Method for manufacturing spot-welded joint

Abstract

A method for manufacturing a spot-welded joint according to one embodiment of the present disclosure comprises: a step for performing main energization on a sheet set including one or more steel members having a tensile strength of 980 MPa or more; and a step for performing post-energization on the sheet set. The main energization is controlled by a means other than adaptive control. An instantaneous heat generation amount per unit volume and unit time is calculated on the basis of the energization duration in the post-energization and from a predetermined cumulative heat generation amount per unit volume with which good post-energization can be performed on the sheet set. The post-energization is adaptively controlled by adjusting the post-energization by a post-heat current or by an inter-electrode voltage or an inter-electrode resistance that causes the calculated instantaneous heat generation amount per unit volume and per unit time to be generated.

Description

Manufacturing method for spot welded joints

[0001] This disclosure relates to a method for manufacturing a spot welded joint. This application claims priority to Japanese Patent Application No. 2024-040451, filed on March 14, 2024, the contents of which are incorporated herein by reference.

[0002] Cold cracking can occur in spot welds of high-strength steel. Cold cracking is a general term for cracks that occur after the temperature of the weld has dropped to near room temperature after welding. Cold cracking in high-strength steel is thought to be caused by delayed fracture due to hydrogen embrittlement of the high-strength steel.

[0003] Various techniques have been proposed to suppress cold cracking in welds of high strength steels.

[0004] Patent Document 1 discloses a resistance spot welding method in which two or more overlapping steel sheets are clamped between a pair of welding electrodes, and current is passed through the steel sheets while pressure is applied to the steel sheets, forming a nugget on the overlapping surfaces of the steel sheets and joining the steel sheets together, characterized in that, after the joining, vibration is applied directly or indirectly to the nugget so that the vibration frequency of the steel sheets is 100 Hz or more and the maximum amplitude of the steel sheets satisfies 10 nm to 500 μm.

[0005] Patent Document 2 discloses a method for welding steel plates using an apparatus in which a high-frequency heating coil is wound around a resistance welding electrode. The material to be welded is melted and solidified by resistance welding to form a nugget, and then the molten solidified portion and the heat-affected portion are tempered by high-frequency heating. In this case, the tempering treatment by high-frequency heating is performed at a heating temperature equal to or lower than the A3 transformation point of the material to be welded. The method for heating to the A3 transformation point of the tempering treatment by high-frequency heating is to detect a change in impedance when the high-frequency heating coil reaches the A2 transformation point, and after a time of 1 second or less has elapsed since the detection, the high-frequency heating is stopped, or the heat input by the high-frequency heating is reduced, and then the material is allowed to cool at a cooling rate of 20 ° C. / s or more, or the resistance welding electrode is held under pressure.

[0006] Patent No. 7243928 Patent No. 5459750

[0007] In the technology described in Patent Document 1, the delayed fracture resistance is improved by applying vibration to the nugget, thereby releasing hydrogen in the nugget to the outside of the steel sheet. However, applying vibration to the nugget requires a vibration applying device, which reduces manufacturing efficiency. It is preferable to suppress delayed fracture using existing spot welding equipment.

[0008] In the technology described in Patent Document 2, the welded portion is heated by high frequency after spot welding to perform tempering. However, tempering by high frequency requires a high frequency heating device, which reduces manufacturing efficiency. It is preferable to suppress delayed fracture using existing spot welding equipment.

[0009] Post-heating is a method of preventing delayed fracture that can be implemented using existing spot welding equipment. Post-heating refers to passing a post-heating current through a sheet assembly. The term "post-heating current" is defined in JIS Z 3001-6:2013, "Welding Terminology - Part 6: Resistance Welding," as "a current passed through resistance welding, such as spot welding, projection welding, and upset welding, of steel that hardens through welding, for the purpose of tempering or annealing the hardened weld after welding. It is also called a tempering current."

[0010] Reducing the hardness of steel also reduces its susceptibility to hydrogen embrittlement, so reducing the hardness of the nugget by post-heating can suppress delayed fracture.

[0011] However, in sheet assemblies including high-strength steel members, the effect of post-current application is often not stably obtained because it is difficult to maintain a constant diameter of the nugget formed by the main current application.

[0012] Main current application refers to the flow of welding current through the sheet assembly. The term "welding current" is defined in JIS Z 3001-6:2013 "Welding Terminology - Part 6: Resistance Welding" as "the (main) current that flows to form a weld." The sheet assembly is melted by the welding current. When the flow of the welding current is stopped, heat transfer from the sheet assembly to the electrode causes the molten metal to solidify and form a nugget. Post-current application is performed on the nugget formed by the main current application.

[0013] In a strictly controlled welding environment, if the welding current, welding time, and welding pressure during the main current are kept constant, the nugget diameter will be constant. However, in a normal welding environment, disturbances exist. Typical examples of disturbances are gaps and shunt currents. A gap is a gap between multiple steel members that make up a plate assembly. A shunt current is a current that flows in a parallel circuit formed by existing welds and the workpieces, in addition to the main welding current. In a welding environment where disturbances exist, the diameters of multiple nuggets obtained by performing multiple main currents are usually not constant.

