JP5206448B2 - Resistance spot welding method for high strength thin steel sheet - Google Patents

Description

  The present invention relates to a method for resistance spot welding of a high strength thin steel sheet.

  In general, a resistance spot welding method, which is a kind of a lap resistance welding method, is used for joining stacked steel plates. For example, in the manufacture of automobiles, several thousand spots are welded per vehicle. This welding method is a method in which two or more steel plates are overlapped, and the surface is directly sandwiched between the upper and lower electrodes and a large welding current is applied between the upper and lower electrodes for a short time to join them. . This electrode is water-cooled in order to extend the life of the electrode, and a point-like melted portion is obtained by a balance between resistance heat generation generated by flowing a large current of welding current and heat removal by the electrode. The dotted melted portion is rapidly cooled after the energization is stopped, and a joint (welded portion) called a nugget is formed.

  In recent years, especially in steel plates used for automobiles, the use of high-strength thin steel plates has been expanding in order to improve collision safety and reduce vehicle weight. However, in the high strength steel sheet, more alloy components are added, and when subjected to rapid heating and rapid thermal cycles such as resistance spot welding, not only melted and solidified nuggets, but also the surrounding thermal effects It is known that the part is also very hard and brittle, and it is difficult to obtain high strength particularly in cross tension.

  In order to obtain such a high cross tensile strength, the cross tensile strength can be improved to some extent by simply increasing the nugget diameter.

  Patent Document 1 discloses a method for spot welding a high-strength steel sheet characterized by separating the electrode from the steel sheet after the welding energization is completed, and after the retention time after welding specified by a function of the plate thickness, and after the welding energization is completed. Further, it is disclosed that the cross tensile strength can be improved by a spot welding method for a high-strength steel sheet characterized by continuing post-energization and adjusting the temperature drop rate during cooling of the spot weld.

  Further, in Non-Patent Document 1, after completion of the main energization, the energization is performed again after cooling for a certain period of time, the hardness is reduced by tempering the nugget portion and the heat-affected zone, and the residual stress is changed. A method for improving the fatigue strength of a weld is disclosed. Among them, it is disclosed that the cross tensile strength is improved by tempering at the same time.

JP 2002-103048 A

"Iron and Steel", Vol. 68, No. 9, pages 1444 to 1451

  However, when the nugget diameter is increased, if an attempt is made to obtain a large nugget, spatter is likely to occur without being suppressed by the electrodes, and the dent remaining in the welded portion also increases. In addition, there is a problem that it is difficult to narrow a flange portion secured for welding.

  In the method described in Patent Document 1, the post-weld holding time is defined as a function of the plate thickness. However, considering welding with an actual robot, the holding time that can be set in the welding timer and the resistance spot welding robot are The time from the end of pressurization to the separation of the electrode from the steel sheet is different, and each welding robot needs to confirm.

In addition, the method described in Non-Patent Document 1 has a problem in practicality because an appropriate temper energization condition range is narrow. In addition, since it is necessary to cool to a temperature below the _Mf point once after the main energization, the diameter of the nugget changes even if a tempering process that takes a long time is added along with the problem that the welding time becomes long. Therefore, it is not possible to obtain a higher strength than in the past by simply recovering to the level of the cross tensile strength obtained in the case of a low-strength steel plate.

  This invention is made | formed in view of the above situations, and it aims at providing the resistance spot welding method of the high intensity | strength thin steel plate excellent in the joint tensile characteristic.

In order to achieve the above-mentioned problems, the present inventors diligently studied a method for improving the cross tensile property in resistance spot welding of a high-strength thin steel sheet. As a result of examining important elements for improving the cross tensile properties based on the temper energization process, it is important to cool to a temperature below the _Mf point during the cooling time between energization and temper energization. It was confirmed. In other words, as long as the temper process is performed, an increase in welding time including cooling time is inevitable. In addition, in order to improve the joint strength (fracture form), it was considered to expand the area of the joint by welding at two points. However, when each welding point is independent, each welding point is individually It breaks, and the joint strength and fracture form cannot be improved. Therefore, after welding the first point, by welding the second point welding so as to overlap the first point welded portion, it is possible to improve the cross tensile characteristics of the resistance spot welded joint of high strength thin steel sheet Thought. That is, after energization of the first point, after holding the melted portion as necessary to solidify, the pressure applied by the electrode is released, and the nugget of the second point is formed next to the nugget of the first point and the position of the electrode. Move to a position within the overlapping range and weld the second point. During the movement and pressurization of the electrode, the first welded part is cooled to a temperature below the _Mf point required before energizing the temper. We thought that we could have the effect of.

