Influence of differential roll rotation speed on evolution of internal porosity in continuous casting bloom during heavy reduction

Heavy reduction (HR), during which a large reduction deformation is implemented around the strand solidification end during continuous casting, is a novel technology that could effectively improve the internal quality of large section continuous casting steel. In the present work, a 3 dimensional (3D) thermal-mechanical coupled model with two pairs of rolls was developed. By setting different rotation speed of the front and the reduction rolls, the effect of differential roll rotation speed on the evolution of internal porosity in continuous casting bloom during HR was investigated. Initial temperature distribution of the 3D thermal-mechanical coupled model was determined by the predicted heat transfer results of a 2 dimensional (2D) heat transfer model. An Arrhenius-type constitutive equation, derived based on the measured true stress-strain curves under different temperatures and strain rates, was adopted in the 3D thermal-mechanical coupled model to describe the metal flow behavior of the casting strand during HR. Accuracy of the 3D thermal-mechanical coupled model was verified by the feedback pressure of the hydraulic cylinders during plant trial of HR. The simulation results indicate that a relatively larger rotation speed of the front rolls (V-f) than that of the reduction rolls (V-r) could effectively suppress the porosity elongation along the casting direction and the bloom width direction, and benefit the decrease of porosity dimension along the bloom thickness direction. As a result, the effect of HR on improving the bloom internal porosity could be effectively promoted. With HR amount of the reduction roll (R-r) decreased or the small reduction amount of the front rolls (r(f)) increased, the improvement effect of differential roll rotation speed on HR becomes more significant. With roll rotation speed ratio of R-v (R-v = V-f / V-r) increased from 1.0 to 2.0, efficiency of differential roll rotation speed on promoting the process effect of HR continuously decreased.