Method for manufacturing spot welded joint and spot welding device

Abstract

To provide a manufacturing method of a spot welded joint which can stably increase the mechanical characteristic of a welding part after post-electrification and provide a spot welding device.SOLUTION: A manufacturing method of a spot welded joint according to an embodiment of the present invention comprises the steps of: spot-welding a plurality of overlapped metal plates by using a spot welding device having a pair of electrodes to form a weld zone; and performing post-electrification on the weld zone by using the pair of electrodes. The method determines a prescribed range for a first feature value before the post-electrification on the basis of a pre-specified correlation between the first feature value being an index of pressing force in the weld zone, an interval between the electrodes or electrical resistance and a second feature value being an index of the joint strength of the spot welded joint, measures the first feature value in the post-electrification, and confirms whether or not the first feature value is within a prescribed range.SELECTED DRAWING: Figure 1A

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 method in which overlapping base materials are sandwiched between the tips of electrodes with appropriately shaped tips, and current and pressure are concentrated in a relatively small area to locally heat the area while simultaneously applying pressure with the electrodes. In spot welding, the molten and solidified part of the weld is called a nugget. In spot welding, the nugget may be post-heat-treated using a welding electrode to improve its mechanical properties. In this case, the current for melting and solidifying the base materials to form the nugget is called the main current, and the current for post-heat-treating the nugget is called the post-current.

[0003] An example of post-current passing is tempering current passing and solidification segregation relaxation current passing, which are mainly applied to spot welding of high-strength steel sheets.

[0004] In spot welding of high-strength steel plates, the decrease in peel strength is considered a problem. Peel strength is the joint strength against stress in the peel direction, which is evaluated by CTS or the like measured by a cross tensile test. Usually, the higher the strength of the base material, the higher the joining strength of the welded joint. However, for high-strength steel plates, especially those with a tensile strength of 980 MPa or more, 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 and P and S segregated in the nugget when the base material melts and solidifies.

[0005] Tempering current tempers the nugget to reduce its hardness and improve the peel strength of the welded joint, while solidification segregation relaxation current heats the nugget to disperse the segregated P and S, thereby relaxing the segregation and improving the peel strength of the welded joint.

[0006] Various proposals have been made regarding a manufacturing method of a spot welded joint including a post-current process. For example, Patent Document 1 discloses a spot welding method for high-strength steel sheets, which is characterized in that it measures the current and interelectrode voltage during spot welding, performs calculations using the measured current, interelectrode voltage, and material property values, and performs one or more of the following based on the calculation results during cooling after the end of welding current: determining the time to remove the electrode from the steel sheet, adjusting the post-current current and post-current time in the post-current that continues after the end of welding current, adjusting the post-current current and post-current time in the post-current that is started after cooling after the end of welding current, and adjusting the electrode pressure.

[0007] Patent Document 2 describes a resistance spot welding method in which a workpiece made of overlapping multiple metal plates is sandwiched between a pair of electrodes and joined by passing current while applying pressure, and the method includes performing main welding and test welding prior to the main welding; (a) in the test welding, a main current for forming a nugget and a post-current for post-heat treatment are performed, and in the main current for 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 passed under constant current control to form an appropriate nugget, and in the post-current for 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, For 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 For tc ≥ 1600 ms: 0.2≦Vtp / Vtm≦1.5 (b) then, in the main welding, a main current for forming a nugget and a post-current for post-heat treatment are performed, and in the main current for the main welding, a time change curve of the instantaneous heat generation per unit volume and a cumulative heat generation per unit volume in the main current for the test welding are set to target values, and adaptive control welding is performed in which the current amount is controlled 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 passed under constant current control under the condition that the following relationship is satisfied:

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

[0009] In a normal spot welding device, the control parameters are the composition and thickness of the base metal plate, the current value, and the applied pressure. In the post-current, the diameter of the nugget obtained by the main current is also a control parameter. However, even if spot welding is performed multiple times with the same control parameters, the mechanical properties of the welded part obtained by the current are not necessarily constant. As will be described later with reference to Figures 2 and 3, the CTS of the welded joint after tempering current or solidification segregation relaxation current may vary greatly even if the current value and other conditions are the same. The cause of this is thought to be various disturbances during welding, but has not been identified. Therefore, it is difficult to optimize the post-current conditions. This difficulty prevents the application of post-current to the manufacturing site of mechanical parts. Note that tempering current tends to cause the CTS to vary more than solidification segregation relaxation current. Hereinafter, the term "post-current" is used as a concept that includes both tempering current and solidification segregation relaxation current.

[0010] The techniques described in the above Patent Documents 1 and 2 address the issue of improving peel strength. However, these documents do not particularly consider the variation in peel strength. Furthermore, these documents do not disclose any specific means for suppressing the variation. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2002-103054 A [Patent Document 2] Republished Publication WO2020 / 004115 Summary of the Invention [Problem to be solved by the invention]

[0012] In view of the above circumstances, an object of the present invention is to provide a method for manufacturing a spot welded joint and a spot welding apparatus that can stably improve the mechanical properties of a welded portion after post-current application. [Means for solving the problem]

