Adjusting the microstructure evolution, mechanical properties and deformation behaviors of Fe-5.95Mn-1.55Si-1.03Al-0.055C medium Mn steel by cold-rolling reduction ratio

Abstract

For a representative medium Mn steel with the actual chemical composition of Fe-5.95 Mn-1.55Si-1.03Al-0.055 C (wt. %), the effect of cold-rolling deformation on microstructural evolution and mechanical properties was investigated systematically. The thickness of coarse δ-ferrite grains decreases with the increase of cold-rolling reduction, and when the cold-rolling reduction ratio reaches up to a certain value, these δ-ferrite grains can be broken into small pieces due to the severe plastic deformation. Additionally, a critical cold-rolling reduction ratio for recrystallization exists. Below this critical reduction value, the medium Mn steel after austenite reverted transformation (ART) annealing remains lath-shaped structure originating from the initial martensitic morphology, and when recrystallization occurs, however, submicron equiaxed grains dominate. The initial microstructure before ART annealing, which is usually determined by cold-rolling reduction, strongly influences not only the martensitic/ferritic matrix, but also reverted austenite grains. Non-recrystallization matrix promotes the formation of acicular reverted austenite, whereas recrystallization forces the austenite grains spherical and promotes the grain size of austenite homogenizing. Under the situation of non-recrystallization, the cold-rolling reduction prior to ART annealing only has a negligible effect on the final mechanical properties. However, the occurrence of recrystallization results in not only the yielding plateau, i.e., discontinuous yielding, but also the remarkable increase of yield strength.