JP3145517B2 - Component for mechanical structure excellent in fatigue strength, especially surface fatigue strength, and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a part for a mechanical structure requiring a fatigue strength, particularly a surface fatigue strength, such as an outer race of a constant velocity joint for an automobile, and a method for manufacturing such a part. Things.

[0002]

2. Description of the Related Art Machine structural parts, particularly power transmission parts for industrial machines such as shafts and constant velocity joints, are required to have excellent fatigue strength. Conventionally, as means for imparting surface fatigue resistance to such parts, carburizing, nitriding, or surface hardening such as induction hardening using medium carbon steel such as S48C and S55C has been widely performed.

[0003]

In recent years, for the purpose of improving the fuel consumption efficiency of industrial machines, especially automobiles, the weight of various steel parts has been reduced by increasing the strength. For example, when the outer race of a constant velocity joint for automobiles is made smaller or thinner, pitching or flaking occurs early due to an increase in contact surface pressure in the ball groove, resulting in a large increase in surface fatigue strength. Improvement is indispensable. It is known that the most effective way to improve the surface fatigue strength is to strengthen the surface layer by increasing the surface hardness.

[0004] However, in the driving force transmitting parts as described above, there is a limit in hardness and toughness in the technology based on the combination of the conventional medium carbon steel and the conventional method, and the fatigue strength, particularly the surface fatigue strength, is greatly improved. It is difficult.

The present invention has been made in order to solve such technical problems, and an object of the present invention is to provide a machine structural component having greatly improved fatigue strength, particularly surface fatigue strength, and such a mechanical structure. It is an object of the present invention to provide a method for manufacturing a component for a vehicle.

[0006]

According to the present invention which has achieved the above objects, C: 0.4 to 0.7% by weight, Si: 0.3% by weight or less (excluding 0% by weight), Mn: 0.2 to 1% by weight, Cr: 0.2 to 3% by weight, Mo: 0.1 to 1% by weight, V: 0.1 to 1% by weight, Al: 0.01 to 0.05
% By weight, N: 0.003 to 0.02% by weight, S: 0.0
7% by weight or less (excluding 0% by weight), Ti: 0.00
2% by weight or less (not including 0% by weight), and the balance consists of Fe and unavoidable impurities. P and O in the impurities are suppressed to 0.02% by weight or less and 0.002% by weight or less, respectively. After forming the material into a predetermined part shape, the material is subjected to nitriding treatment or nitrocarburizing treatment, followed by induction hardening treatment, thereby diffusing nitrogen from the surface, and 0.2 mm from the outermost surface. This is a method for producing a mechanical structure component having excellent fatigue strength, particularly excellent surface fatigue strength, which has a gist in forming a surface hardened layer containing a nitrogen concentration of 0.05% by weight or more at a depth position.

The mechanical structural part of the present invention is manufactured by the above method, and has a C of 0.4 to 0.7.
% By weight, Si: 0.3% by weight or less (excluding 0% by weight), Mn: 0.2 to 1% by weight, Cr: 0.2 to 3% by weight, Mo: 0.1 to 1% by weight, V: 0.1-1% by weight,
Al: 0.01 to 0.05% by weight, N: 0.003 to
0.02% by weight, S: 0.07% by weight or less (excluding 0% by weight), Ti: 0.002% by weight or less (excluding 0% by weight), and the balance of Fe and inevitable impurities And a steel in which P and O in the impurities are suppressed to 0.02% by weight or less and 0.002% by weight or less, respectively, and the nitrogen concentration is set to 0.05 at a depth of 0.2 mm from the outermost surface. % Or more, and the austenite grain size at a depth of 0.2 mm from the outermost surface is No. 1 12
In addition to having the above fine structure, the gist is that the amount of retained austenite in the surface layer portion up to a depth of 0.2 mm is 10 to 25% by volume.

[0008]

In order to achieve the above object, the present inventors have studied both the composition of the steel material and the surface hardening treatment. As a result, the following two points were found to be important in order to improve the fatigue strength, particularly the surface fatigue strength. (1) It is necessary to increase the normal temperature and high temperature hardness of the surface layer of the part. (2) In order to further increase the high-temperature hardness (that is, to increase the softening resistance), it is effective to penetrate and diffuse nitrogen in a larger amount and deeper to form a surface hardened layer by precipitation of fine carbon and nitride. That. In addition, it was also found that, if necessary, the generation of an appropriate amount of retained austenite in the surface layer of the component reduced the hardness increase due to the work-induced transformation and the apparent surface pressure due to the cushion effect.

