DEAD-SOFT CARBON COLD-ROLLED STEEL SHEET EXCELLENT IN SPOT WELDABILITY AND ITS PRODUCTION

摘要

PURPOSE:To obtain a dead-soft carbon cold-rolled steel sheet having superior spot weldability without deteriorating formability, by providing a composition which contains respectively prescribed amounts of C, Si, Mn, P, S, N, Al, Ti, Nb, and B and in which four specific inequalities are satisfied. CONSTITUTION:A dead-soft carbon steel sheet consisting of, 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, 0.0001-0.0010% B, and the balance essentially Fe and satisfying inequalities I-IV is provided. In order to obtain this cold-rolled steel sheet, a steel slab with the above composition is hot-rolled under the conditions of 700-900 deg.C finishing temp. and 300-600 deg.C winding temp., cold-rolled at 60-85% draft, and then subjected to continuous annealing at a temp. between the recrystallization temp. and 780 deg.C. The resulting titled cold-rolled steel sheet is free from deterioration in strength even when subjected to spot welding, and further, it has superior spot weldability.