| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high strength cold-rolled steel sheet having excellent stiffness of≥590MPa tensile strength TS and≥230GPa Young's modulus. SOLUTION: The steel containing, by mass%, 0.05-0.15% C,≤0.5% Si, 1.5-2.5% Mn,≤0.05% P,≤0.01% S,≤0.5% Al,≤0.01% N, 0.005-0.1% Nb and satisfying [Nb]-(93.2/14)[N]≥0.005 and the balance Fe with inevitable impurities, is formed into a slab. A hot-rolling is applied at Ac<SB>3</SB>point-950°C finishing temperature. Further, this hot-rolled steel is coiled at≥500°C and pickling is performed, and when an annealing is performed after cold-rolling at≥50% rolling reduction ratio, A3 is made to the temperature of following formula (1): A3=920-203[C]<SP>0.5</SP>+45[Si]-30[Mn]+80[Al] (°C). The steel is heated at≥1°C/s heating speed to the annealing temperature T of (A3-30)-(A3+30)°C and after soaking for the time t(s) so that P in formula(2): P=t×(T-A3+60) becomes 900-18,000. The steel is cooled at≥5°C/s cooling speed from the T to 750°C, so that the staying time at 750-550°C becomes≥20s. COPYRIGHT: (C)2007,JPO&INPIT
|
