Effect of cold rolling and subsequent heat treatment on microstructural evolution and mechanical properties of Fe-Mn-Al-C-(Ni) based austenitic low-density steels

Thermo-mechanical processing in a two-phase domain of low-density steel (LDS) has the potential of enhancing strength-ductility combination. In the present study, the effects of alloy composition, cold rolling and heat treatment temperature (973–1173 K) on concurrent precipitation of κ-carbide/B2 particles and recrystallization and subsequently, mechanical properties include hardness variation and tensile response are evaluated. In order to understand the synergistic effect of Mn/Ni ratio towards selecting the two-phase processing window, three LDS alloy compositions, namely Fe–28Mn–9Al-0.9C, Fe–28Mn–9Al-0.9C–5Ni and Fe–15Mn–9Al-0.9C–5Ni (in weight percent) are vacuum induction melted, hot rolled and then cold rolled to 50% reduction in thickness at room temperature. The LDS alloys containing Ni showed much delayed recrystallization as compared to Ni-free LDS. The Mn/Ni ratio played a vital role towards controlling precipitation and recrystallization and accordingly, the mechanical properties. All the three alloys exhibited similar hardness, strength and ductility levels after 50% cold reduction. Upon heat treatment, there was precipitation of intergranular and intragranular B2 phase particles along with κ-carbides, whose fraction decreased with increasing the temperature. The tensile response of LDS alloys was influenced primarily by the volume fraction of these precipitates as well as the recrystallized austenite grains. Planar slip activity, dynamic slip band refinement in conjunction with dislocation-twin interaction mechanisms were noticed in fully recrystallized austenite and thereby leading to enhanced strain hardening and ductility. Furthermore, lowering Mn in Ni containing LDS reduced the processing temperature window of the alloy in two-phase region.