Electron backscatter diffraction is used to investigate the crystallographic properties of bainitic and martensitic microstructures obtained after rapid thermal cycles. Numerous low-angle boundaries are found within upper bainite packets, which are in fact sets of finely interlocked groups of variants. Lower bainitic packets contain many high angle boundaries. Micromechanical modelling suggests that self-accommodation of the phase transformation induced strain is more efficient for lower bainite than for upper bainite. Moreover, the fine structure of lower bainite does not necessarily induce high impact toughness properties, owing to the complex three-dimensional shape of the groups of laths.