JP2001040452A - Gear superior in fatigue life strength in contact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To find a simple and effective measure for improving pitting fatigue life of a gear so that the gear withstands increased bearing pressure by load as a result of miniaturization, weight reduction and power increase of automobile parts especially for a gear ground after heat treatment for noise reduction. SOLUTION: A material of the gear consists of, in weight %, 0.1-0.3% C, 0.3-1.5% Mn, 0.01-0.06% S, 0.3-1.5% Cr, and the balance Fe with inevitable impurities. Also the material has the maximum ratio (L/D) of a length of a long diameter L of MnS to that of a short diameter D being not more than 7, or the maximum length being not more than 12 μm, and no oxidation layer of grain boundary on the gear surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gear used for an automobile and a machine tool, and more particularly to a high-strength steel gear used for a drive transmission system such as an automobile transmission.

[0002]

2. Description of the Related Art Gears used in a drive system of an automobile are subjected to heat treatment such as carburizing and quenching and tempering (hereinafter, carburizing) or carbonitriding, quenching and tempering (hereinafter, carbonitriding) after gear cutting. To ensure the contact fatigue life strength of the gears. In recent years, noise in automobiles has become a problem, and it has become necessary to improve the accuracy of gears. To improve the accuracy, shaving as a finishing process is performed before heat treatment. However, further improvement in accuracy requires removal of thermal strain generated during heat treatment, and grinding such as honing is performed after heat treatment. It has become.

[0003] Further, with the downsizing of parts in order to reduce the weight of the vehicle and the increase in the output of the engine, the load on the gears increases, and it is required to improve the fatigue life especially for pitching generated on the tooth surface. It has become so. To improve the pitting fatigue life of a gear, there has been proposed a method of imparting compressive residual stress to a gear by performing shot peening after heat treatment as disclosed in JP-A-1-264727. However, JP-A-3-107
Japanese Patent Publication No. 418 discloses that shot peening shortens the pitting fatigue life of the tooth surface. In addition, shot peening has a problem of noise during use because the surface of the gear is roughened.

A method of depositing a hard coating such as TiC or TiN on the gear surface is disclosed in Japanese Patent Application Laid-Open No. Sho 62-199765 or Transactions of the Japan Society of Mechanical Engineers, Vol.
72, 1993. However, since the film is reheated to 950 ° C. at the time of film formation, heat distortion which is a problem due to noise occurs. In addition, since a hard ceramic film is formed, there is a problem that it is difficult to remove thermal distortion by grinding or the like.

Further, various steel materials having excellent pitting fatigue life have been reported in Special Steel, Vol. 44, No. 3, pp. 39-48 (1995), all of which are surface hardened by carburizing or carbonitriding. The material is processed as is. A grain boundary oxide layer is generated in the vicinity of the tooth surface with the surface hardened during carburization, and pitting occurs from the grain boundary oxidized portion. Therefore, in order to suppress the grain boundary oxide layer, Si, Mn, C
Adjustment of components such as r and Ni is under study.

As described above, with respect to the pitting fatigue characteristics of a gear under a high load, an industrially useful technique for improving the fatigue life has not been found yet. In particular, no effective countermeasure has been found for improving the pitting fatigue life of a gear that is subjected to grinding after surface hardening in order to remove thermal strain.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to improve the pitching fatigue life of a gear used in a drive transmission system such as a transmission of an automobile.

[0008]

Means for Solving the Problems The present inventors have developed a gear that has been subjected to a grinding process after a surface hardening treatment to remove thermal strain,
Alternatively, it has been found that MnS inclusions serve as a starting point of pitting when there is no grain boundary oxide layer on the tooth surface such as a gear that has been carburized by vacuum. MnS stretched during hot rolling exists in a form stretched in the tooth width direction of the tooth surface. FIG. 1 shows the M
Although the drawing direction of nS and the moving direction of the contact load are shown,
In a gear, a load moves on a tooth surface while making contact, and the direction intersects the tooth width in a direction perpendicular to the tooth width. Therefore, the stretched MnS receives a tensile stress in a direction perpendicular to the stretched MnS. When the contact load moves on the tooth surface, MnS has low adhesion at the interface with the ground iron, and hard MnS does not deform but deforms the ground iron. A hole has been created and pitching has taken place. Since the tensile stress generated varies depending on the shape of MnS, and the pitting life is affected, the following means for achieving the pitting fatigue life are clarified, and the present invention has been achieved.

