JP3383349B2 - 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 delay for a microstructure change (deterioration) which occurs under a rolling contact surface due to repeated stress loads. We propose a bearing steel with excellent characteristics.

[0002]

2. Description of the Related Art Conventionally, high-carbon chromium bearing steel (JI
S: SUJ 2) is most often used. In general, bearing steel has one of the important properties that a long rolling contact fatigue life is important. As a factor that affects the rolling contact fatigue life, it is considered that the hard non-metallic inclusions in the steel have a large effect. Was being considered. Therefore, the mainstream of recent research has been to take measures to improve the bearing life by controlling the amount and size of non-metallic inclusions by reducing the amount of oxygen in steel.

For example, as one developed for the purpose of further improving the rolling contact fatigue life of a bearing, Japanese Patent Laid-Open No. 1-306542 has been proposed.
There are proposals such as Japanese Patent Laid-Open Publication No. 3-126839 and Japanese Patent Laid-Open Publication No. 3-126839, which are techniques for controlling the composition, shape, or distribution state of oxide-based nonmetallic inclusions in steel.

[0004]

However, in order to manufacture a bearing steel containing few non-metallic inclusions, it is essential to reduce the amount of oxygen in the steel, but this has already reached the limit, and expensive melting There is a problem in that it requires a large amount of financial burden because it requires installation of equipment or significant improvement of conventional equipment. Further, according to the recent research conducted by the present inventors,
The factors that determine the rolling life are phenomena that have been generally discussed in the past; that is, factors other than the presence of the “decarburized layer” (low C concentration region) and the above-mentioned “non-metallic inclusions” that occur during heat treatment. It turns out that there is. I mean,
Even if the decarburization layer and non-metallic inclusions are simply reduced under the conventional technology, a great effect can be obtained in improving the rolling contact fatigue life of the bearing, especially under severe conditions such as high load or high temperature. Because I experienced a lot of things that didn't happen. From this, we were convinced of the existence of other factors that control the bearing life.

Therefore, the present inventors investigated the cause of the separation of the rolling bearing. As a result, by repeating the shear stress generated during rolling contact between the inner and outer rings and rolling elements of the bearing, the lower layer portion of the rolling contact surface (surface layer portion), FIG. 1
As shown in (a), a microstructure-changed layer consisting of a strip-shaped white product and rod-shaped precipitates is formed, which gradually grows as the number of rolling increases, and at the end fatigue from this microstructure-changed part occurs. It was found that peeling (Fig. 1 (b)) occurs and this leads to bearing life. Furthermore, the harsh bearing operating environment, that is, high surface pressure (miniaturization) and increase in operating temperature, shortens the number of rolling cycles until these microstructural changes occur,
We have found that the conventional bearing steel SUJ2 will significantly reduce the bearing life. In other words, bearing life is
Controlling only the decarburized layer and non-metallic inclusions as in the prior art is not sufficient.For example, if the amount and size of non-metallic inclusions are simply reduced, it will occur under the rolling contact surface described above. It is not possible to delay the time until the microstructure change occurs. As a result, they have found that the bearing life cannot be further improved.

Therefore, an object of the present invention is to delay the microstructure change occurring during the use of the bearing under high load in order to improve the rolling contact fatigue life characteristics under severe operating conditions. The object is to provide a bearing steel capable of significantly improving the bearing life by suppressing the maximum grain size of non-metallic inclusions to be small.

[0007]

