Thermo-Physical Properties of B 2 O 3 -Containing Mold Flux for High Carbon Steels in Thin Slab Continuous Casters: Stru
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MOLD fluxes play various roles in ensuring defectfree steels. During continuous casting, mold flux can absorb various oxide and nitride inclusions; protect liquid steels from the dissolution of atmospheric elements, such as oxygen and nitrogen; control the radiative and conductive heat transfer; insulate the molten steel from excessive heat loss through the exposed steel; and lubricate the partially solidified shell and the water-cooled copper mold to prevent sticking. In particular, lubrication and heat-transfer are highly influenced by the viscosity and crystallization characteristics, and the optimization of these properties is particularly essential in thin-slab casters to ensure uniform heat removal at high casting speeds. Non-uniform heat transfer in the mold is often caused by inappropriate mold flux infiltration and when exacerbated, sticker breakouts and longitudinal surface cracks can occur.[1] High-speed thin slab casting often exhibits this inappropriate infiltration issue and due to its thinner shell growth exiting the mold, thin slab
JUN-YONG PARK, GI HYUN KIM, JONG BAE KIM, and SEWOONG PARK, Graduate Students, and IL SOHN, Associate Professor, are with the Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea. Contact e-mail: [email protected] Manuscript submitted March 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
casting is more sensitive to un-optimized mold fluxes. As the surface area to volume of the mold is high for thin slabs,[2] powder consumption is much lower compared to billets, blooms, and slabs of conventional casters, where the powder consumption is a qualitative index to estimate infiltration of the mold flux and dependent upon mold size,[3,4] break temperature,[2] casting speed,[5] and viscosity.[2,6,7] In addition, thin slab casters can achieve casting speeds in excess of 7 m/min[8,9] resulting in lower powder consumption and greater possibility of a breakout.[9] Thus, optimizing the mold flux to ensure sufficient infiltration by lowering the viscosity and break temperature is essential for thin slab casting.[9] The infiltrated mold flux between the water-cooled copper mold and the solidified steel shell consists of three layers. An initial glassy layer is formed adjacent to the copper mold owing to the large cooling rates during direct contact with the water-cooled mold.[4] A crystallized layer occurs next to the glassy layer, and a molten liquid layer exists adjacent to the partially solidified steel shell.[10] As this multi-layer flux traverses down the mold, the contraction of the solidified steel shell may pull back toward the molten steel and an air gap may form between the water-cooled copper mold and the solidified flux layer. This air gap can be aggravated with an un-optimized taper in the mold. Previous studies have reported that heat transfer from the molten steel to the mold occurs by both conduction and radiation heat transfer.[11,12] Cho et al.[13] found that
the radiative heat transfer within a 1.5-mm-thick flux film was 32 to 47 pct
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