[0014] Variations in nugget diameter cause variations in the cross-sectional area of ​​the current path, which in turn causes variations in the resistance value and the amount of resistance heat generated in the current path, and therefore variations in nugget diameter also cause variations in the mechanical properties of the weld after post-current application.

[0015] If the actual nugget diameter is smaller than the target nugget diameter, the electrical resistance between the electrodes during post-current application increases. This results in excessive resistance heat generation during post-current application, causing the temperature of the weld to rise to the re-hardening temperature range and re-hardening the weld. This hardens the nugget and makes it more susceptible to cold cracking.

[0016] On the other hand, if the actual nugget diameter is larger than the target nugget diameter, the electrical resistance between the electrodes during post-current application is reduced, resulting in an insufficient amount of resistance heat generated during post-current application, which results in the nugget not being softened sufficiently and the occurrence of cold cracking not being sufficiently suppressed.

[0017] As explained above, due to the presence of disturbances in the welding environment and variations in nugget diameter, the effect of suppressing cold cracking cannot be stably obtained with normal post-energization. In view of the above circumstances, an object of the present disclosure is to provide a method for manufacturing a spot-welded joint that can stably reduce the nugget hardness in a welding environment where disturbances are present and suppress cold cracking in high-strength steel members.

[0018] The gist of the present disclosure is as follows.

[0019] (1) A manufacturing method of a spot-welded joint according to one aspect of the present disclosure includes a step of applying a main current to a sheet assembly including one or more steel members having a tensile strength of 980 MPa or more, and a step of post-current application to the sheet assembly, wherein the main current application is controlled by a means other than adaptive control, and an instantaneous heat generation amount per unit volume and unit time is calculated based on a current application time of the post-current from a predetermined cumulative heat generation amount per unit volume that satisfies the post-current application, and the post-current is adaptively controlled by adjusting an interelectrode resistance or interelectrode voltage, or a post-heating current, that generates the calculated instantaneous heat generation amount per unit volume and unit time. (2) Preferably, in the manufacturing method of a spot-welded joint described in (1) above, the cumulative heat generation amount in the first half of the post-current application is set to be at least two-thirds of the cumulative heat generation amount in the entire post-current application. (3) Preferably, in the manufacturing method of a spot-welded joint described in (1) or (2) above, before the main current application, a test plate assembly simulating the plate assembly is subjected to a main test current application and a post-test current application multiple times, the diameters of the nuggets formed in the multiple main test current applications are set to be approximately the same, and the cumulative heat generation amount per unit volume of a post-test current application that successfully post-currents the test plate assembly among the multiple post-test current applications is set to the predetermined cumulative heat generation amount per unit volume that successfully post-currents the plate assembly. (4) Preferably, in the manufacturing method of a spot-welded joint described in (3) above, the cumulative heat generation amount per unit volume of a post-test current application that does not cause cracks in the welds of the test plate assembly is set to the predetermined cumulative heat generation amount per unit volume that successfully post-currents the plate assembly. (5) Preferably, in the manufacturing method of a spot-welded joint described in (3) above, the cumulative heat generation amount in the post-test current application that minimizes the hardness of the nugget of the test plate set is set to the predetermined cumulative heat generation amount per unit volume that allows good post-current application to the plate set. (6) Preferably, in the manufacturing method of a spot-welded joint described in (3) above, the cumulative heat generation amount in the post-test current application that maximizes the cross tensile strength of the weld of the test plate set is set to the predetermined cumulative heat generation amount per unit volume that allows good post-current application to the plate set.(7) Preferably, in the manufacturing method of a spot welded joint described in (3) above, the diameter of the nugget formed in the plate pair is within a range of 0.7 to 1.3 times the diameter of the nugget formed in the test plate pair.

[0020] According to the present disclosure, it is possible to provide a manufacturing method for a spot-welded joint that can stably reduce the hardness of a nugget in a welding environment where external disturbances are present and suppress cold cracking in high-strength steel members.

[0021] Fig. 4 is a schematic diagram of spot welding. Fig. 5 is a graph showing the relationship between current (welding current) in main current application and nugget diameter. Fig. 6 is a graph showing Vickers hardness distributions of various nuggets after adaptively controlled post-current application. Fig. 7 is a cross-sectional view illustrating measurement positions of Vickers hardness shown in the graphs of Fig. 3 and Fig. 4 .

[0022] The method for manufacturing a spot welded joint 1 according to the present disclosure includes a step of applying a main current to a sheet assembly 11 including one or more steel members 111 (high-strength steel members 111H) having a tensile strength of 980 MPa or more, and a step of post-current application to the sheet assembly 11, in which an instantaneous heat generation amount per unit volume and unit time is calculated based on the time of post-current application from a predetermined cumulative heat generation amount per unit volume that will allow good post-current application to the sheet assembly 11, and the post-current application is adaptively controlled by adjusting the interelectrode resistance or interelectrode voltage, or welding current, that generates the calculated instantaneous heat generation amount per unit volume and unit time. The method for manufacturing a spot welded joint 1 according to the present disclosure will be described in detail below.

