Resistance spot welding method

Abstract

Resistance spot welding is performed on a combination of overlapping steel sheets including at least one steel sheet that has, on a surface thereof, a coated layer with zinc as a main component, by (1) starting electric current passage in a state satisfying 0.9tL1.1t; and (2) dividing electric current into main current and initial current that precedes the main current and is two-step current, setting a current value I.sub.1 during current in a first step of the initial current to satisfy I.sub.m1.1I.sub.115.0 kA with respect to a current value I.sub.m during the main current, and setting a current value I.sub.2 in the subsequent second step to no current or low current satisfying 0I.sub.2I.sub.m0.7.

Claims

1. A method of resistance spot welding to join a sheet combination by squeezing the sheet combination between a pair of electrodes and passing current while applying electrode force, the sheet combination being a plurality of overlapping steel sheets including at least one galvanized steel sheet or high tensile strength steel sheet, the galvanized steel sheet having, on a surface thereof, a coated layer with zinc as a main component, the method comprising: (1) starting to apply electric current to the sheet combination when the sheet combination satisfies:
0.9tL1.1t where t is a total thickness of the overlapping steel sheets and L is a distance between tips of the pair of electrodes; and (2) dividing the electric current a into main current period and an initial current period such that the initial current period precedes the main current period and the initial current period is a two-step current, setting a current value I.sub.1 in a first step of the initial current period to satisfy:
I.sub.m1.1I.sub.115.0 kA where I.sub.m is a current value during the main current period, and setting a current value I.sub.2 in a second step of the initial current period to satisfy:
0I.sub.2I.sub.m0.7 setting welding times in the first step and the second step of the initial current period to satisfy:
10 msT.sub.1100 ms
10 msT.sub.2100 ms where T.sub.1 is a welding time in the first step of the initial current period, and T.sub.2 is a welding time or a non-welding time in the second step of the initial current period.

2. The method of claim 1, wherein the initial current period is 2k-step current (where k is an integer greater than or equal to 2).

3. The method of claim 2, wherein when performing the initial current period that is 2k-step current (where k is an integer greater than or equal to 2), a current value I.sub.(2n+1) in a (2n+1).sup.th step (where n is an integer from 1 to k1) of the initial current period satisfies:
I.sub.mI.sub.(2n+1)I.sub.(2n1) with respect to a current value I.sub.(2n1) in a (2n1).sup.th step and the current value I.sub.m during the main current period.

4. The method of claim 1, wherein among the plurality of steel sheets, at least one sheet is a high tensile strength galvanized steel sheet with a tensile strength of 780 MPa or more.

5. The method of claim 2, wherein among the plurality of steel sheets, at least one sheet is a high tensile strength galvanized steel sheet with a tensile strength of 780 MPa or more.

6. The method of claim 3, wherein among the plurality of steel sheets, at least one sheet is a high tensile strength galvanized steel sheet with a tensile strength of 780 MPa or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 illustrates the configuration of resistance spot welding;

(3) FIG. 2 illustrates a state in which an insulating body is wedged in at one side between steel sheets;

(4) FIGS. 3(a) and 3(b) illustrate a total thickness t of steel sheets and a distance L between tips of upper and lower electrodes;

(5) FIG. 4 illustrates the current waveform during an initial current, which is two-step electric current, and a main current; and

(6) FIG. 5 illustrates the current waveform when repeating the initial current, which is two-step current, according to this disclosure,

DETAILED DESCRIPTION

(7) The following describes one of the disclosed embodiments with reference to the drawings.

(8) The method of resistance spot welding of this disclosure is, as illustrated in FIG. 1, for obtaining a weld joint by squeezing a sheet combination 3 that is a plurality of overlapping steel sheets including a galvanized steel sheet or a high tensile strength steel sheet between a pair of upper and lower electrodes 4 and 5 and passing current while applying electrode force to form a nugget 6 of a necessary size. Here, between the lower steel sheet 1 and the upper steel sheet 2, the steel sheet 1 is a high tensile strength galvanized steel sheet.

