JP4127145B2 - Constant velocity joint inner ring with excellent fatigue characteristics and manufacturing method thereof - Google Patents

Description

【００６０】
【表２】

【００６１】
【表３】

【００６２】
表２および表３から明らかなように、本発明で規定した成分組成範囲を満足し、かつ本発明の高周波焼入れ条件を満たす条件でドライブシャフトの嵌合部に焼入れを施した等速ジョイント内輪はいずれも、ドライブシャフトの嵌合部の表面硬化層の旧オーステナイト粒径が全厚にわたって12μm 以下を満たしており、その結果すべり転動疲労寿命100hr 以上、の優れた疲労特性を得ることができた。
なお、表３中のNo. １とNo.39 とを比較すると、ドライブシャフトの嵌合面だけでなく、ボールの転動面についても本発明の高周波焼入れ条件を満たす条件で焼入れを施し、表面硬化層の旧オーステナイト粒径が全厚にわたって12μm 以下を満たすようにすることで、等速ジョイント内輪のボールの転がりによる転動疲労をも向上できることがわかる。
【００６３】
また、表３中のNo.1と２あるいはNo.4と５を比較すると、焼入れ回数を１回から２回に増やすことで、表面硬化層の粒径が微細化し、疲労特性がさらに向上することが分かる。
【００６４】
さらに、No.8, No.37, No.38を比較すると、焼入れ回数を１回から２回に増やした場合において、２回目の焼入れ深さの方が浅い場合（No.37）には、１回しか施さなかった場合よりも疲労特性はむしろ劣化するのに対し、２回目の焼入れ深さを深くした場合（No.38)には、１回しか施さなかった場合に比べてねじり疲労強度は大幅に向上した。No.38 では、硬化層厚方向で、表面から硬化層厚の 4/5位置で最も旧オーステナイト粒径が大きく、3.5 μm であったが、表層近傍（表面から硬化層厚の 1/5位置）では旧オーステナイト粒径は 2.6μm であり、表層の粒径が微細化していることが、疲労特性の向上に寄与したものと考えられる。
【００６５】
これに対し、No.11 は、加工後の冷却速度が小さいため、ベイナイトとマルテンサイトの合計組織分率が10％未満となっており、その結果、表面硬化層粒径が粗大となり、疲労特性が悪い。
No.24 は、表面硬化層粒径は微細であるものの、Ｃ含有量が本発明の範囲より高いため、粒界強度の低下を招き、そのため疲労特性が劣っている。
No,25, 26, 27 は、それぞれＣ, Si, Moの含有量が本発明の適正範囲よりも低いため、表面硬化層粒径が粗大となり、疲労特性が劣っている。
No.28 はＢ含有量が低く、またNo.29 はMn含有量が、No.30 はＳおよびＰ含有量が、No.31 はCr含有量が、それぞれ本発明の適正範囲を超えているため、いずれも粒界強度の低下を招き、疲労特性が劣っている。
No.32 は、Ti含有量が本発明の適正範囲を超えているため、疲労特性が劣っており、逆にNo.35 はTi含有量が低いため、表面硬化層粒径が粗大となり、疲労特性が劣っている。
No.33 は、高周波焼入れ時の加熱温度が高すぎるため表面硬化層の粒径が粗大となり、一方No.34 は、高周波焼入れ時の加熱温度が低すぎるため表面硬化層が形成されず、いずれも疲労特性に劣っている。
No.36 は、Si量が本発明の下限に満たない0.28mass％の場合であるが、この例のように、Si量が本発明の下限をわずかでも下回る場合には、表面硬化層全厚にわたって12μm 以下の粒径を得ることができず、その結果、疲労特性に劣っている。
【００６６】
【発明の効果】
かくして、本発明によれば、優れた疲労特性を有する等速ジョイント内輪を安定して得ることができ、その結果、自動車用部材の軽量化の要求に対し偉功を奏する。
【図面の簡単な説明】
【図１】 等速ジョイントの部分断面図である。
【図２】 等速ジョイント内輪における表面硬化層を示す断面図である。
【符号の説明】
１ ドライブシャフト
２ ハブ
３ 等速ジョイント
４ 内輪
４ａ マウス部
４ｂ ステム部
５ 内輪
５ａ 転動面
５ｂ 嵌合面
６ ボール
７ 表面硬化層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inner ring of a constant velocity joint that has a hardened layer by induction hardening and is used for a drive axle of a power transmission device of an automobile or an industrial machine, and has excellent fatigue characteristics, and a method for manufacturing the same.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in a power transmission device for an automobile, the driving axle on the wheel side is usually via a constant velocity joint 3 to transmit power from the drive shaft 1 to the hub 2 of the wheel as shown in FIG. is there.