[0013] The gist of the present invention is as follows. (1) A method for manufacturing a spot welded joint according to one embodiment of the present invention includes the steps of spot welding a plurality of stacked metal plates using a spot welding device having a pair of electrodes to form a weld, and post-currenting the weld using the pair of electrodes, wherein a predetermined range for the first characteristic value is determined prior to the post-current based on a predetermined correlation between a first characteristic value that is an index of the pressure, inter-electrode spacing, or electrical resistance at the weld, and a second characteristic value that is an index of joint strength of the spot welded joint, and during the post-current, the first characteristic value is measured to confirm whether the first characteristic value is within the predetermined range. (2) The method for manufacturing a spot welded joint described in (1) above may include a test welding process in which, prior to the spot welding, a plurality of stacked metal plates are spot welded using the spot welding device having a pair of the electrodes to form a test weld, and then a current is applied to the test weld using the pair of electrodes while measuring the first characteristic value; a process for measuring the second characteristic value of the test weld; and a process for identifying a correlation between the first characteristic value and the second characteristic value. (3) In the method for manufacturing a spot welded joint described in (1) or (2) above, the first characteristic value may be a difference between an actual pressure between the pair of electrodes measured during the post-current and a set pressure between the pair of electrodes set in the spot welding device during the post-current, or a rate of change of the difference over time. (4) In the method for manufacturing a spot welded joint described in (1) or (2) above, the first characteristic value may be an amount of change in the distance between the pair of electrodes during the post-current passage, or a time rate of change of the amount of change. (5) In the method for manufacturing a spot welded joint according to any one of (1) to (4) above, the second characteristic value may be a cross tensile strength CTS or an L-joint tensile strength LTS of the spot welded joint, or a difference between the cross tensile strength CTS or the L-joint tensile strength LTS of the spot welded joint before and after the post-energization. (6) In the method for producing a spot welded joint according to any one of (1) to (5) above, the plurality of metal plates may be a plurality of steel plates, and one or more of the steel plates may have a tensile strength of 980 MPa or more. (7) The method for producing a spot welded joint according to any one of (1) to (6) above may further include a step of preheating and energizing portions to be welded of the plurality of metal plates before the spot welding. (8) In the method for manufacturing a spot welded joint described in any one of (1) to (7) above, the correlation between the first characteristic value and the second characteristic value may be specified while keeping constant one or more conditions selected from the group consisting of a fracture morphology during a joint strength test of the welded portion, a carbon equivalent of the welded portion, an average thickness of a plurality of the metal plates, and a diameter of a nugget in the welded portion. (9) In the method for manufacturing a spot welded joint described in (6) above, the post-current passing includes one or more selected from the group consisting of temper current passing and segregation relaxation current passing, and in the temper current passing, a cool down time between the current passing immediately before the temper current passing and the temper current passing may be 0.20 seconds or more, and a current ratio obtained by dividing the current value during the temper current passing by the current value during a main current passing which is a current passing for forming the welded portion may be 0.50 to 0.85. In the segregation relaxation current passing, a cool down time between the current passing immediately before the segregation relaxation current passing and the segregation relaxation current passing may be 0.04 seconds or more, and a current ratio obtained by dividing the current value during the segregation relaxation current passing by the current value during the main current passing may be 0.70 to 1.20. (10) In the method for manufacturing a spot welded joint described in (9) above, the post-energization may be the tempering energization, the plurality of metal plates may include two overlapping high-strength steel plates, each having a tensile strength of 980 MPa or more, an average sheet thickness t of the two high-strength steel plates may be set to 1.2 to 2.2 mm, a nugget diameter at an overlapping surface of the two high-strength steel plates in the weld may be set to 4.0×√t to 5.5×√t, and the correlation between the first characteristic value and the second characteristic value may be regarded as following Equation 1. y=ax 2 +bx+c: expression 1 In the above formula 1, a is a value defined by formula 2, b is a value defined by formula 3, c is a value defined by formula 4, x is the increase in actual applied pressure (kN) as the first characteristic value, and y is the cross tensile strength CTS (kN) as the second characteristic value. -500×(mass%C)-150≦a≦-500×(mass%C): Formula 2 800×(mass%C)≦b≦800×(mass%C)+200: Formula 3 -320×(mass%C)-100≦c≦-320×(mass%C)+50: Formula 4 In the formulas 2, 3, and 4, mass%C is the mass concentration of carbon in the nugget. (11) In the manufacturing method of a spot welded joint described in (10) above, in the formula 1, a may be a value defined by formula 5, b may be a value defined by formula 6, and c may be a value defined by formula 7. a=-500×(mass%C)-25: Formula 5 b=800×(mass%C)+40: Formula 6 c=-320×(mass%C)-4: Formula 7 In the formulas 5, 6, and 7, mass%C is the mass concentration of carbon in the nugget. (12) In the method for manufacturing a spot welded joint described in (9) above, the post-energization is the segregation relaxation energization, the plurality of metal plates include two overlapping high-strength steel plates, each having a tensile strength of 980 MPa or more, an average sheet thickness t of the two high-strength steel plates is 1.2 to 2.2 mm, a nugget diameter at an overlapping surface of the two high-strength steel plates in the weld is 4.0×√t to 5.5×√t, and the correlation between the first characteristic value and the second characteristic value may be regarded as following Equation 8. y=ax 2 +bx+c: expression 8 In the above formula 8, a is a value defined by formula 9, b is a value defined by formula 10, c is a value defined by formula 11, x is the increase in actual applied pressure (kN) as the first characteristic value, and y is the cross tensile strength CTS (kN) as the second characteristic value. -50×(mass%C)-50≦a≦-50×(mass%C)+50: Formula 9 70×(mass%C)-80≦b≦70×(mass%C)+50: Formula 10 -30×(mass%C)-80≦c≦-30×(mass%C)+70: Formula 11 In the formulas 9, 10, and 11, mass%C is the mass concentration of carbon in the nugget. (13) In the manufacturing method of a spot welded joint described in (12) above, in the formula 8, a may be a value defined by formula 12, b may be a value defined by formula 13, and c may be a value defined by formula 14. a=-50×(mass%C)-5: Formula 12 b=70×(mass%C): Equation 13 c=-30×(mass%C)+15: Formula 14 In the formulas 12, 13, and 14, mass% C is the mass concentration of carbon in the nugget. (14) In the method for manufacturing a spot welded joint described in any one of (1) to (13) above, the first characteristic value may be set within the predetermined range, and then the post-energization may be terminated when the first characteristic value reaches an end value of the predetermined range, or when a predetermined period of time has elapsed since the first characteristic value reached an end value of the predetermined range. (15) In the method for manufacturing a spot welded joint according to any one of (1) to (14) above, the number of post-current application may be two or more, and the first characteristic value may be measured at a post-current application that is most dominant with respect to the second characteristic value of the spot welded joint, and it may be confirmed whether the first characteristic value is within the predetermined range. (16) A spot welding apparatus according to another aspect of the present invention is a spot welding apparatus for a manufacturing method of a spot welded joint described in any one of (1) to (15) above, comprising a pair of the electrodes and a first characteristic value measuring means configured to be able to measure the first characteristic value. (17) The spot welding apparatus described in (16) above may further include a control unit capable of controlling the first characteristic value within the predetermined range based on the correlation between the first characteristic value and the second characteristic value during the post-current application. Effect of the Invention

[0014] According to the present invention, it is possible to provide a method for manufacturing a spot welded joint and a spot welding apparatus that can stably improve the mechanical properties of a welded portion after post-current application. [Brief description of the drawings]

[0015] [Figure 1A] 10 is a graph showing an example of a change in actual pressurizing force during production of a spot welded joint when post-energization is tempering energization. [Figure 1B] 10 is a graph showing an example of a change in actual pressurized pressure during production of a spot welded joint when post-energization is segregation relaxation energization. [Figure 1C] 1 is a graph showing a schematic example of a change in electrical resistance during production of a spot welded joint. [Diagram 2] 13 is a graph showing a correlation between a first characteristic value and a second characteristic value when a time rate of change of an actual pressurized force is used as a first characteristic value and a CTS is used as a second characteristic value in a case where a post-current is a tempering current. [Diagram 3] 13 is a graph showing a correlation between a first characteristic value and a second characteristic value when the post-energization is a tempering energization, the increase in actual pressurizing force is used as the first characteristic value, and the CTS is used as the second characteristic value. [Figure 4A] 13 is a graph showing a correlation between a first characteristic value and a second characteristic value when the post-current is a segregation relaxation current, the increase in actual applied pressure is used as the first characteristic value, and the CTS is used as the second characteristic value. [Figure 4B] FIG. 4B is a reference diagram in which approximate curves are applied to the data points for a current flow time of 10 cycles and the data points for a current flow time of 20 cycles included in FIG. 4A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The inventors presumed that the reason why the mechanical properties of the welded portion after the post-current are not stable is due to the instability of the temperature of the welded portion during the post-current. From a metallurgical perspective, it is reasonable to consider that the variation in mechanical properties occurring in metals with the same components is due to the variation in the heat treatment temperature. It is also presumed that the current flow state may change due to disturbances during welding, and the heating temperature during the post-current may vary by several tens of degrees Celsius. Note that disturbances include, for example, the surface state of the metal plate, the strike angle of the electrode, and the distance (clearance) between the metal plates. It is difficult to completely eliminate these disturbances, especially in the manufacturing site of mechanical parts, considering the manufacturing costs, etc.

[0017] 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.

[0018] Here, the inventors have focused on thermal expansion that occurs in the welded portion due to a temperature rise during post-current application. When the welded portion thermally expands during post-current application, the distance between the pair of electrodes that clamp the welded portion increases. As a result, the electrodes move in a direction away from the welded portion. This electrode displacement can be detected and measured in existing spot welding devices using indicators such as the welding force, the electrode gap, or the electrical resistance. Therefore, the inventors have come up with the idea of ​​managing spot welding conditions using a first characteristic value that is an indicator of the welding force, the electrode gap, or the electrical resistance of the welded portion, which is closely related to the degree of thermal expansion of the welded portion.

[0019] The inventors produced various spot-welded joints while focusing on the first characteristic value, and evaluated the joint strength. There are various evaluation indices for joint strength. For example, peel strength, which is a type of joint strength, can be evaluated by cross tensile strength CTS measured by a cross tensile test, or L-shaped joint tensile strength LTS measured by an L-shaped joint tensile test. When the inventors investigated the relationship between the second characteristic value, which is an index of joint strength, and the above-mentioned first characteristic value, it was confirmed that there is a good correlation between the two. Furthermore, the inventors selected a second characteristic value according to the required mechanical properties of the spot-welded joint, specified the range of the first characteristic value required to ensure the second characteristic value based on the correlation between the first characteristic value and the second characteristic value, and conceived that the required mechanical properties can be stably ensured by controlling the first characteristic value within a predetermined range and performing post-current application. In addition, even if the first characteristic value is not controlled within the predetermined range, the joint strength of the spot-welded joint can be predicted without destructive testing by confirming whether the first characteristic value was within the predetermined range. Therefore, by checking whether the first characteristic value is within a predetermined range, it is possible to select only spot welded joints having the required mechanical properties.

[0020] The manufacturing method of a spot welded joint according to the present embodiment, which has been completed based on the above findings, includes the steps of spot welding a plurality of overlapping metal plates using a spot welding device having a pair of electrodes to form a weld, and post-currenting the weld using the pair of electrodes, in which a first characteristic value, which is an index of the pressure, inter-electrode spacing, or electrical resistance closely related to the degree of thermal expansion of the weld, is measured in the post-current, and whether or not the first characteristic value is within a predetermined range is confirmed in the post-current based on a pre-specified correlation between a second characteristic value, which is an index of the joint strength of the spot welded joint, and the first characteristic value. The manufacturing method of the spot welded joint according to the present embodiment will be described in detail below.