That is, the present invention provides the following (1) to (4)
I will provide a. (1) By weight%, C: 0.1-0.3%, Mn: 0.3
1.5%, S: 0.01-0.06%, Cr: 0.3
~ 1.5%, the balance being Fe and unavoidable impurities, and the maximum ratio (L / L) of the major axis length L and the minor axis length D of MnS.
A gear excellent in contact fatigue life strength, wherein D) is 7 or less, or the maximum length is 12 μm or less, and there is no grain boundary oxide layer on the tooth surface.

(2) Further, in terms of% by weight, Mo: 0.2 to
(1) The gear excellent in contact fatigue life strength according to (1), wherein one or two kinds of Ni: 0.3 to 1.0% are contained. (3) Further, by weight%, V: 0.05 to 0.3%, T
i: 0.01 to 0.2%, Nb: 0.02 to 0.2%, one or two kinds of which are further contained (1).
Or the gear excellent in contact fatigue life strength according to (2).

(4) Further, in weight%, Ca: 0.001
-0.01%, Te: 0.001-0.01%, Mg:
0.001-0.01%, Zr: 0.005-0.1%
(1) characterized by containing one or more of
A gear excellent in contact fatigue life strength according to any one of (1) to (3).

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
C is an element added to secure the strength required for parts, particularly the strength of the core, but if it is less than 0.1%, such effects cannot be sufficiently obtained, and it exceeds 0.3%. Therefore, the toughness is reduced, so the content is set to 0.1% to 0.3%.

Mn is used as a deoxidizing material and a desulfurizing material at the time of melting, and is an element necessary for ensuring strength, toughness, and hardenability, and is required to be 0.3% or more. But,
If it exceeds 1.5%, a hardened structure such as bainite or martensite will be formed in the cooled home after hot rolling, and it will not be suitable for cutting workability such as gear cutting. S is an element necessary for enhancing machinability. After the gear component is roughly formed by forging, the tooth profile portion is subjected to gear cutting such as hob cutting and shaper cutting, and surface hardening treatment such as carburizing is performed. S is effective in the machinability in the gear cutting and cutting, and needs to be contained at 0.01% or more. However, if it exceeds 0.06%, the working limit during forging is significantly reduced. Further, if the amount of S is large, MnS, which is a starting point of pitting, is increased and the fatigue life is reduced. Therefore, the upper limit is set to 0.06%.

[0014] Cr contributes to the improvement of the mechanical properties, hardenability and wear resistance of steel, but if this element also exceeds 1.5%, the hard structure of bainite or martensite in cooling after hot rolling. And becomes unsuitable for subsequent cutting or other secondary processing, so the content is set to 1.5% or less. But C
If the addition amount of r is less than 0.3%, the effect of hardenability is not sufficient, so the content is made 0.3% or more.

Cr alone is not enough to ensure hardenability, and elements such as Mo and Ni are contained as necessary. Mo is added in an amount of 0.2% or more to provide hardenability equal to or higher than that of conventional steel. However, if the content exceeds 1.0%, the effect is saturated and the economy is impaired, so the content is set to 1.0% or less. Also, N
It is preferable that i is contained in an amount of 0.3% or more in order to obtain the effect of hardenability. However, if the content exceeds 1.0%, the effect is saturated and the economy is impaired. Therefore, the content is set to 1.0% or less.

Regarding the shape of MnS, the present inventors have proposed M
As a result of studying to clarify the relationship between the shape of nS and the pitting life, the maximum ratio (L / D) of the major axis length L and the minor axis length D of MnS is 7 or less, or the maximum length is 12 μm.
It was found that a high pitting fatigue life can be ensured if it is below. When Ca or the like is added, the deformability of MnS decreases, so that MnS does not stretch during rolling and has an elliptical shape. Since the stretched shape becomes an ellipse and a sphere, the stress concentration at the MnS end is reduced, so that the MnS and the self-iron are less likely to be separated, and the pitting fatigue life is improved. Accordingly, the pitching fatigue life is improved as the ratio L / D of the major axis length L to the minor axis length Lm (small to 1) of MnS is improved. ≦ 7. The reason why the value of L / D is limited to the maximum value is that the starting point of pitching is generated from stretched MnS where the highest stress is generated, and it is meaningless even if the average value is small. Furthermore, L / D
Is large, it is difficult to become a starting point of pitching if MnS itself is small. As shown in FIG. 1, if the major axis, which is the length of the MnS in the stretching direction, is short, it is difficult to become a starting point of pitching, so the maximum major axis was set to 12 μm or less. This is because if it exceeds 12 μm, no matter how much the maximum L / D of MnS is reduced, there is a high possibility of becoming the starting point of pitching. Here, as shown in the examples, the major axis L and the short diameter of about 200 MnS were measured from a photomicrograph of 200 to 500 times the longitudinal section in the rolling direction at the radius / 2 part of the round bar as the material. The diameter D was measured to define the maximum value of L / D and the maximum length. Note that MnS includes the case where Ca, Te, Zr, and Mg are contained.

