JP2002121645A - Steel for gear having excellent dedendum bending fatigue characteristic and facial pressure fatigue characteristic and gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a gear in which its dedendum bending fatigue strength is higher than that of the conventional gear, further, facial pressure fatigue characteristics are excellent, and mass production is possible and to provide a gear produced by the same steel. SOLUTION: The gear is composed of the one obtained by forming steel having a composition containing, by mass, 0.10 to 0.35% C, 0.5 to 2.0% Si, 0.2 to 1.0% Mn, 0.01 to 0.50% Ni, 0.5 to 2.5% Cr, 0.01 to 1.00% Mo, 0.005 to 0.200% Al and 0.001 to 0.200% V, and in which the Ac3 temperature parameter is 850 to 940 deg.C, the ideal critical diameter DI is 40 to 400 mm, the carbon equivalent Ceq is <=1.100, and the temper softening resistance parameter is >=100 into a gear shape by forging or machine working, in which the content of retained austenite from the surface layer of the gear to the depth position of 100 μm when carburizing treatment or carbo-nitriding treatment is performed is <=40 vol.%, also, the structure of the core part as a noncarburized part is composed of the dual phase one of ferrite and martensite, and the ratio of ferrite therein is 10 to 45 area %.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for gears, which is used in automobiles, construction machines and other machine parts and has excellent root bending fatigue characteristics and surface pressure fatigue characteristics, and a gear manufactured from the steel. Things.

[0002]

2. Description of the Related Art In recent years, gears used in automobiles and the like have been required to be reduced in size in accordance with a reduction in body weight due to energy saving, and an increase in load on the gears due to an increase in engine output has been required. is happening. The durability of gears is mainly caused by bending fatigue fracture of the tooth root and surface pressure fatigue fracture of the tooth surface. Among them, the surface pressure is 200
For gears exceeding 0 MPa, it is particularly required that, in addition to high bending fatigue strength at the root of the tooth, high pitting resistance of the tooth surface is required.

Conventionally, JIS SCM420H, SCM
Gears have been formed using case-hardened steel such as 822H and subjected to surface treatment such as carburization and used. However, these gears have not been able to withstand use under high stress. Accordingly, proposals have been made to improve the root bending fatigue strength and pitting resistance by changing the steel material, changing the heat treatment method, and work hardening the surface.

[0004] For example, Japanese Patent Publication No. 7-122118 discloses that by reducing Si in steel and controlling Mn, Cr, Mo and Ni, a grain boundary oxide layer on the surface after carburizing heat treatment is reduced. In addition, MnS, which reduces the occurrence of cracks, suppresses the formation of an incompletely quenched layer, suppresses the reduction in surface hardness and increases fatigue strength, and further promotes crack initiation and propagation by adding Ca There is disclosed a technique for controlling stretching. Hereinafter, it is referred to as Prior Art 1.

Japanese Patent No. 2,945,714 discloses a technique for increasing the tempering softening resistance by using a steel material containing 0.25 to 1.50% of Si as a raw material. Hereinafter, this is referred to as prior art 2.

Japanese Patent Application Laid-Open No. 7-54050 discloses a technique for increasing the pitting fatigue strength by defining the distribution of the amount of retained austenite near the surface layer existing after carburizing or carbonitriding. Hereinafter, this is referred to as prior art 3.

[0007] Japanese Patent Publication No. Hei 7-113145 discloses that carbon steel or carbonitriding is carried out using a steel material to which Si and Cr are added to increase the resistance to temper softening, and to contain C and N near the surface. A technique for controlling the amount of retained austenite by limiting the sum of the amounts is disclosed. Less than,
This is referred to as prior art 4.

Japanese Patent Application Laid-Open No. 8-81743 discloses that Si,
By adding V in addition to Cr, the tempering softening resistance is increased, and its variation is reduced.
There is disclosed a technique for reducing MnS, which can be a starting point of a crack, by reducing MnS. Hereinafter, this is referred to as Prior Art 5.

[0009]

However, the above-mentioned prior art has the following problems.

According to the prior art 1, by reducing the amount of Si, the grain boundary oxide layer and the incompletely quenched layer can be reduced, and the occurrence of bending fatigue cracks at the tooth root can be suppressed. However, conversely, the tempering softening resistance is reduced, and the occurrence of fracture shifts from the tooth root to the tooth surface side, making it impossible to suppress tempering softening due to frictional heat on the tooth surface and causing the surface to soften, resulting in pitting. Is more likely to occur.

