JPH06172961A - Parts for machine structure excellent in fatigue resistance, particularly in face fatigue strength and its production - Google Patents

Abstract

PURPOSE:To obtain parts for machine structure excellent in fatigue strength, particularly in face fatigue strength by subjecting steel having a specified componental compsn. to nitriding treatment or soft-nitriding treatment and induction hardening in combination. CONSTITUTION:Steel having a compsn. contg., by weight, 0.4 to 0.7% C, <=0.3% Si, 0.2 to 1% Mn, 0.2 to 3% Cr, 0.1 to 1% Mo, 0.1 to 1% V, 0.01 to 0.05% Al, 0.003 to 0.02% N, <=0.07% S and <=0.002% Ti, and the balance Fe with inevitable impurities, and in which the content of P and that of O as the impurities are respectively limited to <=0.02% and <=0.002% is used as stock. This steel may furthermore be incorporated with one or more kinds among 0.02 to 0.13% Pb, <=1% Zr and <=1% Te. This stock is formed into the shape of parts, and the parts are thereafter subjected to nitriding treatment or soft-nitriding treatment and are successively subjected to induction hardening to form a hardened surface layer contg. >=0.05wt.% nitrogen concn. at the position with a depth of >=0.2mm from the topmost surface layer. This hardened surface layer has a fine structure of No.12 or above, and the amt. of retained austenite in the surface layer part to a depth of 0.2 is regulated to 10 to 25vol.%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine structural component requiring fatigue strength, particularly surface fatigue strength, such as an outer race of a constant velocity joint for an automobile, and a method for producing such a component. It is a thing.

[0002]

2. Description of the Related Art Mechanical structural parts, particularly power transmission parts for industrial machines such as shafts and constant velocity joints, are required to have excellent fatigue strength. 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, particularly automobiles, weight reduction has been aimed at by increasing the strength of various steel parts. For example, when the outer race of a constant velocity joint for automobiles is made smaller or thinner, the contact surface pressure increases in the ball groove portion, which causes early pitching and flaking. Improvement is essential. It is known that strengthening the surface layer by increasing the surface hardness is the most effective for improving such surface fatigue strength.

However, in the driving force transmitting component as described above, there is a limit in hardness and toughness in the conventional technique using a combination of medium carbon steel and a conventional method, and it is necessary to significantly improve the fatigue strength, particularly the surface fatigue strength. It is difficult.

The present invention has been made in order to solve such technical problems, and its purpose is to provide a mechanical structural component having significantly improved fatigue strength, particularly surface fatigue strength, and such a mechanical structure. It is to provide a method for manufacturing a component for use.

[0006]

Means for Solving the Problems The present invention capable of achieving the above object is that C: 0.4 to 0.7% by weight, Si: 0.3% by weight or less, and 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
~ 0.02% by weight, S: 0.07% by weight or less, Ti:
A steel containing 0.002% by weight or less and the balance Fe and unavoidable impurities, and P and O in the impurities being 0.02% by weight or less and 0.002% by weight or less, respectively, After forming the material into a predetermined part shape, nitriding treatment or soft nitriding treatment is performed, and then induction hardening treatment is performed to diffuse nitrogen from the surface, so that nitrogen is formed at a depth of at least 0.2 mm from the outermost surface. This is a method for producing a mechanical structural component excellent in fatigue strength, particularly surface fatigue strength, which is characterized in that a surface-hardened layer containing a concentration of 0.05% by weight or more is formed.

The mechanical structural component of the present invention is manufactured by the above method, and has an austenite grain size of No. 2 at a depth of at least 0.2 mm from the outermost surface. It has a fine structure of 12 or more and has a gist in that the amount of retained austenite in the surface layer portion is 10 to 25% by volume.

[0008]

In order to achieve the above object, the present inventors have studied from both aspects of the steel material composition and the surface hardening treatment. As a result, it was found that the following three points are important for improving the fatigue strength, particularly the surface fatigue strength. (1) It is necessary to increase the room temperature and high temperature hardness of the surface layer of the component. (2) In order to further increase the high-temperature hardness (that is, to increase the softening resistance), it is effective to infiltrate and diffuse a larger amount of nitrogen deeper to form a surface hardened layer by depositing fine carbon and nitride. To be. (3) By generating an appropriate amount of retained austenite in the surface layer of the component, the hardness increase due to the work-induced transformation and the apparent surface pressure due to the cushioning effect are reduced.

In order to obtain the above characteristics, first, nitriding or soft nitriding treatment is required. At that time, from the viewpoint of the material, addition of Mo and V is mainly effective for increasing the nitrogen concentration, and the method It is extremely effective to combine nitriding or soft nitriding with induction hardening in order to diffuse nitrogen, promote solid solution, precipitation of fine carbon / nitride, and strengthen the martensite based on it. I found out.

