JP6073200B2 - Bearing steel and bearing parts with excellent rolling fatigue characteristics - Google Patents
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Description
本発明は、各種産業機械や自動車等に使用される軸受用の転動体（コロ、ニードル、玉、レース等）として用いたときに、優れた転動疲労特性を発揮する軸受用鋼材、およびこのような軸受用鋼材から得られる軸受部品に関するものである。 The present invention relates to a bearing steel material that exhibits excellent rolling fatigue characteristics when used as a rolling element (roller, needle, ball, race, etc.) for bearings used in various industrial machines and automobiles, and the like. The present invention relates to a bearing component obtained from such a bearing steel material.
各種の産業機械や自動車等の分野に使用される軸受用の転動体（コロ、ニードル、玉、レース等）には、高い繰り返し応力が付与される。そのため、軸受用の転動体には転動疲労特性に優れることが求められている。転動疲労特性への要求は、産業機械類の高性能化、軽量化に対応して、年々厳しいものになっており、軸受部品の更なる耐久性向上のため、軸受用鋼材にはより一層良好な転動疲労特性が求められている。 High repetitive stress is applied to rolling elements (rollers, needles, balls, races, etc.) for bearings used in various industrial machines and automobiles. For this reason, rolling elements for bearings are required to have excellent rolling fatigue characteristics. The requirements for rolling fatigue characteristics are becoming stricter year by year in response to the higher performance and lighter weight of industrial machinery. To further improve the durability of bearing parts, the steel materials for bearings are even more demanding. Good rolling fatigue characteristics are required.
従来、転動疲労特性は、鋼中に生成する酸化物系介在物のなかでも、主にＡｌ脱酸鋼を用いたときに生成するＡｌ2Ｏ3等のような、硬質酸化物系介在物の個数密度と深く相関しており、上記硬質酸化物系介在物の個数密度を低減することによって転動疲労特性が改善すると考えられていた。そのため、製鋼プロセスにおいて、鋼中の酸素含有量を低減して転動疲労特性を改善する試みがなされてきた。 Conventionally, the rolling fatigue characteristics are hard oxide inclusions such as Al 2 O 3 produced mainly when using Al deoxidized steel among oxide inclusions produced in steel. It was thought that rolling fatigue characteristics were improved by reducing the number density of the hard oxide inclusions. Therefore, attempts have been made to improve the rolling fatigue characteristics by reducing the oxygen content in the steel in the steelmaking process.
しかしながら近年では、転動疲労特性と、酸化物系介在物に代表される非金属系介在物の関係に関する研究が進み、酸化物系介在物の個数密度と転動疲労特性とは必ずしも相関関係があるわけではないことが判明している。即ち、転動疲労特性は、非金属系介在物のサイズ、例えば非金属系介在物の面積の平方根と密接な相関関係があり、転動疲労特性を改善するには、非金属系介在物の個数密度を低減するよりも、非金属系介在物のサイズを小さくすることが有効であることが明らかになっている。 However, in recent years, research on the relationship between rolling fatigue characteristics and nonmetallic inclusions typified by oxide inclusions has progressed, and the number density of oxide inclusions and the rolling fatigue characteristics are not necessarily correlated. It turns out not to be. That is, the rolling fatigue characteristics are closely correlated with the size of the nonmetallic inclusions, for example, the square root of the area of the nonmetallic inclusions. It has become clear that reducing the size of non-metallic inclusions is more effective than reducing the number density.
