Thermodynamic prediction and experimental verification of phase transformation kinetics in 3Mn steel with Ti and V microadditions

Abstract In this work, the phase transformation kinetics and precipitation processes were studied using thermodynamic calculations and a dilatometric method to determine the optimal chemical composition of medium-Mn steel and its initial parameters of heat treatment. The investigated steel is intended for forgings with the microstructure composed of bainitic matrix and retained austenite (RA). An influence of Al and Si on the M s temperature was characterized to obtain the best chemical composition in terms of RA stabilization. Theoretical continuous cooling transformation (CCT) and time–temperature transformation (TTT) diagrams were determined and compared with dilatometric results. Microstructural observations were compared with the dilatometric results. The hardenability of steel was high due to increased Mn and Mo additions. A very small fraction of cementite is expected in the microstructure due to Al and Si additions. The shortest time to start and finish the bainitic transformation was noted for the isothermal heat treatment at 420 °C. The completion of the bainitic transformation took about 25 min and is acceptable from the industrial point of view. The obtained results constitute a good basis for designing thermomechanical processing routes of bainitic steels with RA.