The Influence of Na 2 O on the Solidification and Crystallization Behavior of CaO-SiO 2 -Al 2 O 3 -Based Mold Flux
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els such as transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have recently gained significant interest owing to their enhanced strength and formability.[1–3] These new grades of steel are characterized by high concentrations of Al within the steel to improve the mechanical properties and decrease the overall weight of the final steel products.[2] However, alloying elements such as Al react with the CaO-SiO2-based mold flux used in the typical continuous casting processes, decreasing the SiO2 content and increasing the Al2O3 content in mold flux,[4] thus converting the original CaO-SiO2-based mold flux into a CaO-SiO2-Al2O3based mold flux. The solidification and crystallization JINXING GAO, Ph.D. Student, GUANGHUA WEN and PING TANG, Professors, QIHAO SUN, Postgraduate Student, and QIANG LIU, Engineer, are with the State Key Laboratory of Mechanical Transmission, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted November 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
behavior changes with this conversion of the mold fluxes can cause the partially solidified steel shell to stick because of decreased lubrication and higher viscosity. Hence, study on the solidification and crystallization behavior of CaO-SiO2-Al2O3-based mold flux is important. Intensive efforts have been carried out to investigate the CaO-SiO2-Al2O3-based mold flux, and most of them are concentrated on the effect of the Al2O3 content in the mold flux, including the variation in the Al2O3 content,[3,4] structure change with the Al2O3 content,[5,6] viscosity change,[4,6] and crystallization behavior with a high Al2O3 content.[7–9] These studies suggested that the viscosity increased with increasing degree of polymerization when the Al2O3 content significantly increased. Otherwise, the mold fluxes with high Al2O3 contents easily crystallized during the casting, hence rapidly deteriorating the mold lubrication.[7] Therefore, adding reasonable fluidizers to adjust the solidification and crystallization behavior of CaO-SiO2-Al2O3-based mold flux is very necessary. Li2O and Na2O are often used as the fluxing agents and network breakers in the traditional mold flux and could decrease the melting temperature and provide O2 ions to be used to
depolymerize the bridged-oxygen (O0) network bonds.[10,11] For the CaO-SiO2-Al2O3-based mold flux system, the [AlO4]5 units, which are the main network structures generated by Al2O3, can be formed because of the charge compensation by Li+ or Na+.[12] Thus, the [AlO4]5 units can be stabilized. The Li2O and Na2O contents are very important in CaO-SiO2-Al2O3-based mold flux, where they are used as the fluxing agents, network breakers, and to stabilize the network structures containing the [AlO4]5 units. Hence, the effect of Li2O and Na2O on CaO-SiO2-Al2O3-based mold flux should be investigated. However, most of the studies have focused on the effect of Li2O or Na2O on the crystallization be
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