JP2020199522A - Resistance-welding method for high strength steel sheet - Google Patents

Abstract

To solve such a problem that crack sensitivity increases due to HAZ softening in resistance-welding of a high strength steel sheet 3 of which a tensile strength exceeds 1 GPa.SOLUTION: According to this method, a heat treatment process, in which a part of a high strength steel sheet 3 to be welded to other steel sheet and surroundings thereof are pinched by a pair of heating electrodes and are electrically conducted while being heated to generate heat thereby increasing ductility, is performed on the high strength steel sheet alone. and thereafter, the high strength steel sheet 3 is overlapped with other steel sheet, and a weld nugget is formed between those steel sheets by pressurized electric conduction with a pair of welding electrodes.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for resistance welding of high-strength steel sheets.

Regarding spot welding of overlapped portions of steel sheets, Patent Document 1 describes that two steps, an energization heating step and a spot welding step, are taken. In the energization heating step, in a state where two steel plates are overlapped, a softened portion by tempering is formed around the planned welding portion by energization by a pair of annular electrodes. In the subsequent spot welding step, spot welding of the planned welding portion is performed by a pair of welding electrodes.

Further, Patent Document 2 describes that, regarding spot welding of aluminum-plated steel sheets, the plating layer between the steel sheets at the planned welding portion is softened and melted by energization control by a pair of welding electrodes and discharged from between the steel sheets. ing. By putting the steel plates in direct contact with each other without going through the plating layer, it is possible to perform spot welding stably without generation of scattering.

Japanese Unexamined Patent Publication No. 2017-131916 Japanese Patent No. 5392142

In recent years, steel sheets have been made thinner and stronger from the viewpoint of environmental protection. For example, in automobiles, the use of high-strength steel plates for vehicle body parts is being studied in order to reduce the weight for the purpose of improving fuel efficiency. However, when a high-strength steel plate is spot-welded, the periphery (HAZ (heat-affected zone)) where the weld nugget is formed is tempered and softened, and a large tensile residual stress due to melting and solidification of the steel plate remains. In the case of an automobile, the HAZ softened zone serves as a stress concentration starting point at the time of a collision, so that the HAZ softened portion is likely to break. In addition, the increase in residual stress increases the risk of delayed cracking due to hydrogen embrittlement.

The present invention solves the problem that the crack sensitivity is increased by the HAZ softened zone in the resistance welding of a high-strength steel plate having a tensile strength exceeding 1 GPa.

In the present invention, in order to solve the above problems, the high-strength steel sheet alone is heat-treated with a pair of heating electrodes before welding.

The resistance welding method for high-strength steel sheets disclosed herein is a method for welding a plurality of stacked steel sheets, and at least one of the plurality of steel sheets has a high strength having a tensile strength exceeding 1 GPa. It is a steel plate
A portion of the high-strength steel plate to be welded to another steel plate among the plurality of steel plates and its surroundings are sandwiched between a pair of heating electrodes and energized while applying pressure to generate heat, and ductility of the portion and its surroundings is generated. The heat treatment process of the above high-strength steel sheet alone and
The plurality of steel plates including the high-strength steel plate subjected to the heat treatment are superposed, and the parts to be welded of the plurality of steel plates are sandwiched between a pair of welding electrodes and energized while applying pressure to generate heat between the steel plates. Equipped with a welding process to form a welding nugget
The tip diameter Dh of the heating electrode in contact with the steel plate is larger than the tip diameter Dw of the welding electrode in contact with the steel plate.

According to this method, a high-strength steel sheet having a tensile strength of more than 1 GPa is a portion to be welded (hereinafter referred to as a “welded portion”) and a portion thereof by heat treatment by pressurization and energization using a pair of heating electrodes. Increased ductility around. Since the tip diameter Dh of the heating electrode is larger than the tip diameter Dw of the welding electrode, the high-strength steel plate is formed with a ductile increasing portion extending from the portion where the welding nugget is formed to the periphery. Therefore, the concentration of stress locally on the high-strength steel plate around the weld nugget is alleviated, which is advantageous in preventing weld cracks.

In addition, compressive stress is applied to the welded portion of the high-strength steel sheet and its surroundings in a state of being softened by pressurization by the heating electrode. By applying this compressive stress, the tensile residual stress associated with the subsequent welding is reduced, which is advantageous in preventing delayed cracking due to hydrogen embrittlement.

In one embodiment, the high-strength steel sheet is a steel sheet having a plating layer on the welded surface and mainly having a martensite structure, and is, for example, a steel sheet formed and hardened by hot stamping.

