| 摘要 |
PURPOSE: A process for preparing a steel composition capable of improving mechanical characteristics such as tensile strength, elongation, impact toughness or the like while maximizing low yield ratio of steel is provided, which improves mechanical characteristics while controlling a blocky type phase fraction of retained austenite in a composite structure of ferrite and retained austenite. CONSTITUTION: This steel of composite structure comprises by weight, C:0.65 to 1.50%, Si:2.0 to 4.0%, Mn:0.1 to 0.8%, Cr:0.1 to 0.8%, P:0.01% or less, S:0.01% or less, N:0.005 to 0.01%, O:0.05% or less, one or two or more selected from Ni:0.3 to 2.0%, B:0.001 to 0.003%, V:0.01 to 0.5%, Nb:0.01 to 0.5%, Mo:0.01 to 0.5%, Ti:0.01 to 0.2%, W:0.01 to 0.5% and Cu:0.01 to 0.2%. The fine structure of the steel is a composite structure of ferrite and retained austenite and a retained austenite phase fraction of blocky type of retained austenite is 20 to 35%.
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