JP2009074110A - Gear part excellent in fitness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gear part having excellent pitting resistance and fitness by reducing any abnormal layer and adequately controlling the distribution of carbide on a surface layer part in a gear. <P>SOLUTION: The gear part is obtained by forming steel having the predetermined composition of chemical components into a predetermined shape, and allowing the steel to be subjected to the carburization or nitrocarburizing. The depth of a surface abnormal layer is ≤5 μm, and the carbide area ratio Fm at the 50 μm depth position from a gear surface is 3-20%. Further, the ratio of the carbide area ratio Fs at the 25 μm depth position from the gear surface to the carbide area ratio Fm (Fs/Fm) is satisfied to be <1.0. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gear component, and particularly to a gear component that exhibits excellent conformability and good pitting resistance when used in a power transmission section.

  For example, gear parts used in power transmission parts such as automobile transmissions are Cr case-hardened steel (SCr material) and Cr-Mo case-hardened steel (SCM material) specified in JIS G4104, JIS G4105, JIS G4103, etc. , Ni-Cr-Mo case-hardened steel (SNCM material) and other mechanical structural steels are formed into a predetermined shape and then carburized or carbonitrided (hereinafter simply referred to as “carburizing / carbonitriding”). In general, it is produced by performing a surface hardening treatment.

  In recent years, demands for higher output and smaller and lighter weight have increased, and therefore, the load stress on steel parts such as gears used in the power transmission section tends to increase. Under such circumstances, the conventional mechanical structural steel and surface-hardened steel as described above are difficult to cope with the above severe use environment.

  For this reason, especially in a contact environment with slip, in order to suppress contact surface peeling damage (ie, pitching damage) due to an increase in contact surface pressure, Si is added to optimize components, It has been studied to improve the surface hardness of steel by optimizing carbonitriding conditions.

  In the carburizing / carbonitriding process, normally, the conditions are controlled so that carbide is not generated in the steel surface layer. For example, in the carburizing / carbonitriding process, C is supplied to the steel surface. By increasing the carbon concentration in the steel surface layer, the carbide is intentionally precipitated to increase the steel surface hardness, so-called high-concentration carburizing treatment or high-concentration carbonitriding (hereinafter simply referred to as “high-concentration carburizing”). (Sometimes referred to as “high-concentration carbonitriding treatment”) (for example, Patent Document 1).

  On the other hand, with the increase in the surface hardness of the gear, there is a problem that the “familiarity” of the tooth surfaces engaged with the gear pair is lowered. Normally, the friction caused by the engagement of the gear pair corrects the tooth surface processing error and the deformation of the tooth profile due to heat treatment distortion, and suppresses an increase in surface pressure on the tooth surface due to excessive gears.

  However, if the surface hardness is increased in order to suppress the above-mentioned pitching damage, the effect on the conformability is reduced, and an excessive increase in the surface pressure occurs on a part of the tooth surface. May occur early.

  By the way, in normal carburizing / carbonitriding treatment or high-concentration carburizing / high-concentration carbonitriding treatment, there is a carburizing abnormal layer (hereinafter, sometimes simply referred to as “abnormal layer”) consisting of a grain boundary oxide layer or an incompletely quenched layer. This will appear as deterioration of the gear characteristics (decrease in tooth root bending fatigue strength, pitting resistance, etc.). From the viewpoint of improving the conformability, a technique for actively using the carburized abnormal layer has also been proposed (for example, Patent Document 2), which is a fundamental measure for suppressing the characteristic deterioration. That's not true.

  Compared to conventional carburizing / carbonitriding and high-concentration carburizing / high-concentrating carbonitriding, carburizing / carbonitriding under reduced pressure or high-concentration / high-concentrating carbonitriding (hereinafter referred to as “vacuum carburizing / carbonitriding” Is also proposed (for example, Non-Patent Documents 1 and 2). In such a vacuum carburizing / carbonitriding process, the abnormal layer is not substantially formed on the gear surface layer, and the hardenability of the surface layer can be secured, so that the surface layer hardness is improved and the pitting resistance is improved.

