Characterization of Roughness, Porosity and Thermal Resistances of Continuous Casting Mold Slag Layers Devitrified and C
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STRAND-TO-MOLD heat transfer is key in determining surface quality and productivity in steel continuous casting, the transfer process is greatly affected by evolution of crystals in the solid layer of the film formed by the slag infiltrated in between. Crystalline phases have thermal—lattice and radiation—conductivities
A. HUMBERTO CASTILLEJOS E. and TANIA M. FLORES F. 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 Saltillo-Ramos Arizpe, 25900, Ramos Arizpe, Coahuila, Mexico. Contact e-mail: [email protected] Manuscript submitted February 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
different from those of the amorphous phase from which they stem and, induce internal pores and surface roughness which play an important role in determining the heat exchange.[1–3] Heat transfer, and therefore, mold powders must suit the operating conditions, i.e., steel grade, casting speed, oscillation frequency and stroke and mold geometry. The slag film is made of liquid and solid layers, the last consist of glassy and crystalline phases that may appear intermingled or layered and its porosity and roughness vary with mold powder composition.[4] Heat is transferred through the slag film by radiation and conduction and it has been reported that the ratio of radiative to total heat flux amounts to ~ 40 pct in the liquid layer and to ~ 6 pct in the crystallized layer, when the total film thickness dt = 1.5 mm.[5] Conduction is also the main heat transfer mechanism operating through the narrow gap formed between the flux solid layer and the mold.[5,6] Heat flux by conduction through
the solid layer and gap is tied to precipitation of crystalline phases because of their intrinsic thermal conductivities and their influence on pore formation and roughness. A comprehensive and recent review points out the need for characterization of porosity, /, and roughness of solid slag layers.[3] The presence of pores—macropores (0.05 £ diameter, lm £ 100) according to IUPAC convention[7]—in samples of films recovered from continuous casting molds and from experimental set-ups has been acknowledged by several authors, who recognized their influence on decreasing the effective thermal conductivity.[8–12] A plant study reported a significant decrease in heat transfer rate to the mold, by ~ 14 pct, as the area percentage of pores in the crystalline layer increased from 4 to 12 pct.[10] The presence of very many irregularshaped pores was reported at the boundaries of crystals and the resistance to heat transfer between shell and mold was attributed to them.[10] A laboratory investigation disclosed the presence of many round voids in crystallized layers of mold slags and indicated that pores were absent in layers of non-crystalizing slags;[11] the shape of the pores suggested they originated from gas evolution, differently from those mentioned in References 8 and 10. The author
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