| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of producing a cold-rolled high tension steel sheet having a tensile strength of≥590 MPa and excellent ductility and stretch-flangeability. SOLUTION: The method of producing the cold-rolled steel sheet having metal structure in which a main phase is a low-temperature transformation-generating phase and a secondary phase includes ferrite, is characterized by comprising the following processes (A)-(C): (A) a hot rolling process in which a slab having a chemical composition containing by mass% >0.020 to <0.20% C, >0.10 to≤2.0% Si, 1.50-3.50% Mn,≤0.10% P,≤0.010% S,≤0.10% sol.Al and≤0.010% N is subjected to hot rolling in which the rolling completes in the temperature range of Ar<SB>3</SB>point or higher, to obtain the hot-rolled steel plate, then the hot-rolled steel plate is cooled to the temperature range of≤720°C within 0.4 sec after completion of the rolling, thereby average number density of iron carbide present on the grain boundary of the ferrite becomes≥5.0×10<SP>-2</SP>pieces/μm<SP>2</SP>; (B) a cold rolling process in which the hot-rolled steel plate is subjected to cold rolling to obtain a cold-rolled steel sheet; and (C) an annealing process in which the cold-rolled steel sheet is subjected to a soaking treatment in the temperature zone of (Ac<SB>3</SB>point -40°C) or higher. COPYRIGHT: (C)2011,JPO&INPIT
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