JP7260761B2 - Manufacturing method of resistance spot welded joint and resistance spot welded joint - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act 1: [Lecture Summary] General Incorporated Association The Welding Society 2018 Spring National Convention Summary of the Welding Society National Convention Lecture No. 102 (2018-4) 2: [Lecture] ・General Incorporated Association The Japan Welding Society 2018 Spring National Convention Lecture at the National Welding Society Conference Lecture slides

TECHNICAL FIELD The present invention relates to a method for manufacturing a resistance spot welded joint of steel plates and the resistance spot welded joint.

BACKGROUND ART In assembling an automobile body, a process of overlapping steel plates and joining them by resistance spot welding is frequently used. On the other hand, in order to reduce the weight and increase the strength of the vehicle body, it is desirable to use a high-strength steel sheet as the steel sheet for frame parts and the like. However, when high-strength steel plates are resistance spot-welded, sufficient joint strength (welded portion strength) may not be obtained. In particular, the tensile strength in the peeling direction of spot-welded joints, for example, the peel strength represented by Cross Tension Strength (hereinafter referred to as "CTS") tends to decrease as the strength of the steel plate increases. . Therefore, application of high-strength steel sheets to the vehicle body and weight reduction of the vehicle body may be hindered.

As measures to improve the peel strength of spot-welded joints, measures such as enlarging the nugget diameter and post-heating are considered effective, but enlarging the nugget diameter makes it difficult to control expulsion, In some cases, there is no choice but to increase the width, and post-energization has the problem of prolonging the total energization time and lowering productivity, so it is sometimes difficult to apply these measures.
Therefore, from these points of view, it is strongly desired to develop a method for improving the peel strength in resistance spot welding of high-strength steel plates without enlarging the nugget diameter or applying post-heating.

By the way, in Patent Document 1, the object is to provide a lap resistance welding method and a lap weld joint that have a low nugget cracking sensitivity and are less likely to break in the nugget. In the lap resistance welding method of spot-welding, a method of inserting a thin iron insert material between two high-strength steel plates and spot-welding in that state has been proposed.

That is, in Patent Document 1, in a lap resistance welding method in which the materials to be joined are overlapped and spot-welded, the materials to be joined have a tensile strength of 340 MPa or more and a plate thickness of 0.5 to 10 mm. (cold-rolled steel sheet or hot-rolled steel sheet), and, in mass%, between the materials to be joined, C: 0.1% or less, Si: 1.4% or less, Mn: 2.0% or less, P: 0 .15% or less, S: 0.03% or less, Al: 0.1% or less, and the plate thickness t1 is 1/50 or more and 1/ A lap resistance welding method has been proposed in which an iron insert material of less than 5 is placed between high-strength steel plates and welded.

In addition, as reported in Non-Patent Document 1, the present inventors used hot stamped steel (HS steel) with a tensile strength of 1470 MPa class as a high-strength steel plate, and two sheets with a plate thickness of 1.6 mm were used. An insert material made of a steel plate with a thickness of 1.6 mm is sandwiched between the HS steel plates, resistance spot welding is performed, and Tensile Shear Strength (hereinafter referred to as “TSS”) is used as the shear strength of the welded joint. ) and the aforementioned CTS and L-type Tension Strength (hereinafter referred to as “LTS”) as peel strength.

JP 2008-173666 A

Seiji Kosako, Yasunobu Miyazaki, “Improvement of strength of spot-welded joints of high-strength steel plates using insert materials” General Incorporated Association Welding Society 2017 Welding Spring National Conference Summary of Lecture (announced on April 21, 2017) Seiji Kosako et al., "Recent Problems in Automobile Body Joining Technology and Countermeasure Technologies - Part 1", Shin Nippon Steel Technical Report No. 393 (2012), p.69-75

In the technique of Patent Document 1, as an insert material, an iron insert material whose plate thickness t1 is 1/50 or more and less than 1/5 of the plate thickness tmin of the thinner high-strength steel plate, that is, the high-strength steel plate to be welded. A remarkably thin foil-like insert material is used. However, according to experiments by the present inventors, it has been found that when such a thin foil-like insert material is used, the peel strength cannot be sufficiently improved. In addition, in the case of Patent Document 1, although a high-strength steel sheet of 780 MPa is used as the object to be welded in the examples, it has not been confirmed what kind of effect can be obtained in the case of a higher-strength steel sheet.

