Abstract: The present invention relates to manufacturing methods of nitrided steel parts, having high tensile strength, high fatigue strength, and excellent bending roughness, through nitriding without thermal refining, to thus-manufactured steel parts, and to steel products for nitriding, and steels for nitriding having a specific chemical composition, serving as steel stock for the manufacture of such steel parts. The chemical composition is as follows: C: over 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.20 to 1.50%, P: 0.08% or less, S: 0.005 to 0.10%, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, Mo: 0.30% or less, V: 0.20% or less, Nb: 0.05% or less, Ti: 0.003 to 0.03%, Al: 0.08% or less, Ca: 0.005% or less, Pb: 0.30% or less, N: 0.008 to 0.030%, and the balance of Fe and unavoidable impurities. Steel products preferably have the ferrite-pearlite microstructure having a ferrite percentage of not less than 10%.

Abstract: Forged, nitrided steel parts having a high fatigue limit and bending toughness are produced without heat treatment for refining between the forging and nitriding. The steel has a composition consisting essentially, on a weight basis, of: 0.30-0.60% of C, 0.05-1.50% of Si, 0.20-2.00% of Mn, less than 0.02% of P, less than 0.04% of S, not greater than 0.30% of Cr, less than 0.01% of Al, 0.01-0.02% of N, and a balance of Fe and incidental impurities in which the content of V as an impurity is 0.02% or less. The steel composition may further contain at least one element selected from P: 0.02-0.07%, S: 0.04-0.10%, Ca: 0.0003-0.003%, and Pb: 0.01-0.20% in order to provide it with improved machinability.