Resistance spot welding method and weld joint

Abstract

Provided is a method for resistance spot-welding at least two overlapping steel sheets. When an electrode force F after an electric current supply is started changes from an initial electrode force Fi to an electrode force F.sub.h.sup.(1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive, a suspension of the electric current supply of from 20 ms to 60 ms inclusive is started. Then the electric current supply is resumed when the electrode force F reaches an electrode force F.sub.c.sup.(1).

Claims

1. A resistance spot welding method for resistance spot-welding at least two steel sheets overlapping each other, the method comprising: starting a suspension of an electric current supply of from 20 ms to 60 ms inclusive when an electrode force F after the electric current supply is started changes from an initial electrode force Fi to an electrode force F.sub.h.sup.(1) represented by formula (1)
1.03FiF.sub.h.sup.(1)1.15Fi,(1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive; and resuming the electric current supply when the electrode force F reaches an electrode force F.sub.c.sup.(1) represented by formula (2):
1.01FiF.sub.c.sup.(1)0.99F.sub.h.sup.(1)(2).

2. The resistance spot welding method according to claim 1, further comprising, after the suspension of the electric current supply, repeating at least once the electric current supply of from 20 ms to 80 ms inclusive and the suspension of the electric current supply of from 20 ms to 60 ms inclusive such that, when the electrode force F during an Nth electric current supply changes from an electrode force F.sub.c.sup.(N1) immediately after an (N1)th suspension of the electric current supply to an electrode force F.sub.h.sup.(N) represented by formula (3),
1.04F.sub.c.sup.(N1)F.sub.h.sup.(N)1.15F.sub.c.sup.(N1),(3) the suspension of the electric current supply is started, and then, the electric current supply is resumed when the electrode force F reaches an electrode force F.sub.c.sup.(N) represented by formula (4):
F.sub.c.sup.(N1)F.sub.c.sup.(N)0.99F.sub.h.sup.(N),(4) wherein N is a natural number of 2 or more.

3. The resistance spot welding method according to claim 2, wherein the period of a last repetition of the electric current supply is from 100 ms to 300 ms inclusive.

4. The resistance spot welding method according to claim 2, wherein at least one of the at least two steel sheets comprises 0.15C0.30 (% by mass), 1.9Mn5.0 (% by mass), and 0.2Si2.0 (% by mass).

5. The resistance spot welding method according to claim 2, wherein at least one of the at least two steel sheets has a tensile strength of 980 MPa or more.

6. The resistance spot welding method according to claim 2, wherein at least one of the at least two steel sheets has, on a surface thereof, a coating layer containing zinc as a main component.

7. The resistance spot welding method according to claim 1, wherein the period of a last repetition of the electric current supply is from 100 ms to 300 ms inclusive.

8. The resistance spot welding method according to claim 7, wherein at least one of the at least two steel sheets comprises 0.15C0.30 (% by mass), 1.9Mn5.0 (% by mass), and 0.2Si2.0 (% by mass).

9. The resistance spot welding method according to claim 7, wherein at least one of the at least two steel sheets has a tensile strength of 980 MPa or more.

10. The resistance spot welding method according to claim 7, wherein at least one of the at least two steel sheets has, on a surface thereof, a coating layer containing zinc as a main component.

11. The resistance spot welding method according to claim 1, wherein at least one of the at least two steel sheets comprises 0.15C0.30 (% by mass), 1.9Mn5.0 (% by mass), and 0.2Si2.0 (% by mass).

12. The resistance spot welding method according to claim 11, wherein at least one of the at least two steel sheets has a tensile strength of 980 MPa or more.

13. The resistance spot welding method according to claim 11, wherein at least one of the at least two steel sheets has, on a surface thereof, a coating layer containing zinc as a main component.

14. The resistance spot welding method according to claim 1, wherein at least one of the at least two steel sheets has a tensile strength of 980 MPa or more.

15. The resistance spot welding method according to claim 14, wherein at least one of the at least two steel sheets has, on a surface thereof, a coating layer containing zinc as a main component.

16. The resistance spot welding method according to claim 1, wherein at least one of the at least two steel sheets has, on a surface thereof, a coating layer containing zinc as a main component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustration showing the outline of resistance spot welding.

(2) FIG. 2 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while the supply of electric current and the suspension of the supply were repeated.

(3) FIG. 3 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while a constant electric current was supplied.

DETAILED DESCRIPTION OF THE INVENTION

(4) An embodiment of the present invention will next be described with reference to the accompanying drawings.

