JP3243322B2 - 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 under a rolling contact surface caused by repeated stress load by adding a large amount of Ni. 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 nonmetallic inclusions under the conventional technology has a great effect on improving the bearing rolling contact fatigue life, especially under severe conditions such as high load or high temperature. Because I experienced many things that I couldn't get. 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, the increase in surface pressure (miniaturization) and the increase in operating temperature shorten the time required for these microstructural changes to occur, which leads to a significant reduction in bearing life. I found it. In other words, the life of the bearing under severe conditions is not sufficient just by controlling the decarburized layer and the nonmetallic inclusions as in the prior art. For example, simply reducing the amount of non-metallic inclusions cannot delay the time required for the above-described microstructural change to occur under the rolling contact surface. As a result, they found that the bearing life could not be further improved.

Accordingly, an object of the present invention is to provide a bearing steel capable of delaying a change in microstructure which is expected to occur during use of a bearing under severe operating conditions, and consequently significantly improving the life of the bearing. Is to do.

[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 found that surprisingly, if a large amount of Ni was added, the above-mentioned microstructure change generated below the rolling contact surface due to repeated stress load could be significantly delayed, and thus 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%, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, O: 0.0020 wt% or less, the balance being Fe and inevitable (1st invention) which is excellent in the delay characteristic of the microstructure change by repetitive stress load which consists of a temporary impurity. (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%, O: 0.0020 wt% or less, and further 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 to 0.01 wt%, Al: 0.005 to 0.07 wt%, and N: 0.0005 to 0.012 wt% A bearing steel containing any one or two or more thereof, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading (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%, O: 0.0020 wt% or less, and N: 0.012 More than 0.050 wt%, Zr: 0.02-0.5 wt%, Ta: 0.02-0.5 wt%, Hf: 0.02-0.5 wt%, and Co: 0.05-1.5 wt% A bearing steel comprising a balance of Fe and unavoidable impurities and having excellent microstructure change delay characteristics due to repeated stress load (third invention). (4) C: 0.5~1.5 wt% , Si: 0.5 ~2.5 wt%, Mn: 0.5 ~2.0 wt%, Ni: 2.00 ~3.0 wt%, O: contains less 0.0020%, furthermore, 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 to 0.01 wt%, Al: 0.005 to 0.07 wt%, and N: 0.0005 to 0.012 wt% N: over 0.012 to 0.050 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, and Co: 0.05 Bearing steel containing at least one selected from the group consisting of -1.5 wt%, and the balance consisting of Fe and unavoidable impurities, and having excellent delay characteristics of microstructure change due to repeated stress loading (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 two kinds of materials to which Ni is added (C: 1.00 wt%, Si: 1.05 wt%, Mn: 0.71 wt%,
Cr: 1.30 wt%, Ni: 1.2 wt%, O: 0.0008 wt%, N: 0.0
046wt%) (C: 1.00wt%,, Si: 1.02wt%, Mn: 0.65wt%,
Cr: 1.31 wt%, Ni: 2.5 wt%, O: 0.0007 wt%, N: 0.0
048 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.
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 Ni 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 3 times B ₁₀ value, to exhibit improved by about 30 times B ₅₀ values were observed compared to. In particular, the addition of a large amount of Ni has a remarkable effect on the delay characteristics of microstructure change generated during high-load rolling, and the damage (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 the addition of Ni 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 Ni becomes extremely significant, harsh 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: 0.5 to 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. Moreover, this Si has a
Fatigue life due to delay of microstructure change under
Has the effect of improving. 0.5 for these effects
More than wt% is required. However, if it exceeds 2.5 wt%,
Of the workability and toughness while the effect of
In the range of 0.5 to 2.5 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.

Mn: 0.5 to 2.0 wt% Mn basically acts as a deoxidizing agent when smelting steel and is an element effective in reducing oxygen in steel. In addition, it improves the toughness and hardness of the base martensite by improving the hardenability of steel, delays the microstructure change due to repeated stress loading, and effectively works to improve the rolling fatigue life. % Or more. However, if the addition exceeds 2.0 wt%, a large amount of residual γ is generated, which decreases the strength and dimensional stability and impairs the machinability.
Limit to wt% range.

