Sintered gear and method of manufacturing the same

Abstract

To provide a sintered gear having high quietness.SOLUTION: A sintered gear includes a body portion and a tooth portion formed on an outer circumferential surface of the body portion, wherein a tooth width direction of the tooth portion is inclined with respect to an axial direction, and the tooth portion includes a densified layer formed at least on a tooth tip and a surface layer of a tooth surface, and a non-densified layer formed at least on a surface layer of a tooth bottom, wherein a thickness of the densified layer is 10 to 1000 μ m, a difference in density between the densified layer and an inside of the tooth portion is 0.2 to 1.0g / cm3, and the non-densified layer has a density of 7.2g / cm3 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a sintered gear and a method for manufacturing the same. [Background technology]

[0002] Sintered gears are generally sintered bodies obtained by sintering a gear-shaped green compact obtained by pressing a metal powder-filled die from above and below with a punch. Because these sintered gears can be mass-produced inexpensively, they are offered as a wide variety of products for a variety of applications. Sintered gears have pores remaining in the metal matrix due to their manufacturing method. Because the pores absorb vibrations, sintered gears are characterized by superior vibration damping compared to gears made from melt-cast steel. Patent Document 1 discloses a sintered gear with controlled porosity in the densified layer of the tooth surface of the tooth portion, as a highly quiet sintered gear. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2004 / 030852 Summary of the Invention [Problem to be solved by the invention]

[0004] Helical gears offer superior driving force and quietness compared to conventional spur gears. Helical gears are used in drive motors for electric vehicles (EVs) and other vehicles to increase the meshing ratio between gears and improve quietness through smooth meshing. Conventional helical gears made from molten steel have been used, but this has led to problems with quietness being compromised as motors rotate at higher speeds.

[0005] An object of the present disclosure is to provide a sintered gear that is highly quiet. [Means for solving the problem]

[0006] The present disclosure includes the following embodiments. [1] A toothed shank includes a main body and teeth formed on the outer peripheral surface of the main body, the teeth having a tooth width direction inclined with respect to the axial direction, the teeth having a densified layer formed on at least the surface layer of the tooth tip and tooth face, and a non-densified layer formed on at least the surface layer of the tooth bottom, the densified layer having a thickness of 10 to 1000 μm, and the densified layer has a density difference of 0.2 to 1.0 g / cm from the inside of the tooth. 3 and the non-densified layer has a density of 7.2 g / cm 3 Below is a sintered gear.

[0007] [2] The sintered gear according to [1], wherein the matrix hardness at a depth of 200 μm from the surface of the tooth flank of the tooth portion is Hv500 or more.

[0008] [3] A method for manufacturing a sintered gear having a main body and teeth formed on the outer peripheral surface of the main body, the teeth being inclined in the tooth width direction with respect to the axial direction, the method comprising: pressurizing a raw material powder to obtain a green compact; sintering the green compact to obtain a sintered body; and rolling the sintered body, wherein a rolling allowance is formed on the tooth surfaces of the teeth of the sintered sintered body, and the thickness of the rolling allowance is 200 μm or less.

[0009] [4] A method for manufacturing a sintered gear having a main body and teeth formed on the outer peripheral surface of the main body and inclined in the tooth width direction with respect to the axial direction, the method comprising: pressurizing a raw material powder to obtain a green compact; sintering the green compact to obtain a sintered body; and rolling the sintered body, wherein the sintered sintered body has a matrix hardness of Hv200 or less on the surfaces of the tooth flanks of the teeth.

[0010] [5] The method for producing a sintered gear according to [3] or [4], further comprising carburizing, quenching, and tempering the rolled sintered body. [Effects of the Invention]

[0011] According to one embodiment of the present disclosure, a sintered gear having high quietness can be provided. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view of an example of a sintered gear. [Figure 2] FIG. 2 is a side view of the sintered gear shown in FIG. [Figure 3] FIG. 3 is a front view of the sintered gear shown in FIG. 1 as viewed from the axial direction. [Figure 4] FIG. 4 is a diametrical end view of the sintered gear shown in FIG. 1 at the axial center thereof. [Figure 5] FIG. 5 is a diametric end view of the teeth of the sintered gear shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] Several embodiments of the present disclosure will be described in detail below, but these are merely examples and the present invention is not limited to these examples.

[0014] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit of a numerical range in a certain stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. In the numerical ranges described in the present disclosure, the upper or lower limit of the numerical range may be replaced with a value shown in the examples. In the present disclosure, the term "step" includes not only an independent step, but also a step that cannot be clearly distinguished from other steps, as long as the intended effect of the step is achieved. Unless otherwise specified, an element refers to either a single element or multiple elements.

[0015] <Density and porosity by Archimedes method> The density and porosity of the sintered body can be determined by measuring the dry weight, oil-soaked weight, and water weight of the sintered body according to the Archimedes method specified in JIS Z2501. The detailed conditions for measuring the density are as follows: Testing machine: Electronic balance (A&D Co., Ltd. "GR-202") Temperature: Room temperature (25℃) The conditions for measuring the oil-immersed mass are as follows. Oil: Killer spindle oil (specific gravity: 0.856) Pressure: 60kPa (vacuum degree) Decompression time: 30 minutes (until no more bubbles appear) After decompression release: Keep in oil for 5 minutes Wipe off the oil on the surface and measure the mass to four decimal places using an electronic balance. When the sintered body is an iron-based sintered body, the porosity is calculated from the density of the sintered body by multiplying the true density of iron by 7.87 g / cm 3 When the sintered body is a sintered body of another metal, the porosity can be calculated using the true density of the appropriate bulk metal.

[0016] <Density and porosity by image analysis> The density and porosity of the sintered body can also be determined by image analysis of a microscope image. More specifically, five or more images of multiple observation fields are randomly taken at 200x magnification on the surface or cross section of the sintered body. It is preferable that the images of each observation field are taken from positions as evenly distributed as possible on the surface. Next, the porosity can be determined and the density can be calculated by analyzing the images using image analysis software.