In resistance welding of such a high-strength steel plate, the welded portion and its surroundings are changed to a tempered martensite structure by the heat treatment step to improve ductility. Alternatively, the heat treatment step forms a softened region around the welded portion to improve its ductility. By improving the ductility around the welded portion, the stress concentration due to the external load is alleviated in the obtained welded product. At the same time, by improving the softening and ductility around the welded portion, the welding residual stress is reduced, so that delayed cracking can be avoided.

By the way, in the hot stamping method, a steel sheet is hot-formed with a mold and then rapidly cooled by a refrigerant and quenched, so that a part having high shape freezing property and high strength can be obtained. In this method, in order to obtain stable quenching strength, the steel sheet is heated to a high temperature equal to or higher than the Ac3 transformation point before forming. At that time, in order to suppress the generation of oxide scale on the surface of the steel sheet, for example, aluminum alloy plating is applied to the surface of the steel sheet.

In such a plated steel sheet, the layer structure of aluminum plating changes depending on heating conditions such as the rate of temperature rise in hot stamping, the ultimate temperature, and the holding time. As a result, there is a problem that an Fe—Al alloy phase having high energization resistance is generated and spot weldability is deteriorated. Further, since the degree of formation of the Fe—Al alloy phase is partially different, the conduction resistance is likely to vary.

Further, since this Fe—Al alloy phase is very hard and brittle, the plating layer may be cracked during hot forming or may be peeled off by rubbing with a mold. Therefore, it is difficult to secure a uniform surface condition suitable for welding. When the surface texture of the steel sheet is not homogeneous, the contact resistance during spot welding is partially different (variation), and as a result, the calorific value is partially different, resulting in a problem that a good welding nugget cannot be obtained. ..

In the welding method described in Patent Document 1, since ductility is obtained around the welded portion by tempering with an annular electrode, it is possible to obtain an effect on suppressing cracks and reducing residual stress due to stress concentration in the HAZ softened portion. However, it does not solve the problem that the plating layer of the high-strength steel sheet adversely affects spot welding.

In the welding method described in Patent Document 2, even if the aluminum layer between the steel plates can be discharged, it is difficult to eliminate the Fe—Al alloy phase, and in the energization control in the state where the steel plates are overlapped, the same. It is actually difficult to make the surface texture of the mating surfaces uniform.

On the other hand, an important feature of the method according to the present invention is that the heat treatment on the high-strength steel sheet is not performed by superimposing the steel sheets on each other, but by applying pressure to the high-strength steel sheet alone and by the heating electrode. This is the point to do. As a result, the plating layer on the welded surface of the high-strength steel sheet is homogenized. That is, the plating layer at the welded portion of the high-strength steel sheet is softened or melted by heat generated by energization, and is crushed and homogenized by being pressurized. For example, cracks in the plating layer are filled with softened and melted plating metal and disappear when pressed. This is the homogenization of the surface texture of the plating layer.

Therefore, the variation in the current-carrying resistance of the welded portion of the high-strength steel sheet is reduced, which is advantageous for producing a good weld nugget.

In one embodiment, the tip diameter Dh of the heating electrode is 2 times or more and 4 times or less the tip diameter Dw of the welding electrode, or the welding nugget formed between the plurality of steel plates by the welding electrode. It is 2 times or more and 4 times or less the diameter.

As a result, the ductility around the welded portion of the high-strength steel sheet can be reliably improved.

In one embodiment, the heat treatment using the heating electrode controls the amount of heat generated by the energization so that the hardness of the high-strength steel sheet gradually changes from the heat-treated portion to the non-heat-treated portion around the high-strength steel sheet. .. For example, the energization is terminated by performing downslope control that gradually reduces the energizing current by the heating electrode.

As a result, stress concentration when an external load is applied to the welded product can be avoided around the weld nugget, which is advantageous in preventing the welded product from breaking.

In one embodiment, after the welding step, the welded portions of the plurality of steel plates are sandwiched by a pair of determination electrodes having a tip diameter in contact with the steel plate larger than the tip diameter Dw in contact with the steel plate of the welding electrode. It is provided with a quality determination step of determining the quality of welding by energizing while pressurizing, measuring the energizing resistance, and comparing the energizing resistance with a reference value.

