Spot welding method of high-strength steel sheets excellent in joint strength

1. A method for spot welding a stack of a plurality of steel sheets, the method comprising:

providing said stack of the plurality of steel sheets, wherein said plurality of steel sheets:

are two steel sheets which both have tensile strength of not less than 780 MPa nor more than 1850 MPa and whose sheet thickness ratio={a sum of sheet thicknesses of the steel sheets}/{the sheet thickness of the thinner steel sheet (when the both have the same thickness, the sheet thickness per one sheet)} is within a range of not less than 2 nor more than 5, or

are three steel sheets which are three steel sheets all having tensile strength of not less than 780 MPa nor more than 1850 MPa or which are two steel sheets both having tensile strength of not less than 780 MPa nor more than 1850 MPa and one steel sheet provided on an outer side of the two steel sheets and having tensile strength of less than 780 MPa, and whose sheet thickness ratio={a sum of sheet thicknesses of the steel sheets}/{the sheet thickness of the thinner steel sheet (when the steel sheets all have the same thickness, the sheet thickness per one sheet)} is within a range of not less than 3 nor more than 6,

applying resistance spot welding to said stack of the plurality of steel sheets by:

a first welding step being pre-welding with a pressurizing force P1 (kN) and a welding current I1 (kA); and

a second welding step being main welding with a pressurizing force P2 (kN) and a welding current I2 (kA),

the resistance spot welding being spot welding that passes a current to an overlapping part of the steel sheets while pressing electrodes against the steel sheets from both sides,

wherein the pressurizing forces P1, P2 are set to a fixed pressurizing force P=P1=P2 all through the first welding step and the second welding step, and are set within a range expressed by the following expression (1), where t (mm) is an average sheet thickness of the plural steel sheets,

0.5≤ P≤ 3.0 t (1/3)   (1),

wherein the welding current I1 is set within a range of not less than 30% nor more than 90% of the welding current I2, and

wherein the second welding step is started within 0.1 (s) after the first welding step is finished.

2. A method for spot welding a stack of a plurality of steel sheets, the method comprising:

providing said stack of the plurality of steel sheets, wherein said plurality of steel sheets:

are two steel sheets which both have tensile strength of not less than 780 MPa nor more than 1850 MPa and whose sheet thickness ratio={a sum of sheet thicknesses of the steel sheets}/{the sheet thickness of the thinner steel sheet (when the both have the same thickness, the sheet thickness per one sheet)} is within a range of not less than 2 nor more than 5, or

are three steel sheets which are three steel sheets all having tensile strength of not less than 780 MPa nor more than 1850 MPa or which are two steel sheets both having tensile strength of not less than 780 MPa nor more than 1850 MPa and one steel sheet provided on an outer side of the two steel sheets and having tensile strength of less than 780 MPa, and whose sheet thickness ratio={a sum of sheet thicknesses of the steel sheets}/{the sheet thickness of the thinner steel sheet (when the steel sheets all have the same thickness, the sheet thickness per one sheet)} is within a range of not less than 3 nor more than 6,

applying resistance spot welding to said stack of the plurality of steel sheets by:

a first welding step being pre-welding with a pressurizing force P1 (kN) and a welding current I1 (kA); and

a second welding step being main welding with a pressurizing force P2 (kN) and a welding current I2 (kA),

the resistance spot welding being spot welding that passes a current to an overlapping part of the steel sheets while pressing electrodes against the steel sheets from both sides,

wherein the pressurizing forces P1, P2 are set within ranges expressed by the following expression (2), expression (3), where t (mm) is an average sheet thickness of the plural steel sheets,

0.5≤ P 2≤3.0 t (1/3)   (2)

1.0× P 2< P 1≤2.0× P 2  (3)

wherein the welding current I1 is set within a range of not less than 30% nor more than 90% of the welding current I2, and

wherein the second welding step is started within 0.1 (s) after the first welding step is finished.

3. The method according to claim 1 , wherein any gap between the stacked steel sheets before the spot welding is less than 0.5 (mm).

4. The method according to claim 2 , wherein at least one of gaps between the stacked steel sheets before the spot welding is 0.5 (mm) or more.

5. The method according to claim 1 , the method comprising a third welding step being post welding after the second welding step being the main welding,

wherein, with a welding current and a welding time of the third welding step being represented by I3 (kA) and T3 (s) respectively, and with a non-welding time between the second welding step and the third welding step being represented by TC (s),

the welding current I3 is set within a range of not less than 3 (kA) nor more than 15 (kA),

the non-welding time TC is set within a range of not less than 0 (s) nor more than 0.2 (s), and

a relation between the welding current I3 and the welding time T3 is set within a range expressed by the following expression (4)

I 3× T 3≤0.7+ TC   (4).

6. The method according to claim 2 , the method comprising a third welding step being post welding after the second welding step being the main welding,

wherein, with a welding current and a welding time of the third welding step being represented by I3 (kA) and T3 (s) respectively, and with a non-welding time between the second welding step and the third welding step being represented by TC (s),

the welding current I3 is set within a range of not less than 3 (kA) nor more than 15 (kA),

the non-welding time TC is set within a range of not less than 0 (s) nor more than 0.2 (s), and

a relation between the welding current I3 and the welding time T3 is set within a range expressed by the following expression (4)

I 3× T 3≤0.7+ TC   (4).

7. The method according to claim 1 , wherein a welding time of the second welding step is longer than a welding time of the first welding step.

8. The method according to claim 2 , wherein a welding time of the second welding step is longer than a welding time of the first welding step.

9. The method according to claim 1 , wherein in the second welding step, a joint area is secured by melting a base metal.

10. The method according to claim 2 , wherein in the second welding step, a joint area is secured by melting a base metal.

11. The method according to claim 1 , wherein:

the first welding step is pre-welding of performing preheating; and

the second welding step is main welding of, subsequently to the first welding step, forming a nugget by welding energization.

12. The method according to claim 2 , wherein:

the first welding step is pre-welding of performing preheating; and

the second welding step is main welding of, subsequently to the first welding step, forming a nugget by welding energization.