Resistance welding joint manufacturing method, resistance welding device, control program for resistance welding device, and control device for resistance welding device

Abstract

To provide a method for manufacturing a resistance welding joint, a resistance welding device, a control program of the resistance welding device, and a control device of the resistance welding device, capable of evaluating joining strength of a welding part or stably improving the joining strength of the welding part.SOLUTION: Acquiring a resistance R1 between the pair of electrodes immediately before the end of the main current passage, acquiring a temporal change in the resistance during cooling down, acquiring the resistance R2 at the start of the subsequent current passage, and using a relationship between Ri (p), Rii (p), and the resistance R2 as an evaluation indicator of the weld. Ri (t) = a * t + b, Rii (t) = a * t + c, Rii (4) = 4 * a + c = R, R = R1 * [13.6 * {Si + Al + 0.4 * (Mn + Cr)} + 12.2] / 140 Here, t is the time elapsed from the acquisition of the resistance R1, and p is the time elapsed from the acquisition of the resistance R1 to the start of the subsequent energization.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a resistance welded joint, a resistance welding apparatus, a control program for the resistance welding apparatus, and a control device for the resistance welding apparatus. [Background technology]

[0002] Resistance welding is a type of welding in which a large current is passed through the weld joint, heating it with the generated resistance heat and applying pressure. Resistance welding can be performed in a short time, so it is used to manufacture a variety of machine parts. A post-heating current may also be passed through the weld formed by resistance welding. Post-heating current is a current passed through the weld after resistance welding for the purpose of performing heat treatments such as tempering, annealing, and segregation relief. The application of a post-heating current is also called post-heating.

[0003] High-strength steel plates are an example of materials for which post-heat treatment is used. High-strength steel plates are applied in various technical fields to reduce the weight and improve the safety of machine parts. However, high-strength steel plates have the problem of being prone to embrittlement at resistance welds. In welded joints made from ordinary steel plates, the higher the strength of the steel plate, the higher the cross tensile strength (CTS). However, in welded joints made from high-strength steel plates, the higher the strength of the steel plate, the lower the CTS.

[0004] Various techniques have been investigated to improve the properties of welds formed by resistance welding.

[0005] Patent Document 1 discloses a method for monitoring a welded portion obtained by spot welding, in which a monitor current is passed immediately after the welding current has been passed, with the electrodes pressurized during the current pass, to detect the voltage and current between the electrodes, and determine whether the detected values ​​are within an allowable range.

[0006] Patent Document 2 discloses a resistance welding method including a heating step of supplying a heating current to electrodes circumscribing the workpieces to Joule heat the welded portion of the workpieces, a first resistance measurement step of measuring a first electrical resistance value (R1) of the welded portion in a heated state before the end of the heating step, a pause step of pausing the supply of the heating current for a predetermined period of time after the end of the heating step, a second resistance measurement step of measuring a second electrical resistance value (R2) of the welded portion in a residual heat state after the pause step, a calculation step of calculating a resistance ratio (R2 / R1 or R1 / R2) which is the ratio of the first electrical resistance value to the second electrical resistance value, a determination step of determining whether the calculated resistance ratio or an index value obtained from the resistance ratio that indicates the welding condition of the welded portion is within a predetermined range, and a reheating step of performing Joule heating again when the resistance ratio or the index value is within the predetermined range, thereby forming a nugget in which at least a portion of the welded portion is melted and solidified. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 61-71189 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-31277 Summary of the Invention [Problem to be solved by the invention]

[0008] The inventors focused on the fact that the joint strength of a weld after post-current application tends to vary. Resistance welding is susceptible to disturbances. Disturbances are factors that affect the characteristics of a weld and are difficult to control during resistance welding. Examples of disturbances include gaps between workpieces and electrode wear. Therefore, even if resistance welding conditions such as current value and welding pressure are constant, the joint strength of a weld after post-current application is unlikely to be constant. Furthermore, because a destructive test such as a cross tensile test is required to accurately measure the joint strength of a weld, it is difficult to evaluate whether the joint strength of a weld included in a mechanical component is within an acceptable range.

[0009] The monitoring method described in Patent Document 1 applies current between electrodes sandwiching the workpieces, detects the voltage and current corresponding to the resistance between the electrodes, and determines the quality of the welded portion, or so-called nugget, from these detected values. However, the technology described in Patent Document 1 does not perform post-current application. In addition, the technology described in Patent Document 1 determines the quality of the nugget size and the quality of the penetration between the workpieces. The technology described in Patent Document 1 does not take into account the embrittlement of the welded portion.

[0010] The resistance welding method described in Patent Document 2 was developed based on the finding that the resistance ratio between the electrical resistance value of the workpieces to be spot-welded just before the end of Joule heating and the electrical resistance value in a residual heat state immediately after the end of heating correlates with the size of the nugget (nugget diameter) formed when the welded portion of the workpieces melts and solidifies. The technology described in Patent Document 2 is also said to be capable of evaluating the weld. However, the technology described in Patent Document 2 uses the resistance ratio as an index value for the nugget diameter. Therefore, the technology described in Patent Document 2 is intended to determine the size of the nugget, but is not intended to determine the joining strength of a welded portion provided on high-strength steel plate.

[0011] The resistance welding method of Patent Document 2 performs reheating as needed based on the resistance ratio or an index value derived from the resistance ratio. However, reheating based on the resistance ratio, which is an index value for nugget diameter, is presumably performed to enlarge the nugget diameter. The reheating described in Patent Document 2 is not post-current application to temper the nugget. Patent Document 2 does not consider suppressing the variation in joint strength that occurs in nuggets tempered by post-current application, nor does it disclose any specific means for suppressing this variation.

[0012] An object of the present disclosure is to provide a method for manufacturing a resistance welded joint, a resistance welding apparatus, a control program for the resistance welding apparatus, and a control device for the resistance welding apparatus, which are capable of evaluating the joint strength of a weld or stably improving the joint strength of a weld. [Means for solving the problem]

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

[0014] (1) A method for manufacturing a resistance welded joint according to a first embodiment of the present disclosure includes a step of applying a main current to a pair of electrodes arranged near a welded joint of workpieces formed by combining a plurality of steel materials, thereby melting the inside of the welded joint of the workpieces; a step of cooling the welded joint using the pair of electrodes, thereby solidifying the inside of the welded joint and forming a weld; and a step of applying a post-current to the welded joint, thereby heating the welded joint without melting it. Immediately before the end of the main current, a resistance value R1 between the pair of electrodes is obtained, and during the cooling, a change in the resistance value over time is obtained. At the start of the post-current, a resistance value R2 is obtained, and a value R is calculated based on the following equations 1 to 4. i (p), R ii The relationship between (p) and the resistance value R2 is used as the evaluation index value for the welded portion. R i (t)=a×t+b……<Formula 1> R ii (t)=a×t+c……<Formula 2> R ii (4)=4×a+c=R……<Formula 3> R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> Here, Equation 1 is a first linear equation that provides a first-order approximation of the change in the resistance value over time in the initial stage of cooling, symbol t is the time elapsed in units of seconds from the time when the resistance value R1 was obtained, symbols a, b, and c are constants, element symbols are the contents in units of mass% of the elements corresponding to the element symbols in the chemical composition of the workpiece material, and symbol p is the time elapsed in units of seconds from the time when the resistance value R1 was obtained to the time when the post-current is started. (2) Preferably, in the method for manufacturing a resistance welded joint described in (1) above, a value αM calculated based on the following formula 5 is used as the evaluation index value for the welded portion. αM={R i (p)-R2} / {Ri (p)-R ii (p)}……<Formula 5> (3) Preferably, in the method for manufacturing a resistance welded joint described in (2) above, the weld is judged to be good when the αM is 40% or more.

[0015] (4) A method for manufacturing a resistance welded joint according to a second embodiment of the present disclosure includes a step of applying a main current to a pair of electrodes arranged near a welded joint of a workpiece formed by combining a plurality of steel materials, thereby melting the inside of the welded joint of the workpiece; a step of cooling the welded joint using the pair of electrodes, thereby solidifying the inside of the welded joint to form a weld; and a step of post-current application to the welded joint, thereby heating the welded joint without melting it. Immediately before the end of the main current application, a resistance value R1 between the pair of electrodes is obtained, and during the cooling, a change in the resistance value over time is obtained, and a value R calculated based on the following equations 1 to 4 is calculated. i (q), R ii When (q) and the resistance value R3 satisfy a predetermined relationship, the post-energization is started. R i (t)=a×t+b……<Formula 1> R ii (t)=a×t+c……<Formula 2> R ii (4)=4×a+c=R……<Formula 3> R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> Here, Equation 1 is a first linear equation that provides a first-order approximation of the change in the resistance value over time in the initial stage of cooling, symbol t is the time elapsed from the time when the resistance value R1 was obtained in unit of seconds, symbols a, b, and c are constants, element symbols are the contents in unit of mass% of the elements corresponding to the element symbols in the chemical composition of the workpiece material, symbol q is the time elapsed from the time when the resistance value R1 was obtained to an arbitrary time in unit of seconds, and symbol R3 is the resistance value at the arbitrary time. (5) Preferably, in the method for manufacturing a resistance welded joint described in (4) above, the post-energization is started when a value αM calculated based on the following formula 6 is within a predetermined range. αM={R i (q)-R2} / {R i (q)-R ii (q)}……<Formula 6> (6) In the method for producing a resistance welded joint described in (5) above, the post-energization is preferably started when the αM is 40% or more. (7) Preferably, in the method for manufacturing a resistance welded joint described in any one of (1) to (6) above, before the main current is applied, a test material having the same configuration as the material to be welded is subjected to test current application and test cooling under the same conditions as the main current application and the cooling, and the slope of a linear approximation equation of the time change in resistance value in the initial stage of the test cooling is set to the constant a.

