Phosphors and Galvanneal Steel

ORNL Wins 1999 R&D 100 Award

Thermometry for the Steel Industry

Cars today don’t rust the way older vehicles did. The reason: The steel industry uses a “galvannealing process” to produce the corrosion-resistant sheet metal now used in virtually all the world’s automobiles. The process combines zinc atoms with iron atoms in a steel surface at high temperatures. The protective layer of zinc-iron alloy that is formed prevents the steel from rusting through. In fact, because of this galvanneal coating, lifetime guarantees against rust through can be offered by the automotive industry.

Getting the galvanneal coating right for automobiles and other products is not easy. First, a sheet of steel is dipped in a liquid bath of zinc at about 450°C. Then the steel sheet passes through a cascade of furnaces, raising its temperature to as much as 700°C. During heating, iron atoms from the molten steel sheet drift into the zinc coating to form the zinc-iron alloy. But, is the molten steel surface always at the right temperature to ensure formation of the best galvanneal coating? Making sure the temperature of galvanneal steel is on the mark has long been a problem for the steel industry, a problem that ORNL is helping to solve. 

ORNL engineer Ruth Ann Abston adjusts the phosphor
deposition device in front of the galvanneal sheet.

The galvannealing process of alloying zinc with iron at the surface must be controlled at production rates of 30 meters per second or higher to ensure the surface quality necessary for the automotive market. When the  galvanneal coating is incorrectly formed, the material is rejected as second-rate steel, costing the U.S. steel industry $4 billion per year and reducing its competitiveness with steelmakers worldwide. Hence,  getting these coatings consistently right was identified by the U.S. steel industry as the key to the future  competitiveness of their galvanneal product line.

Schematic of galvanneal phosphor thermometry components. A thin phosphor layer deposited on the steel strip is illuminated using laser light. The duration of measured fluorescence from the excited phosphor layer indicates the steel’s temperature.

The problem is that the surface alloying process varies as the temperature of the metal surface changes, yielding a product of nonuniform quality.

One challenge has been to devise a method that accurately measures the temperature of the molten material as it forms an alloy and cools. A second challenge has been to relay information instantly to steel producers so they can adjust furnace operation to get the right temperature—and best product. 

To address these challenges, the American Iron and Steel Institute (AISI) accepted a proposal by ORNL and the University of Tennessee at Knoxville (UTK) to develop a totally new, first-principle-based technique for determining the surface temperature of galvannealed steel. 

The ORNL and UTK engineers designed and built a novel instrument system in collaboration with National Steel, the partner steel company. The project is part of the Advanced Process Control Program supported by 15 AISI-member steel companies and the Department of Energy’s Office of Industrial Technologies. Bailey Engineering of Mechanicsburg, Pennsylvania, is now developing the concept of the prototype instrument built at ORNL into a commercial product that will be available soon to the steel industry. 

“Real-time steel temperatures cannot be measured precisely using the conventional method because it assumes that the surface properties are constant,” says Steve Allison of ORNL’s Engineering Technology Division, a principal developer of the technique. “The problem is that properties of the zinc-covered surface rapidly change as the coated steel cools from a molten to solid state, causing errors in the temperature measurement by as much as 40°C. Because our device uses a thermal phosphor method, it has demonstrated accuracy within better than 3°C. Clearly, it is more reliable than the conventional method.” 

How does the thermal phosphor technique work? A steel sheet is partly dusted with white phosphor powder using a computerized phosphor-deposition system. Two optical fibers are positioned between the moving steel sheet and the temperature measurement equipment. As the sheet travels between the furnaces at up to 30 meters per second, short pulses of ultraviolet light are fired from a low-power nitrogen laser through an optical fiber leading to the molten steel. The laser pulses excite the phosphors, which emit light for a short time based on how hot they and the steel substrate are. The emitted light travels through the other optical fiber to a light detector (photomultiplier tube). It measures the time for the phosphorescence to decay, and a computer uses the real-time data to calculate the surface temperature of the galvannealed steel. 

“To apply the phosphor to the moving sheet,” Allison says, “we had to solve some interesting problems in mechanical design, fluid mechanics, and optics. We had to figure out how to illuminate the phosphor and gather the light for temperature measurements. So we assembled a team of diverse skills and expertise from ORNL’s Engineering Technology and Instrumentation and Controls divisions, UTK, and National Steel.” 

The team was asked to determine whether phosphor powder might damage the quality of the steel. Results of tests done by ORNL and National Steel indicated no adverse effects on either the coated steel’s surface appearance or its ability to be painted.

Other ORNL co-developers of the technique were Wayne Manges, Ruth A. Abston, William Andrews, David L. Beshears, Michael Cates, Eric B. Grann, Timothy J. McIntyre, Matthew B. Scudiere, Marc L. Simpson, David N. Sitter, and Todd V. Smith.

Early prototypes were tested at National Steel’s Midwest Steel Division in Portage, Indiana. On May 31, 1998, the final version developed at ORNL was successfully demonstrated on a galvanneal line at the Bethlehem Steel plant in Portage. The demonstration was part of DOE’s Technology Showcase held at this facility, where the system is permanently installed. 

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. Accurate, reliable temperature measurements will ensure a quality product, reducing waste and saving energy. These improvements, if implemented throughout the U.S. steel industry, could save steelmakers as much as $70 million a year, increasing their competitiveness worldwide.

And such a savings might lead to more affordable cars or, at least, larger earnings for the steel industry. 

This article has been reprinted from the ORNL Review.  Click here to read more.