Resistance spot welding method

A resistance spot welding method of joining parts to be welded that are two or more overlapping steel sheets,

• wherein a steel sheet whose Mn content is highest of the overlapping steel sheets has a chemical composition that satisfies
• 0.050%≤C≤0.250%,
• 3.50%≤Mn≤12.00%,
• 0.001%≤Si≤2.000%,
• 0.001%≤P≤0.025%, and
• 0.0001%≤S≤0.0020%,
• the resistance spot welding method comprises
• performing main current passage and subsequent current passage, with a cooling time of 0.01 s or more being provided between the main current passage and the subsequent current passage, and
• Ip/I which is a ratio of a current value of the subsequent current passage to a current value of the main current passage satisfies, depending on a constant A defined by the Mn content and P content in the chemical composition, any of the following Formulas (1) to (3) in relation to Tp, T, and the constant A:
• when A≤0.04,
• (0.41+A)×(1+T)/(1+Tp)≤Ip/I≤(2.04−A)×(1+T)/(1+Tp) (1);
• when 0.04<A≤0.09,
• (0.402+1.2×A)×(1+T)/(1+Tp)≤Ip/I≤(2.08−2×A)×(1+T)/(1+Tp) (2);
• and
• when 0.09<A≤0.155,
• (0.393+1.3×A)×(1+T)/(1+Tp)≤Ip/I≤(2.17−3×A)×(1+T)/(1+Tp) (3),
• where I denotes the current value of the main current passage expressed in kA, Ip denotes the current value of the subsequent current passage expressed in kA, Tp denotes a welding time of the subsequent current passage expressed in s, T denotes the cooling time expressed in s, and the constant A is defined by any of the following Formulas (4) to (6) depending on the Mn content:
• when 3.5%≤Mn≤4.5%,
• A=(Mn+1.5)/200+P (4);
• when 4.5%<Mn≤7.5%,
• A=(Mn−1.5)/100+P (5);
• and
• when 7.5%<Mn≤12.0%,
• A=(Mn−3.9)/60+P (6),
• where Mn and P are respectively the Mn content and the P content in the chemical composition.