Spot-welded joint and spot welding method

A spot welding method of overlapping a plurality of pieces of steel plates and performing spot welding on the steel plates, wherein

• at least one piece of steel plate out of the plurality of pieces of steel plates is a high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, wherein
• a carbon equivalent Ceq of the high-strength steel plate represented by the following expression (A) is 0.20 mass% to 0.55 mass%,
• the spot welding method comprising:
• performing pre-energization of energizing welding electrodes with a pre-energization current If (kA) satisfying the following expression (C) for a pre-energization time tf (msec) satisfying the following expression (D), in a state where the overlapped plurality of pieces of steel plates are pressurized by the welding electrodes at a pressurizing force FE (N) satisfying the following expression (B);
• performing, after the pre-energization is finished, cooling after pre-energization of cooling the plurality of pieces of steel plates for a cooling time after pre-energization tc (msec) satisfying the following expression (E) while retaining the pressurizing force FE (N) satisfying the following expression (B);
• performing, after the cooling after pre-energization is finished, main welding of energizing the welding electrodes with a main welding current IW (kA) while retaining the pressurizing force FE (N) satisfying the following expression (B);
• performing, after the main welding is finished, cooling after main welding of cooling the plurality of pieces of steel plates for a cooling time after main welding ts (msec) satisfying the following expression (F) while retaining the pressurizing force FE (N) satisfying the following expression (B);
• performing, after the cooling after main welding is finished, post-energization of energizing the welding electrodes with a post-energization current IP (kA) satisfying the following expression (G) for a post-energization time tP (msec) satisfying the following expression (H) while retaining the pressurizing force FE (N) satisfying the following expression (B);
• and
• retaining, after the post-energization is finished, the pressurizing force FE (N) satisfying the above expression (B) for a retention time tH (msec) satisfying the following expression (I), and then releasing the pressurizing at the pressurizing force FE (N),
• Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] (A)
• 1960×h≤FE≤3920×h (B)
• 0.40+IW≤If<IW (C)
• 20<tf (D)
• 0≤tC<200+7×h (E)
• 7×h+5≤tS≤300 (F)
• 0.66×IW≤IP<IW (G)
• 48/{(IP/IW)2−0.4}≤tP (H)
• 0≤tH≤300 (I)
• [C], [Si], [Mn], [P], and [S] in the above expression (A) indicate respective contents (mass%) of C, Si, Mn, P, and S, and h in the above expression (B), the above expression (E), and the above expression (F) indicates a plate thickness of the steel plate (mm).