Strain gradient induced grain refinement far below the size limit in a low carbon hypoeutectoid steel (0.19 wt% C) via pipe inner surface grinding treatment

•High strain gradient drives the deformation-induced grain subdivision far below the size limit during traditional severe plastic deformation.•The refinement below size limit was realized through forming novel extended boundaries within ultrafine lamellar and/or nanolaminated ferrite;•To form an extended boundary requires lattice curvature above ∼2.8°, and a minimum strain gradient of 0.25 μm−1 for forming an extended boundary within a lamella of 100 nm thick;•Strain gradient (χ, in μm−1) is correlated with lamellar thickness (dT in nm) by: dT = 12.5/χ.