JP5573128B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a resistance spot welding method which is a kind of lap resistance welding method, and in particular, a plate assembly including a high strength steel plate having a tensile strength of 590 MPa or more is formed in a shorter time and a higher strength joint. The present invention relates to a resistance spot welding method.

  In recent years, steel sheets have been strengthened to achieve both high reliability of the vehicle body and reduction of vehicle body weight for the purpose of reducing emissions. Resistance spot welding is widely used in the assembly of automobile bodies.

  As shown in FIG. 1, resistance spot welding is performed by attaching a plate set 3 of two or more stacked steel plates (here, the lower steel plate 1 and the upper steel plate 2) to a pair of upper and lower electrode tips (lower Are sandwiched between the electrode tip 4 and the upper electrode tip 5), and are melted by pressurization and energization to form a nugget 6 having a necessary size, thereby obtaining a welded joint.

  The quality of the joint obtained in this way is that the diameter and penetration of the nugget are obtained, or the shear tensile strength (strength when the tensile test is performed in the shear direction of the joint) and the cross tensile strength (joint peeling). Strength when a tensile test is performed in the direction) and fatigue strength. Among them, the static strength represented by shear tensile strength and cross tensile strength is regarded as very important as an index of joint quality.

  Among these, the tensile shear strength of the spot welded portion tends to increase as the tensile strength of the steel plate increases. However, the cross tensile strength hardly increases regardless of the increase in the tensile strength of the steel sheet, but decreases. As the cause, the high strength steel sheet has to have a large carbon equivalent Ceq represented by the following formula in order to achieve its strength, and in addition, since welding is a rapid heating and quenching phenomenon, It is also considered that the hardness increases in the heat affected zone and the toughness decreases.

Ceq = C + 1/24 × Si + 1/6 × Mn
However, C, Si, and Mn are content (mass%) of C, Si, and Mn in steel, respectively.

  In order to ensure the joint strength when using a high-strength steel plate, from the viewpoint of the welding method, an increase in the number of hitting points and an increase in the nugget diameter are conceivable. However, an increase in the number of hit points requires a space, which leads to an increase in work time and deteriorates productivity. In order to increase the nugget diameter, the electrode must be enlarged and the applied pressure must be increased to prevent the weld metal from being scattered (chile). There is also a drawback that the properties of the base material are impaired.

  Therefore, various attempts have been made for the post-heat energization method in which energization is performed after the main energization for forming the nugget in order to ensure the strength with the number of hitting points and the nugget diameter that is the same as or less than the conventional one. After this, there are two types of heat energization. There are a martensite temper method in which the weld is once cooled and reheated, and an austemper method in which the weld is re-energized during the cooling of the weld. It is necessary to know the constant temperature transformation curve of the welding material, and it may take a long time for the post-heat energization treatment, and it is difficult to obtain a stable effect, so the martensite temper method is the mainstream in thin steel sheets. The principle of the martensite temper method is to solidify and transform the weld once and then reheat it to soften the nugget and HAZ, thereby improving the toughness of the nugget and reducing stress concentration near the weld. It is considered that the joint strength has been improved.

  As an example, in Patent Document 1, the product of the square of (It / To) and (Tt / To) is calculated using the energization time To and energization current Io in temper energization and the energization time Tt and energization current It in main energization. It is desirable to be in the range of 0.25 / 0.82.

  In Non-Patent Document 1, static strength is improved by conducting temper energization on a 1.05 mm steel plate, and the time required for post-heat energization is 0.4 seconds for cooling time and temper energization time. Is 0.5 seconds, for a total of 0.9 seconds.

In Patent Document 2, in a high-tensile steel sheet having a tensile strength of 35 kg / mm ² or more, in addition to the main energization exceeding the limit current value for generating dust, the temper energization is performed at a current value equal to or less than the main energization to obtain shear strength and fatigue. It is said that the strength can be improved.

  Furthermore, in Patent Document 3, after performing the main energization, the energization is performed at a current value equal to or lower than the main energization, and the holding time after the end of the energization is changed in accordance with the plate thickness, thereby increasing the cross tensile strength of the high-tensile steel plate. It can be improved.

