JPH06287708A - Bearing stress excellent in property of retarding microstructural change due to repeated stress load - Google Patents

Abstract

PURPOSE:To retard microstructural change occurring under a rolling contact surface owing to repeated stress load and to improve rolling fatigue life by specifying respective contents of C, V, O, Si, Mn, Mo, Ni, Cu, etc. CONSTITUTION:The bearing steel has a composition consisting of, by weight, 0.5-1.5% C, 0.05-1% V, <=0.002% O, and the balance Fe or further containing, besides the above, one or more kinds selected from 0.05-0.5% Si, 0.05-2% Mn, 0.05-0.5% Mo, 0.05-1% Ni, 0.05-1% Cu, 0.0005-0.01% B, 0.005-0.07% Al, and 0.0005-0.012% N. In this bearing steel, microstructural change occurring under a rolling contact surface owing to repeated stress load can be retarded and rolling fatigue life can be improved and, as a result, a long-life bearing can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel used as an element member of a rolling bearing such as a roller bearing or a ball bearing, and particularly to a delay for a microstructure change (deterioration) which occurs under a rolling contact surface due to repeated stress loads. We propose a bearing steel with excellent characteristics.

[0002]

2. Description of the Related Art Conventionally, high-carbon chromium bearing steel (JI
S: SUJ 2) is most often used. In general, bearing steel has one of the important properties that a long rolling contact fatigue life is important. As a factor that affects the rolling contact fatigue life, it is considered that the hard non-metallic inclusions in the steel have a large effect. Was being considered. Therefore, the mainstream of recent research has been to take measures to improve the bearing life by controlling the amount and size of non-metallic inclusions by reducing the amount of oxygen in steel.

For example, as one developed for the purpose of further improving the rolling contact fatigue life of a bearing, Japanese Patent Laid-Open No. 1-306542 has been proposed.
There are proposals such as Japanese Patent Laid-Open Publication No. 3-126839 and Japanese Patent Laid-Open Publication No. 3-126839, which are techniques for controlling the composition, shape, or distribution state of oxide-based nonmetallic inclusions in steel.

[0004]

However, in order to manufacture a bearing steel containing few non-metallic inclusions, it is necessary to install expensive melting equipment or to greatly improve conventional equipment, which causes a large economical burden. There was a problem. Further, according to a recent study conducted by the present inventors, as a factor that determines the rolling life, a phenomenon that has been generally discussed in the past;
That is, it has been found that there are factors other than the presence of the "decarburized layer" (low C concentration region) and the above-mentioned "non-metallic inclusions" that occur during heat treatment. The reason is that simply reducing the decarburization layer and non-metallic inclusions under the conventional technology is a major achievement in improving the rolling contact fatigue life of the bearing, especially the bearing life under severe conditions such as high load or high temperature. Because I experienced many things that I could not get. From this, we were convinced of the existence of other factors that control the bearing life.

Therefore, the present inventors investigated the cause of the separation of the rolling bearing. As a result, as shown in Fig. 1 (a), the band-shaped white color on the lower layer (surface layer) of the rolling contact surface is caused by the repeated shear stress generated during the rolling contact between the inner and outer rings of the bearing and the rolling elements. A microstructured layer consisting of products and rod-shaped precipitates is generated, which gradually grows as the number of rolling increases, and at the end, fatigue delamination as shown in Fig. 1 (b) from this microstructured portion. Was found to lead to bearing life. Furthermore, the harsh bearing operating environment, that is, high surface pressure (miniaturization) and rise in operating temperature, shortens the time until these microstructural changes occur, resulting in a significant reduction in bearing life. I stopped. That is, the bearing life under severe conditions is not sufficient by merely controlling the decarburized layer and non-metallic inclusions as in the prior art. For example, simply reducing the amount of non-metallic inclusions cannot delay the time until the above-described microstructure change occurring under the rolling contact surface occurs. As a result, they have found that the bearing life cannot be further improved.

Therefore, an object of the present invention is to delay the microstructural change expected to occur during the use of the bearing under high load in order to improve the rolling contact fatigue life characteristics under severe operating conditions. And to provide a bearing steel that can significantly improve the life of the bearing.