The constant velocity joint 3 is a combination of an outer ring 4 and an inner ring 5. That is, the inner ring 5 is slidably fixed to the inner side of the mouse part 4a via a ball 6 fitted in a ball raceway groove formed on the inner surface of the mouse part 4a of the outer ring 4, and the drive shaft 1 is attached to the inner ring 5. On the other hand, the stem portion 4b of the outer ring 4 is splined to the hub 2, for example, to transmit power from the drive shaft 1 to the hub 2 of the wheel.
[0003]
The spline portion, which is a fitting portion with the drive shaft of the inner ring of the constant velocity joint, is subjected to rolling fatigue accompanied by a slight slip, that is, slip rolling fatigue when performing repeated power transmission. Therefore, the inner ring of the constant velocity joint is subjected to induction hardening and tempering after hot forging, cold forging, rolling, etc. are processed into a predetermined shape, followed by induction hardening and tempering. In general, the sliding rolling fatigue life, which is an important characteristic for this kind of application, is secured (see Patent Document 1).
[0004]
On the other hand, due to environmental problems in recent years, there is a strong demand for weight reduction of automobile members. From this viewpoint, further improvement of the fatigue strength of automobile members is required.
JIS-SUJ2 steel is generally used as a steel material with excellent rolling fatigue life and is widely used. However, SUJ2 steel has low cold workability and high steel costs, so it is rarely applied to constant velocity joint inner rings.
In addition, to improve rolling fatigue strength, a technology has been proposed in which the surface γ grain size is set to 10 or more in JIS G 0551 grain size No. and carbide is finely dispersed by two-stage induction hardening after molding. (For example, Patent Document 2).
However, this technique cannot sufficiently meet the recent demand for rolling fatigue strength because the quenching depth is shallow.
[0005]
Further, Patent Document 3 discloses a technique related to refinement of prior austenite grains. According to it, since fine TiC is dispersed in a large amount at the time of induction quenching heating, the prior austenite grain size is to be refined, so it is necessary to solution TiC before quenching, the hot rolling process The process of heating to 1100 ° C or higher is adopted. Therefore, there is a problem that it is necessary to increase the heating temperature during hot rolling, resulting in poor productivity.
[0006]
Further, in Patent Document 4, the ratio (CD / R) between the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and the CD / R and the surface after induction hardening are used. The value A defined by the austenite grain size γf up to 1 mm, the average Vickers hardness Hf up to (CD / R) = 0.1 as induction-quenched, and the average Vickers hardness Hc at the center of the shaft after induction hardening is expressed as C There has been proposed a shaft object part for a machine structure in which torsional fatigue strength is improved by controlling it within a predetermined range according to the amount.
However, in this part, no consideration is given to the prior austenite grain size over the entire thickness of the hardened hardened layer.
[0007]
[Patent Document 1]
JP 2002-371320 A (Claims)
[Patent Document 2]
JP-A-7-118791 (Claims)
[Patent Document 3]
JP 2000-154819 A (Claims, paragraph [0008])
[Patent Document 4]
JP-A-8-53714 (Claims)
[0008]
[Problems to be solved by the invention]
The present invention has been developed in view of the above-described present situation, and an object thereof is to propose a constant velocity joint inner ring having further improved fatigue strength as compared with the conventional method together with its advantageous manufacturing method.
[0009]
[Means for Solving the Problems]
Now, the inventors have intensively studied to effectively improve the sliding rolling fatigue strength.
As a result, as described below, excellent sliding rolling fatigue is achieved by optimizing the material composition of the constant velocity joint inner ring, the structure, quenching conditions, and the prior austenite grain size over the entire thickness of the hardened layer after quenching. The knowledge that strength was obtained was acquired.
[0010]
(1) Fatigue strength is significantly improved by quenching the constant velocity joint inner ring adjusted to an appropriate chemical composition and setting the prior austenite grain size over the entire thickness of the hardened surface layer to 12 μm or less after quenching. Specifically, regarding the chemical composition, the addition of Si and Mo in an appropriate range increases the number of austenite nucleation sites during induction hardening and suppresses the growth of austenite grains. The particle size of the surface hardened layer is effectively refined, and as a result, the sliding rolling fatigue strength is remarkably improved. In particular, by adding Si in an amount of 0.30 mass% or more, a surface hardened layer having a particle size of 12 μm or less can be obtained over the entire thickness of the surface hardened layer after induction hardening.
[0011]
(2) If the base metal structure of the inner ring of the constant velocity joint, that is, the structure before quenching is made into a structure containing a bainite structure and / or martensite structure in a specific fraction, the bainite structure or martensite structure is ferrite-pearlite. Since the carbide is a finely dispersed structure as compared with the structure, the area of the ferrite / carbide interface that is the nucleation site of austenite increases during quenching heating, and the generated austenite becomes finer. As a result, the particle size of the surface hardened layer becomes fine, thereby improving the grain boundary strength and increasing the sliding rolling fatigue strength.
