Control of heat transfer and growth uniformity of solidifying copper shells through substrate temperature
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TRODUCTION
THE mold-metal interface has a prominent function in twin-roll strip casting, and it is the dominant resistance to heat flow.[1,2,3] Many defects are also produced at this location during the initial contact between the molten metal and the mold.[4–7] The condition of the rolls that act as the mold is determinant in the characteristics of this interface and may need to be tailored to the specific needs of the alloy that is cast. The temperature of the water-cooled rolls is a feature of the process that should be properly adjusted to assist the desired solidification. Their rotation produces temperature cycles with a rise when contact of the strip with the rolls prevails and a fall when it ceases. For a stainless steel strip solidifying on copper rolls, the surface temperature has been cited to cycle between approximately 200 ⬚C and 400 ⬚C.[8] With steel rolls, the temperature cycles for the same alloy were between 400 ⬚C and 800 ⬚C.[8] Tests performed by one of the present authors at the Industrial Materials Institute, using the prototype caster to produce carbon steel strip on rolls with copper sleeves, yielded temperature cycles between 100 ⬚C and 200 ⬚C.[9] Temperature cycles have also been observed in single-roll strip casting.[2,10] To the present, the effect of the roll temperature on the heat transfer and the surface quality of strips has not been reported. Studies dealing with other solidification processes that focused on the issue of substrate temperature are few, and the results may not be necessarily applicable to strip casting. They, nevertheless, indicate that the effect could be significant. For example, in melt spinning, sticking of the solidified amorphous metal ribbon on the wheel has been reported to occur when the wheel temperature was too high.[11] Similarly, in drop-splat experiments, sticking was observed at substrate temperatures between 120 ⬚C and 300 ⬚C, depending on the composition of the solidified metals and the substrates.[11] DOMINIQUE BOUCHARD, JEAN-PAUL NADEAU, DANIEL SIMARD, and FRANC¸OIS G. HAMEL are Research Scientists with the Industrial Materials Institute, National Research Council of Canada, Boucherville, QC, Canada J4B 6Y4. Contact e-mail: dominique.bouchard@ nrc.ca BELINDA HOWES and CLOTILDE PAUMELLE are Undergraduate University Students from the University of British Columbia, Canada and the Institut National des Sciences Applique´e de Rouen, France. Manuscript submitted July 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
With coatings produced by thermal spraying, a drastic change in the pattern of the impinged metallic powder particles has been reported when the temperature of various substrate materials reached the range from 250 ⬚C to 375 ⬚C.[12] In a study with chill blocks immersed in a bath of molten steel, increasing the block temperature from 25 ⬚C to 200 ⬚C reduced the heat flux and was accompanied by an improvement in the surface quality of the solidified shells.[13] In hot-dip coating of steel sheets with an Al-ZnSi alloy, the preheat temperatur
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