| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-strength cold-rolled steel sheet which has such high ductility as having the tensile strength of 590 MPa or higher and a total elongation of 28% or larger, and excellent workability and shape freezability coming from the yield ratio of 0.7 or less, and enables its impact strength after working and formability before working, to be compatible, through increasing its strength after being formed, by imparting strain age hardening characteristics to itself. SOLUTION: This high-strength cold-rolled steel sheet has a composition including, by mass%, 0.05-0.10% C, 1.5-2.5% Mn, 0.005-0.02% Al, 0.010-0.025% N, and 0.10-0.20% Mo, controlling contamination of impurities particularly Si, P, S and Cr, to 0.1% or less Si, 0.03% or less P, 0.01% or less S, and 0.05% or less Cr, respectively, and the balance Fe and other unavoidable impurities. This manufacturing method, when hot rolling a slab having the above composition, cold rolling it, and then continuously cooling it to 300 deg.C from an annealing temperature, at an average cooling rate of 10 deg.C/s or higher but lower than 25 deg.C/s, is characterized by making the annealing temperature before the cooling treatment to be 830 deg.C or higher but 900 deg.C or lower.
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