WO2012073488A1 - 球状化焼鈍後の加工性に優れ、かつ焼入れ・焼戻し後の耐水素疲労特性に優れる軸受鋼 - Google Patents
球状化焼鈍後の加工性に優れ、かつ焼入れ・焼戻し後の耐水素疲労特性に優れる軸受鋼 Download PDFInfo
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/32—Soft annealing, e.g. spheroidising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
- F16C33/00—Parts of bearings; Special methods for making bearings or parts thereof
- F16C33/30—Parts of ball or roller bearings
- F16C33/58—Raceways; Race rings
- F16C33/62—Selection of substances
Definitions
- the present invention can suppress bearing damage derived from a white structure caused by hydrogen, which is a problem in general-purpose bearing steel (JIS-SUJ2) bearings such as automobiles, windmills, and industrial machines, and the amount of added alloy
- JIS-SUJ2 general-purpose bearing steel
- This relates to a bearing steel that has excellent workability after spheroidizing annealing and excellent hydrogen fatigue resistance after quenching and tempering.
- Bearings are components that require excellent rolling fatigue life.
- bearings made of general-purpose bearing steel (JIS-SUJ2) such as automobiles, windmills, and industrial machinery have become a problem with reduced rolling fatigue life. ing. For this reason, various studies on improving the rolling fatigue life have been conducted.
- the bearing for motor vehicle alternators, etc. are mentioned. It is understood that the phenomenon of rolling fatigue failure that determines the rolling fatigue life of a bearing is caused by the occurrence of a structural change called white structure directly under the bearing transfer part, and the occurrence of cracks in the bearing. Yes.
- the white tissue is (1) Irregular white texture (hereinafter also referred to as WEA) that does not have a specific orientation relationship with the transfer unit (2) White structure (butterfly) generated in the 45 ° direction around non-metallic inclusions (3) White structure (white band) with an orientation relationship of about 80 ° and about 30 ° to the transfer unit are categorized.
- WEA Irregular white texture
- Patent Document 1 Regarding the generation mechanism of WEA, as described in Patent Document 1, the lubricating oil used in the bearing or the water that has entered the bearing is decomposed by a tribochemical reaction to generate hydrogen, and into the steel. It is thought to be promoted by invasion and accumulation.
- Patent Document 2 discloses a method of increasing the amount of retained austenite by increasing the amount of Cr and N added
- Patent Document 3 discloses Ni or Ni and Mo. The methods of simultaneously adding are described.
- the bearing steel is required to be excellent in workability such as cutting after spheroidizing annealing while suppressing the generation of WEA.
- consideration is not given to the workability such as cutting for bearing steel, or general consideration is given.
- the present invention was developed in view of the above situation, and not only improves rolling fatigue life by effectively suppressing the generation of WEA, even in an environment where hydrogen enters from a lubricating oil or the like, It is an object of the present invention to provide a bearing steel having excellent workability such as material machinability and forgeability, excellent workability after spheroidizing annealing, and excellent hydrogen fatigue resistance after quenching and tempering.
- the inventors diligently studied to develop a steel material that maintains machinability equivalent to that of JIS-SUJ2, which is a general-purpose steel, and has better hydrogen fatigue resistance than JIS-SUJ2.
- the description which represents content of the element in steel, and what was described only as% means mass%.
- the horizontal axis represents the Si content
- the Si content is more than 0.3%, B 10 life ratio was improved more than quadrupled .
- Cr content is 1.7%, the addition of Mo 0.15% and even Si as 0.3% or more, increase in B 10 life was hardly observed.
- FIG. 2 shows the result of arranging the Si content on the horizontal axis and the tool life ratio on the vertical axis. As shown in FIG.
- the inventors adjusted the amount of Si, Cr, and Mo to improve the rolling fatigue life, and by adding Sb, the rolling performance was maintained while maintaining machinability.
- the knowledge that the fatigue life can be further improved was obtained. The test which acquired the said knowledge is demonstrated below.
- FIG. 3 and 4 are the same as FIG. 1 and FIG. 2 with 0.9% C-1% Mn-0.016% P-0.008% S-0.025% Al-0.003% N-0.0015% O-0.0016% Sb as the base steel. Further, the results of a test in which the amounts of Si, Cr, and Mo are adjusted are shown. That is, the relationship of the Si content and the B 10 life ratio, and the relationship of Si content and the tool life ratio is a graph organizing the results.