Hereinafter, a specific description will be given. In the welding process, since a plurality of stacked steel plates are sandwiched between a pair of welding electrodes and energized while pressurizing, some indentations (dents) are generated on the surface of the steel plates where the welding electrodes come into contact. When the determination electrode is applied to the indented portion of the steel sheet, the determination electrode comes into contact with the indentation on the surface of the steel sheet.

When scattering occurs in the welding process, the melt in the welded portion of the steel sheet is ejected, resulting in a lack of meat. Therefore, the sinking of the welding electrode into the steel plate is larger than that in the case where there is no scattering. That is, the indentation on the surface of the steel sheet due to the welding electrode becomes large. The larger the indentation, the smaller the contact area of the determination electrode with respect to the indentation on the surface of the steel sheet. In addition, the size of the welding nugget becomes smaller when scattering occurs.

Therefore, when scattering occurs in the welding process, the energization resistance is increased because the contact area is small and the welding nugget is small in the energization by the determination electrode after welding. Therefore, by measuring the magnitude of the energization resistance at the time of energization by the determination electrode after welding, it is possible to determine the quality of the welding (welding failure when the energization resistance is larger than the reference value). Therefore, since the quality can be determined without inputting an impact load to the welded portion as in the conventional chisel test, it is possible to obtain the effect of suppressing defects such as cracks around the welded portion generated by the chisel test.

In one embodiment, after the welding step, the welded portions of the plurality of steel plates are sandwiched between the pair of determination electrodes, and the amount of change in the distance between the electrodes of the pair of determination electrodes when pressurized is measured. It is provided with a quality determination step of determining the quality of welding by comparing the amount of change with a reference value.

As described above, when scattering occurs in the welding process, thinning occurs, so that the thickness of the welded portion of the plurality of stacked steel sheets becomes thinner than in the case where there is no scattering. That is, when a welded portion of a plurality of welded steel plates is sandwiched between a pair of determination electrodes and pressurized, the distance between the electrodes of both heating electrodes differs depending on the presence or absence of scattering. Therefore, by measuring the amount of change in the distance between the electrodes when energized by the judgment electrode after welding, it is possible to judge whether the welding quality is good or bad (welding failure when the amount of change in the distance between the electrodes is larger than the reference value). Can be done.

In one embodiment, when a defect in welding quality is determined in the quality determination step, the welded portions of the plurality of steel plates are brought into contact with the steel plates having a tip diameter that contacts the steel plates. It is provided with a repair process in which the welding nugget is grown by sandwiching it between a pair of repair electrodes having a tip diameter larger than Dw and generating heat by energizing while pressurizing.

When a welding defect is determined in the quality determination step, that is, when the welding nugget is small, the energization resistance becomes large. Therefore, in the pressurized energization by the retouching electrode, heat generation becomes large at the welding nugget portion where the energization resistance between the plurality of steel plates is large, and the steel plates melt and the welding nugget grows.

In one embodiment, the heating electrode is a flat electrode having a flat surface in contact with the steel plate. This facilitates heating of the welded portion of the high-strength steel sheet and its surroundings.

In one embodiment, each of the determination electrode and the retouching electrode is a flat electrode having a flat surface in contact with the steel plate. This is advantageous for improving the reliability of quality judgment and repairing welding defects.

For each of the heating electrode, the determination electrode, and the retouching electrode, a radius type electrode having a large tip radius can be used instead of the flat type electrode.

Further, as the determination electrode and the repair electrode, the heating electrode may be used.

As the welding electrode, it is preferable to use a well-known dome radius type electrode in which the surface in contact with the steel plate protrudes in a dome shape, or an electrode having another shape such as a cone flat type electrode can also be used.

According to the present invention, a heat treatment (pressurized energization) of a high-strength steel plate by a pair of heating electrodes before resistance welding forms a ductile increasing portion extending from a portion where a welding nugget is formed on the high-strength steel plate to the periphery. Therefore, it is advantageous in preventing weld cracks, and since the welding residual stress is reduced, it is advantageous in preventing delayed cracks.

The cross-sectional view which shows the heat treatment mode of a high strength steel sheet alone by the heating electrode. The cross-sectional view which shows the mode of superposition welding of a steel plate by a welding electrode. Sectional drawing which shows the heat-treated part of the high-strength steel plate. The front view which shows the electrode for heating and the electrode for welding. The graph which shows the example of the energization control by the heating electrode. The graph which shows the time-dependent change of the resistance between electrodes of a heating electrode. The graph which shows another example of the energization control by a heating electrode. The graph which shows still another example of energization control by a heating electrode. The graph which shows still another example of energization control by a heating electrode. FIG. 5 is a cross-sectional view showing another example of superposition welding of steel plates using welding electrodes. FIG. 5 is a cross-sectional view showing still another example of superposition welding of steel plates using welding electrodes. The cross-sectional view which shows the aspect of the welding quality judgment by the judgment electrode (example of good welding). The cross-sectional view which shows the aspect of the welding quality judgment (weld defect example) by the judgment electrode. The cross-sectional view which shows the aspect of welding rework by the rework electrode (at the start of rework). The cross-sectional view which shows the aspect of welding rework by the rework electrode (at the end of rework).