In order to obtain the above properties, nitriding or nitrocarburizing treatment is first required. At this time, from the viewpoint of the material, the addition of Mo and V is mainly effective in increasing the nitrogen concentration. It is extremely effective to combine nitriding or nitrocarburizing with induction hardening in order to promote the diffusion of nitrogen, the solid solution, and the precipitation of fine carbon and nitride, and the toughening of martensite based on it. I understood that.

The method of induction hardening after the nitrocarburizing treatment is described in, for example, JP-A-2-232353.
A method of performing induction hardening after nitrocarburizing to SCM steel and tool steel is described, but the present invention refers to the optimum alloy component range of the applied steel, and the object of the present invention is to use pitting. The method is different from the above method in that the surface fatigue strength is improved so that the life of flaking can be significantly improved.

Japanese Patent Application Laid-Open No. Sho 60-169544 proposes a technique for defining the components of steel materials suitable for the manufacture of parts for mechanical structures, particularly gears, and performing induction hardening as a means for improving fatigue strength. However, the present invention makes it possible to achieve a significant improvement in fatigue strength by performing induction hardening after nitriding or nitrocarburizing, which is different from the above method. Furthermore, Japanese Patent Application Laid-Open No. 4-66646 discloses a technique relating to a steel material capable of greatly improving fatigue strength by performing nitriding treatment. However, this technique has a technique of reducing fatigue strength by a combination of quenching and tempering treatment and nitriding treatment. The focus is on steel that can be improved and is fundamentally different from the present invention. First, the reasons for limiting chemical components in the present invention are as follows.

C: 0.4 to 0.7% by weight C is an element necessary for obtaining hardness at normal temperature and high temperature.
If the content is less than 0.4% by weight, it cannot withstand high surface pressure during use. However, if the C content is too large, the life of the forging tool is reduced due to an increase in the deformation resistance particularly during the forging of parts, or the life of the tool during the subsequent cutting is reduced. Should. The preferred range of the C content is about 0.5 to 0.6% by weight.

Si: not more than 0.3% by weight (not including 0% by weight) Si is an element necessary for improving the internal quality of steel, but if it is too much, it is forged due to an increase in hardness of the steel material. 0.3% by weight
Should be: The preferred range of the Si content is
It is about 0.15 to 0.3% by weight.

Mn: 0.2 to 1% by weight Mn is an element necessary for improving the internal quality of the steel, improving the induction hardenability, and obtaining the surface hardness. If the amount is less than the weight percentage, such an effect cannot be obtained. However, if the Mn content is too large,
In particular, the life of a forging tool is reduced due to an increase in deformation resistance during forging of a part, or the life of a tool during a subsequent cutting process is reduced. The more preferable range of Mn is about 0.5 to 0.6% by weight.

Cr: 0.2 to 3% by weight Cr is an element necessary for improving the induction hardening property and increasing the hardness due to the formation of carbonitride.
If it is less than the weight percentage, it cannot withstand high surface pressure during use. However, if the Cr content is too large, excessive formation of carbonitride causes excessive increase in surface hardness and lowers fatigue limit. Further, the life of the forging tool is reduced due to an increase in the deformation resistance at the time of forging the part, or the life of the tool during the subsequent cutting is reduced. The more preferable range of Cr is about 0.45 to 0.6% by weight.

Mo: 0.1 to 1% by weight Mo improves the induction hardenability and allows nitrogen to penetrate deeper and more by forming Mo carbonitride.
It is an element necessary for improving the softening resistance by diffusing, and if the Mo content is less than 0.1% by weight, these effects cannot be obtained and it is not possible to withstand high surface pressure during use.
However, when the Mo content is too large, excessive formation of carbonitride causes excessive increase in surface hardness and lowers fatigue limit. Further, since the life of the forging tool is reduced due to an increase in deformation resistance during warm forging, the content should be 1% by weight or less. The more preferable range of Mo is 0.3 to 0.1.
It is about 6% by weight.

