Method for manufacturing spot welded joints and spot welding device

Abstract

To provide a manufacturing method of a spot welded joint which can predict the peel strength (CTS) of a weld zone after post-electrification and a manufacturing device of the spot welded joint.SOLUTION: A manufacturing method of a spot welded joint according to a first embodiment of the present invention comprises the steps of: forming a weld zone by spot-welding two or more laminated steel plates by using a pair of electrodes of a spot welding device; and performing post-electrification to the weld zone by using the pair of electrodes. During the post-electrification, the set pressing force of the pair of electrodes is set to a constant value, the actual pressing force of the pair of electrodes is measured, a change amount of the actual pressing force of the pair of electrodes is set to be equal to or greater than 0.50 kN or an average change speed of the actual pressing force between the pair of electrodes is set to be equal to or higher than 0.20 kN / sec.SELECTED DRAWING: Figure 4A

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a spot welded joint and a spot welding apparatus. [Background technology]

[0002] Spot welding is a resistance welding technique in which overlapping base materials are clamped between the tips of properly shaped electrodes, and current and pressure are concentrated in a relatively small area to locally heat the material while simultaneously applying pressure with the electrode. In spot welding, the molten and solidified part at the weld is called a nugget.

[0003] In spot welding of high-strength steel sheets, a decrease in peel strength is considered a problem. Peel strength is the joint strength against stress in the peel direction, as evaluated by, for example, CTS, which is measured by a cross-tensile test. Normally, the higher the strength of the base material, the higher the bonding strength of the welded joint. However, for high-strength steel sheets, especially those with a tensile strength of 980 MPa or more (so-called high-tensile steel), the higher the strength, the lower the peel strength of the welded joint. The cause of the decrease in peel strength is thought to be embrittlement of the nugget.

[0004] Post-heating is one of the methods to suppress the decrease in CTS caused by embrittlement of the nugget of high-strength steel plate. Post-heating is a technique in which the nugget is heated using a welding electrode to perform heat treatment. This can improve the mechanical properties of the nugget. Note that post-heating does not cause the base metal and nugget to melt and solidify. The current used to melt and solidify the base metal to form the nugget is generally referred to as main current.

[0005] Various methods for manufacturing spot-welded joints, including post-current application, have been proposed. For example, Patent Document 1 discloses a spot welding method for high-strength steel sheets, which comprises measuring the current and interelectrode voltage during spot welding, performing calculations using the measured current, interelectrode voltage, and material properties, and performing one or more of the following based on the calculation results during cooling after the end of welding current application: determining when to remove the electrode from the steel sheet; adjusting the post-current current and post-current time in post-current application that continues after the end of welding current application; adjusting the post-current current and post-current time in post-current application that starts after cooling after the end of welding current application; and adjusting the electrode pressure.

[0006] Patent Document 2 describes 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 current, and the method includes performing main welding and test welding prior to the main welding. (a) In the test welding, a main current is applied to form a nugget and a post-current is applied for post-heat treatment. In the main current of the test welding, a time change curve of the instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume are calculated from the electrical characteristics between the electrodes when a current is applied under constant current control to form an appropriate nugget. In the post-current of the test welding, when an average value of the inter-electrode voltage of the main current in the test welding is Vtm, an average value of the inter-electrode voltage of the post-current in the test welding is Vtp, and a current suspension time between the main current and the post-current in the test welding is tc, If tc<800ms: 0.5≦Vtp / Vtm≦2.0 When 800ms≦tc<1600ms: 0.5-0.3×(tc-800) / 800≦Vtp / Vtm≦2.0-0.5×(tc-800) / 800 If tc≧1600ms: 0.2≦Vtp / Vtm≦1.5 (b) then, in the main welding, a main current is applied to form a nugget and a post-current is applied for post-heat treatment; in the main current for the main welding, target values ​​are set for the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume in the main current for the test welding, and adaptive control welding is performed to control the amount of current applied in accordance with the target values; and in the post-current for the main welding, when the current value of the post-current for the test welding is Itp and the current value of the post-current for the main welding is Imp, 0.8×Itp≦Imp≦1.2×Itp This publication discloses a resistance spot welding method in which current is supplied by constant current control under the condition that the following relationship is satisfied: [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-103054 [Patent Document 2] International Publication No. 2020 / 004115 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in recent years, there has been an increasing demand for the mechanical properties of spot welded joints. As part of this, variations in the quality of welds after post-heat treatment have become a problem in recent years.

[0009] In a typical spot welding device, the control parameters are the composition and thickness of the base steel sheet, the current value, and the welding pressure. In the post-heat process, the diameter of the nugget obtained by the main current is also a control parameter. However, even if these control parameters are the same and spot welding is performed multiple times, the mechanical properties of the resulting weld are not necessarily consistent. That is, even if the current value and other conditions are the same, the CTS of the welded joint after the post-heat process can vary greatly. The cause of this is thought to be various disturbances during welding, but has not yet been identified. Therefore, optimizing the post-heat conditions is difficult.

