Dynamic Restoration Activities in Austenite Matrix and Secondary B2 Phase During Hot Deformation of Austenitic Low-Density Steels

In present study two different austenitic based low-density steel (LDS) grades with compositions of Fe-28Mn-9Al-0.9C and Fe-15Mn-9Al-0.9C-5Ni (in weight percent) were produced by vacuum induction melting. The former alloy was a single-phase austenitic grade, whereas the latter alloy contained coarse B2 ordered Fe(Ni)Al secondary phase along with fine κ-carbides in austenite matrix. Hot deformation behaviour of these two LDS grades was studied in as-cast plus homogenised condition under compression over a range of temperature (1173-1423 K) and strain rate (0.005-5 s-1). The stress-strain curves were analysed to evaluate the role of secondary B2 phase on the stress exponent and activation energy of deformation. The evolution of microstructures in these steels as a function of test temperature, strain rate and strain was examined in-detail by electron backscattered diffraction. The low-density steels exhibited nearly similar stress exponent value of ~4; however, the presence of B2 phase led to an increased activation energy of deformation. The effect of B2 phase on flow behaviour and microstructure evolution during hot deformation was found to be more pronounced at low test temperatures. The B2 containing steel showed slower recrystallization kinetics than that of B2-free alloy due to the strain rate and temperature dependent strain partitioning between γ and B2 phases. The examination of microstructural evolution with progress of deformation revealed that the austenite (γ) matrix grains undergo discontinuous dynamic recrystallization, while B2 grains display continuous dynamic recrystallization along with fragmentation.