Conversely, according to Prior Art 2, a method of adding Si or the like to increase the tempering softening resistance and limiting the carburizing method to vacuum carburizing or plasma carburizing to suppress the progress of grain boundary oxidation is disclosed. However, this method increases the manufacturing cost and is not effective for mass production.

According to Prior Art 3, retained austenite precipitated near the surface layer is transformed into martensite by work-induced transformation, and the gear surface is hardened to improve bending fatigue strength and pitting resistance. . But,
With this alone, the softening of the tooth surface due to frictional heat when driving the gear cannot be suppressed, so that the pitching resistance cannot be increased.

Prior art 4 also controls residual austenite in the surface layer, as in prior art 3, but at the same time, improves the tempering softening resistance by adding Si and Cr. As a result, softening of the gear surface due to frictional heat is reduced, and surface hardening is expected due to work-induced transformation of retained austenite. However, at the same time as the retained austenite becomes martensite due to the work-induced transformation, distortion due to the transformation occurs and the tooth surface is deformed. For this reason, the contact surfaces between the gears are displaced, and pitching is more likely to occur.

According to the prior art 5, the tempering softening resistance is improved and the variation is reduced. But,
When driven as a gear, softening of the tooth surface due to frictional heat is suppressed, but as in the prior art 4, a shift of the gear contact surface due to residual austenite near the surface occurs, resulting in a decrease in pitting resistance. Cannot be suppressed. Also, although it seems that the S reduction is more effective in suppressing the occurrence of pitting on the short time side, Si, Cr and V
In the case where S is reduced in addition to being hardened by the addition of S, cutting workability such as gear cutting and oil hole drilling is remarkably inferior, and the influence on productivity is more greatly reduced.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide mass-produced gear steel having higher tooth root bending fatigue strength than conventional gears and excellent surface pressure fatigue characteristics, and this steel. The object of the present invention is to provide a gear manufactured by:

[0016]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to solve the above problems, and have obtained the following findings.

[0017] Temper softening resistance is increased by increasing the amount of Si and Cr added to the steel material.
If the segregation is suppressed to further improve the tempering softening resistance, and the variation is reduced to suppress the softening due to heat generation at the gear contact surface, the occurrence of surface cracks can be suppressed.

With respect to a grain boundary oxide layer that can be a starting point of a fatigue crack, by adding a certain amount of Si,
The grain boundary oxide layer changes its growth from the growth in the depth direction to the increase in the density of the surface, and the oxide layer grown in the depth direction as a starting point disappears. As a result, it becomes difficult to become a starting point of a fatigue crack.

If the surface structure of the carburized portion of the tooth surface is made of martensite and retained austenite of 40% by volume or less, and the core structure, which is a non-carburized portion, is made of martensite and 10 to 45% by area of ferrite, it becomes a gear. After driving,
Retained austenite in the surface layer causes work-induced transformation,
Even in the case of the martensite structure, the ferrite, which is a relatively soft structure inside, absorbs strain, so that the amount of deformation as a whole is reduced, and the displacement of the gear contact surface is reduced. As a result, occurrence of pitching can be suppressed.

The present invention has been made based on the above findings, and has the following features.

The invention according to claim 1 is characterized in that C: 0.10 to 0.10
0.35, Si: 0.5 to 2.0, Mn: 0.2 to 1.
0, Ni: 0.01 to 0.50, Cr: 0.5 to 2.
5, Mo: 0.01 to 1.00, Al: 0.005 to
0.200, V: 0.001 to 0.200 (above, ma
ss%), and the Ac3 temperature parameter calculated from the following equation (1): Ac3 = 920-203√C-15.2Ni + 44.7Si + 104V-30 Mn-11Cr + 400Al-(1)
In the range of 940 ℃, DI = 7.95√C (1 + 0.715Si-0.50Si 2) (1 + 4.1 Mn) (1 + 2.33Cr) (1 + 0.52Ni) (1 + 3.14Mo) (1+ 5V) FB --- (2) where fB = 1 (when Ti <0.005 mass% or B <0.0005 mass%) or 2
(Ti ≧ 0.005 mass% or B ≧ 0.00
Ideal critical diameter DI calculated in the range of 40 to 400 mm, and Ceq = C + Si / 7 + Mn / 5 + V / 2 + Cr / 9 + Ni / 22 + Mo / 2 --- (3) Ceq is 1.100 or less, and an element S effective for increasing the tempering softening resistance
For i and Cr, the tempering softening resistance parameter calculated by H _SiCr = 90Si + 23.5Cr --- (4) is 10
0 or more, with the balance being Fe and unavoidable impurities.

According to a second aspect of the present invention, the composition of the first aspect further comprises Nb: 0.010 to 0.100 m
It is characterized by containing ass%.

The invention according to claim 3 is the invention according to claim 1 or 2
In addition to the described chemical composition, Ti: 0.005-0.
050, B: 0.0005 to 0.0100 (above, ma
ss%).

[0024] The invention according to claim 4 provides the above-mentioned claims 1-3.
The steel for gears according to any one of the above is formed by forging or machining into a gear shape, and when subjected to carburizing or carbonitriding, from the surface of the gear to a depth of 100 μm. Residual austenite amount of
0% by volume or less, and the structure of the core, which is a non-carburized part, comprises a two-phase structure of ferrite and martensite,
It is characterized in that the ratio of ferrite therein is 10 to 45 area%.

The fifth aspect of the present invention provides the first to third aspects.
The gear steel described in any one of the above is formed into a gear shape by forging or machining, subjected to carburizing or carbonitriding, and shot peening at an arc height of 0.3 mmA or more. When applied, the amount of retained austenite from the surface layer of the gear to a depth of 100 μm is 40% by volume or less, and the structure of the core portion, which is a non-carburized portion, comprises a two-phase structure of ferrite and martensite, Ferrite ratio is 10-4
It is characterized by being 5 area%.

The reasons for limiting numerical values in the present invention will be described below.

C: 0.10 to 0.35 mass% C is an element necessary for securing the strength.
Determine the internal hardness after carburizing and tempering.
At 0 mass%, the hardness of the gear cannot be secured because the internal hardness is too low. On the other hand, 0.35mas
If it exceeds s%, toughness is reduced and workability is deteriorated. Therefore, the C content was limited to the range of 0.10 to 0.35 mass%.

Si: 0.5 to 2.0 mass% Si is an element effective for increasing the tempering softening resistance and has the effect of suppressing the depth of the grain boundary oxide layer, but the amount is 0.5 mass%. If less than, there is no effect of improving the tempering softening resistance, and the grain boundary oxide layer eventually becomes deep,
The root bending fatigue strength is inferior. On the other hand, even if added in excess of 2.0 mass%, not only the effect of improving the tempering softening resistance is saturated, but also the toughness is deteriorated. Therefore, the Si content is
It was limited to the range of 0.5 to 2.0 mass%.

Mn: 0.2 to 1.0 mass% Mn is an element that enhances the hardenability, and for that purpose, it is necessary to add 0.2 mass% or more. However, as the upper limit of the addition for ensuring the hardenability, 1.0 mass% is sufficient. Therefore, the Mn content is 0.2 to 1.0 m
ass%.

Ni: 0.01 to 0.50 mass% Ni is an element that enhances hardenability, but at least 0.01 mass% or more is required to enhance hardenability. However, if the added amount exceeds 0.50 mass%, the hardness becomes too high and the machinability deteriorates. Also, Ni
Is expensive because it is expensive. Therefore,
The Ni content was limited to the range of 0.01 to 0.50 mass%.

Cr: 0.5 to 2.5 mass% Cr is an element that improves the hardenability and an element that increases the tempering softening resistance. In order to exhibit both performances, it is necessary to add 0.5 mass% or more. However, if the addition amount exceeds 2.5 mass%, the effect of increasing the tempering softening resistance is saturated, and the hardenability becomes too high, so that the toughness deteriorates. Therefore, the Cr content is
It was limited to the range of 0.5 to 2.5 mass%.

Mo: 0.01 to 1.00 mass% Mo is an element for improving the hardenability. For that purpose, it is necessary to add 0.01 mass% or more,
Even if it is added in excess of 1.00 mass%, the effect is saturated, and Mo is expensive, so that it is not economical. Therefore, the Mo content is limited to the range of 0.01 to 1.00 mass%.