[0016] (8) A resistance welding apparatus according to a third embodiment of the present disclosure is a resistance welding apparatus for carrying out the method for manufacturing a resistance welded joint according to any one of (1) to (3) above, and includes: a pair of electrodes; a power source that applies current between the pair of electrodes; and a control device that operates the power source to control one or both of a voltage value and a current value between the pair of electrodes, wherein the control device includes: a main current supply unit that operates the power source to apply main current between the pair of electrodes to melt the inside of the welded joint portion of the workpiece; a post-current supply unit that operates the power source to apply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on the voltage value and the current value between the pair of electrodes; and a control device that operates the R i (p), and the R ii a resistance value processing unit that calculates (p), i (p), the R ii (p) and an evaluation unit that evaluates the welded portion based on the relationship between the resistance value R2. (9) Preferably, in the resistance welding apparatus described in (8) above, the evaluation unit evaluates the welded portion using the α M . (10) Preferably, in the resistance welding apparatus described in (9) above, the evaluation unit determines that the weld is good when the αM is 40% or more.

[0017] (11) A resistance welding apparatus according to a fourth embodiment of the present disclosure is a resistance welding apparatus for carrying out the method for manufacturing a resistance welded joint according to any one of (4) to (6) above, and includes: a pair of the electrodes; a power source that applies current between the pair of the electrodes; and a control device that operates the power source to control one or both of a voltage value and a current value between the pair of the electrodes, wherein the control device includes: a main current supply unit that operates the power source to apply main current between the pair of electrodes to melt the inside of the welded joint portion of the workpiece; a post-current supply unit that operates the power source to apply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on the voltage value and the current value between the pair of electrodes; and a control device that operates the R i (q), and the R ii a resistance value processing unit that calculates (q) i (q), the above R ii (q), and a post-energization start unit that activates the post-energization unit at the time point when the resistance value R3 satisfies a predetermined relationship. (12) Preferably, in the resistance welding device described in (11) above, the post-current application starting unit activates the post-current application unit at a time point when the αM is within the predetermined range. (13) Preferably, in the resistance welding device described in (12) above, the post-current application start unit activates the post-current application unit at a time point when αM is 40% or more.

[0018] (14) A control program for a resistance welding device according to a fifth embodiment of the present disclosure is a program for causing the resistance welding device to function as the resistance welding device described in (8) above.

[0019] (15) Alternatively, a control program for a resistance welding apparatus according to a fifth embodiment of the present disclosure is a program for causing the resistance welding apparatus to function as the resistance welding apparatus described in (11) above.

[0020] (16) A sixth embodiment of the present disclosure provides a control device for a chemical resistance welding apparatus, which is a control device for a resistance welding apparatus including a pair of electrodes and a power source for applying current between the pair of electrodes, for carrying out the method for manufacturing a resistance welded joint according to any one of (1) to (3) above, and which includes a main current supply unit that operates the power source to apply main current between the pair of electrodes to melt the inside of the welded joint portion of the material to be welded, a post-current supply unit that operates the power source to apply post-current between the pair of electrodes to heat the welded portion without melting it, a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes, and a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes. i (q), and the R ii a resistance value processing unit that calculates (q) i (q), the above R ii (q), and a post-energization start unit that activates the post-energization unit at the time point when the resistance value R3 satisfies a predetermined relationship.

[0021] (17) A seventh embodiment of the present disclosure provides a control device for a chemical resistance welding apparatus, which is a control device for a resistance welding apparatus including a pair of electrodes and a power source for applying current between the pair of electrodes, for carrying out the method for manufacturing a resistance welded joint according to any one of (4) to (6), and includes a main current supply unit that operates the power source to apply main current between the pair of electrodes to melt the inside of the welded joint portion of the material to be welded, a post-current supply unit that operates the power source to apply post-current between the pair of electrodes to heat the welded portion without melting it, a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes, and a resistance value acquisition unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes. i (q), and the R ii a resistance value processing unit that calculates (q) i (q), the above R ii (q), and a post-energization start unit that activates the post-energization unit at the time point when the resistance value R3 satisfies a predetermined relationship. [Effects of the Invention]

[0022] According to the present disclosure, it is possible to provide a method for manufacturing a resistance welded joint, a resistance welding apparatus, a control program for the resistance welding apparatus, and a control device for the resistance welding apparatus, which are capable of evaluating the joint strength of a weld or stably improving the joint strength of a weld. [Brief explanation of the drawings]

[0023] [Figure 1] 4 is a flowchart of an example of a method for manufacturing a resistance welded joint according to the first embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic diagram showing the change over time in current value and temperature in resistance welding. [Figure 3] FIG. 10 is a cross-sectional view showing the main current application step S1. [Figure 4] 10A and 10B are cross-sectional schematic diagrams illustrating the cooling step S2 and the post-energization step S3. [Figure 5] FIG. 3 is a schematic diagram showing the change in resistance value over time in the method for manufacturing a resistance welded joint according to the first embodiment. [Figure 6] 10 is a graph showing the relationship between the measured resistance between a pair of electrodes and the estimated temperature at the end of the nugget. [Figure 7] FIG. 10 is a schematic diagram showing the change in resistance value over time in the method for manufacturing a resistance welded joint according to the second embodiment. [Figure 8] FIG. 10 is a schematic diagram of an example of a resistance welding machine according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0024] (1. First manufacturing method for resistance welded joint) As shown in FIGS. 1 to 4 , for example, a method for manufacturing a resistance welded joint according to a first embodiment of the present disclosure includes the steps of: applying a main current to a pair of electrodes 11 disposed near a welded joint 22 of workpieces formed by combining a plurality of steel materials 21, thereby melting the inside of the welded joint 22 of the workpieces; cooling the welded joint 22 using the pair of electrodes 11, thereby solidifying the inside of the welded joint 22 and forming a weld 23; and post-current application to the weld 23, thereby heating the weld 23 without melting it. Immediately before the end of the main current application, a resistance value R1 between the pair of electrodes 11 is obtained; during cooling, a change in the resistance value over time is obtained; and at the start of the post-current application, a resistance value R2 is obtained, and a value R is calculated based on the following equations 1 to 4: i (p), R ii The relationship between (p) and the resistance value R2 is used as the evaluation index value for the welded portion 23. R i (t)=a×t+b……<Formula 1> R ii (t)=a×t+c……<Formula 2> R ii (4)=4×a+c=R……<Formula 3> R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> Here, Equation 1 is a first linear equation that provides a first-order approximation of the change in resistance value over time in the initial stage of cooling, symbol t is the time elapsed in units of seconds from the time when the resistance value R1 was obtained, symbols a, b, and c are constants, the element symbols are the contents in units of mass% of the elements corresponding to the element symbols in the chemical composition of the material to be welded, and symbol p is the time elapsed in units of seconds from the time when the resistance value R1 was obtained to the time when post-current application begins. The method for manufacturing a resistance welded joint according to the first embodiment will be described in detail below. The method for manufacturing a resistance welded joint according to the first embodiment will be referred to as the first manufacturing method, and the method for manufacturing a resistance welded joint according to a second embodiment will be referred to as the second manufacturing method.

[0025] (Main energization process S1) In the method for manufacturing a resistance welded joint according to the first embodiment, a workpiece to be welded, which is formed by combining a plurality of steel materials 21, is used as a base material for resistance welding. In the workpiece to be welded, the plurality of steel materials 21 come into contact with each other, and the location where they are joined by resistance welding is referred to as a welded joint portion 22 in the first embodiment.

[0026] In the main current application step S1, a pair of electrodes 11 is placed near the weld joint 22 of the workpieces, and current is applied between the pair of electrodes 11. This causes resistance heating in the weld joint 22 of the workpieces. Figure 2 shows a schematic graph of current value, temperature, and elapsed time. As shown in Figure 2, in the main current application step S1, the resistance heating causes the temperature of the weld joint 22 to rise and exceed the melting point of the workpieces. This causes at least the inside of the weld joint 22 to melt. The main current application step S1 is what is known as resistance welding.

[0027] An example of resistance welding is lap resistance welding such as spot welding. In this case, the plurality of steel materials 21 are a plurality of steel plates. FIG. 3 is a schematic diagram of the main current application step S1 of lap resistance welding. The workpiece is constructed by overlapping a plurality of steel plates made of the steel material 21. A pair of electrodes 11 is arranged to sandwich a weld joint 22. With the plurality of steel plates pressed using the pair of electrodes 11, main current is applied to the pair of electrodes 11, thereby generating molten metal 221 inside the weld joint 22.