On the other hand, in Non-Patent Document 1, when hot stamping steel (HS steel) having a tensile strength of 1470 MPa class is used as the high-strength steel plate, an insert material having the same plate thickness is sandwiched between the HS steel plates to be welded. , conducted resistance spot welding experiments, and reported that in this case, the peel strength (CTS and LTS) of the welded joint was improved compared to the case where the insert material was not inserted. However, it cannot be said that the peel strength has been improved to a sufficient degree yet, and it is not clear whether sufficient welding strength can be reliably obtained when using high-strength steel sheets for automobile bodies and the like. In addition, in the report of Non-Patent Document 1, although the peel strength is slightly improved compared to the case where the insert material is not used, the shear strength of the spot welded portion, that is, the tensile shear strength (TSS) is greatly reduced. A trend has been confirmed.

The present invention has been made against the background of the above circumstances. , to provide a method for producing a spot-welded joint capable of exhibiting sufficiently high peel strength without significantly lowering shear strength, and at the same time, to provide resistance spot welding of a plurality of steel plates, at least one of which has a tensile strength of 780 MPa or more. An object of the present invention is to provide a welded joint having both high peel strength and high shear strength as a joint.

Based on the report of Non-Patent Document 1, the present inventors have found that in resistance spot welding of a steel plate having a tensile strength of 780 MPa or more, the peel strength can be sufficiently improved without significantly reducing the shear strength. We considered measures. As a result, by appropriately adjusting the ratio of the thickness of the insert material to the thickness of the steel plate to be welded (plate thickness ratio), high peel strength can be obtained without causing a significant decrease in shear strength. This finding led to the completion of the present invention.

Specific embodiments of the present invention are shown below.

The basic aspect (first aspect) of the resistance spot welding method of the present invention comprises:
In a method for manufacturing a resistance spot welded joint in which a plurality of steel plates, at least one of which has a tensile strength of 780 MPa or more, are superimposed,
In stacking a plurality of the steel plates by inserting an insert plate made of steel between them and performing resistance spot welding between the plurality of steel plates through the insert plate,
As the insert plate, the plate thickness ti is related to the plate thickness th of the steel plate,
1.0>ti/th>0.20
and the size of the plate surface is 1.1 times or more the diameter dn of the nugget formed by resistance spot welding, and the area of the plate surface is
Sn=π×(dn/2) ²
It is characterized by using a plate material that is 100 times or less the area Sn of the nugget obtained by.
Here, when the plate thicknesses of the plurality of steel plates sandwiching the insert plate are different from each other, the plate thickness of the thinner steel plate is th.

Also, the resistance spot welded joint of the second aspect of the present invention comprises:
In a spot-welded joint in which a plurality of steel plates, at least one of which has a tensile strength of 780 MPa or more, are superimposed,
An insert plate using steel is interposed between the plurality of steel plates, and a nugget by spot welding is formed across the steel plate and the insert plate,
In the insert plate, the plate thickness ti is related to the plate thickness th of the steel plate,
1.0>ti/th>0.20
and the size of the plate surface is 1.1 times or more the nugget diameter dn, and the area of the plate surface is
Sn=π×(dn/2) ²
The plate material is characterized by being 100 times or less of the area Sn of the nugget obtained by.
Here, when the plate thicknesses of the plurality of steel plates sandwiching the insert plate are different from each other, the plate thickness of the thinner steel plate is th.

According to the present invention, it is possible to obtain high peel strength without significantly reducing the shear strength of the welded joint when resistance spot welding is performed by stacking steel sheets, at least one of which has a tensile strength of 780 MPa or more. became.