A method of induction hardening after soft nitriding treatment is disclosed, for example, in Japanese Patent Laid-Open No. 2-232353.
Although a method of performing soft nitriding treatment on SCM steel and tool steel and then performing induction hardening is described, the present invention refers to the optimum alloy composition range of the applied steel, and the object of the present invention is pitching. It is different from the above method in that it is intended to improve the surface fatigue strength so that the flaking occurrence life can be significantly improved and achieved.

Further, Japanese Laid-Open Patent Publication No. 60-169544 proposes a technique for defining the components of a steel material suitable for manufacturing mechanical structural parts, particularly gears, and performing induction hardening as a means for improving fatigue strength. However, the present invention is capable of achieving a significant improvement in fatigue strength by performing induction hardening treatment after nitriding treatment or soft nitriding treatment, which is different from the above method. Further, Japanese Patent Application Laid-Open No. 4-66646 reports a technique relating to a steel material capable of significantly improving fatigue strength by performing nitriding treatment. However, this technique shows that the fatigue strength is improved by a combination of quenching and tempering treatment and nitriding treatment. The focus is on steel materials that can be improved, which is basically different from the present invention. First, the reasons for limiting the 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 ordinary temperature and high temperature,
If the content is less than 0.4% by weight, it cannot withstand a high surface pressure during use. However, if the C content is too large, the life of the forging tool is shortened due to an increase in the deformation resistance during the forging of the parts, or the life of the tool is shortened during the subsequent cutting work. Should be. The preferable range of the C content is about 0.5 to 0.6% by weight.

Si: 0.3 wt% or less Si is an element necessary for improving the internal quality of steel, but if it is too much, the hardness of the steel material increases, which adversely affects forgeability and machinability. 0.3% by weight
Should be: The preferred range of Si content is
It is about 0.15 to 0.3% by weight.

Mn: 0.2-1% by weight Mn is an element necessary for improving the internal quality of the steel, the induction hardenability, and the surface hardness, and the Mn content is 0.2. If it is less than wt%, such an effect cannot be obtained. However, if the Mn content is too high,
In particular, the life of the forging tool is shortened due to the increase of the deformation resistance during the forging of the parts, or the life of the tool during the subsequent cutting is shortened, so the content should be 1% by weight or less. 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 which improves the induction hardenability and is necessary for increasing the hardness due to the formation of carbonitrides, and its content is 0.2.
If it is less than wt%, it cannot withstand high surface pressure during use. However, if the Cr content is too high, excessive carbonitride formation causes an excessive increase in surface hardness, which lowers the fatigue limit. Further, since the life of the forging tool is shortened due to the increase of the deformation resistance during the forging of the parts, or the life of the tool during the subsequent cutting process is shortened, it should be 3% by weight or less. A 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, at the same time, penetrates nitrogen more deeply and more due to the formation of Mo carbonitride.
It is an element necessary to diffuse and improve the softening resistance. If the Mo content is less than 0.1% by weight, these effects cannot be obtained, and a high surface pressure during use cannot be endured.
However, if the Mo content is too large, excessive carbonitride formation causes an excessive increase in surface hardness, which lowers the fatigue limit. Further, since it causes a decrease in forging tool life due to an increase in deformation resistance during warm forging, it should be 1% by weight or less. The more preferable range of Mo is 0.3-0.
It is about 6% by weight.

V: 0.1 to 1 wt% V is an element necessary for improving the softening resistance by penetrating and diffusing nitrogen deeper and in a large amount by forming a carbonitride like Mo. If the content is less than 0.1% by weight, such an effect cannot be obtained and the bearing cannot withstand a high surface pressure during use. However, if the V content is too high,
Excess carbonitride cannot be dissolved completely during quenching and heating, and its effect is saturated, so the amount should be 1% by weight or less. A 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.
This effect is not achieved if the content is less than 10% by weight, and if the content exceeds 0.05% by weight, the hardness of the nitride layer is excessively increased and the fatigue limit is lowered.

N: 0.003 to 0.02% by weight N is combined with Al to form AlN, which has the effect of refining the crystal grain size. If the N content is less than 0.003% by weight, the effect will be insufficient. However, if the N content is too high, cracking tends to occur during rolling, so
It should be 0.02% by weight or less.

S: 0.07 wt% or less S is an element effective for improving the machinability at the time of cutting a part, but if it is contained in excess of 0.07 wt%, the fatigue strength is adversely affected.

Ti: 0.002% by weight or less Ti is an element effective in improving the fatigue strength by forming a nitride by forming a fine grain size. However, if it is contained excessively, the fatigue strength is rather lowered. It should be below 0.002% by weight.