(9) This disclosure targets a sheet combination which at least one sheet is a galvanized steel sheet or a high tensile strength steel sheet. The reason is that as compared to a regular steel sheet, splashing due to a sheet gap occurs more easily with a galvanized steel sheet or a high tensile strength steel sheet. This disclosure is more effective when the sheet combination includes two or more galvanized steel sheets or high tensile strength steel sheets.

(10) A suitable welding device used to perform such spot welding is equipped with a pair of upper and lower electrodes and can apply electrode force and pass current while squeezing the portion to be welded between the pair of electrodes. The welding device should also include an electrode force control unit and a welding current control unit that can freely control the electrode force and the welding current during welding. Any force mechanism (such as an air cylinder or servomotor), current control mechanism (such as AC or DC), form (such as a stationary welder or robot uun), and the like may be used.

(11) In this disclosure, it is important that electric current be started in a state satisfying
0.9tL1.1t
where t is the total thickness of the overlapping steel sheets as illustrated in FIG. 3(a), and L is the distance between tips of the upper and lower electrodes as illustrated in FIG. 3(b).

(12) If L/t is less than 0.9, the electrode contact portion is in an expanded state, and the effect of heat generation due to electric current reduces. Conversely, if L/t exceeds 1.1, the problem of not being able to ensure a contact portion within the sheet combination arises, even when the steel sheets are softened due to heat generation. A preferred range is 0.9tL1.0t.

(13) For the above-described setting, it is assumed that a sheet gap exists between the steel sheets. Such a sheet, gap could, for example, be caused by a sheet gap at a flange due to mismatched shapes in the automotive body. When L/t>1.0, however, it is assumed that there is some sort of current path, such as a contact portion between the steel sheets or a previously welded point near the welding point, or due to the entire materials being conductive. When no current path exists, current passage is impossible, making welding difficult even upon applying this disclosure.

(14) On the other hand, L/t is not restricted after the start of current passage. Normally, however, L/t goes through a process of gradually lowering as the steel sheets soften due to current passage and then increasing somewhat due to expansion of a fused portion (nugget) formed during the latter half of current passage.

(15) Typically, the electrode force applied at the start of current passage is approximately 3.5 kN to 7.0 kN.

(16) The current passage in this disclosure includes a main current for forming a nugget with an appropriate diameter and an initial current, preceding the main current, for ensuring the contact area between the steel sheets.

(17) Furthermore, in this disclosure, the initial current is two-step current. During current in the first step, a high current is passed for a short time to generate heat due to the current density directly below the electrodes and soften the steel sheets, thereby reducing the distance between the electrodes. For the current in the subsequent second step, either no current or low current is performed for a short time to propagate the heat generated by the above-mentioned current density into the steel sheets, further soften the steel sheets, and even further reduce the distance between the electrodes.

(18) Current in the first step of the initial current is high current such that the current value I.sub.1 satisfies
I.sub.m1.1I.sub.115.0 kA
with respect to the current value I.sub.n of the main current, and the current in the subsequent second step is no current. or low current such that the current value I.sub.2 thereof satisfies
0I.sub.2I.sub.m0.7.

(19) FIG. 4 illustrates the current waveform in this embodiment.

(20) As illustrated in FIG. 4, in this disclosure, before the main current constituted by the welding current value I.sub.m and the welding time T.sub.m, initial current constituted by current in the first step at the current value I.sub.1 and the welding time T.sub.1 and current in the second step at the current value I.sub.2 and the welding time T.sub.2 is performed.

(21) When the main current is controlled to be two step or three step, the welding current I.sub.m is defined as the average of the welding current in the main current.

(22) If the current value I.sub.1 in the first step of the initial current does not satisfy I.sub.m1.1, then sufficient heat generation due to the current density is not obtained. Conversely, if I.sub.1 exceeds 15.0 kA, the occurrence of splashing cannot be avoided at least in a high tensile strength steel sheet having a galvanized layer. In order to suppress excessive heat input, I.sub.1 is preferably in the range of I.sub.m1.1I.sub.112.0 kA. If the current value I.sub.2 in the second step of the initial current exceeds I.sub.m0.7, then suitable heat transfer that does not cause an excessive increase in temperature cannot be expected. Accordingly, the current value I.sub.2 of the current in the second step is either no current, i.e. I.sub.2=0, or low current such that the relationship I.sub.2I.sub.m0.7 is satisfied. I.sub.2 is more preferably in the range of 0I.sub.2I.sub.m0.5.

