METHOD FOR MANUFACTURING STEEL PLATES (YIELD STRENGTH: 63KGF/MM2) HAVING SUPERIOR TOUGHNESS EVEN AT ULTRA LOW TEMPERATURE

摘要

PURPOSE: Provided is a method for manufacturing steel plates (yield strength: 63kgf/mm2) having superior toughness even at -196deg.C for the application to LNG container where C and P content in steel is lowered and specific heat treatment is carried out after hot rolling. CONSTITUTION: The manufacturing method of the steel plates comprises the processes of degassing molten steel comprising C 0.04-0.09wt.%, Si 0.30wt.% or less, Mn 0.55-0.85wt.%, P 0.006wt.% or less, S 0.003wt.% or less, Ni 8.5-9.5wt.%, a balance of Fe, and other inevitable impurities for 20min. or more in a vacuum degassing facility; continuous casting the molten steel in order to prepare slab; heating to the temperature range of 1150 to 1220deg.C; finish hot rolling in the temperature range of 800 to 1050deg.C and then air-cooling; performing a first heat treatment where the steel plate is water cooled after heating it in the temperature range of 800 to 840deg.C; performing a second heat treatment where the steel plate is water cooled after heating it in the temperature range of 680 to 700deg.C; a third heat treatment where the steel plate is air-cooled after heated to the temperature range of 570 to 590deg.C.