JP2005042188A - Carbonitrided bearing steel with excellent rolling fatigue life under debris-contaminated environment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbonitrided bearing steel superior in rolling fatigue life to those prepared by applying carbonitriding treatment to conventional steels represented by SUJ2, SCr420 and SCM420 even under debris-contaminated environments. <P>SOLUTION: The carbonitrided bearing steel has a composition consisting of, by weight, 0.1 to 0.4% C, ≤1.0% Si, >1.5 to 3% Mn, ≤0.03% P, ≤0.03% S, 0.3 to 2.5% Cr, 0.005 to 0.050% Al, ≤0.003% Ti, ≤0.0015% O, ≤0.025% N and the balance Fe with inevitable impurities. In the steel, surface hardness after carbonitriding treatment or subsequent secondary quench-and-temper treatment is ≥58 HRC and the amount of surface retained austenite is 20 to 50% . <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a carbonitrided bearing steel having an excellent rolling fatigue life in a lubricating environment containing foreign matter.

Conventionally, bearing steels represented by JIS SUJ2, SCr420, SCM420 and the like have been used as bearing steels for machine structural parts.
However, it is known that even if a normal bearing use environment shows a long service life, the service life is extremely reduced in a lubrication environment containing foreign matter.

  As a typical example, foreign matters such as burrs and wear powder are mixed in the lubricating oil in a bearing used in a transmission or a differential of an automobile. For this reason, in order to extend the life of bearings used in such places, development of foreign material bearing steels suitable for use in a foreign matter mixed environment has been demanded.

In an environment where foreign matter is mixed, surface-origin type separation starting from an indentation due to the mixed foreign matter dominates the rolling fatigue life.
In order to improve the life, it is considered effective to suppress the generation of indentation, reduce the stress concentration in the indentation, and improve the resistance to crack generation.
Therefore, carbonitriding treatment is applied to SUJ2 steel to improve surface hardness and fracture resistance, increase the amount of retained austenite, and relieve stress concentration due to indentation.

However, even when carbonitriding is applied to SUJ2, SCr420, and SCM420 steel, the rolling fatigue life in an environment where foreign matter is mixed is only improved by about 2 to 3 times.
In recent years, the usage environment of bearings has become more severe, and in such a situation, the life of bearings has been required to be further extended.

The thing disclosed by the following patent document 1 is conventionally well-known as the thing aiming at raising the rolling fatigue life in a foreign material mixing environment.
However, the one disclosed in Patent Document 1 relates to a heat-resistant carburized rolling bearing component used in a high-temperature environment, and a bearing used at a high temperature needs to be tempered at a high temperature. Even if tempering, the hardness is not significantly reduced and the dimensional stability can be secured, which is different from the present invention.
JP 2002-60847 A

  The present invention is based on such circumstances, and even in a foreign matter mixed environment, a carbonitriding bearing that has a better rolling fatigue life than conventional carbonitriding treatments such as SUJ2, SCr420, and SCM420. It was made for the purpose of providing steel.

  Thus, the content of claim 1 is by weight, C: 0.1 to 0.4%, Si: ≦ 1.0%, Mn: more than 1.5 to 3%, P: ≦ 0.03%, S: ≦ 0.03%, Cr: 0.3 ~ 2.5%, Al: 0.005-0.050%, Ti: ≤0.003%, O: ≤0.0015%, N: ≤0.025%, the composition consisting of the balance inevitable impurities and Fe, carbonitriding or subsequent secondary The surface hardness after quenching and tempering is 58 HRC or more, and the amount of surface retained austenite is 20 to 50%.

  In addition to the alloy component defined in claim 1, the one according to claim 2 further contains one or two of Ni and Mo by weight%, Ni: 0.25 to 3.5%, Mo: 0.03 to 2.5% The surface hardness after carbonitriding or subsequent secondary quenching and tempering treatment is 58 HRC or more, and the amount of surface retained austenite is 20 to 50%. It is characterized by being.

  In addition to the alloy components defined in any one of claims 1 and 2, the present invention has a composition further containing V by weight and V: 0.05 to 1.0%, with the balance inevitable impurities and Fe The surface hardness after carbonitriding or subsequent secondary quenching and tempering is 58 HRC or more, and the amount of surface retained austenite is 20 to 50%.

