JP7115223B2 - Method for manufacturing resistance spot welded joints - Google Patents

Description

The present invention relates to a method for manufacturing resistance spot welded joints.

Tensile strength and fatigue strength are important properties of resistance spot-welded joints, which are formed by spot-welding a plurality of superimposed steel sheets. In particular, tensile shear strength, which is measured by applying a tensile load in the shear direction It is required to be excellent in both strength (TSS) and cross tensile strength (CTS) measured by applying a tensile load in the peeling direction.

Among resistance spot welded joints, resistance spot welded joints using steel sheets having a tensile strength of 270 MPa or more and less than 750 MPa are unlikely to cause problems related to joint strength because the CTS increases as the tensile strength of the steel sheets increases. On the other hand, in a resistance spot welded joint using a steel sheet having a tensile strength exceeding 750 MPa, if the tensile strength of the steel sheet increases, the CTS does not increase or decreases, so the joint strength may be insufficient.
In particular, in resistance spot-welded joints using high-strength steel sheets with a tensile strength of 980 MPa or more, the carbon equivalent of the welded portion (nugget) is high, and when quenching, the toughness of the nugget decreases, so the CTS of the joint decreases. tends to be easier.

As a method for improving the strength and toughness of resistance spot-welded joints using such high-strength steel sheets, a two-stage current application is known in which post-heating current is applied after main current.
For example, Patent Document 1 discloses a method of tempering a nugget portion and a heat-affected zone of a resistance spot-welded joint by performing tempering energization after a certain period of time has passed since the end of main energization, thereby reducing the hardness of the joint. is disclosed.
Further, Patent Document 2 discloses a method of performing post-heating energization with a current value equal to or higher than the main energization current value after forming a nugget by main energization.
Furthermore, in Patent Document 3, in performing resistance spot welding while sandwiching superimposed high-strength thin steel plates between a pair of electrodes and applying pressure, after welding the first point, the position of the electrode is moved, A method is disclosed for making a second weld partially overlapping the first weld after the second weld has cooled to a temperature below the _Mf point.

WO2014/171495 JP 2010-115706 A JP 2010-172945 A

However, in the methods disclosed in Patent Documents 1 and 2, when the temperature of the electrode varies due to wear of the electrode or adhesion of the alloy, the cooling rate in the cooling process also varies. Strength (in particular, high CTS) cannot be stably obtained.
Furthermore, in the methods disclosed in Patent Documents 1 and 2, since the joint continues to be pressurized even in the cooling process, the thickness of the nugget is greatly reduced, and the joint strength tends to decrease (specifically, in-plane There was a problem that it is easy to break in the nugget in the tensile test).

And, in Patent Document 3, as a specific embodiment, the energizing current at the second point is the same as the energizing current at the first point or is larger than the energizing current at the first point, and is the same. That is, a method is disclosed in which the calculated value of (energization current) ² ×energization time of the second energization is equal to or greater than that of the first energization. With this method, the toughness of the nugget and the heat affected zone (HAZ) near the corona bond adjacent to the nugget could not be sufficiently improved, and there was a limit to the stable improvement of joint strength (especially CTS). .
Furthermore, in such a method, the indentation formed by the first welding causes variation in the area where the current flows when the second welding is performed, resulting in high joint strength (especially, high CTS ) could not be stably obtained.

Therefore, the present invention provides a method for manufacturing a resistance spot welded joint using a plurality of steel plates including a steel plate having a tensile strength of 440 MPa or more, and a resistance spot welded joint that can stably obtain a high joint strength. The object is to provide a manufacturing method.

