JP5293227B2 - Resistance spot welding method for high strength thin steel sheet - Google Patents

Description

  The present invention relates to a resistance spot welding method for 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 resistance spot weldings are performed per vehicle.

  In this resistance spot welding method, as shown in FIG. 3, two or more steel plates (here, two steel plates of steel plate 3 and steel plate 4) are overlapped, and the surface is directly covered by upper and lower electrodes 1 and 2. In this method, a large welding current is applied for a short time between the upper and lower electrodes 1 and 2 while applying the sandwiching pressure. The electrodes 1 and 2 are water-cooled in order to extend the life of the electrodes, and a point-like melted portion can be obtained by the balance between the resistance heat generated by flowing a large welding current and the heat removal by the electrodes. . The dotted melted portion is rapidly cooled after the energization is stopped, and a joint portion called a nugget 6 is formed.

  In recent years, especially in steel plates used for automobiles, the use of high-strength steel plates has been expanding to improve collision safety and reduce vehicle weight. However, more alloy components are added to high-strength steel sheets, 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 heat affected zone However, it is known that the material tends to be 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.

  In Non-Patent Document 1, after completion of the main energization, the energization is performed again after cooling for a certain time, and the nugget part and the heat affected part are tempered to reduce the hardness and change the residual stress. A method for improving the fatigue strength of a spot 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, pp. 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 general, the temper energization is performed by energizing a current lower than the main energization for a long time, and there is a problem that the welding time becomes long.

  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. In particular, we focused on the relationship between the energization conditions and the heat generation position during temper energization.

  FIG. 1 is a schematic diagram of welding conditions for conventional temper energization. As shown in FIG. 1, in the conventional temper energization, after the nugget is formed by the main energization, a long cooling time is set, and then the temper energization is performed. The temper energization is generally performed by flowing a current less than the current value of the main energization for a time equal to or longer than the main energization. In this case, heat is generated from the central portion between the electrodes due to the balance between heat removal by the electrodes and resistance heat generation by energization. If the heat input becomes excessive, the nugget is melted again from the central portion, so that it is not tempered (tempered) but is re-quenched, and in a high-strength thin steel plate, the weld becomes brittle and the joint strength is not improved. On the other hand, when the temper energization current is set low, a long energization time is required to control the temperature near the nugget end that affects the joint strength due to heat generation from the nugget center to the temperature required for the temper. . In addition, when the diameter of the nugget formed by the main energization changes, the appropriate temper energization current also changes. Therefore, considering the implementation, the condition range is narrow and it is difficult to apply.

  On the other hand, the schematic diagram of the welding conditions at the time of applying a high electric current for a short time as temper energization is shown in FIG.

  In the case of the conventional temper energization as shown in FIG. 1, as described above, heat is generated from the central portion between the electrodes due to the balance between heat removal by the electrodes and resistance heat generation by the energization. On the other hand, as shown in FIG. 2, when a high temper current is applied in a short time, a region that generates the most heat is formed in the vicinity of the electrode, and heat is applied from this region to the welded portion by heat conduction. become. In other words, a wide area can be efficiently heated in a short time, and even if the tempering current is excessive, the center of heat generation is different from the boundary between the steel plates, so that scattering does not easily occur between the steel plates, and a wide appropriate We thought that tempering with a range could be performed.

  In addition, even when a high current is applied for a short time as the temper energization described above, if the nugget formed by the main energization is to be energized at a high temperature, the high temperature nugget portion is likely to be remelted and scattered. End up. Furthermore, as long as the process of energizing the temper is taken, it is necessary to start baking once, and thus a long cooling time is required. In this case, it is one of the problems to be solved that the welding time required for each point is significantly increased as compared with the case where temper energization is not performed. The same applies to the heat treatment applied to the welded portion in this short time high current process because it is necessary to cool to the Ac1 transformation point (near 723 ° C.) or less even if it is an austemper.

