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.”