One aspect (aspect 1) of the present invention is a method for manufacturing a resistance spot welded joint using a plurality of steel plates including a steel plate having a tensile strength of 440 MPa or more,
a first step of sandwiching the plurality of superimposed steel plates between a pair of _first electrodes and energizing with a predetermined energizing current I1 and energizing time t1 to form _a nugget;
a second step of releasing the pair of first electrodes and cooling the nugget to a temperature below the _Mf point;
a _third step of tempering the nuggets by sandwiching the portions of the plurality of steel plates corresponding to the nuggets with a pair of second electrodes, and applying current with a predetermined current _I3 and current time t3; including
The manufacturing method is characterized in that energization conditions in the first step and the third step satisfy the following formula (1).
I ₃ ² ×t ₃ <I ₁ ² ×t ₁ (1)

In the manufacturing method of this aspect 1, the third step is a step of tempering the nugget formed in the first step, that is, the edge of the nugget and the vicinity of the steel plate overlapping surface of the heat affected zone adjacent to the nugget are remelted. It is a step of tempering without using and is performed under the energization condition that satisfies the above formula (1), that is, the energization condition with less energy than the first step, so that the toughness of the nugget is improved and stably high. Joint strength (in particular, high CTS) can be obtained.
In addition, in the manufacturing method of this aspect 1, since the cooling in the second step is performed with the pair of first electrodes open, even if the temperature of the electrodes varies, it is unlikely to be affected, and the above-mentioned high joint Strength can be stably obtained. Furthermore, by performing the cooling in the second step with the pair of first electrodes open, the decrease in the thickness of the nugget is suppressed, so the joint strength can be made less likely to decrease (specifically, It is possible to make it difficult to break with a nugget in an in-plane tensile test).
Therefore, the manufacturing method of this aspect 1 can stably obtain a high joint strength even in a resistance spot-welded joint using a plurality of steel plates including a high-strength steel plate having a tensile strength of 440 MPa or more.

In another aspect (aspect 2) of the present invention, the manufacturing method according to aspect 1 includes a plurality of welding target locations where the first step to the third step are performed,
While performing the said 2nd process in one place to be welded, the said 1st process is performed in parallel in another place to be welded.

In still another aspect (aspect 3) of the present invention, the manufacturing method of aspect 1 includes a plurality of welding target locations where the first step to the third step are performed,
While performing the said 2nd process in one place to be welded, the said 3rd process is performed in parallel in another place to be welded.

In still another aspect (aspect 4) of the present invention, in the manufacturing method according to any one of aspects 1 to 3, the first electrode and the second electrode are different electrodes.

In still another aspect (aspect 5) of the present invention, in the manufacturing method according to any one of aspects 1 to 4, the electrode tip diameter D1 of the _first electrode and the electrode tip diameter D2 of the _second electrode are as follows: (2) is satisfied.
D ₁ ≤ D ₂ (2)

In still another aspect (aspect 6) of the present invention, in the manufacturing method according to any one of aspects 1 to 5, the carbon equivalent of the steel sheet having a tensile strength of 440 MPa or more is 0.22% by mass or more and 0.40% by mass or less. In the third step, a portion having a Vickers hardness of 280 or more and 420 or less is formed in the heat-affected zone near the corona bond portion adjacent to the nugget.

In still another aspect (aspect 7) of the present invention, in the manufacturing method according to any one of aspects 1 to 6, the process time of the second step is 3 seconds or more and 600 seconds or less.

According to the method for manufacturing a resistance spot welded joint of the present invention, even in a resistance spot welded joint using a plurality of steel plates including a high-strength steel plate having a tensile strength of 440 MPa or more, high joint strength is stably obtained. be able to.

FIG. 1 is a diagram showing measurement positions of hardness distribution of a resistance spot-welded joint obtained in an example of the present invention. FIG. 2 is a hardness distribution diagram in the direction parallel to the joint interface of the resistance spot welded joint obtained in the example of the present invention.

A preferred embodiment of the method for manufacturing a resistance spot welded joint of the present invention will be described in detail below.