V: 0.1 to 1% by weight V is an element necessary for improving the softening resistance by penetrating and diffusing nitrogen deeper and more by forming carbonitrides, like Mo. If the content is less than 0.1% by weight, such an effect cannot be obtained, and it cannot withstand high surface pressure during use. However, if the V content is too high,
Excess carbonitride does not completely dissolve during quenching and heating, and its effect is saturated. Therefore, the content should be 1% by weight or less. The more preferable range of V is about 0.3 to 0.6% by weight.

Al: 0.01 to 0.05% by weight Al is an element effective for deoxidation and has an Al content of 0.01%.
If the content is less than 0.05% by weight, this effect cannot be achieved, and if the content exceeds 0.05% by weight, the hardness of the nitrided layer is excessively increased and the fatigue limit is lowered.

N: 0.003 to 0.02% by weight N combines with Al to form AlN, and has an effect of reducing the crystal grain size. If the N content is less than 0.003% by weight, the effect becomes insufficient. However, if the N content is too large, cracks are likely to occur during rolling,
It should not exceed 0.02% by weight.

S: 0.07% by weight or less (excluding 0% by weight) S is an element effective for improving the machinability at the time of part cutting, but if contained exceeding 0.07% by weight. Adversely affects fatigue strength.

Ti: 0.002% by weight or less (excluding 0% by weight) Ti is an element effective for improving the fatigue strength by forming nitrides to reduce the crystal grain size. Since the strength is reduced, the content should be 0.002% by weight or less.

The component of the present invention comprises the above elements as basic components and the balance of Fe and unavoidable impurities.
It is necessary to minimize the contents of P and O among the unavoidable impurities. Also, if necessary, P
You may add b, Zr, Te, etc. The reasons for limiting the chemical components of these elements are as follows.

P: not more than 0.02% by weight P is a harmful impurity element that embrittles crystal grain boundaries and has an adverse effect on fatigue strength. Therefore, its content should be suppressed to not more than 0.02% by weight. .

O: not more than 0.002% by weight O forms an oxide and is an impurity element particularly harmful to fatigue strength. Its content must be as low as possible.
It should be kept below 02% by weight.

Pb: 0.02 to 0.13% by weight, Zr:
1% by weight and Te: 1% by weight or less Pb, Zr and Te are elements effective in improving machinability. Pb has the effect of improving machinability during partial cutting, and the Pb content is 0.02% by weight.
If the content is less than 0.13% by weight, the effect cannot be obtained, and even if the content exceeds 0.13% by weight, the fatigue strength is adversely affected. The more preferable range of Pb is about 0.05 to 0.1% by weight.
On the other hand, in addition to the effect of improving machinability, Zr and Te have an effect of improving the toughness by rounding the shape of the sulfide-based inclusions by being added together with S. However, Z
When the content of r and Fe exceeds 1% by weight, the effect is saturated.

The component of the present invention is made by using a steel having the above-mentioned chemical composition as a raw material, forming the raw material into a predetermined shape, performing a nitriding treatment or a soft nitriding treatment, and subsequently performing an induction hardening treatment. Nitrogen is diffused from the surface, and the nitrogen concentration at a depth of 0.2 mm from the outermost surface is set to 0.
It is specified to be at least 05% by weight for the following reasons. That is, if the nitrogen concentration at this position is less than 0.05% by weight, plastic flow occurs during use due to a decrease in softening resistance, and the pitting life is reduced.

In the part of the present invention, 0.2 mm from the outermost surface
Grain size of austenite at depth position
o. It is necessary to have a fine structure of 12 or more. If it is less than 12, the crystal grains are too large, resulting in a decrease in toughness and a decrease in pitting life. In the component of the present invention, the amount of retained austenite in the surface layer up to a depth of 0.2 mm is 10 to 25% by volume.
However, if the amount of retained austenite is out of the above range and less than 10% by volume or more than 25% by volume, the pitting life is reduced.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and any design change in the spirit of the present invention will be described. It is included in the technical scope.

[0029]

EXAMPLES Steels having the chemical compositions shown in Tables 1 (inventive steel) and 2 (comparative steel) were melted using a small vacuum furnace (150 kg / ch), and forged to 30 mm diameter steel bars by forging. Thereafter, a normalizing process was performed. Subsequently, a roller pitching test piece having a total length of 125 mm (diameter: 20 mm) having a cylindrical portion having a diameter of 25 mm and a length of 30 mm at the center was prepared by machining, and gas nitrocarburizing and induction hardening were performed as surface hardening. Was performed. Table 3 shows the processing conditions at that time. Then, a roller pitching test (surface pressure of 3666 N / m) was performed on those test pieces.
m ² ).

