JP3383351B2 - 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]

The inventors of the present invention have newly focused on the "microstructure change delay characteristic" as a factor that determines the bearing life based on the above-mentioned findings. 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 inclusion of a large amount of Mn 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%, Mn: more than 2.0 to 5.0 wt%, the balance consisting of Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions is 8 μm or less, Bearing steel excellent in delay characteristics of microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, Mn: more than 2.0 to 5.0 wt%,
Further, Si: 0.05 to 0.5 wt%, Cr: 0.05 to 2.5 wt%, Mo:
0.05-0.5wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01
wt%及 Beauty N: 0.0005~0.012wt comprise one or any two or more selected from among%, the balance being Fe and unavoidable impurities, and a maximum particle diameter of the oxide-based nonmetallic inclusions Bearing steel of 8 μm or less, which is excellent in delay characteristics of microstructure change due to repeated stress load (second invention). (3) C: 0.5 to 1.5 wt%, Mn: more than 2.0 to 5.0 wt%,
Furthermore , Si: more than 0.5 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%, M
o: more than 0.5 to 2.0 wt%, N: more than 0.012 to 0.050 wt%, V: 0.0
5 to 1.0 wt%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%,
Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.
5 wt%及 Beauty Co: 0.05~1.5wt comprise one or any two or more selected from among%, the balance being Fe and unavoidable impurities, and a maximum particle diameter of the oxide-based nonmetallic inclusions Bearing steel of 8 μm or less, which is excellent in delay characteristics of microstructure change due to repeated stress load (third invention). (4) C: 0.5 to 1.5 wt%, Mn: more than 2.0 to 5.0 wt%,
Furthermore , any one selected from the following group I components
Ingredients or two or more of them
However, the maximum grain size of the oxide-based non-metallic inclusions, which comprises one or more selected from the group (excluding elements selected from Group I) , the balance being Fe and unavoidable impurities Bearing steel having a diameter of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress load (4th invention). Serial group I: Si: 0.05~0.5wt%, Cr: 0.05~2.5wt%, Mo: 0.
05-0.5wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01wt
%及 Beauty N: 0.0005~0.012wt% II group: Si: 0.5 super ~2.5wt%, Cr: 2.5 super ~8.0wt%, Mo:
Over 0.5 ~ 2.0wt%, N: over 0.012 ~ 0.050wt%, V: 0.05 ~
1.0wt%, Nb: 0.05 to 1.0wt%, W: 0.05 to 1.0wt%, 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%

[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: 1.35wt%, Ni: 0.0040wt%, O: 0.0012wt%)
And SUJ 2 (C: 1.01wt%, Si: 0.24wt%, Mn: 0.46wt
%, Cr: 1.32wt%, Ni: 0.0042wt%, O: 0.0015wt%)
And two kinds of materials added with a large amount of Al (C: 1.01 wt%,, Si: 0.21 wt%, Mn: 2.05 wt%,
Cr: 1.30wt%, O: 0.0010wt%, N: 0.0040wt%) (C: 0.99wt%,, Si: 0.28wt%, Mn: 2.10wt%,
Cr: 1.30 wt%, O: 0.0033 wt%, N: 0.0048 wt%) (C: 0.98 wt%,, Si: 0.25 wt%, Mn: 4.22 wt%,
Cr: 1.32wt%, O: 0.0011wt%, N: 0.0040wt%) (C: 0.98wt%, Si: 0.30wt%, Mn: 4.30wt%,
Cr: 1.33 wt%, O: 0.0017 wt%, N: 0.0050 wt%) was prepared. Then, these test materials were subjected to normalizing treatment, spheroidizing annealing and quenching and tempering treatment, and then 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 of simply adding a large amount of Mn without controlling inclusions, the improvement in the B ₁₀ value was small, but the B ₅₀ value was small. It can be seen that the numerical value is considerably high and that it is remarkably improved. That is, the average bearing life is SUJ 2
The B ₁₀ value showed an improvement effect of about 4 times and the B ₅₀ value showed an improvement effect of about 18 times. On the other hand, when the maximum grain size of non-metallic inclusions is controlled together with the addition of a large amount of Mn, a significant improvement effect on the retardation property of the microstructure change generated during high-load rolling is exhibited, and B _As shown in the _10- value, the effect of improving the peeling caused by the non-metallic inclusions was recognized. Above all, the B ₁₀ value is about 14 times better by controlling inclusions despite the high oxygen content in the steel.
It can be seen that the delay of microstructure change and the refinement of inclusions act to improve the B ₁₀ 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 Mn, but it is possible to control non-metallic inclusions. The results of the above also show a remarkable improvement effect.
On the other hand, bearing life indicated by B ₅₀ value with cumulative damage probability of 50%
(Hereinafter, this is referred to as “B ₅₀ high load rolling contact fatigue life”), the effect of improvement is extremely remarkable even if only a large amount of Mn is added, regardless of the control of non-metallic inclusions, and microstructural change generation occurs. It can be seen that it is effective in significantly improving the bearing life under the 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: 0.05 to 0.5 wt%, more than 0.5 to 2.5wt% Si is used as a deoxidizer during the melting of steel and also as a base material.
Quenching and tempering due to an increase in softening resistance
As an element that increases the strength after rolling and improves rolling contact fatigue life
Is effective. Containing Si added for these purposes
The amount is in the range of 0.05 to 0.5 wt%. Also, this Si is
0.5over wt%Is added, the stress is increased under repeated stress loading.
Improves rolling contact fatigue life by delaying the change in microstructure
Has the effect of However, its content exceeds 2.5 wt%
When the effect is saturated, the workability and toughness decrease.
As a result, it is possible to further improve the microstructure change delay characteristics.
For this reason, it is effective to add more than 0.5 to 2.5 wt%.
It

