Spot-welded joint and method for manufacturing spot-welded joint

Abstract

A spot-welded joint according to one aspect of the present disclosure is provided with two or more overlapped steel plates and a nugget joining the steel plates. The metal structure at both ends of the nugget in a virtual line along the joint interface of the steel plates as measured in a cross section of the nugget contains 5-40 area% of fresh martensite, and 95 area% or more of the balance is one or both of tempered martensite and bainite. Preferably, the metal structure at both ends of the nugget contains at least 2 area% of bainite.

Description

Spot welded joint and method for manufacturing spot welded joint

[0001] The present invention relates to a spot welded joint and a method for manufacturing the spot welded joint. This application claims priority to Japanese Patent Application No. 2023-195627, filed on November 17, 2023, the contents of which are incorporated herein by reference.

[0002] Resistance welding is a welding method in which a large current is passed through the weld joint, generating heat by resistance and applying pressure. Resistance welding can be performed in a short time, so it is used to manufacture various machine parts.

[0003] A post-heating current may be passed through a weld formed by resistance welding. The post-heating current is a current passed through the weld after the resistance welding for the purpose of performing heat treatment such as tempering, annealing, and segregation relief. The application of a post-heating current is also called post-heating.

[0004] High-strength steel plates are an example of materials for which post-energization 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 of ordinary steel plates, the higher the strength of the steel plate, the higher the cross tensile strength (CTS). However, in welded joints made of high-strength steel plates, the higher the strength of the steel plate, the lower the CTS.

[0005] Various techniques have been studied to improve the properties of welds formed by resistance welding. For example, Patent Document 1 discloses a spot welding method in which a plate assembly of two or more overlapping steel plates is clamped between a pair of welding electrodes and joined by applying current while applying pressure, in which at least one steel plate of the plate assembly is a high-strength steel plate, and a current value I Ｗ The method includes a main current application process in which current is applied at a current flow rate of 100 kA and a post-tempering heat treatment process in which the post-tempering heat treatment process is performed for a cooling time of t ｃｔ (ms) and the current value I ｔ (kA), and current application time t ｔ (ms), the temperature rise process in which current is applied, and the downslope current application time tｔｍａ (ms), the current is set to a current value I ｔ (kA) to the current value I ｔｍ (kA) and / or current value I ｔｍ (kA), and current application time t ｔｍ and a holding step of energizing the weld for 0.8×I (ms). ｗ ≦I ｔ ≦1.6×I ｗ ... (2)

[0006] Patent No. 6958765

[0007] One of the advantages of spot welding is the short welding time. However, if post-welding is performed, the welding time increases. When manufacturing parts with a large number of welds, it is necessary to shorten the welding time as much as possible.

[0008] The technology described in Patent Document 1 aims to reduce the embrittlement of the nugget and improve the joint strength, but Patent Document 1 does not address the issue of shortening the welding time, nor does it disclose any means for achieving this.

[0009] An object of the present disclosure is to provide a method for manufacturing a spot welded joint that can be performed in a short time, and a spot welded joint that can be manufactured in a short time.

[0010] The gist of the present disclosure is as follows.

[0011] (1) A spot-welded joint according to one aspect of the present invention includes two or more stacked steel sheets and a nugget joining the steel sheets, wherein the metallographic structure of both ends of the nugget, measured on a cross section of the nugget along a virtual line along the joining interface of the steel sheets, contains 5 to 40 area % of fresh martensite, and the remaining 95 area % or more is one or both of tempered martensite and bainite. (2) Preferably, in the spot-welded joint described in (1) above, one or more of the steel sheets is a high-carbon steel sheet having a carbon content of 0.20 mass % or more. (3) Preferably, in the spot-welded joint described in (1) or (2) above, the value obtained by subtracting the Vickers hardness of both ends of the nugget along a virtual line along the joining interface of the steel sheets, measured on a cross section of the nugget, from the estimated maximum Vickers hardness HVmax calculated by the following formula is HV50 or more. HVmax = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) Here, the element symbols included in the formula represent the contents of the elements corresponding to the element symbols in a weighted average of the chemical compositions of the plurality of steel plates, with the plate thickness used as the weight. (4) Preferably, in the spot-welded joint described in any one of (1) to (3) above, the metallographic structure of the both end portions of the nugget contains 2 area % or more of the bainite.

[0012] (5) A manufacturing method of a spot welded joint according to another aspect of the present invention includes the steps of: performing a reproduced thermal cycle test on a test steel sheet multiple times, simulating the main current, cooling, and post-current of spot welding, and varying the end temperature of the cooling and the heating rate in the post-current; measuring the hardness of the test steel sheet after the reproduced thermal cycle test; specifying a correlation between the end temperature of cooling in the reproduced thermal cycle test, the heating rate in the post-current, and the hardness of the test steel sheet after the reproduced thermal cycle test; and specifying a combination of the end temperature and the heating rate that makes the hardness of the test steel sheet equal to or less than a target value and that minimizes the total time required for the cooling and the post-current. (6) In the method for manufacturing a spot-welded joint described in (5), the target value is preferably equal to or less than the estimated maximum Vickers hardness (HVmax-HV150) calculated by the following formula: HVmax = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) Here, the element symbols included in the formula are the contents of the elements corresponding to the element symbols in the weighted average of the chemical components of the test steel plate, with the plate thickness used as the weight. (7) Preferably, in the manufacturing method of a spot-welded joint described in (5) or (6) above, the weighted average of the Mn content, the weighted average of the Cr content, and the weighted average of the Ni content of the steel plate, with the plate thickness of the steel plate used as the weight, are approximately equal to the Mn content, the Cr content, and the Ni content of the test steel plate, respectively.(8) Preferably, the method for manufacturing a spot-welded joint described in any one of (5) to (7) above further includes a step of estimating, by simulation, a cooling time and a post-heating current that can reproduce the combination of the upper limit of the cooling end temperature and the lower limit of the heating rate identified by the reproduced thermal cycle test, and the cooling time and post-heating time identified by the simulation are applied to the cooling and post-heating of the sheet assembly. (9) Preferably, in the method for manufacturing a spot-welded joint described in any one of (5) to (8) above, upslope current is used for the first half of the post-heating. (10) Preferably, in the method for manufacturing a spot-welded joint described in (5) to (9) above, one or more of the steel sheets are high-carbon steel sheets having a carbon content of 0.20 mass% or more.

[0013] According to the present disclosure, it is possible to provide a method for manufacturing a spot welded joint that can be performed in a short time, and a spot welded joint that can be manufactured in a short time.

[0014] 1 is a schematic diagram of current and temperature in spot welding in which the nugget is not cooled to below the Mf point. 2 is a schematic diagram of current and temperature in spot welding in which the nugget is cooled to below the Mf point. 3 is a schematic diagram of current and temperature in spot welding in which the nugget is cooled to below the Mf point. 4 is a schematic diagram of the correlation between the cooling end temperature, the post-energization heating rate, and the hardness of the test steel sheet after post-energization. 5 is a flowchart of a method for manufacturing a spot-welded joint. 6 is a cross-sectional view of a spot-welded joint. 7 is a schematic diagram of a thermal profile in a replicated thermal cycle test. 8 is a graph showing the correlation between the cooling end temperature T2 in a replicated thermal cycle test and the hardness of the steel sheet after the replicated thermal cycle test. 9 is a metallographic photograph of a nugget in a replicated thermal cycle test. 10 is a metallographic photograph of a nugget in a replicated thermal cycle test. 11 is a metallographic photograph of a nugget in a replicated thermal cycle test. 12 is a metallographic photograph of a nugget in a replicated thermal cycle test. 13 is a result of a Formaster test. 14 is a result of a simulation of the relationship between the cooling time and the cooling end temperature T2. 1 is a schematic diagram of a current conduction pattern (a) employed in an experiment by the present inventors; FIG. 2 is a schematic diagram of a current conduction pattern (b) employed in an experiment by the present inventors; FIG. 3 is a result of a spot welding test; FIG. 4 is a result of a spot welding test; FIG. 5 is a result of a spot welding test; FIG. 6 is a simulation result of the weld temperature in the current conduction pattern (a) and the current conduction pattern (b); FIG. 7 is a cross-sectional photograph and hardness measurement results of a spot welded joint obtained by a spot welding test; FIG. 8 is a cross-sectional photograph and hardness measurement results of a spot welded joint obtained by a spot welding test.

[0015] The present inventors conducted extensive research to develop a method for manufacturing a spot-welded joint that allows for post-energization in a short time. As a result, they discovered that by appropriately shortening the cooling time (cooling time t2) performed after the main energization, it is possible to manufacture a spot-welded joint in a short time while ensuring the effects of post-energization. They also discovered that an appropriate cooling time t2 can be identified by conducting a test before spot welding. The technical concept of the present disclosure is described below.

