JP2012187616A - Resistance welding apparatus and resistance welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resistance welding apparatus and a resistance welding method formable of a welding part provided with enough breaking strength when joining the welding part by resistance welding, where superposed similar first steel sheets are further superposed with a second steel sheet different from the first steel sheets in at least one among the carbon content, sheet thickness and hardness.SOLUTION: The resistance welding apparatus 100 is to join a welding part 12 by resistance welding, where superposed similar first steel sheets 10, 11 (for example, high tensile strength steel sheets) are further superposed with a second steel sheet 16 (for example, a soft steel) different from the first steel sheets in at least one among the carbon content, sheet thickness and hardness. Each tip diameter φa, φb of a pair of electrode tips 21a, 21b which pinch the welding part is set to the necessary diameter size to form a nugget between the superposed first steel sheets and the necessary diameter size to form a nugget between the first steel sheet and the second steel sheet.

Description

  The present invention relates to a resistance welding apparatus and a resistance welding method.

  In order to reduce the weight of the vehicle, the thickness of the steel plate to be used is reduced by applying a higher-tensile steel plate. In general, resistance welding (spot welding) is applied to joining high-tensile steel plates (see Patent Document 1). In resistance welding, a joint portion in which two or three high-tensile steel plates are overlapped is sandwiched between a pair of electrodes. The pair of electrodes are configured to be capable of changing the pressure applied to the joint and the energization pattern. The joint is sandwiched between a pair of electrodes, and a current is allowed to flow for a predetermined time while applying pressure, thereby melting the steel sheet using resistance heat generated between the steel sheets and joining the respective steel sheets to obtain a joined body. .

  2. Description of the Related Art Some panel parts of automobile bodies, such as body sides, are formed by stacking and joining dissimilar steel plates in order to ensure strength and freedom of modeling.

JP 2010-115706 A

  As in the case of the body side described above, for example, the superposed high-strength steel plates are joined to each other with another high-strength steel plate and another steel plate (for example, mild steel) that is different in at least one of carbon content, thickness, and hardness. When joining the parts by resistance welding, it is difficult to obtain a joined part having sufficient breaking strength only by adjusting the energization time and the current value. That is, when the energizing time and current value are adjusted first to form a nugget of the required diameter between the stacked high-tensile steel plates, the required diameter between one high-tensile steel plate and the other steel plate Nuggets cannot be secured, and the breaking strength of the joint may be reduced. Contrary to this, the formation of a nugget of the necessary diameter between one high-strength steel plate and the other steel plate is made by adjusting the energization time and the current value first. In some cases, the nugget having a necessary diameter cannot be secured, and the breaking strength of the joint portion is lowered. Since it is difficult to form a nugget having a predetermined diameter, the strength of the joint cannot be improved even if post-heat conduction is performed.

  The present invention has been made in order to solve the problems associated with the above-described prior art, and the first steel plate and the carbon content, the thickness, and the hardness of the first steel plate overlapped with each other are overlapped at least. It is an object of the present invention to provide a resistance welding apparatus and a resistance welding method capable of forming a joint having a sufficient breaking strength when joining joints obtained by superimposing different second steel plates by resistance welding.

  The present invention that achieves the above-described object is the joining of the first steel plates of the same type overlapped with a second steel plate that is different from the first steel plate in at least one of carbon amount, plate thickness, and hardness. This is a resistance welding apparatus for resistance welding of parts. In this resistance welding apparatus, the tip diameter of each of the pair of electrode tips sandwiching the joint portion is set to a nugget of a necessary diameter between the superposed first steel plates and one of the first steel plates. And a dimension for forming a nugget of a necessary diameter between the second steel plate and the second steel plate.

  In the present invention for achieving the above object, the first steel plates of the same type overlapped with each other, and a second steel plate having a carbon amount, a thickness, and a hardness different from those of the first steel plates is overlapped. This is a resistance welding method in which the welded portion is resistance welded. In this resistance welding method, a nugget having a required diameter is formed between the stacked first steel plates, and a nugget having a required diameter is formed between one first steel plate and the second steel plate. A main energization step of energizing the joint portion with a pair of electrode tips having a tip diameter.

