Structure of Solidified Films of Mold Flux for Peritectic Steel
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the continuous casting process, mold flux lubricates the contact between the steel shell and the mold and controls the heat flux from the steel shells to the mold for better surface and sub-surface quality of slabs.[1] For continuous casting of medium-carbon steels, the peritectic transition from delta ferrite to austenite near the meniscus can cause large volume shrinkage, which renders some steel grades very crack-sensitive. Fast or non-uniform cooling conditions tend to cause longitudinal cracks.[1,2] Lower heat flux from the steel shell to the mold near the meniscus is associated with diminished cracking.[3] Previous researchers developed several methods to decrease the heat flux,[4–8] such as coating the upper part of mold surface, adding oxides of transition metals to decrease radiation heat transfer, and decreasing the mold cooling intensity. However, none of these are widely used in industry. A detailed review of industrial data confirmed that higher basicity films give lower mold heat transfer.[9] It appears that using higher basicity mold fluxes (that tend to crystallize when forming films in the mold and to form thick solid slag films) is the most efficient approach to decrease the heat flux in commercial production.[10] Previous researchers proposed that mold fluxes with a high crystallization capacity mainly control heat transfer in three ways:[1,11–14] (1) Large surface roughness is caused by crystallization of the glass layer in the slag film (devitrification),
XIAO LONG is with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China and also with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh PA 15213. SHENGPING HE and QIAN WANG are with the College of Materials Science and Engineering, Chongqing University. P. CHRIS PISTORIUS is with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted September 28, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
increasing the interfacial resistance between the copper mold and slag film. Measurements on films recovered from industrial casters confirmed that higher basicity films had much rougher interfaces.[15] Results from industrial trials with a thin-slab caster support this proposed interfacial effect: changing from a lower basicity flux to a higher basicity flux during casting (apparently retaining lower basicity remnants on the mold surface) had a smaller effect on heat flux than using the higher basicity flux from the start of casting.[16] (2) Precipitation of crystals by solidification can cause the formation of defects in the slag film like pores and cracks which would decrease the thermal conductivity. Crystals tend to decrease the contribution of radiation heat transfer to the total heat flux. It was suggested that the mechanism is that crystallization increases the reflectivity of the film, decreasing radiative heat transfer from th
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