Analysis of the nail board experiment to measure the liquid slag depth in steel continuous casting
The nail board test is an inexpensive technique used to measure the molten steel surface velocity and profile of the molten steel surface in the continuous casting of steel. Aluminum wires are often added to measure the liquid slag layer depth as well. This work investigates the accuracy of the nail board test used for measurement of the liquid slag layer depth. In addition, a new methodology for measuring the liquid slag layer depth is proposed, using just the steel nails themselves without a second wire. This technique is based correlating the temperature associated with the oxide colors observed during tempering on the steel nail with their location along the nail. For this investigation, a transient thermo-fluid model of the nail and wire dipping process has been developed using ANSYS Fluent. In addition, the solidification of the molten steel on the nail is studied through analytical solutions and a computational model developed in ANSYS Fluent. The models are validated for many different trials using experimental data from nail and wire dipping tests in a typical caster. The model is then run for steel, aluminum and copper nails and wires taking the melting behavior into account and the expected difference between the measured and actual liquid slag layer thickness is evaluated. A parametric study to understand the effect of slag consumption, casting conditions and slag properties is also conducted to improve the proposed methodology for different process conditions. For a nail dipping time of 3 s, a reasonable estimate of the liquid slag layer depth is given by taking a fraction (~ 50 %) of the measured distance from the steel lump to the blue-purple color transition line (Liquid slag depth relative to the distance to the blue-purple line ratio). Increasing the dipping time increases the nail temperature and the distance to the transition, so the fraction of distance for the measurement decreases to ~40 % after 5s of dipping. Burning of carbon in the mold powder leads to a higher powder-slag temperature and a deeper liquid slag layer. Changing slag consumption has little effect, but changing the slag composition to increase the softening temperature causes less melting, a thinner liquid slag layer, and a larger fraction (~60%) of distance for the new measurement method. The aluminum wire method for liquid slag depth prediction is less reliable due to the variability of thermal properties in the melted region associated with flow of molten slag dissipating the molten metal. The copper wire method greatly under-predicts the liquid slag depth due to its high melting temperature.
Continuous casting, Nail board, Liquid slag layer measurement, Steel nail, Aluminum wire, Computational model
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign