5th International High Manganese Steel Conference - HMNS 2022

Multiphase medium Manganese TRIP steels offer good combinations of high strength and high ductility and can be designed to be used in automotive industry for light weighting purposes. However, the deformation-driven martensitic transformation associated with the TRIP effect strongly deteriorates the materials’ resistance against hydrogen embrittlement (HE). We studied the hydrogen related damage mechanisms and developed an approach to increase the HE resistance by tailoring of the microstructure. We show by slow strain rate tensile tests that the HE resistance can be significantly increased by adding stable austenite (gamma(stable)) into an ultrafine microstructure. This ductile phase stops cracks by blunting and serves as a dead end for microcracks. In order to increase ductility by a TRIP effect, additional metastable austenite (gamma(metastable)) is required. Such tailored alfa/gamma(stable)/gamma(metastable) microstructures can be produced by simple step annealing: Our atom probe tomography results reveal that during intercritical annealing reverted austenite is formed with the equilibrium partitioning of Mn. This is due to a local equilibrium at the moving alfa/gamma interface. We exploited this for a medium Mn steel (0.2C–10Mn–3Al–1Si in wt.%) to form stable austenite (high Mn partitioning) at 700°C for crack blunting and metastable austenite (lower Mn partitioning) at 750°C to enhance the ductility by a TRIP effect. This approach results in a significantly increased HE resistance with similar high strength and high ductility for the alfa/gamma(stable)/gamma(metastable) microstructure in comparison to the conventional alfa/gamma(metastable) microstructure.