Resistance spot welding method and manufacturing method for welded components

Abstract

To provide a resistance spot welding method capable of stably forming a nugget of a required nugget diameter even when disturbance exists.SOLUTION: Displacement quantity of a pair of electrodes from the time point when pressurization to material to be welded for test is started in a case where there is no disturbance to the time point when fluctuation of pressurizing force disappears is A, displacement quantity of the pair of electrodes from the time point when pressurization to material to be welded of a joining object is started to the time point when fluctuation of a pressurizing force disappears is B, and a displacement quantity of the pair of electrodes at the time point when an increase amount of the pressurizing force to the material to be welded of the joining object arrives at an increase amount of the pressurizing force with respect to the displacement amount of the pair of electrodes in such a case that there is no disturbance is C and, therein, aimed accumulation heat generation quantity is preset based on A, B and C, an energization amount is controlled in accordance with the aimed accumulation heat generation quantity and the material to be welded of the joining object is joined.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] In general, resistance spot welding, which is a type of lap resistance welding, is used to join overlapping metal plates, for example steel plates. This welding method is a method in which two or more overlapping steel plates are sandwiched and pressed from above and below with a pair of electrodes while a high welding current is passed between the upper and lower electrodes for a short period of time to join them, and a spot-like weld is obtained by utilizing the resistance heat generated by passing a high welding current. This spot-like weld is called a "nugget" and is a part where the two steel plates melt and solidify at the contact point when a current is passed through the overlapping steel plates. The steel plates are joined at a spot by this nugget.

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

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

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

[0006] In addition, resistance welding devices equipped with 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 have been used in the past. To use this stepper function, it is necessary to properly set the above-mentioned welding current change pattern in advance.

[0007] However, it takes a lot of time and money to derive welding current change patterns corresponding to many welding conditions and conditions of materials to be welded through tests, etc. In addition, 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, for example, if there is a previously welded point (previously welded point) near the point to be welded, or if the surface unevenness of the material to be welded is large and there is a contact point of the material to be welded near the point to be welded, the current will be shunted to the previously welded point and the contact point during welding. In such a state, even if welding is performed under specified conditions, the current density at the position to be welded directly below the electrode will decrease, and it will not be possible to obtain a nugget of the required diameter. To compensate for this lack of heat generation and obtain a nugget of the required diameter, it is necessary to set a high welding current 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 steel sheets around the welding point, the gap between the steel sheets will become larger, narrowing the contact diameter between the steel sheets and making splashing more likely to occur.

[0010] In order to solve such welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in the welding phenomenon due to the above-mentioned 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 the welding current and voltage are controlled based on the values.

[0011] Patent Document 1 describes a method for controlling welding conditions of 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 by 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 well welded 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 wear state 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 under the electrode, followed by a step for forming a nugget of a specified diameter; 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 under constant current control in test welding to form an appropriate nugget, are each stored as target values; and the current flow is controlled so that the cumulative heat generation in this welding matches the cumulative heat generation previously determined in test welding, thereby obtaining a nugget diameter of a certain size or more.

[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 then 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 obtaining a nugget diameter of a certain size or more.

[0015] Patent Document 5 describes a resistance spot welding method in which pressure is applied until an initial set pressure is reached before current begins to flow for the actual welding, the pressure is similarly measured during the actual welding, and the pressure during current flow is set using a pressure index parameter obtained from the start of current flow until the initial set pressure is reached, thereby enabling a stable production of a desired nugget diameter. [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] JP 2019-34341 A [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 complicates the configuration of the welding control device but also makes the welding control device itself expensive.

[0018] In addition, the technology described in Patent Document 2 is believed to be able to perform good welding even if the electrodes are worn to a certain extent by controlling the cumulative heat generation amount to a target value. However, when the set conditions of the workpieces and the actual conditions of the workpieces are significantly different, for example, when there is a large gap between the metal plates to be welded, even if the final cumulative heat generation amount can be matched to the target value, the form of heat generation, that is, the change over time in the temperature distribution of the welded part, deviates from the heat pattern that results in the desired good welded part, and the required nugget diameter cannot be obtained or expulsion occurs.