[0012]
(3) As mentioned above, the hardened layer particle size is remarkably refined by appropriately controlling the induction hardening conditions (heating temperature, time, number of times of quenching) for the constant velocity joint inner ring whose chemical composition and structure are adjusted. , Grain boundary strength is improved. Specifically, by setting the heating temperature to 800 to 1000 ° C. and the heating time to 5 seconds or less, fine particles having a particle size of 12 μm or less can be stably obtained over the entire thickness of the surface hardened layer. Furthermore, by repeating the quenching process under the above conditions twice or more, a finer surface hardened layer particle size can be obtained as compared with one quenching.
The present invention is based on the above findings.
[0013]
That is, the gist configuration of the present invention is as follows.
1. A constant velocity joint inner ring having a hardened surface layer by induction hardening on the fitting surface with the drive shaft,
C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and the total of these bainite structure and martensite structure A constant velocity joint inner ring having excellent fatigue characteristics, characterized by having a structure fraction of 10% or more and a prior austenite grain size of a hardened surface layer by induction hardening being 12 μm or less over the entire thickness of the hardened surface layer.
[0014]
2. 3. The constant velocity joint inner ring having excellent fatigue characteristics, wherein the hardened surface is provided on a rolling surface of a ball interposed between the outer race and the constant velocity joint outer ring.
[0015]
3. 3. The constant velocity joint inner ring having excellent fatigue characteristics, wherein the thickness of the hardened surface layer is 2 mm or more in 1 or 2 above.
[0016]
4). In any one of the above 1-3, the constant velocity joint inner ring is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A constant velocity joint inner ring excellent in fatigue characteristics, characterized by having a composition containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.
[0017]
5. C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less, with the balance being Fe and inevitable impurities, steel material is hot-worked to form a constant velocity joint inner ring, and then cooled at a rate of 0.2 ° C / s or higher After that, the constant temperature joint with excellent fatigue characteristics is characterized by forming a hardened surface layer on the mating surface with the drive shaft by induction hardening at a heating temperature of 800 to 1000 ° C. Manufacturing method of inner ring.
[0018]
6). 5. The constant velocity joint inner ring having excellent fatigue characteristics, characterized in that the induction hardening is performed on the rolling surface of the ball interposed between the constant velocity joint outer ring and the hardened surface layer. Manufacturing method.
[0019]
7). In the above 5 or 6, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A method for producing a constant velocity joint inner ring having excellent fatigue characteristics, characterized in that the composition contains one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.
[0020]
8). 5. The method for producing a constant velocity joint inner ring having excellent fatigue characteristics, wherein the induction hardening is repeated a plurality of times and the heating temperature at the final induction hardening is 800 to 1000 ° C.
[0021]
9. 8. The method for producing a constant velocity joint inner ring having excellent fatigue characteristics, wherein the heating temperature during induction hardening is set to 800 to 1000 ° C. for all of the plurality of induction hardenings.
[0022]
10. 5. The method for producing a constant velocity joint inner ring having excellent fatigue characteristics, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
That is, as shown in FIG. 2, the constant velocity joint inner ring 5 has a surface on the fitting surface 5b to which the drive shaft 1 is splined, for example, or on the rolling surface 5a of the ball 6, as shown in FIG. A hardened layer 7 is provided, and the prior austenite grain size in the hardened surface layer 7 is 12 μm or less over the entire thickness of the hardened surface layer. Here, the fitting surface 5b is required to have particularly high sliding rolling fatigue strength.
[0024]
Such a constant velocity joint inner ring will first be described for the reason that its component composition is limited to the above range.
C: 0.35-0.7 mass%
C is an element having the greatest influence on the hardenability, and contributes to the improvement of the sliding rolling fatigue strength by increasing the hardness and depth of the hardened hardened layer. However, if the content is less than 0.35 mass%, it becomes difficult to secure the required sliding rolling fatigue strength, so 0.35 mass% or more is added. On the other hand, if the content exceeds 0.7 mass%, the grain boundary strength is lowered, the sliding rolling fatigue strength is lowered accordingly, and the machinability, cold forgeability and fire cracking resistance are also lowered. For this reason, C was limited to the range of 0.35 to 0.7 mass%. Preferably it is the range of 0.4-0.6 mass%.
[0025]
Si: 0.30 to 1.1 mass%
Si has the effect of increasing the number of nucleation sites of austenite during quenching heating, suppressing the grain growth of austenite, and reducing the grain size of the quenched hardened layer. Moreover, carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by carbide | carbonized_material is suppressed. Furthermore, it is an element useful for the formation of a bainite structure, and these improve the sliding rolling fatigue strength.