- the present invention further investigated the Mo content that can ensure machinability, the rolling fatigue life when hydrogen was mixed, and the effects of other component elements on machinability. The result was completed.
- the gist configuration of the present invention is as follows. 1. In mass%, C: 0.85 to 1.10%, Si: 0.30 to 0.80%, Mn: 0.90 to 2.00%, P: 0.025% or less, S: 0.02% or less, Al: 0.05% or less, Cr: 1.8 to 2.5%, Bearing steel containing Mo: 0.15-0.4%, N: 0.0080% or less and O: 0.0020% or less, and further containing Sb: more than 0.0015% and 0.0050% or less, and the balance being Fe and inevitable impurities.
- the steel composition further includes, in mass%, one or more selected from Ti: 0.01% or less, Ni: 0.10% or less, Cu: 0.10% or less, and B: 0.0010% or less. Bearing steel as described in
- the present invention it is needless to say that it is excellent in workability such as cutting, and it is possible to obtain a steel for bearings that has remarkably superior hydrogen fatigue resistance compared to conventional bearing steels. This contributes to the improvement of the rolling fatigue life and brings about an industrially beneficial effect.
- Si on rolling fatigue life B 10 is a graph showing the effect of Cr and Mo content. It is a graph which shows the influence of Si, Cr, and Mo amount on tool life. In the case of addition of Sb, a graph showing Si, the effect of Cr and Mo Content on rolling fatigue life B 10. It is a graph which shows the influence of the amount of Si, Cr, and Mo which gives to the tool life when Sb is added.
- the content (%) of each element means mass%.
- C 0.85 to 1.10%
- C is an element necessary for ensuring the hardness after quenching and tempering and ensuring a high rolling fatigue life of the bearing, and it is necessary to contain 0.85% or more.
- the C content is in the range of 0.85 to 1.10%.
- Si 0.30 to 0.80% Si is an especially important element in the present invention for improving the hydrogen fatigue resistance, and is an important element for securing a rolling fatigue life when hydrogen is mixed. As described above, this effect is not exhibited when Si is less than 0.30%. On the other hand, if the Si content exceeds 0.8%, the machinability deteriorates as described above. Therefore, the Si content is in the range of 0.30 to 0.80%. A more preferred lower limit is 0.40%.
- Mn 0.90 to 2.00%
- Mn is an element necessary for securing the hardness after quenching and tempering and securing the rolling fatigue life of the bearing at a high level. For this purpose, it is necessary to contain 0.90% or more. On the other hand, if the Mn content exceeds 2.00%, the machinability deteriorates. Therefore, the Mn content is in the range of 0.90 to 2.00%, preferably 0.90 to 1.35%. It may be 0.90 to 1.15%.
- P 0.025% or less P segregates at the grain boundaries of austenite and lowers the grain boundary strength, thereby promoting quench cracking during quenching. Therefore, it is desirable to reduce the content as much as possible, but it is allowed up to 0.025%. In addition, Preferably it is 0.020% or less. Industrially, P is contained in an amount exceeding 0%, but may be 0% if possible.
- S 0.02% or less S is an element added to form MnS in steel to improve machinability, but if added over 0.02%, it becomes the starting point of fracture during rolling. Addition of 0.02% or less because fatigue strength decreases. Preferably it is 0.01% or less. The above-mentioned effect due to the formation of MnS can be obtained if S: 0.0003% or more.
- Al 0.05% or less
- Al is an element effective for deoxidation, and is an element useful for reducing oxygen.
- Al oxide present in steel deteriorates rolling fatigue characteristics.
- the amount of Al remaining after deoxidation can be suppressed to a minimum of about 0.004%.
- Cr 1.8-2.5% Cr is an effective element for suppressing the formation of white texture (WEA), and is an important element in the present invention. This is because, as described above, when the addition amount is less than 1.8%, the rolling fatigue life improvement effect by suppressing the formation of the white structure in a hydrogen atmosphere is poor. On the other hand, when added over 2.5%, the cost is increased and the machinability is remarkably deteriorated. Therefore, in the present invention, the range of Cr addition is set to 1.8 to 2.5%.
- Mo 0.15-0.4% Mo is an effective element for suppressing the formation of white texture (WEA), and is an important element in the present invention.
- WEA white texture
- the range of Mo addition is 0.15 to 0.4%.
