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The effects of high speed casting on the mould heat transfer, billet solidification, and mould taper design of continuously cast steel billets Chow, Cindy

Abstract

As the final study in a five-year project, analysing the high speed casting of high quality continuously cast steel billets, an industrial trial was conducted at Company H. The mould was instrumented with thermocouples and a linear variable displacement transducer to study the thermal response of the system to changes in casting conditions. Billet samples were collected to study the effects of high casting speeds on internal and surface quality. Two existing mathematical models were used to approximate the heat transfer response in the mould and predict the billet solidification characteristics. The results of both models were verified using mould water heat transfer calculations, data from the literature, and empirical correlations. The mathematical model of the mould has allowed the quantification of the different characteristics of the heat transfer behaviour at the midfaces and corners of the mould. The analysis has shown that the effect of carbon content on the mould heat transfer was less pronounced at high casting speed due to the dominating effect of the reduced residence time. As well, the magnitude of change in heat transfer was less sensitive at casting speeds exceeding 3.0 m/min. Casting speeds above -3.5 m/min were also found to increase the metal level standard deviation and local mould thermal standard deviation at the meniscus. However, the increased casting speeds were found to decrease the average heat transfer standard deviation at each mould wall, but increase the variability of the responses between each mould wall face. These last two effects have not previously been studied in conventional or high-speed applications. The results of the billet evaluation could not be disclosed for proprietary reasons. However, a qualitative evaluation has established that the internal and surface quality of these particular high speed continuously cast billets were not significantly worse than observed in conventionally cast billets. As well, the observed defects could not be quantitatively linked to metal level standard deviations, casting speed, superheat, or tundish nozzle diameter. In the area of mould design, the longer mould lengths used in high speed casting were found to have a strong effect on the response of the inside curved wall in curved mould machines at casting speeds exceeding 3.0 m/min. This effect was observed at both the midfaces and corners of the mould. This was explained in terms of the interaction of several variables, including: gravity, mould length and curvature, high casting speed, and carbon grade. This phenomenon has not been observed previously, hence mould tapers were recommended for the casting of plain low and high carbon grades at high speed (3.0 to 4.5 m/min) to improve the heat transfer characteristics in the long mould. These mould tapers were found to be relatively insensitive to casting speeds within 3.5 to 4.0 m/min for both high and low plain carbon grades. From the analysis of moulds with sufficient, inadequate, and excessively steep meniscus-level mould tapers, a new mechanism was proposed to explain the high rates of local heat transfer observed in excessively tapered moulds. This phenomenon was described in terms of the mould-strand interaction caused by the dynamic taper during the upstroke of mould oscillation. The local uniformity of the shell, extent of the meniscus taper, and type of mould lubricant used were shown to have an effect on the applicability of the mechanism. This project completes a 5-year study of high speed continuous casting of high quality steel billets, and provides recommendations for further work with a brief look at future trends in the industry.

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