For the past few years, it seems like steel has become the dominant material of choice for automotive applications.
Sure, there are exceptions to this. You can get an aluminum-intensive Audi or Jag, for example. And the Corvette remains a composite-bodied car.
But for the most part, steel rules.
The material is strong (arguably, synonymous with the adjective). The material is familiar (which could actually be one of its weaknesses, given that it has been around so long that people take it for granted and figure that there must be some new material that would be better). The material readily lends itself to stamping and assembly (undoubtedly because those operations were actually set up in the first place to form and weld steel). And, compared with other materials, steel is, generally speaking, less expensive (which should probably put it at the head of this list, rather than at the end).
Still, Ron Krupitzer, vice president, Automotive Applications, Steel Market Development Institute (SMDI; smdisteel.org), a business unit of the American Iron and Steel Institute, and his colleagues are not taking steel’s position in automotive for granted. And one key reason for this is because as OEMs are looking at the forthcoming CAFE 34.1 mpg standard by 2016 and 54.5 mpg by 2025, they are searching, vigorously, not only for improvements under the hood, but also the ways and means to reduce the mass of their vehicles. Which could mean a materials change. And given that steel is arguably the incumbent, this change could lead to something else.
Krupitzer admits, “We’ve spent so much time on the body”—as in developing the steels and applications for both bodies-in-white and closure panels—“that we’ve neglected chassis and suspension parts. We see great opportunities there and even faster implementation than some body applications coming down the road.”
This is because of the way that many of these parts have been specified. That is, Krupitzer explains that generally, “There has been a preference in the chassis and underbody area for thicker steel components, and often that is because the car companies want to keep the cost as low as possible, so they don’t use coated steel. They use bare steel. There is a minimum thickness they’ll allow for parts based on the corrosion testing they do.”
Simply put: In order to assure that the parts will meet, say, a 15-year corrosion requirement, the gauge of the steel underbody parts is greater than it need be if galvanized steel is used in the same applications. The issue is that the galvanized steel is more expensive than the bare steel (which does get some barrier coating as the vehicle goes through assembly, but not a whole lot). But the galvanized steel part can be made thinner, thus saving mass, which then goes to the issue of addressing the CAFE requirements. Krupitzer suggests that the cost increase associated with using galvanized steel “is minimal because they’ll be buying less steel.”
And beyond the galvanizing of steel, there is an entire suite of high- and ultra-high-strength steels that have been developed which lend themselves to application in the underbody and chassis. Given that on the order of 75 to 80% of the chassis and suspension components are still ferrous based, there are lots of opportunities to save.
David Anderson, senior director, Automotive Technical Panel and Long Products Program, SMDI, points out, “Most of our projects have shown that when advanced high-strength steels are used, you can thin up the gauge and reduce overall costs.”
Krupitzer amplifies that by saying, “The premium that might come with using dual-phase or TRIP steel would be offset by a great extent by the weight you save. And it beats the up to 10-times cost-per-component of some low-density materials.”
The “low-density materials” that he is referring to include aluminum, which has become a material of choice in some suspension applications.
So Anderson cites a study that they did on a front lower control arm, with a state-of-the-art forged aluminum OEM component as the baseline of compari-
son. “We provided three steel alternatives”—a forged steel arm, an I-beam design that is an assembly consisting of parts made with different grade dual-phase and HSLA steel, and a clamshell design, which primarily consists of two stampings that are riveted together—“that had the same weight as the aluminum but as much as a 30% cost reduction.”
Their point is that steel can do the job at a cost that is typically less costly than other materials: “There is a potential for us to be half the cost,” Krupitzer says. But this can take some different approaches to designing and engineering the parts and selecting the right types of steel for the parts.
“One thing a lot of people don’t realize is that with the old traditional stamp-ing and assembly processes there is always some redundant mass in the parts because you are always designing for the weak spot in the panel or section of a vehicle,” Krupitzer says. “The engineers do a CAE analysis and look for where the part is overloaded—they see the red zone.” So they have historically addressed that by adding mass to the entire part to accommodate the localized loading. But with the efforts now being taken to take mass out, this is no longer a viable solution.
One of the ways that some of this localized loading has been addressed is through the use of tailor-welding blanks. Briefly, these blanks are laser welded combinations of steels with different thicknesses, strengths and coatings. So if, say, on a door panel where the hinge will be located at the A-pillar, there will be a stronger steel used than in the middle of the door as it doesn’t see the same demands.