- Sb More than 0.0015% and 0.0050% or less Sb is an element effective as a hydrogen trap site and suppresses the formation of white texture. If the amount of Sb added is 0.0015% or less, the effect of addition is poor, so addition of over 0.0015% is necessary. On the other hand, even if Sb exceeds 0.0050%, this effect is saturated. Therefore, the amount of Sb added should be in the range of more than 0.0015% and less than 0.0050%.
- N forms Al, Ti and nitrides or carbonitrides, and has the effect of suppressing austenite growth during heating for quenching.
- coarse nitrides and carbonitrides lead to a decrease in rolling fatigue life, so in the present invention, they are suppressed to 0.0080% or less.
- it is 0.0060% or less.
- the above-mentioned effect due to the formation of nitride / carbonitride can be obtained when N is 0.0015% or more.
- O 0.0020% or less
- O exists as a hard oxide-based nonmetallic inclusion, and an increase in the amount of O coarsens the size of the oxide-based nonmetallic inclusion.
- the coarse inclusions are particularly harmful to rolling fatigue characteristics, so it is desirable to reduce them as much as possible.
- the amount of O needs to be reduced to at least 0.0020% or less. Preferably it is 0.0010% or less.
- O is contained in an amount exceeding 0%, but may be 0% if possible.
- Ti 0.01% or less Ti is preferably added because it becomes nitrogen and TiN in the steel, thereby exhibiting a pinning effect in the austenite region and suppressing grain growth.
- TiN precipitates in a large amount, which may reduce the rolling fatigue life, so the added amount is limited to 0.01% or less. In order to acquire the said effect, it is suitable to add 0.003% or more of Ti.
- Ni 0.10% or less
- Ni is an element that improves hardenability and can be used to adjust hardenability.
- Ni is an expensive element, the steel material price increases as the addition amount increases, so the addition is made 0.10% or less. In order to obtain the above effect, it is preferable to add 0.03% or more of Ni.
- Cu 0.10% or less Cu can be added because it is an element that improves hardenability. However, if it exceeds 0.10%, the hot workability may be impaired, so 0.10% or less is added. In order to acquire the said effect, it is suitable to add 0.03% or more.
- B 0.0010% or less B has an effect of improving hardenability, and therefore can be used when adjusting hardenability. However, even if added over 0.0010%, the effect is saturated, so 0.0010% or less should be added. In order to acquire the said effect, adding 0.0003% or more is suitable.
- the balance other than the above components is inevitable impurities and Fe mixed in the manufacturing process.
- the bearing steel having the component composition described above can be manufactured by a known method. That is, after being melted by a melting method such as a converter and degassing equipment, it is cast, and the obtained slab is made into a steel material having a predetermined size through diffusion annealing, rolling, or forging forming process. This steel material is subjected to conventionally known spheroidizing annealing, and becomes a material for processing into bearing parts. After that, the bearing steel of the present invention is obtained through processing steps such as cutting and forging.
- a melting method such as a converter and degassing equipment
- the spheroidizing annealing is preferably performed at a temperature of 750 to 820 ° C. for about 4 to 16 hours and gradually cooling to about 650 ° C. at about 8 to 20 ° C./h.
- the structure of the processing material after spheroidizing annealing preferably contains ferrite and spherical cementite, and the Vickers hardness is preferably about 180 to 250.
- the processed material is processed into the shape of a bearing part, and then quenched and tempered to become a bearing part. If necessary, it may be processed into a final precision part shape after quenching and tempering.
- the quenching is preferably performed at a temperature of 800 to 950 ° C. for about 15 to 120 minutes, followed by quenching such as oil quenching or water quenching.
- the tempering treatment is preferably performed at 150 to 250 ° C. for about 30 to 180 minutes.
- the steel for bearings (bearing parts) after quenching and tempering preferably contains tempered martensite to the inside and contains about 90% or more by area ratio.
- the Vickers hardness is preferably about 700 to 800.
- the obtained test piece was subjected to a rolling fatigue test using a thrust type rolling fatigue tester and simulating use in a hydrogen environment in which the test was performed within 30 minutes after the hydrogen charge described above.
- This rolling fatigue test was performed under the conditions of Hertzian stress: 3.8 GPa, stress load rate: 3600 cpm, and lubrication (room temperature) of turbine oil (FBK turbine 68, manufactured by JX Nippon Oil & Energy Corporation). Incidentally, it performed 10 times tested for each steel type, by implementing organized by Weibull plots to determine the B 10 life cumulative failure probability of 10%.
