gear

Abstract

To provide a gear having excellent durability.SOLUTION: A gear includes a carburized layer 10 that forms its surface, and a core located inside the carburized layer. The carburized layer 10 includes grain boundary oxidation 14, an incomplete quenched structure 16 and a granular oxide 18. The grain boundary oxidation 14 has a depth Dp of 15 μm or less. The grain boundary oxidation 14 has an average pitch AP of 20 μm or less. The incomplete quenched structure 16 has an average width AW of 4.0 μm or less. A coverage PC of the incomplete quenched structure 16 on the surface 12 is 80% or more. A softening level ST of the gear in a tempered softening test at 300°C is 115 HV or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] This specification discloses a gear. Specifically, this specification discloses a gear having a carburized layer on its surface.

Background Art

[0002] The transmission of a vehicle has a plurality of gears. High-viscosity oil is used as a lubricant in the transmission of a truck (goods vehicle). This high-viscosity oil forms an oil film between gears. With this high-viscosity oil, fluid lubrication can be achieved in the transmission. The high-viscosity oil suppresses the load on the surface of the gear. The high-viscosity oil can suppress pitting peeling in the gear. <00000"12>

[0003] Improvements in the material of gears for the purpose of suppressing pitting peeling have been proposed. Japanese Patent Application Laid-Open No. 2016-222982 discloses a gear subjected to carburizing treatment. In this gear, the depth of grain boundary oxidation and the depth of the incomplete quenched layer are set within a predetermined range.

[0004] Japanese Patent Application Laid-Open No. 2021-055121 also proposes improvements in the material of gears for the purpose of suppressing pitting peeling. This gear is subjected to carburizing treatment. In this gear, the hardness, carbon concentration, and amount of retained austenite in the vicinity of the surface are set within a predetermined range.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] The durability of conventional gears is insufficient. In particular, improving gear durability is urgently needed in transmissions and other applications where low-viscosity oils are used as lubricants to improve vehicle fuel efficiency. This is because the use of low-viscosity oils induces boundary lubrication and mixed lubrication, which can lead to pitting and gear separation. In boundary lubrication, metals come into direct contact with each other. In mixed lubrication, fluid lubrication and boundary lubrication coexist.

[0007] The applicant's intention is to provide a gear with superior durability. [Means for solving the problem]

[0008] The gear according to this embodiment has a carburized layer forming the surface of the gear and a core located inside the carburized layer. The carburized layer includes multiple grain boundary oxidations extending inward from the surface and an incompletely hardened structure located near the surface. The depth Dp of the grain boundary oxidation is 15 μm or less. The average pitch AP of the grain boundary oxidation is 20 μm or less. The average width AW of the incompletely hardened structure is 4.0 μm or less. The coverage PC of the incompletely hardened structure on the surface is 80% or more. The amount of softening in a tempering softening test performed at a temperature of 300°C of this gear is 115 HV or less.

[0009] The material of this gear is hardened steel. Preferably, the core is C: 0.14% by mass or more and 0.45% by mass or less, Si: 0.05% by mass or more and 1.00% by mass or less, Mn: 0.10 mass% or more and 0.90 mass% or less, Cr: 1.30 mass% or more and 3.50 mass% or less, Al: 0.020 mass% or more and 0.200 mass% or less, N: 0.0040 mass% or more and 0.0300 mass% or less, Mo: 2.00% by mass or less, Ni: 2.00% by mass or less, Nb: 0.10% by mass or less, Ti: 0.200% by mass or less, B: 0.0050% by mass or less, and V: 0.500 mass% or less and contains the balance of Fe and impurities.

[0010] Preferably, the core part contains Mo: 0.02 mass% or more and 2.00 mass% or less, and Ni: 0.02 mass% or more and 2.00 mass% or less and contains at least one of them.

[0011] Preferably, the core part contains Nb: 0.02 mass% or more and 0.10 mass% or less, Ti: 0.020 mass% or more and 0.200 mass% or less, B: 0.0010 mass% or more and 0.0050 mass% or less, and V: 0.010 mass% or more and 0.500 mass% or less and contains at least one of them.

