STT and DTT Diagrams of Austenitic Cr-Mn-Ni As-Cast Steels and Crucial Thermodynamic Aspects of γ→α′ Transformation

The mechanical behavior and microstructure evolution during deformation of novel austenitic CrMnNi as-cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation-induced processes, the STT and DTT diagrams were developed for CrMnNi steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation-induced e- and a' martensite formation and twinning the TRIP and TWIP effects occur in the CrMnNi steels depending on the chemical composition and temperature. The differences of deformation-induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of ????a' transformation was determined for the investigated CrMnNi steels.