<Method for manufacturing resistance spot welded joint>
A method for producing a resistance spot welded joint of the present invention is a method for producing a resistance spot welded joint using a plurality of steel plates including a steel plate having a tensile strength of 440 MPa or more. A _first step of forming a nugget by sandwiching it between electrodes and applying a predetermined energizing current I1 and an energizing time t1, and opening the pair of _first electrodes to form the nugget formed in the first step. A second step of cooling to a temperature below the M _f point, and holding a plurality of steel plates at locations corresponding to the nuggets with a pair of second electrodes, with a predetermined energizing current I ₃ and energizing time t ₃ and a third step of tempering the nugget by energizing, and further, the energizing conditions of the first step and the third step are characterized by satisfying the following formula (1). .
I ₃ ² ×t ₃ <I ₁ ² ×t ₁ (1)

As described above, the manufacturing method of the present invention includes the second step and the third step, which are performed under specific conditions, so that a plurality of high-strength steel sheets having a tensile strength of 440 MPa or more High joint strength can be stably obtained even with resistance spot welded joints using steel plates of

Each step in the manufacturing method of the present invention will be specifically described below.

[First step]
In the present invention, the first step is to sandwich a plurality of superimposed steel plates between a pair of first electrodes, apply a current with _a predetermined current I1 and _an energization time t1, and superimpose a plurality of steel plates. It is a step of melting the surface and its neighboring areas to form a nugget.
In this _first step, a predetermined energizing current is applied before energizing with the predetermined energizing current I1 and energizing time t1, so that the steel sheets are blended with _each other and the plating of the steel sheet is removed. Electricity may be applied. Preliminary energization includes current waveforms such as rectangular, trapezoidal, triangular, pulse, and upslope shapes.
In addition, since the nugget is in an unsolidified state at the stage of completing the first step, when the pair of first electrodes is opened in the second step, there is a possibility that the plurality of steel plates will be separated from each other. In order to prevent such separation, it is desirable to set the holding time (that is, the time during which only pressure is applied without opening the electrodes after energization) to 0.05 to 0.4 seconds.

(Electricity condition)
The energization conditions (e.g., energization current, energization time, electrode pressure, etc.) in the first step are conditions according to the size of the nugget to be formed on the overlapping surfaces of the plurality of steel plates and the surrounding area. can be adopted.
The current I ₃ (kA) and the current time t ₃ (seconds) in the third step, which will be described later, are based on the current I ₁ (kA) and the current time t ₁ (seconds) in the first step. The current I ₁ (kA) and the current time t ₁ (seconds) in the first step are limited to those satisfying the above formula (1), but the current value and the current value according to the size of the nugget to be formed. time can be employed.

(first electrode)
As the pair of first electrodes used in the first step, an electrode that can be used for normal resistance spot welding is adopted as long as it can form a nugget on the overlapping surfaces of the sandwiched steel plates and the area in the vicinity thereof. be able to.

[Second step]
In the present invention, the second step is, after the first step, opening a pair of first electrodes sandwiching a plurality of steel plates so that the nugget formed in the first step is below the _Mf point. It is a step of cooling to the temperature of
It should be noted that the fact that the nugget has a temperature below the _Mf point in the second step means that the Vickers hardness of the heat-affected zone in the vicinity of the corona bond portion adjacent to the nugget of the welded portion after the completion of the third step. It can be confirmed by the fact that a softened portion with a value of 280 or more and 420 or less is formed.

This second step is a step for allowing the _nugget to be tempered by the energization of the third step. It cools down to the following temperature.
The second step is to cool the nugget with the pair of first electrodes open, so even if the temperature of the electrodes varies, it is unlikely to be affected, and high joint strength can be stably obtained. be able to. Furthermore, by performing the cooling in the second step with the pair of first electrodes open, the decrease in the thickness of the nugget is suppressed, so the joint strength can be made less likely to decrease. It can be made difficult to break with a nugget in a test.

(Cooling condition)
As the cooling conditions and cooling means for the second step, conditions and means that can cool the nugget formed in the first step to a temperature below the _Mf point can be adopted. For example, the second step may be performed only by opening the pair of first electrodes used in the first step (that is, by cooling), or with the pair of first electrodes open, It may be carried out by spraying a predetermined cooling fluid (for example, cooling water, cooling gas, etc.) on a portion corresponding to the nugget.

