JP7828028B2 - Spot welded joint and method for manufacturing spot welded joint - Google Patents

Description

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

Resistance welding is a welding method in which a large current is passed through the weld joint, heating it with the generated resistance heat and applying pressure. Because resistance welding can be performed in a short time, it is used to manufacture a variety of machine parts.

Post-heating current may be passed through welds 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 post-heating current is also called post-heating.

An example of a material for which post-energization is used is high-strength steel plate. High-strength steel plate is applied in a variety of technical fields to reduce the weight and improve the safety of machine parts. However, high-strength steel plate has the issue of being prone to embrittlement at resistance welds. In welded joints made from ordinary steel plate, the higher the strength of the steel plate, the higher the cross tensile strength (CTS). However, in welded joints made from high-strength steel plate, the higher the strength of the steel plate, the lower the CTS.

Various techniques have been investigated to improve the properties of welds formed by resistance welding. For example, Patent Document 1 discloses a spot welding method in which a sheet set of two or more overlapping steel sheets is clamped between a pair of welding electrodes and joined by passing current while applying pressure, wherein at least one steel sheet of the sheet set is a high-strength steel sheet, and the method includes a main current passing step in which current is passed at a current value _IW (kA) and a post-tempering heat treatment step, wherein the post-tempering heat treatment step includes a cooling step with a cooling time _tct (ms), a heating step in which current is passed _at a current value _It (kA) shown in Formula (2) for a current passing time _tt (ms), and a transition step in which the current passing is continuously reduced from a current value _It (kA) to a current value _Itm (kA) for a downslope current passing time _{ttma (ms) and/or a holding step in which current is passed through the weld at a current value Itm} (kA) for a current passing time _ttm (ms).
0.8×I _w ≦I _t ≦1.6×I _w (2)

Patent No. 6958765

One of the advantages of spot welding is the short time required for welding. However, if post-welding is performed, the time required for welding will increase. When manufacturing parts with a large number of welds, it is necessary to shorten the welding time as much as possible.

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

The objective of the present disclosure is to provide a method for manufacturing a spot welded joint that can be performed in a short period of time, and a spot welded joint that can be manufactured in a short period of time.

The gist of this disclosure is as follows:

(1) A spot-welded joint according to one aspect of the present invention is a spot-welded joint comprising two or more stacked steel plates and a nugget joining the steel plates, wherein the metal 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 plates, 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, at least one of the steel plates is a high-carbon steel plate 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 measured on a virtual line along the joining interface of the steel plates, 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 symbol included in the formula is the content of the element corresponding to the element symbol in the 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 metal structure of the both end portions of the nugget contains 2 area % or more of the bainite.

(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. The method comprises the steps of: applying a main current to a sheet set consisting of two or more steel sheets that have been bonded together using a pair of electrodes for spot welding; cooling the sheet set by reducing the current while applying pressure to the sheet set using the pair of electrodes; and applying a post-current to a nugget formed in the sheet set by the main current and the cooling using the pair of electrodes, wherein a weighted average of the carbon contents of the steel sheets, where the sheet thickness of the steel sheets is used as a weight, is made approximately equal to the carbon content of the test steel sheet, the cooling is terminated after the point in time when it is estimated that the temperature of the nugget has fallen below the end temperature specified by the reproduced thermal cycle test, and the heating rate in the post-current is set to be equal to or less than the heating rate specified by the reproduced thermal cycle test.
(6) Preferably, in the method for manufacturing a spot-welded joint described in (5) above, 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)
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 method for manufacturing a spot-welded joint described in (5) or (6) above, 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, where the plate thickness of the steel plate is used as a weight, are approximately equal to the Mn content, the Cr content, and the Ni content of the test steel plate, respectively.
(8) Preferably, the manufacturing method of 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 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) Preferably, in the method for manufacturing a spot welded joint according to any one of (5) to (8) above, the first half of the post-energization is an upslope energization.
(10) Preferably, in the manufacturing method of a spot welded joint described in (5) to (9) above, at least one of the steel plates is a high-carbon steel plate having a carbon content of 0.20 mass % or more.

The present disclosure provides 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.

