Overlap-welded member, automobile part, method of welding overlapped portion, and method of manufacturing overlap-welded member

1. A method of welding an overlapped portion, including

a resistance spot welding process in which a spot-welded portion is formed through resistance spot welding in an overlapped portion including a plurality of steel sheet members, the spot-welded portion including:

a nugget;

a heat-affected zone formed in the vicinity of the nugget; and

a softest zone having the lowest Vickers hardness in the heat-affected zone,

wherein the heat-affected zone comprises a softening zone and a hardened portion,

the hardened portion being sandwiched between the nugget and the softening zone, and

the softest zone in the softening zone being at an inner peripheral edge of the softening zone, and

a tempering process of forming, in all areas between a central portion of the nugget and the softest zone, a tempered area made out of tempered martensite, the tempered area in which the maximum Vickers hardness is not more than 120% in a case where Vickers hardness of the softest zone is 100%.

2. The method of welding an overlapped portion according to claim 1 , wherein

in the resistance spot welding step, energization is performed so as to satisfy Equation (2) described below, where:

T (second) is an energization time in the resistance spot welding;

if there is only one steel sheet member having the highest tensile strength of the plurality of steel sheet members, t (mm) is a thickness of this steel sheet member, whereas, if there are a plurality of steel sheet members having the highest tensile strength, t (mm) is a thickness of a steel sheet member having the thinnest thickness of these steel sheet members; and

cyc (second) is a period of time for one cycle of energization in the resistance spot welding,

5 t× cyc≤ T ≤(5 t+ 4)×cyc  Equation (2).

3. The method of welding an overlapped portion according to claim 1 , wherein

the method further includes, before the resistance spot welding step,

applying a preheat electric current I (kA) to the overlapped portion in a state where an energization time T 1 (second), a period of time cyc (second) for one cycle of energization, and a thickness t (mm) satisfy Equation (3) described below;

as for the thickness t (mm), if there is only one steel sheet member having the highest tensile strength of the plurality of steel sheet members, a thickness of this steel sheet member is used, whereas, if there are a plurality of steel sheet members having the highest tensile strength, a thickness of a steel sheet member having the thinnest thickness of these steel sheet members is used;

in the resistance spot welding step, a welding electric current I 0 (kA) not more than a splash occurring current is applied to the overlapped portion in a state where Equation (4) described below is satisfied, where T 2 (second) is an energization time, and cyc (second) is a period of time for one cycle of energization in the resistance spot welding; and

the preheat electric current I (kA) and the welding electric current I 0 (kA) satisfy Equation (5) described below,

5 t× cyc≤ T 1 ≤(5 t+ 8)×cyc  Equation (3).

5 t× cyc≤ T 2 ≤(5 t+ 4)×cyc  Equation (4),

0.3I 0≤I≤ 0.7I 0   Equation (5).

4. The method of welding an overlapped portion according to claim 1 , wherein

in the resistance spot welding step, the resistance spot welding is performed so as to satisfy Equation (6) described below, where D (mm) is a distance from a melting boundary portion of the nugget to the softest zone, and if there is only one steel sheet member having the highest tensile strength of the plurality of steel sheet members, t (mm) is a thickness of this steel sheet member, whereas, if there are a plurality of steel sheet members having the highest tensile strength, t (mm) is a thickness of a steel sheet member having the thinnest thickness of these steel sheet members, and

the tempering process is a subsequent energization process in which the tempered area is formed through subsequent energization,

D≤t 0.2   Equation (6).

5. The method of welding an overlapped portion according to claim 4 , wherein

in the resistance spot welding step, energization is applied so as to satisfy Equation (7) described below, where T (second) is an energization time in the resistance spot welding, and cyc (second) is a period of time for one cycle of energization in the resistance spot welding,

5 t× cyc≤ T ≤(5 t+ 4)×cyc  Equation (7).

6. The method of welding an overlapped portion according to claim 4 , wherein

the method further includes, before the resistance spot welding step:

applying a preheat electric current I (kA) to the overlapped portion in a state where an energization time T 1 (second), a period of time cyc (second) for one cycle of energization, and the thickness t (mm) satisfy Equation (8) described below;

in the resistance spot welding step, a welding electric current I 0 (kA) not more than a splash occurring current is applied to the overlapped portion in a state where Equation (9) described below is satisfied, where T 2 (second) is an energization time, and cyc (second) is a period of time for one cycle of energization; and

the preheat electric current I (kA) and the welding electric current I 0 (kA) satisfy Equation (10) described below,

5 t× cyc≤ T 1 ≤(5 t+ 8)×cyc  Equation (8),

5 t× cyc≤ T 2 ≤(5 t+ 4)×cyc  Equation (9),

0.3I 0 ≤I≤0.7I 0   Equation (10).

7. The method of welding an overlapped portion according to claim 1 , wherein the plurality of steel sheet members includes a hot-stamped member having a tensile strength of 1500 MPa or higher.