The following two-dimensional model describes the process of isothermal austenitization in the hypoeutectoid and eutectoid unalloyed steels. The initial lamellar structure of simulations is similar to the real structures. The interlamellar spacing – and at the same time the thickness of lamellas – can be changed arbitrarily and some lamellas can be cracked. The process of nucleus formation is described by a model of free enthalpy basis that makes a difference between the locations of nucleus formation in accordance with their free enthalpy. The nucleus growth is described by the numerical solution calculated by the Fick II. diffusion equation by using the Cellular Automaton method in the course of the simulations.