Gear component and method for manufacturing gear component

Abstract

To provide a gear component excellent in low cycle impact fatigue and wear resistance.SOLUTION: A gear component including a steel core part and a carburized layer is provided. In the gear component, the steel core part has a predetermined chemical composition, the carbon concentration at the bottom surface of the tooth (Crs) is 0.50 to 0.70%, the carbon concentration at a position 0.6 mm from the bottom surface (Cr0.6) is 0.30 to 0.49%, the carbon concentration at the tooth surface (Cfs) is 0.59% or higher, and the carbon concentration at a position 0.6 mm from the tooth surface(Cf0.6) is 0.38% or higher, and the gear component satisfies all of the equations 1 to 3 below. Equation 1: [Si%]+3.3×[Mn%]+ 2.4×[Cr%]+[Mo%]≥4.50 Equation 2: 1.14≤Cfs / Crs≤3.00 Equation 3: 1.18≤Cf0.6 / Cr0.6≤2.00 [Si%], [Mn%], [Cr%], [Mo%] are the contents (mass %) of each element in the steel core part.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to gear components and methods of manufacturing gear components. [Background technology]

[0002] With the trend toward miniaturization of transmissions and e-axles in recent years, gear components are also required to be smaller, and the required strength is becoming stricter. In addition, in the case of e-axles, since the power source is a motor, it is possible to output high torque even at low revolutions, so the impact load received when accelerating suddenly is greater than before. These gear components can be destroyed by a very small number of repeated impacts (low-cycle impact fatigue), such as several tens to several hundred times. Therefore, parts used in these applications are required to have resistance to fatigue damage caused by low-cycle impacts (hereinafter referred to as low-cycle impact fatigue properties). On the other hand, wear resistance is also required because the gear tooth surfaces slide against each other.

[0003] Gear parts that require low-cycle impact fatigue strength and wear resistance are manufactured by subjecting steel to surface hardening treatment. Vacuum carburizing is one type of surface hardening treatment. Vacuum carburizing comprises a carburizing process and a diffusion process. In the carburizing process, a hydrocarbon gas called carburizing gas is introduced to increase the carbon concentration (hereinafter sometimes referred to as "surface carbon concentration" or "surface C concentration") on the surface of steel that has been heated to the carburizing temperature. Examples of hydrocarbon gases include acetylene and propane. In the diffusion process, the introduction of carburizing gas is stopped after the carburizing process, allowing the carbon on the steel surface to diffuse into the interior. The carbon concentration distribution in the steel can be controlled by adjusting the times of the carburizing and diffusion processes.

[0004] In conventional carburized parts, the surface C concentration is generally set to about 0.8%, as disclosed in Patent Document 1, for example. This is because it is believed that if the surface C concentration is less than 0.8%, the hardness of the surface layer of the carburized part decreases and sufficient fatigue strength cannot be obtained. 7High-cycle fatigue strength tests are used. 4 The low-cycle impact fatigue properties have not been fully verified.

[0005] Patent Document 2 discloses a carburized and hardened steel material containing, by mass%, 0.1 to 0.4% C, 0.02 to 1.3% Si, 0.3 to 1.8% Mn, 0.001 to 0.05% Al, and 0.003 to 0.020% N, with the remainder being iron and unavoidable impurities, and having a projected core hardness Hp-core (Hcore: core hardness, t: effective case depth, r: radius of fracture site or half the wall thickness of fracture site) defined by the following formula (1) of HV390 or more. Hp-core=Hcore / (1-t / r) …(1) Patent Document 2 states that this makes it possible to stably improve the low-cycle impact fatigue properties of carburized and quenched steel materials and carburized and quenched parts.

[0006] Patent Document 3 discloses high-strength gears that have been carburized or carbonitrided, in which the effective hardened layer depths of the tooth roots and corner R portions of the tooth roots are controlled to any depth less than 80% of the effective hardened layer depth of the tooth flanks on the pitch circle. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 10-8199 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-332438 [Patent Document 3] Japanese Patent Application Publication No. 2-138554 Summary of the Invention [Problem to be solved by the invention]

[0008] Incidentally, gear parts are constantly sliding when there is an inner ring difference, such as in differential gears, or when a car is turning. As a result, repeated friction can cause parts of the surface of the gear parts to peel off, leading to a fracture known as pitting. Therefore, these gear parts are required to have the above-mentioned low-cycle impact fatigue properties in the tooth roots and wear resistance in the tooth flanks.

[0009] The low-cycle impact fatigue properties of the carburized and quenched parts disclosed in Patent Document 2 have not been fully verified. Furthermore, technology for improving both the low-cycle impact fatigue properties and wear resistance of gear parts has not been fully verified. Therefore, even if the carburized and quenched parts disclosed in Patent Document 2 are used, the low-cycle impact fatigue properties and wear resistance of carburized parts may not be fully achieved.

[0010] Patent Document 3 discloses a high-strength gear that proposes applying an anti-carburizing agent to the relevant areas before carburizing to partially reduce the carbon concentration. However, the process of applying the agent to specific areas during mass production is extremely time-consuming and must be done manually, which could significantly increase costs.

[0011] An object of the present disclosure is to provide a gear component having excellent low-cycle impact fatigue properties and wear resistance, and a method for manufacturing the gear component. [Means for solving the problem]