Advantages of the Invention

[0012] This gear is excellent in durability.

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a perspective view showing a gear according to an embodiment. [Figure 2] FIG. 2 is an enlarged cross-sectional view schematically showing a part of the gear of FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view showing a further enlarged part of the gear of FIG. 2. [Figure 4] FIG. 4 is a cross-sectional view showing a further enlarged part of the gear of FIG. 2. [Figure 5] FIG. 5 is a perspective view showing a small roller test piece for a roller pitching test. [Figure 6] FIG. 6 is a perspective view showing a small roller test piece and a large roller test piece for a roller pitching test. [Modes for carrying out the invention]

[0014] Preferred embodiments will be described below, with reference to drawings as appropriate.

[0015] The gear 2 shown in Figure 1 has a base 4 and a number of teeth 6. The base 4 has a disc shape. Each tooth 6 protrudes from the outer surface of the base 4. This tooth 6 is integral with the base 4. There is a groove 8 between a tooth 6 and another adjacent tooth 6.

[0016] The material of gear 2 is hardened steel, which will be explained in detail later. Gear 2 is obtained by subjecting a gear intermediate, which is obtained by plastic deformation, to a carburizing treatment. In the carburizing treatment, this intermediate is subjected to carburizing, quenching, and tempering, which will be explained in detail later.

[0017] In carburizing, the gear intermediate is held in a high-temperature, carbon-rich atmosphere. Carbon atoms in the atmosphere penetrate the intermediate and gradually diffuse inward. This penetration and diffusion forms a layer with a high carbon content on the surface of the intermediate. This intermediate is then subjected to quenching. In quenching, the intermediate is rapidly cooled. Rapid cooling causes martensitic transformation on the surface of the intermediate. This intermediate is then tempered.

[0018] These processes form a carburized layer, which forms the surface of gear 2. Gear 2 also has a core located inside this carburized layer. The composition of this core is substantially the same as that of the intermediate. The carbon content of the carburized layer is greater than that of the core. The carburized layer is high hardness, while the core is low hardness. In gear 2, the carburized layer may contribute to wear resistance, and the core may contribute to toughness.

[0019] When gear 2 is used in an environment where low-viscosity oil is used as a lubricant, the surface of gear 2 wears down, exposing a new surface. Extreme pressure additives contained in the low-viscosity oil react with this new surface, forming a tribofilm on the surface of gear 2. This tribofilm can be interposed between gear 2 and other metal parts (e.g., other gears). In other words, the metals come into contact with each other via the tribofilm. The tribofilm suppresses direct contact between the metals. This tribofilm can suppress pitting and delamination of gear 2.

[0020] Figure 2 is an enlarged cross-sectional view showing a portion of the gear 2 in Figure 1. The carburized layer 10 is schematically shown in Figure 2. In Figure 2, reference numeral 12 denotes the surface of the gear 2. The carburized layer 10 has grain boundary oxidation 14, an incomplete quenched structure 16, and granular oxide 18.

[0021] Figure 3 shows a magnified view of a portion of the carburized layer 10 of gear 2 in Figure 2. Figure 3 shows one grain boundary oxide 14. The grain boundary oxide 14 is an oxidized area within the grain boundary. Metal oxide is precipitated in the grain boundary oxide 14. The grain boundary oxide 14 is located along the grain boundary. The grain boundary oxide 14 extends generally inward from the surface 12 (towards the bottom in Figure 3).

[0022] In Figure 3, arrow L represents the distance from the surface 12 to the lower end 20 of the grain boundary oxidation 14. The distance L is measured along a direction perpendicular to the surface 12. For measuring the distance L, a small roller test specimen, as described later, is prepared. The cross-section obtained by cutting this specimen is subjected to polishing and nital etching. This cross-section is photographed with an optical microscope to obtain an image at a magnification of 400x. The distance L is measured in this image.