FIG. 1 is a schematic diagram of current and temperature in spot welding in which the nugget is not cooled to the Mf point or below. FIG. 1 is a schematic diagram of current and temperature in spot welding in which the nugget is cooled to the Mf point or below. FIG. 1 is a schematic diagram showing the correlation between the end temperature of cooling, the post-energization temperature rise rate, and the hardness of the test steel sheet after post-energization. 1 is a flowchart of a method for manufacturing a spot welded joint. FIG. 1 is a cross-sectional view of a spot welded joint. FIG. 1 is a schematic diagram of a thermal profile in a reproduced thermal cycle test. 1 is a graph showing the correlation between the cooling end temperature T2 in a reproduced heat cycle test and the hardness of the steel sheet after the reproduced heat cycle test. 1 is a photograph of the metal structure of a nugget in a simulated thermal cycle test. 1 is a photograph of the metal structure of a nugget in a simulated thermal cycle test. 1 is a photograph of the metal structure of a nugget in a simulated thermal cycle test. 1 is a photograph of the metal structure of a nugget in a simulated thermal cycle test. 1 is a photograph of the metal structure of a nugget in a simulated thermal cycle test. These are the results of the Four Masters exam. 10 shows the results of a simulation of the relationship between the cooling time and the cooling end temperature T2. FIG. 1 is a schematic diagram of a current flow pattern (a) employed in an experiment by the present inventors. FIG. 10 is a schematic diagram of a current flow pattern (b) employed in the inventors' experiments. The results of a spot welding test. The results of a spot welding test. The results of a spot welding test. 10 shows simulation results of the weld temperature in the current conduction pattern (a) and the current conduction pattern (b). 1 shows a cross-sectional photograph and hardness measurement results of a spot-welded joint obtained by a spot welding test. 1 shows a cross-sectional photograph and hardness measurement results of a spot-welded joint obtained by a spot welding test.

The inventors conducted extensive research to find a method for manufacturing spot-welded joints that allows for post-energization in a short time. As a result, they discovered that by appropriately shortening the cooling time (cooling time t2) that occurs after the main energization, spot-welded joints can be manufactured in a short time while ensuring the effects of post-energization. They also discovered that by conducting tests before spot welding, it is possible to identify an appropriate cooling time t2. The technical concept of this disclosure is described below.

(1. Changes in metal structure during main current application, cooling, and post-current application)
As shown in Figures 1 and 2, the manufacturing method of a spot-welded joint includes main current application, cooling, and post-current application. During main current application, 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 sheet assembly, while simultaneously applying pressure with the electrodes. The current passed through the sheet assembly during main current application is called the welding current. This main current application raises the temperature of the sheet assembly between the pair of electrodes to above its melting point.

During cooling, the current is stopped or reduced to a very low value while the sheet assembly is still clamped between the tips of the electrodes. Spot welding electrodes have water-cooling holes. With the electrode in contact with the sheet assembly, the resistance heat is reduced to zero or close to zero, allowing heat to transfer from the sheet assembly to the electrode. The temperature of the sheet assembly then drops rapidly between the pair of electrodes. The molten metal formed by this current flow solidifies as it cools to form a nugget 12.

The metal structure of the nugget 12 is austenite immediately after solidification. The nugget 12 is rapidly cooled, which produces 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, as shown in Figure 2, the temperature of the nugget 12 is cooled to below the Mf point, causing the majority of the metal structure of the nugget 12 to become martensite.

In the post-heating process, current is passed through the sheet assembly again to heat the nugget 12. The purpose of the post-heating is to temper the nugget 12, which has hardened due to cooling, and soften it. 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.

The martensite after post-current treatment (i.e., tempering heat treatment) is called tempered martensite. The martensite before post-current treatment is called fresh martensite. Tempered martensite has a lower hardness than fresh martensite. Therefore, post-current treatment reduces the hardness of the nugget 12 and improves its cross-tensile strength. The amount of softening of the nugget 12 is an indicator of the effectiveness of post-current treatment.

(2. Effect of shortening the cooling time t2)
The inventors attempted to shorten the cooling time t2 in order to shorten the time required to manufacture a spot welded joint, but found that shortening the cooling time t2 sometimes increases the hardness of the nugget 12 after post-energization.

The reason for this lies in the temperature of the nugget 12 at the end of cooling (i.e., the start of post-current application). When manufacturing a typical spot-welded joint, the cooling time t2 is set long enough to lower the nugget temperature below the Mf point before starting post-current application (see Figure 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 Figure 1). As a result, post-current application will begin with austenite remaining in the nugget 12.

The austenite remaining in the nugget 12 at the start of the post-heating becomes hard fresh martensite after the post-heating is completed. The fresh martensite increases the hardness of the nugget 12.

From the perspective of reducing the hardness of the nugget 12, it is preferable to lower the temperature of the nugget 12 below the Mf point and convert all austenite to martensite before starting the post-current treatment. This prevents fresh martensite from forming in the nugget 12 after the post-current treatment. However, in order to shorten the manufacturing time of the spot-welded joint, it is desirable to shorten the cooling time t2.

The inventors conducted 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 using a replicated thermal cycle test. A replicated thermal cycle test replicates the thermal cycles experienced by the heat-affected zone of the base metal and the weld metal during welding, and examines changes in base metal properties and evaluates the phase transformation behavior (structure, mechanical performance) of the resulting weld metal. It is difficult to measure the interelectrode temperature during spot welding. However, a 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.

The inventors conducted a simulated thermal cycle test simulating the main current, cooling, and post-current of spot welding on test steel sheets simulating the 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 all the same. However, the cooling time t2 in the cooling (i.e., the cooling end temperature) and the heating rate in the heating simulating the post-current (post-current heating rate) were varied within the range that can be used in actual spot welding.

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, as obtained from the above-mentioned 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, they also found that the nugget can sometimes be softened even when the cooling end temperature is above the Mf point. When the cooling end temperature exceeds the Mf point, the amount of softening 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 maintaining the amount of softening of the nugget 12.

