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TITANIUM is added to ultra-low carbon steel to improve the fracture toughness. Titanium (Ti) reacts with carbon and nitrogen to form TiC, TiN, and Ti (C, N) which are present at the grain boundaries. These carbides prevent the motion of the grains during steel processing.[1,2] Although Ti is beneficial in providing enhanced thermomechanical properties, its addition to liquid steel causes upstream processing problems.[3–12] Ti-bearing molten steel flows from the tundish to the mold through a submerged entry nozzle (SEN). The inconsistent flow of molten steel through the SEN may occur because of clog building up near the exit ports[10] and the upper tundish nozzle.[13] Kaushik et al.[14] and Basu et al.[10] investigated casting productivity problems and suggested solutions to improve the castability of TiSULC (titanium stabilized ultra-low carbon steel) and IF (interstitial free) grade steels, respectively. Analyses of cross-sections of clogged nozzles used for producing IF and TiSULC

MUKESH SHARMA and NESLIHAN DOGAN are with the Department of Materials Science and Engineering, Faculty of Engineering, McMaster University 1280 Main Street West, Hamilton, ON L8S4L7, Canada. Contact e-mail: [email protected].

METALLURGICAL AND MATERIALS TRANSACTIONS B

steel grades show the presence of Al2O3- and Al-Ti-Otype inclusions.[10,14,15] Basu et al.[10] found that aluminum titanate type inclusions are typically reoxidation products. These inclusions are formed when Ti-containing molten steel comes in contact with reducible oxides such as SiO2, present in various refractory materials, ladle slag, or tundish slag, and expressed in Reaction [1].[10] 6þx 6þx ðSiO2 Þ þ 4Al þ Ti ! Si 2 2 þ ðAl2 O3 Þprecipitate þðTiOx  Al2 O3 Þprecipitate

½1

The species in a round bracket in the equation represent species in slag while species with an underline represent those dissolved in steel. From Reaction [1], the dissolved Ti in the steel reacts with oxygen supplied by SiO2 to form the TiOx-Al2O3 complex. Al2O3 inclusions are generally formed during the deoxidation of steel. These alumina inclusions adhere to the wall of the ceramic nozzles and agglomerate to form clusters. The TiOx-Al2O3-type inclusions formed by reoxidation are present alongside Al2O3 inclusions in the clusters. It is difficult to separate liquid steel from TiOx-Al2O3 inclusions because of the high wetting of such inclusion types by steel.[16] The presence of such inclusion types in steel aggravates melt freezing in clogs. An industrial study by Basu et al. shows that the casting of steel with Ti displayed aggravated clogging of ceramic nozzles in comparison with the casting of Ti free steel.[10]

In order to reduce the clogging problem, Al-Ti-O-type inclusions should be prevented from sticking to the wall by using nozzle well packing material or internal lining of submerged entry nozzle.[10] Alternatively, they can be modified by forming a liquid phase[9,17] or removed by dissolution in top slag.[14] Limited studies exist regarding the dissolution of Ti-containing in