The authors present a model of steel under thermomechanical loading with diffusion phase transformations. To construct the model, a two-level approach based on the use of internal variables — the parameters characterizing the state and evolution of meso- and microstructure of the material — has been applied. Formulation of the general problem «split» into several subtasks (the problem of determining the stress-strain state and phase composition, heat conduction and diffusion problems). To describe the inelastic deformation of steels during thermomechanical loading asymmetric crystal plasticity model is used. Numerical experiments performed with developed model for simple loading at high temperature are described and their results are analyzed.