The Safety of Steel
The 2014 Acura MDX has something that no other vehicle did when it was launched. It has the world’s first ultra- high strength steel, one-piece stiffener ring. It’s the A-pillar and the B-pillar. It’s the roof rail and the lower frame member. According to Dr. Blake Zuidema, Director of Automotive Product Applications, ArcelorMittal Global R&D (corporate.arcelormittal.com), that stiffener ring is not only the result of advancements in metallurgy, but involved hot-stamping and laser welding. The purpose of all of that technology is to make the MDX safer. The ring is designed to manage energy not only in the case of side impacts, but also for frontal offset collisions and in rollover situations.
When it comes to safety, stronger is better, and Zuidema points out that when it comes to producing cost-effective structures that can resist intrusion—the sort of thing that you’re looking to do when engineering a passenger compartment—then steel provides the competitive edge.
He cites, for example, the company’s Usibor steel, a press-hardened boron steel with an aluminum-silicon coating that provides a strength of 1,500 MPa after hot-stamping. And suggests that this is much stronger than any alumi- num alloys on the market right now. What’s more, he notes, that this strength provided by the steel doesn’t come at the cost of a mass penalty: “A component is stronger and competitive weight wise.”
But what about composite materials? Strong, light, safe, right?
Yes. But: Zuidema points out, making a door aperture like the one created for the MDX would be far-more labor intensive, which leads to “substantial cost to manufacture.” Which leads to substantial cost to put in a vehicle.
While aluminum and composite materials seem to be bright, shiny and new, and consequently catching a whole lot of attention, Zuidema says that most of the steel grades that are going into cars today weren’t available 10 years ago, so when it comes to contemporary relevancy and capability, it’s not like this is some classic HSLA material. And he says that work is ongoing to produce steels that are stronger with even better formability, which are a couple years away from being commercially available.
Speaking of formability, if some of the materials being formed are “three to four times the strength of the highest- strength aluminum alloys that are available now,” isn’t there a manufacturing challenge presented by these new steels? Zuidema says that that’s not the case, that while there are some new guidelines for stamping, “Steel uses the same press shop and the same body assembly techniques, the same spot welding, MIG welding, and the same basic adhesives.” Whatever changes there are in the production operations, he says, “The cost of converting is far, far lower than it would be for converting to aluminum or carbon fiber.”
The big challenge being faced by OEMs is the 54.5 CAFE target for 2025. Zuidema says that that number can be achieved with the grades of steel that are available now. What’s more, compared with alternatives, it can be done so economically compared with the alternatives. “They might be a little bit lighter,” he admits, “but steel gets you there at a lower cost.”
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