The inventors also discovered that the slower the post-energization heating rate, the softer the nugget 12 becomes after the post-energization. The dashed line graph in Figure 3 shows the correlation between the cooling end temperature T2 and hardness under conditions where the post-energization heating rate is high. The solid line graph in Figure 3 shows the correlation between the cooling end temperature T2 and hardness under conditions where the post-energization heating rate is low. When the post-energization heating rate is low, the amount of softening of the nugget 12 can be increased even if the cooling end temperature T2 is set extremely high.

The cause of this phenomenon is presumed to be bainite transformation. As mentioned above, when post-energization is initiated in a temperature range above the Mf point where austenite remains, the austenite becomes fresh martensite. However, if the post-energization heating rate is small, part of the austenite transforms into bainite. The hardness of bainite is less 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 in turn reduces the hardness of the nugget 12.

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 can minimize the total cooling time and post-energization time. That is,
(1) A range of combinations of the cooling end temperature T2 and the post-energization heating rate that can ensure the required joining strength of the nugget 12 is specified in advance;
(2) Identifying a combination of the cooling end temperature T2 and the post-current heating rate that can minimize the total value of the cooling time t2 and the post-current heating time t3 within the range;
(3) After the temperature of the nugget 12 falls below the cooling end temperature T2, the post-current heating rate is applied and then the current is started promptly. This makes it possible to further shorten the time required to manufacture a spot-welded joint while ensuring the joining strength of the nugget 12.

The method for manufacturing a spot-welded joint according to the present disclosure, which has been obtained based on the above findings, includes the steps of (S1) simulating the main current application, cooling, and post-current application of spot welding, and subjecting a test steel sheet to a reproduced heat cycle test multiple times in which the end temperature of cooling and the heating rate in the post-current application are varied, as shown in FIG. 4 ;
(S2) measuring the hardness of the test steel plate after the test;
(S3) a step of identifying a correlation between the end temperature of cooling in the test, the heating rate of post-energization, and the hardness of the test steel sheet after the reproduced thermal cycle test;
(S4) a step of specifying a combination of an end temperature and a heating rate that makes the hardness of the test steel sheet equal to or less than a target value and minimizes the total time required for cooling and post-current application;
(S5) a step of applying a main current to a sheet set of two or more stacked steel sheets 11 using a pair of electrodes for spot welding;
(S6) A step of cooling the plate set by reducing the current while pressing the plate set using a pair of electrodes;
(S7) a step of post-current-passing the nugget 12 formed in the sheet assembly by the main current-passing and cooling using a pair of electrodes;
The weighted average of the carbon content of the steel plate 11, with the thickness of the steel plate 11 as the weight, is set to be approximately equal to the carbon content of the test steel plate, 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-energization is set to be equal to or lower than the heating rate specified by the reproduced thermal cycle test.

(S1 Multiple Tests)
Prior to spot welding, a test simulating spot welding is conducted using test steel plates that simulate the plate assembly that is the base material for spot welding.

The carbon content of the test steel plate is the same as the carbon content of the sheet assembly. If the carbon contents of the multiple steel plates 11 included in the sheet assembly are different, the weighted average of the carbon contents of the steel plates 11, with the thickness of the steel plates 11 used as the weight, is considered to be the carbon content of the sheet assembly. Carbon content is the element that has the greatest effect on the Ms point and Mf point. By making the carbon content of the sheet assembly and the carbon content of the test steel plate the same, the Ms point and Mf point of the test steel plate can be made to be at the same level as the Ms point and Mf point of the nugget 12 formed in the sheet assembly.

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 replicated thermal cycle test. In a replicated thermal cycle test, the temperature of the test steel plate can be accurately measured and controlled.

In heating to simulate actual spot welding current, the test steel sheet is heated. Note that, while in actual spot welding current, the steel sheet is heated above its melting point, in heating to simulate actual spot welding current, it is not necessary to heat the test steel sheet to its 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 heating to simulate actual spot welding current, the inventors heated the test steel sheet to 900°C and held it there for 5 minutes, thereby austenitizing the structure of the test steel sheet. Heating equivalent to this can be performed in heating to simulate actual spot welding current.

In the cooling process simulating the cooling of spot welds, the temperature of the test steel plate is lowered. The cooling end temperature T2 is varied across 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 for all tests. The cooling end temperature T2 is controlled via the cooling time t2.

In the heating process simulating post-energization of spot welding, the test steel sheet is reheated. The reheating is performed 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 to identify the correlation between the post-energization heating rate and hardness. Hereinafter, the heating rate in the heating process simulating post-energization may also be referred to as the post-energization heating rate for convenience.

(S2 Hardness measurement of test steel plate)
The hardness of the test steel plate is measured after the test is completed. 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 plate. 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 plate.

(S3 Identification of correlation between cooling end temperature T2, post-energization 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-energization 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-energization heating rate.