  In recent years, as seen in Non-Patent Document 2, a method called Spike-Tempering has been proposed in which energization is performed for an extremely short time after constant cooling and the same effect as temper energization can be obtained. The time required for energizing the temper is about 40 cycles (0.8 seconds).

  Further, the post-heat energization is not only used for securing the joint strength as described above, but also used for securing the penetration. In particular, in the combination of a thin plate and two plates thicker than that, there is a problem that it is difficult to form a melted portion between the thin plate and the thick plate. Document 4 states that a sufficient nugget diameter can be secured by performing pulsation energization that repeats energization after energization.

  In Patent Document 5, the welding is divided into two stages, that is, the first stage welding and the second stage welding, and the second stage welding is made to have a lower current and higher pressure than the first stage welding. The suppression effect is said to be obtained.

JP 58-003792 A JP 58-003793 A JP 2002-103048 A JP 2008-093726 A JP 2005-262259 A

1st International Conference Super-high Strength Steels Proceedings, G. Shi et al., Techniques for Improving The Weldability of Trip Steel Using Resistance Spot Welding, 2005 AISI / DOE Technology Roadmap Program, DE-FC36-97ID13554, B.R. Girvin et al., Development of Appropriate Spot Welding Practicing for Advanced High-Strength Steels, 2004

  However, the energization methods such as those described in Patent Documents 1 to 4 and Non-Patent Document 1 can be used in order to select a range in which sufficient resistance heat generation is possible at a current value equal to or less than the main energization. Is narrow and must be greatly affected by slight changes in current flow and current time, and can be mounted at production sites where various disturbance factors exist (for example, a large current drop exceeding 50% of the current flow occurs) In doing so, there is a problem that a margin for performing stable construction is small. In addition, a sufficient welding time (at least 0.5 seconds or more according to Non-Patent Document 1) is required to effectively generate heat at a low current below the main energization, and the total welding is combined with the cooling time. There is also a problem that causes an increase in time (defined from the start of the first energization to the completion of the last energization).

  In addition, a general martensite temper energization method performs tempering by energizing after sufficient cooling, as implemented or described in Non-Patent Document 1 and Non-Patent Document 2. Temper energization, sufficient cooling time (according to Non-Patent Document 1, if the plate thickness is 1.05 mm and at least 20 cycles (0.4 seconds) or more, if you want to obtain a stable effect or increase the plate thickness 20 cycles (a time longer than 0.4 seconds) is required, and the total welding time is increased.

  Furthermore, Patent Document 4 is a method for securing a melted portion with respect to a set of three or more sheets, that is, an object of enlarging a nugget formed by main energization by post-heat energization. Conventionally, from the viewpoint that there is a close relationship between the nugget diameter and the joint strength, the joint strength has been arranged and evaluated with respect to the final nugget diameter regardless of the presence or absence of post-heat conduction. As described above, it is important to improve the strength with a specific nugget diameter. In addition, if the nugget or HAZ is cooled from the molten state, the joint strength cannot be improved.

Similarly, Patent Document 5 is a method for securing a melted portion with respect to a stack of three or more sheets, and it is said that generation of dust can be suppressed. Because it is the second stage of high pressure and low current, the thermal history of the cooling process itself is not different from the condition of only one stage where the energization forms a nugget, and the joint is rapidly cooled, so improving the joint strength Can not.
An object of the present invention is to provide a resistance spot welding method capable of solving the above-mentioned problems and achieving higher joint strength in a shorter welding time in resistance spot welding of a plate set including a high-strength steel plate. And

  In order to solve the above-mentioned problems, the present inventors have intensively studied a method for improving the cross tensile strength in resistance spot welding of a plate assembly including a high-tensile steel plate.

