Simultaneous stress and austenite phase mapping in 3rd generation automotive steels using photons

Important results from the project

The aim was to simultaneously determine residual stresses and phase fractions of martensite and austenite in third-generation high-strength steels with synchrotron light. Thus, the TRIP effect can be studied, which means that austenite is transformed to martensite during deformation. The project has shown that diffraction of synchrotron light is a powerful tool to study the TRIP effect with good lateral resolution and statistical significance, which is important when developing these steels. The project has been carried out in collaboration between SSAB and Swerim.

Expected long term effects

In the project, Swerim has developed a method that allows simultaneous calculation of residual stress and phase maps in two-phase steels based on the processing of a large amount of diffraction data. The method will be important in studies of what happens when forming 3rd generation steels that contain a certain amount of retained austenite. It helps to understand and optimize the TRIP effect and thus achieve an optimal combination of ductility and strength of the material.

Approach and implementation

We utilized high-energy diffraction at the synchrotron radiations source Petra III, beam line P21.2, at Desy in Hamburg. Thin sections were cut from bent samples and diffraction patterns were recorded from the cross section with a resolution of 50 µm in two directions. Macro- and micro-residual stresses as well as fractions of constituent phases were calculated from the same data set and presented as two-dimensional maps. The maps covered the thickness of the steel sheet. The TRIP effect in bending and its effect on residual stress could be explored.