[0016] (1. Changes in Metal Structure During Main Current Passage, Cooling, and Post-Current Passage) As shown in Figures 1 and 2, the manufacturing method for spot-welded joints includes main current passage, cooling, and post-current passage. In main current passage, a sheet assembly formed by overlapping multiple steel sheets 11 is clamped between the tips of spot welding electrodes, and current and pressure are concentrated in a relatively small area to locally heat the assembly, while simultaneously applying pressure with the electrodes. The current passed through the sheet assembly in main current passage is called the welding current. As a result of main current passage, the temperature of the sheet assembly between the pair of electrodes rises to above its melting point.

[0017] During cooling, the current flow is stopped or reduced to a very low value while the sheet assembly is still held between the tips of the electrodes. Spot welding electrodes have water-cooling holes. With the electrodes in contact with the sheet assembly, the resistance heat generation is reduced to zero or a value close to zero, allowing heat to be transferred from the sheet assembly to the electrodes. The temperature of the sheet assembly then drops rapidly between the pair of electrodes. The molten metal formed by this current flow solidifies upon cooling to form a nugget 12.

[0018] The metallographic structure of the nugget 12 is austenite immediately after solidification. The nugget 12 is rapidly cooled to produce martensite in the nugget 12. This hardens the nugget 12. The temperature at which austenite begins to transform into martensite during cooling is called the Ms point. The temperature at which austenite has almost completely transformed into martensite during cooling is called the Mf point. In the production of a typical spot welded joint shown in FIG. 2, the temperature of the nugget 12 is cooled to below the Mf point, and the metallographic structure of the nugget 12 is mostly martensite.

[0019] In the post-heating, current is again passed through the sheet assembly to heat the nugget 12. The purpose of the post-heating is to temper the nugget 12, which has hardened due to cooling, and to soften the nugget 12. This increases the cross tensile strength of the weld. The current passed through the nugget 12 for the purpose of tempering is called post-heating current.

[0020] The martensite after the post-current treatment (i.e., tempering heat treatment) is called tempered martensite. The martensite before the post-current treatment is called fresh martensite. The hardness of tempered martensite is lower than that of fresh martensite. Therefore, the post-current treatment reduces the hardness of the nugget 12, and improves the cross tensile strength of the nugget 12. The amount of softening of the nugget 12 is an indicator of the effect of the post-current treatment.

[0021] (2. Effect of Shortening Cooling Time t2) The present inventors attempted to shorten the cooling time t2 in order to shorten the time required for manufacturing spot welded joints. However, it was found that shortening the cooling time t2 may result in an increase in the hardness of the nugget 12 after post-energization.

[0022] The reason for this lies in the temperature of the nugget 12 at the end of cooling (i.e., at the start of post-current application). In the manufacture of a normal spot-welded joint, the cooling time t2 is set long enough to lower the temperature of the nugget to the Mf point or below before starting post-current application (see FIG. 2). On the other hand, if the cooling time t2 is shortened, the temperature of the nugget 12 at the start of post-current application will be above the Mf point (see FIG. 1). As a result, post-current application will start with austenite remaining in the nugget 12.

[0023] The austenite remaining in the nugget 12 at the start of the post-current conduction becomes hard fresh martensite after the end of the post-current conduction. The fresh martensite increases the hardness of the nugget 12.

[0024] From the viewpoint of reducing the hardness of the nugget 12, it is preferable to start post-energization after lowering the temperature of the nugget 12 to the Mf point or lower and converting all austenite to martensite. This prevents fresh martensite from being generated in the nugget 12 after post-energization. However, in order to shorten the manufacturing time of the spot-welded joint, it is desirable to shorten the cooling time t2.

[0025] The inventors conducted a further investigation into the relationship between the temperature of the nugget 12 at the start of post-current application and the hardness of the nugget 12 after the end of post-current application by a replicated thermal cycle test. The replicated thermal cycle test replicates the thermal cycles that the heat-affected zone of the base metal and the weld metal undergo during welding, and examines changes in base metal properties and evaluates the phase transformation behavior (microstructure, mechanical performance) of the resulting weld metal. It is difficult to measure the interelectrode temperature during spot welding. On the other hand, the replicated thermal cycle test makes it possible to accurately evaluate the relationship between the temperature at the start of post-current application and the hardness after the end of post-current application.

[0026] The inventors conducted a simulated thermal cycle test simulating the main current, cooling, and post-current of spot welding on test steel sheets simulating a sheet assembly that is the base material for spot welding. In the simulated thermal cycle test, the maximum heating temperature in the heating simulating the main current of spot welding, the cooling rate in the cooling simulating the cooling of spot welding, and the maximum heating temperature in the heating simulating the post-current were the same. However, the cooling time t2 in the cooling (i.e., cooling end temperature) and the heating rate in the heating simulating the post-current (post-current heating rate) were varied within a range that can be adopted in actual spot welding.

[0027] Figure 3 shows a schematic diagram of the correlation between the cooling end temperature, the post-current heating rate, and the hardness of the test steel sheet after the post-current heating, obtained from the results of the above experiments. The inventors discovered that when the cooling end temperature is near the Ms point, the post-current heating has almost no effect on softening the test steel sheet. On the other hand, it was also revealed that the nugget can sometimes be softened even when the cooling end temperature is equal to or higher than the Mf point. When the cooling end temperature exceeds the Mf point, the softening amount of the test steel sheet may decrease, but it does not immediately disappear. For example, if the cooling end temperature T2 is below (Ms + Mf) / 2°C, the hardness of the test steel sheet can be reduced. Therefore, it is possible to shorten the cooling time t2 while ensuring the softening amount of the nugget 12.

[0028] Furthermore, the inventors have found that the slower the post-current heating rate is, the softer the nugget 12 becomes after the end of post-current heating. The dashed line graph in Figure 3 shows the correlation between the cooling end temperature T2 and hardness under the condition that the post-current heating rate is high. The solid line graph in Figure 3 shows the correlation between the cooling end temperature T2 and hardness under the condition that the post-current heating rate is low. When the post-current heating rate is low, the softening amount of the nugget 12 can be increased even if the cooling end temperature T2 is extremely high.

[0029] The cause of this phenomenon is presumed to be bainite transformation. As described above, when post-energization is started in a temperature range above the Mf point where austenite remains, the austenite transforms into fresh martensite. However, if the post-energization heating rate is small, part of the austenite transforms into bainite. The hardness of bainite is smaller than that of fresh martensite. Therefore, by reducing the post-energization heating rate, the amount of bainite in the nugget 12 can be increased and the amount of fresh martensite can be reduced. This can reduce the hardness of the nugget 12.

[0030] As explained above, when the post-energization heating rate is set to a low value, the cooling end temperature T2 can be increased and the cooling time t2 can be shortened. However, setting the post-energization heating rate to a low value increases the time required for post-energization (post-energization time t3). If the post-energization heating rate is set too low, the time required to manufacture a spot-welded joint will actually be longer. Therefore, it is most desirable to adopt a post-energization heating rate that minimizes the total cooling time and post-energization time. That is, (1) a range of combinations of cooling end temperature T2 and post-energization heating rate that can ensure the required joint strength of the nugget 12 is identified in advance, (2) a combination of cooling end temperature T2 and post-energization heating rate that can minimize the total value of cooling time t2 and post-energization time t3 within that range is identified, and (3) after the temperature of the nugget 12 falls below the cooling end temperature T2, post-energization is started promptly at the applied post-energization heating rate, thereby further shortening the time required to manufacture a spot welded joint while ensuring the joint strength of the nugget 12.

[0031] The method for manufacturing a spot welded joint according to the present disclosure, which has been obtained based on the above findings, includes, as shown in FIG. 4 , (S1) a step of performing a reproduced thermal cycle test on a test steel sheet multiple times, simulating the main current application, cooling, and post-current application of spot welding, and varying the cooling end temperature and the heating rate in the post-current application; (S2) a step of measuring the hardness of the test steel sheet after the test; (S3) a step of identifying the correlation between the cooling end temperature and the heating rate in the post-current application in the test, and the hardness of the test steel sheet after the reproduced thermal cycle test; (S4) a step of identifying a combination of the end temperature and the heating rate that makes the hardness of the test steel sheet equal to or less than a target value and that minimizes the total time required for cooling and post-current application; (S5) a step of applying a main current to a sheet set consisting of two or more stacked steel sheets 11 using a pair of electrodes for spot welding; and (S6) a step of cooling the sheet set by reducing the current while applying pressure to the sheet set using the pair of electrodes. (S7) A process of applying a post-current to the nugget 12 formed into the sheet assembly by the main current application and cooling, using a pair of electrodes, wherein the weighted average of the carbon content of the steel plate 11, weighted by the thickness of the steel plate 11, is made approximately equal to the carbon content of the test steel plate, the cooling is terminated after the point in time when it is estimated that the temperature of the nugget 12 has fallen below the end temperature specified by the reproduced thermal cycle test, and the heating rate in the post-current application is set to be equal to or lower than the heating rate specified by the reproduced thermal cycle test.

