LOW YIELD RATIO THICK STEEL PLATE EXCELLENT IN TOUGHNESS AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To produce a thick steel plate having high strength and high toughness and low in yield ratio (yield strength/tensile strength) and to provide a method for producig the same. SOLUTION: A steel having a compsn. contg., by weight, 0.02 to 0.35% C, 0.02 to 2.5% Si, 0.30 to 3.5% Mn and 0.002 to 0.10% Al, and the balance Fe with inevitable impurities is, as cast, or after heating to the temp. range of the AC3 point or above, is subjected to hot rolling at >=20% total draft in the temp. range of 900 deg.C to the Ar3 point or above, is swiftly cooled to <=450 deg.C at a cooling rate of R >=5 deg.C/s and is thereafter tempered in the following range, by which the low yield ratio thick steel plate contg. retained austenite having <=0.2μm particle size by 1 to 15% volume ratio can be obtd: T>=Ac1 , T>=-9.1H+740 T<=Ac1 +80 and T<=8.6H+7l9; where T: temp. ( deg.C) and H: temp. rising rate ( deg.C/s).