  In addition to the alloy components defined in any one of claims 1 to 3, the invention according to claim 4 further contains Nb by weight% and Nb: ≦ 0.1%, and the balance is composed of inevitable impurities and Fe. And the surface hardness after carbonitriding or subsequent secondary quenching and tempering is 58 HRC or more, and the amount of retained austenite is 20 to 50%.

Effects and effects of the invention

  As a result of examining the combination of various alloy elements based on carburized steel represented by SCr420 steel, the present invention has the effect of adding Mn within an appropriate range to lower the Ms point and increase the amount of surface retained austenite. In addition, Mn has the following effect alone, that is, to increase the toughness of the matrix, and to reduce the notch sensitivity, thereby suppressing the occurrence of cracks from indentation generated in a foreign matter mixed environment, Obtaining knowledge that the rolling life in a foreign matter-contaminated environment is effectively improved by a combination of setting the surface retained austenite amount to 20 to 50% after carbonitriding or subsequent secondary quenching and tempering, It has been completed.

According to the present invention as described above, it is possible to provide a carbonitrided bearing steel having an excellent rolling fatigue life in an environment where foreign matter is mixed, as compared with the prior art.
In the present invention, in addition to the alloy components specified above, one or two of Ni and Mo can be included in the range of Ni: 0.25 to 3.5% and Mo: 0.03 to 2.5% as necessary ( (Claim 2) Further, V can be contained in a range of V: 0.05 to 1.0% (Claim 3), and Nb can be further contained in a range of Nb: ≦ 0.01% (Claim 4).
As a result, the rolling fatigue life of the carbonitrided bearing steel can be further improved in an environment where foreign matter is mixed.

Next, the reason for limitation of each alloy component in this invention is explained in full detail below.
C: 0.1-0.4%
C affects the core hardness after carbonitriding. In order to secure the core hardness necessary to obtain the strength required for a rolling bearing, the C content needs to be 0.1% or more.
However, if the C content exceeds 0.4%, the toughness, hot workability and machinability deteriorate, so the upper limit of the C content is set to 0.4%.

Si: ≤1.0%
Si is a deoxidizing action during steel melting and improves tempering softening resistance, so it is an element contained for them, but when it increases, the effect is saturated and heat treatment is performed to raise the transformation point of steel. It is necessary to increase the temperature, and the forgeability and machinability are lowered, so the upper limit of the content is set to 1.0%.

Mn: more than 1.5 ~ 3%
Mn is an important component for the present invention, which lowers the martensitic transformation start temperature (Ms point) of steel, increases the retained austenite during quenching, and increases the hot workability of the steel. It is an element to be contained in.
Mn also increases the toughness of the matrix and suppresses notch susceptibility, thereby suppressing the occurrence of cracks from indentations generated in the presence of foreign matter and improving the rolling fatigue life.
In order to obtain these effects, it is necessary to contain more than 1.5%. However, if the amount is too large, it will be difficult to soften and anneal the material, and the machinability will be reduced, so the upper limit of the content is 3%. And

P: ≤ 0.03%
P segregates at the austenite grain boundaries of the steel, leading to a decrease in toughness and rolling fatigue life. Therefore, P needs to be 0.03% or less.

S: ≤0.03%
S impairs the hot workability of steel and forms non-metallic inclusions in the steel to reduce toughness and rolling fatigue life. Therefore, its content is set to 0.03% or less.
Although S has the harmful side as described above, it also has an effect of improving the machinability, so it is desirable to reduce it, but it may be actively added within a range of 0.03% or less.

Cr: 0.3-2.5%
Cr has the effect of improving hardenability, improving temper softening resistance, and improving life. In order to obtain these effects, 0.3% or more is necessary. However, if added over 2.5%, large carbides are formed and the rolling fatigue life is reduced.

Al: 0.005 to 0.050%
When a large amount of Al is contained exceeding 0.050%, hard oxide inclusions are generated, and the rolling fatigue life is significantly reduced. Although Al has such problems, it also has the effect of forming AlN to refine crystal grains, so it is contained in a range of 0.005% or more that does not increase the manufacturing cost of steel.

Ti: ≦ 0.003%
Ti forms nitrides and becomes non-metallic inclusions, which may become the starting point of rolling fatigue failure. Therefore, the content is restricted to 0.003% or less.

O: ≤ 0.0015%
O forms an oxide in the steel and may become a starting point for rolling fatigue failure as a non-metallic inclusion, leading to a decrease in rolling fatigue life, so its content is made 0.0015% or less.

