Processing effects in spray casting of steel strip
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I.
INTRODUCTION
NEARLY 50 pct of the steel produced in the United States is converted to strip and sheet by the hot and cold roiling of billets or blooms. This practice, with the attendant annealing steps, accounts for about 1 pct of the total fuel energy consumed by the domestic steel industry. It is estimated that energy savings of 1.2 x 107 Ll/metric ton (cost saving of $44/metric ton) are possible if rolling and annealing are eliminated, t~] This projected cost reduction has prompted design and development of direct casting processes in which liquid metal contacts a moving mold and solidifies in strip form. Examples are twin-belt, wheel and belt, insidethe-ring, and twin roller casting. To date, short lengths of carbon steel and stainless steel strip 10 to 15 mm thick have been produced in pilot plants by means of direct casting processes.t2'3'4] An alternative technology for the production of strip is spray casting in which a spray of metal droplets impinges on a moving substrate (Figure 1); the droplets are formed by the atomization of a stream of molten metal. Since deposition rates are high (>0.3 kg/s), there is ample throughput capacity. Metallurgically, spray casting is a rapid solidification process that results in reduced segregation of solute elements and a fine-scale homogeneous microstructure. These features enhance workability and mechanical properties, compared to ingot metallurgy processing, tS-j~ Process variables which have a major influence on the metallurgical integrity of a spray deposit are superheat, ratio of the mass flow rates of the melt and the atomizing medium, and the droplet flight distance. Some empirical relations have been developed which describe the effect of at-
S. ANNAVARAPU, Research Assistant, D. APELIAN, Howmet Professor and Associate Dean of the College of Engineering, and A. LAWLEY, Professor, are with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. Manuscript submitted January 6, 1988.
METALLURGICALTRANSACTIONS A
omization conditions on the spray of droplets and hence on the deposit, m'~z'~3j Additional parameters must be considered in the spray casting of strip, in particular the substrate configuration, substrate surface condition, substrate displacement, and uniformity of the spray density. In order to optimize the spray deposition process for the production of strip, a knowledge of the effect of these process variables on the resultant microstructure and strip characteristics is required. Leatham, mj Subramanian, t~4]and Singer r15'~61have examined the strip production of aluminum and zinc alloys via spray rolling. They reported that in order to produce strip with an acceptable microstructure and properties, it was necessary to spray cast using a small flight distance, a high atomization gas pressure, a low superheat, and a high metal flow rate. These studies were made using early spray deposition equipment with a small nozzle diameter (3 mm) and hence a low metal flow rate of 0.01 kg/s. However, a high superheat (>200 ~ was m
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