| 摘要 |
PURPOSE:To obtain a thick steel plate excellent in low-temp. toughness, weldability and tensile strength useful as a member for an LPG tank, by quenching low-N steel immediately after its hot-rolling is finished, and then tempering it. CONSTITUTION:Low-N steel contg., by weight ratio, 0.03-0.20% C, 0.03-0.75% Si, 0.60-2.50% Mn, 0.05-1.50% Ni, 0.05-0.50% Mo, 0.002-0.15% Al and N<= 0.0035% and having the ideal critical diameter D1 defined by the formula above 10mm. is prepared. Said low-N steel is heated at 1,000-1,300 deg.C and hot-rolled into a thick steel plate having predetermined dimensions at a temp. range above Ar3 transformation point. In succession, the steel is then quenched from a temp. above Ar3 transformation point to provide sufficient strength and toughness. After the mixed structure of martensite and bainite is formed in said steel plate, the steel plate is tempered at a temp. below is Ac1 transformation point, to obtain the high-tensile strength thick steel plate not-contg. V but having excellent desired strength and toughness. |