N: ≤ 0.025%
N combines with Al to form AlN and has the effect of refining crystal grains. However, if it is contained in a large amount, the strength of the steel is deteriorated, so the upper limit of the N content is set to 0.025%.

Ni: 0.25-3.5%
Ni increases the amount of retained austenite at the time of quenching and improves the hardenability, and suppresses the formation of a white structure and a carbide structure during the rolling fatigue process, thereby extending the life of the bearing. In order to acquire the effect, it is necessary to contain 0.25% or more, but if it increases, since an effect will be saturated and it will become disadvantageous in terms of cost, let the upper limit of content be 3.5%.

Mo: 0.03-2.5%
Mo increases the amount of retained austenite at the time of quenching and improves the hardenability, so it is contained for them. In order to obtain the action effect, it is necessary to contain 0.03% or more, but if it increases, the effect is saturated and the cost becomes disadvantageous, so the upper limit of the content is set to 2.5%.

V: 0.05-1.0%
V has the function of forming carbides and improving the wear resistance of the bearing. In order to acquire the effect, it is necessary to contain 0.05% or more, but if it increases too much, a giant carbide is generated and the strength is lowered, so the upper limit of the content is set to 1.0%.

Nb: ≤0.1%
Nb is an element that contributes to the refinement of crystal grains. However, if the content is too large, the effect of crystal grain refinement is reduced, so it is contained in a range of 0.1% or less.

Surface hardness after carbonitriding or secondary quenching: 58 HRC or more Bearings are generally tempered at a temperature equal to or higher than the operating environment temperature in order to stabilize the dimensions in the operating environment. Further, it is effective for improving the rolling life to have a surface hardness higher than the hardness of the foreign matter when the foreign matter is mixed.
Further, when the hardness is less than 58 HRC, the rolling life tends to be abruptly reduced, and the variation in the rolling life is increased. Therefore, in the present invention, the surface hardness after quenching and tempering is set to 58 HRC or more.

Surface retained austenite amount: 20 to 50%
Surface-retained austenite is excellent in toughness in rolling fatigue under the environment where foreign matter is mixed, and not only does martensite transformation during use as a bearing and causes high work hardening due to the transformation, but also surface-retained austenite is soft. For this reason, since the shape of the indentation caused by the foreign matter is changed repeatedly by the load, there is an effect of relaxing stress concentration, and an effect of improving the fatigue life.

FIG. 1 shows the relationship between the surface retained austenite amount on the horizontal axis and L ₁₀ on the vertical axis. As is apparent from FIG. 1, the surface retained austenite amount is in the range of 20 to 50%. L ₁₀ indicates a satisfactory value.
Here, L ₁₀ is an index representing the life, and is the number of times of repeated damage at which the cumulative failure probability becomes 10%.
In order to obtain the above-described effects, a surface retained austenite amount of 20% or more is required as shown in FIG. However, if the amount is too large, the predetermined quenching and tempering hardness cannot be obtained, and conversely, the fatigue life is lowered, and the dimensional change due to the decomposition of the surface retained austenite during use of the bearing is increased. And

Next, embodiments of the present invention will be described in detail below.
Bearing steel having the chemical composition shown in Table 1 was melted, and hot rolled steel was used as a test material.
A thrust type rolling fatigue test piece (10 in FIG. 5) having a test part diameter of 63 mm was cut out from this material, carburized under the conditions shown in FIG. 2 (carbon potential CP: 0.8%), and then FIG. Carbonitriding was performed under the conditions shown in FIG.
In this carbonitriding treatment, the treatment was performed under the conditions of CP: 1.1%, NH ₃ : 5%.

Thereafter, the surface was polished by machining and lapping and subjected to a rolling fatigue test.
Further, some of the test pieces were similarly subjected to carbonitriding and then subjected to secondary quenching and tempering under the conditions shown in FIG. 4, followed by surface polishing by machining and lapping, and subjected to a rolling fatigue test.

  Although the carbonitriding process is divided into the carburizing process and the carbonitriding process here, carburizing and carbonitriding (that is, carbonitriding process) can be performed in one process by optimizing the processing temperature. .

The HRC surface hardness of the wrapping surface of the test piece was measured on a 5-point average using a Rockwell hardness meter.
Further, a 5 mm square sample having a thickness of 1 mm was cut from the lapping surface with a microcutter, and the amount of surface retained austenite on the surface layer was measured by X-ray diffraction.