[0017] <Base hardness> The matrix hardness of a sintered body is the Vickers hardness (Hv) of the surface of the matrix excluding the pores of the sintered body. Specifically, it is the Hv when a load of 100 g is applied to the surface of the matrix for high hardness and a load of 10 g is applied to the surface of the matrix for low hardness. Hv is measured by the method specified in JIS Z 2244.

[0018] <Sintered gears> The present disclosure relates to a sintered gear including a main body and teeth formed on the outer peripheral surface of the main body, the teeth being inclined in the tooth width direction relative to the axial direction. An example of a sintered gear will be described below with reference to the drawings. The drawings used in the following description all show schematic views of sintered gears, and the present disclosure is not limited to the specific examples shown in the drawings.

[0019] Fig. 1 is a perspective view of an example of a sintered gear. Fig. 2 is a side view of the sintered gear shown in Fig. 1. Fig. 3 is a front view of the sintered gear shown in Fig. 1 from the axial direction. Fig. 4 is a diametric end view of the axial center of the sintered gear shown in Fig. 1. Fig. 5 is a cross-sectional view of a tooth portion of an example of a sintered gear.

[0020] In this disclosure, referring to FIG. 5, the tooth portion 20 includes a tooth tip 21, which is the tip surface of the tooth portion, a tooth base 22, which is the root of the tooth portion, a tooth bottom 24 between adjacent tooth bases, and tooth surfaces 23a and 23b, which are the meshing surfaces of the teeth. The circle connecting the tooth bases is called the tooth bottom circle, and the bottom surface of the tooth located on the tooth bottom circle is called the tooth bottom. The tooth depth of the tooth portion 20 is the distance from the tooth bottom to the tooth tip, and is the sum of the tooth base and tooth addendum. The thickness of the tooth portion at the tooth base in the outer circumferential direction is the tooth thickness. The tooth tip circle diameter means the diameter of the circle connecting the tips of the tooth portions.

[0021] In FIG. 1, the sintered gear 100 has a main body 10 and a toothed portion 20. The main body 10 is cylindrical, and the toothed portion 20 is formed on its outer circumferential surface. The main body 10 may have a shaft hole 30 in its center. A shaft supporting the sintered gear 100 can be inserted into the shaft hole 30. The toothed portion 20 has a tooth width direction that is inclined with respect to the axial direction of the main body 10. The number and pitch of the toothed portion 20 formed on the outer circumferential surface of the main body 10 are not limited and can be set appropriately depending on the desired sintered gear. The toothed portion 20 also has a tooth depth and a tooth thickness that can be set appropriately depending on the desired sintered gear. Such a sintered gear 100 can be provided as a helical gear. In another example, the main body 10 may be cylindrical, and the main body 10 may be integrally molded with the shaft.

[0022] In some embodiments, the sintered gear 100 is preferably a metal sintered body, and more preferably an iron-based sintered body. The metal sintered body is a sintered body obtained by sintering a compact of metal powder. The main body 10 and the toothed portion 20 are preferably an integrally molded body, but may be formed of separate members joined together. If they are separate members, at least the toothed portion 20 is preferably a metal sintered body. Metal sintered bodies can be manufactured by powder metallurgy and may contain pores derived from the raw material powder. Because metal sintered bodies are porous, they have the advantage of absorbing vibrations and being quieter than metal materials formed through a melting process.

[0023] According to some embodiments, the tooth portion includes a main body portion and a tooth portion formed on an outer peripheral surface of the main body portion and having a tooth width direction inclined with respect to an axial direction, the tooth portion includes a densified layer formed on at least a surface layer of the tooth tip and the tooth face, and a non-densified layer formed on at least a surface layer of the tooth bottom, the densified layer has a thickness of 10 to 1000 μm, and the difference in density between the densified layer and the inside of the tooth portion is 0.2 to 1.0 g / cm 3 and the non-densified layer has a density of 7.2 g / cm 3 The following sintered gear is provided:

[0024] This sintered gear has a densified layer and a non-densified layer on the surface of the tooth portion, so that the density is low in the areas other than the densified layer, and vibrations are easily absorbed by the pores, improving the quietness of the sintered gear as a whole. In addition, the density difference between the densified layer and the inside of the tooth portion is 0.2 to 1.0 g / cm 3 This ensures high strength through the high density of the densified layer, while creating a density difference through the low density inside the teeth. If the density of a sintered gear is uniform throughout, vibrations generated on the tooth surfaces as the sintered gears mesh are transmitted through the sintered gear matrix to the shaft or center where the sintered gear is inserted. As mentioned above, when there is a density difference between the densified layer and the inside of the teeth, vibrations tend to selectively propagate through the high-density areas, so vibrations propagating from the vibration-generating position are diffused along the surface of the sintered gear and gradually attenuated as they propagate. In this way, creating a density difference between the densified layer and the inside of the teeth improves vibration damping, thereby increasing noise reduction.

[0025] In this disclosure, the surface layer of the tooth portion refers to a layer extending from the surface of the tooth portion to a depth of 1000 μm. The densified layer is a region with a higher density than the density of the bulk inside the tooth portion. The non-densified layer is a region with a density similar to the density of the bulk inside the tooth portion. The non-densified layer is defined as a region that shows the same pore distribution as the portion other than the tooth portion (mainly the main body portion) through image analysis. In a cross section of the tooth portion, the non-densified layer is observed as a region with no boundary from the bulk inside the tooth portion. The presence or absence of a non-densified layer is determined by cutting the sintered body in a direction perpendicular to the axial direction and observing an image of the cross section.

