From thermodynamic points of view, the steels are Fe-Me-L-C based closed systems where Me means the metal elements (Cr, Mo, V, Mn, Ni,...) and L are the light elements (N, B,...). The steel phase microstructure depends on the heat treatment during production and exploitation. A major phase is solid solution and minor phases are different stoichiometric and/or non-stoichiometric phases (carbides, carbonitrides, nitrides, intermetallic phases). Thermodynamics deals with equilibrium states, which can be experimentally reached within reasonable annealing time for the steels only at temperatures above approximately 500stC. The equilibrated phases coexisting at given temperature, pressure, and composition can be found on phase diagram of the steel. Another way is to identify equilibrated phases experimentally after long time equilibrating. The carbon activity in steel at given temperature and pressure is very important value, which cannot be evaluate by single experimental way at present.