(5) In the resistance spot welding method according to embodiments of the present invention, a sheet set 3 including overlapping steel sheets 1 and 2 is held between a vertical pair of electrodes 4 and 5 as shown in FIG. 1, and an electric current is supplied to the sheet set 3 under pressure. A nugget 6 of the required size is thereby formed, and a weld joint is obtained.

(6) An embodiment of the present invention is a method for resistance spot-welding overlapping steel sheets, and the electrode force F between the electrodes is controlled as follows.

(7) When the electrode force F after an electric current supply is started changes from an initial electrode force Fi to an electrode force F.sub.h.sup.(1) represented by formula (1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive, a suspension of the electric current supply for from 20 ms to 60 ms inclusive is started.

(8) Then the electric current supply is resumed when the electrode force F reaches an electrode force F.sub.c.sup.(1) represented by formula (2).
1.03FiF.sub.h.sup.(1)1.15Fi(1)
1.01FiF.sub.c.sup.(1)0.99F.sub.h.sup.(1)(2)

(9) If F.sub.h.sup.(1) is less than 1.03F.sub.i, a region in the vicinity of the nugget is not pressurized sufficiently, and the probability of the occurrence of expulsion becomes high. If F.sub.h.sup.(1) is larger than 1.15F.sub.i, the growth of the nugget is inhibited. If F.sub.c.sup.(1) is less than 1.01Fi, cooling proceeds, and therefore the effects of the subsequent heating become small. If F.sub.c.sup.(N) is larger than 0.99F.sub.h.sup.(1), the temperature of the nugget is high, and therefore the possibility of the occurrence of expulsion when the electric current supply is resumed increases.

(10) Before the electric current is supplied, a pressure is applied for about 10 cycles (200 ms) to obtain a stable state, and then the supply of the electric current is started. The initial electrode force Fi used is the average electrode force in the first cycle (20 ms) after the start of the supply. The electrode force immediately after the start of the electric current supply is approximately the same as a set electrode force (designated electrode force) of a servo gun, and therefore the designated electrode force may be used as the initial electrode force Fi. Alternatively, the average electrode force in a period from 0 ms to 20 ms after the start of the electric current supply may be used as the initial electrode force Fi.

(11) In embodiments of the present invention, after the suspension of the electric current supply, the electric current supply for from 20 ms to 80 ms inclusive and the suspension of the electric current supply for from 20 ms to 60 ms inclusive are repeated at least once.

(12) In this case, when the electrode force F during the Nth electric current supply changes from an electrode force F.sub.c.sup.(N-1) immediately after the (N1)th suspension of the electric current supply to an electrode force F.sub.h.sup.(N) represented by formula (3), the suspension of the electric current supply is started. Then, when the electrode force F reaches an electrode force F.sub.c.sup.(N) represented by formula (4), the electric current supply is resumed. N is a natural number of 2 or more.
1.04F.sub.c.sup.(N-1)F.sub.h.sup.(N)1.15F.sub.c.sup.(N-1)(3)
F.sub.c.sup.(N-1)F.sub.c.sup.(N)0.99F.sub.h.sup.(N)(4)

(13) If F.sub.h.sup.(N) is less than 1.04F.sub.c.sup.(N-1), a region in the vicinity of the nugget is not pressurized sufficiently, and the probability of the occurrence of expulsion becomes high. If F.sub.h.sup.(N) is larger than 1.15F.sub.c.sup.(N-1), the growth of the nugget is inhibited. If F.sub.c.sup.(N) is less than F.sub.c.sup.(N-1), cooling proceeds, and therefore the effects of the subsequent heating become small. If F.sub.c.sup.(N) is larger than 0.99F.sub.h.sup.(N), the temperature of the nugget is high, and the possibility of the occurrence of expulsion when the electric current supply is resumed increases.

(14) In embodiments of the present invention, the period of the last repetition of the electric current supply is preferably from 100 ms to 300 ms inclusive. If the period of the last repetition is less than 100 ms, the formation of the nugget is insufficient. If the period of the last repetition of the electric current supply exceeds 300 ms, the workability deteriorates, and the contribution of the electric current supply to the formation of the nugget is small. The period of the last repetition of the electric current supply may be selected optimally within the above range according to the period required for the first electric current supply and the subsequent repetitions of the electric current supply and the suspension of the electric current supply.

