A metallurgical model to describe the formation of ferrite, perlite and bainite during continuous cooling was developed. The model uses classical nucleation and early growth theories to calculate the beginning of the reaction, and Avrami-type equations to evaluate the transformation progress. The materials studied were plain carbon steels with different Mn contents, and microalloyed steels with Nb, Ti and/or V additions. The model predictions were adjusted and validated with experimental results obtained from dilatometric tests. The range of cooling rates considered was between 5 degrees C/sec and 50 degrees C/sec. From the comparison with the experimental data it was found that the model correctly describes the dependence of the reaction kinetic on the cooling rate, as well as on the steel chemistry.