Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps
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A. Origin of Spinel Inclusions in Aluminum-Killed, Low-Carbon Steel
MAGNESIUM spinel inclusions (solid solutions of MgAl2O4 and Al2O3) form in aluminum-killed steels during secondary metallurgy, if the ladle slag is well deoxidized (with low FeO and MnO levels)[1]; arcing during ladle furnace processing also contributes to the spinel formation.[2] Spinel inclusions seem to form in liquid steel mainly by a reaction between the liquid steel and the ladle slag (rather than by reaction between steel and refractory); this conclusion is supported by the following recently reported observations: (1) The magnesium oxide content in inclusions increases in parallel with silicon pickup by the steel (and silicon pickup occurs by reaction between steel and ladle slag)[3]; (2) no difference in spinel formation was found in steel contained in ladles fully lined with MgO-C brick, and ladles with MgO-C at the slag line only[4]; and (3) reaction between aluminum-killed steel and an MgO crucible (in laboratory runs) did yield spinel as product, but the spinel particles remained at the steel–MgO interface and did not enter the steel melt.[5] NEERAV VERMA, formerly PhD Candidate, with the Center for Iron and Steelmaking Research (CISR), Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, and is now Research Engineer, with ExxonMobil Upstream Research Company, Houston, TX 77098. PETRUS C. PISTORIUS and RICHARD J. FRUEHAN, Professors, are with the Center for Iron and Steelmaking Research (CISR), Department of Materials Science and Engineering, Carnegie Mellon University. Contact e-mail: [email protected] MICHAEL S. POTTER, Scientist, is with the RJ Lee Group, Monroeville, PA 15146. HELMUT G. OLTMANN, LMF/VTD Metallurgist, is with Nucor Steel, Mount Pleasant, SC 29465. EUGENE B. PRETORIUS, Manager, is with Steelmaking Technology, Nucor Steel, Mount Pleasant, SC 29465. Manuscript submitted January 17, 2012. Article published online October 4, 2012. 830—VOLUME 43B, AUGUST 2012
B. Calcium Modification of Spinel Inclusions It is now well established that the spinels in the liquid steel are modified readily to liquid inclusions, fully or partially, through calcium treatment.[3,5,6] Important differences between the calcium modification of alumina and of spinel inclusions are as follows: (1) Liquid oxide is the first product when calcium reacts with spinel, whereas the solid calcium aluminates form first when the calcium reacts with alumina (for this reason it has been suggested that spinels are easier to modify than alumina)[7] and (2) the modification of spinels involves a decrease in the MgO content of the inclusions.[3] Figure 1 shows an example of the latter effect. In this figure, the inclusion compositions are plotted on a Mg-Al-Ca diagram; the area of each symbol is proportional to the number fraction of inclusions with that composition.[8] The observed decrease in MgO (as in Figure 1) is real and not an artifact of matrix effects during microanalysis of these micron-sized inclusions[9]; mag
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