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

Using the test piece subjected to the surface hardening treatment, a room temperature hardness and a high temperature hardness (4 mm) at a depth of 0.2 mm from the outermost surface were measured.
00 ° C), nitrogen concentration and austenite grain size N
o. The amount of retained austenite in the surface layer was also investigated. Table 4 (Examples) shows the results of these investigations together with the results of the roller pitting test (pitting occurrence life).
And Table 5 (Comparative Example).

[0034]

[Table 4]

[0035]

[Table 5]

From the results in Tables 4 and 5, the nitrogen concentration and 4
FIG. 1 shows the relationship between the 00 ° C. high-temperature hardness, and it can be seen that the high-temperature hardness increases as the nitrogen concentration increases. In particular, when the nitrogen concentration is less than 0.05% by weight, high-temperature hardness cannot be obtained, and the pitting occurrence life is significantly reduced.

FIG. 2 shows the relationship between the amount of retained austenite and the life of pitting. It can be seen that when the amount of retained austenite is within the range of 10 to 25% by volume, excellent pitting life is exhibited. Among the comparative examples shown in Table 5, although the high-temperature hardness and the nitrogen concentration were high and the austenite crystal grain size was small (No. 12 or more), the pitting occurrence life was low (No. 16). , 18, 20, 22, 25) because the amount of retained austenite exceeds 25% by volume. That is, the austenite crystal grain size at a depth of 0.2 mm from the outermost surface is No. 3; It is important to have a fine structure of 12 or more and to have a retained austenite amount in the surface layer portion up to a depth of 0.2 mm of 10 to 25% by volume.

From these results, the following can be considered.
First, the steels of Nos. 1 to 3, 15, and 16 show the effect of the amount of C, and the steels of the present invention (Nos.
The characteristics of the lower limit, the center and the upper limit of the amount are shown.
Compared to No. 6, the high-temperature hardness, the nitrogen concentration, and the amount of retained austenite were higher in the fine grains, indicating a longer pitting life. In the case of the comparative steel No. 16, although the high-temperature hardness and the nitrogen concentration are high due to the fine grains, the amount of retained austenite is too high beyond an appropriate range, and the pitting generation life is reduced. On the other hand, if the hardness is too high, the deformation resistance during forging is too high, and the tool life is undesirably reduced.

Nos. 4, 5, 8, and 9 show the influence of the Mn content. The steels Nos. 4 and 5 of the present invention show the characteristics of the lower limit and the upper limit of the Mn content. Higher hardness, higher nitrogen concentration, higher retained austenite,
It shows a high pitting life. The comparative steel N
In the case of o. 18, the amount of retained austenite is too high beyond the proper range, and the pitting occurrence life is reduced.

Nos. 6, 7, 19 and 20 show the effect of the Cr content. The steels Nos. 6 and 7 of the present invention show the characteristics of the lower limit and the upper limit of the Cr content. Compared to No. 19, the high-temperature hardness, the nitrogen concentration and the amount of retained austenite were higher in the fine grains, indicating a longer pitting life. Further, in the case of the comparative steel No. 20, although the high-temperature hardness and the nitrogen concentration were both high, the amount of retained austenite was too high beyond an appropriate range, and the pitting occurrence life was reduced.

Nos. 8, 9, 21 and 22 show the effect of the Mo content. The steels Nos. 8 and 9 of the present invention show the characteristics of the lower limit and the upper limit of the Mo content. Compared to No. 21, the high-temperature hardness, the nitrogen concentration and the amount of retained austenite were higher in the fine grains, indicating a longer pitting occurrence life. In the case of the comparative steel No. 22, similarly to the case of the comparative steel No. 23,
The amount of retained austenite is too high, and the pitting occurrence life is shortened.

