In the modern making of steel, molten metal flows from a bathtub-shaped vessel -- called a tundish -- into a water-cooled, bottomless mold in a continuous casting operation. Clogging of the tundish nozzle can lead to hidden defects in the steel or costly halts in production. Researchers at the University of Illinois have developed a mathematical model that successfully predicts when clogs are about to occur, allowing time for remedial action.
"Nozzle clogging has been an industrywide problem that seemed impossible to solve," said Brian Thomas, a professor of mechanical and industrial engineering and director of the Continuous Casting Consortium at the UI. "In the past, steel manufacturers simply had to wait for a clog to occur, and then suffer the consequences. By identifying potential clogging conditions in advance, manufacturers can improve both product quality and plant productivity."
One way manufacturers try to minimize clogging is by injecting inert argon gas through the nozzle wall. The gas reduces contact between metal and atmosphere, which otherwise could create small oxide inclusions in the steel. These bits of oxide, along with other foreign particles, can collect on the nozzle, eventually restricting the flow and causing the steel to freeze into a solid plug.
To maintain constant production, manufacturers use a slide gate to control the flow of molten steel. But moving the slide gate can reduce pressure in the nozzle and suck in air, further aggravating the problem of oxide formation. A necessary step in the model1s creation was determining the pressure drop in the system as a function of both slide-gate position and argon-injection rate. By using principles of fluid dynamics, heat transfer and stress analysis, Thomas and his colleagues simulated the relevant aspects of the steel-making process. The resulting computational model was verified in two independent ways.
First, the model's predictions were compared with experimental results obtained using particle image velocimetry on scale models of the tundish, nozzle and mold. The model predictions and PIV measurements compared very well. Second, actual plant measurements taken during normal steel production were compared with those predicted by the model. Again, the results agreed favorably over a wide range of operating conditions.
"Based upon process variables such as casting speed, tundish depth, slide-gate opening and argon-injection rate, the model predicts the theoretical flow rate through the nozzle," Thomas said. "By comparing the actual flow rate with the predicted flow rate, plant personnel can infer the clogging status of the nozzle and take the appropriate action."
Thomas described the model at the annual meeting of The Minerals, Metals and Materials Society, held March 12-16 in Nashville, Tenn., and the application to clogging at the annual Steelmaking Conference, held March 26-29 in Pittsburgh.