Wetting and Erosion of ZrO 2 -Graphite Refractory by CaO-SiO 2 and CaO-Al 2 O 3 -Based Mold Slags for Submerged Entry No
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TRODUCTION
THE erosion of a submerged entry nozzle (SEN) can result in a necking phenomenon at the slag–metal interface during the continuous casting of steels. Once the residual thickness of the SEN is unable to withstand thermal shock and erosion attacks from both molten slag and liquid steel, the SEN must be replaced, which decreases productivity by limiting the number of ladles that can be continuously cast, diminishes slab quality by enriching them with endogenous inclusions or refractory oxide constituents, and potentially creates emergency situations on the casting platform.[1] Thus, to ensure reliable stability for continuous casting operations, a long service life for SENs has become a requirement considering current prevalent multisequential casting processes.[2]
SHAODA ZHANG, HUAZHI YUAN, MEIJUAN GAN, QIANGQIANG WANG, SHENGPING HE, and QIAN WANG are with the College of Materials Science and Engineering, and Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and Advanced Materials, Chongqing University, Chongqing 400044, China. Contact e-mail: [email protected] Manuscript submitted October 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Most SENs are fabricated from oxide-graphite refractory (such as Al2O3-graphite, ZrO2-graphite, etc.) with inserts of ZrO2 or ZrO2-graphite in the region subjected to extensive wear, which is usually referred to as the slag-line zone,[3] as zirconia exhibits higher resistance to erosion than alumina.[4] Nevertheless, ZrO2 will undergo a phase transition from cubic zirconia to monoclinic zirconia, with an accompanied volume increase of ~ 5 pct at high temperature, resulting in the breakup of the zirconia grains.[2,5] Consequently, reagents, such as CaO, MgO, or Y2O3, are often used for the partial stabilization of zirconia.[6] It has also been reported[6] that adding ZrB2 to the ZrO2-graphite refractory could prevent erosion of SEN by reducing the wettability between the mold slag and the refractory and preventing the slag from penetrating into the microstructure. However, ZrB2 is relatively expensive compared to other materials and is only used for SENs in special cases. It has also been reported that[7–9] CaO could react with the ZrO2 to generate CaZrO3, especially under R > 1.5 [R =(mass pct CaO)/(mass pct SiO2)] condition,[8] which not only inhibits the low melting point phases penetrating into the substrate by increasing the viscosity but also contributes to the formation self-protective coatings on the surface of the refractories.[7] For mold slag, it is preferable to restrict the CaF2 content, as high amounts of CaF2 were found to substantially wear
away ZrO2-graphite refractories due to their attack on the stabilizing oxide.[2,10] Under certain circumstances, when SEN erosion is a significant issue, approximately 2 to 3 pct ZrO2 is added to the mold slag to bring the concentration of ZrO2 in the slag close to the saturated concentration of ZrO2 to decrease the driving force for the dissolution of SEN refractory.[2,11] Some practical operation practices, s
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