[0021] (Spot welding process) In the method for manufacturing a spot welded joint according to the present embodiment, a plurality of metal plates are spot welded using a spot welding device having a pair of electrodes. This forms a weld that joins the plurality of metal plates. The weld is a general term that includes the weld metal (nugget) and the heat-affected zone.

[0022] The conditions for spot welding and the type of electrodes are not particularly limited. Suitable spot welding conditions and electrodes can be appropriately selected according to the type, thickness, and number of metal plates, the required nugget diameter, and the like. A suitable form of a spot welding device for performing spot welding will be described later. Generally, in a manufacturing method for a spot-welded joint including a post-current process, the spot welding process and the post-current process are sometimes collectively referred to as a "spot welding process". However, in this embodiment, the term "spot welding" is used to indicate a concept that does not include post-current.

[0023] Spot welding usually includes a main current and a cool down time. The main current is current for melting the metal plate. The cool down time is the time for cooling the metal plate using a pair of electrodes after the main current is finished. Usually, a coolant flows inside the electrode for spot welding. Therefore, by stopping the current between the electrodes while the metal plate is being clamped by the pair of electrodes, the molten part of the metal plate can be cooled and solidified to form a welded part having a nugget.

[0024] The number of the metal plates stacked for spot welding is not particularly limited. The type and thickness of the metal plates are also not limited. For example, when the spot welded joint is used as an automobile part, the metal plates are preferably steel plates. 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.

[0025] In addition, before spot welding, preheating current may be applied to the portions to be welded in the multiple metal plates. Here, the portions to be welded in preheating current are the locations where the welds will be formed in the subsequent spot welding. Preheating current allows the welds to be formed more stably by spot welding. The conditions for preheating current are not particularly limited. Appropriate spot welding conditions and electrodes can be appropriately selected depending on the type, thickness, number, etc. of the metal plates.

[0026] (Post-energization process) In the manufacturing method of the spot welded joint according to the present embodiment, a weld formed by spot welding is post-currented using a pair of electrodes. Both the post-current and the spot welding can be performed using the same electrodes. In the post-current, a first characteristic value which is an index of the pressure, inter-electrode distance, or electrical resistance of the weld is measured, and a predetermined range for the first characteristic value is determined based on a pre-specified correlation between the first characteristic value and a second characteristic value which is an index of the joint strength of the spot welded joint. Here, the "predetermined range" is a range of the first characteristic value which is set to make the second characteristic value fall within a desired range. In the manufacturing method of the spot welded joint according to the present embodiment, it is confirmed whether the first characteristic value is within the predetermined range in the post-current.

[0027] The conditions of the post-current are not particularly limited and can be appropriately selected depending on the spot welded joint to be manufactured. Examples of post-current include tempering current and segregation relaxation current. Tempering current is current for performing heat treatment to temper the welded part, and segregation relaxation current is current for performing heat treatment to diffuse embrittling elements segregated in the welded part. The temperature range suitable for segregation relaxation is higher than the temperature range suitable for tempering. Therefore, the current value for segregation relaxation current is usually larger than the current value for tempering current. In addition, current with a smaller heat input than tempering current may be performed as post-current.

[0028] When the post-current passing is a tempering current, for example, when the metal plates are steel plates, at least one of which is a high-strength steel plate having a tensile strength of 980 MPa or more or 1500 MPa or more, it is preferable that the cool time between the current passing just before the tempering current and the tempering current is 0.20 seconds or more, and the current ratio, which is the current value during the tempering current divided by the current value during the main current, is 0.50 to 0.85. Here, "main current passing" refers to current passing for forming a welded portion. "Current passing just before the tempering current" refers to main current passing or a post-current passing other than the tempering current.

[0029] In tempering current, when the cooling time is 0.20 seconds or more and the current ratio is 0.50 to 0.85, tempering of martensite occurs. In spot-welded joints manufactured from sheet combinations including high-strength steel plates, a decrease in CTS due to excessive hardening of the welded portion is likely to be a problem. The CTS of the spot-welded joint can be increased by softening the welded portion through tempering current. In addition, in normal tempering current, there is a problem that the CTS is not stable. On the other hand, in the manufacturing method of the spot-welded joint according to the present embodiment, this problem is solved by setting the first characteristic value, which is an index of the pressure, electrode gap, or electrical resistance of the welded portion, within a predetermined range, as described later. In addition, the cooling time is more preferably 0.40 to 0.60 seconds so as not to lengthen the time required to manufacture the spot-welded joint. In addition, the current ratio in tempering current is even more preferably 0.55 to 0.65.

[0030] When the post-current is a segregation relaxation current, for example, when the metal plates are steel plates and at least one of them is a high-strength steel plate having a tensile strength of 980 MPa or more or 1500 MPa or more, it is preferable that the cool time between the current immediately before the segregation relaxation current and the segregation relaxation current is 0.04 seconds or more, and the current ratio, which is the current value during the segregation relaxation current divided by the current value during the main current, is 0.70 to 1.20. "Current immediately before the segregation relaxation current" refers to the main current or a post-current other than the segregation relaxation current.

[0031] When a plate assembly including high-strength steel plates is spot-welded, embrittling elements such as P are segregated in the welded portion immediately after spot welding. The segregated portion of the embrittling element acts as a fracture origin, lowering the CTS. In the segregation relaxation current conduction, if the cooling time is 0.04 seconds or more and the current ratio is 0.70 to 1.20, the embrittling element in the welded portion is diffused. This makes it possible to increase the CTS of the spot-welded joint. Note that even in normal segregation relaxation current conduction, there is a problem that the CTS is not stable. On the other hand, in the manufacturing method of the spot-welded joint according to the present embodiment, this problem is solved by setting the first characteristic value, which is an index of the pressure, electrode gap, or electrical resistance of the welded portion, within a predetermined range, as described later. Note that the cooling time in the segregation relaxation current conduction is more preferably 0.10 to 0.40 seconds so as not to lengthen the time required for manufacturing the spot-welded joint. In addition, the lower limit of the cooling time in the segregation relaxation current conduction may be 0.07 seconds or 0.08 seconds. The upper limit of the current ratio in the segregation relaxation current application may be set to 0.90.

[0032] (First characteristic value that is an index of the pressure, electrode spacing, or electrical resistance of the weld) In the post-current application, the welded portion is heated and thermally expanded. In order to evaluate the degree of this thermal expansion, a first characteristic value, which is an index of the welding force, inter-electrode gap, or electrical resistance of the welded portion, is measured in the post-current application. The welding force, inter-electrode gap, or electrical resistance can be used as the first characteristic value.

[0033] Due to the thermal expansion of the welded portion during the post-current application, the pair of electrodes that sandwich the metal plate move in a direction away from the metal plate. That is, the distance between the electrodes increases due to the thermal expansion. Therefore, the amount of change in the interval between the pair of electrodes during the post-current application may be measured and used as the first characteristic value. Also, the time rate of change in the amount of change in the interval between the pair of electrodes, that is, the speed at which the pair of electrodes separate, may be used as the first characteristic value. The time rate of change may be a time differential value of the first characteristic value at any time after the start of the post-current application. On the other hand, the time rate of change may be an average rate of change of the first characteristic value during any period after the start of the post-current application. That is, when the start time of the arbitrary period is A and the end time is B, the difference between the first characteristic value at time A and the first characteristic value at time B divided by the elapsed time from time A to time B may be used as the time rate of change of the first characteristic value.

[0034] Also, a value related to the pressing force between the pair of electrodes may be used as the first characteristic value. Here, the "pressure" refers to the "actual pressing force" which is the actual pressing force generated between the electrodes. The actual pressing force is measured by a load cell or the like provided between the electrodes. Meanwhile, a typical spot welding device has a function of controlling the force with which the pair of electrodes clamp the metal plate to a predetermined target value. The above-mentioned target value inputted to the control unit of the spot welding device is referred to as the "set pressing force."

[0035] The set pressure and the actual pressure do not necessarily match. During post-energization, a discrepancy occurs between the set pressure and the actual pressure due to thermal expansion of the weld. Figures 1A and 1B show an example of this discrepancy.

[0036] FIG. 1A is a graph showing an example of the change in actual pressure during the manufacture of a spot welded joint when the post-current is tempering current. Specifically, the inventors pressed two metal plates with a pair of electrodes, waited for a while (this waiting time is called the "squeeze time"), then performed main current and cooling, and then performed tempering current as post-current. The set pressure was constant during the cool time and post-current. During this series of operations, the pressure generated between the pair of electrodes was measured using a load cell, and the graph in FIG. 1A was created. Note that no spring was attached to the load cell to reduce measurement noise.