[0019] Furthermore, in the techniques described in Patent Documents 3 and 4, the accumulated heat amount is stored regardless of the presence or absence of disturbance, but there is a problem in that the weldability becomes unstable in response to unknown disturbances.

[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 is a problem in that it may not be an optimal solution for fluctuations due to multiple disturbances, such as when the plate gap is combined with the impact angle.

[0021] The present invention has been made in consideration of the above problems, and has an object 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 determine a condition setting method that can solve the above-mentioned problems and stably form a nugget with a desired diameter even when external disturbances are present, and have obtained the following findings.

[0023] - 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, so the resistance is detected as high and expulsion is more likely to occur. In addition, 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 electrodes increases excessively, the contact resistance in that area decreases, and the electrical characteristics of the plate assembly change significantly. In addition, heat transfer to the electrodes is promoted, resulting in a smaller nugget diameter and thickness. In order to mitigate the effect 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 generation amount according to the contact state between the metal sheets during current application, especially at the start of current application.

[0024] Based on the above findings, the inventors have further investigated a method for setting the cumulative heat generation amount during current flow depending on the size of the plate gap, and have obtained the following findings. The effect of the gap is reflected in a parameter that serves as an index of the electrode position from when pressure begins to be applied until the specified set pressure is reached. For example, if pressure is applied to metal plates under the same conditions when there is a gap between the metal plates and when there is a gap between the metal plates, the change in pressure will be the same in both cases after the plates come into contact. At this time, if there is a gap between the plates, the pressure force is not transmitted directly to the metal plate, so the rate at which the pressure force increases will be slower than the set rate until the gap disappears. Therefore, first, before the start of current flow, in the case where there is no plate gap, the above-mentioned workpieces are pressed until a predetermined set pressure (hereinafter also referred to as the initial pressure) is reached, and the pressure force increase rate (hereinafter referred to as the initial gradient ΔFA) and the position (amount of movement) A of the pair of electrodes from the start to the end of pressing (i.e., the time when the initial pressure force is reached) are recorded. Next, a pressing test is performed on the plate pair in the case where there is a plate gap, and the electrode position (amount of movement) C at which the pressure force increase rate ΔF becomes equal to or greater than ΔFA and the electrode position (amount of movement) C at which pressing is completed (i.e., the time when the set pressure force is reached) are recorded, and the cumulative heat generation amount is set based on the electrode movements A, B, and C. This makes it possible to obtain heat generation that matches the contact area at the start of current flow and during current flow, and as a result, it becomes possible to stably obtain the desired nugget diameter without the occurrence of expulsion, regardless of the influence of the plate gap. Note that the "electrode movement amount" means the sum of the movement amounts of each of the two electrodes. For example, when only one of the two electrodes moves, the "electrode movement amount" is the movement amount of the electrode that moves. Alternatively, when both of the two electrodes move, the "electrode movement amount" means the sum of the movement amounts of the two electrodes that move. In other words, the "electrode movement amount" means the difference between the distance between the two electrodes before the movement and the distance between the two electrodes after the movement.

[0025] It is known that there is a quantitative relationship between the disturbance and the change in applied pressure from when the electrodes come into contact until the pressure is stabilized, as in the method disclosed in Patent Document 5. Therefore, the inventors evaluated the change in applied pressure for various disturbances.

[0026] As a result, the inventors discovered that stable welding is possible even when subjected to complex disturbances by controlling the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume, which are calculated from the electrical characteristics between a pair of electrodes in relation to the amount of electrode movement A from the start of pressure to the time when the pressure fluctuations stop, which is obtained when test pressure is applied prior to actual welding, the amount of electrode movement B from the start of pressure to the time when the pressure fluctuations stop during actual welding, and the amount of electrode movement C to the time when the rate of increase in the pressure in actual welding reaches the rate of increase in the pressure in test pressure, and this has led to the completion of the present invention.