[0023] (Plate assembly 11) In the manufacturing method of the spot-welded joint 1, spot welding is performed on the plate assembly 11. The plate assembly 11 is a workpiece created by stacking steel members 111, which are the welding base material. The shape of the steel members 111 included in the plate assembly 11 is not particularly limited. The steel members 111 can have any three-dimensional shape. For example, if the steel members 111 are hat-shaped members, the plate assembly 11 is formed by stacking the flange portions of the hat-shaped members. The overlapped flange portions are spot-welded to join the hat-shaped members and create the spot-welded joint 1. Alternatively, the steel members 111 may be steel plates. The number of steel members 111 included in the plate assembly 11 can be any value equal to or greater than two.

[0024] At least one of the steel members 111 included in the plate assembly 11 is a high-strength steel member 111H having a tensile strength of 980 MPa or more. The tensile strength of the high-strength steel member 111H may be 1000 MPa or more, 1200 MPa or more, or 1500 MPa or more. There is no particular upper limit to the tensile strength of the high-strength steel member 111H. For example, the tensile strength of the high-strength steel member 111H may be 2500 MPa or less, 2200 MPa or less, or 2000 MPa or less. The plate assembly 11 may also include a low-strength steel member 111L having a tensile strength of less than 980 MPa.

[0025] The high-strength steel member 111H increases the strength of the spot-welded joint 1. On the other hand, low-temperature cracking is likely to occur in the welded portion 12 formed in the high-strength steel member 111H.

[0026] (Main Current Passage) The spot welding performed in the manufacturing method for spot welding according to this embodiment includes a main current passage and a post-current passage. In the main current passage, the sheet pair 11 is sandwiched between a pair of spot welding electrodes 2, and a welding current is passed through the sheet pair 11 while pressure is applied to the sheet pair 11. The welding current is also referred to as a main current. The welding current generates resistance heat in the sheet pair 11, causing the sheet pair 11 to melt.

[0027] After the main current flow is completed, the sheet assembly 11 is sandwiched between the pair of electrodes 2, and the current flowing through the electrodes 2 is set to zero or a value close to zero. Because a coolant flows inside the spot welding electrodes 2, heat is transferred from the sheet assembly 11 to the electrodes 2 while the electrodes 2 are applying pressure to the sheet assembly 11. This cools the sheet assembly 11, solidifying the molten metal and forming a nugget 121. The time from the end of the welding current to the start of the post-heating current is called the cooling time or chill time. The nugget 121 joins the multiple steel members 111 included in the sheet assembly 11. A heat-affected zone 122 is formed around the nugget 121. The portion including the nugget 121 and the heat-affected zone 122 is called the weld 12.

[0028] The main current is controlled by a means other than adaptive control. For example, the main current is controlled by constant current control. Constant current control is a method of controlling the current flow in the main current circuit in resistance welding, which detects fluctuations in the current in the main circuit for each half cycle or each cycle of the main current and automatically corrects these fluctuations (see JIS Z 3001-6:2013). Note that constant current control is a concept that includes both: (1) using a DC power source and controlling the current to be constant, and (2) using an AC power source and controlling the effective current to be constant.

[0029] The welding current may be controlled by up-slope control or down-slope control. Up-slope control refers to control in which the current is continuously increased from a predetermined value or zero within a set time, while down-slope control refers to control in which the current is continuously decreased to a predetermined value or zero within a set time (see JIS Z 3001-6:2013). The waveform of the welding current may be a sine wave, a square wave, or a non-sine wave (a triangular wave, a ramp wave, or a sinc wave).

[0030] The welding current, pressure, and welding time (time for which the welding current is passed) in the main current application are not particularly limited. A person skilled in the art can select welding conditions suitable for the thickness, composition, and number of the steel members 111 included in the sheet assembly 11.

[0031] (Post-energization) After the nugget 121 is formed by the main energization, post-energization is performed on the sheet set 11. In the post-energization, the sheet set 11 is sandwiched between a pair of electrodes, and a post-heating current is passed through the sheet set 11 while pressure is being applied to the sheet set 11. The post-heating current generates resistance heating in the sheet set 11, and tempers the nugget 121.

[0032] The post-current is adaptively controlled. Adaptive control refers to control that changes the characteristics of a control system to meet required conditions in response to disturbances such as the characteristics and environment of the controlled object. In the post-current method for manufacturing a spot welded joint 1 according to this embodiment, the cumulative heat generation amount Q per unit volume is used as an index for adaptive control. The Joule heat generation amount in the post-current is controlled so that the cumulative heat generation amount Q per unit volume is a predetermined value.

[0033] The cumulative heat generation amount Q per unit volume during post-energization is the cumulative value of the value q calculated by the following formula A over the period from the start to the end of post-energization: q = (V × I) / (S × t)... formula A In formula A, V is the inter-electrode voltage, I is the welding current, S is the cross-sectional area of ​​the current path, and t is the total plate thickness (mm) of the steel members 111 included in the plate assembly 11. The value q calculated by formula A is the instantaneous heat generation amount q per unit volume and unit time. This is because the post-heating current passes through a columnar portion with a contact area S and a total plate thickness t, generating resistance heat.

