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STEEL: Staying Strong

There's plenty of buzz about materials other than steel because steel has been used for body applications for so long. But the steel industry is working so that its products will be anything but antique.
door
The ULSAC has three door design concepts: roof integrated, frame integrated and frameless. In all cases, the outer panel is a 0.7-mm sheet that's hydroformed with feature lines to improve dent resistance and to resist oil canning. The frame-integrated and frameless designs use a high-strength steel hydroformed tube for the lower door frame to not only resist sag and to handle loads, but to incorporate the side intrusion beam.

The logo for Fisher Body, the once-upon-a-time division of General Motors that was re-orged into nonexistence, featured a coach. For those of you who don't recall it, that's a coach as in "stage coach." The coach in question was, admittedly, a fancy one. But a coach. That's because when people started building auto bodies, they were often the same people who'd been building the literally horse-powered conveyances. And those coaches were made of wood. As were many engine-powered vehicles early on.

It's hard to imagine mass produced vehicles being manufactured with wood. The main material that replaced wood is steel. Nowadays, steel is so pervasive in the auto industry that all of the talk of alternative body materials notwithstanding, it is hard to imagine that steel could go the way of wood.

According to Daryl Martin, senior director of Automotive Applications for the American Iron and Steel Institute (AISI), "Of all of the body applications out there, about 98% are in steel."

The people involved on the committee—which includes members from AK Steel; Acme Steel;Bethlehem SteelDofascoInland SteelLTV SteelNational SteelRouge SteelStelcoU.S. Steel GroupWCI Steel; and Weirton Steel—feel that they have plenty of reasons—backed with lots of research—on why steel should continue to be the material of choice for body applications for the foreseeable future—and well beyond.

These factors include:

  • Cost. Pound for pound, steel is less expensive than the alternatives. Even the more expensive materials (e.g., the ultrahigh strength steels) can be more cost effective. Martin suggests that what sometimes happens is that a component is switched from steel to a competing material, then, a year or two down the road, it goes back to steel. "The reason is cost," Martin states.
  • Safety. The term to know here is "crash energy management." Pete Peterson, director, Automotive Marketing, U.S. Steel Group, explains that in a crash, all materials absorb energy. But they do so in different ways. According to Peterson, when the speed goes up and there's a harder hit, steel will absorb more energy. Steel is strain-rate sensitive. Peterson says fiber-reinforced plastics don't absorb more energy in a way analogous to steel. And he notes that some aluminums do worse with a harder hit (they're strain-rate negative) and that no aluminums "are as good as steel."
  • Recyclability. This one is very simple. Steel is magnetic. Easy to separate from smashed up, crunched up, ground up, or otherwise scraped out materials. Just turn on the big magnet. Which explains, in large part, why cars are the most recycled consumer goods going.

 

hood
Two hood designs were presented in ULSAC, conventional and grille-integrated. Both have a hydroformed 0.6-mm sheet outer panel. For the inner, a light weight sandwich material - 0.8-mm engineered polypropylene core between two 0.2-mm steel sheets - is proposed. As that's not readily available, a 0.6-mm steel inner is also proposed.

That Was Then

Even though they do have the position of strength in the market, the steel people are hardly taking that position for granted. Peterson candidly admits, "Once the steel industry only wanted to sell what it could make. Now it will make what it can sell." Since vehicle manufacturers are looking for materials that are both light and strong, steel companies have metallurgists even at places like Los Alamos National Labs (where such things as "high-speed straining analyses" are being conducted) working to come up with more desirable materials. Peterson remarks: "Nowadays, all bets are off. Any assumptions that we may have had the metallurgists modify." In other words, although steel has been around for autos for longer than anybody but those people Willard Scott acknowledges on the "Today" show remember and so there might be an assumption that steel is steel is steel, Peterson says that it is as avant garde a material as anything being offered.

There's another piece to this, which is design related. Peterson points out that in the mid-1970s, when the Oil Crisis hit the billfolds of drivers across the land, auto engineers in Detroit feverishly worked to redesign vehicles to make them more fuel efficient. It was an enormous undertaking. Because they worked so rapidly, and because computer-aided tools then weren't anywhere as capable (or as available) as they are now, there wasn't a great deal of structural optimization performed, Peterson comments. At the end of the day there was a sense among some engineers who'd been involved in seven or so years of hard work that as much as could be done with steel had been done. And so alternative materials were—and are—sought.

 

Decklid outer
Hydroforming is also used for the ULSAC decklid outer. The hem flanges are adhesively bonded for structural performance. If the inner is made with the sandwich material, the whole thing comes in as 29% lighter than the benchmarked average.

This Is Now

In 1994, in response to the notion that steel was not the material of tomorrow, the UltraLight Steel Auto Body (ULSAB) Consortium was formed by 35 sheet steel producers from 18 countries (including the 12 companies in the Automotive Applications Committee). They contracted Porsche Engineering Services, Inc. (PES; Troy, MI), to undertake a program to develop a steel body structure that is both light weight and affordable. In March 1998, after $2 million were spent on developing the concept and $20 million were spent in validating that concept by actually manufacturing the proposed body structure (12 models were built), the ULSAB Consortium was able to prove that with existing materials and existing processes a body structure weighing up to 36% less than benchmarked midsize vehicles (from North America, Europe and Japan) could be produced.

Not only does the ULSAB have the structure necessary for ride, handling and safety, but the production of the vehicle wouldn't be at a cost penalty.

Although there is plenty of high-strength and ultrahigh strength steel used in the 203-kg structure—more than 90%—and although there is the use of used-but-not-ubiquitous processes like hydroforming and laser welding of tailored plants, much of the ULSAB's positive performance can be attributed to what is called "holistic design." All body and process engineers are familiar with designing and producing body sides and floor panels and the like. What the people at PES did was, in effect, to forget what they knew and worked with steel as though it wasn't a familiar material but something new. Consequently, the design they developed worked differently than the basic designs that have been used for many years. They had the opportunity to optimize. And so they did in terms of both material and design.

 

hatch
The ULSAC hatch makes use of hydroforming of the skin, adhesive bonding of the hem flanges, hydroforming of a frame, and urethane bonding of glass. This results ina hatch that's 26% lighter than the benchmark.

The Other Pieces

A follow-on study backed by the steel industry and conducted by PES is ULSAC—UltraLight Steel Auto Closures. This study, released at the end of September 1998, looked at the pieces that would be wrapped around the ULSAB: the doors, hoods, decklids, and hatchbacks, the closure panels that account for about 6% of a vehicle's total mass. The study consisted of benchmarking, target setting, conceptual design, finite element analysis, and cost analysis.

Using the same design approach, material types, and processes that were part and parcel of ULSAB, the researchers concluded that 50 lb. could be saved per vehicle through the optimization of closure panel design and construction. These closure components would meet safety and structural performance targets. And they would be cost-competitive compared to conventional closures or those made with alternative materials.

Bottom line: Steel has a long way to go in the auto industry.