As the process time (seconds) of the second step, a time that can cool the nugget to a temperature below the _Mf point can be adopted, but the effect of the above-described second step can be obtained more reliably. The process time of the second step is preferably 3 seconds or more and 600 seconds or less in terms of high productivity and excellent productivity.

[Third step]
In the present invention, the third step is to sandwich a plurality of steel plates at locations corresponding to the nuggets with a pair of second electrodes, and apply a predetermined current I ₃ and an energization time t ₃ to form the nuggets. This is the tempering process.
In the present invention, it is necessary that the energization conditions of the third step and the above-described first step satisfy the above formula (1).

The third step is a step for tempering the nugget, not for remelting or welding the nugget as in the conventional post-heating energization. Since this step is performed under an energization condition that provides less energy than the first step, the nugget is tempered to improve its toughness and to obtain a high joint strength (especially, a high CTS).
In order to obtain such effects more reliably, it is preferable that the energization conditions in the first step and the third step satisfy the following formula (1').
_I12 * _t1 * _0.4≤I32 * ^{t3≤I12 *} _t1 ^* 0.8 _{( 1} ')

(Electricity condition)
The energization conditions (e.g., energization current, energization time, electrode pressure, etc.) in the third step are such that the above nugget can be tempered, and the energization current I ₃ and energization time t ₃ are the above formula (1 ) is satisfied, other conditions are not particularly limited.

(Second electrode)
As the pair of second electrodes used in the third step, any electrode that can be used for normal resistance spot welding can be adopted as long as the portions corresponding to the nuggets of the sandwiched steel plates can be tempered.

In the present invention, the pair of second electrodes used in the third step may be the same electrode as or different from the pair of first electrodes used for the first electrodes.
If the first electrode and the second electrode are the same electrode, the first step and the third step can be performed with the same electrode, so there is an advantage that the manufacturing equipment can be simplified.
On the other hand, if the first electrode and the second electrode are different electrodes, a plurality of cycles can be performed in parallel when the first step to the third step are defined as one cycle as described later. Therefore, productivity can be further improved. Furthermore, since the optimum electrode can be selected for each of the main energization in the first step and the tempering in the third step, each step can be controlled more accurately and efficiently. be.

In this specification, that the first electrode and the second electrode are the same electrode means that the electrode used in the first step and the electrode used in the third step are the same. It means that the electrode used in the process is also used in the third process.
On the other hand, that the first electrode and the second electrode are different electrodes means that the electrode used in the first step is different from the electrode used in the third step, that is, the electrode used in the first step A different electrode is meant to be used in the third step.
Therefore, for example, even if the first electrode and the second electrode have exactly the same structure and size, if the first electrode used in the first step is not used in the third step, they are different electrodes. becomes.

In the present invention, when the first electrode and the second electrode are different electrodes, the electrode tip diameter D1 of the _first electrode and the electrode tip diameter D2 of the _second electrode are obtained by the following formula (2 ) is preferable.
D ₁ ≤ D ₂ (2)
The electrode tip diameter D1 of the first electrode and the electrode tip diameter D2 of the second electrode satisfy the above formula ( ₂ ), that is, the electrode tip diameter D2 of the _second electrode is the electrode tip diameter D of the _first electrode When the tip diameter is ₁ or more, in the third step, not only the nugget formed by the first electrode, but also the corona bond portion and the heat affected zone adjacent to the nugget are easily tempered by energization. The toughness of the joint can be further improved, and as a result, high joint strength (in particular, high CTS) can be obtained more reliably.

Furthermore, in the present invention, when there are a plurality of locations to be welded where the first to third steps are performed, when the second step is performed at one location to be welded, other It is preferable to perform the first step in parallel at the welding target locations. By welding a plurality of locations to be welded in parallel in this manner, welded joints having high joint strength can be continuously produced, thereby further improving productivity.
It should be noted that while the second step is being performed on one portion to be welded, the third step may be performed on another portion to be welded in parallel. Even in this manner, the same effect as described above can be obtained by welding a plurality of welding target portions in parallel.