1 is a schematic vertical cross-sectional view at a stage immediately before spot welding in a resistance spot welding method according to one embodiment of the present invention; FIG. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1; FIG. 4 is a schematic vertical cross-sectional view after spot welding in the resistance spot welding method of one embodiment of the present invention; 4 is a cross-sectional view taken along line IV-IV of FIG. 3; FIG. FIG. 4 is a schematic diagram for explaining the state of stress concentration at the nugget end of the welded joint when no insert plate is used; FIG. 4 is a schematic diagram for explaining the state of stress concentration at the nugget end of the welded joint when using an insert plate with an appropriate plate thickness; FIG. 4 is a schematic diagram showing the shape and dimensions of an LTS test piece without using an insert plate in experiments by the present inventors. FIG. 2 is a schematic diagram showing the shape and dimensions of an LTS test piece using an insert plate in experiments conducted by the present inventors. 4 is a graph showing the relationship between the thickness of the insert plate and the maximum load for the TSS test piece, according to experiments conducted by the inventors. 4 is a graph showing the relationship between the thickness of the insert plate and the maximum load for the CTS test piece, according to experiments conducted by the present inventors. 4 is a graph showing the relationship between the thickness of the insert plate and the maximum load for LTS test pieces, according to experiments conducted by the present inventors. FIG. 4 is a schematic diagram showing another embodiment of a spot-welded joint to which the present invention is applied; FIG. 4 is a schematic diagram showing still another embodiment of a spot-welded joint to which the present invention is applied;

EMBODIMENT OF THE INVENTION Below, embodiment of this invention is described in detail with reference to drawings.

1 to 4 schematically show one embodiment of the resistance spot welding method of the present invention.

1 and 2 show the situation immediately before spot welding, and FIGS. 3 and 4 show the situation after spot welding. In this embodiment, for example, two steel plates 1A and 1B having flange portions 2A and 2B formed by bending the edges in the width direction into an L shape, the plate surfaces of the flange portions 2A and 2B are are overlapped, and resistance spot welding is performed using the flange portions 2A and 2B as welding target portions. In other words, it is shown as an example of forming an L-shaped joint.

In performing resistance spot welding, as shown in FIGS. 1 and 2, two steel plates 1A and 1B, at least one of which has a tensile strength of 780 MPa or more, are welded (flange portions 2A and 2B), and the steel plate is placed between them. are superimposed with an insert plate 3 having a predetermined thickness. Then, resistance spot welding is performed at the position where the insert plate 3 is inserted to form a nugget 5 at a portion corresponding to the inner side of the outer edge of the insert plate 3 as shown in FIGS. In this embodiment, the shape of the plate surface of the insert plate 3 (the shape of the surface in the direction perpendicular to the thickness) is square as an example.

At least one of the steel plates 1A and 1B to be welded should have a tensile strength of 780 MPa or more, preferably 980 MPa or more, and more preferably 1180 MPa or more. Both of the two steel plates 1A and 1B may have a tensile strength of 780 MPa or more.
Here, as the insert plate 3, the plate thickness ti is related to the plate thickness th of the steel plates 1A and 1B to be welded.
1.0>ti/th>0.20
Use a steel plate within the range of In this embodiment, the two steel plates 1A and 1B to be welded have the same thickness th, but in some cases, the two steel plates to be welded may have different thicknesses. , the above relationship should be satisfied with the plate thickness of the thinner steel plate being th.

Furthermore, the size of the plate surface of the insert plate 3 (for example, the dimension W in the width direction of the flange portions 2A and 2B and the dimension L in the longitudinal direction of the flange portions 2A and 2B) is 1 of the diameter dn of the formed nugget 5. .1 times or more, and the plate surface area Si is
Sn=π×(dn/2) ²
100 times or less of the nugget area Sn obtained by Here, the nugget area Sn is a value calculated from the nugget diameter dn by the above formula, assuming that the shape of the nugget 5 on a plane parallel to the plate surface of the insert plate 3 is a perfect circle.

The details of the strength conditions of the steel plates 1A and 1B, the plate thickness conditions of the insert plates, and the plate surface size and area conditions of the insert plates and the reasons for their limitations will be described in the next section.