  In the conventional martensite temper method, the structure is tempered after the joint is formed, and the joint strength is improved. Therefore, it is necessary to allow a sufficient cooling time. Although this cooling time varies depending on the plate assembly and composition, in order to obtain a stable effect, it is longer than at least 20 cycles (0.4 seconds), and in Non-Patent Document 2, about 40 cycles (0.8 seconds) is desirable. It is said. Therefore, in order to shorten the total welding time, a cooling time of 20 cycles (0.4 seconds) or less, which has been considered difficult in the past, was used to consider whether the joint strength could be improved. Furthermore, a method of applying a low current below the main energization for a relatively long time of at least 10 cycles (0.2 seconds) or more as post-heat energization has been taken, whereas 5 cycles (0.1 seconds) or less. We considered whether joint strength could be improved by adding for a short time. Here, as described above, out of the tensile shear strength and the cross tensile strength that represent the static strength of the joint strength of resistance spot welding, the tensile shear strength tends to improve as the strength of the steel plate increases. When considering the joint strength of high-strength steel sheets, the cross tensile strength was more important.

  Therefore, further studies were conducted to solve the problem.

  There is a correlation between the cross tensile strength and fracture mode of resistance spot welded joints, and low-strength welded joints cause peeling fracture that breaks parallel to the steel sheet, so that one steel sheet remains in the shape of a button as the strength increases. It is known to change to a plug rupture that breaks. Therefore, in order to see the change in fracture mode at the same nugget diameter, we created a welded joint with only main energization and a welded joint with temper energization to improve the cross tensile strength in addition to the main energization. When the test was conducted, the joint with only the main current was peeled and fractured, and the joint with temper current was plugged and fractured. Comparison of the two revealed the following.

  That is, the joint fractured and peeled only by this energization was a fracture surface with a cleaved face and a brittle fracture, and the joint fractured with a plug by applying temper current was a smooth and ductile fracture. Then, as shown in FIG. 4, if the structure surrounding the nugget 6 is changed by heating and the hardened part is the heat-affected zone 7, and the softened part of the outer base material is the softened region 8. In the case of peeling and breaking, a portion hardened from the inside of the nugget 6 was seen in the heat-affected zone 7, whereas in the case of plug-breaking, there was no significant change in the softened region 8, but the heat-affected zone 7 Softening was observed. The softening of the heat-affected zone 7 is caused by tempering of the martensite structure by tempering. The softening allows plastic deformation at the outer periphery of the nugget 6 and alleviates stress concentration at the end of the nugget 6. For this reason, it was considered that the fracture mode was changed from the peeling fracture to the plug fracture.

  Therefore, the inventors decided to consider a completely new softening method for the heat-affected zone 7 in order to obtain the same effect as described above when performing post-heat energization. That is, unlike the prior art shown in FIG. 4, the nugget 6 and the heat-affected zone 7 are not hardened by the concept of hardening once and then softened by tempering, but as shown in FIG. 7 was divided into the heat-affected zone 7a on the electrodes 4 and 5 side and the heat-affected zone 7b on the softened region 8 side, and it was thought that it could be controlled separately. The heat affected zone 7b on the softened region 8 side also has heat transfer from the nugget 6, so the cooling rate is relatively slow, whereas the heat affected zone 7a on the electrodes 4 and 5 side has a cooling rate due to heat dissipation to the electrodes. Because it is fast.

  The heat-affected zone 7 is hardened from the nugget 6 when the temper energization is not performed because the resistance spot welding concentrates heating to the center of the nugget 6. The heat-affected zone 7 facing the surface is considered to be caused by a faster cooling rate than the inside of the nugget 6.

  Therefore, in post-heat energization, it was thought that the cooling rate of the heat-affected zone 7b of the heat-affected zone 7, particularly the softened zone 8 side, should be slowed down so as to give an appropriate level of current to the same level as the nugget 6. On the other hand, the heat affected zone 7a on the side of the electrodes 4 and 5 has a short time required for the temperature to be sufficiently lowered, and it is considered that the same effect as the temper treatment can be obtained by heating after waiting for the time.

