Kinetics of Isothermal Melt Crystallization in CaO-SiO 2 -CaF 2 -Based Mold Fluxes
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ntinuous casting of steel, commercial mold fluxes have been mainly used to control the horizontal mold heat transfer and to lubricate the solidified steel shell from oscillating mold.[1] It is well known that the heat transfer and the lubrication are strongly dependent on the crystallization behaviors of mold fluxes such as extent and morphology of crystalline phase,[2–7] which are determined by nucleation and crystal growth. Therefore, it is highly required to understand crystallization kinetics of mold fluxes to improve the performance of continuous casting process. A few studies[8–10] have been conducted to investigate the crystallization kinetics that occurs on heating. However, it should be stressed that these studies[8–10] are limited in revealing the crystallization behavior of commercial mold fluxes which should be regarded as one of melt crystallization during cooling on the copper mold. For the purpose of overcoming the limitation, investigations[11–18] on non-isothermal melt crystallization MYUNG-DUK SEO and JI-YEON BAEK, Ph.D. Candidates, and SEON-HYO KIM, Professor are with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. CHENG-BIN SHI, Assistant Professor, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing (USTB), Beijing 100083, P.R. China. JUNGWOOK CHO, Research Associate Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted December 23, 2014. Article published online April 30, 2015. 2374—VOLUME 46B, OCTOBER 2015
kinetics have been carried out. Choi[11] and Gan et al.[12] had adapted well-known Kissinger and Matusita equations and obtained positive value of activation energy for melt crystallization. However, it is unreasonable to apply those equations on non-isothermal melt crystallization because Kissinger and Matusita equations are initially derived from assumptions in that crystallization occurs on heating. Vyazovkin[19,20] clarified that invalid results will be derived by direct application of these equations to any non-isothermal melt crystallization. Recently, the present authors[21] critically proposed problems of these approaches and concluded that Kissinger and Matusita equation is not applicable in obtaining activation energy of non-isothermal melt crystallization. It was additionally found that Ozawa equation was not suitable to evaluate kinetic parameters of non-isothermal melt crystallization for the mold fluxes[13,14] as well as polymer system.[15–18] As a possible alternative to this problem, the differential iso-conversional method developed by Freidman[22] has been successfully employed to evaluate non-isothermal melt crystallization kinetics.[13,14,23–25] However, it should be interestingly noticeable that aforementioned Friedman method[22] has limitations in obtaining quantitative kinetic
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