[0028] Another example of resistance welding is projection welding, which is a type of resistance welding in which a protrusion shaped on the base material is brought into contact with the welding point and an electric current is passed through it, thereby confining the generation of resistance heat to a relatively small, specific area.

[0029] When the resistance welding is projection welding, at least one of the multiple steel materials 21 is a steel member having a protrusion. Hereinafter, the steel member having the protrusion will be referred to as a first steel member, and the steel member joined to the first steel member will be referred to as a second steel member. In addition, the electrode 11 in contact with the first steel member will be referred to as a first electrode, and the electrode 11 in contact with the second steel member will be referred to as a second electrode.

[0030] When resistance welding is performed as projection welding, a workpiece is constructed by combining multiple steel materials 21 so that the protrusion of a first steel member contacts the second steel member. In this workpiece, the contact area between the protrusion of the first steel member and the second steel member and its surrounding area form a weld joint 22. A first electrode is disposed near the protrusion to support the first steel member. A second electrode is disposed near the contact area with the protrusion to support the second steel member. One or both of the first electrode and the second electrode are used to press the protrusion of the first electrode against the second electrode. In this state, main current is passed through the first electrode and the second electrode to melt the protrusion and its surrounding area.

[0031] Note that a typical resistance welding electrode 11 is configured to allow a coolant to flow therethrough, so that while the electrode 11 is in contact with the welded joint 22, heat transfer from the welded joint 22 to the electrode 11 occurs all the time. As a result, the portion of the welded joint 22 that is in contact with the electrode 11 and its surroundings often do not melt.

[0032] (Cooling process S2) In the cooling step S2, the pair of electrodes 11 are used to cool the welded joint 22. As described above, the electrodes 11 used in ordinary resistance welding are cooled by a refrigerant. Therefore, by reducing or setting to zero the current between the pair of electrodes 11 while keeping them in contact with the welded joint 22, the temperature of the welded joint 22 drops rapidly, as shown in Fig. 2. The temperature of the welded joint 22 starts to drop the moment the amount of resistance heat generation becomes smaller than the amount of heat transferred from the welded joint 22 to the electrodes 11.

[0033] 4 is a schematic diagram showing the cooling step S2 and the post-energization step S3. As shown in FIG. 4, the weld joint 22 is cooled to solidify the molten metal 221, thereby forming a weld 23. The weld 23 is a general term for the portion including the weld metal and the heat-affected zone. The weld metal is the metal that melts and solidifies during welding. The weld metal obtained by resistance welding is called a nugget 231.

[0034] (Post-energization process S3) In the post-current application process S3, post-current is applied to the welded portion 23. Post-current application refers to application of current to the welded portion 23 for the purposes of tempering, annealing, segregation mitigation, etc. As in the main current application process S1, the temperature of the welded portion 23 rises due to resistance heating in the post-current application process S3. The temperature of the welded portion 23 begins to rise the moment the amount of resistance heat becomes greater than the amount of heat transferred from the welded portion 23 to the electrode 11. However, post-current application differs from main current application in that it does not melt and solidify the welded portion 23. If the welded portion 23 melts and solidifies due to post-current application, heat treatment to improve the properties of the welded portion 23 cannot be performed. Therefore, as shown in the schematic diagram of FIG. 2, the maximum temperature of the welded portion 23 in the post-current application process S3 is lower than that in the main current application process S1.

[0035] (Acquisition of resistance value between a pair of electrodes 11) In the method for manufacturing a resistance welded joint according to the first embodiment, the resistance value between the pair of electrodes 11 is obtained. The resistance value between the pair of electrodes 11 can be obtained via the current value and voltage value between the pair of electrodes 11. The resistance value between the pair of electrodes 11 is not the same as the resistance value of the welded joint portion 22 or the resistance value of the welded portion 23, but is a value very close to it. Hereinafter, the resistance value between the pair of electrodes 11 may be simply referred to as the "resistance value."

[0036] (Acquisition of resistance value R1 immediately before the end of main energization process S1) As shown schematically in FIG. 2, in the method for manufacturing a resistance welded joint according to the first embodiment, first, the resistance value R1 is obtained immediately before the end of the main current application step S1. In resistance welding, the temperature of the welded joint 22 is generally within the range of 1400 to 1450°C immediately before the end of the main current application step S1. Furthermore, within the range of 1400 to 1450°C, the resistivity of steel does not depend on the steel's composition. Therefore, in the present disclosure, the resistance value R1 is regarded as an index value of the resistance of the welded joint 22 when the temperature of the welded joint 22 is 1400°C.

[0037] (Acquisition of time change in resistance value during cooling process S2) 2, in the method for manufacturing a resistance welded joint according to the first embodiment, a weak current is then passed between the pair of electrodes 11 in the cooling step S2. Then, the change in resistance value over time in the cooling step S2 is obtained. Hereinafter, the current passed to obtain the resistance value in the cooling step S2 will be referred to as measurement current, and will be distinguished from main current and post-current.

[0038] In normal resistance welding, the current value between the main current and the post-current is typically set to zero in order to quickly solidify the welded portion 23. In the technical field of the present disclosure, it was thought that if current continued after the main current, resistance heating would occur in the welded portion 23, which could hinder cooling of the welded portion 23. However, according to the results of experiments conducted by the present inventors, it was determined that a weak current that allows the resistance value to be measured does not hinder cooling of the welded portion 23.

[0039] (Acquisition of resistance value R2 at the start of post-energization process S3) 2, in the method for manufacturing a resistance welded joint according to the first embodiment, the resistance value R2 between the pair of electrodes 11 is further obtained at the start of the post-energy application step S3. In resistance welding, the temperature of the welded portion 23 begins to rise at the start of the post-energy application step S3. Therefore, the resistance value R2 is an index value of the resistance of the welded portion 23 at the point between the cooling step S2 and the post-energy application step S3, when the temperature of the welded portion 23 is at its lowest.

[0040] (The first linear equation R i (Calculation of (t)) In the method for manufacturing a resistance welded joint according to the first embodiment, a first linear equation R that linearly approximates the time change of the resistance value in the initial stage of cooling is used. i Calculate (t). R i (t) is expressed by the following equation 1. R i (t)=a×t+b……<Formula 1> The symbol a is the first linear expression R iThe symbol b is the slope of (t) and is a constant. The symbol t is the time elapsed in seconds from the time when the resistance value R1 was obtained. The "initial stage of cooling" refers to the period from the time when the main current application is completed until 0.4 seconds have elapsed.

[0041] 5 shows a schematic diagram of the change in resistance over time after the completion of the main current application step S1. In the early stage of the cooling step S2, the elapsed time and the resistance are in a roughly proportional relationship, so that a linear approximation of the change in resistance over time can be easily obtained.

[0042] In the initial stage of cooling, the temperature of the weld 23 exceeds the Ms point. The Ms point is the temperature at which austenite begins to transform into martensite during cooling. Therefore, in the initial stage of cooling, the metal structure of the weld 23 is almost entirely austenite. When the temperature of the weld 23 exceeds the Ms point, the electrical resistivity ρ of the weld 23 is roughly proportional to the temperature of the weld 23, and the electrical resistivity ρ also decreases as the temperature decreases.

[0043] (The second linear equation R ii (Calculation of (t)) In the method for manufacturing a resistance welded joint according to the first embodiment, the second linear equation R ii (t) is calculated. The second linear equation R ii (t) is the linear equation R with the slope i The second linear equation R is defined as the value R obtained by the following equation 4 when t = 4 seconds, which is the same as the slope a of (t). ii (t) is defined by the following formulas 2 to 4. R ii (t)=a×t+c……<Formula 2> R ii (4)=4×a+c=R……<Formula 3> R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> The symbol c is a constant. The element symbol is the content, in mass percent, of the element corresponding to the element symbol in the chemical composition of the workpiece. If the resistance welded joint 2 is a spot-welded joint and the chemical compositions of the multiple steel materials 21 (steel plates) that make up the workpiece are different, the weighted average of the chemical compositions of the steel plates, with the thickness of the steel plates used as the weight, is considered to be the chemical composition of the workpiece. If the resistance welded joint 2 is a projection-welded joint and the chemical compositions of the multiple steel materials 21 (steel members) that make up the workpiece are different, the arithmetic average of the chemical compositions of the multiple steel materials 21 is considered to be the chemical composition of the workpiece.

[0044] As shown in FIG. 5, R calculated by Equation 4 is an index value of the resistance of the welded portion 23 cooled to room temperature when post-energization is omitted. At the time t=4 seconds, the cooling step S2 is usually completed and the temperature of the welded portion 23 reaches room temperature. The second linear equation R ii (t) is a formula that first-order approximates the change in the resistance value of the welded portion 23 over time in the temperature range below the Ms point. R and R ii The technical significance of (t) will be described in detail later.

