Micro-alloying Element Numerical Simulation Study, The Precipitate Dissolved In The Hot Rolling Process

The precipitation and solid solution model of micro-alloying elements during hot rolling process plays a very important role in the microstructure evolution prediction system for hot rolling. Solid solution and precipitation of micro-alloying elements happens during the whole procedure of reheating, rolling and cooling. The size and volume of precipitates has direct impacts on the microstructure and mechanical properties of hot rolled product. And precipitation also affects on other physical metallurgical phenomena. Therefore, it is of great significance to investigate the precipitation and solid solution model of micro-alloying elements during hot rolling process.The mathematical modeling for thermodynamics and kinetics of carbonitrides precipitation in the complex microalloyed steel has been investigated and the following works have been carried out:In MATLAB development environment, a mathematical thermodynamic model which simulates complex precipitation behavior quantitatively in austenite of Nb-V-Ti bearing high strength low alloy (HSLA) steel has been developed on the basis of Henry's law of dilute regular solution and thermodynamic models of Adrian and Rios. By using of the developed model, the precipitation start temperature for complex carbonitride considering AlN was predicted. And the estimation of the equilibrium compositions of the austenite and carbonitride, as well as the mole fractions of each phase in C-Nb-V-Ti-AL-N microalloyed steels at different temperatures and different chemical compositions was simulated and analyzed. Based on the precipitate nucleation and growth mechanism and the additivity rule for simulating continuous cooling process, a mathematical kinetic model which simulates the precipitation behavior of Nb-V-Ti bearing HSLA steel has been developed and kinetics parameters such as free energy of nucleation, nucleation rate, growth rate and volume fraction of precipitates have been calculated and analyzed.Thermo-Calc thermodynamics software has been used to calculate the thermodynamics parameters to verify the developed model. And the precipitation start temperature, compositions of carbonitrides and mole fraction of precipitates have been calculated. It indicates that the results of Thermo-Calc are in good agreement with that of the developed model. Supported by the project of "research of high-efficiency slab continuous casting technology" from the Project of Scientific and Technical Supporting Program of China during the 11th-Five-Year Plan, hot delivery, hot charging and reheating of a Ti microalloyed steel slab at different delivery temperatures have been simulated in laboratory. The microstructure morphologies of the onsite slab and intermediate slab, as well as that of the specimen of laboratory experimental slab at different thermal histories have been observed by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The shape, size, morphology and distribution of precipitates have been analyzed to verify the calculation results of the developed model. And the reason of the slab surface transverse cracking during rough rolling after hot delivery and hot charging has been analyzed. The results show that the allotriomorphic ferrite at the original austenite grain boundary and the precipitation of carbonitrides Ti(C,N) and AlN along the original austenite grain boundary in the slab surface region, which can decrease the high temperature ductility at grain boundaries, might be the main causes for the slab surface cracking during rough rolling after hot delivery and hot charging.