Electron backscatter diffraction was used to investigate the softening effect in low-carbon steel [Fe-0.051C-0.002Si-0.224Mn-0.045Al (wt pct)] during tensile deformation with in situ ultrasonic treatment. A bimodal grain size distribution is observed with relatively small equiaxed grains with an average diameter of 10 ?m at the grain boundaries of large elongated grains. The formation of these relatively small equiaxed grains is interpreted in terms of dynamic recrystallization by lattice and sub-grain rotation.