[0026] The densified layer may be formed at least partially on the tooth tip and tooth flank. The densified layer may be formed partially or entirely on the tooth tip, or partially or entirely on the tooth flank, or a combination of these. From the viewpoint of achieving high strength while maintaining quietness, the entire tooth tip may be formed as a densified layer, or the tooth flank may be at least partially formed as a densified layer, or the entire tooth flank may be formed as a densified layer. A densified layer may also be formed partially on the tooth root of the tooth portion.

[0027] The thickness of the densified layer of the tooth portion is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more. Within these ranges, the effect of selectively transmitting vibration and the effect of improving fatigue strength can be more fully achieved. The thickness of this densified layer is preferably 1000 μm or less, 900 μm or less, 800 μm or less, or 700 μm or less. From the viewpoint of quietness, the thickness of this densified layer is preferably 600 μm or less, more preferably 550 μm or less, and even more preferably 500 μm or less. For example, the thickness of the densified layer may be 10 to 1000 μm, 20 to 900 μm, 30 to 800 μm, or 30 to 700 μm. In other examples, the thickness of the densified layer may be 10 to 600 μm, 20 to 550 μm, or 30 to 500 μm.

[0028] The thickness of the densified layer is determined by observing an image of a cross section of a sintered gear cut perpendicular to the axial direction, measuring the thickness at 5 to 20 randomly selected points in the region where the densified layer is formed, and calculating the arithmetic mean of the measurements. The measurement points are appropriately determined taking into account the standard deviation of the thickness. For example, the thickness is measured at 6 randomly selected points in the region where the densified layer is formed on the tooth tip and tooth surface, and the thickness of the densified layer is calculated from the arithmetic mean of the measurements.

[0029] The non-densified layer of the tooth portion may be formed mainly on the tooth bottom. The non-densified layer may be formed on a portion of the tooth bottom or the entire surface. By forming a non-densified layer on the tooth bottom, it is possible to obtain the effect of absorbing vibrations by the non-densified layer on the nearby tooth bottom when the tooth surfaces mesh with each other. Note that a non-densified layer may be formed partially on at least one of the tooth tip and tooth surface of the tooth portion.

[0030] The non-densified layer of the teeth has a density of 7.2 g / cm 3 It is preferable that the density is 7.15 g / cm or less. This allows vibrations caused by driving to be absorbed and noise reduction to be improved. From this point of view, the density is 7.15 g / cm or less. 3 Below, 7.1g / cm 3 or less, or 7.05 g / cm 3 In this case, the lower limit is 6.8 g / cm or less. 3 , 6.6g / cm 3 , or 6.4 g / cm 3 For example, the density is 6.8 to 7.2 g / cm 3 , 6.8~7.15g / cm 3 , 6.6~7.1g / cm 3 , or 6.4 to 7.05 g / cm 3 It may be. In the present disclosure, the density of the non-densified layer of the tooth portion is determined by the image analysis method described above.

[0031] The density difference between the densified layer and the inside of the tooth is 0.2 to 1.0 g / cm 3 This makes it easier for vibrations caused by driving to be absorbed by the density difference, improving quietness. The density difference is preferably 0.3 to 0.8 g / cm.3 , or 0.4 to 0.6 g / cm 3 It is even more preferable that the area ratio of the densified layer, which is the density difference, to the entire surface of the tooth face may be, for example, 50 to 95%, 60 to 95%, or 70 to 95%.

[0032] In the present disclosure, the densities of the densified layer and the interior of the tooth portion are determined by the image analysis method described above. The density of the densified layer is determined by measuring the density at 5 to 20 points 100 μm deep from the surface of the densified layer and calculating the arithmetic mean value. The density inside the tooth portion is determined by measuring the density at 5 to 20 points 5 mm deep from the surface of the tooth portion or 5 mm deep from the surface of the tooth bottom and calculating the arithmetic mean value. The measurement points are appropriately determined taking into account the standard deviation of the density. For example, the measurement points may be 5 points. The difference (AB) between the average density of the inside of the tooth (A) and the average density of the densified layer (B) is calculated as the density difference.

[0033] In some embodiments, the porosity of the sintered gear as a whole is 8.5 to 18.7%, while the porosity of the densified layer is preferably 10% or less. A low porosity in this range of the densified layer can increase the strength of the teeth while maintaining quieter operation. The porosity of the densified layer is more preferably 10% or less, 8% or less, or 6% or less. Within these ranges, ease of processing and a good overall strength balance are maintained, while a difference in density occurs between the inside of the tooth portion, resulting in improved quietness. The overall porosity of the sintered gear is more preferably 8.5 to 18.7%, 8.5 to 16.1%, or 10 to 13.6%. This level of overall porosity provides strength and excellent vibration absorption, further improving quietness. Furthermore, due to the difference in density between the densified layer and the sintered gear, vibrations are selectively transmitted to the denser densified layer, further improving quietness. From this perspective, the overall density of the sintered gear is 6.4 to 7.2 g / cm. 3 , 6.6~7.2g / cm 3 , or 6.8 to 7.2 g / cm 3 is preferred. In the present disclosure, the overall porosity and overall density of the sintered gear are measured according to the Archimedes method, and the porosity of the densified layer is measured according to an image analysis method.

[0034] The matrix hardness at a depth of 200 μm from the surface of the tooth flank of the tooth portion is preferably Hv500 or more. Having a matrix hardness of this level can further improve noise reduction and wear resistance. From this perspective, the matrix hardness is preferably Hv550 or more, Hv600 or more, or Hv650 or more. A matrix hardness of this level can be obtained by carburizing, quenching, and tempering the sintered body. For example, the matrix hardness may be Hv500 to Hv900, Hv550 to Hv850, Hv600 to Hv800, or Hv650 to Hv750. A specific method for measuring the matrix hardness follows the method described in the Examples.

[0035] <Sintered metal> In some embodiments, the sintered gear is a sintered metal body. Sintered metal bodies can be manufactured by powder metallurgy and may contain porosity due to the raw material powder. Because sintered metal bodies are porous, they have the advantage of absorbing vibrations and being quieter than metal materials formed through a melting process.

