Role of Mn content on processing maps, deformation kinetics, microstructure and texture of as-cast medium Mn (6–10 wt% Mn) steels

Abstract

In the present work, the hot deformation behaviour of as-cast medium Mn steels with nominal Mn content of 6, 8, and 10 wt% were studied using the thermomechanical simulator in the temperature and strain rate range of 1173 to 1373 K and 10−3 - 10 s−1 respectively. The flow curves of the three steels revealed significant work hardening followed by a plateau at strain rates ≥1s−1 in the entire temperature range. At lower strain rates (<10−2s−1) noticeable softening is observed after a strain of 0.2 in all three steels. The extent of softening is lower in 10 wt% Mn steel in comparison to the 6 and 8 wt% Mn steels. Analysis of processing maps reveals that in steels with 6 and 8 wt% Mn, regions of ‘high’ strain rate sensitivity (m) (0.2 - 0.25) are found to occur in similar temperature and strain rate regimes of 1273–1323 K and 10−2s−1 respectively. These steels also showed similar apparent activation energies for hot deformation (380–390 kJmol−1), stress exponents (4.6) and dynamic recovery/recrystallization evolution with strain. The steel with 10 wt% Mn, has higher apparent activation energy for hot deformation (450 kJmol−1), higher stress exponent of 5.1, higher dynamic recovery and lower dynamic recrystallization. The observed activation energies correlate well with the activation energy for self-diffusion (QSD) of Fe in austenite containing Mn as an alloying element. In the ‘low’ m regimes, higher dynamic recovery (evaluated using kinetic analysis) was observed in the 10 % Mn steel as compared to the 6 and 8% Mn steels. In the ‘high’ m regimes, the dynamic recrystallization fraction (estimated from kinetic analysis and from parent grain reconstruction analysis) was lower in the 10 % Mn steel when compared to the 6 and 8 wt % Mn steels. This behaviour can be attributed to the higher dynamic recovery observed in the 10 % Mn steel. From the analysis of processing maps, kinetic analysis and microstructure, the optimum strain, strain rate and temperature for the thermomechanical processing of as-cast medium Mn steels with Mn in the range of 6 to 10 wt% are ≤ 0.6, ≤10−2s−1 and 1273 to 1373 K respectively.