[0012] <1> A gear component including a steel core and a carburized layer, The steel core portion is, in mass%, C: 0.10~0.30%, Si: 0.40 to 2.00% Mn: 0.30 to 1.40% P: less than 0.030% S: less than 0.030% Cr: 0.80~2.00%, Al: 0.010 to 0.100%, N: 0.0010 to 0.0300%, and O: 0.0030% or less, and the balance being Fe and impurities, The C concentration on the tooth bottom surface is 0.50 to 0.70%. The C concentration at a depth of 0.6 mm from the tooth bottom surface is 0.30 to 0.49%, The C concentration on the tooth flank surface is 0.59% or more, The C concentration at a depth of 0.6 mm from the surface of the tooth flank is 0.38% or more, A gear part that satisfies all of the following formulas 1 to 3. Formula 1: [Si%]+3.3×[Mn%]+2.4×[Cr%]+[Mo%]≧4.50 Formula 2: 1.14≦Cfs / Crs≦3.00 Formula 3: 1.18≦Cf 0.6 / Cr 0.6 ≦2.00 In the above formula, the meanings of the symbols are as follows: [Si%]: Si content of the steel core (mass%) [Mn%]: Mn content of the steel core (mass%) [Cr%]: Cr content in steel core (mass%) [Mo%]: Mo content of the steel core (mass%) Cfs: C concentration on the tooth surface (mass%) Crs: C concentration on the tooth bottom surface (mass%) Cf 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth surface Cr 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth bottom surface <2> A gear component including a steel core and a carburized layer, The steel core portion is, in mass%, C: 0.10~0.30%, Si: 0.40 to 2.00% Mn: 0.30 to 1.40% P: less than 0.030% S: less than 0.030% Cr: 0.80~2.00%, Al: 0.010 to 0.100%, N: 0.0010 to 0.0300%, and O: 0.0030% or less, and further containing one or more elements selected from the group consisting of the following Group A to Group C, with the balance being Fe and impurities, [Group A] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group B] Cu: 0.50% or less, Ni: 0.50% or less, and Sn: 0.015% or less, one or more selected from the group consisting of [Group C] Ca: 0.0100% or less, and Mg: 0.0100% or less, one or two selected from the group consisting of The C concentration on the tooth bottom surface is 0.50 to 0.70%. The C concentration at a depth of 0.6 mm from the tooth bottom surface is 0.30 to 0.49%, The C concentration on the tooth flank surface is 0.59% or more, The C concentration at a depth of 0.6 mm from the surface of the tooth flank is 0.38% or more, A gear part that satisfies all of the following formulas 1 to 3. Formula 1: [Si%]+3.3×[Mn%]+2.4×[Cr%]+[Mo%]≧4.50 Formula 2: 1.14≦Cfs / Crs≦3.00 Formula 3: 1.18≦Cf 0.6 / Cr 0.6 ≦2.00 In the above formula, the meanings of the symbols are as follows: [Si%]: Si content of gear parts (mass%) [Mn%]: Mn content of gear parts (mass%) [Cr%]: Cr content of gear parts (mass%) [Mo%]: Mo content of gear parts (mass%) Cfs: C concentration on the tooth surface (mass%) Crs: C concentration on the tooth bottom surface (mass%) Cf 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth surface Cr 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth bottom surface <3> The chemical composition of the steel core contains the A group <2> The gear component according to claim 1. <4> The chemical composition of the steel core contains the B group <2> or <3> The gear component according to claim 1. <5> The chemical composition of the steel core contains the C group <2> ~ <4> 10. A gear component according to any one of the preceding items. <6> It is a differential gear <1> ~ <5> 10. A gear component according to any one of the preceding items. <7> <1> ~ <6> 10. A method for manufacturing a gear component according to any one of the preceding claims, a carburizing-diffusion treatment step in which carburizing treatment and diffusion treatment are alternately repeated multiple times on a gear-shaped steel material having the chemical composition; A method for manufacturing a gear component, in which the following formulas 4 to 6 are all satisfied, when the final n-th carburizing treatment time and diffusion treatment time are A and B, respectively, and the (n-1)-th carburizing treatment time and diffusion treatment time are C and D, respectively. Formula 4: 10 minutes≦A+C<40 minutes Formula 5: 0.05≦B / D<5.00 Formula 6: 20 minutes≦B+D<200 minutes [Effects of the Invention]

[0013] The present disclosure provides a gear component having excellent low-cycle impact fatigue properties and wear resistance, and a method for manufacturing the gear component. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a diagram showing an example of a heat pattern for vacuum carburizing and quenching treatment. [Figure 2A] FIG. 1 is a plan view of a drop weight test piece prepared in an example. [Figure 2B] 2B is a view of the drop weight test specimen of FIG. 2A as seen from the direction of arrow A. FIG. [Figure 3] FIG. 2 is a plan view of a roller pitching test piece prepared in the examples. [Figure 4] FIG. 1 is a schematic diagram of a roller pitching test. DETAILED DESCRIPTION OF THE INVENTION

[0015] A gear component according to an embodiment of the present disclosure will be described below.

[0016] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. However, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.). With respect to the content of elements in the chemical composition, "%" means "mass %." When the content of an element in the chemical composition is stated as "0~", it means that the element does not have to be contained. In addition, in the numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an Example. Furthermore, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an Example. Furthermore, the term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0017] The inventors of the present disclosure have investigated and studied the low-cycle impact fatigue properties and wear resistance of gear components. As a result, the inventors of the present disclosure have obtained the following findings. In this specification, the low-cycle impact fatigue properties refer to the properties of gear components that can withstand an impact load exceeding the yield stress of the material of 10 to 10 4 This refers to the fatigue strength when subjected to stresses of several times.

[0018] [About the tooth base and tooth surface] In the gear component according to the present disclosure, the tooth root and tooth surface are defined as follows. The recess between the root circle and the base circle of a gear is defined as the "root". The convex portion between the base circle and the tip circle of a gear is defined as the "tooth flank."

[0019] The gear component according to the present disclosure comprises a carburized layer formed by carburizing treatment and a steel core portion located inside the carburized layer. The depth of the carburized layer is not particularly limited, but the depth from the surface of the carburized layer is, for example, 0.7 mm to 2.0 mm. The carburized layer and the steel core portion of the gear component can be distinguished by the presence or absence of a difference in the carbon concentration of the steel material.

[0020] [Chemical composition of the steel core of gear parts] The chemical composition of the steel core portion (sometimes referred to as "core portion" in this specification) of the gear component according to the present disclosure contains the following elements.

[0021] C: 0.10 to 0.30% Carbon (C) improves the hardenability of steel and increases the hardness of the core, thereby improving the low-cycle impact fatigue properties of gear components. If the C content is less than 0.10%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forgeability of the steel may decrease. Therefore, the C content is 0.10 to 0.30%. The preferred lower limit of the C content is 0.15%, more preferably 0.18%. The preferred upper limit of the C content is 0.25%, more preferably 0.23%.

[0022] Si: 0.40 to 2.00% Silicon (Si) deoxidizes steel. Furthermore, Si improves the hardenability of steel, thereby increasing low-cycle impact fatigue strength. It also increases temper softening resistance, thereby increasing pitting strength. However, if the Si content is too low, the above effects cannot be obtained. On the other hand, if the Si content is too high, a soft ferrite phase is formed in the core, reducing low-cycle impact fatigue strength. Therefore, the Si content is set to 0.40 to 2.00%. The lower limit of the Si content is preferably 0.60%, and more preferably 0.80%. The upper limit of the Si content is preferably 1.90%, and more preferably 1.80%.

[0023] Mn: 0.30 to 1.40% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability and strength of steel, and improves the low-cycle impact fatigue properties of gear components. If the Mn content is less than 0.30%, this effect cannot be obtained. On the other hand, if the Mn content exceeds 1.40%, the amount of retained austenite becomes excessive, reducing surface hardness and the wear resistance of the gear components. Therefore, the Mn content is 0.30 to 1.40%. The lower limit of the Mn content is preferably 0.50%, more preferably 0.70%. The upper limit of the Mn content is preferably 1.20%, more preferably 1.00%.

[0024] P: Less than 0.030% Phosphorus (P) is an impurity. P segregates at austenite grain boundaries during carburization, reducing the grain boundary strength of the carburized layer. If the grain boundary strength of the carburized layer decreases, the low-cycle impact fatigue properties will decrease. If the P content is less than 0.030%, the P content is low not only in the core but also in the surface layer. This increases the toughness of the surface layer and suppresses the occurrence of grain boundary cracks. As a result, the low-cycle impact fatigue properties will improve. Therefore, the P content is less than 0.030%. The preferred upper limit of the P content is 0.015%. The lower the P content, the better.

[0025] S: Less than 0.030% Sulfur (S) is an impurity. S remains at the grain boundaries and reduces the grain boundary strength of the carburized layer. S also forms coarse MnS at the grain boundaries, which reduces low-cycle impact fatigue properties. Therefore, the S content is less than 0.030%. The preferred upper limit of the S content is 0.015%. The lower the S content, the better.

[0026] Cr: 0.80~2.00% Chromium (Cr) improves the hardenability of steel, increasing core hardness and improving low-cycle impact fatigue properties. Furthermore, Cr increases temper softening resistance. As a result, the wear resistance of gear components is improved. However, if the Cr content exceeds 2.00%, cementite formation at the grain boundaries of the component edges during vacuum carburizing becomes significant, raising concerns about chipping at the component edges. Therefore, the Cr content is 0.80 to 2.00%. The preferred lower limit of the Cr content is 1.00%. The preferred upper limit of the Cr content is 1.70%.

[0027] Al: 0.010 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, Al bonds with N in steel to form AlN, suppressing coarsening of austenite grains during carburization. Suppressing coarsening of austenite grains maintains a high grain boundary area per volume of steel. Cracks initiate at grain boundaries. Therefore, a high grain boundary area per volume of steel distributes loads. This improves low-cycle impact fatigue properties. If the Al content is less than 0.010%, this effect cannot be achieved. On the other hand, if the Al content exceeds 0.100%, the above effect saturates. Therefore, the Al content is 0.010 to 0.100%. The preferred lower limit of the Al content is 0.020%, more preferably 0.025%. The preferred upper limit of the Al content is 0.080%, more preferably 0.065%. In the chemical composition of the core of the gear component according to the present disclosure, the Al content refers to the total amount of Al contained in the steel.

[0028] N: 0.0010~0.0300% Nitrogen (N) combines with Ti, Al, V, and Nb in steel to form nitrides and carbonitrides, which inhibit the coarsening of austenite grains during carburizing. This improves the low-cycle impact fatigue properties of gear components. If the N content is less than 0.0010%, a sufficient effect of inhibiting coarsening cannot be obtained. On the other hand, if the N content exceeds 0.0300%, the above effect saturates. Therefore, the N content is 0.0010 to 0.0300%. The lower limit of the N content is preferably 0.0050%, more preferably 0.0080%. The upper limit of the N content is preferably 0.0250%, more preferably 0.0200%.

[0029] O: 0.0030% or less Oxygen (O) forms coarse oxide-based inclusions in steel. These coarse oxide-based inclusions act as crack initiation sites, reducing the fatigue strength of mechanical components. If the O content exceeds 0.0030%, the fatigue strength of mechanical components will be significantly reduced even if the contents of other elements are within the ranges of the present disclosure. Therefore, the O content is 0.0030% or less. A preferred upper limit of the O content is 0.0025%, more preferably 0.0020%. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0005%, more preferably 0.0010%.

[0030] The remainder of the chemical composition of the core of the gear component according to the present disclosure is Fe and impurities, which are substances that are mixed in from raw materials such as ore and scrap or from the manufacturing environment during industrial production of the gear component and are acceptable to the extent that they do not adversely affect the gear component according to the present disclosure.

[0031] The core of the gear component according to the present disclosure may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo, Ti, Nb, V, B, Cu, Ni, Sn, Ca, and Mg. These elements are optional, and all of them improve the low-cycle impact fatigue properties or machinability of the gear component.

[0032] Mo: 0.80% or less Molybdenum (Mo) is an optional element and does not necessarily need to be contained. When contained, Mo improves the hardenability of steel, increases core hardness, and improves the low-cycle impact fatigue properties of gear components. Mo also improves the toughness of the carburized layer. Mo also improves the temper softening resistance of gear components. Even if even a small amount of Mo is contained, these effects can be obtained. However, if the Mo content exceeds 0.80%, these effects saturate and the raw material cost increases. Therefore, the Mo content is 0 to 0.80%. To stably obtain the above effects, the lower limit of the Mo content is preferably 0.01%, more preferably 0.10%. The upper limit of the Mo content is preferably 0.60%, more preferably 0.40%.

[0033] Ti:0.100% or less Titanium (Ti) is an optional element and may not be included. If included, Ti combines with C and S in the steel to form fine TiC and TiS, which inhibits coarsening of austenite grains during carburization. This improves the low-cycle impact fatigue properties of gear components. Even a small amount of Ti can achieve this effect. However, if the Ti content exceeds 0.100%, TiC coarsens and the toughness of the steel decreases. In this case, the low-cycle impact fatigue properties of gear components decrease. Therefore, the Ti content is 0.100% or less. To stably obtain the above effects, the lower limit of the Ti content is preferably 0.050%, more preferably 0.060%. The upper limit of the Ti content is preferably 0.080%, more preferably 0.070%.

