JP3233719B2 - 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 to a rolling contact by a repetitive stress load by adding a large amount of Mo without containing Cr. We propose a bearing steel with improved delay characteristics against microstructural changes (deterioration) occurring below the surface.

[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 Mo was added, the above-described microstructure change generated below the rolling contact surface due to repeated stress load could be significantly delayed, and the bearing life could 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%, Mo: more than 0.5 to 2.0 wt%, O: 0.002
Bearing steel containing 0 wt% or less, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, O: 0.0020 w
t% or less, Si: 0.05 to 0.5 wt%, Mn: 0.0
5 ~ 2.0wt%, Ni: 0.05 ~ 1.0wt%, Cu: 0.05 ~ 1.0wt%,
B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N:
Bearing steel containing one or more selected from 0.0005 to 0.012 wt%, the balance being Fe and unavoidable impurities, and excellent in delay characteristics of microstructure change due to repeated stress loading (second invention) ). (3) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, O: 0.0020 w
t: up to 2.5 wt%, Ni: 1.0
More than 3.0wt%, N: more than 0.012 to 0.050wt%, V: 0.05 to 1.0wt
%, 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: 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 microstructure change delay characteristics due to repeated stress loading (No. 3 inventions). (4) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, O: 0.0020 w
t% or less, and further contains any one or more selected from the following components (Group I) , and further contains the following components (II) (provided that they are selected from Group I) hand
Are elements 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%, Ni: 0.05 to 1.0 w
t%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt%, Al: 0.0
05 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: more than 0.012 to 0.050 wt%, V: 0.05 to 1.0 wt% %, Nb: 0.05 ~ 1.0wt%,
W: 0.05-1.0 wt%, Zr: 0.02-0.5 wt%, Ta: 0.02-0.5 w
t%, 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%, Ni: 0.0040wt%, O: 0.0012wt%)
And two kinds of materials that are chromium-free while adding Mo (C: 1.00 wt%,, Si: 0.20 wt%, Mn: 0.42 wt%,
Mo: 0.80 wt%, O: 7 ppm, N: 32 ppm) (C: 1.03 wt%,, Si: 0.21 wt%, Mn: 0.41 wt%,
A test steel material (Mo: 1.52 wt%, O: 8 ppm, N: 35 ppm) was prepared. 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 Mo additives, 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, that exhibit improved as about 7 times B ₅₀ values were observed compared to. In particular, Mo
The addition of a large amount of has a remarkable effect on the delay characteristics of the microstructure change generated during high-load rolling, and can be expected to delay the breakage (life) by that much.

[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 the addition of Mo 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 Mo 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: 0.05-2.0 wt% Mn is an element used as a deoxidizing agent at the time of smelting steel and is an element contributing to reducing oxygen in steel. Further, the toughness of the base martensite by improving the hardenability of the steel, in addition to improving hardness, effectively acts on the improvement of B ₁₀ rolling contact fatigue life. However, if the amount of addition is less than 0.05 wt%, the effect is not exhibited, and if it exceeds 2.0 wt%, the machinability and forgeability are remarkably reduced, so the content was limited to the range of 0.05 to 2.0 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.

Mo: more than 0.5 to 2.0 wt% Mo is an element that plays the most important role in the present invention. In particular, when Mo is added in a large amount exceeding 0.5 wt%, high load conversion is achieved. The effect of delaying the microstructure change caused by the load of the repetitive stress accompanying the movement becomes remarkable, and the rolling fatigue life caused by the microstructure change is improved. However, if the addition amount exceeds 2.0 wt%, the machinability and forgeability are reduced and the cost is increased. Therefore, it is necessary to add it in the range of more than 0.5 to 2.0 wt%.

Ni: 0.05-1.0 wt%, more than 1.0-3.0 wt% Ni is used for the purpose of increasing the hardenability and increasing the strength after quenching and tempering to improve the toughness and 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 the strength and impairs the dimensional stability and increases the cost. , More than 1.0 to 3.0 wt%.

