Cellular Automata Simulation Of Microstructure Evolution Of As-cast P91 Alloy Steel During Hot Deformation

Large diameter thick wall seamless steel pipe is widely used as a large-scale component in petrochemical,nuclear energy,national defense science and engineering and other major national projects.At the same time of P91 steel pipe becoming more and more domestic,the research team led by Professor Li Yongtang put forward a new casting extrusion composite forming technology,which takes hollow billet as blank and extrudes at high temperature.The key point of this process is to extrude the hollow billet material into thick wall tube at high temperature,so as to achieve the forged structure meeting the performance requirements.Therefore,in-depth study of the hot deformation behavior and microstructure evolution of as cast P91 alloy steel is of great significance for obtaining the hot extrusion deformation parameters and optimizing the hot working process.In this paper,the as cast P91 alloy steel is taken as the main research object,and the hot compression experiment is carried out with Gleeble-3500thermal simulation test machine under the deformation condition of 900??1250?,strain rate of 0.01s^-1?5s^-1.In order to better describe the rheological behavior of the alloy steel,the traditional hyperbolic sinusoidal constitutive model was modified.The function of material parameters and deformation activation energy on deformation parameters was determined by three-dimensional paraboloid surface fitting,and use R²=0.993 and AARE=3.64%to verify the accuracy of the model.The activation energy decreases with the increase of temperature and strain rate in the range of 82?814k J·mol^-1.Due to the dynamic recovery of steel,the activation energy decreases with the increase of strain rate below 1s^-1,and increases with the increase of strain rate above 1s^-1.According to the theory of thermal activation of metals and the mechanism of grain curvature,a cellular automata model of two-dimensional grain growth of P91 alloy steel with different state transition rules was established.The model was proved to be effective by simulating the shrinkage of circular grains.The normal growth process of austenite grains at different temperatures is analyzed.The initial austenite structure before dynamic recrystallization is obtained.The grain growth index obtained is closer to the theoretical value than the experimental value.The dynamic recrystallization behavior of as-cast P91 alloy steel during hot deformation was successfully predicted by substituting activation energy and initial grain structure into the dynamic recrystallization cellular automaton model,the results are compared with the experimental ones.The results show that the simulated stress-strain curves and microstructure have high accuracy,and the simulation error of grain size is within 10.6%.At low strain rate,the recrystallization process is relatively fast,and the recrystallization grains obtained are small;at high temperature,the incubation period of dynamic recrystallization is short and sufficient.At the same time,it is found that the final steady-state grain size of different initial grain size tends to be the same,and the flow stress curve will also show unimodal and periodic wave shape changes with the change of initial grain size.The topological deformation technology of grain is introduced,and the change of grain shape in the process of deformation is considered.The results show that the grain structure is compact and the morphology is true,and the dislocation density distribution under different strain conditions is calculated.