Thus, Si is a very important element in the present invention, and it is essential to contain 0.30 mass% or more. This is because if the Si amount is less than 0.30 mass%, fine particles having a particle diameter of 12 μm or less cannot be obtained over the entire thickness of the cured layer, regardless of how the manufacturing conditions and quenching conditions are adjusted. However, if the Si content exceeds 1.1 mass%, the hardness increases due to the solid solution hardening of ferrite, leading to a decrease in machinability and cold forgeability. Therefore, Si was limited to the range of 0.30 to 1.1 mass%. Preferably it is the range of 0.40-1.0 mass%.
[0026]
Mn: 0.2 to 2.0 mass%
Mn is an essential component for improving the hardenability and ensuring the hardening depth during quenching, so it is actively added, but if the content is less than 0.2 mass%, the effect of addition is poor, so 0.2% More than mass%. Preferably it is 0.3 mass% or more. On the other hand, if the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the sliding rolling fatigue strength decreases, so Mn is set to 2.0 mass% or less. It should be noted that if the content of Mn is large, the base material is hardened, which may be disadvantageous in machinability. Therefore, it is preferable to set the content to 1.2 mass% or less. More preferably, it is 1.0 mass% or less.
[0027]
Al: 0.005 to 0.25 mass%
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, if the content is less than 0.005 mass%, the effect of addition is poor. On the other hand, even if the content exceeds 0.25 mass%, the effect is saturated, and rather, a disadvantage that causes an increase in the component cost occurs. Limited to a range of ~ 0.25 mass%. Preferably it is the range of 0.05-0.06 mass%.
[0028]
Ti: 0.005 to 0.1 mass%
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and the effect of improving the hardenability of B to disappear, and has the effect of sufficiently exerting the effect of improving the hardenability of B. . In order to obtain this effect, it is necessary to contain at least 0.005 mass%. However, if it exceeds 0.1 mass%, a large amount of TiN is formed, which becomes the starting point of fatigue failure and causes sliding rolling. Ti is limited to the range of 0.005 to 0.1 mass% because it causes a significant decrease in fatigue strength. Preferably it is the range of 0.01-0.07 mass%. Furthermore, it is preferable to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securing N securely and improving the hardenability by B to obtain a bainite and martensite structure.
[0029]
Mo: 0.05-0.6 mass%
Mo promotes the formation of a bainite structure, thereby reducing the austenite grain size during quenching and heating, and reducing the grain size of the surface hardened layer. Moreover, it has the effect | action which refines | miniaturizes the particle size of a surface hardening layer by suppressing the grain growth of austenite at the time of quenching heating. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability. In addition, Mo is an element that suppresses the formation of carbides and effectively prevents a decrease in grain boundary strength due to carbides.
Thus, Mo is a very important element in the present invention, and if the content is less than 0.05 mass%, the particle size over the entire thickness of the surface hardened layer is adjusted no matter how the manufacturing conditions and quenching conditions are adjusted. However, it cannot be made fine particles of 12 μm or less. However, if the content exceeds 0.6 mass%, the hardness of the rolled material is remarkably increased, and the workability is reduced. Therefore, Mo is limited to the range of 0.05 to 0.6 mass%. Preferably it is the range of 0.1-0.6 mass%.
[0030]
B: 0.0003 to 0.006 mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. B also has the effect of improving hardenability by adding a small amount and improving the sliding rolling fatigue strength by increasing the quenching depth during quenching. Further, B preferentially segregates at the grain boundary, reduces the concentration of P segregating at the grain boundary, improves the grain boundary strength, and thus has an effect of improving torsional fatigue strength.
For this reason, in the present invention, B is positively added, but if the content is less than 0.0003 mass%, the effect of addition is poor. On the other hand, if the content exceeds 0.006 mass%, the effect is saturated, rather the component In order to raise the cost, B is limited to the range of 0.0003 to 0.006 mass%. Preferably it is the range of 0.0005-0.004 mass%.
[0031]
S: 0.06 mass% or less S is a useful element that improves the machinability by forming MnS in steel, but if it exceeds 0.06 mass%, it segregates at the grain boundary and lowers the grain boundary strength. , S was limited to 0.06 mass% or less. Preferably it is 0.04 mass% or less.
[0032]
P: 0.02 mass% or less P segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby reducing the sliding rolling fatigue strength. In addition, there is a harmful effect that promotes burning cracks. Therefore, it is desirable to reduce the P content as much as possible, but it is allowed up to 0.02 mass%.
[0033]
Cr: 0.2 mass% or less
Cr stabilizes the carbide and promotes the formation of residual carbide, lowers the grain boundary strength, and degrades the torsional fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 0.2 mass% is acceptable. Preferably it is 0.05 mass% or less.