(23) The ranges
10 msT.sub.1100 ms,
10 msT.sub.2100 ms,
where T.sub.1 is the welding time in the first step of the initial current, and T.sub.2 is the welding time or non-welding time in the second step, are preferred.

(24) If T.sub.1 is less than 10 ms, then sufficient heat generation due to current density is not obtained, whereas if T.sub.1 exceeds 100 ms, splashing is highly likely for a galvanized steel sheet. If T.sub.2 is less than 10 ms, then sufficient heat generation is not obtained, making further softening difficult, whereas if T.sub.2 exceeds 100 ms, the problem of splashing occurs due to excessive heat input.

(25) T.sub.1 and T.sub.2 are more preferably in the following ranges:
10 msT.sub.160 ms,
10 msT.sub.260 ms,

(26) Furthermore, in this disclosure, the above-described initial current may be 2k-step current (where k is an integer greater than or equal to 2). Such 2k-step initial current allows the steel sheets to be softened and the nugget to be expanded while controlling the occurrence of splashing, even if the initial sheet gap is large. FIG. 5 illustrates the current waveform in this embodiment.

(27) When performing such 2k-step initial current, a current value I.sub.(2n+1) in the (2n+1).sup.th step (where n is an integer from 1 to k1) of the initial current preferably satisfies
I.sub.mI.sub.(2n+1)I.sub.(2n+1)
with respect to a current value I.sub.(2n1) in the (2n1).sup.th step and the current value I.sub.m during the main current.

(28) The reason is that if the current value I.sub.(2n+1) becomes larger than the current value I.sub.(2n1), splashing might occur due to sudden beat input.

(29) If the current value I.sub.(2n+2) in the (2n+2).sup.th step (where n is an integer from 1 to k1) of the initial current exceeds I.sub.m0.7, then suitable heat transfer that does not cause an excessive increase in temperature cannot be expected. Accordingly, the current value I.sub.(2n+2) of the current in the (2n+2).sup.th step is preferably either no current, i.e. I.sub.(2n+2)=0, or low current such that the relationship I.sub.(2n+2)I.sub.m0.7 is satisfied. I.sub.(2n+2) is more preferably in the range of 0I.sub.(2n+2)I.sub.m0.5.

(30) The ranges
10 msT.sub.(2n+1)100 ms,
10 msT.sub.(2n+2)100 ms,
where T.sub.(2n+1) is the welding time in the (2n+1).sup.th step (where n is an integer from 1 to k1) of the initial current, and T.sub.(2n+2) is the welding time or non-welding time in the (2n+2).sup.th step, are preferred.

(31) If T.sub.(2n+1) is less than 10 ms, then sufficient heat generation due to current density is not obtained, whereas if T.sub.(2n+2) exceeds 100 ms, splashing is highly likely for a galvanized steel sheet. If T.sub.(2n+2) is less than 10 ms, then sufficient heat generation is not obtained, making further softening difficult, whereas if T.sub.(2n+2) exceeds 100 ms, the problem of splashing occurs due to excessive heat input.

(32) T.sub.(2n+1) are more preferably in the following ranges:
10 msT.sub.(2n+1)60 ms,
10 msT.sub.(2n+2)60 ms,

(33) When performing the 2k-step (where k is an integer greater than or equal to 2) initial current as described above, the nugget formation proceeds gradually. The welding time of the main current can thus be shortened.

(34) In this disclosure, a coated layer with zinc as the main component refers to any widely-known galvanized layer, starting with a hot-dip galvanized layer and an electroalvanized layer, and including a ZnAl coated layer, a ZnNi layer, and the like.

(35) In this disclosure, a high tensile strength steel sheet refers to a steel sheet with a tensile strength of 780 MPa or more.

EXAMPLES

Example 1

(36) As an example of this disclosure, using a resistance welding machine, attached to a C gun, that was of a servomotor pressine type and had a DC power source, the sheet combination 3 in which two steel sheets were overlapped (lower steel sheet 1 and upper steel sheet 2), as illustrated in the above-described FIG. 1, was resistance spot welded to produce a resistance spot weld joint.