[0045] Equations 2 to 4 can be integrated as follows: R ii (t)=a×t+R1×[13.6×{Si+Al+0.4×(Mn+Cr)}+12.2] / 140-4×a……<Formula 2A> As is clear from Equation 2A, the second linear equation R ii (t) can be calculated only from the values ​​listed below. First linear equation R i (t) slope a Resistance R1 between the pair of electrodes 11 just before the end of the main current supply Chemical composition of the material to be welded

[0046] (Weld 23 evaluation index) In the method for manufacturing a resistance welded joint according to the first embodiment, R i (p), R iiThe relationship between the resistance value R1 (p) and the resistance value R2 between the pair of electrodes 11 at the start of post-current application is used as an evaluation index for the welded portion 23. The symbol p is the time, in units of seconds, that has elapsed from the time that the resistance value R1 was acquired to the time that the post-current application started. In other words, the symbol p is the time that has elapsed from the time that the resistance value R1 was acquired to the time that the resistance value R2 was acquired. R i (p) is the first linear equation R when t=p i (t) is the value of R ii (p) is the second linear equation R when t=p ii (t) value.

[0047] For example, R i (p), R ii By substituting (p) and R2 into the following equation 5, αM can be calculated. i (p), R ii An example of a value αM indicating the relationship between (p) and R2 can be used as the evaluation index value for the welded portion 23. αM={R i (p)-R2} / {R i (p)-R ii (p)}……<Formula 5> Martensite is generally abbreviated as "α" or "α'". In the present disclosure, the value α calculated by Equation 5 can be used as an index value for the amount of martensite in the welded portion 23 at the start of the post-energization step S3 and as an evaluation index value for the resistance-welded joint 2. For example, if α is within a predetermined range, it can be determined that the post-energization of the welded portion 23 has been performed well. The predetermined range is, for example, 40% or more. If α is 50% or more or 55% or more, it may be determined that the welded portion 23 is good. The upper limit of the predetermined range for determining that the welded portion 23 is good is not particularly limited, and may be 100% or less, 90% or less, or 80% or less. The technical significance of α will be described in detail below.

[0048] (Actions and Effects of the Method for Manufacturing a Resistance Welded Joint According to the First Embodiment) As described above, post-energization is performed for the purpose of tempering, annealing, and alleviating segregation of the welded portion 23. Therefore, post-energization is a type of heat treatment for steel. Furthermore, in the cooling step S2, the nugget 231 formed by melting and solidifying the steel material 21 is rapidly cooled. By rapidly cooling the steel from a high temperature near its solidification point, a rapidly cooled structure is formed in the steel. Therefore, the cooling step S2 is also a type of heat treatment for steel.

[0049] Generally, controlling the temperature history is extremely important in the heat treatment of steel. Using heat treatment equipment such as a heating furnace and a refrigerant allows precise control of heat treatment parameters, such as the heating start temperature, heating hold temperature, cooling start temperature, cooling stop temperature, and cooling rate. However, controlling the heat treatment parameters during the cooling and post-energization of the weld 23 in resistance welding is extremely difficult. This is because the temperature of the weld 23 cannot be known during the cooling step S2 and the post-energization step S3. Furthermore, resistance welding is susceptible to disturbances. It is not uncommon for the joint strength to vary among multiple welds 23 obtained by resistance welding and post-energization based on the same welding parameters. This is presumably due to disturbances affecting the thermal history of the weld 23.

[0050] The present inventors considered that it is important to keep the amount of martensite in the welded portion 23 constant at the start of post-current application in order to suppress variations in the joint strength of the welded portion 23 after post-current application.

[0051] In the initial stage of the cooling step S2, the weld 23 is at a high temperature exceeding the Ms point, and the metal structure of the weld 23 is substantially entirely austenite. When the post-energization is omitted and the temperature of the weld 23 is lowered to room temperature, the austenite in the weld 23 transforms into fresh martensite. Fresh martensite is martensite that has not been tempered. Fresh martensite is hard and may reduce the toughness of the weld 23, thereby impairing the joint strength.

[0052] When the welded portion 23 is tempered by post-current application, the fresh martensite becomes tempered martensite. Tempered martensite is softer than fresh martensite and does not impair the toughness of the welded portion 23. Post-current application increases the joining strength of the welded portion 23 by changing the fresh martensite into tempered martensite.

[0053] However, if post-heating is started before the martensitic transformation of austenite is completed, the austenite remaining in the welded portion 23 will become fresh martensite after post-heating. This fresh martensite reduces the toughness of the welded portion 23 after post-heating.

[0054] Considering the above metallurgical phenomena, the inventors have considered that it is possible to suppress variations in the joint strength of the welded portion 23 after post-current application by maintaining a constant amount of martensite in the welded portion 23 at the start of post-current application. However, the amount of martensite in the welded portion 23 at the start of post-current application cannot be measured during welding. Furthermore, the amount of martensite in the welded portion 23 at the start of post-current application cannot be measured after the end of post-current application. This is because the structure of the welded portion 23 changes due to post-current application.

[0055] The present inventors have investigated a method for evaluating the amount of martensite in the welded portion 23 at the start of post-energization, and have found that the martensite transformation of austenite in the cooling step S2 affects the rate of change of the resistance value.

[0056] FIG. 6 shows an example of the results of an experiment conducted by the present inventors. The graph shown in FIG. 6 shows the relationship between the resistance value during the cooling step S2 and the estimated temperature at the end of the nugget 231. The vertical axis of the graph in FIG. 6 represents the resistance value. The resistance value was obtained from the actual measured values ​​of the current value and voltage value between the pair of electrodes 11. The horizontal axis of the graph in FIG. 6 represents the temperature at the end of the nugget 231. The temperature at the end of the nugget 231 was estimated using resistance welding simulation software "SORPAS" (registered trademark).

[0057] As shown in FIG. 6, in the temperature range above the Ms point, the temperature and resistance are roughly proportional. Furthermore, in the temperature range above the Ms point, the temperature and resistance are also roughly proportional. However, when the temperature of the edge of the nugget 231 decreases and falls below the Ms point, the slope of the graph showing the relationship between temperature and resistance changes significantly. This is presumably because the resistance value decreases significantly when the temperature of the edge of the nugget 231 falls below the Ms point and martensitic transformation begins. Austenite has a face-centered cubic lattice structure (fcc structure), while martensite has a body-centered cubic structure (bcc structure). Because the crystal structures of the two are completely different, the resistivities of the two also differ significantly.

[0058] Next, the inventors measured the change in resistance value over time in the cooling step S2 and the post-energization step S3. As a result, it was found that the resistance value changes as shown in the schematic diagram of FIG. 5. The horizontal axis of FIG. 5 represents the elapsed time t from the time when the resistance value R1 was acquired. The vertical axis represents the resistance value. The solid line graph in FIG. 5 shows the change in resistance value over time when the post-energization step S3 is omitted and the welded portion 23 is cooled to room temperature. The dashed line graph in FIG. 5 shows the change in resistance value over time when the post-energization step S3 is started p seconds after the resistance value R1 is acquired.

[0059] In the initial stage of the cooling process S2, the slope of the graph of the change in resistance with time is approximately constant. Therefore, the change in resistance with time in the initial stage of the cooling process S2 can be expressed by the first linear equation R i However, as the cooling process S2 progresses, the rate of decrease in resistance increases rapidly. The graph of the change in resistance over time can be approximated by the linear equation R i The graph of the resistance value begins to deviate from that of (t). As the cooling step S2 progresses further, the rate of decrease in the resistance value drops sharply. The slope of the graph of the resistance value change over time then becomes approximately constant again up to room temperature. The resistance value change over time at low temperatures is expressed by the second linear equation R ii It can be approximated by (t).

[0060] The phenomenon of the rapid increase in the rate of decrease in resistance is thought to be due to the onset of martensitic transformation. In addition, the time change in resistance is expressed by the first linear equation R i During the period that can be approximated by (t), the amount of martensite in the weld 23 is estimated to be substantially 0%. The phenomenon of the rate of decrease in the resistance value suddenly decreasing is thought to be due to the completion of martensitic transformation. In addition, the time change in the resistance value is expressed by the second linear equation R ii It is estimated that the amount of martensite in the welded portion 23 is maximized during the period that can be approximated by (t).

[0061] When the steel material 21 contains a large amount of martensite-forming elements, the time change of the resistance value is expressed by the second linear equation R ii In the period that can be approximated by (t), the amount of martensite in the welded portion 23 can be 100%. For example, if the steel material 21 is a high-strength steel such as a high-tensile steel, the amount of martensite in the welded portion 23 to which no post-current was applied will be 100%. For simplicity of explanation, the effects of the present disclosure will be explained below assuming that the amount of martensite in the welded portion 23 to which no post-current was applied is 100%. Figure 5 is created based on this assumption.

[0062] Based on the above estimation, the value R of the first linear equation at the time point p when post-energization started is i (p), the value of the second linear expression R ii Based on the relationship between R2 and the measured resistance value R2, the amount of martensite at time point p can be estimated. i If R2 and R(p) are approximately the same, it is estimated that the amount of martensite at time point p is substantially 0%. ii If the values ​​of R (p) and R (p) are approximately the same, the amount of martensite at time p is estimated to be 100%. ii (p) <R2<R i (p), the amount of martensite at time point p is given by the first linear equation R i (t) and the second linear equation R iiThe simplest estimation method is to calculate αM using Equation 5 above. αM={R i (p)-R2} / {R i (p)-R ii (p)}……<Formula 5> αM is the first linear equation R i (t) and the second linear equation R ii (t) and the first linear equation R i (t) is the ratio of the interval between the coordinates (p, R2) and the first linear equation R i If the coordinates (p, R2) are on the graph of (t), then αM is 0. i As the graph of (t) deviates, αM increases. Then, the coordinate (p, R2) becomes the second linear equation R ii When it is on the graph at (t), αM is 1 (i.e., 100%). Therefore, the amount of martensite in the welded portion 23 at the time p when post-energization starts is estimated to be a value close to αM.