[0034]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cu: 1.0 mass% or less
Cu is effective in improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves the sliding rolling fatigue strength. Further, by suppressing the formation of carbides, the decrease in grain boundary strength due to carbides is suppressed, and the torsional fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracking occurs during hot working, so 1.0 mass% or less is added. In addition, Preferably it is 0.5 mass% or less.
[0035]
Ni: 3.5 mass% or less
Since Ni is an element that improves hardenability, Ni is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves a sliding rolling fatigue strength. However, Ni is an extremely expensive element, and adding more than 3.5 mass% increases the cost of the constant velocity joint inner ring. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Preferably it is 0.1-1.0 mass%.
[0036]
Co: 1.0 mass% or less
Co is an element that suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and improves the sliding rolling fatigue strength. However, Co is an extremely expensive element, and adding more than 1.0 mass% increases the cost of the constant velocity joint inner ring. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 mass% or more. Preferably it is 0.02-0.5 mass%.
[0037]
Nb: 0.1 mass% or less
Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves the temper softening resistance, and these effects improve the sliding rolling fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, 0.1 mass% is made an upper limit. In addition, if less than 0.005% is added, the effect of improving precipitation strengthening and tempering softening resistance is small, so it is desirable to add 0.005 mass% or more. Preferably it is 0.01-0.05 mass%.
[0038]
V: 0.5 mass% or less V combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve sliding rolling fatigue strength. However, since the effect is saturated even if it contains exceeding 0.5 mass%, it shall be 0.5 mass% or less. In addition, since the improvement effect of sliding rolling fatigue strength is small when added less than 0.01 mass%, it is desirable to add 0.01 mass% or more. Preferably it is 0.03-0.3 mass%.
[0039]
The preferred component composition range has been described above. However, in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the matrix structure is also important.
That is, in the present invention, the base material structure of the constant velocity joint inner ring, that is, the structure before quenching (corresponding to the structure other than the surface hardened layer after induction hardening) has a bainite structure and / or a martensite structure, and The total structure fraction of the bainite structure and martensite structure must be 10% or more in terms of volume fraction (vol%). The reason for this is that the bainite structure or martensite structure is a structure in which carbides are finely dispersed compared to the ferrite-pearlite structure, so the area of the ferrite / carbide interface, which is an austenite nucleation site, is increased during quenching heating. This is because the generated austenite is made finer, and thus contributes effectively to making the grain size of the hardened surface hardened layer finer. And by grain refinement | miniaturization of the surface hardened layer, a grain-boundary intensity | strength rises and a sliding rolling fatigue strength improves.
Here, the total structure fraction of the bainite structure and the martensite structure is more preferably 20 vol% or more.
[0040]
The remaining structure other than the bainite structure or martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.
Further, regarding the refinement of the grain size of the hardened surface layer after quenching, the martensite structure has the same effect as the bainite structure, but from an industrial point of view, the bainite structure is better than the martensite structure. Since the amount of the alloy element added is smaller and it can be produced at a low cooling rate, it is advantageous in production.
[0041]
The constant velocity joint inner ring of the present invention has a hardened surface layer by induction hardening on the mating surface with the drive shaft, and the hardened surface layer after induction hardening has a prior austenite grain size of 12 μm or less over its entire thickness. It is necessary to. This is because if the prior austenite grain size over the entire thickness of the surface hardened layer exceeds 12 μm, sufficient grain boundary strength cannot be obtained, and satisfactory improvement in torsional fatigue strength cannot be expected. The thickness is preferably 10 μm or less, more preferably 5 μm or less.
[0042]
Here, the measurement of the prior austenite particle size over the entire thickness of the hardened surface layer is performed as follows.
In the constant velocity joint inner ring of the present invention after induction hardening, the outermost layer of the mating surface with the drive shaft subjected to induction hardening has a martensite structure of 100% in area ratio. And as it goes from the surface to the inside, the area of 100% martensite structure continues at a certain depth, but the area ratio of the martensite structure decreases rapidly from a certain depth.
In the present invention, for the induction-quenched portion, the depth region from the surface of the fitting surface with the drive shaft of the constant velocity joint inner ring to the area ratio of the martensite structure decreasing to 98% is defined as the surface hardened layer. .
Then, for this surface hardened layer, the average prior austenite particle size at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness from the surface was measured, and any average prior austenite particle diameter was 12 μm. In the following cases, it is assumed that the prior austenite grain size over the entire thickness of the surface hardened layer is 12 μm or less.
The average prior austenite grain size is measured 400 times (1 field area: 0.25 mm x 0.225 mm) to 1000 times (1 field area: 0.10 mm x 0.09 mm) at each position using an optical microscope. This is done by observing 5 fields of view and measuring the average particle size with an image analyzer.