(S4) Identifying the Cooling End Temperature T2 and Post-energization Temperature Rise Rate)
Based on the correlation obtained by the above-mentioned procedure, a combination of the upper limit of the cooling end temperature T2 and the lower limit of the post-current heating rate is specified, 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 specifying the cooling end temperature T2 and the post-current heating rate will be explained below with reference to FIG.

The dotted-dash 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-energization heating rate is high. When the post-energization heating rate is high, the hardness of the test steel sheet can be made below 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-energization heating rate is low. When the post-energization heating rate is low, the hardness of the test steel sheet can be made below the target hardness if the cooling end temperature T2 is Y°C or lower.

As shown in Figure 3, increasing the post-energization heating rate shortens the time required for post-energization, i.e., the post-energization time t3. On the other hand, increasing the post-energization 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.

Based on the data obtained from the above-described tests, it is possible to identify a combination of cooling end temperature T2 and post-current heating rate that can achieve the target hardness. Furthermore, because the main current temperature T1 and 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, because the post-current temperature T3 is constant, it is possible to identify the post-current time t3 corresponding to the cooling end temperature T2 and post-current heating rate identified in the above procedure. This makes it possible to identify a combination of cooling end temperature T2 and post-current heating rate that will keep the hardness of the test steel sheet at or below the target value and minimize the cooling time t2 and post-current heating time t3.

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 assembly. Spot welding involves main energization, cooling, and post-energization. As mentioned above, the carbon content of the sheet assembly to be spot welded is the same as the carbon content of the test steel sheet.

(S5 main energization)
First, a pair of electrodes for spot welding is used to apply a 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 conditions are not particularly limited. The main current 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.

(S6 cooling)
Next, the current is reduced while the pair of electrodes is still pressing the sheet assembly. Preferably, the current is stopped and the current flowing through the electrodes is set to zero. The 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.

The time to end cooling is determined based on the combination of the cooling end temperature T2 and the post-energization heating rate determined through the above-mentioned test. Cooling is terminated as soon as possible after the point at which the temperature of the nugget 12 is estimated to have fallen below the cooling end temperature T2 determined through the test.

(Electrification after S7)
A pair of electrodes is used to post-current the nugget 12 formed into a sheet assembly by the main current and cooling. The post-current temperature T3, i.e., the maximum heating temperature in the post-current, is set to the tempering temperature of the nugget 12. This tempers the nugget 12, reducing its hardness. The post-current temperature T3 is set to the same as the maximum heating temperature in the heating simulating the post-current.

The post-current heating rate is determined based on the combination of the cooling end temperature T2 and post-current heating rate that minimizes the cooling time t2 and post-current heating time t3, as determined through the above-mentioned tests. The post-heating current during post-current heating 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. Note that it is not necessary to constantly control the post-current heating rate from the start to the end of post-current heating. It is sufficient that the average heating rate during post-current heating is below the upper limit determined through the tests.

By following the above procedure, once the first weld has been created, the second actual current can be applied immediately. The conditions obtained through the test can, of course, be applied to multiple spot welding runs.

(Action and effect)
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 also allows both the cooling time t2 and the post-energization time t3 to be shortened.

In the manufacturing method for spot-welded joints according to the present disclosure, spot welding is performed to achieve a favorable combination of the cooling end temperature T2 and post-energization heating rate identified through testing. This allows for the manufacturing of spot-welded joints with nuggets 12 that have been suitably tempered in a short period of time.

The above describes the most basic aspect of the method for manufacturing a spot welded joint according to this embodiment. Below, we will explain a more preferred aspect of the method for manufacturing a spot welded joint according to this embodiment.

(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, for example, depending on 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.

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

(Test steel plate)
As described above, the carbon content of the test steel sheet is set to be approximately the same as the carbon content of the sheet pair to be spot-welded. Preferably, the Mn content, Cr content, and Ni content of the test steel sheet are also set to be approximately the same as the Mn content, Cr content, and Ni content of the sheet pair to be spot-welded. More preferably, the chemical composition of the test steel sheet is set to be the same as the chemical composition of the sheet pair to be spot-welded. When the chemical compositions of the multiple steel sheets 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 sheets 11, respectively, with the sheet thickness of the steel sheets 11 used as the weight.

The closer the Ms and Mf points of the test steel plate are to the Ms and Mf points of the sheet assembly, the better. By matching at least the carbon content of the test steel plate with that of the sheet assembly, the closer the Ms and Mf points of the test steel plate can be to the Ms and Mf points of the sheet assembly. On the other hand, by matching not only the carbon content of the test steel plate but also the content of other elements (e.g., Mn, Cr, and Ni, which affect the Ms and Mf points of steel) with that of the sheet assembly, the Ms and Mf points of the test steel plate will be closer to the Ms and Mf points of the sheet assembly. This can further increase the reliability of the combination of end temperature and heating rate identified through testing.

(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, the temperature of the nugget 12 cannot be directly measured. Therefore, in spot welding, cooling is stopped after the temperature of the nugget 12 is estimated to fall below the cooling end temperature T2 determined by testing.

