Bending of high strength steel

The aim of this project, which was done in cooperation with the company SSAB Hard and Special Steels, was to find the minimal bending radius for five different steels with two different bending methods. In order to realise that, the project was divided into two different parts, the material characterisation and the forming operations. The material characterization was composed of different methods, optical and scanning electron microscopy, hardness and tensile test and x-ray diffraction. During the forming part two different methods were used, the corner bending and the press bending, which were performed respectively in the SSAB laboratory in Borlänge and at the company Svetjänst in Arnemark. During the material characterisation various findings were made, eg the deformation level varied with depth into the materials, a high deformation and a strong texture was found at the surface layer of the material, which decreases by going deeper into the middle of the material. Also cracks in the surface layer of the materials were found. The high deformation with the cracks caused the failure during the bending operations, as the surface layer was also the most loaded during bending and consequently, cracks can open and be easily formed. The direction of the surface texture in cooperation with the crack orientation can also explain the different behaviour of the samples bended in different directions. The tensile and hardness test delivered uniform results with a small standard deviation, which would indicate consistency, however this result must be taken with caution, because the tests were made only with material from one position in one roll and therefore may not be representative of a full roll or take into account production variations. A difference in bending behaviour was seen between the different bending directions in both tests, however the magnitude of this difference was very different between the two test methods. This is difficult to explain because the two tests were made in different places and by different people without providing detailed information about the test procedure and without my direct oversight. Therefore it is difficult to directly compare the two tests and thus isolate the method specific factors that influence the results. Some factors that may have a big influence, for example the forming speed, were not controlled in these tests. For the press bending a study was also made about the springback in the materials, which is dependent on the strength and the residual stresses in the material. In general one can say that the springback increases with the increasing strength in the material. But there were also exceptions, which can be explained via defects in the material, which were found in the base and also in the cold rolled material. Also the material with the highest deformation degree by cold rolling had the highest springback despite of the lowest strength, but there the main influence has to be the residual stresses which are already introduced during rolling. The conclusions of this project are the as follows: - The highly deformed surface layer is responsible for defects during the bending sequence - The texture is the highest in the surface layer, which causes the different results in different bending directions - The low statistical variation for the few tests performed here in this project are expected to grow with larger testing sequences with samples taken from across the roll and from different production lines. - The different testing sequences make it difficult to compare the bending methods. These conclusions make it difficult to give a general minimal bending radius for these materials because the radius found is specific to a particular material and a particular test. The results for the tests performed here are given in the body of the report.Validerat; 20101217 (root)