[0036] As the metal sintered body, an iron-based, titanium-based, nickel-based, aluminum-based, copper-based, magnesium-based, alumina-based or other sintered body, or a mixture of these materials, can be used, but an iron-based sintered body is preferably used.

[0037] The iron-based sintered body preferably has a composition that contains iron in the largest amount among the constituent elements, and may have a composition that contains, for example, one or more elements selected from the group consisting of Ni, Mo, Cu, Mn, Cr, and C, with the balance being Fe and inevitable impurities.

[0038] As one composition example, the iron-based sintered body preferably contains 0.1 to 1.4 mass % of C, with the remainder being Fe and inevitable impurities. A low-carbon iron-based sintered body has a structure containing a large amount of soft phase, and is therefore more excellent in workability. By forming a hard phase in the surface layer of this iron-based sintered body by carburizing and quenching, the sintered gear as a whole can absorb vibrations and improve noise reduction, while also ensuring better wear resistance due to meshing in the teeth of the sintered gear. In this composition example, it is more preferable that C be 0.1 to 1 mass %.

[0039] Another example of the composition of the iron-based sintered body is preferably one or more elements selected from the group consisting of 0.1-5% Ni, 0.1-5% Mo, 0.1-3% Cu, 0.1-1% Mn, and 0.1-5% Cr, and 0.1-1.4% C, with the remainder consisting of Fe and unavoidable impurities. This composition improves hardenability, resulting in an appropriate amount of hard phase being contained throughout the sintered gear, thereby increasing the material strength of the entire sintered gear. Furthermore, carburizing and quenching make it easier to form a hard phase in the surface layer, thereby enabling more sufficient wear resistance of the teeth. In this composition example, 0.1-1% C by mass is more preferable.

[0040] As the raw material powder, a mixed powder in which each alloying element powder is blended with pure iron powder, a pre-alloyed steel powder in which each element is completely alloyed, and a partially diffused alloyed steel powder (also called a composite alloyed steel powder) in which each alloying element powder is partially adhered and diffused on the surface of pure iron powder or pre-alloyed steel powder can be used.

[0041] The particle size of the raw material powder is preferably 5 μm or more and 200 μm or less in terms of the particle diameter (D50) at which the integrated value in the volume-based particle size distribution is 50%. More preferably, the particle size of the raw material powder made of metal is 5 μm or more and 100 μm or less, or 10 μm or more and 50 μm or less in terms of D50. Here, D50 can be measured using a laser diffraction particle size distribution analyzer or the like.

[0042] An example of the composition of an iron-based sintered body will be described below. In the following description, % of the content ratio indicates % by mass. Ni: 0.1 to 5% Ni improves the hardenability of the iron-based sintered body, and after sintering and cooling, has the effect of including a hardened structure in the iron-based sintered body and the effect of remaining as austenite. Ni content of 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more can increase the strength of the material. Ni content is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less, and may be 2% or less, or 1% or less.

[0043] Mo: 0.1 to 5% Mo improves the hardenability of the iron-based sintered body and has the effect of making the iron-based sintered body contain a hardened structure after sintering and cooling. Mo content of 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more can increase the strength of the material. Mo content is preferably 5% or less, and may be 3% or less, 2% or less, or 1% or less.

[0044] Cu: 0.1 to 3% Cu diffuses into Fe to increase the strength of the material. By making the Cu content 0.1% or more, preferably 0.5% or more, and more preferably 1% or more, the diffusion into Fe can be promoted. Cu content is preferably 3% or less. This suppresses the generation of a soft Cu phase, preventing a decrease in material strength, and also suppresses the generation of a Cu liquid phase during sintering, improving the dimensional accuracy of the entire product. Cu content is preferably 3% or less, and may be 2% or less, 1% or less, or 0.5% or less.

[0045] Mn: 0.1 to 1% Mn improves the hardenability of the iron-based sintered body and has the effect of making the iron-based sintered body contain a hardened structure after sintering and cooling. Mn content of 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more can increase the strength of the material. Mn content is preferably 1% or less, and may be 0.9% or less, 0.8% or less, or 0.5% or less.

[0046] Cr: 0.1 to 5% Cr improves the hardenability of the iron-based sintered body and has the effect of imparting a hardened structure to the iron-based sintered body after sintering and cooling. Cr content of 0.1% or more, preferably 1.2% or more, and more preferably 1.5% or more can increase the strength of the material. Cr content is preferably 5% or less, more preferably 4.5% or less, and even more preferably 4% or less, and may be 3.5% or less, 3% or less, 2.5% or less, or 2% or less.

[0047] C: 0.1 to 1.4% A portion of the carbon dissolves in the iron to improve strength, while another portion remains in the pores of the iron-based sintered body. A carbon content of 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more, produces a metal structure with high matrix hardness, thereby enhancing material strength. A carbon content of 1.4% or less increases the amount of carburization during carburizing and quenching, facilitating the production of a hard surface structure. It is more preferable that the carbon content be 1.3% or less, 1.2% or less, 1.1% or less, or 0.5% or less. A low-carbon sintered gear has a soft phase throughout its structure, making it easy to machine. The strength of the teeth can be improved by subjecting the sintered gear to carburizing and quenching. Carbon can be added in the form of graphite powder to enhance the compressibility of the compact. From this perspective, the carbon content may be 1% or less, 0.5% or less, or 0.4% or less.

[0048] To obtain sufficient vibration absorption within the sintered gear, it is preferable that the interior of the sintered gear has a sufficient soft structure. Therefore, the C content in the overall composition of the sintered gear is preferably 0.1 to 1.0 mass%. Furthermore, to obtain sufficient hardenability during hardening, the overall composition of the sintered gear preferably contains at least one element selected from the group consisting of Ni, Mo, Cu, Mn, Cr, and C. For example, the sintered gear may be an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy. It is more preferable that the proportion of each element in these compositions is within the above-mentioned range. The iron-based sintered body is the remainder Fe, and may contain unavoidable impurities.