[0037] As described above, the set pressure was constant during the cool down time and post-current. However, as shown in FIG. 1A, the actual pressure gradually increased immediately after the start of post-current. This is believed to be due to the thermal expansion of the welded part caused by the post-current. When the set pressure is constant, the control unit of the spot welding device tries to control the distance between the electrodes to be constant. Therefore, it is believed that when the distance between the electrodes increases due to thermal expansion, the actual pressure between the electrodes increases.

[0038] Fig. 1B is a graph showing an example of the change in actual applied pressure during the manufacture of a spot welded joint when the post-current is segregation relaxation current. The conditions during current application were the same as those shown in Fig. 1A, except that the post-current was tempering current. Even in the segregation relaxation current application, as shown in Fig. 1B, the actual applied pressure increased immediately after the start of the post-current application. That is, even in the segregation relaxation current application, the distance between the electrodes increases due to thermal expansion, which affects the actual applied pressure.

[0039] The behavior of the pressing force during the segregation relaxation current passing shown in FIG. 1B is slightly different from the behavior during the tempering current passing shown in FIG. 1A. This is presumably due to the fact that the cooling time provided between the segregation relaxation current passing and the main current passing just before it is shorter than the cooling time provided before the tempering current passing. As shown in FIG. 1A, the tempering current passing starts after the actual pressing force drops once after the main current passing and then stabilizes. On the other hand, as shown in FIG. 1B, the segregation relaxation current passing starts at the timing when the actual pressing force drops after the main current passing. It is considered that this difference causes a difference in the behavior of the actual pressing force during current passing. However, despite the existence of this difference, the inventor was able to use the pressing force increase amount as the first characteristic value in the segregation relaxation current passing as in the tempering current passing. It is presumed that the reason for this is that the spot welding machine applies a constant force to the electrode in order to control the pressing force to a constant value, and that since the pressing force increase amount due to the temperature rise is large, the effect of the force that the spot welding machine uses to keep the pressing force constant can be ignored.

[0040] For the above reasons, a value related to the actual pressure generated between the electrodes can be used as a first characteristic value that is an index of the pressure, electrode gap, or electrical resistance of the weld. Specifically, for example, the difference between the actual pressure and the set pressure may be used as the first characteristic. Also, the time rate of change of the difference between the actual pressure and the set pressure, i.e., the rate of increase of the difference between the actual pressure and the set pressure, may be used as the first characteristic value. When the set pressure is kept constant during post-energization, the actual pressure itself or the time rate of change of the actual pressure may be used as the first characteristic value.

[0041] Furthermore, a value related to the electrical resistance between a pair of electrodes may be used as the first characteristic value. Here, "electrical resistance" refers to the ratio of the voltage and current between a pair of electrodes. Due to thermal expansion of the welded portion during post-current, the vibration of atoms in the steel sheet increases, and the electrical resistance changes in an increasing direction. In other words, the electrical resistance increases due to thermal expansion. Therefore, the value or change in the electrical resistance between the pair of electrodes during post-current may be measured and used as the first characteristic value. In addition, the time rate of change of the electrical resistance may be used as the first characteristic value. The time rate of change may be a time differential value of the first characteristic value at any time after the start of post-current. On the other hand, the time rate of change may be an average rate of change of the first characteristic value during any period after the start of post-current. The electrical resistance is measured using an ammeter and a voltmeter.

[0042] A case where the electrical resistance is used as the first characteristic value will be specifically described with reference to FIG. 1C. The left side of FIG. 1C is a graph that shows a schematic change in the electrical resistance between the electrodes during the manufacture of a spot-welded joint, and the right side of FIG. 1C is a graph that shows a schematic relationship between the electrical resistance increase and the CTS. When spot welding and post-current are performed while measuring the electrical resistance between the electrodes, the electrical resistance fluctuates as shown on the left side of FIG. 1C. In the main current, the current path expands due to the formation of a nugget, and the electrical resistance decreases. However, in the post-current, the current path does not change, so the electrical resistance increases with the rise in temperature. Therefore, a correlation that draws an upward convex curve as shown on the right side of FIG. 1C is also seen between the increase in the electrical resistance in the post-current and the CTS. Therefore, the electrical resistance can be used as the first characteristic value. Whether the post-current is tempering current or segregation relaxation current, the behavior shown in FIG. 1C is exhibited.

[0043] The above-described examples of the first characteristic value do not limit the manufacturing method of the spot welded joint according to the present embodiment. A parameter related to the pressing force, the electrode gap, or the electrical resistance can be appropriately selected as the first characteristic value according to the characteristics of the control unit of the spot welding device and the pressing force set during welding. Two or more types of parameters can also be appropriately combined and used as the first characteristic value.

[0044] (Second characteristic value, which is an index of joint strength of spot welded joint) The second characteristic value is a value that is an index of the joint strength of the spot-welded joint obtained after the post-energization. There are various methods for evaluating the joint strength, and for example, JIS Z 3001-6:2013 "Welding Terminology - Part 6: Resistance Welding" lists cross tensile test, tensile shear test, fatigue test, peel test, chisel test, and torsion test as examples of test methods for the welded portion of a spot-welded joint. For example, the evaluation values ​​obtained by these tests can be used as the second characteristic value. When the spot-welded joint is used as an automobile part, it is preferable to set the second characteristic value to the cross tensile strength CTS of the spot-welded joint or the L-joint tensile strength LTS.

[0045] Furthermore, the difference in joint strength of the spot-welded joint before and after post-energization (e.g., cross tensile strength CTS or L-joint tensile strength LTS of the spot-welded joint) may be used as the second characteristic value. The difference in joint strength of the spot-welded joint before and after post-energization can be obtained by producing a spot-welded joint with and without post-energization under the same spot welding conditions and comparing the joint strengths of both.

[0046] The above-described examples of the second characteristic value do not limit the manufacturing method of the spot welded joint according to the present embodiment. An effective parameter can be appropriately selected as the second characteristic value depending on the application of the spot welded joint and the required characteristics. Two or more types of parameters can be appropriately combined and used as the second characteristic value.

[0047] (Correlation between the first and second characteristic values) In the manufacturing method of the spot welded joint according to the present embodiment, a predetermined range is determined for the first characteristic value based on a correlation between the first characteristic value and the second characteristic value that is specified in advance. Here, the "predetermined range" is a range of the first characteristic value that is set to make the second characteristic value within a desired range. Then, in the post-current, the first characteristic value is measured to confirm whether or not at least the first characteristic value is within the predetermined range. Preferably, in the post-current, the first characteristic value is controlled to be within the predetermined range. An example of the correlation is a graph shown in FIG. 2, in which the horizontal axis represents the time rate of change of the actual press force during the post-current and the vertical axis represents the cross tensile strength CTS. The time rate of change of the actual press force is the average rate of change from the start of the post-current to the time when 20 cycles (0.4 sec) have elapsed. That is, the difference between the actual press force at the start of the post-current and the actual press force at the time when 20 cycles have elapsed from the start of the post-current divided by 20 cycles is defined as the time rate of change of the actual press force. As described later, Fig. 2 was obtained by producing various spot welded joints while measuring the time rate of change of the actual applied pressure during post-heating, measuring the CTS of these joints, and plotting the measurement results. Here, the post-heating was a tempering heat.

[0048] In the graph of Fig. 2, the first characteristic value is the time rate of change of the actual applied pressure during post-current application, and the second characteristic value is the cross tensile strength (CTS). The graph of Fig. 2 shows that if the first characteristic value is made the same, the variation in the second characteristic value can be suppressed. The graph of Fig. 2 also shows that the correlation between the first characteristic value and the second characteristic value can be approximated by an upwardly convex quadratic function.

[0049] Based on the correlation shown in Fig. 2, it is possible to specify an appropriate range of the first characteristic value for making the second characteristic value equal to or greater than a predetermined value. For example, when it is required that the CTS, which is the second characteristic value, is 11.0 kN or more, it is found that the appropriate range of the time rate of change of the actual applied pressure, which is the first characteristic value, is 1.38 to 3.13 kN / sec. Based on this correlation, it is possible to achieve a desired second characteristic value by making the first characteristic value fall within the appropriate range during post-current application.

[0050] The means for keeping the first characteristic value within the predetermined range is not particularly limited. For example, the first characteristic value may be kept within the predetermined range, and then the post-current application may be terminated when the first characteristic value reaches an end value (e.g., an upper limit value) of the predetermined range. When the post-current application is performed two or more times, the above-mentioned control may be performed in one or more post-current applications.

