Stronger steels build better bodies

A pair of virtual concept cars designed by Porsche Engineering Services Inc. and the American Iron and Steel Institute (AISI) may drive away any doubts about the benefits of using the most advanced steels for automotive body structures.

If built, these closely related two- and four-door cars would weigh 17% less and have 40% fewer body parts than benchmark vehicles made from conventional mild and high-strength steels, reports Marcel van Schaik, AISI's director of advanced material technology. "And neither design would increase manufacturing costs," he says.

The two virtual cars represent the latest ULSAB effort to come up with a lightweight auto body design that can improve fuel economy without sacrificing crash worthiness. ULSAB, a consortium of 33 steel makers, previously built a concept vehicle that replaced conventional mild steels with high strength steels, whose yield strengths typically exceed 210 MPa. "Containing nearly 90 percent high-strength steel, the first ULSAB body-in-white fully validated the principle that with the latest materials, coupled with innovative design, it is realistic to significantly reduce mass and meet crash standards--while keeping costs under control," says Peter Southwick, president of Ispat Inland Inc., one of the consortium members.

The new virtual cars, called "Advanced Vehicle Concepts," get an even bigger strength boost. They make extensive use of high-tech steels whose yield strengths can top 550 MPa. This diverse family of "advanced high strength steels" (AHSS) includes dual-phase, martensite, complex-phase, and transformation-induced-plasticity grades. All feature tightly controlled multi-phase micro-structures that not only contribute to the added strength but also impart beneficial forming characteristics, according to van Schaik. Work- and bake-hardening, for example, can boost the yield strengths of some AHSS grades by up to 50%, he says. "We have understood the superb attributes of Advanced High-Strength Steels since the late 1970s, but it is only recently that we have been able to produce them in commercial quantity," says Southwick.

Overall, 80% of the body and closures on both concept cars are constructed from AHSS, while the rest are made from conventional high strength steels. And the advanced steels didn't only go into the main body structures. AHSS also saw use on a variety of closures and suspension components.

At first glance, this strength boost from AHSS doesn't look like it will come cheap. Van Schaik estimates that the advanced steels could carry a price premium of roughly 10% over traditional mild steels. Yet the new concept cars still manage to hold the line on manufacturing costs through parts count reduction. Van Schaik points out that the four-door car's main body consists of just 81 major parts. "Normally a car this size would have 135 or so," he says.

Better steels, better designs. Much of the parts integration springs from smart design. Rather than trying to shoehorn the higher strength steels into existing designs, AISI and Porsche engineers wisely started with a blank page in order to take full advantage of advanced steel's strength and forming properties. On the front-end, for example, the strength of the steels allowed engineers to come up with a design built around a pair of 100-mm diameter hydroformed rails. Made from dual phase steel with a 500 MPa yield and an 800 MPa tensile strength, these rails run from the bumper back to a cross member at the rear of the car. Van Schaik points out that this design does away with shock towers and related components found on conventional front ends. "The rails create a single load path and manage crash energy with fewer parts," he explains.

The ability to reduce parts count also resulted from the heavy use of modern forming and joining technologies. Tailored blanks account for nearly 40% of the body and closure structures, hydroforming contributed another 20%, and tailored tubes make up 6%. All of these technologies also allow mass optimization through selective down-gauging of areas that bear lower loads-as opposed to a uniformly thick part whose gauge is dictated by the part's area of maximum stress. "These forming methods allow us to place material just where it's needed," he says. This strategy can be seen in the concept cars' outer body sides, which are made from a five-piece tailored blank of variable thickness.

As for assembly, the concept vehicles rely on laser welding, which not only promises productivity gains but also helps improve overall stiffness. Van Schaik says the new concepts have about 100 m of laser welds compared to just 18 m on the previous ULSAB concept.

Tallying up the influence of the design, forming, and assembly factors, AISI cost models show that the body structure for its four-door car would cost $972 versus $979 for a benchmark vehicle. What's more, the parts integration and improved strength-to-weight ratio of the ADHSS made mass reduction possible. With a body weight of 481 lbs, the four-door vehicle weighs in at 2,200 lbs, contributing to the car's projected fuel economy of 52 mpg. And because of the strength of AHSS, both concept cars will likely meet future European and U.S. "Five Star" ratings-something that Porsche and AISI validated with virtual crash tests.

For more information about Advanced Vehicle Concepts from AISI: Enter 533