[0023] In this specification, the depth Dp of grain boundary oxidation 14 is calculated according to the following formula. Dp = (L1 + L2 + L3) / 3 In this formula, L1 represents the distance L of the first grain boundary oxidation 14, L2 represents the distance L of the second grain boundary oxidation 14, and L3 represents the distance L of the third grain boundary oxidation 14. The first grain boundary oxidation 14 is the grain boundary oxidation 14 with the largest distance L in the aforementioned image. The second grain boundary oxidation 14 is the grain boundary oxidation 14 with the second largest distance L in this image. The third grain boundary oxidation 14 is the grain boundary oxidation 14 with the third largest distance L in this image. The calculated depth Dp is shown in Figures 2 and 3. The distance L of the grain boundary oxidation 14 shown in Figure 3 is smaller than the depth Dp.

[0024] The depth Dp of the grain boundary oxide 14 is preferably 15 μm or less. In a gear 2 with a depth Dp of 15 μm or less, a large amount of grain boundary oxide 14 can be eliminated in the initial stages of wear. Therefore, pitting originating from the grain boundary oxide 14 can be suppressed, and initial cracks can be suppressed. Furthermore, a uniform tribofilm is formed in this gear 2. This tribofilm suppresses pitting delamination. From these viewpoints, a depth Dp of 14 μm or less is more preferable, and 13 μm or less is particularly preferable. The ideal depth Dp is zero, but in reality, the gear 2 has grain boundary oxide 14 with a distance L of 1 μm or more. By adjusting the composition of the hardened steel and the carburizing conditions, a gear 2 in which the depth Dp of the grain boundary oxide 14 is within the above range can be obtained.

[0025] In Figures 2 and 3, the symbol S1 represents a line segment that is located at a distance from the surface 12 that is half the depth Dp. This line segment S1 is substantially parallel to the surface 12. In Figure 3, the symbol 22 represents the intersection of this line segment S1 and the grain boundary oxidation 14. At this intersection 22, the grain boundary oxidation 14 intersects with the line segment S1. In the aforementioned images, a line segment S1 with a length of 100 μm is randomly assumed. The number N of grain boundary oxidation 14 that intersect with this line segment S1 is counted. Grain boundary oxidation 14 whose distance L to its lower end 20 is less than (Dp / 2) is not counted in the number N. In this specification, the average pitch AP (μm) of the grain boundary oxidation 14 is calculated by the following formula. AP = 100 / N

[0026] The average pitch AP is preferably 20 μm or less. Wear of gear 2 with an average pitch AP of 20 μm or less is uniform. A uniform tribofilm is formed in this gear 2. This tribofilm suppresses pitting and delamination. From this viewpoint, an average pitch AP of 18 μm or less is more preferable, and 17 μm or less is particularly preferable. An average pitch AP of 5 μm or more is preferable. By adjusting the composition of the hardened steel and the carburizing conditions, a gear 2 with an average pitch AP within the above range can be obtained.

[0027] Figure 3 shows a single incompletely hardened structure 16. The incompletely hardened structure 16 is a structure that has not undergone martensitic transformation even after quenching. Specifically, the incompletely hardened structure 16 is bainite, pearlite, or a mixture thereof. The incompletely hardened structure 16 is located near the surface 12 of the gear 2. In Figure 3, the incompletely hardened structure 16 is along the grain boundary oxidation 14 and extends generally inward from the surface 12.

[0028] Figure 4 shows another cross-section of the carburized layer 10 of gear 2. Figure 4 shows a single granular oxide 18. Metal oxide is precipitated on the granular oxide 18. The granular oxide 18 is a precipitation defect. The granular oxide 18 is located near the surface 12. Figure 4 also shows an incompletely hardened structure 16. The incompletely hardened structure 16 is located near the surface 12 of gear 2. In Figure 4, the incompletely hardened structure 16 surrounds the granular oxide 18.

[0029] In Figure 3, arrow W1 represents the width of the incompletely hardened structure 16. The width W1 is measured along the line segment S1. All possible widths W1 measured in the aforementioned image are averaged to calculate the average width AW of the incompletely hardened structure 16. The size of the incompletely hardened structure 16 whose depth from the surface 12 is less than (Dp / 2) and therefore does not intersect the line segment S1 is not reflected in the average width AW. For example, the size of the incompletely hardened structure 16 shown in Figure 4 is not reflected in the average width AW.