[0034] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained. When contained, Nb forms precipitates (carbonitrides) and, due to its pinning effect, suppresses the coarsening of austenite grains during carburizing. This improves the low-cycle impact fatigue properties of gear components. Even if even a small amount of Nb is contained, this effect can be obtained. However, if the Nb content exceeds 0.100%, the carbonitrides become coarse and the toughness of the steel decreases. Therefore, the Nb content is 0.100% or less. To stably obtain the above effect, the lower limit of the Nb content is preferably 0.010%, more preferably 0.020%. The upper limit of the Nb content is preferably 0.070%, more preferably 0.050%.

[0035] V: 0.50% or less Vanadium (V) is an optional element and may not be contained. When contained, V forms precipitates (carbides, nitrides, carbonitrides, etc.) and, due to its pinning effect, suppresses coarsening of austenite grains during carburization. This improves the low-cycle impact fatigue properties of gear components. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.50%, the hardness of the steel material becomes excessively high. In this case, the machinability of the steel material decreases. Therefore, the V content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the V content is preferably 0.05% or more, more preferably 0.10% or more. The upper limit of the V content is preferably 0.40% or less, more preferably 0.30% or less.

[0036] B: 0.0100% or less Boron (B) is an optional element and does not necessarily need to be contained. When contained, B improves the hardenability of steel, increasing the core hardness and grain boundary strength of carburized steel parts manufactured using the steel as a raw material. This improves the low-cycle impact fatigue properties of gear parts. Even if even a small amount of B is contained, the above effects can be obtained to a certain extent. However, if the B content exceeds 0.0100%, the effect saturates. Therefore, the B content is 0.0100% or less. The preferred lower limit of the B content is 0.0010% or more, more preferably 0.0020% or more, and even more preferably 0.0030% or more. The preferred upper limit of the B content is 0.0090% or less, and even more preferably 0.0080% or less.

[0037] Cu: 0.50% or less Copper (Cu) is an optional element and does not necessarily need to be contained. When contained, Cu improves the hardenability of the steel, increases the core hardness, and improves the low-cycle impact fatigue properties of gear components. This effect can be achieved even if even a small amount of Cu is contained. On the other hand, if the Cu content exceeds 0.50%, hot workability decreases. Therefore, the Cu content is 0.50% or less. To stably obtain the above effects, the lower limit of the Cu content is preferably 0.10%, more preferably 0.15%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.25%.

[0038] Ni: 0.50% or less Nickel (Ni) is an optional element and does not necessarily need to be contained. When contained, Ni improves the hardenability of the steel, increasing the core hardness, and further improves the toughness of the carburized layer, thereby improving the low-cycle impact fatigue properties of gear components. Even if even a small amount of Ni is contained, these effects can be obtained. However, if the Ni content exceeds 0.50%, the amount of retained austenite increases and workability decreases. Therefore, the Ni content is 0.50% or less. To stably obtain the above effects, the preferred lower limit of the Ni content is 0.05%, more preferably 0.10%. The preferred upper limit of the Ni content is 0.45%, more preferably 0.40%.

[0039] Sn: 0.015% or less Tin (Sn) is an optional element and does not necessarily need to be contained. When contained, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.015%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges according to the present disclosure. Therefore, the Sn content is 0.015% or less. The preferred lower limit of the Sn content is 0.001% or more, and more preferably 0.005% or more. The preferred upper limit of the Sn content is 0.013% or less, and more preferably 0.010% or less.

[0040] Ca:0.0100% or less Calcium (Ca) is an optional element and does not necessarily need to be contained. When contained, Ca modifies sulfides in the steel material and inhibits the elongation of the sulfides during hot working. This improves the low-cycle impact fatigue properties of gear components. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0100%, the above effects saturate. Therefore, the Ca content is 0.0100% or less. The preferred lower limit of the Ca content is 0.0001% or more, and more preferably 0.0002% or more. The preferred upper limit of the Ca content is 0.0075% or less, and more preferably 0.0050% or less.

[0041] Mg: 0.0100% or less Magnesium (Mg) is an optional element and does not necessarily need to be contained. When contained, Mg modifies sulfides in the steel material and suppresses the elongation of the sulfides during hot working. This improves the low-cycle impact fatigue properties of gear components. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0100%, the above effects saturate. Therefore, the Mg content is 0.0100% or less. The preferred lower limit of the Mg content is 0.0010% or more, and more preferably 0.0020% or more. The preferred upper limit of the Mg content is 0.0075% or less, and more preferably 0.0050% or less.

[0042] [C concentration on the tooth root surface of gear components] C concentration on the tooth root surface of gear parts: 0.50 to 0.70% The C concentration at the tooth root surface of a gear component (hereinafter referred to as tooth root surface C concentration) is 0.50 to 0.70% by mass. If the tooth root surface C concentration exceeds 0.70%, the toughness of the carburized layer decreases, resulting in a shorter crack initiation life in a low-cycle impact fatigue test. On the other hand, if the tooth root surface C concentration is less than 0.50%, the surface hardness of the gear component is too low, resulting in a lower plastic deformation resistance. In this case, the low-cycle impact fatigue properties deteriorate. If the tooth root surface C concentration is 0.50 to 0.70%, excellent low-cycle impact fatigue properties can be obtained. The preferred lower limit of the tooth root surface C concentration is 0.54%, more preferably 0.56%. The preferred upper limit of the tooth root surface C concentration is 0.66%, more preferably 0.64%.

[0043] [C concentration at a depth of 0.6 mm from the tooth root surface of a gear component] C concentration at a depth of 0.6 mm from the surface of the tooth root of a gear component: 0.30 to 0.49% The C concentration at a depth of 0.6 mm from the tooth bottom surface of a gear component (hereinafter referred to as the C concentration at a depth of 0.6 mm at the tooth bottom) is 0.30 to 0.49% by mass. If the C concentration at a depth of 0.6 mm at the tooth bottom exceeds 0.49%, the crack length when an initial crack occurs due to low-cycle fatigue becomes too long, resulting in a reduced fracture life in a low-cycle impact fatigue test. On the other hand, if the C concentration at a depth of 0.6 mm at the tooth bottom is less than 0.30%, the hardened region of the gear component is too small, reducing its plastic deformation resistance. In this case, the low-cycle impact fatigue properties are reduced. If the C concentration at a depth of 0.6 mm at the tooth bottom is 0.30 to 0.49%, excellent low-cycle impact fatigue properties are obtained. The lower limit of the C concentration at a depth of 0.6 mm at the tooth bottom is preferably 0.32%, more preferably 0.34%. The upper limit of the C concentration at a depth of 0.6 mm at the tooth bottom is preferably 0.47%, more preferably 0.45%.