Beyond that, there is now the oppor-tunity to source from companies like TWB (twbcompany.com) tailor-welded coils. Again, this is an approach that combines different steels in coil form; the coils can be used for progressive die applications or for roll forming. Lighter-weight parts can be achieved by using this material because the requirements can be met by the material characteristics in a more precise manner than has historically been the case.
But what are the consequences of using advanced high-strength steels (AHSS) in the factory? According to Krupitzer, “In the car plant, there is almost no difference.” The big difference is in the steel mill, where there needs to be highly sophisticated heating and cooling stations in order to achieve the required microstructure.
Still, there are some changes, especially when stamping materials that are 980 MPa, which is approximately three times the strength of mild steel.
One issue that has to be addressed is springback. The characteristics of AHSS are different, so there needs to be consideration of things including the hold-down pressure, bushing pressure, and shut height. More overbend is likely to be designed in the die. And speaking of the dies, there is likely the need to have inserts in the die to protect it from excessive wear.
Trimming and piercing operations can require different clearances or different materials for the cutting edges. Weld cycles can require different parameters, such as current, hold time, and pressure. But as Krupitzer notes, “These are just minor adjustments. There’s no need to recapitalize in the plants.”
Again: savings is key.
Lifecycle Considerations & Chemical Bonding
From model year 1990 to model year 2010, the CAFE standard for automobiles was 27.5 mpg. As you can well imagine, OEMs managed to nail that sometime early on in that 20-year period. Consequently, there wasn’t a whole lot of need to do things like aggressively pursue light-weighting strategies. And from the governmental point of view, there was a lot of regulatory attention paid to things like stricter requirements for crash safety, be it for roof crush or offset collisions. Ron Krupitzer, vice president, Automotive Applications, Steel Market Development Institute, suggests that if it wasn’t for the development of new high-strength steels, there would have been an addition of 400 to 500 lb. to the weight of vehicles in order to provide the necessary structure to meet these safety requirements.
But now, with the CAFE standards making an aggressively steady increase for the next 13 years, there is a need to consider not only alternative powertrains, but vehicle mass reductions, as well. (“Mass is still important no matter what powertrain technology you use,” Krupitzer notes.)
There is another consequence of this improved fuel efficiency of vehicles, which is a concomitant reduction in emissions.
Krupitzer and his colleagues think that with this change it becomes more important to consider the lifecycle effects of a vehicle. Whereas driving has accounted for 85% or more of the total emissions of a vehicle if the manufactur-ing, driving, and end-of-life stages are assessed, the more efficient powertrains and vehicles are going to reduce that contribution. “If you want to reduce a vehicle’s impact on emissions, you need to consider all of the principal parts,” he says. Meaning that the emissions related to the produc-tion of the materials, parts and components up front, as well as the recycling at the end of life, need to be accounted for.
He notes that National Resources Canada (nrcan.gc.ca) has initiated a program in which they are developing the means by which a lifecycle assessment for auto parts can be made. When complete, this would mean that it would be possible to make choices on not only what type of material to use for a particular vehicle application, but the associated processing of the part, as well.
(As the NRC describes its undertaking: “Natural Resources Canada, through its CanmetMATERIALS Advanced Materials for Transportation program, is providing technical support in the development of a methodology for assessing life cycle carbon emissions with respect to vehicle light-weighting. The process involves development of product category rules under the ISO standard ISO 14025:2006 using a consensus based approach with a broad cross-section of industrial materials and component suppliers. The methodology, when developed, can be applied on a voluntary basis by users interested in assessing the effects of vehicle lightweighting strategies from a life cycle assessment perspective. It is intended that the assessment will include raw material production, semi-finished goods fabrication, component manufacturing, the vehicle use phase and the beneficial effects of recycling. The methodol-ogy will be made broadly available to Canadians through the Canadian Standards Association, when completed in late 2012.”)
While Krupitzer acknowledges that steel isn’t the right solution for everything in the production of a vehicle, that a variety of other materials are certainly necessary, he thinks that from an environmental lifecycle point of view, the steel industry is in a good position, not only because many plants have undergone transformations that make them more energy-efficient but because when you get down to the fundamentals, “It is easier to free up iron from iron oxide than aluminum from aluminum oxide because of the bonding energy.” Less energy is required in that up-front stage.