Effect of Nb and Ti addition on activation energy for austenite hot deformation

Constitutive equations are employed in several algorithms to predict load during hot working. The peak stress (σp) can be correlated to the temperature and strain rate at which the metal was deformed using the known expression: Z = A(sinh(ασp))n, where, Z is the Zener–Hollomon parameter, given by Z=ε˙exp(Qdef/RT). The activation energy for hot deformation, Qdef, magnifies the dependence of the stress on temperature in an exponential way requiring accurate measurement of this quantity. Empirical expressions published in the literature can be used to evaluate Qdef. Unfortunately, these equations were derived for alloys with relatively high contents of C and N and when applied to the case of IF austenite, they yield unreasonable values. This is so because IF steels have, simultaneously, very low C and N contents and substantial amounts of microalloying elements leading to over prediction of values of Qdef. This paper investigates, therefore, how the chemical composition of IF alloys can influence the values of the activation energy for hot deformation allowing suitable predictions to be made. Measurements of Qdef were carried out for IF steels containing Ti and a combination of Ti-Nb additions. Predicted and measured values of Qdef were compared to those calculated from published equations and a corrected expression was derived in order to better fit the experimental results here presented.