  Since the welded portion is rapidly cooled by heat radiation to the electrodes sandwiching the steel plate, the welded portion can be cooled faster as the contact area between the electrode and the steel plate is wider. In general, an electrode for resistance spot welding has a shape in which the tip of the electrode has a curvature, as schematically shown in FIG. Therefore, as a method of accelerating the cooling after the main energization, it is only necessary to perform welding with a high pressure and widen the contact area between the electrode and the steel plate. That is, if the applied pressure is increased during the main energization or after the main energization, the contact area between the electrode and the steel plate can be increased, the cooling rate can be increased, and the cooling time can be shortened. When the temper is energized, the applied pressure does not need to be high, so after the cooling is completed, the applied pressure may be reduced and then energized with a high current for a very short time. Due to the expansion, the contact surface between the electrode and the steel sheet is reduced, and there is an advantage that the current value of temper energization for passing a high current for an extremely short time can be reduced.

  Also, looking at the parts structure of automobiles, for example, the center pillar employs a structure in which a panel outer using a soft thin steel plate is combined outside a combination of a pillar outer using a high strength steel plate and a pillar inner. Therefore, it is necessary to weld this three-layer structure.

  When a high-tempering current is applied to this structure for a short time, the heat generation position is in the vicinity between the soft thin steel plate and the high strength steel plate, but the soft thin steel plate has a lower specific resistance than the high strength steel plate and is less likely to generate heat. Therefore, since the problem of occurrence of scattering does not easily occur, there is no problem at all, and the joint strength between the high-strength steel sheets can be improved. Furthermore, since the heat generation position is in the vicinity between the soft thin steel plate and the high strength steel plate, there is an advantage that the joining state between the thin plate and the thick plate is improved, and the thin plate is difficult to peel off.

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

[1] A resistance spot welding method in which a plate assembly in which two or more high-strength thin steel plates are stacked is sandwiched between a pair of electrodes and a current is applied while welding is applied.
A first step of forming a nugget by main energization, a second step of cooling the welded portion by holding it with an electrode after completion of the first step, and a pole after completion of the second step A resistance spot welding method for a high-strength thin steel sheet, characterized in that resistance spot welding is performed by a process including a third step in which a current higher than the main energization is applied within a short time without scattering.

  [2] The resistance spot welding method for a high-strength thin steel sheet according to [1], wherein the shape of the tip of the electrode is a curved surface.

  [3] The resistance spot welding of the high-strength thin steel sheet according to [1] or [2], wherein the applied pressure is increased from the middle of the first step or after the end of the first step. Method.

  [4] The resistance of the high-strength thin steel sheet according to any one of [1] to [3], wherein the third step is performed after the applied pressure is reduced after completion of the second step. Spot welding method.

  [5] A plate assembly in which a thin steel plate having a thickness smaller than that of the high-strength thin steel plate and having a lower strength is further superimposed on at least one side of the plate assembly in which the two or more high-strength thin steel plates are stacked. Resistance spot welding of the high-strength thin steel sheet according to any one of [1] to [4], wherein resistance spot welding is performed.

  In the resistance spot welding of a high-strength thin steel sheet, the temper energization condition is such that a high current is passed in an extremely short time as the temper energization process as opposed to the conventional temper energization condition. Can be controlled in a different form from conventional tempering, and tempering can be performed efficiently in a short time with a wide appropriate current range. This is a problem in resistance spot welding of high-strength steel sheets. It becomes possible to improve the joint strength, especially the cross tensile strength.

  In addition, the use of an electrode with a curved tip makes it possible to shorten the cooling time until the temper energization is performed after the main energization. Time can be shortened.

  Also, welding of a three-layered plate assembly having a large thickness ratio (for example, a plate assembly in which a thin plate having a thickness of 0.7 mm, a thick plate having a thickness of 1.6 mm, and a thick plate having a thickness of 1.6 mm are stacked) Since the heat generation position is near the boundary between the thin plate and the thick plate, there is also an effect of improving the bonding state between the thin plate and the thick plate.

It is a schematic diagram which shows the welding conditions of the conventional temper energization. It is a schematic diagram which shows the welding conditions of temper electricity supply in one Embodiment of this invention. It is a schematic diagram of resistance spot welding. It is a figure which shows the electrode from which the front end became a curved surface. It is a figure which shows the board assembly in Example 1 of this invention.

  Embodiments of the present invention will be described below.

  The resistance spot welding method for a high-strength thin steel sheet according to an embodiment of the present invention is the same as the schematic diagram shown in FIG. The welding conditions are the same as those in the schematic diagram shown in FIG.