[0030] The average width AW is preferably 4.0 μm or less. Wear of gear 2 with an average width AW of 4.0 μm or less is uniform. A uniform tribofilm is formed in this gear 2. This tribofilm suppresses pitting and peeling. From this viewpoint, an average width AW of 3.7 μm or less is more preferable, and 3.5 μm or less is particularly preferable. An average width AW of 1.0 μm or more is preferable. By adjusting the composition of the hardened steel and the carburizing conditions, a gear 2 with an average width AW within the above range can be obtained.

[0031] In Figures 3 and 4, arrow W2 represents the surface width of the incompletely hardened structure 16. The surface width W2 is measured along the surface 12 of the gear 2. In the aforementioned image, a zone of surface 12 with a length of 100 μm is randomly selected. The total TW of all possible surface widths W2 measured in this zone is calculated. The coverage PC is calculated using the following formula. PC(%) = TW(μm) / 100(μm) * 100

[0032] A coverage PC of 80% or more is preferable. Wear of gear 2 with a coverage PC of 80% or more is uniform. A uniform tribofilm is formed in this gear 2. This tribofilm suppresses pitting and peeling. From this viewpoint, a coverage PC of 83% or more is more preferable, and 85% or more is particularly preferable. The ideal coverage PC is 100%. By adjusting the composition of the hardened steel and the carburizing conditions, a gear 2 with a coverage PC within the aforementioned range can be obtained.

[0033] In this specification, a tempering softening test is performed on a test specimen, and the amount of softening ST is measured. As mentioned above, gear 2 is obtained through heat treatment (quenching and tempering). The tempering softening test is performed at a temperature higher than the tempering temperature of this heat treatment. In the tempering softening test in this specification, gear 2 is held at a temperature of 300°C for 90 minutes. After that, gear 2 is air-cooled. The amount of softening ST is calculated by the following formula. ST = H1 - H2 In this formula, H1 represents the Vickers hardness of gear 2 before the tempering softening test, and H2 represents the Vickers hardness of gear 2 after the tempering softening test. The amount of softening ST can, for convenience, be measured on a test specimen described later, rather than on gear 2. Hardness H1 and H2 are the average values ​​of the hardness from the first to the sixth point on the test specimen. The distances from the surface 12 to each point are as follows. First point: 0.05mm Second point: 0.10mm Third point: 0.15mm Fourth point: 0.20mm Fifth point: 0.25mm Sixth point: 0.30mm

[0034] The softening amount ST is preferably 115 HV or less. Gear 2 with a softening amount ST of 115 HV or less is less prone to cracking even when heated by friction. In this gear 2, pitting after reaching contact with other metals via the tribofilm is suppressed. This gear 2 has excellent durability. From this viewpoint, a softening amount ST of 105 HV or less is more preferable, and 100 HV or less is particularly preferable. A softening amount ST as close to zero as possible is preferable. By adjusting the composition of the hardened steel and the carburizing conditions, a gear 2 with a softening amount ST within the above range can be obtained.

[0035] As mentioned above, the material of gear 2 is hardened steel (alloy steel). The preferred composition of the core of gear 2 is as follows: C: 0.14% by mass or more and 0.45% by mass or less Si: 0.05 mass% or more and 1.00 mass% or less Mn: 0.10 mass% or more and 0.90 mass% or less Cr: 1.30 mass% or more and 3.50 mass% or less Al: 0.020 mass% or more and 0.200 mass% or less N: 0.0040 mass% or more and 0.0300 mass% or less Mo: 2.00% by mass or less Ni: 2.00% by mass or less Nb: 0.10% by mass or less Ti: 0.20% by mass or less B: 0.0050% by mass or less V: 0.500% by mass or less Remainder: Fe and impurities Preferably, the remainder consists of Fe and unavoidable impurities. Each element will be described in detail below.

