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TECH TRAK Unlocking the world of dual-phase, trip and twip steels

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Technological advances in the production of new types of steel are driving steel producers forward in their quest to develop new products, but the needs of customers have settled behind the wheel as dual-phase and transfer-induced plasticity (trip) steels come to the fore.

Steelmakers such as ArcelorMittal, the world's largest producer, are heavily involved in the development of new advanced high-strength steels (AHSS), including dual-phase steel, trip steel and the next stage, which is coming to be known as twinning-induced plasticity (twip) steel.

"With our operations in the Americas and in Europe, ArcelorMittal is focused on the high-value-added products," Pinakin Chaubal, director of process research for Mittal Steel USA Inc., Chicago, said. "What we are continuing to look for is ways to make steel a more formable product. That really is being driven by increasing needs for safety and fuel efficiency on the part of our (automotive) customers."

Pittsburgh-based Integ Process Group Inc. is mining some of the same territory with its Hot Strip Mill Model (HSMM), an off-line PC software product that allows steelmakers to simulate the steel rolling process for hot mills. In partnership with the American Iron and Steel Institute (AISI), Integ is marketing the HSMM to mills worldwide—and finding customers that are seeking ways to use more dual-phase and trip steels.

"We are now adding features to the model, including dual-phase steels," Richard Shulkosky, president of Integ Group, said. "We're finding that's what the customers want."

Such steels are considered advanced high-strength steels and are seen to a large degree as key to steel potentially gaining market share among automotive customers.

The AISI, through its Market Development Committee and various customer partnerships, is promoting the use of AHSS—lighter, more formable steels—in vehicle construction. The use of such steels maintains vehicle efficiency but improves crashworthiness.

Chaubal said ArcelorMittal wants to maintain its position as one of the leading suppliers to the North American automotive industry through a multifaceted approach using more AHSS. The company in May announced plans to upgrade the continuous heat-treating line at its Burns Harbor, Ind., facility so it can produce more such steels.

"We are trying to be aggressive not only in terms of (increasing) the number of products we produce, but also in the timing of bringing these concepts to the market," he said. "The customers want to see these more formable steels brought to the market more quickly. That's one of the things that is driving our capital investment (in the upgrade of the continuous heat-treating line). It's all being driven by the needs of the automotive industry."

ArcelorMittal's involvement goes beyond its automotive customers to working with stampers. As new steels are developed, stampers must learn of their formability characteristics and have to adapt to use new tools and/or materials in their processes. ArcelorMittal is among those employing Integ's HSMM.

"We are involved with customers at every stage of the game," Chaubal said. "You cannot be a high-end player in steel without being able to understand what will be happening to your steels at the customer's manufacturing plant. Our focus right now is on the dual-phase and trip steels, but we are not going to stop there. We are now looking at the twip steels, which provide higher strength and higher formability than you get with the trip steels. It's an entirely new product."

The HSMM is not new, but is moving into stages of additional development. The project was launched in 2003 and the first model was sold midway through 2004 to the Decatur, Ala., mill of Nucor Corp., Charlotte, N.C.

The hot-rolled development group at Stelco Inc., Hamilton, Ontario, has relied heavily on the Integ HSMM to support recent developments involving advanced high-strength steels with good success. Stelco used it most recently during the Phase II hot-strip mill upgrade at its Lake Erie Steel plant in Nanticoke, Ontario.

"In many instances, in part due to the HSMM, our initial mill development trial process conditions are very close to the final optimized manufacturing conditions, thus dramatically reducing time to market of new products and development costs," Peter Badgley, Stelco's manager of product development and applications, said.

"Near-optimal process conditions can be identified off-line, eliminating trial and error in the manufacturing process," Badgley added. "This has dramatically reduced the development time for new products."

The model was used in the upgrade of the Lake Erie hot-strip mill to examine the impact of new equipment and mill practices on product performance, thus allowing countermeasures to be identified so product performance could be maintained.

