Microstructure Evolution and Tensile Behaviour of a Cold Rolled 8 Wt Pct Mn Medium Manganese Steel

Abstract

A novel medium manganese steel with composition Fe–8.3Mn–3.8Al–1.8Si–0.5C–0.06V–0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41 pct. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20 pct reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but without a significant drop in ductility during subsequent tensile tests. The microstructural evolution with cold rolling reduction was analysed to understand the mechanisms behind this phenomena. It was found that cold rolling activated additional twinning systems which provided a large number of potent nucleation sites for strain induced martensite to form during subsequent tensile tests in what can be described as an enhanced TRIP effect.

The relative contributions of TWIP and TRIP to strength in fine grained medium-Mn steels

“…The SFE of FC and WQ steels were both within the predicted twinning regime of TWIP steels [1] and also medium Mn steels [20]. On the other hand, because of the large C content in both steels (> 1 wt%), the Md 30 and Ms temperatures were very low, suggesting that the austenite was very stable.…”

“…Since the TWIP effect does little in terms of strength for medium Mn steels, it is probably better to not pursue the TWIP + TRIP effect in alloy design. In order to enable the TWIP + TRIP effect, a relatively high SFE and austenite stability is needed [20]. This is enabled by a combination of either low Mn and high C (current alloy), or high Mn and low C. If the TWIP effect can be forgone during alloy design, it would be possible to enable low to medium Mn and low C (3-6 wt% Mn, 0.05-0.2 wt% C) compositions that exhibit the TRIP effect only.…”

“…the Transformation Induced Plasticity (TRIP) effect. Additionally, if the Stacking Fault Energy (SFE) can be raised into the twinning regime (15-35 mJ m -2 ) through element partitioning during an Intercritical Annealing (IA) heat treatment, the TWIP effect may also be activated [19,20]. Medium Mn steels can therefore also be grouped according to their plasticity enhancing mechanism, i.e.…”

“…This means that the austenite grains do not deform homogeneously and such steels often exhibit serrations in the strain hardening curve. Therefore, the combined TWIP + TRIP effect, which occurs in a single austenite grain, is of particular interest as it allows for homogeneous deformation, sustained hardening during deformation and elongations in excess of 40% [19,20,[24][25][26][27]. However, the interplay between TWIP and TRIP can be very different.…”

“…It is still uncertain what factors determine whether the austenite phase deforms via successive or simultaneous TWIP + TRIP although factors such as microstructure, SFE and austenite stability are likely to play a large role [20,33,34]. Furthermore, the majority of medium Mn steels shown to exhibit the TWIP + TRIP effect possess equiaxed microstructures, i.e.…”