Abstract: A steel for carburizing or carbonitriding use consisting of, by mass %, C: 0.1 to 0.3%, Si: 0.01 to 0.15%, Mn: 0.6 to 1.5%, S: 0.012 to 0.05%, Cr: 0.5 to 2.0%, Al: 0.030 to 0.050%, Ti: 0.0006 to 0.0025%, N: 0.010 to 0.025%, and O: 0.0006 to 0.0012%, and, optionally, at least one selected from Mo?0.5%, Ni?1.5% and Cu?0.4%, and the balance of Fe and impurities. P and Nb are P?0.025% and Nb?0.003% respectively. Formulas of [?5.0?log (Ti×N)??4.4] and [?12.5?log (Al2×O3)??11.7] are satisfied. Austenite grain coarsening is prevented even when the steel is heated in the process of carburizing or carbonitriding, and even under various hot forging temperatures. The steel has excellent bending fatigue strength after carburizing or carbonitriding.

Abstract: A rolled steel bar or a wire rod for hot forging capable of coping with both bending/surface fatigue strength of components and machinability at a high level includes: a composition containing, in mass %, C: 0.1 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.4 to 1.0%, S: 0.003 to 0.05%, Cr: 1.60 to 2.00%, Mo: 0.10% or less (including 0%), Al: 0.025 to 0.05%, and N: 0.010 to 0.025%, where a value of fn1 represented in a following formula (1) satisfies 1.82?fn1?2.10: fn1=Cr+2×Mo (1); impurities containing P: 0.025% or less, Ti: 0.003% or less, and O (oxygen): 0.002% or less; and a cross section in which a maximum value/a minimum value of an average ferrite grain diameter is 2.0 or less when measurement by observation is randomly carried out in 15 visual fields with an area per visual field set to be 62500 ?m2.

Abstract: A rolled steel bar or wire rod having hot surface fatigue strength, wear resistance, and machinability even after hot forging has a composition containing C, Si, Mn, S, Cr, Mo (optional), Al, and N, with the balance being Fe and impurities. The chemical composition satisfies that fn1 defined by Formula (1) is 1.60 to 2.10. The structure of the rolled steel bar or wire rod for hot forging includes a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. A maximum value/a minimum value of average ferrite grain size, which is observed and measured randomly in 15 visual fields each having an area of 62500 ?m2 in a cross section, is not more than 2.0. fn1=Cr+2×Mo (1), and each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.

Abstract: A steel for carburizing or carbonitriding use consisting of, by mass %, C: 0.1 to 0.3%, Si: 0.01 to 0.15%, Mn: 0.6 to 1.5%, S: 0.012 to 0.05%, Cr: 0.5 to 2.0%, Al: 0.030 to 0.050%, Ti: 0.0006 to 0.0025%, N: 0.010 to 0.025%, and 0: 0.0006 to 0.0012%, and, optionally, at least one selected from Mo?0.5%, Ni?1.5% and Cu?0.4%, and the balance of Fe and impurities. P and Nb are P?0.025% and Nb?0.003% respectively. Formulas of [?5.0?log(Ti×N)??4.4] and [?12.5?log(Al2×O3??11.7] are satisfied. Austenite grain coarsening is prevented even when the steel is heated in the process of carburizing or carbonitriding, and even under various hot forging temperatures. The steel has excellent bending fatigue strength after carburizing or carbonitriding.

Abstract: A rolled steel bar or a wire rod for hot forging capable of coping with both bending/surface fatigue strength of components and machinability at a high level includes: a composition containing, in mass %, C: 0.1 to 0.25%, Si: 0.01 to 0.10%, Mn: 0.4 to 1.0%, S: 0.003 to 0.05%, Cr: 1.60 to 2.00%, Mo: 0.10% or less (including 0%), Al: 0.025 to 0.05%, and N: 0.010 to 0.025%, where a value of fn1 represented in a following formula (1) satisfies 1.82?fn1?2.10: fn1=Cr+2×Mo (1); impurities containing P: 0.025% or less, Ti: 0.003% or less, and O (oxygen): 0.002% or less; and a cross section in which a maximum value/a minimum value of an average ferrite grain diameter is 2.0 or less when measurement by observation is randomly carried out in 15 visual fields with an area per visual field set to be 62500 ?m2.

Abstract: A hot-rolled steel bar or wire rod consisting of C: 0.1 to 0.3%, Si: 0.05 to 1.5%, Mn: 0.4 to 2.0%, S: 0.003 to 0.05%, Cr: 0.5 to 3.0%, Al: 0.02 to 0.05%, and N: 0.010 to 0.025%, the balance being Fe and impurities, and the impurities containing P: 0.025% or less, Ti: 0.003% or less, and O: 0.002% or less, wherein the structure thereof is composed of a ferrite-pearlite structure, ferrite-pearlite-bainite structure, or ferrite-bainite structure; the standard deviation of ferrite fractions at the time when randomly selected 15 viewing fields of a transverse cross section are observed and measured with the area per one viewing field being 62,500 ?m2 is 0.10 or less; and in a region from the surface to one-fifth of the radius and a region from the center to one-fifth of the radius in the transverse cross section, the amount of Al precipitating as AlN is 0.005% or less, and the density in terms of the number of AlN having a diameter of 100 nm or larger is 5/100 ?m2 or less.

Abstract: A hot-rolled steel bar or wire rod consisting of C: 0.1 to 0.3%, Si: 0.05 to 1.5%, Mn: 0.4 to 2.0%, S: 0.003 to 0.05%, Cr: 0.5 to 3.0%, Al: 0.02 to 0.05%, and N: 0.010 to 0.025%, the balance being Fe and impurities, and the impurities containing P: 0.025% or less, Ti: 0.003% or less, and O: 0.002% or less, wherein the structure thereof is composed of a ferrite-pearlite structure, ferrite-pearlite-bainite structure, or ferrite-bainite structure; the standard deviation of ferrite fractions at the time when randomly selected 15 viewing fields of a transverse cross section are observed and measured with the area per one viewing field being 62,500 ?m2 is 0.10 or less; and in a region from the surface to one-fifth of the radius and a region from the center to one-fifth of the radius in the transverse cross section, the amount of Al precipitating as AlN is 0.005% or less, and the density in terms of the number of AlN having a diameter of 100 nm or larger is 5/100 ?m2 or less.

Abstract: A wheel has both wear resistance and heat-crack resistance sufficient for use in a high-speed railway vehicle. The chemical composition of a base steel comprises 0.4 to 0.75 % C, 0.4 to 0.95% Si, 0.6 to 1.2% Mn and less than 0.2% Cr. The wheel has a pearlitic microstructure in a region of at least 50 mm in depth from the tread surface of the wheel. When the tread of the wheel is quenched, a particular cooling process is applied in order to obtain the pearlitic structure.