The flow behaviour of material is strongly influenced by the microstructure evolution during hot deformation processes. In this work, a comprehensive mathematical modelling of heat transfer and plastic deformation was carried out employing finite element analysis based on rigidviscoplastic formulation. Semi-empirical models of dynamic recovery and recrystallization were utilized to develop the microstructure dependent constitutive equations. They were then integrated into the finite element code to simulate stress-strain curve of API-X70 steel during hot deformation process. Hot torsion tests were carried out at various deformation conditions for characterization of microstructure equations and model validation. The good agreement between experimental data and simulation results were achieved. The model predicts work hardening, dynamic recovery and recrystallization simultaneously and it considers their effects on the flow stress of the material during hot deformation.