ISIJ International, Vol. 46 (2006), No. 5, pp. 660–669

In continuous casting of steel, the interfacial gap between the mold and the solidifying shell controls heat transfer in the process and is responsible for lubrication and the prevention of cracks and other defects. Mold slag forms when mold powder added to the top surface of the steel melts, and is the best current practice to perform these important functions. The properties and behavior of the slag depend greatly on its microstructure, which ranges from crystalline to glassy. Thus, there is a great incentive to predict the slag microstructure, by quantitative understanding of the phenomena which occur in the interfacial gap. Figure 1 shows the typical structure of interfacial slag layers in samples obtained from the mold walls during operation. The slag layer adjacent to the cold mold wall cools and greatly increases in viscosity, thus acting as a re-solidified solid layer. Its thickness increases greatly just above the meniscus, where it is called the “slag rim”. Depending on its composition and cooling history, the microstructure of this layer could be glassy, crystalline or mixtures of both. The slag is consumed into the gap, and carried downwards in a highly transient and non-uniform manner. The solid layer often remains stuck to the mold wall, but it sometimes may be dragged intermittently downward at an average speed far less than the casting speed. However, the mechanism of slag layer flow, fracture, and attachment is not yet understood well. The behavior of the interfacial slag layers is revealed experimentally only through indirect means, including analysis of slag samples, monitoring of mold heat transfer, and characterization of the mold slag properties. Heat transfer is inferred from the heat-up of the mold cooling water and thermocouples measurements of the mold wall temperature together reveal the heat flux profile down the mold. Breakout shell thickness measurements reveal the corresponding growth profile of the solidified steel shell. The rate of consumption of the mold slag into the gap can be measured, and shows a decrease with increasing casting speed. Optimal design of the mold slag is needed to help avoid surface defects such as longitudinal, transverse and star cracks; enhance surface quality with the formation of uniform and shallow oscillation marks; prevent breakouts; and enable increased casting speed. Many properties of the mold slag must be measured in order to understand and quantify these phenomena, and improve casting quality. Traditionally, the mold slag properties are characterized by the temperature-dependent viscosity of the liquid slag, which has been quantified reliably as a function of powder composition. However, the powder melting rate, solidification temperature and crystallinity of the slag are more important for predicting lubrication, heat transfer, and steel quality. The microstructure of the slag layer can be determined by measuring its Time-TemperatureISIJ International, Vol. 46 (2006), No. 5, pp. 660–669