Nos. 10, 11, 23, and 24 show the effect of the amount of V. The steels Nos. 10 and 11 of the present invention show the characteristics of the lower limit and the upper limit of the V amount. 23,
As compared with No. 24, the high-temperature hardness, the nitrogen concentration, and the amount of retained austenite were higher in the fine grains, indicating a longer pitting occurrence life. N
In the case of o.25, the amount of Ti is too large.
The steel of the present invention restricted to not more than 02% by weight (for example, No. 2)
The pitting occurrence life is shorter than that of the above.

The steels of the present invention of Nos. 12 to 14 were Pb, Z, respectively.
The graph shows the effect of adding r and Te. All of them show fine grains with appropriate ranges of hardness, nitrogen content and residual austenite content, and excellent pitting life.

The comparative steel No. 26 shows the characteristics when the gas soft nitriding treatment was omitted as the heat treatment and only the induction hardening treatment was performed. In this case, the hardness is slightly reduced, and the amount of nitrogen is extremely reduced, almost reaching the nitrogen concentration of the matrix, and accordingly, the amount of retained austenite is also reduced, leading to a reduction in pitting life.

[0045]

As described above, the present invention is constructed as described above, and can realize a part for a mechanical structure having extremely excellent fatigue characteristics, particularly excellent surface fatigue strength, and this part is most suitable as an outer race of a constant velocity joint.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between nitrogen concentration and high-temperature hardness (400 ° C.) at a depth of 0.2 mm from the outermost surface.

FIG. 2 is a graph showing the relationship between the amount of retained austenite in a surface layer and the life of pitting.

Continued on the front page (72) Inventor Masayoshi Kokura 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. In-house (72) Inventor Toyofumi Hasegawa 2 Nadahama-Higashi-cho, Nada-ku, Kobe City, Hyogo Prefecture Inside Kobe Steel, Ltd.Kobe Works (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 8/26

Claims (5)

(57) [Claims] 1. C: 0.4 to 0.7% by weight, Si: 0.
3% by weight or less (not including 0% by weight) , Mn: 0.2 to
1% by weight, Cr: 0.2 to 3% by weight, Mo: 0.1 to 1%
%, V: 0.1-1% by weight, Al: 0.01-0.
05% by weight, N: 0.003 to 0.02% by weight, S:
0.07% by weight or less (excluding 0% by weight) , Ti:
0.002% by weight or less (not including 0% by weight) , and the balance consists of Fe and unavoidable impurities, and P and O in the impurities are 0.02% by weight or less and 0.002% by weight, respectively.
After the steel is formed into a predetermined part shape, a nitriding treatment or a nitrocarburizing treatment is performed, followed by an induction hardening treatment to diffuse nitrogen from the surface, and A method for producing a machine structural component having excellent fatigue strength, particularly excellent surface fatigue strength, comprising forming a surface hardened layer containing a nitrogen concentration of 0.05% by weight or more at a depth of 0.2 mm from the surface. 2. A steel material containing at least one member selected from the group consisting of Pb: 0.02 to 0.13% by weight, Zr: 1% by weight or less, and Te: 1% by weight or less. The manufacturing method according to claim 1, wherein 3. The method according to claim 1, wherein the hardened surface layer is formed by depositing fine carbon and nitride. 4. C: 0.4-0.7% by weight, Si: 0.
3% by weight or less (not including 0% by weight), Mn: 0.2 to
1% by weight, Cr: 0.2 to 3% by weight, Mo: 0.1 to 1%
%, V: 0.1-1% by weight, Al: 0.01-0.
05% by weight, N: 0.003 to 0.02% by weight, S:
0.07% by weight or less (not including 0% by weight), Ti:
0.002% by weight or less (not including 0% by weight)
And the balance consists of Fe and unavoidable impurities.
The content of P and O is 0.02% by weight or less and 0.002% by weight, respectively.
The material is made of steel that is suppressed to less than 10% by weight, and is 0.1% from the outermost surface.
Nitrogen concentration at 0.05mm% or less at 2mm depth
At the depth of 0.2 mm from the outermost surface
The grain size of austenite is no. More than 12 fine structures
And the residual in the surface layer up to a depth of 0.2 mm
A machine structural component having excellent fatigue strength, particularly excellent surface fatigue strength, characterized in that the retained austenite amount is 10 to 25% by volume . (5) Further, Pb: 0.02 to 0.13 weight
%, Zr: 1% by weight or less and Te: 1% by weight or less
Steel containing at least one selected from the group consisting of
The component for a machine structure according to claim 4, which is a component.