そこで、従来のようなＡｌ脱酸鋼を用いるのではなく、鋼中のＡｌ含有量を極力抑えると共に、Ｓｉ脱酸鋼にすることで、生成する酸化物の組成を、Ａｌ2Ｏ3主体ではなくＳｉＯ2、ＣａＯなどを主体とする組成に制御し、これにより、圧延工程で非金属系介在物を延伸、分断させて非金属系介在物のサイズを低減し、転動疲労特性を改善する方法が提案されている。 Therefore, instead of using the conventional Al deoxidized steel, while suppressing the Al content in the steel as much as possible, and making it a Si deoxidized steel, the composition of the oxide produced is mainly Al 2 O 3 Control to a composition mainly composed of SiO 2 , CaO, etc., thereby reducing the size of the nonmetallic inclusions by extending and dividing the nonmetallic inclusions in the rolling process and improving the rolling fatigue characteristics. A method has been proposed.
例えば特許文献１には、酸化物の平均組成を質量％で、ＣａＯ：１０〜６０％、Ａｌ2Ｏ3：２０％以下、ＭｎＯ：５０％以下及びＭｇＯ：１５％以下で残部ＳｉＯ2及び不純物からなると共に、鋼材の長手方向縦断面の１０箇所の１００ｍｍ2の面積中に存在する酸化物の最大厚さの算術平均の値と硫化物の最大厚さの算術平均の値が、それぞれ、８．５μｍ以下であることを特徴とする軸受鋼材が提案されている。このような軸受鋼材を製造する方法として、二次製錬の過程におけるスラグの主要構成成分を厳密に制御し、酸化物を微細化する方法が開示されている。 For example, Patent Document 1 discloses that the average composition of oxides is% by mass, CaO: 10 to 60%, Al 2 O 3 : 20% or less, MnO: 50% or less, and MgO: 15% or less, and the remaining SiO 2 and impurities. The arithmetic average value of the maximum thickness of oxides and the arithmetic average value of the maximum thickness of sulfide existing in 10 areas of 100 mm 2 in the longitudinal section of the steel material in the longitudinal direction are 8 respectively. A bearing steel material having a diameter of 5 μm or less has been proposed. As a method for producing such a bearing steel material, a method for finely controlling an oxide by strictly controlling main constituent components of slag in the process of secondary smelting is disclosed.
一方、母相の鋼と非金属系介在物のヤング率に着目した技術として、特許文献２が挙げられる。上記特許文献２には、鋼のヤング率と介在物のヤング率の差が小さくなるように制御することで転動疲労特性の向上を図る技術が提案されている。上記特許文献２は、「従来のＡｌ脱酸工程で製造された鋼中のＡｌ量は０．０１５〜０．０２５％程度となり、その結果、Ａｌを多く含有する介在物は、鋼（母相）に比べてヤング率が非常に高くなるため、母相と介在物との界面に空洞が形成されやすく、この空洞の形成を経て、その空洞の周囲に引張応力が作用して容易にき裂が発生する。また鋼中に空洞に近いような軟質な非金属介在物が存在する場合には、その周囲に引張応力の作用で容易にき裂が発生する。」との知見に基づいて完成されたものであり、Ａｌが０．０１０％未満であり、鋼のヤング率をＥ1とし、かつ介在物の大きさを（縦方向の最大長さ×横方向の最大長さ）1/2で定めるときの鋼中の検鏡面積３，０００ｍｍ2に存在する（縦方向の最大長さ×横方向の最大長さ）1/2が１５μｍ以上の介在物の平均ヤング率をＥ2とするとき、それらのヤング率比Ｅ2／Ｅ1を、０．３＜Ｅ2／Ｅ1＜１．６なる範囲に制御した鋼を開示している。すなわち、上記特許文献２では、Ｅ2／Ｅ1が０．３以下の空洞に近い軟質な非金属介在物や、Ｅ2／Ｅ1が１．６以上の空洞を形成し易い非金属介在物に関しては、母相の鋼と非金属介在物の界面に引張応力が作用して容易にき裂が発生し、転動疲労特性が悪化するとの認識の下で、ヤング率比のＥ2／Ｅ1を、０．３＜Ｅ2／Ｅ1＜１．６の範囲に制御している。 