The microstructural evolution of interstitial-free (IF) steel subjected to Φ = 90° equal channel angular extrusion (ECAE) for up to 4 passes via routes A, BA, C and up to 8 passes via routes BC was studied using electron back-scattering diffraction (EBSD). Routes BC and BA recorded the smallest grain size and aspect ratios and the largest average misorientation and area fraction of high-angle grain boundaries (HAGBs). During multiple passes, microstructure refinement continues until a convergence in effective subgrain and grain diameters occurs; following which the rate of HAGB formation reduces slightly. The percentage rise in the number of Σ3 and random boundaries should be correlated with the operation of recovery mechanisms in ultra-fine grained IF-steel rather than linking such special boundaries with twinning during ECAE. Compared to the scaling factor Hall–Petch (H–P) equation, the composite H–P equation indicates that although the low-angle grain boundaries (LAGBs) provide the maximum strengthening up to 8 passes, the contribution from HAGBs also increases with greater pass number.