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

[1] When carrying out resistance spot welding while sandwiching two or more high strength thin steel plates stacked between a pair of electrodes and applying pressure, the position of the electrode is moved after welding the first point. A resistance spot welding method for a high-strength thin steel sheet, wherein the second point welding is performed so that the second point welding partially overlaps with the first point welding portion after the welded portion is cooled to a temperature below the _Mf point.

  [2] When performing the second point welding so as to partially overlap the first point welded portion, before performing the second point welding, it is necessary to perform only the first point welding at another location. The resistance spot welding method for a high-strength thin steel sheet according to the above [1].

  In the present invention, by performing welding so that two welds (nuggets) partially overlap each other, it is possible to improve the effect of temper energization and the cross tensile property by expanding the nugget area.

  Moreover, since the total nugget diameter is expanded by welding at two points, the nugget diameter at each point does not need to be increased more than necessary, and it is easy to apply to welding to a narrow flange portion. is there. Since it is not necessary to increase the nugget diameter of each point more than necessary, the set current can be suppressed low, the occurrence of scattering can be suppressed, and the indentation of the welded portion can be reduced.

  Further, in the normal temper energization process, the process stops for a long cooling time between the main energization and the temper energization. However, the present invention moves the welding position after welding the first point. Since this is a process of welding the second point from the beginning, the first point of the next hit point is further welded at this time, and then the second point is returned to welding, thereby reducing the time. There is also.

It is a schematic diagram which shows the resistance spot welding method which concerns on one Embodiment of this invention. It is a drawing substitute photograph of the cross section which cut | disconnected the fracture | rupture part after the cross tension test of the comparative example in Example 1 of this invention. It is a drawing substitute photograph of the cross section which cut | disconnected the fracture | rupture part after the cross tension test of the example of this invention in Example 1 of this invention.

  Embodiments of the present invention will be described below.

  FIG. 1 is a schematic diagram of a resistance spot welding method according to an embodiment of the present invention.

  As shown in FIG. 1A, the high-strength thin steel plate 13 and the high-strength thin steel plate 14 are overlapped, and the electrode 11 and the electrode 12 sandwich the portion where the thin steel plate 13 and the thin steel plate 14 are joined. A current is passed between the electrodes 11 and 12. Of course, this current may be either direct current or alternating current. As a result, the portion where the current flows generates resistance heat, and a melted portion (nugget) 15 is formed in the vicinity of the central portion between the electrodes due to the balance with heat removal by the electrodes. At this time, if the diameter of the nugget 15 is large, a high cross tensile strength can be obtained even at only one point. Therefore, a nugget having a large diameter is preferable in a range where no scattering occurs. A feature of this welding method is that a joint with excellent cross tensile properties can be obtained even if the nugget diameter at each point is small, but the nugget diameter is less than 3√t when the plate thickness is t. In this case, it is difficult to obtain a sufficient bonding area even if two points are overlapped, so that the nugget diameter is preferably 3√t or more.

After the energization at the first point, the pressure is maintained as much as necessary for the nugget 15 to solidify, and then the pressure is released and the electrodes 11 and 12 are moved as shown in FIG. During this movement, the nugget 15 is cooled to a temperature not higher than the _Mf point, so that the temper effect in the second welding is effectively performed. Accordingly, since the time until the second point welding may be increased, as shown in FIG. 1 (b ′), the next point is further welded to form the nugget 16 during this period. For example, when the steel plate is thick and a long cooling time is required between the first point and the second point, it may be preferable to shorten the welding process.

  Thereafter, as shown in FIG. 1C, the second point is welded to the first point nugget 15 at a position where the second point nugget 17 partially overlaps. By this second welding, the first weld is tempered, the toughness of the first nugget 15 and the heat-affected zone is improved, and the total nugget diameter is increased to increase the cross tensile property. Improved. In this second welding, since the current is diverted to the first welding portion, the diameter of the second melting portion is reduced under the same welding conditions. Therefore, the second welding current is increased or the energization time is extended. It is more preferable. However, in the condition that the melted part by the second point welding remelts all the first point nuggets, the effect of the temper is lost, resulting in a brittle nugget and the cross tensile properties are not improved. In addition, when the first nugget 15 and the second nugget 17 do not overlap at all, each weld point is individually broken, and in this case, the cross tensile property is not improved. The first nugget 15 is not completely remelted by the second welding and the two nuggets 15 and 17 are overlapped so that the welding positions of the first and second welds are shifted by 2 What is necessary is just to select the welding conditions of the point.

