How High Can High-Strength Steel Go?
As research begins on a third generation of advanced high-strength steels, the high-strength alloys already available are poised to gain widespread acceptance.
By Myra Pinkham, Contributing Editor
Development of new high-strength and advanced high-strength steels represents a big opportunity for steel producers and processors in the next decade as the new alloys find further applications in automotive and beyond.
Designed initially to reduce the weight and improve the fuel efficiency of the nation’s cars and light trucks, there are numerous potential uses in other markets, as well. “Improved fuel efficiency mandates also are driving weight reduction efforts in on-highway trucks and trailers. Demand for lighter, more durable rail cars represents another current and future opportunity for high-strength steels,” says John Ferriola, president and chief operating officer of Nucor Corp., Charlotte, N.C.
Use of high-strength steels also could allow for cranes with longer boom lengths and increased lifting capacity. Farm machinery, military equipment, line pipe and pressure vessels that are lighter and easier to transport are among other possible uses.
But today, well over 90 percent of the true advanced high-strength steel grades are used by the automotive industry in its quest to meet tightening government fuel efficiency standards, says Jim Mortensen, director of quality assurance and process technology for Severstal North America, Dearborn, Mich. Until the supply of AHSS is further developed, automakers will have first dibs on the specialized material, making other OEMs stand in line.
The most advanced high-strength steels on the market today are three to five times stronger than the conventional steels they replace, says Larry Kavanagh, president of the American Iron and Steel Institute’s Steel Market Development Institute (SMDI) in Washington. The composition of the new steels enables advances in two critical performance factors, strength and formability, both of which are crucial to automakers striving to remove weight from vehicles without affecting their safety attributes.
“Higher strength means less material is needed,” says John Surma, chairman and chief executive officer of Pittsburgh-based United States Steel Corp. New steel grades available today can yield parts as much as 39 percent lighter than parts made from traditional steels, he adds. Their higher formability gives auto designers latitude to use more sophisticated shapes and improve vehicle aerodynamics, which further improves fuel economy.
Prompted by federal miles-per-gallon targets of over 50 mpg, the United States leads the world in development of AHSS, says Christopher Plummer, managing director of Metal Strategies Inc., West Chester, Pa. The market for AHSS has nearly doubled from 860,000 tons in 2008 to 1.6 million tons last year. Metal Strategies forecasts it to be a 2.5 million ton market by 2020.
Obviously, new-generation high-strength steels are more costly than conventional grades, but their application can be surprisingly economical, in part because mild steel can be replaced with less high-strength steel. According to the steel industry’s FutureSteelVehicle program and recent National Traffic Safety Administration studies, vehicle mass reduction using advanced high-strength steels can be achieved at almost no additional cost. In comparison, replacing mild steel with aluminum adds a cost around $2.75 per pound, while substituting carbon fiber adds $7 per pound, claim steel industry estimates.
The science of high-strength steels
Defining “high-strength steel” can be difficult. What one manufacturer considers high-strength might be considered conventional by another. Generally, the term high strength is used for steel grades ranging from tensile strengths of 270 megapascals up to 590 MPa, (or roughly 40,000 to 85,000 psi), says Blake K. Zuidema, director of automotive product applications for ArcelorMittal Global Research and Development in East Chicago, Ind. Advanced high-strength steels—grades with ferrite plus other constituents such as martensite, bainite, retained austenite or a combination, formed through special thermal processing—can achieve tensile strengths of 590 MPa to ultra-high strengths of 1,000 MPa (nearly 150,000 psi) or greater.
Mortensen at Severstal says the key to making steels increasingly stronger is to create alloys with microstructures that are smaller and finer. This is accomplished not only by adding alloying ingredients, but also by heating the steel to a very high temperatures and then cooling it very quickly, much as a blacksmith does when making a knife blade.
According to the SMDI’s report “AHSS 101: The Evolving Use of Advanced High Strength Steels for Automotive Applications,” steels with only a modest amount of strengthening compared with mild steels include interstitial free steels, bake hardenable steels and high-strength low-alloy steels. Of this group, HSLA is among the most widely used in automotive applications because it is tough, corrosion resistant, formable and weldable.