[0044] [C concentration on the tooth surface of gear parts] C concentration on the tooth surface of gear parts: 0.59% or more The C concentration on the tooth flank surface of a gear component (hereinafter referred to as tooth flank surface C concentration) is 0.59% or more by mass. If the tooth flank surface C concentration is less than 0.59%, the surface hardness of the gear component is too low, resulting in reduced wear resistance. The lower limit of the tooth flank surface C concentration is preferably 0.61%, and more preferably 0.63%. On the other hand, although there is no particular upper limit for the C concentration on the tooth flank surface, if it exceeds 1.50%, the toughness of the carburized layer decreases, resulting in a shortened crack initiation life in low-cycle impact fatigue tests. The upper limit of the C concentration on the tooth flank surface is preferably 1.40%, and more preferably 1.30%.

[0045] [C concentration at a depth of 0.6 mm from the tooth flank surface of gear parts] C concentration at a depth of 0.6 mm from the tooth surface of gear parts: 0.38% or more The C concentration at a depth of 0.6 mm from the tooth flank surface of a gear component (hereinafter referred to as the C concentration at a depth of 0.6 mm from the tooth flank) is 0.38% or more by mass. If the C concentration at a depth of 0.6 mm from the tooth flank is less than 0.38%, the hardened region of the gear component is too small, resulting in reduced wear resistance. The lower limit of the C concentration at a depth of 0.6 mm from the tooth flank is preferably 0.40%, and more preferably 0.42%. On the other hand, although there is no particular upper limit for the C concentration at a depth of 0.6 mm from the tooth surface, if it exceeds 1.20%, the crack length when an initial crack occurs due to low cycle fatigue becomes too long, resulting in a decrease in fracture life in a low cycle impact fatigue test. A preferred upper limit for the C concentration at a depth of 0.6 mm from the tooth surface is 1.10%, and more preferably 1.00%.

[0046] The carbon concentration of a gear component is measured using the following method. Three or more measurement positions are selected on the tooth root and tooth surface of the gear component. The gear component is cut perpendicular to the rotation axis, and three measurement positions are selected on each of the tooth root and tooth surface. At each measurement position, the carbon concentration (mass%) is measured at three points, 30 μm and 0.6 mm deep, in the vertical direction from the surface, with measurement intervals of 3 μm, using an electron probe microanalyzer (EPMA). The carbon concentration at a depth of 30 μm is defined as the surface carbon concentration. The EPMA measurement is performed using an acceleration voltage of 15 kV, a probe current of 500 nA, and a beam diameter of 3 μm. The average carbon concentration at the tooth root and tooth surface obtained by EPMA is defined as the carbon concentration (mass%) of the gear component's tooth root and tooth surface.

[0047] The gear component according to the present disclosure satisfies the following formulas 1 to 3 in addition to the above chemical composition and C concentrations in the tooth bottom and tooth flank.

[0048] Formula 1: [Si%]+3.3×[Mn%]+2.4×[Cr%]+[Mo%]≧4.50 In Equation 1, [Si%], [Mn%], [Cr%], and [Mo%] represent the Si, Mn, Cr, and Mo contents, respectively, in mass percent, of the steel core. The alloying elements included in Equation 1 are components that increase the ease of carburization (hereinafter referred to as carburization resistance) during vacuum carburization. Therefore, if the value of [Si%] + 3.3 × [Mn%] + 2.4 × [Cr%] + [Mo%] is lower than Equation 1, i.e., if the carburization resistance is less than 4.50, it is difficult to increase the carbon concentration of the gear component, making it difficult to achieve the desired carbon concentration distribution economically. The preferred lower limit of the value of [Si%] + 3.3 × [Mn%] + 2.4 × [Cr%] + [Mo%] is 5.0, and even more preferably 5.5. The preferred upper limit is 10.00, and even more preferably 9.50.

[0049] Formula 2: 1.14≦Cfs / Crs≦3.00 Equation 2 is the surface carbon concentration on the tooth surface (Cfs) divided by the surface carbon concentration on the tooth root (Crs). If this ratio is below 1.14 (the surface carbon concentration on the tooth surface is low), the wear resistance will be low, and the gear component cannot be said to have both low-cycle fatigue strength and wear resistance. On the other hand, if it exceeds 3.00 (the surface carbon concentration on the tooth root is significantly low), the low-cycle fatigue strength will be low, and the gear component cannot be said to have both low-cycle impact fatigue strength and wear resistance. The preferred lower limit of Cfs / Crs is 1.16, and the more preferred lower limit is 1.18. The preferred upper limit is 2.70, and the even more preferred upper limit is 2.40.

[0050] Formula 3: 1.18≦Cf 0.6 / Cr 0.6 ≦2.00 Surface carbon concentration at a depth of 0.6 mm on the tooth surface (Cf 0.6 ) is the surface carbon concentration (Cr 0.6 ) is Equation 3, and if this is below 1.18 (the hardened area on the tooth surface is small), the wear resistance will be low, and the gear component cannot be said to have both low cycle fatigue strength and wear resistance. On the other hand, if it is above 2.00 (the hardened area on the tooth bottom is small), the low cycle fatigue strength will be low, and the gear component cannot be said to have both low cycle impact fatigue strength and wear resistance. Cf 0.6 / Cr 0.6 The preferred lower limit is 1.20, and the more preferred lower limit is 1.22. The preferred upper limit is 1.80, and the more preferred upper limit is 1.60.

[0051] [Manufacturing method for gear parts] Although the manufacturing method of the gear component according to the present disclosure is not particularly limited, the gear component can be suitably manufactured by a vacuum carburizing process. A vacuum carburizing process suitable for manufacturing the gear component according to the present disclosure will be described in detail below.

[0052] The gear component according to the present disclosure can be manufactured by the vacuum carburizing method. An example of a manufacturing process for the gear component according to the present disclosure, which includes the vacuum carburizing method, is as follows.