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of preferred embodiments is merely exemplary and is not intended to limit the invention, its applications or its uses.

The resistance welding method for high-strength steel sheets of the present embodiment is a method for welding a plurality of superposed steel sheets. At least one of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of more than 1 GPa and an Al alloy plating layer on the welded surface, and is obtained by a hot stamping method.

In the resistance welding method of the present embodiment, the heat treatment step using the pair of heating electrodes 1 and 1 shown in FIG. 1 and the welding step using the pair of welding electrodes 2 and 2 shown in FIG. 2 are executed. Further, if necessary, after the welding step, the welding quality determination step by the pair of determination electrodes 11 and 11 shown in FIGS. 12 and 13 is executed, and if necessary, after the welding quality determination step, FIG. The welding reworking step using the pair of reworking electrodes 12 and 12 shown in 14 and 15 is executed.

<About heat treatment process>
As shown in FIG. 1, the high-strength steel sheet 3 having the Al alloy plating layer 5 on the surface of the steel base material 4 is heat-treated by the high-strength steel sheet 1 alone. Specifically, a portion of the high-strength steel sheet 3 to be welded to another steel sheet and its surroundings are sandwiched between a pair of heating electrodes 1 and 1, and heat is generated by energizing while pressurizing. As a result, as shown in FIG. 3, a softened structure portion 4a having increased ductility is formed on the steel base material 4 at the welded portion 3a of the high-strength steel plate 3 and its surroundings 3b, and the surface layer portion 3c of the welded portion 3a is formed. Make it homogenized.

As shown in FIG. 4, the heating electrode 1 is a flat electrode in which the surface 1a in contact with the high-strength steel plate 3 is a flat surface, and the welding electrode 2 has a surface 2a in contact with the steel plate protruding like a dome. It is a dome radius type electrode. The tip diameter Dh of the heating electrode 1 in contact with the steel plate is larger than the tip diameter Dw of the welding electrode 1 in contact with the steel plate. It is preferable that 4 × Dw ≧ Dh ≧ 2 × Dw.

[Example of energization control by heating electrodes 1 and 1]
FIG. 5 shows an example of energization control in the heat treatment of the high-strength steel sheet 3 by the heating electrodes 1 and 1. In this example, a DC inverter type spot welder with a power frequency of 50 Hz or 60 Hz is used to energize the high-strength steel sheet 3 by the heating electrodes 1 and 1, and the first energization control, cooling, and second energization control are performed. ..

In the first energization control, an upslope energization period US for gradually increasing the energization current is taken for 10 cycles or more, followed by a constant current energization period C of 2 to 5 kA for 10 to 20 cycles, and a downslope for gradually reducing the energization current. The energization is completed in the constant current energization period C without removing the current. Then, after a cooling period of several cycles from the first energization control, the second energization control is started.

In the second energization control, the upslope energization period US is 5 to 10 cycles, followed by the constant current energization period C with a current value of 5 to 10 kA for 10 to 20 cycles, and the downslope period DS is 10 cycles or more. , The energization is finished.

After the end of the second energization control, a hold period for cooling the heat-treated portion is provided while holding the pressing force on the steel sheet by the heating electrode 1, and after that, the pressing force is released and the heating electrode 1 is used as the high-strength steel sheet 3. Keep away from.

[Significance of energization control]
Here, in the high-strength steel plate 3 formed and hardened by hot stamping, the base material 4 has a martensite structure, while the surface layer portion (plating layer 5 portion) has an Al—Fe alloy phase. There are variations in energization resistance due to formation and cracks.

The first energization control is a process for stabilizing the contact state between the heating electrode 1 and the high-strength steel plate 3. Therefore, instead of applying a high current suddenly, an upslope energization period US is provided so that scattering does not occur, and the current value is also suppressed to a low level.

