PRODUCTION OF EXTREMELY LOW CARBON COLD-ROLLED STEEL SHEET HAVING EXCELLENT FATIGUE CHARACTERISTIC IN SPOT WELDED PART

摘要

PURPOSE:To produce an extremely low carbon cold-rolled steel sheet having an excellent fatigue characteristic in a spot welded part by subjecting a steel slab consisting of a specific compsn. successively to hot rolling, cold rolling, continuous annealing and temper rolling under specific conditions. CONSTITUTION:The steel slab contg., by weight %, =0.004% C, =0.1% Si, =0.5% Mn, =0.025% P, =0.025% S, =0.004% N, 0.01-0.10% Al, 0.01-0.04% Ti, 0.001-0.010% Nb, and 0.0001-0.0010% B, satisfying the formula I - the formula IV, and consisting of the balance substantially Fe is prepd. This steel slab is hot rolled at 700-900 deg.C finishing temp. and 300-600 deg.C coiling temp. and is then cold rolled at 60-85% draft. The steel sheet is further subjected to the continuous annealing in the temp. range of the recrystallization temp. or above and =780 deg.C and is thereafter subjected to the temper rolling at =[sheet thickness (mm)-0.1]% and =3.0% draft. The extremely low carbon cold rolled steel sheet having the high fatigue strength of the welded part is thereby obtd. without impairing the formability.