[0027] 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 multiple metal plates is sandwiched between a pair of electrodes and joined by passing electricity through the electrodes while applying pressure, Prior to the actual welding, a test welding material, which is a test workpiece made of overlapping metal plates, is sandwiched between a pair of electrodes under a condition without disturbance, and a test pressure is applied until the fluctuation of the pressure force disappears, to obtain a first relationship between the movement amount of the pair of electrodes and the pressure force; In the main welding, A workpiece to be welded, which is a plurality of overlapping metal plates, is sandwiched between the pair of electrodes, and pressure is applied until the fluctuation in the pressure force is eliminated to obtain a second relationship between the movement amount of the pair of electrodes and the pressure force; a movement amount of the pair of electrodes from the start of applying pressure to the test workpieces to a time when the fluctuation in the pressing force stops in the first relationship is defined as A; a movement amount of the pair of electrodes from the start of applying pressure to the workpieces to be joined to a time when the fluctuation in the pressing force stops in the second relationship is defined as B; and a movement amount of the pair of electrodes at a time when an increase in the pressing force on the workpieces to be joined relative to the movement amount of the pair of electrodes in the second relationship reaches an increase in the pressing force relative to the movement amount of the pair of electrodes in the first relationship is C, wherein a target cumulative heat amount is set based on A, B, and C, and the amount of current is controlled in accordance with the target cumulative heat amount to join the workpieces to be joined.

[0028] [2] Prior to the actual welding, a test welding is performed under the same conditions as the test pressure. In the test welding, a time change curve of an instantaneous heat generation amount per unit volume and an accumulated heat generation amount per unit volume are calculated from an electrical characteristic between the pair of electrodes when a current is passed under constant current control to form a nugget having the predetermined nugget diameter, The resistance spot welding method according to [1], wherein, in the current supply for the main welding, the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume obtained in the test welding are set as a target cumulative heat generation amount, and the amount of current supplied is controlled in accordance with the target cumulative heat generation amount.

[0029] [3] The resistance spot welding method according to [2], wherein E is a cumulative heat generation amount capable of forming a nugget having the specified nugget diameter in the test welding, and E' is a target cumulative heat generation amount in the main welding, and E' is set so as to satisfy the following formula (1): C / B≦E' / E≦(1-0.01×(BC) / A) (1)

[0030] [4] A method for producing a welded component, comprising joining a plurality of overlapping metal plates to be welded by the resistance spot welding method according to any one of [1] to [3] above, to obtain a welded component. Effect of the Invention

[0031] According to the present invention, even when disturbances exist, a nugget having a desired diameter can be stably formed. [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 is a diagram for explaining an outline of a resistance spot welding method. [Diagram 2] FIG. 13 is a diagram showing the inter-electrode distance L from when pressure starts to be applied until a predetermined set pressure is reached, 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. [Diagram 3] 5A and 5B are diagrams showing the relationship between the electrode movement amount and the applied pressure, in which (a) is for the case where there is no plate gap as an external disturbance, and (b) is for the case where there is a plate gap as an external disturbance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0034] As described 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. In addition, the effects of disturbances are reflected in a parameter that is an index of the electrode position (electrode movement amount) from the start of pressing to the end of pressing. Therefore, before starting the current flow for the main welding, it is important to pressurize the workpiece until it reaches the initial set pressure, and then set the cumulative heat generation amount during the current flow for the main welding using a parameter that is an index of the electrode position obtained from the start of pressing before the current flow for the main welding until the initial set pressure is reached.

[0035] For example, if pressure is applied to metal plates 1 and 2 under the same conditions in a case where there is no gap between metal plates 1 and 2 as shown in Figure 2(a) and a case where there is a gap between metal plates 1 and 2 as shown in Figure 2(b), the inter-electrode distance (i.e., inter-electrode position) L from the start of pressure application to the time when the initial pressure setting is reached will be different for each case.

