P 645 - Calculation and design principles for sandwich-like composite girders with trapezoidal sheet metal webs in bridge construction

P 644 - Design of a process window for sheet metal forming of ultra-high-strength heat-treatable steels at elevated temperatures Reducing weight while at the same time meeting increasing safety requirements is a key issue in automotive engineering. This has led to a wide variety of solutions from the automotive and supplier industry. In recent years, press hardening technology has proven to be a good option for the production of crash-relevant body components. This involves a combination of hot forming and hardening, which are carried out simultaneously in a single process step. As part of the research project "Design of the process window for sheet metal forming of ultra-high-strength heat-treatable steels at elevated temperatures", the objective was to contribute to a better understanding of the process and to a more robust process design. For the investigations, the press hardening process was divided into the four process steps of austenitising, transfer, forming and hardening. The process window for austenitising was examined with regard to an upper and a lower limit for the austenitising time. It was found that the material exhibited similar hardness values regardless of the annealing time. Even a faster heating rate, which was realised by conductive sample heating, had no influence on the hardness values. A test set-up was developed specifically to determine the cooling rates typical of the process during hardening, with which the cooling can be determined by metallic contact on both sides. The resulting cooling rates as a function of the contact pressure could thus be determined. This contact condition was also modelled in finite element simulations and the heat transfer coefficient occurring in each case was detected by comparing the results from the experiment and the finite element simulation. Hot tensile tests were carried out to record the thermo-mechanical parameters of the steel 22MnB5 (1.5528) generally used for press hardening. With the aid of the Gleeble universal testing machine, which is equipped with a conductive heating unit, the typical time-temperature curve for press hardening was observed. The influence of temperature, strain rate, cooling rate and batch or sheet thickness was determined. DaimlerChrysler AG provided the mould for pressing a B-pillar as a demonstrator component. The influence of the hold-down distance and the holding time on the moulding result was determined both during the pressing process and afterwards by means of thermographic images, hardness and sheet thickness measurements. The demonstrator component was modelled with the finite element software LS Dyna using the process and material data previously determined in the experiments. The failure mode of the component and the temperature distribution could thus be reproduced well. Authors: M. Geiger, M. Merklein, C. Hoff, K. Roll, D. Lorenz Publication: 2006

P 642 - Improvement of the notch geometry to increase the fatigue strength of high-strength and stainless steels through process control and innovative post-treatment in laser and arc welding and brazing The aim of the project was to achieve higher fatigue strengths (ideally up to that of the base material) by improving the seam geometry of fillet welds at lap joints (sheet thickness s = 2 mm). For this purpose, the process control of already known and established joining processes (laser beam welding and brazing, MSG welding, MSG brazing, laser-MSG hybrid welding) for two steels, a high-strength ferritic-martensitic dual-phase steel (DPW600/ 1.0936) and a stainless austenitic steel (H400/1.4376), was first optimised. The influence of the energy intensity and its distribution during welding, the influence of the welding and brazing consumables and the shielding gas on the seam transition radius and seam rise were investigated. Furthermore, an attempt was made to reduce the influence of the seam geometry on the MSG brazing processes by means of thermal post-treatment using the combined MIG-TIG process. In order to ensure the realisation of the results in industrial production, the transferability of the processes to a demonstrator component was also to be tested. For an initial assessment of the fatigue strength, Wöhler tests were first carried out on flat samples with overlap seams for both materials. In the next step, gas welding tests (variable load amplitude) were carried out on flat specimens with lap seams. The welding processes used for both materials were laser-MSG hybrid welding, laser beam welding and MIGWIG brazing. The latter represents an improvement on GMAW brazing in terms of the seam geometry due to the thermal post-treatment by the TIG process and was used for the DP-W600 using the filler material AlBz5Ni2 and for the H400 using the filler material CuAl8, in accordance with the results of the Wöhler tests. In addition, the corresponding Wöhler tests were also carried out for these variants for reasons of comparability. The feasibility of optimising the joining process was verified on a real component at the end of the project. For this purpose, instrument panel carriers with a welded steering column attachment were manufactured at an industrial partner of the group of supervisors in accordance with the knowledge gained in the project. Publication: October 2020 Authors: Dr J. Laakmann, Dr.-Ing. M. Küppers, Prof. Dr.-Ing. M. Sonsino, Dipl.-Ing. A. Sevim, Dipl.-Ing. St. Longerich, Dipl.-Ing. D. Piontek, Dr.-Ing. J. Gollnik, Dr.-Ing. A. Zwick, Dipl.-Ing. R. Imhoff

P 640 - Development and testing of cathodic corrosion protection systems for ultra-high-strength steels In addition to reliability and quality, vehicle safety and cost-effectiveness are decisive criteria for car manufacturers. Corrosion protection plays a key role here, as it increases the service life and service life. The advantages, disadvantages and areas of application of different corrosion protection measures in the automotive sector were compared. With the provision of high-strength steel grades, materials are available which, due to their mechanical properties, have a high potential for lightweight construction and very good energy absorption capacity. In conjunction with the possibility of hot forming, they are therefore predestined for the production of complicated, stress-compliant moulds in the crash-relevant bodywork area. The various conventional corrosion protection systems for press-hardened high-strength steels are described, characterised and compared. At present, there is no production-ready solution for the subsequent corrosion protection of components that have already been hot-formed. For this reason, a corrosion protection system based on the conventional hot-dip galvanising process was developed and applied. The development was based on an extensive investigation of the softening behaviour of the ultra-high-strength substrate 22MnB5. The newly developed product was compared with various conventional corrosion protection systems. The evaluation was carried out with regard to surface and coating properties, coating adhesion, corrosion behaviour, tribological properties and weldability. In addition to the tests on flat samples, component tests were also carried out. A section of a B-pillar was used as a component. Scanning electron microscopy and optical microscopy were used for evaluation. The investigations were carried out in close interdisciplinary co-operation between steel manufacturers, manufacturers/processors of coating systems, the automotive industry and research institutes. The research project was carried out at the VDEh-Betriebsforschungsinstitut GmbH, Düsseldorf and the Ingolstadt University of Applied Sciences Materials Technology Laboratory for Surface Technology with technical support and financial assistance from the Forschungsvereinigung Stahlanwendung e. V., Düsseldorf, with funds from the Stiftung Stahlanwendungsforschung, Essen. Authors: J. Kurzynski, C. Strobl, V. Merklinger, R. Salomon Publication: 2013