  After the cooling time as described above, it is necessary to load a high current in order to exert a sufficient effect by the post-heat conduction for a short time of 5 cycles (0.1 seconds) or less. However, the addition of a high current may cause no change in joint strength due to remelting and rapid cooling, or may decrease. Further, since the specific resistance value of the steel sheet decreases as the temperature decreases, when the cooling time is extended, heat generation does not occur sufficiently even at the same current value. On the other hand, if dust is generated when a high current is applied, it is considered that sufficient joint strength cannot be ensured due to a decrease in weld metal from the melted portion. For this reason, it is necessary to load a high current while suppressing the generation of dust. In order to prevent this, the idea was to improve the limit of dust generation by increasing the contact pressure after the main energization and increasing the contact area.

  From these studies, the inventors increased the applied pressure after the main energization to form the melted part, and then energized the heat energization current at a current value higher than the main energization current and up to about 4 times the main energization. As a result, it was found that dust generation does not occur, and that even if there is a certain range in cooling time, a sufficient heat generation effect can be obtained.

  Therefore, the present invention improves the cross tensile strength according to the following principle. That is, after the formation of the nugget 6 by the main energization, as a post-heat energization, a short cooling time (rest time) of 20 cycles (0.4 seconds) or less is used, and 5 cycles (0.1 seconds) or less. By applying a current with a current value equal to or greater than the main energization current value in a short time, the heat input suppresses rapid cooling of the heat affected zone 7b on the nugget 6 and the softened region 8 side, and at the same time suppresses hardening. The heat-affected zone 7a on the 5th side is tempered and softened by the temper effect. Unlike the temper energization, the tempering effect or the like is not given to the entire joint, but the effect is different in the joint portion. For this reason, according to this method, joint strength can be improved without increasing the nugget diameter. Furthermore, generation | occurrence | production of dust can be suppressed by improving a pressurizing force after this electricity supply. These features are not found in the prior art.

  In order to effectively establish the above principle, it is necessary to pay attention to the following points. That is, the heat-affected zone 7a on the electrodes 4 and 5 side needs to be appropriately heated after being sufficiently cooled. For this purpose, the object can be achieved by setting the cooling time (resting time) to at least one cycle (0.02 seconds) or more and at most 20 cycles (0.4 seconds) or less. Thereafter, by increasing the applied pressure during post-heat energization as compared to during main energization, not only the current value margin in post-heat energization is increased, but also dust can be suppressed. Furthermore, in the heat-affected zone 7b on the softened region 8 side, if the energization time of the post-heat energization is too long, it will be heated more than necessary and will be re-cooled rapidly, and conversely will be a factor of hardening. And it also causes dust. Therefore, the energization time is about 5 cycles (0.1 seconds) at the longest. Also, the current value should be set for the same reason, and a current value up to about three times the current value in the main energization is appropriately selected according to the object to be welded.

  However, since the temperature difference between the nugget and the surrounding area is large, the above effect may not be obtained in one post-heat energization process. To ensure the effect, the rest and energization are repeated. There is a need. This number of times needs to be repeated one or more times according to the plate thickness and the like. However, even if the number of repetitions is excessively increased, the joint strength is not improved, and the total welding time is increased. For this reason, it is desirable that it is usually once or twice, and at most 3 times.

  These are markedly hardened by rapid cooling, and exhibit a remarkable effect when applied to a high-tensile steel plate having a tensile strength of 590 MPa or more where the cross tensile strength is greatly deteriorated.

  In this way, in post-heat energization, the above-described rest time and energization were alternately repeated one or more times, thereby succeeding in improving joint strength without fouling or electrode welding.

  Based on the above, the present invention has the following features.

[1] A resistance spot welding method in which a plate assembly in which two or more steel plates are overlapped is sandwiched between a pair of welding electrodes and a current is applied while welding and welding is performed.
Consists of at least two steps,
A main energization step of forming a nugget of a predetermined diameter by energization;
A post-heat energization process consisting of a pause of 1 cycle or more and 20 cycles or less and a short-time energization by increasing the applied pressure from the main energization process,
The resistance spot welding method characterized by the above-mentioned.

  [2] The resistance spot according to [1], wherein the post-heat energization step includes a pause of 6 cycles or more and 20 cycles or less and a short time energization by increasing the applied pressure from the main energization step. Welding method.

