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The slag layer added on top of the molten steel plays an important role in the continuous casting process. It provides thermal insulation, prevents steel re-oxidation, controls the horizontal heat transfer and improves the cleanliness of the steel by entrapping inclusions. The powder added on top of the molten steel melts to form a sintered layer, eventually forming a liquid slag layer. The depth of the liquid slag layer influences the consumption of slag into the gap between the mold and the shell, its effectiveness at lubrication, and the extraction of detrimental inclusions from the molten steel into the slag [1].

The nail board test is an inexpensive technique used to measure the molten steel surface velocity and profile of the molten steel surface [2, 3]. The test is performed by inserting one or two rows of~ 10 steel nails in a long wooden board. The nails and wires are dipped straight down through the top surface powder layers into the molten steel. A lump of steel is formed due to solidification of the molten steel on the cold nail surface. The nail lump positions plotted together can be used to observe the steel meniscus shape. The nails are generally dipped for 3-4 s to prevent re-melting and solidification of the lump formed. The direction of the flow on the steel top surface can be determined by observing the high end of the lump formed as it represents the instantaneous direction from which the steel flow impinges on the nail. The height difference between the high and low ends of the lump can be used to measure the surface velocity [4].