[0007]

The inventors of the present invention focused on a new "microstructure change delay characteristic" as the bearing life based on the above-mentioned knowledge, and of course, it is necessary to improve it. With the recognition that a new alloy design (composition composition) of (1) is necessary and the life of the bearing cannot be further improved without realizing this, various experiments and studies were further conducted. As a result, they have surprisingly found that the addition of an appropriate amount of V can significantly delay the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading, and have conceived the present invention bearing steel.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt%, O:
A bearing steel (first invention) containing 0.0020 wt% or less, and the balance being Fe and inevitable impurities, which is excellent in delay characteristics of microstructure change due to repeated stress load. (2) C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt%, O:
Contains 0.0020 wt% or less, and Si: 0.05-0.5 wt
%, Mn: 0.05 to 2.0 wt%, Mo: 0.05 to 0.5 wt%, N
i: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005
~ 0.01wt%, Al: 0.005-0.07wt% and N: 0.0005-0.0
12 wt%, containing one or more selected from the group consisting of Fe and inevitable impurities.
Bearing steel excellent in delay characteristics of microstructure change due to repeated stress load (second invention). (3) C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt%, O:
Contains less than 0.0020 wt%, and Si: more than 0.5 to 2.5 wt
%, Ni: over 1.0 to 3.0 wt%, N: over 0.012 to 0.050 wt%,
Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%, Zr: 0.02
~ 0.5 wt%, Ta: 0.02 ~ 0.5 wt%, Hf: 0.02 ~ 0.5 wt
% And Co: 0.05 to 1.5 wt% selected from the group consisting of one or more, and the balance being Fe and unavoidable impurities, the balance of which is excellent in delay characteristics of microstructure change due to repeated stress loading. (Third invention). (4) C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt%, O:
Contains 0.0020 wt% or less, and Si: 0.05-0.5 wt%,
Mn: 0.05 to 2.0 wt%, Mo: 0.05 to 0.5 wt%, Ni: 0.0
5 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01
wt%, Al: 0.005-0.07 wt% and N: 0.0005-0.012 wt
%, At least one selected from the group consisting of more than 0.5 to 2.5 wt%, Ni: more than 1.0 to 3.0 wt%, N: more than 0.012 to 0.050 wt%, Nb: 0.05-1.0
wt%, W: 0.05 to 1.0 wt%, Zr: 0.02 to 0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and Co:
A bearing steel containing one or more selected from 0.05 to 1.5 wt% and the balance consisting of Fe and unavoidable impurities and having excellent delay characteristics of microstructure change due to repeated stress load (the fourth invention ).

[0009]

The background to the idea of the bearing steel of the present invention having the above alloy design will be described below based on the results of experiments conducted by the present inventors. First, in the experiment, SUJ 2 (C: 1.02 wt%, Si: 0.25 wt%, Mn: 0.45 wt
%, Cr: 1.35wt%, Ni: 0.0040wt%, O: 0.0012wt%)
And two materials with V added (C: 1.00 wt%, Si: 0.28 wt%, Mn: 0.49 wt%,
O: 0.0009wt%, V: 0.46wt%, N: 0.0051wt%) (C: 1.00wt%, Si: 0.30wt%, Mn: 0.48wt%,
O: 0.0008 wt%, V: 0.91 wt%, N: 0.0048 wt%) were prepared as test steel materials. Then, these test materials were subjected to normalizing treatment, spheroidizing normalizing treatment, quenching and tempering treatment, and 12 mmφ × 22 mm cylindrical test pieces were produced from the respective test materials.

Next, these test pieces were subjected to a Hertz maximum contact stress: 60 using a radial type rolling fatigue life tester.
A rolling fatigue life test was conducted under load conditions of kgf / mm ² and 46500 cpm. The test results are plotted on Weibull distribution establishment paper, and are considered to be the numerical values showing the improvement of rolling contact fatigue life due to the increase of material strength. B ₁₀ (10% cumulative failure probability) and micro stress due to repeated stress loading during high load rolling. B ₅₀ (50% cumulative failure probability), which is considered to be a numerical value showing the improvement of rolling fatigue life by delaying the occurrence of microstructural change, was determined.