  [3] The resistance spot according to [1], wherein the post-heat energization step includes a pause of 6 cycles or more and 10 cycles or less and a short time energization by increasing the applied pressure from the main energization step. Welding method.

[4] In resistance spot welding of a plate assembly in which two or more high-strength steel plates are superposed, a main energizing step for forming a nugget of a predetermined diameter, and then a main force is applied by increasing the applied pressure from the main energizing step. It has a post-heat energization process that performs multi-stage energization at a higher current value than the energization process
In this energization process, the nugget diameter d (mm) is the thickness t (mm) of the thinnest high-strength steel plate in the plate assembly,
3 × √t ≦ d ≦ 6 × √t
After setting the energization time and current value so that it is in the range represented by
In the post-heat energization step, multi-stage energization is performed by repeating energization and deactivation one or more times with energization time of 1 to 5 cycles and deactivation time of 1 to 10 cycles by increasing the applied pressure.

[5] The applied pressure Pb in the post-heat energizing step is greater than the applied pressure Pa in the main energizing step.
Pa <Pb ≦ 3 × Pa
The lap resistance spot welding method for high-strength steel sheets according to [4] above, wherein

[6] The current value Ib in the post-heat energization step is equal to the current value Ia in the main energization step.
Ia <Ib ≦ 3 × Ia
The resistance spot welding method according to [4] or [5] above, wherein

  [7] The resistance spot welding method according to any one of [1] to [6], wherein in the post-heat energization step, the pause and energization are repeated in the range of 1 to 2 times.

  [8] The resistance spot welding method according to any one of [1] to [7], wherein the tensile strength of at least one of the two or more steel plates is 590 MPa or more.

  [9] The resistance spot welding method according to any one of [1] to [8], wherein the tensile strength of at least one of the two or more steel plates is 980 MPa or more.

  According to the present invention, a resistance spot welded joint having a high cross tensile strength can be created for two or more sheet sets including at least one high-tensile steel sheet, and generation of dust is suppressed. Since it is possible to create a joint in a shorter time than conventional temper energization, there is a remarkable industrial effect.

It is a figure which shows typically the board assembly and the arrangement position of an electrode in one Embodiment of this invention. It is a figure which shows typically the construction procedure in one Embodiment of this invention. The electrode tip shape used in the example of the present invention is shown. It shows the heat affected zone and the softened zone in resistance spot welding. 1 illustrates the principle of the present invention.

  An embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the resistance spot welding method according to an embodiment of the present invention includes one or more high-strength steel plates that are superposed (here, the lower steel plate 1 and the upper steel plate 2). Among them, the steel plate 1 is a high-strength steel plate) and the plate assembly 3 is sandwiched between a pair of upper and lower electrode tips (the lower electrode tip 4 and the upper electrode tip 5) and welded by resistance spot welding to be pressurized and energized. This is a resistance spot welding method in which a nugget 6 having a necessary size is formed to obtain a resistance spot welded joint.

  A welding device that can be suitably used in this embodiment includes a pair of upper and lower electrode tips, can sandwich and pressurize and energize a portion to be welded, and can arbitrarily apply pressure and welding current during welding. If you have a controllable pressure control device and welding current control device, pressurization mechanism (air cylinder, servo motor, etc.), current control mechanism (AC, DC, etc.), type (stationary, robot gun, etc.) Etc. are not particularly limited.

  And the construction procedure in this embodiment is shown in FIG.

  First, the energization time (main energization time) Ta and the current value (main energization current value) Ia of the main energization process are set so as to obtain a predetermined nugget diameter in the state of the applied pressure Pa. obtain. However, in order to increase the nugget diameter d, it is necessary to flow a large current, and the expansion of the heat-affected zone and the accompanying strength reduction may be problematic. In order to avoid these problems, the nugget diameter d is 3 × √t or more and 6 × using the thickness t (mm) of the thinnest high-tensile steel plate (here, the lower steel plate 1). A range of √t or less is considered to be more preferable.