  In the resistance welding apparatus of the present invention, the diameters of the tips of the pair of electrode tips sandwiching the joint portion are nuggets of a necessary diameter between the superposed first steel plates and one of the first steel plates. Since it is set to a dimension that forms a nugget of a necessary diameter with the second steel plate, a welding condition (energization time, current value, etc.) that can simultaneously secure a nugget of a necessary diameter between each steel plate is set. As a result, it is possible to form a joint having sufficient breaking strength.

  According to the resistance welding method of the present invention, a nugget having a necessary diameter is formed between the stacked first steel sheets, and a nugget having a necessary diameter is formed between one of the first steel sheet and the second steel sheet. Since it has a main energization process that energizes by sandwiching the joint between a pair of electrode tips with a tip diameter, welding conditions (such as energization time and current value) that can simultaneously secure a nugget of the required diameter between each steel plate As a result, it is possible to form a joint having a sufficient breaking strength.

It is a schematic block diagram which shows the resistance welding apparatus of this embodiment. It is an enlarged view which shows the principal part of FIG. It is a figure which shows the electricity supply pattern and pressure pattern for obtaining the joined body of the high-tensile steel plate of this embodiment. It is a graph which shows the relationship between the cool time in a re-energization process, and an electric current value. In this embodiment, it is a graph which shows the relationship between the electric current value in 1st electricity supply, and a nugget diameter. It is a graph which shows the relationship between a nugget diameter and a cross tensile strength (CTS intensity | strength). It is a figure which shows the combination of the tip diameter of the electrode tip in contrast 1. 5 is a graph showing a relationship between a current value in first energization and a nugget diameter in contrast 1; It is a figure which shows the combination of the front-end | tip diameter of the electrode tip in contrast 2. 5 is a graph showing the relationship between the current value and the nugget diameter in the first energization in contrast 2;

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. The dimensional ratios in the drawings are exaggerated for convenience of explanation, and are different from the actual ratios.

  With reference to FIG. 1 and FIG. 2, the resistance welding apparatus 100 shown in the drawing is briefly described in that the first steel plates 10, 11, the carbon content, and the thickness of the first steel plates 10, 11 are overlapped with each other. And at least one of the hardnesses is used for resistance welding of the joint portion 12 in which the second steel plates 16 having different hardnesses are stacked. In this resistance welding apparatus 100, the diameters necessary between the first steel plates 10, 11 overlapped with each other at the tip diameters φa, φb of the pair of electrode tips 21 a, 21 b sandwiching the joint portion 12 ( Dimensions to form a nugget 13 (indicated by arrow b in FIG. 2) and a nugget 13 having a required diameter (indicated by arrow a in FIG. 2) between one of the first steel plate 11 and the second steel plate 16 It is set to. The case where the first steel plate to be superposed is a high-tensile steel plate and the second steel plate to be superposed is mild steel will be described as an example. For convenience of explanation, the second steel plate 16 is also referred to as “another steel plate 16”. Details will be described below.

  In the automobile body, resistance welding (spot welding) is applied in order to join the pressed parts 10, 11, 16 to each other. The illustrated press parts 10 and 11 are made of a high-tensile steel plate, and the press part 16 is made of a mild steel such as an alloyed hot-dip galvanized steel plate. The press parts 10, 11, and 16 are provided with flanges 10a, 11a, and 16a for joining, respectively. The joint 12 where the flanges 10a, 11a, 16a are overlapped is locally pressurized and energized by the welding gun 20. Thus, resistance heat (Joule heat) is generated between the steel plates, and the steel plates constituting the pressed parts 10, 11, 16 are melt-bonded by the heat to assemble the vehicle body. The code | symbol "15" in FIG. 1 has shown the joined body formed by resistance welding the press components 10, 11, and 16 mutually.

  The welding gun 20 includes a pair of electrode tips 21a and 21b, and a pressure cylinder 22 that urges the joining portion 12 through the electrode tips 21a and 21b. The electrode tips 2121 a and 21 b are connected to the current supply device 30. The current supply device 30 is configured such that the current value and the voltage value can be adjusted. The current supply device 30 is connected to a controller 40 for controlling a pattern to be energized through the electrode tips 21a and 21b.

