Microstructural Evolution During Hot Deformation Of316LN Austenitic Stainless Steel For AP1000Nuclear Primary Pipe

316LN nitrogen alloyed ultralow carbon stainless steel is a good candidate material for primary loop-pipes for AP1000nuclear power plant, due to its excellent manufacturing performance and good corrosion resistance. However, the manufacturing process of such pipes has been changed from sequence welding of castings to integral forging, which leads to the great difficulties. As ultra-large-scale forging, the controlling of both shapes and microstructures of the pipes is the ultimate goal during manufacturing. Thus, it is of great significance to comprehensively and deeply study the microstructure evolution of316LN steel based on experiments and simulation, and to understand the relationship between manufacturing parameters and microstructures. This work is valuable for predicting the microstructure evolution of studied steel during actual production, and providing scientific basis for optimization of manufacturing parameters.In the present work, both physically-based simulation experiments and multi-scale modelling have been employed to conduct investigations on the microstructure evolution of the316LN stainless steel.Single pass axial hot deformation experiments were conducted on a Gleeble-3500thermal-mechanical simulator in order to simulate the hot forging process of the studied steel. Based on the experimental results, with the introduction of Zener-Hollomon parameter, the Arrhenius-type constitutive model was established, which can be used to predict the true strain-true stress relationship of the studied steel during hot deformation. Also, the empirical formula of grain size evolution was developed to predict the grain sizes of the studied steel under certain deformation conditions.According to microstructure analysis of the deformed and quenched specimens, at the last stepof the hot forging process, which is the most important step of all, the reduction should be as large as possible, to obtain fully completed fine dynamic recrystallization (DRX) microstructure. The suggested optimized forging temperature is1273-1423K, and the strain rate is0.1s-1. In this case, the completed DRX grain size of the studied steel is15～20μm, which is far lower than the engineering requirement of ASTM Level-2. Besides, the interaction of work hardening, dynamic recovery and dynamic recrystallization was studied, along with the DRX nucleation mechanism. This provided basis for physical metallurgical modelling.Unfortunately, the Arrhenius-type constitutive model is a kind of empirical model, and its applied scope is consequently narrow. In order to predict the microstructure evolution under a relatively wider deformation conditions, a physical metallurgy based model was established, which included the DRX model composed of the effects of chemical composition, work hardening and microstructure evolution. This model consists of modules of dislocation density, nucleation, DRX and precipitation, and also input module and material constants module etc. The flow behavior and DRX behavior of the studied steel under different deformation conditions has been predicted. It shows a good agreement between predicted and experimental results, which indicates that the established model is capable for predicting microstructure evolution of316LN steel.In addition, the application scope of the physical metallurgy based model has been broaden to Nb microalloyed steel, in which carbonitrides play an important role. The DRX behavior has been successfully predicted with different compositionand deformation processes. The results show that the prediction has a good accuracy, which indicates that the established physical metallurgy based model could be applied broadly for different kinds of steels.