Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 1 Conventional Mould Powders

Abstract

The physical properties of mould slags are key to their performance in the continuous casting process. The magnitudes of key properties (viscosity, break temperature, f cryst and optical properties) are determined by the mould dimensions, casting conditions and the steel grade being cast. However, a range of other properties (e.g. interfacial tension, density) are needed to minimise defects and process problems. The extant data for thermo-physical properties of conventional mould slags are reviewed here and those for specialist powders (e.g. F-free or for casting TRIP steels) are reviewed in Part 2. It was concluded that there is a need for (i) resolution of the huge differences in thermal conductivity of mould slags for T > 1 050 K obtained with the LP and THW methods (ii) more data for some properties (e.g. C p and density) and more accuracy for others (viscosity, surface tension) (iii) standardised procedures for the determination of f cryst and (iv) characterisation of the porosity in slag films. It was also concluded that (i) gaseous convection makes a significant contribution to the heat transfer in the powder bed and (ii) glassy slag films are probably optically-thin.

Physical and Numerical Investigations of Mould Flux Entrainment into Liquid Steel

“…During the casting process, the mould powder is given on the surface of liquid steel, located in the mould working volume. As a result of contact with hot, liquid metal, layer of mould powder melts and forms a slag layer, wchich fulfills series of tasks, affecting the proper operation of mould and having a major impact on the semifinished steel product quality: prevents welding of continuous ingot to mould`s walls due to providing of lubrication, assimilates some of the nonmetallic inclusions, protects the liquid steel from oxidation as a result of contact with air atmosfere, controls the intensity of heat dissipation [1,2,3]. Hydrodynamic conditions in the mould have a significat impact, whether slag properly fulfills its role.…”

Key Lubrication Concepts to Understand the Role of Flow, Heat Transfer and Solidification for Modelling Defect Formation during Continuous Casting

“…A considerable amount of research has been focused on the properties of slags and the present manuscript is not an attempt to review such properties. Instead, the reader is referred to Mills' et al 29,30,[37][38][39] and Sohn's et al 40,41) work which provide further information on slag properties. However, the main role of these properties will be discussed below in relative terms to distinguish their influence on the casting process.…”

“…However, the conductivity of crystalline and partially crystalline slags is around double the value for the glassy phase due to increased long range ordering. 30) Figure 4 shows the typical behaviour of thermal conductivity as a function of the temperature and cooling/heating history. 43) Crystalline slags typically have higher thermal conductivities since conduction is dependent on the transport of phonons through lattice vibrations; 44) however, this is counter intuitive to current industrial practices where crystalline slags are used to decrease the overall heat transfer to the mould.…”

“…Typically, CC slag viscosities range from 0.5 dPa-s (high speed casting) to 30 dPa-s (billet casting). 30) "Process-wise", low-viscosity slags are able to infiltrate more easily into the shell-mould channel. However, this behaviour is strongly affected by the melting behaviour (i.e.…”

“…[25][26][27][28] Unfortunately, real slag behaviour is not so simple and important knowledge gaps persist in areas such as crystallization, radiation properties, wettability and lubrication extent in the mould. 29,30) Last but not least, CC deals with liquid-metal-flows that are inherently turbulent, since casting conditions are constantly changing. This is due to the fact that liquid metal available for casting relies on prior steelmaking processes, such as, ladle refining, degassing, alloying etc.…”