[0001]The present invention relates to a steel shaft component, and more particularly relates to a steel shaft component equipped with one or a plurality of shaft parts, as typified by a crankshaft and a camshaft. BACKGROUND ART [0002] A steel shaft component typified by a crankshaft is equipped with one or a plurality of shaft parts. In a case where the steel shaft component is a crankshaft, a crank pin and a crank journal correspond to shaft parts. Such kind of steel shaft components are utilized as machine components of industrial machinery, construction machinery, and machinery for transportation as typified by an automobile. [0003] A steel shaft component is produced by the following process. A steel material that is the starting material for the steel shaft component is hot-forged to produce an intermediate product. As necessary, the produced intermediate product is subjected to a thermal refining treatment. The non-heat treated intermediate product in the state after hot forging or the intermediate product after thermal refining treatment is subjected to machining into a component shape by cutting or piercing or the like. A surface hardening heat treatment such as high frequency quenching is performed on the machined intermediate product. After the surface hardening heat treatment, the intermediate product is finished by grinding to produce a steel shaft component. [0004] -2- A steel shaft component to be utilized for the aforementioned uses is required to have excellent fatigue strength. Techniques for increasing the fatigue strength of a machine component that is subjected to high frequency quenching after hot forging are proposed, for example, in Japanese Patent Application Publication No. 2013-7098 (Patent Literature 1), Japanese Patent Application Publication No. 2010-270346 (Patent Literature 2), and Japanese Patent Application Publication No. 2004-137237 (Patent Literature 3). [0005] In Patent Literature 1, a steel material for use in a machine component that is subjected to high frequency quenching after hot forging is proposed. A steel for hot forging disclosed in Patent Literature 1 consists of, by mass%, C: more than 0.30 to less than 0.60%, Si: 0.10 to 0.90%, Mn: 0.50 to 2.0%, P: 0.080% or less, S: 0.010 to 0.10%, Al: more than 0.005 to 0.10%, Cr: 0.01 to 1.0%, Ti: 0.001 to less than 0.040%, Ca: 0.0003 to 0.0040% Te: 0.0003 to less than 0.0040%, N: 0.0030 to 0.020%, and O: 0.0050% or less, with the balance being Fe and impurities, and satisfies the expression Ca/Te > 1.00, and in which the equivalent circular diameter of sulfide inclusions is 20 um or less. [0006] In Patent Literature 2, a steel material for use in a machine component that is subjected to high frequency quenching after hot forging and which is a steel material that is excellent in bending fatigue strength is proposed. A non-heat treated steel for hot forging disclosed in Patent Literature 2 has a chemical composition consisting of, by mass%, C: 0.25 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.60 to 1.80%, P < 0.030%, S < 0.060% Cr: 0.50% or less, Mo: 0.03% or less, V: 0.050 to 0.250% Ti: 0.005 to 0.020%, Al: 0.050% or less, and N: 0.008 to 0.015%, and that satisfies Formula (1), with the balance being Fe and unavoidable impurities, and in which, in the microstructure of the raw steel material after hot forging, a ferrite volume fraction (F%), an average ferrite grain size (um), and a content (%) of V in the steel satisfy Formula (2). Here, Formula (1) is 3.10 < 2.7 x Mn + 4.6 x Cr + V < 5.60, and Formula (2) is 0.04 < ferrite volume fraction (F%) x V/average ferrite grain size (Um) < 0.18. [0007] -3- In Patent Literature 3, a steel material for use in a machine component that is subjected to high frequency quenching after hot forging and which is a steel material excellent in fatigue strength is proposed. A high strength and high workability steel for high frequency quenching disclosed in Patent Literature 3 consists of, by mass%, C: 0.5 to 0.7%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, Cr: 0.4% or less, S: 0.035% or less, V: 0.01 to 0.15%, Al: more than 0.015 to less than 0.050%, and N: more than 0.010 to less than 0.025%, with the balance being Fe and unavoidable impurities, and the steel material is used by performing high frequency quenching of a part of the component after forging. CITATION LIST PATENT LITERATURE [0008] Patent Literature 1: Japanese Patent Application Publication No. 2013-7098 Patent Literature 2: Japanese Patent Application Publication No. 2010-270346 Patent Literature 3: Japanese Patent Application Publication No. 2004-137237 SUMMARY OF INVENTION TECHNICAL PROBLEM [0009] Conventionally, as in the aforementioned Patent Literatures 1 to 3, it has been attempted to improve the fatigue strength of a steel shaft component that is an end product after hot forging and high frequency quenching, by adjusting the chemical composition and microstructure of the steel material that serves as the starting material of the steel shaft component. However, even when the aforementioned steel material is used, in some cases high fatigue strength is not sufficiently obtained in the steel shaft component that is the end product. [0010] An objective of the present disclosure is to provide a steel shaft component that has excellent fatigue strength. SOLUTION TO PROBLEM -4- [0011] A steel shaft component according to the present disclosure includes: one or a plurality of shaft parts, each of the shaft part having a circular shape in a cross section perpendicular to an axial direction, and having a hardened layer having a Vickers hardness of 620 HV or more in an outer layer; wherein: a chemical composition of the steel shaft component consists of, by mass%, 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 a surface of the shaft part is 620 HV or more; in the cross section perpendicular to the axial direction of the shaft part, a Vickers hardness Hb at an R/2 position that corresponds to a center position of a radius R of the shaft part satisfies Formula (1); a microstructure at the R/2 position is composed of ferrite and pearlite; a depth Hr (mm) of the hardened layer having a Vickers hardness of 620 HV or more satisfies Formula (2); and in the hardened layer of the cross section perpendicular to the axial direction of the shaft part, a number of density of V-containing precipitates which contain V and which have an equivalent circular diameter of more than 100 nm is 10 particles/276 um2 or less: Hs/2.3