Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part II. Results and
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NTRODUCTION
AS discussed in Part I, this research tested the hypothesis that, upon calcium injection into liquid steel, CaS forms and plays a central role in subsequent modification of alumina inclusions. This suggestion is based on previous research that demonstrated CaS to be an intermediate reaction product.[1–4] The reaction responsible for modification of alumina is hence proposed to be as follows: 3CaS þ Al2 O3 ! 3ðCaOÞ þ 2½Al þ 3½S
½1
If this mechanism held, then the inclusions would be a mixture of CaS and Al2O3 immediately after calcium injection, converting to calcium aluminates by reaction [1]. In reaction [1], [Al] and [S] represent the species dissolved in Fe and (CaO) indicates CaO in the inclusion. The proposed reaction path is illustrated in Figure 1. If the proposed mechanism held, the inclusion compositions would lie along the CaS-Al2O3 join after calcium injection; after subsequent reaction, Al and S would be N. VERMA, Ph.D. Candidate, and PETRUS C. PISTORIUS and RICHARD J. FRUEHAN, Professors, are with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] MICHAEL POTTER, Senior Scientist, is with the RJ Lee Group, Monroeville, PA 15146. MINNA LIND, formerly Visiting Researcher, Carnegie Mellon University, is now Postdoctoral Researcher with Aalto University, FI-00076 Aalto, Finland. SCOTT R. STORY, Research Consultant, is with the Department of Process Technology—Steelmaking and Casting, United States Steel Corporation Research and Technology Center, Munhall, PA 15120. Manuscript submitted February 25, 2011. Article published online April 22, 2011. 720—VOLUME 42B, AUGUST 2011
returned to the steel melt by reaction [1], causing the inclusion compositions to follow the arrows in Figure 1. The molar ratio of Ca/Al in the non-CaS portion of the inclusion (indicated by [Ca/Al]mod in the figure, and as explained in Part I) would be increased by the reaction. Successful modification would require the resulting inclusion composition to lie within the shaded region in Figure 1; this shaded region is the range of compositions that are at least 50 pct liquid, at 1823 K (1550 °C) (as calculated with FactSage 6.2 [ThermFact Inc., Montreal, Canada] using the FSstel and FToxid databases[5]). To test this possibility, aluminum-killed heats containing 7 ppm to 100 ppm sulfur were calcium treated in the laboratory, taking steel samples before and at various times after calcium treatment. In all cases, as demonstrated previously in Part I, the sulfur content of the steel was sufficiently low to ensure that solid CaS was not stable in contact with alumina or CaAl2O4saturated liquid calcium aluminate inclusions. Several samples from industrial heats were also analyzed, and a typical example is given in this paper. For the industrial sample, the heat size was 200 metric tons, and the amount of calcium silicide injected (for calcium treatment) was 170 kg. The steel composition of the industrial
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