Key Engineering Materials Vols. 410-411

Abstract: In the sheet metal processing industry bending is one of the common metal forming processes. Depending on the process state, a differentiation has to be made between free bending (air bending) in the die and coining (die bending). Because of its flexibility the air bending process is nowadays one of the widely applied processes for sheet metal bending, but the springback phenomenon is still a great challenge for the industrial application. At the Institute of Forming Technology and Lightweight Construction (IUL) of the Technische Universität Dortmund, Germany, a new method has been developed allowing the compensation of springback effects in air bending of sheet metals. This method is based on the incremental and local superposition of stresses in the forming zone. The superposition occurs after the bending operation but before unloading along the sheet metal width. The advantage of this new method is that a minimal force is required to compensate the springback. This paper describes the springback compensation method in detail and presents first experimental results.

Abstract: Stainless steel sheet metals were laser bent by means of a high power diode laser at different values of power and scan velocity. The laser power ranged from 100 to 300 W (with an increment of 50 W); two scan speeds were used, 4 and 8 mm/s, and the number of passes was 2, 4 or 6. In the experimentation, the values of bending angle, microstructure and residual stresses of the laser bended sheet metals were analyzed with regard to the input variables. In particular, residual stresses were evaluated by means of X-ray analysis in terms of first and second order stress. Measurements were performed on the convex surface of the sample in the laser beam action zone. The bending process was numerically simulated by means of a thermo-mechanical finite element model, implemented to predict the sheet metal bending angle as a function of the laser power and scan velocity. The residual stress distribution was extracted from the numerical simulations and its agreement with the experimental observations was discussed. As a general conclusion, the effect of multiple scans is hardly simulated by thermo-mechanical models which do not take into account the material annealing during forming.

Abstract: This paper presents a model for prediction of forming roll profile wear during the cold roll forming process. The method is based on contact pressure distribution, geometrical description of the initial forming roll profile and NURBS representation of the contact area.

Abstract: Ultra-high-strength (UHS) steels are very interesting materials for many applications where high strength can be utilized to create lighter and more effective constructions. The poor formability of UHS steels, however, may limit the usefulness of these materials in many applications. In this work, some experiments using a flexible roll forming machine developed at the Chair of Manufacturing Technology (LFT), University of Erlangen-Nuremberg were carried out using 4mm-thick bainitic martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and the outcomes have been analysed. Results of these experimental tests show that using roll forming it is possible to bend test materials to an angle of 90º without damages with an evidently smaller radius than in air bending. The radius obtained using roll forming can be as small as 40% of the value used in air bending. Tests also show that with the method used in these tests it is possible to make roll forming for the whole length of the plate. The tests proved that the NC-controlled single-step roll forming method has potential for manufacturing small batches of bend profiles; however, more development has to be carried out if the process is to be made suitable for industry.

Abstract: Steel strips covered by a tertiary thermal oxide scale from a hot rolling process were studied in this research work. In case that the clean strip is required, i.e. for the cold rolling process, this tertiary scale formed on the steel surface is typically removed by a pickling process. In this work, a finishing temperature – the temperature at the exit of strip from a finishing mill – was kept constant at 830 °C. A coiling temperature – the temperature that the strip was coiled at a down coiler – was varied in a real hot rolling line to obtain different types of tertiary scales formed on low carbon steel strips. Physico-chemical characteristics and pickling behaviour of the scales were further investigated. It was found from a scanning electron microscope (SEM) that the thickness of scale at the centre of strip was in the range of 3-5 m and reduced with the decrease in coiling temperature. A laser Raman spectroscopy was also applied to make a depth profile of scale. Additionally, with the results observed from SEM and X-ray diffraction (XRD), it is concluded that the tertiary scale is a non-homogeneous single layer of a eutectoid structure consisting of magnetite and iron. Moreover, the samples were further immersed in a 10%v/v HCl pickling solution at 80 °C. It was found that the time needed to remove the tertiary scale was reduced with the decreased scale thickness, which was obtained by lowering the coiling temperature. Kinetics of scale removal during pickling is discussed in the paper.