<Steel plate>
In the present invention, the plurality of steel plates to be welded includes at least one steel plate having a tensile strength of 440 MPa or more, and the other steel plates are steel plates according to the desired joint strength and the application of the welded joint. can be used. Such steel sheets include, for example, steel sheets having a tensile strength of 270 MPa to 2500 MPa class, and such steel sheets may be steel sheets plated with zinc or the like (that is, plated steel sheets).

In addition, as long as the steel plate has a tensile strength of 440 MPa or more, a steel plate suitable for the desired joint strength and the application of the welded joint can be used. A steel plate having a tensile strength of 1470 MPa or more, and particularly preferably a steel plate having a tensile strength of 1500 MPa or more.
Among such steel sheets, a steel sheet having a carbon equivalent of 0.22% by mass or more and 0.40% by mass or less can be suitably used. When a steel sheet with such a specific carbon equivalent is used, in the above-described third step, the heat-affected zone near the corona bond portion adjacent to the nugget has a Vickers hardness of 280 or more and 420 or less. As a result, it is possible to more stably and reliably obtain a high joint strength that is excellent in CTS and that the nugget is less likely to break in an in-plane tensile test. Although the Vickers hardness will be described later, as shown in FIG. 1, the Vickers hardness is measured substantially parallel to the surface of the steel sheet at a position 0.2 mm away from the surface of the steel sheet on the overlapping surface side. In the case of a weld of different steel sheets, the steel sheet with the highest tensile strength shall be measured. The position of the heat affected zone can be confirmed by, for example, picral etching. Moreover, it is preferable to measure the Vickers hardness with a load of about 1 kgf.

In the present invention, if the plurality of steel plates includes at least one steel plate having a tensile strength of 440 MPa or more, even if all the steel plates are the same type, only some of the steel plates are the same type. , or all the steel plates may be different types of steel plates.

Furthermore, the number of steel sheets is not particularly limited, and two or more sheets can be employed depending on the use of the welded joint.

In addition, in the present invention, predetermined treatment steps may be provided before and after each step of the first to third steps.

The method of manufacturing a resistance spot-welded joint of the present invention is not limited to the above-described embodiments and examples described later, and can be appropriately combined, substituted, changed, etc. within the scope of the purpose and gist of the present invention. It is possible.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited only to such examples.

(Example 1)
A pair of DR-type electrodes having an electrode tip diameter (diameter) of 6 mm and an electrode tip curvature radius of R40 were provided as electrodes (that is, first and second electrodes) commonly used in the first step and the third step. , a stationary spot welder was prepared.

Furthermore, as test materials, steel plate 1 (plate thickness 1.6 mm) with a tensile strength of 1800 MPa, steel plate 2 (plate thickness 1.6 mm) with a tensile strength of 1180 MPa, and steel plate 3 (plate thickness 0.75 mm) with a tensile strength of 270 MPa ) are prepared, and these three steel plates are stacked in order of steel plate 1, steel plate 2, and steel plate 3 to form a plate set, and then the plate set is connected to a pair of electrodes of the stationary spot welder. (applied force: 450 kgf), and energized with an energizing current I ₁ of 8.0 kA and an energizing time t ₁ of 0.40 sec to form a nugget (first step).
After the first step, the pair of electrodes was opened, and this open state was maintained for 10 seconds to cool the nugget to a temperature below the _Mf point (second step).
After the second step, the portions of the three steel plates corresponding to the nuggets were sandwiched by the same pair of electrodes as those used in the first step (pressing force: 450 kgf), and the current I ₃ was 5.5. The nugget was tempered by energizing at 0 kA and an energizing time t ₃ : 0.40 sec (third step), and a resistance spot welded joint of Example 1 was obtained.
The energization conditions in the first step and the third step are I ₁ ² ×t ₁ =25.6 and I ₃ ² ×t ₃ =10.0, which satisfy the above formula (1).
Further, in manufacturing the resistance spot welded joint of Example 1, the first step of a different cycle was carried out during the second step.

