Spot welded joint and spot welding method

1. A spot welding method for forming a nugget in a joint surface of two or more thin steel plates, wherein at least one of the two or more thin steel plates is a high-strength steel plate having a tensile strength of 750 MPa to 1850 MPa and having a carbon equivalent Ceq represented by Formula (1) of 0.22 mass % to 0.55 mass %:

Ceq=[C]+[Si]/30 +[Mn]/20 +2[P]+4[S]  Formula (1),

wherein [C], [Si], [Mn], [P], and [S] each respectively represent a content, in mass %, of C, Si, Mn, P, and S in the high-strength steel plate, the method comprising the steps of, in order:

a) performing welding energization under a condition that a pressurizing force EF, in N, of a welding electrode against the two or more thin steel plates satisfies Formula (2):

1960 ×h ≦EF≦3430 ×h   Formula (2),

wherein h represents a quotient, in mm, which is obtained when a sum of thicknesses of all the thin steel plates is divided by two

b) cooling the thin steel plates for a cooling time after welding Ct, in ms, satisfying Formula (3), while holding the pressurizing force EF:

16≦Ct≦300  Formula (3);

(c) while holding the pressurizing force EF, performing post-heating energization of the welding electrode with a post-heating energization current PC, in kA, which satisfies Formula (4):

0.40×WC≦PC≦0.95×WC  Formula (4)

wherein WC is measured in kA, and represents a welding current during the welding energization for a post-heating energization time Pt,

wherein Pt is measured in ms, and is defined by Formula (5):

30≦Pt≦200  Formula (5);

and

d) releasing pressurizing at the pressurizing force EF,

wherein a holding time Ht, in ms, during which the pressurizing force EF is held between the end of the step of performing post-heating energization and the step of releasing pressurizing at the pressurizing force EF satisfies Formula (6):

0≦Ht≦240  Formula (6).

2. A spot welding method for forming a nugget in a joint surface of two or more thin steel plates, wherein at least one of the two or more thin steel plates is a high-strength steel plate having a tensile strength of 750 MPa to 1850 MPa and having a carbon equivalent Ceq represented by Formula (1) of 0.22 mass % to 0.55 mass %:

Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]  Formula (1),

wherein [C], [Si], [Mn]. [P], and [S] each respectively represent a content, in mass %, of C, Si, Mn, P, and S in the high-strength steel plate, the method comprising the steps of, in order:

a) performing welding energization under a condition that a pressurizing force EF, in N, of a welding electrode against the two or more thin steel plates satisfies Formula (7):

1470 ×h ≦EF≦1960 ×h   Formula (7),

wherein h represents a quotient, in mm, which is obtained when a sum of thicknesses of all the thin steel plates is divided by two;

b) cooling the thin steel plates for a cooling time after welding Ct, in ms, satisfying Formula (3), while holding the pressurizing force EF:

16≦Ct≦300  Formula (3),

c) while holding the pressurizing force EF, performing post-heating energization of the welding electrode with a post-heating energization current PC, in kA, which satisfies Formula (4):

0.40×WC≦PC≦0.95×WC  Formula (4)

wherein WC is measured in kA, and represents a welding current during the welding energization for a post-heating energization time Pt,

wherein Pt is measured in ms, and is defined by Formula (5):

30≦Pt≦200  Formula (5):

and

d) releasing pressurizing at the pressurizing force EF,

wherein a holding time Ht, in ms, during which the pressurizing force EF is held between the end of the step of performing post-heating energization and the step of releasing pressurizing at the pressurizing force EF satisfies Formula (6):

0≦Ht≦240  Formula (6).

3. The spot welding method of a high-strength steel plate according to claim 1 , further comprising a step, after the step of releasing pressurizing, of holding the thin steel plates at a heat treatment temperature after welding AT (° C.) satisfying Formula (8):

120≦AT≦220  Formula (8),

for a heat treatment time after welding At (s) of 100≦At≦6000.

