JP2008121075A - Gear part excellent in fitness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide gear parts improving a pitting resistance and also, fitness by reducing the abnormal layer and suitably controlling carbon distribution on the surface layer part in the gear. <P>SOLUTION: The gear part in this invention, is the one, in which after forming a steel material having a prescribed chemical component composition into a prescribed shape, a decarburizing treatment or a decarburize-nitriding treatment is applied. The depth of the surface abnormal layer, is ≤5 μm and also, C concentration [Cm] at 50 μm depth position from the gear surface, is 0.4-1.5% and further, the ratio [Cs]/[Cm] of C concentration [Cs] at 25 μm depth position from the gear surface and the above C concentration [Cm], is satisfied to be <1.0. <P>COPYRIGHT: (C)2008,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, the demand for higher output and smaller size and lighter has been increasing, and therefore, the load stress on steel parts such as gears used in the power transmission section tends to increase. Under such circumstances, conventional mechanical structural steels and surface-hardened steels as described above are difficult to cope with the severe use environment as described above.

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

  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 a normal carburizing / carbonitriding process, a carburized abnormal layer (hereinafter, sometimes simply referred to as “abnormal layer”) consisting of a grain boundary oxide layer or an incompletely quenched layer is generated, which causes deterioration of gear characteristics (tooth The original bending fatigue strength, pitting resistance and the like are reduced). From the viewpoint of improving the conformability, a technique that actively uses the carburized abnormal layer has also been proposed (for example, Patent Document 1), but this is a fundamental measure for suppressing the characteristic deterioration. That's not true.

  It has also been proposed to perform carburizing / carbonitriding under reduced pressure (hereinafter sometimes referred to as “vacuum carburizing / carbonitriding”) in contrast to conventional gas carburizing / carbonitriding (eg, non-carburizing / carbonitriding). 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 the amount of surface layer C is particularly excessive. The actual situation is that there is a tendency for the compatibility to deteriorate.
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 Co., Ltd.

  The present invention has been made in view of such circumstances, and the object thereof is to reduce the abnormal layer and appropriately adjust the carbon distribution of 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 This is a gear part formed by molding a steel material made of inevitable impurities into a predetermined shape and then carburized or carbonitrided. The depth of the surface abnormal layer is 5 μm or less, and the C concentration at a depth of 50 μm from the tooth surface [Cm] is 0.4 to 1.5%, and the ratio [Cs] / [Cm] of the C concentration [Cs] and the C concentration [Cm] at a depth of 25 μm from the tooth surface is 1. It has a gist in that it satisfies less than 0. 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 wire is specified, the depth of the abnormal surface layer is reduced by appropriately controlling the carburizing / nitriding conditions, and the C concentration distribution in the tooth surface layer is optimized. As a result, a gear part that improves not only the pitching resistance but also the conformability can be realized, and such a gear part is extremely useful as a gear part used in a power transmission unit.

  The present inventors have studied from various angles in order to improve the “familiarity” in gear parts on the premise that the abnormal layer is reduced by applying vacuum carburizing / carbonitriding. As a result, if a steel material having a predetermined chemical composition is used, and the conditions of vacuum carburizing / nitriding treatment are optimized and the C concentration distribution on the tooth surface is controlled as described above, a gear suitable for the above purpose is obtained. The present invention has been completed by finding that parts can be realized.

  In the gear component of the present invention, basically, the C concentration [Cm] at a depth of 50 μm from the tooth surface is 0.4 to 1.5%, and the C concentration at a depth of 25 μm from the tooth surface. The ratio [Cs] / [Cm] between the concentration [Cs] and the C concentration [Cm] 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 strength of the tooth surface that affects pitching damage etc. depends on the C concentration [Cm] 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 C concentration [Cm] at a depth of 50 μm from the tooth surface to at least 0.4% or more. However, when the C concentration [Cm] exceeds 1.5%, coarse carbides are precipitated, and the strength of the tooth surface is reduced. From the viewpoint of further reducing pitting damage in gear parts, 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% (range). 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, when the N concentration [Nm] exceeds 1.0%, coarse nitrides are deposited, 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 [Cs] / [Cm] of the C concentration [Cs] and the C concentration [Cm] at a position 25 μm deep from the tooth surface needs to be less than 1.0. .

  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 parts of the present invention are manufactured by vacuum carburizing / carbonitriding (details will be described later), the depth of the surface abnormal layer is basically deeper than 5 μm. There is no. 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 becomes excessive, the amount of retained austenite after carburizing / carbonitriding increases and the hardness decreases. There is a need to. 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, when 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 machinability and are contained as necessary. However, if the content of these elements is excessive, not only the effect is saturated, but also the fatigue strength is lowered.

  In the gear component of the present invention, the C concentration distribution in the tooth surface layer is appropriately controlled. For example, the gear component can be manufactured as follows. After forming a steel material satisfying the above chemical composition into a predetermined shape (gear shape), a carburizing gas (for example, acetylene gas) is used in a furnace during vacuum carburizing / carbonitriding at a high temperature (for example, about 930 ° C.). The time ratio (Dx / Cx) of the flow time (carburizing time: Cx) and the time (flow time: Dx) in which the carburizing gas does not flow in the high temperature state for the purpose of diffusing carbon inside after flowing the carburizing gas ) As a value greater than 2.2.

  The time ratio (Dx / Cx) has been conventionally set to about 1.5 to 2.0 from the viewpoint of carburization efficiency (carburization time shortening / optimization) (for example, Non-Patent Document 2). ) By increasing the ratio of the diffusion time Dx, the target carbon distribution can be obtained. That is, by increasing the ratio of the diffusion time Dx, the C concentration [Cm] 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). The C concentration [Cs] at the position) can be reduced, and the ratio [Cs / Cm] 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, the furnace atmosphere during the diffusion may be an ammonia atmosphere (NH ₃ atmosphere) (other conditions are the same as above).

  After performing the carburizing / carbonitriding treatment as described above, from the viewpoint of suppressing heat treatment distortion, etc., after cooling to about 860 ° C. (usually furnace cooling with heater OFF), quenching may be performed 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 carburizing / carbonitriding treatments. Then, C concentration ([Cm], [Cs]) and N concentration [Nm] in the gear surface layer were 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
Oils: 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 = A, 2/4 = B, 3/4 = 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 2 below together with the carburizing / carbonitriding conditions, C concentration ([Cm], [Cs]), N concentration [Nm], depth of abnormal layer, and the like.

  As is clear from these results, the surface roughness Ra is small in all of the products with appropriate manufacturing conditions and appropriate C concentration distribution in the surface layer portion (test Nos. 1 to 7). The compatibility improvement 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 [Cs] / [Cm] ratio 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 [Cs] / [Cm] ratio 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 [Cs] / [Cm] ratio has on tooth 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, the depth of the surface abnormal layer is 5 μm or less, and the C concentration [Cm] at a depth of 50 μm from the tooth surface is 0.4 to 1.5%, and The ratio [Cs] / [Cm] of the C concentration [Cs] and the C concentration [Cm] at a depth of 25 μm from the tooth surface satisfies less than 1.0. Excellent gear parts.   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.

Images