P 1128 - Thermal joining of arc surface-treated steels with fibre-reinforced plastics

P1127 - Materials science-based development of simulation strategies for the use of adiabatic cutting in sheet metal part production In order to reduce the amount of material used and at the same time pursue the concept of lightweight construction, high-strength steels with tensile strengths greater than 1000 MPa are increasingly being used in automotive engineering. Practically every sheet metal component undergoes a trimming operation during the manufacturing process. However, conventional shear cutting or fine blanking cannot be used for ultra-high-strength steels, or only at great expense in terms of pre- and post-treatment. Here, adiabatic shear cutting offers an alternative to conventional shear cutting or cost-intensive laser trimming. It has already been shown that this process has a high quality in terms of straightness, roundness and surface finish of the shear-cut edge. However, for the industrial use of adiabatic shear cutting, there is a lack of fundamental investigations for relevant high-strength steels, such as frequently used press-hardening steels. Furthermore, the microstructural processes occurring during adiabatic shear cutting and their effect on technological properties have not been described for these steels. Basic approaches for estimating the necessary process energy, process speed and a classification regarding the suitability of the materials to be used are also lacking. The aim of the project is to answer these open questions and to close the gap to industrial application by means of a fundamental characterisation of the process. Furthermore, a methodology for the simulation of the strongly temperature- and strain rate-dependent process is required. Extensive material characterisation tests (temperatures up to 1000 °C and strain rate ?̇ up to 1000 1/s) were carried out to achieve the objectives. Tensile tests with notch geometry were used to determine the failure behaviour. A total of three different steel grades were analysed. Based on these investigations, numerical and analytical models were developed to estimate the locally generated temperature and the process energy required to perform the separation operation. Adiabatic shear cutting tests were carried out to produce cylindrical components and various demonstrators (rectangular plates with rounded edges). Furthermore, it was shown that two of the investigated steels (20MnB5 and C75S), despite almost similar mechanical properties at room temperature under quasi-static load, showed significant differences during adiabatic cutting. In particular, it was shown that the strain rate sensitivity of the steels has a major influence on the initiation and formation of adiabatic shear bands, but at the same time the required process energy is also strongly influenced by this factor. Using finite element simulations, the resulting characteristic shape of the shear-cut surfaces could be predicted with a high degree of accuracy. The methods developed can be used for process design. Publication: May 2020 Authors: Prof. Dr.-Ing. A. E. Tekkaya, Dr.-Ing. T. Clausmeyer, Dipl.-Ing. D. Staupendahl, F. Schmitz M.Sc., Univ.-Prof. Dr.-Ing. habil. M. F.-X. Wagner, Dipl.-Ing. S. Winter

P 1126 - Determination of the load-bearing capacity of novel steel shell structures under realistic consideration of welding distortion Recently, stiffened shell structures (so-called monocoque) have been increasingly used in steel construction. Due to the slender construction methods, buckling is a limit state that must be taken into account. A geometrically and materially non-linear analysis of the imperfect shell structure (GMNIA) has the highest theoretically achievable degree of reality. Its verification quality depends crucially on whether the imperfections introduced into the calculation model describe the effect of the imperfections of real structures equivalently. In principle, it is now possible to reliably predict the effects of the welding process using welding simulation. The analytical-numerical hybrid model developed at the LFT offers a practice-relevant methodology for the welding simulation of large components. From a design perspective, an interface to the welding simulation is to be realised and subsequently verified. The aim of the project was to provide a robust and reliable calculation method for a numerically supported GMNIA verification of welded shell structures. In contrast to previous approaches, this is the first time that this has been done directly using hybrid approaches to capture the influences of the welding process. The analytical numerical hybrid model was used to calculate the welding imperfections. In order to be able to use the model to calculate the welding imperfections that occur when welding curved structures in space, the underlying analytical model had to be extended. Based on numerical variation calculations, an acquisition function was derived that takes into account the curvature in space and the resulting reduced stiffness. The hybrid model has already been successfully applied to a large number of differently complex components. However, the focus was on welding distortions. Little attention has been paid to the localised stress state in the near-seam area. However, it influences the load-bearing behaviour of welded components. Both the verification of the stresses calculated using the hybrid model with the results from thermomechanical calculations and the validation with radiographic measurements showed good agreement. Only a new meshing strategy was developed to improve this. Since in most cases the stiffeners are welded on both sides, an influence function was derived based on the source method, which takes into account the intermediate time between the two welds and records its effect on the plastic strains. Publication: August 2020 Authors: Prof. Dr.-Ing. habil. H. Pasternak, Dr.-Ing. Ch.Stapelfeld, Dr.-Ing. Launert, Prof. Dr.-Ing. habil. V. G. Michailov, Dr.-Ing. N. Doynov, C. A. Kaya

