Evolution of microstructure in rod and bar rolling

Rod and bar rolling schedules have been executed on a variety of steel grades using laboratory torsion simulation. For very short interpass times of less than 100 ms, it has been shown that dynamic recrystallization takes place due to strain accumulation in the first one or two passes of the finishing schedule. Studies of the complete post dynamic softening behaviour reveal only a metadynamic process, with no indication of a static stage. This is in contrast with other investigations, which show post dynamic recrystallization softening to be comprised of a rapid, but relatively insignificant, metadynamic stage, followed by completion due to nucleation and growth processes. These contrasting behaviours are rationalized in terms of differences in the respective dynamically recrystallized microstructures. No apparent effect of temperature on the post dynamic softening kinetics is observed between 900 and 1200°C. However, there is a strong effect of strain rate. These contradictory observations can be reconciled from a consideration of the individual effects of strain rate and temperature on nuclei density and growth rate. Since close to 50% postdynamic softening occurs within 200 to 300 ms, at a strain rate of 3 s-1, it is predicted that the much higher strain rates encountered in the mill will lead to considerable metadynamic softening at all stages of finishing. Fortunately, there are only marginal differences between the dynamically and metadynamically recrystallized grain sizes. Therefore, either of these metallurgical processes can be used to obtain a very fine austenite grain size at the end of rolling. The main effect of an addition of 0.03 wt% Nb is to retard the post dynamic softening kinetics at temperatures lower than 1000°C. Mathematical models have been developed to predict the rate of metadynamic recrystallization, and the resultant grain size.