[0036] [Carbon (C)] Carbon (C) contributes to the hardness and strength of the core. From this viewpoint, the C content is preferably 0.14% by mass or more, more preferably 0.17% by mass or more, and particularly preferably 0.20% by mass or more. Excessive C impairs the processability (machinability and cold plastic workability) of the intermediate. From the viewpoint of processability, the C content is preferably 0.45% by mass or less, more preferably 0.35% by mass or less, and particularly preferably 0.26% by mass or less.

[0037] [Silicon (Si)] Si suppresses softening of gear 2 during use. Si can also contribute to deoxidation during the melting of the alloy. From these viewpoints, the Si content is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, and particularly preferably 0.13 mass% or more. Excess Si inhibits the workability (machinability and cold plastic workability) of the intermediate. Excess Si further inhibits the diffusion of C during carburizing. From the viewpoint of workability and ease of carburizing, the Si content is preferably 1.00 mass% or less, more preferably 0.80 mass% or less, and particularly preferably 0.74 mass% or less.

[0038] [Manganese (Mn)] Mn contributes to the hardenability of gear 2. From this viewpoint, the Mn content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and particularly preferably 0.20% by mass or more. Excess Mn inhibits the machinability (machinability and cold plastic machinability) of the intermediate. From the viewpoint of machinability, the Mn content is preferably 0.90% by mass or less, more preferably 0.80% by mass or less, and particularly preferably 0.75% by mass or less.

[0039] [Chromium (Cr)] Cr suppresses softening of gear 2 during use. Furthermore, Cr contributes to the hardenability of gear 2. From these viewpoints, the Cr content is preferably 1.30% by mass or more, more preferably 1.40% by mass or more, and particularly preferably 1.48% by mass or more. Excess Cr inhibits the machinability (machinability and cold plastic workability) of the intermediate. Excess Cr further inhibits the diffusion of C during carburizing. From the viewpoint of machinability and ease of carburizing, the Cr content is preferably 3.50% by mass or less, more preferably 3.20% by mass or less, and particularly preferably 2.80% by mass or less.

[0040] [Aluminum (Al)] Al can cause the precipitation of fine nitrides and fine carbides. These nitrides and carbides can contribute to the toughness and fatigue characteristics of gear 2. From these viewpoints, the Al content is preferably 0.020 mass% or more, more preferably 0.025 mass% or more, and particularly preferably 0.030 mass% or more. Excess Al leads to the precipitation of coarse nitrides. These nitrides impair the machinability of the intermediate and the fatigue characteristics of gear 2. From the viewpoint of machinability and fatigue characteristics, the Al content is preferably 0.200 mass% or less, more preferably 0.100 mass% or less, and particularly preferably 0.040 mass% or less.

[0041] [Nitrogen (N)] N can cause the precipitation of fine nitrides and fine carbides. These nitrides and carbides can contribute to the toughness and fatigue characteristics of gear 2. From these viewpoints, the N content is preferably 0.0040 mass% or more, more preferably 0.0060 mass% or more, and particularly preferably 0.0075 mass% or more. Excess N leads to the precipitation of coarse nitrides. These nitrides impair the machinability of the intermediate and the fatigue characteristics of gear 2. From the viewpoint of machinability and fatigue characteristics, the N content is preferably 0.0300 mass% or less, more preferably 0.0280 mass% or less, and particularly preferably 0.0260 mass% or less.

[0042] [Molybdenum (Mo)] Mo suppresses softening of gear 2 during use. Mo also contributes to the hardenability of gear 2. From these viewpoints, the Mo content is preferably 0.02 mass% or more, more preferably 0.20 mass% or more, and particularly preferably 0.30 mass% or more. Excess Mo hinders the workability (machinability and cold plastic workability) of the intermediate. From the viewpoint of workability, the Mo content is preferably 2.00 mass% or less, more preferably 1.00 mass% or less, and particularly preferably 0.40 mass% or less. In the hardened steel of this embodiment, Mo is not an essential element. Therefore, the Mo content may be substantially zero. In other words, the Mo content may be below the detection limit.

