Abstract: An upper member of a steel piston has a chemical composition which consists of, in mass %, C: 0.15 to 0.30%, Si: 0.02 to 1.00%, Mn: 0.20 to 0.80%, P: 0.020% or less, S: 0.028% or less, Cr: 0.80 to 1.50%, Mo: 0.08 to 0.40%, V: 0.10 to 0.40%, Al: 0.005 to 0.060%, N: 0.0150% or less, O: 0.0030% or less, and the balance: Fe and impurities, and satisfies Formula (1) and Formula (2), in which, at a cross section parallel to the axial direction of the upper member, the number of Mn sulfides is 100.0 per mm2 or less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 ?m or more is within a range of 1.0 to 10.0 per mm2, and the number of oxides is 15.0 per mm2 or less. 0.42?Mo+3V?1.50??(1) V/Mo?0.

Abstract: A track link which is a tracked undercarriage component is made of a steel having a specific component composition, and includes a high hardness portion having a hardness of HRC 57 or more and HRC 60 or less, and a low hardness portion. The high hardness portion includes a first matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the first matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN, and it does not include a M23C6 carbide. The low hardness portion includes a second matrix including a martensite phase, and second nonmetallic particles dispersed in the second matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN, and it does not include a M23C6 carbide.

Abstract: A rolled wire rod for spring steel contains, as a chemical composition, by mass %: C: 0.42% to 0.60%; Si: 0.90% to 3.00%; Mn: 0.10% to 1.50%; Cr: 0.10% to 1.50%; B: 0.0010% to 0.0060%; N: 0.0010% to 0.0070%; Mo: 0% to 1.00%; V: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 0.50%; Al: 0% to 0.100%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; P: limited to less than 0.020%; S: limited to less than 0.020%; and a remainder including Fe and impurities, the carbon equivalent (Ceq) is 0.75% to 1.00%, the area fraction of tempered martensite and bainite included in a microstructure is 90% or greater, the tensile strength is 1,350 MPa or less, and the reduction of area is 40% or greater.

Abstract: An upper member of a steel piston has a chemical composition which consists of, in mass %, C: 0.15 to 0.30%, Si: 0.02 to 1.00%, Mn: 0.20 to 0.80%, P: 0.020% or less, S: 0.028% or less, Cr: 0.80 to 1.50%, Mo: 0.08 to 0.40%, V: 0.10 to 0.40%, Al: 0.005 to 0.060%, N: 0.0150% or less, O: 0.0030% or less, and the balance: Fe and impurities, and satisfies Formula (1) and Formula (2), in which, at a cross section parallel to the axial direction of the upper member, the number of Mn sulfides is 100.0 per mm2 or less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 ?m or more is within a range of 1.0 to 10.0 per mm2, and the number of oxides is 15.0 per mm2 or less. 0.42?Mo+3V?1.50??(1) V/Mo?0.

Abstract: A rolled wire rod wherein the contents of Ti, N, and S (mass %) are respectively [Ti], [N], and [S], and, if [S]?0.0010, [Ti] is (4.5×[S]+3.4×[N]) or more and (0.008+3.4×[N]) or less, while if [S]?0.0010, [Ti] is (4.5×[S]+3.4×[N]) or more and (8.0×[S]+3.4×[N]) or less, the internal structure is a mixed structure of ferrite and pearlite with an area ratio of a ferrite fraction of 40% or more, and a mean area of sulfides present in a range from a surface of the wire rod to a depth position D/8 from the surface of the wire rod is 6 ?m2 or less, wherein D represents a diameter, in mm, in a cross-section of the wire rod at a plane including the axis of the wire rod, and a mean aspect ratio of the sulfides is 5 or less.

Abstract: A steel material has a chemical composition which consists of, in mass %, C: 0.15 to 0.30%, Si: 0.02 to 1.00%, Mn: 0.20 to 0.80%, P: 0.020% or less, S: 0.028% or less, Cr: 0.80 to 1.50%, Mo: 0.08 to 0.40%, V: 0.10 to 0.40%, Al: 0.005 to 0.060%, N: 0.0150% or less, O: 0.0030% or less, and the balance: Fe and impurities, and satisfies Formulae (1) and (2), in which, at a cross section parallel to the axial direction of the steel material for a steel piston, the number of Mn sulfides is 100.0 per mm2 or less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 ?m or more is in a range of 1.0 to 10.0 per mm2, and the number of oxides is 15.0 per mm2 or less. 0.42?Mo+3V?1.50??(1) V/Mo?0.

