Microstructure and tensile behaviors for a medium Mn steel with δ-ferrite phase under different annealing temperatures

In this experiment, we systematically investigated the microstructure evolution, mechanical properties, and deformation behaviors as functions of annealing temperature using cold-rolled Fe-0.05C-6Mn-1Al-1.5Si steel. Almost all annealed specimens are composed of the equiaxed and granular alpha-ferrite (alpha) grains due to recrystallization, reversed austenite (gamma) grains because of reversed transformation from deformed martensite, and fibrous delta-ferrite (delta) grains only undergoing recovery during intercritical annealing except for 760 degrees C. It is interesting that several annealing twins are formed within gamma grains. The outstanding combination of strength and ductility is obtained for annealing at 740 degrees C. The tensile strength and total elongation are 980 MPa and 32.1%, which are attributed to the ultrafine grains and optimal TRIP effect associated with rational g content and stability. A phenomenon that yield point elongation (YPE) gradually shortens with increased gamma content and decreased gamma stability is observed. What's more, the mechanism that active alpha' formation can promote the improvement of work hardening rate and further curtail YPE is verified using deep-cryogenic treatment. Based on SEM observation, microscopic strain of delta grains appears nearly consistent with matrix (alpha and gamma grains) during tensile process, although delta phase presents as fibrous morphology with coarse size. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).