JP3233718B2 - 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. In particular, by adding a large amount of Cu, the steel is generated under a rolling contact surface due to a repeated stress load. We propose a bearing steel with improved delay characteristics against the changing microstructure (deterioration).

[0002]

2. Description of the Related Art Rolling bearings used in automobiles, industrial machines, and the like are conventionally known as high carbon chromium bearing steel (JI).
S: SUJ 2) is used the most. In general, bearing steel has one of the important properties that the rolling fatigue life is long. One of the factors affecting the rolling fatigue life is that hard non-metallic inclusions in steel have a large effect. Was thought. Therefore, the mainstream of recent research has been to improve the bearing life by controlling the amount and size of nonmetallic inclusions by reducing the amount of oxygen in steel.

For example, Japanese Unexamined Patent Publication (Kokai) No. 1-306542 has been developed with the aim of further improving the rolling fatigue life of a bearing.
And Japanese Patent Application Laid-Open No. 3-126839, which are techniques for controlling the composition, shape or distribution of oxide-based nonmetallic inclusions in steel.

[0004]

However, in order to produce bearing steel with less nonmetallic inclusions, it is necessary to install expensive smelting equipment or to significantly improve the conventional equipment, which imposes a large economic burden. There was a problem. According to the results of recent research conducted by the present inventors, factors that determine the rolling life are phenomena generally discussed in the past; that is, the "decarburized layer" generated during heat treatment. (Low C concentration region) and other factors other than the existence of the above-mentioned "non-metallic inclusions". This is because simply reducing the decarburized layer and non-metallic inclusions under the conventional technology will greatly improve the rolling contact fatigue life of bearings, especially under severe conditions such as high loads or high temperatures. This is because they have experienced many inefficiencies.
From this, I was convinced that there were other factors that govern the specific bearing life.

Therefore, the present inventors investigated the cause of the occurrence of peeling of the rolling bearing. As a result, due to the repetitive shear stress generated when the inner and outer races of the bearing and the rolling element are brought into rotational contact, the lower layer (surface layer) of the rolling contact surface in FIG.
As shown in (a), a microstructure change layer consisting of a band-like white product and a rod-like precipitate is generated, which gradually grows as the number of rolling increases, and finally from this microstructure change portion, It was found that fatigue peeling as shown in FIG. Furthermore, the harsh operating environment of the bearing, that is, high surface pressure (miniaturization) and increase in operating temperature, shorten the number of rollings before these microstructure changes occur, leading to a significant reduction in bearing life. I found it. In other words, the bearing life is not sufficient just by controlling the decarburized layer and non-metallic inclusions as in the prior art. For example, simply reducing the non-metallic inclusions reduces the above-mentioned rolling contact surface. It is not possible to delay the time until the underlying microstructural changes occur. As a result, they found that the bearing life could not be further improved.

Accordingly, an object of the present invention is to delay the microstructure change expected to occur during use of a bearing under a high load in order to improve the rolling fatigue life characteristics under severe use conditions. Accordingly, it is an object of the present invention to provide a bearing steel which can significantly improve the bearing life.

[0007]

The inventors of the present invention have focused on a new "microstructure change delay characteristic" as a bearing life based on the above-mentioned knowledge, and it is natural to aim for improvement of the characteristic. Based on the recognition that a new alloy design (component composition) is required, and further improvement in bearing life cannot be achieved without realizing this, various experiments and studies were further conducted. As a result, it was surprisingly found that if a large amount of Mn is added, the above-described microstructure change generated below the rolling contact surface due to repeated stress load can be significantly delayed, and thus the bearing life can be significantly improved. Invented bearing steel was developed.

