Introduction
During continuous casting of steel, surface quality requirements of the produced slabs increase, whereas the mold slags have a tremendous impact on the surface quality. For a control of the melting behavior, mold powders contain free carbon; however, a carburization of certain steel grades is not desired. Furthermore, mold powders can contain fluorine for viscosity adjustment, and the fluorine can induce corrosion of the casting plant or the slag band. Beside this, occurring redox-reactions lead to a changed mold slag composition. This results in a massive change of the slag viscosity and the solidification behavior.
The slag viscosity is important not only for the continuous casting, but also for other metallurgical processes. Increasing CPU performance capacities enable augmented process simulation possibilities. The knowledge of slag viscosity is relevant for both, the current operation as well as for process prediction. Since viscosity measurement is time and cost intensive, it is necessary to develop viscosity models for large ranges of validity.
The planned methodology of the current project consists of theoretical studies, thermochemical model calculations and experimental activities in lab and pilot scale. Since 2004, the scientific partner of the project, Chair of Ceramics from Montanuniversität Leoben, is active in the research on mold powders and mold slags. Therefore, several evaluation methods exist to develop improved mold slags such as Double and Single Hot Thermocouple Technique (DHTT and SHTT) to characterize the crystallization behavior as well as a high temperature rheometer to measure slag viscosity (see figures).
Potentially suitable slag compositions will be tested in a pilot-scale continuous casting plant of our industrial project partner and the obtained results should help to further optimize the slag compositions. In addition to this, dissolution tests of certain particles in the mold slag are planned by using High Temperature Laser Scanning Confocal Microscopy (HT-LSCM, Chair of Ferrous Metallurgy of Montanuniversität Leoben) to estimate the corrosion behavior of the slag band.
Regarding the development of a mold powder without free carbon, research work will focus on a raw material concept using different oxides and the target will also be to quantify suitable components as substitutes for the free carbon. The corresponding methodology contains stepwise annealing, simultaneous thermal analysis and heating microscopy.
Viscosity measurements will be done with a high temperature rheometer. Synthetic slags will be produced with varying composition to systematically quantify the influence of the different components on the final slag viscosity. Finally, viscosity models should be generated based on the results from the rheometer measurements.
Results and application
Optimized mold slag compositions are expected for the continuous casting process to produce certain steel qualities. The viscosity of the mold slags should stay within a certain range, and beside this, requirements on the solidification behavior will be given. Based on the pilot plant tests, knowledge should be generated regarding slag film generation, slag behavior during operation and regarding the heat transfer.
For viscosity modelling, an evaluation is expected on viscosity calculation models being available in literature. Based on that, a model should be developed, which enables the viscosity calculation for different slag systems. Beside this, single sets of slag viscosity values are expected. Furthermore, a full set of data for one slag type should be available being used to generate a viscosity model.