Effects of martensitic transformability and dynamic strain age hardenability on plasticity in metastable austenitic steels containing carbon

摘要

We investigated the effects of solute carbon concentration on the mechanical properties of Fe-19Cr-8Ni-0.05C and Fe-19Cr-8Ni-0.14C metastable austenitic steels. These steels showed an FCC(gamma) -> HCP(epsilon) -> BCC(alpha') martensitic transformation, resulting in transformation-induced plasticity (TRIP). The presence of excess solute carbon reduced the transformability because of an increase in the austenite stability. However, the work hardening capability was enhanced by a combined effect of the TRIP and dynamic strain aging (DSA). DSA requires a high diffusivity of carbon. Thus, the FCC (low diffusivity) to BCC (high diffusivity) transformation favors DSA. The hardening capability of BCC-martensite per volume is enhanced by the dislocation pinning and solution hardening effect of the carbon atmosphere, despite a decrease in the transformation rate per strain by carbon addition. Moreover, carbon addition stabilizes the deformation-induced HCP-martensite against the BCC-martensite, improving the hardening capability of the HCP-martensite through suppression of the window effect, which affects the plastic accommodation mechanism. According to our study, the steel with a low carbon content demonstrated extraordinary work hardening rates owing to a high transformation rate per strain. In contrast, the steel with a high carbon content showed sustained and high work hardening rates because of DSA. Both the steels showed approximately the same tensile strength, but completely different work hardening behavior.