That is, the bearing steel of the present invention has the following gist configuration. (1) C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5 wt%, O: 0.0020 w
A bearing steel containing at most t%, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading (first invention). (2) C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5 wt%, O: 0.0020
wt% or less, Si: 0.05 to 0.5 wt%, Mn: 0.
05 ~ 2.0wt%, Cr: 0.05 ~ 2.5wt%, Mo: 0.05 ~ 0.5wt%, N
i: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005 to
0.07 wt% and N: one or more selected from 0.0005 to 0.012 wt%, the balance consisting of Fe and unavoidable impurities, and excellent in the delay characteristics of microstructure change due to repeated stress loading. Bearing steel (second invention). (3) C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5 wt%, O: 0.0020
wt% or less, Si: more than 0.5 to 2.5 wt%, Cr: 2.5
More than 8.0wt%, N: more than 0.012 to 0.050wt%, V: 0.05 to 1.0wt
%, Nb: 0.05-1.0 wt%, W: 0.05-1.0 wt%, Zr: 0.02-0.5 wt%, Ta: 0.02-0.5 wt%, Hf: 0.02-0.5 w
t% and Co: Bearing containing one or more selected from 0.05 to 1.5 wt%, the balance being Fe and unavoidable impurities, and excellent in the delay characteristics of microstructure change due to repeated stress loading. Steel (third invention). (4) C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5 wt%, O: 0.0020
wt% or less, and further contains one or more selected from the following components (Group I) , and further contains the following components (Group II) (provided that they are selected in Group I).
That element is excluded) comprises one kind or two or more species selected from among, the balance being Fe and unavoidable impurities, bearing steel excellent in delay characteristics of the microstructure changes due to repeated stress loading. (Group I) Si: 0.05 to 0.5 wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 w
t%, Mo: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, B: 0.0005
~ 0.01wt%, Al: 0.005 ~ 0.07wt% and N: 0.0005 ~ 0.0
12 wt% (Group II) Si: more than 0.5 to 2.5 wt%, Mn: more than 2.0 to 5.0 wt%, Cr: more than 2.5 to 4.0 wt%, N: more than 0.012 to 0.050 wt%, V: 0.05 to 1.0 wt%
%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%, Zr: 0.02 to
0.5wt%, Ta: 0.02-0.5wt%, Hf: 0.02-0.5wt% and C
o: 0.05-1.5wt%

[0009]

The background that led to 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%, N: 0.0040wt%, O: 0.0012wt%)
And two kinds of materials to which Cu is added (C: 0.98 wt%, Si: 0.25 wt%, Mn: 0.42 wt%, C
r: 1.32 wt%, Cu: 1.20 wt%, O: 8 ppm, N: 38 ppm) (C: 0.96 wt%, Si: 0.25 wt%, Mn: 0.44 wt%, C
(r: 1.31 wt%, Cu: 1.83 wt%, O: 9 ppm, N: 43 ppm). Next, these test materials were subjected to normalizing, spheroidizing normalizing, and quenching and tempering, and cylindrical test pieces of 12 mmφ × 22 mm were prepared from the respective test materials.

Next, these test pieces were subjected to a rolling contact fatigue life tester of a radial type using a maximum contact stress of 60 Hz.
The rolling fatigue life was tested under the load conditions of kgf / mm ² and 46500 cpm. Test results are plotted in Weibull distribution establishment paper, micro by repeated stress load at B ₁₀ seen a numerical value indicating the improvement in rolling fatigue life due to an increase in the material strength (10% cumulative failure probability) a high load rolling tissue change numeric and found B ₅₀ (50% cumulative failure probability) indicating the improvement in rolling fatigue life by bringing generate delayed and was determined.

[0011] As a result, as shown in Table 1, for the high Cu additive material, wherein at not so large improvement for the B ₁₀ value, B ₅₀ value exhibited significantly higher numbers for bearing life expectancy SUJ 2 about 1.5-fold in B ₁₀ value, to exhibit improved by about 20 times B ₅₀ values were observed compared to. In particular, Cu
Is very effective for delaying microstructural change generated during high-load rolling, and correspondingly, breakage (life)
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 caused by non-metallic inclusions and the change in life caused by microstructure change.
As is clear from this figure, the cumulative failure probability
% Of bearing life represented by B ₁₀ value (hereinafter referred to as "B ₁₀ rolling contact fatigue life"), according to a large amount its effect by adding Cu is not manifested to the extent expected. But this is B
Looking at the ₅₀ values, the effect of addition of a large amount of Cu becomes extremely significant, severe conditions, i.e. as long as to be aware of the bearing life under microstructural change generation environment, evaluation of excellent such B ₅₀ Turned out to be essential.