[0036] That is, if there is no gap between the metal sheets 1 and 2, after the electrodes 4 and 5 start applying pressure, the initial pressure setting 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 are deformed in the initial stage of pressure setting to bring the metal sheets 1 and 2 into contact with each other. Therefore, the rate at which the pressure increases differs before and after contact, and the amount of movement of the electrodes 4 and 5 until the initial pressure setting is reached is large.

[0037] The parameters that serve as indicators of the electrode positions are as follows: - Electrode displacement, electrode displacement speed, and pressure measured directly by an external sensor - Electrode displacement and pressure indirectly measured by the distortion of the gun or case 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 etc.

[0038] Figure 3 shows the relationship between the electrode movement and the applied pressure when a workpiece made of overlapping metal plates is sandwiched between a pair of electrodes and pressure is applied, with (a) being for the case where there is no plate gap (the gap between metal plates 1 and 2) as an external disturbance, and (b) being for the case where there is a plate gap as an external disturbance. As shown in Figure 3(a), when there is no plate gap, the applied pressure increases at a roughly constant rate and stops when it reaches a specified set value.

[0039] In contrast, as shown in Fig. 3(b), when there is a disturbance such as a gap between the plates, even if a pressing force is detected, the load is not transmitted to the metal plates, and the pressing force does not increase according to the amount of movement of the electrode. When the metal plates come into contact with each other with continued application of pressure, the pressing force is transmitted to the metal plates as in Fig. 3(a). In the present invention, by comparing and controlling the difference in the fluctuation of the pressing force, stable spot welding is achieved even when a disturbance is present.

[0040] That is, first, prior to actual joining of the workpieces 3 to be joined, which are made up of multiple overlapping metal plates, a test press is performed. Specifically, under conditions that do not include disturbances such as plate gaps, the workpieces for testing, which are made up of multiple overlapping metal plates, are sandwiched between a pair of electrodes, and pressurized until the fluctuation in the pressing force disappears, to obtain the relationship (first relationship) between the movement amount of the pair of electrodes and the pressing force.

[0041] The test pressure is the same as that of the workpieces to be welded, which are made up of multiple overlapping metal plates, to be welded in the actual joining described below, but does not include disturbances such as plate gaps.

[0042] In this specification, the "amount of electrode movement" refers to the amount of electrode movement associated with an increase in the applied pressure, and means the change in the distance between the pair of electrodes. Also, the "point at which the change in applied pressure disappears" means the point at which the change in applied pressure becomes equal to or less than 1 / 100 of the set applied pressure, or equal to or less than 0.05 kN, whichever is higher.

[0043] The amount of movement A in FIG. 3(a) means the amount of movement of the pair of electrodes from the time when pressure is started to be applied to the test workpieces to the time when the fluctuation in the applied pressure stops.

[0044] Next, in this joining, as in the test pressure application described above, the workpieces to be joined, which are multiple overlapping metal plates, are clamped between a pair of electrodes and pressure is applied until there is no fluctuation in the pressure force, and the relationship (second relationship) between the movement amount of the pair of electrodes and the pressure force is obtained.

[0045] When determining the second relationship, the workpieces may include disturbances such as plate gaps. As described above, Fig. 3(b) is for a case including disturbances such as plate gaps, and the movement amount B in Fig. 3(b) means the movement amount of the pair of electrodes from the time when pressure is started to be applied to the workpieces to be joined to the time when the fluctuation in the pressing force disappears. Moreover, the movement amount C means the movement amount of the pair of electrodes at the time when the increase in the pressing force on the workpieces to be joined relative to the movement amount of the pair of electrodes reaches the increase in the pressing force relative to the movement amount of the pair of electrodes in the first relationship shown in Fig. 3(a).

[0046] In the present invention, a target cumulative heat generation amount in the main welding is set based on the movement amount A obtained in the test pressurization and the movement amounts B and C obtained in the main welding. Then, the amount of current is controlled according to the set target heat generation amount to spot weld the workpieces. This makes it possible to stably form a nugget with a desired nugget diameter even when disturbances exist.

