Modeling And Simulation Of Dynamic Recrystallization Behavior For Materials With Second Phase Particles Using Cellular Automata Method

Along with the development of materials science and computer simulation technology, materials fabrication science is changing from traditional design manner to its reverse one. By the aid of computer simulation, material properties can be predicted based on the components and microstructures, and the optimum manufacturing technology can be determined to produce materials which can satisfy the actual requirements.The second phase particles are the products to improve materials performance by adding alloy elements. With the application of alloying technology and controlled rolling and controlled cooling technology, alloyed and microalloyed steels with high performance are widely developed. The precipitation of second phase particles will occur during thermo-mechanical working of this kind of steels. The effects of these precipitated second phase particles on dynamic recrystallization behavior and corresponding microstructure evolution largely determine the microstructures and mechanical properties of the final product.Cellular automata (CA) model, which is of simplity, short run time and can directly investigate the local interaction and its influence, has been successfully used in the simulation of grain growth, static recrystallization and dynamic recrystallization in recent years. However, it is has not been applied in the simulation of dynamic recrystallization for hot deforming materials with second phase particles. Establishing dynamic recrystallization CA model for hot deformation materials with second phase particles can not only investigate the effects of deformation temperature, strain rate, initial grain size, heterogeneous deformation and the volume fraction and size of second phase particles on flow stress, recrystallization volume fraction and steady state mean grain size of dynamic recrstallization to understand and control the microstructure evolutions of hot deformation materials, in order to provide guidance for hot deformation technology optimization, but also extend the range of application for CA model. Therefore, it is of theoretical and practical significance to carry out the modeling and simulation research of dynamic recrystallization CA model for materials with second phase particles.In the present thesis, based on the CA model and metallurgical principles of hot deformation, dynamic recrystallization CA models with homogeneous distributed dislocation model and heterogeneous distributed dislocation model for single phase materials were established, respectively. On the basis of the two models, dynamic recrystallization CA model for materials with second phase particles was proposed. According to the CA models and the corresponding key technologies of dynamic recrystallization simulation, applications were written in the Visual Fortran6programming language. Related research on simulation and experiment of dynamic recrystallization were employed to verify the rationality of the CA models.Firstly, under the homogeneous deformation and homogeneous dislocation distribution hypothesis, based on the metallurgical principles of hot deformation, the dynamic recrystallization CA model with dependable physical mechanism for single phase materials was constructed and corresponding application was developed..Taking oxygen-free high-conductivity copper (OFHC) for example, the single peak/multiple peak phenomenon of stress-strain curves for dynamic recrystallization was successfully represented and explained.Secondly, taking consideration of the effects of grain size, grain boundary and local defect on dislocation movement and distribution, a new mesoscopic heterogeneous distributed dislocation model was established and dynamic recrystallization CA model with this heterogeneous distributed dislocation model for single phase materials was established. Compared with the homogeneous distributed dislocation model, the heterogeneous distributed dislocation model could simulate dynamic recrystallization more effectively. Then, the impact of deformation parameters on dynamic recrystallization of HPS485wf steel was simulated and discussed by using the heterogeneous distributed dislocation model.Then, according to the different acting mechanism of small and large second phase particles on dynamic recrystallization behavior, base on dynamic recrystallization CA model with heterogeneous distributed dislocation model for single phase materials, dynamic recrystallization CA model with heterogeneous distributed dislocation model for materials with second phase particles was constructed. In this model, the influence of second phase particles on dislocation accumulation, dynamic recrystallization nucleation and growth of recrystallized grain was considered. The simulated results are in good agreement with related theory and experiment results.To verify the rationality and application effect of dynamic recrystallization CA model for materials with second phase particles, experimental researches on hot compressive dynamic recrystallization behavior of Q420qE steel were carried out. Then, the CA model was applied to simulate the dynamic recrystallization of Q420qE steel. The comparison of simulated and experimental results indicates that the nucleation site, recrystallized microstructure and steady state microstructure can be accurately simulated by the CA model. Simulated dynamic recrystallization mechanics, kinetics rules and the effects of deformation parameters on dynamic recrystallization behavior are in agreement with related dynamic recrystallization theory and experimental results. Meanwhile, there is certain error between simulated and experimental dynamic recrystallization rate because of the hypothesis of CA model and the precision of experiment. To perfect the model further is helpful to improve its simulation accuracy and practical application.