Role of B 2 O 3 on the Viscosity and Structure in the CaO-Al 2 O 3 -Na 2 O-Based System
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IN the continuous casting process of steels, the control of the thermo-physical properties of mold fluxes is essential in enhancing the efficiency of the overall steelmaking process. In particular, the viscosity of the mold flux ensures appropriate lubrication at the mold/ metal interface and prevents caster breakouts beyond the mold.[1] Typical mold flux compositions include CaO, SiO2, Na2O, CaF2, Li2O, MnO, and other oxides and halides. However, recent developments in high Al containing steels for ultra-light vehicle applicants have resulted in significant issues with mold flux chemistry control as the Al reduces the SiO2-based mold fluxes. The initial SiO2-rich fluxes are transformed into Al2O3rich fluxes reducing the lubrication ability and the radiative heat transfer across the copper mold. Thus, additional fluxes to optimize the viscosity and form new crystalline phases beyond the typical cuspidine (3CaOÆ 2SiO2ÆCaF2) to control the heat transfer for high Al containing steels has been emphasized in recent research efforts.[2,3] Furthermore, as the environmental restrictions for fluorine continue to limit its use in steelmaking and casting, mold flux designers have suggested several possible family of fluxes that may be possible candidates for fluorine-free Al2O3-based mold fluxes. The majority of these fluxes are based on the quaternary CaO-Al2O3Na2O-B2O3 slag systems and thus fundamental studies regarding this system is of significant importance.[3] GI HYUN KIM, Graduate Student, and IL SOHN, Associate Professor, are with the Department of Materials Science and Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Korea. Contact e-mail: [email protected] Manuscript submitted May 15, 2013. Article published online October 4, 2013. 86—VOLUME 45B, FEBRUARY 2014
According to Wang et al.,[4] the role of CaF2 and B2O3 was similar when both are added at comparable amounts, but B2O3 lowered the break temperature more than CaF2 for the highly basic CaO-Al2O3-MgO-SiO2 slag system. It was also observed that the viscosity decreases due to the lower melting point of B2O3 and in spite of B2O3 behaving as a network forming acidic oxide, B2O3 reduces the viscosity in the fluorine-free slags.[5] According to Lu and Hahapatra,[6] Al2O3 is well known to predominantly act as a network forming [AlO4]-tetrahedral structural units in the presence of significant basic oxides such as CaO, MgO, Li2O, and others. Figure 1 shows the various B2O3-based structural units and the depolymerization of the slag structure with free oxygen ions (O2 ) in the alumino-borate slag system adapted from Wright[7] and expanded to 3-D. According to Klyuev and Pevzner,[8–10] in general the borate structures predominantly form boroxol rings composed of [BO3]-triangular structural units which are interconnected forming 2-dimensional layers held by van der Waals forces. As alkali or alkali earth oxides are added, the [BO3]-triangular structural units can transform to [BO4]-tetrahedral structural units, where the additional negative charges are compensat
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