The cooling period between the first energization control and the second energization control is appropriately set according to the state of the high-strength steel sheet 3 and the processing conditions, and is not necessarily required to be provided. However, if this cooling period is provided, it is advantageous in stabilizing the contact state of the electrodes 1 and 1 with respect to the high-strength steel plate 3.

In the second energization control, the welded portion 3a of the high-strength steel plate 3 and the base metal 4 around it are tempered back to form a tempered martensite structure (softened structure portion 4a), and the plating layer 5 is softened and melted. It is a process of crushing to homogenize the surface layer portion 3c of the welded portion 3a of the high-strength steel plate 3. Therefore, the current value in the constant current control period C is higher than that in the first energization control.

In the second energization control, a sudden hardness change point is prevented from occurring between the tempered martensite structure (softened structure portion 4a) of the welded portion 3a and its surrounding 3b and the hardened martensite structure of the base metal 4 on the outside thereof. .. That is, the hardness is gradually changed from the heat-treated portion by the second energization control to the non-heat-treated portion around the heat-treated portion. Therefore, as described above, it is preferable to provide a downslope energization period in which the energization current is gradually reduced to end the energization.

In the second energization control, it is not always necessary to provide the upslope period, but it is advantageous to provide it to prevent the occurrence of scattering.

The pressing force from the start of the first energization control to the end of the hold period can be, for example, about 2.45 to 5.88 kN. This pressing force is one of the important conditions for stably homogenizing the surface layer portion, and is the state of the high-strength steel sheet 3 due to hot stamping, that is, the degree of surface damage, the plating basis weight, and the plate. Set in consideration of the thickness, degree of Al—Fe alloying, etc. A higher pressing force can be set depending on the state of the high-strength steel plate 3.

The current value and the energization time (number of cycles) of each of the first energization control and the second energization control are examples, and can be appropriately set according to the state of the high-strength steel sheet 3 as in the case of the pressing force.

During the hold period, the heat-treated portion is cooled to stabilize the surface layer portion of the high-strength steel sheet 3 so that the plating layer is not adhered or welded to the electrode 1 and peeled off. The hold period does not necessarily have to be provided from the viewpoint of stabilizing the conductive resistance during welding (because the current-carrying resistance decreases if the plating layer is not peeled off), but it is disadvantageous in terms of corrosion resistance. , It is preferable to provide.

[Confirmation of effect of heat treatment process]
FIG. 6 shows the waveform of the energization resistance between the heating electrodes 1 and 1 when the same portion of the high-strength steel sheet 3 is repeatedly heat-treated under the conditions shown in Table 1. As the high-strength steel sheet 3, an Al alloy-plated steel sheet having a tensile strength of 1.8 GPa (plate thickness: 1.6 mm, plating amount: 70/70 (g / m ² )) was used.

According to FIG. 6, when the waveforms of the resistance between the electrodes of the first time and the second and subsequent times are compared, the resistance between the electrodes in the first energization control period (0 to 20 cycles) is greatly reduced in the second time as compared with the first time. There is. Then, from the second time onward, the waveforms of the resistance between the electrodes are substantially the same. This means that the current-carrying resistance of the surface layer portion 3c of the high-strength steel sheet 3 was reduced by the first heat treatment. That is, the homogenization of the surface of the welded portion of the high-strength steel sheet 3 is achieved and the contact resistance is stabilized by performing the heat treatment only once.

Since the formation of the softened structure portion 4a and the homogenization of the surface layer portion 3a are processed by the second energization control after the contact resistance is stabilized, the resistance between the electrodes depends on the intrinsic resistance of the steel sheet 3, and the first and second times. There is no big difference from the following. However, it can be seen from FIG. 6 that the heat generated by energization is stably obtained, that is, the heat treatment effect is obtained.

[Other energization control examples using heating electrodes 1 and 1]
In the energization control example shown in FIG. 7, as the first energization control, pulsation energization is adopted in which the same current is energized a plurality of times or more while pressurizing the same portion of the high-strength steel sheet 3 by pulsation control. .. The second energization control and the setting of the hold period thereafter are the same as the controls shown in FIG. 5 described above. A cooling period is provided between the first energization control and the second energization control as necessary.

Even when the pulsation is energized, the contact state between the heating electrode 1 and the high-strength steel plate 3 can be stabilized without causing scattering.

The energization control example shown in FIG. 8 is basically the same as the energization control example shown in FIG. 5, but the current value of the constant current control period C in the first energization control is made lower than that of the control example shown in FIG. , The current value of the constant current control period C in the second energization control is made higher than that of the control example shown in FIG. The setting of the hold period is the same as the control shown in FIG. 5 described above.

