Structure and Crystallization Kinetics of Glassy CaO-Al 2 O 3 -SiO 2 -CaF 2 -Na 2 O Mold Fluxes with Varying Basicity
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MOLD fluxes are usually used in continuous casting of steel and play a crucial role in the continuous casting operation of steel due to its multifunction.[1] The mold flux is used to perform the following functions[1,2]: (1) protecting the meniscus of the steel from oxidation; (2) providing thermal insulation; (3) providing liquid fluxes to lubricate the strand; (4) providing the optional level of heat transfer; and (5) absorbing inclusions from the steel. The quality of casting slabs is closely related to these functions. The mold flux crystallization is regarded as one of the most important properties of mold fluxes. It is generally accepted that heat transfer in continuous casting process is mainly controlled by the crystallization behaviors of mold fluxes. The crystallization of cuspidine (3CaO 2SiO2 CaF2 ) from the traditional mold flux is deemed as an optimal crystallization product to control the heat transfer. After infiltrating into the gap between mold and steel shell, molten fluxes would solidify to form solid layer JIANGLING LI, Ph.D. Student, BAIJUN YAN and KUOCHIH CHOU, Professors, and QIFENG SHU, Associate Professor, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 1, 2014. Article published online July 14, 2015. 2458—VOLUME 46B, DECEMBER 2015
due to the cooling of copper mold. The solid layer could be mixture of glassy and crystalline phases. The crystalline phases have an impact on the heat transfer between mold and steel shell; therefore, crystallization from liquid mold fluxes is of vital importance to optimal continuous casting. On the other hand, solid film of mold flux can inevitably absorb some heat released from the solidification of liquid steel, thereby producing reheating to the mold flux.[3] Moreover, the reheating process would lead to the crystallization of glassy slag film, near the wall of the copper mold, which affects the thickness of the crystalline layer and the surface roughness of the solidified mold flux, and it has a strong influence on the horizontal heat transfer and lubrication. Therefore, the crystallization of glassy mold flux is also very important for the continuous casting process. A series of techniques have been developed to study the crystallization process of mold flux, such as the differential thermal analysis (DTA)[4] or differential scanning calorimeter (DSC),[5] single hot thermocouple technique (SHTT),[6] double hot thermocouple technique (DHTT),[7] and so on.[8] The basicity (R = w(CaO)/w(SiO2), where w(CaO) and w(SiO2) are the weight percentages of CaO and SiO2, respectively) of mold flux has been regarded as one of the most important influential factors on the mold flux crystallization. Zhou et al.[9] found that the crystallization was enhanced with the increase of basicity in mold fluxes.
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