Ultrafine grained ferrite-martensite dual phase steels fabricated via equal channel angular pressing: Microstructure and tensile properties

Ultrafine grained (UFG) ferrite-martensite dual phase steels containing different amounts of vanadium were fabricated by equal channel angular pressing (ECAP) and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of a coarse grained counterpart. The formation of UFG martensite islands of ~1 (mu)m was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG ferrite matrix. A diffusion analysis showed that this specific microstructure may result from dissolution of carbon atoms from pearlitic cementite and their concurrent diffusion into UFG ferrite during ECAP, making the average carbon content reach the equilibrium content to form austenite during subsequent intercritical annealing. The strength of UFG dual phase steels was much higher than that of the coarse grained counterpart, but uniform and total elongations were not degraded. More importantly, the present UFG dual phase steels exhibited extensive rapid strain hardening unlike most UFG materials. The addition of vanadium slightly increased the strength and elongation of the present UFG dual phase steels, but it was found that excessive vanadium addition did not lead to further improvement of their mechanical properties. An excellent combination of strength, elongation and strain hardening of the present UFG dual phase steels was explained in terms of their specific microstructural features.