Advanced High-Strength Steels Show Potential

DETROIT, MI, June 6, 2001 - Advanced High-Strength Steels (AHSS) combined with sheet hydroforming show potential for further mass reduction in steel auto body outer panels, the UltraLight Steel Auto Closures (ULSAC) Consortium revealed today.

To explore the benefits of advanced high-strength steels, the ULSAC Consortium successfully manufactured door outer panels using 0.6 mm Dual Phase (DP) 600 steel sheet. The Consortium specified sheet hydroforming to increase work hardening in the center of the panel, which hydroforming does more effectively than stamping. The AHSS approach resulted in an additional .7 kg of mass savings over the already significant amount achieved in the original study, which produced lightweight frameless doors (with stamped outer panels) weighing 10.47 kg. The additional saving of .7 kg boosted the mass improvement from 42 percent to 46 percent over the average benchmarked frameless door.

These doors are lightweight, efficient structures that offer superb crash performance, and they are affordable to manufacture in large quantities," said Darryl Martin, Senior Director Automotive Applications, American Iron and Steel Institute. "The use of advanced high-strength steels demonstrates ongoing potential for reducing mass in automotive body panels, while maintaining dent resistance and eliminating 'oil canning' tendency. The results show that the unique work hardening qualities of Dual Phase steels yield excellent performance when a sufficient amount of stretch is achieved in the center area of the panel."

While advanced high-strength steels are being used with other processes, like tube hydroforming or stamping, the Consortium conducted research on the sheet hydroforming process, gaining valuable documentation about this developing technology.

Background

The ULSAC program, which began in 1998, included a Concept Phase that produced designs for lightweight doors, hoods, decklids and hatches. A Validation Phase followed with the manufacture of a frameless door to demonstrate the feasibility of the lightweight closure concepts. The first doors produced weighed just 10.47 kg, which included a stamped outer panel of 0.7 mm thickness Bake Hardenable (BH) 260 steel. The ULSAC Consortium reported on these results in May 2000.

ULSAC's design and engineering team, Porsche Engineering Services, Inc. (PES), Troy, Mich., USA, accomplished these significant weight savings though the use of high- and ultra high-strength steels, combined with technologies such as tailored blanks and hydroforming of tubular structural components. Sheet hydroforming was still a developing technology

Hoods, roofs and outer panels produced by conventional forming methods could, depending on the vehicle styling, lack sufficient stretch in the center of the part, resulting in insufficient work hardening. Normally engineers would increase material thickness to improve stiffness, but this results in an increase in mass.

Sheet Hydroforming Enhances Work Hardening of Large Panels

The ULSAC program explored sheet hydroforming as a possible alternative to stamping, which would increase the overall work hardening benefit. The process used was active hydromechanical sheet hydroforming (AHM), a multi-stage forming technology with a liquid working medium. The closures program actually encouraged a development of this process to accommodate the high- and advanced high-strength steel materials and the small radii required by the door design. The press environment was modified to attain a higher working medium pressure, and the working medium chamber was redesigned and updated to resist the increased pressure.

For applying advanced high-strength steels and sheet hydroforming to the manufacture of outer panels, the Consortium specified a range of materials: BH 210, BH 260 and Dual Phase (DP) 600 in both 0.6 and 0.7mm thicknesses. All three of these high-strength steel grades in both thicknesses were successfully hydroformed into door outers. Ultimately, the consortium chose 0.6 mm DP 600 steel to build demonstration hardware. The Consortium specified this material because it enabled above-average dent resistance and tendency for "oil canning". These qualities were a result of the dual phase steel work hardening in the center of the panel because the sheet hydroforming process distributes stress more evenly across the broad expanse of a sheet.

Structural Testing In the ULSAC Validation Phase, testing of the door confirm structural performance. With today's stringent safety standards, it was important to demonstrate that the low-mass frameless door design could still provide sufficient side intrusion protection. A quasi-static side intrusion test (similar to the U.S. federal regulation FMVSS214) was performed. The test results indicated better than average safety performance. Also conducted was a computer analysis of longitudinal crush, results of which indicated that the ULSAC door would contribute significantly to crash energy management in a frontal crash.

Dent Resistance and Oil Canning

The intent of the dent resistance and oil canning testing was to investigate stiffness of the new panels to establish optimum grade and thickness. Because the 0.6 mm DP 600 steel (coupled with hydroforming) demonstrated above average performance in oil canning and met the requirements for static and dynamic dent resistance, it was chosen for the ULSAC door outer panels.

Lighter Weight At No Cost Penalty

The results of the economic analysis revealed that this door structure, with significant weight savings and comparable performance relative to state-of-the-art generic doors, has the potential to be made at a rate of 225,000 units per year without cost penalty. The material costs for the hydroformed outer panel compared to the stamped outer panel are almost identical. Additionally, labor costs, along with equipment costs, depend on the cycle time. Details follow:

Economic Analysis Results

 

ULSAC Door

"State of the Art" Generic Door

Stamped Outer

Sheet Hydroformed Outer

Parts Fabrication

$39.50

$43.00

$45.00

Material

$14.00

$14.00

$24.00

Stamping

$7.00

$3.00

$8.00

Tailored Blank Stamping

$6.00

$6.00

$10.00

Sheet Hydroforming

 

$8.00

 

Tube Hydroforming

$7.50

$7.50

 

Purchased Parts

$4.50

$4.50

$3.50

Assembly

$27.00

$27.00

$23.50

Total Cost of Door (1)

$66.50

$70.00

$69.00

Note - costs shown are for one door

Details of the ULSAC study are available online at www.autosteel.org or on CD ROM, which contains the complete ULSAC January 2001 Engineering Report. Requests for the CD can be made by calling 1-800-STEEL-WORKS.

The Automotive Applications Committee (AAC) is a subcommittee of the Market Development Committee of AISI. The AAC scope focuses on advancing the use of steel in the highly competitive automotive market. With offices and staff located in Detroit, cooperation between the automobile and steel industries has been significant to its success. This industry cooperation resulted in the formation of the Auto/Steel Partnership, a consortium of DaimlerChrysler, Ford and General Motors and the member companies of the AAC.

This news release and supporting PowerPoint Presentation, the complete ULSAB Feasibility Study, and other steel-related information are available for viewing and downloading at American Iron and Steel Institute/Automotive Applications Committee's website at www.autosteel.org.

American Iron and Steel Institute/
Automotive Applications Committee:

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Dofasco Inc.
Ispat Inland Inc.
National Steel Corporation
Rouge Steel Company
United States Steel Corporation
LTV Steel Company
WCI Steel, Inc.
Weirton Steel Corporation