[0010] To promote the application of post-heating to the manufacturing sites of machine parts, it is desirable to be able to easily predict the CTS of welds after post-heating. For welds where the CTS is predicted to be insufficient, the mechanical properties can be improved by performing post-heating again.

[0011] The techniques described in the above-mentioned Patent Documents 1 and 2 address the issue of improving peel strength. However, these documents do not particularly consider variations in peel strength. Furthermore, these documents do not disclose any specific means for predicting the CTS of a weld after post-energization.

[0012] An object of the present invention is to provide a method and an apparatus for manufacturing a spot-welded joint that are capable of predicting the peel strength (CTS) of a welded portion after post-energization. [Means for solving the problem]

[0013] The gist of the present invention is as follows.

[0014] (1) A manufacturing method of a spot-welded joint according to a first embodiment of the present invention includes the steps of spot-welding two or more stacked steel plates using a pair of electrodes of a spot welding device to form a weld, and applying a post-current to the weld using the pair of electrodes, wherein during the post-current, the set pressure of the pair of electrodes is set to a constant value, the actual pressure of the pair of electrodes is measured during the post-current, and the amount of change in the actual pressure of the pair of electrodes is set to 0.50 kN or more, or the average rate of change in the actual pressure between the pair of electrodes is set to 0.20 kN / sec or more. (2) The method for manufacturing a spot-welded joint described in (1) above may further include a step of determining a correlation between the change in the actual pressure during the post-current and the peel strength of the weld after the post-current; a step of determining, based on the correlation, a predetermined range of the change in the actual pressure within which the peel strength is predicted to be equal to or greater than a predetermined value; and a step of determining whether the change in the actual pressure measured during the post-current is within the predetermined range. (3) In the method for manufacturing a spot-welded joint described in (2) above, the post-energization may be terminated when the amount of change in the actual applied pressure exceeds the predetermined range. (4) The method for manufacturing a spot-welded joint described in (1) above may further include a step of identifying a correlation between the change in the actual pressure during the post-current and the peel strength of the weld after the post-current, and a step of determining the peel strength corresponding to the change in the actual pressure measured during the post-current by referring to the correlation. (5) In the method for manufacturing a spot-welded joint according to any one of (1) to (4) above, at least one of the steel plates may be a high-strength steel plate having a tensile strength of 980 MPa or more.

[0015] (6) A spot welding apparatus according to a second embodiment of the present invention is a spot welding apparatus for carrying out the method for manufacturing a spot welded joint described in any one of (1) to (5) above, and includes a pair of electrodes, a pressure measuring unit that measures the actual pressure between the pair of electrodes, and a control unit that operates the pair of electrodes based on the set pressure between the pair of electrodes. (7) In the spot welding apparatus described in (6) above, the control unit may be configured to record a correlation between the amount of change in the actual welding force during post-current application and the peel strength of the weld after the post-current application, which correlation is identified before the manufacturing of the spot welded joint, and the control unit may be configured to control the amount of current flowing through the pair of electrodes in accordance with the actual welding force. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method and an apparatus for manufacturing a spot-welded joint that are capable of predicting the peel strength (CTS) of the weld after post-energization. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic diagram of a spot welding device and a spot-welded joint at a stage after spot welding is completed and before post-energization begins. FIG. [Figure 2] 1 is a schematic diagram of a spot welding device and a spot-welded joint during post-current application. FIG. [Figure 3A] 10 is an example of the change over time in actual welding pressure during post-energization in the manufacturing method of a spot-welded joint according to the present embodiment. [Figure 3B] 10 is an example of the change in actual applied pressure over time during post-energization in a conventional spot-welded joint manufacturing method. [Figure 4A] 10 is a scatter diagram showing the correlation between the amount of change ΔF in the actual welding pressure during post-current application and the CTS of the weld after the post-current application in the method for manufacturing a spot-welded joint according to the present embodiment. [Figure 4B] 1 is a scatter diagram showing the correlation between the change in actual applied pressure ΔF during post-energization and the CTS of the weld after the post-energization in a conventional manufacturing method of spot-welded joints. [Figure 5] 10 is a graph showing the correlation between the current ratio and CTS in post-current application where the average rate of change ΔF / T of the actual applied force is 0.44 kN and in post-current application where the average rate of change ΔF / T of the actual applied force is 0.17 kN. [Figure 6A] 4B is a graph showing the range of ΔF that can make the CTS of the weld after post-current heating 13 kN under the post-current heating conditions of FIG. 4A. [Figure 6B] 4C is a graph showing the range of ΔF that can make the CTS of the weld after post-current heating 13 kN under the post-current heating conditions of FIG. 4B. DETAILED DESCRIPTION OF THE INVENTION

