Prediction of ferrite grain size in niobium microalloyed controlled rolled steels

The ferrite grain size in hot rolled steels is determined by the austenite grain size and conditioning prior to transformation, the cooling rate through transformation and the steel composition. To successfully predict the ferrite grain size after rolling and cooling all these factors must be taken into account. An empirical equation has been developed which accurately predicts the grain size for controlled rolled Nb steels containing 0.05 to 0.15 wt% carbon at cooling rates up to 30 C/s. The data was obtained from simulation of controlled rolling and cooling using hot torsion. The equation has been integrated into a full microstructural model which has been applied to plate, structural and bar mills. The influence of the accuracy of the ferrite grain size equation on the accuracy of prediction of the final yield strength is also discussed.