Model Study Of Annealing Process And Related Key Technologies With The Coupling Of FEM And MC Method

With the development of material science and computer technology, science of material preparation is changing from traditional design to reverse manner. By the aid of computer, material properties can be predicted based on its components and microstructure, and the optimum manufacturing techniques can be selected to derive some functional materials which can meet the needs of actual production. The microstructure of metallic materials is formed during the process of smelting, casting, forming and heat treating, in which forming and heat treating are critical to materials' microstructure and properties. However, the inhomogeneous pre-deformation induces the inhomogeneous distribution of the stored energy, which, as the driving force for recrystallization, leads to different recrystallization kinetics and different properties of the annealed materials.With the aid of finite element model (FEM), the process of plastic deformation of metallic materials can be simulated effectively, and the detailed information (such as the distribution of stress and strain) can be obtained. Meanwhile, by the simulation of annealing process of deformed materials, the information which is not easily observed in experiments can be reproduced quantitatively, continuously and dynamically. This method is helpful for better understanding of microstructure evolution and its inner mechanism, controlling the characteristic parameters of annealed microstructure and forecasting the properties of materials. In this way a solid foundation can be laid for optimizing annealing process and developing high performance materials. Therefore, the macroscopic and mesoscopic inhomogeneous distribution of stored energy, microstructure evolution and the corresponding recrystallization kinetics can be studied comprehensively and thoroughly with the coupling model of FEM and MC method, which is of great theoretical and practical value.MC method is a numerical calculation technique based on probability and statistics theories. Firstly, a probability model of special events associating to the investigated subject is established; secondly, a computer sampling plan is determined; finally, by means of the sampling plan, the occurrence frequencies of the events are derived as solutions of the subject investigated. In present thesis, Monte Carlo Potts model is used as the simulating tool to analyze complicated microstructure and to visualize the process of the emulation. A new coupling model of FEM with MC method, a new mesoscopic stored energy density distribution model and a new recovery model are constructed based on the experimental and theoretical researches. According to the new models and the corresponding key technologies, a computer program is compiled to simulate the isothermal annealing process of cold worked materials. Finally, the experimental results of extra-low carbon and high strength bake-hardening steel plate (ELC-BH steel plate) and industrial pure aluminum plate are used to certificate the rationality of the models.Firstly, the limitations of previous stored energy distribution models, especially the RSRP model, are well analyzed. According to the relationship among stress, dislocation density and stored energy density in poly-crystal undergoing plastic deformation, the coupling model of FEM with Monte Carlo method is built through the translating of the state variables between forming simulation and annealing simulation, the mapping of the data from FEM to MC nodes, the establishing of inhomogeneous stored energy distribution in mesoscale and the constructing of a appropriate nucleation model. Then the macroscopic and mesoscopic inhomogeneous stored energy distributions in cold working materials are derived. A simulated program of isothermal annealing process of cold working metallic materials is compiled based on the coupling model.Taken the cold rolled industrial pure aluminum plate as an example, the recrystallization process of both the maximum and the minimum stored energy regions of the blank are simulated with the simulated program, and the rational simulated results are derived: 1) The variation in recrystallization kinetics and microstructures in vary parts of the blank resulted from the difference in stored energy is simulated effectively. The region with higher stored energy, which is close to the surface of the blank, possesses faster recrystallization kinetics and smaller average recrystallized grain size. 2) The statistical results of recrystallization kinetics accord with the intuitive results of microstructure evolution in the simulation; meanwhile, the simulated results are consistent with the experimental and theoretical ones, which certificate the rationality of the new structured coupling model.Secondly, the mechanism of various mesoscopic structures' effect on the distribution of dislocations and stored energy density is deeply studied, and several models and hypotheses are constructed based on the experimental and theoretical researches. 1) Based on Kocks composite model, the problem of stored energy distribution among grains is then preliminarily solved by the foundation of a new model about grain size dependent stored energy distribution. 2) Starting from the action mechanism of grain boundaries on the dislocations, the changing rule of stored energy along with the distance to grain boundary is established based on Mughrabi's shearing stress distribution model. 3) A hypothesis is established considering the effect of various kinds of local defects (e.g. cell structure, twins, microbands or shearbands et al) on the dislocation distribution in different materials. Based on the models and the hypothesis above, a new mesoscopic stored energy distribution model is constructed.Taken the cold rolled pure ferrite steel plate as an example, the application of the mesoscopic stored energy distribution model in annealing simulation is deeply studied. 1) The recrystallization process is simulated under different stored energy distribution models. It can be seen from the simulated results, on the one hand, the new model can simulate the recrystallization process, especially the nucleation process, more efficiently because of the consideration of local defects inside grains on the condition of large deformation; on the other hand, the sigmoid relation of recrystallization volume fraction curve and the linear relation of its logarithm analysis curve are derived as expected. 2) With the new model, the effect of rolling reduction and annealing temperature on the microstructure evolution and the recrystallization kinetics are discussed. And the results are reasonable: with the increase of rolling reduction, the defects inside grains multiply rapidly, which leads to the transition from boundary nucleation to core nucleation and then the refinement of recrystallized grains; the annealing temperature can speed up recrystallization kinetics but has little effect on Avrami index and the size of recrystallized grains.Thirdly, the limitations of the previous nucleation model, which can not reflect the recovery process, are further studied. A new relationship between initial subgrain mean misorientation and strain is constructed based on experimental results. The reducing of stored energy during recovery is well considered, and a new viewpoint is proposed that the reduction of stored energy during recovery is the main driving force of the subgrain growth; based on the new viewpoint, a converse model from Monte Carlo step to real time during recovery is constructed. A new viewpoint is proposed that the critical size of recrystallization nucleus increases with the inducing of stored energy during recovery in subgrain abnormal growth nucleation model. Then, a recovery model is built on the basis of subgrain abnormal growth nucleation model and the modification mentioned above.Based on the recovery model and the recrystallization MC Potts model, a recovery-recrystallization model is constructed, with which, the annealing process of cold rolled pure ferrite steel plate is simulated efficiently, and the expectable simulated results are attained. 1) Recovery process is simulated successfully with the new model, which reflects the appropriate relationship between strain and initial subgrain mean misorientation; the reducing of stored energy slows down the recrystallization kinetics, which assures the simulated results accord with the experimental ones better. 2) The rise of the temperature results in the shortening of the incubation time; similarly, under a certain temperature, the increase of the rolling reduction leads to the shortening of the incubation time. The simulated results can be verified by the existing theoretical and experimental results.To certificate the rationality and the application effect of the model constructed in present thesis, annealing processes of ELC-BH steel plate and industrial pure aluminum (1060) plate are investigated in detailed. The comparison of experimental and simulated results indicates that: 1) the initial microstructure (especially with a large deformation), the nucleation sites, recrystallized microstructure and the subsequent grain growth microstructure can be simulated with the model constructed in present thesis; 2) the sigmoid relation of recrystallization volume fraction curve is derived, which is consistent with the experimental results; 3) the recovery process is simulated successfully, and the simulated influential rules of rolling reduction and annealing temperature on incubation time and recrystallization kinetics are consistent with the experimental ones: the increase of rolling reduction shortens incubation time and speeds up recrystallization velocity. The results mentioned above certificate the rationality of the coupling model of FEM and MC method, the mesoscopic stored energy density distribution model and the recovery model constructed in present thesis.