[0047] As described above, the present invention is characterized by the method of setting the target cumulative heat generation amount when performing main welding, and other configurations are not particularly limited.

[0048] 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 pressure and welding current during welding. There are no particular limitations on the type (fixed type, robot gun, etc.), electrode shape, etc.

[0049] The multiple metal sheets constituting the workpiece to be welded, which is the object of joining, may be, for example, steel sheets. The steel sheets may be steel sheets that have not been surface-treated. The steel sheets may also be surface-treated steel sheets that have been subjected to a surface treatment such as plating, or two or more of the multiple metal sheets may be steel sheets having a plating layer. Furthermore, all of the multiple steel sheets may be steel sheets having a plating layer. The plating layer may be provided on only one side (one surface) of the steel sheet, or on both sides.

[0050] An example of the steel sheet having the above-mentioned plating layer is a zinc-based plated steel sheet. The zinc-based plated steel sheet is a steel sheet in which a zinc-based plating is coated on the steel sheet by various methods such as hot-dip plating, electroplating, vapor deposition plating, and thermal spraying. Examples of the zinc-based plating include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, electrogalvanizing, and electrogalvanizing-nickel alloy plating, but are not limited thereto, and all known zinc-based platings containing zinc can be applied. Examples of such zinc-based plated steel sheets include hot-dip galvanized steel sheets (GI) that have not been subjected to alloying treatment, and alloyed hot-dip galvanized steel sheets (GA) that have been subjected to alloying treatment.

[0051] In the present invention, it is preferable to perform test welding under the same conditions as the test pressurization prior to actual welding, and in the test welding, obtain the cumulative heat generation amount per unit volume calculated from the electrical characteristics between a pair of electrodes when current is passed under constant current control to form a nugget of a predetermined nugget diameter, and in passing current for actual welding, set the cumulative heat generation amount per unit volume obtained in the test welding as the target cumulative heat generation amount, and control the amount of current passed in accordance with the target cumulative heat generation amount.

[0052] In addition, when the cumulative heat generation amount capable of forming a nugget of a predetermined diameter in the above test welding is E, and the target cumulative heat generation amount in actual welding is E', it is preferable to set E' so as to satisfy the following formula (1). C / B≦E' / E≦(1-0.01×(BC) / A) (1)

[0053] For example, the amount of movement of the electrode position from the start of pressure application before the start of current application to the start of current application in this welding can be divided into [the amount of movement until the plate gap is eliminated] (between 0C in Figure 3(b)) and [the amount of movement from the elimination of the plate gap to the arrival of a specified pressure force] (between CB in Figure 3(b)).

[0054] Therefore, when the electrode position fluctuation from the start of pressure application before the start of current application in this welding to the start of current application is used as the pressure force, it is possible to set the pressure force within a range that satisfies the above formula (1), for example, by calculating the ratio of the [pressure force until the sheet gap is eliminated] to the [pressure force from the sheet gap being eliminated until the specified pressure force is reached].

[0055] In the present invention, it is necessary to determine the electrode contact time point, 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 point of each electrode with the metal plate may be measured using the change in pressure.

[0056] By setting the cumulative heat generation ratio E' / E during current flow in the main welding to C / B or more, it is possible to obtain a sufficient amount of heat for welding and obtain a nugget with the desired diameter. Furthermore, by setting E' / E to (1-0.01 x (BC) / A) or less, it is possible to properly grasp the resistance change due to the presence of disturbances such as plate gaps, and to prevent excessive heat generation during adaptive control welding. It is more preferable that the above E' / E satisfies the following formula (2). 1.1×C / B≦E' / E≦(1-0.02×(BC) / A) (2)

[0057] Here, the method for calculating the calorific value is not particularly limited, but an example is disclosed in Patent Document 3, and the above method can be adopted in the present invention as well. 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 disclosed in Patent Document 3 is as follows.

