Advanced Steels Driving Vehicle—Lightweighting and Technology Adoption

Federally mandated Corporate Average Fuel Economy (CAFE) standards demand 54.5-mpg automotive and light-truck fleet averages by 2025, and automakers are leaving no stones unturned in striving to comply.

As newer, stronger steels find their way into passenger vehicles to reduce weight in order meet these stringent mileage standards, and improve crashworthiness and occupant safety, the automotive supply chain has become more familiar with how to process these steels. In addition, forming the latest generation of advanced high-strength steels (AHSS) into complex components productively has required adoption of varying technologies.

New Steels in New Vehicles Everywhere

Within the past two model years, vehicle weight reductions attributed to the use of AHSS over more traditional steels or other materials are in evidence across all vehicle types and models.

Noting just a few examples, redesigns including incorporation of AHSS have trimmed 200 lb. from the 2015 Chevy Colorado, 71 lb. from the 2015 Chrysler 200, 300 lb. from the 2016 Chevy Malibu, 300 lb. from the 2016 Honda Pilot, 129 lb. from the 2015 Ford Edge and 185 lb. from the 2016 Volkswagen Passat.

AHSS Offers Unique Blanking and Forming Options

Weight reductions in the Chevy Colorado owe plenty to tailor-rolled AHSS blanks formed into B-pillar outer reinforcements. Precise control of roller pressure allows the blanks to vary between 1 and 2 mm thick, with extra thickness provided in areas targeted for increased crash-energy management.

Weight reductions in the Colorado owe plenty to tailor-rolled AHSS blanks formed into B-pillar outer reinforcements. Precise control of roller pressure allows the blanks to vary between 1 and 2 mm thick, with extra thickness provided in areas targeted for increased crash-energy management. While tailor-rolled blanking is not brand new, it is a more recent introduction to North America, having found greater use in Europe. Rolling a single sheet to multiple thicknesses, a process primarily performed by blanking specialists or independent fabricators, offers benefits over tailor-welded blanks. Rolling requires less material handling, without the need to transport and place multiple pieces of material, and does not require use of the time-, quality-control- and cost-intensive welding process. As more automotive designers and part suppliers become familiar with tailor-rolled blanks, expect their increased use for placement of AHSS in vehicles, according to Dave Anderson, senior director of the Automotive Market and Long Products Program at the Steel Market Development Institute.

But don’t discount the continued use of tailor-welded blanks, he says.

The 2016 Acura TLX employs tailor-welded blanks that are hot-stamped to form door rings for A and B pillars, roof rails and rocker sections. The process yields a one-piece door-ring stiffener, replacing four separate pieces that had to be welded together, saving 9 lb. and eliminating weld-flange overlaps.

“A couple of Honda vehicles, the 2016 Pilot and Acura TLX, use tailor-welded blanks, which then are hot-stamped (another European technology that has gained traction in North American automotive production) to form door rings for A and B pillars, roof rails and rocker sections,” Anderson explains.

Hot stamping, via an in-process quench, imparts martensitic properties upon the part material, leaving it with very high strength but increased formability, in contrast to traditional martensitic steels. Metalformers are using hot stamping to produce complex parts, including those in some of the above-mentioned vehicles, that carry the benefits of newer ultra-high-strength material. With the Acura TLX, hot stamping has yielded a one-piece door-ring stiffener, replacing four separate pieces welded together, saving 9 lb. and eliminating weld-flange overlaps.

Rollforming is another process on the upswing for forming AHSS for automotive applications.

“We are starting to see it used for martensitic grades to about 1500-MPa tensile strength,” Anderson notes.

Hydroforming, too, he says, is employed to a greater degree to form AHSS. Case in point is the 2015 Ford Edge, where DP 1000 grades are tube-hydroformed for roof rails.