The most important factors affecting the dispersed-phase transformation toughening in steels are the stability of the dspersed austenhe and the transformation volume change. Both depend on the composition of the steel. The THERMOCALC computer program was used to describe the effect of composition on austenite stability while a formulation based on the two-gamma states model allowed the description of the compositional dependence of the lattice parameters taking into account magneto-volume effects in the austenite.