Interaction of alumina inclusions in steel with calcium-containing materials
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I. INTRODUCTION
In steel,
ALUMINUM-KILLED steels routinely clog tundish well nozzles and submerged entry nozzles (SENs) during continuous casting. Clogging initiates when alumina attaches to the nozzle wall and restricts the flow of steel. Typically, the accretion accumulates throughout a casting sequence, decreasing the casting speed and resulting in frequent nozzle changes or, in the most severe instances, termination of the cast. Portions of the accretion layer often erode and enter the steel stream as inclusions that subsequently impact quality. Accretions typically consist of a friable alumina layer sintered to the nozzle wall. Occasionally, a dense alumina layer has been observed between the alumina layer and the nozzle refractory. Figure 1 schematically represents a typical accretion structure with a dense alumina layer present.[1] The alumina necessary for accretions may be derived from a number of sources. Aluminum is added to the steel to deoxidize the metal by reacting with dissolved oxygen to form alumina. Steel is exposed to a number of oxygen sources during processing, including air during steel transfer, leaks in ladle shrouds or SENs, and out-gassing from refractory materials.[2,3,4] Oxidizing gases diffuse to the steel/refractory interface, where they can react with the soluble aluminum in the steel. Equations [1] through [4] present the reactions theorized to occur in industrial alumina-graphite refractories, the most common SEN refractory. Silica is typically available because it is a major impurity phase in these nozzles. In the alumina-graphite refractory, Al2O3(s) 2C(s) Al2O(g) 2CO(g)
[1]
SiO2(s) C(s) SiO(g) CO(g)
[2]
ROBERT B. TUTTLE, Assistant Professor, is with the Department of Mechanical Engineering, Saginaw Valley State University, Saginaw, MI 48710. Contact e-mail: [email protected] JEFFREY D. SMITH and KENT D. PEASLEE, Associate Professors, are with the Department of Materials Science and Engineering, University of Missouri–Rolla, Rolla, MO 65409. Manuscript submitted February 20, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
Al2O(g) 2CO(g) Al2O3(s) 2C
[3]
3SiO(g) 2Al Al2O3(s) 3Si
[4]
Most buoyant deoxidation products are assimilated by the slag; however, smaller alumina particles may ultimately be cast with the steel. Research conducted by Dawson determined that accretions occur in regions of stagnant steel.[5] Small alumina particles generated by the reaction between soluble aluminum and oxygen are transported with the steel and accumulate within these stagnate flow zones. Once trapped, the alumina particles sinter to the nozzle wall and to each other, forming the porous alumina growth typically associated with nozzle clogs. Preventing alumina accretions in steelmaking nozzles may not be possible; however, it may be possible to inhibit or destabilize the problematic growths. The most meticulous oxygen-control measures cannot prevent all oxygen from reaching the steel. Ultimately, alumina at some concentration will have to be managed. A novel appro
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