  That is, in this embodiment, as shown in FIGS. 3 and 2, a plate assembly in which two high-strength thin steel plates 3 and 4 are overlapped is sandwiched between a pair of upper and lower electrodes 1 and 2 while applying pressure. When performing the resistance spot welding method by passing an electric current, as a first step, the nugget 6 is formed by main energization, and as a second step, the welded portion is cooled by being held between the electrodes, and the third step is performed. As a step, a current higher than that of the main energization is made to flow within a range in which scattering does not occur in an extremely short time.

  Hereinafter, each step will be described in detail.

  First, in the first step, the high-strength thin steel plate 3 and the high-strength thin steel plate 4 are overlapped, and the electrode 1 and the electrode 2 sandwich the portion where the high-strength thin steel plate 3 and the high-strength thin steel plate 4 are joined. A current is passed between the electrodes 1 and 2. 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) 6 is formed in the vicinity of the central portion between the electrodes 1 and 2 due to balance with heat removal by the electrodes. At this time, if the diameter of the nugget 6 is sufficiently 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. It is a feature of this welding method that even if the nugget diameter is small, a joint having excellent cross tensile properties can be obtained by conducting tempering with a high current for a short time in the third step.

  Next, in the second step, the cooling time needs to be taken until the nugget is sufficiently cooled (cooled to the Ac1 point or less), and if the cooling is insufficient, the effect of the temper of the third step cannot be obtained, Scattering occurs. In order to shorten the cooling time, it is preferable to increase the contact pressure between the electrodes 1 and 2 and the steel plates 3 and 4 during the cooling time. In order to obtain this effect, it is preferable that the tips of the electrodes 1 and 2 are curved surfaces (having curvature) as shown in FIG.

  Next, in the third step, in determining the current value of the temper energization, the nugget of the minimum diameter (for example, about 3√t to 4√t when the plate thickness is t) for which the strength is to be guaranteed. It is preferable to determine the tempering condition based on the main energizing condition where the slab is formed. At this time, since the center of heat generation is different from the boundary portion between the steel plates 3 and 4, it is difficult for scatter to occur, and temper energization having a wide appropriate range can be performed. Is preferred. By setting the temper current on the high current side, the effect of temper energization can be obtained even when the nugget diameter formed by the main energization is large. At this time, if the temper energization time is long, the position of the center of heat generation is near the boundary between the steel plates, and therefore energization for an extremely short time is preferable. Specifically, a temper energization time of 0.1 sec or less is preferable. On the other hand, when the temper energizing time is 0.02 sec or less, it becomes difficult to control the current at a high current. Accordingly, the temper energization time T1 is preferably 0.02 sec <T1 ≦ 0.1 sec.

  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. Moreover, 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, work-induced transformation type, etc.), and is a plate assembly (combination with mild steel or three-layer superposition). Etc.).

  In order to confirm the effect of the present invention, the present invention example and the comparative example were carried out.

  At that time, the plate assembly shown in Table 1 and FIG. 5 was used. That is, the plate sets A to E are plate sets obtained by superposing two high-strength steel plates of the same type and the same plate thickness on the first and second sheets (FIG. 5 (a)). The second and third sheets are a set of two high strength steel sheets of the same type and the same sheet thickness, and one soft thin steel sheet as the first sheet (FIG. 5B). )).

  Table 2 shows the welding conditions for the inventive example and the comparative example.

  The welding apparatus used was a single-phase AC servo motor pressure resistance spot welding robot.

  And the cross-tension test prescribed | regulated to JISZ3137 was done about the test body after welding, and it evaluated. Those that resulted in plug fracture were judged as acceptable (◯), and those that resulted in interface fracture were judged as unacceptable (x).

  Table 2 shows the results of the present invention and comparative examples (whether or not scattering occurs, cross tensile strength, fracture mode).

  First, No. 10 (comparative example) is a case where temper energization was not performed. When a cross tension test was performed, interface fracture occurred and the cross tensile strength was low.

