Effects of Transition Metal Oxides ZrO 2 , Y 2 O 3 , and Sc 2 O 3 on Radiative Heat Transfer of Low-Reactive CaO-Al 2 O
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HEAT transfer is mainly directed from a solidifying shell to a water-cooled mold during the continuous casting of steel. To ensure that the newly formed shell has an appropriate thickness, it is essential to control the heat extraction across the slag film depending on the steel grades being cast and the casting conditions.[1] For peritectic steel with a carbon content in the range of 0.08 to 0.17 pct, a 4 pct mismatch in the thermal shrinkage coefficient from the d to c phases occurs during solidification,[2] and a corrugated shell structure is
MEIJUAN GAN, ZENGKUN DAN, SHAODA ZHANG, QIANGQIANG WANG, XUBIN ZHANG, SHENGPING HE, and QIAN WANG are with the College of Materials Science and Engineering, and Chongqing Key Laboratory of Vanadium–Titanium Metallurgy and Advanced Materials, Chongqing University, Chongqing 400044, P.R. China. Contact e-mail: [email protected]. Manuscript submitted November 3, 2019.
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created. This behavior results in a considerable heat flux variation and stress concentration within the solidified shell, and even longitudinal cracking. Thus, mild cooling of the shell is desirable for crack-sensitive steel grades. It is well known that heat is transferred in three ways, namely conduction, radiation, and convection. Because the liquid slag film sandwiched between the shell and the solid film is sufficiently thin (ca. 0.1 mm thick),[3] the effects of convection can be ignored.[4] To achieve mild cooling condition, it is common practice to increase the fraction of the crystalline phase in the slag film. Greater crystallization not only attenuates the radiative heat transfer through the transparent glassy slag, but also greatly weakens lattice vibrations by forming crystal grain boundaries and micro pores in the film, which reduces the conduction heat transfer.[5–8] However, a reduction in heat transfer by increased crystallization occurs at the expense of worsened lubrication of the shell surface. Furthermore, crystalline phases tend to be more rigid and are prone to fracture,[9] leading to fracture of the slag film at the lower part of
the mold and star and spongy cracking.[10] Thus, balancing the roles of the mold slag between heat transfer and lubrication is key to improving the quality of continuous casting slabs. Despite the magnitude of the radiation, the total heat flux across the slag film is not known with certainty. The reduction in radiative heat flux, approximately accounting for 20 to 40 pct,[5,7,11] indeed would result in mild cooling in continuous casting and affect the cracking level.[12] Another approach to control heat transfer is the addition of transition metal oxides (e.g., FeO, MnO, NiO, Fe2O3) into the mold slag to improve the ability to absorb infrared radiation. Several typical studies[4,12–15] have examined the effects of transition metal oxides on the heat flux, as summarized in Table I. Table I indicates that addition of transition metal oxides into mold slag reduces the radiative heat transfer by enhancing the ability of the slag to absor
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