[0049] The raw powder of the iron-based sintered body may contain a molding lubricant. The inclusion of a molding lubricant prevents seizure when the powder compact is removed from the die. On the other hand, the inclusion of a molding lubricant hinders densification. By reducing the amount of molding lubricant, a high-density powder compact can be obtained. As the molding lubricant, metal soaps such as lithium stearate, zinc stearate, barium stearate, calcium stearate, and magnesium stearate can be used. Other examples of the molding lubricant that can be used include fatty acid amides such as lauric acid amide, stearic acid amide, and palmitic acid amide, and higher fatty acid amides such as ethylene bisstearic acid amide.

[0050] <Manufacturing method for sintered gears> According to some embodiments, a method for manufacturing a sintered gear can be provided, which includes a main body portion and teeth formed on the outer peripheral surface of the main body portion and whose tooth width direction is inclined with respect to the axial direction, the method including pressurizing a raw material powder to obtain a green compact, sintering the green compact to obtain a sintered body, and rolling the sintered body, wherein a rolling allowance is formed on the tooth surfaces of the teeth portion of the sintered body, and the thickness of the rolling allowance is 200 μm or less.

[0051] According to this manufacturing method, by including a step of rolling the sintered body, it is expected that the processing accuracy of the tooth portion will be improved, leading to quieter operation. Furthermore, by setting the thickness of the rolling allowance to 200 μm or less, a densified layer with an appropriate porosity, which leads to quieter operation as described above, can be obtained on the surface layer of the tooth portion. From this perspective, the thickness of the rolling allowance may be 200 μm or less, 180 μm or less, or 150 μm or less, with the lower limit being 10 μm or more, 30 μm or more, or 50 μm or more, respectively. The method may further include carburizing, quenching, and tempering the rolled sintered body, which can further increase the strength of the teeth.

[0052] The thickness of the rolling allowance is determined by measuring the tooth profile of the sintered body before rolling, comparing it with the tooth profile after rolling, and calculating the difference. In this process, the shape of the tooth part at five points in the area where the rolling allowance is formed is measured, and the thickness is determined as the arithmetic average value.

[0053] Furthermore, according to some embodiments, it is possible to provide a method for manufacturing a sintered gear including a main body and teeth formed on the outer peripheral surface of the main body and having tooth portions whose tooth width direction is inclined with respect to the axial direction, the method including pressurizing a raw material powder to obtain a green compact, sintering the green compact to obtain a sintered compact, and rolling the sintered compact, wherein the sintered sintered compact has a matrix hardness of Hv200 or less on the surfaces of the tooth flanks of the tooth portions.

[0054] According to this manufacturing method, by including a step of rolling the sintered body, it is expected that the processing accuracy of the tooth portion can be improved, leading to quieter operation. Furthermore, since the matrix hardness of the tooth flank surface of the sintered body after sintering is Hv200 or less, an appropriate amount of densified layer can be formed in the subsequent rolling step, and an appropriate densified layer that leads to quieter operation as described above can be obtained in the surface layer of the tooth portion. From this perspective, the matrix hardness of the tooth flank surface of the tooth portion may be Hv200 or less, Hv190 or less, or Hv180 or less, with the lower limit being Hv70 or more. For example, the matrix hardness may be Hv70 to Hv200. The matrix hardness is measured according to the method described in the Examples.

[0055] The method may further include carburizing, quenching, and tempering the rolled sintered body. By carburizing, quenching, and tempering the sintered body having a low matrix hardness, carbon can penetrate deep into the teeth, thereby further increasing the strength of the teeth.

[0056] The method for producing a sintered gear includes a main body and teeth formed on the outer peripheral surface of the main body, the teeth being inclined in the tooth width direction with respect to the axial direction, and includes press-molding a raw material powder to obtain a green compact, sintering the green compact to obtain a sintered body, and rolling the sintered body, and the sintered body may have a rolling allowance formed on the tooth flanks of the teeth, the rolling allowance having a thickness of 200 μm or less, and the sintered body may have a matrix hardness of Hv200 or less on the surface of the tooth flanks of the teeth. This makes it possible to more appropriately form a densified layer and a non-densified layer, thereby providing a sintered gear that is quieter.

[0057] Hereinafter, a method for producing an iron-based sintered gear will be described as an example of a method for producing a sintered gear. An iron-based sintered gear can be obtained by mixing raw material powders to obtain a desired composition, pressurizing the mixture to produce a green compact, and then sintering the green compact. Other metal sintered bodies can also be produced in the same way.

[0058] First, a method for obtaining a powder compact by pressure molding of raw material powder will be described. Raw material powder having the composition described above can be used for powder molding. The powder compact can be typically produced by using a press machine having a mold capable of uniaxial pressure. A typical mold includes a die having a through hole, and an upper punch and a lower punch that are fitted into the upper and lower openings of the through hole, respectively. The inner peripheral surface of the die and the end face of the lower punch form a cavity. The raw material powder is filled into the cavity. The powder compact can be produced by compressing the raw material powder in the cavity with the upper and lower punches at a predetermined molding pressure. The powder compact may also be molded by using a press machine having a mold capable of biaxial pressure.

[0059] An example of a method for manufacturing a powder compact of a sintered gear is described below. A die is prepared that forms a cavity in the shape of the sintered gear, and a punch that fits into the inner peripheral surface of the die. The inner peripheral surface of the die is formed with teeth and tooth grooves that conform to the shape of the teeth of the sintered gear. In other words, the inner peripheral surface of the die is formed with the longitudinal direction of the teeth and tooth grooves inclined relative to the axial direction. The outer peripheral surface of the punch is formed with a shape that fits into the inner peripheral surface of the die. The die and punch are pressed in the axial direction and rotated in the outer peripheral direction along the shape of the teeth, so that the punch fits into and is inserted into the die.

