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Processing High-Strength Steel

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‘The Stronger the Steel, the More It Fights You’ What does it take the level, cut-to-length and slit the new generation of advanced high-strength steels? That depends on the steel and your equipment, say the experts. By Tim Triplett, Editor-in-Chief Processing new high-strength and advanced high-strength steels poses some serious engineering challenges for both machine makers and steel processors—challenges that will only become more common as use of the new alloys migrates from automotive to other sectors in the coming decade. To date, most of the new high-strength steels are consumed by the automakers in their quest to make vehicles lighter and more fuel efficient (see related story on page 20). Advanced high-strength steels allow auto designers to use thinner gauges in structural components and other parts without compromising passenger safety. To date, toll processors serving the auto industry have the most experience at leveling, cutting to length and slitting high-strength steel sheet, but experts say it’s only a matter of time before the new steel products spread to other markets and service centers find themselves having to deal with a different animal. “If there is a need for this steel, the mills will fill it. So it falls on the equipment manufacturers and the processors to come up with a solution to process it every day,” says Greg Santillo, director of sales and engineering at Herr-Voss Stamco, Callery, Pa. The challenges in processing high-strength steel increase exponentially with the strength of the material. “There is a direct correlation between yield strengths and what it takes to work a particular piece of steel. The higher the strength, the more the steel fights you when you level and slit it,” says Dave Lerman, president of Steel Warehouse in South Bend, Ind., whose company specializes in processing heavy gauge materials. The term “high-strength” is defined differently by various industries. The automotive industry uses steels with well over 200,000 psi tensile strength and 160,000 psi yield, two to three times stronger than conventional steels. “The auto guys would not consider 100,000 psi to be high strength anymore, but the agriculture equipment guys would. It depends on who you are talking to and their framework,” Santillo says. So, depending on just how high a customer’s “high- strength” material is, and how heavy duty the processing line, a service center may be able to level, slit or cut to length with its existing equipment by just making a few adjustments. But in many cases, the equipment will need to be modified or replaced. Such modifications could include more rigid structural members, stronger motors, gear reducers and hydraulics, and new tooling and slitter knives designed especially for very hard steels. The entry and exit ends of the line must have more powerful uncoilers and recoilers to handle the stiffer material, and stronger tensioning to produce solidly rewrapped coils, experts say. One issue with slitting high-strength steel is the tooling and clearances, says Dean Linders, vice president of marketing and sales at Red Bud Industries, Red Bud, Ill. “You have to educate yourself on the proper clearances for that tooling. The old rule of thumb to set your clearances at 10-12 percent of the material thickness does not hold true for high-strength steels. The horizontal clearance is extremely critical to getting a good cut. Otherwise, it can damage the material and damage the knives. If you don’t get those clearances right, you are setting yourself up for failure.” Cutting to length is a different challenge because the steel has to be leveled and sheared. The shear must be more powerful and, like a slitter knife, shear blades can chip if not right for the high-strength material. Leveling high-strength steel is perhaps the trickiest task, say the experts. Bending, stretching or compressing the material beyond its yield point—the point at which it deforms plastically and no longer returns to its original shape—takes enormous pressures. Applying too much pressure can exceed the metal’s tensile strength and cause it to break. The debate between advocates of roller levelers, stretcher levelers and temper mills about which is the best option for leveling mild steel carries over to high-strength steel, as well. Designing a roller leveler for high-strength steels presents a big engineering challenge, explain equipment makers. The stronger steel complicates the interplay between roll geometry, separating force and torque. Normally, light-gauge material is leveled with smaller work rolls and heavy-gauge with larger work rolls. With high-strength material, the work rolls need to be larger to withstand the separating forces even at thinner gauges, and so they may not bend the metal sufficiently to remove all the defects. Some equipment makers are now attempting to engineer machines with small rolls that have the gearing and horsepower to withstand the forces required to level high-strength material. Stretcher levelers relieve steel’s internal stresses by stretching the material beyond its yield point, leaving it flat. Stretchers can apply enough force to stretch the highest strength materials, but high-strength steel allows less elongation than mild steel, so processors have to be careful not to stretch it beyond its tensile and damage the material. Temper mills use enormous compressive forces to flatten steel. Proponents claim tempering is the best solution for difficult-to-level materials including high-strength steel, especially at heavier gauges. Temper mills cost considerably more than roller levelers and stretchers, however, though they can make up for that higher initial capital cost in productivity advantages. Steel Warehouse uses both stretcher leveler and temper mill. Generally speaking, stretcher leveling works well for material of 50,000 psi yield at one-half inch or less, Lerman says. For material that is higher strength or thicker with more shape correction issues, the company relies on its temper mills. “The moral of the story is that as the tensiles and yields go up, the thickness you can run on any given machine has to go down,” says Linders. “No matter what approach we are talking about, leveling high-strength steel is a challenge.” Joe Savariego, president of Delta Steel Technologies in Irving , Texas, notes that as high-strength steels migrate from automotive to other end-use sectors, such as ag or construction equipment where fuel economy is not the main driving force, they may well be used at heavier gauges. This will be a challenge for service centers and other processors. “Take a look at your equipment,” he says. “If it is not up to it, there are some real engineering issues you will have to address.” How the mills market and sell their high-strength grades will determine how far and how rapidly they spread, says Santillo at Herr-Voss. “Right now we see the toll processors going after the ultra high-strength automotive grades, which they slit for the mills. We believe it will trickle down to other places, but how long that will take, who knows?” Marketing the next generation of high-strength steels represents an opportunity for the mills that produce it, the tollers and service centers that process it, and the makers of machinery who will equip them. “It is going to be a new and exciting world for equipment manufacturers. It is the next place for engineers to play,” Santillo says. “If you are used to processing commercial materials, be ready for a good challenge as you step up to these ultra high-strength grades,” Lerman concludes.

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