However, under the conditions of the vacuum carburizing and nitriding treatments that have been proposed so far, a gear having a surface property that provides good “fitability” is not always obtained, and in particular, the amount of C in the surface layer is excessive. In reality, there is a tendency for compatibility to deteriorate. Also in high-concentration carburization and high-concentration carbonitriding under reduced pressure, the carbide area ratio of the surface layer is excessive under the conditions reported so far, leading to a decrease in conformability.
JP 2004-285384 A JP 2000-322536 A "Heat treatment", Volume 37, No. 3, P154-159, June 1997, published by Japan Heat Treatment Technology Association "Ishikawajima Harima Technical Report" Vol. 45 No. 1, P15-20 (2005-3), issued by Ishikawajima-Harima Heavy Industries Ltd.

  The present invention has been made in view of such circumstances. The purpose of the present invention is to reduce the abnormal layer and appropriately control the carbide distribution in the surface layer portion of the gear, thereby improving the compatibility with the pitting resistance. It is also to provide an improved gear part.

  The gear parts of the present invention that could achieve the above-mentioned object are: C: 0.10 to 0.30% (meaning of mass%, the same shall apply hereinafter), Si: 1.5% or less (not including 0%) , Mn: 1.5% or less (excluding 0%), Cr: 0.5 to 2.5%, Mo: 0.5% or less (excluding 0%), respectively, the balance: iron and A gear part obtained by forming a steel material made of inevitable impurities into a predetermined shape and then carburizing or carbonitriding, and having a surface abnormal layer depth of 5 μm or less and a carbide area at a depth of 50 μm from the tooth surface The rate Fm is 3 to 20%, and the ratio (Fs / Fm) of the carbide area ratio Fs and the carbide area ratio Fm at a position 25 μm deep from the tooth surface satisfies less than 1.0. It has a gist. In this gear part, the N concentration [Nm] at a depth of 50 μm from the tooth surface is preferably 0.05 to 1.0%.

  In the steel material constituting the gear part of the present invention, if necessary, as another element, (a) Ni: 2.0% or less (not including 0%), (b) B: 0.005% or less (0 %), (C) Al: 0.05% or less (not including 0%), Nb: 0.1% or less (not including 0%), Ti: 0.1% or less (0% 1) or more selected from the group consisting of 0.1% or less (not including 0%) and (d) N: 0.03% or less (not including 0%) (e) S : 0.04% or less (not including 0%), Ca: 0.01% or less (not including 0%), Mg: 0.01% or less (not including 0%), Pb: 0.1% It is also effective to include one or more selected from the group consisting of the following (not including 0%) and Bi: 0.05% or less (not including 0%). So that the characteristics of the gear part is further improved according to the type of elements to be.

  According to the present invention, the chemical component composition of the steel material is specified, and the depth of the abnormal surface layer is reduced by appropriately controlling the high-concentration carburizing / high-concentration nitriding treatment conditions, and the carbide distribution in the tooth surface layer. By optimizing the gear ratio, it is possible to realize a gear part that improves not only the pitching resistance but also the conformability, and such a gear part is extremely useful as a gear part used in a power transmission unit.

  The present inventor has long studied to improve the “familiarity” of gear parts under the idea of applying vacuum carburizing / carbonitriding to reduce abnormal layers. And, while using a steel material having a predetermined chemical composition, it was possible to improve the “familiarity” if the carbon concentration distribution on the tooth surface surface layer was appropriately controlled by appropriately adjusting the conditions of vacuum carburizing / nitriding treatment. It has been found that gear parts can be realized and its technical significance has been recognized, so it has been filed earlier (Japanese Patent Application No. 2006-307093).

