Effect of Annealing on Microstructure and Mechanical Properties of Ultrafine-Grained Low-Carbon Medium-Manganese Steel Produced by Heavy Warm Rolling

An ultrafine-grained (UFG) low-carbon medium-manganese steel was fabricated by the heavily warm rolling (HWR) and subsequent quenching, and the effects of annealing temperatures on microstructure and mechanical properties of the UFG HWRed steel were investigated. The results show that the HWRed steel exhibits simultaneous improvements in strength, uniform elongation and work hardening, which is mainly attributed to the refinement of martensitic microstructures. The HWRed steels comprise only α-phase when annealing at lower temperatures below to 550 °C and at higher temperatures above to 700 °C. Whereas, UFG γ-austenite is formed by reverse transformation when the HWRed steel was annealed at intermediate temperatures from 550 to 700 °C and the volume fraction increases with increasing annealing temperatures, consequently resulting in a dramatic increase in ductility of the annealed HWRed steels. It was found that the transformed UFG austenite and ferrite remained ~ 500 nm and ~ 800 nm in size when the HWRed steel was annealed at 650 and 700 °C for 1 h, respectively, showing an excellent thermal stability. Moreover, the HWRed steel annealed at 650 °C exhibits high strength-ductility combinations with a yield strength of 906 MPa, ultimate tensile strength (UTS) of 1011 MPa, total elongation (TEL) of 51% and product of strength and elongation (PSE: UTS × TEL) of 52 GPa%. It is believed that these excellent comprehensive mechanical properties are closely associated with the UFG austenite formation by reverse transformation and principally attributed to the transformation-induced plasticity (TRIP) effect.