[0034] If we rewrite equation A using equation B, we get equation C. R = (r × t) / S...equation B q = (V × I × R) / (r × t ２ ) = (V ２ ) / (r × t ２ )...Equation C In Equation B, r is the resistivity of the material to be welded (plate assembly 11). R calculated by Equation B is the resistance of a columnar portion having a cross-sectional area S and a total plate thickness t.

[0035] As is clear from Equation C, the instantaneous heat generation amount q per unit volume and unit time can be calculated from the inter-electrode voltage V, the total thickness t of the plate assembly 11, and the resistivity r of the plate assembly 11, and is not affected by the cross-sectional area S of the current path. On the other hand, the resistivity of the plate assembly 11 changes depending on the temperature of the plate assembly 11. Even if I and V are constant from the start to the end of post-current application, q will not be constant. Furthermore, when expulsion occurs, the total thickness t of the plate assembly 11 at the portion clamped by the electrodes 2 is significantly reduced.

[0036] The instantaneous heat generation amount q per unit volume and unit time is accumulated from the start to the end of post-current application to obtain the cumulative heat generation amount Q per unit volume in post-current application. This cumulative heat generation amount Q per unit volume can also be calculated without using the cross-sectional area S of the current path.

[0037] In the manufacturing method of the spot-welded joint 1 according to this embodiment, the cumulative heat generation amount Q per unit volume is used as an index for adaptive control of post-heating. Specifically, (1) first, the cumulative heat generation amount Q per unit volume that allows for satisfactory post-heating of the sheet pair 11 is determined in advance. (2) Next, based on the cumulative heat generation amount Q per unit volume that allows for satisfactory post-heating of the sheet pair 11 and the post-heating time (post-heating time), the instantaneous heat generation amount q per unit volume and unit time that can reproduce the cumulative heat generation amount Q is calculated. (3) Then, the post-heating is adaptively controlled so as to obtain the calculated instantaneous heat generation amount q per unit volume and unit time. The adaptive control is performed by adjusting the interelectrode resistance, interelectrode voltage, or post-heating current. A predetermined instantaneous heat generation amount q per unit volume and unit time may be achieved by adjusting the post-heating current. Alternatively, a predetermined instantaneous heat generation amount q per unit volume and unit time may be achieved by adjusting the interelectrode resistance or interelectrode voltage. For example, it is preferable to feed back the voltage between the electrodes during current application in real time and automatically control the current value and current application time in order to reproduce the optimum conditions.

[0038] There is no particular limitation on the method for determining the cumulative heat generation amount Q that will allow for good post-energization in the sheet assembly 11. The cumulative heat generation amount Q can be determined by simulation. Alternatively, the cumulative heat generation amount Q can be determined by post-test energization. Post-test energization is a post-energization that is performed prior to the main energization and post-energization that obtain the spot welded joint 1 to be manufactured, in order to find appropriate conditions. An example of post-test energization will be described below.

[0039] Prior to conducting post-test energization, a plate assembly 11 simulating the plate assembly 11 to be welded is first prepared. Hereinafter, the plate assembly 11 to be welded will be referred to as the "production plate assembly" or simply as the "plate assembly," and the plate assembly 11 simulating the production plate assembly will be referred to as the "test plate assembly." The production plate assembly and the test plate assembly only need to be the same in terms of the base metal steel type, base metal plate thickness, number of base metal sheets, and stacking order of the base metal sheets. For example, the plate assembly 11 to be welded may be created by stacking flat portions of steel members 111 obtained by pressing steel plates. The plate assembly 11 simulating this may also be created by stacking steel plates before pressing or the like.

[0040] When conducting main current to the test plate assembly, disturbances are suppressed as much as possible to suppress variations in nugget diameter. Examples of disturbances include sheet gaps and shunt currents. The diameter of the nugget 121 produced by conducting main current to the test plate assembly is the same as the target diameter of the nugget 121 of the spot-welded joint 1 to be produced. Preferably, the welding current, current application time, and welding pressure in the main current to be conducted to the test plate assembly (hereinafter referred to as "main current to be tested") are the same as those in the main current to be conducted to the production plate assembly (hereinafter referred to as "main current to be produced"). The main current to be tested is conducted multiple times.

[0041] A post-test current is applied to the nugget 121 formed in the test plate assembly by the main test current application. The main test current application and the post-test current application are performed multiple times. The cumulative heat generation amount is changed during the multiple post-test current applications. The multiple nuggets 121 produced in this manner are evaluated. The cumulative heat generation amount during the post-test current application that resulted in the production of a good nugget 121 is used as the predetermined cumulative heat generation amount per unit volume that will allow the plate assembly 11 to be post-currented successfully.

[0042] For example, by observing the weld 12 formed on the test plate assembly, it can be determined whether or not cold cracking occurs after post-test current application. The cumulative heat generation amount in any post-current application that did not cause cold cracking in the weld 12 may be used as a predetermined cumulative heat generation amount per unit volume that allows good post-current application of the plate assembly 11.

