Microstructure and mechanical property of ultra-fine (ferrite + cementite) duplex structure in high carbon steel

The characteristics and mechanical properties of the (α+θ) microduplex structures formed by various thermomechanical processings were studied in an ultrahigh carbon steel (Fe-1.4Cr-1.0C). When the pearlite was austenitized in the (γ+θ) region, quenched and tempered at the temperature below AI, an (α+θ) microduplex structure in which most of α boundaries are of high-angle one is formed through the recovery of the fine (lath martensite + undissolved θ) mixture during tempering. Such (α+θ) microduplex structure with high-angle α boundaries exhibits superior superplasticity compared with those formed by the heavy warm rolling of pearlite, or by the annealing of heavily cold-rolled pearlite which contain higher fraction of low-angle α grain boundaries. Although the microduplex structures obtained various thermomechanical processings exhibit nearly the same tensile strength (about 900MPa), the ductility is markedly influenced by the nature of α grain boundaries. The specimens which contain large portion of high-angle α grain boundaries exhibit better ductility such as 25% in total elongation.