(15) The welding device on which the spot welding method according to embodiments of the present invention is performed may be any welding device, so long as it includes: a vertical pair of electrodes which holds parts to be welded and through which the electrode force and the electric current are applied; and a welding current controller that can control the welding current freely during welding. No particular limitation is imposed on the pressure mechanism (such as an air cylinder or a servo motor), the current control mechanism (such as an AC or DC current control mechanism), the type (such as a stationary type or a robot gun), etc.

(16) To perform the spot welding method according to embodiments of the present invention, a unit that can measure the electrode force F is installed in the welding device described above and is configured as follows. While the electrode force F is measured during the electric current supply, the electrode force F is controlled according to the measurement results. Specifically, a strain gauge is mounted to an arm of a C gun type welding device that holds the upper and lower electrodes. The strain of the arm during the electric current supply is detected to detect the force applied between the electrodes, and the force applied between the electrodes may be used as the electrode force.

(17) When the spot welding method according to embodiments of the present invention is performed, the real-time measurement of the electrode force F during the electric current supply is not essential. An experiment may be performed on a sheet set in advance to obtain a current supply-suspension pattern that allows appropriate control of the electrode force F. Then the current supply-suspension pattern obtained may be used to weld a sheet set similar to the sheet set tested in advance.

(18) In certain embodiments, the present invention is applied to a welding method for a sheet set of a plurality of sheets including a galvanized steel sheet or a high-strength steel sheet. Galvanized steel sheets and high-strength steel sheets are more likely to cause expulsion due to a sheet gap than ordinary steel sheets. However, since certain embodiments of the present invention has the effect of preventing the occurrence of expulsion, it is more effective to apply certain embodiments of the present invention to welding of a sheet set including at least one sheet selected from those steel sheets.

(19) Therefore, even when at least one of the steel sheets included in the sheet set to be welded is a high-strength steel sheet having a tensile strength of 980 MPa or more, the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed.

(20) Even when at least one of the steel sheets included in the sheet set to be welded is a high-strength steel sheet containing components including 0.15C0.30 (% by mass), 1.9Mn5.0 (% by mass), and 0.2Si2.0 (% by mass), the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed.

(21) Moreover, even when at least one of the steel sheets included in the sheet set to be welded is a galvanized steel sheet, the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed. The galvanized steel sheet is a steel sheet including a coating layer containing Zn as a main component and is intended to encompass any conventionally known galvanized layer. Specific examples of the coating layer containing Zn as a main component include a hot-dip galvanized layer, an electrogalvanized layer, an Al coating layer, a ZnAl coating layer, and a ZnNi layer.

(22) In the resistance spot welding method according to embodiments of the present invention, the electric current is supplied and suspended while the electrode force during the electric current supply is controlled appropriately as described above. In this manner, the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed. Therefore, even when work disturbances such as a sheet gap are present, the diameter of the nugget can be ensured stably.

Example 1

(23) In Examples of the present invention, a servo motor pressurizing-type resistance welding device attached to a C gun and including a DC power source was used to perform resistance spot welding on a sheet set 3 including two overlapping hot-dip galvannealed steel sheets (a lower steel sheet 1 and an upper steel sheet 2) as shown in FIG. 1 to thereby produce a resistance spot-welded joint.

(24) In this case, the electric current was supplied under conditions shown in Table 1.

(25) The electrodes 4 and 5 used were DR type electrodes made of alumina-dispersed copper and having a tip radius of curvature R of 40 mm and a tip diameter of 8 mm. The test pieces used were high-strength steel sheets having a 980 MPa-class tensile strength and sheet thicknesses of 1.2 mm and 2.0 mm and a high-strength steel sheet having a 1,470 MPa-class tensile strength and a sheet thickness of 2.0 mm. Two steel sheets of the same type and with the same thickness were stacked and welded.

(26) The electrode force during the electric current supply was measured using a strain gauge attached to the C gun. The electrode force was changed such that the measured electrode force was adjusted to a prescribed value.

(27) Table 1 shows the results of studies on the occurrence of expulsion during welding and the diameter of the nugget. The diameter of the nugget was evaluated based on the structure of an etched cross section as follows. Let the sheet thickness be t (mm). Then a Good rating was given when the diameter of the nugget was equal to or larger than 5.5 t. A Poor rating was given when the diameter of the nugget was less than 5.5 t. Specifically, a nugget diameter equal to or larger than 5.5 t was set to be an appropriate diameter.