[0018] (1. Manufacturing method of spot welded joints) The method for manufacturing a spot-welded joint according to the first embodiment of the present invention includes the steps of spot welding two or more stacked steel plates 21 together using a pair of electrodes 11 of a spot welding device 1 to form a weld 22, and applying a post-current to the weld 22 using the pair of electrodes 11, wherein the set pressure of the pair of electrodes 11 is set to a constant value during the post-current, the actual pressure of the pair of electrodes 11 is measured during the post-current, and the change in the actual pressure of the pair of electrodes 11, ΔF, is set to 0.50 kN or more, or the average rate of change in the actual pressure between the pair of electrodes 11, ΔF / T, is set to 0.20 kN / sec or more during the post-current.

[0019] The inventors have hypothesized that the unstable peel strength (CTS) of the weld after post-current application is due to the instability of the temperature of the weld during post-current application. From a metallurgical perspective, it is reasonable to consider that the variation in mechanical properties that occurs in metals with the same composition is due to variations in the heat treatment temperature. It is also hypothesized that external disturbances during welding can cause changes in the current application state, resulting in variations in the heating temperature during post-current application by several tens of degrees Celsius. Examples of external disturbances include the surface condition of the steel sheet, the electrode strike angle, the clearance between the steel sheet and the lower electrode, and the gap between the steel sheets. Completely eliminating these external disturbances is difficult, especially in the manufacturing of mechanical parts, given factors such as manufacturing costs.

[0020] However, there is currently no means for measuring the temperature of the welded portion during post-current application. Therefore, it is not possible to prove by temperature measurement that there is variation in the heating temperature of the welded portion during post-current application. Furthermore, since there is no means for measuring the temperature of the welded portion during post-current application, it is not possible to control the temperature of the welded portion during post-current application. Therefore, the inventors focused on the thermal expansion that occurs in the welded portion due to the temperature increase during post-current application.

[0021] FIG. 1 is a schematic diagram of a spot welding apparatus 1 and a spot-welded joint 2 after a nugget 221 has been formed by spot welding and before the start of post-current application. FIG. 2 is a schematic diagram of the spot welding apparatus 1 and the spot-welded joint 2 during post-current application. As shown in FIG. 1 , the spot welding apparatus 1 uses a pair of electrodes 11 to clamp a welded portion 22 of overlapping steel sheets 21. That is, the pair of electrodes 11 apply pressure to the welded portion 22. Furthermore, electrodes 11 used for normal spot welding have internal piping through which a coolant flows. Therefore, the welded portion 22 clamped by the electrodes 11 is constantly cooled by the electrodes 11.

[0022] 2, when post-energization begins, the welded portion 22 is heated and thermally expands. As a result, the pair of electrodes 11 holding the welded portion 22 move in directions away from each other. The spot welding device 1 detects the displacement of the electrodes 11 and moves the electrodes 11 so as to offset the displacement of the electrodes 11. As a result, the actual measured value of the pressure applied by the electrodes 11 to the welded portion 22, i.e., the actual pressure, increases.

[0023] The change in the actual applied pressure is a value that corresponds to the amount of thermal expansion of the welded portion 22. The amount of thermal expansion of the welded portion 22 is a value that corresponds to the temperature of the welded portion 22. Therefore, the actual applied pressure measured by the applied pressure measuring unit 12 during post-energization can be used as an index of the temperature of the welded portion 22. The inventors considered that by using the actual applied pressure, it is possible to predict the CTS of the welded portion 22 after post-energization.

[0024] In the manufacturing method of the spot welded joint 2 according to this embodiment, the "actual pressure" refers to the actual pressure generated between the electrodes 11. The actual pressure is measured by a pressure measuring unit 12, such as a load cell, provided between the electrodes 11. On the other hand, a typical spot welding device 1 has a function of controlling the pressure mechanism so that the force with which the pair of electrodes 11 clamps the steel plate is set to a predetermined target value. In the manufacturing method of the spot welded joint 2 according to this embodiment, the control target value of the pressure input to the control unit 13 of the spot welding device 1 is referred to as the "set pressure."

[0025] The set pressure and the actual pressure do not necessarily coincide. During post-current application, a discrepancy occurs between the set pressure and the actual pressure as heat input progresses. Figures 3A and 3B show examples of the change in actual pressure over time during post-current application. In both Figures 3A and 3B, the set pressure is set to 3.8 kN, but the actual pressure increases as post-current application progresses. The actual pressure then reaches its maximum when post-current application is completed. This is thought to be because the temperature of the weld reaches its maximum when post-current application is completed, i.e., when heat input is completed.

