| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet with tensile strength of a 340-390 MPa class, of which yield stress increases by 30 MPa or higher by heat treatment at 170°C for 20 minutes, even without receiving predistortion, and to provide a manufacturing method therefor. SOLUTION: This cold-rolled steel sheet comprises having a chemical composition of, by mass%, 0.001-0.01% C, 0.5% or less Si, 0.5-2.5% Mn, 0.05% or less P, 0.01% or less S, 0.005-0.1% sol.Al, 0.01% or less N, 0.0025% or less B, 0.02-1.5% Mo, further, 0.003-0.15% Ti and/or 0.01-1.5% Cr, as needed, while satisfying the relation given by the expression (1): B≥1.5×10<SP>-4</SP>×(Mn<SP>2</SP>+1), and the balance Fe with impurities, and having a structure consisting of a primary phase of ferrite phases, and a secondary phase containing low- temperature transformation forming phases. The method for manufacturing the steel sheet comprises hot-rolling, cold-rolling, and annealing it at a temperature of the Ac<SB>1</SB>transformation temperature or higher but lower than the Ac<SB>3</SB>transformation temperature. COPYRIGHT: (C)2003,JPO
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