| 摘要 |
PURPOSE: A method for manufacturing high strength working product having low yield ratio is provided in which graphitization heat treatment for securing graphite structure is performed on steel (wire rod) before cold forming, and heat treatment for securing composite structure of ferrite and residual austenite is performed on the steel working product after forming. CONSTITUTION: The method for manufacturing high strength working product having low yield ratio comprises heat treatment process of graphitizing a steel comprising 0.40 to 0.60 wt.% of carbon, 2.0 to 4.0 wt.% of silicon, 0.1 to 0.8 wt.% of manganese, 0.01 wt.% or less of phosphorus, 0.01 wt.% or less of sulfur, 0.002 to 0.01 wt.% of nitrogen, 0.002 wt.% or less of oxygen, selective one or more elements selected from the group consisting of 0.3 to 2.0 wt.% of nickel, 0.001 to 0.003 wt.% of boron, 0.01 to 0.5 wt.% of vanadium, 0.01 to 0.5 wt.% of niobium, 0.01 to 0.5 wt.% of molybdenum, 0.01 to 0.2 wt.% of titanium, 0.01 to 0.5 wt.% of tungsten and 0.01 to 0.2 wt.% of copper and a balance of Fe and other impurities in the temperature range of Ac1 to(60±30 deg.C) and air cooling the graphitized steel; cold forming process of cold forming the heat treated steel into steel working product; and a heat treatment process of obtaining composite structure consisting of dual phase steel of ferrite and austenite by heating the steel working product at a temperature from Ac3-(Ac3-Ac1)/1.3 to Ac3-(Ac3-Ac1)/5.5, rapidly cooling the heated steel working product to a temperature of Ms+120 to Ms+180 deg.C in a cooling rate of 70 deg.C/sec or more, isothermal heat treating the rapidly cooled steel working product for 20 minutes or more, and oil cooling or air cooling the isothermal heat treated steel working product so that microstructure of the steel working product has a composite structure of ferrite and bainite(ferrite+retained austenite), and fraction of the total retained austenite in the bainite is 10% or more.
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