[0032] (S1 Multiple Tests) Prior to spot welding, a test simulating spot welding is performed on test steel sheets simulating a sheet assembly that is a base material for spot welding.

[0033] The carbon content of the test steel plate is set to be the same as the carbon content of the sheet assembly. When the carbon contents of the multiple steel plates 11 included in the sheet assembly are different, the weighted average value of the carbon contents of the steel plates 11, with the plate thickness of the steel plates 11 used as the weight, is considered to be the carbon content of the sheet assembly. The carbon content is the element that has the greatest effect on the Ms point and the Mf point. By setting the carbon content of the sheet assembly and the carbon content of the test steel plate to be the same, the Ms point and the Mf point of the test steel plate can be set to values ​​at the same level as the Ms point and the Mf point of the nugget 12 formed in the sheet assembly.

[0034] The test involves heating to simulate the actual current flow during spot welding, cooling to simulate the cooling of spot welding, and heating to simulate the post-current flow during spot welding. The test is a simulated thermal cycle test, which allows the temperature of the test steel to be accurately measured and controlled.

[0035] In the heating simulating actual current application for spot welding, the test steel sheet is heated. Note that, during actual current application for spot welding, the steel sheet is heated to or above its melting point, but in the heating simulating actual current application, it is not necessary to heat the test steel sheet to the melting point. It is sufficient to heat the test steel sheet to an extent that the structure of the test steel sheet is austenitized. For example, in the heating simulating actual current application for spot welding, the inventors heated the test steel sheet to 900°C and held it for 5 minutes, thereby austenitizing the structure of the test steel sheet. Heating equivalent to this may be performed in the heating simulating actual current application for spot welding.

[0036] In the cooling process simulating the cooling of spot welds, the temperature of the test steel sheet is lowered. The cooling end temperature T2 is varied in multiple tests. This allows data to be obtained to identify the correlation between the cooling end temperature T2 and hardness. Preferably, the cooling rate is the same in all tests. The cooling end temperature T2 is controlled via the cooling time t2.

[0037] In the heating simulating post-energization of spot welding, the test steel sheet is reheated. The reheating is performed up to the tempering temperature of the test steel sheet. The tempering temperature of the test steel sheet is, for example, 700°C. The heating rate is varied in multiple tests. This makes it possible to obtain data for identifying the correlation between the post-energization heating rate and hardness. Hereinafter, the heating rate in the heating simulating post-energization may also be referred to as the post-energization heating rate for convenience.

[0038] (S2 Hardness measurement of test steel sheet) The hardness of the test steel sheet after the test is completed is measured. The hardness measurement may be performed in accordance with, for example, JIS Z 2244:2009 "Vickers hardness test - Test method." The test force and other test conditions are the same for all hardness measurements. Specific test conditions are not particularly limited, and appropriate conditions can be selected depending on the hardness of the test steel sheet, etc. In addition, the Vickers hardness measurement conditions for both end portions 12E of the nugget 12 described below may be applied to the measurement of the hardness of the test steel sheet.

[0039] (S3 Identifying the Correlation Between Cooling End Temperature T2, Post-Current Heating Rate, and Hardness of Test Steel Sheet) Following the test and hardness measurement, the correlation between the cooling end temperature T2 in the test, the post-current heating rate, and the hardness of the test steel sheet after the test is identified. For example, as shown in Figure 3, a graph showing the correlation between the cooling end temperature T2 and hardness can be created for each post-current heating rate.

[0040] (S4 Identification of Cooling End Temperature T2 and Post-Current Heating Rate) Based on the correlation obtained by the above procedure, a combination of an upper limit value of the cooling end temperature T2 and a lower limit value of the post-current heating rate is identified, which sets the hardness of the test steel sheet at or below the target value and minimizes the total time required for cooling and post-current heating. The procedure for identifying the cooling end temperature T2 and the post-current heating rate will be described below with reference to FIG.

[0041] The dashed-dotted line graph illustrated in Figure 3 shows the correlation between the cooling end temperature T2 and the hardness of the test steel sheet when the post-current heating rate is high. When the post-current heating rate is high, the hardness of the test steel sheet can be made equal to or less than the target hardness if the cooling end temperature T2 is X°C or lower. The solid line graph illustrated in Figure 3 shows the correlation between the cooling end temperature T2 and the hardness of the test steel sheet when the post-current heating rate is low. When the post-current heating rate is low, the hardness of the test steel sheet can be made equal to or less than the target hardness if the cooling end temperature T2 is Y°C or lower.

[0042] 3, increasing the post-current heating rate shortens the time required for post-current heating, i.e., the post-current heating time t3. On the other hand, increasing the post-current heating rate lowers the upper limit of the cooling end temperature T2 at which the target hardness can be achieved. This lengthens the time required for cooling, i.e., the cooling time t2.

[0043] Based on the data obtained by the above-described tests, it is possible to identify a combination of the cooling end temperature T2 and the post-current heating rate that can achieve the target hardness. Furthermore, since the main current temperature T1 and the cooling rate are constant, it is possible to identify the cooling time t2 corresponding to the cooling end temperature T2 identified in the above procedure. Furthermore, since the post-current temperature T3 is constant, it is possible to identify the post-current heating time t3 corresponding to the cooling end temperature T2 and the post-current heating rate identified in the above procedure. This makes it possible to identify a combination of the cooling end temperature T2 and the post-current heating rate that makes the hardness of the test steel sheet equal to or less than the target value and minimizes the cooling time t2 and the post-current heating time t3.

[0044] After determining the combination of the upper limit of the cooling end temperature T2 and the lower limit of the post-energization heating rate using the above procedure, spot welding is performed on the sheet pair. The spot welding includes main energization, cooling, and post-energization. As described above, the carbon content of the sheet pair to be spot welded is set to be the same as the carbon content of the test steel sheets.

[0045] (S5 Main current application) First, a pair of electrodes for spot welding is used to apply main current to a sheet assembly consisting of two or more stacked steel sheets 11. This causes the sheet assembly to melt between the pair of electrodes. The main current application conditions are not particularly limited. Main current application conditions suitable for the sheet assembly can be selected as appropriate. Generally, conditions that ensure the nugget diameter and keep the amount of expulsion within an acceptable range are applied to the main current application.

[0046] (S6 Cooling) Next, the current is reduced while the pair of electrodes are still pressing the sheet assembly. Preferably, the current is stopped and the current flowing through the electrodes is set to zero. Spot welding electrodes have water-cooling holes, and the tip of the electrode is cooled by a refrigerant during spot welding. By setting the resistance heat value to zero or a value close to zero while the electrodes are in contact with the sheet assembly, the molten metal can be cooled and a nugget 12 can be formed.

[0047] The cooling end point is determined based on the combination of the cooling end temperature T2 and the post-energization heating rate determined through the above-described test. Cooling is ended as early as possible after the point in time when the temperature of the nugget 12 is estimated to have fallen below the cooling end temperature T2 determined through the test.

[0048] (S7 Post-current application) A pair of electrodes is used to apply a post-current to the nugget 12 formed into a sheet assembly by the main current application and cooling. The post-current application temperature T3, i.e., the maximum heating temperature in the post-current application, is set to the tempering temperature of the nugget 12. This tempers the nugget 12, thereby reducing its hardness. The post-current application temperature T3 is set to the same as the maximum heating temperature in the heating that simulates the post-current application.

[0049] The post-current heating rate is determined based on the combination of the cooling end temperature T2 and the post-current heating rate that minimizes the cooling time t2 and the post-current heating rate, as determined through the above-mentioned tests. The post-heating current in the post-current is controlled so that the post-current heating rate is equal to or less than the post-current heating rate determined through the tests and is as large as possible. It is not necessary to constantly control the post-current heating rate from the start to the end of the post-current. It is sufficient that the average heating rate during the post-current is below the upper limit determined through the tests.

[0050] The above procedure allows the second welding to begin immediately after the first weld is created. The conditions obtained through the test can of course be used for multiple spot welding.

[0051] (Effects) In the manufacturing method of a spot-welded joint according to the present disclosure, a test simulating spot welding is first conducted. Then, through the test, a favorable combination of the cooling end temperature T2 and the post-energization heating rate is identified. This combination allows the hardness of the nugget 12 to be equal to or less than the target hardness. Furthermore, this combination allows both the cooling time t2 and the post-energization time t3 to be shortened.

