JPH05179401A - Bearing steel - Google Patents

Abstract

PURPOSE:To provide a bearing steel capable of remarkably prolonging the service life of bearing by increasing the resistance to the foreign matter which is a factor practically prescribing the service life of bearing. CONSTITUTION:The bearing steel can be obtained by subjecting a steel having a composition consisting of, by weight ratio, 0.60-1.50% C, 0.15-1.00% Si, 0.15-0.50% Mn, 3.0-19.0% Cr, 0.50-6.50% Mo, 0.05-1.50% V, 0.020-1.50% Al, and the balance Fe with inevitable impurities to nitriding at 500-600 deg.C. By this method, the surface can be greatly hardened by means of nitriding and provided with resistance to the biting of foreign matter, and further, even in the inner part, tempering is done at the time of nitriding treatment and high hardness can be obtained by means of secondary hardening.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is strong against foreign substances mixed in lubricating oil and the like (the rate of decrease in rolling contact fatigue strength is small).
Bearing steel

[0002]

2. Description of the Related Art The manufacturing process of bearing steel has made great progress in recent years, and the cleanliness of steel itself has been greatly improved by introducing various out-of-core refining equipment and degassing equipment. As a result, according to laboratory data obtained under ideal conditions, the rolling fatigue life of a single material has been significantly improved, but when actually used as a bearing, May be significantly shorter. This is the case when the cause of the life of the bearing (which becomes unusable) is due to an external factor rather than the material itself. At present, it has been found that the largest cause thereof is foreign matter such as mechanical wear powder mixed in the lubricating oil, but such foreign matter occurs at every mechanical contact portion. For example, in the case of an automobile transmission, fine iron powder is generated due to contact and sliding between gears. The foreign matter thus generated also penetrates through the lubricating oil into the inside of the bearing which is lubricated with the lubricating oil common to the mechanical sliding portion (transmission gear).
The foreign matter that has entered the inside of the bearing scratches the rolling surface, especially when it is caught in the rolling surface of the ball and the race, and promotes peeling to significantly shorten the rolling fatigue life.

[0003]

As described above, it is necessary to consider the life of the bearing practically on the assumption that foreign matter is present. In this case, the most effective measure is to prevent scratches due to foreign matter.

Another factor that significantly reduces bearing life is temperature. For example, in recent years, in response to a strong demand for quietness of automobiles, measures are being taken to cover the engine part with a heat insulating material or the like to shut off sound. However, such sealing of the engine part increases the temperature of the engine part. Will be invited. Since bearings are originally rolling and sliding,
Although the temperature rises more easily than the other portions, such a temperature rise of the entire engine section causes a further temperature rise in the bearing section.

The present invention provides a bearing steel capable of significantly extending the life of the bearing by taking measures against the factors that actually define the life of these bearings.

[0006]

The present invention, which has been made to solve the above problems, has a weight ratio of C: 0.60 to 1.50.
%, Si: 0.15 to 1.00%, Mn: 0.15 to 0.50%, Cr:
3.0 to 19.0%, Mo: 0.50 to 6.50%, V: 0.05 to 1.50
%, Al: 0.020 to 1.50%, the balance Fe and unavoidable impurities are steels for bearings which are nitrided at a temperature of 500 to 600 ° C.

[0007]

[Function] The reason for limiting each chemical component is as follows.

C: 0.60 to 1.50% Carbon forms carbides with Cr, Mo and V. This carbide imparts the necessary wear resistance to the bearing steel and has the effect of causing secondary hardening during tempering.
In order to effectively obtain such effects and satisfy the hardness of the bearing steel, it is necessary to contain carbon of 0.60% or more. However, if the content exceeds 1.50%, the amount of pro-eutectoid carbides increases, and it becomes difficult to obtain a uniform carbide distribution, resulting in a reduction in rolling contact fatigue life even at room temperature. In addition, hot workability is reduced.