  The previously proposed technology is based on the premise that carbide is not formed on the steel surface layer, and the gear density is improved by optimizing the carbon concentration distribution in the steel surface layer (particularly the tooth surface layer). It is a part realized. The present inventor calls high-concentration carburization / high-concentration carbonitriding under reduced pressure (hereinafter referred to as “vacuum high-concentration carburizing / high-concentration carbonitriding”), which provides higher hardness than ordinary vacuum carburizing / carbonitriding. And on the premise of positively forming carbides on the steel surface layer, we examined the requirements for exhibiting excellent “familiarity”. As a result, while using a steel material having a predetermined chemical composition, processing by optimizing the conditions of vacuum high-concentration carburization / high-concentration carbonitriding, and controlling the carbide distribution in the tooth surface layer as described above, The present invention has been completed by finding that gear parts suitable for the purpose can be realized.

  In the gear part of the present invention, basically, the carbide area ratio Fm at the position of 50 μm depth from the tooth surface is 3 to 20%, and the carbide area ratio Fs at the position of 25 μm depth from the tooth surface is The ratio of the carbide area ratio Fm (Fs / Fm) satisfies less than 1.0. The reason for defining these requirements is as follows.

  The 25 μm depth position from the tooth surface is easily removed by wear due to the engagement of the gear pair, and the tooth surface strength that affects pitching damage and the like depends on the carbide area ratio Fm at the 50 μm depth position from the tooth surface. Become. From such a viewpoint, it is necessary to ensure the strength of the tooth surface by setting the carbide area ratio Fm at a depth of 50 μm from the tooth surface to at least 3% or more. However, if the carbide area ratio Fm exceeds 20%, the carbides are coarsened, and the strength of the tooth surface is lowered. The preferable lower limit of the carbide area ratio Fm is 5%, and the preferable upper limit is 17%.

  Further, from the viewpoint of further reducing the pitching damage in the gear part, it is preferable to control the N concentration [Nm] at a depth of 50 μm from the tooth surface to be 0.05 to 1.0% (the range thereof). The reason for setting is the same as in C). By setting the N concentration [Nm] at a depth of 50 μm from the tooth surface to 0.05% or more, the strength of the tooth surface can be further increased. However, if the N concentration [Nm] exceeds 1.0%, coarse nitrides are precipitated, and the strength of the tooth surface is lowered.

  On the other hand, the conformability of the gear is influenced by the properties at a position (25 μm depth position from the tooth surface) that is easily removed by wear due to the engagement of the gear pair. That is, it is considered that unevenness caused by machining or the like is reduced by wear due to fitting (that is, the surface roughness is reduced), excessive stress generated on the surface is reduced, and as a result, conformability is improved. . In order to ensure such a state, the ratio (Fs / Fm) between the carbide area ratio Fs and the carbide area ratio Fm at a position 25 μm deep from the tooth surface needs to be at least less than 1.0. In addition, the preferable minimum of the said ratio (Fs / Fm) is 0.3, and a preferable upper limit is 0.9.

  In the gear part of the present invention, the abnormal layer is reduced as much as possible from the viewpoint of reducing pitching damage. However, in order to exert such an effect, the depth of the surface abnormal layer must be 5 μm or less. It is. However, since the gear part of the present invention is manufactured by vacuum high-concentration carburization / high-concentration carbonitriding (details will be described later), the depth of the surface abnormal layer is basically less than 5 μm. Will not be deep. The above-described “depth position of 25 μm (or 50 μm) from the tooth surface” is intended to include the thickness of the abnormal surface layer.

  In the gear part of the present invention, it is necessary to appropriately control the chemical component composition of the steel material as the raw material. The reason for limiting the basic component range is as follows.

[C: 0.10 to 0.30%]
C is an element necessary for ensuring the hardness of the core portion as the steel part for machine structural use. In order to exert such an effect, it is necessary to contain at least 0.10%. However, when the C content is excessive, the hardness of the core is excessive and the toughness of the gear is deteriorated. For these reasons, the C content needs to be 0.30% or less. The minimum with preferable C content is 0.15%, and a preferable upper limit is 0.25%.