[0043]
Furthermore, in the present invention, the thickness of the surface hardened layer by induction hardening is preferably 2 mm or more. This is because, when the thickness of the cured layer is too small, the sliding rolling stress also affects the non-cured layer, causing damage from the non-cured layer.
[0044]
Moreover, in this invention, it is preferable to have also the surface hardened layer mentioned above also on the rolling surface of the ball interposed between constant velocity joint outer rings.
The rolling surface of the ball is subject to rolling fatigue due to the rolling of the ball, but the above-mentioned surface hardened layer whose average prior austenite grain size is 12 μm or less over the entire surface hardened layer thickness is also effective in improving this rolling fatigue strength. It is. For this reason, the durability can be further improved by forming the above-mentioned hardened layer not only on the fitting surface of the drive shaft but also on the rolling surface of the ball.
[0045]
Next, conditions for manufacturing the constant velocity joint inner ring of the present invention will be described.
First, a steel bar obtained by rolling a steel material adjusted to a predetermined component composition is cut into a predetermined length, then a constant velocity joint inner ring is formed by hot forging, and then subjected to normalization as necessary, After forming a groove for spline coupling of the drive shaft to the mating surface by rolling, induction hardening is applied to the surface layer of the mating surface to obtain a product.
[0046]
In the present invention, the base material structure has the above-described bainite structure and / or martensite structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol% or more. The material before quenching needs to be cooled at a rate of 0.2 ° C / s or higher after being processed into a predetermined shape by hot working such as rolling and forging. This is because when the cooling rate is less than 0.2 ° C./s, it becomes difficult to obtain a bainite or martensite structure, and the total structure fraction of these structures may not reach 10 vol%. The preferable range of the cooling rate after hot working is 0.3 to 30 ° C./s.
[0047]
In addition, it is preferable to perform hot working in a temperature range of over 900 ° C. to 1150 ° C. If the temperature is 900 ° C. or lower, the necessary bainite structure and / or martensite structure cannot be obtained. On the other hand, if it exceeds 1150 ° C., the heating cost increases, which is economically disadvantageous.
[0048]
Next, in the present invention, induction hardening is performed on the contact surface with the drive shaft, and preferably on the rolling surface of the ball interposed between the constant velocity joint outer ring to obtain the above-described surface hardened layer. However, the heating temperature range during induction hardening must be 800-1000 ° C. This is because when the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and the surface hardened layer structure described above is insufficiently generated. As a result, sufficient fatigue strength cannot be ensured. When the heating temperature exceeds 1000 ° C., the growth of austenite grains is promoted and becomes coarse, and the grain size of the surface hardened layer becomes coarse, so that the fatigue strength is also lowered. A more preferable heating temperature range is 800 to 950 ° C.
[0049]
When the above-described induction hardening is repeated a plurality of times, at least the final induction hardening may be performed at a heating temperature of 800 to 1000 ° C. Here, when induction hardening is repeated a plurality of times, it is most desirable to set the heating temperature to 800 to 1000 ° C. for all induction hardening. Further, by performing repeated quenching twice or more, a finer surface hardened layer particle size can be obtained as compared with the single quenching.
[0050]
When induction hardening is repeated a plurality of times, it is preferable that at least the final quenching depth by induction hardening be equal to or greater than the previous quenching depth by induction hardening. This is because the crystal grain size of the hardened surface layer is most strongly affected by the last induction hardening, so if the quenching depth by the last induction hardening is smaller than the previous quenching depth by induction hardening, the surface This is because the average grain size over the entire thickness of the hardened layer is rather increased, and the fatigue strength tends to decrease.
[0051]
Further, in the present invention, the induction hardening is preferably performed with the heating time in the above heating temperature range being 5 seconds or less. This is because when the heating time is set to 5 seconds or less, the austenite grain growth can be further suppressed as compared with the case where the heating time exceeds 5 seconds, and a very fine surface hardened layer particle size can be obtained. it can. A more preferable heating time is 3 seconds or less.
[0052]
【Example】
Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400mm. This slab was rolled into a 150 mm square billet through a breakdown process, and then rolled into a 40 mmφ steel bar.
[0053]
Next, after cutting this steel bar to a predetermined length, a constant velocity joint inner ring (outer diameter: 45 mm and inner diameter: 20 mm) is formed by hot forging, and then a groove for spline connection to the mating surface by rolling. Formed. The finishing temperature for hot forging was over 900 ° C as a suitable temperature from the viewpoint of bainite or martensite structure formation. The conditions for cooling in hot forging were as shown in Table 2.