Si: 0.15-1.00% Silicon is used as a deoxidizer for molten steel during steelmaking. When the deoxidation of steel is insufficient, oxide-based inclusions increase in the steel, which becomes a stress concentration source and reduces the rolling fatigue life. In order to surely deoxidize the steel in order to avoid such an adverse effect, it is necessary to add 0.15% or more of silicon. However, silicon generally has a function of stabilizing retained austenite after quenching of alloy steel. As will be described later, the bearing steel according to the present invention is tempered at a relatively high temperature (simultaneously with nitriding), but when silicon exceeding 1.00% is contained, the decomposition temperature of retained austenite shifts to the high temperature side. Retained austenite lowers the hardness and shortens the rolling contact fatigue life, and its decomposition causes dimensional changes over time, so it is necessary to keep the amount of retained austenite small especially when used at high temperatures. In addition, since silicon has a function of promoting decarburization, etc., the content of silicon in excess of 1.00% makes steel production difficult.

Mn: 0.15-0.50% Like manganese, manganese is also used as a deoxidizer during steelmaking. Further, it has a great effect of improving hardenability, and is an element useful for performing complete quenching on a relatively large component. In order to exert these effects, it is necessary to contain at least 0.15% manganese. However, if it exceeds 0.50%, the amount of retained austenite after quenching increases, quenching hardness decreases, and rolling life decreases. Also, the machinability is reduced.

Cr: 3.0 to 19.0% Chromium not only acts as a spheroidized carbide during annealing, which is important as a bearing steel, but also increases the hardness of the surface nitrided layer during the nitriding treatment, which is an essential requirement of the present invention. However, if it is 3.0% or less, those effects cannot be sufficiently exhibited. However, the chromium content is 19.0
If it exceeds 0.1%, a chromium-based giant carbide is generated and remains in the final structure after heat treatment, so that the rolling fatigue life is deteriorated. It also makes the manufacturability during hot working extremely difficult.

Mo: 0.50 to 6.50% Molybdenum has the effect of causing softening resistance and secondary hardening during tempering of steel, and, like chromium, has the effect of increasing the hardness of the surface nitrided layer during nitriding. .. In order to obtain such an effect, it is necessary to contain 0.50% or more of molybdenum. But 6.50
If it is contained in excess of%, a large carbide is generated as in the case of chromium, and the rolling fatigue life is deteriorated. Further, it makes the manufacturability during hot working difficult.

V: 0.05 to 1.50% Vanadium, like molybdenum, imparts temper softening resistance and secondary hardenability to steel by precipitation of fine carbides, and also has the effect of refining crystal grains. Although vanadium increases the hardness of carbides, in the case of the steel of the present invention, the effect of increasing the hardness of the surface nitrided layer improves the wear resistance of the steel and extends the rolling fatigue life. In order to obtain such an effect, it is necessary to contain vanadium in an amount of 0.05% or more. However, even if the content exceeds 1.50%, such an effect is saturated and the material price is unnecessarily increased. In addition, a large amount of vanadium carbide is precipitated at the grain boundaries, which deteriorates hot workability.

Al: 0.02 to 1.50% The steel of the present invention is to increase the surface hardness by nitriding treatment, and aluminum is an element forming the nitrided layer. It is also an element necessary for deoxidation during steel production. In order to obtain these effects, it is necessary to contain 0.02% or more of aluminum. However, if the content of Al exceeds 1.50%, it becomes brittle during hot work, making rolling difficult.

In the present invention, the steel containing the above alloy components is heated in the range of 950 ° C to 1200 ° C and quenched. The reason why the temperature range is set to this range is that at 950 ° C. or less, the carbide does not sufficiently dissolve in the matrix, and a sufficient secondary hardening cannot be obtained during the subsequent tempering. Also, the quenching temperature is
This is because if the temperature exceeds 1200 ° C, the crystal grains become coarse and decarburization becomes remarkable.

After quenching, nitriding treatment is performed in the temperature range of 500 ° C to 600 ° C. The nitriding treatment may be either a gas nitriding method using ammonia or a salt bath nitriding method using molten KCN or the like. By nitriding treatment, it is about 0.
A hardened layer with a maximum hardness of about Hv1000 is formed up to a depth of about 3 mm. As a result, even when foreign matter such as metal powder is caught in the rolling surface, the surface is not easily scratched, and the reduction in rolling contact fatigue life can be minimized.
Further, since the wear of the bearing balls, rollers, etc. is reduced, the rolling contact fatigue life of the bearing becomes longer than before. In addition,
After nitriding, a very brittle ε phase called a white layer is formed on the outermost surface, which is preferably polished and removed before it is used as a bearing.

