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

Abstract

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