JP3243326B2 - 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 more particularly to a delay against a microstructural change (deterioration) occurring under a rolling contact surface due to a repeated stress load. We propose a bearing steel with excellent characteristics.

[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 manufacture a bearing steel having a small amount of nonmetallic inclusions, it is essential to reduce the oxygen content in the steel. There is a problem that the installation of the equipment or a significant improvement of the conventional equipment is required, and the economic burden is large. Also, according to a recent study conducted by the present inventors,
Factors that determine the rolling life include phenomena that have been generally discussed in the past; that is, factors other than the presence of the “decarburized layer” (low C concentration region) generated during heat treatment and the aforementioned “non-metallic inclusions”. It turned out that there was. I mean,
Even simply reducing the decarburized layer and nonmetallic inclusions under the prior art has a significant effect on improving the rolling contact fatigue life of bearings, especially under severe conditions such as high loads or high temperatures. Because he has experienced many things that are not. 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, the rolling elements, and the rolling elements are in rotational contact, the lower part (surface layer) of the rolling contact surface
Then, as shown in FIG. 1 (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 rollings increases, and finally this microstructure changes. It was found that fatigue exfoliation (Fig. 1 (b)) occurred from the changed part, which led to the life of the bearing. In addition, the harsh operating environment of the bearing, that is, high surface pressure (small size) and an increase in operating temperature, reduce the number of rollings before these microstructure changes occur, and the conventional bearing steel SUJ2 has a remarkable bearing life. Was found to decrease. That is, the bearing life is not enough to control only the decarburized layer and the non-metallic inclusions as in the prior art.For example, simply reducing the amount and size of the non-metallic inclusions is not enough. It is not possible to delay the time until the microstructural change occurs under the rolling contact surface. As a result, they found that the bearing life could not be further improved.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to improve the rolling fatigue life under severe operating conditions by delaying the microstructural change occurring during use of the bearing under a high load. Another object of the present invention is to provide a bearing steel that can significantly improve the bearing life by suppressing the maximum particle size of nonmetallic inclusions.

[0007]

On the basis of the above findings, the present inventors have newly focused on "microstructure change delay characteristic" as a factor that determines the bearing life. In order to improve these characteristics, it is natural that a new alloy design (composition composition) is necessary, and from the recognition that the life of the bearing cannot be further improved without realizing this, Various experiments and studies were performed. As a result, it has been found that if a large amount of Ni is added in combination with appropriate amounts of Si and Mn, the above-mentioned microstructure change generated below the rolling contact surface due to repeated stress loading can be significantly delayed, and the present inventors have conceived the bearing steel of the present invention. .

That is, the bearing steel of the present invention has the following gist configuration. (1) C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, the balance consists of Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions is 8 µm or less. Microstructure by repeated stress loading Bearing steel excellent in delay characteristics of change (first invention). (2) C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%.
Cr: 0.05 or more and less than 1.0 wt% , Mo: 0.05-0.5 wt%, C
u: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.00
5 to 0.07 wt% and N: 0.0005 to 0.012 wt%, containing at least one selected from the group consisting of Fe and unavoidable impurities, and the largest grain of oxide-based nonmetallic inclusions A bearing steel having a diameter of 8 μm or less and excellent in delaying microstructure change due to repeated stress load (second invention). (3) C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%
r: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to
0.5 wt%, Co: 0.05 to 1.5 wt% and N: more than 0.012 to 0.
Contains at least one selected from 050 wt%, the balance being Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions is 8 μm or less, cyclic stress load Bearing steel which is excellent in the delay characteristic of microstructure change due to (3rd invention). (4) C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%.
Cr: 0.05 or more and less than 1.0 wt% , Mo: 0.05-0.5 wt%, C
u: 0.05-1.0 wt%, B: 0.0005-0.01 wt%,
Al: 0.005 to 0.07 wt% and N: one or more selected from 0.0005 to 0.012 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%
%, Hf: 0.02 to 0.5 wt%, Co: 0.05 to 1.5 wt%, and N: one or more selected from more than 0.012 to 0.050 wt%, with the balance being Fe and unavoidable impurities. A bearing steel in which the maximum particle size of the oxide-based nonmetallic inclusions is 8 μm or less, and which is excellent in delay characteristics of microstructure change due to repeated stress load (a fourth invention).