[0051] This means is exemplified as follows, using FIG. 1A. The first characteristic value is the actual applied pressure, which is the vertical axis of FIG. 1A. From the correlation between the first characteristic value and the second characteristic value previously obtained by test welding, the upper and lower limits of the first characteristic value required to obtain a desired second characteristic value can be specified. The actual applied pressure is measured during post-current application, and it is detected that the actual applied pressure has increased and exceeded the lower limit. Then, it is detected that the actual applied pressure has further increased and reached the upper limit, and the post-current application is stopped at that point. When the post-current application is stopped, the welded portion is cooled and contracted by the electrode, so that the actual applied pressure decreases. This makes it possible to control the first characteristic value within a predetermined range. When performing such control, it is preferable that the spot welding device has a control unit configured to stop the post-current application when the first characteristic value falls within the predetermined range and then reaches an end value of the predetermined range.

[0052] Alternatively, the first characteristic value may be set within a predetermined range, and then the post-current may be terminated when a predetermined period of time has elapsed since the first characteristic value reached an end value (e.g., an upper limit or a lower limit) of the predetermined range. If the speed of power supply control by the control unit is insufficient, it is difficult to stop the post-current at the time the first characteristic value reaches an end value of the predetermined range. Therefore, depending on the performance of the control unit, a predetermined period of time may be set as a time lag between the time the first characteristic value reaches an end value of the predetermined range and the time the post-current is terminated. In this case, it is preferable to determine an appropriate range of the first characteristic value taking into consideration the time lag. The length of the time lag may be constant, or may be a predetermined range having an upper limit and a lower limit.

[0053] However, it is not essential to perform control during post-current application to bring the first characteristic value into the predetermined range. This is because, even if the first characteristic value cannot be brought into the predetermined range, it is sufficient to perform post-current application again. For example, a spot welded joint having a plurality of welds may be formed by performing spot welding and post-current application multiple times on one plate assembly. If control to bring the first characteristic value into the predetermined range is not performed during post-current application, the spot welded joint may include a welded joint whose first characteristic value is within the predetermined range and a welded joint whose first characteristic value is outside the predetermined range. In the case of normal spot welding, it is difficult to judge the quality of these welds without destructive testing. However, if the first characteristic value is measured and recorded during post-current application, only the welded parts whose first characteristic value is outside the predetermined range can be regarded as defective parts of post-current application, and post-current application can be performed again only on these parts. In this way, by performing post-current application multiple times, the first characteristic value can be brought into the predetermined range, and a spot welded joint having good joint strength can be obtained.

[0054] In addition, it is not essential that the first characteristic value is within the predetermined range in the post-current application, and it is sufficient to at least check whether the first characteristic value is within the predetermined range in the post-current application, because it becomes possible to dispose of a spot welded joint including a defective current application portion where the first characteristic value could not be brought into the predetermined range as a defective product.

[0055] 1A, 1B, and 2, a specific embodiment for bringing the first characteristic value into a predetermined range based on a pre-specified correlation between the first and second characteristic values ​​in the post-energization has been described. However, the above-mentioned example does not limit the manufacturing method of the spot welded joint according to the present embodiment. Since various parameters can be adopted as the first and second characteristic values, the correlation between the first and second characteristic values ​​and the method for pre-specifying this correlation can be various.

[0056] (Methods for Preliminarily Identifying Correlations) In the manufacturing method of the spot welded joint according to the present embodiment, it is necessary to specify the correlation between the first characteristic value and the second characteristic value before performing spot welding and post-energization. Although the method for specifying the correlation is not particularly limited, an example will be shown below.

[0057] First, before spot welding, one or more test weldings are performed to specify the correlation between the first characteristic value and the second characteristic value in advance, thereby making it possible to specify the correlation in advance. Hereinafter, spot welding for producing a spot welded joint is referred to as "main welding", and welding for specifying the correlation before main welding is referred to as "test welding" to distinguish between the two.

[0058] The test welding is performed by spot welding a plurality of overlapping metal plates using a spot welding device having a pair of electrodes to form a test weld, and then applying a post-current to the test weld using the pair of electrodes while measuring the first characteristic value. The plurality of metal plates, the pair of electrodes, and the spot welding device are preferably the same as those used in the actual welding. The number of test welding times may be one, but is preferably two or more in order to obtain an accurate correlation.

[0059] Next, the second characteristic value of the test weld obtained by the test welding is measured. Then, the first characteristic value during the test welding is compared with the second characteristic value measured after the test welding, and the correlation therebetween is identified. For example, if the first characteristic value is the time rate of change of the actual applied pressure and the second characteristic value is the CTS, a graph like that shown in FIG. 2 can be created. The correlation obtained by the test welding also holds true in actual welding, which uses the same metal plate and spot welding device as in the test welding. Therefore, based on the correlation previously identified by the test welding, a desired second characteristic value can be ensured by performing post-current application while controlling the first characteristic value within a predetermined range in actual welding.

[0060] On the other hand, instead of test welding, the correlation between the first characteristic value and the second characteristic value may be determined by simulating the thermal expansion of the welded portion during spot welding and post-energization. The thermal expansion coefficient of the welded portion is determined according to the chemical components of the heat-affected zone (i.e., the chemical components of the metal plate that is the base material), the chemical components of the nugget, and the shape of the nugget. Therefore, it is possible to predict the correlation between the first characteristic value and the second characteristic value by using parameters such as the chemical components of the metal plate, the plate thickness, and the spot welding conditions. It is also possible to combine test welding and simulation. This makes it possible to predict the correlation between the first characteristic value and the second characteristic value with high accuracy while reducing the number of test weldings. In addition, under certain conditions described later, the correlation between the first characteristic value and the second characteristic value can be determined in advance by using the following formula 1 without performing test welding or simulation.

[0061] When the correlation between the first characteristic value and the second characteristic value is identified by test welding, simulation, or the like, the welding conditions are not particularly limited. Considering the purpose of performing test welding, simulation, or the like, it is preferable to change the first characteristic value in various ways, but it is also assumed that there is no means for directly controlling the selected first characteristic value. In addition, if the first characteristic value is easily affected by disturbance, it is difficult to control it. Therefore, it is preferable to identify the correlation between the first characteristic value and the second characteristic value while appropriately selecting and changing welding conditions that can affect the first characteristic value.

[0062] According to the studies made by the present inventors, it is considered preferable to specify the correlation between the first characteristic value and the second characteristic value while keeping constant one or more conditions selected from the group consisting of the fracture mode during a joint strength test of the welded portion, the carbon equivalent of the welded portion, the average sheet thickness of a plurality of metal sheets, and the diameter of a nugget in the welded portion. By keeping constant one or more of these conditions and varying the other welding conditions, it is possible to preferably specify the correlation between the first characteristic value and the second characteristic value.

[0063] (An example of the correlation between the first characteristic value and the second characteristic value) The correlation between the first characteristic value and the second characteristic value varies depending on the type of metal plate, welding conditions, etc. Therefore, it is basically preferable to specify the correlation according to the spot welded joint to be manufactured each time. On the other hand, according to the results of experiments conducted by the present inventors, when the post-energization is tempering energization, the multiple metal plates include two overlapping high-strength steel plates each having a tensile strength of 980 MPa or more, the average plate thickness t of the two high-strength steel plates is 1.2 to 2.2 mm, and the diameter of the nugget on the overlapping surface of the two high-strength steel plates in the weld is 4.0×√t to 5.5×√t, the correlation between the first characteristic value and the second characteristic value can be considered to follow Formula 1. y=ax 2 +bx+c :Formula 1 In formula 1, a is a value defined by formula 2, b is a value defined by formula 3, and c is a value defined by formula 4. x is the increase in actual applied pressure (kN) as a first characteristic value, more specifically, the difference between the actual applied pressure at the start time of post-current and the maximum actual applied pressure during post-current. y is the cross tensile strength CTS (kN) as a second characteristic value. -500×(mass%C)-150≦a≦-500×(mass%C): Formula 2 800×(mass%C)≦b≦800×(mass%C)+200: Formula 3 -320×(mass%C)-100≦c≦-320×(mass%C)+50: Formula 4 In Equation 2, Equation 3, and Equation 4, mass%C is the mass concentration of carbon in the nugget. Also, instead of formulas 2 to 4, a may be a value defined by formula 5, b may be a value defined by formula 6, and c may be a value defined by formula 7. a=-500×(mass%C)-25: Formula 5 b=800×(mass%C)+40: Formula 6 c=-320×(mass%C)-4: Formula 7 If the type of metal plate is within the above-mentioned specific range, the correlation between the first characteristic value and the second characteristic value in tempering current can be specified in advance by using Equation 1 without performing test welding or simulation. For other types of metal plates, it is possible to specify a correlation that can be generally used, such as Equation 1, by accumulating data. The reason why the diameter of the nugget at the mating surface of two high-strength steel plates is set within a predetermined range is because the joining strength at this mating surface has the greatest effect on the joint strength. For example, the correlation between the first characteristic value and the second characteristic value can be suitably used in spot-welded joints for automobile parts, which are made of two overlapping high-strength thick steel plates, or three steel plates, which are two overlapping high-strength thick steel plates and a low-strength thin steel plate.