[0026] The inventors then produced various spot-welded joints while recording the fluctuations in the actual pressure during the post-current and evaluated the CTS. As a result, it was confirmed that there is a correlation between the amount of change in the actual pressure during the post-current and the CTS of the weld after the post-current. In addition, the inventors found that by reducing the responsiveness of the pressure control, the correlation between the amount of change in the actual pressure during the post-current and the CTS of the weld after the post-current becomes even better. This matter will be explained below with reference to the experimental data obtained by the inventors.

[0027] Generally, the lower the responsiveness of the pressure control, the greater the deviation between the set pressure and the actual pressure. This is shown, for example, in the graphs of FIGS. 3A and 3B. FIG. 3A shows the change in the actual pressure over time during the post-current in the manufacturing method of a spot-welded joint according to this embodiment. FIG. 3B shows the change in the actual pressure over time during the post-current in the manufacturing method of a conventional spot-welded joint. In the post-current according to this embodiment shown in FIG. 3A, the responsiveness of the pressure control was set to a level lower than that generally considered preferable. As a result, the average rate of change in the actual pressure during the post-current in FIG. 3A was higher than that during the post-current in FIG. 3B. The amount of change in the actual pressure during the post-current in FIG. 3A was also higher than that during the post-current in FIG. 3B. In both FIGS. 3A and 3B, the set pressure was set to 3.8 kN, but the deviation between the set pressure and the actual pressure during the post-current in FIG. 3A was larger than that during the post-current in FIG. 3B.

[0028] Based on these facts, a person skilled in the art would consider a decrease in the responsiveness of the pressure control to be undesirable. This is because the deviation between the set pressure and the actual pressure caused by a decrease in responsiveness is considered to be a disturbance factor that has an unexpected effect on the post-heat results. However, the present inventors investigated the relationship between the amount of change in the actual pressure during the post-heat and the CTS of the weld after the post-heat, and as a result, the following facts became clear. (i) The lower the responsiveness of the pressure control, the stronger the correlation between the change in actual pressure during post-heat and the CTS of the weld after post-heat. (ii) The decrease in the responsiveness of the welding force control does not adversely affect the CTS of the weld after post-heat application.

[0029] (i) Fig. 4A is a scatter diagram showing the correlation between the change in actual pressure during post-current and the CTS of the welded portion 22 after post-current in a method for manufacturing a spot-welded joint according to this embodiment. Fig. 4B is a scatter diagram showing the correlation between the two during post-current in a conventional method for manufacturing a spot-welded joint. The change in actual pressure during post-current is a value obtained by the following formula. ΔF=F2-F1 Here, F1 is the actual applied pressure at the start of post-energization, and F2 is the actual applied pressure at the end of post-energization.

[0030] These graphs were obtained by first performing spot welding and post-heating multiple times, and then evaluating the CTS of various welds after post-heating using a peel test. Post-heating was performed while monitoring the change in actual applied pressure over time, thereby measuring the change in actual applied pressure during post-heating. The dashed lines in Figures 4A and 4B are polynomial approximation curves of the data points. The degree is 2. "R2" in Figures 4A and 4B is the coefficient of determination obtained based on the polynomial approximation curve. The larger the R2, the stronger the correlation between the change in actual applied pressure and CTS.

[0031] In the post-current according to this embodiment, which was performed to create Fig. 4A, the responsiveness of the pressure control was reduced, so that the average rate of change ΔF / T of the actual pressure during the post-current was 0.44 kN. In the normal post-current performed to create Fig. 4B, the average rate of change ΔF / T of the actual pressure during the post-current was 0.17 kN. The average rate of change ΔF / T of the actual pressure during the post-current is the value obtained by dividing the amount of change ΔF in the actual pressure during the post-current by the post-current time T.

[0032] As a result, the change in actual pressure ΔF during the post-current according to this embodiment was larger than that of the normal post-current. Meanwhile, the coefficient of determination R2 for the post-current according to this embodiment was significantly larger than that of the normal post-current. In other words, the correlation between the change in actual pressure ΔF during the post-current and the CTS of the weld after the post-current increased as the responsiveness of the pressure control decreased. The post-current according to this embodiment, which has a lower responsiveness of the pressure control, improves the prediction accuracy of the CTS based on the change in actual pressure ΔF compared to the normal post-current.

[0033] (ii) Figure 5 is a graph showing the correlation between the current ratio and the CTS for a post-heat treatment in which the average rate of change of the applied pressure ΔF / T is 0.44 kN and a post-heat treatment in which the average rate of change of the applied pressure ΔF / T is 0.17 kN. The current ratio is the ratio of the current applied to the spot welding (main current) to form the nugget to the current applied to the post-heat treatment. The higher the current ratio, the greater the heat input in the post-heat treatment. For reference, a dashed line indicating the CTS of a weld in which no post-heat treatment was performed, i.e., the single-heat CTS, is also shown in Figure 5.

