gear

Abstract

To provide a gear in which surface fatigue strength and flexure fatigue strength are high, and further dimensional accuracy over time is also high.SOLUTION: Concerning a gear, in the range from the outermost surface of the tooth surface to a depth of 50 μm, (a) the average carbon concentration is 0.85 Cwt%-1.15 Cwt%, (b) the content of cementite is 3-9 area%, and the maximum diameter is 3 μm or less, and (c) the content of retained austenite is 35 area% or less. Thus, a gear can be provided, in which both surface fatigue strength and flexure fatigue strength are high, and further dimensional accuracy over time is also high.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to gears, and more particularly to gears to which high-concentration carburizing treatment has been applied. [Background technology]

[0002] Carburizing and quenching is a process in which a part made of steel is held in a high-temperature carburizing atmosphere, carbon is impregnated into the surface of the steel to form an austenite phase, and the austenite is then transformed into martensite by rapid cooling at a rate equal to or greater than the critical quenching rate.

[0003] Parts that have been hardened by carburizing and quenching have a martensite structure formed on the surface, improving their strength.

[0004] A high-concentration carburizing process (also known as hyper-eutectoid carburizing) is known that further improves strength compared to the above-mentioned carburizing and quenching.

[0005] High-concentration carburizing is a process in which a large amount of carbon is impregnated to make the steel supersaturated, and then the carbon is precipitated to form cementite. This cementite structure is harder than the martensite and does not soften even at high temperatures.

[0006] The cementite improves the hardness of the surface of the part, thereby reducing wear, and also increases the temper softening resistance in a high-temperature environment during use of the part, thereby improving the contact fatigue strength.

[0007] However, while cementite improves the hardness of parts, it also causes a decrease in bending fatigue strength because stress during operation of the parts is concentrated at the interface between cementite and martensite.

[0008] Furthermore, the cementite tends to precipitate preferentially at austenite grain boundaries and around existing cementite precipitation nuclei.

[0009] Cementite precipitated at the austenite grain boundaries takes on a plate-like shape, and cementite precipitated around the precipitation nuclei tends to coarsen, resulting in stress concentration at the grain boundaries between cementite and martensite transformed from austenite.

[0010] In addition, if a large amount of carbon is impregnated to precipitate cementite, transformation deformation during martensite formation increases, resulting in increased quenching distortion and deteriorating the dimensional accuracy of the finished part.

[0011] Patent Document 1, Japanese Patent Application Laid-Open No. 2008-115427, describes that the Si content of a steel material is set to 0.40 to 0.80 wt%, the Mo content is set to 0.3 to 0.8 wt%, and the Cr content is set to 1.25 to 2.0 wt%, and that the cementite can be refined by the composition of the steel material, thereby preventing a decrease in bending fatigue strength. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-115427 Summary of the Invention [Problem to be solved by the invention]

[0013] However, if the hardenability is improved by adjusting the dispersion state of cementite through the addition of Si, Cr, Mo, and other steel components, the composition of the steel must be changed depending on the required strength, which increases the cost of steel procurement.

[0014] Furthermore, Si, Cr, and Mo are all components that increase the deformation resistance during forging and the resistance during cutting, and therefore increase the cost of forming parts before carburizing and quenching.

[0015] The present invention has been made in view of the problems associated with the prior art. Its object is to provide gears that have high surface fatigue strength and bending fatigue strength and that have high dimensional accuracy over time, not by developing a specialized steel material with a new composition, but by using a general steel material that has conventionally been used for gears. [Means for solving the problem]

[0016] As a result of extensive research into achieving the above object, the inventors have discovered that the above object can be achieved by forming desired amounts of refined cementite and retained austenite on the surface of a gear, and have thus completed the present invention.

