Influence of manganese content on ε-/α'-martensitic transformation and tensile properties of low-C high-Mn TRIP steels

Low carbon steels containing 15-19 wt% Mn were processed to study the influence of Mn content on the thermally induced and deformation-induced epsilon-/alpha'-martensitic transformation and tensile properties of high-Mn TRIP steels. The stability of austenite and epsilon-martensite was studied in terms of thermodynamics, and the work hardening behavior during tensile deformation was divided into two stages using Hollomon analysis. The results indicated that Mn increased the stability of austenite and c-martensite and austenite grain refinement had a larger effect on gamma -> epsilon than gamma -> alpha' transformation. During early stages of tensile deformation, the steel having similar to 15 wt% Mn continued the gamma -> epsilon, epsilon -> alpha' and gamma -> alpha' transformation because deformation energy compensated the Gibbs free energy required for phase transformation. But alpha'-martensitic transformation was difficult in steel having similar to 19 wt% Mn even after fracture because of the high stability of epsilon-martensite. On account of high density of dislocations in alpha'-martensite, the dynamic strain aging process was obvious when deformation-induced alpha'-martensitic transformation occurred in steel. The significant alpha'-martensitic transformation and intense dynamic strain aging improved work hardening exponent and ultimate tensile strength of steel, while the coordinated transformation of gamma -> epsilon and epsilon -> alpha' during tensile deformation was beneficial to improve ultimate elongation. (c) 2018 Elsevier Ltd. All rights reserved.