Simulation is one method for determining the point in time when 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, electrode configuration, and pressure, it is possible to estimate the change in the temperature of the nugget 12 over time, as illustrated in Figure 10 (described below). Therefore, the manufacturing method for a spot-welded joint according to the present disclosure preferably further includes a step S8 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 determined by the replicated thermal cycle test. In this case, it is preferable to apply the cooling time and post-heating time determined by simulation to the cooling and post-heating of the plate assembly.

Another means for determining the point at which the temperature of the nugget 12 is estimated to be below the cooling end temperature T2 determined by testing is the inter-electrode 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 clamping the sheet assembly and calculating the inter-electrode resistance. Note that, in order to obtain the inter-electrode resistance, a current must be passed through the electrodes. However, the current flow required to obtain the inter-electrode resistance does not interfere with cooling. This is because only a very small current is required to obtain the inter-electrode resistance.

(Post-energization pattern)
As illustrated in Fig. 1 and Fig. 11A, the post-heating current flowing during the post-heating may be constant. On the other hand, as illustrated in Fig. 11B, the post-heating current may be up-slope controlled during at least a portion of the post-heating. Up-slope control refers to control in which the current is continuously increased. It is particularly preferable to use up-slope current flow during at least the first half of the post-heating, and more preferably, to use up-slope current flow 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.

While the reason why upslope current application increases the amount of bainite is unclear, the inventors speculate as follows: Bainite nucleation and growth are important for transforming steel into bainite 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 Figure 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 Figure 11A), which has a fast heating rate in the low-temperature region.

(Steel type)
The type of steel plate 11 in the plate 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 with 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.

(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 in 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.

(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.

When observing the cross section of the nugget 12, it can be seen that fresh martensite is present at both ends 12E of the nugget 12 along the imaginary line 13L that runs along the joint interface 13. In the spot-welded joint 1 according to this embodiment, the area fraction of fresh martensite is specified at both ends 12E of the nugget 12 along the imaginary line 13L that runs along the joint interface 13.

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

The end 12E of the nugget 12 at the imaginary line 13L along the bond interface 13 is an extremely important area for improving the bond strength of the nugget 12. This is because the end 12E along the bond interface 13 becomes a stress concentration area during a cross-tension test.

(Metal structure of both ends 12E of nugget 12)
The metal structure of each end 12E of the nugget 12 includes 5 to 40 area % of fresh martensite. The region other than the fresh martensite is mainly composed of tempered martensite and/or bainite. For example, 95 area % or more of the region other than the fresh martensite is composed of one or both of tempered martensite and bainite.
That is, 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, respectively, in the metal structure of both end portions 12E of the nugget 12. 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.

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 structure of martensite.

If cooling is performed to below the Mf point and no post-current is applied, all of the austenite in the nugget 12 will transform into martensite and remain in that state. Therefore, if no post-current is applied, all of the martensite in the nugget 12 will become fresh martensite.

When post-current is applied after cooling to below the Mf point, all of the austenite in the nugget 12 at the start of post-current is transformed into martensite. When post-current is applied to such a nugget 12, all of the martensite in the nugget 12 becomes tempered martensite.

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 either transforms into bainite during post-current or transforms into martensite and becomes 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, 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.

In the metal structure of both end portions 12E of the nugget 12, the area fraction 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 fraction 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 remaining structure), the proportion of tempered martensite and bainite may be 96% or more, 98% or more, or 100% by area.

(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.

The metal structure revealed by etching is observed using a SEM. The magnification of the SEM observation is 3000 times. The area ratio of the metal structure is measured in an observation field that satisfies the following requirements. Note that the end 12E shown in FIG. 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 ends 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 for 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.

Areas where the substructure is not revealed and where the brightness is low 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 where the brightness is high are considered to be fresh martensite or retained austenite. Areas where the substructure is revealed are considered to be tempered martensite or bainite. When determining 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 determining the area fraction of bainite will be described later.

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 using the above procedure is considered to be the area fraction.

(Action and effect)
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.

In addition, in the spot-welded joint according to this embodiment, the amount of fresh martensite in the metal structure of both end portions 12E of the nugget 12 is 40 area % or less, with the remaining structure being primarily composed of tempered martensite and/or bainite. This increases the toughness of the nugget 12 and the cross-tensile strength of the spot-welded joint.

The above describes the most basic aspect of the spot welded joint according to this embodiment. Below, we will explain more preferred aspects of the spot welded joint according to this embodiment.

(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 with 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.

(Hardness of Nugget 12)
The nugget 12 of the spot-welded joint according to this embodiment is preferably softer than the nugget 12 to which no post-energization is performed. 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.

The estimated maximum Vickers hardness HVmax is a value calculated by the following formula.
HVmax=217+1080×(C+Si/70+Mn/113+Cr/93+Mo/30)
The element symbols included in the above formula represent the content of the corresponding element in the chemical composition of the steel plate 11. When a spot-welded joint includes multiple steel plates 11 with different chemical compositions, the chemical composition of the steel plate 11 is a weighted average of the chemical compositions of the steel plate 11, with the plate thickness used as the weight. C, Mn, Cr, and Mo included in the above formula are elements that improve the hardenability of steel. The estimated maximum Vickers hardness HVmax calculated by the above formula 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.