<Strength of steel plate to be welded>
For example, as shown in FIG. 1 of Non-Patent Document 2, the tensile shear strength (TSS), which is an index of the shear strength of a spot welded joint, increases as the tensile strength of the two steel plates to be welded increases. showing an upward trend. On the other hand, the cross tensile strength (CTS), which is one index of peel strength, tends to increase slightly as the tensile strength of the steel sheets increases when the tensile strength of the two steel sheets to be welded is less than 780 MPa. However, when the tensile strength of the two steel plates is 780 MPa or more, it tends to decrease as the tensile strength of the two steel plates increases. In addition, L-shaped tensile strength (LTS), which is another indicator of peel strength, is not shown in FIG. It has been confirmed that the tensile strength of the two steel sheets tends to decrease as the tensile strength increases.

As is clear from the above, when the tensile strength of the two steel plates to be welded is less than 780 MPa for both sheets, the decrease in peel strength is not observed or is slight, so measures against decrease in peel strength There is little need to take Therefore, in the present invention, at least one steel sheet having a tensile strength of 780 MPa or more is targeted. If the tensile strength of at least one of the steel plates to be welded is 980 MPa or more, the peel strength is greatly reduced. do.

<Action by inserting the insert plate>
The main cause of the low peel strength of spot-welded joints of steel plates is thought to be the large stress concentration at the edge of the nugget (weld metal) when tension in the peeling direction acts on the welded joint. Here, FIG. 5 schematically shows a cross-sectional state near the end of the nugget 5 when the two steel plates 1A and 1B are overlapped and spot-welded without inserting the insert plate 3 . FIG. 6 schematically shows the cross section of the nugget 5 near the end when two steel plates 1A and 1B are superimposed and spot-welded with an insert plate 3 having a certain thickness interposed therebetween.

In the case of FIG. 5 where the insert plate 3 is not interposed, if a tensile load acts, the region from the end 5a of the nugget 5 to the heat affected zone (HAZ) of the two steel plates 1A and 1B adjacent thereto (the A large stress concentrates on the portion P) enclosed by the dashed line. On the other hand, in the case of FIG. 6 in which the insert plate 3 having a certain thickness or more is inserted, if a tensile load acts, the two steel plates 1A and 1B adjacent to the nugget 5 from the two ends 5b and 5c are pulled apart. Although stress concentrates in each of the regions (portions Q and R enclosed by the dashed line in FIG. 6) leading to the heat affected zone (HAZ), the stress concentration points are dispersed in two places, so the insert plate 3 is not inserted (FIG. 5). The concentrated stress at each location is much smaller than in the case of . As a result, it is considered that high peel strength can be ensured by inserting an insert plate having a certain thickness or more and performing spot welding.

<Thickness of insert plate>
The plate thickness ti of the insert plate is related to the plate thickness th of the steel plate to be welded,
1.0>ti/th>0.20
within the range that satisfies When the two steel plates to be welded have different plate thicknesses, the above relationship should be satisfied with the plate thickness of the thinner steel plate being th.
Here, when the plate thickness ratio ti/th is 0.20 or less, the effect of dispersing the stress concentration at the nugget edge is not sufficiently obtained, and it becomes difficult to prevent the peel strength from decreasing. On the other hand, if the plate thickness ratio ti/th is 1.0 or more, the shear strength (tensile shear strength: TSS) of the spot-welded joint will decrease. Therefore, the plate thickness ratio ti/th was determined as described above.
The plate thickness ratio ti/th is preferably within the range of 0.3 to 0.8, even within the above range.
The relationship between the plate thickness ratio ti/th and the peel strength and shear strength as described above was found through experiments by the present inventors, and the outline of the experiments will be described below.

<Experiment on thickness of insert plate>
As the steel plate to be welded, a 1470 MPa class hot stamped steel (hereinafter referred to as HS steel) with a plate thickness of 1.6 mm is used, and as an insert plate, a 270 MPa class carbon steel (hereinafter referred to as HS steel) with a plate thickness of 0.2 to 1.6 mm , 270 steel), the following experiment was conducted.
That is, spot welding was performed for two modes, one in which two sheets of HS steel were superimposed without an insert plate, and the other in which 270 steel of various thicknesses were sandwiched between two HS steels as an insert plate and superimposed. Then, three types of joint tensile test pieces were prepared.

