JP3379780B2 - Bearing steel with excellent microstructure change delay characteristics due to repeated stress loading - Google Patents

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 when a large amount of Mn is added, it is generated below a rolling contact surface by repeated stress loading. We propose a bearing steel with improved delay characteristics against changes in microstructure (deterioration).

[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 summary of recent research results 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, a "decarburized layer" generated during heat treatment It was found that there are other factors besides the (low C concentration region) and the presence of the above-mentioned "non-metallic inclusions". This is because simply reducing the decarburization layer and non-metallic inclusions under the conventional technology is not enough to improve the rolling contact fatigue life of the bearing, especially the bearing life under severe conditions such as high load or high temperature. This is because I experienced many things that I could not get the effect. 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, due to the repeated shear stress generated during the rolling contact between the inner and outer races of the bearing and the rolling element, a band-shaped white color was generated in the lower layer (surface layer) of the rolling contact surface as shown in Fig. 1 (a). A microstructure-changed layer consisting of solids and rod-shaped precipitates is formed, which gradually grows as the number of rolling increases, and at the end, fatigue delamination from this microstructure-changed part as shown in Fig. 1 (b). Was found to lead to bearing life. In addition, the harsh bearing operating environment, that is, higher surface pressure (miniaturization) and higher operating temperature, will shorten the time until these microstructural changes occur, resulting in a marked reduction in bearing life. I found him. 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 a bearing under a 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, it was unexpectedly found that by adding a large amount of Mn, the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading can be significantly delayed, and by extension, the bearing life can be significantly improved. Invented bearing steel developed.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Mn: over 2.5 to 5.0 wt%, O: 0.00
A bearing steel containing 20 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 (first invention). (2) C: 0.5 to 1.5 wt%, Mn: over 2.5 to 5.0 wt%, O: 0.00
Contains less than 20wt%, Si: 0.05-0.5wt%, C
r: 0.05 to 2.5 wt%, Mo: 0.05 to 0.5 wt%, Cu: 0.05 to 1.0
wt%, B: 0.0005~0.01wt% 及 beauty N: 0.0005~0.012wt
%, And a balance of Fe and unavoidable impurities, the balance of which is excellent in delay characteristics of microstructure change due to repeated stress load (second invention). (3) C: 0.5 to 1.5 wt%, Mn: over 2.5 to 5.0 wt%, O: 0.00
It contained the following 20 wt%, further, Si: 0.5 super ~2.5wt%, C
r: over 2.5 ~ 8.0wt%, Mo: over 0.5 ~ 2.0wt%, N: over 0.012 ~ 0.050wt%, V: 0.05 ~ 1.0wt%, Nb: 0.05 ~ 1.0wt
%, W: 0.05 to 1.0 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02
~ 0.5wt%, Hf: 0.02-0.5wt% and Co: 0.05-1.5wt%
A bearing steel which contains 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 load (third invention). (4) C: 0.5 to 1.5 wt%, Mn: over 2.5 to 5.0 wt%, O: 0.00
20 wt% or less, and further contains any one kind or two or more kinds selected from the following group I components, and further contains the following group II components (provided that they are selected in group I). Yuan
(Excluding elements) , and any 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 (4th invention). Serial group I: Si: 0.05~0.5wt%, Cr: 0.05~2.5wt%, Mo: 0.
05-0.5wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01wt
% And N: 0.0005 to 0.012 wt% Group II: Si: more than 0.5 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, Mo:
Over 0.5 ~ 2.0wt%, N: over 0.012 ~ 0.050wt%, V: 0.05 ~
1.0wt%, Nb: 0.05 to 1.0wt%, W: 0.05 to 1.0wt%, Z
r: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5
wt% and Co: 0.05 to 1.5 wt%

[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 containing Mn (C: 1.01 wt%,, Si: 0.21 wt%, Mn: 2.05 wt%,
Cr: 1.30 wt%, O: 0.0010 wt%, N: 0.0040 wt%) (C: 0.98 wt%,, Si: 0.25 wt%, Mn: 4.22 wt%,
Cr: 1.32 wt%, O: 0.0011 wt%, N: 0.0040 wt%) was prepared. 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 high Mn-added material, the B ₁₀ value is not so much improved, but the B ₅₀ value is remarkably high, and the average bearing life is SUJ 2 It was confirmed that the B ₁₀ value showed an improvement of about 4 times, and the B ₅₀ value showed an improvement of about 18 times, compared with the above. In particular, the addition of a large amount of Mn has a remarkable effect on the delaying property of the microstructure change generated during high-load rolling, and the damage (life) is correspondingly increased.
Can be expected to be delayed.

