PRODUCTION OF HOT ROLLED STEEL PRODUCTS HAVING HIGH STRENGTH AND HIGH TOUGHNESS

PURPOSE:To specify the structure in a surface layer part and central part and to permit as-rolled production of steel products having high strength and high toughness by subjecting a middle-carbon steel contg. specific ratios of Si, Mn, Al, and N to hot rolling then to quick cooling and self-tempering by the internal heat retained therein. CONSTITUTION:An ingot consisting, by weight %, of 0.20-0.60% C, 0.10-0.35% Si, 0.30-1.80% Mn, 0.010-0.045% Al, and 0.005-0.020% N, and consisting of the balance Fe and unavoidable impurities is hot rolled at finishing temp. A3-A3+150 deg.C. The rolled steel is then quickly cooled by water cooling, etc., down to the range of >=50 deg.C and Ms point or below. The steel is subjected to the self-tempering by the internal heat retained therein, by which the surface part is tempered to the martensite structure and the central part is tempered to the martensite, bainite and/or supercooled ferrite and pearlite structure. The hot rolled steel products having the fine crystal structure and excellent strength and toughness and obtd. by this method without requiring a thermal refining treatment.