PRODUCTION OF COLD ROLLED STEEL SHEET EXCELLENT IN DEEP DRAWABILITY AND AGEING CHARACTERISTIC BY CONTINUOUS ANNEALING

摘要

PURPOSE:To produce a cold rolled steel sheet excellent in deep drawability and ageing characteristic by rolling a cast slab of Al killed steel at a temp. in a specific region, exerting cold rolling, and then carrying out continuous annealing having specific thermal hysteresis. CONSTITUTION:A slab of an Al killed steel having a composition containing, by weight, 0.010-0.038% C, 0.04% Si, 0.05-0.25% Mn, 0.003-0.020% S, 0.010-0.10% Al, and 0.0060% N is hot rolled at =1100 deg.C into a plate and rolling is performed at 50% reduction of area based on the plate thickness when the surface temp. of the hot rolled plate reaches the upper limit of the temp. range T1 represented by an inequality I, and hot rolling is finished at =900 deg.C. This plate is coiled at 600-800 deg.C, cooled, and cold-rolled to the final sheet thickness, and the resulting sheet is annealed at 700-900 deg.C and successively cooled from the above annealing temp. down to 690-650 deg.C at 3-20 deg.C/sec cooling rate. Further, this sheet is cooled down rapidly to 200-340 deg.C at 50-200 deg.C/sec cooling rate, heated up to 345-450 deg.C, and cooled down slowly to 200-330 deg.C in =100sec.