[0043] [Nickel (Ni)] Ni contributes to the toughness and hardenability of gear 2. From these viewpoints, the Ni content is preferably 0.02 mass% or more, more preferably 0.10 mass% or more, and particularly preferably 0.30 mass% or more. Excess Ni impairs the workability (machinability and cold plastic workability) of the intermediate. From the viewpoint of workability, the Ni content is preferably 2.00 mass% or less, more preferably 1.80 mass% or less, and particularly preferably 1.60 mass% or less. In the hardened steel of this embodiment, Ni is not an essential element. Therefore, the Ni content may be substantially zero. In other words, the Ni content may be below the detection limit.

[0044] [Mo and Ni] The hardened steel preferably contains at least one of Mo and Ni. The total content of Mo and Ni is preferably 0.02% by mass or more and 4.00% by mass or less.

[0045] [Niobium (Nb)] Nb can cause the precipitation of fine nitrides and fine carbides. These nitrides and carbides can contribute to the toughness and fatigue characteristics of gear 2. From these viewpoints, the Nb content is preferably 0.02 mass% or more, more preferably 0.03 mass% or more, and particularly preferably 0.04 mass% or more. Excess Nb inhibits the machinability (machinability and cold plastic workability) of the intermediate. From the viewpoint of machinability, the Nb content is preferably 0.10 mass% or less, more preferably 0.09 mass% or less, and particularly preferably 0.08 mass% or less. In the heat-hardened steel of this embodiment, Nb is not an essential element. Therefore, the Nb content may be substantially zero. In other words, the Nb content may be below the detection limit.

[0046] [Titanium] Ti can cause the precipitation of fine nitrides and fine carbides. These nitrides and carbides can contribute to the toughness and fatigue properties of gear 2. Ti suppresses the formation of BN and contributes to the hardenability of gear 2. Ti can further suppress grain coarsening. From these viewpoints, the Ti content is preferably 0.020 mass% or more, more preferably 0.030 mass% or more, and particularly preferably 0.040 mass% or more. Excess Ti inhibits the workability (machinability and cold plastic workability) of the intermediate. From the viewpoint of workability, the Ti content is preferably 0.200 mass% or less, more preferably 0.18 mass% or less, and particularly preferably 0.16 mass% or less. In the heat-hardened steel of this embodiment, Ti is not an essential element. Therefore, the Ti content may be substantially zero. In other words, the Ti content may be below the detection limit.

[0047] [Boron (B)] B contributes to the hardenability of gear 2. From this viewpoint, the B content is preferably 0.0010 mass% or more, more preferably 0.0013 mass% or more, and particularly preferably 0.0015 mass% or more. Excess B impairs the machinability (machinability and cold plastic workability) of the intermediate. From the viewpoint of machinability, the B content is preferably 0.0050 mass% or less, more preferably 0.0030 mass% or less, and particularly preferably 0.0020 mass% or less. In the hardened steel of this embodiment, B is not an essential element. Therefore, the B content may be substantially zero. In other words, the B content may be below the detection limit.

[0048] [Vanadium (V)] V can cause the deposition of fine nitrides and fine carbides. These nitrides and carbides can contribute to the toughness and fatigue characteristics of gear 2. From these viewpoints, the V content is preferably 0.010 mass% or more, more preferably 0.050 mass% or more, and particularly preferably 0.100 mass% or more. Excess V inhibits the machinability (machinability and cold plastic workability) of the intermediate. From the viewpoint of machinability, the V content is preferably 0.500 mass% or less, more preferably 0.440 mass% or less, and particularly preferably 0.380 mass% or less. In the heat-hardened steel of this embodiment, V is not an essential element. Therefore, the V content may be substantially zero. In other words, the V content may be below the detection limit.

[0049] [Nb, Ti, B, V] The hardened steel preferably contains one or more elements selected from the group consisting of Nb, Ti, B, and V. The total content of Nb, Ti, B, and V is preferably 0.0010% by mass or more and 0.80% by mass or less.

[0050] [Iron (Fe)] The base material of this hardened steel is Fe. Fe can contribute to the strength and toughness of gear 2. From these viewpoints, the Fe content is preferably 85.0% by mass or more, more preferably 90.0% by mass or more, and particularly preferably 92.0% by mass or more.

