| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of producing a cold rolled steel sheet for press forming by using an extra low carbon steel minimal in the additive quantities of carbonitride-forming elements. SOLUTION: A slab, which has a composition consisting of, by weight, <=0.0015% C, <=0.02% Si, <=0.2% Mn, <=0.02% P, 0.002-0.015% S, 0.01-0.05% sol.Al, <=0.002% N, 0.0002-0.0015% B, either of Ti and Zr in the amount satisfying inequality 1<=(M/[M]-S/32-N/14)×12/C<=8, and the balance Fe with inevitable impurities, is cast. This cast slab is cooled down to <=600 deg.C so that the average cooling rate X in the central part is not lower than 0.004 deg.C/sec and satisfies inequality K/X<=-200. After reheating to 1200 deg.C, hot rolling is performed at a temp. not lower than the Ar3 point, followed by coiling at <=720 deg.C. After cold rolling, recrystallization annealing is carried out at <=850 deg.C. In the above inequalities, K=4log(M/[M]-N/14)+2log(C/12)+2log(S/32) is satisfied, M is the additive quantity of Ti or Zr, [M] is the atomic weight of Ti or Zr, and the unit of M, S, N, and C is wt.%.
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