- a test for obtaining the tool life was performed on each steel after spheroidizing annealing.
- the content of Cr, Mo and Si in steel components is controlled within an appropriate range, and by adding an appropriate amount of Sb, the workability such as cutting is excellent.
- a steel for bearings having significantly superior hydrogen fatigue resistance compared to the bearing steel can be obtained.
- the rolling fatigue life of the bearing is significantly improved, and a great industrial effect is brought about.
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Abstract
Description
軸受の転動疲労寿命を決める転動疲労破壊の現象は、軸受転送部直下に白色組織と呼ばれる組織変化が発生して、軸受に亀裂が発生・伸展することで引き起こされていることが分かっている。
(1) 転送部に対し特定の方位関係を持たず、不規則に発生する白色組織(以下、WEAともいう)
(2) 非金属介在物の周辺に45°方向に発生する白色組織(バタフライ)
(3) 転送部に対し、約80°と約30°の方位関係をもつ白色組織(ホワイトバンド)
に分類される。
特に、WEAが生成すると、想定している軸受寿命よりも短時間で疲労破壊が生じるために、最も対策が求められている白色組織である。
かようなWEAの生成を抑制する方法として、例えば、特許文献2には、CrおよびNの添加量を高めて残留オーステナイト量を増加する方法が、また、特許文献3にはNiあるいはNiとMoを同時添加する方法がそれぞれ記載されている。
しかしながら、前掲した特許文献1~3では、軸受鋼に対する切削などの加工性に関して、考慮が払われていないか、一般的な配慮がなされている程度である。
なお、以下の説明において、鋼中の元素の含有量を表す表記で、単に%と表記したものは全て質量%を意味する。
また、比較のために汎用鋼であるJIS-SUJ2相当鋼(1.05%C-0.25%Si-0.45%Mn-0.016%P-0.008%S-0.025%Al-1.45%Cr-0.003%N-0.0010%O)についても転動疲労寿命B10を調査した。なお、試験片の作成条件および転動疲労試験条件については、後述する実施例に記載したものと同様とした。
図1に、Si含有量を横軸に、B10寿命比を縦軸にして整理した結果を示す。
同図から明らかなように、Crの含有量が1.8%以上で、かつMoを0.15%含有させた鋼では、Si含有量が0.3%以上になると、B10寿命比が4倍以上に向上した。これに対し、Cr含有量が1.7%の場合は、Moを0.15%添加し、かつ、Siを0.3%以上としても、B10寿命の上昇はほとんど認められなかった。また、Cr含有量が1.8%以上であっても、Mo含有量が0%の鋼では、Siを0.3%以上添加しても、B10寿命の向上が認められなかった。
また、転動疲労寿命に達した試験片の剥離部分の組織を観察した結果、剥離した部分には、WEAが観察され、剥離は全てWEAに起因することが判明した。
以上のことから、Crを1.8%以上、Siを0.3%以上、Moを0.15%以上含有させることで、WEAの発生を遅らせて、転動疲労寿命を向上できることが分かった。
図2に、Si含有量を横軸に工具寿命比を縦軸にして整理した結果を示す。
図2に示したように、転動疲労寿命B10が高い値を示した1.8%Cr-0.15%Mo鋼、および2.5%Cr-0.15%Mo鋼は、Si含有量が0.8%超になると、工具寿命が急激に低下した。また、2.7%Cr-0.15%Mo鋼は、Si含有量に関わらず、工具寿命は低い値を示した。
以上のことから、Moを含有させた鋼において、Cr含有量を2.5%以下で、かつSi含有量を0.8%以下とすることにより、汎用鋼JIS-SUJ2並の被削性を確保できることが分かった。
上記知見を得た試験を、以下に説明する。