[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 SUJ 2 (C: 1.01wt%, Si: 0.24wt%, Mn: 0.46wt
%, Cr: 1.32wt%, N: 0.0042wt%, O: 0.0015wt%)
And two kinds of materials with a large amount of Ni added together with Si and Mn (C: 1.00 wt%,, Si: 1.05 wt%, Mn: 0.71 wt%,
Cr: 1.30 wt%, O: 0.0008 wt%, Ni: 1.20 wt%, N: 0.
(0046 wt%) (C: 1.02 wt%,, Si: 1.08 wt%, Mn: 0.75 wt%,
Cr: 1.31 wt%, O: 0.001 wt%, Ni: 1.22 wt%, N: 0.
0044 wt%) (C: 1.00 wt%, Si: 1.02 wt%, Mn: 0.65 wt%,
Cr: 1.31 wt%, O: 0.0007 wt%, Ni: 2.53 wt%, N: 0.
0048 wt%) (C: 1.01 wt%,, Si: 1.05 wt%, Mn: 0.67 wt%,
Cr: 1.33 wt%, O: 0.0011 wt%, Ni: 2.52 wt%, N: 0.
0042 wt%). 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.
A rolling fatigue life test was performed under a load condition of 0 kgf / mm ² and a cyclic stress number of 46500 cpm. Test results are plotted in Weibull distribution establishment paper, and B ₁₀ that appear to numerical value indicating the improvement in rolling fatigue life due to the increase of the material strength is influenced by the control of the non-metallic inclusions (10% cumulative failure probability), It was determined and B ₅₀ seen a numerical value indicating the improvement in rolling fatigue life by delaying the microstructure change caused by repeated stress load during high-load rolling (50% cumulative failure probability).

[0011] As a result, as shown in Table 1, without the inclusion control, for merely by the addition of Ni in a large amount, although improvement of the B ₁₀ value is small,
It can be seen that an improved show a considerably high value for the B ₅₀ value. That is, the average life of the bearing is B compared to SUJ 2.
An improvement of about 3 times at ₁₀ values and about 30 times at _B50 value was shown.
In contrast, the present invention having a controlled maximum particle size of the non-metallic inclusions with addition of a large amount of Ni, with show significant improvement for the delay of microstructural changes that produced under high-load rolling, further B ₁₀ As shown in the values, an improvement effect on peeling due to nonmetallic inclusions was observed.

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the bearing life caused by the particle size of nonmetallic inclusions and the change in life caused by a change in microstructure. As is evident in this figure, the bearing life represented by conventional cumulative failure probability of 10% B ₁₀ value as (hereinafter referred to as "B ₁₀ rolling contact fatigue life") is the addition of Ni in large amounts A great effect cannot be expected by only using the compound alone, but one that also performs control of nonmetallic inclusions shows a remarkable improvement effect. On the other hand, bearing life indicated by the cumulative failure probability of 50% B ₅₀ value (hereinafter referred to as "B ₅₀ high load rolling contact fatigue life") Looking at, the Ni addition of a large amount regardless of the non-metallic inclusions control The effect becomes extremely remarkable, and it can be seen that the bearing life under a microstructure change generating environment is significantly improved.

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. To ensure strength after quenching and tempering and to improve the rolling fatigue life due to it. To be contained. 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 machinability and forgeability deteriorate, so the range was limited to the range of 0.5 to 1.5 wt%.

Si: 0.05 to 2.5 wt% Si is used as a deoxidizing agent in steel smelting, and is also dissolved in a matrix to increase temper softening resistance due to an increase in softening resistance, thereby increasing strength after quenching and tempering. It is effective as an element for improving the rolling fatigue life. The content of Si added for such a purpose is in the range of 0.05 to 2.5 wt%.

Mn: 0.5 to 2.0 wt% Mn acts as a deoxidizing agent during melting of steel and is an element effective 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. These effects require at least 0.5 wt% addition, while 2.
Addition exceeding 0 wt% saturates the effect, so 0.5-2.0 %
Add in the range of wt%.