Al: 0.005 to 0.200 mass% Al is an effective element for deoxidation, and its effect is 0.0%
It is exhibited by addition of 05 mass% or more. Also, Al
Has the function of forming AlN by combining with N, suppressing the coarsening of the crystal grains and increasing the toughness.
It is effective to add up to mass%, and if it exceeds that, coarse grains are generated and toughness is reduced. Therefore, the Al content was limited to the range of 0.005 to 0.200 mass%.

V: 0.001 to 0.200 mass% V is an element that increases the tempering softening resistance like Si and Cr. At the same time, it also has the effect of forming carbonitrides to refine the crystal grains and thereby suppress the segregation of Si.
It is necessary to add ss% or more. However, this effect
Even if it is added in excess of 0.200 mass%, it saturates, a sufficient effect cannot be obtained, and only the production cost increases. Therefore, the V content is 0.001 to 0.200.
It was limited to the range of mass%.

Nb: 0010 to 0.100 mass% Nb is an element for making crystal grains fine by the formation of carbonitrides, thereby improving the root bending fatigue strength. 0.010 mass% to refine crystal grains
The above addition is necessary. On the other hand, even if added in excess of 0.100, the effect is saturated. Therefore, the Nb content is
It was limited to the range of 0.010 to 0.100 mass%.

B: 0.0005 to 0.0100 mass
%, Ti: 0.005 to 0.050 mass% B is an element effective for improving the hardenability, but the effect is obtained at 0.0005 mass% or more,
If it exceeds 100 mass%, it saturates. Therefore, the B content was limited to the range of 0.0005 to 0.0100 mass%. However, the effect of improving the hardenability of B is that B is N
Since it is ineffective if present as a compound, Ti is added to fix N while adding B. The proper addition amount of Ti varies depending on the amount of N, but is 0.005 to 0.050 m
The range of ass% is preferable.

In the steel of the present invention, the contents of P and oxygen as unavoidable impurities are preferably as low as possible. Also, N is for the purpose of refining crystal grains,
If necessary, the addition is allowed up to 0.20 mass%. Also, if necessary to improve machinability,
Free-cutting elements such as S, Pb, Se, and Ca may be contained.

Ac3 temperature parameter: 850-940
FIG. 1 shows a heat treatment pattern in a conventional carburizing treatment. In the conventional method, the steel is carburized at 930 ° C., and carbon is diffused and permeated therein, and then quenched at 850 ° C. lower than the carburizing temperature to reduce distortion. Therefore, when the Ac3 temperature parameter calculated by the following equation (1) is less than 850 ° C., the ferrite structure cannot be precipitated in austenite even if the temperature is maintained at 850 ° C. after carburization. On the other hand, when the temperature exceeds 940 ° C., the area ratio of ferrite, which is a soft structure, becomes too large, resulting in insufficient strength. Therefore,
The Ac3 temperature parameter calculated by the following equation (1) should be limited to the range of 850 to 940 ° C.

Ac3 = 920-203√C-15.2Ni + 44.7Si + 104V-30 Mn-11Cr + 400Al --- (1) Ideal critical diameter (DI): 40 to 400 mm The ideal critical diameter (DI) indicates the hardenability of steel. Value. If the ideal critical diameter (DI) calculated by the following formula (2) is less than 40 mm, the hardness inside the gear becomes too low, so that the desired bending fatigue strength at the tooth root cannot be secured. On the other hand, if it exceeds 400 mm, the toughness inside the gear deteriorates. Therefore, the ideal critical diameter (DI) calculated by the following equation (2) should be limited to the range of 40 to 400 mm.

DI = 7.95 ° C. (1 + 0.715Si−0.50Si ² ) (1 + 4.1 Mn) (1 + 2.33Cr) (1 + 0.52Ni) (1 + 3.14Mo) (1 + 5V) · fB (2) However, fB = 1 (when Ti <0.005 mass% or B <0.0005 mass%), or 2
(Ti ≧ 0.005 mass% or B ≧ 0.00
Carbon equivalent (Ceq) ≦ 1.100 Carbon equivalent (Ceq) is an important index for determining hardness before carburizing and internal fatigue of a gear. When the carbon equivalent (Ceq) calculated by the following formula (3) exceeds 1.100,
Since the hardness before carburization becomes too high, cutting becomes difficult and productivity is reduced. Therefore, the following (3)
The carbon equivalent calculated by the formula should be limited to 1.100 or less.

