Microstructural Evolution of Nb-V-Mo and V Containing TRIP assisted Steels during Thermomechanical Processing

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb-V-Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite. The TEM analysis showed that a significant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the finishing stage in Nb-V-Mo steel (i.e. 830 °C). It was also found that the V(C,N) precipitation primarily occurred in both ferrite and deformed austenite below the finishing stage. The results suggested that Nb-Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a significant precipitation strengthening in both Nb-V-Mo and V steels at room temperature.