UBC Theses and Dissertations
Mould behaviour and product quality in continuous casting of slabs Mahapatra, Rama Ballav
An extensive study has been conducted to elucidate mould behaviour and to examine its influence on product quality during continuous casting of slabs. The study essentially comprised of industrial measurements, mathematical modelling and metallographic examination. The industrial measurements consisted of mould temperature measurements; an operating slab mould was instrumented with 114 thermocouples and the temperature of the mould at different locations was successfully measured for a wide range of casting conditions. A three-dimensional heat flow model of the mould was developed to quantitatively characterize the heat fluxes in the mould from a knowledge of the mould temperature data. Futhermore, a one-dimensional solidification model was developed to simulate solidification of steel and also, a heat flow model was developed to examine the mould flux behaviour by characterizing the slag rim thickness at the meniscus. Slab samples collected during the industrial trial campaign were metallographically examined to study the different aspects of solidification in the mould, sub-surface structure, solidification bands, cracks and oscillation marks. The thermocouple measurements revealed the occurrence of metal level fluctuation in the mould, the magnitude of which was appreciable. Thus, implementation of a metal level control system has been recommended. The time-averaged mould temperature data was converted into heat fluxes and it has been well demonstrated that a three-dimensional model of the mould wall was essential for accurate computation of heat fluxes in the mould. The measurements have clearly established the strong dependence of heat transfer in the mould on the mould flux employed during casting. A reduction in the viscosity and melting temperature of the mould flux will lead to enhancement of heat transfer in the mould. It was also found that the heat transfer in the mould can be influenced by changes in casting speed, submergence depth, steel carbon content; the effect of these variables on heat transfer has been explained mostly on the basis of their influence on the mould flux behaviour at the meniscus. Furthermore, heat extraction characteristics on the two broad faces were different which was a consequence of differences in mould flux behaviour resulting from differences in mould wall thickness between the two broad faces. The heat-flux profiles were employed as a boundary condition in the solidification model to compute the shell thickness in the mould for a wide range of casting conditions. The casting speed has a significant influence on the shell profile in the mould; an increase in the casting speed led to a reduction in the shell thickness. The non-uniformity of the shrinkage of the solid shell in the mould was evident from the slab surface-temperature profile which clearly revealed the advantages of a non-linear taper of the narrow face compared to the conventional single taper. From a knowledge of heat-flux profile and metallographic analysis, a mechanism towards formation of longitudinal cracks/depression was formulated. Mathematical analysis performed on the mould flux at the meniscus revealed the presence of a slag rim adjacent to the mould wall; the dimensions of the slag rim thickness were computed at different casting speed and mould wall thickness. It was shown that oscillation marks are formed by the interaction of the slag rim with the partially solidified meniscus; the depth of the oscillation marks is strongly governed by the thickness of the slag rim at the meniscus. This study has unambiguously shown that the dimension of the slag rim at the meniscus is quite critical from the standpoint of heat transfer and product quality of slabs. Based on the findings of the present study, for the first time, links have been established between the mould wall thickness and heat transfer in the mould. The slag rim thickness at the meniscus can be reduced by increasing the mould wall thickness. It is anticipated that an increase in the casting speed and thus, a corresponding increase in production rate can be accomplished by changing the design of the mould.
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