Effect of Slag Composition on Inclusions in Si-Deoxidized 18Cr-8Ni Stainless Steels
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TRODUCTION
NON-METALLIC inclusions in steel have a detrimental effect on the property of steels, such as their strength, toughness, and fatigability.[1] Control of inclusions is one of the main tasks of steelmakers.[2–6] In the stainless steel, spinel inclusions resulted in sliver-like defects on the cold sheet of stainless steel. Inclusions of Al2O3 might also cause nozzle clogging due to their high-melting temperature.[7–9] The Al2O3-rich Al2O3SiO2-CaO-MnO inclusions had poor deformability, causing spot defects on the surface of stainless steel plate after polishing.[10] It was reported that undeformable inclusions can be avoided by controlling of the dissolved aluminum in steel.[11] In order to avoid the generation of high-Al2O3 inclusions, silicon or manganese deoxidation was used and aluminum deoxidation was excluded. Miki and Hino[12,13] investigated Si deoxidation of molten Fe, Ni, Fe-Ni, Fe-Cr, Fe-Cr-Ni, Ni-Cu, and Ni-Co alloys. The deoxidation equilibrium of Si in the molten Fe, Ni, Fe-Ni, Fe-Cr, Fe-Cr-Ni, Ni-Cu, and Ni-Co alloys could be quantitatively expressed by binary interaction parameters determined on basis of the quadratic formalism. Suzuki et al.[14,15] YING REN, Ph.D. Student, LIFENG ZHANG, Professor, and WEN FANG, Master Student, are with the State Key Laboratory of Advanced Metallurgy and the School of Metallurgical and Ecological Engineering at University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact e-mail: [email protected] SHIJIE SHAO, JUN YANG, and WEIDONG MAO, Engineers, are with Baoshan Iron & Steel Co., Ltd. Stainless Steel Business, Shanghai 200431, China. Manuscript submitted April 15, 2015. Article published online December 18, 2015. 1024—VOLUME 47B, APRIL 2016
studied the deoxidation equilibrium of Cr-Ni stainless steel with silicon, and the phase stability diagrams of Si-Al complex deoxidation of Fe-Cr melts were at 1823 K (1600 C). Tanahashi et al.[16] measured activities of MnO and SiO2 in the MnO-SiO2-CrOx slags and calculated the composition of non-metallic inclusions under the various conditions of Si deoxidation process. Since there was aluminum in ferrosilicon alloy added to deoxidation, the formation of Al2O3-rich inclusions could hardly be avoided in stainless steel.[17,18] Inclusion composition was greatly influenced by the composition of the top slag and the refractory material.[19] Meanwhile, the MgO-based refractory such as dolomite, MgO-C, and MgO-chromite bricks were commonly employed for refining of stainless steel. Therefore, the precise control of inclusions composition in stainless steel could be achieved by choosing the proper top slag composition. The high FeO and MnO contents in the ladle slag were the potential for reoxidation and the corresponding generation of alumina inclusions in Al-deoxidized steel. The contamination of FeO and MnO should be lowered through countermeasures, such as minimization slag carryover and reduction treatment of ladle slag.[20] The technology of inclusions control by adjusting slag basicity (CaO/ Si
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