Design of a low density Fe-Mn-Al-C steel with high strength-high ductility combination involving TRIP effect and dynamic carbon partitioning

Abstract A novel process involving hot rolling and air cooling followed by dynamic carbon partitioning is proposed to design a low density Fe-Mn-Al-C steel with high strength-high ductility combination. The low density alloy 0.25C-3Mn-2Al (wt%) steel was designed to control the phase transformation and achieve dynamic carbon partitioning, thereby obtaining bainite/martensite matrix embedded with nano-sized retained austenite (RA). The effect of different air-cooling finish temperatures on the microstructures and mechanical properties is elucidated in the study described here. Multi-phase microstructures of ferrite, martensite/bainite and RA were obtained during air-cooling in the temperature range of 360–510 °C. It was interesting that bainite matrix was obtained at finish temperature of 400 °C, while the martensite matrix including lath and twin martensite was obtained on air cooling temperature to 510 °C. The twin martensite resulted in higher tensile strength of ~1096 MPa in sample air cooled to 510 °C. The RA in samples subjected to dynamic partitioning was high, significantly approaching 27.3%. Additionally, RA was characterized into two types, film and blocky. A large amount of blocky RA in sample air cooled to 510 °C led to 21.4% transformed RA during uniform deformation. Consequently, excellent combination of high tensile strength of ~1096 MPa and uniform elongation of ~16% was attained in sample air cooled to 510 °C. The study simplifies the existing processes and breaks the constraint for quenching and partitioning treatment limited by quenching temperature below M s . It has important implications for developing the new generation hot rolled high strength steels.