If 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 higher, both end portions 12E of the nugget 12 are sufficiently tempered. Therefore, the cross tensile strength of the spot welded joint is further increased.

The chemical composition of steel plate 11, which is used to calculate the estimated maximum Vickers hardness, can be measured using a common method. For example, a portion of steel plate 11 is cut and chips are collected. The chemical composition of the chips can then be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), for example, in accordance with JIS G 1201:2014. The carbon content, which is difficult to measure using ICP-AES, can be measured using a combustion-infrared absorption method.

As described in JIS G 0417:1999, the analysis sample is taken so as to obtain the 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 widthwise ends of the steel plate 11.

The Vickers hardness of both end portions 12E of the nugget 12 is measured as follows. The hardness of the nugget 12 is continuously measured on the 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 the region within 400 μm from the edge of the nugget 12 is considered to be 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. If 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.

(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 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.

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 high amount of bainite, the cooling time t2 can be shortened. In other words, a spot welded joint with a higher amount of bainite can be manufactured in a shorter time.

There is no particular upper limit to the amount of bainite. For example, in the metal structure at both ends of the nugget, the remaining amount of bainite may be 50 area percent or less, 45 area percent or less, or 40 area percent or less.

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, using the above-mentioned method, the area where the substructure is exposed is identified in the observation field of both end portions 12E of the nugget 12. The area 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.

Electron backscatter diffraction (EBSD) analysis is used to obtain crystal orientation information for the metal structure in the observation field. The measurement interval is 0.2 μm. An EBSD analysis device is used for the measurement. The EBSD analysis device used by the inventors consists of a thermal field emission scanning electron microscope (JEOL JSM-7200F) 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. During the measurement, the vacuum level inside the device is set to 9.6 x 10-5 Pa or less, the acceleration voltage is set to 25 kV, and the probe current level is set to 16.

The obtained crystal orientation information is analyzed using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL Solution. Measurement points between which the crystal orientation difference is 15° or more are considered to be grain boundaries. Areas surrounded by grain boundaries are 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 a crystal grain. The average value of the difference in crystal orientation is the GAM value of the crystal grain.

Next, we display the boundaries with a crystal misorientation of more than 5° in the crystal grains with a GAM value of more than 0.5°. We calculate the density of boundaries within the crystal grains with a crystal misorientation of more than 5°, i.e., the length of grain boundaries with a crystal misorientation of more than 5° per unit area. The density of boundaries with a crystal misorientation of more than 5° is referred to as the 5° boundary density of the crystal grain.

In the region where the substructure is revealed, crystal grains having a GAM value of more than 0.5° and a 5° boundary density of 0.4 μm/μm2 or ^less are regarded as bainite. By measuring the area ratio of crystal grains regarded as bainite, the area ratio of bainite contained in the remainder in the metal structure at both ends of the nugget can be calculated.

(Other configurations)
Although 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.

(Tensile strength, hardness, thickness, and surface treatment of steel plate 11)
The tensile strength 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 at least one 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, at least one of the plurality of steel plates 11 may be a mild steel having a tensile strength of 500 MPa or less.

The hardness of the steel plates 11 is not particularly limited. From the perspective of improving the rigidity and fracture resistance of the spot-welded joint 1, it is preferable that one or more of the multiple steel plates 11 be a high-carbon steel plate with a hardness of HV 330 or more. It is even 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 multiple steel plates 11 may be mild steel.

The thickness of the steel plates 11 is not particularly limited. The thickness ratio of the plate assembly is also not particularly limited. The thickness ratio of the plate assembly is the total plate thickness of the plate assembly divided by the thickness of the thinner of the steel plates 11 arranged on the surface of the plate assembly. For example, if 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 components. The mild steel plates are automobile exterior components. The thickness of the high-carbon steel plates is preferably 1.0 to 2.5 mm, for example. The thickness of the mild steel plates is preferably 0.4 to 1.2 mm, for example.

To improve corrosion resistance and aesthetics, it is preferable that the surface of the steel sheet 11 be plated. Examples of types of plating include Al-based plating, Al-based alloy plating, Zn-based plating, and Zn-based alloy plating.

(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.

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 thickness of the two steel plates 11 that make up the joint interface 13 at which the diameter of the nugget 12 is measured. When there are three or more steel plates 11 and two or more joint interfaces 13, 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.

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

(Experiment 1: Reproduced thermal cycle test)
The present inventors conducted a simulated thermal cycle test simulating spot welding. The test steel sheets and the simulated thermal cycle test conditions were as follows:
・Steel type of test steel plate: 2 GPa-class hot stamping steel plate ・Thickness of test steel plate: 2.0 mm
Ms point of test steel plate: approx. 360°C
Mf point of test steel plate: about 180°C
- Thermal profile in the reproduced thermal cycle test: as shown in Figure 6

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 rate after the main current and the cooling rate after 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.