On the other hand, Patent Document 2 is cited as a technique focusing on the Young's modulus of the parent phase steel and the nonmetallic inclusions. Patent Document 2 proposes a technique for improving rolling fatigue characteristics by controlling the difference between the Young's modulus of steel and the Young's modulus of inclusions to be small. The above Patent Document 2 states that “the amount of Al in steel produced by the conventional Al deoxidation process is about 0.015 to 0.025%, and as a result, inclusions containing a large amount of Al are steel (matrix). ), The Young's modulus is very high, and cavities are likely to be formed at the interface between the parent phase and inclusions. After the formation of these cavities, tensile stress acts on the periphery of the cavities and cracks easily. If there are soft nonmetallic inclusions in the steel that are close to cavities, cracks are easily generated around them due to the action of tensile stress. " are those which are, Al is less than 0.010%, the Young's modulus of the steel as E 1, and the size of the inclusions (vertical maximum length × the maximum horizontal length) 1/2 Existing in a speculum area of 3,000 mm 2 in steel as defined in (maximum length in the vertical direction x maximum length in the horizontal direction) When the average Young's modulus of inclusions whose 1/2 is 15 μm or more is E 2 , the Young's modulus ratio E 2 / E 1 is controlled in a range of 0.3 <E 2 / E 1 <1.6. Steel is disclosed. That is, in the above-mentioned Patent Document 2, a soft non-metallic inclusion close to a cavity having E 2 / E 1 of 0.3 or less, or a non-metallic inclusion that easily forms a cavity having E 2 / E 1 of 1.6 or more. With respect to the Young's modulus ratio E 2 / E with the understanding that tensile stress acts on the interface between the parent phase steel and the non-metallic inclusions to easily generate cracks and deteriorate rolling fatigue properties. 1 is controlled in a range of 0.3 <E 2 / E 1 <1.6.
しかしながら、上記特許文献１では、「［００４４］…酸化物は全体的に軟質であり、圧延等の加工によって容易に延伸、分断されて微細になるため、転動疲労寿命を低下させることがなく、したがって、過酷な使用環境下においても優れた転動疲労寿命を確保できる」と記載されており、最大厚さの低減にのみ着眼しており、アスペクト比低減への取組みはなされていない。本発明者らの知見からすると、最大厚さが低減したとしても、延伸性の高い酸化物はアスペクト比が大きくなって、転動疲労寿命が低下しやすいと思われる。その結果、転動疲労特性を安定的に改善できず、近年の転動疲労特性の要望を満足するものではないと考えられる。 However, in the above-mentioned Patent Document 1, “[0044]... Oxide is generally soft, and is easily stretched and divided by processing such as rolling to become fine, so that the rolling fatigue life is not reduced. Therefore, it is described that an excellent rolling fatigue life can be secured even in a harsh use environment ”, focusing only on the reduction of the maximum thickness, and no effort has been made to reduce the aspect ratio. According to the knowledge of the present inventors, even when the maximum thickness is reduced, it is considered that an oxide having high stretchability has a large aspect ratio, and the rolling fatigue life is likely to decrease. As a result, it is considered that the rolling fatigue characteristics cannot be stably improved, and it does not satisfy the recent demand for rolling fatigue characteristics.