When the surface state of the high-strength steel sheet 3 is relatively homogeneous, the contact state between the heating electrode 1 and the high-strength steel sheet can be stabilized even if the current value of the constant current control period C in the first energization control is lowered. it can. When the high-strength steel sheet 3 has a large plating basis weight, or when the high-strength steel sheet 3 has a large plate thickness, the current value in the constant current control period C in the second energization control is increased as shown in the control example shown in FIG. do it.

In the control example shown in FIG. 9, the first energization control is the same as the control example shown in FIG. 8, but the pulsation energization is adopted in the second energization control. However, the current value of the pulsation energization is higher than the current value of the pulsation energization related to the first energization control shown in FIG. 7 from the viewpoint of the above-mentioned purpose (significance) of the second energization control. The setting of the hold period is the same as the control shown in FIG. 5 described above.

The pulsation energization is useful for heating the welded portion of the high-strength steel sheet 3 without causing scattering, and like the downslope energization, the hardness is from the heat-treated portion toward the non-heat-treated portion around it. Can be gradually changed.

<Welding process>
As shown in FIG. 2, the high-strength steel sheet 3 heat-treated using the heating electrode 1 is superposed on the other steel sheet 6. The parts to be welded of both steel plates 3 and 6 are sandwiched between a pair of welding electrodes 2 and 2, and heat is generated by energizing while pressurizing to form a welding nugget 7 between the steel plates 3 and 6. FIG. 2 shows an example in which the high-strength steel plate 3 and the ordinary steel plate 6 are overlapped and welded.

For welding, for example, a DC inverter type spot welder having a power frequency of 50 Hz or 60 Hz is used. To explain an example, first, both steel plates 3 and 6 are sandwiched between welding electrodes 2 and 2 to start pressurization, and the contact state and pressurization state between the electrodes 2 and the steel plates 3 and 6 are stabilized (squeeze period). .. Following the squeeze period, energization control is started. That is, the energization is terminated after the upslope energization period, the constant current energization period, and the downslope energization period for preventing scattering and other defects. Then, after a predetermined hold period elapses, the pressing force is released to separate the electrode 2 from the steel plates 3 and 6.

The surface layer portion 3c of the welded portion 3a of the high-strength steel sheet 3 is homogenized including the surface texture by the pressure heat treatment by the heating electrode 1. Further, due to the pressurization, the Al—Fe alloy phase of the surface layer portion 3c is crushed and spreads to the surroundings, and the thickness thereof is reduced. That is, the energization resistance of the surface layer portion 3c is lower than that before the heat treatment.

Therefore, in the pressurized energization by the welding electrode 2, the occurrence of concentrated resistance (current is locally concentrated and flows to increase the resistance) is alleviated, and the variation in film resistance is not small. Therefore, the welding nugget 7 is easy to grow without causing scattering, and stable and uniform welding can be achieved.

Next, in the high-strength steel plate 3, the softened structure portion (tempered martensite structure) 4a is formed in the base metal 4 at the welded portion 3a and the surrounding 3b by the pressure heat treatment by the heating electrode 1, and the ductility is improved. There is. Further, the softened structure portion 4a extends from the welded portion 3a toward the periphery thereof so as not to cause a sudden change point in hardness. Therefore, a portion that easily causes stress concentration due to HAZ softening is not generated around the weld nugget 7, which is advantageous in preventing the welded product from being destroyed by an external force.

Further, compressive stress is applied to the welded portion 3a of the high-strength steel sheet 3 and its surroundings 3b by pressure heat treatment by the heating electrode 1. Therefore, by applying this compressive stress, the tensile residual stress generated by welding is reduced, which is advantageous in preventing delayed cracking due to hydrogen embrittlement.

[Other welding examples]
The welding example shown in FIG. 2 is the welding of the high-strength steel plate 3 and the ordinary steel plate 6 obtained by hot stamping, but as shown in FIG. 10, even in the case of the superposition welding of the high-strength steel plates 3 to each other. After performing the above heat treatment step on each welded portion of both high-strength steel plates 3, both can be welded by executing the above welding step. Alternatively, as shown in FIG. 11, even in the case where two high-strength steel plates 3 and 3 and one ordinary steel plate 6a are overlapped and welded, the heat treatment step is executed on each welded portion of both high-strength steel plates 3. After that, the three steel plates 3, 6a can be overlapped and welded by executing the above welding step.