(28) TABLE-US-00001 TABLE 1 Second Test piece Set First current current Tensile Sheet electrode supply First suspension supply Evaluation strength thickness force Fi I.sub.1 T.sub.1 T.sub.c1 I.sub.2 T.sub.2 Occurrence of nugget No. (MPa) (mm) (kN) (kA) (ms) F.sub.h(1)/F.sub.i (ms) F.sub.c(1)/F.sub.i F.sub.c(1)/F.sub.h(1) (kA) (ms) of expulsion diameter Remarks 1 980 1.2 5.0 10 60 1.05 20 1.03 0.98 8.5 280 No Good Inventive Example 2 980 1.2 5.0 10 60 1.05 40 1.00 0.95 8.5 280 Yes Poor Comparative Example 3 980 1.2 5.0 10 60 1.01 20 1.00 0.99 8.5 280 No Poor Comparative Example 4 980 1.2 5.0 8.5 60 1.03 0 8.5 220 Yes Poor Comparative Example 5 1470 2.0 6.0 9 60 1.07 20 1.04 0.98 7.8 300 No Good Inventive Example 6 1470 2.0 6.0 9 60 1.08 60 1.00 0.92 7.8 320 Yes Poor Comparative Example 7 1470 2.0 6.0 9 60 1.03 20 1.00 0.97 7.8 320 No Poor Comparative Example 8 1470 2.0 6.0 8 60 1.05 0 7.8 260 Yes Poor Comparative Example 9 980 1.2 5.0 12 20 1.03 20 1.02 0.99 8.5 280 No Good Inventive Example 10 980 1.2 5.0 9 80 1.08 60 1.05 0.97 8.5 280 No Good Inventive Example 11 980 1.2 5.0 9 80 1.10 200 1.00 0.91 8.5 280 Yes Poor Comparative Example 12 980 2.0 5.5 9.5 60 1.05 20 1.04 0.98 8 280 No Good Inventive Example 13 980 2.0 5.5 9 60 1.05 200 1.00 0.95 8 280 Yes Poor Comparative Example 14 980 2.0 5.5 11 60 1.18 20 1.07 0.91 8 280 Yes Poor Comparative Example 15 980 2.0 5.5 8 60 1.02 0 8 220 Yes Poor Comparative Example

(29) In Table 1, I.sub.1 (kA) is the current value in the first electric current supply, T.sub.1 (ms) is the current supply time in the first electric current supply, and F.sub.h.sup.(1)/F.sub.i is the ratio of the electrode force F.sub.h.sup.(1) to the initial electrode force Fi. Tc.sub.1 (ms) is the first suspension time, and F.sub.c.sup.(1)/F.sub.i is the ratio of the electrode force F.sub.c.sup.(1) to the initial electrode force Fi. F.sub.c.sup.(1)/F.sub.h.sup.(1) is the ratio of the electrode force F.sub.c.sup.(N) at which the electric current supply is resumed to the electrode force F.sub.h.sup.(N) at which the suspension of the electric current supply is started. I.sub.2 (kA) is the current value in the second electric current supply, and T.sub.2 (ms) is the current supply time in the second electric current supply.

(30) As can be seen from Table 1, when the resistance spot welding was performed according to embodiments of the present invention, no expulsion occurred, and each nugget formed had an appropriate diameter, in contrast to Comparative Examples.

Example 2

(31) In Examples of the present invention, a servo motor pressurizing-type resistance welding device attached to a C gun and including a DC power source was used to perform resistance spot welding on a sheet set including three overlapping hot-dip galvannealed steel sheets to thereby produce a resistance spot-welded joint.

(32) In this case, the electric current was supplied under conditions shown in Table 2.

(33) The electrodes 4 and 5 used were DR type electrodes made of alumina-dispersed copper and having a tip radius of curvature R of 40 mm and a tip diameter of 8 mm. The test pieces used were a 980 MPa class high-strength steel sheet having a sheet thickness of 1.2 mm and a 1,470 MPa class high-strength steel sheet having a sheet thickness of 1.2 mm. Three steel sheets of the same type and with the same thickness were stacked and welded.

(34) The electrode force during the electric current supply was measured using a strain gauge attached to the C gun. The electrode force was changed such that the measured electrode force was adjusted to a prescribed value.

(35) Table 2 shows the results of studies on the occurrence of expulsion during welding and the diameter of the nugget. The diameter of the nugget was evaluated based on the structure of an etched cross section as follows. Let the sheet thickness be t (mm). Then a Good rating was given when the diameter of the nugget was equal to or larger than 5.5 t. A Poor rating was given when the diameter of the nugget was less than 5.5 t. Specifically, a nugget diameter equal to or larger than 5.5 t was set to be an appropriate diameter.

