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 as least twice as high as conventional stamping steels. Their tensile strenghts, for instance, start at roughly 500 MPa. So AHSS can certainly help automakers cut weight of body structures without sacrificing strength. Yet the advanced steels do have some cost and manufacturing barriers to overcome before they can more widely displace their lower-strength predecessors. The Great Designs in Steel seminar, held last month in Livonia, MI, served as 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. The presentations covered the full variety of steel applications, including fuel tank and wheel designs. "For the first time, the seminar also included engine 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 of 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 company's 2002 Cayenne sports utility vehicle. About 65percent of its body structure uses advanced steels, including DP 600, TRIP 700, and CPW 800. Compare that to the 33percent in the 1996 Porsche Boxster. What's significant about this vehicle, according to Krupizter, is that actually embodies many of the concepts developed for the institute's Ultralight Steel Auto Body (ULSAB) concept car. This 2001 concept used AHSS for about 98percent of the body structures, mostly in the form of tailor-welded blanks with a smattering of 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 come of those concepts have come to fruition," says Krupitzer
Another key presentation came from Volvo engineer Jonas Bernquist, who described the body structure for the company's 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 with 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