Therefore, in the present invention, the range of the component composition as described below is determined from the viewpoint of improving the microstructure change delay characteristic due to the repeated stress load.

C: 0.5-1.5 wt% C is an element which forms a solid solution in the matrix and effectively acts to strengthen martensite, and secures strength after quenching and tempering and improves rolling fatigue life. To be included.
If the content is less than 0.5 wt%, such effects cannot be obtained. On the other hand, if the content exceeds 1.5 wt%, the non-oxidizing property and the forgeability deteriorate, so the content is limited to the range of 0.5 to 1.5 wt%.

Si: 0.05-0.5 wt%, more than 0.5-2.5 wt%%  Si is used not only as a deoxidizer when smelting steel, but also as a base.
Solid solution and tempering Quenching and tempering due to increase in softening resistance
Element to increase rolling strength and improve rolling fatigue life
Effective. Inclusion of Si added for such purpose
The amount ranges from 0.05 to 0.5 wt%. Furthermore, this Si
is 0.5 wt% More than under repeated stress loading
To delay rolling micro-structure change to improve rolling fatigue life
Has the effect of raising. However, its content is 2.5wt%
If it exceeds, the effect saturates while workability and toughness decrease.
The microstructure change delay characteristics.
For the above, it is effective to add more than 0.5 to 2.5 wt%
It is.

Mn: 0.05 to 2.0 wt%, more than 2.0 to 5.0 wt% Mn acts as a deoxidizing agent during melting of steel and is an effective element for reducing oxygen in steel. In addition, by improving the hardenability of steel, the toughness and hardness of the base martensite are improved, which effectively works to improve the rolling fatigue life. For these purposes, the addition of 0.05-2.0 wt% is sufficient. However, when Mn is added in excess of 2.0 wt%, it has the effect of significantly delaying microstructural changes due to the application of repeated stress during rolling, similar to Al, and improves rolling fatigue life. However, if the addition exceeds 5.0 wt%,
Since a large amount of residual γ is generated and strength and dimensional stability are reduced, for this purpose, it is added in a range of more than 2.0 to 5.0 wt%.

O: 0.0020 wt% or less O forms hard non-metallic inclusions. Therefore, even if the control of other components can delay the microstructure change due to repeated stress loading, the rolling fatigue life can be reduced. Therefore, it is desirable to be as low as possible. However, a content of 0.0020 wt% or less is acceptable.

Cr: 0.05-2.5 wt%, more than 2.5-8.0 wt% Cr is formed by improving hardenability and forming stable carbides.
It is a component that improves strength and abrasion resistance, and thereby improves rolling fatigue life. To obtain this effect, the addition of 0.05 to 2.5 wt% is sufficient. Furthermore, when this Cr is added in a large amount exceeding 2.5 wt%, it is effective to delay the microstructure change caused by the repeated stress load and to improve the rolling fatigue life on this surface. . And the effect of adding Cr for this purpose is
If the content exceeds 8.0 wt%, not only saturation occurs, but also the strength of solid solution C decreases at the time of quenching. Therefore,
When added for this purpose, it must be greater than 2.5 to 8.0 wt%.

Mo: 0.05-0.5 wt% Mo is an element which improves wear resistance by stabilizing residual carbides. Particularly, when 0.05 to 0.5 wt% is added, hardenability is increased to contribute to improvement in strength after quenching and tempering, and precipitation of stable carbides improves wear resistance and rolling fatigue life.