(36) The same test was repeated 10 times, and the variations in nugget diameter were evaluated. When the diameters obtained were appropriate and the range of variations in nugget diameter was equal to or less than 0.1 Nit, an Excellent rating was given.

(37) TABLE-US-00002 TABLE 2 Test piece Set First current Second current Tensile Sheet electrode supply First suspension supply strength thickness force Fi I.sub.1 T.sub.1 T.sub.c1 I.sub.2 T.sub.2 No. (MPa) (mm) (kN) (kA) (ms) F.sub.h(1)/F.sub.i (ms) F.sub.c(1)/F.sub.i F.sub.c(1)/F.sub.h(1) (kA) (ms) F.sub.h(2)/F.sub.c(1) 1 980 1.0 4.5 9.5 60 1.07 20 1.04 0.98 9 60 1.06 2 980 1.0 4.5 9.5 60 1.07 20 1.04 0.98 5 60 1.02 3 980 1.0 4.5 9.5 60 1.07 20 1.04 0.98 4 980 1.0 4.5 6 100 1.01 60 1.00 0.99 5 980 1.0 4.5 8 60 1.02 6 1470 1.2 5.0 9.5 60 1.15 20 1.12 0.97 9 60 1.05 7 1470 1.2 5.0 9.5 60 1.15 20 1.12 0.97 6 60 1.02 8 1470 1.2 5.0 9.5 60 1.15 20 1.12 0.97 9 1470 1.2 5.0 6 100 1.01 20 1.00 0.99 10 1470 1.2 5.0 7 60 1.02 Third current Second suspension supply Evaluation T.sub.c2 I.sub.3 T.sub.3 Occurrence of nugget No. (ms) F.sub.c(2)/F.sub.c(1) F.sub.c(2)/F.sub.h(2) (kA) (ms) of expulsion diameter Remarks 1 20 1.02 0.96 8 300 No Excellent Inventive Example 2 20 1.00 0.98 8 300 No Good Inventive Example 3 8 300 No Good Inventive Example 4 8 300 Yes Poor Comparative Example 5 8 300 Yes Poor Comparative Example 6 20 1.02 0.97 7 300 No Excellent Inventive Example 7 20 1.00 0.98 7 300 No Good Inventive Example 8 7 300 No Good Inventive Example 9 7 300 Yes Poor Comparative Example 10 7 300 Yes Poor Comparative Example

(38) In Table 2, I.sub.1 (kA) is the current value in the first electric current supply, T.sub.1 (ms) is the current supply time in the first electric current supply, and F.sub.h.sup.(1)/F.sub.i is the ratio of the electrode force F.sub.h.sup.(1) to the initial electrode force Fi. Tc.sub.1 (ms) is the first suspension time, and F.sub.c.sup.(1)/F.sub.i is the ratio of the electrode force F.sub.c.sup.(N) to the initial electrode force Fi. F.sub.c.sup.(1)/F.sub.h.sup.(1) is the ratio of the electrode force at which the electric current supply is resumed to the electrode force at which the suspension of the electric current supply is started. Similarly, F.sub.h.sup.(2)/F.sub.c.sup.(1) is the ratio of the electrode force at which, after the second electric current supply, the suspension of the electric current supply is started to the electrode force immediately after the first suspension. F.sub.c.sup.(2)/F.sub.c.sup.(1) is the ratio of the electrode force immediately after the second suspension to the electrode force immediately after the first suspension, and F.sub.c.sup.(2)/F.sub.h.sup.(2) is the ratio of the electrode force immediately after the second suspension to the electrode force at which, after the second electric current supply, the suspension of the electric current supply is started. I.sub.2 (kA) and I.sub.3 (kA) are the current values in the second and third electric current supplies, respectively, T.sub.2 (ms) and T.sub.3 (ms) are the current supply times in the second and third electric current supplies, respectively, and Tc.sub.2 (ms) is the second suspension time.

(39) As can be seen from Table 2, when the resistance spot welding was performed according to embodiments of the present invention, no expulsion occurred, and each nugget formed had an appropriate diameter, in contrast to Comparative Examples. As can also be seen, when the second electric current supply was performed under the conditions of embodiments of the present invention, the effect of stabilizing the nugget diameter was obtained, in contrast to other cases.

REFERENCE SIGNS LIST

(40) 1 lower steel sheet 2 upper steel sheet 3 sheet set 4 lower electrode 5 upper electrode 6 nugget