Here, as the joint tensile test piece, a tensile shear test piece (TSS test piece) conforming to JIS Z 3136 (confirmed in 2013, 2014 edition), a cross tension conforming to JIS Z 3137 (confirmed in 2013, 2014 edition) Tensile tests were performed on each of three types of test piece (CTS test piece) and L-shaped tensile test piece (LTS test piece). Although there is no standard for the LTS test piece, the shape and dimensions shown in FIGS. 7 and 8 were used.
Each tensile test on a test piece in which two HS steels are superimposed without an insert plate (that is, a test piece in which the thickness of the insert plate is zero) and a test piece in which insert materials of various plate thicknesses are interposed. 9 to 11 show the relationship between the maximum tensile load and the plate thickness of the insert material. FIG. 9 shows results for TSS specimens, FIG. 10 shows results for CTS specimens, and FIG. 11 shows results for LTS specimens.

From FIG. 9, in the TSS test piece, when the insert plate is inserted, the maximum load tends to be slightly lower than when the insert plate is not inserted (when the plate thickness of the insert plate is zero), that is, the shear strength TSS as a tendency to decrease was observed.
On the other hand, in the CTS test piece and the LTS test piece, when the insert plate is inserted, the maximum load tends to increase more than when the insert plate is not inserted (when the plate thickness of the insert plate is zero). Especially in the LTS test piece, if the plate thickness of the insert material exceeds 0.32 mm (thus, if the plate thickness ratio ti/th exceeds 0.20), the maximum load stably becomes large, that is, stably It was observed that high peel strength was obtained.
From these results, it is possible to newly recognize the effectiveness of improving the peel strength by inserting the insert plate, especially the effectiveness of improving the peel strength by the plate thickness ratio ti/th exceeding 0.20. I was able to do it.
Regarding the TSS test piece, the case where the plate thickness of the insert plate exceeds 1.6 mm (that is, the case where the plate thickness ratio ti/th exceeds 1.0) is not shown in the above experiment, but another Experiments have confirmed that the TSS is further reduced.

<Shape of insert plate>
The shape of the insert plate (the shape of the plate surface along the direction orthogonal to the thickness) may be square, rectangular, circular, oval, or elliptical, and the shape of the part to be spot-welded, etc. The most suitable shape should be selected according to the

<Size and surface area of insert plate>
The size of the plate surface of the insert plate should be 1.1 times or more the target nugget diameter dn to be formed by spot welding.
Here, the size of the plate surface of the insert plate means the length of each side when the plate surface shape of the insert plate is square or rectangular. Therefore, in this case, the minimum length of each side of the insert plate should be at least 1.1 times the nugget diameter dn. Further, when the plate surface shape of the insert plate is circular, oval, or elliptical, the size of the insert plate means its diameter (diameter in all directions passing through the center). That is, in this case, the minimum diameter of the insert plate should be 1.1 times or more the nugget diameter dn. Preferably, the size of the plate surface of the insert plate is 1.5 times or more the nugget diameter.

If the size of the plate surface of the insert plate is less than 1.1 times the nugget diameter dn, a phenomenon in which the molten metal protrudes from the outer edge of the insert plate due to the pressure during resistance spot welding, that is, expulsion occurs, and the nugget diameter increases. may fall short of the target value.
On the other hand, the area Si of the insert plate is the area Sn of the nugget, that is, Sn=π×(dn/2) ²
If the value of Sn exceeds 100 times the value obtained by , there is concern that weight reduction of automobile bodies and the like will be hindered. Preferably, the area Si of the insert plate is 80 times or less the area Sn of the nugget.

Here, the reason why the insert plate is used in the present invention is to improve the peel strength of the spot welded portion. This is because the peel strength is increased by Therefore, in the case of the present invention, it is sufficient that the insert material is locally present only in the region that includes the nugget and surrounds the outside of the nugget.