[0043] Furthermore, the hardness of the nugget 121 of the test plate assembly can be measured by cutting the nugget 121 formed in the test plate assembly and performing a Vickers hardness test on the cross section. The cumulative heat generation amount in post-energization that minimizes the hardness of the nugget 121 of the test plate assembly may be used as a predetermined cumulative heat generation amount per unit volume that allows good post-energization of the plate assembly 11. Furthermore, any cumulative heat generation amount that allows the hardness of the nugget 121 of the test plate assembly to be equal to or less than a predetermined value may be used as a predetermined cumulative heat generation amount per unit volume that allows good post-energization of the plate assembly 11.

[0044] Alternatively, the cross tensile strength of the weld 12 can be measured by performing a cross tensile test on the weld 12 provided in the test plate set. The cumulative heat generation amount during post-energization that maximizes the cross tensile strength of the weld 12 of the test plate set may be used as the predetermined cumulative heat generation amount per unit volume that allows for satisfactory post-energization of the plate set 11. Alternatively, any cumulative heat generation amount that allows the cross tensile strength of the weld 12 of the test plate set to be equal to or greater than a predetermined value may be used as the predetermined cumulative heat generation amount per unit volume that allows for satisfactory post-energization of the plate set 11. The cross tensile test is performed, for example, in accordance with JIS Z 3137:1999 "Test specimen dimensions and test method for cross tensile tests of resistance spot and projection welded joints."

[0045] (Effects) In the manufacturing method of the spot-welded joint 1 according to this embodiment, the sheet assembly 11 includes high-strength steel members 111H. This makes it possible to manufacture a spot-welded joint 1 having high strength. However, cold cracking is likely to occur in the weld 12 provided in the high-strength steel members 111H. Furthermore, because the nugget diameter is likely to vary during the main current application, it is difficult to stably suppress cold cracking by performing an appropriate post-current application.

[0046] Therefore, in the manufacturing method of the spot welded joint 1 according to this embodiment, the cumulative heat generation amount Q per unit volume that allows for satisfactory post-energization of the sheet assembly 11 is determined in advance. Then, the post-energization is adaptively controlled to reproduce this cumulative heat generation amount. This makes it possible to stably suppress cold cracking.

[0047] The reason why the effect of the post-heating is unstable is that the nugget diameter changes due to external disturbances, which changes the resistance of the current path. In conventional post-heating, an optimal post-heating current and current application time (post-heating time) are specified, and constant current control is performed in the actual post-heating to reproduce this. However, if the diameter of the nugget 121 in the actual post-heating is smaller than that in the post-test current, the resistance increases. When the post-heating is controlled with a constant current, the interelectrode voltage increases in response to the increase in resistance. This results in excessive heat input during the post-heating.

[0048] However, the inventors discovered that this problem can be avoided by adaptively controlling the post-heating current with the cumulative heat generation amount as the control target. When adaptively controlling the post-heating current with the cumulative heat generation amount as the control target, the post-heating current decreases in response to an increase in resistance. The cumulative heat generation amount during the post-heating current in the actual welding process substantially coincides with the cumulative heat generation amount during the post-heating current in the test welding process. Experiments conducted by the inventors have shown that by maintaining a constant cumulative heat generation amount during the post-heating current, the effect of the post-heating current can be maintained within a certain range. In other words, by using a post-heating current in the actual welding process that reproduces the cumulative heat generation amount during the post-heating current, which can suppress cold cracking, cold cracking can be stably suppressed. In other words, in the manufacturing method of the spot-welded joint 1 according to this embodiment, even when the nugget diameter varies due to disturbances during the main currenting, optimal post-heating conditions can be automatically applied to the sheet assembly 11 during the post-heating current, thereby suppressing cold cracking.

[0049] In the prior art, adaptive control of the main current is believed to stabilize the nugget diameter. However, in the manufacturing method of the spot-welded joint 1 according to the present embodiment, the main current is not adaptively controlled. According to the knowledge of the inventors, when significant disturbances exist in the welding environment, adaptive control of the main current can cause significant expulsion. When cold cracking needs to be suppressed, adaptive control of the main current is of little benefit. In fact, from the viewpoint of suppressing the risk of significant expulsion during the main current and enhancing the cold cracking suppression effect, it is preferable not to adaptively control the main current. Note that adaptive control of the post-current does not increase the risk of expulsion. This is because a nugget, which is a stable current path, is present when the post-current is performed.

[0050] The most basic aspect of the method for manufacturing the spot welded joint 1 according to this embodiment has been described above. A more preferred aspect will now be described.

[0051] (Cumulative heat generation amount in the first half of post-current) It is preferable that the cumulative heat generation amount in the first half of post-current is at least two-thirds of the cumulative heat generation amount in the entire post-current. The cumulative heat generation amount in the first half of post-current is the value obtained by accumulating the instantaneous heat generation amount q per unit volume and unit time over the period from the start of post-current to the point where half of the post-current time (post-heating time) has elapsed. The cumulative heat generation amount in the entire post-current is, in other words, the value obtained by accumulating the instantaneous heat generation amount q per unit volume and unit time over the period from the start to the end of post-current.