[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”), even if a large amount of Mn is added, the effect is not as expected. But this is B
_{The 50} value shows that the effect of adding a large amount of Mn becomes extremely remarkable, and it is evaluated that the B ₅₀ is excellent as long as the bearing life under severe conditions, that is, under the microstructure change generation environment, is taken into consideration. Proved to be essential.

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 on the strengthening of martensite, and secures the strength after quenching and tempering and improves the rolling contact fatigue life. Is included for
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 non-oxidizing property and the 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 2.5wt% Si is used as a deoxidizer during the melting of steel and also as a base material.
Quenching and tempering due to an increase in softening resistance
As an element that increases the strength after rolling and improves rolling contact fatigue life
Is effective. Containing Si added for these purposes
The amount is in the range of 0.05 to 0.5 wt%. Furthermore, this Si
is 0.5wt% Addition, under repeated stress loading
The microstructure change of the
It has the effect of increasing the quality. However, its content is 2.5 wt%
If it exceeds, the effect will be saturated, but the workability and toughness will be reduced.
As a result, it is possible to further improve the microstructure change delay property.
For the above, it is effective to add more than 0.5 ~ 2.5wt%
is there.

O: 0.0020 wt% or less O forms a hard non-metallic inclusion, so even if the delay of 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.

Mn: more than 2.0 to 5.0 wt% This Mn is an element that plays the most important role in the present invention, and especially when it is added in a large amount exceeding 2.0 wt%, it is Under cyclic stress loading, it promotes the delay of the above-mentioned microstructural change and significantly improves the B ₅₀ rolling contact fatigue life. However, if the amount added exceeds 5.0 wt%, a large amount of residual γ is generated and the strength and dimensional stability are reduced. Add in range.

Cr: 0.05 to 2.5 wt%, more than 2.5 to 8.0 wt% Cr improves the hardenability and forms stable carbides.
It is a component that improves strength and wear resistance, and eventually improves rolling contact fatigue life. To obtain this effect, addition of 0.05 to 2.5 wt% is sufficient. Furthermore, when Cr is added in a large amount exceeding 2.5 wt%, it is effective in delaying the microstructure change due to repeated stress loading and improving the rolling fatigue life in this aspect. The effect of Cr addition for this purpose is not only saturated when it exceeds 8.0 wt%, but rather causes a decrease in the amount of solid solution C during quenching, resulting in a decrease in strength. Therefore, when it is added for this purpose, it must be more than 2.5 to 8.0 wt%.

Mo: 0.05 to 0.5 wt%, more than 0.5 to 2.0 wt% Mo is an element which 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. Further, when Mo is contained in a large amount of more than 0.5 wt%, the effect of delaying the microstructure change during rolling becomes remarkable, and the rolling fatigue life in this aspect is improved. However, if the amount exceeds 2.0 wt%, the machinability and forgeability will be reduced, and this will cause cost increase. Therefore, for this purpose, addition within the range of more than 0.5 to 2.0 wt% is necessary. Is.

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.

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%.

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 contact 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.

As described above, the main components (Mn and Si, Cr, Mo, for improving rolling fatigue life by delaying the microstructure change due to repeated stress loading and improving rolling fatigue life by increasing strength). The reason for limiting Cu, B, N) and C, P, S has been described, but the present invention further includes any one or two selected from V, Nb, W, Zr, Ta, Hf and Co. By adding the above, 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.

[0029]

EXAMPLE Steels having the compositions shown in Tables 3, 4, and 5 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. And for each of the above test pieces,
A test of B ₅₀ rolling contact fatigue life, which is the average bearing life, was conducted.
This B ₅₀ rolling contact fatigue life test uses a radial type rolling contact fatigue life tester and the Hertz maximum contact stress: 600 kgf /
The test was performed under the condition of mm ² , cyclic stress number of about 46500 cpm. The test results are summarized on the probability paper according to the Weibull distribution, and the average life of steel material No. 40 (conventional steel SUJ2) (cumulative failure probability: 50%, the total number of loads before delamination) is set to 1, and other The steel grades were evaluated in comparison. The evaluation results are also shown in Tables 3, 4, and 5, respectively.

