Effect of thermal conditions and alloying constituents (Ni, Cr) on macrosegregation in continuously cast high-carbon (0.
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INTRODUCTION
T H E morphology arid spacing of the dendritic structure in cast steel is largely dependent on the composition and thermal conditions during casting. Variations in the fineness of the dendritic structure and the distribution of solute elements in the cast product, coupled with porosity m the interdendritic region, result in a deterioration in mechanical properties. Also, the mode of solidification, i.e., columnar or equiaxed, strongly controls the resulting distribution of the solutes. This latter point is more important in continuous casting than in ingot casting because of the increased directionality of the solidification process, producing increased columnar zones with their associated increased interdendritic fluid flow of the soluterich liquid, t*] Such segregation is difficult to remove by postsolidification heat treatment or mechanical working. Therefore, the thermal conditions in continuous casting need to be controlled in order to provide the required mode and fineness of the solidification structure and minimum segregation. Major problems can occur in high-carbon, low-alloy steels and are largely concerned with the prevailing thermal conditions, e.g., thermal gradient, G, and the rate of solidification, R. The present investigation has examined the influence of heat transfer and thermal conditions on the solidification and segregation in a nominal 0.8 pet C steel with additions of up to I pet Ni and 1 pet Cr.
NITIN A. SHAH, Research Scientist. is with Micro Semiconductor Corporation (a Siemens Company), t00 School House Road, Somerset, NJ 08873. JOHN J. MOORE, Professor and Head, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 8040t. Manuscript submitted August 3, ]988. METALLURGICAL TRANSACTIONS B
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EXPERIMENTAL WORK
A. Casting Details A series of 13.6 kg, nominal 0.8 pet C steel castings was unidirectionally solidified in a static water-cooled copper mold, cast under varying conditions of water flow rates through the copper chill in an attempt to simulate conditions in an industrial billet caster. Superheat is known to be a major factor on segregation; therefore, extremely low superheats, i.e.,
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