[0052] In the method for manufacturing a spot-welded joint according to the present disclosure, spot welding is performed to achieve a favorable combination of the cooling end temperature T2 and the post-energization heating rate identified through testing, thereby enabling a spot-welded joint having a nugget 12 that has been favorably tempered to be manufactured in a short period of time.

[0053] The most basic aspect of the method for manufacturing a spot welded joint according to this embodiment has been described above. Below, more preferred aspects of the method for manufacturing a spot welded joint according to this embodiment will be described.

[0054] (Target Hardness Value) The target hardness value of the test steel sheet used when specifying the combination of the cooling end temperature T2 and the post-energization heating rate is not particularly limited. The cross tensile strength required for the spot-welded joint is determined depending on, for example, the application, material, and shape of the spot-welded joint. The target hardness can be selected appropriately depending on the cross tensile strength required for the spot-welded joint.

[0055] The target hardness value can be determined, for example, based on the estimated maximum Vickers hardness HVmax of the test steel plate. The estimated maximum Vickers hardness HVmax is a value calculated using the following formula: HVmax = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30). The element symbols in the formula above represent the contents of the corresponding elements in the chemical composition of the test steel plate. C, Mn, Cr, and Mo in the formula above are elements that improve the hardenability of the steel. The estimated maximum Vickers hardness HVmax calculated using the formula above is the estimated hardness of the steel when quenched to maximize its hardness. For example, the target hardness value may be a value equal to or less than "HVmax of the test steel plate - HV150." This further improves joint strength (especially CTS).

[0056] (Test Steel Plate) As described above, the carbon content of the test steel plate is approximately the same as the carbon content of the spot-welded sheet pair. Preferably, the Mn content, Cr content, and Ni content of the test steel plate are also approximately the same as the Mn content, Cr content, and Ni content of the spot-welded sheet pair. More preferably, the chemical composition of the test steel plate is the same as the chemical composition of the spot-welded sheet pair. When the chemical compositions of the multiple steel plates 11 in the sheet pair are not the same, the Mn content, Cr content, and Ni content of the sheet pair are weighted average values ​​of the Mn content, Cr content, and Ni content of the steel plates 11, respectively, with the sheet thickness of the steel plates 11 used as the weight.

[0057] The closer the Ms and Mf points of the test steel sheets are to the Ms and Mf points of the sheet pair, the more preferable. By matching at least the carbon content of the test steel sheets with the carbon content of the sheet pair, the Ms and Mf points of the test steel sheets can be made closer to the Ms and Mf points of the sheet pair. On the other hand, by matching not only the carbon content of the test steel sheets but also the contents of other elements (e.g., Mn, Cr, and Ni, which affect the Ms and Mf points of steel) with the sheet pair, the Ms and Mf points of the test steel sheets become even closer to the Ms and Mf points of the sheet pair. This can further increase the reliability of the combination of end temperature and heating rate identified through the test.

[0058] (Method of Determining Cooling Time t2 and Post-heating Current) In spot welding, it is desirable to end cooling after the temperature of the nugget 12 actually falls below the cooling end temperature T2 determined by testing. However, during spot welding, it is not possible to directly measure the temperature of the nugget 12. Therefore, in spot welding, cooling is stopped after the point in time when it is estimated that the temperature of the nugget 12 has fallen below the cooling end temperature T2 determined by testing.

[0059] Simulation is one method for determining the time point at which the temperature of the nugget 12 is estimated to be below the cooling end temperature T2 determined by testing. By performing a simulation using variables such as the plate assembly configuration, the electrode configuration, and the welding pressure, it is possible to estimate the change in the temperature of the nugget 12 over time, as illustrated in FIG. 10 (described later). Therefore, the method for manufacturing a spot-welded joint according to the present disclosure preferably further includes a step S8 of estimating, by simulation, a cooling time and a post-heating current that can reproduce the combination of the upper limit of the cooling end temperature and the lower limit of the heating rate determined by the replicated thermal cycle test. In this case, the cooling time and post-heating time determined by the simulation are preferably applied to the cooling and post-heating of the plate assembly.

[0060] Another means for determining the time when the temperature of the nugget 12 is estimated to be below the cooling end temperature T2 determined by testing is the interelectrode resistance. The electrical resistance of steel is an indicator of the steel temperature. Therefore, the temperature of the nugget 12 can be estimated by measuring the current and voltage between the electrodes that sandwich the sheet assembly and calculating the interelectrode resistance. Note that, in order to obtain the interelectrode resistance, a current must be passed through the electrodes. However, the current flowing to obtain the interelectrode resistance does not interfere with cooling, because an extremely small current is sufficient to obtain the interelectrode resistance.

[0061] (Post-heating pattern) As exemplified in Fig. 1 and Fig. 11A, the post-heating current flowing during the post-heating may be constant. On the other hand, as exemplified in Fig. 11B, the post-heating current may be up-slope controlled during at least a portion of the post-heating period. Up-slope control refers to control in which the current is continuously increased. It is particularly preferable to use up-slope current for at least the first half of the post-heating, and more preferably to use up-slope current throughout the entire post-heating. This increases the amount of bainite in the nugget 12, thereby further increasing the amount of softening of the nugget 12.

[0062] Although the reason why the amount of bainite increases with upslope current application is unclear, the inventors speculate as follows. Bainite nucleation and growth are important for bainite transformation of steel and increasing the amount of bainite. According to classical nucleation theory, as the temperature of steel decreases, the driving force for bainite transformation increases, making nucleation more likely. Meanwhile, bainite growth occurs due to element diffusion. Therefore, as the temperature of steel increases and the element diffusion rate increases, bainite growth becomes more likely. Therefore, both the low-temperature region (nucleation) and the high-temperature region (growth) are important for increasing the amount of bainite. Current application pattern (b) (see FIG. 11B), which has a constant heating rate, is thought to be more advantageous for increasing the amount of bainite than current application pattern (a) (see FIG. 11A), which has a fast heating rate in the low-temperature region.

[0063] (Steel Type) The type of steel plate 11 in the sheet assembly is not particularly limited, but preferably, at least one steel plate 11 is a high-carbon steel plate. A high-carbon steel plate is a steel plate 11 having a carbon content of 0.20 mass% or more. This allows the spot-welded joint to be applied to mechanical parts that require high strength, such as automobile frame parts. It is known that spot welds in high-carbon steel plates are prone to embrittlement and a decrease in cross tensile strength. However, the manufacturing method for a spot-welded joint according to this embodiment allows the nugget 12 to be softened in a short time, thereby improving the cross tensile strength.

[0064] (Spot-welded joint) Next, a spot-welded joint 1 according to another aspect of the present disclosure will be described. The spot-welded joint according to this embodiment includes two or more stacked steel sheets 11 and a nugget 12 joining the steel sheets 11, and the metal structure of both end portions 12E of the nugget 12 at an imaginary line 13L along a joining interface 13 of the steel sheets 11, as measured at a cross section of the nugget 12, contains 5 to 40 area % of fresh martensite, and the remaining 95 area % or more is one or both of tempered martensite and bainite.

[0065] (Steel Plate 11 and Nugget 12) A spot welded joint includes two or more stacked steel plates 11 and a nugget 12. The nugget 12 is a molten and solidified portion that joins the steel plates 11. In the spot welded joint 1 according to this embodiment, the configuration of the nugget 12 is defined using a virtual line 13L along the joint interface 13. As shown in FIG. 5 , the joint interface 13 refers to the surfaces where the steel plates 11 contact and face each other. The joint interface 13 is sometimes referred to as a mating surface. However, the joint interface 13 disappears inside the nugget 12. In this embodiment, the virtual line 13L along the joint interface 13 is used to evaluate the nugget 12. When there are two steel plates 11, there is one joint interface 13 and one virtual line 13L. When there are three or more steel plates 11, there are two or more joint interfaces 13 and two or more virtual lines 13L.

[0066] When a cross section of the nugget 12 is observed, it can be seen that fresh martensite is contained in both end portions 12E of the nugget 12 on the imaginary line 13L along the joint interface 13. In the spot welded joint 1 according to this embodiment, the area fraction of fresh martensite in each of both end portions 12E of the nugget 12 on the imaginary line 13L along the joint interface 13 is specified.

[0067] The end 12E of the nugget 12 on the imaginary line 13L along the joint interface 13 refers to the region near the intersection of the outer edge of the nugget 12 (i.e., the fusion boundary) and the imaginary line 13L, as shown in Fig. 5. Two end portions 12E exist on one imaginary line 13L. When there are two or more imaginary lines 13L, two end portions 12E exist on each of the multiple imaginary lines 13L.