[Si: 1.5% or less (excluding 0%)]
Si effectively acts as a deoxidizing element during the melting of steel, and also effectively acts to improve wear resistance and pitting resistance. These effects increase as the content increases. However, if the Si content is excessive, not only the effects are saturated but also the machinability deteriorates. The minimum with preferable Si content is 0.3%, and a preferable upper limit is 1.0%.

[Mn: 1.5% or less (excluding 0%)]
Mn acts effectively as a deoxidizer / desulfurizer and a hardenability improving element. These effects increase as the content increases. However, if the Mn content is excessive, the amount of retained austenite after the high-concentration carburizing / high-concentration carbonitriding treatment increases and the hardness decreases. It is necessary to make it 5% or less. The minimum with preferable Mn content is 0.3%, and a preferable upper limit is 1.2%.

[Cr: 0.5 to 2.5%]
Cr is an element effective for improving the hardenability, and in order to exert such an effect, it is necessary to contain 0.5% or more. However, if the Cr content is excessive, the hardness of the outermost surface increases and good conformability cannot be ensured, so 2.5% or less is necessary. The minimum with preferable Cr content is 0.8%, and a preferable upper limit is 1.7%.

[Mo: 0.5% or less (excluding 0%)]
Mo, like Cr, is an element effective in improving hardenability, and an effect of reducing abnormal layers in the gear surface layer is also recognized. These effects increase as the content increases, but if the Mo content becomes excessive, the hardness of the outermost surface increases and good conformability cannot be secured, so it is necessary to make it 0.5% or less. is there. The minimum with preferable Mo content is 0.05%, and a preferable upper limit is 0.3%.

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities (for example, P, O, etc.). If necessary, the following elements are actively contained to further enhance the characteristics. Is also effective. The reasons for limiting the range when these elements are contained are as follows.

[Ni: 2.0% or less (excluding 0%)]
Ni is an element effective for improving the fatigue strength of gear parts, and is contained if necessary. However, if the Ni content is excessive, the conformability of the outermost layer is lowered, so that it is preferably 2.0% or less (more preferably 1.0% or less). In addition, the preferable minimum for exhibiting the said effect is about 0.4%.

[B: 0.005% or less (excluding 0%)]
B is a hardenability improving element, works for strengthening the grain boundary of the surface layer portion, and is an effective component for improving fatigue strength. However, if the B content is excessive, the grain boundaries are embrittled, so it is preferable to set the content to 0.005% or less (more preferably 0.003% or less). In addition, the preferable minimum for exhibiting the said effect is about 0.001%.

[Al: 0.05% or less (not including 0%), Nb: 0.1% or less (not including 0%), Ti: 0.1% or less (not including 0%), and V: 0.0. 1 or more selected from the group consisting of 1% or less (excluding 0%)]
Al, Nb, Ti, and V have the effect of refining crystal grains by forming fine carbonitrides, and contribute to improving the fatigue strength of gear parts. However, when the content of these elements is excessive, the carbonitride becomes coarse and the above effect is lowered. Therefore, it is preferable that the content is within the above range.

[N: 0.03% or less (excluding 0%)]
N forms carbonitride with the carbonitride-forming elements (Al, Nb, Ti and V) and the like, and has the effect of refining crystal grains. However, if the N content is excessive, not only the effect is saturated but also the toughness is lowered, so 0.03% or less is preferable.

[S: 0.04% or less (not including 0%), Ca: 0.01% or less (not including 0%), Mg: 0.01% or less (not including 0%), Pb: 0.0. 1% or less (not including 0%) and Bi: one or more selected from the group consisting of 0.05% or less (not including 0%)]
S, Ca, Mg, Pb and Bi are elements that improve the machinability and are contained if necessary. However, when the content of these elements is excessive, not only the effect is saturated, but also the fatigue strength is lowered.

