| 摘要 |
PURPOSE: To produce a medium carbon steel sheet particularly excellent in workability, such as bore-expandability and secondary workability. CONSTITUTION: This sheet is a steel sheet excellent in workability, which has a composition containing 0.20-0.60% C, 0.05-0.50% Si, 0.05-0.30% Mn, <=0.02% P, <=0.01% S, 0.02-0.05% Cr, 0.05-0.50% Al, 0.003-0.015% N, 5-50ppm B, <=2.0% Ni, and <=0.01% Ca and in which 10-50% of C content is graphitized and the steel structure at a cross section contains graphite grains of >=3μm size by Cwt.%×10<2> pieces/mm<2> to Cwt.×10<3> pieces/mm<2> and is composed of ferritic phase where spheroidal cementite is dispersed. This steel sheet can be produced by the following methods: (1) after hot rolling by finishing at 750-900 deg.C and coiling at 400-650 deg.C, annealing is done at 600-720 deg.C; (2) after the same hot rolling as in (1), cold rolling at 40-80% and annealing at 600-720 deg.C are done; (3) after the same hot rolling as in (1), annealing is done at 600-720 deg.C and then cold rolling at 40-80% and annealing at 600-720 deg.C are performed.
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