Based on the above-mentioned findings, the present inventors have newly focused on the "microstructure change delay characteristic" as a factor that controls the bearing life.
In order to improve these characteristics, of course, a new alloy design (ingredient composition) is necessary for that purpose, and with the recognition that it is impossible to further improve the life of the bearing without realizing this, Various experiments and studies were conducted. As a result, they have found that the addition of an appropriate amount of Si and Cr in combination can significantly delay the above-described microstructural change generated under the rolling contact surface due to repeated stress loading, and have conceived the present invention bearing steel.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, the balance consisting of Fe and unavoidable impurities, and maximum of oxide-based nonmetallic inclusions A bearing steel having a grain size of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, Mn: 0.05 to 2.0 wt%, Ni:
0.05-1.0wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01
wt%, Al: 0.005-0.07 wt% and N: 0.0005-0.012 wt%
A microstructure containing any one or more selected from among them, the balance consisting of Fe and unavoidable impurities, and having a maximum grain size of oxide-based nonmetallic inclusions of 8 μm or less, by repeated stress loading Bearing steel with excellent change delay characteristics (second invention). (3) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, Ni: more than 1.0 to 3.0 wt%, Z
r: 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% and any one or more selected from the group consisting of Fe and unavoidable impurities and the maximum particle size of oxide-based nonmetallic inclusions is 8 μm. The following bearing steels (third invention) which are excellent in delay characteristics of microstructure change due to repeated stress loading. (4) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, and any one or two selected from the following group I components In addition to the above, the following Group II components (provided that the elements selected in Group I are
(Excluding element) , and one or more selected from the group consisting of Fe and inevitable impurities.
A bearing steel having a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress load (4th invention). Serial group I: Mn: 0.05~2.0wt%, Ni: 0.05~1.0wt%, Cu: 0.
05-1.0wt%, B: 0.0005-0.01wt%, Al: 0.005-0.07
wt% and N: 0.0005 to 0.012 wt% Group II: Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt%, Ta:
0.02-0.5wt%, Hf: 0.02-0.5wt% and Co: 0.05-1.5w
t%

[0009]

The background to the idea of 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: 1.35 wt%, N: 0.0040 wt%, O: 0.0012 wt%)
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 in which Si and Cr are added in combination (C: 1.00 wt%, Si: 1.28 wt%, Mn: 0.46 wt%, C
r: 3.51wt%, O: 0.0009wt%, N: 0.0046wt%) (C: 1.02wt%, Si: 1.29wt%, Mn: 0.48wt%, C
r: 3.49wt%, O: 0.0037wt%, N: 0.0048wt%) (C: 1.00wt%, Si: 1.32wt%, Mn: 0.48wt%, C
r: 7.23wt%, O: 0.0008wt%, N: 0.0052wt%) (C: 1.02wt%, Si: 1.32wt%, Mn: 0.50wt%, C
r: 7.21 wt%, O: 0.0014 wt%, N: 0.0050 wt%) was prepared. Then, these test materials were subjected to normalizing treatment, spheroidizing normalizing treatment, quenching and tempering treatment, and 12 mmφ × 22 mm cylindrical test pieces were produced from the respective test materials.

Next, these test pieces were subjected to a Hertz maximum contact stress: 60 using a radial type rolling fatigue life tester.
A rolling fatigue life test was carried out under a load condition of 0 kgf / mm ² and a cyclic stress number of 46500 cpm. The test results are plotted on Weibull distribution establishment paper, and B ₁₀ (10% cumulative failure probability), which is considered to be a numerical value showing the improvement of rolling fatigue life due to the increase of material strength affected by the control of non-metallic inclusions, B ₅₀ (50% cumulative failure probability), which is considered to be a numerical value showing the improvement of rolling fatigue life by delaying the occurrence of microstructure change due to repeated stress load during high load rolling, was determined.

As a result, as shown in Table 1, in the case where Cr is added in a large amount and Si is contained without controlling inclusions, the improvement in B ₁₀ value is small, but _{As for the 50} value, it shows a considerably high value, and it can be seen that the value is significantly improved. That is, the bearing life is SUJ 2
Compared with the above, the improvement effect was about twice as much as the B ₁₀ value and about 30 times as much as the B ₅₀ value. On the other hand, when the maximum grain size of non-metallic inclusions is controlled together with the addition of Si and a large amount of Cr, a significant improvement effect is shown for the retardation property of the microstructure change generated during high load rolling. Further, as shown in the B ₁₀ value, the effect of improving peeling caused by non-metallic inclusions was recognized. Among them, the ₁₀ value B by high despite inclusions control in the steel oxygen content is excellent also about 38 times, finer delay the inclusion of microstructure change this B ₁₀
It can be seen that it is working to improve the value.

