5th International High Manganese Steel Conference - HMNS 2022

The intercritical annealing approach is widely used as a heat treatment for the formation of a dual-phase structure, composed of ferrite and austenite in medium-Mn alloys. By controlling the temperature, it is possible to control a fraction of both phases. Then, during deformation, the austenite undergoes martensite transformation, enhancing the plasticity of the steel. Depending on the mechanical stability of it, the transformation can occur at different stages of the deformation.
This work presents the effect of the double intercritical annealing of 5% medium-Mn steel on the grain size and mechanical stability of retained austenite. During the second intercritical annealing, the material was heated at a rate of 150°C/s to 850°C. This fast heating leads to formation of new, small austenite grains. The temperature was then lowered to 750°C, resulting in the formation of ferrite at austenite grain boundaries. As the newly formed austenite was small, the ferrite was also refined. Finally, the microstructure should be composed of large retained austenite (formed during the first intercritical annealing) and some fraction of small austenite and ferrite grains (formed during the second step). This could result in the continuous transformation of retained austenite into martensite during cold straining. In this way, it is possible to further enhance the plasticity of the steel during deformation.