  Note that even if the energization for forming the nugget is controlled in multiple stages in practice, the energization that plays a central role in the nugget formation is assumed to be the main energization, and the current value at this time is Ia.

  Next, after forming the nugget diameter, the applied pressure is increased to Pb. At this time, if the setting of Pb is higher than Pa and set to 3 times or less of Pa, it is considered that a sufficient effect can be obtained.

  Further, as the energization after the second stage (post-heat energization), multi-stage energization is performed in which energization and deactivation with an energization time Tb of 1 to 5 cycles and a deactivation time Th of 1 to 20 cycles are repeated one or more times.

  First, it is desirable that the timing of increasing the pressurization is completed before the first energization of the multi-stage energization is added so that the effect of the increase of the pressurization can be obtained. Moreover, since an excessively high applied pressure may cause deterioration of the electrode, it is more preferable that the applied pressure be within a range of Pa <Pb ≦ 2 × Pa.

  Next, since the specific resistance of the steel sheet decreases as the temperature decreases, the rest time Th, the energization time Tb, and the post-heat conduction current value Ib of the post-heat conduction process must be balanced accordingly. If the current is applied before the cooling proceeds, the nugget 6 may be completely remelted or excessively raised to a high temperature, which may cause a decrease in strength. However, depending on the thickness of the steel sheet, when it is cooled to more than 20 cycles, the transformation proceeds sufficiently, and not only the effect of the present invention is not obtained by the subsequent energization, but it also causes an increase in welding time. For this reason, it is desirable that the pause time Th is 6 cycles or more and 20 cycles. Also, an energization time that is too long or a current value that is too high causes dust and reduces the life of the electrode. Therefore, the energization time Tb is up to 5 cycles, the post-heat energization current value Ib is up to 4 times the main energization current value Ia, and is appropriately selected depending on the combination.

  In order to improve the joint strength, it is necessary to repeat the post-heat energization step one or more times according to the plate thickness and the like. However, even if the number of repetitions is excessively increased, the joint strength is not improved, and the total welding time is increased. For this reason, the number of repetitions is preferably 1 to 2 times, and at most 3 times. In consideration of implementation, 1 to 2 times is most preferable.

  Furthermore, in an implementation where there are various disturbance factors, it is most preferable that the post-heat conduction current value Ib be within the range of Ia <Ib ≦ 3 × Ia in order to stably obtain the effect. . When the post-heat energization current value in this range is adopted, the energization time Tb is 2 to 4 cycles if the number of repetitions is 1, and the downtime Th is 6 to 20 cycles, and the energization time Tb if the number of repetitions is 2 times. Is preferably set to 2 to 4 cycles, and the rest time Th is set to 6 to 10 cycles.

  In view of stability in construction and generation limit of dust, the energization time Tb should be 2 to 4 cycles since the energization time not too long, the downtime not too short, and the energization current not too high should be selected. It is considered most preferable that the rest time Th is 6 to 10 cycles and the post-heat conduction current value Ib is within the range of Ia <Ib ≦ 3 × Ia.

  However, it is conceivable that the cooling will be delayed due to the influence of the construction atmosphere such as temperature and humidity and the base material temperature. At this time, even if the cooling time exceeds 20 cycles, the current Ib having a current value higher than the main energizing current Ia is applied twice or more, and the post-heat energization is performed without melting the whole nugget. It can be said that it is within the scope of the present invention.

  Further, in the case of a steel sheet having a tensile strength of less than 590 MPa, it is preferable to use it for a high-strength steel plate having a tensile strength of 590 MPa or more from the viewpoint that sufficient joint strength can be achieved by ordinary welding, and in particular, a tensile strength of 980 MPa or more. A remarkable effect can be obtained with the high strength steel plate.

  Further, in order to achieve the effect of the present invention based on the principle of the present invention described above, when the post-heat energization process is repeated twice, the rest time Th, the post-heat energization current value Ib, and the post-heat energization time Tb are 2 It doesn't have to be the same. For example, if the cooling does not proceed sufficiently during the first downtime, but the cooling proceeds too much during the second downtime, the first downtime Th1 should be longer than the second downtime Th2. Is also possible. Similarly, the current value for the first time may be reduced or the energization time may be shortened. For these reasons, the rest time Th, the current value Ib, and the energization time Tb of the post-heat energization process are individually changed. This does not depart from the intent of the present invention.

