Abstract: A machine component includes a core made up of a steel for machine structural use, and a medium carbon-containing layer and a high carbon-containing layer formed of the steel for machine structural use, the medium carbon-containing layer covering the core, the high carbon-containing layer covering the medium carbon-containing layer and having a carbon concentration of 0.8-1.5%. The high carbon-containing layer is made up of a martensitic structure having carbides dispersed therein and a residual austenitic structure, wherein spheroidized carbides with an aspect ratio of 1.5 or less constitute 90% or more of a total number of the carbides, and the number of spheroidized carbides on prior austenite grain boundaries is 40% or less of the total number of the carbides.

Abstract: A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030° C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5° C./sec or lower from a temperature higher than the Acm point of a surface layer to a cooling end temperature that is at least 50° C. lower than the A1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the Acm point at the surface layer; heating the material to not higher than the Acm point at the surface layer; and performing tempering.

Abstract: A steel with high hardness and excellent toughness contains, in mass %, 0.55-1.10% C, 0.10-2.00% Si, 0.10-2.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-2.50% Cr, and 0.010-0.10% Al, with the balance consisting of Fe and unavoidable impurities. The structure of the steel after quenching is a dual phase structure of martensitic structure and spheroidized carbide. Spheroidized cementite particles with an aspect ratio of 1.5 or less constitute at least 90% of all cementite particles. The proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to a total number of cementite particles is 20% or less.

Abstract: A steel with high hardness and excellent toughness contains, in mass %, 0.40-1.00% C, 0.10-2.00% Si, 0.10-1.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-3.20% Cr, 0.010-0.10% Al, and 0.15-0.50% V, and further contains at least one or two of 2.50% or less Ni and 1.00% or less Mo, with an amount of (C+V) being 0.60% or more in mass %, with the balance consisting of Fe and unavoidable impurities. The steel has a microstructure which is a martensitic structure with finely dispersed Fe-based ? carbides, with its prior austenite grain size being 20 ?m or less.

Abstract: A machine component includes a core made up of a steel for machine structural use, and a medium carbon-containing layer and a high carbon-containing layer formed of the steel for machine structural use, the medium carbon-containing layer covering the core, the high carbon-containing layer covering the medium carbon-containing layer and having a carbon concentration of 0.8-1.5%. The high carbon-containing layer is made up of a martensitic structure having carbides dispersed therein and a residual austenitic structure, wherein spheroidized carbides with an aspect ratio of 1.5 or less constitute 90% or more of a total number of the carbides, and the number of spheroidized carbides on prior austenite grain boundaries is 40% or less of the total number of the carbides.

Abstract: A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030° C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5° C./sec or lower from a temperature higher than the Acm point of a surface layer to a cooling end temperature that is at least 50° C. lower than the A1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the Acm point at the surface layer; heating the material to not higher than the Acm point at the surface layer; and performing tempering.

Abstract: A steel with high hardness and excellent toughness contains, in mass %, 0.55-1.10% C, 0.10-2.00% Si, 0.10-2.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-2.50% Cr, and 0.010-0.10% Al, with the balance consisting of Fe and unavoidable impurities. The structure of the steel after quenching is a dual phase structure of martensitic structure and spheroidized carbide. Spheroidized cementite particles with an aspect ratio of 1.5 or less constitute at least 90% of all cementite particles. The proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to a total number of cementite particles is 20% or less.

Abstract: A steel with high hardness and excellent toughness contains, in mass %, 0.40-1.00% C, 0.10-2.00% Si, 0.10-1.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-3.20% Cr, 0.010-0.10% Al, and 0.15-0.50% V, and further contains at least one or two of 2.50% or less Ni and 1.00% or less Mo, with an amount of (C+V) being 0.60% or more in mass %, with the balance consisting of Fe and unavoidable impurities. The steel has a microstructure which is a martensitic structure with finely dispersed Fe-based ? carbides, with its prior austenite grain size being 20 ?m or less.