Steel: It's Not What It Used to Be

Gary S. Vasilash

Perhaps because steel has long been used in the auto industry, some engineers think that the material is old-hat, that different materials have to be used in order to develop vehicles that are lighter and so more fuel efficient. But 35 steel companies from 18 different countries are proving that steel is as high tech and as capable as anything available, and they're building a car to prove it. And, oh, by the way—steel is a whole lot more economical than the competition, too.

Novelty has a tremendous appeal for most people. Although there may be something to be said for the old-tried-and-true, there is an inexorable tug from the brand new. (One could make the case that the entire consumer products industry is predicated on this appeal. Doubt it? Visit a Best Buy or Circuit City and note the endless proliferation of computers, audio systems, TVs, VCRs, etc.)

Although the auto industry was originally built, from a body standpoint, on composite materials (wood, after all, is a composite, albeit a natural one), for the better part of this century the industry has been one built on ferrous materials. Cars are, with few and not wide-spread exceptions, steel. Quibble about Saturns and Corvettes and NSXs; note the aluminum panels here and there and plastic fascias everywhere. But mainly it's steel.

Engineers are problem solvers. Most, if not all, of them look for the best solutions to problems and challenges. But engineers are also like their next-door neighbors. Many of them feel the force of novelty, of difference. One of the consequences of this is that a number of engineers are looking for alternatives to what's been done so many times in the past: engineering with steel.

In fact, when the announcement was made about the establishment and goals of the Partnership for the Next Generation Vehicle (PNGV), the industry-government program established to get a vehicle on the road by the end of the first decade of the next century that will provide 3X the fuel efficiency of a current full-size car, the implication was clear that the PNGV platform would be nonferrous.

Understandably, the people in the steel industry have been observing these and other efforts with some concern. And so they decided to do something about it. In 1994 the Automotive Applications Committee of the American Iron and Steel Institute (AISI) decided to see what steel could really do with regard to addressing the needs of auto manufacturers. On the one hand, the vehicle manufacturers need lighter weight vehicles in order to meet fuel-efficiency and emissions requirements. Given that the public isn't particularly inclined to buy small cars, this lightening is crucial. But the car just can't be light; it must also be safe, so crash energy management is a key concern. At the same time, the auto makers are not in a position to be able to offer more-efficient vehicles to a mass market at a premium price. Economic issues are significant. So the steel people figured that a clean-sheet approach would be necessary.

In April, 1994, AISI representatives described their plan to their Asian and European colleagues at a meeting of the International Iron & Steel Institute. And so ULSAB—the UltraLight Steel Auto Body program—was started. There are 35 steel companies from 18 different countries involved in funding, managing, and developing ULSAB.

This is not a trivial undertaking. The budget for the two phases of the program is approximately $22-million.


ULSAB Background

There are two phases in ULSAB. The first, which ended in September, 1995, after 15 months of work performed primarily by Porsche Engineering Services, Inc. (Troy, MI), which is undertaking ULSAB for the steel companies, focused on developing the design platform for the vehicle. The goal was to come up with a vehicle that could be produced with existing technologies and from commercially available materials that would be light, roomy, safe, and affordable. They bench-marked vehicles including the Taurus, Accord, Lumina, and BMW 5-series. When the exercise was complete, the Porsche engineers had come up with a design for a vehicle that is significantly lighter than the benchmarks (205 kg versus an average 271 kg for the benchmarks) and which exhibits better rigidity and frequency numbers. What's more, through the holistic design practices (i.e., looking at the vehicle as a system, not just working with individual parts; historically there has been a tendency to try to lighten each piece in a vehicle; here, with the focus on function rather than mass, it is possible to develop a vehicle that has less mass overall, without necessarily lightening every part), they came up with a design that used fewer parts and less material which, coupled with innovative-but-available assembly processes, would cost nearly 11% less to produce than a conventional car.

Phase II, which was kicked off shortly after the completion of the first phase (it began on November 1, 1995), and which will be complete Spring, 1998, has validation as its goal. This means validation of both the design (after all, the "vehicle" during Phase I was digital) and of the manufacturing processes. Which means that cars are actually being built at the Porsche facilities in Weissach, Germany. Sourcing parts, processes and materials is an international initiative. Whereas Phase I accounted for about $2-million of the budget, Phase II costs some $20-million.

Ed Opbroek is the program manager for ULSAB. An industry veteran who took early retirement from AK Steel, where he had been general manager of Technical Resources, Opbroek has been involved in the project from the beginning.


Beyond the Smokestacks

Opbroek openly acknowledges that steel isn't always perceived by people in the most ideal light: they think that it is a heavy, stodgy material that comes from a "smokestack industry." No, ULSAB isn't about changing the density of the material. And yes, there are still smokestacks on steel mills—just as there are on the roofs of microprocessor manufacturing facilities (which, incidentally, are referred to as "foundries" by the digitally inclined). The stodginess may be a characteristic that some people in the steel industry may have once earned—but it is one that most of the people populating the firms no longer deserve.