Mn: over 2.0 to 5.0 wt% Mn is an element that basically acts as a deoxidizing agent during the melting of steel and effectively acts to reduce the oxygen content of steel. Further, by improving the hardenability of steel, the toughness of martensitic matrix,
By improving the hardness, it effectively acts to improve the rolling contact fatigue life. However, in the present invention, this Mn has a more important role, and particularly when it is added in a large amount such that its addition amount exceeds 2.0 wt%, it has an effect of remarkably delaying the microstructure change due to the load of cyclic stress. Improve the rolling contact fatigue life in this aspect. However, if the amount exceeds 5.0 wt%, a large amount of residual γ will be generated and the strength and dimensional stability will decrease.
Add in the range of.

Cr: 0.05 to 2.5 wt%, 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 0.05 to 2.5 wt% is sufficient. Furthermore, when Cr is added in a large amount exceeding 2.5 wt%, it is effective in delaying the microstructure change due to repeated stress loading and improving the rolling fatigue life in this aspect. 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, when it is added for this purpose, it must be more than 2.5 to 8.0 wt%.

Mo: 0.05-0.5 wt%, more than 0.5-2.0 wt% Mo is an element that improves wear resistance by stabilizing residual carbides. In particular, the addition of 0.05 to 0.5 wt% increases the hardenability and contributes to the improvement of the strength after quenching and tempering, and the precipitation of stable carbide improves the wear resistance and rolling fatigue life. Furthermore, when Mo is added in a large amount of more than 0.5 wt%, the effect of delaying the microstructure change during rolling becomes remarkable, and the rolling fatigue life in this aspect is improved. However, if the amount exceeds 2.0 wt%, machinability,
Forgeability is deteriorated and it causes a cost increase. Therefore, for this purpose, it is necessary to add it in the range of more than 0.5 to 2.0 wt%.

Cu: 0.05-1.0 wt% Cu enhances the strength after quenching and tempering due to the increase in quenching, and thereby improves the rolling fatigue life.
This action effect becomes remarkable with the addition of 0.05 wt%, and 1.0 wt%
Since it will be saturated when it exceeds 0.1%, 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%.

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 to form a solid solution in the matrix to enhance the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
Add within 0.012 wt%. Also, this N is 0.
When added in excess of 012 wt%, rolling fatigue life is improved by delaying microstructural change due to repeated stress. However, if the amount exceeds 0.05 wt%, the workability decreases, so for this purpose it exceeds 0.012 to 0.
Add 05wt%.

P ≦ 0.025 wt% P is desired to be as low as possible because it lowers the toughness and rolling contact fatigue life of steel, and the upper limit of its allowable limit is
It is 0.025 wt%.

S ≦ 0.025 wt% S combines with Mn to form MnS and improves 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, main components (Mn and Si, Cr, Mo, for improving rolling fatigue life by delaying microstructure change due to repeated stress loading and improving rolling fatigue life through increase in strength). The reason for limiting Cu, B, N) and C, P, S has been described, but the present invention further includes any one or two selected from V, Nb, W, Zr, Ta, Hf and Co. By adding the above, the rolling fatigue life under high load may be improved.

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 non-metallic 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 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.

[0030]

[Examples] Steels having the chemical compositions shown in Tables 3, 4 and 5 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 normalize-
Heat treatment was performed in the order of spheroidizing annealing-quenching-tempering, and 12 mmφ x 22 mm cylindrical rolling fatigue life test pieces were prepared by lapping finish. The test for non-metallic inclusions is 4
Measure oxide-based non-metallic inclusions in 800 fields at 00 times and summarize the maximum size of inclusions in each field on Gumbel probability paper.
An extreme value equivalent to mm ² was calculated and used as the maximum particle size of oxide-based nonmetallic inclusions present in the steel. In addition, the rolling fatigue life test
Using a radial type rolling fatigue life tester, Hertz maximum contact stress: 600 kgf / mm ² , cyclic stress number about 46500
It went under the condition of cpm. The test results are summarized on the probability paper as being in accordance with the Weibull distribution, and the steel material No. 1 (conventional steel
JIS-SuJ2) average life (cumulative damage probability: 10% and 50
%, The total number of loads until peeling occurs) is 1, and
It is evaluated by comparing other steel types. The evaluation results are also shown in Tables 3 and 4, respectively.