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

As is clear from the results shown in Tables 3, 4 and 5, steel No. 41 and Mn in steel having a C content in the steel outside the range of the present invention.
The average life of steel materials No. 42 and 43 whose amounts are outside the scope of the present invention and steel materials No. 44 whose amount of O in the steel is outside the scope of the present invention are both lower than that of conventional steel (steel material No. 40). . On the other hand, steel materials No. 1 to 36 of the present invention (however, No. 2 , 25, 2
The average life of 6, 28, 29 and 31 is the same as that of conventional steel (steel material No. 4).
It is 2.3 to 26.9 times better than 0). That is, it can be seen that the addition of Mn to the bearing steel significantly retarded the microstructural change, and as a result, effectively acted to improve the rolling contact fatigue life.

Among them, Si, Cr, Mo, W, V, Zr, Ta,
Steel No. with positive addition of Hf, Cu, Co, N in a specified amount or more 27
In the case of ~ 39, the above average life (B ₅₀ rolling contact fatigue life)
Was confirmed to be further improved.

[0035]

As described above, according to the present invention,
Basically, by using a bearing steel with a high Mn content of more than 2.0 wt%, it is possible to achieve an improvement in rolling contact fatigue life by delaying the microstructural change due to repeated stress loading, and for bearings with long life. 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]

1A and 1B are micrographs of a metal structure showing a microstructure change occurring under repeated stress loading.

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

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (56) Reference JP-A-3-138332 (JP, A) Kaihei 3-271319 (JP, A) JP-A-3-254339 (JP, A) JP-A-1-283430 (JP, A) JP-A-3-53021 (JP, A) (58) Fields investigated ( Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. A C: 0.5~1.5wt%, Mn: 2.5 Ultra 5.0 wt%,
O: 0.0020 wt% or less, the balance being Fe and unavoidable impurities, and a bearing steel excellent in delay characteristics of microstructure change due to repeated stress load. Wherein C: 0.5~1.5wt%, Mn: 2.5 Ultra 5.0 wt%,
O: contains 0.0020 wt% or less, and further Si: 0.05 to 0.5 w
t%, Cr: 0.05 to 2.5 wt%, Mo: 0.05 to 0.5 wt%, Cu: 0.0
5~1.0wt%, B: 0.0005~0.01wt% 及 beauty N: 0.0005~0.
A bearing steel containing at least one selected from the group consisting of 012 wt% and the balance consisting of Fe and unavoidable impurities and having excellent delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5~1.5wt%, Mn: 2.5 Ultra 5.0 wt%,
O: 0.0020 wt% or less is contained, and Si: more than 0.5 to 2.5
wt%, Cr: over 2.5 to 8.0 wt%, Mo: over 0.5 to 2.0 wt%, N:
Over 0.012 ~ 0.050wt%, V: 0.05 ~ 1.0wt%, Nb: 0.05 ~
1.0wt%, W: 0.05-1.0wt%, Zr: 0.02-0.5wt%, T
a: 0.02-0.5wt%, Hf: 0.02-0.5wt% and Co: 0.05-
A bearing steel that contains one or more selected from 1.5 wt% and the balance is Fe and inevitable impurities, and that is excellent in delay characteristics of microstructure change due to repeated stress loading. 4. C: 0.5~1.5wt%, Mn: 2.5 Ultra 5.0 wt%,
O: 0.0020 wt% or less, further contains any one or more selected from the following group I components, and further contains the following group II components (provided that in group I Selected
And which elements are excluded) include any one or more selected from among, the balance being Fe and unavoidable impurities, repetitive stress load by microstructural changes in the delay characteristics excellent bearing steel. Serial group I: Si: 0.05~0.5wt%, Cr: 0.05~2.5wt%, Mo: 0.
05-0.5wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01wt
% And N: 0.0005 to 0.012 wt% Group II: Si: more than 0.5 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, Mo:
Over 0.5 ~ 2.0wt%, N: over 0.012 ~ 0.050wt%, V: 0.05 ~
1.0wt%, Nb: 0.05 to 1.0wt%, W: 0.05 to 1.0wt%, Z
r: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5
wt% and Co: 0.05 to 1.5 wt%