Furthermore, the hardness of the steel sheets after the simulated thermal cycle test was measured. The correlation between the cooling end temperature T2 and the hardness of the steel sheets after the simulated thermal cycle test was identified, and is shown in Figure 7.

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 during post-current application. On the other hand, it was confirmed that the test steel sheet softened sufficiently when the cooling end temperature was approximately 280°C or below. When the test steel sheet was cooled to the Mf point (approximately 180°C) or below and then tempered, the hardness of the test steel sheet was approximately 300 HV. If the cooling end temperature was set to 280°C above the Mf point, the hardness of the test steel sheet could be made approximately 300 HV.

Furthermore, by reducing the heating rate during reheating to simulate post-energization, it was possible to expand the range of cooling end temperatures 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 reduced to approximately HV300 if the cooling end temperature T2 was approximately 320°C or less.

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:
8A: Cooling end temperature T2: room temperature, no post-energization. FIG. 8B: Cooling end temperature T2: 160° C., post-energization temperature rise rate: 100° C./sec.
8C: Cooling end temperature T2: 320°C, post-energization heating rate: 100°C/sec
8D: Cooling end temperature T2: 360°C, post-energization heating rate: 100°C/sec
8E: Cooling end temperature T2: room temperature, post-energization heating rate: 100°C/sec

In the test steel plate in Figure 8A, all of the martensite was fresh martensite. No carbide precipitation was observed in the microstructure photograph of this test steel plate.

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

In the replicated thermal cycle test to obtain the test steel plate in Figure 8C, the cooling end temperature T2 was 320°C, slightly exceeding the Mf point. However, the test steel plate in Figure 8C was sufficiently softened after the replicated thermal cycle test. The presence of precipitated carbides can also be confirmed in the structural photograph in Figure 8C, as in Figure 8B.

In the replicated thermal cycle test to obtain the test steel plate shown in Figure 8D, the cooling end temperature T2 was 360°C, far exceeding the Mf point. However, the test steel plate shown in Figure 8D did not soften sufficiently after the replicated thermal cycle test. Almost no carbides can be seen in the structural photograph of Figure 8D.

In the replicated thermal cycle test to obtain the test steel plate shown in Figure 8E, the cooling end temperature T2 was room temperature. The test steel plate shown in Figure 8E was sufficiently softened after the replicated thermal cycle test. The presence of precipitated carbides can be confirmed in the structural photograph of Figure 8E.

These experimental results show that tempered martensite and bainite can be formed within the nugget even if the cooling end temperature T2 is slightly above 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.

(Experiment 2: Four Masters Test)
The inventors conducted a Formaster test to investigate the effect of the heating rate during reheating, which simulates 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, which simulates 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.

In the graph of Figure 9, the section marked with symbol (1) corresponds to heating simulating the actual current flow of spot welding. In the heating simulating the actual current flow, 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 the cooling of spot welding. Cooling was performed to 320°C. The section marked with symbol (3) corresponds to reheating simulating post-current flow. Reheating was performed to 700°C. The section marked with symbol (4) corresponds to cooling after post-current flow. Cooling after post-current flow was performed to room temperature.

In the graph of Figure 9, noteworthy is the increase in the 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.

(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 plates: 2 Pressure: 3.92 kN
Actual welding time: 0.36 seconds Welding current during actual welding: 8.0 kA
・Nugget diameter: 7 mm
The simulation results are shown in FIG.

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 optimal current conditions for the test steel plate are as shown in Table 1.

The (D) post-energization time shown in Table 1 is a value calculated by the following formula.
(D) = 50 cycles/sec × {700 ° C − (B)} / (A)
In the above formula, "700°C" is the maximum temperature during post-energization (see Figure 6). "700°C - (B)" is the temperature rise required for post-energization. "50 cycles/sec" in the above 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, it is not always preferable to reduce the temperature rise rate during post-energization. For the test steel sheets used in this experiment, it was estimated that setting the temperature rise rate during post-energization to 100°C/sec would enable a shorter welding time.

(Experiment 4: Spot welding test)
Taking into consideration the results of Experiments 1 to 3, spot welding experiments were carried out under the following conditions.
Plate assembly: Two test steel plates from Experiment 1 and Experiment 2 stacked together. Main current conditions: Main current value 8.4 kA, current time 0.36 seconds. Cooling time: (A) 40 cycles, (B) 50 cycles, or (C) 60 cycles. Post-heating time: 1.98 seconds. Maximum post-heating current: 4 to 8 kA.
Current waveform in post-heating: 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 upslope controlled. After the spot welding test, a cross tensile test was performed on the spot-welded joint, and the cross tensile strength (CTS) was measured.

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

As shown in Figure 12A, when the cooling time was 40 cycles, no significant improvement in CTS was observed regardless of the post-heat conditions. On the other hand, as shown in Figure 12C, when the cooling time was 60 cycles, an improvement in CTS was confirmed. In Figure 12C, it is believed that tempered martensite was created because the nugget was cooled to below the Mf point.