Ni: 0.05 to 1.0 wt% Ni enhances the strength after quenching and tempering by increasing the hardenability, thereby improving the toughness and, thereby, the rolling fatigue life. This effect is manifested by the addition of 0.05 wt% or more. At 1.0 wt%, saturation occurs and a large amount of residual γ is generated, leading to a decrease in hardness and, consequently, a deterioration in rolling fatigue life. It is added within the range of 1.0 wt%.

Cu: more than 1.0 to 2.5 wt% Cu is an element that plays the most important role in the present invention. In particular, when this Cu is contained in a large amount exceeding 1.0 wt%, the content of Cu is high. by delaying the microstructure changes described above caused by repeated stress load during rolling under load, it will significantly improve the B ₅₀ rolling Do疲 labor life. However, if the amount exceeds 2.5% by weight, the effect of the addition is saturated and the forgeability is rather lowered. Therefore, it is necessary to contain Cu in a range of more than 1.0 to 2.5 wt%.

B: 0.0005 to 0.01 wt% B is added in an amount of 0.0005 wt% or more because B increases the strength after quenching and tempering due to an increase in hardenability and improves the rolling fatigue life. However, if added in excess of 0.01 wt%, the workability is degraded, 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 at the time of melting steel,
It combines with N in the steel to refine the crystal grains and contribute to improving the toughness of the steel. In addition, it effectively acts to improve the rolling fatigue life by increasing the strength after quenching and tempering. For such an effect, it is effective to add 0.005 to 0.07 wt% of Al.

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 dissolves in the matrix to increase the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
It is added within the range of ~ 0.012 wt%. This N is 0.
When added in excess of 012 wt%, the rolling fatigue life in this aspect is improved by delaying the microstructural change caused by repeated stress loading. However, if the amount exceeds 0.05% by weight, the workability is reduced. For this purpose, more than 0.012 to 0.05% by weight is added.

P ≦ 0.025 wt% P is desirably as low as possible from the viewpoint of lowering the toughness and rolling fatigue life of steel.
0.025 wt%.

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

As described above, the main components (Cu and Si, Mn, Cr, and Cr) for improving the rolling fatigue life by delaying the microstructure change due to the repeated stress load and improving the rolling fatigue life by increasing the strength. Mo, Ni, B, Al,
N) and the reasons for limiting C, P, and S have been described.
In the present invention, the rolling fatigue life under a high load is improved by adding one or more selected from V, Nb, W, Zr, Ta, Hf and Co. You may.

The preferred addition range and purpose of each of the above elements,
Table 2 summarizes the reasons for limiting the upper and lower limits.

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, M
Even if g, P, Sn, As, etc. are added, the above-mentioned object of the present invention does not inhibit the retardation characteristics due to the change in microstructure due to the repeated stress load, and the machinability can be easily improved. Therefore, they may be added as needed.

[0031]

EXAMPLES Steels having the composition shown in Tables 3, 4 and 5 were smelted by a conventional method, and the obtained steel was subjected to diffusion annealing at 1240 ° C. for 30 hours and then rolled into a 65 mmφ steel bar. Next, heat treatment was performed in the order of normalizing, spheroidizing annealing, quenching, and tempering, and a 12 mmφ × 22 mm cylindrical rolling contact fatigue life test piece was prepared by lapping. Then, for each of the above test pieces,
It was tested for B ₅₀ rolling contact fatigue life is bearing life expectancy.
The B ₅₀ rolling fatigue life test using a rolling fatigue life tester of the radial type, Hertzian maximum contact stress: 600 kgf /
mm ² and the number of repetitive stresses was about 46500 cpm. The test results are summarized on a probability paper assuming that they follow the Weibull distribution, and the average life of steel material No. 1 (SUJ2, a conventional steel) (cumulative failure probability: 50%, the total number of loads up to the occurrence of peeling) is set to 1, and other Of steel types were evaluated. The evaluation results are also shown in Tables 3, 4, and 5, respectively.