In a gear in which a grain boundary oxidized layer formed during carburizing treatment remains, pitting occurs from the oxidized grain boundary part, and thus there is almost no effect of suppressing the MnS shape. Therefore, it is necessary that there is no grain boundary oxide layer after the surface hardening treatment, and after the surface hardening treatment, grinding is performed to also remove thermal strain. Alternatively, when heat distortion removal is not required, a step of performing surface hardening treatment by vacuum carburization may be considered. The grain boundary oxide layer on the tooth surface cross section was 10
Observation was made at a magnification of 00 or more, and it was confirmed that there was no grain boundary oxide layer in the example in the same manner.

Further, in reducing the weight of the gear, the bending fatigue strength at the root of the tooth is also important. Since the root bending fatigue also occurs from the surface of the tooth surface, the influence of the grain boundary oxide layer has been studied. However, the present invention is limited to a gear without a grain boundary oxide layer, and it is necessary to refine the crystal grains after carburizing in order to further improve the root fatigue strength. V, Ti, and Nb are elements that form carbonitrides and are effective in refining carburized crystal grains, and can be arbitrarily added. To obtain the effect, V is 0.05% or more, and Ti is 0.01%.
As described above, Nb must be contained at 0.02% or more. However, if the content exceeds 0.3% in V, exceeds 0.2% in Ti, and exceeds 0.2% in Nb, the effect is saturated, so V is 0.05 to 0.3%. %, Ti is 0.01 to
0.2% and Nb are 0.02 to 0.2%.

For controlling the morphology of MnS, Ca, Te, M
g, Zr, or Ca, T
e is preferably contained at 0.001% or more, and Zr is preferably contained at 0.005% or more. On the other hand, Ca and Te are 0.01%, Z
Even if the content of r exceeds 0.1%, the effect is saturated. Therefore, Ca and Te are set to 0.001 to 0.01%, and Zr is set to 0.005 to 0.1%. Mg has an effect of uniformly dispersing sulfides such as MnS in steel. If it is less than 0.001%, the refining effect of MnS is small, and if it exceeds 0.01%, the effect is saturated.
001 to 0.01%.

Incidentally, shot peening may be performed after carburizing or carbonitriding as a surface hardening treatment. Also in this case, if the grain boundary oxide layer is removed by the grinding after the shot peening, the roughness of the tooth surface due to the shot peening is eliminated, so that the pitting starting from MnS occurs. Therefore, the effects of the present invention can also be achieved by a method in which shot peening is performed after carburizing or carbonitriding and then ground.

[0021]

EXAMPLES Steel having the chemical composition shown in Table 1 was smelted, then ingot-formed, then slab-rolled and rolled into a steel bar to produce a 70 mm diameter (rolling ratio 50), and further rolled into a round bar having a diameter of 32 mm. did. The form of MnS was controlled by adding Ca, Te, Mg, Zr and the like. Normalize each rolled material at 925 ° C,
Then, from a round bar with a diameter of 32 mm, a diameter of 26 mm and a width of 2 mm
A roller-shaped test piece having an 8 mm cylindrical portion was prepared.
After forging a round bar having a diameter of 70 mm to a diameter of 150 mm, a normalizing process was performed at 925 ° C. to form a large roller having a diameter of 130 mm and a width of 18 mm.

Heat the roller-shaped test piece and the large roller in a carburizing gas atmosphere at 930 ° C. × 5 hours → 130 ° C. oil quenching →
Carburizing was performed under the condition of tempering at 180 ° C. for 1 hour. Thereafter, grinding with a diameter of 50 μm was performed. The depth of the grain boundary oxide layer generated during carburization was about 15 μm when the cross section of the small roller was observed with a scanning electron microscope at a magnification of 2000 times, and the grain boundary oxide layer was removed by this grinding. The roughness after the grinding was Rmax 2 μm or less.