Furthermore, the nugget diameter varies depending on the welding conditions.
3√th≦dn≦5√th
Welding conditions are generally selected so that a nugget diameter dn that satisfies is obtained. Therefore, the size of the plate surface of the insert plate may also be set according to the target nugget diameter dn determined by the welding conditions and the plate thickness th of the steel plate to be welded.

Here, the nugget diameter dn is the diameter of the nugget, which is cut perpendicular to the plate surface so as to pass through the center of the indentation (dent) that appears on the joint surface after spot welding, and the cut surface is polished. , is obtained by chemical etching (for example, immersion in a picric acid alcohol solution) to reveal a nugget, and measuring the length (nugget diameter) parallel to the plate surface of the nugget with an optical microscope. Spot welding under conditions that cause expulsion means that the welding conditions are inappropriate. In this case, the maximum diameter including the flash portion is taken as the nugget diameter.

In carrying out the spot-welded joint manufacturing method of the present invention, for example, as a preliminary experiment, the steel plate and the insert plate are overlapped and spot-welded in advance, and the nugget diameter dn is measured by the above method. And when actually manufacturing a spot-welded joint, the size of the plate surface is 1.1 times or more of dn, and the area of the plate surface is 100 times or less of the nugget area Sn = π × (dn / 2) ² Insert A spot-welded joint may be produced using a plate under the same conditions as in the preliminary experiment. Here, the conditions are the conditions that determine the diameter of the nugget, such as current, time, pressure, and the like.

<Insert plate material>
As the insert material, an appropriate steel plate can be used, such as a steel plate having a strength comparable to that of the steel plate to be welded, or a steel plate having a lower strength than that. However, as the steel plate for the insert material, it is desirable to use steel with a carbon content (C content) exceeding 0.1% by mass. This is because there is concern that if the amount of C in the insert plate is 0.1% or less, the shear strength (TSS) of the welded joint may become low.

<Form of welded joint>
In the example shown in FIGS. 1 to 4, two steel plates 1A and 1B having flange portions 2A and 2B formed by bending L-shaped edges in the width direction are provided with an insert plate 3 between them. An example is shown in which the plate surfaces of the flange portions 2A and 2B are sandwiched and resistance spot welding is performed using the flange portions 2A and 2B as welding target portions. The form is not limited. For example, as shown in FIG. 12, the end portions of bent steel plates 1A and 1B without flange portions may be overlapped with an insert plate 3 interposed therebetween, and resistance spot welded to form a joint portion. Alternatively, as shown in FIG. 13, a joint portion may be formed by crossing bent steel plates 1A and 1B without flange portions, overlapping them with an insert plate 3 interposed therebetween, and applying resistance spot welding to the overlapping portion. .

Moreover, the steel plates to be welded are not limited to two-ply, but may be three or more multi-ply. In this case, the insert material may be sandwiched between all the steel plates, or the insert plate may be sandwiched only between some (for example, two) steel plates. In this case, between which steel plates the insert plate is sandwiched may be selected depending on which steel plates are desired to have a high peel strength.

<Spot welding conditions>
In resistance spot welding, the superimposed members are arranged so that the center of the insert plate coincides with the spot welding point, and the nugget is formed across the steel plates on both sides of the insert plate. As the welding conditions at this time, the conditions for welding a normal set of three steel plates can be adopted. For example, the relationship between the nugget diameter dn and the plate thickness th of the steel plate to be welded is
3√th≦dn≦5√th
Welding conditions may be selected so as to obtain a nugget diameter dn that satisfies

Examples of the present invention are described below together with comparative examples.

A steel plate with a thickness of 1.6 mm (C content is 0.22% by mass) is used as a steel plate to be welded, and steel plates with a tensile strength of 1520 MPa and various thicknesses (C content is 0.22% by mass) are used as insert plates. , were combined and resistance spot welded to prepare an LTS test piece as shown in FIG. 7 or 8 and a TSS test piece conforming to JIS Z 3136. Here, the shape of the plate surface of the insert plate was square (therefore, the width W and the length L were the same).

