Microstructural evolution of two low-carbon steels with a medium manganese content

In the present paper, the influence of the intercritical annealing temperature and cooling rate on the final microstructure of a 0.1C3.5Mn and 0.1C5Mn steel was elaborately investigated. Dilatometric experiments were carried out and additionally supported by microstructural observations, micro-hardness testing and X-ray diffraction. During soaking in the two-phase ferrite-austenite region and subsequent slow cooling the C and Mn concentration in the austenite increased and resulted in its chemical stabilization. The variation of the annealing temperature and cooling rate altered the amounts of ferrite, retained austenite, bainite and martensite in the steel microstructure. Furthermore, two thermodynamical models for the prediction of the maximum retained austenite content and the optimal annealing temperature have been thoroughly evaluated in this work. It can be stated that the comparison with the experimental data revealed a shift of the maximum retained austenite to higher annealing temperatures. This effect was even more pronounced as the cooling rate decreased, thereby showing the need for model adaptions.