No. 1 in Table 1. As shown in FIGS. 17 and 18, instead of carburizing, carbonitriding and vacuum carburizing were also performed. As carbonitriding conditions, gas carbonitriding (93
Heating at 0 ° C. × 5 hours → furnace cooling → nitriding with NH 3 gas → oil quenching at 840 ° C. × 2 hours) and tempering at 180 ° C. × 1 hour. The vacuum carburizing was performed at a heating temperature of 1100 ° C. due to the carbon potential in the furnace, and the subsequent grinding was not performed.

No. 1 in Table 1. As shown in FIG. 19, a test piece subjected to shot peening after carburizing and a large roller were also prepared. The shot peening conditions are as follows.
Using an 8 mm steel ball, the irradiation speed of the shot ball is 85 m /
s and arc height of 0.6 mmA. As a pitting fatigue life evaluation, a roller pitting test using the above roller-shaped test piece and a large roller was performed. The test conditions were as follows: the rotational speed of the test piece was 1000 rpm, the slip ratio was 40%, the oil used was an automatic oil as a lubricant, and the oil temperature was about 80 ° C.

In the evaluation, the maximum surface pressure that can be rotated up to 10 ⁷ times while maintaining the soundness was defined as the pitting fatigue strength of the steel material. The surface pressure was calculated based on the Hertz surface pressure. A single tooth bending fatigue test was performed as a tooth root fatigue test. 70m diameter mentioned above
Using a round bar of m, it was processed to a diameter of 120 mm by hot forging and normalized at 925 ° C. Module 4 by cutting, number of teeth 27, pitch circle diameter 108mm, tooth width 9m
After preparing a test spur gear of m and carburizing each gear, it was ground by a hob into a gear having a precision of JIS 0 to 1 class. Using a hydraulic servo tensile testing machine, a fatigue test was conducted in which a combination of the test gear was applied to one tooth of the test gear from the other gear and a bending load was applied to the tooth base.

Further, the outer periphery of a round bar having a diameter of 70 mm was cut to evaluate the machinability. The processing conditions are tool: SKH5
7, cutting speed 75-84 mm / rev, tool feed speed 0.02 mm / rev, cutting oil: water-insoluble oil 120 l /
min. At that time, the cutting workability was evaluated based on the criteria that a tool with a flank wear of the front flank after 3000 cycles of less than 300 μm was acceptable.

Table 1 shows the components of various test materials and the maximum major axis / minor axis ratio and the maximum major axis as MnS shapes. The maximum major axis / minor axis ratio and the maximum major axis of MnS can be determined by measuring the longitudinal section in the rolling direction of radius / 2 part in each of the above-mentioned rolled materials having a diameter of 32 mm with an optical microscope at a magnification of 500 times. With respect to MnS, the major axis L and the minor axis D are actually measured and determined from the measured values.

Table 2 shows the results of evaluation of the pitting fatigue strength, the one-tooth bending fatigue strength, and the cutting workability in the roller pitting test. From Table 1, it can be seen that No. 1 of the present invention. 1 to No.
19, No. 19 4 and No. 4. L / D ≦ 7 other than 12. No. 4 and No. 4. 12, L / D> 14, but the maximum major axis is L ≦ 12, which satisfies the condition of the present invention. From Table 2, the pitting fatigue strength using these test materials is 320 kgf / mm ² or more in each case, indicating a long life. In addition, No. 11-No. In the test material to which V, Ti, Nb, etc. were added, the one-tooth bending fatigue strength was 110 kgf / mm ² or more, which was higher than the test material to which these components were not added. Table 2
Thus, in the examples of the present invention, the L / D of MnS was small irrespective of the amount of S, so that the cutability was acceptable in each case.

On the other hand, in Comparative Example No. 20-No. 26, L / D> 15 and L> 14 from Table 1, all of which do not satisfy the conditions of the method of the present invention. Thus, 60 kgf / mm ² or more was small compared to the pitting fatigue strength 260kgf / mm ² or less and the present invention method. Consider the difference of 60 kgf / mm ² in terms of fatigue life. For example, no. Pitting fatigue strength of 21 is 260kgf / mm ^2, pitching to over 10 ⁷ times when the surface pressure 260kgf / mm ² is not generated. However, in the case of 60 kgf / mm ² higher surface pressure 320 kgf / mm ^2, the fatigue life was 2.4 × 10 ⁶ times. Therefore, the fatigue life differs by about four times, and there is a great difference in the life.