Much as the name implies, dual phase steel has two phases of steel present in its microstructure—a soft ferrite matrix with discreet hard martensitic islands. Dual phase steel is currently one of the most rapidly growing alloys, as automakers increasingly use it to down-gauge HSLA components. Common applications include beams and cross members; rocker, sill and pillar reinforcements; cowls, crush cans; shock towers, fasteners and wheels. It is not only strong but ductile, so it is helpful in absorbing energy in a crash.
Martensitic steels are among the highest strength of the first-generation AHSS—up to an incredible 1,800 MPa or 260,000 psi. They don’t have a great amount of formability, however, making them most suitable for vehicle crash load structures like door beams and bumpers.
Hot-formed steels are sometimes referred to as boron hardenable steels. A tiny percentage of boron content gives the material great hardenability. According to Mark Blankenau, generation manager of process development for Severstal North America, these steels are designed to allow martensite to be formed after moderate cooling rates from elevated temperatures. They are used in hot stamping operations in which a part blank is heated and then transferred while still hot into a stamping press. It is then stamped in water-cooled dies. The part is held in the dies until the transformation to martensite is completed. This process reduces the challenges of forming complex shapes with this very high-strength material. Some automotive applications for ultra high-strength hot-formed steels include reinforcements for A- and B-pillars, roof blows, sidewall members, beams and other parts that carry severe loads.
Ronald Krupitzer, SMDI’s vice president of automotive applications, says the fanciest of the first-generation AHSS alloys is TRIP steel, which could actually be the secret to the third generation of steels. It not only contains ferrite and martensite, but also retained austenite. Initially the material is low in strength, but the retained austenite allows it to strengthen further as it transforms to martensite during cold working. This transformation also creates plasticity, which is a desirable property for forming automotive parts.
“It is the most popular grade for tough parts, and is one that we are trying to work on with austenite to make the next generation of steels,” Krupitzer says. “If we can put in more retained austenite and have more controlled stability, we can get steel that has strength and formability at levels we haven’t seen yet.”
Research is now commencing on the third generation of advanced high-strength steels. Virtually all of the AHSS used today is such first-generation alloys as dual phase, ferritic bainitic, complex phase, martensitic and hot formed. “There really isn’t a second generation of AHSS,” says Kavanagh at SMDI. “We did some experimentation a few years ago, but that family of high-end AHSS was very expensive to manufacture.” That included such grades as twinning induced plasticity (TWIP), lightweight induced plasticity (L-IP) and austenitic stainless steels. “So we stopped that research path and now we are focusing on the third generation of research.”
In mid-February the U.S. Automotive Materials Partnership LLC, in collaboration with the Auto/Steel Partnership, received a $6 million grant from the U.S. Department of Energy for a four-year, $8 million research project to develop third-generation AHSS. The aim of this research, which will involve five universities, three car companies, six steel companies and other experts, is to develop new AHSS grades by manipulating the steel’s molecular structure and then taking that development all the way through the part manufacturing process. “We are looking to come up with a good intermediate solution that is both affordable and very formable,” Krupitzer says.
Most of the major mills have been making investments to prepare to supply both higher volumes of AHSS and the new grades to come. “The investments made by the industry focus on supplying the steels needed for the new car requirements of safety and light weighting,” Zuidema says. “We see new coating lines, annealing lines and upgrades in steelmaking capabilities.”
Not all high-strength products are the same. Currently, there is a fair amount of variability in the properties of the newer AHSS grades from producer to producer, says Ferriola at Nucor. But he believes that as time passes, product quality will become more uniform. “As one OEM said, the producer with the best properties raises the bar for the rest. Either they work to improve their properties or they will not be supplying those grades.”
“I see demand continuing to grow for AHSS, especially for automotive applications,” Kavanagh says. “We will continue to enhance the properties and performance going forward to meet what is a very demanding and competitive market.”