Transition slabs occur because the initial flow into the tundish from a new heat -- for about the first minute and a half -- is much faster than the exit flow from the tundish to the strand. This initial high flow rate creates turbulence that draws impurities from the bath surface down into the liquid being cast, resulting in lower quality steel for one or two slabs.
The turbulence suppressor pad, designed (and recently patented) by U.S. Steel through a combination of physical and computer modeling, suppresses this turbulence. "Modeling on the CRAY," notes Vassilicos, "helped to verify physical model observations of how the distribution of turbulence in the pouring region varies. Without the pad, the highest level turbulence is at the surface of the bath, where we don't want it. These computations verified that the design was doing what we expected." Plant testing has shown that using the pad in conjunction with special refractory baffles improves quality of the transition slabs to the point that they meet the criteria of D & I steel. The pad is now used for all steel produced at Gary, resulting in quality improvements for all products.
This graphic shows computed results from modeling turbulence in the Gary No. 2 tundish equipped with the turbulence suppressor pad. The colored surfaces represent turbulence kinetic energy, with red representing the highest values decreasing to blue/gray, which represents the lowest values. High values of turbulence occur near the pad, and the bath surface is relatively undisturbed.
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