[0060] In this example, the punch at one axial end of the die may be fixed to the bottom surface of the die, and the punch at the other end may be inserted by being pressed and rotated against the die. In another configuration, punches at both axial ends of the die may be inserted by being pressed and rotated against the die. In this configuration, the metal powder filled in the die is compressed from both axial ends, allowing for more precise control of the compression ratio to produce a green compact. Furthermore, since control of the density distribution of a green compact also depends on the relative movement of the die and punch, being able to insert punches into the die from both axial ends allows for more precise control of the compression ratio of the green compact. Furthermore, in this configuration, a load is applied to the tooth portion not only linearly from the axial direction but also in the rotational direction, allowing for appropriate control of the density and porosity of the tooth surface.

[0061] The powder compact may be larger than the final product in consideration of processing allowances, and the teeth of the powder compact may be larger than the teeth of the final product in consideration of rolling allowances.

[0062] The pressure (surface pressure) of the uniaxial pressing can be 600 MPa or more. Increasing the surface pressure can increase the relative density of the powder compact. A preferred surface pressure is 700 MPa or more, and a more preferred surface pressure is 800 MPa or more. Furthermore, the surface pressure is preferably 1000 MPa or less. This allows an appropriate non-densified layer to be formed in the powder compact, improving vibration absorption and noise reduction.

[0063] To prevent the metal powder from sticking to the mold, an external lubricant may be applied to the inner peripheral surface of the mold (the inner peripheral surface of the die or the pressing surface of the punch). Examples of external lubricants that can be used include metal soaps such as lithium stearate and zinc stearate. Other examples include fatty acid amides such as lauric acid amide, stearic acid amide, and palmitic acid amide, and higher fatty acid amides such as ethylene bisstearic acid amide. By reducing the amount of external lubricant, the density of the surface can be increased.

[0064] Next, a method for sintering the powder compact to obtain a sintered body will be described. The powder compact is preferably sintered in a non-oxidizing atmosphere at a maximum holding temperature of 900°C to 1250°C. This maximum holding temperature is preferably 900°C or higher, more preferably 1000°C or higher. This promotes the diffusion of elements such as Ni, Mo, Cu, Mn, and Cr into Fe, resulting in a metal structure with high matrix hardness and increased tensile strength. Furthermore, this maximum holding temperature is preferably 1250°C or lower, more preferably 1200°C or lower. This suppresses excessive diffusion of other elements into Fe, preventing a decrease in material strength. The powder compact is preferably held at the maximum holding temperature for 10 to 90 minutes.

[0065] After firing, the sintered body is preferably cooled at a cooling rate of 2°C / min to 150°C / min. At this cooling rate, it is preferable to cool the body from the maximum holding temperature to a temperature range of 900 to 200°C.

[0066] This cooling rate may be 2°C / min or more, more preferably 5°C / min or more, and even more preferably 10°C / min or more. This allows the base structure to contain an appropriate amount of ferrite phase, pearlite phase, bainite phase, martensite phase, or a combination thereof, thereby further increasing the material strength. This cooling rate may be 150°C / min or less, preferably 100°C / min or less, and more preferably 50°C / min or less. This prevents the base structure from containing an excessive amount of martensite phase, and further improves the workability due to the soft structure.

[0067] As described above, the sintered body after sintering preferably has a matrix hardness of Hv200 or less at least on the surface of the tooth flank of the tooth portion before rolling. The sintered body after sintering preferably has a matrix hardness of Hv200 or less on the tooth flank of the tooth portion and on at least one of the tooth tip and tooth bottom surfaces before rolling.

[0068] In sintered compacts, the metal material does not completely melt, so gaps form between the particles, which become pores. The pores absorb vibrations in the sintered gear, which can contribute to improving noise reduction.

[0069] The sintered body obtained as described above may be optionally post-processed to a shape similar to that of the final product. This optional post-processing can eliminate the effects of dimensional errors, thermal expansion, burrs, etc., and can also process additional structures such as steps, grooves, and screw holes. This post-processing can be performed more easily if the sintered gear is made of a low-carbon material.

[0070] The sintered body obtained as described above may be subjected to pressure processing to densify the surface of the tooth portion. Examples of pressure processing include sizing, rolling, and extrusion. These pressure processing methods reduce the porosity of the surface and form a highly dense densified layer. Pressure processing may be performed in stages in multiple steps, or multiple types of processing may be combined. The shape of the sintered body may be determined taking into account the reduction in processing allowance due to pressure processing in the final gear shape. The reduction in processing allowance that can be achieved by a single pressure processing operation varies depending on the processing conditions, i.e., the material being processed, the processing pressure, and the processing temperature. For example, when rolling is used, the desired densified layer can be formed in a single process by adjusting the rolling allowance, rolling pressure, and temperature according to the thickness of the densified layer to be formed. Preferably, the pressure processing is rolling.

[0071] Next, we will explain the method of rolling the sintered body. Rolling can be performed using gear-shaped rolling dies. The gear to be rolled is sandwiched between two rolling dies, and the dies are rotated in the same direction while being pressed toward the center, thereby forming a densified layer. Rolling not only makes the shape of the teeth more precise, but also improves the strength of the tooth surfaces of the teeth.

[0072] The sintered body obtained as described above may have a rolling allowance in the tooth portion. By providing an extra rolling allowance in the shape of the tooth portion of the final product, a densified layer is formed on the surface of the tooth portion by the rolling process, allowing for more precise control of the density and porosity of the surface layer of the tooth portion. As described above, the rolling allowance may be 200 μm or less. This makes it easier to form an appropriate amount of densified layer after rolling, making it possible to provide a sintered gear that is quieter and has excellent strength.

