Modeling microstructural evolution of microalloyed forging steels during thermomechanical processing

Thermomechanical processing has been extensively studied in the past decades. At the same time, modeling microstructural evolution has become a powerful tool for materials and process design by providing quantitative relations between microstructure, composition and processing. Much less attention has been paid to the thermomechanical processing of hot forging. This process is different from flat rolling. In addition to the different deformation geometry, the steels have higher carbon contents and the reheat treatment is faster. Induction heating is widely used in forging, which has a much higher heating rate and shorter holding time than conventional furnace heating.; This study focused on the microstructural evolution of microalloyed forging steels during induction reheat and after hot deformation. Four materials were studied: 1541Nb (0.41C-1.67Mn-0.038Nb-0.013S-0.005N), 1541VTi (0.40C-1.49Mn-0.11V-0.014Ti-0.042S-0.009N), 1541A1 (0.40C-1.48Mn-0.023A1-0.026S-0.006N), and 1141Nb (0.39C-1.51Mn-0.046Nb-0.12S-0.005N).; Three investigations were carried out: (1) Grain growth and precipitate evolution during induction reheat; (2) Effect of initial microstructure on grain growth during induction reheat; and (3) Recrystallization kinetics following hot deformation.; A physical model was established to predict the evolution of the precipitate size distribution during continuous heating, resulting from diffusion-controlled precipitate dissolution followed by coarsening. The Zener equation for particle pinning was modified to incorporate the effect of a distribution of precipitate sizes. Starting from the initial precipitate size distribution in the bar stock, the models predict the precipitate size distribution and the mean grain diameter at all stages of the heating cycle. The predicted results agree well with the experimental observations.; Different initial microstructures were produced by preheating as-rolled 1141Nb steel bar, and the grain growth behaviour was determined for each preheated condition. Reprecipitation occurred in the pre-quenched samples at 900–1000°C which resulted in smaller grain sizes at intermediate reheat temperatures. Grain growth and precipitate evolution were accurately predicted by the models provided no additional precipitates formed during reheat.; The measured recrystallization kinetics were fitted to the Avrami equation and a generalized equation for the recrystallization rate in the absence of strain-induced precipitation was determined for 1541Nb and 1541VTi. Classical nucleation theory was extended for complex carbonitrides and the onset of strain-induced precipitation was calculated to explain the observed recrystallization behaviour.