[0063] However, when post-energization is performed, the second linear equation R is calculated based on the measured resistance value. ii (t) cannot be obtained because the second linear expression R ii This is because (t) is an equation that approximates the change in resistance over time when post-current is omitted and cooling is performed to room temperature. When post-current is performed, the second linear equation R ii (t) needs to be estimated by some means.

[0064] As a result of repeated experiments under various conditions, the present inventors discovered the following. Second linear equation R ii The slope of (t) is the first linear equation R i It is almost the same as the slope a of (t). The resistance value of the welded portion 23 at room temperature can be estimated by the above-mentioned formula 4. R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> The temperature of the welded portion 23 becomes approximately room temperature at t = 4 seconds, so the value R obtained by substituting 4 seconds for t in the second linear equation Rii(t) is ii (4) can be considered to be equal to R.

[0065] Based on the above, the second linear equation R ii (t) can be considered to be a linear equation with a slope of a and a value R obtained by substituting 4 seconds for t. Therefore, R1 is measured during the main current application step S1, and the first linear equation R i By identifying the slope a of (t), the second linear equation R ii Then, R2 is measured at the start of the post-energization step S3, and the first linear equation R i (t) and the second linear equation R ii A value (e.g., αM) obtained by comparing with (t) can be used as an index value for the amount of martensite in the welded portion 23 at the start of post-current application. The amount of martensite at the start of post-current application is closely related to the amount of fresh martensite in the welded portion 23 after post-current application is completed, and to the joint strength. Therefore, the joint strength of the resistance welded joint 2 can be easily evaluated using the index value for the amount of martensite.

[0066] In the resistance welded joint 2 obtained by the manufacturing method according to the first embodiment, the joint strength can be evaluated without conducting a destructive test such as a cross tension test. Therefore, post-energization can be performed again on the welded portion of the resistance welded joint 2 that is estimated to have poor joint strength. This makes it possible to easily ensure the joint strength of the resistance welded joint 2.

[0067] (2. Second manufacturing method of resistance welded joint 2) In the above-described method for manufacturing a resistance welded joint, the start time of post-current application is determined before the start of main current application. Various resistance values ​​obtained during main current application, cooling, and post-current application, and various calculated index values, are used to evaluate the weld 23 after post-current application. On the other hand, the start time of post-current application can also be selected based on the various resistance values ​​obtained during main current application and cooling, and various calculated index values. This allows the effect of post-current application to be stably exerted.

[0068] That is, the manufacturing method of a resistance welded joint according to the second embodiment of the present disclosure, i.e., the second manufacturing method, includes a step of applying a main current to a pair of electrodes 11 arranged in the vicinity of a welded joint 22 of materials to be welded, which is formed by combining a plurality of steel materials 21, thereby melting the inside of the welded joint 22 of the materials to be welded; a step of cooling the welded joint 22 using the pair of electrodes 11, thereby solidifying the inside of the welded joint 22 and forming a weld 23; and a step of post-current being applied to the weld 23, thereby heating the weld 23 without melting it. Immediately before the end of the main current application, a resistance value R1 between the pair of electrodes 11 is obtained, and during cooling, a change in the resistance value over time is obtained, and a value R calculated based on the following equations 1 to 4 is obtained. i (q), R ii When (q) and the resistance value R3 satisfy a predetermined relationship, post-energization is started. R i (t)=a×t+b……<Formula 1> R ii (t)=a×t+c……<Formula 2> R ii (4)=4×a+c=R……<Formula 3> R=R1×〔13.6×{S i +Al+0.4×(Mn+Cr)}+12.2〕 / 140……<Formula 4> Here, Equation 1 is a first linear equation that first-order approximates the change in resistance over time during the initial stage of cooling, where t is the time elapsed in seconds from the time when resistance R1 was obtained, a, b, and c are constants, the element symbol is the content in mass percent of the element corresponding to the element symbol in the chemical composition of the workpiece, q is the time elapsed in seconds from the time when resistance R1 was obtained to an arbitrary time point, and R3 is the resistance value at the arbitrary time point. The second manufacturing method will be described in detail below. However, the same configuration as the first manufacturing method described above will not be described again.

[0069] (Main energization process S1) (Cooling process S2) (Post-energization process S3) The second manufacturing method includes a main current application step S1, a cooling step S2, and a post-current application step S3. These steps can be configured in the same manner as in the first manufacturing method.

[0070] (Acquisition of resistance value R1 immediately before the end of main energization process S1) (Acquisition of time change in resistance value during cooling process S2) In the second manufacturing method, the resistance value R1 immediately before the end of the main current application step S1 and the change in the resistance value over time during the cooling step S2 are obtained. These resistance values ​​can be obtained in the same way as in the first manufacturing method.

[0071] (The first linear equation R i (Calculation of (t)) (The second linear equation R ii (Calculation of (t)) In the second manufacturing method, the first linear formula R i (t), and the second linear equation R ii These linear expressions can be calculated using Equations 1 to 4 in the initial stage of the cooling step S2, as in the first production method.

[0072] (Determining the start time of the post-energization process S3) In the second manufacturing method, the first linear formula R i (t), and the second linear equation Rii The start time of post-current application is determined based on (t). A method for determining the start time of post-current application will be described below with reference to the graph in Fig. 7 for convenience. However, it should be noted that when carrying out the second manufacturing method, the control device 13 of the resistance welding apparatus 1 does not need to draw a graph.

[0073] In the second manufacturing method, the first linear formula R i (t) and the second linear equation R ii Even after calculating (t), the change in resistance over time during the cooling process S2 continues to be measured. The solid line graph in Figure 7 shows the change in resistance over time during the cooling process S2. Hereinafter, the measured resistance at any time point q during the cooling process S2 is defined as R3. Furthermore, the coordinate (q, R3) is defined as coordinate Q. Furthermore, the resistance values ​​at times q1, q2, and q3 are denoted as R31, R32, and R33, respectively. The coordinates (q1, R31), (q2, R32), and (q3, R33) are denoted as coordinates Q1, Q2, and Q3. The coordinates Q1, Q2, and Q3 are shown in Figure 7.

[0074] The coordinate Q moves from the top left to the bottom right of the solid line graph in Figure 7. Therefore, the coordinate Q is initially calculated by the first linear equation R i (t) (see Q1 in FIG. 7). Then, when the temperature of the welded portion 23 falls below the Ms point, the coordinate Q is calculated as follows: i Away from the graph of (t), the second linear equation R ii (t) (see Q2 in Figure 7). If post-energization is omitted, the coordinate Q is calculated using the second linear equation R ii It overlaps with the graph of (t) and moves along it (see Q3 in Figure 7).

[0075] In the second method, the coordinate Q and the first linear equation R i Graph of (t) and the second linear equation R ii The numerical value corresponding to the positional relationship with the graph of (t) is always calculated. Then, the first linear equation R i Graph of (t) and the second linear equation R iiWhen it is determined that the coordinate Q has moved to a predetermined position between the graphs (t) and (t), post-energization is started. For example, the value R of the first linear expression at the time q2 i (q2), the value of the second linear equation R ii If (q2) and the measured resistance value R32 satisfy a predetermined relationship, post-energization is initiated when the coordinate Q reaches Q2 in Figure 7. This allows the production of a welded portion 23 with the desired CTS.

[0076] An example of an index for determining the time point at which post-energization starts is αM calculated by the following equation 6. αM={R i (q)-R2} / {R i (q)-R ii (q)}……<Formula 6> The content of Equation 6 is substantially the same as the content of Equation 5 above. αM calculated by Equation 6 is an index value of the martensite amount in the weld 23 at an arbitrary time point q. When αM is used, it is not necessary to plot a graph such as that illustrated in FIG. 7. αM is also calculated while measuring the change in resistance over time in the cooling step S2. Then, post-current application can be started when it is determined that αM is within a predetermined range. For example, it is preferable to define the predetermined range as 40% or more. That is, it is preferable to start post-current application when αM is 40% or more. Post-current application may also be started when αM is 50% or more or 55% or more. The upper limit of αM when starting post-current application is not particularly limited, but αM may be 100% or less, 90% or less, or 80% or less.

[0077] (Test welding, first linear equation R i (Identifying the slope a of (t)) In both the first and second production methods described above, the first linear formula R i Both the constant a and the constant b included in (t) = a × t + b are supposed to be calculated from the time change of the resistance value in the initial stage of the cooling process S2. However, in the first linear equation R iThe constant a, which is the slope of (t), can be found before the start of the cooling step S2. Another example of a method for calculating the constant a will be described below.