(Examples 2 to 20)
Resistance spot welded joints of Examples 2 to 20 were obtained in the same manner as in Example 1, except that the electrodes, energization conditions and cooling conditions were changed to the conditions shown in Table 1 below.
In Examples 4 and 5, different electrodes were used in the first step and the third step.

(Examples 21-23)
Example 1 was carried out in the same manner as in Example 1, except that two steel plates shown in Table 1 below were used as the steel plates, and that the electrodes, energization conditions, and cooling conditions were changed to the conditions shown in Table 1 below. 21-23 resistance spot welded joints were obtained.
Further, in manufacturing the resistance spot welded joints of Examples 21 to 23, the third step of a different cycle was carried out during the second step.

(Comparative example 1)
A resistance spot-welded joint of Comparative Example 1 was obtained in the same manner as in Example 1, except that the second step and the third step were not performed.

(Comparative example 2)
A resistance spot-welded joint of Comparative Example 2 was obtained in the same manner as in Example 1, except that the current I3 in the _third step was changed to 8.0 kA.
The energization conditions in the _first step and the _third step ^are I12*t1=25.6 and _I32 *t3=25.6 _, which do not satisfy the above formula ( ¹ ).
Further, in manufacturing the resistance spot welded joint of Comparative Example 2, the welding points are shifted by 3 mm between the first step and the third step.

(Comparative Example 3)
In the second step, the electrode was cooled while being pressed for 2 seconds without being opened, and in the _third step, the current I3 was changed to 5.8 kA, except that it was the same as in Example 1. A resistance spot-welded joint of Comparative Example 3 was obtained in the same manner.

In Examples 1 to 23 and Comparative Examples 1 to 3 described above, the retention times in the first step are as follows.
Retention time of Examples 1-8 and Comparative Examples 1-3: 0.05 seconds Retention time of Examples 9-16: 0.2 seconds Retention time of Examples 17-23: 0.4 seconds

Regarding the properties of the resistance spot welded joints of Examples 1 to 23 and Comparative Examples 1 to 3 thus obtained, the heat affected zone (HAZ ) hardness (the hardness of the harder one of the two HAZs near the corona bond), cross tensile strength (CTS) between steel plates 1 and 2, and fracture in the nugget in the in-plane tensile test The presence or absence of each was confirmed.
In addition, these confirmations were all performed with n number of 5, and the average values are shown in Table 1 below.

Here, FIG. 1 is a diagram showing the measurement positions of the hardness distribution of the resistance spot welded joint obtained in the example of the present invention, and FIG. 2 is the resistance spot welded joint obtained in the example of the present invention. FIG. 2 is a hardness distribution diagram in a direction parallel to the bonding interface of (Example 1 and Comparative Example 1).
The hardness distribution of the resistance spot welded joint was measured using a Vickers hardness meter, as shown in FIG. measured to The measurement load was 1 kgf, and the measurement was performed at a predetermined pitch with one nugget end as a base point, and particularly the vicinity of the nugget boundary was measured at a pitch of 0.2 mm or 0.25 mm.
Then, the average hardness in the range of 0.5 mm from the boundary of the nugget to the heat affected zone (HAZ) was obtained as the hardness of the HAZ near the corona bond adjacent to the nugget.

Moreover, the CTS between the steel plates 1 and 2 was measured by a cross tension test based on JIS Z 3137. As the measurement sample, two steel plates having a length of 150 mm and a width of 50 mm were stacked in a cross shape and the overlapping portions were joined by spot welding.

