Inclusion Characterization in Aluminum-Deoxidized Special Steel with Certain Sulfur Content Under Combined Influences of
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ODUCTION
SPECIAL steels were widely used as bearings, gearings, axis, wires, etc. in infrastructures and manufacturing industry. Nonmetallic inclusions are often observed as origins of various problems during the production, fabrication, and service of steel. To improve productivity and performances of steels, number, size, chemistry, etc. of inclusions should be strictly controlled.
ZE-WANG HOU, MIN JIANG, EN-JIAO YANG, SHENG-YA GAO and XIN-HUA WANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing City, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Using Al deoxidation and basic slag refining, oxygen in steel can be reduced to a very low level. The advantage of this technical choice is that the number of inclusions can be greatly decreased. However, the disadvantage is that high melting point inclusions, such as Al2O3 or MgO-Al2O3, can be easily formed, to reduce castability and the antifatigue property of steel.[1–12] To minimize the negative effects of those inclusions, calcium treatment was often applied to modify them into low melting point calcium aluminate or calcium magnesia aluminate.[13–32] However, formation and evolution of inclusions in Ca treatment are very complex and low melting point inclusions are not absolutely formed.[15,17,18,21,23,32,37–45] According to the CaO-Al2O3 binary phase diagram, calcium aluminates can be reversibly changed between solid and liquid state with the variations of CaO contents. It was widely accepted that modification of Al2O3 by [Ca] during Ca treatment followed the unreacted core model, and accumulation of CaO in inclusions against reaction time contributed to the
formation of liquid calcium aluminate.[25,26,43] Compared to Al2O3, modification of spinel is more difficult. Because Mg in inclusion should be reduced by [Ca] and diffusion of the reduced Mg to inclusion surface would be the limiting step, incomplete modification of spinel can occur, especially when they are large in size. In this case, inclusion would be featured by a semiliquid/liquid calcium aluminate surface layer surrounding an unreacted spinel core.[46] According to Pretorius et al., effective modification of spinel by Ca treatment involves two steps, viz. reacting with [Ca] and keeping the reaction, in which low oxygen content in steel is very important.[20] Second, CaS inclusions can be easily formed in Ca treatment. CaS can exist as singular particles or clusters and it can also coexist with calcium aluminates/calcium magnesia aluminates as oxy-sulfides or together with MnS as (Ca, Mn)S.[32–36] Verma et al. also found CaS can transiently coexist with spinel or Al2O3 and further react with Al2O3 and spinel to shift them into liquid inclusions.[17–19] Moreover, CaO-CaStype inclusions can be formed if strong Ca treatment is applied (intensive calcium addition).[15,23,47] Optimal steel chemistry is very important to inclusion modification in Ca treatment. Prev