[0052] By setting the cumulative heat generation amount in the first half of the post-current to at least two-thirds of the cumulative heat generation amount in the entire post-current, adaptive control of the post-current can be further stabilized. This is because setting the cumulative heat generation amount in the first half of the post-current within the above range makes it easier to avoid excessive heat generation in the second half of the post-current. Furthermore, by ensuring most of the required heat generation amount in the first half of the post-current and fine-tuning the heat generation amount in the second half of the post-current, more precise control of the heat generation amount can be achieved.

[0053] (Method for Specifying a Predetermined Cumulative Heat Generation Amount Per Unit Volume That Allows for Good Post-Current Conduction of the Plate Assembly 11) The method for specifying a predetermined cumulative heat generation amount per unit volume that allows for good post-current conduction of the plate assembly 11 is not particularly limited. As described above, the heat generation amount can be estimated by simulation. On the other hand, as described above, the heat generation amount can also be estimated by performing main test current conduction and post-test current conduction on a test plate assembly that simulates the production plate assembly. That is, before the production main current conduction, main test current conduction and post-test current conduction are performed multiple times on a test plate assembly that simulates the production plate assembly, with the diameters of the nuggets 121 formed in the multiple main test current conductions being approximately the same, and the cumulative heat generation amount per unit volume of the post-test current conduction that allowed for good post-current conduction of the test plate assembly among the multiple post-test current conductions can be determined as the predetermined cumulative heat generation amount per unit volume that allows for good post-current conduction of the plate assembly 11. When the diameter of the nugget 121 formed in the multiple main test current passes is within a range of ±10% of the target diameter, the diameter of the nugget 121 formed in the multiple main test current passes is considered to be substantially the same. Preferably, the diameter of the nugget 121 formed in the multiple main test current passes is within a range of ±5% of the target diameter.

[0054] When post-test current is applied, for example, any of the following values ​​can be used as the cumulative heat generation amount per unit volume that is determined in advance to allow good post-test current application to the plate assembly 11: (1) The cumulative heat generation amount in post-test current application that does not cause cracks in the welded portion 12 of the test plate assembly; (2) The cumulative heat generation amount in post-test current application that minimizes the hardness of the nugget 121 of the test plate assembly; or (3) The cumulative heat generation amount in post-test current application that maximizes the cross tensile strength of the welded portion 12 of the test plate assembly.

[0055] (Difference between the nugget diameter of the test plate assembly and the nugget diameter of the actual plate assembly) Disturbances may be present in the environment in which the manufacturing method for the spot-welded joint 1 according to this embodiment is performed. Therefore, the diameter of the nugget 121 formed by the actual current application may vary from the target value. This is because adaptively controlled post-current application can sufficiently mitigate the effects of variations in the diameter of the nugget 121. On the other hand, by bringing the diameter of the nugget 121 formed by the actual current application closer to the diameter of the nugget 121 formed by the test current application, the cold cracking resistance of the spot-welded joint 1 can be further improved. For example, it is preferable that the diameter of the nugget 121 formed in the actual plate assembly be within a range of 0.7 to 1.3 times the diameter of the nugget 121 formed in the test plate assembly.

[0056] In addition, JIS Z 3001-6:2013, "Welding Terminology - Part 6: Resistance Welding," defines the nugget diameter as "the diameter of the nugget measured at the joint interface by a cross-sectional test of a spot weld or a projection weld." When the number of steel members 111 is three or more and the steel members 111 have two or more joint interfaces, the diameter of the nugget 121 is measured at two locations. The manufacturing method for the spot-welded joint 1 according to this embodiment aims to suppress cold cracking of the high-strength steel members 111H. Therefore, when comparing the nugget diameter of the actual plate assembly with the nugget diameter of the test plate assembly, the nugget diameter measured at the joint interface of the high-strength steel members 111H is used for comparison. For example, when the high-strength steel members 111H have two or more joint interfaces, such as when three high-strength steel members 111H are stacked, it is preferable that the nugget diameter measured at at least one joint interface be within the above-mentioned range, and it is even more preferable that the nugget diameters measured at all joint interfaces be within the above-mentioned range.

[0057] While the embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit thereof. A more preferred example of the method for manufacturing the spot welded joint 1 according to the present embodiment will now be described.

[0058] (Vickers hardness and thickness of high-strength steel member 111H) The Vickers hardness of the high-strength steel member 111H is not particularly limited. For example, it is preferable that the Vickers hardness of the high-strength steel member 111H be HV300 or more, HV400 or more, HV500 or more, or HV650 or more. The Vickers hardness of the high-strength steel member 111H may be HV500 or less, HV650 or less, or HV850 or less. The thickness of the high-strength steel member 111H is also not particularly limited. For example, the thickness of the high-strength steel member 111H may be 1.0 mm or more, 1.4 mm or more, or 1.8 mm or more. The thickness of the high-strength steel member 111H may be 1.8 mm or less, 2.0 mm or less, or 2.6 mm or less.

