CONTINUOUSLY ANNEALED COLD-ROLLED STEEL SHEET EXCELLENT IN BALANCE BETWEEN DEEP DRAWABILITY AND RESISTANCE TO SECONDARY-WORK EMBRITTLEMENT AND METHOD FOR MANUFACTURING SAME

摘要

A continuously annealed and cold rolled steel sheet substantially comprisingthe following components and having excellent balance between deep drawabilityand secondary process brittleness resistance: less than 0.003 wt. % of C, lessthan 0.05 wt. % of Si, 0.05-0.20 wt. % of Mn, not more than 0.02 wt. % of P,not more than 0.15 wt. % of S, 0.025-0.06 wt. % of sol. Al, not more than0.0030 wt. % of N, 0.02-0.10 wt. % of Ti, 0.0003-0.0010 wt. % of B, and Fe andunavoidable impurities for the rest, in which a value of an index Xrepresentative of the contents of Ti and B is in the range of 9.2-11.2. Thefinish rolling of this continuously annealed and cold rolled steel sheetaccording to the present invention in the hot rolling thereof is done so thata rolling reduction allocation function Y expressed by a specific equation issatisfied. This finish rolling is completed at a temperature in the range of880·-920 ·C, and a hot rolled steel strip thus obtained is taken up. This hotrolled steel strip is then subjected to cold rolling at a total rollingreduction of not less than 70 % to produce a cold rolled steel strip, which isthen continuously annealed at a temperature in the range of 750 ·C to nothigher than an AC3 transformation temperature.