As automakers work to improve both fuel efficiency and safety, they increasingly need to add some lean muscle to their vehicles. Strong, lightweight structural components made from Advanced High Strength Steels (AHSS) may be just what the doctor ordered.
This diverse group of multiphase steels with high strain hardening rates generally offers yield and tensile strengths at least twice as high as conventional stamping steels. Their tensile strengths, for instance, start at roughly 500 MPa. So AHSS can help automakers cut the weight of body structures without sacrificing strength. Yet the advanced steels do have some cost and manufacturing barriers to overcome before they can displace their lower-strength predecessors. The Great Designs in Steel seminar, held last month in Livonia, MI, served as a good progress report on these advanced steels.
The American Iron and Steel Institute's Automotive Applications Committee (AAC) sponsored the seminar, which consisted of 27 presentations and drew more than 1,200 automotive engineers. Steel applications included fuel tank and wheel designs. "For the first time, the seminar also included engine and powertrain components," notes Ron Krupitzer, the institute's senior director of automotive applications. But AHSS grabbed the lion's share of the attention, reflecting an increase in high-strength steel development activities over the past few years.
And much of that activity has come from European automakers. "Offshore companies are pushing to higher and higher strength levels faster than domestic companies," Krupitzer says. A couple of key presentations illustrated this point.
Porsche engineer Michael Mehrkens gave a talk outlining the use of AHSS in the Cayenne sports utility vehicle, introduced in 2002. About 65 percent of its body structure uses advanced steels, including DP 600, TRIP 700, and CPW 800. Compare that to the 33 percent in the 1996 Porsche Boxster. What's significant about the Cayenne, according to Krupitzer, is that it actually embodies many of the concepts developed for the institute's Ultra Light Steel Auto Body (ULSAB) concept car. This 2001 concept used AHSS for about 98 percent of the body structures, including tailor-welded blanks and hydroformed sections. The Cayenne likewise made liberal use of advanced steels in tailor-welded blanks for a variety of components, including its bodyside inner subassembly and substantial parts of its front- and rear rails. "Porsche played a strong role in that project, and you can now see some of those concepts that have come to fruition," says Krupitzer.
Another key presentation came from Volvo engineer Jonas Bernquist, who described the body structure for the XC-90 sports utility vehicle. It features a new front structure to handle the higher forces of an SUV crash and to minimize the impact of collisions on smaller vehicles. It incorporates a "safety cage"—a collection of reinforced side, roof, and seat structures designed to protect occupants from rollovers and side impacts. According to Krupitzer, this body design serves as an object lesson in how to "match a wide range of steels to specific tasks." The requirements for some cage components—the A-pillar upper, for example—were satisfied by high-strength rephosphorized steels whose 380 MPa tensile strength falls a bit short of the AHSS range. But the company applied dual phase steels with tensile strengths in the neighborhood of 600 MPa, for selected reinforcements. And for components with the most extreme load cases, such as rear seat frame, the company used boron steel with a minimum tensile strength of roughly 800 MPa.
The coming revolution
North American automakers have not adopted true AHSS to the same degree as the Europeans, Krupitzer points out. "They use proportionately more steel in the intermediate range," he says. He cites bake hardenable (BH) and solid solution strengthened (SSS) steels with tensile strengths up to about 350 MPa as two examples. Still, even stronger steels have sparked "a great deal of interest and some aggressive plans" among the domestic manufacturers, Krupitzer says.
Ford gave two presentations on the use of AHSS—one covering the Ford Mustang and the other most recent Ford F-150. Another presentation at the seminar outlined the extensive use of AHSS in the new Chrysler LX sedan. "It has a larger assortment of high strength steel parts," Krupitzer reports. These include relatively large amounts of high-strength-low-alloy (HSLA) materials, whose tensile strengths can reach about 500 MPa, but also a smattering of dual phase components with tensile strengths above 600 MPa.
As for those "aggressive plans," look no further than a presentation given by General Motors development engineer Curt Horvath. He described an evolutionary path in which higher strength materials continue to gradually displace the low-carbon steel that made up as much as 70 percent of a typical body in the early 1990s. GM has already incorporated significant amounts of HSLA—as much as 34 percent in current midsized luxury models. Dual phase steels, meanwhile, have made an appearance too. The Chevy Malibu body structure, for example, has a 12 percent dual phase and 5 percent HSLA content.
In the near future, Horvath predicts a typical body structure will continue to shift up to AHSS—as much as 35 percent dual phase steels and another 8 percent martensite. This near term scenario is based primarily on the strengthening of existing components such as rails, pillars, and rocker inners to improve crash performance. As newer body designs appear, ones designed with AHSS from the get go, Horvath sees dual phase usage climbing to as much as 45 percent and martensite to 12 percent.
For all their promise, AHSS do still have some barriers to overcome. At current production volumes, these materials do cost more than conventional steels and face a growing threat from aluminum. "All the manufacturers want five-star crash ratings," Krupitzer says. "But a low-cost solution is always preferred."
AHSS steels may also be somewhat more difficult to fabricate—at least given
the current state of knowledge about them. Krupitzer says they do exhibit more
springback than conventional stamping steels. And their strain hardening
tendencies, so beneficial from a mechanical properties standpoint, tend to cause
a lot of die wear. Finally, the industry has less experience welding these
materials. "You have to be careful how you stamp them and weld them." Krupitzer
admits. But the seminar included presentations that addressed all these
difficulties. "We've already gained a tremendous amount of knowledge in how to
work with these steels," he says.
Stronger Steels: This body-in-white for
the new Chrysler LX sedan, on display at the recent Great Designs in Steel
seminar, illustrates the pending shift to higher strength steels for
automotive body structures.