JP3233727B2 - 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,
Simply reducing the decarburized layer and non-metallic inclusions under the conventional technology 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 Mo is contained in an appropriate amount, 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 requires the following
That is, it has a configuration to the effect. (1) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%
And the balance consists of Fe and unavoidable impurities and is oxidized
The maximum particle size of non-metallic inclusions is less than 8 μm.
Excellent in microstructure change delay characteristics due to reverse stress loading
Bearing steel (first invention). (2) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%
And Si: 0.05-0.5 wt%, Mn: 0.05-2.0 wt%
%, Ni: 0.05-1.0 wt%, Cu: 0.05-1.0 wt%, B: 0.0
005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005
Any one or 2 selected from ~ 0.012 wt%
Species and the remainder is made up of Fe and unavoidable impurities.
And the maximum particle size of oxide-based nonmetallic inclusions is 8 μm or less
, Delay of microstructure change due to repeated stress loading
Bearing steel with excellent properties (second invention). (3) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%
Si: 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%, V: 0.05 to 1.0 wt%, N
b: 0.05 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
And Co: any one selected from 0.05 to 1.5 wt%
Or two or more, the balance being Fe and unavoidable impurities
And the maximum particle size of the oxide-based nonmetallic inclusions is 8 μm.
m or less, microstructure change due to repeated stress loading
Bearing steel having excellent retardation characteristics (third invention). (4) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%
And thenIngredients of the following (Group I)Which one to choose from
One or more of them, andThe following (II
Group) (excluding the elements selected in group I)
H)Any one or two or more selected from
And the balance consists of Fe and unavoidable impurities and
The maximum particle size of the non-metallic inclusions is 8 μm or less.
Excellent microstructure change delay characteristics due to repeated stress loading
Bearing steel.Record (Group I)  Si: 0.05-0.5 wt%, Mn: 0.05-2.0 wt%, Ni: 0.05-
1.0 wt%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt%,
Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%(Group II) Si: more than 0.5 to 2.5 wt%, Ni: more than 1.0 to 3.0 wt%, N: 0.0
More than 12 to 0.050 wt%, V: 0.05 to 1.0 wt%, Nb: 0.05 to 1.
0 wt%, W: 0.05-1.0 wt%, Zr: 0.02-0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and Co: 0.05-
1.5 wt%

[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 materials to which a large amount of Mo was added (C: 1.00 wt%, Si: 0.20 wt%, Mn: 0.42 wt%, M
o: 0.80 wt%, O: 0.0007 wt%, N: 0.0032 wt%) (C: 1.00 wt%, Si: 0.25 wt%, Mn: 0.43 wt%, M
o: 0.79 wt%, O: 0.0042 wt%, N: 0.0040 wt%) (C: 1.03 wt%, Si: 0.21 wt%, Mn: 0.41 wt%, M
o: 1.52 wt%, O: 0.0008 wt%, N: 0.0035 wt%) (C: 0.98 wt%, Si: 0.22 wt%, Mn: 0.42 wt%, M
o: 1.48 wt%, O: 0.0018 wt%, N: 0.0038 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 46,500 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 Mo in a large amount, although improvement of the B ₁₀ value is small,
The _B50 value shows a considerably high value, indicating that the value is significantly improved. That is, the bearing life expectancy showed improvement as about 7 times at about 2-fold, B ₅₀ value B ₁₀ value compared to SUJ 2. On the other hand, when the maximum grain size of the nonmetallic inclusions was controlled together with the addition of a large amount of Mo, the remarkable improvement effect on the delay characteristics of the microstructure change generated during high-load rolling was exhibited, and further, B _As shown in the ₁₀ values, an improvement effect on peeling due to nonmetallic inclusions was observed. Among them, the ₁₀ value B by high despite inclusions control in the steel oxygen content is excellent about 9 times, finer delay and inclusions microstructure changes acts on the improvement of the B ₁₀ value You can see that it is.

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the life of a bearing caused by the particle size of nonmetallic inclusions and the life caused by a change in microstructure. As is clear from this figure, the cumulative failure probability is 10%
The bearing life represented by B ₁₀ value (hereinafter referred to as "B ₁₀ rolling contact fatigue life"), although large effect can not be expected only by the addition of Mo in a large amount, the nonmetallic inclusions control In addition, the effect of the improvement is more remarkable.
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 effect of improvement by only Mo addition of a large amount regardless of the non-metallic inclusions control becomes very prominent, microstructure changes generated It turns out that it is effective in remarkably improving the bearing life under the environment.

Therefore, in the present invention, the range of the component composition as described below is determined mainly from the viewpoint of improving the microstructure change delay characteristic due to 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%, machinability and 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% or less Si is used as a deoxidizing agent when steel is melted, and is also dissolved in a matrix and tempered due to an increase in tempering resistance. It is effective as an element for increasing the strength after tempering and improving the rolling fatigue life. The content of Si added for such a purpose is in the range of 0.05 to 0.5 wt%. In addition, this Si
Addition of more than 0.5 wt% has an effect of delaying microstructure change under repeated stress load and improving rolling fatigue life. However, if the content exceeds 2.5 wt%, the effect is saturated and the workability and toughness are reduced. Therefore, in order to further improve the microstructure change delay characteristics, it is necessary to use more than 0.5 to 2.5 wt%. Is effective.

Mn: 0.05-2.0 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. For this purpose, Mn is added within the range of 0.05 to 2.0 wt%.

