A Unified Mechanism for the Formation of Oscillation Marks
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ion has been long implemented in continuous casting to avoid sticking of the shell to the mold walls if a lack of lubrication occurs. Thus, the original purpose of mold oscillation is as a preventive measure to avoid breakouts. However, oscillation leads inevitably to the formation of regularly spaced indentations along the slab width known as oscillation marks (OMs) (Figure 1). The nature of these marks has long been a subject of analysis in the continuously cast (CC) literature and several mechanisms have been proposed to explain their formation.[1–3] The most fundamental works in the area are authored by Tomono[4] and Ackerman,[5] where the overflow and folding mechanisms were proposed after observations on scaled models using organic compounds and steel. Figure 2 shows both mechanisms schematically. PAVEL E. RAMIREZ LOPEZ, Research Engineer, is with the Casting and Flow Simulation Group, Process Metallurgy Department, Swerea MEFOS, SE97 437 Lulea˚, Sweden. Contact e-mail: pavel. [email protected] KENNETH C. MILLS, Professor, is with the Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom. PETER D. LEE, formerly Professor, the Department of Materials, Imperial College London, is now Co-Director, Manchester X-Ray Imaging Facility, School of Materials, The University of Manchester, Research Complex at Harwell, Rutherford Appleton Laboratory, Didcot, Oxon OX11 OFA, United Kingdom. BEGON˜A SANTILLANA, Group Leader, is with the Casting Metallurgy Flat Products, Steelmaking & Continuous Casting Department, Tata Steel Research Development & Technology, 1970 CA IJmuiden, The Netherlands. Manuscript Submitted December 8, 2010. Article published online October 6, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
In both mechanisms, the shell solidifies along the curved meniscus profile to form a ‘‘pointy’’ tip with a hook shape. The critical difference lies in the thickness of the shell. In the first case, the shell is strong enough to avoid deformation, causing the steel meniscus to overflow the tip (i.e., overflow mechanism). Otherwise, if the shell is too thin, its tip bends back under the rim pressure (i.e., folding mechanism). From this point on, different theories have been proposed to discard or confirm these mechanisms and to demonstrate their applicability to describe marks in the final product. Deformation of the initial shell caused by contact with the slag rim, bending caused by thermal stresses, and increased mold flux pressure can be mentioned among these theories. More recently, Thomas et al.[6] analyzed the overflow mechanism in more detail and highlighted the influence of OMs on powder consumption and product quality. Moreover, Sengupta and Thomas[7] updated the mechanism for hooks to fit with the microstructure in the final slab based on an overflow mechanism. A common factor in all these works is that the analysis of the marks in the product is carried out after casting and not during the casting. This leaves many open questions regarding the precise moment of their creation and subseque
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