  When the energization time and the current value are adjusted based on the formation of a nugget of a necessary diameter between the superposed high strength steel plates 10, 11, the one high strength steel plate 11 and the other steel plate 16 A nugget of a necessary diameter cannot be secured in between, and the breaking strength of the joint portion 12 may decrease. On the contrary, when the energization time and current value are adjusted based on the formation of a nugget having a required diameter between one high-strength steel plate 11 and the other steel plate 16, the superposed high-tensile steel plate 10, A nugget having a required diameter cannot be secured between the two members 11, and the breaking strength of the joint portion 12 may be reduced.

  Therefore, in the present embodiment, not only the energization time and the current value are adjusted, but also the tip diameter φa of the electrode tip 21a is set to a necessary diameter between one high-tensile steel plate 11 and the other steel plate 16 ( 2 is set to the dimension for forming the nugget 13 (indicated by the arrow a), and the tip diameter φb of the electrode tip 21b is set to a necessary diameter between the superposed high-tensile steel plates 10 and 11 (arrow in FIG. 2). The dimension of the nugget 13 (shown by b) is set. A broken line i in FIG. 2 is a boundary line for indicating a current flowing range.

  According to the resistance welding apparatus 100 having such a configuration, a nugget having a necessary diameter between the stacked high-tensile steel plates 10 and 11, and between the one high-tensile steel plate 11 and the other steel plate 16 are necessary. Welding conditions (such as energization time and current value) that can ensure a nugget of diameter at the same time can be set. As a result, it is possible to form the joint portion 12 having sufficient breaking strength.

  The tip diameter φa of one (21a) of the pair of electrode tips 21a, 21b may be different from the tip diameter φb of the other (21b) of the pair of electrode tips 21a, 21b. For example, the tip diameter φa of the electrode tip 21a is 6 mm, and the tip diameter φb of the electrode tip 21b is 8 mm. In various combinations of the high-tensile steel plates 10 and 11 and the high-strength steel plates 10 and 11 and other steel plates 16 that differ in at least one of carbon content, plate thickness, and hardness, nuggets having a necessary diameter between the respective steel plates. This is because it is possible to easily set welding conditions (such as energization time and current value) that can simultaneously ensure the above, and to form the joint 12 having sufficient breaking strength.

  The high-tensile steel plates 10 and 11 are harder to be deformed because they are harder than the other steel plates 16 made of mild steel, although they have a high carbon content and good conductivity. When the joining portion 12 is sandwiched between the pair of electrode tips 21a and 21b, the superposed high-tensile steel plates 10 and 11 are difficult to adhere to each other, but one high-tensile steel plate 11 and the other steel plate 16 are different from each other. It is greatly deformed and comes into close contact. Therefore, the electrical resistance between the superposed high-tensile steel plates 10 and 11 is greater than the electrical resistance between one high-tensile steel plate 11 and the other steel plate 16. In such a case, it is preferable that the tip diameter φb of the electrode tip 21b on the side in contact with the superposed high-tensile steel plates 10 and 11 is larger than the tip diameter φa of the electrode tip 21a on the side in contact with the other steel plate 16. . Since the contact diameter of the electrode tip 21a on the side having a small electrical resistance does not increase, the current density does not decrease, and the other steel plate 16 (soft steel) having a relatively low electrical resistance can be melted. And it is possible to easily set the welding conditions (energization time, current value, etc.) that can simultaneously secure a nugget of a necessary diameter between the steel plates, and the joint 12 having sufficient breaking strength can be formed. It is.

  The carbon content of the high-tensile steel plate is, for example, 0.1 to 0.3% by mass, and the other steel plate is, for example, mild steel having a carbon content of 0.05% by mass or less. A plate assembly of a high-strength steel plate and a mild steel is often applied to a panel part of an automobile body, for example, a body side, and manufactures a panel part of an automobile body formed with a joint 12 having sufficient breaking strength. Because you can. In addition, these carbon content is only an example, and this invention is not limited to this range.

  The joined body 15 of the high-tensile steel plates 10 and 11 and the mild steel 16 of the present embodiment can be obtained as follows.

  FIG. 3 is a diagram schematically showing an energization pattern and a pressurization pattern for obtaining a joined body of high-tensile steel plates according to the present embodiment.

