Analysis of Different Continuous Casting Practices Through Numerical Modelling

Abstract: Fluid flow accompanied by heat transfer, solidification and interrelated chemical reactions play a key role during Continuous Casting (CC) of steel. Generation of defects and production issues are a result of the interaction between mould flux, steel grade and casting conditions. These issues are detrimental to both productivity and quality. Thus, the development of reliable numerical models capable of simulating fluid flow coupled to heat transfer and solidification are in high demand to assure product quality and avoid defects. The present work investigates the influence of steel grade, mould powder and casting conditions on process stability by including heat and mass transfer through liquid steel, slag film layers and solidifying shell. The thesis addresses the application of a numerical model capable of coupling the fluid flow, heat transfer and solidification developed by Swerea MEFOS; based on the commercial CFD code FLUENT v12. The Volume of Fluid (VOF) method, which is an interface tracking technique, is coupled to the flow model for distinction of the interface between steel and slag. The current methodology not only allows the model to describe the behaviour of molten steel during solidification and casting but also makes the assessment of mould powders performance possible.  Direct prediction of lubrication efficiency, which is demonstrated by solid-liquid slag film thickness and powder consumption, is one of the most significant advantages of this model. This prediction is a direct result of the interaction between metal/slag flow, solidification and heat transfer under the influence of mould oscillation and transient conditions.  This study describes the implementation of the model to analyse several steel and mould powder combinations. This led to detection of a combination suffering from quality problems (High Carbon Steel + High Break Temperature Powder) and one, which provides the most stable casting conditions (Low Carbon Steel + Low Break Temperature Powder).  Results indicate the importance of steel pouring temperature, mould powder break temperature and also solidification range on the lubrication efficiency and shell formation. Simulations illustrate that Low Carbon Steel + Low Break Temperature Powder delivers the best lubrication efficiency and thickest formed shell. In contrast, High Carbon Steel + High Break Temperature Powder conveys the minimum lubrication efficiency. Therefore, it was concluded that due to absence of proper powder consumption and solidification rate the latter combination is susceptible to production defects such as stickers and breakouts during the casting sequence.