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

The intercritical annealing treatment at 650 and 700 degrees C results in two ultrafine-grained (UFG) dual-phase ferrite-austenite steels. The two steels exhibit different and special discontinuous yielding and pronounced Luders-like strain phenomena with large yielding strain which are related to their retained gamma-austenite (RA) volume fractions and RA stabilities. The steel annealed at 650 degrees C shows an absent or very small strain hardening, while the steel annealed at 700 degrees C shows an obvious strain hardening upward curvature with increasing strain. The results show that before and during straining, the steel annealed at 650 degrees C exhibits a mixture of equiaxed and elongated UFG alpha-ferrite and austenite phases; however, the steel annealed at 700 degrees C exhibits only elongated UFG alpha and gamma phases. It was found that most of the gamma-austenite to alpha '-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 (gamma-alpha ') 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 (sigma(b)) hardening effect. It is believed that the pronounced strain hardening behavior at the later deformation stage is mainly associated with sigma(b) enhancement induced by the strain partitioning between the soft and hard phases due to SIM transformation during tensile deformation.