Successful Engineering Thanks to Hot Forming
After both project partners had jointly developed the concept and KIRCHHOFF Automotive had implemented the design, thyssenkrupp Steel assisted KIRCHHOFF Automotive in working out the process design and launched initial forming tests. “We chose hot forming for the bending crossbeam,” says Rolf Röttger, Senior Engineer Crash at thyssenkrupp Steel, in explanation of the development approach. “We specifically optimized the geometry of the crash management system to suit process.”
Press Hardening Delivers High-Strength Steel
As regards the material, the decision was made in favor of the high-strength, uncoated MBW-K® 1900 hot-forming steel – a lightweight construction material that KIRCHHOFF Automotive is already using for series production in other projects. “The MBW-K® 1900 hot stamping steel is perfect for this,” says Melanie Dinter, thyssenkrupp Steel Customer Service. “In addition to the advantages relating to the crash concept, we can create very complex geometries using these hot-formed steels.” Dr. Stephane Graff, Senior Engineer Hot Forming at thyssenkrupp, explains why: “At the high forming temperatures of around 800 °C in the press die, the MBW-K® 1900, like all hot-forming steel grades, is characterized by a high breaking elongation and, thanks to its low flow properties, can be formed at very low press forces. Thanks to its strength of up to 2,000 megapascals after press hardening, the MBW-K® 1900 offers an even greater potential for lightweight construction than our classic hot-forming grade MBW® 1500 as it allows manufacturers to further reduce the sheet thickness.”
Hot Forming Provides the Basis for Special Geometries
The construction of the prototype tool and the pressing tests took place on the premises of automotive supplier KIRCHHOFF Automotive in Attendorn, Germany. “In the case of the bending crossmember, what made the forming process challenging was the special geometry, which is characterized by several shafts along the height of the component as well as interlocking shafts along the longitudinal axis of the component,” says Lena Kremer, Advanced Product Development Specialist at KIRCHHOFF Automotive. “The special feature of our bending crossbeam concept is that the direction in which the profile opens changes across the length of the system. The profile opens inwards in the center, which has a positive effect on the deflection of the crossbeam. At the edges, the profile opens to the outside, which in turn increases the length of the connected crash boxes and thus allows more energy to be absorbed. At the same time, the open, wave-shaped profile is resistant to torsion.”
Less Weight and Lower Costs Thanks to Hot Forming
The facts speak in favor of the MBW®-K 1900 hot-forming steel: Given the same crash performance of the overall system, the high-strength steel grades from thyssenkrupp Steel result in a weight reduction of almost 20 percent compared with the reference design. Moreover, this helps to cut costs by around 8 percent. “In addition,” says Kremer, “the initial force peak can be fine-tuned by adjusting the local length and directly connecting the crash box to the bending crossbeam.”
Test Program for Crash Management System Has Been Launched
The innovative crash management system from KIRCHHOFF Automotive meets all predefined reference requirements.
Melanie Dinter: “Together, we have developed an innovative bending crossbeam design which, in combination with the outstanding strength of our hot forming-capable steels, offers many advantages. Such a successful project is only possible if the roles and interfaces are clearly defined. At every stage of the project, we exchanged ideas and were updated in workshops and on digital platforms.”
A real crash test based on the component will soon be carried out; towing tests and a quasi-static three-point bending test will complete the testing regimen.