| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-rolled dual-phase steel sheet having excellent strength, ductility and toughness, and its manufacturing method. SOLUTION: The high-strength hot-rolled steel sheet has a composition consisting of 0.05 to <0.20% C, 0.5 to <1.5% Mn, 0.002 to <0.05% sol.Al, <0.1% Si, <0.1% Cr,≤0.01% Ti, <0.005% Nb,≤0.01% V, <0.005% N and the balance Fe with impurities and also has a structure which contains a ferrite phase as a main phase and also contains 10 to 30 vol.% martensite phase and in which respective average grain sizes of the ferrite phase and the martensite phase are 1.1 to 3.0μm and≤3.0μm, respectively. This steel sheet can be manufactured by performing hot rolling where rolling is finished at a temperature between [(Ae<SB>3</SB>point)-20°C] and [(Ae<SB>3</SB>point)+20°C], starting cooling within 0.3 sec, applying rapid cooling at≥400°C/s average cooling rate down to 580 to 680°C and then reheating the resultant steel sheet at≥10°C/s temperature-rise rate up to 710 to 880°C, followed by cooling at≥200°C/s average cooling rate down to room temperature. COPYRIGHT: (C)2007,JPO&INPIT
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