[0051] [impurities] This hardened steel may contain impurities (or unavoidable impurities). Examples of impurities include phosphorus (P) and sulfur (S). P segregates at grain boundaries and inhibits the toughness of gear 2. From the viewpoint of toughness, the P content is preferably 0.030 mass% or less, more preferably 0.025 mass% or less, and particularly preferably 0.020 mass% or less. S leads to the precipitation of MnS. MnS inhibits the toughness and fatigue properties of gear 2. From the viewpoint of toughness and fatigue properties, the S content is preferably 0.030 mass% or less, more preferably 0.025 mass% or less, and particularly preferably 0.020 mass% or less.

[0052] [Manufacturing method] The following describes an example of a manufacturing method for gear 2. In this manufacturing method, first, a base material having the aforementioned composition is obtained by melting. This base material is forged to obtain a bar. This bar is hot forged to form a gear intermediate. This gear intermediate is heat-treated. A typical heat treatment is normalizing. The surface of this intermediate is machined to adjust its dimensions and surface condition. This intermediate is then carburized. During carburizing, the intermediate is held in a high-temperature, carbon-rich atmosphere. The temperature of the atmosphere is higher than the A3 transformation point. During carburizing, carbon atoms in the atmosphere penetrate the intermediate and gradually diffuse inward. This penetration and diffusion forms a layer with a high carbon content on the surface of the intermediate. Solid carburizing, liquid carburizing, and gas carburizing methods can be used for carburizing. This intermediate is then subjected to quenching. During quenching, the intermediate is rapidly cooled. Rapid cooling causes martensitic transformation on the surface of the intermediate. This intermediate is then tempered to complete gear 2. The tempering temperature is between 150°C and 250°C.

[0053] [Applications of gears] This gear 2 is suitable for transmissions used in high-load environments (such as truck transmissions). In particular, this gear 2 is suitable for transmissions that use low-viscosity oil as a lubricant. For example, if the kinematic viscosity at 300°C is 1.50 mmHg. 2 This gear 2 is suitable for transmissions that use lubricants with a value of / S or less.

[0054] [Gear configuration] Figure 1 shows a so-called spur gear. In addition to this spur gear, the gear 2 according to the present invention includes internal gears, helical gears, screw gears, thread gears, bevel gears, crown gears, worm gears, rack gears, sprockets, and the like. [Examples]

[0055] The effects of the gears according to the embodiments will be described below, but the scope disclosed herein should not be interpreted as limiting based on the description of these embodiments.

[0056] [Example 1] A base material having the composition shown in Table 1 was obtained in a vacuum melting furnace. This base material was forged to obtain a rod with a diameter of 32 mm. This rod was hot forged to obtain a gear intermediate. This intermediate was heated to 925°C and subjected to a normalizing treatment. This intermediate was held in a carburizing gas atmosphere at a temperature of 930°C for 360 minutes to allow carbon atoms to enter and diffuse into the intermediate. This intermediate was quenched by water cooling. This intermediate was heated to 180°C and tempered by air cooling to obtain the gear of Example 1.

[0057] [Examples 2-13 and Comparative Examples 1-6] Gears for Example 2-13 and Comparative Example 1-6 were obtained in the same manner as in Example 1, except that the composition was as shown in Table 1.

[0058] [Cross-sectional observation] Small roller test specimens for roller pitting testing were obtained from the same material as the gears. These specimens were obtained by normalizing, carburizing, quenching, and tempering under the same conditions as the gears. This small roller test specimen 24 is shown in Figure 5. This specimen has a main body 26 and a pair of grips 28. The main body 26 has a cylindrical shape. The main body 26 has a diameter of 26 mm and a length of 28 mm. Each grip 28 has a cylindrical shape. The grips 28 have a diameter of 24 mm and a length of 51 mm. The main body 26 was cut to obtain a cross-section. This cross-section was polished and nital etched. This cross-section was photographed with an optical microscope to obtain an image at a magnification of 400x. From this image, the depth Dp of grain boundary oxidation, the average pitch AP of grain boundary oxidation, the average width AW of the incompletely quenched structure, and the coverage PC of the incompletely quenched structure were measured using the method described above. The results are shown in Table 2 below.

