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The Epsilon-based architecture for the 2004 Chevy Malibu has a stiffness of 27 Hz thanks to the careful utilization of steel types and grades throughout the structure.
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.
But there is something else that Krupitzer acknowledges about the move toward AHSS, which is that when it comes to issues of weldability and formability, materials are different, and so too are the characteristics of those key processes. What’s lacking, but being developed, he explains, is a database of information that people can use to better understand how the materials perform under a variety of conditions as well as when used in varying gages and combinations. Not only is this an issue related to designing parts for manufacture, but also when it comes to repairing vehicles that may have been in a crash. Some of the AHSS grades, for example, because they obtain their properties during rapid cooling during production, can’t be heated and straightened in the local garage.
Consequently, one of the major focus areas during the Great Designs in Steel seminar was on process parameters, work that is being done by both steel companies and auto manufacturers; some of the research is being supported by funding from the U.S. Department of Energy. Typical of the sort of research that is occurring at steel companies, work that was reported during the seminar, is that occurring at U.S. Steel and at Ispat Inland. Alex A. Konieczny, who is with the U.S. Steel Automotive Center, gave a presentation on the formability of AHHS. Among the conclusions reached are: “DP [dual-phase] and TRIP steels display excellent formability as compared to conventional HSS, and better capacity to distribute strain over the part surface” and “Due to excellent combination of strength with formability, TRIP steel has potential for economic application—possibly for the reduction of the number of parts in the car.” Benda Yan of Ispat Inland described work that is being done regarding fatigue behavior of AHSS. Among the conclusions cited: “AHSS exhibited much higher fatigue strength over conventional HSS” and “Compared with HSLA350, DP600 exhibits a 30% higher and TRIP600 a 70% higher endurance limit.” Other papers were presented on spot weld fatigue, strain rates, weldability, and repairability. It will only be with further understanding of the behavior of these materials under various conditions that they will find increased application.
DESIGN IMPORTANCE. One important point that Krupitzer makes is that while the AHSS materials are more expensive than low-carbon steels, “the amount of mass saved by using AHSS is approximately equal to the cost increment per ton.” But, and this should be seen as a big BUT, design is a big consideration. “You couldn’t have an efficient [from a materials utilization point of view] or low-cost vehicle if you simply took conventional parts and assigned new steel to them. Part substitution savings would be minimal in mass reduction and efficiency,” Krupitzer says. “Design is critical to everything we do.”
In order to get great cars and trucks with great designs in steel, it is essential to take a holistic look at material capability and performance and then to apply the materials accordingly. This may mean part consolidation. It may mean employing different processes. And it can mean better, safer, more cost-effective vehicles.
During the “official” public introduction of the 2004 Chevy Malibu at the North American International Auto Show in Detroit this past January, Kurt Ritter, then-Chevrolet general manager, said, “The Malibu is the first North American car based off GM’s new global Epsilon architecture, which creates a vehicle with superb driving dynamics.
“The story begins with an all-new body structure, giving Malibu stiffness comparable to that of the best midsize European sedans.”
It isn’t entirely surprising that there is stiffness comparable to Euro sedans because the Epsilon platform, which is used for the Opel Vectra and the Saab 9-3 sport sedan, was largely developed in Europe by Opel and Saab engineers. As Gene Stefanyshyn, GM midsize vehicle line executive, who is responsible for the Malibu, the yet-to-be-introduced Malibu Maxx (a larger version with a significant amount of room in the rear for passengers and/or cargo), and the for the forthcoming Pontiac Grand Am, puts it, “Part of the beauty of the Epsilon architecture is that it’s flexible enough to be ‘tuned’ to various models and markets. For the Malibu, we were able to work within that flexibility to come up with a body structure tuned for American roads that also delivers the kind of stringent performance, durability and refinement objectives more often associated with European and Japanese cars.” The structural stiffness of the Malibu is a highly respectable 27 Hz. And it should be noted that steel is the basis of that structure.
According to Michael F. Weber, Engineering Group Manager, Body Structures-Epsilon, the steel grade distribution based on mass for the Malibu is:
Weber acknowledges that the Epsilon doesn’t necessarily have a whole lot of high-strength steel content, part of this is a consequence of the fact that when the program was under development, and it started six years ago, the steels weren’t widely available so there wasn’t a good working knowledge vis-à-vis performance and processing characteristics. Nowadays, he says, “As a general rule, we are continuing to use higher strength steels.” He amplifies that remark by saying, “We are pushing ourselves with regard to dual-phase and HSLA.”
Weber says that for the Malibu body structure, careful attention was paid to using the right steels for the right application areas. For example, bake hardenable material is used in such places as the bumper-to-bumper rail structure for crash energy management and for reinforcing the B-pillar and roof rail. Dual-phase steel is used for simple part shapes, such as in the rocker section, to provide increased compressive strength for key crash members. HSLA is used primarily as a means to provide pieces to strengthen the underbody; it is used for such things as the rocker bulkheads, roof bow, and lower hinge reinforcement. Solution strengthen material is used for reinforcement and structural support where there isn’t a need for the kind of strength provided by HSLA. And then there is the extensive use of low-carbon steel. This material, he explains, is used in such places as closeout panels.
According to Weber, one of the reasons why there is an extensive use of low-carbon steel relates to its formability, especially as compared with, say, dual-phase materials. “You can hit low-carbon steel several times to get the shape you’re looking for.” As examples, he cites the sharp corner and sharp radius of the base of the A-pillar, where it comes down to meet the front quarter and door, and the tight radius (~1.5 to 2 mm) necessary for the tail lamp pocket. Low-carbon steel offers the sort of formability and stiffness that’s sought. “Formability factors are greatly reduced the higher strength you go,” he says, noting that American-based vehicles tend to be highly styled with regard to forms than vehicles from other parts of the world. At the same time, however, the higher-strength materials provide the kind of strength and energy absorption that they’re looking for.
While they are moving toward an increased use of higher strength steel at GM, Weber notes that they’re still developing the process knowledge that is necessary for such things as welding (e.g., it isn’t a good thing, he suggests, to weld dual-phase to dual-phase because of resultant brittleness) and forming. One of the concerns for high-volume operations is that things keep running reliably and predictably, which means a stable process, which results from a good understanding of material behavior.
One more thing: “We don’t do anything that doesn’t work from a business-case perspective,” Weber says. Some new materials may be interesting, but if they don’t make economic sense for the application, they’re unlikely to see extensive use.