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Sticking-type breakouts during the continuous casting of steel slabs

University of British Columbia

Sticking of the shell in the mold, which often occurs in a high-speed continuous slab casting machine, can be detected with thermocouples in the mold copper plates and prevented from developing into a breakout by reduction of the casting speed. However, a rapid reduction of the casting speed causes some quality problems and a low slab temperature. Thus, sticking-type breakouts remain a concern to the steel industry, and it is still not clear how and why the sticking initiates at the meniscus. The objectives of this study were to identify the causes of sticking at the meniscus, to elucidate the mechanism of sticking and finally to propose methods to reduce the occurrence of sticking. In order to identify the causes of sticking, it was necessary to examine a sticking-type breakout shell metallurgically, especially the dendrite structure at the shell surface. To link the metallurgical information to the casting conditions, the validity of a correlation in the literature between secondary arm spacing and local cooling rate has been examined. The secondary dendrite arm spacing in the subsurface of a slab has been measured and linked to a local cooling rate calculated from the measured mold heat-flux with this correlation. From this analysis, it was confirmed that Suzuki's correlation between secondary dendrite arm spacing and local cooling rate can be applied for a high cooling rate such as in continuous casting. A sticking-type breakout slab exhibiting five sticking events of 0.08% carbon steel, has been studied and it has been found that small holes exist at the surface in the sticking shells (most likely the site of entrapment, of solid mold flux). The shell which initially sticks exhibits a coarse dendrite structure and, in a longitudinal section, the shape of the initial sticking shell is parabolic. Moreover, with one exception, segregation lines typically 1-3 mm below the surface and almost parallel to the surface have been found in most of the sticking shell. From secondary ion mass spectroscope studies, the solutes concentrating in these segregation lines were determined to be Mn and S. Apparently, the sticking occurs at the meniscus where heat extraction is greatest and molten mold flux flows between the shell and solid mold flux rim oscillating with the mold. Therefore, to explain these meniscus phenomena, mathematical models of heat transfer at the meniscus and fluid flow in the mold flux channel have been formulated. To analyze the initial sticking event, the meniscus level has been changed in the computer simulations and the following mechanism has been proposed to explain the initiation of a sticking-type breakout. If the meniscus level rises, a deep notch forms in the shell due to the interaction between the mold flux rim and the shell. When a thick mold flux rim moves downward, it contacts the shell above the notch and the shell sticks to the mold flux rim. During the upstroke motion of the mold, tensile forces on the shell cause a rupture at the deep notch which is the hottest and weakest. The predicted solid flux rim profile agrees well with the parabolic shell shape measured in a longitudinal section of the sticking shell. Since the hot spot is the most likely point to be ruptured, conditions which minimize the hot spot were sought with the models. It was found that most of the conditions required to reduce hot-spot formation are exactly opposite to those required to minimize oscillation mark depth. Notwithstanding this, there are a few techniques to reduce the occurrence of sticking and to improve the surface quality: use a low melting point mold flux and, probably, maintain a deep mold flux pool. An interesting finding with respect to oscillation mark formation is that, if the mold flux rim is thick, the oscillation mark is caused by the interaction of the flux rim with the solidifying shell, while the fluid pressure development in the molten flux film dominates the mark formation in the case of a thin flux rim. For the analysis of the segregation line, a mass transfer model has been formulated based on a consideration of δ — γ transformation. From this analysis, it was found that the segregation observed in the sticking shell is a band of interdendric segregation enhanced by enlarged primary dendrite arm spacing which, probably, is caused by the appearance of an air gap due to the shell shrinkage.

Text

2010-08-30

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http://hdl.handle.net/2429/27941

Materials Engineering

University of British Columbia

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