[0068] An end 12E of the nugget 12 on the imaginary line 13L along the joint interface 13 is an extremely important region for improving the joint strength of the nugget 12. This is because the end 12E along the joint interface 13 becomes a stress concentration portion in a cross tension test.

[0069] (Metal Structure of Both Ends 12E of Nugget 12) The metal structure of both end portions 12E of the nugget 12 contains 5 to 40 area % of fresh martensite. Furthermore, the regions other than fresh martensite are mainly composed of tempered martensite and / or bainite. For example, 95 area % or more of the regions other than fresh martensite is one or both of tempered martensite and bainite. In other words, the metal structure of both end portions 12E of the nugget 12 satisfies both of the following formulas: 5≦FM≦40 95≦(TM+B) / (100%−FM) Note that FM, TM, and B are the area fractions of fresh martensite, tempered martensite, and bainite in the metal structure of both end portions 12E of the nugget 12, respectively. The area fraction FM of fresh martensite is the value obtained by dividing the area of ​​fresh martensite in an observation field described below by the area of ​​the observation field. Similarly, the area fraction TM of tempered martensite is the value obtained by dividing the area of ​​tempered martensite in the observation field by the area of ​​the observation field, and the area fraction B of bainite is the value obtained by dividing the area of ​​bainite in the observation field by the area of ​​the observation field.

[0070] Martensite is a metal structure that is formed when austenite is rapidly cooled and transformed without diffusion. Fresh martensite is martensite that has not been tempered. Tempered martensite is a tempered martensite structure.

[0071] When cooling is performed to the Mf point or below and no post-current is applied, all of the austenite in the nugget 12 is transformed into martensite and maintained as such. Therefore, when no post-current is applied, all of the martensite in the nugget 12 becomes fresh martensite.

[0072] When post-energization is performed after cooling to the Mf point or lower, all of the austenite in the nugget 12 at the start of post-energization is transformed into martensite. When post-energization is performed on such a nugget 12, all of the martensite in the nugget 12 becomes tempered martensite.

[0073] When post-current is applied after cooling is stopped in a temperature range below the Ms point and above the Mf point, the nugget 12 at the start of post-current is a mixture of martensite and austenite that has not been transformed into martensite. The martensite present at the start of post-current becomes tempered martensite after post-current. The austenite present at the start of post-current is transformed into bainite during post-current or martensite and transformed into fresh martensite after post-current. Therefore, when post-current is applied after cooling is stopped in a temperature range below the Ms point and above the Mf point, the nugget 12 is a mixture of fresh martensite and tempered martensite and / or bainite. In other words, the spot-welded joint according to this embodiment can be manufactured by stopping cooling in a temperature range below the Ms point and above the Mf point before post-current application.

[0074] In the metal structure of both end portions 12E of the nugget 12, the area ratio of fresh martensite may be 6% or more, 8% or more, 10% or more, 15% or more, or 18% or more. In the metal structure of both end portions 12E of the nugget 12, the area ratio of fresh martensite may be 35% or less, 30% or less, 28% or less, 25% or less, or 20% or less. In the structure other than fresh martensite (the remainder structure), the proportion of tempered martensite and bainite may be 96 area% or more, 98 area% or more, or 100 area%.

[0075] (Method for Evaluating Metal Structure) The method for evaluating the content of fresh martensite is as follows. First, the nugget 12 is cut. The cut surface is positioned as close as possible to the center of the indentation 14 formed on the surface of the steel sheet 11. The cut surface is also perpendicular to the surface of the steel sheet 11. The cut surface is then polished and etched with nital.

[0076] The metal structure revealed by etching is observed using an SEM. The magnification of the SEM observation is 3000x. The area ratio of the metal structure is measured in an observation field that satisfies the following requirements. The end 12E shown in Figure 5 satisfies the following requirements: - It has a rectangular shape of 400 µm square - Its center is on the virtual line 13L along the joint interface 13 - One vertex coincides with the outer edge (fusion boundary) of the nugget 12 - Two sides are parallel to the virtual line 13L along the joint interface 13 From the observed image, the area ratio of each structure at both end parts 12E is measured and the average value is calculated. When the number of steel plates 11 is three or more, the area ratio of the structure is measured at both ends of each of two or more joint interfaces 13 and the average value is calculated. The method for measuring the area ratio of the structure is as follows.

[0077] Areas where the substructure is not revealed and have low brightness are considered to be ferrite. Areas that are a lamellar structure of ferrite and cementite are considered to be pearlite. Areas where the substructure is not revealed and have high brightness are considered to be fresh martensite or retained austenite. Areas where the substructure is revealed are considered to be tempered martensite or bainite. When specifying the total area fraction of tempered martensite and bainite in the remainder of the metal structure, i.e., the areas other than fresh martensite in the metal structure, it is not necessary to distinguish between tempered martensite and bainite. A method for distinguishing between tempered martensite and bainite for the purpose of specifying the area fraction of bainite will be described later.

[0078] Fresh martensite and retained austenite cannot be clearly distinguished by SEM observation. Therefore, the area fraction of fresh martensite is calculated by subtracting the volume fraction of retained austenite from the area fraction of the structure determined to be fresh martensite or retained austenite. The volume fraction of retained austenite is quantified from the (200) and (210) area integral intensities of ferrite and the (200), (220), and (311) area integral intensities of austenite using MoKα radiation. Note that although area fraction and volume fraction are different concepts, they can be considered to be essentially the same value. Therefore, when calculating the area fraction of fresh martensite, the volume fraction of retained austenite obtained by the above procedure is considered to be the area fraction.

[0079] (Effects) In the spot-welded joint according to this embodiment, the metal structure of both end portions 12E of the nugget 12 contains 5 area % or more of fresh martensite. Such a nugget 12 can be produced by stopping cooling in a temperature range below the Ms point and above the Mf point, and then applying post-current. Therefore, the spot-welded joint according to this embodiment can be produced by spot welding with a shortened cooling time t2, resulting in high production efficiency.

[0080] In the spot-welded joint according to this embodiment, the amount of fresh martensite in the metallographic structure of each end 12E of the nugget 12 is 40 area % or less, and the remaining structure is mainly composed of tempered martensite and / or bainite, thereby increasing the toughness of the nugget 12 and the cross tensile strength of the spot-welded joint.

[0081] The most basic aspect of the spot welded joint according to this embodiment has been described above. Below, more preferred aspects of the spot welded joint according to this embodiment will be described.

[0082] (Steel Type) The type of steel plate 11 in the spot-welded joint is not particularly limited, but preferably, at least one steel plate 11 is a high-carbon steel plate. A high-carbon steel plate is a steel plate 11 having a carbon content of 0.20 mass% or more. This allows the spot-welded joint to be used in mechanical parts that require high strength, such as automobile frame parts. It is known that spot welds in high-carbon steel plates are prone to embrittlement and a decrease in cross tensile strength. However, the method for manufacturing a spot-welded joint according to this embodiment allows the nugget 12 to be softened in a short time, thereby improving the cross tensile strength.

[0083] (Hardness of Nugget 12) The nugget 12 of the spot-welded joint according to this embodiment is preferably softer than a nugget 12 that is not subjected to post-energization. For example, it is preferable that the value obtained by subtracting the Vickers hardness of both end portions 12E of the nugget 12 from the estimated maximum Vickers hardness HVmax be HV50 or more.

[0084] The estimated maximum Vickers hardness HVmax is a value calculated using the following formula: HVmax = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30). The element symbols in the formula above represent the content of the corresponding elements in the chemical composition of the steel sheet 11. When a spot-welded joint includes multiple steel sheets 11 with different chemical compositions, the chemical composition of the steel sheet 11 is a weighted average of the chemical compositions of the steel sheets 11, with the sheet thickness used as the weight. C, Mn, Cr, and Mo in the formula above are elements that improve the hardenability of steel. The estimated maximum Vickers hardness HVmax calculated using the formula above is the estimated hardness of steel when quenched to maximize the hardness of the steel. Therefore, the estimated maximum Vickers hardness HVmax is an indicator of the hardness of the nugget 12 obtained by spot welding without post-heat application.

[0085] When the value obtained by subtracting the Vickers hardness of both end portions 12E of the nugget 12 from the estimated maximum Vickers hardness HVmax is HV50 or more, both end portions 12E of the nugget 12 are sufficiently tempered. Therefore, the cross tensile strength of the spot welded joint is further increased.

[0086] The chemical components of the steel plate 11 used to calculate the estimated maximum Vickers hardness may be measured by a common method. For example, a portion of the steel plate 11 is cut to collect chips. Then, the chemical components of the chips may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) in accordance with JIS G 1201:2014, for example. Note that the C content, which is difficult to measure using ICP-AES, may be measured using a combustion-infrared absorption method.

