| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile strength cold rolled steel sheet having a tensile strength of≥440 MPa and a high yield ratio of≥0.8 and also to provide its manufacturing method. SOLUTION: This steel sheet has: a composition containing 0.005 to 0.025% C,≤1.0% Si, 0.5 to 2.5% Mn, 0.01 to 0.10% P,≤0.01% S, 0.01 to 0.10% Al and≤0.005% N, also containing Nb and Ti in amounts satisfying [Nb]×[C]≥5×10<SP>-4</SP>and 3.5≤[Ti]/[N]≤7.0, and having the balance Fe with inevitable impurities; and a structure in which≥50% of precipitatable Nb content exists in the form of precipitates having a size smaller than 20 nm and a ferrite phase having 5 to 30μm average crystal grain size is contained as a main phase. The steel sheet can be manufactured, e.g., by carrying out cold rolling, heating and holding at a temperature between (Ac<SB>3</SB>transformation point) and [(Ac<SB>3</SB>transformation point)+100]°C, cooling at≥10°C/s cooling rate, holding at 500 to 700°C for≥30 s and cooling again. COPYRIGHT: (C)2009,JPO&INPIT
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