[0073] The rolling allowance is preferably formed at least on the tooth flank of the tooth portion. This allows a densified layer with sufficient thickness and density difference to be formed on the tooth flank after rolling. Furthermore, by rolling from the tooth tip, a densified layer with sufficient thickness and density difference can also be formed on the tooth bottom after rolling. Preferably, a rolling allowance is also formed on the tooth tip. The rolling allowance on the tooth tip should be in the same thickness range as the rolling allowance on the tooth flank. The rolling allowance on the tooth tip and the rolling allowance on the tooth flank may be the same thickness or different thicknesses.

[0074] When rolling from the tooth tip, the rolling die is pressed against the tooth flank from the tooth tip. In this case, the rolling die does not need to be pressed at least partially against the tooth root, and does not need to be pressed against the tooth root over the entire surface. This allows a non-densified layer to be formed on the tooth root.

[0075] After rolling, a densified layer is formed on the surface of the tooth flank and tooth tip of the sintered body. Forming a densified layer on the tooth flank and tooth tip not only increases wear resistance but also creates a density difference with the interior of the tooth, improving noise reduction. The thickness of the densified layer may be 10 to 1000 μm. In particular, it is even more preferable if the thickness of the densified layer formed on the tooth tip and tooth flank of the tooth is 10 to 1000 μm.

[0076] Next, methods for quenching and tempering the rolled sintered body will be described. The sintered gear may be optionally quenched after rolling. Quenching is preferably performed to strengthen the metal structure. Quenching is preferably performed by a process accompanied by rapid cooling. For example, strengthening processes such as carburizing and quenching, bright hardening, induction hardening, and carbonitriding heat treatment can be performed, but carburizing and quenching are particularly preferred.

[0077] Carburizing and quenching can be performed by heat treating the gear at 850-1050°C in a carburizing gas atmosphere, followed by rapid cooling. During carburizing and quenching, carbon penetrates through the pores on the surface of the teeth and the inner surface of the body, forming a hard structure on the surface. This increases the hardness of the matrix on the surface of the sintered gear, making it resistant to wear caused by gear meshing. Meanwhile, because the soft structure remains inside, the vibration-damping properties of the sintered gear as a whole are maintained, improving noise reduction.

[0078] In carburizing and quenching, the cooling rate is preferably 50°C / sec to 300°C / sec, and the material is preferably cooled from the maximum holding temperature to a temperature range of 25 to 150°C at this cooling rate.

[0079] In carburizing and quenching, after heat treatment, the gear is preferably quenched in oil at 30 to 150°C, and more preferably at 50 to 120°C. This prevents the hard phase from being excessively incorporated into the matrix, improving the vibration absorption of the soft structure and noise reduction. If the oil temperature is higher than 200°C, the cooling rate will be insufficient, and the surface will not harden, remaining a soft phase, and the internal matrix will have a low hardness. On the other hand, if the oil temperature is lower than 30°C, the cooling rate will be too fast, causing quench cracks, which may become the starting point for gear chipping or fracture.

[0080] After carburizing and quenching, the sintered body may be subjected to a treatment to restore impact resistance, such as tempering. The tempering temperature is preferably about 100 to 300° C. The tempering holding time can be, for example, 10 to 180 minutes.

[0081] The sintered gear of the present disclosure can be used in vehicles that use electricity as a drive source. Examples of vehicles powered by electricity include electric vehicles (EVs), hybrid vehicles, plug-in hybrid vehicles, trains, motorcycles, electric bicycles, mopeds, and construction vehicles. Among these, EVs are rapidly gaining popularity as a product with the potential to bring about major changes in global energy issues. As EVs gain increasing attention, electric powertrains are becoming increasingly important. Electric powertrains consist of a drive motor, which serves as the drive source, an inverter that drives the motor, and power transmission mechanisms such as a reduction gear and a differential gear unit (diff). Furthermore, among electric powertrains, electric axles, which integrate a drive motor, a power transmission mechanism, and in some cases, an inverter, have become increasingly common in recent years. The sintered gear of this embodiment can be used in these power transmission mechanisms.

[0082] The electric axle is key to differentiating EVs, as it affects the driving range from a full charge and power performance. For example, by increasing the rotation speed of the drive motor in the electric axle, the motor can be made smaller while maintaining the same output. On the other hand, in EVs without engines, the gear noise of the reducer and differential becomes noticeable due to the increased rotation speed, resulting in a loss of quietness. To address this issue, using the sintered gear of this embodiment in the reducer can increase strength by improving the meshing ratio between the gears, and improve quietness through smooth meshing. [Example]

[0083] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0084] Example 1 Graphite powder was added to iron alloy powder (particle size: 80 mesh) with a composition of Fe-0.5%Ni-0.5%Mo by mass ratio to prepare a mixed powder (composition: Fe-0.5%Ni-0.5%Mo-0.3%C) with a graphite powder ratio of 0.3 mass%. This mixed powder was used as the raw material powder to carry out the following operations.

[0085] [Sample 1] A die with helical teeth and tooth spaces was prepared, along with upper and lower punches shaped to fit into the die. The raw material powder was filled into the die, and the upper and lower punches were pressed against the die while rotating from top to bottom, resulting in a helical gear-shaped green compact. The green compact was prepared so that the final sintered gear would have a tooth tip circle equivalent diameter of 65 mm. The green compact was also prepared so that the tooth width direction of the final sintered gear would be inclined 30° relative to the axial direction. The teeth were formed so that the rolling allowance during rolling would be 100 μm. The resulting green compact was sintered in a non-oxidizing atmosphere at 1195°C for 30 minutes to obtain a sintered gear. This sintered gear was then subjected to rolling. The rolling process was performed by sandwiching the sintered gear between a pair of rolled gears and applying pressure. The rolled sintered gear was maintained at 900°C for 60 minutes in a carburizing gas atmosphere, then rapidly cooled at 100°C / second to be carburized and quenched. It was then maintained in air at 150°C for 100 minutes for tempering, yielding the sintered gear of Sample 1.