Ceq = C + Si / 7 + Mn / 5 + V / 2 + Cr / 9 + Ni / 22 + Mo / 2 (3) Temper Softening Low Resistance Parameter (H _SiCr ) ≧ 100 The temper softening resistance parameter determines the degree of softening after tempering of steel. Value. If the tempering softening resistance parameter (H _SiCr ) calculated by the following equation (4) is less than 100, the softening at the contact surface when driven under a high surface pressure becomes too large, so that pitting is likely to occur, and the surface pressure is increased. The fatigue strength deteriorates. Therefore, the tempering softening resistance parameter (H _SiCr ) calculated by the following equation (4) is 100
It should be limited to the above.

H _SiCr = 90Si + 23.5Cr --- (4) Amount of retained austenite in tooth surface layer ≦ 40% by volume The retained austenite is soft by itself, but is present in the tooth surface layer. Since it is transformed into martensite by the stress during driving of the gear and hardened, it has an effect of suppressing crack growth against bending and surface pressure fatigue. However, when the retained austenite exceeds 40% by volume, on the contrary, the hardness of the surface layer is excessively reduced, and the surface pressure fatigue properties are deteriorated. Therefore, the amount of retained austenite is 4
It should be limited to 0% by volume or less.

Ferrite area% of the structure inside the tooth (non-carburized portion): 10 to 45% When the gear that has been carburized and quenched and tempered is driven,
The retained austenite existing in the carburized part changes to a martensitic structure due to work-induced transformation. At this time, a strain is generated due to volume expansion, a shift occurs in a contact surface between the gears, and the surface pressure fatigue strength is reduced. Therefore, in order to reduce this strain and eliminate the decrease in the surface pressure fatigue strength, it is effective to absorb the strain by ferrite which is a soft structure, and in order to obtain the effect, at least 10% of ferrite is contained in the structure.
% Is required. However, when the ferrite content is 45% or more, the hardness of the non-carburized portion is reduced, so that the strength becomes insufficient and the durability is deteriorated. Therefore, the ferrite area ratio of the non-carburized portion should be limited to the range of 10 to 45%.

Arc height of shot peening (S / P): 0.3 mmA or more By performing shot peening, compressive residual stress is applied near the surface layer to further increase bending fatigue strength and surface pressure fatigue strength. . When this treatment is performed, if the arc height is less than 0.3 mmA, the application of compressive residual stress is too small, so that the bending fatigue strength and the surface pressure fatigue strength cannot be further increased. Therefore, shot peening is preferably performed at an arc height of 0.3 mmA or more.

The arc height is an index indicating the strength of shot peening according to SAE (standard of the American Automobile Technology Association) J442a. 19mm × 77m
After fixing a plate-shaped test piece of m and projecting a shot on the entire surface of one side of the test piece, removing the test piece, the test piece is warped (bent). This amount of warping (bending) is called arc height. In addition, A is the kind of thickness of a test piece, and means 1.3 mm.

[0046]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to embodiments.

No. 1 having the chemical composition shown in Table 1 1
~ 15 steels were each melted to prepare ingots.
In addition, No. shown in Table 1 Nos. 1 to 7 are steels of the present invention. 8
No. 13 to Comparative Steel No. 13 14 and 15 are conventional steels. Conventional steel is No. 14 is JIS SCM42
0H, No. 15 is the SCM822H. Ac of each steel
3 temperature parameters, ideal critical diameter DI, carbon equivalent Ce
Table 1 shows the values of q and the tempering softening resistance parameter H _SiCr together.

[0048]

[Table 1]

Each of the ingots of the steel of the present invention, the comparative steel and the conventional steel prepared as described above was hot-rolled to obtain a sample No. of the present invention consisting of round bars of 32 mm and 70 mm in diameter. Nos. 1 to 7; Nos. 8 to 13 and conventional test piece Nos. 14, 15 were prepared. Next, a normalizing process is performed on each of the specimens thus obtained,
Each specimen after normalizing was subjected to the following tests to determine the amount of quenching strain, surface layer retained austenite, ferrite area ratio of non-carburized portion, rotational bending characteristics, surface pressure fatigue characteristics, internal hardness, internal toughness, grain boundary The austenitic grain size of the oxide layer and the carburized part was examined. Table 2 shows the results.

