Shell Thinning Phenomena Affected by Heat Transfer, Nozzle Design and Flux Chemistry in Billets Moulds

Shell thinning affected by nozzle design, flux chemistry, heat transfer and steel flow was simulated through a mathematical model. Two nozzles were studied, the first, S60 with outer and inner diameters of 60 mm and 36 mm and the second, S73, with outer and inner diameters of 73 mm and 36 mm, respectively. Casting conditions include a casting speed of 1.3 m/min, use of a basic flux and a superheat of 36 K. Simulations included hypothetical isothermal casting conditions compared with casting conditions under thermal gradients. Simulation results indicated that the buoyancy forces, generated by thermal fields, exert a braking effect on the discharging jet whose magnitude was approximately 1/4 of the inertial forces. Shell growth along the curved mould walls suffers considerably thinning effects through the transport of sensible heat by convective mechanisms. Both nozzles induce shell thinning although, predictions of this mathematical model, using nozzle S73, indicate severe shell thinning effects in a region located in the inner radius side, down the second half of the mould length. This final shell thickness is very small making possible the existence of a strand breakout. Steel solidification along the flat mould walls leads to thick and uniform shells using either of these nozzles. The present numerical results indicate that in the field of flux design steel chemistry must be taken into account together with nozzle design.