Since the nitriding temperature is 500 to 600 ° C., the surface is nitrided and at the same time, the quenched structure is tempered inside. As described above, the steel of the present invention contains secondary hardening elements such as Cr, Mo and V, and the temperature at which the fine carbides of these elements are deposited is exactly in this temperature range, so the surface is nitrided. At the same time, a high hardness can be obtained internally by secondary curing.

[0018]

EXAMPLES Next, the characteristics of the steel of the present invention will be described with reference to examples. The main chemical components of the sample steel are shown in Table 1. This sample steel has been conventionally used as a high temperature bearing steel, AISI-.
It falls within the composition range of M50 steel.

[0019]

[Table 1]

In order to conduct a rolling fatigue test, a flat test piece of φ60 × 10 mm was prepared from a test steel rolled to φ65 mm and subjected to the following heat treatment. First, test piece 11
After holding at 20 ° C. for 30 minutes, it was oil-quenched and immediately immersed in liquid nitrogen (−78 ° C.) for subzero treatment. Then, gas nitriding treatment was performed at 550 ° C. for 600 minutes (10 hours). As a result, the surface is nitrided, and the tempering process proceeds inside, so that secondary hardening by Cr, Mo, and V occurs. After nitriding, the surface (0.1 mm below the surface) had a hardness of Hv1000 and an internal hardness of Hv780. After the nitriding treatment, the white layer generated on the outermost surface is removed.
0.05 mm was polished.

In addition to the above test steel (A), for comparison,
Test pieces (B) obtained by tempering the test steels shown in Table 1 under the same temperature conditions (550 ° C x 600 minutes) without nitriding treatment.
And conventional heat treatment (840 ℃ × 40
A test piece (C) prepared by subjecting to partial quenching and tempering at 180 ° C. for 90 minutes) was prepared.

Three kinds of test pieces A produced in this way,
A rolling fatigue test was performed on B and C under the conditions shown in Table 2. Here, in order to bring the test conditions closer to the actual load conditions of the bearing, iron powder with a particle size of about 100 μm (0.1 mm) is mixed in the lubricating oil, and the test piece is immersed in the lubricating oil to roll the load ball. Moved. The results of the rolling fatigue test are shown in Table 3, and the test results are expressed as a ratio to the L10 life (life with a cumulative failure rate of 10%) of the test piece C of the conventional steel SUJ2 as 1. As shown in Table 3, the surface-nitrided test piece A has a rolling fatigue life that is at least twice that of ordinary bearing steel under such severe conditions. This is because the rolling surface is very hard (Hv1000), so even if foreign matter (iron powder mixed in the lubricating oil) gets caught between the load ball and the rolling surface, the rolling surface is less likely to be damaged. Conceivable.

[0023]

[Table 2]

[0024]

EFFECTS OF THE INVENTION In a transmission of an automobile, for example, abrasion powder and the like generated in the meshing portion of a transmission gear may be mixed with lubricating oil and enter the inside of the bearing. Since the bearing steel according to the present invention is used by nitriding the surface, even if such a foreign substance enters between the bearing balls or rollers and the race, the surface may be deformed or flawed. And the decrease in life is minimized. Therefore, the bearing can have a very long life under actual use conditions. The steel of the present invention can be applied to various rolling parts such as constant velocity joints other than bearings.

[Table 3]

Claims (1)

[Claims] 1. A weight ratio of C: 0.60 to 1.50%, S
i: 0.15 to 1.00%, Mn: 0.15 to 0.50%, Cr: 3.0 to 1
9.0%, Mo: 0.50 to 6.50%, V: 0.05 to 1.50%, A
1: Steel for bearings, characterized by nitriding steel consisting of 0.020 to 1.50%, balance Fe and unavoidable impurities at a temperature of 500 to 600 ° C.