また、上記特許文献２のようにヤング率比を０．３＜Ｅ2／Ｅ1＜１．６の範囲に制御しているが、下記内容からアスペクト比は非常に大きく、転動疲労特性を安定的に改善できるとはいえないという問題があると考えられる。まず、特許文献２では、「［００１４］…Ａｌの添加を極力少なくし、かつ脱酸も十分に行うために、ＳｉやＭｎなどの脱酸元素を有効に利用するものとする」との記載内容から、ＳｉやＭｎ以外の脱酸元素は使用されていないと考えられる。そのため、ＳｉＯ2を主体とする酸化物系介在物が生成していると推定される。本発明者らの知見からすると、ＳｉＯ2を主体とする酸化物系介在物は熱間加工などの高温域で容易に延伸し、アスペクト比が大きくなると考えられる。更に、「［００１８］…表１は、本発明の鋼およびその比較鋼のそれぞれの供試鋼の化学組成で、Ｆｅおよび不可避的不純物を除いて示す。」との記載内容からも、脱酸元素として、本発明で活用しているようなＣａ、Ｔｉを含んでいる可能性は低い。加えて特許文献２の本文中には、ＭｇＯの活用に関する記載は無く、本発明で活用しているような、酸化物系介在物中のＭｇＯに関して、検討されていないと考えられる。上記考えより、特許文献２は、ヤング率比に関する検討はなされているものの、生成する酸化物系介在物のアスペクトに関する配慮はなされていない。さらに、特許文献２では、Ｓｉ、Ｍｎ脱酸で得られるＳｉＯ2を主体とする酸化物系介在物が得られ、それらは熱間加工などの高温域で容易に延伸し、アスペクト比の大きな酸化物系介在物となってしまう。その結果、転動疲労特性を安定的に改善できず、近年の転動疲労特性の要望を満足するものではないと考えられる。 Moreover, although the Young's modulus ratio is controlled in the range of 0.3 <E 2 / E 1 <1.6 as in the above-mentioned Patent Document 2, the aspect ratio is very large from the following contents, and the rolling fatigue characteristics are There seems to be a problem that it cannot be said that it can be stably improved. First, Patent Document 2 states that “[0014] ... deoxidation elements such as Si and Mn are effectively used in order to reduce addition of Al as much as possible and sufficiently perform deoxidation”. From the contents, it is considered that deoxidation elements other than Si and Mn are not used. Therefore, it is estimated that oxide inclusions mainly composed of SiO 2 are generated. From the knowledge of the present inventors, it is considered that oxide inclusions mainly composed of SiO 2 are easily stretched in a high temperature region such as hot working, and the aspect ratio is increased. Furthermore, from the description of “[0018]... Table 1 shows the chemical composition of each steel of the present invention and its comparative steel, excluding Fe and unavoidable impurities.” The possibility of containing Ca and Ti as utilized in the present invention as an element is low. In addition, there is no description regarding utilization of MgO in the text of Patent Document 2, and it is considered that MgO in oxide inclusions as utilized in the present invention has not been studied. From the above-mentioned idea, Patent Document 2 does not consider the aspect of the oxide inclusions to be generated, although the Young's modulus ratio has been studied. Furthermore, in Patent Document 2, oxide inclusions mainly composed of SiO 2 obtained by deoxidation of Si and Mn are obtained, and they are easily stretched in a high temperature region such as hot working and oxidized with a large aspect ratio. It becomes a material inclusion. As a result, it is considered that the rolling fatigue characteristics cannot be stably improved, and it does not satisfy the recent demand for rolling fatigue characteristics.
このように、非金属系介在物の最大厚み（いわゆる最大短径）のみを制御する方法（特許文献１）、鋼と非金属介在物のヤング率比を制御する方法（特許文献２）では、転動疲労特性の改善が不十分である。特に、本発明者らの検討によると、アスペクト比が大きな酸化物系介在物が形成された場合、非金属系介在物の最大長径が粗大化し、転動疲労特性の向上が不十分になることが分かった。 Thus, in the method (Patent Document 1) for controlling only the maximum thickness (so-called maximum minor axis) of nonmetallic inclusions, and the method for controlling the Young's modulus ratio of steel and nonmetallic inclusions (Patent Document 2), Insufficient improvement of rolling fatigue properties. In particular, according to the study by the present inventors, when oxide inclusions having a large aspect ratio are formed, the maximum major axis of nonmetallic inclusions becomes coarse, and the rolling fatigue characteristics are not sufficiently improved. I understood.