[0059] (Tensile strength, Vickers hardness, and thickness of low-strength steel member 111L) The plate assembly 11 may include a low-strength steel member 111L having a tensile strength of 980 MPa or less. The configuration of the low-strength steel member 111L is not particularly limited. A configuration appropriate for the application of the spot-welded joint 1 can be applied to the low-strength steel member 111L. For example, the tensile strength of the low-strength steel member 111L may be 270 MPa or more, 440 MPa or more, or 590 MPa or more. The tensile strength of the low-strength steel member 111L may be 440 MPa or less, 590 MPa or less, or 780 MPa or less. The Vickers hardness of the low-strength steel member 111L may be HV80 or more, HV120 or more, or HV180 or more. The Vickers hardness of the low-strength steel member 111L may be HV150 or less, HV200 or less, or HV280 or less. The thickness of the low-strength steel member 111L is not particularly limited. For example, the thickness of the low-strength steel member 111L may be 0.5 mm or more, 0.6 mm or more, or 0.7 mm or more. The thickness of the low-strength steel member 111L may be 1.2 mm or less, 1.4 mm or less, or 1.6 mm or less.

[0060] (Surface Treatment) The steel member 111 may be subjected to a surface treatment. For example, in order to improve corrosion resistance, aesthetics, and the like, it is preferable that a plating is provided on the surface of the steel member 111. Examples of types of plating include Al-based plating, Al-based alloy plating, Zn-based plating, and Zn-based alloy plating. Al-based plating refers to plating with an Al content of 50% by mass or more. Al-based alloy plating refers to plating with an Al content of 50% by mass or more, a portion of which is alloyed with the underlying steel member 111. Zn-based plating refers to plating with a Zn content of 50% by mass or more. Zn-based alloy plating refers to plating with a Zn content of 50% by mass or more, a portion of which is alloyed with the underlying steel member 111.

[0061] (Thickness ratio of plate set 11) The thickness ratio of the plate set 11 is not particularly limited. The thickness ratio is the value obtained by dividing the total thickness of the plate set 11 by the thickness of the thinner of the steel members 111 arranged on the surface of the plate set 11. For example, the thickness ratio of the plate set 11 may be 3 or more, 4 or more, or 5 or more. The thickness ratio of the plate set 11 may be 6 or less, 7 or less, or 9 or less.

[0062] The tensile strength of the steel member 111 is measured in accordance with JIS Z 2241:2011 "Method for tensile testing of metallic materials." The shape of the test piece can be selected appropriately depending on the shape of the steel member 111. If it is difficult to obtain a test piece from the steel member 111, the tensile strength of the steel member 111 may be estimated by measuring the Vickers hardness of the steel member 111 and converting the Vickers hardness to tensile strength using a known conversion table. The hardness of the steel member 111 is measured in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method." During measurement, the test force is 0.5 kg.

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

[0064] (1. Nugget hardness evaluation) Various spot welded joints were created in a strictly controlled welding environment with virtually no external disturbances. The conditions were as follows. Two spot welding runs were performed for each set of conditions. ● Sheet assembly Two 1.5 GPa-class cold-rolled steel sheets (30 mm length, 50 mm width, 1.6 mm thickness) were stacked together ● Optimal conditions for the above sheet assembly Main current: welding time 360 ​​msec, welding current 4.4 kA Cooling time: 1000 msec Nugget diameter: 3√t (= 3.79 mm) Post-current: post-heating time 980 msec, post-heating current 3.7 kA ● Spot welding conditions Main current Optimal condition ±0.2, ±0.4, or ±0.8 kA Cooling time 1000 msec Post-current (1) Adaptive control with the cumulative heat generation amount under the optimal conditions as the control target value (2) Constant current control under the same conditions as the post-current under the optimal conditions

[0065] By applying the optimal conditions to the sheet assembly and performing the main and post-heating processes, a nugget free from cold cracking was obtained. Then, spot welding experiments simulating actual welding were conducted based on the optimal conditions. In the spot welding experiments, the welding current was varied within a range of ±0.8 kA relative to the optimal conditions. This simulated fluctuations in nugget diameter caused by external disturbances. The post-heat in the spot welding experiments was adaptively controlled or constant current controlled. In the spot welding experiments with adaptively controlled post-heating, the instantaneous heat generation rate per unit volume and unit time was calculated based on the post-heating time from the cumulative heat generation rate per unit volume in the post-heating under the optimal conditions. The post-heat in the spot welding experiments was adaptively controlled by adjusting the interelectrode resistance, interelectrode voltage, or post-heating current that generated the calculated instantaneous heat generation rate per unit volume and unit time. In the spot welding experiments with constant current controlled post-heating, the post-heating under the optimal conditions was reproduced.

[0066] The welded portion of the spot-welded joint obtained by the above experiment was cut, and the diameter of the nugget and the Vickers hardness of the welded portion were measured. The hardness measurement of the nugget was performed as shown in FIG. 5. The hardness measurement of the nugget 121 was performed continuously along a dashed line X that was parallel to the joining interface of the steel sheets and slightly separated from the joining interface. The starting point of the continuous hardness measurement was the center of the nugget 121. The ending point of the continuous hardness measurement was outside the heat-affected zone 122.

[0067] The relationship between the welding current and the nugget diameter in this current application is shown in Figure 2. It was confirmed that the nugget diameter varies depending on the welding current.

