The capabilities of the numerical simulation of technological processes are limited by the accuracy and efficiency of determining the properties of materials continuously changing under repeated heating and cooling. The parameters of structural transformations are the principal factors affecting the properties of alloyed steels. In this paper, we present a method for determining the parameters of relationships describing C-shaped curves in the experimental diagrams of isothermal decomposition of austenite. The proposed approach makes it possible to reconstruct the entire C-shaped curve using a relatively small fragment near the “nose” (based on three points). The joint processing of a series of curves provides determining the parameters of ferritic, pearlitic and bainitic transformation kinetics. However, one should take into account the distinctive features of the diffusion decomposition of austenite. For example, ferrite and pearlite are formed in overlapping temperature ranges and have similar mechanical properties, but their combining into a single ferrite-pearlitic structure complicates the construction of a mathematical model for the transformation. The bainitic transformation is a transient one between diffusion and diffusionless transformations. In a part of the transformation temperature range the limit of conversion level is a function of temperature (just as in the case of martensitic transformation). It has been shown that, for the case of ferrite-pearlitic transformation, the best results can be obtained with the use of Kolmogorov–Avrami equation, whereas for the case of bainitic transformation, the best results can be obtained with the use of Austin–Rickett equation modified to take into account an incomplete conversion level of the transformation.