(37) The current at this time had the current waveform illustrated in FIG. 4 (the initial current being performed in two steps) and was performed under the conditions listed in Table 1. The electrode force was constant at 4.5 kN, and the welding time T.sub.m of the main current was constant at 14 cycles (280 ms). In the experiment, the insulating body 7 was inserted between the steel sheets to adjust the distance between the electrodes to a predetermined distance.

(38) DR-type electrodes made of alumina-dispersed copper and each having a curvature radius of R40 at the tip and a tip diameter of 6 mm were used as the electrodes 4 and 5. Furthermore, as the test pieces, 1.0 mm to 1.2 mm galvanized steel sheets that were 780 MPa grade to 1470 MPa grade were used.

(39) Table 1 illustrates the results of verifying whether splashing occurred and verifying the nugget diameter upon performing welding. The nugget diameter was evaluated by the etching structure of a sliced section. A nugget diameter of 5t or greater was evaluated as excellent, 4t or greater to less than 5t as good, and less than 4t as poor, where t is the sheet thickness. Since sufficient joint strength is obtained if the nugget diameter is 4t or greater, a diameter of 4t or greater was deemed to be an appropriate diameter.

(40) TABLE-US-00001 TABLE 1 Steel sheet 1 Steel sheet 2 Sheet Sheet Occur- Tensile thick- Tensile thick- rence strength ness strength ness L t I.sub.1 T.sub.1 I.sub.2 T.sub.2 I.sub.m of Nugget No. (MPa) (mm) (MPa) (mm) (mm) (mm) L/t (kA) (ms) (kA) (ms) (kA) I.sub.1/I.sub.m I.sub.2/I.sub.m splashing diameter Notes 1 793 1 793 1 2.05 2 1.03 9.6 60 3.6 20 6 1.60 0.60 no good Example 2 793 1 793 1 2.05 2 1.03 9.6 60 3 40 6 1.60 0.50 no good Example 3 793 1 793 1 2.05 2 1.03 9.6 60 0 20 6 1.60 0.00 no good Example 4 793 1 793 1 2.05 2 1.03 5.4 60 4.8 20 6 0.90 0.80 yes poor Comparative Example 5 793 1 793 1 3.8 2 1.90 9.6 60 3.6 20 6 1.60 0.60 yes poor Comparative Example 6 1210 1.2 1210 1.2 2.4 2.4 1.00 9.6 60 3.6 20 6 1.60 0.60 no good Example 7 1210 1.2 1210 1.2 2.4 2.4 1.00 9.6 60 3 40 6 1.60 0.50 no good Example 8 1210 1.2 1210 1.2 2.4 2.4 1.00 9.6 60 0 20 6 1.60 0.00 no good Example 9 1210 1.2 1210 1.2 2.4 2.4 1.00 5.4 60 4.8 20 6 0.90 0.80 yes poor Comparative Example 10 1210 1.2 1210 1.2 4.4 2.4 1.83 9.6 60 3 20 6 1.60 0.50 yes poor Comparative Example 11 1530 1.4 1530 1.4 2.76 2.8 0.99 9.6 60 3 20 6 1.60 0.50 no good Example 12 1530 1.4 1530 1.4 2.76 2.8 0.99 9.6 60 3.6 40 6 1.60 0.60 no good Example 13 1530 1.4 1530 1.4 2.76 2.8 0.99 9.6 60 0 20 6 1.60 0.00 no good Example 14 1530 1.4 1530 1.4 2.76 2.8 0.99 5.4 60 4.8 20 6 0.90 0.80 yes poor Comparative Example 15 1530 1.4 1530 1.4 5.1 2.8 1.82 9.6 60 3 20 6 1.60 0.50 yes poor Comparative Example Nugget diameter: 5t or greater is excellent, 4t or greater to less than 5t is good, and less than 4t is poor, where t is sheet thickness

(41) Table 1 shows that when performing resistance spot welding according to this disclosure, as compared to the Comparative Examples, splashing does not occur, and a nugget of an appropriate diameter can be formed.