[0068] FIG. 3 shows the hardness distribution of the nugget that was adaptively controlled and then energized and its surrounding area. FIG. 4 shows the hardness distribution of the nugget that was subjected to constant current control and then energized.

[0069] In spot-welded joints after adaptively controlled post-heat application, the hardness increased from the center to the edge of the nugget, then rapidly decreased outside the nugget (heat-affected zone), and then recovered outside the heat-affected zone. Because the nugget diameters varied, the location of the softest zone varied for each spot-welded joint. However, in spot-welded joints that underwent adaptively controlled post-heat application, the hardness of the nugget interior, the hardness of the nugget edge, and the hardness of the heat-affected zone were all at the same level, regardless of the nugget diameter. In other words, adaptively controlled post-heat application consistently exerted its effects regardless of variations in nugget diameter.

[0070] On the other hand, in the spot-welded joints after post-energization under constant current control, the hardness inside the nugget varied greatly. Due to the variation in nugget diameter, post-energization under constant current control was unable to stably exert its effect.

[0071] (2. Evaluation of cold cracking resistance) Steel plate 1 and steel plate 2 shown in Table 1 were stacked together to form a sheet assembly. This sheet assembly was subjected to main current and post-current to produce a spot-welded joint. The welded portion of the spot-welded joint was then observed to check for the presence or absence of cold cracking. The control method for post-current and the presence or absence of cold cracking are shown in Table 1.

[0072] In all conditions, the target nugget diameter for the main welding was 3.48 mm. The conditions for the post-current performed at a constant current were optimized for a nugget with a diameter of 3.48 mm. In the post-current performed under adaptive control, the cumulative heat generation amount during the post-current performed under constant current control optimized for a nugget with a diameter of 3.48 mm was set as the control target value.

[0073]

[0074] In Example 1, no post-current was applied, resulting in cracks in the weld. In Examples 2 and 6, the welding environment was strictly controlled, and the nugget diameter was set to the same value as the target value. As a result, cracks in the weld were suppressed. In Examples 3, 4, and 5, the nugget diameter was smaller than the target value due to disturbances. In Example 3, the heat input during post-current application was excessive, resulting in cracks in the weld. In Examples 7, 8, and 9, the nugget diameter was also smaller than the target value due to disturbances. However, in Examples 7, 8, and 9, the post-current was adaptively controlled, resulting in no cracks in the weld.

[0075] DESCRIPTION OF SYMBOLS 1 Spot welded joint 11 Plate assembly 111 Steel member 111H High strength steel member 111L Low strength steel member 12 Welded portion 121 Nugget 122 Heat affected zone 2 Electrode X Hardness measurement portion

Claims

1. A method for manufacturing a spot weld joint, comprising: a step of initially energizing a plate assembly including one or more steel members having a tensile strength of 980 MPa or more; and a step of subsequently energizing the plate assembly, wherein the initial energization is controlled by means other than adaptive control, the instantaneous heat generation amount per unit volume and per unit time is calculated based on the energization time of the subsequent energization from a previously determined cumulative heat generation amount per unit volume that enables good subsequent energization of the plate assembly, and the subsequent energization is adaptively controlled by adjusting the resistance between electrodes or the voltage between electrodes that generates the calculated instantaneous heat generation amount per unit volume and per unit time, or the post-heating current.

2. The method for manufacturing a spot weld joint according to claim 1, wherein the cumulative heat generation amount in the first half of the subsequent energization is 2 / 3 or more of the cumulative heat generation amount in the entire subsequent energization.

3. Before the initial energization, a test plate assembly simulating the plate assembly is subjected to test initial energization and test subsequent energization a plurality of times, the diameters of the nuggets formed in the plurality of test initial energizations are made substantially the same, and the cumulative heat generation amount per unit volume of the test subsequent energization in which the test plate assembly can be successfully subsequently energized among the plurality of test subsequent energizations is set as the previously determined cumulative heat generation amount per unit volume that enables good subsequent energization of the plate assembly. The method for manufacturing a spot weld joint according to claim 1 or 2, characterized in that.

4. The method for manufacturing a spot weld joint according to claim 3, wherein the cumulative heat generation amount in the test subsequent energization that does not cause cracks in the welded portion of the test plate assembly is set as the previously determined cumulative heat generation amount per unit volume that enables good subsequent energization of the plate assembly.

5. The method for manufacturing a spot weld joint according to claim 3, wherein the cumulative heat generation amount in the test subsequent energization that minimizes the hardness of the nugget of the test plate assembly is set as the previously determined cumulative heat generation amount per unit volume that enables good subsequent energization of the plate assembly.

6. The method for manufacturing a spot weld joint according to claim 3, wherein the cumulative heat generation amount of the test subsequent energization that maximizes the cross-tensile strength of the welded portion of the test plate assembly is set as the previously determined cumulative heat generation amount per unit volume that enables good subsequent energization of the plate assembly.

7. The method for manufacturing a spot weld joint according to claim 3, wherein the diameter of the nugget formed in the plate assembly is in the range of 0.7 times to 1.3 times the diameter of the nugget formed in the test plate assembly.

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