In the above-mentioned in-plane tensile test, three steel plates with a length of 200 mm and a width of 15 mm were superimposed so that the length direction and the width direction respectively corresponded, and the center part and both ends in the length direction Measurement was performed by a tensile test using a measurement sample in which points were joined by spot welding. The spot-welded portions at both ends in the length direction are for fixing the grip portions, and portions 30 mm apart from both ends in the length direction of the measurement sample were used as the grip portions for the tensile test.

In Comparative Example 1 in which the second step and the third step were not performed, as shown in Table 1 and FIG. 2, the hardness of the HAZ near the corona bond portion adjacent to the nugget was high, and the toughness was not improved. Therefore, the CTS was low. In addition, in Comparative Example 2 in which the nugget was not tempered under the energization conditions satisfying the above formula (1) in the third step, as shown in Table 1, the HAZ near the corona bond portion adjacent to the nugget had a high hardness and a toughness. was not improved, the CTS was low, and fracture occurred in the nugget even in the in-plane tensile test.
In addition, in Comparative Example 3, in which the electrodes were cooled in a pressurized state without opening in the second step, the hardness of the HAZ near the corona bond portion adjacent to the nugget was low and the toughness was improved, as shown in Table 1. Therefore, although the CTS was high, breakage occurred in the nugget in the in-plane tensile test.
On the other hand, in Examples 1 to 23, which are examples of the present invention, the hardness of the HAZ near the corona bond portion adjacent to the nugget is low as shown in Table 1, and the toughness is improved, so the CTS is high. Furthermore, no breakage occurred in the nugget in the in-plane tensile test.
Therefore, according to the present invention, even a resistance spot welded joint using a plurality of steel plates including a high-strength steel plate having a tensile strength of 440 MPa or more is tested at n number 5, and high joint strength is stabilized. It was found that

According to the present invention, even a resistance spot welded joint using a plurality of steel plates including a high-strength steel plate having a tensile strength of 440 MPa or more can stably obtain a high joint strength. It can be applied to the manufacture of various structural parts such as vehicle body parts and building structures.
Therefore, the method for manufacturing a resistance spot welded joint of the present invention has high industrial applicability.

Claims (7)

A method for manufacturing a resistance spot welded joint using a plurality of steel plates including a steel plate having a tensile strength of 440 MPa or more,
a first step of sandwiching the plurality of superimposed steel plates between a pair of _first electrodes and energizing with a predetermined energizing current I1 and energizing time t1 to form _a nugget;
a second step of releasing the pair of first electrodes and cooling the nugget to a temperature below the _Mf point;
a _third step of tempering the nuggets by sandwiching the portions of the plurality of steel plates corresponding to the nuggets with a pair of second electrodes, and applying current with a predetermined current _I3 and current time t3; including
The manufacturing method, wherein the energization conditions of the first step and the third step satisfy the following formula (1).
I ₃ ² ×t ₃ <I ₁ ² ×t ₁ (1) The manufacturing method has a plurality of welding target locations where the first step to the third step are performed,
2. The manufacturing method according to claim 1, wherein said first step is concurrently performed at another welding target location while said second step is being performed at one welding target location. The manufacturing method has a plurality of welding target locations where the first step to the third step are performed,
2. The manufacturing method according to claim 1, wherein the third step is performed at another welding target portion while the second step is being performed at one welding target portion. The manufacturing method according to any one of claims 1 to 3, wherein the first electrode and the second electrode are different electrodes. The manufacturing method according to any one of claims 1 to 4, wherein the electrode tip diameter D1 of the _first electrode and the electrode tip diameter D2 of the _second electrode satisfy the following formula (2).
D ₁ ≤ D ₂ (2) The carbon equivalent of the steel sheet having a tensile strength of 440 MPa or more is 0.22% by mass or more and 0.40% by mass or less,
6. The method according to any one of claims 1 to 5, wherein in the third step, a portion having a Vickers hardness of 280 or more and 420 or less is formed in the heat-affected zone near the corona bond portion adjacent to the nugget. Production method. The manufacturing method according to any one of claims 1 to 6, wherein the second step has a process time of 3 seconds or more and 600 seconds or less.

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