[0086] [Matrix hardness of the tooth surface after sintering] After sintering and before rolling, the matrix hardness of the tooth surface of the tooth portion of the sintered body was measured by the following procedure. The matrix hardness, Hv, is measured by applying a load of 100g to the surface of the matrix for high hardness and a load of 10g to the surface of the matrix for low hardness. Hv is measured by the method specified in JIS Z 2244. The measurement conditions are as follows: Testing machine: Mitutoyo Corporation "HM-200" Test temperature: Room temperature (25°C) Test load: 100g, 10g

[0087] Densified layer thickness After carburizing, quenching and tempering, the thickness of the densified layer was measured by the following procedure. The thickness of the densified layer is determined by cutting the sintered body in a direction perpendicular to the axial direction, measuring the thickness at six random measurement points in the region where the densified layer is formed, and calculating the arithmetic mean. The measurement points are appropriately determined taking into account the standard deviation of the thickness.

[0088] [Non-densified layer of the teeth] After carburizing, quenching and tempering, the presence or absence of a non-densified layer in the teeth was measured by the following procedure. The presence or absence of a non-densified layer was determined by cutting the sintered body perpendicular to the axial direction and observing the image of the cross section. The non-densified layer was defined as the part that showed the same pore distribution as the part other than the tooth part (mainly the main body part) through image analysis. In sample 1, the non-densified layer was observed at the tooth root, and the densified layer was observed at the tooth tip and tooth surface.

[0089] Density of undensified layer After carburizing, quenching and tempering, the density of the non-densified layer was measured by the following procedure. The density of the non-densified layer is determined by image analysis of the surface of the sintered body. More specifically, five randomly selected images of multiple observation fields are taken of the surface of the sintered body at a magnification of 200x. The images of each observation field are taken from positions that are as evenly distributed as possible on the surface. Next, the image is analyzed using image analysis software to determine the porosity, and the true density of iron, 7.87 g / cm 3 The density distribution is calculated using

[0090] [Density difference between the densified layer and the inside of the tooth] After carburizing, quenching and tempering, the density difference between the densified layer and the inside of the tooth portion was measured by the following procedure. The density of the densified layer and the interior of the teeth is measured by cutting the sintered body in a direction perpendicular to the axial direction and performing image analysis of the cross section. The density of the densified layer is determined as the arithmetic mean value of measurements taken at five points 100 μm deep from the surface of the densified layer. The density inside the tooth is determined by measuring the density at five points 5 mm deep from the surface of the tooth or 5 mm deep from the surface of the tooth bottom, and calculating the arithmetic mean value. The difference (AB) between the average density of the inside of the tooth (A) and the average density of the densified layer (B) is calculated as the density difference.

[0091] [Matrix hardness at a depth of 200 μm from the tooth surface] After carburizing, quenching and tempering, the matrix hardness at a depth of 200 μm from the surface of the tooth flank of the tooth portion was measured by the following procedure. The sintered body is cut in a direction perpendicular to the axial direction, and the matrix hardness is measured at a depth of 200 μm from the surface of the tooth flank of the tooth portion in the cross section. The matrix hardness, Hv, is measured by applying a load of 100 g to the surface of the matrix for high hardness and a load of 10 g for low hardness. Hv is measured by the method specified in JIS Z 2244. The measurement conditions are the same as above.

[0092] [Table 1]

[0093] [Evaluation method] (Sound pressure level evaluation) The sound pressure level was measured using Sample 1. The conditions for measuring the sound pressure level were as follows: Testing machine: Gear tester manufactured by Ono Sokki Co., Ltd. Temperature: Room temperature (25℃) Lubricating oil: ATF (Automatic Transmission Fluid) dripping Torque: 15Nm Rotation speed: Adjustable in the range of 0 to 2,000 rpm Counterpart material: SCM420H

[0094] The sound pressure level of the sintered gear of Sample 1 was 85 dB, demonstrating excellent quietness. This sintered gear had densified layers on the tooth tips and tooth surfaces and a non-densified layer on the tooth roots. The thickness of the densified layer, the density difference between the densified layer and the interior of the tooth, and the density of the non-densified layer were all within the desired ranges. [Explanation of symbols]

[0095] 10 Main body 20 Tooth 21 Tooth tip 22 Tooth dedendum 23a Tooth surface 23b Tooth surface 24 Root 30 Shaft hole 100 Sintered Gears

Claims

1. a main body portion; and teeth formed on an outer peripheral surface of the main body portion, the teeth width direction of which is inclined with respect to an axial direction, the tooth portion includes a densified layer formed on at least a surface layer of the tooth tip and tooth face, and a non-densified layer formed on at least a surface layer of the tooth bottom, The thickness of the densified layer is 10 to 1000 μm, The density difference between the densified layer and the inside of the tooth portion is 0.2 to 1.0 g / cm ３ and The non-densified layer has a density of 7.2 g / cm ３ Below is a sintered gear.

2. 2. The sintered gear according to claim 1, wherein the matrix hardness at a depth of 200 μm from the surface of the tooth flank of the tooth portion is Hv 500 or more.

3. A method for manufacturing a sintered gear including a main body and teeth formed on an outer peripheral surface of the main body, the teeth having a tooth width direction inclined with respect to an axial direction, the method comprising: Pressing the raw material powder to obtain a green compact; sintering the powder compact to obtain a sintered body; and rolling the sintered body; a rolling allowance formed on the tooth surface of the tooth portion of the sintered body after sintering, the rolling allowance having a thickness of 200 μm or less.

4. A method for manufacturing a sintered gear including a main body and teeth formed on an outer peripheral surface of the main body, the teeth having a tooth width direction inclined with respect to an axial direction, the method comprising: Pressing the raw material powder to obtain a green compact; sintering the powder compact to obtain a sintered body; and rolling the sintered body; the sintered body after sintering has a matrix hardness of Hv200 or less on the surface of the tooth flank of the tooth portion.

5. 5. The method for producing a sintered gear according to claim 3, further comprising carburizing, quenching, and tempering the rolled sintered body.

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