Ni: 0.05-1.0 wt%, more than 1.0-3.0 wt% Ni is used for the purpose of enhancing the strength after quenching and tempering by increasing the hardenability, improving the toughness, and improving the rolling fatigue life. Is added in the range of 0.05 to 1.0 wt%. Furthermore, when this Ni is added in excess of 1.0 wt%, the microstructure change during rolling is delayed, thereby improving the rolling fatigue life. However, even in this case, if it is added in excess of 3 wt%, a large amount of residual γ is precipitated, which lowers strength and impairs dimensional stability.
If this effect is expected, it is necessary to add in a range of more than 1.0 to 3.0 wt% because the cost is increased.

Mo: more than 0.5 to 2.0 wt% Mo is an element which basically improves the wear resistance by stabilizing the residual carbides, and particularly increases the hardenability to improve the strength after quenching and tempering. While contributing, precipitation of stable carbides improves wear resistance and rolling fatigue life. However, in the present invention, Mo plays a more important role, and particularly when a large amount of Mo is added in an amount exceeding 0.5 wt%, the effect of delaying the microstructure change due to the load of the repetitive stress described above is remarkable. This improves the rolling fatigue life in this aspect. But the amount
If the content exceeds 2.0 wt%, the machinability and forgeability are reduced and the cost is increased. Therefore, it is necessary to add the content within the range of more than 0.5 to 2.0 wt%.

Cu: 0.05 to 1.0 wt% Cu improves rolling fatigue life by increasing the strength after quenching and tempering by increasing quenching. This effect becomes remarkable when 0.05 wt% is added, and when it exceeds 1.0 wt%, the effect is saturated, so that it is added in the range of 0.05 to 1.0 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%, 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 contact 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 (Mo and Si, Mn, Ni, Ni, 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. Although the reasons for limitation of Cu, Al, B, N) and C, P, S have been described, the present invention further provides any one selected from V, Nb, W, Zr, Ta, Hf and Co or By adding two or more kinds, the rolling fatigue life under a high load may be improved.

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, 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 types of 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.

[0031]

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.

[0032]

[Table 3]

[0033]

[Table 4]

As apparent from the results shown in Tables 3 and 4,
Steel No. amount in the steel C is outside the range present invention, or B ₅₀ rolling fatigue life of the steel No.6 amount in the steel Mo is outside the present invention is the same as the conventional steels (steel No.1) Rather bad. The maximum diameter inclusions in No.4 exceeds 8 [mu] m, B ₁₀ rolling fatigue life resulted poor. In contrast, B ₁₀ value of the steel No.6 and 7 a first invention steel, B ₅₀ values are all excellent 5-7 times compared with the conventional steels (steel No.1). In other words, it can be seen that the addition of a large amount of Mo to the bearing steel significantly delayed the microstructural change, and effectively controlled the maximum rolling fatigue life of the bearing by controlling the maximum diameter of the inclusions.

Among them, in addition to Mo, Si, W, V, Nb, Zr,
Ta, Hf, Co, when taken alone added and their combined addition example (third invention steels) No. 18 to 28 of the N, said life expectancy (B ₅₀ rolling fatigue life) is to further improve Was confirmed.

Further, the second invention example in which the addition of improved lifetime components by increasing strength suit inclusion particle diameter control alone or combined to (No.9~17) is very well towards the B ₁₀ bearing life It showed a high degree of improvement. Further, in the case of the fourth invention steel in which all the life improving components are selectively added, the life improving tendency becomes more remarkable.

[0037]

As described above, according to the present invention,
Basically, by using a high Mo content bearing steel of more than 0.5 to 2.0 wt%, the rolling fatigue life can be improved by delaying the microstructure change due to repeated stress loading, and high Long life bearing steel 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 diameter of nonmetallic inclusions and the rolling contact fatigue life.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd. Technology Research Division (56) References JP-A-6-264188 JP-A-63-57749 (JP, A) JP-A-2-156045 (JP, A) JP-A-5-271866 (JP, A) JP-A-5-306432 (JP, A) (58) Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%
%, With the balance consisting of Fe and unavoidable impurities, and having a maximum particle size of oxide-based nonmetallic inclusions of 8 μm or less, and having excellent characteristics of delaying microstructure change due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0
wt.%, Si: 0.05-0.5 wt%, Mn: 0.
05-2.0 wt%, Ni: 0.05-1.0 wt%, Cu: 0.05-1.0 w
t%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: any one selected from 0.0005 to 0.012 wt%
Or two or more, the balance being Fe and unavoidable impurities, and the maximum particle size of the oxide-based nonmetallic inclusions is 8
Bearing steel excellent in delay characteristics of microstructure change due to repeated stress loading of less than μm. (3) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0
wt%, Si: more than 0.5 to 2.5 wt%, Ni: 1.0
Ultra-3.0 wt%, N: 0.012 Ultra-0.050 wt%, V: 0.05-
1.0 wt%, 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 wt%, and Co: one or more selected from 0.05-1.5 wt%, the balance being 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%, Mo: more than 0.5 to 2.0 wt%
%Ingredients of the following (Group I)Choose from
Including one or more of the above,
Ingredients of the following (Group II) (however, the elements selected in Group I
Excluding prime)Any one or two selected from
Including the above, the balance consists of Fe and unavoidable impurities,
And the maximum particle size of the oxide-based nonmetallic inclusion is 8 μm or less.
Characteristics of microstructure change due to repeated stress loading
Excellent bearing steel.Record (Group I)  Si: 0.05-0.5 wt%, Mn: 0.05-2.0 wt%, Ni: 0.05-
1.0 wt%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt%,
Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%(Group II) Si: more than 0.5 to 2.5 wt%, Ni: more than 1.0 to 3.0 wt%, N: 0.0
More than 12 to 0.050 wt%, V: 0.05 to 1.0 wt%, Nb: 0.05 to 1.
0 wt%, W: 0.05-1.0 wt%, Zr: 0.02-0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and Co: 0.05-
1.5 wt%