No abstract

a b s t r a c tStatic recrystallisation of three steels having different Al, V and N contents (one of them without V) has been studied by means of hot torsion tests. It has been found that strain-induced precipitates of AlN in the austenite have a mean size of approximately 86 nm. These particles barely inhibit the static recrystallisation, as associated pinning forces are very weak. However, mean size of VCN particles is equal to 11 nm. This fine size leads to a temporary inhibition of recrystallisation, revealed by the characteristic "plateau" of the plots of recrystallised fraction versus holding time after deformation. Besides, activation energy for recrystallisation considerably augments when VCN precipitation occurs, but it hardly increases when AlN particles precipitate. On the other hand, diffusion coefficient of Al in austenite is two orders of magnitude higher than for V. Furthermore, according to thermodynamic calculations based on Hillert and Staffanson method, precipitation of AlN particles starts at much higher temperatures than VCN. Aforesaid reasons make AlN particles to be much coarser than VCN precipitates. From the results it can be concluded that low Al contents would lead to more intense precipitation of VCN that is beneficial for V-microalloyed steels.

No abstract

No abstract

Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructure under isothermal and nonisothermal hot deformation of a Nb microalloyed steel. Thanks to these tests and with the assistance of microstructural characterization, it has been verified that norecrystallization temperature (T nr ) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling from stress-strain data and with no need of metallographic studies is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional 3 softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests is better understood.