Transient Asymmetric Flow and Bubble Transport Inside a Slab Continuous-Casting Mold
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nuous-casting process, argon gas is injected into the submerged entry nozzle (SEN) to prevent attachment of nonmetallic inclusions to the inner wall of the SEN, to prevent air from entering the SEN, to promote desired flow patterns, and to promote the flotation of nonmetallic inclusions.[1–3] Most of the bubbles float up because of density difference with molten steel, but some of the bubbles are dragged deep into the mold because of the inertial force. However, fine argon bubbles were sometimes observed in the continuous-casting slabs.[4] In the subsequent rolling process, these argon bubbles could lead to formation of the pinholes, blowholes, and ‘‘pencil pipe’’ blister defects in the final products.[5–7] Until now, these defects have not been successfully and completely eliminated from the continuous-casting slab. Argon gas disintegrates into many uneven-sized bubbles as it emerges out from the SEN. They are characterized by a size distribution function. Bubbles can collide as the spectrum shifts toward larger objects, ZHONGQIU LIU, Ph.D. Candidate, BAOKUAN LI, Professor, and MAOFA JIANG, Professor, are with the School of Materials and Metallurgy, Northeastern University, Wenhua Road 3-11, Heping District, Shenyang 110819, P.R. China. Contact e-mail: libk@smm. neu.edu.cn Manuscript submitted May 23, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
and there are more bubbles with higher terminal velocity and thus with better tendency to escape. Bubbles also can break up and disintegrate into smaller bubbles, influenced by shear and turbulence.[3] Bubble size distribution in the mold after argon injection through the SEN have been studied using water model in some previous studies.[8–11] Recently, Lee and Thomas[11] used water model to investigate initial bubble behavior using specially coated porous samples with different permeabilities; they had found that the number of active sites increases with the increasing gas flow rate, permeability, and velocity of the downward-flowing water, and lower contact angle. However, many small bubble defects have been found in solidified slabs, and the diameters of bubbles[7,12,13] are between 60 and 1000 lm, and even ~20 lm. Argon bubbles are thought of as carrying media for nonmetallic inclusions. There are two ways for bubbles and inclusions to interact.[3] First, bubbles can entrain inclusions in their wake, and inclusions are thus lifted upward without touching the bubbles.[12] The second mechanism is the attachment of inclusions to bubbles. Larger bubbles are capable of taking along hundreds of inclusions, and the diameters of the attached inclusions were ranging from 2 to 15 lm.[7,13] The process of inclusions attachment to a bubble has been widely studied.[14–16] The recent slab ultrasonic testing (UT) results have shown that sizes of many defects are around 100 and 200 lm. Many of the defects are bubbles with a
lot of inclusions attachment. Therefore, it is important to study the motion of argon bubbles inside the solidifying shell as well as the way to control it. The complex t
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