  With reference to FIG. 3, the resistance welding method for obtaining the joined body 15 of the high-tensile steel plates 10 and 11 and the mild steel 16 of the present embodiment has a required diameter between the superposed high-tensile steel plates 10 and 11. Main energization in which the joint 12 is sandwiched between the nugget and a pair of electrode tips 21a and 21b having tip diameters φa and φb that form a nugget of a necessary diameter between one high-strength steel plate 11 and the other steel plate 16. Step (S1) is included. Furthermore, a re-energization step (S2) that repeats at least one stop of energization and re-energization while energizing the joint 12 with the same pressure as in the main energization step (S1), and a main energization step (S1) ) And a hold step (S3) for stopping energization after the re-energization step (S2) with the same pressure applied.

  In the re-energization step (S2) of the present embodiment, the energization stop and re-energization are repeated twice, and a total of three energizations are performed, including one energization in the main energization step (S1). ing. Hereinafter, for convenience of explanation, the energization in the main energization step (S1) is referred to as “first energization”, the first energization in the re-energization step (S2) is referred to as “second energization”, and the re-energization step (S2). The second energization is referred to as “third energization”.

  The high-tensile steel plate is, for example, a 1.2 GPa class steel having a carbon content of 0.18% by mass. The above-described high-tensile steel plate having a plate thickness of 1.4 mm is overlaid, and further, GA soft steel (alloyed hot-dip galvanized steel plate) having a plate thickness of 0.75 mm is overlaid to form the joint 12. The total thickness of the three sheets is 3.55 mm. As described above, a plate set of high-tensile steel plate and mild steel is widely applied to panel parts of automobile bodies, such as body sides.

  In the main energization step (S1), in order to obtain a limit nugget diameter at which stable welding strength can be obtained, the applied pressure in the main energization step (S1), the energization time in the first energization, the current value in the first energization, and the electrode tip 21a The tip diameters φa, φb, and the like of 21b are set under the condition that a nugget 13 having a size larger than the limit nugget diameter is generated.

  In the re-energization step (S2), the energization pause and the re-energization are repeated at least once while the same pressure is applied to the joint 12 as in the main energization step (S1). The energization is stopped, that is, the cooling is performed only for a time during which the second energization and the third energization can be performed before the molten metal in the nugget 13 is sufficiently cooled. For example, the suspension of energization is 4 cycles (0.08 seconds). The energization time in the second energization and the third energization, and the current value in the second energization and the third energization are set to an energization time and a current value at which rapid cooling and post-heating can be performed without melting the nugget 13 again. . For example, the energization time in the second energization and the third energization is 3 cycles (0.06 seconds), and the current value in the second energization and the third energization is 10.5 kA.

  The current value in the second energization and the third energization is equal to or greater than the current value in the first energization.

  The degree of rapid cooling and post-heating in the re-energization step (S2) depends on the total thickness of the materials. That is, when the nugget diameters are equal, the volume of the molten metal changes depending on the total plate thickness. Therefore, it is necessary to increase the amount of heat input for reheating even with substantially the same initial energization pattern (first energization).

  Therefore, in the re-energization step (S2), the amount of heat input by re-energization per unit volume of the molten metal in the nugget 13 is determined, and the volume of the molten metal in the nugget 13 increases or decreases with respect to the unit volume. It is preferable to change the amount of heat input by re-energization. This is because, even in various plate assemblies, it is possible to obtain bonded bodies with sufficiently improved tensile strength in practical use.

  FIG. 4 is a graph showing the relationship between the cool time and the current value in the re-energization step (S2). In the graph, “◯” indicates that the tensile strength in the joint 12 including the high-strength steel plates that are superposed can be sufficiently improved in practice, and “x” indicates that the tensile strength cannot be improved. The “◯” area has a characteristic of rising to the right because the cooling is constant as shown by being surrounded by a two-dot chain line. From this graph, it can be seen that the current value needs to be slightly increased as the cool time increases. Therefore, the amount of heat input due to the above re-energization takes the cool time into consideration.

  In this embodiment, the applied pressure is not released simultaneously with the completion of the third energization. That is, in the hold step (S3), the state where the same pressure is applied to the joint 12 as in the main energization step (S1) is maintained for a short time (hold time) after the energization is stopped. The hold time may be a short time for completing the rapid cooling, and is, for example, one cycle (0.02 seconds).