  In the gear part of the present invention, the carbide distribution in the tooth surface layer is appropriately controlled. To manufacture such a gear part, for example, the following may be performed. After forming a steel material satisfying the above chemical composition into a predetermined shape (gear shape), carburizing gas (for example, acetylene) during vacuum high-concentration carburizing / high-concentration carbonitriding at a high temperature (for example, about 1000 ° C.) The time ratio between the time when the gas is flown into the furnace (carburizing time: Cx) and the time when the carburizing gas is not flowed in the high temperature state for the purpose of diffusing carbon after flowing the carburizing gas (diffusion time: Dx) (Dx / Cx) is operated as a value larger than 2.7.

  In normal vacuum carburizing and carbonitriding, the processing temperature is about 900 to 950 ° C, and if the processing temperature is increased, carbides are formed, so the gas flow rate can be reduced or the processing time can be shortened. In this way, precipitation of carbides is avoided. On the other hand, in the present invention, the gas flow rate and the processing time are adjusted and the processing is performed in a high temperature state. Further, in the normal vacuum carburizing / carbonitriding process, the gas flow rate is set for the atmosphere without using the concept of “carbon potential” (see Examples below).

  On the other hand, the time ratio (Dx / Cx) has conventionally been set to about 1.5 to 2.5 from the viewpoint of carburization efficiency (carburization time shortening and optimization) (for example, the non-patent document). The target carbon distribution can be obtained by increasing the ratio of the literature 2) and the diffusion time Dx. That is, by increasing the ratio of the diffusion time Dx, the carbide area ratio Fm at a deeper position from the tooth surface (50 μm depth position from the tooth surface) is increased and a relatively shallow position (25 μm depth from the tooth surface). By reducing the carbide area ratio Fs at the position), the ratio (Fs / Fm) can be made less than 1.0.

In the gear part of the present invention, the N concentration [Nm] at a position deeper than the tooth surface (50 μm depth position from the tooth surface) is preferably about 0.05 to 1.0%. In order to achieve the N concentration, ammonia (NH ₃ ) may be appropriately introduced at the time of diffusion (other conditions are the same as described above: “vacuum carbonitriding treatment” in Examples described later).

  After performing vacuum high-concentration carburization / high-concentration carbonitriding as described above, from the viewpoint of suppressing heat treatment strain, etc., once cooled to about 860 ° C. (the temperature at this time is referred to as “quenching temperature”), It may be quenched by oil cooling or the like. Further, in order to prevent the coarsening of crystal grains, it may be once cooled to 600 ° C., heated again to about 860 ° C., and quenched by oil cooling or the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Steel materials with the chemical composition shown in Table 1 below are melted in a vacuum melting furnace, forged and normalized, blanked into a shape close to the parts, machined such as gear cutting, and gear parts Was made.

  The obtained gear parts were subjected to various vacuum high-concentration carburizing and high-concentration carbonitriding processes. Vacuum high-concentration carburization / high-concentration carbonitriding conditions [carburizing furnace, carburizing method, carburizing atmosphere (gas type, gas flow rate), carburizing temperature, carburizing time [Cx], diffusion time [Dx], ratio ([Dx ] / [Cx])] is shown in Table 2 below.

  Then, the carbide area ratio (Fm, Fs) in the surface layer portion of the gear tooth surface was obtained by scanning microscope (SEM) analysis (2000 times), and N concentration [Nm] was measured by analysis using EPMA (Electron Probe Microanalyzer). . The depth of the abnormal layer was determined by observing the tooth surface cross section with a scanning microscope (SEM).

  A power circulation gear test was performed in which the test gear (the gear part produced above) and the mating gear were combined. The specifications of the gear pair used at this time and the gear test conditions are as follows.