[0050]

[Table 2]

Amount of Quenching Strain A navy C test piece was processed from each of the normalized specimens having a diameter of 70 mm. FIG. 2 shows a front view of the navy C test piece, and FIG. 3 shows a side view thereof. As shown in both figures, the navy C test piece 1 has an opening 2 and a disc-shaped space 3 in a disk-shaped body, and the dimensions of each part of the test piece are as follows.

Test specimen diameter (a): 60 mm Thickness (b): 12 mm Diameter of disc-shaped space (c): 34.8 mm Opening interval (d): 6 mm Distance between test specimen center and opening circular center ( p): 10.2 m
m The test for measuring the amount of quenching strain by carburizing quenching and tempering was performed by preparing 10 pieces of the above-mentioned navy C test pieces 1 for each test piece, carburizing and tempering these test pieces,
The rate of change of the opening distance (d) between the test pieces before and after carburizing and tempering was measured, and this value was defined as the amount of quenching strain. When a carburized gear was manufactured using a steel material whose deformation after carburizing quenching and tempering with a navy C test piece exceeded 2.0%, the residual austenite existing near the tooth surface of the gear was obtained. But shot peening,
Due to the stress after operating as a gear, when the martensitic transformation occurs, the deformation of the tooth surface becomes too large, and the surface pressure fatigue characteristics deteriorate. Therefore, the quenching distortion amount is 2.0
% Is desirable.

Furthermore, both the deformation due to the transformation from the austenitic structure to the martensite structure due to carburizing and tempering and the deformation of the tooth surface due to the transformation from the residual austenite structure to the martensitic structure driven by gears are reduced, and carburizing and quenching are performed. -In order to obtain a gear having good surface pressure fatigue characteristics without performing tooth profile modification grinding after tempering, it is desirable that the strain amount by this test be 1.0% or less. The test result of the quenching strain amount is an average value of the test repetition number n = 10.

Surface layer residual austenite% Using a test piece for which the amount of carburizing and quenching strain was measured, two out of ten test pieces of each specimen were extracted, and an arc height of 0.1% was placed on the front part of one of the test pieces. Shot peening of 6 mmA was performed. Then, one shot peened test piece and one shot peened test piece of each specimen
For each of the pieces, the amount of retained austenite was measured at every 20 μm depth from the surface layer of the front portion to a depth of 100 μm, and the average value was obtained. Note that electrolytic polishing was used for polishing the measurement surface, and an X-ray diffractometer was used for measurement.

Ferrite Area% of Non-Carburized Part One of the test pieces for which the amount of carburizing and quenching and the amount of residual austenite in the surface layer have been measured is cut, and the ferrite inside (non-carburized part) in the carburizing and quenching and tempering of each specimen. The martensite structure was observed for each specimen in 10 visual fields by a microscopic test, and the ratio of ferrite in each visual field was measured by an image processing apparatus. The average value of the 10 visual fields was obtained and defined as ferrite area%. .

Rotational Bending Fatigue Characteristics A test piece having a diameter of 10 mm in a parallel part was sampled from each specimen having a diameter of 32 mm, and a notch having a depth of 3 mm (a notch coefficient: 1.4) in a direction perpendicular to the parallel part was taken. A rotating bending fatigue test piece attached over the entire circumference was prepared. Carburizing quenching and tempering were performed on all the test pieces under the conditions applied to the navy C test pieces. Thereafter, a shot peening treatment (arc height: 0.6 mmA) was performed on half of the test pieces. The shot peening embodiment sample, the incomplete product, performs rotating bending fatigue test as fatigue limit of 10 ⁷ times using fatigue tester Ono-type rotating bending was measured rotating bending fatigue strength.

Pitching Test (Surface Contact Fatigue Property) Pitching test pieces 4 each having a cylindrical portion having a test surface diameter of 26 mm and a width of 28 mm as shown in FIG. 4 were prepared from each of the test pieces having a diameter of 32 mm. Further, as shown in FIG.
Using each specimen of 0mm, 135mm in diameter by forging
After normalizing, a diameter of 130 mm and a width of 18
The large roller 5 of mm was produced. Next, the carburizing and tempering treatment was performed on the pitched test piece 4 and the large roller 5 under the same conditions as the navy C test piece and the rotating bending fatigue test piece. Thereafter, a shot peening treatment (arc height: 0.6 mmA) was performed on the half of each under the same conditions as the rotating bending fatigue test pieces. And
Shot peening embodiment sample, about the un-embodied products, and the test was conducted as a fatigue limit of 10 ⁷ times using a roller pitching test machine. The test conditions at that time were as follows.