P 1125 - Increasing technological properties by cryo-treatment of tool steels As part of the project, the effects of cryogenic treatment on the microstructure, mechanical properties and wear resistance of the high-alloy tool steels X38CrMoV5-3 (electroslag remelting process (ESR)), X153CrMoV12 (powder metallurgy (PM) and fusion metallurgy (SM)) and ~X190CrVMo20-4 (PM) were investigated. The tempering parameters of the samples for the mechanical tests and those of the wear samples were selected on the basis of the tempering curves so that a similar hardness was present for both the conventionally heat-treated and the cryo-treated samples. Although a finer, martensitic microstructure and a more homogeneous distribution of secondary carbides in the X38CrMoV5-3 was achieved by including a cryogenic treatment in the conventional heat treatment or an increased amount and a more homogeneous distribution of secondary and nano carbides in the X153CrMoV12 and ~X190CrVMo20-4 steels could be attributed to the cryogenic treatment, no differences were found in the mechanical properties and in various wear tests (sandpaper method, wear cup and cavitation erosion). The wear resistance of the high-alloy tool steels depends to a large extent on the primary carbides and the macro hardness, less on the secondary or nano carbides. Both cryo-treatment and tempering three times at high temperatures achieved an almost complete conversion of the retained austenite into martensite in the small samples. Cryogenic treatment of tool components with large wall thicknesses would have a positive effect, as a uniform and complete transformation of the retained austenite is achieved over the entire cross-section. This allows the user to better assess the cryogenic treatment technology and thus utilise it in a more targeted manner, thereby achieving improved results or saving costs. Publication: 2021 Authors: Dr.-Ing. A. Schulz, Dr.-Ing. F. Nürnberger, Dipl.-Ing. T. Ümit

P 1131 - Alloying concepts and process strategies for press-hardening steels with increased residual formability One effective way of reducing emissions from passenger cars is to reduce the weight of the vehicle. The greatest savings potential lies in the area of car body construction, where, for example, high-strength steels are now replacing conventional thin sheet materials. However, the use of high-strength steels to increase vehicle safety in the event of a crash is particularly challenging. One of the most efficient processes for manufacturing crash-relevant components, such as the B-pillar, is press hardening, in which forming and hardening are combined in a single process step. The material 22MnB5 (1.5528) is currently used in the industrial series production of press-hardened components, but its ductility in the hardened state is limited due to the alloy concept. An improved property profile consisting of strength and residual formability is being sought for increased vehicle occupant safety in the event of a crash. This project is therefore concerned with the development of new alloy concepts and process strategies to improve the residual formability of press-hardened steels. Three different alloying concepts were investigated: the dual-phase concept (DP), the grain refinement concept (KF) and the retained austenite concept (RA). Depending on their chemical composition, different heat treatment cycles were combined with press hardening for a total of 6 steels. The investigations focussed on the microstructure development and the resulting characteristic properties in the tensile and bending test. In addition, the coatability and further processability by welding were investigated. It was demonstrated that an improvement in the ductility of press-hardened materials in the RA concept can be achieved by complex Q&P structures (quench&partitioning). In contrast, the DP and KF concepts, which were also investigated, showed only a marginal improvement in the property profile. The best properties in the RA concept result for a material with 0.3 wt% C and 2.5 wt% Mn with tensile strength values of 1400-1500 MPa and elongation at break values of 14-19 % depending on the respective heat treatment. Further tests on this alloy show that the material is temper resistant and therefore suitable for stove enamelling in vehicle production. Joining can be carried out by both laser beam and spot welding. Hot-dip coating with both AlSi and zinc coatings is possible in principle. This project has thus succeeded in demonstrating possibilities for improving the material properties of press-hardened steels using a Q&P heat treatment strategy. The crash absorption potential of the materials produced was doubled compared to the reference material 22MnB5. Publication: 2021 Authors: Prof. Dr.-Ing. W. Bleck, Dipl.-Ing. A. Rüskamp, M.Sc. Y. Sparrer