Depth of oscillation marks forming in continuous casting of steel
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I. INTRODUCTION
THE surface of continuously cast steel strands exhibits oscillation marks, which are caused by the mold oscillation. The oscillation marks have an important effect on the surface quality of the strands, being often the sites of transverse cracks. In each period of mold oscillation, one mark develops, which typically has a depth of 0.5 mm and a pitch of 1.5 cm. The primary reason for the formation of the marks is the pressure fluctuation in the layer of casting flux located between strand and mold. When the mold moves downwards, casting flux is pulled into the gap between strand and mold. The pressure in the gap increases and, during a certain period, the tip of the solid steel shell is bent away from the mold. Afterwards, when the mold moves upwards, flux is sucked out of the gap, the pressure decreases again, and the tip of the shell moves towards the mold. Two different types of oscillation marks may result, the depression type or the hook type. The depression type is a mere surface indentation. The hook type consists of an indentation with a subsurface welding plane (looking like a hook on an etched sample) extending from the indentation into the interior. It originates by overflow of liquid steel over the tip of the shell. The present article deals with the oscillation marks of the depression type. Figure 1 presents schematical illustrations of the mechanism of mark formation from two classical articles.[1,2] Figure 1(a) shows the various stages of shell bending during one oscillation cycle according to Takeuchi and Brimacombe.[1] Figure 1(b) is given by Emi et al.[2] and draws attention to the slag rim freezing at the mold wall in the meniscus region. This rim plays an important role because it narrows the entrance to the gap (slit) and, therefore, enlarges the extent of the pressure variations. Models for the fluid flow in the gap and its interaction KLAUS SCHWERDTFEGER, Professor, is with the Institut fu¨r Allgemeine Metallurgie, Technische Universita¨t Clausthal, D-38678 ClausthalZellerfeld, Germany. HONG SHA, Scientist, formerly with the Institut fu¨r Allgemeine Metallurgie, Technische Universita¨t Clausthal, is with Volkswagen AG, D-38436 Wolfsburg, Germany. Manuscript submitted June 11, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
with the steel shell have been published by several researchers.[1,3–10] The fluid flow has been treated by solving the one-dimensional equation of motion, assuming a certain gap profile.[1,3–5] The magnitude of the pressure fluctuations was usually taken to be a measure for the depth of the resulting oscillation marks.[5–9] In more sophisticated models, the hydrodynamic model was coupled to a mechanical model for the shell deformation using the beam bending theory and elastic or elastoplastic material behavior.[7,8,9] Or, the movement of the steel shell was modeled as that of a very viscous liquid.[10] However, in reality, the deformation of the solid steel shell under a load is viscoplastic (creep mechanism) at elevated temperatures.[11] We have develope
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