Method of manufacturing low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness

摘要

PROBLEM TO BE SOLVED: To provide a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness.SOLUTION: A production method of low yield ratio high strength hot rolled steel sheet is provided in which a steel raw material containing, C:0.03 to 0.10%, Si:0.10 to 0.50%, Mn:1.4 to 2.2%, P:0.025% or less, S:0.005% or less, Al:0.005 to 0.10%, Nb:0.02 to 0.10%, Ti:0.001 to 0.030%, Mo:0.05 to 0.50%, Cr:0.05 to 0.50%, Ni:0.001 to 1.00% and the balance Fe with inevitable impurities is heated to a heating temperature:1050 to 1300°C, the heated steel is rough-rolled to produce a sheet bar, the sheet bar is subjected to finish rolling with a cumulative draft of 50% or more in a temperature region of 930°C or less to obtain hot rolled steel sheet, cooling of the hot rolled sheet bar is stared just after completion of finish rolling, cooled to a temperature region of cooling stop temperature: Bs point to 450°C at average cooling rate in a sheet thickness center part of 5 to 60°C/s, and the cooled sheet is wound to a coil shape, held for 60 to less than 600 sec. and is cooled.SELECTED DRAWING: None