The component of the present invention comprises the above elements as basic components and the balance Fe and unavoidable impurities.
Of the unavoidable impurities, it is necessary to suppress the content of P and O as much as possible. If necessary, the parts of the present invention may have P
You may add b, Zr, Te, etc. The reasons for limiting the chemical composition of these elements are as follows.

P: 0.02 wt% or less P is a harmful impurity element that embrittles the grain boundaries and adversely affects fatigue strength, so its content should be kept to 0.02 wt% or less. .

O: 0.002% by weight or less O forms an oxide, and is an impurity element that is particularly harmful to fatigue strength, and 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% or more selected from the group consisting of 1% by weight or less Pb, Zr and Te are all effective elements for improving machinability. Pb has the effect of improving machinability during partial cutting, and the Pb content is 0.02% by weight.
If it is less than 0.1%, the effect cannot be obtained, and if it exceeds 0.13% by weight, the fatigue strength is adversely affected. A more preferable range of Pb is about 0.05 to 0.1% by weight.
On the other hand, Zr and Te have the effect of improving the machinability and the effect of rounding the shape of the sulfide-based inclusions and improving the toughness when added together with S. However, Z
If the content of r and Fe exceeds 1% by weight, the effect is saturated.

The parts of the present invention are made by using steel having the above chemical composition as a raw material, shaping the raw material into a predetermined shape, subjecting it to nitriding treatment or soft nitriding treatment, and then performing induction hardening treatment. The nitrogen concentration is defined as 0.05 wt% or more at the depth of at least 0.2 mm from the outermost surface by diffusing nitrogen from the surface, for the following reason. That is, if the nitrogen concentration at this position is less than 0.05% by weight, the softening resistance is lowered, causing plastic flow during use, which shortens the pitching life.

In the component of the present invention, the austenite grain size at the depth position of at least 0.2 mm from the outermost surface is no. It is necessary to have a fine structure of 12 or more, but the grain size at that position is No. If it is less than 12, the crystal grains are too large and the toughness is lowered, and the pitching life is shortened. Further, in the component of the present invention, the amount of retained austenite in the surface layer portion at least to a depth of 0.2 mm needs to be 10 to 25% by volume, but the amount of retained austenite is out of the above range and is less than 10% by volume or 25% by volume. %, The pitching life is shortened.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and following paragraphs. It is included in the technical scope.

[0029]

EXAMPLES Steels having chemical compositions shown in Tables 1 (inventive steels) and 2 (comparative steels) were melted using a small vacuum furnace (150 kg / ch) and forged into a steel bar having a diameter of 30 mm. After that, 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 in the central portion was produced by machining, and a gas soft nitriding treatment and an induction hardening treatment were performed as a surface hardening treatment. Was done. Table 3 shows the processing conditions at that time. Then, a roller pitching test (contact pressure: 3666 N / m) was performed on these test pieces.
m ² ) was performed.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

Using a surface-hardened test piece, room temperature hardness and 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. The results of these investigations are shown in Table 4 (Examples) together with the results of the roller pitching test (pitting occurrence life).
And Table 5 (comparative example).

[0034]

[Table 4]

[0035]

[Table 5]

From the results shown 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 high temperature hardness cannot be obtained, and the pitting occurrence life is significantly reduced.

FIG. 2 shows the relationship between the retained austenite amount and the pitting occurrence life. It can be seen that when the retained austenite amount is in the range of 10 to 25% by volume, excellent pitting occurrence life is exhibited. In addition, among the comparative examples shown in Table 5, those having a high high temperature hardness and a high nitrogen concentration and a fine austenite grain size (No. 12 or more) but a short pitting occurrence life are recognized (No. 16). , 18, 20, 22, 25) because the amount of retained austenite exceeds 25% by volume.

From these results, the following can be considered.
First, Nos. 1 to 3, 15, and 16 show the influence of the amount of C, and the steels of the present invention (No. 1 to No. 3) are C.
It shows the characteristics of the lower limit, the center, and the upper limit of the amount.
Compared with No. 6, fine grains have high high-temperature hardness, nitrogen concentration, and retained austenite amount, and have a long pitting occurrence life. Further, in the case of Comparative Steel No. 16, fine particles have high temperature hardness and high nitrogen concentration, but the amount of retained austenite becomes too high beyond the proper range, and the pitting occurrence life is reduced. Further, if the hardness is excessively high, the deformation resistance during forging becomes too high, and the tool life is shortened, which is not preferable.

