In-situ Crystallization of Highly Volatile Commercial Mold Flux Using an Isolated Observation System in the Confocal Las
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an important role in the continuous casting of steels and understanding the crystallization behavior of mold fluxes provides the quintessential basis for the design of new casting powders. It is well known that mold flux can protect the molten steel surface from the atmosphere, absorb inclusions within the steel, thermally insulate and control the heat transfer between the shell molds, and provide sufficient lubrication preventing sticking of the partially solidified shell.[1] It can be assumed that within the flux film two distinguishable layers exist. An amorphous layer near the water-cooled copper mold and a crystalline layer near the steel. At high temperatures, much of the heat transfer can be dominated by the crystalline film. According to the past literature regarding the reduction
JUN-YONG PARK, Graduate Student, and IL SOHN, Associate Professor, are with the Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea. Contact e-mail: [email protected] JAE WOOK RYU, Director, is with the Korea Metal Material Research Association, Seoul 138-950, Korea. Manuscript submitted January 31, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
of heat transfer,[2] a decrease in heat transfer occurs as the flux crystallizes and also as volumetric contraction increases the air gap, which significantly inhibits heat transfer[3–5] and increases the thermal resistance at the interface.[4,5] In addition, the crystallized mold flux layer lowers the transmissivity, which in turn reduces the radiative-heat transfer across the film toward the copper mold.[6,7] The crystallization behavior of mold fluxes has been examined actively by many authors using a mold simulator,[8,9] differential thermal analysis (DTA),[10,11] single and double hot thermocouple technique (SHTT, DHTT),[12,13] and confocal laser scanning microscopy (CLSM).[14,15] Each method has direct and in-direct advantages and disadvantages in the study of flux crystallization, but all methods have provided important contributions in the fundamental understanding of high temperature crystallization. The CLSM method allows the continuous cooling transformation (CCT) and time– temperature transformation (TTT) diagram to be obtained and an in situ observation of crystal formation and growth can usually be easily acquired. However, mold fluxes have significant halides such as CaF2, which can lower the liquidus of the flux system and the viscosity for better lubrication and accelerate the formation of cuspidine (3CaOÆ2SiO2ÆCaF2) crystals to optimize heat transfer, but can be highly volatile depending on the overall composition of the flux making the in situ observation practically impossible in the CLSM. In this study, the crystallization behavior of commercial mold fluxes for medium carbon steels was investigated using the CLSM by employing a modified quartz cover isolated observation, which was particularly useful in handling highly volatile compounds and suppress continuous evaporation allowing in situ observation of crystallization. The morphology
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