  When the holding step (S3) ends, the applied pressure is released.

  FIG. 5 is a graph schematically showing the relationship between the current value and the nugget diameter in the first energization in the present embodiment. In FIG. 5, the nugget diameter between the superposed high-tensile steel plates 10 and 11 is indicated by white circles, and the nugget diameter between one high-tensile steel plate 11 and the other steel plate 16 is indicated by black rhombuses. Yes. A solid line indicates a characteristic line. A broken line L1 indicates a necessary reference nugget diameter between the superposed high-tensile steel plates 10 and 11, and a broken line L2 indicates a necessary reference nugget diameter between one high-tensile steel plate 11 and the other steel plate 16. Show.

  Factors that control the nugget diameter are the applied pressure in the main energization step (S1), the energization time in the first energization, the current value in the first energization, and the tip diameters φa and φb of the electrode tips 21a and 21b. As shown in FIG. 5, suitable welding conditions related to the current value in the first energization could be found. That is, by appropriately selecting the current value in the first energization, the reference nugget diameter L1 can be secured as the nugget diameter between the superposed high-tensile steel plates 10 and 11, as shown by being surrounded by a one-dot chain line in FIG. At the same time, it was found that the reference nugget diameter L2 can be secured as the nugget diameter between one high-tensile steel plate 11 and the other steel plate 16.

  FIG. 6 is a graph schematically showing the relationship between the nugget diameter and the cross tensile strength (CTS strength). The cross tensile strength (CTS strength) is the strength when a tensile load is applied between the superposed high-tensile steel plates in the peeling direction. The nugget diameter is a nugget diameter between the superposed high-tensile steel plates 10 and 11. The nugget diameter obtained by the combination of the tip diameters (φa and φb) of the electrode tip in the present embodiment is indicated by a black rhombus, and the nugget diameter obtained by the combination of the tip diameters of the electrode tip (φa and φa) in a proportional manner Is indicated by an “x”. A broken line L3 indicates a necessary reference nugget diameter between the superposed high-tensile steel plates 10 and 11, and a broken line L4 indicates a required cross tensile strength (CTS strength). A region above the broken line L4 is a region that satisfies the required strength. As shown in FIG. 6, the variation width d of the nugget diameter is reduced by the combination of the tip diameters (φa and φb) of the electrode tip in this embodiment, and the joint 12 that satisfies the required strength can be obtained.

  FIG. 7 is a diagram showing a combination (φa and φa) of the tip diameters of the electrode tips 51a and 51b in the comparative example 1. FIG. 8 schematically shows the relationship between the current value and the nugget diameter in the first energization in the comparative example 1. FIG. In FIG. 7, the reference nugget diameter required between one high-strength steel plate 11 and the other steel plate 16 is indicated by an arrow a, and the required reference nugget diameter between the superposed high-tensile steel plates 10 and 11 is shown. Is indicated by an arrow b. A broken line i is a boundary line for indicating a current flowing range. In FIG. 8, the nugget diameter between the superposed high-tensile steel plates 10 and 11 is indicated by black circles, and the nugget diameter between one high-tensile steel plate 11 and the other steel plate 16 is indicated by black diamonds. Yes. A solid line indicates a characteristic line. A broken line L1 indicates a necessary reference nugget diameter between the superposed high-tensile steel plates 10 and 11, and a broken line L2 indicates a necessary reference nugget diameter between one high-tensile steel plate 11 and the other steel plate 16. Show.

  The high-tensile steel plate and other steel plates in the comparative example 1 are the same as in the embodiment. That is, the high-tensile steel plate is a 1.2 GPa class steel having a carbon content of 0.18% by mass. The above-described high-tensile steel plate having a plate thickness of 1.4 mm is overlaid, and further, GA soft steel (alloyed hot-dip galvanized steel plate) having a plate thickness of 0.75 mm is overlaid to form the joint 12.

