High-volume automotive body structures using Advanced High-Strength Steel (AHSS) grades offer the potential for low cost and weight, high strength performance, and competitive life-cycle and sustainability attributes.
Reducing the number of individual parts within an automotive body structure can yield further cost, weight, and sustainability benefits without compromising performance.
WorldAutoSteel’s latest engineering demonstrator project, Steel E-Motive, delivered a clean-sheet body structure concept for a fully autonomous Mobility as a Service vehicle. The body structure design features components and sub-assemblies where the number of individual parts (i.e., stampings) have been reduced by applying fabrication methods such as hydroforming and tailor welded blanks, combined with the latest AHSS grades such as Press Hardened/Hot Formed and 3rd generation/Retained Austenite grades.
Integrating multiple body structure parts yields more efficient material utilization (reduced scrap), enabling cost & weight reduction, structural performance improvement, and life-cycle Greenhouse Gas (GHG) benefits.
Some examples of steel body structure part integration applied to the Steel E-Motive concept design follow:
Part Integration Through Hydroformed B and D Pillars
Tube hydroforming enables the creation of complex geometries by using internal pressure to expand a tube against a die cavity. The result is a single tubular component with no weld flanges, offering uniform properties with higher overall strength and stiffness than a component fabricated (i.e., welded) from multiple parts. Hydroformed parts have high material utilization rates (low scrap), giving good cost and weight efficiency. The Steel E-Motive body structure features hydroformed tubes for the B and D pillars.
Steel E-Motive B Pillars
The B pillar acts as one of the main structural members protecting the vehicle occupants and propulsion battery in the event of a high-speed side impact collision. Crash simulations demonstrate that the Steel E-Motive SEM1 vehicle has the potential to achieve IIHS “good” (highest) side crash rating, and the battery is well protected in the event of a collision. Steel E-Motive B pillars are positioned on the closing edges of the front and rear side closures. In the event of a high-speed side collision, the B pillar section profiles ensure that both B pillars deform, contact, and combine to produce an effective box section that reacts to the side impact crash loads, minimizing intrusion.
A compact and efficient section profile enables overlapping and interlocking features and maximizes the windows’ size, enhancing occupants’ visibility. Tube hydroforming enables the achievement of such complex geometric profiles. A TRIP690 (CR400Y690T-RA) grade AHSS was selected for the B pillars. Its high yield and UTS strength deliver side crash performance, and up to 25% elongation enables the complex geometry profiles to be achieved.
The hydroformed tube approach for the Steel E-Motive B pillars has enabled an integrated part solution, with a 10-15% cost and weight saving compared to a cold stamped and spot welded design.
Steel E-Motive D pillars
The Steel E-Motive D pillars are an integral part of the rear torsion ring structure, which significantly contributes to the static and NVH torsional stiffness of the BIW structure. The tube hydroformed D pillars effectively enable 2 to 3 cold stamped and spot-welded parts to be integrated into a flange-less single component, achieving higher overall stiffness, improved material utilization, and improved overall performance.
The hydroformed D pillars of the Steel E-Motive BIW are another example of part efficiency and integration, providing cost, weight, and performance benefits.
Find further details on tube hydroforming using steel: https://ahssinsights.org/forming/hydroforming/hydroforming/
The newest AHSS grades and fabrication techniques enable engineers to streamline automotive body structures by reducing the number of parts or blanks needed. The Steel E-Motive concept showcases several successful part consolidation processes, which lead to lower scrap rates, better material utilization, reduced costs, and decreased GHG emissions. Additionally, these integrated structures enhance overall stiffness and strength.
Thanks go to Neil McGregor for his contribution of this article to the AHSS Insights blog. As Chief Engineer, Systems Integration at Ricardo, Neil has extensive knowledge of lightweight, advanced materials across all major vehicle sub-systems and leads the Steel E-Motive vehicle engineering program at Ricardo.