Cr: 0.05Not more than 1.0 wt%  Cr improves hardenability and forms stable carbides.
Improves strength and abrasion resistance, and eventually rolls
It is a component that improves fatigue life. To get this effect
Has 0.05Not more than 1.0 wt%Add within the range.

Ni: 2.00 to 3.0 wt% Ni is an element that plays the most important role in the present invention. In particular, when Ni is contained in an amount exceeding 2.00 wt%, high load rolling occurs. prompting repeated stress load delay microstructural changes described above under the significantly improve B ₅₀ rolling contact fatigue life. However, even in this case, an excessively large amount of Ni exceeding 3.0 wt% causes precipitation of a large amount of residual γ, lowering the strength and impairing the dimensional stability and increasing the cost. Therefore, it is necessary to add Ni in the range of 2.00 to 3.0 wt% in order to improve the bearing life during high-load rolling.

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.

Cu: 0.05-1.0 wt% Cu is added to increase the strength after quenching and tempering by increasing the quenching and to improve the rolling 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 B increases the strength after quenching and tempering due to the 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. These effects cannot be obtained at less than 0.005 wt%. On the other hand, 0.07wt
%, The effects described above become saturated. Therefore, Al is added in the range of 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 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%, rolling fatigue life is improved by delaying microstructural changes due to repeated stress. However, if the amount exceeds 0.05 wt%, the workability is reduced, and for this purpose, it exceeds 0.012 to 0.
Add 05 wt%.

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 (Ni and Si, Mn, Cr, and Ni) for improving the rolling fatigue life by delaying the microstructural change due to the repeated stress load and improving the rolling fatigue life by increasing the strength. Mo, Cu, Sb, Al,
B, N) and the reasons for limiting C, P, and S have been described. In the present invention, however, the addition of one or more selected from Zr, Ta, Hf, and Co further enhances the efficiency. The rolling fatigue life under load may be improved.

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

[0029]

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P,
Even if Sn, As, etc. are added, the above-mentioned object of the present invention does not hinder the retardation characteristics due to the change in microstructure due to the repeated stress load, and the machinability can be easily improved. You may add according to it.

Next, in the present invention, in addition to the limitation of the chemical composition, it'll be done in the form (size) control of oxide-based nonmetallic inclusions in the steel mainly above B ₁₀ rolling fatigue life Has been decided to be further improved.

Therefore, first, the present inventors have proposed two materials having different amounts of oxide-based nonmetallic inclusions and different component compositions:
High carbon chromium bearing steel (JIS-SUJ2) (A) , by using a bearing steel having the composition within the above adaptation range (B), the oxide-based nonmetallic inclusions maximum diameter and B ₁₀ rolling fatigue life of the steel The relationship with was investigated. As a result, as shown in FIG. 3, regardless of the oxide-based nonmetallic inclusions amount or composition of the steel, the maximum diameter of the non-metallic inclusions exceeds 8 [mu] m, B ₁₀ rolling contact fatigue life decreases noticeably From this, it can be seen that it is necessary for the bearing steel of the present invention to have a maximum particle size of 8 μm or less.

[0033]

EXAMPLES Steels having the component compositions shown in Tables 3 and 4 were melted 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. Testing of non-metallic inclusions at 400x8
00 field oxide-based nonmetallic inclusions was measured, the inclusion maximum diameter of each field are summarized in the paper Gumbel probability, calculates the 50000 mm ² corresponding extremes, oxide-based nonmetallic present in the steel The maximum inclusion particle size was used. The rolling fatigue life test was performed using a radial type rolling fatigue life tester under the conditions of a Hertz maximum contact stress: 600 kgf / mm ² and a repetitive stress number of about 46500 cpm. The test results are summarized on a probability paper assuming that they follow the Weibull distribution, and steel No. 1 (JIS-SuJ, a conventional steel)
The average life of 2) (the total number of loads until the occurrence of peeling at 10% and 50% of the cumulative failure probability) is set to 1, and the other steel types are evaluated in comparison. The evaluation results are shown in Tables 3 and 4, respectively.