  The electrode tips 51a and 51b having the same tip diameter of the pair of electrode tips as in Comparative Example 1 were welded while changing the current value in the first energization, and the obtained nugget diameter was observed. In some cases, a reference nugget diameter L1 can be secured as a nugget diameter between one high-tensile steel plate 11 and another steel plate 16. However, even at this time, the electrical resistance is high between the stacked high-tensile steel plates 10 and 11, and the diameter of the contact surface between the plates is a reference nugget based on the plate thickness of the steel plates 10 and 11. Since it is smaller than the diameter, significant spatter occurs and the nugget cannot be grown. Therefore, it is impossible to form the joint portion 12 having sufficient breaking strength.

  In addition, in order to ensure continuous spotting performance, a new chip (or just after dressing the tip of the chip) needs to have a nugget diameter that is smaller than the reference nugget diameter, so the above-described problem is likely to occur.

  In the region indicated by reference numeral A1 in FIG. 8, the heat generation of the other steel plate 16 that is mild steel is slow, and the nugget diameter does not reach the reference nugget diameter L2. Further, in the region indicated by reference numeral A2, heat is discarded due to sputtering generated between the stacked high-tensile steel plates 10, 11, and the nugget diameter on the thin plate side almost reaches the reference nugget diameter L2. Absent. Therefore, it is not possible to form a joint having a sufficient breaking strength.

  FIG. 9 is a diagram showing combinations (φb and φb) of the tip diameters of the electrode tips 52a and 52b in the comparative example 2, and FIG. 10 schematically shows the relationship between the current value and the nugget diameter in the first energization in the comparative example 2. FIG. In FIG. 9, the reference nugget diameter required between one high-tensile steel plate 11 and the other steel plate 16 is indicated by an arrow a, and the required reference nugget diameter between the superposed high-tensile steel plates 10 and 11 is shown. Is indicated by an arrow b. A broken line i is a boundary line for indicating a current flowing range. In FIG. 10, the nugget diameter between the superposed high-tensile steel plates 10 and 11 is indicated by a black circle, and the nugget diameter between one high-tensile steel plate 11 and the other steel plate 16 is indicated by a black rhombus. Yes. A solid line indicates a characteristic line. A broken line L1 indicates a necessary reference nugget diameter between the superposed high-tensile steel plates 10 and 11, and a broken line L2 indicates a necessary reference nugget diameter between one high-tensile steel plate 11 and the other steel plate 16. Show.

  The high-tensile steel plate and other steel plates in the comparative 2 are the same as those in the embodiment. That is, the high-tensile steel plate is a 1.2 GPa class steel having a carbon content of 0.18% by mass. The above-described high-tensile steel plate having a plate thickness of 1.4 mm is overlaid, and further, GA soft steel (alloyed hot-dip galvanized steel plate) having a plate thickness of 0.75 mm is overlaid to form the joint 12.

  Using electrode tips 52a and 52b having both tip diameters φb as in Comparative Example 2, welding was performed while changing the current value in the first energization, and the obtained nugget diameter was observed. Since the diameter of the contact surface between the superposed high-tensile steel plates 10 and 11 is increased, the reference nugget diameter L1 may be ensured without causing significant spattering. However, even at this time, since the contact diameter between one high-strength steel plate 11 and the other steel plate 16 is also increased, the current density is reduced and another steel plate which is a mild steel having a relatively low electrical resistance. 16 cannot be melted, and the reference nugget diameter L2 cannot be ensured. Therefore, it is impossible to form the joint portion 12 having sufficient breaking strength.

  In particular, when the mild steel is a plated steel plate, the melted plating spreads between the one high-tensile steel plate 11 and the other steel plate 16, thereby further expanding the current-carrying diameter, making it difficult to melt the mild steel plate.

  Since the tip tip diameter increases as welding is continued, the above-described problem is likely to occur immediately before dressing the tip tip.

  In the region indicated by reference numeral A1 in FIG. 10, the heat generation of another steel plate 16 that is mild steel is slow, and the nugget diameter does not reach the reference nugget diameter L2. Further, in the region indicated by reference numeral A2, heat is discarded due to sputtering generated between the stacked high-tensile steel plates 10, 11, and the nugget diameter on the thin plate side almost reaches the reference nugget diameter L2. Absent. Therefore, it is not possible to form a joint having a sufficient breaking strength.

