Spray casting of steel strip: Process analysis
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INTRODUCTION
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response to increasing global competition, major changes in manufacturing philosophy are taking place in the metals processing industries. The traditional strategy has been to increase productivity by utilizing high plant capacity and mass production. Manufacturing efficiency achieved through economy of scale must now be replaced or complemented by flexible manufacturing methods. This new climate has resulted in a strong interest in net- or near-net shape manufacturing (NNSM) processes which are materials and energy efficient. The NNSM of flat products in the domestic steel industry is particularly timely. Experts predict that NNSM practices will increase yields, improve efficiency, enhance product quality and generate higher profits, ttl The primary incentive is cost reduction, achieved by circumventing, in part, the hot rolling step in the production of continuous strip. In addition, flat products of materials not amenable to hot rolling may be produced by NNSM. Near-net shape casting of nonferrous alloy strip and sheet is commercially viable. Processes include the single roller, twin roller, inside-the-ring, wheel-and-belt, the Hazelett twin belt method, and planar flow casting. These processes utilize moving mold systems which travel with the cast strand and avoid relative motion or friction between the mold and the product. This allows high casting speeds to compensate for the small cross-sectional SURESH ANNAVARAPU, Research Associate, and ALAN LAWLEY, Professor, are with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. DIRAN APELIAN, formerly Howmet Professor of Materials Engineering at Drexel University, is Provost and Howmet Professor of Mechanical Engineering at Worcester Polytechnic Institute, Worcester, MA 01609. Manuscript submitted July 31, 1989. METALLURGICAL TRANSACTIONS A
area. In contrast, direct strip casting of high melting temperature ferrous alloys is not a commercial practice. The major problem is that the low thermal diffusivity of the cast steel gives rise to moderate solidification rates, and in turn, the throughput rate is too low for large-scale production. In comparison to the commercial processes utilized in the NNSM of nonferrous alloy flat products, spray deposition is unique since it eliminates casting as an independent step in the process. Spray deposition has an intrinsic beneficial component of rapid solidification which results in a fine-scale homogeneous microstructure devoid of macroscopic segregation. Further, the process is capable of high throughput rates in excess of 0.3 to 2.5 kg/s per nozzle. In the spray casting process, the substrate (mold) surface comes into contact with a spray which consists of droplets in different stages of solidification;t2,3,aj the enthalpy of the spray differs from that of the molten metal. The microstructure of the deposit is dictated by the fraction of liquid delivered by the spray to the surface of the substrate/deposit. To date, three principal mechanisms have been proposed for micro
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