Study of Shell-Mold Thermal Resistance: Laboratory Measurements, Estimation from Compact Strip Production Plant Data, an
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THE mold is the most critical component of the steel continuous casting machine; the rate and uniformity of heat removal from the strand, especially near the meniscus, controls the surface quality of the product, as well as the productivity and safety of the process. An excessive or uneven rate of heat extraction from the J. MANUEL GONZA´LEZ DE LA C., Doctoral Candidate, TANIA M. FLORES F., M.Sc. Graduate, and A. HUMBERTO CASTILLEJOS E., Professor, are with the Laboratory of Process Metallurgy, Department of Metallurgical Engineering, Centro de Investigacio´n y de Estudios Avanzados, CINVESTAV Unidad Saltillo, Av. Industria Metalu´rgica 1062, Parque Industrial SaltilloRamos Arizpe, 25900, Ramos Arizpe, Coahuila, Mexico. Contact email: [email protected] Manuscript submitted January 7, 2016. Article published online June 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
newly solidified shell may induce thermal stresses, which ultimately might cause longitudinal cracks.[1] On the other hand, insufficient heat removal may lead to relatively frail spots in the shell that could culminate in breakouts.[2] Rejection or reconditioning of cracked slabs causes considerable economic losses,[3] and slab breakouts are major upsets, which bring about safety hazards, machine damage, and line stoppages.[4] Thus, heat transfer from the shell to the mold must match the operating conditions, e.g., steel composition, casting speed, and mold design, by choosing the mold powder that has adequate crystallization tendency, solidification temperature, and effective (phononic plus photonic) thermal conductivity, and that produces a slag film and solid slag layer-mold gap with the right thicknesses. The slag film is made of a liquid layer beside the shell and a solid layer next to the Cu mold. The latter generally has both glassy and crystalline phases VOLUME 47B, AUGUST 2016—2509
accommodated in complex fashion.[5,6] Through the liquid and solid layers, heat is transferred by conduction and radiation and reaches the mold by conduction through an air gap that is claimed to form between the solid layer and the mold.[7–9] Normally, it is stated that the gap arises because the crystalline phase has a higher density than the glass, and therefore crystallization results in shrinkage of the solid flux layer.[10,11] In an investigation carried out to determine the surface roughness of solidified mold fluxes, it was found that this was in the range of 10 to 30 lm, when the crystalline phase precipitated;[12] this surface roughness was claimed to agree with the calculated thickness of the air gap assumed to form between the mold and the solidified slag layer. Furthermore, it was disclosed that the surface roughness of mold powder slags for casting medium-carbon steels is larger than that of mold slags used for low-carbon steels.[12] Other authors indicated that substantial shrinkage occurs in mold fluxes for medium-carbon steels due to larger volumetric ratio and faster growth rate of cuspidine, as compared to what occurs in mold fluxes
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