Evolution of austenite static recrystallization and grain size during hot rolling of a V-microalloyed steel

Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-microalloyed steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature (Tnr) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below Tnr, where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T < Tnr. A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and Tnr or RLT temperatures can be better understood.