Development Of Microstructure Evolution And Property Prediction Model For Hot Rolled Strip And Preliminary Exploration Of Ultra Fast Cooling Process

The physical metallurgy model and ultra fast cooling (UFC) in hot strip rolling have been studied in this paper. The core computational models in the microstructure evolution and property prediction system for hot rolled steel strip were developed, modified and improved, thus the scope of predictable steel grades and processing conditions of the microstructure evolution and property prediction system was extended and the prediction precision was improved. The ferrite phase transformation and carbonitride precipitation in the UFC process were numerically simulated by the microstructure and property prediction system for hot rolled strip steel. And the thermo-mechanical coupled modeling on UFC was conducted by using ANSYS FEM software. Then influences of UFC on phase transformation and precipitation were investigated by experimental simulation in the laboratory and the pilot trials of hot rolled strip using UFC technology was implemented in the pilot plant. The numerical simulation and experimental simulation results provide the theoretical basis and experimental evidence for industrial application of UFC process.By establishing, modifying or improving some physical metallurgy models and anslysis methods, progresses were made in the microstructure and property prediction of hot rolled strip steel. On the basis of the suppose of minor force when deformated at melting temperature, a simple and practical way to correcte the errors in flow stress of single-hit hot compression test caused by thermal effect was proposed. On the basis of the evolution rule of deformation stored energy in thermal compression process, methods for calculating fraction and critical strain of dynamic recrystallization and softening fraction between passes and modeling flow stress from the measured stress-strain curves were achieved. According to large amounts of data of Ae3, a modified superelement model to calculate thermodynamics of phase transformation for Fe-ΣXi-C alloy systems were developed by the modification of Zener’s two-parameter for substitutional elements, and the interactions between alloying elements have been taken into account. Using the similar way to calculate unit interfacial energy of small-angle grain boundaries, the unit interfacial energy of austenite ferrite interface was computed and the classical nucleation equations of different nucleation sites and nucleation shapes were constructed. A method calculated precipitation kinetics and particle size distribution of microalloyed carbonitride in Fe-Nb-V-Ti-Al-C-N system was stated.A two-dimensional model of thermo-mechanical coupling simulation on UFC for hot rolled steel plate was established and the distribution of temperature and thermal stress in thickness direction was calculated by ANSYS FEM software. The effect of initial temperature, thinkness, carbon content and surface heat transfer coefficient on the plate temperature and thermal stress were analyzed. And the layout of inter-stand UFC device that can be used in CSP process was discussed.Through the numerical simulation of UFC process by the further improvement of microstructure and property prediction system and laboratory simulation and verification of UFC process by test, as well as the result analysis of microstructure and properties of hot rolled strip in the UFC condition, it shows that the cooling way of front UFC and subsequent slow cooling is helpful to enhance the amount of ferrite transition and carbonitride precipitation, and increase significantly the small size precipitation particles, but back UFC only has litte effect on precipitation behavior. It also indicates that when using high temperature rolling and subsequent alternating UFC process, refined ferrite grain can be got. It reduces the mill load and improves strip strength, especially tensile strength, and at the same time increases product of tensile strength and ductility, reduces yield ratio, and increases impact absorbed energy. Generally, the UFC process can help to realize "Resources Reducing" production of strip steel.