"Steel is a high-tech, engineered product," Opbroek states, adding, "And it has to be used that way."

Although it might be naturally assumed that he would make a statement of this nature—after all, he is drawing a paycheck from the steel industry—it is worth considering what his points are, starting with the second one first. If steel is concerned to be a generic material that's been used pretty much the same way in the production of automobiles for as long as anyone can remember, then it is unlikely that a whole lot of thought will be given to those methodologies—especially not as much as will be given, say, to aluminum or composites. Those materials will be clearly perceived as being new or at least different; consequently, there is likely to be more attention on—and funding for—the alternative material. It is the case of experience versus novelty. If you assume that something is the same as it ever was, you won't pay a great deal of attention to it.

As for Opbroek's assessment of the nature of the material: the steel industry has recognized for some time now that there are plenty of other materials manufacturing concerns that would like to get a bigger piece of the automotive body-in-white. Competition means two things. One is that you get better. The other is that you keep doing what you are doing until you find yourself out of business. The steel industry decided to improve rather than abdicate.


Product & Process Improvements

Case in point: high-strength steel (HSS). More than 90% of the ULSAB as configured is HSS. HSS per se is not new in the auto industry. Opbroek notes that auto manufacturers have been working with HSS since the early `80s. He candidly admits, "Most of the car companies have gotten bloody noses the past several years with high-strength steels." There are at least two reasons for these bad experiences. One goes to the steel companies. The uniformity of the materials' properties were not, well, all that uniform. One response to that issue was the formation of the Auto/Steel Partnership, which is a joint effort between U.S. car makers and AISI member companies. The Auto/Steel Partnership has been monitoring steel uniformity during the past several years, and its data indicate overall improvements, especially, Opbroek points out, for HSS.

The second aspect to the bloodied noses could be found within the auto manufacturing organizations. Platform teams and concurrent engineering aren't (a) all that old and (b) extensively used by all companies. So there were (or are) part designers in one place, tool designers in another, and production people somewhere else. The prevailing experience base for all concerned was with mild steel, long the staple in auto production. HSS doesn't have the same forming characteristics as mild steel (e.g., there's a greater amount of springback). So if steel is treated as "that material we've been designing parts with/creating tooling for/stamping since as long as anyone can remember" and not as a particular material with particular properties, then there are going to be problems. Take the springback, for example. If the tools aren't designed and built to accommodate it, then there are going to be serious problems. Problems that could result in the figurative bloodied noses. But more organizations are now cross-organized, so the communications about what's happening (or not) can be more readily communicated, thus ameliorating problems.

Given the facts, then, that the materials are better and the organizations are better aligned to communicate, a vehicle with 90% HSS is now a real-world possibility.

When ULSAB is introduced to the public in North America, Europe and Asia, it will give credence to Opbroek's contention that steel is (1) a high-tech material that (2) can do some impressive things when applied and processed in the appropriate manners.

Putting It Together. As mentioned, the vehicle is fundamentally based on high-strength steel, a material that's generally defined as that which has a yield strength or 25 ksi or more. In the case of the materials selected for ULSAB, the yield strengths are well in excess of that, as they range from 210 to 800 Mpa. The material thicknesses used: from 0.65 to 2.0 mm.

As for just what is being made with HSS: it is easier to note what isn't:

•back panel

•wheelhouse (right and left) outer panel

•rear header panel

•member pass through

•panel skirt (right and left)

•panel gutter deck lid (right and left)

•support panel rear header (right and left).

Tailored blanks—single blanks that are produced by welding (typically with a laser) two or more pieces of material together—are taking the auto industry by storm. ULSAB is simply ratcheting up the amount: almost 50% of the mass is accounted for by tailored blanks. The advantage of tailor welded blanks compared with conventional blanks is that conventional blanks are one thickness, one material. But by welding individual materials together, it is possible to vary the blanks' physical properties (including thicknesses and grades).

One of the major ULSAB components that's a tailored blank: the body side outer. This was a challenging part to design and produce in that it includes the rear quarter panel, a class A surface part, so it was necessary to develop it so that the welds wouldn't be apparent. The component is produced from a blank that measures 1,600 mm by 3,500 mm. The thickness of the blank ranges from 0.7 mm to 1.7 mm.

Hydroforming is another process that, like tailored blanks, is finding increased use in automotive. (It is such a coming thing that in some Corvette ads the fact that the side rails are hydroformed pieces—stiffer, stronger and lighter than rails produced by conventional methods [i.e., assembling an array of individual components]—is actually pointed out as one of the beneficial features of the C5.)