It goes without saying that the present invention is not limited to the welding examples shown in FIGS. 2, 10 and 11, and can be used in cases where a plurality of steel plates including at least one high-strength steel plate 3 are laminated and welded.

<Welding quality judgment process>
As shown in FIG. 12, a welded portion of a welded product (a laminated welded product of a plurality of steel plates 3 and 6 that has undergone the heat treatment step and the welding step) is sandwiched between the pair of determination electrodes 11 and 11 and pressed. While energizing, measure the energization resistance. The determination electrode 11 of this example is a flat electrode having a tip diameter in contact with the steel plates 3 and 6 larger than the tip diameter Dw of the welding electrode 2, similar to the heating electrode 1. The heating electrode 1 may be used as the determination electrode 11.

In the previous welding step, since the steel plates 3 and 6 are energized while being pressurized by the welding electrodes 2 and 2, indentations (dents) 8 are generated on the surfaces of the steel plates 3 and 6 where the welding electrodes 2 come into contact. When the determination electrode 11 is applied to the portion of the steel plates 3 and 6 where the indentation 8 is generated, the determination electrode 11 comes into contact with the indentation 8 on the surface of the steel plates 3 and 6. On the other hand, when there is scattering in the welding process, the melt in the welded portion of the steel plates 3 and 6 is ejected, resulting in a lack of meat. Therefore, the sinking of the welding electrode 2 into the steel plates 3 and 6 is larger than that in the case where there is no scattering.

FIG. 12 shows a case where there was no scattering, and FIG. 13 shows a case where there was scattering. In the case of FIG. 13 in which scattering occurs, the indentation 11 becomes larger (deeper) than in the case of FIG. 12 in which there is no scattering, corresponding to the size of the sinking of the welding electrode 2 into the steel plates 3 and 6. .. As a result, the contact area of the determination electrode 11 with respect to the indentation 8 is smaller in the case of FIG. 13 in which scattering occurs than in the case of FIG. 12 in which there is no scattering. Further, when scattering occurs (FIG. 13), the size of the welding nugget 7 is also smaller than when there is no scattering (FIG. 12).

Therefore, in the energization by the determination electrode 11 after welding, when scattering occurs (FIG. 13), the contact area of the determination electrode 11 with the steel plate is small, and the welding nugget 7 is small, so that the energization resistance is high. growing. Therefore, by measuring the magnitude of the resistance when energized by the heating electrode after welding, it is possible to determine whether the welding quality is good or bad (welding failure when the energization resistance is larger than the reference value).

Further, when scattering occurs in the welding process, thinning occurs, so that the thickness of the welded portion of the laminated steel plates 3 and 6 is thinner than that in the case where there is no scattering. That is, when the welded portions of the steel plates 3 and 6 are sandwiched between the pair of determination electrodes 11 and 11 and pressurized, the distance between the electrodes of both determination electrodes 11 and 11 differs depending on the presence or absence of scattering. Therefore, by measuring the amount of change in the distance between the electrodes when energized by the determination electrode 11 after welding, the quality of welding (welding failure when the amount of change in the distance between electrodes is larger than the reference value) is judged. be able to.

12 and 13 show welding examples of the high-strength steel plate 3 and the ordinary steel plate 6, but the welding quality determination is based on the welding example of the high-strength steel plates 3 and 3 shown in FIG. 10 and the height of the two sheets shown in FIG. It can be carried out in a lap welded product of a plurality of steel plates including at least one high-strength steel plate 3, such as lap welding of high-strength steel plates 3 and 3 and one ordinary steel plate 6a.

<Welding repair process>
When a welding defect is determined in the quality determination step, welding can be repaired by a pair of repair electrodes 12 and 12, as shown in FIG. The retouching electrode 12 of this example is a flat electrode having a tip diameter in contact with the steel plates 3 and 6 larger than the tip diameter Dw of the welding electrode 2, similar to the heating electrode 1. As the retouching electrode 12, the heating electrode 1 may be used.

In the welding reworking step, the welded portion of the laminated welded product of a plurality of steel plates is sandwiched between a pair of reworking electrodes 12 and 12, and heat is generated by energizing while pressurizing to grow the welding nugget 7. FIG. 14 shows a case where the welding nugget 7 between the high-strength steel plate 3 and the ordinary steel plate 6 is smaller than planned in the superposition welding of two high-strength steel plates 3 and 3 and one thin ordinary steel plate 6a. is there.