[0053] [Preparation process] The steel material to be carburized is prepared. For example, the steel material is manufactured and prepared by the following method. Molten steel having the aforementioned chemical composition is produced, and a cast piece (slab or bloom) is produced using the molten steel by a continuous casting method. The molten steel may be used to produce an ingot by an ingot casting method. The cast piece or ingot is hot worked to produce a billet. The billet is hot worked to produce a steel bar or wire rod. The hot working may be hot rolling or hot forging.

[0054] The produced steel material is then forged. The forging method may be hot forging or cold forging. Mechanical processing, typically cutting, is performed on the forged steel material as needed to produce the steel material to be vacuum carburized. Steel material produced by other manufacturers may also be used. In other words, the manufacturer that performs the carburizing method of the present disclosure does not need to produce the steel material.

[0055] [Heating process] The produced steel material is subjected to a heating step in which the produced steel material is placed in a vacuum carburizing furnace and heated to a carburizing temperature.

[0056] The carburizing temperature is preferably 900 to 1130°C. If the carburizing temperature is 900°C or higher, heat transfer by radiation increases and the temperature inside the vacuum carburizing furnace becomes uniform. As a result, carburizing variation is reduced. If the carburizing temperature is 1130°C or lower, the grain size of the steel can be prevented from becoming coarse, and a decrease in low-cycle impact fatigue strength can be suppressed.

[0057] It is preferable to keep the pressure inside the vacuum carburizing furnace low. Low pressure means, for example, 10 Pa or less. If the pressure inside the vacuum carburizing furnace is low, the frequency of collisions between carburizing gas molecules decreases. In other words, the frequency of decomposition of the carburizing gas in the atmosphere decreases. Therefore, if the gas is sprayed onto the steel surface as quickly as possible at low pressure, the generation of soot and tar can be suppressed. As a result, the surface carbon concentration of the steel can be increased quickly.

[0058] [Soaking process] The heated steel material is subjected to a soaking step in which the steel material is soaked at the carburizing temperature.

[0059] The soaking time is adjusted appropriately depending on the size of the steel material, and is preferably 30 minutes or more per 25 mm diameter steel bar, calculated as the size of the steel material in terms of a round bar.

[0060] The pressure inside the furnace during the soaking step is not particularly limited, and may be 100 Pa or less.

[0061] [Carburizing and diffusion treatment process] Carburizing gas is introduced into a vacuum carburizing furnace, and the soaked steel undergoes vacuum carburizing. Figure 1 shows an example of a heat pattern for vacuum carburizing and quenching in this disclosure. The vacuum carburizing process consists of multiple carburizing and diffusion processes. This is because it is difficult to create a difference in the carbon concentration distribution between the tooth surface and the tooth root of a gear with a single carburizing and diffusion process, making it impossible to achieve both excellent low-cycle fatigue strength and excellent wear resistance. The carburizing and diffusion processes may be repeated any number of times, provided that each is two or more times, but the diffusion process must always be performed after the carburizing process. Preferably, the carburizing and diffusion processes are performed alternately three times each.

[0062] (Carburizing time) The carburizing time is determined by the target carbon concentration distribution. Alternatively, a vacuum carburizing test can be conducted to determine the carburizing time to achieve the target carbon concentration distribution. The final carburizing time, C, for the nth carburizing step and A for the n-1th carburizing step are particularly important. If the carburizing time is too long, the surface carbon concentration may become too high, resulting in a decrease in low-cycle impact fatigue strength. On the other hand, if the carburizing time is too short, the carbon concentration may become too inconsistent and the surface carbon concentration of the tooth surface may become insufficient, resulting in a decrease in wear resistance. Therefore, the total carburizing time, A+C, for the n-1th and nth carburizing steps should be between 10 and 40 minutes, i.e., satisfy the following formula 4: Formula 4: 10 minutes≦A+C<40 minutes The preferred lower limit of the total time of the (n-1)th and nth carburizations is 11 minutes, and the preferred upper limit is 35 minutes.

[0063] Furthermore, if the n-th carburization time C is too long compared to the (n-1)th carburization time A, it becomes difficult to achieve both excellent low-cycle fatigue strength and excellent wear resistance. Therefore, it is preferable to satisfy the following formula 7. Formula 7:0.5≦A / C

[0064] Furthermore, if the nth carburizing time D is extremely short compared to the nth carburizing time C, there is a risk of reduced wear resistance, and if the carburizing time C is longer than the diffusion time D, there is an increased risk of chipping at the edges of the gear component. Therefore, it is preferable to satisfy the following formula 8: Formula 8: 0.02≦C / D≦1

[0065] (diffusion time) In the diffusion process, the steel is held at the carburizing temperature for a specified time. Like the carburizing time, the holding time in the diffusion process is determined by the target carbon concentration distribution. The final nth diffusion time, D, and the n-1th diffusion time, B, are particularly important. A longer holding time in the diffusion process reduces the surface carbon concentration, thereby improving low-cycle impact fatigue strength. If the holding time is too short, the risk of chipping at the edges of gear components increases. On the other hand, if the holding time is too long, the surface carbon concentration on the tooth surface decreases, potentially reducing wear resistance. Therefore, the sum of the n-1th and nth diffusion times, B+D, should be between 20 and 200 minutes, i.e., satisfy the following formula 6: Formula 6: 20 minutes≦B+D<200 minutes The preferred lower limit of the sum (B+D) of the diffusion times of the (n-1)th and nth times is 40 minutes, and the preferred upper limit is 180 minutes.

[0066] Furthermore, if the (n-1)th diffusion time B is extremely short compared to the nth diffusion time D, it becomes difficult to achieve both excellent low-cycle fatigue strength and excellent wear resistance, and if it is extremely long, the possibility of chipping at the edge of the gear component increases. Therefore, the following formula 5 is satisfied. Formula 5: 0.05≦B / D<5.00

[0067] [Cooling process] The steel material may be cooled after the carburizing and diffusion treatment process. When cooling is performed before quenching, the cooling rate is not particularly limited. However, a higher cooling rate is preferable because it has the effect of shortening the treatment time. The upper limit of the cooling rate is 30°C / sec, and the lower limit is preferably 0.02°C / sec. The cooling rate here is the difference between the carburizing temperature and the holding temperature before quenching divided by the time required for cooling. When gas cooling is used, the time required for cooling is the time spent circulating gas using a cooling fan after transporting the material to a location separate from the carburizing chamber, excluding the time required for transport.