[0078] To calculate the constant a, first prepare a test material having the same structure as the material to be welded. Then, test welding is performed on the test material before the actual current is passed through. Hereinafter, the resistance welding for producing the resistance-welded joint 2 will be referred to as the actual welding. The actual welding and the test welding are to be distinguished.

[0079] Test welding includes test current application and test cooling. The conditions for test current application are the same as those for actual current application in the actual welding. The conditions for test cooling are the same as those for cooling in the actual welding. In the test current application, there is no need to apply current corresponding to the post-current application in the actual welding. Then, the change in resistance over time in the early stage of test cooling is measured, and a linear approximation equation is calculated. The slope of the linear approximation equation that approximates the early stage of test cooling can be used as the constant a in the actual welding.

[0080] According to the results of experiments conducted by the inventors, the resistance value immediately before the end of test current application does not necessarily match the resistance value R1 immediately before the end of actual current application in the actual welding. This is presumably because the diameter of the nugget 231 varies due to the influence of disturbances, which in turn varies the cross-sectional area of ​​the current path. On the other hand, the inventors have found that the slope of the first linear equation Ri(t) is less susceptible to the influence of disturbances. The slope of the linear equation that approximates the initial stage of test cooling is the slope of the first linear equation R that approximates the initial stage of actual welding cooling. i This roughly coincides with the slope a of the linear equation Ri(t) and the second linear equation Ri(t). Therefore, the slope of the linear equation obtained by test welding can be used in actual welding. This eliminates the need to calculate the constant a in actual welding, and allows the first linear equation Ri(t) and the second linear equation Ri(t) to be calculated quickly.

[0081] Various configurations other than those described above can be appropriately adopted in the methods for manufacturing a resistance welded joint according to the first embodiment and the second embodiment. Below, modified examples of the method for manufacturing a resistance welded joint according to the first embodiment and the method for manufacturing a resistance welded joint according to the second embodiment will be described.

[0082] (Composition of Steel Material 21) The material, shape, size, and number of the steel material 21 constituting the workpiece are not particularly limited. When the resistance welding is spot welding and the resistance-welded joint 2 is a spot-welded joint, the steel material 21 is a steel plate, as described above. The steel plate may be, for example, a high-strength steel plate. The tensile strength of the high-strength steel plate is, for example, 980 MPa or more, 1500 MPa or more, or 2000 MPa or more. In the resistance welding of high-strength steel plates, excessive quench hardening of the welded portion 23 and segregation of embrittling elements such as P in the welded portion 23 are likely to be problems. According to the manufacturing method for a resistance-welded joint disclosed herein, it is possible to evaluate whether the post-energization of the welded portion 23 of the resistance-welded joint 2 made of a high-strength steel plate was appropriate, and preferably, the post-energization of the welded portion 23 can always be performed appropriately.

[0083] When the resistance welding is projection welding and the resistance-welded joint 2 is a projection-welded joint, at least one of the plurality of steel materials 21 is a first steel member having a protrusion, as described above. The first steel member is, for example, a bolt or a nut. The second steel member is, for example, a steel plate. Both the first steel member and the second steel member may be made of high-strength steel, for example, of 980 MPa or more, 1500 MPa or more, or 2000 MPa or more.

[0084] (Electrification conditions) The conditions for the main current and post-current are not particularly limited. Various conditions such as the current value, heat input, and pressure during the main current and post-current may be set to known values ​​according to the material, size, shape, and number of the steel material 21. When the resistance welding is spot welding, the current ratio, which is the ratio of the current value during the main current to the current value during the post-current, may be set to 0.55 to 0.90, or 0.55 to 0.85, for example. When the purpose of the post-current is tempering, the current ratio is preferably set to 0.55 to 0.70.

[0085] It is preferable to use a small current value in the measurement current flow performed to measure the resistance value during cooling. This can promote cooling of the welded portion 23. For example, the current value in the measurement current flow is preferably 3.5 kA or less, and more preferably 3.0 kA or less. It is also acceptable to provide a short period of time without current flow between the actual current flow and the measurement current flow.

[0086] The measurement current may be applied continuously or intermittently. By applying the measurement current continuously, the approximation accuracy of the first linear equation Ri(t) and the second linear equation can be improved. On the other hand, by applying the measurement current intermittently, the cooling of the welded portion 23 can be further promoted. Intermittent application of current is sometimes called pulse application.

[0087] (3. First resistance welding device 1) Next, a resistance welding apparatus 1 according to a third embodiment of the present disclosure will be described. The resistance welding apparatus 1 according to the third embodiment is also referred to as a first resistance welding apparatus 1. The resistance welding apparatus 1 according to the third embodiment is an apparatus for carrying out the method for manufacturing a resistance welded joint according to the first embodiment, and includes a pair of electrodes 11, a power source 12 that applies current between the pair of electrodes 11, and a control device 13 that operates the power source 12 to control one or both of the voltage value and the current value between the pair of electrodes 11. The control device 13 includes a main current supply unit 131 that operates the power source 12 to apply main current between the pair of electrodes 11 to melt the inside of a weld joint portion 22 of a workpiece, a post-current supply unit 134 that operates the power source 12 to apply post-current between the pair of electrodes 11 to heat a weld portion 23 without melting it, a resistance value acquisition unit 132 that acquires a resistance value between the pair of electrodes 11 based on the voltage value and the current value between the pair of electrodes 11, and an R i (p), and R ii (p) and a resistance value processing unit that calculates R i (p), R iiand an evaluation unit 135 that evaluates the welded portion 23 based on the relationship between (p) and the resistance value R2. Hereinafter, a resistance welding device 1 according to a third embodiment will be described with reference to Fig. 8. Note that Fig. 8 is a schematic diagram of a spot welding device, but the resistance welding device 1 according to the third embodiment may be a resistance welding device 1 other than a spot welding device.

[0088] (Pair of electrodes 11) The pair of electrodes 11 clamps the workpieces to be welded and passes current through the workpieces. The pair of electrodes 11 also has the function of cooling the welded portion 23. Therefore, the pair of electrodes 11 may have a flow path for circulating a coolant. The configuration of the pair of electrodes 11 is not particularly limited. Ordinary electrodes 11 for resistance welding can be used as the electrodes 11 of the resistance welding apparatus 1 according to the third embodiment. For example, if the resistance welding apparatus 1 is a spot welding apparatus, various spot welding electrodes 11 specified in JIS C 9304:1999 can be used.

[0089] (power supply 12) The power source 12 passes current between the pair of electrodes 11. The power source 12 is simply a power supply source, and its configuration is not particularly limited. The power source 12 may be built into the resistance welding apparatus 1 or may be provided externally to the resistance welding apparatus 1. The resistance welding apparatus 1 according to the third embodiment can be considered a resistance welding system consisting of multiple devices operating in coordination with one another. Therefore, even if the power source 12 is provided externally to the resistance welding apparatus 1, the entirety of these devices is considered to be the resistance welding apparatus 1. A device incorporated into the resistance welding system, operating upon receiving power from an external source, and supplying power between the pair of electrodes 11 may also be considered the power source 12. For example, a transformer connected to a commercial power source may also be considered the power source 12. On the other hand, a commercial power source connected to the resistance welding apparatus 1 may also be considered the power source 12.

[0090] (Control device 13) The control device 13 operates the power source 12 and controls one or both of the voltage value and the current value between the pair of electrodes 11. Like the power source 12, the control device 13 may be built into the resistance welding apparatus 1 or may be provided externally to the resistance welding apparatus 1. Even if the control device 13 is provided externally to the resistance welding apparatus 1, the entirety of these devices is considered to be the resistance welding apparatus 1.

[0091] (Main energizing portion 131 and rear energizing portion 134) The control device 13 has a main current-carrying unit 131 and a post-current-carrying unit 134. The main current-carrying unit 131 operates the power source 12 to apply a main current between the pair of electrodes 11 to melt the inside of the weld joint 22 of the workpiece. That is, the main current-carrying unit 131 controls the resistance welding performed by the main current-carrying. The post-current-carrying unit 134 operates the power source 12 to apply a post-current between the pair of electrodes 11 to heat the weld 23 without melting it. That is, the post-current-carrying unit 134 controls the post-current that performs a post-heat treatment on the weld 23 obtained by resistance welding.

[0092] (Resistance value acquisition unit 132) The control device 13 further includes a resistance value acquiring unit 132. The resistance value acquiring unit 132 acquires the resistance value between the pair of electrodes 11 based on the voltage value and current value between the pair of electrodes 11. Specifically, the resistance value acquiring unit 132 acquires the resistance value R1 immediately before the end of main current application, the resistance value R2 at the start of post-current application, and the change in the resistance value over time in the cooling step S2. Furthermore, the resistance value acquiring unit 132 may pass a measurement current to the pair of electrodes 11 in the cooling step S2 to acquire the resistance value.

[0093] (Resistance value processing unit 133) The control device 13 has a resistance value processing unit 133. The resistance value processing unit 133 calculates a first linear equation Ri(t) and a second linear equation Rii(t) based on various resistance values ​​acquired by the resistance value acquisition unit 132. The resistance value processing unit 133 may further include a recording unit that records the slope of the first linear equation Ri(t). The resistance value processing unit may use the value recorded in the recording unit as the slope a of the first linear equation Ri(t). The value recorded in the recording unit can be calculated, for example, by the test welding described above. Furthermore, the resistance value processing unit 133 substitutes the time point p of the start of post-energization into the first linear equation Ri(t) and the second linear equation Rii(t) to calculate R i (p), and R ii Calculate (p).