  In the present embodiment, the tip diameter of the electrode tip 21a is set to φa and the tip diameter of the electrode tip 21b is set to φb in the present embodiment with respect to the above-described comparatives 1 and 2, and therefore, between the high-tensile steel plates 10 and 11 Since the diameter of the contact surface increases, the reference nugget diameter L1 can be ensured without causing significant sputtering. Moreover, since the contact diameter between one high-strength steel plate 11 and the other steel plate 16 does not increase, the current density does not decrease, and the other steel plate 16 that is a mild steel having a relatively low electrical resistance can be melted. The reference nugget diameter L2 can be ensured. As a result that a nugget 13 having a necessary diameter between the high-tensile steel plates 10 and 11 and a nugget 13 having a necessary diameter between one high-tensile steel plate 11 and the other steel plate 16 can be secured at the same time. It is possible to form the joint 12 having a sufficient breaking strength.

  As described above, in the present embodiment, the tip diameters φa and φb of the pair of electrode chips 21a and 21b sandwiching the joint 12 are necessary between the superposed high-tensile steel plates 10 and 11. Since the nugget having a necessary diameter and the nugget having a necessary diameter are formed between one high-strength steel plate 11 and the other steel plate 16 which is a mild steel, the nugget having a necessary diameter is simultaneously formed between the steel plates. It is possible to set welding conditions (such as energization time and current value) that can be ensured, and as a result, it is possible to form the joint 12 having sufficient breaking strength.

  The tip diameter φa of one (21a) of the pair of electrode tips 21a, 21b is different from the tip diameter φb of the other (21b) of the pair of electrode tips 21a, 21b. In various combinations of the high-tensile steel plates 10 and 11 and the high-strength steel plates 10 and 11 and other steel plates 16 that differ in at least one of carbon content, plate thickness, and hardness, nuggets having a necessary diameter between the respective steel plates. It is possible to easily set welding conditions (such as energization time and current value) that can simultaneously ensure the above, and it is possible to form the joint 12 having sufficient breaking strength.

  Since the electrical resistance between the superposed high-tensile steel plates 10 and 11 is greater than the electrical resistance between one high-tensile steel plate 11 and the other steel plate 16, it contacts the superposed high-tensile steel plates 10 and 11. The tip diameter φb of the electrode tip 21b on the side is set larger than the tip diameter φa of the electrode tip 21a on the side in contact with the other steel plate 16. Since the contact diameter of the electrode tip 21a on the side having a small electrical resistance does not increase, the current density does not decrease, and the other steel plate 16 (soft steel) having a relatively low electrical resistance can be melted. And the welding conditions (energization time, electric current value, etc.) which can ensure simultaneously the nugget of a required diameter between each steel plate can be set easily, and the junction part 12 provided with sufficient fracture strength can be formed.

  The carbon content of the high-tensile steel plate is, for example, 0.1 to 0.3% by mass, and the other steel plate is, for example, mild steel having a carbon content of 0.05% by mass or less. A plate assembly of a high-strength steel plate and a mild steel is often applied to a panel part of an automobile body, for example, a body side, and manufactures a panel part of an automobile body formed with a joint 12 having sufficient breaking strength. Can do.

  In the resistance welding method for obtaining the joined body 15 of the high-strength steel plates 10 and 11 and the mild steel 16 according to the present embodiment, a nugget having a necessary diameter between the superposed high-tensile steel plates 10 and 11 and one of them. A main energization step (S1) in which the joint 12 is sandwiched between the pair of electrode tips 21a and 21b having tip diameters φa and φb that form a nugget of a necessary diameter between the high-tensile steel plate 11 and another steel plate 16 have. It is possible to set welding conditions (such as energization time and current value) that can simultaneously secure a nugget of a necessary diameter between the steel plates, and as a result, it is possible to form the joint 12 having sufficient breaking strength.

  Furthermore, a re-energization step (S2) that repeats at least one stop of energization and re-energization while energizing the joint 12 with the same pressure as in the main energization step (S1), and a main energization step (S1) ) And a hold step (S3) for stopping energization after the re-energization step (S2) with the same pressure applied. The cooling history at the joint 12 is controlled to sufficiently exert the softening effect on the heat affected zone around the nugget 13, and the progress of the crack toward the interface of the nugget 13 is difficult to proceed. Thereby, the interfacial fracture is suppressed, and the fracture strength of the joint 12 in the joined body 15 can be improved.