Table 1A shows the plate thickness th and tensile strength of the two steel plates to be welded (the upper steel plate and the lower steel plate). In addition, the plate thickness ti, tensile strength and C content of the insert plate, the size (W, L) and area Si of the insert plate, and the diameter dn and area Sn of the nugget are similarly calculated for the LTS test piece and the TSS test piece. Shown in Table 1A.
In addition, the ratio [ti/th] between the plate thickness ti of the insert plate and the plate thickness th of the steel plate to be welded, and the ratio [ W/dn], [L/dn], and the ratio of the insert plate area Si to the nugget area Sn [Si/Sn] for the LTS and TSS specimens are shown in Table 1B.

Furthermore, a tensile test was performed on each of the LTS test piece and the TSS test piece. Table 2 shows the results of evaluating the L-shaped tensile strength (LTS) for the LTS test piece and the tensile shear strength (TSS) for the TSS test piece, and examining the presence or absence of expulsion during spot welding. show. The evaluation of LTS and TSS is indicated by the ratio (strength ratio) to the LTS and TSS values of Comparative Example (Treatment No. 1) in which no insert material was used. Here, if the LTS strength ratio is 1.3 or more, it can be determined that a high peel strength has been obtained, and if the TSS strength ratio is 0.8 or more, it is determined that the decrease in shear strength is small. can do.

As shown in Tables 1A, 1B, and 2, in treatment numbers 3 to 5 and 7 within the conditions of the present invention, there was little decrease in shear strength, high peel strength was obtained, and spattering occurred. There was no
On the other hand, in the comparative example (Treatment No. 2) having a plate thickness ratio ti/th of 0.18, the LTS value was not sufficient, and the improvement in peel strength was insufficient.
Furthermore, in the comparative example (treatment number 6) in which the plate thickness ratio ti/th is 1.00 and the insert plate size (W, L) is less than 1.1 times the nugget diameter dn, the TSS value is low and the shear strength is The decrease in the value was large, and scattering occurred.

Preferred embodiments and experimental examples of the present invention have been described above, but these embodiments and experimental examples are merely examples within the gist of the present invention, and are within the scope of the gist of the present invention. Additions, omissions, substitutions, and other changes are possible. That is, the present invention is not limited by the foregoing description, but is limited only by the scope of the appended claims, and can of course be modified within the scope thereof.

1A, 1B High-strength steel plate to be welded
2A, 2B Flange (Welded part)
3 insert plate 5 nugget

Claims (2)

In a method for manufacturing a resistance spot welded joint in which a plurality of steel plates, at least one of which has a tensile strength of 780 MPa or more, are superimposed,
In stacking a plurality of the steel plates by inserting an insert plate made of steel between them and performing resistance spot welding between the plurality of steel plates through the insert plate,
As the insert plate, the plate thickness ti is related to the plate thickness th of the steel plate,
1.0>ti/th>0.20
and the size of the plate surface is 1.1 times or more the diameter dn of the nugget formed by resistance spot welding, and the area of the plate surface is
Sn=π×(dn/2) ²
A method for manufacturing a resistance spot welded joint, characterized by using a plate material that is 100 times or less than the nugget area Sn obtained by.
Here, when the plate thicknesses of the plurality of steel plates sandwiching the insert plate are different from each other, the plate thickness of the thinner steel plate is th. In a spot-welded joint in which a plurality of steel plates, at least one of which has a tensile strength of 780 MPa or more, are superimposed,
An insert plate using steel is interposed between the plurality of steel plates, and a nugget by spot welding is formed across the steel plate and the insert plate,
In the insert plate, the plate thickness ti is related to the plate thickness th of the steel plate,
1.0>ti/th>0.20
and the size of the plate surface is 1.1 times or more the nugget diameter dn, and the area of the plate surface is
Sn=π×(dn/2) ²
A resistance spot welded joint characterized by being a plate material 100 times or less than the area Sn of the nugget obtained by.
Here, when the plate thicknesses of the plurality of steel plates sandwiching the insert plate are different from each other, the plate thickness of the thinner steel plate is th.

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