[0064] Furthermore, according to the results of experiments conducted by the present inventors to date, when the post-energization is segregation relaxation energization, the plurality of metal plates include two overlapping high-strength steel plates, each having a tensile strength of 980 MPa or more, the average plate thickness t of these two high-strength steel plates is 1.2 to 2.2 mm, and the diameter of the nugget at the overlapping surface of the two high-strength steel plates in the weld is 4.0×√t to 5.5×√t, the correlation between the first characteristic value and the second characteristic value can be considered to follow Formula 8. y=ax 2 +bx+c :Formula 8 In Equation 8, a is a value defined by Equation 9, b is a value defined by Equation 10, c is a value defined by Equation 11, x is the increase in actual applied pressure (kN) as a first characteristic value, and y is the cross tensile strength CTS (kN) as a second characteristic value. -50×(mass%C)-50≦a≦-50×(mass%C)+50: Formula 9 70×(mass%C)-80≦b≦70×(mass%C)+50: Formula 10 -30×(mass%C)-80≦c≦-30×(mass%C)+70: Formula 11 In Equation 9, Equation 10, and Equation 11, mass%C is the mass concentration of carbon in the nugget. Also, instead of formulas 9 to 11, in formula 1, a may be a value defined by formula 12, b may be a value defined by formula 13, and c may be a value defined by formula 14. a=-50×(mass%C)-5: Formula 12 b=70×(mass%C): Equation 13 c=-30×(mass%C)+15: Formula 14 In Equations 12, 13, and 14, mass%C is the mass concentration of carbon in the nugget. If the type of metal plate is within the above-mentioned specific range, the correlation between the first characteristic value and the second characteristic value in the segregation relaxation current can be specified in advance by using Equation 8 without performing test welding or simulation. For other types of metal plates, it is possible to specify a correlation that can be generally used, such as Equation 8, by accumulating data. The reason why the diameter of the nugget at the mating surface of two high-strength steel plates is set within a predetermined range is because the joining strength at this mating surface has the greatest effect on the joint strength. For example, the correlation between the first characteristic value and the second characteristic value can be suitably used in spot-welded joints for automobile parts, which are made of two overlapping high-strength thick steel plates, or three steel plates, which are two overlapping high-strength thick steel plates and a low-strength thin steel plate.

[0065] The number of post-current passes performed after the main current pass to form the welded portion may be one or more. When the number of post-current passes is two or more, all of the post-current passes may be segregation relaxation current passes or tempering current passes. Also, tempering current passes and segregation relaxation current passes may be combined.

[0066] As described above, in the manufacturing method of a spot welded joint according to this embodiment, the first characteristic value is measured and it is confirmed whether it is within a predetermined range. When post-current is performed multiple times, there is no particular limitation on which post-current is used to measure and confirm the first characteristic value. For example, it is preferable to consider the first characteristic value of the post-current that is most dominant in terms of the characteristics of the spot welded joint as the first characteristic value in the entire series of procedures for manufacturing the spot welded joint by the following procedure, and to confirm whether it is within the predetermined range. The post-current that is most dominant in terms of the characteristics of the spot welded joint can be identified by the following procedure, for example.

[0067] (When only tempering current is applied multiple times) When only tempering current is performed multiple times and the first characteristic value for all tempering currents is within the appropriate range, it is considered that the first characteristic value of the tempering current performed under the best conditions has the best correlation with the second characteristic value of the spot welded joint finally obtained. In other words, in the above case, it is considered that the tempering current performed under the best conditions is most dominant in terms of the characteristics of the spot welded joint. Therefore, in the above case, the first characteristic value of the tempering current performed under the best conditions is regarded as the first characteristic value in the manufacturing process of the spot welded joint, and it is judged whether it is within the appropriate range.

[0068] Here, the "best condition" refers to a condition that can make the second characteristic value the best. The first characteristic values ​​in all tempering currents are measured, and a pre-specified correlation such as that shown in Fig. 2 is applied to each of these first characteristic values ​​to estimate and compare the second characteristic values ​​in each of all tempering currents, thereby making it possible to specify the tempering currents performed under the best condition.

[0069] Up to this point, the explanation has been given considering only the case where the temper current consists only of temper currents whose first characteristic value is within the appropriate range. However, in reality, when temper currents are performed multiple times, the first characteristic values ​​of some of the temper currents may be outside the appropriate range. Hereinafter, for convenience, temper currents whose first characteristic value is below the appropriate range will be referred to as "temper current in state 1", temper currents whose first characteristic value is within the appropriate range will be referred to as "temper current in state 2", and temper currents whose first characteristic value is above the appropriate range will be referred to as "temper current in state 3", and the relationships between them will be explained.

[0070] The tempering current conduction in states 1 to 3, which are carried out successively, is considered to have the following relationship. · The tempering current in state 1 is presumed not to be able to raise the temperature of the weld to the tempering temperature range, so in principle it does not affect the results of the tempering current performed immediately before it. However, the relationship between tempering currents in state 1 is similar to the relationship between tempering currents in state 2. The tempering current in state 2 is assumed to raise the temperature of the weld to the tempering temperature range, so it overwrites the results of the previous tempering current in state 1 or state 3. If the tempering current in state 2 is performed twice in succession, as described above, the tempering current performed under the better conditions will dominate the resulting spot welded joint. The tempering current in state 3 is estimated to raise the temperature of the weld beyond the tempering temperature range, so it overwrites the results of the tempering current performed immediately before. By measuring the first characteristic value of each of the multiple tempering current passes and applying the above-mentioned relationship to these, it is possible to identify the first characteristic value of the tempering current that is most dominant with respect to the second characteristic value of the resulting spot-welded joint. Then, the first characteristic value of the tempering current that is most dominant with respect to the second characteristic value of the spot-welded joint is regarded as the first characteristic value for the entire series of manufacturing steps for the spot-welded joint, and it is checked whether this is within a predetermined range.

[0071] (When only segregation relaxation current is applied multiple times) When only segregation relaxation current passing is performed multiple times, it can be considered the same as when only tempering current passing multiple times. Hereinafter, for convenience, segregation relaxation current passing in which the first characteristic value is below the appropriate range will be referred to as "segregation relaxation current passing in state 1", segregation relaxation current passing in which the first characteristic value is within the appropriate range will be referred to as "segregation relaxation current passing in state 2", and segregation relaxation current passing in which the first characteristic value is above the appropriate range will be referred to as "segregation relaxation current passing in state 3". The segregation relaxation current passing in states 1 to 3 that are performed consecutively is considered to have the following relationship. Since it is assumed that the segregation relaxation current in state 1 cannot raise the temperature of the weld to the segregation relaxation temperature range, in principle it does not affect the results of the segregation relaxation current performed immediately before it. However, the relationship between segregation relaxation currents in state 1 is the same as the relationship between segregation relaxation currents in state 2. The segregation relaxation current in state 2 is estimated to raise the temperature of the weld to the segregation relaxation temperature range, so it overwrites the results of the segregation relaxation current in state 1 or state 3 that was performed immediately before it. If the segregation relaxation current in state 2 is performed twice in succession, the segregation relaxation current performed under the more favorable conditions will be dominant in the resulting spot welded joint. The segregation relaxation current conduction in state 3 is presumed to raise the temperature of the weld beyond the segregation relaxation temperature range, melting and re-solidifying the weld, and forming segregations of embrittlement elements, so it overwrites the results of the segregation relaxation current conduction performed immediately before this. By measuring the first characteristic value for each of the multiple segregation relaxation current conductions and applying the above-mentioned relationship to these, it is possible to identify the first characteristic value of the segregation relaxation current conduction that is most dominant with respect to the second characteristic value of the resulting spot-welded joint. Then, the first characteristic value of the segregation relaxation current conduction that is most dominant with respect to the second characteristic value of the spot-welded joint is regarded as the first characteristic value for the entire series of manufacturing steps of the spot-welded joint, and it is confirmed whether or not this is within a predetermined range.