Example 2

(42) Resistance spot welding was performed similarly as in Example 1 to produce a resistance spot weld joint.

(43) However, the current at this time had the current waveform illustrated in FIG. 4 or FIG. 5 (the initial current being performed in two or four steps) and was performed under the conditions listed in Tables 2-1 and 2-2. The electrode force was 4.5 kN, and the welding time of the main current was 10 to 14 cycles (200 ms to 280 ms). An insulating body was inserted between the steel sheets to adjust the distance between the electrodes to a predetermined distance.

(44) The same electrodes as in Example 1 were used. The steel sheets were a combination of three overlapping sheets: one mild steel sheet having a galvannealing layer, and two high strength steel sheets having a hot-dip galvanizing laver.

(45) Table 2-2 illustrates the results of verifying whether splashing occurred and verifying the nugget diameter upon performing welding. A nugget diameter of 5t or greater was evaluated as excellent, 4t or greater to less than 5t as good, and less than 4t as poor, where t is the sheet thickness. A diameter of 4t or greater is an appropriate diameter.

(46) TABLE-US-00002 TABLE 2-1 Steel sheet 1 Steel sheet 2 Steel sheet 3 Sheet Sheet Sheet Tensile thick- Tensile thick- Tensile thick- strength ness strength ness strength ness L t I.sub.1 T.sub.1 I.sub.2 T.sub.2 I.sub.3 T.sub.3 I.sub.4 T.sub.4 No. (MPa) (mm) (MPa) (mm) (MPa) (mm) (mm) (mm) L/t (kA) (ms) (kA) (ms) (kA) (ms) (kA) (ms) Notes 1 298 1 1210 1.6 1210 1.6 4.4 4.2 1.05 9.6 60 4 20 9 60 4 20 Example 2 298 1 1210 1.6 1210 1.6 4.4 4.2 1.05 9.6 60 3 20 9 60 3 20 Example 3 298 1 1210 1.6 1210 1.6 4.4 4.2 1.05 9.6 60 0 20 9 60 0 20 Example 4 298 1 1210 1.6 1210 1.6 4.4 4.2 1.05 9.6 60 0 20 Example 5 298 1 1210 1.6 1210 1.6 4.4 4.2 1.05 6 60 5 20 6 60 5 20 Comparative Example 6 298 1 1210 1.6 1210 1.6 7.2 4.2 1.71 9.6 60 3 20 9 60 3 20 Comparative Example 7 298 1 1210 1.6 1210 1.6 4.1 4.2 0.98 8.4 60 0 20 8.4 60 0 20 Example 8 298 1 1210 1.6 1210 1.6 4.1 4.2 0.98 12 40 0 40 8.4 60 0 20 Example 9 298 1 1210 1.6 1210 1.6 4.1 4.2 0.98 8.4 60 0 20 Example 10 298 1 1210 1.6 1210 1.6 4.1 4.2 0.98 6 60 0 20 Comparative Example

(47) TABLE-US-00003 TABLE 2-2 Total welding Nugget diameter I.sub.m T.sub.m time Occurrence of (between steel No. (kA) (ms) (ms) I.sub.1/I.sub.m I.sub.2/I.sub.m I.sub.3/I.sub.m I.sub.4/I.sub.m splashing sheets 2-3) Notes 1 6.5 200 360 1.48 0.62 1.38 0.62 no excellent Example 2 6.5 200 360 1.48 0.46 1.38 0.46 no excellent Example 3 6.5 200 360 1.48 0.00 1.38 0.00 no good Example 4 6.5 280 360 1.48 0.00 no good Example 5 6.5 200 360 0.92 0.77 0.92 0.77 yes poor Comparative Example 6 6.5 200 360 1.48 0.46 1.38 0.46 yes poor Comparative Example 7 7 200 360 1.20 0.00 1.20 0.00 no excellent Example 8 7 200 360 1.71 0.00 1.20 0.00 no excellent Example 9 7 280 360 1.20 0.00 no good Example 10 7 280 360 0.86 0.00 yes poor Comparative Example Nugget diameter: 5t or greater is excellent, 4t or greater to less than 5t is good, and less than 4t is poor, where t is sheet thickness

(48) Table 2 shows that when performing resistance spot welding according to this disclosure, as compared to the Comparative Examples, splashing does not occur, and a nugget of an appropriate diameter can be formed.

