Superior strength-ductility combination in a Ni-modified medium manganese lightweight steel with the determining role of phase transformation and recrystallization behaviour

The present work elucidates the complex interplay between the changes in the microstructural features and tensile properties in a Ni-modified medium manganese steel under the combined influence of cold rolling (CR) and annealing treatment. The annealing temperature within 973 K–1273 K showed a wide range of microstructures, varying from single phase ferrite in CR-973 K to duplex microstructure (ferrite and austenite) in CR-1073 K and to multiphase structure (ferrite, austenite and martensite) in CR-1173 K and CR-1273 K annealed conditions. Additionally, B2 (NiAl) and intermetallic carbide precipitates were also found to be heterogeneously distributed in ferrite matrix of the CR-973 K annealed specimen. However, these precipitates dissolved and concurrently resulted in evolution of the austenite phase at an annealing temperature of ≥1073 K. Apart from the phase transformation, the recrystallization fraction increased and grain size became coarser at higher annealing temperature. The presence of duplex phase microstructure, optimum grain size of austenite and ferrite, and co-occurrence of recrystallized and deformed grains resulted in excellent strength-elongation synergy in CR-1073 K annealed specimen in comparison to the other annealing variants. More importantly, the austenite to martensite transformation during tensile test promoted the transformation induced plasticity (TRIP) effect, which improved the work hardening and ductility of the CR-1073 K annealed specimen. In contrast, the presence of pre-existing martensite in CR-1173 K and CR-1273 K annealed specimens adequately masked the TRIP effect leading to predominant brittle failure in these specimens without rendering any considerable ductility.