[0054]
Using a high-frequency quenching device with a frequency of 15 Hz, the surface of the constant velocity joint inner ring fitted with the drive shaft and the rolling surface of the ball interposed between the constant velocity joint outer ring were quenched under the conditions shown in Table 2. Then, tempering was performed using a heating furnace under the conditions of 180 ° C. × 2 h to obtain a product. The constant velocity joint inner ring thus obtained has a drive shaft fitted to its fitting surface and is mounted on the mouse portion of the constant velocity joint outer ring via a ball (steel ball), while the stem portion of the constant velocity joint outer ring. A constant velocity joint unit was obtained by fitting a hub to the head (see FIG. 1). The specifications of the ball, outer ring, drive shaft, and hub are as follows.
Ball: Quenched and tempered steel of high carbon chrome bearing steel SUJ2 Outer ring: Induction-quenched and tempered steel of carbon steel for machine structure Hub: Induction-quenched and tempered steel of carbon steel for machine structure Drive shaft: Induction-hardened and tempered steel of carbon steel for machine structure [0055]
Next, using this constant velocity joint unit, in the power transmission system that transmits the rotational movement of the drive shaft to the hub via the inner ring and inner ring of the constant velocity joint, the sliding rolling fatigue strength of the fitting portion of the drive shaft, and Durability tests were conducted on the rolling fatigue strength of the rolling surface of the ball.
[0056]
Here, the sliding rolling fatigue test uses a torsional fatigue tester with a maximum torque of 4900 N · m, and the frequency between the hub of the constant velocity joint unit and the drive shaft under the condition of a maximum torque of 700 N · m. The torsional force was repeatedly applied by swinging at 2 Hz, and the time until the spline part (fitting surface with the drive shaft) of the constant velocity joint inner ring was damaged due to sliding rolling was evaluated as sliding rolling fatigue strength.
[0057]
In the rolling fatigue test, power is transmitted under the conditions of torque: 900 N · m, operating angle (angle formed by the inner ring axis and drive shaft axis): 20 °, and rotation speed: 300 rpm. The time until separation occurred on the rolling surface was evaluated as the rolling fatigue strength. Here, the dimensions and shapes of the drive shaft, constant velocity joint outer ring, etc. were set so that the constant velocity joint inner ring would be the weakest part during the durability test.
[0058]
Moreover, about the constant velocity joint inner ring | wheel produced on the same conditions, the average prior austenite particle size obtained over the base material structure | tissue, the surface hardening layer thickness after hardening, and the full thickness of a surface hardening layer was measured using the optical microscope.
Table 2 shows the results of the investigation of the base material structure, and Table 3 shows the surface hardened layer thickness, the average prior austenite particle size (hardened layer particle size), the sliding rolling fatigue strength, and the rolling fatigue strength of the ball rolling surface. The results of the survey are also listed.
Here, as described above, the thickness of the surface hardened layer was set to the depth at which the area ratio of the martensite structure was reduced to 98% from the surface of the rolling surface and the fitting surface of the constant velocity joint inner ring. Moreover, about what performed induction hardening several times, the surface hardening layer thickness after each hardening was measured. Furthermore, for the surface hardened layer particle size, the average prior austenite particle size at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness from the surface was measured and the maximum value was shown. . In addition, about what performed induction hardening several times, the average prior-austenite particle size after final hardening was measured.
[0059]
[Table 1]

[0060]
[Table 2]

[0061]
[Table 3]

[0062]
As is apparent from Tables 2 and 3, the constant velocity joint inner ring that satisfies the component composition range defined in the present invention and that has been quenched into the fitting portion of the drive shaft under the conditions satisfying the induction hardening condition of the present invention is as follows. In all cases, the old austenite grain size of the hardened surface layer of the mating part of the drive shaft satisfied 12 μm or less over the entire thickness, and as a result, excellent fatigue characteristics with a sliding rolling fatigue life of 100 hours or more could be obtained. .
When comparing No. 1 and No. 39 in Table 3, not only the fitting surface of the drive shaft but also the rolling surface of the ball was hardened under the conditions satisfying the induction hardening condition of the present invention. It can be seen that rolling fatigue due to rolling of the ball of the constant velocity joint inner ring can be improved by making the prior austenite grain size of the hardened layer satisfy 12 μm or less over the entire thickness.
[0063]
In addition, when comparing No. 1 and 2 or No. 4 and 5 in Table 3, increasing the number of times of quenching from 1 to 2 makes the surface hardened layer finer and further improves the fatigue characteristics. I understand that.
[0064]
Furthermore, comparing No.8, No.37, and No.38, when the number of times of quenching was increased from 1 to 2 and the second quenching depth was shallower (No.37), Fatigue properties deteriorate rather than the case where it was applied only once, whereas the torsional fatigue strength is higher when the second quenching depth is increased (No. 38) than when it is applied only once. Improved significantly. In No. 38, in the thickness direction of the hardened layer, the oldest austenite grain size was 3.5 μm at the 4/5 position from the surface to the hardened layer thickness. ), The prior austenite grain size is 2.6μm, and the finer grain size of the surface layer is thought to have contributed to the improvement of fatigue properties.
