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The double hot thermocouple technique (DHTT) enables simulations of the temperature gradient at near-service conditions during continuous casting of steel. With the equipment applied so far, a rectangular slag film of even thickness often cannot be achieved. Further, the minimum temperature frequently lies within the slag film. Modified equipment can avoid these disadvantages via the following design features. The entire furnace chamber is heated to the selected temperature of the cold wire, and the minimum temperature is not located within the slag film. Furthermore, the shape of the heating wire is improved, which enables mounting of a thin, rectangular slag film between four platinum wires. This modification allows for investigations on transparent and translucent slags. So far, the results from DHTT investigations were represented via snapshots of the samples at certain experimental times. Therefore, appropriate methods for the graphical representation of the results were suggested: the maximum crystallinity, the time related to certain crystallinities with a dependence on the position within the slag film, and the crystal growth rate. The CaO-MgO-Al 2 O 3 -SiO 2 slag investigated with this equipment was mineralogically examined additionally, and based on thermodynamic calculations, the allocation of temperatures to certain positions within the crystallized slag film was possible.

Methods for the characterization of mould slag crystallization with special emphasis on the single/double hot thermocouple technique (SHTT/DHTT) are reviewed. In the continuous casting process of steels, horizontal heat transfer is mainly influenced by the crystallization behaviour of the mould flux film. Here, both precipitation of crystals out of a liquid phase and devitrification of the glassy film in contact with the mould are of main interest. Therefore, various investigation methods are implemented to characterize different slag properties related to crystallization: a viscometer for determining the break temperature, differential thermal analysis (DTA), confocal scanning laser microscopy, and the water-cooled copper finger test. For near-service conditions, DHTT reveals the most detailed information, including not only the crystallization or devitrification temperature but also the morphology as well as the crystallization velocity. Due to improvements in the device and the representation of the results, a comparison of different samples is possible. Nevertheless, the application field of SHTT/DHTT is restricted to slag systems with low contents of evaporating components. Furthermore, the time required for data analysis is significantly longer than that required for other methods, e.g. DTA. Therefore, the application of DHTT is mainly advisable for mould slag research and development, whereas DTA can also be used for incoming inspections. Keywords Single hot thermocouple technique Á Double hot thermocouple technique Á Mould slag Á Near-service condition Á In situ investigation Á Crystallization

Commercial mold powders use a limited number of main mineral constituents, but may differ significantly in chemical composition. The main mineral raw materials of specimens investigated here are quartz, fluorite and free carbon, as well as wollastonite and carbonates. The investigations revealed the use of secondary raw materials like blast furnace slag, fly ash, glass scrap and phosphorous slag as further components. Since the formation of cuspidine was one major point of interest, the influence of the silica source on its formation was identified. A replacement of wollastonite by blast furnace slag reduced the temperature of the first precipitation of cuspidine by about 100°C; the dissociation of sodium carbonate was lowered by * 40°C. The lowest temperature of the first Na 2 CO 3 dissociation could be achieved by using fluorine in combination with blast furnace slag. Cuspidine formation from the melt is further decreased if sodium and fluorine are both present. The use of glass scrap and phosphorous slag strongly reduced the temperature of first melt formation and enhanced cuspidine formation.

The continuous casting process of steels is mainly influenced by the infiltration of mould slag into the gap between the strand and the mould and the temperature-dependent solidification behaviour of the slag. Thus, measurements of the temperature-dependent viscosity and the determination of the break temperature, TBr, which is related to the formation of the first solids in the liquid slag, based on the recorded viscosity data are essential to characterizing mould slags. To enable automatic, person-independent data analysis method, a procedure must be developed that calculates TBr based on the log η versus 1/T representation. This aim was achieved by detecting the maximum absolute value of the second derivative of log η after regression and relating the associated temperature to TBr; comparisons between these values and the manually determined values showed good agreement. Furthermore, the error was assessed by statistical analysis and the effect of sampling frequency was analysed.