  By the way, the welding equipment used in the present invention is the type of pressure mechanism (by air cylinder, by servo motor), shape (stationary, robot gun), and type of power source (single-phase AC, AC inverter, DC inverter). There is no particular limitation. Further, the high-strength steel plate to be welded is not limited to its type (solid solution strengthening type, precipitation strengthening type, two-phase structure type, processing induced transformation type, etc.). Etc.).

  In order to confirm the effect of the present invention, the following examples of the present invention and comparative examples were carried out as Example 1.

  The plate assembly at that time is a double stack of 980 MPa class high strength thin steel plates with a plate thickness of 1.6 mm. As the electrode, a DR type electrode having a tip diameter of 6 mm was used. For welding, a single-phase AC servo motor pressure resistance spot welding robot was used. The welding conditions are as shown in Table 1. Welding was performed in the case of the present invention in which two points were welded and in the case of a comparative example in which only one point was welded. In addition, the welding conditions of the 1st point of this invention example and the welding conditions of a comparative example are the same, and the nugget diameter was welded on the conditions used as about 4√t. The second point of the inventive example was welded under the same welding conditions as the first point at a position shifted by 2.5 mm from the position of the first point.

  Each of the test specimens after welding was evaluated by performing a cross tension test defined in JIS / Z / 3137. FIG. 2 shows a cross section obtained by cutting the fracture portion after the cross tension test in the case of the comparative example, and FIG. 3 shows a cross section obtained by cutting the fracture portion after the cross tension test in the case of the present invention example.

  In the case of the comparative example, as shown in FIG. 2, the interface fractured and the cross tensile strength was 9.9 kN, whereas in the example of the present invention, as shown in FIG. The cross tensile strength is improved to 13.2 kN, and the effectiveness of the present invention can be confirmed.

  In order to confirm the effect of the present invention, the following examples of the present invention and comparative examples were carried out as Example 2.

Here, welding was performed on each two-layered sheet set of 980 MPa class, 1180 MPa class, and 1470 MPa class steel sheets using a single-phase AC servo motor pressure resistance spot welding robot. Welding was performed under the welding conditions shown in Table 2. Here, a chisel test defined in JIS / Z / 3144 was performed to evaluate the strength of the welded portion. In addition, when the temperature of the weld zone at the first point was cooled to the _Mf point temperature or lower before the second point was welded, it was marked (◯), and when it was not cooled (x). The _Mf point in the present invention is measured from a transformation expansion change using a high-frequency heating type fully automatic transformation recording measuring device (formastor-EDF, FTM-1000). Further, in the rattan test, it was judged that the case where the rattan was driven and not broken was good (◯), and the case where the interface was broken was judged as bad (x).

As a result, as shown in Table 2, it can be seen that the example of the present invention does not break in the chisel test, and the joint has good strength. On the other hand, the conventional one that does not perform the second point welding (No. 2, 16), the cooling time between the first point and the second point is short, and the welded portion at the first point is below the _Mf point temperature. In comparative examples such as those that were not cooled (No. 10, 11) and those that did not overlap the nugget because the distance between the first and second points was too far (No. 4, 14) It is easy to confirm that the interface breaks and the weld is brittle and weak.

  In order to confirm the effect of the present invention, after the first point welding, only the first point welding is performed at another location, and then the second point welding is performed so as to partially overlap the first point welding portion. Went. As a result, the cooling time between the first point and the second point, which was necessary when welding the first point and the second point continuously, can be shortened, and the time required for the entire welding operation can be shortened, that is, the tact time. It was confirmed that it was possible to shorten.

DESCRIPTION OF SYMBOLS 11 Electrode 12 Electrode 13 High-strength thin steel plate 14 High-strength thin steel plate 15 Melting part (nugget)
16 Melting part (nugget)
17 Melting part (nugget)

Claims (2)

When carrying out resistance spot welding with two or more superposed thin steel plates sandwiched between a pair of electrodes and applying pressure, the position of the electrode is moved after welding the first point. Is cooled to a temperature equal to or lower than the M _f point, the second point welding is performed so as to partially overlap the first point welded portion.   When performing the second point welding so as to partially overlap the first point weld, only the first point welding is performed at another location before the second point welding is performed. The resistance spot welding method of the high-strength thin steel sheet according to claim 1.