[0059] [Softening amount ST] A base material of the same material as that used for gears was prepared. This base material was forged to obtain a rod with a diameter of 32 mm. This rod was subjected to normalizing, carburizing, and quenching under the same conditions as for gears. This rod was heated to 180°C and tempered by air cooling to obtain a test specimen. The amount of softening ST of this test specimen was measured by the tempering softening test described above. The results are shown in Table 2 below.

[0060] [Fatigue limit] A small roller test specimen 24 with the same specifications as the specimen used for cross-sectional observation was prepared. A roller pitting test was performed using this specimen 24. The test procedure is shown in Figure 6. Figure 6 shows the small roller test specimen 24 and the large roller 30. In Figure 6, arrow R1 represents the rotation direction of the small roller test specimen 24, and arrow R2 represents the rotation direction of the large roller 30. The test conditions were as follows. Slip ratio: -40% Surface pressure: 2.0 to 4.0 GPa Small roller rotation speed: 2000 rpm Large roller 30 material: SUJ2 Processing of large roller 30: Polishing after deep hardening. Crowning amount for large roller 30: R150mm Lubricant material: 5W30 Lubricant temperature: 90℃ The number of cycles is 1.0 × 10 7 The fatigue limit at a given point in time was measured in accordance with the provisions of "JIS Z2273 1978". The results are shown in Table 2 below.

[0061] [Amount of surface softening] A small roller test specimen 24 was prepared after the fatigue limit test described above. The Vickers hardness Hm was measured at a point 0.05 mm from the surface of the main part 26 of this specimen, at the point where it rubbed against the large roller 30. Furthermore, the Vickers hardness Hg was measured at a point 0.05 mm from the surface of the main part 26 of this specimen, at a point where it was not in contact with the large roller 30. The surface softening amount SS(HV) was calculated using the following formula. SS = Hg - Hm These results are shown in Table 2 below.

[0062] [Table 1]

[0063] The remainder of each alloy shown in Table 1 consists of Fe and unavoidable impurities.

[0064] [Table 2]

[0065] As shown in Table 2, the gears in each embodiment exhibit excellent performance in various aspects. The superiority of these gears is evident from these evaluation results. [Industrial applicability]

[0066] The gears described above can be used as components in various machines. [Explanation of Symbols]

[0067] 2. Gears 4. Base 6 teeth 8... Valley 10...carburized layer 12...Surface 14. Grain boundary oxidation 16. Incompletely hardened structure 18..Grain oxides 24... Small roller test specimens 26... Main part 28... Grip 30...Large roller

Claims

[Claim 1] A gear comprising a carburized layer forming its surface and a core located inside this carburized layer, The above carburized layer includes multiple grain boundary oxidations extending inward from the surface, and an incomplete quenched structure located near the surface. The depth Dp of the above grain boundary oxidation is 15 μm or less. The average pitch AP of the above grain boundary oxidation is 20 μm or less. The average width AW of the above incompletely quenched structure is 4.0 μm or less. The coverage rate PC of the incompletely hardened structure on the above surface is 80% or more. In a tempering softening test conducted at a temperature of 300°C, the softening amount ST was 115HV or less. The material of the above gear is hardened steel. The core part mentioned above is C: 0.14% by mass or more and 0.45% by mass or less, Si: 0.05% by mass or more and 1.00% by mass or less, Mn: 0.10% by mass or more and 0.90% by mass or less, Cr: 1.30% by mass or more and 3.50% by mass or less, Al: 0.020% by mass or more and 0.200% by mass or less, and N: 0.0040% by mass or more and 0.0300% by mass or less, Includes, Mo: 0.02% by mass or more and 2.00% by mass or less, and Ni: 0.02% by mass or more and 2.00% by mass or less Includes at least one of the following: Nb: 0.02% by mass or more and 0.10% by mass or less, Ti: 0.020% by mass or more and 0.200% by mass or less, B: 0.0010% by mass or more and 0.0050% by mass or less, and V: 0.010% by mass or more and 0.500% by mass or less Includes at least one of the following: A gear whose remainder is Fe and impurities.

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