[0034] As shown in Figure 5, the CTS of the weld obtained by the post-current according to this embodiment was at the same level as the CTS of the weld obtained by normal post-current and even exceeded the CTS of the weld obtained by single-current. In other words, the decrease in responsiveness of the welding force control did not adversely affect the CTS of the weld after post-current.

[0035] Note that by reducing the responsiveness of the pressure force control, it is possible to obtain the effect of widening the preferable range of the change ΔF in the actual pressure force during post-energization, as shown in FIGS. 6A and 6B.

[0036] 6A and 6B are graphs showing the relationship between the change in actual applied pressure ΔF during post-heat and the CTS of the weld after post-heat. The data points and approximation curves in FIGS. 6A and 6B are the same as those in FIGS. 4A and 4B. The double-headed arrows in FIGS. 6A and 6B indicate the range of ΔF where the CTS is predicted to be 13 kN or greater. Both ends of the double-headed arrows are aligned with the intersections of the approximation curve and the dashed line indicating CTS = 13 kN.

[0037] It is estimated that a weld with a CTS of 13 kN or more can be produced by performing post-heat so that ΔF is within the range indicated by the double-headed arrows in Figures 6A and 6B. The double-headed arrows in Figure 6A are longer than the double-headed arrows in Figure 6B. In other words, by reducing the responsiveness of the pressure control, it becomes easier to keep the change in actual pressure during post-heat ΔF within the range indicated by the double-headed arrows, thereby producing a weld with a CTS of 13 kN or more.

[0038] The method for manufacturing a spot welded joint according to this embodiment, which was obtained based on the above findings, will be described in detail below.

[0039] (Spot welding process) In the method for manufacturing a spot welded joint according to this embodiment, two or more steel plates 21 are spot welded together using a spot welding device 1 having a pair of electrodes 11. This forms a weld 22 that joins the multiple steel plates. The weld 22 is a general term that refers to a portion including a nugget 221 and its surrounding heat-affected zone (HAZ).

[0040] The spot welding conditions and the type of electrode are not particularly limited. Suitable spot welding conditions and electrodes can be selected as appropriate depending on the type, thickness, and number of steel sheets, the desired nugget diameter, and the like. A suitable form of the spot welding device 1 for performing spot welding will be described later. Note that in a manufacturing method for a spot-welded joint that includes post-energization, the spot welding process and the post-energization process may be collectively referred to as the "spot welding process." However, in this embodiment, the term "spot welding" does not include post-energization.

[0041] Spot welding typically includes a main current application and a cool-down period. The main current application is the application of current to melt the steel sheet. The cool-down period is the time required to cool the steel sheet using a pair of electrodes after the main current application is complete. A coolant typically flows inside the electrodes used for spot welding. Therefore, by stopping the application of current between the electrodes while the steel sheet is still held between the pair of electrodes, the molten portion of the steel sheet can be cooled and solidified, forming a weld with a nugget.

[0042] The number of stacked steel plates to be spot-welded is not particularly limited. The type and thickness of the steel plates are also not particularly limited. It is more preferable that at least one of the steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more or 1500 MPa or more.

[0043] (Post-energization process) In the method for manufacturing a spot welded joint according to this embodiment, a weld 22 formed by spot welding is post-current-applied using a pair of electrodes 11. Both post-current-applied and spot welding can be performed using the same electrodes 11.

[0044] During the post-current, the set pressure between the pair of electrodes 11 is kept constant. Furthermore, during the post-current, the responsiveness of the pressure control of the spot welding device is reduced. Specifically, during the post-current, the average rate of change of the actual pressure between the pair of electrodes 11 is set to 0.20 kN / sec or more, or the amount of change in the actual pressure between the pair of electrodes 11 is set to 0.50 kN or more. The amount of change in the actual pressure is a value ΔF obtained by the following formula. ΔF=F2-F1 3A, F1 is the actual applied pressure at the start of post-current application, and F2 is the actual applied pressure at the end of post-current application. The average rate of change of the actual applied pressure is the value obtained by dividing ΔF by the post-current application time T.

[0045] By setting the responsiveness of the pressure control so that the amount of change or average rate of change of the actual pressure falls within the above-mentioned range, the correlation between the amount of change of the actual pressure during the post-current and the CTS of the weld after the post-current becomes strong, as shown in Fig. 4A, for example, and it becomes possible to predict the peel strength (CTS) of the weld after the post-current with high accuracy. Note that the amount of change or average rate of change of the actual pressure can also be kept within the above-mentioned range by increasing the set pressure during the post-current, but in this case, the effect of improving the accuracy of CTS prediction cannot be obtained.

