Heat-Transfer Behavior of Mold Fluxes for Continuous Casting of Invar Alloy
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during continuous casting.[1,2] The most important function of mold fluxes is to control horizontal heat transfer from the strand to the mold. Mold fluxes are fed onto the top of liquid steel in the mold, and then they melt and infiltrate into the gap between the mold and strand to form a structure with a glassy layer, a crystalline layer, and a liquid layer. The differences of structure and properties of slag layers cause inconsistent influence on heat transfer across the flux film. Some studies have showed that heat transfer from the strand to the mold is mainly controlled by interfacial thermal resistance[3–5] and the crystallization property of the flux film.[6–8] Invar alloy Fe-36Ni is a kind of typical low-expansion and low-temperature toughness high-alloy steel, which has increasing use in liquefied natural gas tanks, precise instruments, electronic industry, and special structural materials. Traditionally, it is produced by ingot cast or electroslag remelting, which have a high cost, low yield, and low efficiency. Recently, it is produced by vertical continuous casting in Japan and China. During recent continuous casting of Invar alloy Fe-36Ni, coarse columnar grains in WEI YAN, Ph.D. Candidate, WEIQING CHEN, Professor, and HONGCHENG XU, Graduate Student, are with the State Key Laboratory of Advanced Metallurgy and School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] CARSTEN LIPPOLD, Technical Director, is with the Qingdao Stollberg & Samil Co. Ltd., Qingdao 266300, P.R. China. Manuscript submitted October 26, 2012. Article published online August 14, 2013. 1466—VOLUME 44B, DECEMBER 2013
edge of slab and severe hot cracking were found frequently when Fe-36Ni slabs were cast by using the conventional mold fluxes. It is well known that hot cracking is caused by a combination of low strength and low ductility of material at a high temperature. Invar alloy Fe-36Ni is sensitive to hot cracking due to its low transition temperature of brittle to ductile fracture even though it has narrow solidification interval. Kim et al.[9] also reported that low transition temperature of brittle to ductile fracture of Invar alloy increases its sensitivity to hot cracking. An increased cooling rate in Invar alloy can increase the transition temperature of brittle to ductile fracture due to the transformation of solidification structure from the columnar solidification structure to the equiaxed solidification structure with higher strength. Accordingly, heat transfer of mold fluxes for continuous casting of Invar alloy Fe-36Ni is given the primary consideration to release hot cracking. More laboratory and field studies on heat transfer of mold fluxes for plain carbon steels, such as low carbon and middle carbon steels, have been reported.[3,4,6–8] The study method and some factors such as basicity and B2O3 influenced heat transfer across mold fluxes were explored,[3,10–12] but these studies were only conducted in laboratory and on mold fluxes
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