Naturally Deposited Oxide Films in Near-Net-Shape Casting: Importance, Mechanisms of Formation, and Prediction of Their
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ear-net-shape casting of steel products (i.e., strip casting) the use of mold powder is eliminated and the melt solidifies in direct contact with the mold. In a twinroll strip caster, for example, the mold consists of a water-cooled rotating copper roll. This mold design is necessary as high cooling rates are needed in order to achieve the high casting speeds necessary to maintain a reasonable level of productivity in the industrial process. When steel solidifies in direct contact with a watercooled copper mold, the thermal resistance at the steelmold interface initially controls the heat extraction rate and thus the productivity of such a casting machine, when casting thin sections.[1] This resistance arises from the imperfect contact conditions between solidifying steel and mold, which are due to mold roughness and texture, entrapped gas, incomplete spreading of the melt on the mold surface and, finally, steel contraction. Of course, this resistance can be further complicated by shell buckling due to phase transformation as the shell PAOLO NOLLI, formerly Graduate Research Assistant, with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA, is Development Engineer, Vesuvius Research, Pittsburgh, PA 15205, USA. Contact e-mail: paolo. [email protected] ALAN W. CRAMB, Clark and Crossan Professor of Engineering and Dean, School of Engineering, is with the School of Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. Manuscript submitted January 17, 2007. Article published online December 28, 2007. 56—VOLUME 39B, FEBRUARY 2008
cools, but in this article, only issues related to initial solidification will be discussed. The conditions of direct contact between solidifying steel and a water-cooled mold also lead to the formation of deposits on the mold surface in the form of thin films. These films deposit naturally and directly from the liquid steel being cast onto the mold surface. A number of investigators have noted the presence of both oxide and sulfide films during strip casting.[2–8] Given their presence at the steel-mold interface, these naturally deposited oxide films can have a major effect on the inmold heat extraction rate. The presence and the effect on heat transfer of naturally deposited oxides was first documented by Strezov et al.[2] in a number of seminal publications during the casting of a manganese-silicon (Mn-Si) killed steel in both experimental and commercial casting operations. These results were confirmed by Phinichka[5] and Misra et al.,[6] using a droplet solidification technique. The researchers performed a series of casting experiments—either chill dipping experiments or droplet ejection experiments—in which steel, as in strip casting, solidified in direct contact with a water-cooled mold. The same substrates were used for all the experiments in the series and their surface was not cleaned between experiments. Black films were observed to deposit and then build up on the mold surface. This was found to lead to a significant increase in
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