The rolling fatigue test was performed under the conditions shown in Table 2. Specifically, high-speed steel gas atomized powder 12 classified to a hardness of about 750 Hv and a particle size of 100 to 180 μm was used as the foreign matter.
In addition, the housing 16 of the thrust tester 14 (see FIG. 5) has a curved slope on the inner surface, so that foreign matter agitation is sufficiently performed during the test, and foreign matter is appropriately caught on the rolling surface. did.

Table 3 when carbonitriding, surface retained austenite amount in the case of further secondary quenching and tempering treatment after carbonitriding in Table 4, L ₁₀ in rolling contact fatigue life test in surface hardness and under contaminated environment, L ₅₀ Life is shown.

As can be seen from these results, when carbonitriding was performed, the amount of retained austenite in the example of the present invention was as high as 30% or more compared to the comparative example due to the effect of Mn addition, and both the L ₁₀ and L ₅₀ lifetimes were excellent. .

On the other hand, as shown in Comparative Examples 23 and 45, when the Mn amount exceeds 3% (Steel Type U), the surface retained austenite amount increases, but the surface hardness decreases greatly, and the L ₁₀ and L ₅₀ lifetimes are also inferior.

Further, as shown in Comparative Examples 17 and 18, when the steel types O and P were only carburized, the L ₁₀ and L ₅₀ lifespans were obtained even though the surface retained austenite amount was about 30% or more. It is inferior to the case of carbonitriding (Invention Examples 15 and 16).
This is considered to be due to the improvement in hardness by nitrogen solid solution, the improvement in temper softening resistance and the stabilization of surface retained austenite.
The steel type Q in Table 1 is JIS SCr420.

In the example of the present invention, when secondary quenching is performed after carbonitriding, the amount of surface retained austenite is slightly reduced compared to the case of only carbonitriding, but the surface hardness is increased, and L ₁₀ , L ₅₀ Both lifespan is excellent.

Even in the comparative example, when the secondary quenching is performed after the carbonitriding process, the L ₁₀ and L ₅₀ lifespan is superior to the case of only the carbonitriding process, but the improvement is relatively small, and only the carbonitriding process of the present invention example. This is not even the case.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Is a diagram showing the relationship between L ₁₀ life of rolling fatigue test of the surface retained austenite amount and the contaminated environment at the time of heat-treated at various conditions. It is a figure which shows the temperature of the carburizing process performed to the test material in embodiment of this invention, holding | maintenance time, and cooling conditions. It is a figure which shows the temperature, holding time, and cooling conditions of the carbonitriding process performed to the test material in embodiment of this invention. It is a figure which shows the temperature of the secondary quenching process performed to the test material in embodiment of this invention, holding time, and cooling conditions. It is the figure which showed the thrust tester used for the rolling fatigue life test under a foreign material mixing condition with a housing shape, a test piece, etc.

Claims (4)

% By weight
C: 0.1-0.4%
Si: ≤1.0%
Mn: more than 1.5 ~ 3%
P: ≤0.03%
S: ≦ 0.03%
Cr: 0.3-2.5%
Al: 0.005 to 0.050%
Ti: ≦ 0.003%
O: ≦ 0.0015%
N: ≦ 0.025%
It has a composition consisting of the balance inevitable impurities and Fe, the surface hardness after carbonitriding treatment or subsequent secondary quenching and tempering treatment is 58 HRC or more, and the amount of surface retained austenite is 20 to 50%. Carbonitrided bearing steel. In addition to the alloy components defined in claim 1, either one or two of Ni and Mo in weight percent,
Ni: 0.25-3.5%
Mo: 0.03-2.5%
The surface hardness after the carbonitriding process or the subsequent secondary quenching and tempering process is 58 HRC or more, and the surface retained austenite amount is 20 to 20%. Carbonitrided bearing steel characterized by 50%. In addition to the alloy components defined in any one of claims 1 and 2, and V in wt%,
V: 0.05-1.0%
The surface hardness after the carbonitriding process or the subsequent secondary quenching and tempering process is 58 HRC or more, and the surface retained austenite amount is 20 to 20%. Carbonitrided bearing steel characterized by 50%. In addition to the alloy components defined in any one of claims 1 to 3, Nb in weight percent,
Nb: ≤0.1%
And has a composition consisting of the balance inevitable impurities and Fe, the surface hardness after carbonitriding or subsequent secondary quenching and tempering is 58 HRC or more, and the amount of surface retained austenite is 20 to 50% Carbonitrided bearing steel characterized by that.

Images