Abstract: Provided are a roughly-shaped steel material for a nitrided part, and a nitrided part obtained by nitriding the roughly-shaped steel material for a nitrided part, having a determined chemical composition, in which the portion with a diameter or width ranging from 60 to 130 mm of the roughly-shaped steel material for a nitrided part has a microstructure at a depth of 14.5 mm from a surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 70 to 100%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite; and has a microstructure at a depth of 15 mm or more from the surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 0 to less than 50%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite.

Abstract: The steel shaft component consists of C: 0.40 to 0.60%, Si: 0.05 to 1.00%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.005 to 0.100%, Cr: 0.10 to 0.50%, V: 0.10 to 0.30%, Al: 0.005 to 0.050%, N: 0.0050 to 0.0200%, Ti: 0 to 0.050%, and the balance: Fe and impurities. A Vickers hardness Hs of the surface of the shaft part is 620 HV or more. A Vickers hardness Hb at an R/2 position satisfies Formula (1). A depth of the hardened layer Hr (mm) having a Vickers hardness of 620 HV or more satisfies Formula (2). The microstructure at the R/2 position is composed of ferrite and pearlite. Within the hardened layer, the number of density of V-containing precipitates having an equivalent circular diameter of more than 100 nm is 10 particles/276 ?m2 or less. Hs/2.3?Hb?350??(1) 0.05?Hr/R?0.

Abstract: A hot forged steel material in the present embodiment includes a chemical composition that consists of, in mass %, C: 0.14 to 0.20%, Si: 0.20 to 1.00%, Mn: 1.00 to 1.90%, P: 0.030% or less, S: 0.030% or less, V: 0.16 to 0.30%, Al: 0.015 to 0.050%, N: 0.0050 to 0.0250%, Cr: 0.10 to 0.30%, Cu: 0 to 0.10%, and Nb: 0 to 0.10%, with the balance being Fe and impurities, and that satisfies Formula (1) and Formula (2), wherein a grain size number of ferrite in the steel is 9.0 or more, and an absorbed energy at ?30° C. is 100 J or more in the Charpy impact test using a V notch specimen. 0.36?C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68??(1) 51/12×C?V?0.

Abstract: Provided are a roughly-shaped steel material for a nitrided part, and a nitrided part obtained by nitriding the roughly-shaped steel material for a nitrided part, having a determined chemical composition, in which the portion with a diameter or width ranging from 60 to 130 mm of the roughly-shaped steel material for a nitrided part has a microstructure at a depth of 14.5 mm from a surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 70 to 100%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite; and has a microstructure at a depth of 15 mm or more from the surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 0 to less than 50%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite.

Abstract: A hot forged steel material in the present embodiment includes a chemical composition that consists of, in mass %, C: 0.14 to 0.20%, Si: 0.20 to 1.00%, Mn: 1.00 to 1.90%, P: 0.030% or less, S: 0.030% or less, V: 0.16 to 0.30%, Al: 0.015 to 0.050%, N: 0.0050 to 0.0250%, Cr: 0.10 to 0.30%, Cu: 0 to 0.10%, and Nb: 0 to 0.10%, with the balance being Fe and impurities, and that satisfies Formula (1) and Formula (2), wherein a grain size number of ferrite in the steel is 9.0 or more, and an absorbed energy at ?30° C. is 100 J or more in the Charpy impact test using a V notch specimen. 0.36?C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68??(1) 51/12×C?V?0.

Abstract: There is provided a hot forged product having excellent wear resistance and fatigue strength even when the hot forged product is produced with a thermal refining treatment and a case hardening thermal treatment after hot forging omitted, and having a chemical composition consisting of, in mass %, C: 0.45 to 0.70%, Si: 0.01 to 0.70%, Mn: 1.0 to 1.7%, S: 0.01 to 0.1%, Cr: 0.05 to 0.25%, Al: 0.003 to 0.050%, N: 0.003 to 0.02% with the balance being Fe and impurities. The matrix at the depth of 500 ?m to 5 mm from an unmachined surface of the forged product is a ferrite-pearlite structure, in which a pro-eutectoid ferrite area fraction is 3% or less or a pearlite structure, and the average diameter of pearlite colonies in the pearlite structure at the depth of 500 ?m to 5 mm from the unmachined surface is 5.0 ?m or less.

Abstract: There is provided a hot forged product having excellent wear resistance and fatigue strength even when the hot forged product is produced with a thermal refining treatment and a case hardening thermal treatment after hot forging omitted, and having a chemical composition consisting of, in mass %, C: 0.45 to 0.70%, Si: 0.01 to 0.70%, Mn: 1.0 to 1.7%, S: 0.01 to 0.1%, Cr: 0.05 to 0.25%, Al: 0.003 to 0.050%, N: 0.003 to 0.02% with the balance being Fe and impurities. The matrix at the depth of 500 ?m to 5 mm from an unmachined surface of the forged product is a ferrite-pearlite structure, in which a pro-eutectoid ferrite area fraction is 3% or less or a pearlite structure, and the average diameter of pearlite colonies in the pearlite structure at the depth of 500 ?m to 5 mm from the unmachined surface is 5.0 ?m or less.