In the comparative example, since the MnS was stretched, the one-tooth bending fatigue strength was also low by the method of the present invention. N
o. 27 shows that MnS, which is a starting point of pitting, is small because the amount of S added is small from Table 1. Therefore, the pitting fatigue strength is 340 kgf / mm ² from Table ^2, which is the same as that of the present invention. However, since the amount of MnS is small, the machinability is poor and a problem occurs during gear manufacturing.

No. No. 28 is No. The amount of S added opposite to 27 is large. For this reason, there was a concern that cracks would occur during forging, but no particular problem occurred during the production of the test pieces. However, the pitting fatigue strength was about 280 kgf / mm ² , which was lower than that of the present invention. This is thought to be due to the large amount of MnS serving as the starting point of pitching.

No. 29, although the MnS shape also satisfies the conditions of the present invention, the carburized state, that is, the grain boundary oxide layer remains, and the pitting occurs from the grain boundary oxide layer, so that the pitting fatigue strength is as low as 240 kgf / mm ² . Only after the grain boundary oxide layer is removed, the suppression of the MnS shape has an effect on the pitting life.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

According to the present invention, the pitting during use is controlled by controlling the morphology of MnS in a gear having no grain boundary oxidized layer, such as by grinding after performing a surface hardening treatment such as carburizing or carbonitriding. The life can be significantly improved. As a result, the load applied to the gears can be increased, or the gears themselves can be reduced in size and weight, and automobiles and construction machines that use many gears can be reduced in size and weight.
It brings great effects such as improved fuel efficiency.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an extending direction of MnS and a moving direction of a contact load in a gear.

[Procedure amendment]

[Submission date] July 30, 1999 (July 7, 1999)
0)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claim 2

[Correction method] Change

[Correction contents]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction contents]

(2) Further, in terms of% by weight, Mo: 0.2 to
(1) The gear having excellent contact fatigue life strength according to (1), wherein one or two kinds of Ni: 0.2 to 1.0% are contained. (3) Further, by weight%, V: 0.05 to 0.3%, T
i: 0.01 to 0.2%, Nb: 0.02 to 0.2%, one or two kinds of which are further contained (1).
Or the gear excellent in contact fatigue life strength according to (2).

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction contents]

Cr alone is not enough to ensure hardenability, and elements such as Mo and Ni are contained as necessary. Mo is added in an amount of 0.2% or more to provide hardenability equal to or higher than that of conventional steel. However, if the content exceeds 1.0%, the effect is saturated and the economy is impaired, so the content is set to 1.0% or less. Also, N
It is preferable that i is contained in an amount of 0.2 % or more in order to obtain the effect of hardenability. However, if the content exceeds 1.0%, the effect is saturated and the economy is impaired. Therefore, the content is set to 1.0% or less.

 ──────────────────────────────────────────────────の Continued on front page (72) Inventor Seiji Ito 12 Nakamachi, Muroran, Hokkaido Nippon Steel Corporation Muroran Factory (72) Inventor Hideo Kanisawa 12 Nakamachi, Muroran, Hokkaido Nippon Steel Corporation Muroran Manufacturing In-house F-term (reference) 3J030 BA01 BC03 BC06 CA10

Claims (4)

[Claims] C .: 0.1 to 0.3% by weight, M:
n: 0.3 to 1.5%, S: 0.01 to 0.06%,
Cr: 0.3 to 1.5%, the balance being Fe and inevitable impurities, and the major axis length L and the minor axis length D of MnS.
The maximum ratio (L / D) is 7 or less, or the maximum length is 12
A gear with excellent contact fatigue life strength, characterized by having a grain boundary oxide layer on the tooth surface of not more than μm. 2. Mo: 0.2 to 1.0 in weight%.
%, Ni: 0.3 to 1.0%, the gear having excellent contact fatigue life strength according to claim 1. 3. The composition according to claim 1, wherein V: 0.05 to 0.5% by weight.
3%, Ti: 0.01-0.2%, Nb: 0.02-
3. The gear excellent in contact fatigue life strength according to claim 1 or 2, wherein the gear contains 0.2% of one or two kinds. 4. The composition according to claim 1, further comprising:
0.01%, Te: 0.001 to 0.01%, Mg:
0.001-0.01%, Zr: 0.005-0.1
The gear according to any one of claims 1 to 3, wherein the gear has excellent contact fatigue life strength.