| 摘要 |
PURPOSE: A process for preparing steel having low yield ratio and high strength by forming fine ferrite and a bainite structure of a suitable phase fraction without adding a large quantity of an alloy and reheating after hot rolling is provided, thereby providing steel having excellent weldability. CONSTITUTION: This high strength steel comprises by weight, C:0.07 to 0.16%, Si:0.05% or less, Mn:1.2 to 1.6%, Al:0.005 to 0.1%, Ti:0.005 to 0.02%, V:0.03 to 0.08%, P:0.03% or less, S:0.03% or less, N:0.03 to 0.014% and the balance of Fe with other inevitable impurities. The fine structure of steel is bainite having a phase fraction of 20 to 35% and the remainder being ferrite and the steel has a low yield ratio and grain size of ferrite of 2 to 4 micrometer.
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