[0058] That is, let t be the total thickness of the materials to be welded, such as steel plates, r be the electrical resistivity of the materials to be welded, V be the voltage between the electrodes, I be the welding current, and S be the area of ​​contact between the electrodes and the materials to be welded. In this case, the welding current I passes through a columnar portion with a cross-sectional area of ​​S and thickness t, generating resistance heat. The amount of heat q per unit volume and unit time in this columnar portion can be calculated using the following equation (3). q=(V I) / (S t) (3)

[0059] Moreover, the electrical resistance R of the columnar portion is calculated by the following formula (4). R = (r t) / S (4)

[0060] By solving equation (5) for S and substituting it into equation (3), the amount of heat generated q is given by the following equation (5). q=(V I R) / (r t 2 )=(V 2 ) / (r t 2 ) (5)

[0061] As is clear from the above formula (5), the amount of heat generated per unit volume and unit time q can be calculated from the interelectrode voltage V, the total thickness t of the metal plates, and the electrical resistivity r of the metal plates, and is not affected by the contact area S between the electrodes and the metal plates. Although the formula (5) calculates the amount of heat generated q from the interelectrode voltage V, it can also be calculated from the interelectrode current I, and in this case, it is not necessary to use the contact area S between the electrodes and the metal plates. Then, by accumulating the amount of heat generated q per unit volume and unit time over the current application period, the accumulated amount of heat generated per unit volume Q applied during welding can be obtained. As is clear from the formula (5), this accumulated amount of heat generated per unit volume Q can also be calculated without using the contact area S between the electrodes and the metal plates.

[0062] The pressure P set in the present invention may be appropriately set depending on the material and thickness of the metal plates 1 and 2 constituting the workpiece 3. For example, when a plate assembly of two stacked 270-2000 MPa class steel plates having a thickness of 1.4 mm and a material of which the surface is plated with Zn is used as the workpiece, the initial pressure is preferably set to 1.0-7.0 kN. When a plate assembly of three stacked 270-2000 MPa class steel plates having a thickness of 1.4 mm and a material of which the surface is plated with Zn is used as the workpiece, the initial pressure is preferably set to 2.0 kN-10.0 kN.

[0063] The present invention performs adaptive control welding, in which welding is performed using the above-mentioned 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 is controlled so that the deviation in the amount of heat generated will be compensated for within the remaining current flow time, so that the accumulated amount of heat generated per unit volume in this welding will match the accumulated amount of heat generated per unit volume set as the target value.

[0064] The desired nugget diameter of the nugget formed by the second-stage current flow need only be greater than or equal to what is required for production, but it is desirable to aim for a nugget diameter between the sheets that is greater than or equal to 4√T, where T is the thickness of the thinner sheet of the two sheets that make up the gap.

[0065] (Method of manufacturing welded components) A method for producing 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.

[0066] As described above, in the resistance spot welding method according to the present invention, a target cumulative heat generation amount is set based on the electrode movement amount A from the start of application of pressure to the time when the applied pressure stops fluctuating, which are obtained when a test application of pressure is performed prior to the actual welding, the electrode movement amount B from the start of application of pressure to the time when the applied pressure stops fluctuating during the actual welding, and the electrode movement amount C to the time when the increase rate of the applied pressure in the actual welding reaches the increase rate of the applied pressure in the test application, and the current flow is controlled in accordance with the target cumulative heat generation amount, thereby making it possible to perform stable welding even under complex disturbances. In this way, by joining the workpieces consisting of overlapping multiple metal plates, a welded member with a desired nugget size and high weld strength can be produced. EXAMPLES

[0067] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0068] For plate pairs 1 to 18 composed of the metal plates shown in Table 1, welded joints were produced by performing main welding under the conditions shown in Table 1 when there was no plate gap or when there was a plate gap. For each of the obtained welded joints, the welded part was cut, the cross section was etched, and then observed with an optical microscope to measure the nugget diameter. For the plate gap, for laboratory experiments, a spacer was inserted between the steel plates so that the gap between the steel plates at a point 25 mm away from the welding target position on both sides was 0.1 to 3 mm. The measured nugget diameter was evaluated as follows based on the deviation from the target nugget diameter. At that time, the target nugget diameter was compared with the deviation when there was a plate gap, based on the case when there was no plate gap. The obtained evaluation results are shown in Table 1. For plate pairs 9, 10, 17, and 18, the plate gap shown in Table 1 was provided only between metal plate 2 and metal plate 3.