As described above, when the welding nugget 7 is small, the energization resistance is large. Therefore, in the pressurized energization by the repair electrode 12, heat generation increases at the portion of the welding nugget 7 having a small diameter, and as shown in FIG. 15, the steel plates 3 and 6a melt and the growth of the welding nugget 7 is promoted. Will be done.

The welding repair step is not limited to the case shown in FIG. 14, and can be executed in a laminated welded product of a plurality of steel plates including at least one high-strength steel plate 3, such as the welding example shown in FIGS. 2 and 10. ..

1 Heating electrode 2 Welding electrode 3 High-strength steel plate 3a Welding part 3b Around the welding part 4 Base material 4a Softened structure part 5 Plating layer 6 Ordinary steel plate 7 Welding nugget 8 Indentation (dent)
11 Judgment electrode 12 Rework electrode

Claims (11)

It is a method of welding a plurality of stacked steel sheets, and at least one of the plurality of steel sheets is a high-strength steel sheet having a tensile strength exceeding 1 GPa.
A portion of the high-strength steel plate to be welded to another steel plate among the plurality of steel plates and its surroundings are sandwiched between a pair of heating electrodes and energized while applying pressure to generate heat, and ductility of the portion and its surroundings is generated. The heat treatment process of the above high-strength steel sheet alone and
The plurality of steel plates including the high-strength steel plate subjected to the heat treatment are superposed, and the parts to be welded of the plurality of steel plates are sandwiched between a pair of welding electrodes and energized while applying pressure to generate heat. It has a welding process to form a welding nugget between them.
A resistance welding method for a high-strength steel sheet, wherein the tip diameter Dh of the heating electrode in contact with the steel sheet is larger than the tip diameter Dw of the welding electrode in contact with the steel sheet. In claim 1,
The high-strength steel sheet is a resistance welding method for a high-strength steel sheet, which has a plating layer on the surface to be welded and is a steel sheet mainly having a martensite structure. In claim 2,
The high-strength steel sheet is a resistance welding method for high-strength steel sheets, which is a steel sheet that has been formed and hardened by hot stamping. In claim 2 or 3,
A method for resistance welding of a high-strength steel plate, characterized in that, in the heat treatment step, the portion to be welded and the periphery thereof of the high-strength steel plate are tempered to form a martensite structure. In any one of claims 1 to 4,
The tip diameter Dh of the heating electrode is twice or more and four times or less the tip diameter Dw of the welding electrode, or twice or more the diameter of the welding nugget formed between the plurality of steel plates by the welding electrode. A resistance welding method for high-strength steel sheets, which is characterized by being four times or less. In any one of claims 1 to 5,
The heat treatment using the heating electrode is characterized in that the amount of heat generated by energization is controlled so that the hardness of the high-strength steel plate gradually changes from the heat-treated portion to the non-heat-treated portion around the high-strength steel plate. Resistance welding method for high-strength steel sheets. In claim 6,
The heat treatment using the heating electrode is performed by performing a downslope control that gradually reduces the energizing current so that the hardness gradually changes from the heat-treated portion of the high-strength steel plate to the non-heat-treated portion around the high-strength steel plate. A resistance welding method for a high-strength steel plate, characterized in that the energization is terminated. In any one of claims 1 to 7,
After the welding step, the welded portions of the plurality of steel plates are sandwiched by a pair of determination electrodes having a tip diameter in contact with the steel plate larger than the tip diameter Dw in contact with the steel plate of the welding electrode, and energization is performed while pressurizing. A resistance welding method for a high-strength steel plate, which comprises a quality determination step of measuring the current-carrying resistance and determining the quality of welding by comparing the current-carrying resistance with a reference value. In any one of claims 1 to 7,
After the welding step, the welded portions of the plurality of steel plates are sandwiched between the pair of determination electrodes, the amount of change in the distance between the electrodes of the pair of electrodes when pressurized is measured, and the amount of change is compared with the reference value. A resistance welding method for high-strength steel sheets, which comprises a quality judgment process for judging the quality of welding by performing resistance welding. In claim 8 or 9,
In the quality determination step, when a defect in the welding quality is determined, the tip diameter of the welded portion of the plurality of steel plates in contact with the steel plate is larger than the tip diameter Dw in contact with the steel plate of the welding electrode. A resistance welding method for a high-strength steel plate, which comprises a repair process in which the welding nugget is grown by sandwiching it between a large pair of repair electrodes and generating heat by energizing while pressurizing. In any one of claims 1 to 10.
The heating electrode is a resistance welding method for a high-strength steel sheet, characterized in that the surface in contact with the steel sheet is a flat surface.

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