Strain-induced precipitation behaviour in hot rolled strip steel

The strain-induced precipitation of Nb(C,N) into the austenite in a Nb-micr oalloyed steel was investigated both experimentally and using a predictive model. The precipitation of Nb(C,N) was measured indirectly from the hardne ss at room temperature after thermomechanical treatment. The predictive mod el combined the precipitation start model of Dutta and Sellars with the Avr ami 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 temper ature 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 deform ations. The fit between the experimental data and calculated results was fo und to be good in all cases with the exception of the slow cooling rate res ults of schedule (ii). It was concluded that the model could, once calibrat ed, successfully predict the hardness and strength of thermomechanically pr ocessed Nb-microalloyed steels. (C) 2001 Elsevier Science B.V. All rights r eserved.