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Modeling of solidification microstructure to predict CET (columnar to equiaxed transition) in continuously cast steels

M. Uddagiri.

Master Thesis (2019)

Abstract
Based on Rappaz and Thevoz’s solute diffusion model for equiaxed dendritic growth [1,2] a new numerical model is developed to predict Columnar to Equiaxed Transition (CET) in continuously cast steel slabs. In addition to the solute diffusion, the model also takes the effects of melt convection into account. The model consists of solute and heat bal- ance modules, growth kinetics, nucleation, and a solute mixing module. Apart from CET, the model can also predict the thickness of chill zone that forms at the outer cool- ing surface. The model uses a critical undercooling to predict the boundaries between chill zone to columnar and columnar to equiaxed dendritic region. The model is applied to Fe-C binary system with the thermophysical properties obtained from literature and CALPHAD simulations. Finite difference scheme is employed to solve the temperature distribution. Using the nodal temperatures and equilibrium phase diagram, all relevant microscopic quantities are obtained by solving the corresponding equations in a staggered scheme. The model successfully predicts recalescence, phase fraction evolution, and con- centration profiles in different phases. When applied to temperature data obtained from macroscopic FEM simulation with realistic boundary conditions of the continuous casting process, the model is able to predict the CET and chill zone thickness. Finally, from the model predictions, it is established that by incorporating the solute mixing effects to the established solute distribution models and by tracking undercooling in the system ahead of solidification front, the CET and chill zone thickness can be predicted.

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