[0031]

[Table 3]

[0032]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Steel No. 5 with a C content outside the scope of the present invention and steel No. 6 with a Mn content outside the scope of the present invention have the same B ₅₀ rolling contact fatigue life as the conventional steel (steel No. 1). Bad compared to. Further, in No. 4 in which the maximum inclusion diameter exceeds 8 μm, the B ₁₀ rolling contact fatigue life is worse. On the other hand, the B ₁₀ and B ₅₀ values of the steel materials No. 7 and 8 of the present invention (first invention) are about 2 to 4 times better than those of the conventional steel (steel material No. 1). ing. That is, it can be seen that the addition of a large amount of Mn alone to the bearing steel significantly retarded the microstructural change, and effectively controlled all the rolling fatigue life of the bearing by controlling the maximum inclusion diameter.

Among them, addition of Mn and Sb, addition of Si, Cr,
In the case of adding Mo, W, V, Nb, Zr, Ta, Hf, Co, N alone and their compounding examples (third invention steel) No. 17 to 29, the above average life (B ₅₀ rolling It was confirmed that the fatigue life) was further improved.

In addition, N is obtained by adding a life-improving component for increasing the strength alone or in combination in accordance with the control of the inclusion particle size.
Nos. 9 to 16 (Examples of the second invention) showed a much higher degree of improvement in the B ₁₀ bearing life. Furthermore, No. 3 is obtained by selectively adding all the life improving components recommended in the present invention.
In the case of 0 to 42 (4th invention steel), the life improvement tendency is B ₁₀ , B.
₅₀ is even more prominent.

[0036]

As described above, according to the present invention,
Basically, by using a bearing steel containing more than 2.0 to 5.0 wt% Mn, the rolling fatigue life is improved by delaying the microstructural change due to repeated stress loading, and in this aspect A long-life bearing steel can be provided. 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 effect of Mn 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 Steel Works Ltd. Technical Research Division (56) Reference JP-A-3-254342 (JP, A) Kaihei 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: 0.5-1.5 wt%, Mn: more than 2.0-5.0 wt%, the balance consisting of Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions being 8 μm or less. A bearing steel with excellent delay characteristics for microstructural changes due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, Mn: more than 2.0 to 5.0 wt%, Si: 0.05 to 0.5 wt%, Cr: 0.05 to 2.5 wt%
%, Mo: 0.05 to 0.5 wt%, Cu: 0.05 to 1.0 wt%, B: 0.00
05～0.01Wt%及 Beauty N: maximum any one selected from among 0.0005～0.012Wt% comprises one or two or more, the balance being Fe and unavoidable impurities, and an oxide-based nonmetallic inclusions Bearing steel with a grain size of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5-1.5 wt%, Mn: more than 2.0-5.0 wt% , Si: more than 0.5-2.5 wt%, Cr: more than 2.5-8.0
wt%, Mo: more than 0.5 to 2.0 wt%, N: more than 0.012 to 0.050 wt%,
V: 0.05 to 1.0 wt%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.
0wt%, Zr: 0.02-0.5wt%, Ta: 0.02-0.5wt%, Hf:
0.02～0.5Wt%及 Beauty Co: up either selected from among 0.05～1.5Wt% comprises one or two or more, the balance being Fe and unavoidable impurities, and an oxide-based nonmetallic inclusions Bearing steel with a grain size of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Mn: more than 2.0 to 5.0 wt%, further containing any one or more selected from the following group I components , and Of group II below
It contains any one or two or more selected from the components (excluding the elements selected in Group I) , the balance being Fe and inevitable impurities, and oxide-based nonmetallic inclusions Bearing steel with a maximum grain size of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress loading Serial group I: Si: 0.05~0.5wt%, Cr: 0.05~2.5wt%, Mo: 0.
05-0.5wt%, Cu: 0.05-1.0wt%, B: 0.0005-0.01wt
%及 Beauty N: 0.0005~0.012wt% II group: Si: 0.5 super ~2.5wt%, Cr: 2.5 super ~8.0wt%, Mo:
Over 0.5 ~ 2.0wt%, N: over 0.012 ~ 0.050wt%, V: 0.05 ~
1.0wt%, Nb: 0.05 to 1.0wt%, W: 0.05 to 1.0wt%, 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%