[0087] As described in JIS G 0417:1999, the analysis sample is taken so as to obtain an average chemical composition across the entire thickness of the steel plate 11. Specifically, the analysis sample is taken from the t / 4 position, avoiding the ends of the steel plate 11 in the width direction.

[0088] The Vickers hardness of both end portions 12E of the nugget 12 is measured in the following manner. The hardness of the nugget 12 is continuously measured on an imaginary line 13L along the joining interface 13 of the steel sheets 11. The measurement interval is 100 μm. The test force is 200 gf. The minimum hardness value in a region within 400 μm from the end of the nugget 12 is regarded as the hardness of the end portion 12E of the nugget 12. Then, the average value of the hardness at both end portions 12E of the nugget 12 is calculated. When the number of steel sheets 11 is three or more, the hardness is measured at both ends of each of two or more joining interfaces 13, and the average value is calculated.

[0089] (Area Ratio of Bainite) The metal structure of both end portions 12E of the nugget 12 preferably contains bainite. For example, the metal structure of both end portions of the nugget preferably contains 2 area % or more of bainite. More preferably, the metal structure of both end portions of the nugget contains 5 area % or more, 10 area % or more, or 15 area % or more of bainite. The area ratio B of bainite is the value obtained by dividing the area of ​​bainite in the observation field by the area of ​​the observation field.

[0090] The hardness of bainite is lower than that of fresh martensite. Therefore, by replacing fresh martensite with bainite, the hardness of the nugget 12 can be reduced. Furthermore, when manufacturing a nugget 12 with a large amount of bainite, the cooling time t2 can be shortened. In other words, a spot welded joint with an increased amount of bainite can be manufactured in a shorter time.

[0091] For example, in the metal structure at both ends of the nugget, the amount of remaining bainite may be 50 area % or less, 45 area % or less, or 40 area % or less.

[0092] The method for measuring the amount of bainite in the metal structure of both end portions 12E of the nugget 12 is as follows. First, by using the above-mentioned means, a region where the substructure is exposed is identified in the observation field of both end portions 12E of the nugget 12. The region where the substructure is exposed is either tempered martensite or bainite. Therefore, by distinguishing between tempered martensite and bainite, the area ratio of bainite can be identified.

[0093] Crystal orientation information of the metal structure in the observed field of view is obtained by electron backscatter diffraction. The measurement interval is 0.2 μm. An EBSD analyzer is used for the measurement. The EBSD analyzer used by the inventors consists of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector). Naturally, the area fraction of bainite can also be measured using an equivalent device to the above. During the measurement, the degree of vacuum in the device is 9.6 × 10-5 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16.

[0094] The obtained crystal orientation information is analyzed using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution. The measurement points between which the crystal orientation difference is 15° or more are considered to be crystal grain boundaries. The area surrounded by the crystal grain boundaries is considered to be crystal grains. The GAM value (Grain Average Misorientation value) of each of the multiple crystal grains is then measured. The GAM value is obtained by calculating the difference in crystal orientation between all measurement points within the crystal grain. The average value of the difference in crystal orientation is the GAM value of the crystal grain.

[0095] Next, in the grains with a GAM value of more than 0.5°, the boundaries with a misorientation of more than 5° are displayed. The density of the boundaries with a misorientation of more than 5° within the grain, i.e., the length of the grain boundaries with a misorientation of more than 5° per unit area, is calculated. The density of the boundaries with a misorientation of more than 5° is referred to as the 5° boundary density of the grain.

[0096] In the area where the substructure is exposed, the GAM value is greater than 0.5° and the 5° boundary density is 0.4 μm / μm ２ Crystal grains that satisfy the following conditions are considered to be bainite. By measuring the area ratio of crystal grains that are considered to be bainite, the area ratio of bainite contained in the remainder in the metal structure at both ends of the nugget can be calculated.

[0097] (Other Configurations) While the embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit of the disclosure. More preferred examples of the spot-welded joint 1 and the method for manufacturing the spot-welded joint 1 according to the present embodiments will be described below. Unless otherwise specified, the preferred aspects described below can be applied to both the spot-welded joint 1 and the method for manufacturing the spot-welded joint 1.

[0098] (Tensile strength, hardness, thickness, and surface treatment of steel plate 11) The tensile strength of the steel plate 11 is not particularly limited. From the viewpoint of improving the rigidity and fracture resistance of the spot-welded joint 1, it is preferable that one or more of the plurality of steel plates 11 be a high-carbon steel plate having a tensile strength of 980 MPa or more. It is more preferable that the tensile strength of the high-carbon steel plate be 1000 MPa or more, 1200 MPa or more, or 1500 MPa or more. On the other hand, one or more of the plurality of steel plates 11 may be a mild steel having a tensile strength of 500 MPa or less.

[0099] The hardness of the steel plates 11 is not particularly limited. From the viewpoint of improving the rigidity and fracture resistance of the spot-welded joint 1, it is preferable that one or more of the plurality of steel plates 11 be a high-carbon steel plate having a hardness of HV 330 or more. It is more preferable that the hardness of the high-carbon steel plate be HV 350 or more, HV 400 or more, or HV 450 or more. On the other hand, one or more of the plurality of steel plates 11 may be mild steel.

[0100] The thickness of the steel plates 11 is not particularly limited. The plate thickness ratio of the plate assembly is also not particularly limited. The plate thickness ratio of the plate assembly is the value obtained by dividing the total plate thickness of the plate assembly by the thickness of the thinner of the steel plates 11 arranged on the surface of the plate assembly. For example, when the spot welded joint 1 is an automobile part, it is preferable that the steel plates 11 be two thick high-carbon steel plates and one thin mild steel plate. The high-carbon steel plates are automobile frame members. The mild steel plates are automobile exterior members. The plate thickness of the high-carbon steel plates is preferably 1.0 to 2.5 mm, for example. The plate thickness of the mild steel plates is preferably 0.4 to 1.2 mm, for example.

[0101] In order to improve corrosion resistance, aesthetics, etc., it is preferable that a plating is provided on the surface of the steel sheet 11. Examples of the type of plating include Al-based plating, Al-based alloy plating, Zn-based plating, and Zn-based alloy plating.

[0102] (Diameter of nugget 12) The diameter of the nugget 12 is not particularly limited and can be appropriately selected within a range that does not cause poor bonding. For example, the diameter of the nugget 12 may be within a range of 2.5√t to 5.0√t. From the viewpoint of preventing poor bonding, it is more preferable that the diameter of the nugget 12 be 2.8√t or more, 3.0√t or more, or 3.2√t or more. Furthermore, from the viewpoint of suppressing the amount of heat input and preventing expulsion, it is more preferable that the diameter of the nugget 12 be 4.8√t or less, 4.5√t or less, or 4.2√t or less.

[0103] The diameter of the nugget 12 refers to the diameter of the nugget 12 measured at the joint interface 13 by a cross-sectional test of the weld. t refers to the average value of the plate thicknesses of the two steel plates 11 constituting the joint interface 13 at which the diameter of the nugget 12 is measured. When the number of steel plates 11 is three or more and the number of joint interfaces 13 is two or more, it is preferable that the diameter of the nugget 12 at one or more joint interfaces 13 is within the above-mentioned range, and it is even more preferable that the diameter of the nugget 12 at all joint interfaces 13 is within the above-mentioned range.

[0104] The effects of one embodiment of the present disclosure will be explained in more detail using examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure. The present disclosure is not limited to this example of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0105] (Experiment 1: Reproduced Heat Cycle Test) The present inventors conducted a reproduced heat cycle test simulating spot welding. The test steel sheet and the conditions for the reproduced heat cycle test were as follows: Type of test steel sheet: 2 GPa-class hot stamping steel sheet Thickness of test steel sheet: 2.0 mm Ms point of test steel sheet: approximately 360°C Mf point of test steel sheet: approximately 180°C Thermal profile in the reproduced heat cycle test: as shown in Figure 6

[0106] As shown in Figure 6, the heating simulating the main current was performed under constant conditions. Furthermore, in the heating simulating the post-current, the maximum heating temperature was kept constant. Furthermore, the cooling rates after the main current and the post-current were also kept constant. Meanwhile, the cooling end temperature T2 (i.e., the post-current temperature) was varied. The post-current heating rate was set to either 400°C / sec or 100°C / sec.

[0107] Furthermore, the hardness of the steel sheet after the simulated thermal cycle test was measured, and the correlation between the cooling end temperature T2 and the hardness of the steel sheet after the simulated thermal cycle test was identified and shown in FIG.

[0108] When the cooling end temperature was near the Ms point (approximately 360°C), post-current application had almost no effect on softening the test steel sheet, regardless of the heating rate in the post-current application. On the other hand, it was also confirmed that the test steel sheet was sufficiently softened when the cooling end temperature was approximately 280°C or lower. When the test steel sheet was cooled to the Mf point (approximately 180°C) or lower and then tempered, the hardness of the test steel sheet was approximately 300 HV. When the cooling end temperature was set to 280°C or higher than the Mf point, the hardness of the test steel sheet could be made approximately 300 HV.

