Advanced high-strength steels have shown plenty of promise in automotive applications for years now. With tensile and yield strengths two to five times those of mild steel and tendency to work harden, these multiphase steels resonate with automakers trying to reduce vehicle weight and improve crash performance. What, then, has held these beneficial materials back? Economics played a role initially—the higher costs associated with new materials whose volumes haven't ramped up yet. Another important factor has to do with manufacturing issues. These steels, behave differently when it comes time to stamp or weld them. But these materials have gained ground over the last couple of years as automakers have gained manufacturing experience with them.For proof, take a look at the "Great Designs in Steel Seminar," held in March by the American Iron and Steel Institute (AISI). This year's seminar featured 30 presentations, 10 of them from OEM representatives who talked about their progress in fabricating these materials and their plans to increase their usage in current and future vehicles. Here's a look at some highlights:
BMW 3 Series
Discussion of advanced high-strength steels often revolves around crash performance—with good reason. But these materials also have important performance attributes, such as improving stiffness and mass balance. All of these benefits came into play in the new BMW 3 Series, according to a presentation given by Markus Pfestorf, BMW's manager of body-in-white materials. This car contains a huge amount of high-strength steels. Defining the cutoff between low- and high-strength steels as a yield strength of 180 N/mm2, Pfestorf reports that the average minimum yield strength of steels on the 3 Series is 294 N/mm2 up from 178 N/mm2 on the previous 3 Series.
This new sedan incorporates its share of advanced high-strength steel, too. DP 600 represents 15 percent of the steel, by mass, on the Ford 500's body. This strong steel isn't applied in a radical way. In fact, the vehicles architecture and energy management evolved from the Volvo P2X platform. The front-end rails and load path also recall the design spelled out in the AISI's ULSAB-Advanced Vehicle Concepts study. But Michael Lee, manager of Ford's body CAE department, notes in his presentation that the 500 and Freestyle do represent an implementation success story. To incorporate the DP600, Ford overcame supply, stamping, and welding challenges and got improvements to crash performance and stiffness in return.
Volkswagen Golf V
With 56m of laser-based joints, this vehicle showcases the potential for laser welding and brazing, two crucial technologies as automakers move to use more advanced high-strength steels. "No other company does more with laser welding than Volkswagen," says Ron Krupitzer, senior automotive director for the American Iron and Steel Institute. These welds include a variety of body components, including roof joints, side panels, and a mounting plate for the front end module. Not all of these joints currently involve advanced high strength steels, but Krupitzer notes that laser-based process will grow more important with the growth of these steels. "Laser welding has a smaller heat affected zone and less embrittlement than traditional welding methods," he says, explaining that these factors can help preserve the strength of advanced steels. "You want to be careful not to undo their strength with welding processes that change their microstructure."