[0046] There are no particular limitations on the means for predicting the CTS of the weld after post-energization based on the amount of change in actual applied pressure during post-energization, but specific means will be described below as an example.

[0047] The primary method for predicting CTS is (a) Create a graph like Figure 6A. (b) Based on the graph, determine the acceptable range of the actual applied pressure during the post-energization; (c) Predict the pass / fail of the CTS of the weld based on the pass / fail of the actual applied pressure during post-energization. That is, the manufacturing method to which the first means for predicting CTS is applied further includes: (a) a step of determining the correlation between the change in actual pressure ΔF during post-current application and the peel strength of the welded portion 22 after post-current application; (b) a step of determining, based on the correlation, a predetermined range of the change in actual pressure ΔF within which the peel strength is predicted to be equal to or greater than a predetermined value; and (c) a step of determining whether the change in actual pressure ΔF measured during post-current application is within the predetermined range.

[0048] A first method for predicting CTS will be described with reference to Fig. 6A, which is a graph showing the correlation between the change ΔF in the actual applied pressure during energization and the peel strength (CTS) of the welded portion 22 after post-energization. The "predetermined value" for CTS is 13kN in the graph shown in Figure 6A. Welds with a CTS of 13kN or greater are considered to pass with respect to CTS. 6A, the predetermined range of the change in actual applied pressure ΔF within which CTS is predicted to exceed the predetermined value is the range of change in actual applied pressure indicated by the double-headed arrows, whose left and right ends are aligned with the intersections of the straight line representing CTS=13 kN and the approximation curve.

[0049] In the method for manufacturing a spot-welded joint according to this embodiment, the responsiveness of the pressure control is reduced, so there is a high correlation between the change in actual pressure during the post-current, ΔF, and the CTS of the weld after the post-current. Therefore, if the change in actual pressure during the post-current, ΔF, is within the range indicated by the arrow in the graph of FIG. 6A, there is a high probability that the CTS of the weld after the post-current will be 13 kN or greater. Therefore, the acceptability of the weld after the post-current can be determined based on whether the change in actual pressure during the post-current, ΔF, is within a predetermined range.

[0050] The energization control may be performed based on a predetermined range of the change amount ΔF in the actual applied pressure, i.e., an acceptable range. For example, the post-energization may be terminated when the change amount ΔF in the actual applied pressure exceeds the predetermined range.

[0051] By measuring the actual welding pressure during the post-current application, it is possible to detect when the change in actual welding pressure ΔF reaches the upper limit of a predetermined range for the actual welding pressure. At that point, the post-current application may be stopped. When the post-current application is stopped, the weld is cooled by the electrode and contracts, causing the actual welding pressure to decrease. This allows the change in actual welding pressure ΔF to be controlled within the above-mentioned predetermined range. This allows a weld with a CTS equal to or greater than a predetermined value to be consistently obtained. To achieve this control, the spot welding apparatus 1 preferably includes a control unit 13 configured to stop the post-current application when the change in actual welding pressure ΔF exceeds the upper limit of the predetermined range.

[0052] The second method for predicting CTS is (A) Create a graph like Figure 6A. (B) The change in actual applied pressure ΔF during post-energization is applied to the graph to predict the CTS of the weld. That is, the manufacturing method to which the second means for predicting CTS is applied further includes: (A) a step of identifying the correlation between the change ΔF in the actual applied force during post-current application and the peel strength of the weld after post-current application; and (B) a step of determining the peel strength corresponding to the change ΔF in the actual applied force measured during post-current application by referring to the correlation.

[0053] In the second method for predicting CTS, a graph such as that shown in Fig. 6A is created, as in the first method. In the second method, the change in actual applied pressure ΔF during post-current is applied to the graph to predict a specific value of the peel strength (CTS) of the weld after post-current. For example, if the change in actual applied pressure ΔF during post-current is 0.80 kN, the CTS of the weld after post-current is predicted to be approximately 13.5 kN according to the approximation curve in Fig. 6A.

[0054] The correlation between the change in actual pressure ΔF during post-current application and the CTS is preferably determined before the spot-welded joint is manufactured. This allows a quick determination of whether the post-current application is performed properly. Alternatively, the correlation between the change in actual pressure ΔF during post-current application and the CTS may be determined after the spot-welded joint is manufactured. In this case, too, the acceptability of the manufactured weld can be determined without subjecting the weld to destructive testing.

[0055] There are no particular limitations on the method for determining the correlation between the change in actual press force ΔF during post-heat and CTS, as illustrated in Fig. 6A. For example, the correlation can be determined by performing one or more test productions to determine the correlation between the change in actual press force ΔF during post-heat and CTS in advance. Hereinafter, the production of a spot-welded joint for determining the correlation will be referred to as "test production," and the production of a spot-welded joint for which CTS prediction is based on the correlation will be referred to as "actual production," to distinguish between the two.

