The U.S. Steel research team wants to know what happens when a "heat" of steel -- a huge ladle containing more than 200 tons of molten metal at close to 3,000 degrees F. -- empties its fiery brew into a continuous casting "tundish." In particular, they want to know as precisely as possible what happens inside the tundish as the molten steel churns and swirls around. The tundish holds the white-hot liquid and feeds it out the bottom into a continuous casting mold, where it forms a moving strand of steel that eventually cools from white to red hot and gets cut into slabs for further processing.
Continuous casting is the most up-to-date technology available for producing high-quality steel at low cost, and good understanding of what goes on in the tundish is critical because it affects the purity and chemistry of the output steel. Impurities, such as oxides of aluminum, calcium and iron, tend to float to the top of the tundish bath. The steel flow must be controlled to enhance this flotation and to prevent turbulence from drawing impurities back down into the bath. Furthermore, you need to know how the chemistry of the mix feeding out the bottom of the tundish varies as a new heat pours in the top.
"The objective is to have the caster running continuously," says Vassilicos, "and you usually aim for a string of several hundred heats. The chemistry often varies significantly from heat to heat. If you know exactly what is happening in the tundish in real time, you can precisely and intelligently disposition the output steel to meet the specifications of customer orders."
Using a combination of computer and laboratory modeling of tundish flows, the U. S. Steel team developed an automated process control method for predicting the chemistry of output steel at its Gary, Ind. plant. Another research effort led to a "turbulence suppressor pad," a patented device that controls the quality of the very high quality steel used in thin-wall beverage cans.
In recent calculations at the Pittsburgh Supercomputing Center, A. K. Sinha and Achilles Vassilicos compared physical measurements of tracer response in scale model and real tundishes to results from computer modeling. Tracers such as a pulse of copper are added to a tundish mix to give a reading of residence time -- how long it takes for the tracer to exit the bath -- and tracer density over time at the exit. This information gives a valuable index of the flow characteristics of a tundish.
The study shows that commonly used numerical techniques are not sufficiently accurate. The researchers developed FORTRAN code that adapts a more accurate algorithm, known as QUICK (quadratic upstream interpolation), for efficient use on the CRAY.
The fast response of the CRAY as compared to in-house workstations -- a day turnaround versus as much as a week -- is important to the U. S. Steel researchers. "These computations sometimes require a lot of tweaking and adjustments to parameters," notes Vassilicos, "sometimes with several restarts. With the CRAY, we can see right away what we're getting, and if something needs to be changed, we can do it."
Researchers: Achilles Vassilicos & A. K. Sinha, U.S. Steel Technical Center
Hardware: CRAY Y-MP C-90
Software: QUICK (quadratic upstream interpolation)
Keywords: tundish, steel manufacturing, flow patterns, molten metal, continuous casting, metallurgy, turbulence, transition slabs, tracer effects, process design, drawn and iron (D&I) steel, technology transfer, U.S. Steel, USX.
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