Investigation of the effect of low temperature on the mechanical behavior of an ultrafine-grained metastable steel using a physically based analytical model

Abstract

A duplex steel consisting of an ultrafine-grained austenite (γ) matrix and a residual strain-induced α′-martensite (SIMα′), i.e., ultrafine-grained (γ + SIMα′) steel, was fabricated by the cryogenic-rolling of a 304 austenitic stainless steel followed by annealing. Tensile tests of the ultrafine-grained (γ + SIMα′) steel were carried out at 77–298 K, the corresponding mechanical properties, austenite stability and kinetics of SIMα′ were investigated, and a physically-based analytical model was established to explore its temperature-dependent mechanical behavior. The results showed that the increase in the tensile strength caused by the decrease in temperature was significantly greater than that of the yield strength and was accompanied by a slight decrease in elongation. In addition, the strength-ductility synergy increased with reducing temperature. The work-hardening of the ultrafine-grained (γ + SIMα′) steel changed from continuous reduction in the work-hardening rate to the three-stage work-hardening behavior with a hardening peak as the temperature decreased from 298 K to 77 K. The analytical model revealed that the contribution of austenite to the work-hardening rate (Θγ) had a positive effect during the early uniform plastic deformation (UPD) stage and then a negative effect during the middle-late UPD stage. Both the positive and negative effects were enhanced with decreasing temperature. However, the contribution of SIMα′ to the work-hardening rate (Θα′) had only a positive impact throughout the whole UPD stage and was also improved with decreasing temperature. The work-hardening rate of ultrafine-grained (γ + SIMα′) steel was initially attributed mainly to the austenite during the early UPD stage and then was governed by the SIMα′ during the middle-late UPD stage. Moreover, the enhanced growth rate of Θγ and Θα′, resulting from the temperature decrease, led to the change from the continuous reduction of work-hardening between 298 K and 253 K to a three-stage work-hardening behavior between 173 K and 77 K.