As a result, as shown in Table 1, with respect to the V-added material, the improvement in the B ₁₀ value was not so large, but the B ₅₀ value showed a remarkably high value, and the average bearing life was SUJ 2. It was confirmed that the B ₁₀ value showed an improvement of about 1.5 times and the B ₅₀ value showed an improvement of about 6 times. In particular, the addition of V has a remarkable effect on the delay characteristic of the microstructure change generated during high-load rolling, and it can be expected that the damage (lifetime) is delayed by that amount.

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the bearing life due to non-metallic inclusions and the life change due to microstructural changes.
As is clear from this figure, the cumulative damage probability 10
According to the bearing life indicated by the B ₁₀ value of% (hereinafter, referred to as “B ₁₀ rolling contact fatigue life”), the addition of V does not provide the expected effect. However, looking at this in terms of B ₅₀ value, the effect of adding V becomes extremely remarkable, and the bearing life under the so-called microstructure change generation environment (B ₅₀ rolling fatigue life with cumulative failure probability 50%) ), Very good results have been obtained.

Therefore, in the present invention, the range of the composition of components as described below is determined from the viewpoint of improving the microstructure change delaying property due to repeated stress loading.

C: 0.5 to 1.5 wt% C is an element which forms a solid solution in the matrix and effectively acts to strengthen the martensite, and in order to secure the strength after quenching and tempering and to improve the rolling fatigue life by it. Contained in. If the content is less than 0.5 wt%, such effects cannot be obtained. On the other hand, if it exceeds 1.5 wt%, the machinability and forgeability deteriorate, so the range was limited to 0.5 to 1.5 wt%.

Si: 0.05 to 0.5 wt%, more than 0.5 to 2.5 wt% or less Si is used as a deoxidizing agent during the melting of steel, and is also solid-dissolved in the matrix to increase the resistance to tempering and quenching, It is effective as an element that increases the strength after tempering and improves the rolling fatigue life. The content of Si added for this purpose is in the range of 0.05 to 0.5 wt%. Furthermore, this Si
Has the effect of delaying the microstructural change under cyclic stress loading and improving the rolling fatigue life by adding more than 0.5% by weight. However, its content is 2.5 wt%
If it exceeds 0.1%, the effect is saturated while the workability and toughness are lowered. Therefore, in order to further improve the microstructure change retardation property, it is effective to add more than 0.5 to 2.5 wt%.

Mn: 0.05 to 2.0 wt% Mn is an element that acts as a deoxidizer during the melting of steel and is effective in reducing the oxygen content of steel. Also, by improving the hardenability of steel, it improves the toughness and hardness of the base martensite, and effectively acts to improve the rolling fatigue life. However, if this content is less than 0.05 wt%, the effect will not be realized, and 2.
If it exceeds 0 wt%, machinability and forgeability will decrease, so 0.05
Limit to ~ 2.0 wt%.

Ni: 0.05 to 1.0 wt%, more than 1.0 to 3.0 wt% Ni increases the hardenability to increase the strength after quenching and tempering, improve toughness, and improve rolling contact fatigue life. Is added in the range of 0.05 to 1.0 wt%. Further, this Ni, when added in excess of 1.0 wt%, delays the microstructure change during rolling, thereby improving rolling fatigue life. However, even in this case, if it is added in excess of 3 wt%, a large amount of residual γ will be precipitated, resulting in reduced strength and impaired dimensional stability, as well as cost increase. , Addition of more than 1.0 to 3.0 wt% is necessary.

Mo: 0.05 to 0.5 wt% Mo is an element that improves wear resistance by stabilizing residual carbides. In particular, the addition of 0.05 to 0.5 wt% increases the hardenability and contributes to the improvement of the strength after quenching and tempering, and the precipitation of stable carbide improves the wear resistance and rolling fatigue life.

Cu: 0.05 to 1.0 wt% Cu is added in order to enhance the strength after quenching and tempering due to the increase in quenching and to improve the rolling contact fatigue life. When added for this purpose, a range of 0.05-1.0 wt% is sufficient.