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the bearing life due to the particle size of non-metallic inclusions and the life due to microstructural changes. As is clear from this figure, the cumulative damage probability is 10% as in the past.
The bearing life indicated by the B ₁₀ value (hereinafter referred to as “B ₁₀ rolling contact fatigue life”) cannot be expected to have a great effect only by adding a large amount of Cr and containing Si.
The one in which non-metallic inclusion control is also performed shows a remarkable improvement effect. On the other hand, looking at the bearing life indicated by the B ₅₀ value with a cumulative damage probability of 50% (hereinafter referred to as “B ₅₀ high load rolling contact fatigue life”), Cr-Si has no relation to non-metallic inclusion control. It can be seen that the effect of the improvement becomes extremely remarkable only by adding the compound, and it is effective for significantly improving the bearing life under the microstructure change generation environment.

Therefore, in the present invention, the range of the composition of components as described below is determined mainly from the viewpoint of improving the microstructure change retarding property due to the repeated stress load.

C: 0.5 to 1.5 wt% C is an element which forms a solid solution in the matrix and effectively acts to strengthen the martensite, and in order to secure the strength after quenching and tempering and to improve the rolling fatigue life by it. Contained in. If the content is less than 0.5 wt%, such effects cannot be obtained. On the other hand, if it exceeds 1.5 wt%, machinability and forgeability will deteriorate, so it is limited to the range of 0.5 to 1.5 wt%.

Si: 1.0 to 2.5wt% Si is basically used as a deoxidizer during the melting of steel
In addition, quenching due to an increase in tempering softening resistance as a solid solution in the base
To improve the rolling fatigue life by increasing the strength after tempering.
Is effective as an element. However, in the present invention, this
Role of SiIsIt is important that if 1.0 wt% or more is added,
Has a delay in microstructural evolution under cyclic stress loading
This has the effect of improving rolling fatigue life. But,
If its content exceeds 2.5wt%, its effect will be saturated.
On the other hand, it reduces the workability and toughness, so the microstructure changes.
1.0 to 2.5 wt% is required to further improve the delay characteristics.
It is effective to add.

Mn: 0.05 to 2.0 wt% Mn is an element that acts as a deoxidizer during the melting of steel and is effective in reducing the oxygen content of steel. Also, by improving the hardenability of steel, it improves the toughness and hardness of the base martensite, and effectively acts to improve the rolling fatigue life. For this purpose, Mn is added within the range of 0.05 to 2.0 wt%.

Cr: More than 2.5 to 8.0 wt% Cr improves the hardenability and forms stable carbides.
It is a component that improves strength and wear resistance, and eventually improves rolling contact fatigue life. To obtain this effect, addition of 2.5 wt% is necessary . That is, Cr is 2.5
when added pressure beyond the wt% is allowed delays microstructure change caused by repeated stress load is an element effective for improving the rolling contact fatigue life in this area. The effect of Cr addition for this purpose is not only saturated when it exceeds 8.0 wt%, but rather causes a decrease in the amount of solid solution C during quenching, resulting in a decrease in strength. Therefore, Cr exceeds 2.5
Add within the range of 8.0 wt%.

Ni: 0.05 to 1.0 wt%, more than 1.0 to 3.0 wt% Ni increases the hardenability to enhance the strength after quenching and tempering, improve the toughness, and improve the rolling contact fatigue life. Is added in the range of 0.05 to 1.0 wt%. Further, this Ni, when added in excess of 1.0 wt%, delays the microstructure change during rolling, thereby improving rolling fatigue life. However, even in this case, if it is added in excess of 3 wt%, a large amount of residual γ will be deposited, resulting in a decrease in strength and a loss of dimensional stability.
If this effect is expected, it is necessary to add it in the range of more than 1.0 to 3.0 wt% because it increases the cost.

Cu: 0.05-1.0 wt% Cu improves rolling fatigue life by increasing quenching and strengthening after quenching and tempering. This effect becomes remarkable when 0.05 wt% is added, and becomes saturated when it exceeds 1.0 wt%, so 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 it increases the hardenability and thereby enhances the strength after quenching and tempering and improves the rolling contact fatigue life. However, if added in excess of 0.01 wt%, the workability deteriorates, so the range is limited to 0.0005 to 0.01 wt%.

