The aim of the work was to determine the diagrams of phase evolution under equilibrium conditions and numerical simulation of austenite phase transformations under non-equilibrium conditions, as well as to determine CCT (Continuous Cooling Transformation) and TTT (Temperature Time Transformation) diagrams with the use of JMatPro software. The subject of the analysis were two newly elaborated multiphase steels assigned for production of forgings: steel A, containing of 0.165% C, 2% Mn, 1.11% Si and steel B, containing 0.175% C, 1,87% Mn, 1% Si, 0.22% Mo and Ti and V microadditions at a concentration of 0.031% and 0.022%, respectively. Performed simulation revealed that investigated steels have similar critical temperatures under equilibrium conditions: Ac1 ~ 680°C, Ac3 ~ 830°C. Chemical composition of steel B and the interaction of Mo, Ti and V in particular, determines that diffusion transformations, i.e. ferritic and pearlitic, in the elaborated CCT and TTT diagrams are significantly shifted to longer times in relation to the position of these transformations in the diagrams for steel A. A distinct delay concerns also bainitic transformation. Moreover, it was found that the MS temperature of steel B is slightly lower. Determined CCT and TTT diagrams are essentially helpful in the development of heat and thermo-mechanical treatment conditions for new steel grades.