V: 0.05 to 1.0 wt% V is a component that plays the most important role in the present invention, and the content of V has the effect of delaying the occurrence of the microstructure change due to the repeated stress loading, On the other hand,
It also has the function of improving wear resistance by stabilizing residual carbides. It also contributes to the refinement of crystal grains and improves toughness. These actions and effects become remarkable by the content of 0.05 wt%. On the other hand, if this amount exceeds 1.0 wt%, the amount of solid solution C decreases during quenching, which causes a decrease in strength, and also to improve the rolling fatigue life that affects both the B ₁₀ and B ₅₀ values described above. The effect is saturated, so 0.05-1.0w
Include in the range of t%.

B: 0.0005 to 0.01 wt% B is added in an amount of 0.0005 wt% or more because it increases the hardenability to increase the strength after quenching and tempering and improves the rolling fatigue life. However, if added in excess of 0.01 wt%, the workability deteriorates, so the range is limited to 0.0005 to 0.01 wt%.

Al: 0.005 to 0.07 wt% Al is used as a deoxidizing agent during the melting of steel, and at the same time, it combines with N in the steel to refine the crystal grains and contribute to the improvement of the toughness of the steel. Also, since it effectively works to improve the rolling fatigue life by increasing the strength after quenching and tempering,
Although 0.005 wt% is added, the effect is saturated when it exceeds 0.07 wt%, so the range is limited to 0.005 to 0.07 wt%.

N: 0.0005 to 0.012 wt%, more than 0.012 to 0.05
wt% N combines with the nitride-forming element to refine the crystal grains and to form a solid solution in the matrix to enhance the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
Add within 0.012 wt%. Also, this N is 0.
When added in excess of 012 wt%, rolling fatigue life is improved by delaying microstructural change due to repeated stress. However, if the amount exceeds 0.05 wt%, the workability decreases, so for this purpose it exceeds 0.012 to 0.
Add 05wt%.

P ≦ 0.025 wt% P is desirable because it lowers the toughness and rolling fatigue life of the steel, so it is desirable to be as low as possible.
It is 0.025 wt%.

S ≦ 0.025 wt% S combines with Mn to form MnS and improves the machinability. However, if it is contained in a large amount, the rolling contact fatigue life will be reduced, so 0.025 wt% must be the upper limit.

O: 0.0020 wt% or less O forms a hard non-metallic inclusion, so even if a delay in microstructure change due to repeated stress loading is obtained by controlling other components, rolling fatigue life Therefore, it is desirable to be as low as possible. However, a content of 0.0020 wt% or less is acceptable.

As described above, the main components (V and Si, Mn, Mo,) for improving the rolling fatigue life by delaying the microstructure change due to the repeated stress loading and improving the rolling fatigue life by increasing the strength. Ni, Cu, B, Al,
N, O) and C, P, S have been described above, but in the present invention, any one or more selected from Nb, W, Zr, Ta, Hf and Co is added. Thus, the rolling fatigue life under high load may be improved.

The preferred range of addition of each element and the purpose of addition,
The reasons for limiting the upper limit and the lower limit are summarized in Table 2.

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P,
The addition of Sn, As, etc. does not hinder the retardation property due to the change in microstructure due to the repeated stress load, which is the object of the present invention, and the machinability can be easily improved. You may add according to it.

[0031]

[Examples] Steels having the compositions shown in Tables 3 and 4 were melted by a conventional method, and the obtained steel materials were diffusion annealed at 1240 ° C for 30 hours and then rolled into steel bars of 65 mmφ. Then, heat treatment was performed in the order of normalizing-spheroidizing annealing-quenching-tempering, and a 12 mmφ x 22 mm cylindrical rolling fatigue life test piece was prepared by lapping finish. Then, a B ₅₀ rolling contact fatigue life, which is an average bearing life, was tested for each of the above test pieces. This B ₅₀ rolling contact fatigue life test uses a radial type rolling contact fatigue life tester and the maximum contact stress of Hertz: 600 kgf / mm.
^{2. The} test was performed under the condition of cyclic stress number of about 46,500 cpm. The test results are summarized on the probability paper according to the Weibull distribution, and the average life of steel material No. 1 (conventional steel SUJ2) (cumulative failure rate: 50% total load until peeling) is set to 1 and others The steel grades were evaluated in comparison. The evaluation results are also shown in Tables 3 and 4, respectively.