Abstract: A steel according to one aspect of the present invention has a chemical composition containing, by unit mass %, C: 0.25 to 0.55%, Si: 0.20 to 1.30%, Mn: 0.50 to 1.50%, P: 0.010 to 0.200%, S: 0.010 to 0.100%, Cr: 0 to 1.20%, V: 0.25 to 0.40%, Ti: 0.010 to 0.070%, Mo: 0 to 0.15%, N: 0.0010 to 0.0060%, Al: 0 to 0.200%, Ca: 0 to 0.005%, Zr: 0 to 0.005%, Mg: 0 to 0.005%, Bi: 0 to 0.0050%, Pb: 0 to 0.50%, Nb: 0 to 0.05%, Cu: 0 to 0.05%, and Ni: 0 to 0.05%, with the balance being Fe and impurities, wherein 0.002<Ti?3.4×N<0.050 is satisfied.

Abstract: Steel according to one embodiment of the present invention has predetermined chemical components, wherein in a region where a distance r from the center of a cross-section perpendicular to the length direction satisfies 0.7R?r?0.9R, structures include ferrite and bainite, the average fraction of the ferrite is in the range of 40 to 70% in terms of area ratio, the total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and the balance includes bainite; and the standard deviation of a ferrite fraction in the region is 4% or less.

Abstract: A bearing steel part having a predetermined chemical composition, in which the number density of oxide particles having an equivalent circle diameter of 5 ?m or more, and containing CaO, Al2O3 and SiO2, such that the content ratio of Al2O3 with respect to the total mass of CaO, Al2O3, and SiO2 is 50% by mass or more, is 3.0/cm2 or less in an arbitrary cross section of the part, and in which the Vickers hardness at a depth of 50 ?m from a rolling surface is 750 or more, and in which the compressive residual stress at the rolling surface is 900 MPa or more. Also provided is a steel bar for a bearing steel part that is suitable for obtaining the foregoing bearing steel part.

Abstract: In a rolled steel bar or rolled wire rod for a cold-forged component having a predetermined chemical composition, Y1 represented by Y1=[Mn]×[Cr] and Y2 represented by Y2=0.134×(D/25.4?(0.50×?[C]))/(0.50×?[C]) satisfy Y1>Y2, the tensile strength is 750 MPa or less, an internal structure is a ferrite-pearlite structure, and the ferrite fraction in the internal structure is 40% or greater. AMOUNT IS 0.

Abstract: In a rolled steel bar or rolled wire rod for a cold-forged component having a predetermined chemical composition, Y1 represented by Y1=[Mn]×[Cr] and Y2 represented by Y2=0.134×(D/25.4?(0.50×?[C])/(0.50×?[C]) satisfy Y1>Y2, the tensile strength is 750 MPa or less, an internal structure is a ferrite-pearlite structure, and the ferrite fraction in the internal structure is 40% or greater.

Abstract: A carburized bearing steel part has a predetermined chemical composition, and, in the carburized bearing steel part, the number density of oxide particles having an equivalent circle diameter of 5 ?m or more, and containing CaO, Al2O3 and SiO2, such that the content ratio of Al2O3 with respect to the total mass of CaO, Al2O3, and SiO2 is 50% by mass or more, is 3.0/cm2 or less in an arbitrary cross section of the part, the Vickers hardness at a depth of 50 ?m from a rolling surface is 750 or more, and the compressive residual stress at the rolling surface is 900 MPa or more. Also provided is a steel bar for a carburized bearing steel part that is suitable for obtaining the foregoing carburized bearing steel part.

Abstract: A steel for induction hardening according to the present invention includes a chemical composition consisting of, in mass percent: C: 0.58 to 0.68%, Si: 0.70 to 1.40%, Mn: 0.20 to 1.40%, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, and Ca: 0.0005 to 0.005%, with the balance being Fe and impurities, and satisfies Formulae (1) to (3). The steel microstructure is made up of ferrite and by area fraction, 85% or more of pearlite. In the steel, a ratio of a number of composite inclusions to a total number of Al2O3 inclusions and the composite inclusions that contain 2.0% or more of SiO2 and 2.0% or more of CaO is 20% or more. C+Si/7+Mn/5+Cr/9+Mo/2.5?1.05??(1) C+Si/10+Mn/20+Cr/25?0.70??(2) Cr/Si?0.

Abstract: A steel for induction hardening according to the present invention includes a chemical composition consisting of, in mass percent: C: 0.58 to 0.68%, Si: 0.70 to 1.40%, Mn: 0.20 to 1.40%, P: less than 0.020%, S: less than 0.020%, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, O: 0.0015% or less, V: 0.01 to 0.25%, B: 0.0003 to 0.0040%, Ti: 0.010 to 0.050%, and Ca: 0.0005 to 0.005%, with the balance being Fe and impurities, and satisfies Formulae (1) to (3). The steel microstructure is made up of ferrite and pearlite. A ratio of a number of composite inclusions is 20% or more. C+Si/7+Mn/5+Cr/9+Mo/2.5?1.05??(1) C+Si/10+Mn/20+Cr/25?0.70??(2) Cr/Si?0.