[0069] [Table 1]

[0070] Diameter Judgment: ○ (Pass): The nugget diameter is larger than the target nugget diameter. × (Fail): The nugget diameter is smaller than the target nugget diameter.

[0071] Diameter Variation: ○ (Pass): The nugget diameter varies from the target nugget diameter by less than 0.5 × the square root of (plate thickness). × (Fail): The nugget diameter varies from the target nugget diameter by more than the square root of 0.5 × (plate thickness). Note that (plate thickness) here refers to the plate thickness (mm) of the thinnest steel plate in the plate assembly.

[0072] As is clear from Table 1, in all of the inventive examples, not only were the desired nugget diameters obtained, but the variation was small, regardless of the sheet gap. On the other hand, in the comparative examples, depending on the sheet gap, a sufficient nugget diameter was not obtained, and the variation was large. [Industrial Applicability]

[0073] According to the present invention, even when disturbances exist, a nugget having a desired diameter can be stably formed. [Explanation of symbols]

[0074] 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 passing current through the workpiece while applying pressure, Prior to the actual welding, a test pressurization is performed under a condition without disturbance, in which a test workpiece made of overlapping multiple metal plates is sandwiched between a pair of electrodes, and a pressurization force is applied until the fluctuation of the pressurization force is eliminated to obtain a first relationship between the movement amount of the pair of electrodes and the pressurization force; In the main welding, A workpiece to be welded, which is a plurality of overlapping metal plates, is sandwiched between the pair of electrodes, and pressure is applied until the fluctuation in the pressure force is eliminated to obtain a second relationship between the movement amount of the pair of electrodes and the pressure force; a movement amount of the pair of electrodes from the start of applying pressure to the test workpiece to a time when the fluctuation in the pressing force stops in the first relationship is defined as A; a movement amount of the pair of electrodes from the start of applying pressure to the workpiece to be joined to a time when the fluctuation in the pressing force stops in the second relationship is defined as B; and a movement amount of the pair of electrodes at a time when an increase in the pressing force on the workpiece to be joined relative to the movement amount of the pair of electrodes in the second relationship reaches an increase in the pressing force relative to the movement amount of the pair of electrodes in the first relationship is defined as C, wherein a target cumulative heat amount is set based on A, B, and C, and the amount of current flow is controlled in accordance with the target cumulative heat amount to join the workpieces to be joined.

2. Prior to the actual welding, a test welding is performed under the same conditions as the test pressure. In the test welding, a cumulative heat generation amount per unit volume is calculated from electrical characteristics between the pair of electrodes when a nugget having the predetermined nugget diameter is formed by passing current under constant current control, and 2. The resistance spot welding method according to claim 1, wherein, in the current supply for the main welding, the cumulative heat generation amount per unit volume determined in the test welding is set as a target cumulative heat generation amount, and the amount of current supplied is controlled in accordance with the target cumulative heat generation amount.

3. 3. The resistance spot welding method of claim 2, wherein E is a cumulative heat generation amount capable of forming a nugget having the predetermined nugget diameter in the test welding, and E' is a target cumulative heat generation amount in the main welding, and E' is set so as to satisfy the following formula (1): C / B≦E' / E≦(1-0.01×(B-C) / A) (1)

4. A method for manufacturing a welded member, comprising joining a plurality of overlapping metal plates to be welded by the resistance spot welding method according to any one of claims 1 to 3 to obtain a welded member.

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