The nitrogen alloyed ultralow carbon stainless steel is a good candidate material for primary loop pipes of AP1000 nuclear power plant.These pipes are manufactured by hot forging,during which dynamic recrystallization acts as the most important microstructural evolution mechanism.A physically based model was proposed to describe and predict the microstructural evolution in the hot forging process of those pipes.In this model,the coupled effects of dislocation density change,dynamic recovery,dynamic recrystallization and grain orientation function were considered.Besides,physically based simulation experiments were conducted on a Gleeble-3500 thermo-mechanical simulator,and the specimens after deformation were observed by optical metallography(OM)and electron back-scattered diffraction(EBSD)method.The results confirm that dynamic recrystallization is easy to occur with increasing deformation temperature or strain rate.The grains become much finer after full dynamic recrystallization.The model shows a good agreement with experimental results obtained by OM and EBSD in terms of stress-strain curves,grain size,and recrystallization kinetics.Besides,this model obtains an acceptable accuracy and a wide applying scope for engineering calculation. The nitrogen alloyed ultralow carbon stainless steel is a good candidate material for primary loop pipes of AP1000 nuclear power plant. These pipes are manufactured by hot forging, during dynamic recrystallization acts as the most important microstructural evolution mechanism. A physically based model was proposed to describe and predict the microstructural evolution in the hot forging process of those pipes. In this model, the coupled effects of dislocation density change, dynamic recovery, dynamic recrystallization and grain orientation function were considered.Besides, physically based simulation experiments were conducted on a Gleeble -3500 thermo-mechanical simulator, and the specimens after deformation were observed by optical metallography (OM) and electron back-scattered diffraction (EBSD) method. The results confirm that dynamic recrystallization is easy to occur with increasing deformation temperature or strain rate. grains become much finer after full dynamic recrystallization.The model shows a good agreement with experimental results obtained by OM and EBSD in terms of stress-strain curves, grain size, and recrystallization kinetics .esides. this model is an acceptable accuracy and a wide application scope for engineering calculations.