Better vehicles through steel: advances are being made in steel that can lead to better, safer, more fuel-efficient vehicles—without taking a huge economic hit

Automotive Design & Production,  April, 2003  by Gary S. Vasilash

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Historically--at least for the better part of the 20th century-the notion of "designing with steel" was pretty much a foregone conclusion. There wasn't a whole lot of point in talking about it. Steel is simply what automotive designers (and engineers, and production people) worked with. Sure, there were the occasional exceptions. The Corvette. The Saturn. Or an aluminum body panel here and there. (or sometimes lots of them, like on the Ford F Series). Magnesium shows up every now and then, although typically in places that aren't evident, as, say, it is used as a structural beam beneath the instrument panel.

But the steel industry, battered by things from imports to energy costs, recognizes that for the past several years, even though composites, aluminum and magnesium haven't made huge inroads into the market--although some awfully interesting ones: the Nissan 350Z has an aluminum hood, engine, and suspension; more exotically, there's the Ferrari Enzo, with aluminum honeycomb body and braking system--needs to keep the attributes and potentials of its material front and center in the minds of designers. So the Automotive Applications Committee of the American Iron and Steel Institute (AISI) has initiated a program titled "Great Designs in Steel Seminar." The second event was held this past February in a suburb of Detroit. According to Ronald P. Krupitzer, senior director, Automotive Applications, AISI), last year there were approximately 450 attendees. This year, the number was in excess of 750. Evidently, there's interest in the ways and means of creating better designs in steel.

A NEW CLASS. What's happening is that there is a comparatively new class of steel, ultra high strength steel, or advanced high strength steel (AHSS), materials with yield strengths in excess of 550 MPa. The conventional high strength steels (HSS) fall within the range of 210 to 550 MPa. The differences between the two types of material are a result of the way they are produced, with the traditional HSS resulting from alloying and the AHSS resulting from controlled cooling. Krupitzer suggests that the new materials are causing designers and engineers to take notice of some new potential that can be realized through the use of AHSS. "The industry is moving faster than we expected," he admits.

For example, he cites Honda. During a meeting that he and some of his colleagues recently had with Honda engineers, they learned that Honda is talking about using as much as 60% of AHSS in vehicles it is building within a short period of time. Given the expansion of Honda's production in North America in both cars and trucks, that's some significant steel

UNDER PRESSURE. Krupitzer says 'that the number-one reason why people are currently interested in such things as dual-phase and TRIP steel is cost. "There's a lot of cost pressure on both the new and traditional North American manufacturers," he notes. "Because cost is so critical, they have to address new requirements--requirements for such things as crash and fuel efficiency--"with materials they can afford. Even with the advanced steels, there is a cost basis they can be successful with. The costs of some competitive materials have taken them out of the equation." In other words, the aforementioned aluminum and composites and the like tend to be economically more dear than steel. Which means that there is good opportunity for the AHSS.

About the dual phase and TRIP steels, the two materials that now typically pepper any conversation between people conversant in steel: Essentially, the dual-phase steels allow the stamping of parts that have a comparative thin gage but formability. Typical application areas for these steels include front and rear rails, cross members, pillar reinforcements, door intrusion beams, and closures. TRIP steels--and the material gets its name from Transformation Induced Plasticity--have even greater formability, which permits the creation of more complex shapes.

LIMITED DATA. One of the mechanisms that the AISI has used to convince people of the applicability and appropriateness of AHSS materials is the Ultralight Steel Auto Body-Advanced Vehicle Concepts (ULSAB-AVC) program. Essentially, this was an engineering exercise in the application of holistic design and HSS. In fact, the design consisted of 100% HSS, of which more than 80% was AHSS. Essentially, they demonstrated that it was possible to engineer and manufacture a car that would be safe, affordable, and fuel efficient with steel, that it wasn't necessary to use non-ferrous materials to achieve those goals. While the technologies that would be needed to make an ULSAB-AVC are all being used to some extent (e.g., hydroforming; laser welding), the amount of that utilization in general industry is rather limited--one might say that with few exceptions (e.g., hydroformed truck rails; Volvo's roof welding with lasers), the kind of technology implementation that ULSAB-AVC would call for is not wholly unlike the kind of changes that Audi has had to make for its aluminum vehicle programs.