[0017] That is, in the gear of the present invention, the range from the outermost surface of the tooth surface to a depth of 50 μm is Meet the following requirements (a) to (c). death, (a) Average carbon concentration is 0.85Cwt% to 1.15Cwt% (b) the cementite content is 3 to 9 area percent; and maximum of Cementite diameter is 3 μm or less, (c) The content of retained austenite is 12% or more 35are% or less and , Inside, C content is 0.13 to 0.33 mass %, The Si content is 0.15 to 0.35 mass %; Mn content is 0.6 to 0.9 mass %, Cr content is 0.9 to 1.20 mass %; P content is 0.030 mass% or less, S content of 0.030% by mass or less, Ni content is 0.25 mass% or less, The Mo content is 0.30 mass% or less, The remainder is iron. It is characterized by: [Effects of the Invention]

[0018] According to the present invention, the surface of a gear contains desired amounts of refined cementite and retained austenite, and therefore it is possible to provide a gear using ordinary steel that has both good surface fatigue strength and good bending fatigue strength and that has high dimensional accuracy over time. DETAILED DESCRIPTION OF THE INVENTION

[0019] <Gear> The gear of the present invention will now be described in detail. The gear satisfies the following requirements (a) to (c) in the range from the outermost surface of the tooth surface to a depth of 50 μm. (a) Average carbon concentration is 0.85Cwt% to 1.15Cwt% (b) The cementite content is 3 to 9 area % and its maximum diameter is 3 μm or less. (c) The retained austenite content is 35 area % or less.

[0020] (average carbon concentration) In the gear of the present invention, the average carbon concentration in the range from the outermost surface of the tooth surface to a depth of 50 μm (hereinafter sometimes referred to as the "surface portion") is 0.85 Cwt% to 1.15 Cwt%, and more preferably 0.95 Cwt% to 1.10 Cwt%.

[0021] By setting the average carbon concentration to 0.85 Cwt% or more, a desired amount of cementite can be precipitated to increase hardness, thereby improving the contact fatigue strength.

[0022] Furthermore, by keeping the average carbon concentration at 1.15 Cwt% or less, excessive cementite precipitation is prevented, improving bending fatigue strength. Furthermore, deformation during martensitic transformation is suppressed, improving the dimensional accuracy of the finished gear and reducing the amount of retained austenite.

[0023] Furthermore, when the average carbon concentration increases, the martensite transformation start temperature decreases while the bainite transformation start temperature increases, so the critical cooling rate for martensite transformation increases, and hardenability decreases.

[0024] The carbon concentration was determined by measuring the carbon concentration distribution in the gear cross section using wavelength dispersive type X-ray fluorescence analysis (WDX), and averaging the carbon concentration in the region between the tooth root and tip (half of the tooth tip) from the outermost surface of the tooth surface to a depth of 50 μm.

[0025] (Cementite content and maximum of Cementite diameter) present on the surface maximum of Cementite The diameter is 3 μm or less.

[0026] Both cementite and martensite are hard and are difficult to deform even when subjected to stress during gear operation, so stress tends to concentrate at the grain boundaries between cementite and martensite.

[0027] The maximum cementite grain size is By keeping the grain size at 3 μm or less, the stress acting on the grain boundaries is dispersed and reduced, improving the bending fatigue strength.

[0028] The cementite content in the surface portion is 3 to 9 area %, and more preferably 5 to 9 area %.

[0029] When the content of cementite with a grain size of 3 μm or less is 3 area% or more, the hardness of the surface increases and the contact fatigue strength improves, and when it is 9 area% or less, there are no excessive grain boundaries where stress tends to concentrate, and the bending fatigue strength improves.

[0030] The grain size of cementite was measured by etching the cross section of the surface with a 3% picral solution (picric acid + alcohol) and then photographing it using an SEM. 2 It can be measured from the particle size of the carbides present in the steel.

[0031] (Retained austenite content)

[0032] The content of retained austenite in the surface area is 35 area% or less. Retained austenite is austenite that did not transform into martensite by the rapid cooling of quenching. This retained austenite undergoes phase transformation over time, causing changes in the dimensions and shape of the finished part after manufacture over time.

[0033] By keeping the content of retained austenite at 35% or less, the dimensional accuracy of the gear over time (hereinafter sometimes referred to as "shape fixability") is improved. Furthermore, because retained austenite is soft, if the content of retained austenite increases, the hardness decreases and the surface fatigue strength decreases.

