Evolution of austenite microstructure and microalloy precipitation state during double-twist torsion testing on Nb-Ti-bearing steels

Double-twist torsion testing was applied to two Nb-Ti-bearing microalloyed steels to develop the relationship between the mechanical response and the resulting microstructure. The double-twist torsion test applies pairs of isothermal deformation passes to a single specimen at progressively lower temperatures. The test captures the effect of multiple deformation and recrystallization events and allows changes in recrystallization behavior to be assessed with fractional softening calculations. Conventional methods for determining fractional softening were applied to the torsional data to infer changes in recrystallization behavior and estimate the temperature regime where partial recrystallization occurs. The non-recrystallization temperature (Tnr) was determined with multistep hot torsion testing and compared to the partial softening regime from the double-twist torsion test. The traditional concept of Tnr as a single temperature was thereby assessed with respect to the fractional softening calculations and corresponding microstructural changes at the micro- and nano- scales. Tnr was within the partial softening regime assessed with double-twist torsion testing, suggesting partial recrystallization near Tnr. Measurements of grain aspect ratio matched the trends observed in the fractional softening data and confirmed that strain accumulation occurred throughout the partial softening regime. Higher Nb contents corresponded to greater strain accumulation at lower temperatures within the partial softening regime. The precipitation of Nb(C, N) occurred at high temperatures in the form of caps precipitating on pre-existing Ti-rich cores, while fine-scale precipitation occurred at lower temperatures. Increasing the Nb-content increased both the extent of cap formation and the extent of fine-scale precipitation.