Approximately 36% of the baseline vehicle's BIW is made of PHS and AHSS. In the optimized S-in motion solution, PHS and AHSS equate to 54% of the BIW weight. Although the AHSS amount remains nearly identical, the PHS amount in the S-in motion is 25% vs. the baseline's 6%.
The world's largest steel producer recently unveiled a portfolio of weight-saving designs for front, side, and rear body panels as well as for the chassis, doors, and other hang-on parts to spur increased usage of advanced high-strength steels (AHSS).
"Any of these independent component or submodule solutions can be integrated into a system. The intent is to show what can be done and also reinforce that we are a capable partner to assist in providing effective solutions," Dr. Blake Zuidema, Director of Automotive Product Applications for ArcelorMittal Global R&D in East Chicago, told AEI at a January press briefing showcasing the results of the "S-in motion" demonstration project.
The lightest S-in motion solution reduces body-in-white (BIW) and closures weight by 57 kg (126 lb), while the lightest chassis offering nets 16 kg (35 lb) in weight savings compared to a baseline C segment vehicle.
According to Dr. Greg Ludkovsky, ArcelorMittal's Vice President of Global R&D, the baseline model "incorporates all of the state-of-the-art solutions that ArcelorMittal developed at the beginning of the [two-year] S-in motion program."
Production processes were evaluated in tandem with lightweight metals, including dual phase, complex phase, trip 780, martensitic, and other AHSS as well as press-hardened steel (PHS). Multiple versions of the same parts, modules, and closures were produced to illustrate various lightweight strategies.
Explained Zuidema, "ArcelorMittal's commercially available Usibor 1500P with a 1500-MPa tensile strength and Ductibor 500P—a product just entering commercial production—were used for the front and rear rails and the B-pillars in a North American market-targeted design approach focused primarily on hot stampings. Another design approach primarily focused on cold stampings using dual phase and trip dual steels."
One of three front door designs represents a novel departure from a laser-welded blank door inner.
"The monolithic mild-steel door inner uses a hot stamp reinforcement to form a tube, which is essentially a thin-walled, closed structure beam element that provides stiffness. In the beltline reinforcement area, an AHSS grade transfers energy from the front structure to the rear.
"The upper waistline reinforcement and the forward hinge area form a very strong tube section, which along with the diagonal door beam equates to an extremely stiff structure. An intermediate reinforcement that spans between the waistline reinforcement and the door beam provides support for the door outer, which enabled further gauge reduction without compromising stiffness," Zuidema explained.
S-in motion's innovative door ring concept strays from the conventional approach of using a one-piece cold stamping.
"Although much of the side structure needs high strength for intrusion resistance, not every part in the door ring requires the same stiffness. Some areas need greater stiffness and a higher gauge, and some areas don't require that.
"For example, the junction between the lower part of the B-pillar and the rocker panel needs high ductility in order to withstand substantial deformation during a side impact. Because we incorporated different gauges and different grades, we were able to achieve different thicknesses as well as different mechanical properties after hot stamping," said Zuidema.
The two-year S-in motion project is the culmination of nearly a decade of product design analysis by ArcelorMittal researchers and engineers. Beyond the concepts that S-in motion demonstrated, additional weight savings of 29 kg (64 lb) will be realized with the creation of new alloys and the development of other scientific concepts. Said Ludkovsky, "We intend to pursue this quite aggressively in the next couple of years."
ArcelorMittal's current market-available products include stainless steels for body structure applications such as B-pillars and lower A-pillars. According to Zuidema, the 17-7 (17% chrome, 7% nickel) stainless steel grade's formability provides an alternative to hot stamping. "This stainless steel is not currently in a production vehicle application, but it is used for the S-in motion project's body-in-white," said Zuidema.
The BIW and other S-in motion modules were verified in-house through crash and stiffness assessments as well as forming and assembly feasibility studies that meet European and North American requirements.
S-in motion's worldwide show-and-tell tour for ArcelorMittal customers continues for the next several months. "Simply put: We used a design-materials-manufacturing interaction to come up with solutions," said Zuidema.