The Mathematic Models For Flow Stress And Kinetics For Proeutectoid Ferrite Formation Of Hot-deformed Austenite Of Structural Steels

With the development of modern deformation theory, the research on the hot-deformation of structural steels focus not only on the behavior of flow stress, but also on the microstructural evolution and the forecast and control of mechanical properties. During the hot-deformation process, steels bear a series of microstructural evolution, such as dynamic/static recovery or recrystallization, grain growth, phase transformation after deformation, and so on. Therefore, to find the quantitative relationship between the microstructural parameters and the macro-parameters during hot-deformation process of structural steels, i.e. to establish the mathematical model, is an important part to develop the modern plastic shaping techniques, and is very worthy for application. From the view of grain size, a parameter of Grain Distortion Degree (GDD) is brought forward creatively in this paper. And a mathematical model is established to describe the flow stress during the hot deformation of structural steels. Since this mathematical model is based on the microstructure, it can calculate the flow stress not only before, but also after the occurrence of dynamic recrystallization. The calculated results of several structural steels show that this model agrees well with the experimental data and indicates a higher accuracy than other models. Especially when dealing with the flow stress after the dynamic recrystallization, this model exhibits a distinctive accuracy. The existed macro-models lack physical basis and micro-models concerning the microstructural parameters are too complicated to be widely applied in practice, while the current model solves these problems. The current model can obtain smooth simulated flow stress curves without broken points even when the dividing point is chosen randomly near the critical strain, and thus expects a wider application in practice. The austenitic grain evolution of 35CrMo steel in the process of work hardening, dynamic recovery and dynamic recrystallization during hot deformation, as well as the influence of strain and holding time after deformation on the austenitic grain size, was investigated through physical simulation experiment using the Gleeble-1500 thermal simulator. The results show that during the whole process of hot deformation and the holding time after deformation, the shape of the austenitic grain almost remains unchanged, displaying a uniform size distribution without evident deformation. The austenitic grain size decreases with the strain either in the process of dynamic recovery or in dynamic recrystallization. And within 10s at high temperature after deformation, the austenitic grain size changes negligibly when the strain is small, while it increases sharply when the strain is bigger but the strain rate...