| 摘要 |
PURPOSE: A steel whose cold formability is improved using graphiting characteristics, a steel working product whose delayed fracture resistance and mechanical properties are improved using composite structure characteristics of ferrite and residual austenite as its fine structure, and a method for manufacturing the steel working product are provided. CONSTITUTION: The method for manufacturing high strength steel working product having superior delayed fracture resistance comprises a process of cold forming into a steel working product a wire drawing material which comprises 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, 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, and in which graphite grains having size of 50 μm or less are contained in fine structure in a phase fraction of 0.1% or more; and a process of heating the steel working product at a temperature of Ac3 transformation temperature or more so that austenite single phase is obtained, rapidly cooling the heated steel working product to the temperature range of Ms+30 deg.C to Ms+90 deg.C in a cooling rate of 70 deg.C/sec or more, and isothermal heat treating the rapidly cooled steel working product so that a composite structure of ferrite and residual austenite having fraction of 15% or more is obtained.
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