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ORNL system helping U.S. steel industry get tougher

With a puff of powder and a flash of nitrogen laser, United States manufacturers of galvannealed steel can improve product quality, lower costs and save energy, according to the American Iron and Steel Institute and Oak Ridge National Laboratory (ORNL).

An advanced temperature sensor originally developed to refine uranium for national defense programs by the Department of Energy's (DOE) ORNL is allowing producers of the corrosion-resistant steel to tell in an instant if it is being processed correctly. That's important because the system reduces the amount of material that isn't up to specifications, thus improving overall production efficiency, engineers say.

Galvannealed steel, used in automobiles and a number of other products, is made by dipping a roll of steel in a liquid zinc bath at about 800 degrees Fahrenheit. Then, the steel strip passes through a series of furnaces that raise the temperature to 1,000 degrees. During the process, iron atoms from the sheet move into the zinc coating to form a zinc-iron layer. If the temperature isn't right, the galvanneal coating won't be right.

ORNL's system ensures that the temperature is on the mark, said David Beshears of the Engineering Technology Division.

"We use a low-power laser beam to read the surface after we've dusted it with a small amount of phosphor powder, which is like a white dust," Beshears said. "Using short pulses of light from the laser, we can more accurately measure the temperature of the strip of galvannealed steel as it travels through the furnaces at up to 350 feet per minute."

Conventional methods of measuring the temperature of the steel's surface have been unreliable because the zinc-covered surface has rapidly changing characteristics that make it difficult to obtain an accurate measurement. ORNL's system uses fiber optics and pulses from a small ultraviolet laser to excite the phosphor. Light detectors measure the time it takes the fluorescence to decay, giving operators real-time data on surface temperature.

Joe Vehec, director of the American Iron and Steel Institute, summarized the benefits of the system and the reasons people in the steel industry are excited. "Unlike conventional measurement techniques, this system operates independent of the finish," Vehec said. "It's also more accurate."

An early concern was whether the phosphor would damage the quality of the steel. Results of tests done by National Steel, the sponsoring steel company for the project, indicated no adverse effects on either the strip surface appearance or paintability, Vehec said.

Recently, ORNL engineers successfully demonstrated this new technique on line at National Steel's Midwest Steel Division in Portage, Ind. This was the first time the phosphor thermometry system had been used on an operating galvanneal line.

"The next step is to permanently install the system at a production plant," Beshears said, "and that's going to happen next month in Portage."

The new process should result in less second-rate material and eliminate the need for costly off-line tests to determine if the galvanneal coating is correct. These improvements should increase the U.S. steelmakers' competitiveness worldwide.

The Advanced Process Control Program was funded by 15 American Iron and Steel Institute member steel companies and DOE's Office of Industrial Technologies. Researchers in ORNL's Instrumentation and Controls Division assisted in developing this technology. ORNL, one of DOE's multiprogram research facilities, is managed by Lockheed Martin Energy Research Corporation.