Effect of Slag-Steel Reaction on the Initial Solidification of Molten Steel during Continuous Casting
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continuous casting, mold flux is usually added on the top of molten steel surface to protect the steel from oxidation, moderate the mold heat transfer, absorb the inclusions rise from molten steel, lubricate the newly formed shell, and control the heat transfer between the shell and the mold.[1] The contents of Al2O3 and TiO2 were found to increase in the mold flux with the progress of continuous casting of Interstitial-free (IF) steels (Ti-bearing, Al-killed ultra-low carbon steel).[2] The sources of Al2O3 and TiO2 pickup were partly from the absorbed inclusions of Al2O3 and TiO2 (original from steelmaking) rising from molten steel; however, the majority was from the reaction between the silica in liquid slag and the alloying element in molten steel as shown in the following equations:
WANLIN WANG, ZHICAN LOU, and HAIHUI ZHANG are with the School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan, China. Contact e-mail: [email protected] Manuscript submitted August 10, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
4½Al þ 3ðSiO2 Þ ¼ 3½Si þ 2ðAl2 O3 Þ
½1
½Ti þ ðSiO2 Þ ¼ ðTiO2 Þ þ ½Si:
½2
The pickup of Al2O3 content would lead to the increase of the break temperature (Tbr), solidification temperature, fraction of crystalline phase (fcrys), and viscosity (ls) of mold flux, and then introduce the reduction of slag consumption (Qslag = 0.55/l0.5 s ÆVc or Qslag = 0.6/lsÆVc).[3] Additionally, with the increase of Al2O3 and TiO2 in the spent mold flux, the most common crystal cuspidine (Ca4Si2O7F2) would partly be replaced by nepheline (NaAlSiO4) and gehlenite (Ca2Al2SiO7).[3,4] Once the amount of alumina exceeds a critical value (about 15 to 20 wt pct), the serious operational problems, such as poor mold lubrication, severe slag rim formation, surface defects, sticking, and breakouts, would arise.[3,5–7] Thus, the clear understanding of the effect of slag-steel reaction on mold flux performances during continuous casting would be of great importance for the control of the quality of final slab. Previous works suggested that the thickness of the liquid slag film (dl) between the oscillating mold and the solidifying shell determines mold lubrication, where an increase of break temperature and solidification temperature of mold flux would lead to a reduction of dl that could deteriorate the mold lubrication.[8]
Additionally, mold heat flux is usually decreased with increasing the thickness of solid slag film (ds) and fcrys.[9] Moreover, crystallization of slag film will decrease the heat transfer by scattering radiation at the grain boundaries,[10–15] and lead to the formation of interfacial thermal resistance (Rint) between the mold and the slag film.[16] In practice, Rint increases with increasing fcrys and ds.[17,18] Works by Esaka[19] and Yamauchi et al.[20] indicated that the thickness of infiltrated mold/ shell slag film decreased with increasing the basicity (CaO/SiO2) and solidification temperature of mold flux. Hanao et al.[21] has investigated the heat transfer behavior of slag fi
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