[Specifications of gear pair]
Module: Test gear (2.5mm), mating gear (2.5mm)
Pressure angle: Test gear (20 °), mating gear (20 °)
Number of teeth: Test gear (22), mating gear (25)
Tooth width: Test gear (12 mm), mating gear (25 mm)
Helix angle: Test gear (30 °), mating gear (30 °)

[Gear test conditions]
Input shaft torque: 300 N · m
Input shaft speed: 1000rpm
Oil type: ATF (automatic transmission oil)
Oil temperature: 80 ° C

  At each tooth position of 1/4, 2/4, and 3/4 of the tooth width on the tooth surface of the tooth every 90 ° (90 ° with respect to the axis) of the test gear rotated 2 million times which is the specified number of times. The surface roughness Ra (arithmetic average roughness) was measured. The measurement position in the width direction at this time is shown in FIG. 1 (corresponding to 1/4 position = A, 2/4 position = B, and 3/4 position = C). And these average values were made into tooth surface roughness Ra.

Furthermore, the gear test was continuously carried out to determine the pitching life. At this time, the pitching life was defined as the number of times when pitching of 1 mm ² or more occurred on any one tooth surface. These test results are shown in Table 3 below together with the N concentration [Nm] and the abnormal layer depth.

  As is apparent from these results, the surface roughness Ra is small in all of the products (test Nos. 1 to 7) in which the production conditions are appropriate and the carbide distribution in the surface layer part is appropriate. Can be confirmed. The pitching life is also good.

  In contrast, test no. 8 to 17 do not satisfy any of the requirements defined in the present invention, the surface roughness Ra shows a large value, and the pitching life is also reduced.

  Based on these results, FIG. 2 shows the influence of the ratio (Fs / Fm) on the tooth surface roughness Ra (surface roughness Ra after 2 million rotations) and the pitching life. As can be seen from FIG. 2, by adjusting the ratio (Fs / Fm) to an appropriate range (less than 1.0), it is understood that the adaptability and the pitting resistance are improved.

It is the schematic diagram which showed the measurement position of the width direction when measuring surface roughness Ra on a tooth surface. It is a graph which shows the influence which ratio (Fs / Fm) has on tooth surface surface roughness Ra and pitching lifetime.

Claims (7)

  C: 0.10 to 0.30% (meaning mass%, the same shall apply hereinafter), Si: 1.5% or less (not including 0%), Mn: 1.5% or less (not including 0%), Each of Cr: 0.5 to 2.5%, Mo: 0.5% or less (not including 0%) is contained, and the balance: steel material composed of iron and inevitable impurities is formed into a predetermined shape, and then carburized or A carbonitrided gear part having a surface abnormal layer depth of 5 μm or less, a carbide area ratio Fm at a depth of 50 μm from the tooth surface of 3 to 20%, and a depth of 25 μm from the tooth surface. A gear part having excellent conformability, characterized in that the ratio (Fs / Fm) of the carbide area ratio Fs at the vertical position and the carbide area ratio Fm satisfies less than 1.0.   The gear part according to claim 1, wherein the N concentration [Nm] at a depth of 50 µm from the tooth surface is 0.05 to 1.0%.   The gear part according to claim 1 or 2, wherein the steel material further contains Ni: 2.0% or less (not including 0%) as another element.   The gear part according to any one of claims 1 to 3, wherein the steel material further contains B: 0.005% or less (not including 0%) as another element.   The steel material further contains, as other elements, Al: 0.05% or less (not including 0%), Nb: 0.1% or less (not including 0%), Ti: 0.1% or less (0%) And V: 0.1% or less (not including 0%) and at least one selected from the group consisting of 0.1% and 5%.   The gear part according to any one of claims 1 to 5, wherein the steel material further contains N: 0.03% or less (not including 0%) as another element.   The steel material further contains, as other elements, S: 0.04% or less (not including 0%), Ca: 0.01% or less (not including 0%), Mg: 0.01% or less (0%) ), Pb: 0.1% or less (not including 0%), and Bi: 0.05% or less (not including 0%). The gear part in any one of Claims 1-6.

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