Even Lotus considers high-strength steel a lightweight option06 July 2010
Lotus Engineering conducted a study replacing mild steels with high-strength steels in the body in white (BIW) of a crossover utility vehicle—a 2009 Toyota Venza. The HSS-intensive BIW was about 16% lighter—and 2% cheaper.
“Some of you are probably wondering what Lotus is doing at a steel seminar,” Lotus Engineering’s Senior Technology Specialist said to begin his presentation at the recent Great Designs in Steel (GDIS) seminar in Livonia, MI. “The overriding mission for Lotus Engineering is basically performance through light weight. It’s not performance through intensive use of aluminum or nonferrous materials, it’s not performance through carbon fiber, and it’s not performance through composites…but performance through lightweight materials.”
For example, the current Lotus Elise and Exige production cars use high-strength steel (HSS) for the rear bulkhead—which is saying something considering that even the automaker’s cup holder is machined from billet aluminum and includes some carbon fiber, noted Gregory Peterson.
“High-strength steel was the best material from a cost standpoint, a functional standpoint, as well as from a mass standpoint for this particular application,” he said. “It certainly has great applications for many vehicles especially for the near term.”
Peterson presented a study at GDIS that was a subset of a broader study recently published by the International Council on Clean Transportation, which addressed both long-term and near-term scenarios. The near-term scenario—defined as production-ready in 2017 with a 2014 technology-readiness level—involved replacing mild steel with HSS in a crossover utility vehicle’s all-steel body in white (BIW).
A 2009 Toyota Venza was selected for the analysis. The target was to reduce overall BIW mass by 20%, with a 20% plus cost allowance for the BIW piece cost, while using equivalent manufacturing and assembly processes. “What we also did on a component and subsystem level was not define any constraints. In other words, we could have used magnesium, titanium, carbon fiber—and that’s one of the more interesting aspects of this study, what we ended up with,” Peterson said.
Spectrometer analysis was performed on the BIW to categorize the steel types, and a bill of materials was created—a total of 419 parts for the all-steel BIW. Dimensional and volumetric targets were kept identical—so from a NHTSA standpoint, the HSS-intensive vehicle was the same as the baseline vehicle, according to Peterson.
The baseline BIW mass was determined to be 382.5 kg (843.3 lb). Its material breakdown consisted of 8% high-strength steel (DP 590), 2% Quiet Steel, 12% interstitial-free mild steel (IFMS), and 78% cold-rolled mild steel (CRMS). The HSS-intensive BIW (about 89% HSS) ended up weighing just shy of 325 kg (716.5 lb)—about a 16% mass reduction. The underbody floor alone went from roughly 114 kg (251 lb) on the baseline crossover vehicle to about 94 kg (207 lb).
The material balance consisted of 5% mild steel, 2% magnesium, and 4% paint/NVH materials.
“We ended up using steel for all panels to ensure manufacturing compatibility,” Peterson said. “The interesting thing was that we also ended up with about a 2% cost saving.”
The mass reduction came solely from gauge-thickness reductions; there were no design changes, he said.
Peterson conceded that some weight might have to be added back in for NVH characteristics due to the switch to thinner gauge HSS. He also noted that repairability could be more difficult with such a high amount of HSS, but that those issues were outside the scope of this study.
So the question becomes, is 89% HSS for a production BIW feasible in the near term? Peterson answered this question by referencing the 2010 Mercedes-Benz E-Class, which reportedly uses 72% HSS. “The bottom line is that’s a 2.4% per year increase in high-strength steel to go from 72% today to 89% in the 2017 time frame,” he said. “So we think it’s a fairly conservative value.”
Peterson concluded that a greater than 10% mass reduction by switching from mild steel to HSS appears feasible “based on the conservative estimates that we use, at near or little plus cost to the BIW structure.”
So look for Lotus to continue incorporating some HSS components into its sports cars—but likely not for the cup holder.
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