Manufacturing method of low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a low yield ratio high-strength hot rolled steel sheet excellent in low-temperature toughness.SOLUTION: A method for producing a low yield ratio high strength hot rolled steel sheet comprises: heating, to a heating temperature:1,050 to 1,300°C, a steel raw material including predetermined components, with a Pvalue defined in Formula (1) satisfying a range between 0.16 and 0.22%, and comprising the balance Fe with inevitable impurities; performing a finish-rolling for the heated steel sheet with a cumulative draft of 60% or more in a temperature region of 930°C or less to obtain a hot rolled steel sheet; upon completion of the finish-rolling, starting a first cooling to a cooling-stop temperature: 600 to 450°C, at an average cooling rate of 30°C/s or more in a sheet thickness center part; performing a second cooling from the cooling-stop temperature to winding temperature, at an average cooling rate of 2°C/s or less in the sheet thickness center part; and winding the cooled sheet at a winding temperature of 450°C or more. Formula (1): P(%)=C+Si/30+Mn/20+Cu/20+nickel/60+Cr/20+Mo/15+V/10+5×B (where each element symbol represents a content (mass%)).SELECTED DRAWING: None