Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part I. Background,
- PDF / 575,346 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 175 Views
THE
production of clean steel involves the control, elimination, or modification of nonmetallic inclusions.[1] In Al-killed steels, alumina (Al2O3) is the main inclusion type. In some cases, these inclusions also can contain some MgO,[2,3] but the work presented here focuses on alumina inclusions and their modification. Calcium treatment, by powder injection or wire feeding, is commonly used to modify solid alumina inclusions to form calcium aluminates, which can be fully or partially liquid, depending on the CaO content.[4] Liquid inclusions improve steel castability. The following reaction sequence [1] summarizes the conventional view[5,6] of the reactions after calcium injection, which include melting, vaporization, and dissolution (in liquid steel) of the calcium as well as a reaction with alumina, dissolved oxygen, and dissolved sulfur: CaðsÞ ! CaðlÞ ! CaðgÞ ! ½Ca ½Ca þ ½O ! CaO ½Ca þ ½S ! CaS ½Ca þ ðx þ 1=3Þ Al2 O3 ! CaO xAl2 O3 þ 2=3 ½Al ½1
NEERAV VERMA, Ph.D. Candidate, and PETRUS C. PISTORIUS and RICHARD J. FRUEHAN, Professors, are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, 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 at Carnegie Mellon University, is now with Aalto University, FI-00076 Aalto, Finland. SCOTT STORY, Research Consultant, is with the United States Steel Corporation Research and Technology Center, Munhall, PA 15120. Manuscript submitted February 24, 2011. Article published online April 12, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
The importance and role of CaS formation are not clear from previous works. CaS is not stable in contact with unmodified or CaAl2O4-saturated modified inclusions in aluminum-killed steel if the dissolved sulfur content is less than approximately 300 ppm at 1873 K (1600 °C) (Figure 1). In Figure 1, CaS is an equilibriumstable phase for steel compositions above the line. Based on these facts, it can be concluded that the extent of CaS formation at typical sulfur levels in steel is negligible.[6] However, CaS was reported to form as a transient phase[9–11] immediately upon calcium injection,[3] or perhaps only after calcium aluminates have formed.[11] The subsequent reaction mechanism of CaS is similarly unclear; suggestions include the evaporation of calcium[10] or an unspecified reaction leading to equilibration (i.e., consumption of CaS).[11] Our preliminary work supported a direct role of CaS in the modification of alumina for 3-kg laboratory steel melts containing 45 ppm S and 0.04 to 0.05 pct Al. The transient CaS inclusions in contact with unmodified alumina were observed in steel samples taken immediately after calcium addition, whereas steel samples that were taken later on contained partially modified (calcium aluminate) inclusions with much lower sulfur contents.[3] It is hence proposed that the CaS can react with the alumina to yi
Data Loading...
