1 Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

Based on Dutta and Sellars's expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate and grain size. Although the equation given by these authors reproduces well the typical "C " shape of the precipitation start time (Ps) curve, the expression is not reliable for all cases. Recrystallization-precipitation-time-temperature (RPTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (Ks) and constant B, which is no longer a constant but a function of Ks when the latter is calculated at the nose temperature (TN) of the Ps curve. The value of parameter B is deduced from the minimum point or nose of the Ps curve, where t0.05/T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters derive from the latest studies undertaken by the authors and this work represents an attempt to improve the model. The expressions are now more consistent and predict the precipitation-time-temperature (PTT) curves with remarkable accuracy. The model for strain induced precipitation kinetics is completed by means of Avrami's equation