Structure Evolution of Slag Films of Ultrahigh-Basicity Mold Flux During Solidification
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old fluxes (with binary basicity, defined as pct CaO/pct SiO2, in the range 1.2 to 1.4) are often used in casting peritectic carbon steels; the fluxes maintain relatively low heat fluxes in the mold and so help to avoid longitudinal cracking. Recently, fluxes with even higher binary basicity (1.5 to 1.8) were developed[1–4]; industrial tests showed these to yield better lubrication and better slab surface qualities than conventional high-basicity mold fluxes.[1] The work presented here examined the structure of mold flux films solidified onto a water-cooled probe (‘‘cold finger’’[5]), which provides rapid film growth and has been shown to give similar structures to films from industrial casters.[7] The copper probe used here was 20 mm wide by 15 mm high (immersed 12 mm into the molten flux) by 6.35 mm thick. This is smaller than the probe used in previous work[5,6] and was found to give more consistent flux film thicknesses and heat fluxes.[7]
XIAO LONG is with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China and also with the Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA, 15213. QIAN WANG and SHENGPING HE are with the College of Materials Science and Engineering, Chongqing University. P. CHRIS PISTORIUS is with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA, 15213. Contact e-mail: [email protected] Manuscript submitted November 7, 2016. 1938—VOLUME 48B, AUGUST 2017
Of particular interest in this work was the development of the morphology of the interface between the solidified mold flux layer and the copper probe, since development of a rough interface would be expected to lead to a larger contact resistance. Higher-basicity fluxes (which also readily crystallize) often lead to rough interfaces, but it is not clear whether there is a causal link between crystallization and roughness. The mold flux tested in this work had a binary basicity of 1.74; details of its composition and physical properties are given in Tables I and II. The melting point was estimated with the hemisphere point method.[2] Viscosity was measured with a rotary viscometer (using graphite crucibles with 55 mm inner diameter and graphite bobs with 15 mm diameter); the temperature below which the viscosity of the molten slag increased sharply upon cooling [at 6 K/ minutes (6 °C/minutes)] was defined as the break temperature. Laser flash measurement of thermal conductivity was performed on disks (3 mm thick) cut from cylinders (12.7 mm diameter; approximately 30 mm tall) prepared by melting the flux in a graphite crucible (12.7 mm inner diameter). As Table II shows, the break temperature, viscosity, and melting temperature are similar to those of conventional fluxes for peritectic steels. Samples for cold-finger measurements were prepared with analytical reagents (CaF2, CaCO3, SiO2, Na2CO3, Li2CO3, MgO, and Al2O3). A 300 g sample for each experiment was placed in a graphite crucible (60 mm internal
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