Better Car Building Through Steel (But Not the Material You’re Familiar With)


Automotive Chassis

Automotive Materials

The global steel industry is nothing if not proactive when it comes to providing information to the auto industry about how they, the vehicle manufacturers, can do a better job of applying steel.

Perhaps it comes as no surprise that the steel industry’s newest conceptual vehicle design initiative (which is a technical way of stating that this vehicle exists only in math, not in, well, steel), ULSAB-AVC (Advanced Vehicle Concepts), shows how steel—particularly advanced high-strength steel (AHSS), which is used today in only a few vehicles (e.g., Honda Civic, Nissan Altima, Cadillac CTS)—can be used to great advantage by automakers who are looking for safety, fuel efficiency and production economy.

A definition is in order regarding these “advanced” steels, especially as their use is limited. These are materials that are both strong and formable. Historically, as steels became stronger, they became less formable. But these new steels have what is known as “multi-phase microstructures,” which means that they have a variety of iron phases (e.g., ferrite, martensite, etc.) within each type. Their development has been made possible by the ability to carefully control the cooling rate of the steel. As J. Paul Kadlic, executive vice president-Sheet Products, U.S. Steel Corp., puts it, “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.” Because these AHSS’s are strong—higher strengths can be achieved through work-hardening (as in stamping) as well as through baking (as in paint ovens)—it is possible to use thinner gage AHSS in place of thicker HSS, so the resulting structure can be just as strong but lighter.

And a bit of background regarding the ULSAB-AVC. Back in the 90s, when the Partnership for a New Generation of Vehicles (PNGV) was initiated (1993), it seemed that steel was not going to be part of the equation when it came to developing the cars of the future that were roomy, fuel-efficient, and affordable (as distinct from being small, fuel-efficient and not particularly cost-effective either for the builder or the consumer). So the Automotive Applications Committee of American Iron and Steel Institute (AISI) initiated the UltraLight Steel Auto Body (ULSAB) program, which was joined by a number of steel companies from other countries. Porsche Engineering Services (Troy, MI) went to work at engineering a new vehicle that was based on a holistic design (i.e., each element was developed to contribute to the good of the whole such that, in effect, the result is greater than the sum of its parts) and which used steels that were commercially available and manufacturing processes that were in use somewhere. Not only did they do the calculations to prove that an ULSAB vehicle was possible, but in 1998 they actually built a body-in-white to show that is was more than conceivable to cost-effectively build a car that is light (25% lighter than standard sedans) and which could realize significant fuel savings. There were other related studies conducted, looking at things from lightweight trucks to closure panels. PNGV—which was a program supported by the U.S. government and General Motors, Ford, and DaimlerChrysler—was something that the steel industry took very seriously. So they kept at it, and one of the results of their work was ULSAB-AVC, which was officially announced on January 30, 2002. On January 9, 2002, Secretary of Energy Spencer Abraham announced at the North American International Auto Show, “The PNGV wasn’t cost effective and it wasn’t moving a competitive automobile to the showroom. It certainly had a desirable goal—an 80-mile-per-gallon vehicle—but it wasn’t at all clear this vehicle would appeal to consumer tastes.” PNGV was referred to as “the old PNGV program” during that speech. It is now replaced by FreedomCAR, which is a public-private partnership, but more focused on the means of powering vehicles—fuel cells, in particular—rather that the materials and methods by which vehicles are built.

But when the ULSAB-AVC task force went to work in 1999 on behalf of 33 steel companies, PNGV was still viable. So when Porsche Engineering was given the ULSAB-AVC brief, it developed two vehicles: a two-door hatchback that fits within the European C-Class category (the participants are also interested in the European CO2 reduction program, EUCAR) and a midsize-sedan, which, because it is so much lighter than a conventional midsize, they chose to call a “PNGV-Class” vehicle, based on their target. (One interesting note: Because both were designed together and because there was a goal of commonality, the two vehicles have 50% common parts, which account for 22% of the vehicles’ mass.) Diesel- and gasoline-powered variants were developed for each of the vehicles.

Let’s look at some of the manufacturing processes/materials that would be deployed to manufacture the ULSAB-AVC’s body structure and closures:

  • 38% stamping
  • 31% stamped tailor-welded blanks
  • 13% stamping or sheet hydroforming
  • 7% hydroformed tube
  • 6% tailor welded tube
  • 3% straight tube
  • 2% other

Then there are the body structure joining processes that are anticipated for putting together the PNGV-Class vehicle:

  • Laser welding: 100 m
  • Spot welds: 814
  • Adhesive bonding: 1.6 m
  • MIG welding: <1 m

Finally, consider the steels for the vehicle. Overall, the body-in-white is more than 80% AHSS, with the remain-ing being high-strength steels. More specifically:

  • 74% dual phase
  • 10% bake hardenable
  • 4% high strength IF
  • 4% martensite
  • 2% transformation induced plasticity (TRIP)
  • 1% HSLA
  • 1% complex phase
  • 4% other.

And it should be noted that the body structure consists of just 81 major parts.

Given some of the results that the ULSAB-AVC study shows, people in the industry—both accountants and engineers, alike—ought to give some serious thought to the benefits of taking these materials and processes and design approaches and replacing the old-but-standard ways and means. For example, every car manufacturer is touting its safety (or would like to). According to the computer simulations, both ULSAB-AVCs would receive “Five Stars” in the U.S. NCAP and SINCAP crash safety tests. The standards of the Insurance Institute for Highway Safety (IIHS) would be met with flying colors. And they would receive a “Five-Star” rating in European New Car Assessment Program (NCAP) tests.

This safety is matched with efficiency. It is calculated that the PNGV-Class vehicle would have a combined driving cycle fuel economy between 52 and 68 mpg (4.5L and 3.4L/100km), depending on specific configuration, and up to 78 mpg in highway driving.But then there’s the cost. The Achilles heel of ULSAB-AVC? Quite the contrary. At least for those people who look at total cost. That is, compared to a reference structure, to build the body structure, the cost of the steel is $369. For the PNGV-Class model, the steel is more expensive, $468. If someone stopped there, obviously, ULSAB-AVC would be an interesting study in comparatively expensive steel. But the forming cost for the ULSAB-AVC is just $213, compared with the reference $282, and the assembly costs are $291, compared with $328 for the reference structure. The total body structure costs for the ULSAB-AVC PNGV-Class is $972, vs. $979 for the reference car. What’s more, the tooling investment cost for the ULSAB-AVC is significantly lower: $40.3-million vs. $68.0 million. Overall, the manufacturing cost estimates range from $9,500 for PNGV-Class with a gasoline engine to $10,500 for one with a diesel engine.