Reduction of CaO and MgO Slag Components by Al in Liquid Fe
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RECENT development of new Advanced HighStrength Steel (AHSS) grades has resulted in higher levels of the alloying elements Si, Mn, Al, and Cr than traditional low-C Al-killed grades.[1–3] Particularly important is Al, as increased dissolved Al content can reduce MgO, CaO, and other oxides that comprise ladle slags and refractories at liquid steel refining temperatures. Reduction of these slag components leads to Ca or Mg transfer to inclusions in the steel and uncontrolled changes to inclusions, leading to castability or performance problems.
HAOYUAN MU is with the Advanced Refractories Division of Vesuvius, Bettsville, OH, 44815. TONGSHENG ZHANG is with the Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA, 15213 and with the School of Metallurgy and Environment, Central South University, Changsha, China and also with the National Center for International Research of Clean Metallurgy, Central South University. RICHARD J. FRUEHAN is with the Center for Iron and Steelmaking Research, Carnegie Mellon University. BRYAN A. WEBLER is with the Center for Iron and Steelmaking Research, Carnegie Mellon University and also with the Materials Science and Engineering Department, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted September 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Several previous studies have investigated the reduction of slag and refractory components in Al-killed (approx. 0.03 wt pct Al) steels. Spinel (MgOAl2O3) formation due to MgO reduction has been the most extensively studied, often in stainless steels since spinel inclusions are detrimental to production processes and final properties.[4–11] Mass transfer of Mg in liquid steel has been shown to be the rate determining step in the MgO reduction and spinel formation processes.[10–13] Researchers have investigated the thermodynamics of MgO reduction from slags and spinel inclusion formation and suggested to optimize the time of Al additions for deoxidation and to minimize MgO activity in slags.[4,9] If the steel is Ca treated, the spinel inclusions can be modified to CaO-MgO-Al2O3, where the MgO in the inclusions is reduced by the added Ca.[7,14–17] Metallic Mg additions can likewise modify CaO-Al2O3 inclusions to CaO-MgO-Al2O3 with possible CaO-Al2O3 cores.[18] Even without Ca treatment, previous work has suggested that sufficient Ca can be transferred from the slag and modify spinel inclusions to liquid calcium aluminates when slag was reduced by metallic Al additions.[8,9] Liquid calcium aluminate, usually formed at the interfaces of original inclusion and metal, has been shown to be stable at 1873 K (1600 C) even though dissolved Ca content in liquid steel is very low.[7–9,14–17] However, reduction of CaO has not been as extensively studied as has reduction of MgO. Some of
the challenges in quantifying the reduction of CaO is due to the large variability in the thermodynamics of dissolved Ca in liquid iron.[19] It is possible, however, that some inferences about the dissolv
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