Effect of the CaO-Al 2 O 3 -Based Top Slag on the Cleanliness of Stainless Steel During Secondary Metallurgy
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STAINLESS steel products, due to their high corrosion resistance, have been widely used in areas ranging from the process, to construction and transport industries. However, the product performance can be significantly affected by the steel cleanliness, e.g., the nonmetallic inclusion number density, size distribution, morphology and chemistry. For instance, lowering the inclusion number density and size can extend the bearing and fatigue life of steel products.[1,2] The inclusion chemistry has a more complex impact. Oxide inclusions, such as Al2O3, SiO2, and TiO2, can cause cracking, while coating a layer of sulfide (MnS) on these oxides may reduce their detrimental effect.[2–4] On the other hand, the sulfide can also act as the starting point of pitting corrosion and weaken the corrosion resistance of stainless steel.[2–4] Top slag, because of its considerable effect on deoxidation and desulfurization, influences the inclusion chemistry during the secondary metallurgy.[5,6] Meanwhile, the top slag can also reduce the inclusion number density and area fraction through the slag absorption effect.[7] Therefore, it is of significant importance to investigate the influence of top slag on steel cleanliness. CaO-CaF2 (CSF)-based slag is commonly used in stainless steel production. This slag has a good ability of desulfurization/inclusion removal due to its high basicity, which allows enough (1) free oxygen ion for sulfur PENGCHENG YAN, Ph.D. Student, SHUIGEN HUANG and LIEVEN PANDELAERS, Academic Staff, JORIS VAN DYCK, Technician, MUXING GUO and BART BLANPAIN, Professors, are with the Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, Bus 2450, 3001 Leuven, Belgium. Contact e-mail: [email protected] Manuscript submitted February 21, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
removal[5,6]; and (2) CaO to absorb Al2O3 inclusions.[7,8] However, the CSF slag is so basic that C2S is formed during the cooling of post-metallurgical process. The phase transformation of b-C2S to c-C2S leads to slag disintegration and consequently limits the slag valorization potential.[9] In addition, leaching of fluorinecontaining compounds is also considered to be a potential environmental issue. On the other hand, CaO-Al2O3 (CA)-based slag (about 50 to 55 pct CaO, 40 to 45 pct Al2O3, £5 pct SiO2, and 1.0 lm) is observed in the test with Ar blowing (test 4) than that without it (test 3). Moreover, the statistical properties (Table V) show that the average size of the inclusions is much smaller in test 4 than in test 3, substantiating an effective removal of large inclusions with bottom Ar blowing. B. Inclusion composition Figure 5 depicts a typical evolution of the inclusions during the treatment in test 5. It shows that (1) the SiO2-Cr2O3-MnO complex inclusions are already present in the steel before the start of the experiment. These inclusions consist mainly of spherical and angular shapes (Figure 5(a)). Apparently, this is the result of a reduction (and/or alloying) operation at the end of AOD
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