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Steel Never Sleeps

The use of hydroformed components, bake hardenable steel stampings and other design and process innovations can provide vehicle manufacturers with the means to produce cost-effective, lightweight products-so says the American Iron and Steel Institute.

Steel's dominance of the automotive materials market has been under attack by lightweight competitors like aluminum and composites for years, but the steel industry isn't about to slip into the annals of Rust Belt history without a good fight. The American Iron and Steel Institute's Automotive Applications Committee (AISI-AAC) won its first major battle when it showed the UltraLight Steel Auto Body (ULSAB) in 1998. This showpiece of high-strength steel (HSS) and tailor-welded blanks purported to offer a lower weight at a lower manufactured cost than a conventional unibody design; some of these technologies are now increasingly finding their way into production vehicles. Building on the ULSAB's success, AISI-AAC has been busy developing a number of new research projects. The most important of these, the ULSAB Advanced Vehicle Concept, is being developed to directly compete with the ongoing and aluminum-intensive Partnership for the Next Generation Vehicle (PNGV) project and won't be completed until mid-2001. However, this spring saw the completion of the UltraLight Steel Auto Suspensions (ULSAS) and UltraLight Steel Auto Closures (ULSAC) projects. Both studies build on the basic ULSAB concept of a holistic, "clean sheet of paper" design with a heavy use of HSS and manufacturing processes like hydroforming. This war is far from over.

 

Suspension of Disbelief
Lotus Engineering Services provided the engineering might for the ULSAS project, which began with a benchmarking study conducted on suspensions used in B-, C-, D-, E- and PNGV-class vehicles. Lotus then used a variety of CAE tools for the design and analysis work necessary to create five different suspension designs: twistbeam, strut and link, double wishbone, multi-link, and an entirely new system. These designs had a very simple objective: reduce the mass of the suspension system without incurring any cost penalty, based on a high-volume manufacturing scenario.

The results as published show weight savings from 17 to 34% when compared to similar steel-intensive designs. In the case of the multi-link design, the only one whose benchmark was aluminum-intensive, the weight is said to be comparable to aluminum but at a 30% lower cost. (It is important to point out that this study is just a concept study and did not involve actually building any of the designs. All the work for this study was performed with digital models, using packages including Catia for design and ADAMS for dynamic modeling.)

The keys to making these suspension designs perform while still reducing weight primarily involved substituting hollow HSS tubes for solid bars and HSS sheet stampings and hydroformed parts for forged components. By specifying HSS tubular anti-roll bars instead of solid ones, for instance, the ULSAS study shows that mass can be reduced from 36 to 48%. Even substituting solid ultra HSS rod (1300 MPa) for spring material is said to allow for mass reductions.

AISI-AAC describes the twistbeam design pictured here as its "star of the show," weighing in at 32% less than a conventional steel-intensive design with no cost penalty. Its transverse beam is made from 600 MPa ultra HSS tube with a 2.8- to 4.1-mm wall thickness (depending on vehicle application) and a plasma-trimmed cutout. The bend radius is 125 mm using wiper die and ball mandrel. Trailing arm is a 70- by 2-mm HSS tube (400 MPa) that's hydroformed and MIG-welded to the twistbeam; an end flaring or punch point operation is required. The spring pan is stamped from a 3.2 to 4.0-mm ultra HSS (500 MPa) blank. The hub mounting plate is forged from 600 MPa ultra HSS.

Doors of Perception

 

door

The ULSAC study was conducted by Porsche Engineering Services and involved a number of designs for hoods, deck lids, hatchs, and doors. Unlike the ULSAS project, it actually resulted in prototypes that could be physically validated, but only for the door design (chosen because it was the most complicated of the closures). Like ULSAS, this study began by benchmarking doors, both framed and frameless designs (all of which were steel-intensive). Porsche then developed a frameless door that weighs only 23.15 lb. (10.5 kg), which is 42% less than the benchmarked average for frameless de-signs and 22% lighter than the lightest benchmarked door, a framed design.

This significant reduction in weight came about primarily as a result of the elimination of the door inner and a reduction in the number of major parts to just nine. This functional consolidation comes without any sacrifice in performance, as Porsche claims dent resistance, oil canning, door sag, side intrusion and torsional performance to be similar to conventional doors. Most importantly, Porsche studies indicate that the door could be built in high volumes (over 225,000 per year) at a cost of $66.50 per unit, slightly less than the cost of building a conventional door.

As can be seen in the photo, the ULSAC door design looks radically different without the structural need of a full inner door panel. The vertical latch and hinge are both tube-hydroformed from high strength steel (1.0 mm, 280 MPa; 1.2 mm, 280 MPa). These eliminate the need for reinforcements at the hinge and mirror flag attachment points. The lower tube and outer belt reinforcement are Dual Phase ultra HSS and perform the duties of absorbing crash impact in both frontal and side impact collisions while also providing basic structural support for the door. The inner front and mirror flag inner is a stamped, tailor-welded blank; the upper portion is 1.0-mm, 140-MPa mild steel, while the lower portion is 1.2 mm. Adhesive bonding and laser, spot and MIG welding are all used in the assembly process.

The outer door skin uses Bake Hardenable (BH) 260-MPa HSS in a 0.7-mm thickness. This material was selected from a number of other steels in both 0.6- and 0.7-mm thickness: BH210, Dual Phase 500, Dual Phase 600, Isotropic 260, and Interstitial Rephosphorized 260. While all of these were successfully stamped into door outers, the 0.7-mm BH 260 was selected due to its superior performance in the dent resistance and oil canning testing.

This research is said to advance the state-of-the-art in stamping of these grades and thicknesses of steel, however AISI-AAC isn't content with the results using conventional stamping. Hydroforming expert Schuler SMG is conducting further research in active sheet hydroforming for the door skin outers, in an attempt to be able to use thinner, higher-strength materials. Active sheet hydroforming pre-stresses the blank with water pressure that balloons the blank up towards the die. This creates a more uniform strain distribution in the center of the panel, which is thought to improve oil canning performance. The downside is that there are certain design limitations to this type of forming due to the increased strength of the steel. AISI-AAC is looking forward to the results of these trials, eager to fire another salvo in defense of all things ferrous.