Nos. 4, 5, 8 and 9 show the effects of the amount of Mn, and the steels No. 4 and 5 of the present invention show the characteristics of the lower and upper limits of the amount of Mn. Compared with No. 17, high temperature hardness, nitrogen concentration, and high retained austenite amount,
It shows a high pitting life. Also, the comparative steel N
In the case of o.18, the residual austenite amount is too high beyond the proper range, and the life for occurrence of pitting is reduced.

Nos. 6, 7, 19 and 20 show the influence of the Cr content, and the steels No. 6 and 7 of the present invention show the characteristics of the lower limit and the upper limit of the Cr content. Compared with No. 19, the high-temperature hardness, the nitrogen concentration, and the amount of retained austenite are high in the fine grains, and the pitting occurrence life is high. Further, in the case of the comparative steel No. 20, although the high temperature hardness and the nitrogen concentration are both high, the retained austenite amount becomes too high beyond the proper range, and the pitching occurrence life is reduced.

Nos. 8, 9, 21, and 22 show the influence of the amount of Mo, and the steels No. 8 and 9 of the present invention show the characteristics of the lower limit and the upper limit of the amount of Mo. Compared with No. 21, the fine grains were high in high temperature hardness, nitrogen concentration and retained austenite amount, and showed a long pitting occurrence life. In the case of comparative steel No. 22, as in the case of comparative steel No. 23,
The amount of retained austenite becomes too high and the pitting occurrence life is reduced.

Nos. 10, 11, 23 and 24 show the influence of the V content, and the steels No. 10 and 11 of the present invention show the characteristics of the lower limit and the upper limit of the V content. 23,
Compared with No. 24, the fine particles have higher high-temperature hardness, nitrogen concentration, and retained austenite amount, and show a longer pitting occurrence life. N
O.25 has too much Ti content, but Ti content is 0.0
The steel of the present invention limited to 02% by weight or less (for example, No. 2)
The pitting occurrence life is reduced as compared with.

The steels of the present invention Nos. 12 to 14 are Pb and Z, respectively.
The effects of the addition of r and Te are shown, and all of them show the hardness, the amount of nitrogen, and the amount of retained austenite in a proper range in the fine grains, and show the excellent pitting occurrence life.

The comparative steel No. 26 shows the characteristics when the gas soft nitriding treatment is omitted as the heat treatment and only the induction hardening treatment is performed. In this case, the hardness is slightly lowered, the nitrogen amount is extremely lowered, and the nitrogen concentration of the matrix is almost reached, and accordingly, the retained austenite amount is also lowered, and the pitching occurrence life is shortened.

[0045]

As described above, the present invention can realize a machine structural component having extremely excellent fatigue characteristics, particularly surface fatigue strength, and this component is optimal as an outer race for a constant velocity joint.

[Brief description of 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 the surface layer and the pitting life.

Front page continuation (72) Inventor Masayoshi Ogura 2 Takaracho, Kanagawa-ku, Kanagawa Prefecture Nissan Motor Co., Ltd. (72) Inventor Takehiko Kato 2 Nadahama-Higashicho, Nada-ku, Kobe-shi, Hyogo Kobe Steel Works Kobe Steel In-house (72) Inventor Toyofumi Hasegawa 2 Nadahamahigashi-cho, Nada-ku, Kobe-shi, Hyogo Stock Company Kobe Steel Works Kobe Steel Works

Claims (4)

[Claims] 1. C: 0.4 to 0.7% by weight, Si: 0.
3% by weight or less, Mn: 0.2 to 1% by weight, Cr: 0.2
~ 3 wt%, Mo: 0.1-1 wt%, V: 0.1-1
% By weight, Al: 0.01 to 0.05% by weight, N: 0.0
03-0.02% by weight, S: 0.07% by weight or less, T
i: a steel containing 0.002% by weight or less and the balance Fe and unavoidable impurities, and P and O in the impurities being 0.02% by weight or less and 0.002% by weight or less, respectively. After molding the material into a predetermined part shape, nitriding treatment or soft nitriding treatment is performed, and then induction hardening treatment is performed to diffuse nitrogen from the surface and to a depth position of at least 0.2 mm from the outermost surface. 2. A method for manufacturing a machine structural component having excellent fatigue strength, particularly surface fatigue strength, which comprises forming a surface-hardened layer having a nitrogen concentration of 0.05 wt% or more. 2. A steel material containing at least one 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 surface hardened layer is formed by depositing fine carbon / nitride. 4. The method according to any one of claims 1 to 3, wherein the austenite grain size at a depth position of at least 0.2 mm from the outermost surface is No. A mechanical structural component excellent in fatigue strength, particularly surface fatigue strength, having a fine structure of 12 or more and having a retained austenite amount of 10 to 25% by volume in a surface layer portion to a depth of at least 0.2 mm.