Thermodynamic-kinetic and dilatometric characterization of the influence of Mo and Cu additions on phase transition behavior in high-strength medium-Mn steels

Abstract Theoretical and experimental critical temperatures as well as phase transition phenomena during continuous cooling in 4 mass. % Mn advanced steels with and without Mo and Cu additions were investigated in the present study. The thermodynamic–kinetic calculations were performed using JMatPro software and phase transformations were recorded using a DIL805 dilatometer. The research covered slow austenitization and continuous cooling rates from 60 to 0.05 °C s −1 . The issues related to both modelling and dilatometric methodology were discussed. All analyzed steels exhibited very high hardenability as at a cooling rate of 1 °C s −1 all microstructures were martensitic. Only decreasing the cooling rate to 0.05 °C s −1 resulted in the formation of full bainitic microstructure in a base material and partly bainitic–martensitic microstructure in an alloy enriched by 0.3 mass.% Mo. The steel with the addition of 0.3 mass.% Mo and 1 mass.% Cu exhibited fully martensitic microstructure even after cooling at 0.05 °C s −1 indicating the strong effect of both Mo and Cu on steel’s hardenability increase. The obtained microstructures were very homogeneous and ultrafine.