Fabrication of ultrafine grained ferrite/martensite dual phase steel by severe plastic deformation

Processing of ultrafine grained ferrite/martensite dual phase (UFG F/M DP) steel by equal channel angular pressing (ECAP) and subsequent intercritical annealing was described in detail. Along with strain gradient plasticity concept in which an introduction of a high density of geometrically necessary dislocations attributes to enhance strain hardenability, the aim of the present study was fabrication of UFG F/M DP steel exhibiting extensive strain hardenability, which other UFG materials hardly exhibit, in addition to ultrahigh strength and good uniform elongation. By selecting the optimum ECAP and intercritical annealing conditions, it was possible to fabricate UFG F/M DP steel in which isolated UFG martensite islands were uniformly embedded into UFG ferrite matrix. The formation of such a unique microstructure under the present processing conditions was discussed in terms of microstructural evolution during ECAP. Room temperature tensile properties of UFG F/M DP steel were superior to those of coarse grained counterpart. More importantly, in spite of an UFG structure, the present UFG F/M DP steel exhibited extensive strain hardenability from the onset of plastic deformation in association with grain size independent strain gradient plasticity, unlike other UFG materials.