[0094] (Evaluation Section 135) The control device 13 includes an evaluation unit 135. The evaluation unit 135 evaluates the R calculated by the resistance value processing unit 133. i (p) and R ii (p) and the resistance value R2 acquired by resistance value acquisition unit 132. Evaluation unit 135 may evaluate welded portion 23 using α calculated by Equation 5, for example. α may be calculated by evaluation unit 135 or by resistance value processing unit 133. For example, evaluation unit 135 can determine that welded portion 23 is good when α is within a predetermined range. The predetermined range is, for example, 40% or more.

[0095] (4. Second resistance welding device 1) Next, a resistance welding apparatus 1 according to a fourth embodiment of the present disclosure will be described. The resistance welding apparatus 1 according to the fourth embodiment is also referred to as a second resistance welding apparatus 1. The resistance welding apparatus 1 according to the fourth embodiment is an apparatus for carrying out the method for manufacturing a resistance welded joint according to the second embodiment, and includes a pair of electrodes 11, a power source 12 that applies current between the pair of electrodes 11, and a control device 13 that operates the power source 12 to control one or both of the voltage value and the current value between the pair of electrodes 11. The control device 13 includes a main current supply unit 131 that operates the power source 12 to apply main current between the pair of electrodes 11 to melt the inside of a weld joint portion 22 of a workpiece, a post-current supply unit 134 that operates the power source 12 to apply post-current between the pair of electrodes 11 to heat a weld portion 23 without melting it, a resistance value acquisition unit 132 that acquires a resistance value between the pair of electrodes 11 based on the voltage value and the current value between the pair of electrodes 11, and a resistance value acquisition unit 133 that acquires a resistance value between the pair of electrodes 11 based on the voltage value and the current value between the pair of electrodes 11. i (q), and R ii (q) and a resistance value processing unit for calculating R i (q), R ii (q), and a post-energization start unit 136 that activates the post-energization unit 134 at the time when the resistance value R3 satisfies a predetermined relationship. The resistance welding apparatus 1 according to the fourth embodiment will be described below. However, the description of the same configuration as that of the first resistance welding apparatus 1 described above will be omitted.

[0096] (Pair of electrodes 11) (power supply 12) (Control device 13) (Main energizing portion 131 and rear energizing portion 134) (Resistance value acquisition unit 132) The second resistance welding apparatus 1 has a pair of electrodes 11, a power source 12, and a control device 13. The control device 13 has a main current supply unit 131, a post-current supply unit 134, and a resistance value acquisition unit 132. These can have the same configuration as the first resistance welding apparatus 1.

[0097] (Resistance value processing unit 133) The control device 13 has a resistance value processing unit 133. The resistance value processing unit 133 calculates a first linear equation Ri(t) and a second linear equation Rii(t) based on various resistance values ​​acquired by the resistance value acquisition unit 132. The resistance value processing unit 133 of the second resistance welding device 1 may further include a recording unit that records the slope of the first linear equation Ri(t), similar to the first resistance welding device 1. Furthermore, the resistance value processing unit 133 substitutes an arbitrary time point q into the first linear equation Ri(t) and the second linear equation Rii(t) to calculate R i (q), and R ii The resistance value processing unit 133 calculates R i (q), and R ii (q) is calculated repeatedly. R i (q) and R ii The more times (q) is calculated, the better.

[0098] (Post-energization start part 136) The control device 13 has a post-energization start unit 136. The post-energization start unit 136 calculates the R calculated by the resistance value processing unit 133. i (q) and R ii The post-energization start unit 136 determines the time point at which to activate the post-energization unit 134 based on the relationship between the resistance value R (q) calculated by the resistance value processing unit 133 and the resistance value R3 acquired by the resistance value acquisition unit 132. i (q) and R ii (q), and the resistance value R3 measured at time q are determined to satisfy a predetermined relationship. For example, R i (q), R ii (q) and the resistance value R3 are substituted into the above equation 6. If αM is within a predetermined range, R i (q), R ii It is determined that (q) and the resistance value R3 satisfy a predetermined relationship. For example, at any time when αM is 40% or more, the post-current applying unit 134 can be activated.

[0099] R i (q), R iiIf it is determined that (q) and the resistance value R3 satisfy a predetermined relationship, the post-energization start unit 136 activates the post-energization unit 134. i (q), R ii If it is determined that the resistance value R3 does not satisfy the predetermined relationship, the post-energization starting unit 136 determines that the resistance value R3 is a newly calculated resistance value R i (q) and R ii (q), and whether the resistance value R3 newly acquired by the resistance value acquisition unit 132 satisfies a predetermined relationship. The post-energization start unit 136 repeats the above-described determination until the predetermined relationship is satisfied. The number of determinations per unit time is preferably as large as possible.

[0100] 8, the control device 13 of the resistance welding apparatus 1 may also include both the evaluation unit 135 and the post-energization start unit 136. Such a resistance welding apparatus 1 can execute both the first method for manufacturing a resistance welded joint and the second method for manufacturing a resistance welded joint.

[0101] (First power source and second power source) The power source for main current application and post-current application may be different from the power source for measurement current application. That is, in the resistance welding apparatus 1 according to the present disclosure, the power source 12 may have a first power source and a second power source, the main current application unit 131 and the post-current application unit 134 may operate the first power source, and the resistance value acquisition unit 132 may operate the second power source. The control device 13 can switch the power source 12 system depending on the purpose of current application.

[0102] The current required for the main and post-current application is much greater than the current required for measuring the resistance value. To prevent the temperature of the welded portion 23 from rising during resistance value measurement, it is preferable to minimize the current required for measuring the resistance value. When the power source 12 for the main and post-current application is different from the power source 12 for measuring the resistance value, a power source 12 for a conventional spot welding device can be used as the first power source for the main and post-current application, and a power source 12 capable of applying a weak current can be used as the second power source for measuring the resistance value. This can further promote cooling of the welded portion 23. For example, the resistance welding device 1 may have a second transformer for applying a weak current in addition to a first transformer for applying the welding current. In this case, the first transformer may be considered the first power source, and the second transformer may be considered the second power source.

[0103] The resistance welding apparatus 1 according to the present disclosure can employ various configurations other than those described above. For example, the resistance welding apparatus 1 may include a pressure device capable of applying a desired pressure to the pair of electrodes 11. This allows the pair of electrodes 11 to firmly clamp the workpieces, stabilizing the flow of current. The control device 13 may include a pressure control unit that operates the pressure device to control the pressure between the pair of electrodes 11. The resistance welding apparatus 1 may also include a coolant supply means for supplying a coolant to the electrodes 11. This can suppress wear at the tips of the electrodes 11 and promote cooling of the welded portion 23.

[0104] (5. Control program for resistance welding device 1) The control program for the resistance welding apparatus 1 according to the fifth embodiment of the present disclosure is a program for causing the resistance welding apparatus 1 to function as the resistance welding apparatus 1 according to the third embodiment or the resistance welding apparatus 1 according to the fourth embodiment. The control program according to the fifth embodiment causes the resistance welding apparatus 1 to function, and can suitably execute the method for manufacturing a resistance welded joint according to the first embodiment or the method for manufacturing a resistance welded joint according to the second embodiment.

[0105] (6. First control device 13 of resistance welding device 1) A control device 13 for a resistance welding apparatus 1 according to a sixth embodiment of the present disclosure is a control device 13 for a resistance welding apparatus 1 for executing the method for manufacturing a resistance welded joint according to the first embodiment. The resistance welding apparatus 1 includes a pair of electrodes 11 and a power source 12 that applies current between the pair of electrodes 11. The control device 13 includes a main current supply unit 131 that operates the power source 12 to apply main current between the pair of electrodes 11 to melt the inside of a weld joint portion 22 of a workpiece, a post-current supply unit 134 that operates the power source 12 to apply post-current between the pair of electrodes 11 to heat a weld portion 23 without melting it, a resistance value acquisition unit 132 that acquires a resistance value between the pair of electrodes 11 based on a voltage value and a current value between the pair of electrodes 11, and an R i (q), and R ii (q) and a resistance value processing unit for calculating R i (q), R ii and a post-energization start unit 136 that activates the post-energization unit 134 at the time when the resistance value R3 satisfies a predetermined relationship. The control device 13 according to the sixth embodiment can control the resistance welding device 1 so as to suitably perform the method for manufacturing a resistance welded joint according to the first embodiment.