[0109] Furthermore, by reducing the heating rate in the reheating simulating post-energization, it was possible to expand the range of the cooling end temperature T2 at which the hardness of the test steel sheet could be reduced. When the heating rate was 400°C / sec, the hardness of the test steel sheet increased when the cooling end temperature T2 exceeded approximately 280°C. On the other hand, when the heating rate was 100°C / sec, the hardness of the test steel sheet could be approximately HV300 as long as the cooling end temperature T2 was approximately 320°C or less.

[0110] 8A to 8E show SEM photographs of the test steel sheets after the simulated thermal cycle test. The simulated thermal cycle test conditions for these test steel sheets were as follows: Fig. 8A: Cooling end temperature T2: room temperature, no post-current application; Fig. 8B: Cooling end temperature T2: 160°C, post-current application heating rate: 100°C / sec; Fig. 8C: Cooling end temperature T2: 320°C, post-current application heating rate: 100°C / sec; Fig. 8D: Cooling end temperature T2: 360°C, post-current application heating rate: 100°C / sec; Fig. 8E: Cooling end temperature T2: room temperature, post-current application heating rate: 100°C / sec

[0111] In the test steel plate of Fig. 8A, all of the martensite was fresh martensite, and no carbide precipitation was observed in the microstructure photograph of this test steel plate.

[0112] In the replicated thermal cycle test to obtain the test steel plate of Fig. 8B, the cooling end temperature T2 was equal to or lower than the Mf point. The test steel plate of Fig. 8B was sufficiently softened after the replicated thermal cycle test. The presence of precipitated carbides can be confirmed in the microstructure photograph of Fig. 8B. The areas where carbides precipitated are tempered martensite or bainite.

[0113] In the replicated thermal cycle test to obtain the test steel sheet of Fig. 8C, the cooling end temperature T2 was 320°C, which is slightly above the Mf point. However, the test steel sheet of Fig. 8C was sufficiently softened after the replicated thermal cycle test. In the microstructure photograph of Fig. 8C, as in Fig. 8B, the presence of precipitated carbides can be confirmed.

[0114] In the replicated thermal cycle test to obtain the test steel plate of Fig. 8D, the cooling end temperature T2 was 360°C, which was significantly higher than the Mf point. However, the test steel plate of Fig. 8D did not soften sufficiently after the replicated thermal cycle test. Almost no carbides can be confirmed in the structural photograph of Fig. 8D.

[0115] In the replicated thermal cycle test to obtain the test steel sheet shown in Fig. 8E, the cooling end temperature T2 was room temperature. The test steel sheet shown in Fig. 8E was sufficiently softened after the replicated thermal cycle test. The presence of precipitated carbides can be confirmed in the microstructure photograph of Fig. 8E.

[0116] The above experimental results show that tempered martensite and bainite can be formed in the nugget even if the cooling end temperature T2 is slightly higher than the Mf point. On the other hand, if the cooling end temperature T2 is near the Ms point, tempered martensite and bainite cannot be obtained.

[0117] (Experiment 2: Formaster Test) The inventors conducted a Formaster test to investigate the effect of the temperature rise rate during reheating simulating post-energization on hardness. In the Formaster test, the thickness of the test steel sheet was measured while the test steel sheet was subjected to a simulated thermal cycle test simulating spot welding. The steel sheet used in the Formaster test was the same as the test steel sheet in Experiment 1. The results of the Formaster test are shown in Figure 9. The horizontal axis of the graph shown in Figure 9 represents temperature, and the vertical axis represents the change in thickness of the steel sheet.

[0118] In the graph of FIG. 9 , the section marked with symbol (1) corresponds to heating simulating actual current conduction in spot welding. In the heating simulating actual current conduction, the temperature of the test steel sheet was held at 900°C for 5 minutes. The section marked with symbol (2) corresponds to cooling simulating cooling in spot welding. Cooling was performed to 320°C. The section marked with symbol (3) corresponds to reheating simulating post-current conduction. Reheating was performed to 700°C. The section marked with symbol (4) corresponds to cooling after post-current conduction. Cooling after post-current conduction was performed to room temperature.

[0119] What should be noted in the graph of Figure 9 is the increase in thickness of the test steel plate in the sections marked with symbols (3) and (4). At the beginning of these sections, the thickness of the test steel plate increased or decreased monotonically. However, in the middle of these sections, the thickness of the test steel plate increased suddenly. This is presumably due to bainite transformation.

[0120] (Experiment 3: Estimation of Cooling Conditions Using Spot Welding Analysis Software) The relationship between the cooling time and the cooling end temperature T2 in spot welding of the test steel sheets (hot stamped steel sheets with a thickness of 2.0 mm and a tensile strength of 2 GPa) used in Experiments 1 and 2 was simulated using spot welding analysis software SORPAS (registered trademark). The simulation conditions for spot welding were as follows: Number of test steel sheets: 2 Pressing force: 3.92 kN Main current time: 0.36 seconds Welding current during main current: 8.0 kA Nugget diameter: 7 mm The simulation results are shown in FIG. 10 .

[0121] According to the simulation results, the time required to raise the temperature of the test steel plate to 320°C is 50 cycles (1.0 second). The time required to raise the temperature of the test steel plate to 280°C is 55 cycles (1.1 seconds). Considering the results of Experiment 1 and the simulation results, it was estimated that the suitable current application conditions for the test steel plate are as shown in Table 1.

[0122]

[0123] The post-energization time (D) listed in Table 1 is a value calculated using the following formula: (D) = 50 cycles / sec × {700°C - (B)} / (A). In the formula, "700°C" is the maximum temperature during post-energization (see FIG. 6). "700°C - (B)" is the temperature rise required for post-energization. "50 cycles / sec" in the formula is the number of cycles per second. As shown in Table 1, the cooling time can be shortened by reducing the temperature rise rate during post-energization. However, reducing the temperature rise rate during post-energization increases the post-energization time. Therefore, reducing the temperature rise rate during post-energization is not always desirable. For the test steel sheets used in this experiment, it was estimated that a temperature rise rate of 100°C / sec during post-energization would shorten the welding time.

[0124] (Experiment 4: Spot Welding Test) Taking into consideration the results of Experiments 1 to 3, a spot welding experiment was conducted under the following conditions. Plate assembly: Two test steel plates from Experiments 1 and 2 stacked together. Main current conditions: Main current value of 8.4 kA, current time of 0.36 seconds. Cooling time: (A) 40 cycles, (B) 50 cycles, or (C) 60 cycles. Post-current time: 1.98 seconds. Maximum post-heating current: 4 to 8 kA. Current waveform during post-current: either pattern a ( FIG. 11A ) or pattern b ( FIG. 11B ). Pattern a ( FIG. 11A ) is a current waveform in which the post-heating current is constant. Pattern b ( FIG. 11B ) is a current waveform in which the post-heating current is up-slope controlled. After the spot welding test, a cross tensile test was conducted on the spot-welded joint, and the cross tensile strength (CTS) was measured.

[0125] The correlation between the post-heating current setting and the cross tensile strength is shown in Figures 12A to 12C. Figure 12A shows the results of a spot welding test in which the cooling time was (A) 40 cycles. Figure 12B shows the results of a spot welding test in which the cooling time was (B) 50 cycles. Figure 12C shows the results of a spot welding test in which the cooling time was (C) 60 cycles.

[0126] As shown in Fig. 12A, when the cooling time was 40 cycles, no significant improvement in CTS was confirmed regardless of the post-heating conditions. On the other hand, as shown in Fig. 12C, when the cooling time was 60 cycles, an improvement in CTS was confirmed. In Fig. 12C, it is considered that tempered martensite was able to be created because the nugget was cooled to below the Mf point.

[0127] Furthermore, as shown in Figure 12B, even when the cooling time was 50 cycles, an improvement in CTS was confirmed under the conditions of current application pattern (b). In Figure 12B, the nugget cooling end temperature T2 exceeds the Mf point, and martensitic transformation was not completed at the start of post-current application. Therefore, it is presumed that fresh martensite was formed in the nugget. However, by suppressing the amount of fresh martensite, the CTS improvement effect of post-current application can be enjoyed.

[0128] The reason why better results were obtained with current conduction pattern (b) than with current conduction pattern (a) in the spot welding experiment of Figure 12B is thought to be as shown in Figure 13. Figure 13 shows the results of a simulation of the temperature of the weld in spot welding using current conduction pattern (a) or current conduction pattern (b). According to the simulation results, upslope control of the post-heating current reduces the amount of fluctuation in the heating rate. This is presumably what promoted the formation of bainite.