[0032]

[Table 3]

[0033]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Average of steel material No. 2 with C content outside the scope of the present invention, steel material No. 3 with V content outside the scope of the present invention, and steel material No. 4 with O content outside the scope of the present invention The service life is lower than that of conventional steel (No. 1 steel material). On the other hand, the average life of the steel material No. 5 of the present invention is about three times as excellent as that of the conventional steel material (steel material No. 1). That is, it can be seen that the addition of V to the bearing steel significantly retarded the microstructural change, and as a result, effectively acted to improve the rolling fatigue life.

Among them, in addition to V, Si, Mn, Mo, W, Z
Steel in which more than a specified amount of one or more of rolling fatigue life improving components such as r, Ta, Hf, Ni, Cu, Co and N is positively added.
It was confirmed that in the cases of Nos. 6 to 39, the average life (B ₅₀ rolling contact fatigue life) was further improved.

[0037]

As described above, according to the present invention,
Basically, by using ≧ 0.05% V-containing bearing steel,
The rolling fatigue life can be improved by delaying the microstructural change associated with cyclic stress loading, and a long-life bearing steel can be provided. Therefore, it is necessary to further reduce the oxygen content in steel or control the composition, shape, and distribution state of oxide-based nonmetallic inclusions present in steel, which was indispensable under the conventional technology. It is not necessary to improve or construct steelmaking equipment. Further, the development of the bearing steel according to the present invention enables downsizing of the rolling bearing and further increase of the bearing operating temperature.

[Brief description of drawings]

1 (a) and 1 (b) are under cyclic stress loading,
A micrograph of the metal structure showing the appearance of the microstructure change that occurs.

FIG. 2 is an explanatory diagram showing the influence of V on the bearing life due to inclusions and the bearing life due to microstructural changes.

Front page continuation (72) Inventor Akihiro Matsuzaki 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture, Kawasaki Steel Corporation Technical Research Division (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Iron & Steel Co., Ltd.

Claims (4)

[Claims] 1. C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt
%, O: 0.0020 wt% or less, the balance being Fe and inevitable impurities, and a bearing steel excellent in delay characteristics of microstructure change due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt
%, O: 0.0020 wt% or less, Si: 0.05 to
0.5 wt%, Mn: 0.05 to 2.0 wt%, Mo: 0.05 to 0.5 wt%,
Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.
0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005
˜0.012 wt%, a bearing steel containing one or more selected from the group consisting of Fe and unavoidable impurities with the balance being excellent in delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt
%, O: 0.0020 wt% or less, and Si: more than 0.5 ~
2.5 wt%, Ni: over 1.0 to 3.0 wt%, N: over 0.012 to 0.050
wt%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%, Z
r: 0.02-0.5 wt%, Ta: 0.02-0.5 wt%, Hf: 0.02-
0.5 wt% and Co: 0.05 to 1.5 wt% selected from the group consisting of one or more selected from the group consisting of Fe and unavoidable impurities with the balance being excellent in delaying microstructural change due to repeated stress loading. Bearing steel. 4. C: 0.5 to 1.5 wt%, V: 0.05 to 1.0 wt
%, O: 0.0020 wt% or less, Si: 0.05 to 0.5
wt%, Mn: 0.05 to 2.0 wt%, Mo: 0.05 to 0.5 wt%,
Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.000
5 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.
012 wt%, one or more selected from the group consisting of Si: more than 0.5 to 2.5 wt%, Ni:
Over 1.0-3.0 wt%, N: 0.012 Over-0.050 wt%, Nb: 0.0
5 to 1.0 wt%, W: 0.05 to 1.0 wt%, Zr: 0.02 to 0.5 wt
%, Ta: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and C
o: A bearing steel containing one or more selected from 0.05 to 1.5 wt% and the balance being Fe and inevitable impurities, and having excellent delay characteristics of microstructure change due to repeated stress loading.