また、図4から明らかなように、Moを含有させた鋼で、Cr含有量を2.5%以下で、Si含有量を0.8%以下とし、そしてさらにSbを含有させることにより、汎用鋼JIS-SUJ2並の被削性を確保できることが分かる。ここに、Crを1.8%以上、Siを0.3%以上、Moを0.15%以上としたときのB10寿命の向上幅は、Sbを添加した場合のほうが、Sbを添加しない場合の時よりも格段に大きいことが分かる。
1. 質量%で、C:0.85~1.10%、Si:0.30~0.80%、Mn:0.90~2.00%、P:0.025%以下、S:0.02%以下、Al:0.05%以下、Cr:1.8~2.5%、Mo:0.15~0.4%、N:0.0080%以下およびO:0.0020%以下を含み、さらにSb:0.0015%超0.0050%以下を含有し、残部Fe及び不可避的不純物からなる軸受鋼。
C:0.85~1.10%
Cは、焼入れ焼戻し後の硬さを確保し、軸受の転動疲労寿命を高位に確保する上で必要な元素であり、0.85%以上含有する必要がある。一方、C含有量が1.10%を超えると、粗大な炭化物が残存してかえって転動疲労寿命が劣化する。したがって、C含有量は0.85~1.10%の範囲とする。
Siは、耐水素疲労特性を向上させる上で本発明では特に重要な元素であり、水素が混入した場合における転動疲労寿命を確保する上で重要な元素である。前述したとおり、Siが0.30%未満では、この効果が発現しない。一方、Si含有量が0.8%を超えると、前述したとおり被削性が劣化する。したがって、Si含有量は0.30~0.80%の範囲とする。より好ましい下限は0.40%である。
Mnは、焼入れ・焼戻し後の硬さを確保し、軸受の転動疲労寿命を高位に確保する上で必要な元素であり、そのためには0.90%以上含有する必要がある。一方、Mn含有量が2.00%を超えると、被削性が劣化する。したがって、Mn含有量は0.90~2.00%、好ましくは0.90~1.35%の範囲とする。なお、0.90~1.15%としてもよい。
Pは、オーステナイトの粒界に偏析し、粒界強度を低下させることにより、焼入時に焼割れを助長する。したがって、その含有量は極力低下させるのが望ましいが、0.025%までは許容される。なお、好ましくは0.020%以下とする。工業的には、Pは0%を超える量含有されるが、可能であれば0%として良い。
Sは、鋼中でMnSを形成して切削性を向上させるために添加する元素であるが、0.02%を超えて添加すると、転動時の破壊の起点となって転動疲労強度が低下するため、0.02%以下の添加とする。好ましくは0.01%以下とする。MnS形成による上記効果は、S:0.0003%以上であれば得ることができる。
Alは、脱酸に有効な元素であり低酸素化のために有用な元素であるが、鋼中に存在するAl酸化物は転動疲労特性を低下させるため、本発明では0.05%以下の添加とする。脱酸後に残存するAl量は、最小で0.004%程度に抑制することができる。
Crは、白色組織(WEA)の生成を抑制するために有効な元素であり、本発明では重要な元素である。というのは、前述したように、添加量が1.8%未満の場合では、水素雰囲気下での白色組織の生成抑制による転動疲労寿命向上効果が乏しい。一方、2.5%を超えて添加した場合では、コストアップになるとともに、被削性を著しく劣化させる。したがって、本発明では、Cr添加量の範囲を1.8~2.5%とする。
Moは、白色組織(WEA)の生成を抑制するために有効な元素であり、本発明では重要な元素である。ここに、添加量が0.15%未満の場合は、水素雰囲気下での転動疲労寿命の向上効果が乏しい。一方、0.4%を超えて添加すると、コストアップになるだけでなく、被削性を著しく劣化させる。したがって、本発明では、Mo添加量の範囲を0.15~0.4%とする。
Sbは、水素のトラップサイトとなり、白色組織の生成を抑制するために有効な元素である。Sb添加量が0.0015%以下では、添加効果が乏しいため0.0015%超の添加が必要である。一方、0.0050%を超えてSbを添加しても、この効果は飽和する。そのため、Sbの添加量は0.0015%超0.0050%以下の範囲とする。
Nは、Al,Tiと窒化物あるいは炭窒化物を形成し、焼入れのための加熱時に、オーステナイトの成長を抑制する効果がある。一方で、粗大な窒化物、炭窒化物は転動疲労寿命の低下を招くことになるため、本発明では0.0080%以下に抑制する。好ましくは0.0060%以下とする。窒化物・炭窒化物形成による上記効果は、N:0.0015%以上であれば得ることができる。