[0106] (7. Second control device 13 of resistance welding device 1) A control device 13 for a resistance welding apparatus 1 according to a seventh embodiment of the present disclosure is a control device 13 for a resistance welding apparatus 1 for executing the method for manufacturing a resistance welded joint according to the second embodiment. The resistance welding apparatus 1 includes a pair of electrodes 11 and a power source 12 that applies current between the pair of electrodes 11. The control device 13 includes a main current supply unit 131 that operates the power source 12 to apply main current between the pair of electrodes 11 to melt the inside of a weld joint portion 22 of a workpiece, a post-current supply unit 134 that operates the power source 12 to apply post-current between the pair of electrodes 11 to heat a weld portion 23 without melting it, a resistance value acquisition unit 132 that acquires resistance values ​​R1 and R2 between the pair of electrodes 11 based on the voltage value and current value between the pair of electrodes 11, and an R i (q), and R ii (q) and a resistance value processing unit for calculating R i (q), R iiand a post-energization start unit 136 that activates the post-energization unit 134 at the time when the resistance value R3 satisfies a predetermined relationship. The control device 13 according to the seventh embodiment can control the resistance welding device 1 so as to suitably perform the method for manufacturing a resistance welded joint according to the second embodiment. [Explanation of symbols]

[0107] S1 Main energization process S2 cooling process S3 post-energization process R1 Resistance value just before the end of main current R i (t) First linear equation t is the time elapsed since the resistance value R1 was obtained R2 Resistance value just before the end of main current R ii (t) Second linear equation 1 Resistance welding equipment 11 electrodes 12 Power supply 13 Control device 131 Main power supply 132 Resistance value acquisition unit 133 Resistance value processing section 134 Rear energizing part 135 Evaluation Department 136 Post-energization start section 2. Resistance welded joints 21 Steel materials 22 Welded joint 221 Molten Metal 23 Welded section 231 Nuggets

Claims

1. a step of applying a current to a pair of electrodes arranged near a weld joint portion of a workpiece formed by combining a plurality of steel materials, thereby melting the inside of the weld joint portion of the workpiece; a step of cooling the welded joint using the pair of electrodes, thereby solidifying the inside of the welded joint to form a weld; post-energizing the welded portion, thereby heating the welded portion without melting it; Equipped with Immediately before the end of the main energization, a resistance value R1 between the pair of electrodes is obtained; During the cooling, a change in resistance value over time is obtained; At the start of the post-energization, a resistance value R2 is obtained; The value R calculated based on the following formulas 1 to 4 ｉ (p), R ｉｉ (p) and the relationship between the resistance value R2 and the evaluation index value of the welded portion. A method for manufacturing resistance welded joints. R ｉ (t) = a × t + b ... <Equation 1> R ｉｉ (t) = a × t + c ... <Equation 2> R ｉｉ (4) = 4 × a + c = R ... <Equation 3> R = R1 × [13.6 × {S ｉ + Al + 0.4 × (Mn + Cr)} + 12.2] / 140... <Equation 4> where: Equation 1 is a first linear equation that first-order approximates the change in the resistance value over time in the initial stage of the cooling, The symbol t is the elapsed time in seconds from the time when the resistance value R1 was acquired, Symbols a, b, and c are constants, The element symbol represents the content, in unit mass%, of the element corresponding to the element symbol in the chemical composition of the workpiece, The symbol p is the time, in units of seconds, that has elapsed from the time when the resistance value R1 is acquired to the time when the post-energization starts.

2. 2. The method for manufacturing a resistance welded joint according to claim 1, wherein a value αM calculated based on the following formula 5 is used as the evaluation index value of the welded portion: αM={R ｉ (p)-R2} / {R ｉ (p)-R ｉｉ (p)}...<Formula 5>

3. The method for manufacturing a resistance welded joint according to claim 2, wherein the weld is judged to be good when the αM is 40% or more.

4. a step of applying a current to a pair of electrodes arranged near a weld joint portion of a workpiece formed by combining a plurality of steel materials, thereby melting the inside of the weld joint portion of the workpiece; a step of cooling the welded joint using the pair of electrodes, thereby solidifying the inside of the welded joint to form a weld; post-energizing the welded portion, thereby heating the welded portion without melting it; Equipped with Immediately before the end of the main energization, a resistance value R1 between the pair of electrodes is obtained; During the cooling, a change in resistance value over time is obtained; The value R calculated based on the following formulas 1 to 4 ｉ (q), R ｉｉ (q) and the resistance value R3 satisfy a predetermined relationship, the post-energization is started. A method for manufacturing resistance welded joints. R ｉ (t) = a × t + b ... <Equation 1> R ｉｉ (t) = a × t + c ... <Equation 2> R ｉｉ (4) = 4 × a + c = R ... <Equation 3> R = R1 × [13.6 × {S ｉ + Al + 0.4 × (Mn + Cr)} + 12.2] / 140...... <Equation 4> where: Equation 1 is a first linear equation that first-order approximates the change in the resistance value over time in the initial stage of the cooling, The symbol t is the elapsed time in seconds from the time when the resistance value R1 was acquired, Symbols a, b, and c are constants, The element symbol represents the content, in unit mass%, of the element corresponding to the element symbol in the chemical composition of the workpiece, The symbol q is the time, in seconds, that has elapsed from the time the resistance value R1 was acquired to an arbitrary time point, The symbol R3 is the resistance value at the arbitrary point in time.

5. 5. The method for manufacturing a resistance welded joint according to claim 4, wherein the post-energization is started at a point in time when the value αM calculated based on the following formula 6 is within a predetermined range. αM = {R ｉ (q) - R2} / {R ｉ (q) - R ｉｉ (q)}... <Equation 6>

6. The method for producing a resistance welded joint according to claim 5, wherein the post-energization is started when the αM is 40% or more.

7. Before the main current application, a test material having the same configuration as the material to be welded is subjected to test current application and test cooling under the same conditions as the main current application and cooling, The slope of the linear approximation of the time change in resistance value in the initial stage of the test cooling is defined as the constant a.

7. The method for manufacturing a resistance welded joint according to claim 1.

8. A resistance welding apparatus for carrying out the method for manufacturing a resistance welded joint according to any one of claims 1 to 3, A pair of the electrodes; a power source that applies current between the pair of electrodes; a control device that operates the power source to control one or both of a voltage value and a current value between the pair of electrodes; Equipped with The control device a main current supply unit that operates the power source to supply a main current between the pair of electrodes to melt the inside of the weld joint portion of the workpiece; a post-current supply unit that operates the power source to supply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquiring unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes; The R ｉ (p), and the R ｉｉ a resistance value processing unit that calculates (p); The R ｉ (p), the R ｉｉ (p) and an evaluation unit that evaluates the welded portion based on the relationship between the resistance value R2; Resistance welding apparatus having:

9. The resistance welding device according to claim 8, wherein the evaluation unit evaluates the welded portion using the αM.

10. 10. The resistance welding device according to claim 9, wherein the evaluation unit determines that the weld is good when the αM is 40% or more.

11. A resistance welding apparatus for carrying out the method for manufacturing a resistance welded joint according to any one of claims 4 to 6, A pair of the electrodes; a power source that applies current between the pair of electrodes; a control device that operates the power source to control one or both of a voltage value and a current value between the pair of electrodes; Equipped with The control device a main current supply unit that operates the power source to supply a main current between the pair of electrodes to melt the inside of the weld joint portion of the workpiece; a post-current supply unit that operates the power source to supply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquiring unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes; The R ｉ (q), and the R ｉｉ a resistance value processing unit that calculates (q); The R ｉ (q), the R ｉｉ (q) and the resistance value R3 satisfy a predetermined relationship, a post-energization start unit that activates the post-energization unit; Resistance welding apparatus having:

12. 12. The resistance welding device according to claim 11, wherein the post-current application start unit activates the post-current application unit when the αM is within the predetermined range.

13. 13. The resistance welding device according to claim 12, wherein the post-current application start unit activates the post-current application unit when the αM is 40% or more.

14. A control program for a resistance welding apparatus, for causing the resistance welding apparatus to function as the resistance welding apparatus according to claim 8.

15. A control program for a resistance welding apparatus, for causing the resistance welding apparatus to function as the resistance welding apparatus according to claim 11.

16. A control device for a resistance welding apparatus including a pair of electrodes and a power source that applies current between the pair of electrodes, for performing the method for manufacturing a resistance welded joint according to any one of claims 1 to 3, a main current supply unit that operates the power source to supply a main current between the pair of electrodes to melt the inside of the weld joint portion of the workpiece; a post-current supply unit that operates the power source to supply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquiring unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes; The R ｉ (q), and the R ｉｉ a resistance value processing unit that calculates (q); The R ｉ (q), the R ｉｉ (q) and the resistance value R3 satisfy a predetermined relationship, a post-energization start unit that activates the post-energization unit; A control device for a resistance welding device comprising:

17. A control device for a resistance welding apparatus including a pair of electrodes and a power source that applies current between the pair of electrodes, for performing the method for manufacturing a resistance welded joint according to any one of claims 4 to 6, a main current supply unit that operates the power source to supply a main current between the pair of electrodes to melt the inside of the weld joint portion of the workpiece; a post-current supply unit that operates the power source to supply post-current between the pair of electrodes to heat the welded portion without melting it; a resistance value acquiring unit that acquires the resistance value between the pair of electrodes based on a voltage value and a current value between the pair of electrodes; The R ｉ (q), and the R ｉｉ a resistance value processing unit that calculates (q); The R ｉ (q), the R ｉｉ (q) and the resistance value R3 satisfy a predetermined relationship, a post-energization start unit that activates the post-energization unit; A control device for a resistance welding device comprising:

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