  Furthermore, in the re-energization step (S2), re-energization is performed for a short time by applying a current equal to or greater than the current value in the main energization step (S1), and the temperature of the heat affected zone around the nugget 13 is controlled. Will not be long. Therefore, the number of dots that can be welded in the same process is not reduced, and the productivity of the joined body does not decrease.

  Therefore, according to this embodiment, the high-strength steel plates 10 and 11 overlapped with each other, and the high-strength steel plates 10 and 11 are overlapped with another steel plate 16 that is different in at least one of carbon amount, plate thickness, and hardness. When joining the joining part 12 by resistance welding, a resistance welding apparatus and a resistance welding method capable of forming the joining part 12 having sufficient breaking strength can be provided.

  In addition, although the case where the first steel plate is a high-strength steel plate and the second steel plate is a mild steel has been described as an example, the first steel plate is a mild steel and the second steel plate is a high-tensile steel plate. In this case, the present invention can be applied. Although the embodiment of three plate assemblies has been described, it goes without saying that the present invention can be applied to four or more plate assemblies. In addition, even if the second steel plate is the same high-tensile steel plate as the superimposed first steel plate and the carbon amount and hardness are not different, the necessary nugget diameter is different when the plate thickness is different. Therefore, the present invention can be applied.

10, 11 Press parts (first steel plate, high-tensile steel plate),
12 joints,
13 Nuggets,
15 Joints of high-tensile steel and mild steel,
16 Other steel plates (second steel plate, mild steel),
20 welding gun,
21a electrode tip,
21b electrode tip,
22 pressure cylinder,
30 current supply device,
40 controller,
100 resistance welding equipment,
S1 main energization process,
S2 re-energization process,
S3 hold process,
φa The tip diameter of the electrode tip 21a,
φb The tip diameter of the electrode tip 21b.

Claims (8)

In a resistance welding apparatus for resistance welding a joined portion in which at least one of the first steel plate and the second steel plate different in carbon amount, plate thickness, and hardness is further overlapped with the first steel plates of the same type There,
A tip diameter of each of the pair of electrode chips sandwiching the joint portion is set between a nugget having a necessary diameter between the superposed first steel plates, and between the one first steel plate and the brother 2 steel plate. Resistance welding equipment set to dimensions that form a nugget of the necessary diameter.   The first steel plate is a high-strength steel plate and the second steel plate is mild steel, or the first steel plate is mild steel and the second steel plate is a high-tensile steel plate. Resistance welding equipment.   The resistance welding apparatus according to claim 1 or 2, wherein a tip diameter of one of the pair of electrode tips is different from a tip diameter of the other of the pair of electrode tips. The electrical resistance between the superposed first steel plates is greater than the electrical resistance between one of the first steel plates and the second steel plate,
The tip diameter of the electrode tip on the side in contact with the superimposed first steel plate is larger than the tip diameter of the electrode tip on the side in contact with the second steel plate. Resistance welding equipment.   The resistance welding apparatus according to claim 2, wherein the high-tensile steel sheet has a carbon content of 0.1 to 0.3 mass%, and the mild steel has a carbon content of 0.05 mass% or less. In a resistance welding method in which a welded portion in which at least one of the first steel plate and the second steel plate different in carbon amount, plate thickness, and hardness is further overlapped with each other by overlapping the same type of first steel plates is resistance-welded. There,
A pair of electrodes having a tip diameter that forms a nugget of a necessary diameter between the superposed first steel plates and a nugget of a necessary diameter between one of the first steel plates and the second steel plate. A resistance welding method comprising a main energization step of energizing the joint portion with a tip.   The first steel plate is a high-strength steel plate and the second steel plate is mild steel, or the first steel plate is mild steel and the second steel plate is a high-tensile steel plate. Resistance welding method. A re-energization step that repeats at least one stop of energization and re-energization while energizing the joint with the same pressure as in the main energization step;
8. The resistance welding method according to claim 6, further comprising: a hold step of stopping energization after the re-energization step while energizing the joint with the same pressure as in the main energization step.

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