High-alloy CrMnNi cast TRIP/TWIP steels with variable content of nickel exhibit excellent mechanical properties in terms of concurrently high strength and high ductility. However, the yield strength of the as-cast state is quite low (about 250 MPa) due to the large grain size with a scatter between 50 µm and 2000 µm. An enhancement of the yield strength can be achieved by the generation of an ultrafine grained (UFG) microstructure. Beside techniques of severe plastic deformation, the thermo-mechanical controlled processes (TMCP) are favorable for the generation of UFG material with high yield strength and reasonable ductility.
The investigated material was a metastable, low carbon, high-alloy CrMnNi cast steel with about 3 % Ni after solution annealing treatment. The initial microstructure with a grain size of about 60 µm up to 500 µm contained about 10% of ?-ferrite as well as about 10% of cooling martensite. The applied TMCP consisted of a cold rolling process up to 90 % of thickness reduction and a subsequent reversion annealing treatment at different temperatures and holding times. The enhanced mechanical properties of the obtained UFG material were tested during tensile deformation at room temperature. These tests were complemented by infrared thermography and digital image correlation. Furthermore, the microstructure of the UFG material was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after the tensile tests. In addition, measurements of the ferromagnetic phase fraction and nanoindentation experiments at different deformation states were performed.
The results show that due to the applied TMCP an UFG austenitic microstructure with a mean grain size between 0.7 µm and 1 µm was generated. The UFG microstructure led to a significant increase of yield strength up to 900 MPa. In addition, a Lüders-phenomenon was observed, which is characterized by four states (1) upper yield point, (2) stress drop, (3) unusual increase of strain (about 10%) at nearly constant stress level and (4) work hardening. In state (3) the plastic deformation is localized in bands propagating along the whole gauge length of the specimen. Furthermore, a pronounced stress-assisted martensitic phase transformation around 300 MPa was observed.
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