Essentially, hydroforming is a process that involves putting a tube in a forming die, then filling it with fluid under pressure. The fluid (or the "hydro") forms the part—it forces the tube to follow the shape of the die cavity. As you can imagine, the die in question needs to be fairly robust. Whereas the tools produced to make most of the pieces of the ULSAB are "soft" tools (e.g., produced with materials such as kirksite), the hydroforming tooling is steel.

The ULSAB features a hydroformed side roof rail that starts at the A-pillar and makes its way back down the C-pillar and to the rear rail. One interesting aspect of the development of this part was not the hydroforming (actually, this was comparatively simple, as the amount of change in the perimeter and shape of the part was just 5%) but of getting the required thin-walled HSS tubing needed to make the part. Apparently, the wall thickness as compared with the circumference of the tube is much smaller than is the norm: the tube has a wall thickness of 1 mm and an outside diameter of 96 mm. Things like the stiffness and straightness of the tubing are typically concerns, which is not the case with the material needed for the hydroforming, as it is destined to be significantly shaped by the process. The tube processor had to contend with the greater springback of the HSS material (it has a yield strength of 280 Mpa) as compared with the mild steel usually employed. Also, it was important that the high-frequency-generated weld beam was accurately positioned.

One surprise: ULSAB contains polypropylene—a plastic. But the plastic is in the middle of a sandwich that has sheets of HSS on either side. The sandwich material is used to produce the spare tire hub. The reasons: not only is the piece light and strong (50% stronger than a steel-only hub would be), but there's sound-deadening qualities to boot. Originally, it was thought that the tub would be part of the rear floor panel. But based on analyses performed by steel suppliers on the forming characteristics, it was determined by the design engineers that it would be better to produce two separate pieces: the tub and a flat panel integral to the body in white. Note that interaction and feedback were important to the development of this component, as was the case for other elements of the ULSAB.

"We're not proposing this as the solution for the year 2020. This is a solution for today," Opbroek says. He points out, "The whole manufacturing infrastructure in automotive bodies is geared for steel. People may have to think differently and make some changes, but it is still in the sweet spot of what they do best."


Beyond High Strength

Looking for something more than high-strength steel (HSS)? If so, you might want to consider UHSS from Inland Steel (Chicago)—that's ultra high-strength steel.

The steels that Inland has in its line-up—which they're calling Generation 2000—are all cold-rolled and continuously annealed.

The basic benefit of the materials: a higher strength-to-weight ratio.

Included are:

•CAL HI-FORM, which combines strength and formability characteristics. Applications can include door intrusion beams, bumper reinforcement beams, and seat parts such as tracks, pillars and risers.

•DI-FORM, which has a low yield strength to tensile strength ratio and high strain hardening and high bake hardening in strained areas. This material can be used for door intrusion beams, bumper reinforcement beams, seating components, and structural cross members.

•MartINsite, which contains fully martensitic microstructures, which makes this material extremely strong. Not only does this range of materials lend themselves to bumper reinforcement beams and door intrusion beams, other applications are side sill reinforcements, belt line reinforcements, springs, and clips.

•Electrosite is similar to MarINsite, but this is an electrogalvanized material, so it not only provides high strength, but also corrosion resistance. Bumper reinforcement beams are a typical application.


Cold Rolled Stainless from Crawfordsville

The best book we've ever read on steel making is certainly American Steel by Richard Preston (Prentice Hall; 1991). Although Preston didn't garner quite as much public recognition for the processing of ferrous material as he did for the Ebola virus in The Hot Zone, (i.e., no Hollywood movie with Dustin Hoffman as Ken Iverson), the book is not one to be missed.

The geographic center of American Steel is Crawfordsville, Indiana. Specifically, a steel plant built by Nucor Steel in Crawfordsville. Preston writes: "'The Crawfordsville Project is immensely complicated but immensely simple,' he [Ken Iverson, Nucor chairman] said. `We are constructing one million square feet of building space. This is a unique process. We're going to cast a very thin slab of steel, two inches thick, and fifty-two inches wide, then we're going to roll it down into a sheet, in one continuous process.' Iverson's machine, the heart of the steel mill, was the world's first continuous steelmaking machine. It was known as the Compact Strip Production Facility, the CSP." The story of American Steel is the story of the Crawfordsville Project.

That interview with Iverson took place in 1988. Now, 10 years later, the Crawfordsville plant, which has been rolling out steel for quite some time, is adding a new offering to the industry: 409 stainless material.

According to Michael J. Wagner, sales manager at the facility, the company is offering widths from 36 to 52 in.; the gages range from 0.018 to 0.04.

While the metallurgy of 409 stainless from supplier to supplier may be consistent, Wagner notes, "When you look at coils of material, they may appear to be the same, but when you start running the materials, you can see the differences," with the differences resulting from the gage control that they are providing at Crawfordsville.

More cost-competitive than some of the existing sources? "We would think so," Wagner answers.

And they have established a good bit of capacity at the Crawfordsville plant: 100,000 tons per year in a market that Wagner estimates is from 375,000 to 400,000 tons.