[0056] The test manufacturing is performed by spot welding multiple stacked steel plates using a spot welding device with a pair of electrodes to form a test weld, and then post-energizing the test weld using the pair of electrodes while measuring the actual applied pressure. The multiple steel plates, pair of electrodes, and spot welding device are preferably the same as those used in this manufacturing. The test manufacturing may be performed once, but it is preferable to perform it two or more times to obtain an accurate correlation.

[0057] Next, the CTS of the test weld obtained by the test manufacturing is measured. The change in actual applied force ΔF during the post-test manufacturing process is compared with the CTS measured after the test manufacturing process to determine the correlation. This results in a graph like that shown in Figure 6A.

[0058] Alternatively, instead of test manufacturing, the correlation between the change in actual applied pressure ΔF during post-heat and CTS may be determined by simulating the thermal expansion of the weld during post-heat. The thermal expansion coefficient of the weld is determined depending on the chemical composition of the heat-affected zone (i.e., the chemical composition of the base steel sheet), the chemical composition of the nugget, and the shape of the nugget. Therefore, by using parameters such as the chemical composition and thickness of the steel sheet and the spot welding conditions, it is possible to predict the correlation between the change in actual applied pressure ΔF during post-heat and CTS. Test manufacturing and simulation can also be combined. This makes it possible to accurately predict the correlation between the change in actual applied pressure ΔF during post-heat and CTS while reducing the number of test manufacturing runs.

[0059] When determining the correlation between the change in actual applied pressure ΔF during post-heat and CTS by test manufacturing, simulation, or the like, the spot welding conditions and post-heat conditions are not particularly limited. It is preferable to determine the correlation between the change in actual applied pressure and CTS while appropriately selecting and changing welding conditions that can affect the actual applied pressure. According to the inventors' previous studies, it is considered preferable to determine the correlation between the change in actual applied pressure ΔF during post-heat and CTS while keeping constant one or more conditions selected from the group consisting of the fracture morphology during a joint strength test of the weld, the carbon equivalent of the weld, the average thickness of multiple steel sheets, and the nugget diameter at the weld.

[0060] (2. Spot welding equipment) Next, a spot welding apparatus according to a second embodiment of the present invention will be described. The spot welding apparatus according to the second embodiment is an apparatus for carrying out the method for manufacturing a spot welded joint according to the first embodiment, and as illustrated in Figures 1 and 2, includes a pair of electrodes 11, a pressing force measuring unit 12 that measures the actual pressing force between the pair of electrodes 11, and a control unit 13 that operates the pair of electrodes 11 based on the set pressing force between the pair of electrodes 11. Operating the electrodes 11 means applying pressure to and passing current through a steel sheet 21.

[0061] According to the spot welding apparatus 1 of this embodiment, it is possible to maintain the set welding force of the pair of electrodes 11 at a constant value during post-current application using the control unit 13, and furthermore, it is possible to measure the actual welding force of the pair of electrodes 11 during post-current application using the welding force measuring unit 12. Furthermore, by setting the control parameters input to the control unit 13 to appropriate values ​​or the spring constant of the spring provided in the electrode 11 to an appropriate value, it is possible to set the amount of change in the actual welding force during post-current application to 0.50 kN or more, or to set the average rate of change in the actual welding force to 0.20 kN / sec or more.

[0062] A typical control unit of a spot welding apparatus does not have a function for changing welding conditions based on the actual welding pressure. In contrast, in the spot welding apparatus 1 according to this embodiment, the control unit 13 may be configured to record a correlation between the change in actual welding pressure ΔF during post-current and the CTS of the welded portion 22 after post-current, which is determined before the manufacturing of the spot-welded joint 2. In other words, the control unit 13 may have a correlation recording unit that records the correlation between the change in actual welding pressure ΔF during post-current and the CTS of the welded portion 22 after post-current, which is determined before the manufacturing of the spot-welded joint 2. Furthermore, the control unit 13 may be configured to control the amount of current applied to the pair of electrodes 11 based on the actual welding pressure. In other words, the control unit 13 may have a current amount control unit that controls the amount of current applied to the pair of electrodes 11 based on the actual welding pressure. Such a control unit 13 can terminate post-current when the change in actual welding pressure ΔF during post-current exceeds a predetermined range. The predetermined range of change ΔF in actual welding force during post-energization is, for example, a predetermined range of change ΔF in actual welding force within which CTS is predicted to be equal to or greater than a predetermined value, and is identified based on the correlation described above. Spot welding apparatus 1 having such control unit 13 can execute a manufacturing method for consistently obtaining welded portion 22 having a CTS equal to or greater than a predetermined value. [Example]

[0063] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0064] Various spot-welded joints were created by applying various pressure control responsiveness to a spot welding device. The spot-welded joints were produced by continuously performing spot welding and post-heat application. The change in actual pressure and the average rate of change in actual pressure during post-heat application were measured. In addition, the CTS of the welded portion of the spot-welded joints was measured. The R2 value of the quadratic function approximation between the average rate of change in actual pressure and CTS was calculated. When the R2 value is 0.75 or higher, there is a high correlation between the average rate of change in actual pressure and CTS, and CTS prediction based on actual pressure can be performed with high accuracy.

