New methodology for numerical springback compensation

Fig. 1: Springback response on components made of steel grades with different strengths

High-strength and ultra high-strength steels offer light-weight construction potential for structural parts in modern car body engineering. However, the increased yield strength will lead to pronounced springback effects on component geometry in the forming process (Fig. 1), it is not possible without compensation to map the tool geometry on a 1:1 basis. These factors will extend the tool development process. Specific software tools that predict springback effects on the components by means of finite elements simulation are therefore employed in the automotive and supplier industries in order to reduce development times. Based on these results, the tool geometry is adapted automatically in order to compensate for springback. But the currently available programs for springback compensation still disclose certain deficiencies.
 
In the course of the joint EC project "Sprincom - An efficient and effective methodology and simulation tools for die design and springback compensation for HSS and UHSS", an improved method for simulating springback compensation was developed and validated in cooperation with the steel and automotive industries, as well as test institutes.
Salzgitter Flachstahl supplied two types of material (SZBS800 and HCT780XD) for this project, while Salzgitter Mannesmann Forschung GmbH contributed with their expertise in the field of forming technology. All real parts were produced in the process on the 1000 ton try-out press and then measured digitally.

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Fig 2: Steps for validation of the new simulation methodology

n order to validate the predictions derived from numerical springback compensation, it was necessary to select suitable tool geometries with an open profile structure that produces the maximum springback effect. For this purpose, the B-pillar tool owned by Salzgitter Mannesmann Forschung GmbH was used and two additional geometries (S-Rail and bumper) were developed and implemented in the tool technology. The example of a bumper part in Fig. 2 demonstrates the various steps in successful validation of the new springback compensation software. The formed parts and tools were digitized in high-res 3D format with the help of the ATOS measuring system and the real part was then compared with the springback simulation. The newly developed simulation tool returned the new tool contour for springback compensation. Following the reworking of the tools, including a repeated press-forming and digitization process, it was possible to compare the properties of the parts produced before and after compensation. The result demonstrated that the new simulation methodology returned improved springback compensation compared to standard systems.

After three years, the joint project was successfully concluded in June this year and disclosed the possibilities and limits of the newly developed simulation methodology for springback compensation. The final report will be published and be available at the beginning of the next year at the RFCS (Research Fund for Coal and Steel).