from the target nugget diameter by 0.3 × (plate thickness (mm)) square root (mm) or less. × (Fail): The nugget diameter varies from the target nugget diameter by more than the square root (mm) of 0.5 × (plate thickness (mm)). Note that (plate thickness) here refers to the thickness (mm) of the thinnest steel plate in the plate assembly.

[0067] As is clear from Table 1, in all of the inventive examples, not only was a predetermined nugget diameter obtained, but the variation was small, regardless of the sheet gap. On the other hand, in the conventional examples, although a sufficient nugget diameter was obtained, the variation in nugget diameter was large. [Industrial Applicability]

[0068] According to the present invention, even when disturbances exist, a nugget having a desired nugget diameter can be stably formed. [Explanation of symbols]

[0069] 1, 2 metal plate 3. Welding material 4, 5 electrodes 6 Welding section (nugget)

Claims

1. A resistance spot welding method in which a workpiece made of overlapping metal plates is sandwiched between a pair of electrodes and joined by applying pressure and passing current through the electrodes, In a pressure application step before welding, a voltage is applied between the pair of electrodes so that a current flows when the plurality of metal plates contact each other, measuring a time T from the time when the pressure force from the metal plate to the electrode is detected to the time when current conduction between the pair of electrodes is confirmed; A target cumulative heat generation amount per unit volume is set based on the measured time T, and the amount of current is controlled in accordance with the target cumulative heat generation amount, thereby joining the workpieces to be joined; Prior to the actual welding, under conditions without disturbance, a test workpiece made of overlapping metal plates is sandwiched between a pair of electrodes, and a time Ts is measured from the time when a pressure applied from the metal plate to the electrode is detected to the time when current is passed between the pair of electrodes, and the target cumulative heat generation amount is set based on the time T and Ts. a resistance spot welding method characterized in that, when T / Ts>1, a target cumulative heat generation amount E is set so as to satisfy E<Es, where Es is a cumulative heat generation amount that forms a desired nugget in the absence of external disturbances.

2. 2. A method for manufacturing a welded component, comprising joining a plurality of overlapping metal plates to be welded by the resistance spot welding method of claim 1 to obtain a welded component.

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