[0068] [Quenching process] The steel material after the carburizing and diffusion treatment process or the steel material after the cooling process is reheated to the quenching temperature and subjected to the quenching process. The quenching temperature is not particularly limited, but a higher temperature is preferable. In this case, the steel material can be quenched from the state where the interior is in the γ phase. When quenching with oil, a lower quenching temperature is preferable. In this case, heat treatment distortion can be reduced. Therefore, the upper limit of the quenching temperature is preferably 1130°C, and more preferably 880°C. The lower limit of the quenching temperature is preferably 800°C.

[0069] A known method can be used as the cooling method in the quenching process. For example, the cooling method in the quenching process may be water quenching, gas cooling, or other methods. When gas cooling is used for cooling, it is preferable to use an inert gas as the cooling gas. For example, nitrogen gas or helium gas is preferable as the inert gas. In particular, nitrogen gas, which is inexpensive and easily available, is preferable as the inert gas. By using an inert gas as the cooling gas, oxidation of the steel material can be prevented.

[0070] After the quenching treatment, tempering treatment may be carried out by a known method.

[0071] Through the above steps, the gear component according to the present disclosure is manufactured using the vacuum carburizing method.

[0072] [Application] The gear component according to the present disclosure is not limited to any particular application, but is suitable for use as a differential gear due to its excellent low-cycle impact fatigue properties and wear resistance. [Example]

[0073] Examples of the gear component according to the present disclosure will be described below. Note that the following examples do not limit the scope of the gear component according to the present disclosure.

[0074] Steel bars (diameter: 35 mm) were prepared having the chemical compositions shown in Table 1. Underlines in Table 1 indicate compositions outside the scope of the present disclosure, and blank spaces indicate that the content of the corresponding element is 0% in the significant figures (numbers down to the least significant digit) described above. For example, a blank space in the Mo column means that the Mo content is 0.00%. The remainder is Fe and impurities.

[0075] Each steel bar was machined into a drop weight test specimen having a shape simulating the tooth root of a gear component shown in Figures 2A and 2B, and a roller pitting test specimen having a shape simulating the tooth flank of a gear component shown in Figure 3, and then subjected to a carburizing diffusion treatment under the carburizing conditions shown in Table 2. After the carburizing and diffusion treatment, the following carbon concentrations were measured by the method described above. Cfs: C concentration on the tooth surface (mass%) Crs: C concentration on the tooth bottom surface (mass%) Cf 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth surface Cr 0.6 : C concentration (mass%) at a depth of 0.6 mm from the tooth bottom surface In Table 1, "carburization resistance" is a value calculated by [Si%] + 3.3 × [Mn%] + 2.4 × [Cr%] + [Mo%].

[0076] [Table 1]

[0077] [Table 2]

[0078] [Evaluation test] (Low cycle impact fatigue test) The steel bar before the carburizing and diffusion treatment was machined to prepare a drop weight test specimen with a shape simulating the tooth root of the gear component shown in Figures 2A and 2B. The numbers in Figures 2A and 2B indicate dimensions in mm. "R2" indicates that the radius of curvature of the R portion is 2 mm. The thickness of the drop weight test specimen was 14 mm at the thick part, 10 mm at the thin part, and 20 mm wide. A low-cycle impact fatigue test was performed using a drop-weight impact fatigue tester on test pieces with test numbers each having a shape simulating the tooth root of a gear component as shown in Figures 2A and 2B. Specifically, a 61 kg weight was allowed to fall freely from a specified range of heights (20 to 80 mm) to impact the drop-weight test piece at a position 10 mm from the end of the 10 mm thick side, applying an impact stress load. The white arrow in Figure 2A indicates the direction of impact of the weight. This impact was repeated, and the stress at which the drop-weight test piece broke after the 100th stress load (referred to as the 100-cycle fracture strength) was determined. Test pieces with a 100-cycle breaking strength of 3400 MPa or more were rated A, test pieces with a 100-cycle breaking strength of 3100 MPa to less than 3400 MPa were rated B, test pieces with a 100-cycle breaking strength of 2800 to less than 3100 MPa were rated C, and test pieces with a 100-cycle breaking strength of less than 2800 MPa were rated ×. Test pieces rated A to C were judged to have excellent low-cycle impact fatigue properties. Test pieces rated × were judged to have poor low-cycle impact fatigue properties. The results are shown in Table 3 under "Low Cy fatigue."

[0079] (Wear resistance evaluation test) A 50 mm diameter steel bar before carburizing and diffusion treatment was machined to prepare a roller pitting test specimen having the shape shown in Figure 3. The roller pitting test specimen was cylindrical and had a parallel section in the center with a diameter of 26 mm. The diameter of the roller pitting test specimen other than the parallel section was 22 mm. The roller pitting test specimen was the small roller 200 used in the roller pitting test described below. A wear resistance test was carried out on the test pieces having the respective test numbers each having a shape simulating the tooth surface of such a gear component. Specifically, the test was carried out under the following conditions using a two-disk rolling tester manufactured by Komatsu Engineering Co., Ltd. Slip rate: -40% Lubricant: Automatic transmission oil Lubricant temperature: 90℃ Lubricant flow rate: 2L / min Rotation speed: 1500 rpm Surface pressure: 2000MPa

[0080] As shown in Figure 4, the large roller 100 was pressed against the small roller 200 with the above-mentioned surface pressure while the small roller 200 was rotated. The small roller 200 was a roller pitting test specimen prepared in the above-mentioned test specimen preparation. The large roller 100 was made of steel meeting the SCM420 standard of JIS G 4053 (2016), and was subjected to low-temperature tempering after eutectoid carburization and surface polishing. The radius of the large roller 100 was 130 mm. The rotation speed was 1 x 10 6 The wear depth Dw of each test piece was measured after each run. A stylus-type surface roughness tester was used to measure the wear depth Dw. The measurement length was set to 24 mm, and the stylus was scanned in the axial direction of each test piece to obtain a cross-sectional curve.