4. The spot welding method of a high-strength steel plate according to claim 3 , further comprising, between the step of releasing pressurizing and the step of holding the thin steel plates at the heat treatment temperature after welding, cooling the thin steel plates to reduce a surface temperature of a portion contacting the welding electrode of each of the thin steel plates to a temperature CT (° C.) satisfying Formula (10):

CT≦150  Formula (10).

5. The spot welding method of a high-strength steel plate according to claim 1 , wherein the high-strength steel plate contains:

C: 0.10 mass % to 0.40 mass %;

Si: 0.01 mass % to 2.50 mass %; and

Mn: 1.5 mass % to 3.0 mass %, and

wherein, in the high-strength steel plate,

P content is equal to or less than 0.03 mass %,

S content is equal to or less than 0.01 mass %,

N content is equal to or less than 0.0100 mass %,

O content is equal to or less than 0.007 mass %,

Al content is equal to or less than 1.00 mass %, and

the balance comprises Fe and an inevitable impurity.

6. The spot welding method of a high-strength steel plate according to claim 5 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of:

Ti: 0.005 mass % to 0.10 mass %;

Nb: 0.005 mass % to 0.10 mass %; and

V: 0.005 mass % to 0.10 mass %.

7. The spot welding method of a high-strength steel plate according to claim 5 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of:

B: 0.0001 mass % to 0.01 mass %;

Cr: 0.01 mass % to 2.0 mass %;

Ni: 0.01 mass % to 2.0 mass %;

Cu: 0.01 mass % to 2.0 mass %; and

Mo: 0.01 mass % to 0.8 mass %.

8. The spot welding method of a high-strength steel plate according to claim 5 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of Ca, Ce, Mg, and REM within a range of 0.0001 mass % to 0.5 mass % in total.

9. The spot welding method of a high-strength steel plate according to claim 2 , further comprising a step, after the step of releasing pressurizing, of holding the thin steel plates at a heat treatment temperature after welding AT (° C.) satisfying Formula (8):

120≦AT≦220  Formula (8),

for a heat treatment time after welding At (s) of 100≦At≦6000.

10. The spot welding method of a high-strength steel plate according to claim 9 , further comprising, between the step of releasing pressurizing and the step of holding the thin steel plates at the heat treatment temperature after welding, cooling the thin steel plates to reduce a surface temperature of a portion contacting the welding electrode of each of the thin steel plates to a temperature CT (° C.) satisfying Formula (10):

CT≦150  Formula (10).

11. The spot welding method of a high-strength steel plate according to claim 2 , wherein the high-strength steel plate contains:

C: 0.10 mass % to 0.40 mass %;

Si: 0.01 mass % to 2.50 mass %; and

Mn: 1.5 mass % to 3.0 mass %, and

wherein, in the high-strength steel plate,

P content is equal to or less than 0.03 mass %,

S content is equal to or less than 0.01 mass %,

N content is equal to or less than 0.0100 mass %,

O content is equal to or less than 0.007 mass %,

Al content is equal to or less than 1.00 mass %, and

the balance comprises Fe and an inevitable impurity.

12. The spot welding method of a high-strength steel plate according to claim 11 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of:

Ti: 0.005 mass % to 0.10 mass %;

Nb: 0.005 mass % to 0.10 mass %; and

V: 0.005 mass % to 0.10 mass %.

13. The spot welding method of a high-strength steel plate according to claim 11 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of Ca, Ce, Mg, and REM within a range of 0.0001 mass % to 0.5 mass % in total.

14. The spot welding method of a high-strength steel plate according to claim 11 , wherein the high-strength steel plate further contains at least one element selected from the group consisting of:

B: 0.0001 mass % to 0.01 mass %;

Cr: 0.01 mass % to 2.0 mass %;

Ni: 0.01 mass % to 2.0 mass %;

Cu: 0.01 mass % to 2.0 mass %; and

Mo: 0.01 mass % to 0.8 mass %.