PURPOSE:To develop steel for cryogenic temp. pressure vessel having superior cryogenic temp. toughness, by applying hot rolling, quenching and tempering to 9% Ni steel under specified conditions. CONSTITUTION:The titled steel is superior in toughness in cryogenic temp. as material for pressure vessel of cryogenic temp. gas such as liquefied natural gas. the steel is composed of, by weight 0.01-0.10% C, 0.02-0.50% Si, 0.10-2.00% Mn, 8.00-10.00% Ni, 0.005-1.10% Al or further at least one kind among =1.00% Cu, =1.00% Cr, =0.50% Mo, =0.10% V, =0.005% B, =0.05% Ti, =0.01% Ca, =0.05% Ce, =0.01% Mg, =0.05% Zr, =0.05% La and the balance Fe. The steel slab is heated to 900-1,000 deg.C, hot rolled at =700 deg.C to plate, then said plate is immediately cooled rapidly, quenched, successively tempered at 560-620 deg.C.
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