[0081] Measurements were performed at two locations every 180° in the circumferential direction for each test piece to obtain a profile curve. From the obtained profile curve, the average height of the profile curve elements in the portion of each test piece that was not in contact with the large roller 100 and the average height of the profile curve elements in the portion that was in contact with the large roller 100 and was worn away were calculated. The difference in height between the portion that was not in contact with the large roller 100 and the portion that was in contact with the large roller 100 was then calculated. The average value of the obtained height differences at the measurement locations was taken as the wear depth Dw (μm) for each test number. Test pieces with a wear depth Dw of less than 20 μm were rated A, test pieces with a wear depth Dw of 20 to less than 40 μm were rated B, test pieces with a wear depth Dw of 40 to less than 60 μm were rated C, test pieces with a wear depth Dw of 60 to less than 80 μm were rated D, and test pieces with a wear depth Dw of 80 μm or more were rated ×. In the cases of evaluations A to D, it was judged that the abrasion resistance was excellent. In the case of evaluation x, it was judged that the abrasion resistance was poor. The results are shown in Table 3 under "Abrasion Resistance."

[0082] (Edge chipping) Test pieces with a shape simulating the edge of a gear component were evaluated for the presence or absence of chipping at the edge under each carburizing condition. The shape of the test piece was a square with a 90° edge. If there was no chipping at the edge after vacuum carburizing and tempering, it was rated as ◯, as it was deemed that there was a low possibility of chipping at the edge of a gear component. If there was chipping at the edge, it was rated as ×, as it was deemed that there was a high possibility of chipping at the edge of a gear component. The results are shown in Table 3 under "Edge θ (90 degrees)".

[0083] [Table 3]

[0084] As shown in Table 3, the comparative test pieces that did not meet the requirements of the present disclosure were rated "x" in at least one of the three evaluations. On the other hand, the test pieces of the examples simulating gear components that meet the requirements of the present disclosure did not receive any "x" marks in the three evaluations, indicating that they had excellent low-cycle impact fatigue properties and wear resistance.

Claims

1. A gear component including a steel core and a carburized layer, The steel core portion comprises, in mass %, C: 0.10-0.30%, Si: 0.40-2.00%, Mn: 0.30-1.40%, P: less than 0.030% S: less than 0.030% Cr: 0.80-2.00%, Al: 0.010-0.100%, N: 0.0010 to 0.0300%, and O: 0.0030% or less, and the balance being Fe and impurities, The C concentration at the tooth bottom surface is 0.50 to 0.70%; the C concentration at a depth of 0.6 mm from the tooth bottom surface is 0.30 to 0.49%, The C concentration on the tooth flank surface is 0.59% or more, the C concentration at a position 0.6 mm deep from the surface of the tooth flank is 0.38% or more, A gear part that satisfies all of the following formulas 1 to 3. Formula 1: [Si%]+3.3×[Mn%]+2.4×[Cr%]+[Mo%]≧4.50 Formula 2: 1.14≦Cfs / Crs≦3.00 Equation 3: 1.18≦Cf ０．６ / Cr ０．６ ≤2.00 In the above formula, the meanings of the symbols are as follows: [Si%]: Si content of the steel core (mass%) [Mn%]: Mn content of the steel core (mass%) [Cr%]: Cr content in steel core (mass%) [Mo%]: Mo content of the steel core (mass%) Cfs: C concentration on the tooth flank surface (mass%) Crs: C concentration on the tooth bottom surface (mass%) Cf ０．６ : C concentration (mass%) at a position 0.6 mm deep from the tooth flank surface Cr ０．６ : C concentration (mass%) at a position 0.6 mm deep from the tooth bottom surface

2. A gear component including a steel core and a carburized layer, The steel core portion comprises, in mass %, C: 0.10-0.30%, Si: 0.40-2.00%, Mn: 0.30-1.40%, P: less than 0.030% S: less than 0.030% Cr: 0.80-2.00%, Al: 0.010-0.100%, N: 0.0010 to 0.0300%, and O: 0.0030% or less, and further containing one or more elements selected from the group consisting of the following Group A to Group C, with the balance being Fe and impurities: [Group A] Mo: 0.80% or less, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group B] Cu: 0.50% or less, Ni: 0.50% or less, and Sn: 0.015% or less, one or more selected from the group consisting of [Group C] Ca: 0.0100% or less, and Mg: 0.0100% or less, one or two selected from the group consisting of The C concentration at the tooth bottom surface is 0.50 to 0.70%; the C concentration at a depth of 0.6 mm from the tooth bottom surface is 0.30 to 0.49%, The C concentration on the tooth flank surface is 0.59% or more, the C concentration at a position 0.6 mm deep from the surface of the tooth flank is 0.38% or more, A gear part that satisfies all of the following formulas 1 to 3. Formula 1: [Si%]+3.3×[Mn%]+2.4×[Cr%]+[Mo%]≧4.50 Formula 2: 1.14≦Cfs / Crs≦3.00 Equation 3: 1.18≦Cf ０．６ / Cr ０．６ ≤2.00 In the above formula, the meanings of the symbols are as follows: [Si%]: Si content of the steel core (mass%) [Mn%]: Mn content of the steel core (mass%) [Cr%]: Cr content in steel core (mass%) [Mo%]: Mo content of the steel core (mass%) Cfs: C concentration on the tooth flank surface (mass%) Crs: C concentration on the tooth bottom surface (mass%) Cf ０．６ : C concentration (mass%) at a position 0.6 mm deep from the tooth flank surface Cr ０．６ : C concentration (mass%) at a position 0.6 mm deep from the tooth bottom surface

3. 3. The gear component according to claim 2, wherein the chemical composition of the steel core contains the group A elements.

4. 3. The gear component according to claim 2, wherein the chemical composition of the steel core contains the B group.

5. 3. The gear component according to claim 2, wherein the chemical composition of the steel core contains the C group.

6. The gear component according to any one of claims 1 to 5, which is a differential gear.

7. The method for manufacturing a gear component according to any one of claims 1 to 6, a carburizing-diffusion treatment step in which carburizing treatment and diffusion treatment are alternately repeated multiple times on a gear-shaped steel material having the chemical composition; A method for manufacturing a gear component, in which the following formulas 4 to 6 are all satisfied, when the carburizing treatment time and diffusion treatment time for the nth final stage are A and B, respectively, and the carburizing treatment time and diffusion treatment time for the (n-1)th final stage are C and D, respectively. Formula 4: 10 minutes≦A+C<40 minutes Formula 5: 0.05≦B / D<5.00 Formula 6: 20 minutes≦B+D<200 minutes

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