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the volume-to-surface ratio of continuously cast products, it is clear that billet casting involves low mold flux consumption rates measured in kg of powder per m2 of strand.[1] Therefore, since

CLAUDIA BARRAZA DE LA P., A. HUMBERTO CASTILLEJOS E., SERGIO RODRI´GUEZ A., and FE´LIX ORTEGA C. 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, Ramos Arizpe, 25900 Coahuila, Me´xico. Contact e-mail: humberto. [email protected] Manuscript submitted August 18, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS B

consumption is constrained, it is possible to use mold powders which form high-viscosity slags and thus, minimize slag entrapment that would otherwise result from the high level of turbulence caused by the steel stream feeding the mold.[2–4] Viscosities of approximately 0.7[5] and 2.1[3] Pa/s, at 1573 K (1300 C), have been reported for mold slags used for casting medium carbon steels, MC, round billets. It could be conjectured that the high viscosity of the slags would impose a kinetic barrier for the formation of crystalline phases, favoring retention of amorphous phase as glass or super-cooled liquid, with little or no presence of crystals, even when cooled slowly. Percentages of amorphous phase between 76 and 87 wt pct were reported for a fluorine-free (F-free) slag held at 1223 K (950 C) for 1200 seconds.[3] The main crystalline phase which appeared was gehlenite, Ca2Al2SiO2, and based on plant

trials it was claimed that the lubrication capacity of the slag layer, formed between mold and strand, resulted from the nature and proportion of crystalline and amorphous phases present.[3] Generically melilite refers to a group of minerals which appear along the binary solid solution of gehlenite, Ca2Al2SiO2, and akermanite, Ca2MgSi2O7, but which normally contains significant amounts of iron and sodium.[6] Mold fluxes whose composition lead to formation of minerals of this group have been called melilite slags.[2,4] These slags in general have higher viscosities than slags which lead to formation of cuspidine (Ca4Si2O7F1.5(OH)0.5) as the main crystalline phase, and maintain high proportion of glass in the slag film.[7] There is great interest in arriving to mold powder compositions which ensure that the right phases appear in the slag film to provide adequate lubrication and heat extraction of the strand.[8,9] With this aim, it has been reported that Al2O3 promotes melilite precipitation and inhibit cuspidine formation,[10,11] and that melilite is also favored by the presence of moderate amounts of Li2O, Ti2O and Fe2O3 in the mold powder formulation.[12] In the design of a F-free mold powder, for replacing a fluoride-containing flux for billet casting, it was considered vital to replicate three characteristics: (1) viscosity at 1573 K (1300 C); (2) break temperature; and (3) crystallinity percentage of the solid slag layer.[13]