  Next, numbers 1 to 4 correspond to a case where temper energization with a high current is applied for an extremely short time, which is changed only for the cooling time. Under this main energization and cooling condition in this plate assembly, No. 4 (comparative example) is scattered because the cooling time is too short, and the cross tensile fracture strength is low due to interfacial fracture. On the other hand, numbers 1 to 3 (examples of the present invention) are plug fractures, indicating high strength.

  Next, Nos. 5 to 9 were obtained by changing the current value of the temper energization. In No. 9 (comparative example) with a current of 18 kA, the current was too high to cause scattering, and interfacial fracture caused cross tension fracture. Although the strength is low, it can also be seen that in Nos. 5 to 8 (examples of the present invention), a high strength is obtained by plug rupture, and the appropriate range of the temper current is wide.

  Next, Nos. 11 to 19 increase the cooling rate by increasing the electrode tip diameter and further increasing the pressure during cooling. In No. 11 (comparative example) where temper energization is not applied, the strength is low due to interfacial fracture, but in Nos. 12 to 19 (examples of the present invention), not only high strength is exhibited due to plug fracture, but also the cooling time is shortened. You can see that

  Next, Nos. 20 to 24 are obtained by changing the energization time of temper energization. However, in No. 20 (comparative example), the energization time is too short. Not. On the other hand, in numbers 23 and 34 (comparative examples) having a long temper energization time of 0.12 sec, scattering occurs, interface fracture occurs, and the strength is low.

  Next, numbers 25 to 32 are for examining the influence of the thickness, strength, and presence / absence of plating of the steel sheet to be applied. In numbers 26, 28, 30, and 32 (comparative examples) where temper energization is not performed, In the case of numbers 25, 27, 29, and 31 (examples of the present invention) that had low strength at break, high strength was obtained at plug break by performing tempering with a high current for an extremely short time. The effect of the present invention can be confirmed.

  Numbers 33 to 35 correspond to a case of a three-layered plate set including one sheet of mild steel plated steel plate (plate thickness 0.7 mm) and two 980 MPa class high-tensile plated steel plates (plate thickness 1.6 mm). . In No. 35 (comparative example) in which temper energization was not performed, the cross tensile strength between 1.6 mm high-strength steel sheets was low and interface fracture occurred, but in No. 33 and 34 (invention example), plug fracture occurred and was high. The strength was obtained, and the effect of the present invention was confirmed even for a three-layered plate assembly.

  Furthermore, although not described in Table 2, in No. 35 (comparative example), the joining state between the mild steel plated steel plate (thin plate) and the high-tensile plated steel plate (thick plate) was poor, and the interface was easily broken. However, in Nos. 33 and 34 (examples of the present invention), the plug is broken between the mild steel plated steel plate (thin plate) and the high strength plated steel plate (thick plate). It can be seen that the problem of thin plate peeling, which is a problem, can be solved by using the present invention.

1 electrode 2 electrode 3 steel plate (high strength steel plate)
4 Steel plate (high strength steel plate)
6 Nuggets

Claims (5)

A resistance spot welding method in which a plate assembly in which two or more high-strength thin steel plates are stacked is sandwiched between a pair of electrodes and current is applied while welding is applied, and welding is performed.
A first step of forming a nugget in this energization, after completion of the first step, a second step of cooling the welded portion by holding while sandwiched by the electrodes, after completion of the second step, 0 A resistance spot welding is performed by a process including a third step in which temper energization is performed at a higher current than the main energization within a range in which no scattering occurs in an extremely short time of .02 sec and 0.1 sec or less. Resistance spot welding method for high strength thin steel sheet.   2. The resistance spot welding method for a high strength thin steel sheet according to claim 1, wherein the shape of the tip of the electrode is a curved surface.   3. The resistance spot welding method for high-strength thin steel sheets according to claim 1 or 2, wherein the applied pressure is increased from the middle of the first step or after the end of the first step.   The resistance spot welding method for a high-strength thin steel sheet according to any one of claims 1 to 3, wherein the third step is performed after the applied pressure is reduced after the second step.   At least one side of the plate assembly in which the two or more high-strength thin steel plates are overlapped is resistance spot welded to a plate assembly in which the thin steel plates that are thinner and lower in strength than the high-strength thin steel plates are further overlapped A resistance spot welding method for a high-strength thin steel sheet according to any one of claims 1 to 4.

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