The phase transformations occurring during continuous cooling in Nb-V microalloyed steels have been analyzed in depth on this thesis. In particular, three compositions of niobium microalloyed steels and three steels microalloyed with vanadium have been characterized. The work is divided into six chapters and begins with a brief introduction which aims to value the importance of the steel in the modern world. Then, on the second chapter, the most important metallurgical concepts related especially to low carbon steels in the literature have been reviewed and summarized. Chemical compositions and experimental techniques carried out are detailed on the third chapter. Experimental techniques have consisted mainly in two types of tests (dilatometry and multipass torsion tests). The data obtained from the continuous cooling tests have been used to develop two mathematical models. These models aim to summarize the behavior of the analyzed steels according to three leading variables: the austenite grain size, accumulated deformation and cooling rate. At the fourth chapter, the results obtained and the principles governing the two developed models have been described. This chapter is divided into four blocks, the first one consists on a summary of the microstructural characterization performed and continuous cooling diagrams (CCT diagrams) obtained by dilatometry tests. These tests were carried out with the three Nb microalloyed steels and C-Mn-V1. In the second block of results, a predictive model of the austenite transformation kinetics is developed. The microstructures obtained are composed of one or more phases (ferrite, pearlite and bainite) upon cooling from different austenite microstructures (recrystallized and deformed). On the third block the study has been focused on the austenite-ferrite transformation (low cooling rates). Besides the improvement in predicting the mean ferrite size, a model that takes into account the heterogeneity of the austenite grain size distributions before the phase transformation has been developed. The effect of heterogeneity in the austenite-ferrite transformation has been studied in terms of different variables (Effective Austenite grain size and cooling rate). The objective of the model is to predict the grain size distribution of transformed ferrite from different austenite conditions. Finally, a study of the hot rolling process on different industrial profiles is described with two steels similar to C-Mn-V1 composition. The effect of varying temperatures and strain distribution has been studied by hot torsion multi-pass testing. Then, after quantifying the microstructural changes associated to these changes in the rolling process, the austenite-ferrite transformation model has been validated again. These microstructures correspond to a similar range of cooling rates compared to the microstructures obtained by dilatometry trials, which have been adjusted the model parameters. In the fifth chapter of this work the conclusions and future lines of work have been presented. Finally, the three publications developed during the course of this thesis have been attached. Specifically, an international journal publication (ISIJ International, Vol 55, n.9, 2015), an international conference proceeding contribution (British Columbia, Canada, June 2015) and a contribution to the XIII National Materials Congress (Barcelona, June 2014).