12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

High strength steels are increasingly used, particularly by the automotive industry where reduction of vehicle weight and emission of greenhouse gases have high priority. With increasing strength, the material becomes more sensitive to hydrogen embrittlement. Hydrogen can enter the steel during the production process, for instance when electro galvanizing. The combination of a sensitive material, high stresses, and hydrogen in the material constitutes a risk for fractures. Slitting and punching operations in a manufacturing process will introduce additional stresses at the cut edges. The stress levels depend on cutting method, cutting parameters and tool conditions. Poor edge quality increases the risk for fractures initiated at the edges. Increased knowledge about how cutting methods and cutting conditions affect residual stresses are therefore important. In this work, we have used synchrotron radiation at beamline P21.2 of PETRA III at DESY Hamburg, to study residual stresses at cut edges. Cold rolled martensitic steel was cut using three types of cutting operations; shearing, shearing followed by milling, and laser cutting. Long range, macro stresses were determined using measured shifts in Bragg diffraction peaks and short range, micro stresses using the widths of diffraction peaks. Micro Vickers hardness measurements were used to map the hardness close to the cut edges. Constant load tests combined with in-situ hydrogen charging were performed to compare the sensitivity of different edge conditions to hydrogen embrittlement. The evaluation of residual stresses shows that synchrotron radiation is a powerful tool to map stresses at cut edges with good lateral resolution. Shearing is the cutting operation that affects the material the most and causes the largest amount of stress underneath the cut surface. The affected zone extends to a depth of approximately 1 mm. Milling after shearing removes this zone and creates a shallower stress zone formed by the milling operation. Laser cut edges have a thermally induced stress profile. Fresh martensite forms at the laser cut surface and a heat affected zone forms underneath. The micro hardness measurements show that the hardness is higher near the surface of the edge than in the bulk. The laser cut edge shows a lower hardness in the heat affected zone than in the bulk.