Rotation speed: 1500 rpm Slip ratio: 40% Lubricant: mission oil Oil temperature: 120 ° C. Carburizing effective hardened layer depth, austenite grain size of carburized part, grain boundary oxide layer, internal hardness and internal toughness Carburizing quenching / tempering The Vickers hardness of the subsequent test piece having a diameter of 32 mm was measured at a predetermined pitch from the surface layer portion in the vertical cross section, and the depth at which the hardness became 550 Hv was defined as the carburized effective hardened layer depth. Next, the Vickers hardness inside the non-carburized portion was measured, and the strength of the gear core portion was evaluated based on the measured value. Further, a JIS No. 3 impact test piece was prepared from the non-carburized portion, and an impact test was performed. The result was used to evaluate the toughness of the gear core portion. Further, the same test piece was partially shot-peened, and the grain boundary oxide layer on the surface layer in the vertical cross-section was examined by microscopic observation in each of the sections where shot peening was not performed and the section where the shot peening was performed. Further, the austenite grain size of the carburized part was also examined.

As is clear from Tables 1 and 2, the steel of the present invention, No. 1 Nos. 1 to 7 are conventional steel Nos. 14 and 1
As compared with No. 5, although the internal toughness, the grain size of the carburized portion, and the amount of retained austenite are the same, the internal hardness is high and the grain boundary oxide layer is small. Therefore, the rotating bending fatigue strength is improved. Further, since the tempering softening resistance parameter (H _SiCr ) is high and the amount of heat deformation due to ferrite in the non-carburized portion is small, the surface pressure fatigue strength is significantly increased.

On the other hand, the comparative steel No. In No. 8, the C content is lower than the range of the present invention, and the Si content is higher than the range of the present invention. For this reason, ferrite precipitates in the interior beyond the range of the present invention, and the internal hardness is low, so that the rotational bending fatigue strength is reduced.

Comparative steel No. In No. 9, the C content, the Mn content, and the V content exceed the range of the present invention, and the carbon equivalent (Ceq) and the ideal critical diameter (DI) are higher than the range of the present invention. Accordingly, the internal toughness is reduced, and as a result, the rotational bending fatigue strength is reduced. The Ac three-point temperature parameter is lower than the range of the present invention, and the area ratio of ferrite is lower than the range of the present invention. As a result, the amount of quenching strain increased, and the surface pressure fatigue strength decreased.

Comparative steel No. No. 10 shows that the Cr content is higher than the range of the present invention, the carbon equivalent (Ceq) and the ideal critical diameter (DI) are higher than the range of the present invention, and the internal impact value is low. ing. Also, A
Since the l content is lower than the range of the present invention, the crystal grains become large, and the Ac three-point temperature parameter is lower than the range of the present invention, and the area ratio of ferrite is lower than the range of the present invention, so that the amount of quenching strain increases, Therefore, the surface pressure fatigue strength is reduced.

Comparative steel No. In No. 11, the Si content was lower than the range of the present invention, and therefore, the grain boundary oxide layer became large. Further, the tempering softening resistance parameter (H _SiCr ) is lower than the range of the present invention. Therefore, both the rotational bending fatigue strength and the surface pressure fatigue strength are reduced.

Comparative steel No. In No. 12, since the Cr content is lower than the range of the present invention, the softening resistance parameter (H _SiCr ) is lower than the range of the present invention. Therefore, the surface pressure fatigue strength is reduced.

Comparative steel No. In No. 13, the ideal critical diameter (DI) was lower than the range of the present invention because the Mn content was lower than the range of the present invention. Accordingly, the rotational bending fatigue strength is also reduced due to the reduced internal hardness.

Conventional steel No. 14 and 15 are JIS
Although SCM420H and SCM822H were used, the Si content was lower than the range of the present invention, the Ac three-point temperature parameter was lower than the range of the present invention, and the tempering softening resistance parameter H _SiCr was lower than the range of the present invention. As a result, the internal hardness is lower, the crystal grains are larger than the steel of the present invention, and no ferrite exists in the non-carburized portion. In addition, the grain boundary oxide layer is large. Therefore, the rotational bending fatigue strength and the surface pressure fatigue strength are reduced.