[0034] The content of retained austenite in the surface portion is preferably 13 area % or more. Retained austenite is softer than cementite and martensite, and improves the toughness of the gear.

[0035] The content of retained austenite was determined by observing the cross section of the surface portion with a scanning electron microscope (SEM) and identifying martensite, cementite, and austenite.

[0036] (residual stress) The gear preferably has a residual stress on the outermost surface of the tooth surface of 600 MPa or more and less than 950 MPa.

[0037] Fatigue failure occurs when cracks form and grow. By ensuring that the compressive residual stress on the outermost surface is 600 MPa or higher, even if cracks form, the compressive residual stress closes them, suppressing their growth and improving fatigue strength.

[0038] Furthermore, if the residual stress on the outermost surface is 950 MPa or more, the compressive residual stress becomes too large, making it easier for initial defects (cracks) to occur inside the tooth surface.

[0039] The compressive residual stress can be imparted by shot peening, and the compressive residual stress caused by shot peening increases with increasing depth from the surface up to a predetermined depth, and decreases once the predetermined depth is exceeded.

[0040] In the gear of the present invention, the maximum value of residual stress within a depth range of 25 to 100 μm from the surface is preferably 1000 MPa or more and 1500 MPa or less.

[0041] By keeping the maximum value of the residual stress in the depth range of 25 to 100 μm within the above range, it is possible to prevent cracks from occurring inside the tooth surface during the manufacturing stage, and the fatigue strength is improved.

[0042] Residual stress was measured by X-ray diffraction while etching the tooth surface from the outermost surface in the depth direction.

[0043] <Gear manufacturing> The gear of the present invention can be manufactured by forming a steel material into a gear shape and then subjecting it to a high-concentration carburizing treatment. The high-concentration carburizing treatment includes a primary carburizing and quenching step in which the gear is impregnated with carbon, and a secondary carburizing and precipitation quenching step in which cementite is precipitated.

[0044] (First carburizing and quenching process) The primary carburizing and quenching process is a process in which carbon is dissolved so that the average carbon concentration on the surface of the gear reaches the desired concentration. The carburizing atmosphere is adjusted so that the average carbon concentration in the range from the outermost surface of the tooth surface to a depth of 50 μm is 0.85 Cwt% to 1.15 Cwt%.

[0045] Carbon dissolved in steel by the primary carburizing and quenching, in excess of the solubility limit, forms cementite (Fe3C). This cementite is supplied with carbon from the carburizing atmosphere and is introduced via grain boundaries, so it grows at the grain boundaries and tends to coarsen.

[0046] In the present invention, the carbon concentration in the surface portion reaches a desired concentration and is equilibrated with the carbon concentration in the carburizing atmosphere, so that cementite does not exist in the structure, and the carburizing atmosphere is maintained to allow the carbon forming cementite to be fully dissolved in the matrix.

[0047] The time for which the primary carburizing and quenching is carried out depends on the carbon potential and temperature of the carburizing atmosphere, but it is preferable to carry out the process for at least one hour so that the carbon diffuses and dissolves and no cementite remains, and it is even more preferable to carry out the process for at least eight hours.

[0048] If coarse cementite remains at the grain boundaries during the primary carburizing and quenching, this remaining cementite reduces the fatigue strength of the gear.

[0049] The cooling rate in the primary carburizing and quenching is preferably 20 to 50°C / sec, and more preferably 27 to 33°C / sec.

[0050] By keeping the cooling rate within the above range, there is no time for cementite to form, so cementite does not form after the primary carburizing and quenching. In addition, less martensite is formed, which helps to suppress heat treatment deformation after the primary carburizing and quenching.

[0051] If the rate exceeds 50°C / sec, the lattice defects in the parent phase increase, and cementite tends to coarsen during the secondary carburization treatment. If the rate is less than 20°C / sec, the lattice defects in the parent phase decrease, the content of cementite generated during the secondary carburization treatment decreases, and the amount of retained austenite increases.

[0052] There are no particular restrictions on the pressure for the primary carburizing and quenching, but it is preferable to carry out the carburizing under reduced pressure of 2000 Pa or less, as this allows for high-concentration carburizing and prevents uneven carburizing and oxidation of the gear surface.

