In most severely deformed ultra-high strength steels, grains are of anisotropic shape and the following numbers correspond to the smallest dimensions. To my knowledge, the smallest grains are less than 10 nm and are obtained in severely cold-drawn pearlitic wires, that are also the strongest steels known today (doi.org/10.1103/PhysRevLett.113.106104). By high-pressure torsion of a martensitic low carbon steel, we have recently obtained about 30 nm (doi.org/10.1002/adem.201800202 , a more detailed analysis of the as-deformed state is under preparation). For bainitic steels, see for example doi.org/10.4028/www.scientific.net/MSF.584-586.655 and references therein. In my opinion, the challenges are to realize deformation conditions, which allow severe grain refinement without fracture, on the one hand and to have tools, that are not deformed themselves during severe plastic deformation (especially if you combine it with solid solution strengthening with higher carbon contents in medium/high carbon steels), on the other hand.

Thank you sir, for a very nice answer to my question.