| 摘要 |
PURPOSE: A process for preparing YP355 grade(N/mm¬2) steel plates having high strength and toughness and excellent yield ratio by adding an alloy element of Cu, Ni and V to steel, control rolling and quenching a hot rolled steel sheet is provided. Whereby, steel plates can be used for structural material corresponding with ultra high-rise and scale-up trend of a building. CONSTITUTION: A steel slab comprising by weight, C:0.136 to 0.164%, Mn:1.00 to 1.25%, Si:0.40 to 0.50%, P:0.012% or less, Nb:0.025 to 0.035%, Ti:0.015 or less, Cu:0.030 or less, Ni:0.03% or less, V:0.020% or less, Ceq:0.42% or less and the balance of Fe with other inevitable impurities is reheated, drawn, quenched at a speed of 5 to 6deg.C and air cooled to produce steel plates having a final temperature of 600 to 550deg.C.
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