Patent ID: 7824508

Claim:
A fine grain surface layer steel part containing, by mass %, C: 0.45% to 0.70%, Nb: 0.01% to 0.60%, Si: 0.10% to 1.50%, Mn: 0.40% to 2.0%, P: 0.10% or less, S: 0.001% to 0.15%, N: 0.003% to 0.025% and having a balance of Fe and unavoidable impurities, said fine grain surface layer steel part characterized in that the surface layer and inside at all or part of the part have structures of different average particle sizes of ferrite crystal grains surrounded by high angle grain boundaries of a misorientation angle of 15 degrees or more, the structure from the surface to a depth of at least 1.0 mm is a structure comprised of ferrite having an average particle size of ferrite crystal grains surrounded by high angle grain boundaries of a misorientation angle of 15 degrees or more of 4 μm or less and of pearlite and/or cementite, while the structure of the location from the center of thickness of the part to at least ⅙ thickness is a structure comprised of ferrite having an average particle size of ferrite crystal grains surrounded by high angle grain boundaries of a misorientation angle of 15 degrees or more of 15 μm or more and of pearlite.