Al: 0.005 to 0.07 wt% Al is used as a deoxidizer during the melting of steel, and at the same time,
Combines with N in the steel to refine the crystal grains and contribute to the improvement of the toughness of the steel. Further, it also effectively works to improve the rolling contact 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% N combines with the nitride-forming element to refine the crystal grains, and forms a solid solution in the matrix to increase the strength after quenching and tempering.
Improves rolling fatigue life. For this reason, N is 0.0005-
Add within 0.012 wt%.

P ≦ 0.025 wt% P is desirable because it lowers the toughness and rolling contact fatigue life of the steel, so it is desirable to be as low as possible.
It is 0.025 wt%.

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

As described above, the main components (Cr and Si, and then Mn, Ni, and Mn, Ni, to improve the rolling fatigue life by increasing the strength as well as to improve the rolling fatigue life by delaying the microstructural change due to repeated stress loading. Cu, Al, B,
N) and the reason for limiting C, P, S is explained,
In the present invention, one or more selected from Zr, Ta, Hf and Co may be added to improve the rolling fatigue life under high load.

The preferred range of addition of each element and the purpose of addition,
The reasons for limiting the upper limit and the lower limit are summarized in Table 2.

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P,
The addition of Sn, As, etc. does not hinder the retardation property due to the change in microstructure due to the repeated stress load, which is the object of the present invention, and the machinability can be easily improved. You may add according to it.

Next, in the present invention, in addition to the above-mentioned compositional limitation, the morphology (size) of the oxide-based nonmetallic inclusions in the steel is controlled, so that the above-mentioned B ₁₀ rolling contact fatigue life is mainly obtained. It was decided to further improve.

Therefore, first of all, the present inventors have made two kinds of materials having different amounts of oxide-based non-metallic inclusions and component compositions:
Using a high carbon chromium bearing steel (JIS-SUJ2) (A) and a bearing steel (B) with a composition within the above applicable range, the maximum diameter of oxide non-metallic inclusions in the steel and B ₁₀ rolling contact fatigue life I investigated the relationship with. As a result, as shown in FIG. 3, regardless of the amount or composition of oxide nonmetallic inclusions in the steel, when the maximum diameter of the nonmetallic inclusions exceeds 8 μm, the B ₁₀ rolling contact fatigue life is markedly reduced. From this, it is necessary for the bearing steel of the present invention to have a maximum grain size of 8 μm or less.

[0031]

[Examples] Steels having the compositions shown in Tables 3 and 4 were melted by a conventional method, and the obtained steel materials were diffusion annealed at 1240 ° C for 30 hours and then rolled into steel bars of 65 mmφ. Then, heat treatment was performed in the order of normalizing-spheroidizing annealing-quenching-tempering, and a 12 mmφ x 22 mm cylindrical rolling fatigue life test piece was prepared by lapping finish. The test for non-metallic inclusions is 400 times 8
The oxide-based non-metallic inclusions in the field of view of 00 were measured, the maximum diameter of the inclusions in each field of view was summarized on the Gumbel probability paper, and the extreme value equivalent to 50,000 mm ² was calculated. The maximum particle size of inclusions was used. The rolling fatigue life test was conducted using a radial type rolling fatigue life tester under the conditions of Hertz maximum contact stress: 600 kgf / mm ² and repeated stress number of about 46500 cpm. The test results are summarized on the probability paper as being in accordance with the Weibull distribution and the steel material No. 1 (JIS-SuJ
The average life of (2) (cumulative damage probability: total load count until peeling at 10% and 50%) was set to 1 and evaluated in comparison with other steel types. The evaluation results are also shown in Tables 3 and 4, respectively.

