The high carbon which is superior in the hole expansion property and secondary processability the black sheet and its production manner

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.%×10² pieces/mm² to Cwt.×10³ pieces/mm² 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°C and coiling at 400-650°C, annealing is done at 600-720°C; (2) after the same hot rolling as in (1), cold rolling at 40-80% and annealing at 600-720°C are done; (3) after the same hot rolling as in (1), annealing is done at 600-720°C and then cold rolling at 40-80% and annealing at 600-720°C are performed.;COPYRIGHT: (C)1996,JPO