Investigation and Modelling of Magnesium Alloy Grain Size during Hot Strip Rolling with Inter-Pass Annealing

Abstract

Hot rolling combined with inter-pass annealing is an important method for increasing plastic deformation and refining the grain size of magnesium alloys. In this study, we statistically analyzed the influence of the number of annealing treatments and the annealing holding time (5–20 min) between hot rolling passes on the microstructure of AZ31 alloy after four rolling passes and inter-pass annealing. In addition, an exponential model was proposed to predict the functional relationship between the average grain size and the number of annealing treatments and holding time. After a single rolling pass, the average grain size increased exponentially with increasing holding time. However, increasing the annealing holding time to more than 15 min resulted in a nonhomogeneous microstructure owing to grain coarsening and secondary recrystallization. The grain refinement effect weakened with the increasing number of rolling passes, whereas the microstructural uniformity was significantly improved by multi-pass rolling deformation under the same annealing conditions. The higher amount of grain boundary energy accumulated during multi-pass rolling clearly increased the grains size of the dynamic recrystallized in the early stages with increasing holding time in the subsequent annealing processes. The average grain size was determined to be an exponential function of both the number of annealing treatments and holding time. The predicted average grain diameters were in good agreement with the measured results, thus validating the proposed method for establishing the average grain size in hot deformation processes.

Fig. 5 Grain size after the first rolling pass and annealing treatment with different holding times.