[0032]

[Table 3]

[0033]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Steel No. 6 having a C content outside the scope of the present invention and steel No. 7 having a Cr content outside the scope of the present invention have the same B ₅₀ rolling contact fatigue life as the conventional steel (steel No. 1). Or rather bad. Further, in No. 4 in which the maximum inclusion diameter exceeds 8 μm, the B ₁₀ rolling contact fatigue life was poor. Further, steel material No. 5 in which the Si content in the steel is out of the range of the present invention also has a slightly better rolling contact fatigue life, but the improvement is small at B ₁₀ life of 2.5 and B ₅₀ life of 2.8. On the other hand, the B ₁₀ value and B ₅₀ value of the steel materials No. 8 and 9, which are the first invention steels, are 25 to 40 times better than those of the conventional steel (steel material No. 1). That is, Si for bearing steel and
It can be seen that the combined addition of Cr significantly retarded the microstructural change and effectively controlled the improvement of all rolling contact fatigue life of the bearing by controlling the maximum diameter of inclusions.

In addition to Si and Cr, Mn, Ni and C
In the case of adding u, Al, B, Zr, Ta, Hf, Co, N alone or in combination with No. 10 to 36, even if the amount of Cr added is suppressed, the above-mentioned average bearing life (B ₁₀ , B ₅₀ rolling fatigue life)
Was confirmed to be quite high.

[0036]

As described above, according to the present invention,
Basically, by using 1.0 to 2.5 wt% of Cr as a bearing steel containing Si, the rolling fatigue life was improved by delaying the microstructural change due to repeated stress loading. In view of the above, it is possible to provide steel for bearings having a long life. Moreover, by increasing the material strength by controlling the particle size of the non-metallic inclusions, it is possible to improve the rolling fatigue life in this aspect. The development of the bearing steel according to the present invention makes it possible to downsize the rolling bearing and further increase the bearing operating temperature.

[Brief description of drawings]

1 (a) and 1 (b) are under cyclic stress loading,
A micrograph of the metal structure showing the appearance of the microstructure change that occurs.

FIG. 2 is an explanatory diagram showing the influence of Si—Cr on the bearing life due to inclusions and the bearing life due to microstructural changes.

FIG. 3 is a graph showing the relationship between the maximum diameter of non-metallic inclusions and bearing rolling fatigue life.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd. Technical Research Division (56) Reference JP-A-55-145158 (JP, A) Kaihei 3-122255 (JP, A) JP-A 2-156045 (JP, A) JP-A-3-56640 (JP, A) JP-A 63-62847 (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: 1.0 to 2.5 wt%, C
r: containing more than 2.5 to 8.0 wt%, the balance consisting of Fe and unavoidable impurities, and the maximum grain size of oxide-based non-metallic inclusions is 8 μm or less, for delaying microstructural change due to repeated stress loading Excellent bearing steel. 2. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, C
r: more than 2.5-8.0wt%, Mn: 0.05-2.0wt%
%, Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.00
05-0.01wt%, Al: 0.005-0.07wt% and N: 0.0005-
0.012 wt% of any one or more selected from the rest, the balance is Fe and inevitable impurities,
A bearing steel having a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less and excellent in retarding microstructural changes due to repeated stress loading. 3. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, C
r: more than 2.5 to 8.0 wt%, and Ni: more than 1.0 to 3.0w
t%, Zr: 0.02-0.5wt%, Ta: 0.02-0.5wt%, Hf: 0.0
2 to 0.5 wt% and Co: 0.05 to 1.5 wt% and one or more selected from the rest, the balance consisting of Fe and inevitable impurities, and the maximum grain size of oxide-based non-metallic inclusions Bearing steel with a diameter of 8 μm or less and excellent in delay characteristics of microstructural changes due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, C
r: more than 2.5 to 8.0 wt% and further contains any one or more selected from the following group I components , and further contains the following group II components (provided that I: Selected in groups
The element has a maximum particle size of 8 μm, and the balance consists of Fe and inevitable impurities, and the oxide-based non-metallic inclusions have a maximum particle size of 8 μm.
The following bearing steels have excellent delay characteristics for microstructural changes due to repeated stress loading. Serial group I: Mn: 0.05~2.0wt%, Ni: 0.05~1.0wt%, Cu: 0.
05-1.0wt%, B: 0.0005-0.01wt%, Al: 0.005-0.07
wt% and N: 0.0005 to 0.012 wt% Group II: Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt%, Ta:
0.02-0.5wt%, Hf: 0.02-0.5wt% and Co: 0.05-1.5w
t%