Resistance spot welding method
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 0.9×t≦L≦1.1×t; 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 1 during current in a first step of the initial current to satisfy I m ×1.1≦I 1 ≦15.0 kA with respect to a current value I m during the main current, and setting a current value I 2 in the subsequent second step to no current or low current satisfying 0≦I 2 ≦I m ×0.7.
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:
0.9× t≦L≦ 1.1× t
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 1 in a first step of the initial current period to satisfy:
I m ×1.1≦ I 1 ≦15.0 kA
where I m is a current value during the main current period, and
setting a current value I 2 in a second step of the initial current period to satisfy:
0≦ I 2 ≦I m ×0.7
setting welding times in the first step and the second step of the initial current period to satisfy:
10 ms≦ T 1 ≦100 ms
10 ms≦ T 2 ≦100 ms
where T 1 is a welding time in the first step of the initial current period, and T 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 (2n+1) in a (2n+1) th step (where n is an integer from 1 to k−1) of the initial current period satisfies:
I m ≦I (2n+1) ≦I (2n−1)
with respect to a current value I (2n−1) in a (2n−1) th step and the current value I 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.