Effect of intercritical annealing and subsequent ageing on the microstructure and mechanical properties of a medium Ni–Mn low carbon steel

The aim of this research was to develop a medium Ni–Mn dual phase steel with a combination of relatively high strength and good formability for industrial applications. Accordingly, this research was conducted to examine the effects of intercritical annealing and subsequent ageing treatments on the microstructure and mechanical properties of an Fe-9.5Ni-6.5Mn (wt.%) low carbon martensitic steel. Electron back scattering diffraction (EBSD) analysis revealed that intercritical annealing treatment at 610 °C in the ferrite + austenite (α+γ) dual phase region for various holding times led to formation of the reversed austenite phase in the martensite matrix, producing a medium Ni–Mn dual phase steel. It was resulted that the volume fraction of austenite and its grain size were significantly increased with increasing the holding time. Also, high resolution transmission electron microscope (HRTEM) images illustrated the formation of nano-sized θ-NiMn precipitates within the martensite phase after subsequent ageing at 450 °C for 7.2 ks. Intercritical annealing followed by ageing yielded a good combination of an ultimate tensile strength (∼1160 MPa) and fracture strain (∼11.5%) in the developed dual phase steel compared to the initial martensitic steel. The high strength and good ductility of the developed dual phase steel were respectively attributed to the presence of θ-NiMn precipitates with a hard character and reversely formed austenite with a ductile nature in the microstructure. Furthermore, the fully brittle fracture mode in the aged initial steel changed to a mixture of brittle and ductile behavior in the dual phase steel produced by intercritical annealing and subsequent ageing.