A multiscale approach for fluid flow effect on microstructure and segregation

Maintaining competitiveness in steel manufacturing requires improving process efficiency and production volume whilst enhancing product quality and performance. This is particularly demanding for producing value-added advanced steel grades. In today’s world of high quality steels, cast in near net shape where the ability to control microstructure through thermo-mechanical processes is limited, understanding of the dependence of the solidification structure on the process parameters like fluid flow is of technical importance. Variations of phase evolution across different length scales during solidification resulting from a continuous casting process define the macrosegregation (at the scale of casting) and hence the final properties of the solid steel. Macro scale (100 to 10-3 m) fluid flow during continuous casting washes away the rejected solute ahead of the micro scale (10-6 to 10-5 m) solid/liquid interface giving rise to different undercooling levels at different positions of the moving solidification front. With the progress of solidification, the intensity of the washing effect will decrease and the influence of diffusion will come into play, thereby contributing to the macro scale solute profile. Understanding the competition between the crystallographic growth direction and solute transport with casting parameters during the progress of casting will provide an important perspective towards reducing the macrosegregation in the cast product. Stringent quality requirements for the present generation steel grades for automotive applications demand more information into the growing micro scale solute profile mechanism and how it relates to the phenomenon occurring at the macro scale. In order to address the translation of micro scale information into the macro scale, a combined theoretical and experimental approach had been undertaken. Starting with a single component system, open source phase-field method based solidification model coupled with fluid flow have been developed. Quantitative validation of the solidification model for single component system with experimental results in literature have been done. The developed micro scale model in presence of fluid flow gives an account of the preferred solid/liquid interface growth direction. At different degrees of undercooling, the model predicts the transient nature of the evolving solute profile. The effects of flow velocity and dendrite growth speed on the interface growth direction were separated. Improved theoretical formulations for estimation of the bending angle (defined as deviation from the original growth direction of primary dendrite in absence of fluid flow) were put forward which extends the current knowledge available from literature. On the experimental part, dendrite bending angle measurements were made in the industrial steel slab samples from a conventional slab caster at Tata Steel in IJmuiden, The Netherlands. The dendrites were found to undergo a change in the growth direction indicating the transition in the fluid flow profile occurring within the mould. Also, the magnitude of the bending angle was found to decrease away from the slab surface. Through the proposed approach of micro-macro coupling an attempt was made to correlate the macro scale fluid flow profile within the continuous casting mould with that of the developed micro scale bending angle formulation. The proposed formulation based on the anisotropy in solid/liquid interface energy was found to fit the experimental deflection angles better than the few available empirical correlations in literature. Line scanning measurements were performed to determine the composition profile of industrial slab samples proving the influence of fluid flow on macrosegregation