[0053] Furthermore, when carburizing is performed under reduced pressure, the amount of carbon introduced into the surface of the tooth flank and tooth bottom varies depending on the R of the tooth bottom of the gear, so the amount of cementite at the tooth bottom can be reduced compared to the tooth flank, resulting in the production of gears with high surface fatigue strength at the tooth flank and high bending fatigue strength at the tooth bottom.

[0054] (Secondary carburizing and precipitation hardening process) Secondary carburizing precipitation quenching is a process in which the carbon dissolved in the primary carburizing quench is precipitated to form cementite. This is done with a carburizing potential of 0Cwt% so that carbon does not dissolve into the surface and decarburization does not occur.

[0055] In addition, secondary carburizing and precipitation quenching allows cementite to precipitate only in areas where it is likely to precipitate, and forms fine cementite by slowly precipitating it in a temperature range close to the temperature at which precipitation begins, so that the cementite does not grow rapidly and become coarse.

[0056] When the carbon concentration is 0.85 Cwt% to 1.15 Cwt%, the heat treatment temperature is preferably 820 to 900°C.

[0057] The heat treatment time for the secondary carburizing and quenching is preferably 3 to 5 hours, although it depends on the average carbon concentration in the surface portion and the heat treatment temperature. If the heat treatment time is too short, the cementite content will decrease, and if the heat treatment time is too long, the cementite will grow and become coarse.

[0058] There is no particular restriction on the pressure of the secondary carburizing and quenching, but if the carburizing is carried out under a reduced pressure of, for example, 2000 Pa or less, it is possible to prevent grain boundary oxidation in the surface portion.

[0059] The cooling rate in the secondary carburizing and precipitation quenching is preferably 15°C / sec or more, and more preferably 100°C / sec to 150°C / sec. A cooling rate of 15°C / sec or more forms martensite, improving the strength of the gear, and also reduces retained austenite, improving the shape fixability of the gear.

[0060] The gear of the present invention can be made using steel materials that are conventionally used for gears, such as SCM430, SCM420, SCM415, SCR430, SCR420, and SCR415.

[0061] Preferably, the steel contains iron as its main component and Si, Cr, and Mo, with the Si content being 0.35 mass% or less, the Cr content being 1.20 mass% or less, and the Mo content being 0.30 mass% or less. When the Si, Cr, and Mo contents are within the above ranges, the steel has low deformation resistance, making it easier to form gears.

[0062] The steel material is preferably produced using a steel material conventionally used for gears, which has a C content of 0.13 to 0.33 mass%, a Si content of 0.15 to 0.35 mass%, a Mn content of 0.6 to 0.9 mass%, a Cr content of 0.9 to 1.20 mass%, a P content of 0.030 mass% or less, a S content of 0.030 mass% or less, a Ni content of 0.25 mass% or less, a Mo content of 0.30 mass% or less, and the remainder being iron. [Example]

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0064] [Example 1] Steel (SCM420) was formed into a gear shape and then placed in a heating furnace. Acetylene gas was supplied to the furnace so that the average carbon concentration of the surface was 1.0 Cwt%, and the furnace was maintained at 1500 Pa and 1050°C for 10 hours. The furnace was then cooled to 20°C at a cooling rate of 30°C / s for the first carburizing and quenching.

[0065] Then, while acetylene gas was supplied to the heating furnace so that the carburizing potential was 0 mass%, the material was held at 850°C under a pressure of 1500 Pa for 4 hours, and then cooled to 20°C at a cooling rate of 30°C / s for secondary carburizing and quenching, and then annealed at 140°C.

[0066] The gears that had been carburized and quenched were then shot peened so that the residual stress on the outermost surface was 900 MPa, thereby obtaining gears of Example 1.

[0067] The treatment temperature was measured using a CA thermocouple as the radiation temperature at a point 10 cm from the thermocouple workpiece. The quenching cooling rate was measured by photographing the steel surface over time using a two-color temperature measurement system (manufactured by Mitsui Photonics, product name: Thermera) and measuring the rate of temperature decrease.