(49) Furthermore, as compared to Example 1, in which two-step initial current was only performed once, Example 2 is superior in that depending on the conditions, a large nugget diameter could be obtained even when the current value in the final main current was low.

Example 3

(50) Resistance spot welding was performed similarly as m Example 1 to produce a resistance spot weld joint.

(51) However, the current at this time had the current waveform illustrated in FIG. 4 or FIG. 5 (the initial current being performed in two, four, or six steps) and was performed under the conditions listed in Tables 3-1 and 3-2. The electrode force was 5.5 kN, and the welding time T.sub.m of the main current was 10 to 18 cycles (200 ms to 360 ms). An insulating body was inserted between the steel sheets to adjust the distance between the electrodes to a predetermined distance.

(52) The same electrodes and steel sheets as in Example 1 were used.

(53) Table 3-2 illustrates the results of verifying whether splashing occurred and verifying the nugget diameter upon performing welding. A nugget diameter of 5t or greater was evaluated as excellent, 4t or greater to less than 5t as good, and less than 4t as poor, where t is the sheet thickness. In particular, a diameter of 5.5 greater was evaluated as excellent>5.5. A diameter of 4t or greater is an appropriate diameter.

(54) TABLE-US-00004 TABLE 3-1 Steel sheet 1 Steel sheet 2 Tensile Sheet Tensile Sheet strength thickness strength thickness L t I.sub.1 T.sub.1 I.sub.2 T.sub.2 No. (MPa) (mm) (MPa) (mm) (mm) (mm) L/t (kA) (ms) (kA) (ms) 1 1210 1.6 1210 1.6 3.3 3.2 1.03 9 60 0 20 2 1210 1.6 1210 1.6 3.3 3.2 1.03 11 40 0 20 3 1210 1.6 1210 1.6 3.3 3.2 1.03 9 60 0 20 4 1210 1.6 1210 1.6 3.3 3.2 1.03 9 60 0 20 5 1210 1.6 1210 1.6 3.3 3.2 1.03 7 60 0 20 6 1210 1.6 1210 1.6 7 3.2 2.19 9 60 0 20 7 1210 1.6 1210 1.6 3.14 3.2 0.98 9.5 60 0 20 8 1210 1.6 1210 1.6 3.14 3.2 0.98 8.8 100 0 20 9 1210 1.6 1210 1.6 3.14 3.2 0.98 11.5 40 0 20 10 1210 1.6 1210 1.6 3.14 3.2 0.98 9.5 60 0 20 11 1210 1.6 1210 1.6 3.14 3.2 0.98 9.5 60 0 20 12 1210 1.6 1210 1.6 3.14 3.2 0.98 7.5 60 0 20 13 1004 1.2 1004 1.2 2.3 2.4 0.96 10.5 60 0 20 14 1004 1.2 1004 1.2 2.3 2.4 0.96 10.5 60 0 20 15 1004 1.2 1004 1.2 2.3 2.4 0.96 10.5 60 0 20 16 1004 1.2 1004 1.2 2.3 2.4 0.96 8 60 0 20 17 1032 2 1032 2 4 4 1.00 9.6 60 0 20 18 1032 2 1032 2 4 4 1.00 9.6 60 0 20 19 1032 2 1032 2 4 4 1.00 9.6 60 0 20 20 1032 2 1032 2 4 4 1.00 7 60 0 20 21 1502 1.6 1502 1.6 3.18 3.2 0.99 9 60 0 20 22 1502 1.6 1502 1.6 3.18 3.2 0.99 9 60 0 20 23 1502 1.6 1502 1.6 3.18 3.2 0.99 9 60 0 20 24 1502 1.6 1502 1.6 3.18 3.2 0.99 7 60 0 20 I.sub.3 T.sub.3 I.sub.4 T.sub.4 I.sub.5 T.sub.5 I.sub.6 T.sub.6 No. (kA) (ms) (kA) (ms) (kA) (ms) (kA) (ms) Notes 1 8.5 60 0 20 8.5 60 0 20 Example 2 8.5 60 0 20 8.5 60 0 20 Example 3 8.5 60 0 20 Example 4 Example 5 7 60 0 20 7 60 0 20 Comparative Example 6 8.5 60 0 20 8.5 60 0 20 Comparative Example 7 9 60 0 20 9 60 0 20 Example 8 8.4 60 0 20 8.4 60 0 20 Example 9 9 60 0 20 9 60 0 20 Example 10 9 60 0 20 Example 11 Example 12 7.5 60 0 20 7.5 60 0 20 Comparative Example 13 9 60 0 20 9 60 0 20 Example 14 9 60 0 20 Example 15 Example 16 8 60 0 20 8 60 0 20 Comparative Example 17 8.6 60 0 20 8.6 60 0 20 Example 18 8.6 60 0 20 Example 19 Example 20 7 60 0 20 7 60 0 20 Comparative Example 21 8.5 60 0 20 8.5 60 0 20 Example 22 8.5 60 0 20 Example 23 Example 24 7 60 0 20 7 60 0 20 Comparative Example

