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 Ms. It has important implications for developing the new generation hot rolled high strength steels.