DETERMINATION OF STATIC RECRYSTALLIZATION CRITICAL-TEMPERATURE OF AUSTENITE IN MICROALLOYED STEELS

A method is developed which makes it possible to determine with precision the static recrystallization critical temperature (SRCT) of austenite in microalloyed steels, in other words the temperature at which recrystallization starts to be inhibited. Using torsional tests and applying the back extrapolation method the recrystallized fraction of the austenite has been determined at two strains (0.20 and 0.35) and several temperatures in three steels with Nb, V and Ti respectively. The graphic representation of the activation energy vs. the inverse of the temperature makes it possible to determine the SRCT of each steel as a function of the strain and the austenite grain size. The comparison of the SRCT with the solubility temperature calculated for nitrides and carbides indicates the nature of the precipitates. With the results obtained a model had been constructed for the recrystallized fraction both at temperatures above and below SRCT, and this, together with other equations already published, calculates the residual strain and the austenite grain size which the steel will have at the end of rolling and before the gamma --> alpha transformation. The graphs of the recrystallized fraction vs. time show the kinetics of the induced precipitation and the PTT diagrams (precipitation-time-temperature) have been determined for the three steels.