[0067]

As described above, according to the present invention, a conventional gear used for an automobile, an industrial machine or the like has a bending fatigue characteristic higher than that of the conventional gear without changing the conventional manufacturing process. In addition, a useful effect such as mass production of gears requiring a high surface pressure of 2000 MPa or more is provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a carburizing treatment pattern.

FIG. 2 is a front view showing a navy test piece.

FIG. 3 is a side view showing a navy test piece.

FIG. 4 is a front view showing a pitching test piece.

FIG. 5 is a perspective view showing a mechanism of a pitching test.

[Explanation of symbols]

 1: Navy test piece 2: opening 3: space 4: pitching test piece 5: large roller

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuaki Fukuoka 2-12-8 Shinkawa, Chuo-ku, Tokyo NKK Article Steel Inc. (72) Inventor Mitsuhiko Itaya 3-25 Tonomachi, Kawasaki-ku, Kawasaki-shi, Kawasaki No. 1 Isuzu Motors Corporation Kawasaki Plant F-term (reference) 3J030 AC10 BC02 BC03

Claims (5)

[Claims] 1. C: 0.10 to 0.35, Si: 0.5 to 2.0, Mn: 0.2 to 1.0, Ni: 0.01 to 0.50, Cr: 0.5 ~ 2.5, Mo: 0.01 ~ 1.00, Al: 0.005 ~ 0.200, V: 0.001 ~ 0.200 (more than mass%), and further the following (1) Ac3 = 920-203√C-15.2Ni + 44.7Si + 104V-30 Mn-11Cr + 400Al --- (1) The Ac3 temperature parameter calculated by the following equation is 850 to 850.
In the range of 940 ℃, DI = 7.95√C (1 + 0.715Si-0.50Si 2) (1 + 4.1 Mn) (1 + 2.33Cr) (1 + 0.52Ni) (1 + 3.14Mo) (1+ 5V) FB --- (2) where fB = 1 (when Ti <0.005% by mass or B <0.0005% by mass) or 2
(Ti ≧ 0.005 mass% or B ≧ 0.00
Ideal critical diameter DI calculated in the range of 40 to 400 mm, and Ceq = C + Si / 7 + Mn / 5 + V / 2 + Cr / 9 + Ni / 22 + Mo / 2 --- (3) Ceq is 1.100 or less, and an element S effective for increasing the tempering softening resistance
For i and Cr, the tempering softening resistance parameter calculated by H _SiCr = 90Si + 23.5Cr --- (4) is 10
A gear steel excellent in tooth root bending fatigue characteristics and surface pressure fatigue characteristics, characterized by being 0 or more, with the balance being Fe and unavoidable impurities. 2. The gear steel according to claim 1, further comprising Nb: 0.010 to 0.100 mass% as a chemical component composition. 3. The composition according to claim 1, further comprising: Ti: 0.005 to 0.050, B: 0.0005 to 0.0100 (more than mass%) as a chemical component composition. 2. The steel for gears according to 2. 4. The gear steel according to any one of claims 1 to 3, wherein the gear steel is formed into a gear shape by forging or machining, and is subjected to carburizing or carbonitriding. The amount of retained austenite from the surface layer of the gear to a depth of 100 μm is 40% by volume or less;
In addition, the root portion bending fatigue characteristics and the structure of the core portion, which is a non-carburized portion, comprise a two-phase structure of ferrite and martensite, and the ratio of ferrite therein is 10 to 45 area%. Gears with excellent surface pressure fatigue characteristics. 5. The gear steel according to any one of claims 1 to 3, which is formed into a gear shape by forging or machining, and is subjected to a carburizing treatment or a carbonitriding treatment. And 10 mm from the surface of the gear when shot peening with an arc height of 0.3 mmA or more.
The amount of retained austenite up to a depth of 0 μm is 40% by volume or less, and the structure of the core portion, which is a non-carburized portion, is composed of a two-phase structure of ferrite and martensite. A gear excellent in tooth root bending fatigue characteristics and surface pressure fatigue characteristics, characterized by being 45% by area.