The Structure and Solidification Behaviour of a Mould Slag Used in Continuous Casting of Steels

Abstract

Continuous casting is the dominant manufacturing method of steels, accounting for over 90% of global steel production. Mould slags, which are calcium silicate glasses, play a crucial role in this process due to their multifunctional purpose, directly impacting the safety, quality, and efficiency of steel production. As the steel industry transitions toward greener practices, new challenges arise, including the need for fluorine-free mould slags to meet increasingly stringent safety and environmental regulations. Historically, mould powder development has relied heavily on trial-and-error approaches, limiting the development rate of these materials for new demands. This thesis is part of a broader research effort aimed at understanding the structural transformations that drive the desired material properties of mould slags. In this study, various characterization techniques—such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-temperature Raman spectroscopy (RS)—were employed to investigate the structure of an industrial mould slag, with a focus on its heat transfer properties. Samples were melted and then solidified followed by three different isothermal holding temperatures to replicate the thermal cycling experienced during casting. The multi-method characterization revealed that the industrial mould slag (S1) consists of three distinct phases: cuspidine, combeite, and a glassy matrix. These phases were present regardless of the isothermal holding temperature. The microstructure after solidification featured large faceted cuspidine crystals embedded within a mixture of smaller dendritic combeite crystals and the glassy phase. High-temperature RS measurements showed that the formation kinetics of the S1 mould slag were faster than anticipated based on previous research. This highlights the necessity of multi-method characterization, and not to rely only on the chemical composition to fully understand mould slags and adapt them to future industry requirements. While there is room for improving the accuracy of the utilized technique, high temperature RS measurements have demonstrated their value in mould slag research and, with further refinement, hold the potential to unlock new research opportunities.