Steel sheet assembly, method of manufacturing steel sheet assembly, and spot welding process

A welding process used in a method of manufacturing a steel sheet assembly that comprises:

• applying, in advance, an adhesive and a carbon-supplying agent to a surface of either or both of steel sheets to be lapped, the steel sheets having a composition containing:
• C:
• 0.4% or less,
• Si:
• 3.0% or less,
• Al:
• 3.0% or less,
• Mn:
• 0.2% to 6.0%,
• P:
• 0.1% or less, and
• S:
• 0.07% or less
• on a mass basis, the remainder being Fe and inevitable impurities, and having a tensile strength of 1,470 MPa or less and a thickness of 0.3 mm to 5.0 mm;
• lapping the steel sheets;
• and then
• welding the steel sheets,
• wherein a weld of the assembly has a nugget diameter of 2.8√t (mm) or more, where t (mm) denotes the thickness of a thinner one of the steel sheets on both sides of a weld interface, and
• the amount of C is increased by 0.02% by mass or more as compared to the steel sheets before being applied with the adhesive and the carbon-supplying agent, the welding process comprising:
• spot welding the steel sheets performed for a heat time of 0.08 seconds or more using a convex electrode with a tip radius of curvature of 20 mm or more or a flat electrode such that the weld force F (kN) for initial 0.03 seconds of the heat time satisfies formula:
• F<0.00125×TS×(1+0.75×tall)+3
• where TS (MPa) denotes an average strength of the steel sheets and represents a weighted mean value of a thickness of each of the steel sheets, and
• tall (mm) denotes a total thickness of the steel sheets (the sum of the thicknesses of the steel sheets).