UBC Theses and Dissertations
Mould thermal response, billet surface quality and mould-flux behaviour in the continuous casting of steel billets with powder lubrication Pinheiro, Carlos A. M.
The main objectives of this study were to examine mould thermal response and billet surface quality during continuous casting of steel billets with powder lubrication, and to compare with oil lubrication. Measurements were carried out on an operating billet caster to determine mould-wall temperature profiles for different mould-flux types, mould coolingwater velocity, oscillation frequency and steel grade. The trial involved data acquisition on mould displacement, casting speed, metal level and mould powder temperature field. In addition, mould powder consumption and liquid flux pool depth were also measured. An inverse heat conduction model was developed to determine mould heat flux from measured mould wall temperatures. Existing mathematical models were utilized to investigate mould/billet binding and mould taper. Results from plant measurement, mathematical models and billet sample evaluation were used to compare mould-powder and oil casting in terms of mould thermal response, transverse depression, rhomboidity, oscillation mark and mould level variation. Finally, a mathematical model was developed to analyze the influence of mould-flux properties and feeding strategies on melting behaviour. This work has led to a very comprehensive understanding of mould thermal response and mould-related quality problem in billet casting with powder lubrication. Transverse depressions were found to be formed in steel grades with high coherency temperature due to metal level fluctuation. For Boron(Ti)-alloyed medium-carbon steel cast with powder transverse depressions were eliminated due to a substantial decrease in meniscus heat flux, thus producing a thinner, hotter, more flexible shell, and also due to lower metal level fluctuations on account of pouring with SEN. An understanding of the role of titanium and nitrogen on transverse depressions resulted in the establishment of maximum values for these elements. In order to minimize transverse depression in billet casting with oil lubrication the nitrogen content of the steel must be kept below 60 ppm and the titanium content below 0.019%. Mathematical modelling of billet shrinkage, corroborated by billet inspection, showed that excessive mould taper caused the mould to squeeze the solidifying shell which led to the formation of longitudinal depressions. To eliminate this problem a double mould taper with 1.8% m⁻¹ up to 450 mm from the mould top and 0.9% m⁻¹ for the rest of the mould is recommended. Mathematical modelling of mould powder melting has led to further understanding of the response of the molten-flux pool to changes in powder properties, feeding strategies and casting speed.
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