  As Example 1 of the present invention, as shown in FIG. 1 above, a servo motor attached to a C gun is added to a plate set 3 in which two steel plates (lower steel plate 1 and upper steel plate 2) are stacked. Resistance spot welding was performed by using a pressure single-phase alternating current (50 Hz) resistance welder to produce a resistance spot welded joint. As shown in FIG. 3, the pair of electrode tips used (lower electrode tip 4 and upper electrode tip 5) are composed of an alumina-dispersed copper DR type electrode having a tip radius of curvature R40 and a tip diameter of 6 mm. did.

  As the test pieces, steel plates 1 and 2 were both 1.6 mm thick and the same steel type, and a bare steel plate of tensile strength 980 MPa class duplex steel, 1180 MPa class duplex steel, and tensile strength 1470 MPa class full martensite steel was used. Welding and tensile tests were performed based on JIS Z3137. In the first embodiment, the main energization conditions were such that the applied pressure Pa was 3.5 kN and the main energization time Ta was constant at 14 cycles. No other squeeze time or slope time was set.

  And as an example of this invention, resistance spot welding was performed based on one Embodiment of said this invention. At that time, as a post-heat energization after the end of the main energization, a set in which the energization current Ib is energized during the energization time Tb after the pressurization pressure Pb is increased from the main energization pressure Pa and the rest time Th is set. Was performed three times. The holding time until the gun was opened after the end of energization and the pressure became zero was set to one cycle.

  On the other hand, as Comparative Example (1), resistance spot welding in which only post-energization was not performed and post-heat energization was performed, and in Comparative Examples (2) and (3), post-heat energization was performed without changing the applied pressure. In Comparative Example (3), the post-heating current value Ib was set higher than in Comparative Example (2).

  Table 1 shows welding conditions and welding results of the inventive examples and the comparative examples. From the viewpoint of joint strength, the cross tensile strength was compared with Comparative Example (2). Furthermore, the presence or absence of dust was evaluated. For the nugget diameter, condition no. It was observed to be about 4 mm (about 3.2√t) in 6 to 10, and about 5 mm (about 4√t) in other cases. In the inventive examples and comparative examples (1) and (2), generation of dust was not observed. In addition, the measured value of the holding time after the end of energization was about 9 cycles.

  In the examples of the present invention, an improvement in the cross tensile strength was recognized as compared with Comparative Examples (1) and (2). Furthermore, in the example of the present invention, after the generation of dust in Comparative Example (3), a condition was obtained in which dust does not occur at a current value higher than the thermal current value Ib.

  As Example 2 of the present invention, as shown in FIG. 1 above, a servomotor attached to a C gun is added to a plate set 3 in which two steel plates (lower steel plate 1 and upper steel plate 2) are stacked. Resistance spot welding was performed by using a pressure single-phase alternating current (50 Hz) resistance welder to produce a resistance spot welded joint. As shown in FIG. 3, the pair of electrode tips used (lower electrode tip 4 and upper electrode tip 5) are composed of an alumina-dispersed copper DR type electrode having a tip radius of curvature R40 and a tip diameter of 6 mm. did.

  As test pieces, steel plates 1 and 2 were both 1.6 mm thick and the same steel type was used, and 1180 MPa grade duplex steel was used. Welding and tensile tests were performed based on JIS Z3137. In the second embodiment, the main energization conditions are set such that the applied pressure Pa is 3 kN, the main energization current Ia is 5 kA, and the main energization time Ta is constant for 20 cycles. Further, the post-heat energization time Ib was fixed at 3 cycles. No other squeeze time or slope time was set.

  And as an example of this invention, resistance spot welding was performed based on one Embodiment of said this invention. At that time, as a post-heat energization after the end of the main energization, a set in which the energization current Ib is energized during the energization time Tb after the pressurization pressure Pb is increased from the main energization pressure Pa and the rest time Th is set. Was performed once or twice. The holding time until the gun was opened after the end of energization and the pressure became zero was set to one cycle.

  On the other hand, as a comparative example, after the main energization, post-heat energization was performed without changing the applied pressure.

  Table 2 shows welding conditions and welding results of the inventive examples and the comparative examples. From the viewpoint of joint strength, the cross tensile strength was compared with that of the comparative example. Furthermore, the presence or absence of dust was evaluated. The nugget diameter was about 4.5 mm. The measured value of the holding time after the end of energization was about 9 cycles.

  In the example of the present invention, an improvement in the cross tensile strength was recognized as compared with the comparative example. Furthermore, in the example of the present invention, even under the same post-heat conduction current Ib, a condition that dust does not occur at a higher current value was obtained.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Lower steel plate 3 Board set 4 Lower electrode tip 5 Upper electrode tip 6 Nugget 7 Heat affected zone 7a Heat affected zone on electrode side 7b Heat affected zone on softened zone 8 Softened zone

Claims (9)

It is a resistance spot welding method in which a set of two or more steel plates including at least one high-strength steel plate is overlapped, sandwiched between a pair of welding electrodes, and welded by applying a current while applying pressure,
Consists of at least two steps,
A main energization step of forming a nugget of a predetermined diameter by energization;
The applied pressure is increased from the main energization process, and rapid cooling of the heat affected zone on the softening region side is suppressed, which includes a pause of 1 cycle or more and 20 cycles or less and a short time energization at a higher current value than the main energization process. Having a post-heat energization step,
A resistance spot welding method using a single-phase alternating current resistance welding machine .   The post-heat energization process in which rapid cooling of the heat-affected zone on the softened region side is suppressed increases the applied pressure from the main energization process, pauses for 6 cycles or more and 20 cycles or less, and a shorter current value than the main energization process. The resistance spot welding method according to claim 1, wherein the resistance spot welding method comprises time energization.   The post-heat energization process in which rapid cooling of the heat-affected zone on the softened region side is suppressed increases the applied pressure from the main energization process, pauses for 6 cycles or more and 10 cycles or less, and a shorter current value than the main energization process. The resistance spot welding method according to claim 1, wherein the resistance spot welding method comprises time energization. In resistance spot welding of a set of two or more high-strength steel plates, a main energization process for forming a nugget with a predetermined diameter, and then a pressure increase from the main energization process to increase the applied pressure from the main energization process. Multi-stage energization at a high current value, having a post-heat energization process that suppresses rapid cooling of the heat affected zone on the softened region side,
In this energization process, the nugget diameter d (mm) is the thickness t (mm) of the thinnest high-strength steel plate in the plate assembly,
3 × √t ≦ d ≦ 6 × √t
After setting the energization time and current value so that it is in the range represented by
In the post-heat energization step, a single-phase AC resistance characterized by performing multi-stage energization that repeats energization and deactivation one or more times with energization time of 1 to 5 cycles and deactivation time of 1 to 10 cycles by increasing the applied pressure Resistance spot welding method using a welding machine . The applied pressure Pb in the post-heat energizing step is relative to the applied pressure Pa in the main energizing step.
Pa <Pb ≦ 3 × Pa
5. The resistance spot welding method according to claim 4, wherein the resistance spot welding method is within the range. The current value Ib in the post-heat energization step is compared with the current value Ia in the main energization step.
Ia <Ib ≦ 4 × Ia
The resistance spot welding method according to claim 4, wherein the resistance spot welding method is within a range of the above.   The resistance spot welding method according to any one of claims 1 to 6, wherein in the post-heat energization step, the pause and energization are repeated in the range of 1 to 2 times.   The resistance spot welding method according to any one of claims 1 to 7, wherein the tensile strength of at least one of the two or more steel plates is 590 MPa or more.   The resistance spot welding method according to any one of claims 1 to 8, wherein a tensile strength of at least one of the two or more steel plates is 980 MPa or more.

Images