High strength-elongation balance in ultrafine grained ferrite-martensite dual phase steels developed by thermomechanical processing

Abstract

Ultrafine grained dual phase (UFG DP) steels were processed using a thermomechanical treatment designed to obtain UFG ferrite/nano carbide aggregates prior to intercritical annealing. The influences of intercritical annealing temperature on the microstructures and deformation behaviors of DP steels were investigated. The grain average misorientation maps confirmed the prescience of higher amount of strains in martensite phases. In addition, it was shown that the geometrically necessary dislocations which mainly formed at ferrite/martensite interfaces are responsible for continuous yielding of DP structures. The results showed around 3 times improvement in strength-elongation balance (about 33286 MPa% for the UFG DP, compared with the 11501 MPa% for the ferritic-pearlitic structure). The Hollomon, differential Crussard-Jaoul (DC-J) and modified C-J (MC-J) analyses were used to evaluate the deformation behavior of DP steels. The Hollomon and C-J analyses proposed two and three-stage deformation for the present DP steels, respectively, while the modified C-J analysis illustrated four-stage deformation. A relatively high strain hardening rate was observed for all the DP steels in the initial stages of deformation. The initial strain hardening rate was enhanced as a result of decreasing the ferrite grain size and increasing the martensite volume fraction at lower intercritical annealing temperatures and then it began to decrease with further increasing the intercritical annealing temperature.