Microstructure evolution in large billet during reduction pretreatment based on laboratory experiments

The reduction pretreatment process has been proposed to improve the center quality of large billet and reduce the rolling ratio. The microstructure evolution during the reduction pretreatment was further understood. The austenite grains were refined after the reduction pretreatment experiment, especially those at the center of the billet. The effects of strain and strain rate on the average grain size were dependent on the deformation temperature. At a strain rate of 0.01 s(-1) and 1200 degrees C, the newly formed strain-free austenite grains grew very fast as the strain continued to increase, which resulted in the coarsening of austenite grains. The calculation results of the microstructure evolution model showed that at the same deformation temperature, the evolution curves of average grain size with different strain rates had the intersection points. With the increase in temperature, the position of intersection point moved to the downward direction of strain. The simulation results showed that when the reduction amount increased to 20%, the average grain size at the center was smaller than that near the surface. It could be inferred that when the reduction amount greatly exceeded 20%, the dynamic recrystallization at the center was mostly completed, and the austenite grain growth would become the main mechanism.