Mold Simulator Study of Heat Transfer Phenomenon During the Initial Solidification in Continuous Casting Mold
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is usually added on the top of liquid steel surface that protects the steel meniscus from oxidation, inhibits heat loss from steel, and absorbs the inclusions rising from liquid steel.[1] The liquid slag from the top molten slag layer infiltrates into the mold/shell gap and lubricates the newly formed shell. Then the liquid slag is getting cooled quickly and leads to the formation of a solid slag film (1 to 2 mm thick)[2] with glass, crystal, or a mixture of both phases, and a liquid slag film (~0.1 mm thick) next to the solidified steel shell.[3–6] Lubrication of liquid slag film throughout the shell is preferred to prevent the sticking of the shell to the mold and decrease the occurrences of the longitudinal surface cracks and the star cracks.[6] Jenkins[3] pointed out that the liquid film slag thickness could be estimated from 1/(3V0.5 c ). As the liquid slag film travels with the strand, the average thickness of liquid slag film can be estimated from the slag consumption by assuming that the solid slag film remains stuck to the mold. The calculated thickness of liquid slag film would be thicker than that of the exact
HAIHUI ZHANG is with the School of Metallurgical Science and Engineering, Central South University, Changsha 410083, Hunan, China, and also with School of Metallurgical and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China. WANLIN WANG is with the School of Metallurgical Science and Engineering, Central South University. Contact e-mail: [email protected] Manuscript submitted September 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
value as the solid slag could be dragged intermittently downward at an average speed slower than the casting speed.[7,8] In this study, a novel method for estimating the liquid slag film thickness is proposed. Occurrences of shell surface longitudinal cracks and off squareness are accelerated when the heat flux at the meniscus area exceeds a critical value (depending on the steel grade).[9–11] Hence the decrease of heat flux is expected to alleviate the longitudinal facial cracks and the off squareness, especially for the hypo-peritectic steels. The major heat transfer mechanism across the slag film is heat conduction and radiation, where the conduction dominates the heat transfer in the solid slag and the radiative heat transfer plays an important role in the transparent glassy and the liquid layer.[2] The control of the heat flux through the mold can be achieved by adjusting the slag film thickness (controlled by slag break temperature and viscosity etc), the fraction of crystals, and the optical properties of slag.[4,12–15] Generally, the high basicity mold flux (high tendency for crystallization) is applied to the crack-sensitive steel grades to optimize the mold heat flux and the liquid/glassy slag film with transition metal oxides is desired to suppress the radiative heat transfer and provide the lubrication.[16–18] In principle, a glass film adjacent to mold is formed at first, as the cooling rate is high enough to prevent the growth of
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