HIGH STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME

摘要

A high strength cold rolled steel sheet which has the TS of 340 to 400 MPa and the r value of 1.8 or more, or has the TS of 400 to 590 MPa and the r value of 1.6 or more, preferably 1.7 or more without requiring any special manufacturing processes, and a method for manufacturing the same are provided. A high strength cold rolled steel sheet comprises, by mass percent, 0.015% or less of C, 1.5% or less of Si, 0.4 to 3% of Mn, 0.15% or less of P, 0.02% or less of S, exceeding 0.2 to 0.7% of sol.Al, 0.01% or less of N, 0.2% or less of Ti, and the balance of Fe and inevitable impurities, wherein the following equation(1) is satisfied: 1<=([Ti]/48)/([C]/12+[N]/14), where [M] represents the content(mass percent) of an element M. A method for manufacturing a high strength cold rolled steel sheet comprises the steps of: heating a slab having a composition comprising, by mass percent, 0.015% or less of C, 1.5% or less of Si, 0.4 to 3% of Mn, 0.15% or less of P, 0.02% or less of S, exceeding 0.2 to 0.7% of sol.Al, 0.01% or less of N, 0.2% or less of Ti, and the balance of Fe and inevitable impurities at 1,080 to 1,350 deg.C; hot rolling the heated steel slab at a finishing temperature from (Ar3 transformation temperature-20) to (Ar3 transformation temperature+150)deg.C to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a coiling temperature CT satisfying the following equation(5) or (6): 480<=CT<=580+0.17/{([Ti]+0.08x[sol.Al])x[P]} and 480<=CT<=580+0.17/{(0.6x[Nb]+[Ti]+0.08x[sol.Al])x[P]}, where [M] represents the content (mass percent) of an element M; cold rolling the hot rolled steel sheet with a reduction ratio of 50 to 90 % to obtain a cold rolled steel sheet; and continuously annealing the cold rolled steel sheet at a temperature of 750 to 870 deg.C or box annealing the cold rolled steel sheet at a temperature of 600 to 750 deg.C.