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

As is clear from the results shown in Tables 3, 4, and 5, steel No. 2 having a C content in steel outside the range of the present invention, steel No. 3 having a Cu content outside the range of the present invention, and The average life of steel material No. 4 in which the amount of O in steel is out of the range of the present invention is lower than that of the conventional steel (steel material No. 1). On the other hand, the average life of steel materials No. 5 to 47 of the present invention steel was
2.6-58.9 times better than 4). That is,
Addition of Cu to bearing steel significantly delays microstructural change,
As a result, it can be seen that it effectively worked to improve the rolling fatigue life.

In particular, in the case of steels Nos. 32 to 47 in which a life improving component due to an increase in strength and a life improving component due to a delay in microstructure change are added in addition to Cu, the average life (B ₅₀ rolling fatigue life) showed a life ratio of at least 23 times.

[0037]

As described above, according to the present invention,
Basically, by using a high Cu content bearing steel of more than 1.0 wt%, the rolling fatigue life can be improved by delaying the microstructural change due to repeated stress loading, and it can be used for bearings with high life. Steel can be provided. Therefore, it is necessary to further reduce the oxygen content in steel or control the composition, shape, and distribution of oxide-based nonmetallic inclusions present in steel, which were 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 makes it possible to reduce the size of the rolling bearing and further increase the operating temperature of the bearing.

[Brief description of the drawings]

FIG. 1 is a micrograph of a metal structure showing a state of a microstructure change occurring under a repeated stress load.

FIG. 2 is an explanatory diagram showing the influence of Cu on the bearing life caused by inclusions and the bearing life caused by microstructure change.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (56) References JP-A-63-143239 (JP, A) JP-A-3-122255 (JP, A) JP-A-6-256906 (JP, A) JP-A-6-264186 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38 / 00-38/60

Claims (4)

(57) [Claims] C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: A bearing steel containing 0.0020 wt% or less, with the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: 0.0020 wt% or less, and further, Si: 0.05
~ 0.5 wt%, Mn: 0.05 ~ 2.0 wt%, Cr: 0.05 ~ 2.5 wt
%, Mo: 0.05-0.5 wt%, Ni: 0.05-1.0 wt%,
B: 0.0005 to 0.01 wt% Al: 0.005 to 0.07 wt% and N: 0.
A bearing steel containing one or more selected from 0005 to 0.012 wt%, the balance being Fe and unavoidable impurities, and excellent in delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: 0.0020 wt% or less, Si: more than 0.5 to 2.5 wt%, Mn: more than 2.0 to 5.0 wt%, Cr: more than 2.5 to 8.0 w
t%, N: more than 0.012 to 0.050 wt%, V: 0.05 to 1.0 wt%,
Nb: 0.05-1.0 wt%, W: 0.05-1.0 wt%, Zr: 0.0
2 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt
% And Co: a bearing steel containing one or more selected from 0.05 to 1.5 wt%, the balance being Fe and unavoidable impurities, and having excellent delay characteristics of microstructure change due to repeated stress loading. . 4. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5 wt%,
O: contains 0.0020 wt% or less, and further has the following composition (group I):
It includes one kind or two or more species selected from among minute, furthermore, in component (However, I the following group (II group)
(Excluding selected elements) , and the balance is Fe and unavoidable impurities, the balance being excellent in the delay characteristic of microstructure change due to repeated stress loading. (Group I) Si: 0.05 to 0.5 wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 w
t%, Mo: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, B: 0.0005
~ 0.01wt%, Al: 0.005 ~ 0.07wt% and N: 0.0005 ~ 0.0
12 wt% (Group II) Si: more than 0.5 to 2.5 wt%, Mn: more than 2.0 to 5.0 wt%, Cr: more than 2.5 to 4.0 wt%, N: more than 0.012 to 0.050 wt%, V: 0.05 to 1.0 wt%
%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%, Zr: 0.02 to
0.5wt%, Ta: 0.02-0.5wt%, Hf: 0.02-0.5wt% and C
o: 0.05-1.5wt%