PROBLEM TO BE SOLVED: To obtain a steel having high strength by suing ferrite in which the average grain size is regulated to a specified value or below and surrounded by a large angle boundary in which the orientation difference angle in the boundary is regulated to a specified value or above as a mother layer.
SOLUTION: This steel having an ultrafine structure is the one using ferrite in which the average grain size is regulated to ≤30μm and surrounded by a large angle boundary in which the orientation difference angle in the boundary is regulated to ≥15° as a mother layer. The content of C is preferably regulated to ≤0.3%. It contains C, Si, Mu, Al, P, S and N, and the balance Fe with inevitable impurities. It is heated to the Ac3 point or above, is austenitized, is thereafter subjected to anvil compression working of ≥50% draft at the Ar3 point or above and is next cooled to produce the steel with an ultranfine structure in which ferrite having ≤3 μm average grain size is used as a mother layer. The cooling is executed preferably at a rate of ≥3 K/s. The compression working is preferably executed simultaneously or continuously as working from at least two planes among the three planes of X, Y and Z of the material to be worked.