Formation Mechanism of Oxide-Sulfide Complex Inclusions in High-Sulfur-Containing Steel Melts
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LFURIZED steel has been widely employed in automotive parts. The [S] content in these grades is controlled in the range of 200 to 800 ppm to improve their machining properties due to the formation of MnS inclusions in the steel.[1,2] It is well known that oxide-sulfide complex inclusions are formed in molten steel during the ladle refining process for high-S-containing steel. Many researchers have reported that the ‘‘oxide (mainly CaO-Al2O3 based system)+CaS’’ complex inclusions were formed when calcium treatment was carried out,[2–19] and these inclusions cause nozzle clogging during continuous casting. Thermodynamic calculations have been conducted by several researchers to predict the precipitation of oxide-sulfide complex inclusions to improve the castability of middle grade S-containing steel (0.02 to 0.03 mass pct S). Holappa et al.[7] reported a thermodynamic evaluation of inclusion formation and modification in Ca-treated steels. They proposed that CaS precipitation is promoted with increasing concentrations of [S] in the residual liquid steels during solidification. Gatellier et al.[3] calculated the influence of the (pct S)/
JAE HONG SHIN and JOO HYUN PARK are with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea. Contact e-mail: [email protected] Manuscript submitted June 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
(pct O) ratio in the inclusions on the formation of oxide and CaS. Choudhary and Ghosh[9] developed a thermodynamic model for predicting the formation of oxide-sulfide complex inclusions arising out of competitive reactions among [O], [S], and [Ca] in Al-killed steel. Recently, thermodynamic calculations that include the activity of CaS in the MnS-CaS solid solution have been conducted to predict the precipitation of CaS-bearing inclusions.[18] Alternatively, many researchers have studied the formation mechanism of sulfide inclusions, such as CaS in Ca-treated and low-S-containing steels (S < 0.02 mass pct), to improve castability. Pires and Garcia studied the modification of oxide inclusions present in Al-killed low-carbon steel by adding calcium.[8] They reported that the [S] content in the Al-killed molten steel must be controlled not more than 100 ppm to fully modify the alumina inclusions. Verma et al.[10–12] studied the modification of spinel inclusion by Ca treatment. They reported that spinel-CaS complex inclusions formed due to a reaction between spinel and CaS during the modification of spinel to liquid oxide inclusions in Ca-treated steels. Inclusions in casting slab samples with various compositions of molten steels were investigated in Al-killed and Ca-treated steels with low S content based on industrial trials by Zhao et al.[19] They proposed that the formation mechanism of Al2O3-CaS complex inclusion is based on the shape of the CaS layer of the aluminate inclusions as follows. (1) A crescent-shaped CaS layer surrounding calcium aluminate is generated by the
reaction between dissolved [Al], [S], and calcium aluminate inclusion or diffusion o
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