In Situ Observation and Investigation of Mold Flux Crystallization by Using Double Hot Thermocouple Technology
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flux is widely used in modern continuous casting of steel, and its main function can be divided into two categories: (1) protecting the steel from oxidation, insulating the steel from freezing and absorbing inclusions, when it floats on the top of liquid steel, and (2) lubricating the shell and moderating the heat transfer in the mold, when it infiltrates into the mold/ shell channel. The crystallization of mold flux is regarded as one of the most important properties of mold flux, as it primarily controls the heat transfer and influences the lubrication of steel in continuous casting.[1,2] However, the actual situation in the mold cannot be observed directly because of the high temperature, transient fluid flow, complicated phase transition, chemical reaction etc. Thus, many kinds of technology were developed to study the crystallization behavior of mold flux. Through using the exothermic or endothermic property of the crystallization process, differential thermal analysis was adopted to investigate the crystallization behavior of mold flux.[3,4] Unfortunately, the low heating and cooling rate limited its application. The single hot thermocouple technology (SHTT) and double hot thermocouple technology (DHTT) were first developed by Kashiwaya et al.[5] for the in situ observation of LEJUN ZHOU, Ph.D. Student, WANLIN WANG, Professor, DAOYUAN HUANG, Postdoctor, and JUAN WEI and JIN LI, Graduate Students, are with the School of Metallurgical Science and Engineering, Central South University, Changsha 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 24, 2011. Article published online May 8, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
mold flux crystallization, and these processes were favored by many other researchers[6–9] because of the visual as well as high heating and cooling rates. However, they were mainly used to construct the temperature-time-transformation (TT) and CCT diagrams of mold slag. A confocal scanning laser microscope was also used to study in situ the crystallization behavior of mold flux,[10,11] but it cannot obtain the desired temperature gradient that occurred in the mold. Furthermore, some other technologies were also tried to study the crystallization behavior of mold flux under conditions close to real mold, such as water-cooled probe technique,[12,13] infrared radiation emitter technique,[14] etc. However, they still have their own limitations. In this article, the DHTT was used for the in situ observation of the crystallization processes of a typical low-carbon (LC) and medium-carbon (MC) mold fluxes under the simulated thermal conditions in the mold. One thermocouple was controlled at the temperature of 1773 K (1500 °C) to simulate the thermal condition of initial shell; the other thermocouple was set as 1073 K (800 °C), simulating the temperature of the solid mold flux layer next to the hot face of the copper mold wall. The distance between the two thermocouples was set as 2 mm, which corresponded to the width of mold/shell gap. The article provides a better under
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