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

A method is developed which makes it possible to determine with precis ion the static recrystallization critical temperature (SRCT) of austen ite in microalloyed steels, in other words the temperature at which re crystallization starts to be inhibited. Using torsional tests and appl ying the back extrapolation method the recrystallized fraction of the austenite has been determined at two strains (0.20 and 0.35) and sever al temperatures in three steels with Nb, V and Ti respectively. The gr aphic representation of the activation energy vs. the inverse of the t emperature 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 ob tained a model had been constructed for the recrystallized fraction bo th at temperatures above and below SRCT, and this, together with other equations already published, calculates the residual strain and the a ustenite grain size which the steel will have at the end of rolling an d before the gamma --> alpha transformation. The graphs of the recryst allized fraction vs. time show the kinetics of the induced precipitati on and the PTT diagrams (precipitation-time-temperature) have been det ermined for the three steels.