5th International High Manganese Steel Conference - HMNS 2022

The plastic deformation of solution annealed Fe-17Cr-8.6Mn-4Ni-0.17N-steel (concentrations in wt.%) and the resulting strain-induced α´-martensite evolution were investigated at different tensile test temperatures to provide an estimate of martensite formation during tube manufacture. For this purpose, the onset and evolution of α´-martensite were determined by in situ magnetic measurement during tensile straining at -40 °C and 20 °C. The triggering stress for martensite formation at 20 °C was 754 MPa, whereas the triggering stress at -40 °C declined to 680 MPa. In addition, the triggering strain decreased with decreasing test temperature. The strain-induced α´-martensite volume fraction was obtained by ex situ volumetric magnetic measurements. The microstructure characterization was done by light optical microscope (LOM) and scanning electron microscope (SEM) using electron backscatter diffraction (EBSD). Using EBSD, strain-induced α´-martensite platelets were detected within straight deformation bands in the austenite. The strain hardening curve at -40 °C shows a curve progression typical of metastable austenitic steels with pronounced strain-induced martensite formation and can be divided into 4 hardening stages. At this temperature, the studied steel exhibited the highest strain hardening rate. The amount of α´-martensite increased with decreasing test temperature, and reached a maximum volume fraction of 76 vol.% at -40 °C. The highest ductility of 83% was achieved at 40 °C, accompanied by a tensile strength of 732 MPa.

Keywords: TRIP/TWIP effect; strain-induced martensite formation; mechanical properties; strain hardening