Heat transfer, oil lubrication and mould tapers in steel billets casting machines

This study examines in detail the factors that influence mould-billet interaction and heat transfer during the continuous casting of steel billets. In an extensive three-year project, major industrial trials were held in three Canadian steel plants involving in each case an operating mould instrumented with arrays of thermocouples to record mould wall and mould cooling water tem peratures. Additionally, load cells were installed between the mould housing and the oscillator table to record mould-billet interaction. Linear variable displacement transducers were attached to the mould wall in order to monitor mould displacement. Measurements were made under different casting conditions - steel grades, types and flow rates of lubricating oils and mould tapers - and were recorded on a computer controlled data acquisition system. The liquid steel surface in the mould was also filmed during casting. Two existing mathematical models of the mould were modified and used to calculate the axial heat flux profiles and the dynamic distortion of the mould during service. A two-dimensional, finite-difference, heat-flow, mathematical model of the billet was developed to simulate solidifi cation and shrinkage as a function of axial position in the mould. The coefficient of thermal con traction of steel was estimated as a function of steel carbon content and temperature from experimental data in the literature Ofl the lattice parameter of and ‘ unit cells; this was particularly important to model the shrinkage of low-carbon steels. It has been shown that in theory, the low carbon steels (C <0.15%) should experience the largest contraction due to δ- γ phase transformation; but in practice, they shrink less because heat transfer to the mould is low compared to higher carbon grades. A computer programme was developed to analyse the load cell response as a function of mould displacement. Finally billet samples collected during the trials were metallographically examined to study the different aspects of the solidification in the mould e.g., cracks, oscillation mark depth and rhomboidity. The most important result of the research work has been the finding that the heat transfer in the mould is significantly influenced by the taper of the mould wall in the meniscus region. A high initial taper (2.5-3.0%/m) in the meniscus region can compensate for the outward bulging of the mould wall during operation preventing it from acquiring a negative taper. This absence of negative taper has been shown to decrease mould-billet interaction during the negative strip period thereby leading to a decrease in the heat extracted in the meniscus region. This finding has been corroborated by an analysis of the load cell signals. It has been shown unambiguously that, for high mould heat transfer, a shallow initial taper of the mould, that permits the wall to acquire a bulged shape, is required. High heat transfer in the mould is likely to result in adverse lubrication condition for casting high-carbon steel billets. Filming of the steel surface has shown that only some of the lubricating oil flowing down the mould wall reaches below the meniscus while the remainder collects on the liquid steel surface and burns. As a result an increase in the flow rate of the oil is not reflected in a commensurate increase in lubrication or heat transfer. In fact the industrial trials have clearly revealed that the existing flow rate of oil at all three plants could be reduced at least by half without any visible deleterious effect on billet quality. It has also been possible to link various sensor signals to the generation of defects in the billet, in particular to the formation of off-corner internal cracks, transverse depressions and billet rhomboidity. This together with the linkages between mould heat transfer and operating variables now makes it possible to conceive of a control system consisting of an instrumented mould and an expert system that not only can asses billet quality on-line but can also initiate corrective action by changing operating conditions that alter the heat transfer in the mould.

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