Cr: 0.05-Less than 1.0wt%  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 0.05-Less than 1.0wt%Need to be added.

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: 2.00 to 3.0 wt% Ni is an element which plays the most important role in the present invention. In particular, when this Ni is contained in excess of 2.00 wt% or more , high load rolling occurs. It promotes the delay of the above-mentioned microstructure change under repeated stress load of time, and significantly improves the B50 rolling fatigue life. However, even in this case, 3wt%
If the amount of Ni is too large, the amount of residual γ will be precipitated in a large amount to lower the strength and impair the dimensional stability and increase 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.

Cu: 0.05 to 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, an addition of 0.05-1.0 wt% is necessary.

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. These effects cannot be obtained at less than 0.005 wt%. On the other hand, 0.07wt%
If the addition exceeds 3, the above-mentioned effects are saturated. Therefore,
Al is limited to 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%, 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 (Ni, Si and Mn, Cr, and Cr) 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, B, Al,
(N, O) and the reasons for limiting C, P, and S have been described. However, in the present invention, the addition of one or more selected from Zr, Ta, Hf, and Co further improves the efficiency. The rolling fatigue life under 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,
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.

[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. 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: 600 kgf / mm
^{2. The} test was performed under the conditions of a repetitive stress number of about 46500 cpm. The test results are summarized on probability paper assuming that they follow the Weibull distribution, and the average life of steel No. 1 (conventional steel, SUJ2) (cumulative failure probability: 50%, the total number of loads up to the occurrence of peeling) is 1, and other Of steel types were evaluated. The evaluation results are also 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. 4 in which C content in steel is out of the range of the present invention, Steel Nos. 5 and 6 in which Ni content in steel is out of the range of the present invention, and Steel No. 7 in which O content in steel is out of the range of the present invention The average lifespans are lower than those of conventional steel (steel No. 1). On the other hand, the average life of steel material No. 10 , which is the steel of the present invention, is about 13 times better than the conventional steel (steel material No. 1). That is, it can be seen that the addition of Ni to the bearing steel significantly delayed the microstructural change, and as a result, effectively acted on the improvement of the rolling fatigue life.

Incidentally , in addition to Si, Mn, Ni, Cr, Mo,
In the case of steel that actively adds one or more of Cu, B, Al, N and W, V, Zr, Ta, Hf, Co, etc., which increase the strength or improve the rolling fatigue life by delaying microstructure change The average life (B50 rolling fatigue life) tends to be further improved.

[0036]

As described above, according to the present invention,
By using a bearing steel with a basic composition containing 2.00wt% or more of high Ni together with Si and Mn, the rolling fatigue life has been improved by delaying the microstructure change due to repeated stress loading, Long life steel for bearings 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 effect of Ni on bearing life caused by inclusions and bearing life caused by microstructure change.

────────────────────────────────────────────────── ─── Continued from 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-2-30733 (JP, A) JP-A-3-122255 (JP, A) JP-A-49-47212 (JP, A) (58) Fields investigated (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%, O: 0.0020 wt% or less, the balance being Fe Bearing steel that is excellent in delaying microstructural change due to repeated stress load and composed of unavoidable impurities. 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%, O: 0.0020 wt% or less, and 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 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% A bearing steel containing one or more selected ones, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading. 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%, O: 0.0020 wt% or less. : At least 0.012 to 0.050 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, and Co: 0.05 to 1.5 wt%. A bearing steel that contains more than one species, 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%, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.0 wt%, Ni: 2.00 to 3.0 wt%, O: 0.0020 wt% or less. : 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 to 0.01 wt%, Al: 0.005 to 0.07 wt%, and N: 0.0005 to 0.012 wt% And at least one selected from the group consisting of N: more than 0.012 to 0.050 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co : Bearing steel containing one or more selected from 0.05 to 1.5 wt%, the balance being Fe and unavoidable impurities, and having excellent characteristics of delaying microstructure change due to repeated stress loading.