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 a-phase when annealing at lower temperatures below to 550 °C and at higher temperatures above to 700 °C. Whereas, UFG c-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 and700 °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) of1011 MPa, total elongation(TEL) of 51% and product of strength and elongation(PSE: UTS 9 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.

Tensile behavior of ultrafine-grained low carbon medium manganese steel by intercritical annealing treatment

The intercritical annealing treatment at 650 and 700 ℃ results in two ultrafine-grained (UFG) dual-phase ferrite-austenitesteels. The two steels exhibit different and special discontinuous yielding and pronounced Lüders-like strain phenomenawith large yielding strain which are related to their retained γ-austenite (RA) volume fractions and RA stabilities. The steelannealed at 650 ℃ shows an absent or very small strain hardening, while the steel annealed at 700 ℃ shows an obviousstrain hardening upward curvature with increasing strain. The results show that before and during straining, the steel annealedat 650 ℃ exhibits a mixture of equiaxed and elongated UFG α-ferrite and austenite phases;however, the steel annealed at700 ℃ exhibits only elongated UFG α and γ phases. It was found that most of the γ-austenite to α′-martensite transformationoccurred at the initial deformation stage and very small or almost no transformation occurred afterward. This demonstratesthat the strain-induced martensite (SIM) transformation (γ-α′) or transformation-induced plasticity (TRIP) effect dominatesonly at the initial deformation stage. RA remained stable, and no TRIP effect was observed at the final deformation stage. Theload-unload-reload test was performed to evaluate the back stress (σb) hardening effect. It is believed that the pronouncedstrain hardening behavior at the later deformation stage is mainly associated with σb enhancement induced by the strainpartitioning between the soft and hard phases due to SIM transformation during tensile deformation.