PRODUCTION OF THICK STEEL PLATE HAVING =48KGF/MM2 AND 75 KGF/MM2 TENSILE STRENGTH AND EXCELLENT WELDABILITY

摘要

PURPOSE:To produce a high tensile thick steel plate having excellent weldability by subjecting a steel ingot having a slight ratio of Nb and a specific ratio of carbon equiv. to a heat treatment and rolling under specific conditions. CONSTITUTION:The steel ingot which contains, by weight %, 0.03-0.23% C, 0.3-1.5% Mn, 0.005-0.04% Nb, 0.08% Al, 0.001-0.005% T, N or further contg. =1 kinds among B, Ti, V, Cr, Ni, Mo and Ca at a specific ratio each and of which the carbon equiv. expressed by formula (I) is =0.36% when the tensile strength of the steel plate is 48-58kg/cm2 or =0.40% when 58-75kgf/ cm2 is manufactured. Such steel ingot is heated and soaked to and at 1,000-1,200 deg.C as it is or while the surface temp. after the reduction rolling does not fall yet down to 500 deg.C. The ingot is then rolled at least at 20% rolling reduction at the temp. down to 920 deg.C - start of water cooling and is cooled with water immediately after the rolling. The water cooling is suspended when the surface temp. of the steel plate decreases to 350-600 deg.C and the steel plate is immediately held for 5-60min at 500-650 deg.C. The thick steel plate having the excellent weldability and 48-75kgf/cm2 tensile strength is produced.