Oは、硬質の酸化物系非金属介在物として存在し、O量の増大は、酸化物系非金属介在物のサイズを粗大化させる。この粗大化介在物は、特に転動疲労特性にとって有害であるため、極力低減することが望ましい。そのためには、O量を、少なくとも0.0020%以下に低減する必要がある。好ましくは0.0010%以下とする。工業的には、Oは0%を超える量含有されるが、可能であれば0%として良い。
Ti:0.01%以下
Tiは、鋼中の窒素とTiNになることで、オーステナイト域でピンニング効果を発揮し、粒成長を抑制する効果があるため添加することが好ましい。一方、多量に添加するとTiNが多量析出することで転動疲労寿命を低下させてしまうおそれがあるため、添加量を0.01%以下に制限する。前記効果を得るためにはTiを0.003%以上添加することが好適である。
Niは、焼入性を向上させる元素で、焼入性を調整する場合に用いることができる。しかしながら、Niは高価な元素であるので添加量が多くなると鋼材価格が高くなるため、0.10%以下の添加とする。前記効果を得るためにはNiを0.03%以上添加することが好適である。
Cuは、焼入性を向上させる元素であるため添加することができる。しかしながら、0.10%を超えると、熱間加工性を阻害するおそれが生じるために0.10%以下の添加とする。前記効果を得るためには0.03%以上添加することが好適である。
Bは、焼入性を向上させる効果があるので、焼入性を調整する場合に用いることができる。しかし、0.0010%を超えて添加しても効果が飽和するので、0.0010%以下の添加とする。前記効果を得るためには0.0003%以上添加することが好適である。
すなわち、転炉、脱ガス設備等溶製方法にて溶製された後、鋳造され、得られた鋳片を、拡散焼鈍、圧延あるいは鍛錬成形工程を経て所定寸法の鋼材とされる。この鋼材に対し、従来公知の球状化焼鈍が施され、軸受部品への加工用の素材となる。その後、切削加工や鍛造加工などの加工工程を経て、本発明の軸受鋼になる。
球状化焼鈍は750~820℃で4~16時間程度保持し、650℃程度まで8~20℃/h程度で徐冷する処理を施すことが好ましい。球状化焼鈍後の加工用素材の組織はフェライトおよび球状セメンタイトを含むことが好ましく、ビッカース硬度は180~250程度となることが好ましい。
球状化焼鈍後の鋼から、直径:60mm、厚さ:5.5mmの疲労試験片を粗加工した。得られた粗加工試験片を840℃に30分保持後、油焼入れし、さらに180℃、2時間の焼戻しを行なった。焼戻し後の粗加工試験片を、直径:60mm、厚さ:5.0mmの試験片に仕上げ加工した。かかる仕上げ加工後の試験片に対し、水素チャージを行った。水素チャージは、50℃の20%チオシアン酸アンモニウム(NH4SCN)水溶液中に、24時間保持することで行なった。この条件では、汎用鋼JIS-SUJ2(表1中のNo.1鋼)で鋼中に水素が0.5質量ppm侵入することを、昇温式水素分析を用いて600℃までの水素量を測定することにより確認している。
それぞれの鋼について得られた転動疲労寿命B10をJIS-SUJ2相当鋼についての転動疲労寿命B10の値で除すことによって、汎用鋼に対する寿命の向上度(B10寿命比=B10寿命/ JIS-SUJ2相当鋼のB10寿命)を求め、評価した。
・切削速度:120m/min(潤滑剤なし)
・送り:0.2mm/rev
・切込み:1.0mm
・工具材種:超硬P10種相当
上記の切削条件で、切削工具の逃げ面摩耗量が0.2mmになるまでの時間を工具寿命として調査した。それぞれの鋼について得られた工具寿命を、JIS-SUJ2相当鋼についての工具寿命の値で除すことで、汎用鋼に対する工具寿命の向上度(工具寿命比=工具寿命/JIS-SUJ2相当鋼の工具寿命)を求め、評価した。
得られた結果を、表1、表2にそれぞれ併記する。
これに対し、本発明の成分組成範囲を外れる比較例は、転動疲労寿命B10または工具寿命のいずれかが、発明例よりも劣っていた。
Claims (2)
- 質量%で、C:0.85~1.10%、Si:0.30~0.80%、Mn:0.90~2.00%、P:0.025%以下、S:0.02%以下、Al:0.05%以下、Cr:1.8~2.5%、Mo:0.15~0.4%、N:0.0080%以下およびO:0.0020%以下を含み、さらにSb:0.0015%超0.0050%以下を含有し、残部Fe及び不可避的不純物からなる軸受鋼。
- 前記鋼組成が、さらに、質量%で、Ti:0.01%以下、Ni:0.10%以下、Cu:0.10%以下およびB:0.0010%以下のうちから選ばれる1種または2種以上を含有する請求項1に記載の軸受鋼。
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