Casting Simulation of Calcium Titanate and Calcium Zirconate Nozzles for Continuous Casting of Aluminum-Killed Steels
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DURING the continuous casting of aluminum-killed steels, alumina accretions frequently form on the nozzle wall obstructing the flow of steel and causing quality and operation problems. Typically, accretions are composed of small micron-sized alumina particles that attach to the nozzle wall and form a powdery friable alumina layer. Many researchers report that the alumina densifies at the nozzle interface resulting in a dense alumina layer between the powdery layer and the nozzle refractory (Figure 1 for a typical accretion structure).[1] In some cases, portions of the accretion layer detach and enter the steel stream, which degrades steel quality. In other cases, the accretion continuously grows throughout the casting sequence and decreases the flow rate until eventually the nozzle must be changed or the casting strand plugs. There are several possible methods of forming alumina. Oxygen can dissolve in the steel when air contacts the liquid metal surface during transfer between steelmaking vessels or from leaks in the ladle shroud or submerged entry nozzle (SEN).[2] The dissolved oxygen reacts with dissolved aluminum in the steel forming micron-sized particles of alumina that can eventually migrate to the nozzle surface to form accretions. Many nozzle refractories produce small amounts of suboxide gasses during casting at steelmaking temperatures.[1,3,4] These suboxide gases are transported to the steel/ refractory interface where they react with aluminum in the steel to form alumina. One possible solution to ROBERT B. TUTTLE, Assistant Professor, is with the Department of Mechanical Engineering, Saginaw Valley State University, University Center, MI 48710, USA. JEFFREY D. SMITH, Associate Professor, and KENT D. PEASLEE, Professor, are with the Department of Materials Science and Engineering, University of Missouri– Rolla, Rolla, MO, USA. Contact: e-mail: [email protected] Manuscript submitted February 8, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
eliminate nozzle clogging would be to modify the solid alumina particles attaching to the walls to form liquid inclusions. Liquid inclusions would detach from the refractory and flow with the molten steel preventing accretion formation. A common industrial steelmaking practice for modifying alumina inclusions is calcium wire injection.[5] Sufficient calcium wire is injected to form liquid calcium aluminates through a reaction with the alumina in the steel. To maintain liquid calcium aluminate inclusions, a CaO content of 37 to 54 wt pct must be achieved in the inclusion (Figure 2). Solid calcium aluminate phases are reported to undergo the same agglomeration and sintering processes as solid alumina particles and therefore can cause nozzle clogs.[6] Despite reported reductions in clogging by forming liquid calcium aluminates, calcium treatment does not always eliminate clogs. Steels with sulfur levels above 0.012 pct form CaS. Calcium sulfide particles are solid at steelmaking temperatures and agglomerate and sinter together to form a powdery layer, which obstructs
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