[0068] The composition of the steel (SCM420) used for this gear is as follows: Carbon (C): 0.18~0.23% by mass Silicon (Si): 0.15 to 0.35 mass% Manganese (Mn): 0.60 to 0.90 mass% Phosphorus (p): 0.030% by mass or less Sulfur (S): 0.030% by mass or less Nickel (Ni): 0.25% by mass or less Chromium (Cr): 0.90 to 1.2 mass% Molybdenum (Mo): 0.15 to 0.25 mass% Iron (Fe): Remainder

[0069] [Examples 2 to 18] [Comparative Examples 1 to 6] Gears of Examples 2 to 18 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1, except that the conditions for the primary carburizing and quenching, the secondary carburizing and quenching, and the shot peening were changed to those shown in Table 1.

[0070] [Table 1]

[0071] <Evaluation> The produced gears were evaluated by the following methods. The evaluation results are shown in Table 2 along with the surface structure.

[0072] (shape freezing) Using an eccentricity tester (manufactured by Obishi Keiki Seisakusho, trade name: Universal Bench Center MV type), the shape fixability was measured from the amount of axial contact of the gear one week after production.

[0073] (surface fatigue strength) The test was carried out in accordance with JIS Z 2275 using a roller pitching tester (manufactured by Komatsu Engineering Co., Ltd., trade name: RPT401 type), and the load at which the specimen broke was measured at a repetition rate of 2000 times per minute and 100,000 repetitions.

[0074] (bending fatigue strength) The test was carried out using a rotating bending fatigue strength tester in accordance with JIS Z 2274, and the load at which the specimen broke after 1 million cycles at a cycle rate of 1,350 cycles per minute was measured.

[0075] (Bending fracture absorption energy) The energy was calculated from the elongation breaking load in the bending test, and shown as a relative value with Example 11 as the reference (1.0).

[0076] [Table 2]

[0077] In Comparative Example 1, the maximum diameter of cementite was 7 μm, so the contact fatigue strength and bending fatigue strength were reduced. In Comparative Example 2, the average carbon concentration was low, and therefore the contact fatigue strength was reduced. In Comparative Example 3, the average carbon concentration was high, so the bending fracture absorbed energy was low and the impact fatigue was low. In Comparative Example 4, the amount of cementite was small, and therefore the contact fatigue strength was reduced. In Comparative Example 5, the amount of cementite was small, and therefore the impact fatigue was reduced. In Comparative Example 6, the shape fixability was reduced due to the large amount of retained austenite.

[0078] Comparison between the Examples and Comparative Examples confirmed that the gears of the present invention have high surface fatigue strength and bending fatigue strength, and also high dimensional accuracy over time.

Claims

1. The area from the outermost surface of the tooth surface to a depth of 50 μm is Meet the following requirements (a) to (c): (a) Average carbon concentration is 0.85 Cwt% to 1.15 Cwt% (b) The cementite content is 3 to 9 area%, and the maximum cementite diameter is 3 μm or less; (c) The content of retained austenite is 12 area% or more and 35 area% or less, Inside, The C content is 0.13 to 0.33 mass %, The Si content is 0.15 to 0.35 mass %, The Mn content is 0.6 to 0.9 mass %, The Cr content is 0.9 to 1.20 mass %; The P content is 0.030 mass% or less, S content of 0.030% by mass or less, The Ni content is 0.25 mass% or less, The Mo content is 0.30 mass% or less, A gear characterized in that the remainder is made of iron.

2. 2. The gear according to claim 1, wherein the content of retained austenite is 13 area % or more.

3. 3. The gear according to claim 1, wherein the average carbon concentration is 0.95 Cwt% to 1.10 Cwt%.

4. 4. The gear according to claim 1, wherein the cementite content is 5 to 9 area %.

5. 5. The gear according to claim 1, wherein the residual stress on the outermost surface of the tooth flank is 600 MPa or more and less than 950 MPa.

6. 6. A gear according to claim 1, wherein the maximum value of residual stress in a depth range of 25 to 100 μm from the outermost surface of the tooth surface is 1000 MPa or more and 1500 MPa or less.

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