## 1 Introduction  
Medium-Mn bainitic steels with retained austenite belong to a group of steels dedicated to the automotive industry due to a great combination of mechanical and technological properties. These steels can be produced as cold-rolled--when the required microstructure is obtained during heat treatment after cold rolling--or they can be obtained as thermomechanically rolled [1]. A higher amount of retained austenite can be achieved for hot-rolled steels; however, the optimal ductility of steel sheets is not always obtained for the steels containing the highest fraction of \(\gamma\) phase. The critical factor is the optimal kinetics of strain-induced martensitic transformation related to the mechanical stability of retained austenite [2]. Medium-Mn sheet steels offer the best combination of strength and ductility at reasonable cost. They are much cheaper compared to fully austenitic high-Mn steels. The intermediate Mn content allows to stabilize a fraction of retained austenite between the high-Mn steels (2 nd generation Advanced High-Strength Steels) and the low-Mn automotive multiphase steels (1 st generation Advanced High-Strength Steels). Moreover, the mixture of ultra-fine ferrite and austenite or carbide-free bainite-austenite enables us to reduce the hardness difference between microstructural constituents compared to a mixture of ferrite, bainite, and retained austenite [2; 3]. It results in better edge formability, stretch flangeability, and mechanical properties of medium-Mn sheet steels. In order to provide the highest number of mechanical properties of steel, it is crucial to select proper conditions of bainitic transformation, which ensure obtaining the optimal amount and stability of the retained austenite. The conditions (time and temperature) of isothermal holding in a bainitic transformation region have a significant effect on the stability of this structural constituent. If the duration of isothermal holding is too short, the retained austenite is characterized by low stability due to the small carbon content. This leads to an increase in the temperature, at which point martensitic transformation begins (M\({}_{\mathrm{s}}\) temperature). Conversely, if the time of isothermal holding is too long, carbon is trapped in carbides, which also leads to a negative increase in M\({}_{\mathrm{s}}\)[3; 4; 5]. Incorrect time and temperature of isothermal holding during bainitic transformation result in obtaining some fraction of martensite, usually as martensite-austenite (MA) constituents. [PERSON] et al. [6] reported that the temperature of isothermal holding during bainitic transformation for C-Mn-Si steels strongly affects the amount of retained austenite and carbon concentration in this phase. They found that the maximum amount of RA was obtained at \(\sim\)425 \({}^{\circ}\)C, while the maximum C content in RA was detected at \(\sim\)375 \({}^{\circ}\)C for the steels containing 0.1-0.2 wt. % C. [PERSON] et al. [7] noted that the carbon content in retained austenite depends on the chemical composition of steel, that is, silicon and aluminum contents. Our previous research [8] regarding the effect of bainitic transformation temperature on the thermodynamic stability of retained austenite was carried out using a medium-C bainitic steel containing 1.5% Mn. Currently, there is a large amount of interest in bainitic steels with manganese content from 3 to 12%. The increased Mn amount enables to obtain a high fraction of retained austenite (\(\sim\)10-30%) [9; 10]. In order to improve the mechanical properties of steels with a TRIP (TRansformation Induced Plasticity) effect, microadditions of Nb and Ti can be also added [11]. The influence of bainitic transformation conditions on the microstructure consisting of ferrite, bainite, and retained austenite have been analyzed by many authors [12; 13; 14]. [PERSON] et al. [15; 16] analyzed the factors affecting the stability of retained austenite in nanostructured bainitic steels.