Sumitomo Metal Industries, Inc.

Harunori Kakimi

Masato Kurita

Koji Watari

Toru Kato

Yoshihiko Kamada

Mitsuo Uno

Masanori Sakamoto

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%. Nitrided steel parts are formed from the steel products serving as steel stock therefor, and have a nitrided case formed in the surface portion thereof.

Steels, steel products for nitriding, nitrided steel parts

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.

Forged, non-heat treated, nitrided steel parts and process of making

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Sakai, Japan

Harunori Kakimi