| Dynamic recrystallization controlled rolling(DRCR) process was simulated by multistage torsion tests in the range of 1000∼900℃, 0.1/sec∼3.0/sec with decreasing temperature and increasing strain rate(ε). The critical strain(ε_c) for the initiation of dynamic recrystallization(DRX) was determined by plots of the work hardening rate(θ) against stress(σ) and the relationship ε_c/ε_p was found to be around 0.7. Grain refinement was accelerated by the penultimate pass in the DRCR. After the final deformation at 900℃, grain growth was restricted from controlling the cooling rate. When the cooling rate was 7℃/sec, refined ferrite, pearlite and bainite were observed and grains were refined down to 20∼30 ㎛ which gave tensile strength of 780MPa. The precipitates of V(C, N) distributed homogeneously at higher cooling rate and improvement of tensile strength confirmed precipitation hardening. |
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