Method of resistance spot welding of high tensile strength steel sheet and welding joint manufactured by the method
Provided is a method of resistance spot welding of welding a sheet set including a high strength steel sheet with which a high joint strength can be achieved even for a high Ceq material. A method of resistance spot welding a sheet set of two or more lapped steel sheets by weld the sheet set while clamping and pressing the sheet set between a pair of welding electrodes includes a first welding step of applying an weld current Im (kA) and forming a nugget having a nugget diameter d (mm) that satisfies the following inequality (1); a cooling step, subsequent to the foregoing first welding, of cooling the sheet set while continuing to press the sheet set; and a second welding step of performing two-step welding, 3×√ t m ≦d ≦6×√ t m (1), where t m is the thickness (mm) of the thinnest one of the foregoing two or more steel sheets.
1. A method of resistance spot welding a sheet set of two or more lapped steel sheets by welding the sheet set while clamping and pressing the sheet set between a pair of welding electrodes, the method sequentially comprising:
a first welding step of applying an weld current Im (kA) and forming a nugget having a nugget diameter d (mm) that satisfies the following inequality (1);
a cooling step (A), subsequent to the first welding step, of cooling the sheet set while continuing to press the sheet set; and
a second welding step of performing two-step welding of applying weld currents that satisfy the following inequalities (2) and (3),
3 ×√t m ≦d≦ 6 ×√t m (1),
where t m is the thickness (mm) of the thinnest one of the two or more steel sheets,
Im<I 21 <Im× 2.0 (2)
I 22 <I 21 (3),
where I 21 and I 22 are respectively the weld currents (kA) of a pre-step and a post-step of the two-step welding.
2. The method of resistance spot welding according to claim 1 , wherein a time Tc 1 of the cooling step (A) subsequent to the first welding step satisfies the following inequality (4),
t/ 2< Tc 1 <t× 5 (4),
where t is the total thickness (mm) of the sheet set and the unit of time is (cycles/50 Hz).
3. The method of resistance spot welding according to claim 1 , wherein a total weld time T 2 of the second welding step satisfies the following equation (5) and inequality (6),
T 2 =T 21 +T 22 (5),
where T 21 and T 22 are respectively weld times of the pre-step and the post-step,
T 2< t× 5 (6),
where t is the total thickness (mm) of the sheet set and the unit of time is (cycles/50 Hz).
4. The method of resistance spot welding according to any one of claim 1 , further sequentially comprising:
a cooling step (B) of continuing to press the sheet set after the second welding step; and
a third welding step of applying an weld current that satisfies the following inequality (7),
I 3 33 I 3 ×T 3 ≧I 21 ×I 21 ×T 21 +I 22 ×I 22 ×T 22 (7),
where I 3 is the weld current (kA) of the third welding step, I 21 and I 22 are respectively the weld currents (kA) of the pre-step and the post-step of the two-step welding, T 3 is an weld time of the third welding step, T 21 and T 22 are respectively the weld times of the pre-step and the post-step, and the unit of time is (cycles/50 Hz).
5. The method of resistance spot welding according to claim 4 , wherein a time Tc 2 of the cooling step (B) subsequent to the second welding step satisfies the following inequality (8),
Tc ⅕< Tc 2 ≦Tc 1 (8),
where Tc 1 is the time of the cooling step (A) subsequent to the first welding step, and the unit of time is (cycles/50 Hz).
6. The method of resistance spot welding according to claim 4 , wherein two-step welding is performed in the third welding step instead of one welding.
7. The method of resistance spot welding according to claim 4 , wherein the cooling step (B) subsequent to the second welding step and the third welding step are repeated once or twice.
8. The method of resistance spot welding according to claim 1 , wherein at least one of the two or more steel sheets satisfies the following inequality,
0.25<Ceq<0.6,
where Ceq=C+ 1/30×Si+ 1/20×Mn+2×P+4×S (%), in which the right side is the contents (mass %) of elements included in the at least one of the steel sheets.