Microstructural evolution and tensile properties of 70 GPa.% grade strong and ductile hot-rolled 6Mn steel treated by intercritical annealing

An ultrahigh tensile strength (over 1000 MPa) and enhanced elongation (65%) of hot-rolled 6Mn steel is designed through controlling the combination of fraction and stability of austenite by intercritical annealing (IA). During IA, austenite and ferrite duplex phases with plate-like and equiaxed morphologies are formed between initial martensite lath and junction. With the increase of IA temperature, grain size enlarges, plate-like grains transform into equiaxed and austenite fraction increases, which all reduce the stability of austenite. The austenite stability determines the three types of work hardening behavior. The increasing fraction and decreasing stability of austenite corporately affect the mechanical properties, so the product of strength and elongation increases firstly and then decreases as IA temperature increases. Austenite with high fraction (65 vol%) is obtained in sample after IA at 700 degrees C and efficiently transforms into martensite (40%) from 0.035 to fracture. The gradient of austenite stability inside IA700 leads to the discontinuous TRIP effect and work hardening rate (WHR) curve with both monotical decrease and fluctuation. Moreover, ferrite slip and cooperation of transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) in austenite act together, producing 70 GPa.% grade 6Mn (medium-Mn) steel through facile hot rolling (HT) and IA process.