[0065] Furthermore, the range of applied pressure during post-current was evaluated. In this example, the applied pressure range refers to the range of actual applied pressure within 1 kN of the peak CTS. The peak CTS refers to the amount of change in actual applied pressure at which CTS is maximized in a quadratic function approximation curve between the amount of change in actual applied pressure and CTS. The applied pressure range is the range of actual applied pressure that is predicted to enable the CTS of the weld after post-current to be (peak CTS - 1) kN or more. The larger the applied pressure range, the easier it is to keep the actual applied pressure during post-current within the acceptable range.

[0066] The welding conditions were as follows: Welding machine: Servo motor pressure welding machine Squeeze time: 0.60 seconds Main energization time: 0.36 seconds Main current: 6.5kA Cooldown: 1.98 seconds Post-energization time: 1.98 seconds Post-energization current: 4.3kA ·Holding time: 0.20 seconds The above evaluation results after welding are shown in Table 1.

[0067] [Table 1]

[0068] Under conditions 1 to 3, the responsiveness of the pressure control of the spot welding device was high and the average pressure change rate was low, resulting in an insufficient R2 value and a narrow pressure range.

[0069] On the other hand, under conditions 4 to 7, in which the responsiveness of the spot welding device's pressure control was set low, the average pressure change rate was within an appropriate range. Under these conditions, the R2 value was high. Therefore, according to the manufacturing method for spot-welded joints under these conditions, it is possible to predict the peel strength (CTS) of the weld after post-energization with high accuracy. [Explanation of symbols]

[0070] 1 Spot welding equipment 11 electrodes 12 Pressure measurement unit 13 Control Unit 2 Spot welded joints 21 Steel plate 22 Welded section 221 Nugget F1 Actual pressure at the start of post-energization F2 Actual pressure at the end of post-energization ΔF: Change in actual applied force during post-current application T post-energization time ΔF / T Average rate of change of actual applied force during post-energization

Claims

1. a step of spot welding two or more stacked steel plates using a pair of electrodes of a spot welding device to form a weld; a step of post-currently applying a current to the welded portion using the pair of electrodes; A manufacturing method of a spot welded joint comprising: During the post-energization, the set pressure of the pair of electrodes is set to a constant value, During the post-energization, an actual applied force of the pair of electrodes is measured; During the post-energization, the amount of change in the actual applied force between the pair of electrodes is set to 0.50 kN or more, or the average rate of change in the actual applied force between the pair of electrodes is set to 0.20 kN / second or more. A method for manufacturing spot welded joints.

2. determining a correlation between the change in the actual applied pressure during the post-current application and the peel strength of the welded portion after the post-current application; specifying a predetermined range of the change amount of the actual applied pressure within which the peel strength is predicted to be equal to or greater than a predetermined value based on the correlation; determining whether the change in the actual applied pressure measured during the post-energization is within the predetermined range; The method for manufacturing a spot welded joint according to claim 1 , further comprising:

3. 3. The method for manufacturing a spot-welded joint according to claim 2, wherein the post-energization is terminated when the amount of change in the actual applied pressure exceeds the predetermined range.

4. determining a correlation between the change in the actual applied pressure during the post-current application and the peel strength of the welded portion after the post-current application; determining the peel strength corresponding to the change in the actual applied force measured in the post-energization by referring to the correlation; The method for manufacturing a spot welded joint according to claim 1 , further comprising:

5. At least one of the steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more. The method for manufacturing a spot welded joint according to claim 1 .

6. A pair of electrodes; a pressure measuring unit that measures an actual pressure between the pair of electrodes; a control unit that operates the pair of electrodes based on a set pressure between the pair of electrodes; A spot welding apparatus for carrying out the method for manufacturing a spot welded joint according to any one of claims 1 to 5, comprising:

7. the control unit is configured to record a correlation between an amount of change in the actual applied pressure during post-current application, which is specified before manufacturing of the spot welded joint, and a peel strength of the weld after the post-current application; The control unit is configured to control the amount of current flowing through the pair of electrodes in accordance with the actual applied pressure.

7. The spot welding device according to claim 6.

Images