[0072] (When tempering current and segregation relaxation current are combined) When tempering current and segregation relaxation current are combined, by considering these relationships, it is possible to identify the post-current that is most dominant for the second characteristic value of the resulting spot welded joint. Since the segregation relaxation temperature is higher than the tempering temperature, it is considered that the following relationship exists between the successive tempering current and segregation relaxation current. Segregation relaxation current passes overwrite the results of the previous temper current. Tempering current in state 1 or state 3 does not affect the results of the segregation relaxation current performed immediately before. The tempering current in state 2 overwrites the results of the previous segregation relaxation current. By measuring the first characteristic value of each of the multiple post-currents and applying all of the above-mentioned relationships to these, it is possible to identify the first characteristic value of the post-current that is most dominant with respect to the second characteristic value of the resulting spot-welded joint. Then, the first characteristic value of the post-current that is most dominant with respect to the second characteristic value of the spot-welded joint is regarded as the first characteristic value for the entire series of manufacturing processes for the spot-welded joint, and it is confirmed whether or not this is within a predetermined range.

[0073] Next, a spot welding device according to another aspect of the present invention will be described. The spot welding device according to this embodiment is for use in the manufacturing method of the spot welded joint according to the above-mentioned embodiment, and includes a pair of electrodes and a first characteristic value measuring means configured to be able to measure the first characteristic value.

[0074] The pair of electrodes is not particularly limited, and may be appropriately selected depending on the type, thickness, number, and required nugget diameter of the metal plates. The first characteristic value measuring means is also not particularly limited, and may be appropriately selected depending on the type of the first characteristic value. For example, when the first characteristic value is a value related to the actual applied pressure, the first characteristic value measuring means may be a load cell provided between the pair of electrodes. When the first characteristic value is a value related to the displacement between the electrodes, the first characteristic value measuring means may be a displacement meter capable of measuring the distance between the pair of electrodes. When the first characteristic value is a value related to the electrical resistance between the electrodes, the first characteristic value measuring means may be an ammeter and a voltmeter capable of measuring the electrical resistance between the pair of electrodes. The first characteristic value measuring means may be incorporated in the spot welding device, or may be a device detachable from the spot welding device.

[0075] The spot welding apparatus may further include a control unit capable of controlling the first characteristic value within a predetermined range based on the correlation between the first characteristic value and the second characteristic value during post-energization. This makes it easy to keep the second characteristic value of the welded portion of the spot welded joint within an appropriate range. Naturally, the control unit may be configured to perform appropriate control for the various aspects of the manufacturing method for a spot welded joint according to the present embodiment described above. EXAMPLES

[0076] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0077] Example 1 The stacked metal plates were spot-welded using a spot welding device having a pair of electrodes to form a welded portion. The welded portion was then post-current-welded using a pair of electrodes. The post-current was tempering current, and the welding conditions were as follows: Metal plate: 1.5GPa-class hot stamped steel plate (thickness 2mm), two sheets of the same steel overlapped Power supply: Single-phase AC (50Hz) ·Electrode: CrCu DR type φ6-R40 -Pressure setting: 3.92kN Squeeze time: 0.60sec ·Main energization time: 0.36sec Main current: 6.5kA Cooldown time: 1.98sec Post-energization time: 1.98 sec ·Post-energizing current: 3.9kA~4.3kA ·Holding time: 0.13sec In this embodiment, the cooling time was set long so as to ensure that martensitic transformation occurred, but as described above, the cooling time may be 0.20 seconds or more, and is preferably 0.40 to 0.60 seconds.

[0078] During post-current application, the actual applied force between the pair of electrodes was measured. In this experiment, the set applied force was constant, so (1) the time rate of change of the actual applied force or (2) the amount of increase in the actual applied force was used as the first characteristic value. The time rate of change of the actual applied force was taken as the average rate of change from the start of post-current application until the time 20 cycles (0.4 sec) had elapsed. In other words, the time rate of change of the actual applied force was taken as the difference between the actual applied force at the start of post-current application and the actual applied force at the time 20 cycles had elapsed from the start of post-current application, divided by 20 cycles. The amount of increase in the actual applied force was taken as the difference between the actual applied force at the start of post-current application and the maximum value of the actual applied force during post-current application.

[0079] According to the above-mentioned procedure, a plurality of test pieces of spot welded joints were prepared. These test pieces were subjected to a cross tension test according to JIS Z 3137:1999 "Test piece dimensions and test method for cross tension test of resistance spot and projection welded joints" to measure CTS. CTS was used as the second characteristic value.

[0080] (1) FIG. 2 shows the correlation between the first and second characteristic values ​​when the rate of change of the actual applied pressure is used as the first characteristic value and the CTS is used as the second characteristic value. As shown in FIG. 2, even though the welding conditions were constant as described above, the CTS of the test pieces varied widely within a range of about 6.0 kN to 14.0 kN. On the other hand, a good correlation was observed between the rate of change of the actual applied pressure and the CTS. If the rate of change of the actual applied pressure was the same, the variation in the CTS could be kept within a maximum range of about 2.0 kN. Therefore, by controlling the rate of change of the actual applied pressure within a predetermined range or by performing a second post-current on a welded portion whose rate of change of the actual applied pressure does not fall within the predetermined range while keeping the welding conditions within the above-mentioned range, the mechanical properties of the welded portion after the post-current can be stably improved.

[0081] (2) Figure 3 shows the correlation between the first and second characteristic values ​​when the increase in actual applied pressure is used as the first characteristic value and CTS is used as the second characteristic value. When the post-current is tempering current, a good correlation was also observed between the increase in actual applied pressure and CTS. It is presumed that in the test piece where the increase in actual applied pressure was insufficient, the hardness of the weld could not be sufficiently reduced due to incomplete tempering, resulting in insufficient CTS. On the other hand, in the test piece where the increase in actual applied pressure was excessive, it is presumed that the temperature of the weld rose above the Ac1 point, causing quench hardening in the weld, resulting in insufficient CTS.

[0082] 3, for example, when it is required to set the CTS, which is the second characteristic value, to 11.0 kN or more by using temper current, it is found that the appropriate range of the increase in actual applied pressure, which is the first characteristic value, during temper current is 0.72 to 0.95 kN. Based on this correlation, it is possible to achieve a desired second characteristic value by setting the first characteristic value within the appropriate range during temper current, which is the post-current.

[0083] "State 2" in Figure 3 shows the appropriate range of the first characteristic value. This "State 2" is based on a fitting curve that absorbs measurement variability, not on measured values. A joint that has a first characteristic value within a predetermined range has a CTS of 90% or more of the appropriate CTS, so the numerical range of 0.72 to 0.95 kN based on the fitting curve is understood to be valid.

[0084] In this experiment, both (1) the time rate of change of the actual applied pressure and (2) the increase in the actual applied pressure could be effectively used as the first characteristic value. However, when experiments are performed under different welding conditions, there is a possibility that one of them is superior to the other. In this case, too, the first characteristic value can be appropriately selected depending on the welding conditions.

[0085] Example 2 The stacked metal plates were spot-welded using a spot welding device having a pair of electrodes to form a welded portion. Then, a post-current was applied to the welded portion using the pair of electrodes. The post-current was a segregation relaxation current, and the welding conditions were as follows: Metal plate: 1.5GPa-class hot stamped steel plate (thickness 2mm), two sheets of the same steel overlapped Power supply: Single-phase AC (50Hz) ·Electrode: CrCu DR type φ6-R40 -Pressure setting: 3.92kN Squeeze time: 0.60sec ·Main energization time: 0.36sec Main current: 6.5kA Cooldown time: 0.12sec ·Post-energization time: 0.20~0.40sec ·Post-energizing current: 5.2kA~6.5kA ·Holding time: 0.30sec

[0086] During the post-current application, the actual applied pressure between the pair of electrodes was measured. In this experiment, the set applied pressure was constant, so the increase in the actual applied pressure was used as the first characteristic value. The increase in the actual applied pressure was defined as the difference between the actual applied pressure at the start time of the post-current application and the maximum actual applied pressure during the post-current application.

[0087] According to the above-mentioned procedure, a plurality of test pieces of spot welded joints were prepared. These test pieces were subjected to a cross tension test according to JIS Z 3137:1999 "Test piece dimensions and test method for cross tension test of resistance spot and projection welded joints" to measure CTS. CTS was used as the second characteristic value.

