COLD-ROLLED THICK SHEET STEEL WITH GOOD DEEP DRAWABILITY, AND METHOD FOR PRODUCING IT

摘要

A thick cold rolled steel sheet having an r value of not less than 2.9 even when the thickness is not less than 1.2 mm; and a method of manufacturing the same, comprising: rough hot-rolling a steel slab which contains not more than 0.008 wt.% of C, not more than 0.5 wt.% of Si, not more than 1.0 wt.% of Mn, not more than 0.15 wt.% of P, not more than 0.02 wt.% of S, 0.01-0.10 wt.% of Al, not more than 0.008 wt.% of N, 0.035-0.20 wt. % of Ti, 0.001-0.015 wt.% of Nb, the contents of the C, S, N, Ti and Nb satisfying the relation of 1.2 (C/12+N/14+S/32) < (Ti/48+Nb/93), and the balance consisting of Fe and unavoidable impurities at a temperature in a range from not lower than the Ar3 transformation point to not higher than 950 ~C and at a draft of not less than 85 %; finish hot-rolling the sheet, while lubricating the same, at a temperature in a range from not lower than 600 ~C to not higher than the Ar3 transformation point at a draft of not less than 65 % so that the average shearing strain may be not more than 0.06; pickling the steel sheet; subjecting the sheet to mother plate annealing at 700 to 920 ~C; cold rolling the sheet at a draft of not less than 65 %; and annealing the sheet steel for recrystallization at 700 ~-920 ~C.