(55) TABLE-US-00005 TABLE 3-2 Total welding I.sub.m T.sub.m time Occurrence of No. (kA) (ms) (ms) I.sub.1/I.sub.m I.sub.2/I.sub.m I.sub.3/I.sub.m I.sub.4/I.sub.m I.sub.5/I.sub.m I.sub.6/I.sub.m splashing Nugget diameter Notes 1 8 200 440 1.13 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 2 8 200 420 1.38 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 3 8 280 440 1.13 0.00 1.06 0.00 no excellent Example 4 8 360 440 1.13 0.00 no excellent Example 5 8 200 440 0.88 0.00 0.88 0.00 0.88 0.00 yes poor Comparative Example 6 8 200 440 1.13 0.00 1.06 0.00 1.06 0.00 yes poor Comparative Example 7 8.5 200 440 1.12 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 8 8 200 480 1.10 0.00 1.05 0.00 1.05 0.00 no excellent >5.5 Example 9 8.5 200 420 1.35 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 10 8.5 280 440 1.12 0.00 1.06 0.00 no excellent Example 11 8.5 360 440 1.12 0.00 no excellent Example 12 8.5 200 440 0.88 0.00 0.88 0.00 0.88 0.00 yes poor Comparative Example 13 8.5 200 440 1.24 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 14 8.5 280 440 1.24 0.00 1.06 0.00 no excellent Example 15 8.5 360 440 1.24 0.00 no excellent Example 16 8.5 200 440 0.94 0.00 0.94 0.00 0.94 0.00 yes poor Comparative Example 17 8 240 480 1.20 0.00 1.08 0.00 1.08 0.00 no excellent >5.5 Example 18 8 280 440 1.20 0.00 1.08 0.00 no excellent Example 19 8 320 400 1.20 0.00 no excellent Example 20 8 240 480 0.88 0.00 0.88 0.00 0.88 0.00 yes poor Comparative Example 21 8 240 480 1.13 0.00 1.06 0.00 1.06 0.00 no excellent >5.5 Example 22 8 300 460 1.13 0.00 1.06 0.00 no excellent Example 23 8 360 440 1.13 0.00 no excellent Example 24 8 240 480 0.88 0.00 0.88 0.00 0.88 0.00 yes poor Comparative Example Nugget diameter: 5.5t or greater is excellent >5.5, 5t or greater is excellent, 4t or greater to less than 5t is good, and less than 4t is poor, where t is sheet thickness

(56) Table 3-2 shows that when performing resistance spot welding according to this disclosure, as compared to the Comparative Examples, splashing does not occur, and a nugget of an appropriate diameter can be formed.

(57) Furthermore, as compared to Examples 1 and 2, in which two-step initial current was only performed once or twice, a superior effect was obtained in that the nugget diameter was increased.

REFERENCE SIGNS LIST

(58) 1, 2 Steel sheet

(59) 3 Sheet combination

(60) 4, 5 Electrode

(61) 6 Nugget

(62) 7 Insulating body