Abstract: The steel material for a carburized bearing part according to the present invention contains, by mass %, C: 0.25 to 0.45%, Si: 0.15 to 0.45%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.60 to 2.00%, Mo: 0.10 to 0.35%, V: 0.20 to 0.40%, Al: 0.005 to 0.100%, Ca: 0.0002 to 0.0010%, N: 0.0300% or less and O: 0.0015% or less, with the balance being Fe and impurities, and satisfies Formulae (1) to (3). 1.20<0.4Cr+0.4Mo+4.5V<2.75??(1) A1/A2>0.50??(2) 2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V>2.55??(3) Formula (2) shows an area fraction of sulfides containing Ca in an amount of 1 mol % or more among sulfides having an equivalent circular diameter of 1 ?m or more.

Abstract: A bolt is provided that has high strength and excellent hydrogen embrittlement resistance characteristics. A bolt according to an embodiment of the present invention consists of, in mass %, C: 0.32 to 0.39%, Si: 0.15% or less, Mn: 0.40 to 0.65%, P: 0.020% or less, S: 0.020% or less, Cr: 0.85 to 1.25%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0010 to 0.0030%, N: 0.0015 to 0.0080%, O: 0.0015% or less, Mo: 0 to 0.05%, V: 0 to 0.05%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, and Nb: 0 to 0.05%, with the balance being Fe and impurities. The bolt satisfies Formula (1) and Formula (2), and has a tensile strength of 1000 to 1300 MPa and satisfies Formula (3). 4.9?10C+Si+2Mn+Cr+4Mo+5V?6.1??(1) Mn/Cr?0.55??(2) [dissolved Cr]/Cr?0.

Abstract: A rolled wire rod for spring steel contains, as a chemical composition, by mass %: C: 0.42% to 0.60%; Si: 0.90% to 3.00%; Mn: 0.10% to 1.50%; Cr: 0.10% to 1.50%; B: 0.0010% to 0.0060%; N: 0.0010% to 0.0070%; Mo: 0% to 1.00%; V: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 0.50%; Al: 0% to 0.100%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; P: limited to less than 0.020%; S: limited to less than 0.020%; and a remainder including Fe and impurities, the carbon equivalent (Ceq) is 0.75% to 1.00%, the area fraction of tempered martensite and bainite included in a microstructure is 90% or greater, the tensile strength is 1,350 MPa or less, and the reduction of area is 40% or greater.

Abstract: A gear keeping the bending stress at the dedendum from becoming locally high at the time of power transmission and thereby raising the dedendum bending strength. The radius of curvature is maximum at a critical section position determined by the Hofer's 30° tangent method. Both radii of curvature from the critical section position to the first and second connecting points X1 and Y1 are constant or decreasing. In dedendum line segment 23, there are points A1 and B1 where radius of curvature is smaller than the critical section position. In the dedendum line segment 23, the maximum radius of curvature is 3 times or less the minimum radius of curvature. By a cross-sectional view, the critical section position is part of an arc, and the arc extends to the two sides of the critical section position. Thus, the maximum bending stress becomes smaller and dedendum bending strength is improved.

Abstract: Steel improved in all of the hardenability, toughness, surface-originated micropitting life, and bending fatigue strength and a part manufactured using such steel are provided having predetermined constituents having an Fn1 defined by the following formula (1) of 0.20 to 0.65 and having an Fn2 defined by the following formula (2) of 0.50 to 1.00: Fn1=4.2×[Cr]/(7.0×[Si]+16.0×[Mn])??(1) [Element]: mass % of element Fn2=A1/A2??(2) A1: total area (?m2) of sulfide-based inclusions containing 1.0 mol % or more of Ca with respect to the total number of moles in the sulfides and having a circle equivalent diameter of 1.0 ?m or more in observation regions of a total area of 4.0 mm2 A2: total area (?m2) of sulfide-based inclusions having a circle equivalent diameter of 1.0 ?m or more in observation regions of a total area of 4.

Abstract: A cold forged part having a high cold forgeability and having a high endurance ratio due to work hardening by cold forging and age-hardening after cold forging, characterized by having a predetermined chemical composition, having an amount of solute Nb/amount of solute V of 0.03 or more and having a structure, by area ratio, of ferrite of 85% or more and a total of bainite and martensite of 5% or less.