CFD-Modeling of Inclusion Behavior During Ladle Exchange in a Continuous Casting System

The production efficiency in continuous slab casting strongly depends on the ability to cast large numbers of ladles in sequence without severe interruptions. The amount of steel necessary to continue the operation during ladle changes at least at a reduced casting speed is supplied by the tundish, which is usually a bath-tube like vessel. During ladle change, ladle and tundish slag is entrained in the steel, which leads to an increase of the concentration of nonmetallic inclusions in the slab. Slag particles with a diameter of 40 – 100 µm or even larger may not be tolerated in material requiring a high level of cleanness. Therefore, the part of a slab which has been produced close to a ladle change has to be downgraded and in order to avoid either waste of material or the risk of unapt quality, it is important to know exactly how large this region is. Consequently, a mathematical model is sought which describes the location of the affected region with sufficient precision. Thus, turbulent three-dimensional numerical calculations are carried out with the CFD code Fluent. A model of the whole tundish and mould configuration is used to simulate the steel flow behaviour. The whole liquid domain inside the strand is modelled. The shape of the solidification interface is calculated via the square root law for the thickness of the solidified casting strand. A multiphase model is implemented to calculate the particle behaviour inside the liquid steel respectively the particle removal at boundaries (tundish and mould slag as well as solidification in the mould). Both steady and unsteady simulations of the particle behaviour are performed. In the unsteady simulations, a particle concentration impulse is specified at the tundish inlet, and the resulting particle concentration is calculated as a function of the strand length in the completely solidified strand.