Shape, size and crystallographic orientation of the ferrite grains formed at grain boundaries of deformed austenite in a low carbon steel

The influence of plastic strain on the shape, size and crystallographic orientation of ferrite (α) grains formed at the grain boundaries of the deformed austenite (γ) was studied in a 0.17C-0.3Si-1.5Mn steel. Specimens with a coarse γ grain size of 300 μm were compressed at 1023 K and cooled at 10 K/s. When the plastic strain increased to 0.4, the shape of α grains changed from plate like to equiaxed, and the average length of α grains decreased from 12 to 6 μm. However, the average length did not change in the larger plastic strains up to 1.1. On the other hand, the average thickness of α grains was constant regardless of the plastic strain. The crystallographic orientations of the α grains formed at one γ grain boundary were almost the same when the plastic strain was smaller than 0.2 and the α grain shape was plate like. However, the orientations were widely distributed, and most of the α/α boundaries were high angle ones, when the plastic strain was larger than 0.4 and the α grain shape was equiaxed. The shape change and α grain refinement by the deformation resulted from the wide distribution of crystallographic orientations of α grains rather than from the increase in the nucleation rate. The wide distribution of crystallographic orientation of α grains is closely associated with the serrated austenite grain boundaries induced by the deformation.