Laser-welded lap joint, method for producing laser-welded lap joint, and automobile frame component

1. A laser-welded lap joint comprising a weld zone formed by laser welding in a lapped portion including a plurality of steel sheets lapped one over another,

wherein the weld zone includes a main weld zone that penetrates the steel sheets in the lapped portion and a final weld zone formed at one end of the main weld zone and having a crater, and

wherein the weld zone satisfies formulas (1) to (4):

L≥ 15.0;  (1)

10.0≥ L 2≥2 l c ;  (2)

t 1≥2 d c ;  (3)

w c >d c ,  (4)

where L is the full length (unit: mm) of the weld zone, L 2 is the length (unit: mm) of the final weld zone, l c is the length (unit: mm) of the crater in the final weld zone, t 1 is the thickness (unit: mm) of an uppermost steel sheet in the lapped portion, d c is the depth (unit: mm) of the crater in the final weld zone, and w c is the width (unit: mm) of the crater in the final weld zone.

2. The laser-welded lap joint according to claim 1 , wherein the total size of a gap between the steel sheets in the lapped portion is 0% or more and 15% or less of the total thickness of the plurality of steel sheets.

3. The laser-welded lap joint according to claim 2 , wherein at least one steel sheet of the plurality of steel sheets has a chemical composition comprising, in mass %:

C: more than 0.07% and 0.25% or less;

P+S: less than 0.03%;

Mn: 1.8% or more and 3.0% or less; and

Si: more than 1.2% and 2.5% or less,

with the balance being Fe and unavoidable impurities.

4. The laser-welded lap joint according to claim 3 , further comprising, in addition to the chemical composition, one or two selected from the following groups A and B:

group A: in mass %, one or two selected from Ti: 0.005% or more and 0.01% or less and Nb: 0.005% or more and less than 0.050%, and

group B: in mass %, one or two or more selected from Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.10% or less.

5. The laser-welded lap joint according to claim 4 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

6. The laser-welded lap joint according to claim 3 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

7. The laser-welded lap joint according to claim 2 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

8. A method for producing the laser-welded lap joint according to claim 2 , the method comprising:

lapping the plurality of steel sheets vertically one over another; and

performing laser welding in which a surface of the uppermost steel sheet in the lapped portion including the plurality of lapped steel sheets is irradiated with a laser beam to form the weld zone in the lapped portion to thereby join the plurality of lapped steel sheets together.

9. The method for producing the laser-welded lap joint according to claim 8 , wherein the laser welding includes: a main welding step of forming the main weld zone;

and a final welding step of forming the final weld zone having the crater, and wherein at least one of laser power, welding speed, a focal position, and a beam diameter in the final welding step is controlled under conditions that formula (5) to (7) are satisfied such that the weld zone formed satisfies formulas (1) to (4):

L≥ 15.0;  (1)

10.0≥ L 2≥2 l c ;  (2)

t 1≥2 d c ;  (3)

w c >d c ,  (4)

P i ≥P f ≥(¼) P i ;  (5)

v i ≥v f ≥(¼) v i ;  (6)

f i ≤f f ≤20.0,  (7)

where L is the full length (unit: mm) of the weld zone, L 2 is the length (unit: mm) of the final weld zone, l c is the length (unit: mm) of the crater in the final weld zone, t 1 is the thickness (unit: mm) of the uppermost steel sheet in the lapped portion, d c is the depth (unit:

mm) of the crater in the final weld zone, w c is the width (unit: mm) of the crater in the final weld zone, P i is laser power (unit: kW) in the main welding step, P f is the laser power (unit: kW) in the final welding step, v i is welding speed (unit: m/min) in the main welding step, v f is the welding speed (unit: m/min) in the final welding step, f i is a focal position (unit: mm) in the main welding step, and f f is the focal position (unit: mm) in the final welding step.

10. An automobile frame component comprising the laser-welded lap joint according to claim 2 .

11. The laser-welded lap joint according to claim 1 , wherein at least one steel sheet of the plurality of steel sheets has a chemical composition comprising, in mass %:

C: more than 0.07% and 0.25% or less;

P+S: less than 0.03%;

Mn: 1.8% or more and 3.0% or less; and

Si: more than 1.2% and 2.5% or less,

with the balance being Fe and unavoidable impurities.

12. The laser-welded lap joint according to claim 11 , further comprising, in addition to the chemical composition, one or two selected from the following groups A and B:

group A: in mass %, one or two selected from Ti: 0.005% or more and 0.01% or less and Nb: 0.005% or more and less than 0.050%, and

group B: in mass %, one or two or more selected from Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.10% or less.

13. The laser-welded lap joint according to claim 12 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

14. The laser-welded lap joint according to claim 11 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

15. The laser-welded lap joint according to claim 1 , wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more.

16. A method for producing the laser-welded lap joint according to claim 1 , the method comprising:

lapping the plurality of steel sheets vertically one over another; and

performing laser welding in which a surface of the uppermost steel sheet in the lapped portion including the plurality of lapped steel sheets is irradiated with a laser beam to form the weld zone in the lapped portion to thereby join the plurality of lapped steel sheets together.

17. The method for producing the laser-welded lap joint according to claim 16 , wherein the laser welding includes: a main welding step of forming the main weld zone; and a final welding step of forming the final weld zone having the crater, and

wherein at least one of laser power, welding speed, a focal position, and a beam diameter in the final welding step is controlled under conditions that formula (5) to (7) are satisfied such that the weld zone formed satisfies formulas (1) to (4):

L≥ 15.0;  (1)

10.0≥ L 2≥2 l c ;  (2)

t 1≥2 d c ;  (3)

w c >d c ,  (4)

P i ≥P f ≥(¼) P i ;  (5)

v i ≥v f ≥(¼) v i ;  (6)

f i ≤f f ≤20.0,  (7)

where L is the full length (unit: mm) of the weld zone, L 2 is the length (unit: mm) of the final weld zone, l c is the length (unit: mm) of the crater in the final weld zone, t 1 is the thickness (unit: mm) of the uppermost steel sheet in the lapped portion, d c is the depth (unit: mm) of the crater in the final weld zone, w c is the width (unit: mm) of the crater in the final weld zone, P i is laser power (unit: kW) in the main welding step, P f is the laser power (unit: kW) in the final welding step, v i is welding speed (unit: m/min) in the main welding step, v f is the welding speed (unit: m/min) in the final welding step, f i is a focal position (unit: mm) in the main welding step, and f f is the focal position (unit: mm) in the final welding step.

18. An automobile frame component comprising the laser-welded lap joint according to claim 1 .