[0088] The correlation between the first and second characteristic values ​​when the increase in actual applied pressure is used as the first characteristic value and the CTS is used as the second characteristic value is shown in Fig. 4A. Note that Fig. 4A includes both the results of segregation relaxation current passing with a current passing time of 10 cycles and the results of segregation relaxation current passing with a current passing time of 20 cycles. For reference, Fig. 4B also shows a graph with approximation curves added to the measurement results under the two conditions included in Fig. 4A.

[0089] Even when the post-current was a segregation relaxation current, a good correlation was observed between the increase in actual applied pressure and CTS. In Figures 4A and 4B, in the test piece where the increase in actual applied pressure was insufficient, segregation relaxation was incomplete, so embrittled areas remained in the weld, and CTS was insufficient. On the other hand, in Figures 4A and 4B, in the test piece where the increase in actual applied pressure was excessive, CTS was insufficient because the temperature of a part of the weld rose above the melting point of the nugget.

[0090] 4A, for example, when it is required to make the CTS, which is the second characteristic value, 9.0 kN or more by using the segregation relaxation current passing, it is found that the appropriate range of the increase in the actual applied pressure, which is the first characteristic value, during the segregation relaxation current passing is 0.2 to 1.1 kN. Based on this correlation, it is possible to achieve a desired second characteristic value by setting the first characteristic value within the appropriate range during the segregation relaxation current passing, which is the post-current passing.

Claims

1. A step of spot welding the stacked metal plates using a spot welding device having a pair of electrodes to form a weld; A step of post-current-passing the welded portion using the pair of electrodes; A method for manufacturing a spot welded joint comprising: a predetermined range is determined for the first characteristic value before the post-current is applied based on a predetermined correlation between a first characteristic value which is an index of the welding force, the electrode gap, or the electrical resistance at the welded portion, and a second characteristic value which is an index of the joint strength of the spot-welded joint; In the post-energization, the first characteristic value is measured, and it is confirmed whether the first characteristic value is within a predetermined range. A method for manufacturing spot welded joints.

2. Prior to the spot welding, a test welding process in which the stacked metal plates are spot-welded by the spot welding device having a pair of the electrodes to form a test weld, and then, while measuring the first characteristic value, a post-current is applied to the test weld using the pair of the electrodes; measuring the second characteristic of the test weld; determining a correlation between the first feature value and the second feature value; The method for manufacturing a spot welded joint according to claim 1, further comprising the steps of:

3. The first characteristic value is a difference between an actual pressing force between the pair of electrodes measured during the post-current and a set pressing force between the pair of electrodes set in the spot welding apparatus during the post-current, or a time change rate of the difference.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

4. The first characteristic value is a change amount of the gap between the pair of electrodes during the post-current application, or a time rate of change of the change amount.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

5. The method for manufacturing a spot welded joint according to any one of claims 1 to 4, characterized in that the second characteristic value is a cross tensile strength CTS or an L-joint tensile strength LTS of the spot welded joint, or a difference between the cross tensile strength CTS or the L-joint tensile strength LTS of the spot welded joint before and after the post-energization.

6. The plurality of metal plates are a plurality of steel plates, The tensile strength of one or more of the steel plates is 980 MPa or more. The method for manufacturing a spot welded joint according to any one of claims 1 to 5.

7. The method for manufacturing a spot welded joint according to any one of claims 1 to 6, further comprising a step of preheating and energizing portions to be welded of the plurality of metal plates before the spot welding.

8. A fracture mode during a joint strength test of the welded portion; the carbon equivalent of the weld; The average thickness of the metal plates; and Diameter of the nugget in the weld and determining the correlation between the first feature value and the second feature value while keeping constant one or more conditions selected from the group consisting of: The method for manufacturing a spot welded joint according to any one of claims 1 to 7.

9. The post-current passing includes one or more selected from the group consisting of temper current passing and segregation relaxation current passing, In the tempering current passing, a cool time between the current passing immediately before the tempering current passing and the tempering current passing is set to 0.20 seconds or more, and a current ratio, which is a value obtained by dividing a current value during the tempering current passing by a current value during main current passing, which is current passing for forming the welded portion, is set to 0.50 to 0.85, In the segregation relaxation current conduction, the cool time between the current conduction immediately before the segregation relaxation current conduction and the segregation relaxation current conduction is 0.04 seconds or more, and the current ratio, which is the current value during the segregation relaxation current conduction divided by the current value during the main current conduction, is 0.70 to 1.

20. The method for manufacturing a spot welded joint according to claim 6 .

10. The post-current passing is the temper current passing, The plurality of metal plates include two overlapping high-strength steel plates each having a tensile strength of 980 MPa or more; The average plate thickness t of the two high-strength steel plates is 1.2 to 2.2 mm, The diameter of the nugget at the overlapping surface of the two high-strength steel plates of the weld is 4.0×√t to 5.5×√t, The correlation between the first feature value and the second feature value is considered to follow Equation 1. The method for manufacturing a spot welded joint according to claim 9 . y = ax ２ +bx+c :Formula 1 In the above formula 1, a is a value defined by formula 2, b is a value defined by formula 3, c is a value defined by formula 4, x is an increase in actual applied pressure (kN) as the first characteristic value, and y is a cross tensile strength CTS (kN) as the second characteristic value. -500×(mass%C)-150≦a≦-500×(mass%C): Formula 2 800×(mass%C)≦b≦800×(mass%C)+200: Formula 3 -320×(mass%C)-100≦c≦-320×(mass%C)+50: Formula 4 In the formulas 2, 3, and 4, mass% C is the mass concentration of carbon in the nugget.

11. The method for manufacturing a spot welded joint according to claim 10, characterized in that, in Formula 1, a is a value defined by Formula 5, b is a value defined by Formula 6, and c is a value defined by Formula 7. a=-500×(mass%C)-25: Formula 5 b=800×(mass%C)+40: Formula 6 c=-320×(mass%C)-4: Formula 7 In the formulas 5, 6, and 7, mass% C is the mass concentration of carbon in the nugget.

12. The post-current application is the segregation relaxation current application, The plurality of metal plates include two overlapping high-strength steel plates each having a tensile strength of 980 MPa or more; The average plate thickness t of the two high-strength steel plates is 1.2 to 2.2 mm, The diameter of the nugget at the overlapping surface of the two high-strength steel plates of the weld is 4.0×√t to 5.5×√t, The correlation between the first feature value and the second feature value is considered to follow Equation 8. The method for manufacturing a spot welded joint according to claim 9 . y = ax ２ + bx + c: Formula 8 In the above formula 8, a is a value defined by formula 9, b is a value defined by formula 10, c is a value defined by formula 11, x is an increase in actual applied pressure (kN) as the first characteristic value, and y is a cross tensile strength CTS (kN) as the second characteristic value. -50×(mass%C)-50≦a≦-50×(mass%C)+50: Formula 9 70×(mass%C)-80≦b≦70×(mass%C)+50: Formula 10 -30×(mass%C)-80≦c≦-30×(mass%C)+70: Formula 11 In the formulas 9, 10, and 11, mass% C is the mass concentration of carbon in the nugget.

13. The method for manufacturing a spot welded joint according to claim 12, characterized in that, in formula 8, a is a value defined by formula 12, b is a value defined by formula 13, and c is a value defined by formula 14. a=-50×(mass%C)-5: Formula 12 b=70×(mass%C): Formula 13 c=-30×(mass%C)+15: Formula 14 In the formulas 12, 13, and 14, mass% C is the mass concentration of carbon in the nugget.

14. The first characteristic value is within the predetermined range; Next, the post-energization is terminated when the end value of the predetermined range is reached or when a predetermined period of time has elapsed since the first characteristic value reached the end value of the predetermined range. The method for manufacturing a spot welded joint according to any one of claims 1 to 13.

15. The number of times of the post-energization is two or more, The first characteristic value is measured at a post-energization time that is most dominant with respect to the second characteristic value of the spot welded joint, and it is confirmed whether the first characteristic value is within the predetermined range. The method for manufacturing a spot welded joint according to any one of claims 1 to 14.

16. A spot welding apparatus for the manufacturing method of the spot welded joint according to any one of claims 1 to 15, A pair of the electrodes; a first characteristic value measuring means configured to be able to measure the first characteristic value; A spot welding device comprising:

17. a control unit capable of controlling the first characteristic value within the predetermined range based on the correlation between the first characteristic value and the second characteristic value during the post-energization. The spot welding device of claim 16 further comprising:

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