Microstructure – property relationship in medium-Mn steels

  3D atom maps of ferrite/austenite interface in medium-Mn steel Fe-5Mn-0.2C ART annealed at 650 °C for 2 hours. IEHK 3D atom maps of ferrite/austenite interface in medium-Mn steel Fe-5Mn-0.2C ART annealed at 650 °C for 2 hours.

Growing demands of weight saving, emission reduction and passenger safety in the automotive industry have promoted new concepts for modern steel design to achieve an improved combination of high strength and superior ductility. Medium-Mn steels pertain to the 3rd generation advanced high strength steels (AHSS) and exhibit an excellent balance of mechanical properties and cost effectiveness. Industrial implementation of these materials, requires a fundamental knowledge of the mechanical properties which are strongly related to the stability of retained austenite in the microstructure.

This project aims to improve the mechanical properties of Fe-Mn-Al-C medium-Mn steels by controlling the austenite stability and thereby the deformation behavior. The role of intrinsic and extrinsic parameters on the austenite stability is studied by combining material simulation tools and various characterization methods. Phase diagram calculations based on the CALPHAD are employed to predict the deformation mechanisms of austenite and identify the process window for Austenite Reverted Transformation (ART). The microstructure evolution and elemental partitioning between ferrite and austenite during ART annealing is simulated by phase field approach. An experimental validation is carried out by Electron Backscatter Diffraction (EBSD) and Atom Probe Tomography (APT), respectively. Required input data on the nucleation conditions is measured by synchrotron radiation and utilized for the calibration of the phase field model. In addition, synchrotron radiation enables the measurement of the phase fractions and the characterization of the deformation mechanisms. The gained knowledge is employed to optimize the mechanical properties by the control of austenite stability and the respective deformation mechanisms.