Strain-induced precipitation behaviour in hot rolled strip steel

The strain-induced precipitation of Nb(C,N) into the austenite in a Nb-microalloyed steel was investigated both experimentally and using a predictive model. The precipitation of Nb(C,N) was measured indirectly from the hardness at room temperature after thermomechanical treatment. The predictive model combined the precipitation start model of Dutta and Sellars with the Avrami equation and the additivity principle to allow prediction of the volume fraction of Nb(C,N) precipitated. The effects of several thermomechanical schedules were studied. These were (i) the effect of isothermal hold temperature and duration; (ii) the effect of deformation temperature at high and low cooling rates; (iii) the effect of cooling rate prior to the austenite to ferrite transformation; and (iv) the effect of multiple pancaking deformations. The fit between the experimental data and calculated results was found to be good in all cases with the exception of the slow cooling rate results of schedule (ii). It was concluded that the model could, once calibrated, successfully predict the hardness and strength of thermomechanically processed Nb-microalloyed steels.