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. 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 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) for improving the rolling fatigue life by delaying the microstructural change generated by the repeated stress load and improving the rolling fatigue life by increasing the strength. , Ni, Cu,
B, Al, N) and the reasons for limiting C, P, and S have been described. However, in the present invention, any one or two of V, Nb, W, Zr, Ta, Hf, and Co are further selected. By adding the above, the rolling fatigue life under a high 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.

[Table 2]

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

[0030]

EXAMPLES Steels having the component compositions 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. Then, normalizing-
Heat treatment was performed in the order of spheroidizing annealing-quenching-tempering, and a cylindrical rolling fatigue life test specimen of 12 mmφ × 22 mm was prepared by lapping. Then, for each test piece were 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: 60
The test was performed under the conditions of 0 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.
The average life of 2) (the total number of loads until the occurrence of peeling at a cumulative failure probability of 50%) was set to 1, and evaluations were made in comparison with those of other steel types. The evaluation results are shown in Tables 3, 4, and 5, respectively.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

As is evident from the results shown in Tables 3, 4 and 5, steel No. 2 and steel No.
The average lifespan of the steel materials No. 3, 5 whose Mo content is outside the range of the present invention and the steel material No. 4 whose O content is outside the range of the present invention are both lower than those of the conventional steel (steel No. 1). . On the other hand, the average life of steel Nos. 6 to 46 of the present invention steel was
It is 3.1 to 15.4 times better than o.1). That is, it can be seen that the addition of Mo to the bearing steel significantly delayed the microstructural change, and as a result, effectively acted on the improvement of the rolling fatigue life.

Particularly, in the case of steels Nos. 34 to 46 to 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 together, the above average life (B ₅₀
Rolling fatigue life) showed a life ratio of at least 7.6 times.

[0036]

As described above, according to the present invention,
Basically, by using a high Mo content bearing steel with Mo: more than 0.5 wt%, the rolling fatigue life can be improved by delaying the microstructure change due to repeated stress loading,
Long life steel for bearings can be provided. Therefore,
Necessary for further reduction of oxygen content in steel or control of composition, shape, and distribution of oxide-based nonmetallic inclusions present in steel, which were indispensable under the conventional technology No improvement or construction of steelmaking equipment is required. 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 effect of Mo on bearing life caused by inclusions and bearing life caused by microstructure change.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd. (56) References JP-A-63-57749 (JP, A) 2-156045 (JP, A) JP-A-5-271866 (JP, A) JP-A-5-306432 (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%, Mo: more than 0.5 to 2.0
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%, Mo: more than 0.5 to 2.0
wt%, O: 0.0020 wt% or less, and further, 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%, A
l: Delay of microstructure change due to repeated stress loading, including one or more selected from among 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%, with the balance being Fe and unavoidable impurities. Bearing steel with excellent properties. (3) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0
wt%, O: 0.0020 wt% or less, 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: 0.05-1.0 wt%, Nb: 0.05-1.0 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-
Bearing steel containing one or more selected from 1.5 wt%, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%,
O: 0.0020 wt% or less,The following (group I)
componentAny one or two or more selected from
Including, and also,Components of the following (Group II) (however, Group I
Excluding the element selected in)Which one to choose from
Including one or two or more, the balance being Fe and inevitable
Microstructure due to repeated stress loading composed of chemical impurities
Bearing steel with excellent change delay characteristics. Record(Group I)  Si: 0.05-0.5 wt%, Mn: 0.05-2.0 wt%, Ni: 0.05-1.0 w
t%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt%, Al: 0.00
5 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: more than 0.012
0.050 wt%, V: 0.05-1.0 wt%, Nb: 0.05-1.0 wt%, W: 0.
05-1.0wt%, Zr: 0.02-0.5wt%, Ta: 0.02-0.5wt%, H
f: 0.02-0.5 wt% and Co: 0.05-1.5 wt%