Furthermore, as shown in Figure 12B, even when the cooling time was 50 cycles, an improvement in CTS was confirmed when current flow pattern (b) was used. In Figure 12B, the nugget's cooling end temperature T2 exceeds the Mf point, and martensitic transformation was not complete at the start of post-current flow. 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 flow can be enjoyed.

The reason why better results were obtained with current pattern (b) than with current pattern (a) in the spot welding experiment in Figure 12B is thought to be as shown in Figure 13. Figure 13 shows the simulation results of the temperature of the weld in spot welding using current pattern (a) or current 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.

Finally, Figure 14 shows the cross-sectional observation and hardness measurement results of the nugget with the highest CTS in a spot welding test with a cooling time of 50 cycles and a post-heat time of 55 cycles, while Figure 15 shows the cross-sectional observation and hardness measurement results of the nugget with the highest CTS in a 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.

(Experiment 4: Amount of fresh martensite in nuggets)
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.

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

No post-heat treatment was performed in the spot welding of Example 1. In the spot-welded joint of Example 1, the nugget structure became entirely fresh martensite.

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. This reduced the amount of fresh martensite in the spot-welded joint of Example 8, 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 perspective 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.

In the spot welding of Examples 2, 3, and 5, post-energization was performed. 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, in the spot welding of Examples 2, 3, and 5, the cooling time and total value were significantly reduced compared to Example 8.

In the spot welding of Example 4, post-energization was performed. The total cooling time and post-energization time for the spot welding of Example 4 were at the same level as in Example 3. However, in the spot welding of Example 4, the heating rate during post-energization was set to a value that exceeded the heating rate determined by the simulated 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.

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 in Examples 2 to 4. However, in the spot welding of Examples 6 and 7, cooling was completed and post-energization was initiated before the point at which the nugget temperature was estimated to have fallen below the end temperature determined by the simulated 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.

1 Spot welded joint 11 Steel plate 12 Nugget 12E End 13 Joint interface 13L Virtual 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 (10)

Two or more stacked steel plates;
a nugget that joins the steel plates;
A spot welded joint comprising:
A spot welded joint, wherein the metal structure at both ends of the nugget on a virtual line along the joining interface of the steel sheets, 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, wherein 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, wherein a value obtained by subtracting the Vickers hardness of both ends of the nugget measured on a virtual line along the joining interface of the steel sheets, measured on a cross section of the nugget, from an 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 symbol included in the formula is the content of the element corresponding to the element symbol in the weighted average of the chemical compositions of the plurality of steel plates, with the plate thickness used as the weight. 3. The spot welded joint according to claim 1, wherein the metal structure at each end of the nugget contains 2 area % or more of the bainite. A method for manufacturing a spot welded joint, comprising:
a step of subjecting a test steel sheet to a plurality of replicated heat cycle tests, which simulate the main current application, cooling, and post-current application of spot welding, and which vary the end temperature of the cooling and the heating rate in the post-current application;
measuring the hardness of the test steel plate after the replicated thermal cycle test;
a step of identifying a correlation between the end temperature of the cooling and the heating rate of the post-energization in the replicated thermal cycle test, and the hardness of the test steel sheet after the replicated thermal cycle test;
a step of 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 minimizes the total time required for the cooling and the post-current application;
A step of applying a main current to a plate set of two or more stacked steel plates using a pair of electrodes for spot welding;
a step of cooling the plate set by reducing the current while applying pressure to the plate set using the pair of electrodes;
a step of post-current-applying a current to the nugget formed in the sheet assembly by the main current application and the cooling using the pair of electrodes;
Equipped with
A weighted average of the carbon contents of the steel plates, where the thickness of the steel plates is used as a weight, is set to be approximately equal to the carbon content of the test steel plates,
The cooling is terminated after the time when the temperature of the nugget is estimated to be lower than the end temperature specified by the reproduced thermal cycle test;
A method for manufacturing a spot welded joint, wherein the temperature rise rate in the post-energization is equal to or less than the temperature rise rate specified by the reproduced thermal cycle test. 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 an 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. The method for manufacturing a spot welded joint according to claim 5 or claim 6, wherein a weighted average of the Mn amount, the weighted average of the Cr amount, and the weighted average of the Ni amount of the steel plate, where the plate thickness of the steel plate is used as a weight, are approximately equal to the Mn amount, the Cr amount, and the Ni amount of the test steel plate, respectively. and estimating, by simulation, a cooling time and a post-heating current that can reproduce the combination of the upper limit value of the cooling end temperature and the lower limit value of the heating rate identified by the reproduced thermal cycle test.
7. The method for manufacturing a spot welded joint according to claim 5, wherein the cooling time and post-energization time specified by the simulation are applied to the cooling and post-energization of the plate assembly. 7. The method for manufacturing a spot welded joint according to claim 5, wherein the first half of the post-energization is an up-slope energization. 7. The method for manufacturing a spot welded joint according to claim 5, wherein at least one of the steel plates is a high-carbon steel plate having a carbon content of 0.20 mass % or more.