[0129] Finally, Fig. 14 shows the cross-sectional observation results and hardness measurement results of the nugget with the maximum CTS in the spot welding test with a cooling time of 50 cycles and a post-heat time of 55 cycles, and Fig. 15 shows the cross-sectional observation results and hardness measurement results of the nugget with the maximum CTS in the spot welding test with a cooling time of 60 cycles and a post-heat time of 55 cycles. Under both post-heat conditions, the edge of the nugget was sufficiently softened, and the CTS was 10 kN or more. The CTS of the spot-welded joint produced without post-heat was 6.5 kN. Therefore, it was confirmed that the effect of post-heat was exerted under both post-heat conditions.

[0130] (Experiment 4: Amount of Fresh Martensite in Nugget) Spot-welded joints were manufactured under various conditions, and their properties were evaluated. Specifically, the amounts of fresh martensite, tempered martensite, and bainite contained in the metallographic structure at both ends of the nugget, and the Vickers hardness at both ends of the nugget along a virtual line along the joint interface of the steel sheets were measured using the methods described above. The CTS of the spot-welded joints was also measured in accordance with JIS Z 3137:1999, "Specimen dimensions and test methods for cross tension tests on resistance spot- and projection-welded joints." The manufacturing conditions and evaluation results are shown in Table 2. The "FM" column indicates the area fraction of fresh martensite. The "TM" column indicates the area fraction of tempered martensite. "B" indicates the area fraction of bainite. "Hardness" indicates Vickers hardness.

[0131] Prior to the production of the spot-welded joints, a simulated thermal cycle test was conducted to simulate the main current, cooling, and post-current application of spot welding. Through the simulated thermal cycle test, a combination of the end temperature and heating rate was identified that would maintain the hardness of the test steel sheet at or below the target value and minimize the total time required for cooling and post-current application. In the spot welding of Examples 2, 3, and 5, cooling was completed after the nugget temperature was estimated to have fallen below the end temperature identified by the simulated thermal cycle test. Furthermore, the heating rate during post-current application was set to be equal to or lower than the heating rate identified by the simulated thermal cycle test.

[0132]

[0133] Post-energization was not performed in the spot welding of Example 1. In the spot-welded joint of Example 1, the structure of the nugget was entirely fresh martensite.

[0134] In the spot welding of Example 8, post-energization was performed. Furthermore, in the spot welding of Example 8, the cooling time was extended to allow the nugget to completely transform into martensitic metal before post-energization was initiated. As a result, the amount of fresh martensite in the spot-welded joint of Example 8 was reduced, maximizing the effect of post-energization. However, in the spot welding of Example 8, the total time required for the process of cooling the sheet assembly (cooling time) and the process of performing post-energization (post-energization time) was 4 seconds. From the viewpoint of improving manufacturing efficiency, it is desirable to reduce the total value of the cooling time and post-energization time while maintaining the effect of post-energization.

[0135] Post-energization was performed in the spot welding of Examples 2, 3, and 5. As a result, the amount of fresh martensite in the nuggets was reduced in the spot-welded joints of Examples 2, 3, and 5. The CTS of the spot-welded joints of Examples 2, 3, and 5 was significantly improved compared to Example 1. Furthermore, the spot welding of Examples 2, 3, and 5 allowed for significantly reduced cooling times and total values ​​compared to Example 8.

[0136] In the spot welding of Example 4, post-energization was performed. The total value of the cooling time and post-energization time in the spot welding of Example 4 was at the same level as in Example 3. However, in the spot welding of Example 4, the temperature rise rate in the post-energization was set to a value exceeding the temperature rise rate determined by the reproduced thermal cycle test. In the spot-welded joint of Example 4, the amount of fresh martensite was not sufficiently reduced. The CTS of the spot-welded joint of Example 4 was only slightly improved compared to Example 1. In other words, in the spot welding of Example 4, the effect of post-energization was hardly obtained.

[0137] In the spot welding of Examples 6 and 7, post-energization was performed. The temperature rise rate of post-energization in the spot welding of Examples 6 and 7 was the same as that of Examples 2 to 4. However, in the spot welding of Examples 6 and 7, cooling was completed and post-energization was initiated before the nugget temperature was estimated to have fallen below the end temperature specified by the reproduced thermal cycle test. In the spot-welded joints of Examples 6 and 7, the amount of fresh martensite was not sufficiently reduced. The CTS of the spot-welded joints of Examples 6 and 7 was only slightly improved compared to Example 1. In other words, in the spot welding of Examples 6 and 7, the effect of post-energization was hardly obtained.

[0138] REFERENCE SIGNS LIST 1 spot welded joint 11 steel plate 12 nugget 12E end 13 joint interface 13L imaginary line 14 indentation 15 sheet separation T1 main current temperature T2 cooling end temperature T3 post current temperature t2 cooling time t3 post current time

Claims

1. A spot welded joint comprising two or more overlapping steel plates and a nugget joining the steel plates, wherein the metal structure of both ends of the nugget on a virtual line along the joining interface of the steel plates, measured on a cross section of the nugget, contains 5 to 40 area % of fresh martensite, and the remaining 95 area % or more is one or both of tempered martensite and bainite.

2. The spot welded joint according to claim 1, characterized in that one or more of the steel plates is a high carbon steel plate having a carbon content of 0.20 mass% or more.

3. The spot welded joint according to claim 1 or 2, characterized in that the value obtained by subtracting the Vickers hardness of both ends of the nugget on a virtual line along the joint interface of the steel plates, measured on the cross section of the nugget, from the estimated maximum Vickers hardness HVmax calculated by the following formula is HV50 or more: HVmax = 217 + 1080 x (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) Here, the element symbol included in the formula is the content of the element corresponding to the element symbol in the weighted average of the chemical components of the multiple steel plates, weighted by the plate thickness.

4. The spot welded joint according to claim 1 or 2, characterized in that the metal structure at both ends of the nugget contains 2 area % or more of bainite.

5. A method for manufacturing a spot welded joint, comprising the steps of: subjecting a test steel plate to a plurality of replicated heat cycle tests, which simulate the main current application, cooling, and post-current application of spot welding, and in which the end temperature of the cooling and the heating rate in the post-current application are varied; measuring the hardness of the test steel plate after the replicated heat cycle test; identifying a correlation between the end temperature of the cooling in the replicated heat cycle test, the heating rate in the post-current application, and the hardness of the test steel plate after the replicated heat cycle test; identifying a combination of the end temperature and the heating rate that makes the hardness of the test steel plate equal to or less than a target value and that minimizes the total time required for the cooling and the post-current application; applying a main current to a plate set consisting of two or more overlapping steel plates using a pair of electrodes for spot welding; and cooling the plate set by reducing the current while applying pressure to the plate set using the pair of electrodes. a post-current is applied to the nugget formed in the plate assembly by the main current and the cooling, using a pair of the electrodes; a weighted average of the carbon content of the steel plate, weighted by the thickness of the steel plate, is made substantially equal to the carbon content of the test steel plate; the cooling is terminated after the temperature of the nugget is estimated to have fallen below the end temperature specified by the reproduced thermal cycle test; and a heating rate in the post-current is set to be equal to or lower than the heating rate specified by the reproduced thermal cycle test.

6. The method for manufacturing a spot welded joint according to claim 5, wherein the target value is set to a value equal to or less than the estimated maximum Vickers hardness HVmax-HV150 calculated by the following formula: HVmax=217+1080×(C+Si / 70+Mn / 113+Cr / 93+Mo / 30), where the element symbol included in the formula is the content of the element corresponding to the element symbol in the weighted average of the chemical composition of the test steel plate, with the plate thickness used as the weight.

7. A method for manufacturing a spot welded joint as described in claim 5 or claim 6, characterized in that a weighted average of the Mn content, the weighted average of the Cr content, and the weighted average of the Ni content of the steel plate, weighted by the plate thickness of the steel plate, are approximately equal to the Mn content, Cr content, and Ni content of the test steel plate, respectively.

8. A method for manufacturing a spot welded joint as described in claim 5 or claim 6, further comprising a step of estimating, by simulation, a cooling time and post-heating current that can reproduce the combination of the upper limit of the cooling end temperature and the lower limit of the heating rate identified by the reproduced thermal cycle test, and applying the cooling time and post-heating time identified by the simulation to the cooling and post-heating of the plate assembly.

9. A method for manufacturing a spot welded joint as claimed in claim 5 or 6, characterized in that the first half of the post-energization is an up-slope energization.

10. A method for manufacturing a spot welded joint as described in claim 5 or 6, characterized in that one or more of the steel plates is a high carbon steel plate having a carbon content of 0.20 mass% or more.

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