Method for producing resistance-welded member

1 . A method for producing a resistance-welded member made of three or more steel sheets including at least one plated high-tensile steel sheet having a base metal strength of 980 MPa or more, the method comprising:

a main energizing by performing energization with a first current value while compressing the steel sheets between a pair of electrodes with a first compressive force to form a nugget;

a subsequent energizing by performing, after the main energizing, energization such that a current value gradually decreases from the first current value while compressing the steel sheets between the pair of electrodes with a second compressive force greater than the first compressive force; and

holding the pair of electrodes against the steel sheets while maintaining the second compressive force after the subsequent energizing, wherein:

the steel sheets are joined under conditions satisfying formulae (1) to (3):

A≥ 1.9  Formula (1)

wherein A=P 2 /t;

P 2 represents the second compressive force [kN]; and

t represents a total sheet thickness [mm] of the steel sheets, respectively;

0≤ Tdl≤ 200  Formula (2)

wherein Tdl represents a compression rise delay time [ms] which is a time difference between an end of energization with the first current value and a start of compression with the second compressive force; and

B≤Tds≤C   Formula (3)

wherein B=0.0011Tht 2 −2.64Tht+1284.8, C=0.0015Tht 2 −2.52Tht+1268.6;

Tds represents a down slope time [ms] in the subsequent energizing; and

Tht represents an electrode holding time [ms], which is a time for holding the pair of electrodes against the steel sheets in the holding the electrode, respectively.

2 . The method for producing a resistance-welded member according to claim 1 , wherein the Tht satisfies formula (4):

Tht< 700  Formula (4).

3 . The method of producing a resistance-welded member according to claim 1 , wherein a tip end diameter of the electrode is from 4 mm to 10 mm.

4 . The method for producing a resistance-welded member according to claim 1 , wherein:

a servo compression welding machine is employed as a welding machine; and

when a depth of an indentation on the steel sheet by the electrode becomes 0.15 mm or more, a control is performed to forcibly terminate only the main energizing or both the main energizing and a compression.

5 . The method for producing a resistance-welded member according to claim 2 , wherein:

a servo compression welding machine is employed as a welding machine; and

when a depth of an indentation on the steel sheet by the electrode becomes 0.15 mm or more, a control is performed to forcibly terminate only the main energizing or both the main energizing and a compression.

6 . The method of producing a resistance-welded member according to claim 2 , wherein a tip end diameter of the electrode is from 4 mm to 10 mm.

7 . The method for producing a resistance-welded member according to claim 3 , wherein:

a servo compression welding machine is employed as a welding machine; and

when a depth of an indentation on the steel sheet by the electrode becomes 0.15 mm or more, a control is performed to forcibly terminate only the main energizing or both the main energizing and a compression.

8 . The method for producing a resistance-welded member according to claim 6 , wherein:

a servo compression welding machine is employed as a welding machine; and

when a depth of an indentation on the steel sheet by the electrode becomes 0.15 mm or more, a control is performed to forcibly terminate only the main energizing or both the main energizing and a compression.