[0034]

[Table 3]

[0035]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Steel No. 6 with C content outside the range of the present invention, Si, Mn, Ni in steel
B ₅₀ Bearing life expectancy of the steel amount is outside the range present invention No.2 is worse than the both the conventional steels (steel No.1). Also,
In No.3 inclusions maximum diameter exceeds 8 [mu] m, B ₁₀ bearing life resulted poor. In contrast, the steel of the present invention (
The average lifespan of B _10, B ₅₀ value of the steel No.7~10 a first invention), both of which better 10 times as compared with the conventional steels (steel No.1). That is, it can be seen that the addition of Mn and Ni to the bearing steel significantly delayed the change of the microstructure, and that the control of the maximum diameter of the inclusion effectively acted on the improvement of all rolling fatigue lives of the bearing.

Among them, in addition to Mn and Ni, Cr, Mo,
Cu, B, Al, Zr, Ta, Hf, in the case of steel No.11~36 plus Co and N aggressively than the predetermined amount, the average life (B _{5 0} rolling fatigue life) It was confirmed that it was further improved.

[0038]

As described above, according to the present invention,
Basically, by using a high Ni-containing bearing steel of 2.00 wt% or more, the rolling fatigue life is improved by delaying the microstructure change due to the repeated stress load, and a long life in this aspect is achieved. Steel for bearings can be provided. In addition, by increasing the material strength by controlling the particle size of the non-metallic inclusions, it is possible to improve the rolling fatigue life on this surface. 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]

1 (a) and 1 (b) are views under a repeated stress load.
5 is a micrograph of a metal structure showing a change in the microstructure that occurs.

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

FIG. 3 is a graph showing the relationship between the maximum particle size of nonmetallic inclusions and the rolling contact fatigue life.

────────────────────────────────────────────────── ─── 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-2-30733 JP-A-3-122255 (JP, A) JP-A-49-47212 (JP, A) JP-A-3-56640 (JP, A) JP-A-4-26752 (JP, A) JP-A-6-271977 (JP JP, A) Ogawa et al. "Technology to reduce inclusions in high-carbon steel" 4 Vol. 4 (1991) -1206 Ibuki et al. “Technology to reduce non-metallic inclusions in high clean steel” Materials and Process No. 4 Vol. 4 (1991) -1210 Shiroyama et al. “Development of High Purity Steel Smelting Technology” Material and Process No. 4 4 Vol. 4 (1991) -1214 (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] (1) C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%
%, Mn: 0.5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, the balance being Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions is 8 µm or less, cyclic stress Bearing steel with excellent microstructure change delay characteristics due to load. 2. C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%
%, Mn: 0.5 to 2.0 wt%, Ni: 2,000 to 3.0 wt%, Cr: 0.05 to less than 1.0 wt% , Mo: 0.05 to 0.5 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005-0.01wt
%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%, and the balance consists of Fe and unavoidable impurities and the oxide nonmetallic Bearing steel with a maximum particle size of 8 μm or less and excellent in delaying microstructural change due to repeated stress loading. 3. C: 0.5-1.5 wt%, Si: 0.5-2.5 w
t%, Mn: 0.5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, Co: 0.05 -1.5 wt% and N: more than 0.012-0.050 wt%, containing at least one selected from the group consisting of Fe and unavoidable impurities, and the largest grains of oxide-based nonmetallic inclusions Bearing steel with a diameter of 8 μm or less and excellent in delaying microstructural change due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Si: 0.5 to 2.5 wt%
%, Mn: 0.5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, Cr: 0.05 to less than 1.0 wt% , Mo: 0.05 to 0.5 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005-0.01wt
%, Al: 0.005 to 0.07 wt%, and N: 0.0005 to 0.012 wt%, and one or more selected from the group consisting of: Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt% %, Hf: 0.02 to 0.5 wt%, Co: 0.05 to 1.5 wt%, and N: one or more selected from more than 0.012 to 0.050 wt%, with the balance being Fe and unavoidable impurities A bearing steel in which the maximum particle size of oxide-based nonmetallic inclusions is 8 μm or less, and which is excellent in delaying microstructure change due to repeated stress loading.