[0065]
In contrast, No. 11 has a low cooling rate after processing, so the total structural fraction of bainite and martensite is less than 10%. As a result, the surface hardened layer has a coarse grain size and fatigue characteristics. Is bad.
No. 24 has a fine surface-hardened layer particle size, but its C content is higher than the range of the present invention, which leads to a decrease in grain boundary strength, and therefore fatigue properties are inferior.
In Nos. 25, 26, and 27, the contents of C, Si, and Mo are lower than the appropriate ranges of the present invention, respectively, so that the surface hardened layer particle size is coarse and fatigue characteristics are inferior.
No. 28 has a low B content, No. 29 has an Mn content, No. 30 has an S and P content, and No. 31 has a Cr content exceeding the proper range of the present invention. For this reason, both of them cause a decrease in grain boundary strength and inferior fatigue properties.
No.32 is inferior in fatigue characteristics because the Ti content exceeds the appropriate range of the present invention, and conversely No.35 has a low Ti content, resulting in a coarse surface hardened layer grain size and fatigue. The characteristics are inferior.
In No.33, the particle size of the surface hardened layer becomes coarse because the heating temperature during induction hardening is too high, while in No.34, the surface hardening layer is not formed because the heating temperature during induction hardening is too low. Is also inferior in fatigue properties.
No. 36 is the case where the Si amount is 0.28 mass% which is less than the lower limit of the present invention. However, when the Si amount is slightly below the lower limit of the present invention as in this example, the total thickness of the surface hardened layer is The particle size of 12 μm or less cannot be obtained over the entire range, and as a result, the fatigue properties are inferior.
[0066]
【The invention's effect】
Thus, according to the present invention, it is possible to stably obtain a constant velocity joint inner ring having excellent fatigue characteristics, and as a result, it is excellent for the demand for weight reduction of automobile members.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view of a constant velocity joint.
FIG. 2 is a cross-sectional view showing a hardened surface layer in a constant velocity joint inner ring.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Drive shaft 2 Hub 3 Constant velocity joint 4 Inner ring 4a Mouse | mouth part 4b Stem part 5 Inner ring 5a Rolling surface 5b Fitting surface 6 Ball 7 Surface hardening layer

Claims (10)

A constant velocity joint inner ring having a hardened surface layer by induction hardening on the fitting surface with the drive shaft,
C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and the total of these bainite structure and martensite structure A constant velocity joint inner ring having excellent fatigue characteristics, characterized by having a structure fraction of 10% or more and a prior austenite grain size of a hardened surface layer by induction hardening being 12 μm or less over the entire thickness of the hardened surface layer. 2. The constant velocity joint inner ring having excellent fatigue characteristics according to claim 1, further comprising the surface hardened layer on a rolling surface of a ball interposed between the constant velocity joint outer ring. 3. The constant velocity joint inner ring having excellent fatigue characteristics according to claim 1, wherein the thickness of the surface hardened layer is 2 mm or more. The constant velocity joint inner ring according to any one of claims 1 to 3, further comprising:
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A constant velocity joint inner ring excellent in fatigue characteristics, characterized by having a composition containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less. C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less, with the balance being Fe and inevitable impurities, steel material is hot-worked to form a constant velocity joint inner ring, and then cooled at a rate of 0.2 ° C / s or higher After that, the constant temperature joint with excellent fatigue characteristics is characterized by forming a hardened surface layer on the mating surface with the drive shaft by induction hardening at a heating temperature of 800 to 1000 ° C. Manufacturing method of inner ring. 6. The constant velocity joint excellent in fatigue characteristics according to claim 5, further comprising forming a hardened surface layer by subjecting the rolling surface of the ball interposed between the constant velocity joint outer ring and the induction hardening to form a hardened surface layer. Manufacturing method of inner ring. The steel material according to claim 5 or 6, further comprising:
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A method for producing a constant velocity joint inner ring having excellent fatigue characteristics, characterized in that the composition contains one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less. The method for producing a constant velocity joint inner ring with excellent fatigue characteristics according to any one of claims 5 to 7, wherein the induction hardening is repeated a plurality of times, and the heating temperature at the final induction hardening is set to 800 to 1000 ° C. . 9. The method for manufacturing a constant velocity joint inner ring having excellent fatigue characteristics according to claim 8, wherein the heating temperature during induction hardening is set to 800 to 1000 [deg.] C. for all of the plurality of induction hardenings. The method for producing a constant velocity joint inner ring having excellent fatigue characteristics according to any one of claims 5 to 9, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening.

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