Prediction and Measurement of the Kinetics of Isothermal Crystallization and Devitrification of Mold Powder Slags
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and/or uneven heat extraction from the solidifying steel shell may induce thermal stresses that ultimately lead to longitudinal cracks during continuous casting, particularly in slabs of medium and hypo-peritectic steels.[1–3] One way found to soften cooling of the solidifying shell has been the use of mold
TANIA M. FLORES F. and A. HUMBERTO CASTILLEJOS E. 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 email: [email protected] SERGIO RODRI´GUEZ A. and FE´LIX ORTEGA C. are with the Laboratory of X-ray Diffraction, Department of Metallurgical Engineering, Centro de Investigacio´n y de Estudios Avanzados, CINVESTAV – Unidad Saltillo Manuscript submitted December 11, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
powders with a large tendency to crystallize.[2,3] The mechanisms proposed for explaining how crystallization reduces heat flux from the strand to the mold have been: (1) the formation of denser phases causes porosity in the slag film,[4] as well as the development of a gap at the mold/slag interface[5] which reduces conduction heat transfer through both media, and (2) the presence of crystals in the film solid layer lowers transmissivity and enhances scattering of infrared radiation through it.[6] Equally important to knowing how crystalline phases reduce heat transfer is being able to determine the rate and extent at which glassy or liquid slags form crystalline phases under conditions that provide reasonable estimation of in-mold behavior using an easily available technique [e.g., differential thermal analysis (DTA) or differential scanning calorimetry (DSC)]. The reason is that selection of the correct powder is essential for a successful casting operation.[7]
As an approximation, it can be conjectured that crystallization and devitrification of the slag film within the mold-strand gap occur under locally isothermal conditions, i.e., under constant temperature gradient conditions. Which of the two transformation routes takes place might depend on the position within the slag film. In the compact strip process (CSP), molds operate with surface temperatures between ~ 653 K and 803 K (~ 380 C and 530 C)[8] and thus upon contact the liquid slag would quench and form a glassy layer that with time devitrifies as it receives heat from the strand. Furthermore, the cold surface of the solid slag layer would be hotter than the hot surface of the mold because of a contact resistance between the two;[9] hence, under sufficiently high temperatures devitrification would proceed. On the other hand, the liquid layer in contact with the strand is exposed to temperatures ranging from the steel liquidus temperature to ~ 1273 K and 1423 K (~ 1000 C and 1150 C);[10] hence, crystals may precipitate from the undercooled liquid. It has been estimated that the cooling rates betw
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