The model also has been integral to the development program for dual-phase steel undertaken by Stelco. "Hot-roll dual-phase steels are part of a class of new advanced high-strength steels that achieve their product properties through the thermal cooling trajectory in the run-out table, or cooling section, in the hot strip mill," Badgley said. "These products are processed in the final stage at temperatures as low as 100 degrees Celsius (212 degrees Fahrenheit) vs. traditional cooling temperatures above 500 degrees Celsius (932 degrees Fahrenheit)."

Badgley said Stelco has achieved a number of successes with use of the HSMM and likely will see more as improvements to the model are developed.

"The Integ HSMM has been used to help successfully develop value-added hot-roll products such as dual-phase and stretch-flange steel for automotive applications as well as new line pipe steels for the oil and gas industry," he said. "As the software is continually improved with enhanced capabilities, it may be possible in the future that the software will evolve from an off-line simulative development tool to one that is used on-line in real-time in the manufacturing environment."

The software model is one of several technologies developed under AISI's advanced process control program, a cooperative effort between steelmakers and the U.S. Energy Department. Integ sells the model to steelmakers, research organizations and universities. Eighteen models are in use around the world, four from its original partnership group and 14 sold by Integ since the program launch. Sales of the model have generated $90,000 in royalties to AISI thus far. One quarter of those are returned to the Energy Department.

The model simulates the hot-rolling process for a variety of steel grades and forecasts microstructures and properties, allowing users to achieve deeper insight into operations. The program also predicts temperatures, forces, microstructure evolution and final mechanical properties of plate or strip steel.

Shulkosky said customers have become more interested in the model in recent years and now want to see two more developments the ability to use more AHSS, including the dual-phase and trip steels, as well as a migration from the current off-line model to an on-line model.

"A lot of people are interested in it," Shulkosky said recently. "Right now they want to add more steels to it," he said. "That's happening globally. They want to see more dual-phase and trip steels and that's what we are working to give them."

Much of that interest springs from the runout table on a hot-strip mill. Shulkosky said customers want to use HSMM in conjunction with a runout table control model to measure qualities and adjust spray banks to ensure temperatures hit pre-established targets.

"With advanced high-strength steels, a lot of those measurements aren't clear anymore," Shulkosky said. "You go through a period of cooling, then a pause and then cooling again. The transfer is more complex and mills want to be able to model those things to get better quality."

Shulkosky said that Integ wants to take the process further and now is talking with a company that provides runout table software to see if that software can be linked to the HSMM. "The mills used to worry more about rolling," he said. "Now they are more worried about issues related to the runout table. Those (issues) are the keys to success with advanced high-strength steels."

ArcelorMittal, Chaubal said, is focused on other developments as well. Many are in the areas of iron-making and blast furnace technology as well as the re-use of secondary materials. "One of the challenges in iron-making, when you have the new steel grades, comes after you cast the slab," he said. "There are challenges in hot-rolling and cold-rolling that you face with these higher-end products, so we are directing a significant focus to the cleanliness and quality of the slab.

"The second area of focus has to do with cost, quality and productivity. By cost, we are looking at the impact of the cost of raw materials, environmental regulations and processing. We are making significant investment in our production equipment in order to become more efficient and lessen the cost (of iron-making) on the environment."

Blast furnace research is yielding results in the form of better refractory performance and the extension of furnace campaigns, Chaubal said. "Now we can say we are trying to design blast furnaces for a reline every 20 years. We also are making improvements in the level of carbon injection. I would say that 15 years ago we were at 15 kilograms (of carbon injected) per ton of hot metal produced. Now we are at close to 200 kilograms and on the way to 250."

Environmental stewardship also is the focus of considerable research. "The big challenge for ArcelorMittal is on the primary side and the re-use of secondary materials without losing productivity or efficiency," Chaubal said, citing such secondary materials as wet and dry dust and sludge as well as mill scale from steelmaking operations. "That's a very important challenge. We're looking at ways to reduce the first generation of those materials and looking at ways to improve our operations to further the re-use of materials such as blast furnace dust."a


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