本発明は上記事情に鑑みてなされたものであり、その目的は、転動疲労特性に極めて優れており、早期剥離を抑制することのできる新規な軸受用鋼材を提供することにある。 This invention is made | formed in view of the said situation, The objective is to provide the novel steel material for bearings which is very excellent in rolling fatigue characteristics and can suppress early peeling.
本発明に係る転動疲労特性に優れた軸受用鋼材は、Ｃ：０．８〜１．１％（％は質量％の意味、以下、特に断らない限り、同じ）、Ｓｉ：０．１５〜０．８％、Ｍｎ：０．１〜１．０％、Ｃｒ：１．３〜１．８％、Ｐ：０．０５％以下（０％を含まない）、Ｓ：０．０１５％以下（０％を含まない）、Ａｌ：０．０００２〜０．００５％、Ｃａ：０．０００２〜０．００２０％、Ｔｉ：０．０００５〜０．０１０％、Ｏ：０．００２５％以下（０％を含まない）を含有し、残部は鉄及び不可避的不純物からなり、
鋼材の長手方向に平行する断面に存在する、式：（長径×短径）1/2で求まる介在物寸法が５μｍ以上となる酸化物系介在物のヤング率の平均値をＥ1とし、母相のヤング率の平均値をＥ2とした場合に式：Ｅ1／Ｅ2で求まるヤング率比が０．５超１．５未満であり、
鋼材の長手方向に平行する断面に存在する、短径が１μｍ以上の酸化物系介在物のアスペクト比の平均値が３．０以下であるところに要旨を有するものである。
本発明の好ましい実施形態では、上記鋼中に含まれる短径が１μｍ以上の酸化物系介在物が、平均組成の質量％で、ＣａＯ：２０〜５０％、Ａｌ2Ｏ3：２０〜５０％、ＳｉＯ2：２０〜５７％と、ＴｉＯ2：３〜１０％およびＭｇＯ：３〜１０％のいずれか１種または両方とを含有し、残部は不純物からなるものであってもよい。前記ヤング率比及びアスペクト比は、例えば、球状化焼鈍と、それに次ぐ焼入れ・焼戻し後において決定してもよい。
本発明には、上記軸受用鋼材を用いて得られる軸受部品も包含される。
Steels for bearings excellent in rolling fatigue characteristics according to the present invention are: C: 0.8 to 1.1% (% means mass%, hereinafter the same unless otherwise specified), Si: 0.15 0.8%, Mn: 0.1 to 1.0%, Cr: 1.3 to 1.8%, P: 0.05% or less (excluding 0%), S: 0.015% or less ( 0% not included), Al: 0.0002 to 0.005%, Ca: 0.0002 to 0.0020%, Ti: 0.0005 to 0.010%, O: 0.0025% or less (0% The remainder consists of iron and unavoidable impurities,
E 1 is the average value of Young's modulus of oxide inclusions having an inclusion size of 5 μm or more, which is obtained by the formula: (major axis × minor axis) 1/2 , existing in a cross section parallel to the longitudinal direction of the steel material, If the average value of the Young's modulus of the phase and the E 2 wherein: Young's modulus ratio determined by the E 1 / E 2 is less than 0.5 ultra 1.5,
The gist is that the average value of the aspect ratio of oxide inclusions having a minor axis of 1 μm or more present in a cross section parallel to the longitudinal direction of the steel material is 3.0 or less.
In a preferred embodiment of the present invention, the oxide inclusions having a minor axis of 1 μm or more contained in the steel are CaO: 20 to 50%, Al 2 O 3 : 20 to 50% in mass% of the average composition. , SiO 2: from 20 to 57% and, TiO 2: 3-10% and MgO: contains and any one or both of 3-10%, the balance may be made of an impurity. The Young's modulus ratio and the aspect ratio may be determined, for example, after spheroidizing annealing and subsequent quenching / tempering.
The present invention also includes a bearing component obtained using the bearing steel material.
本発明の軸受用鋼材によれば、鋼材の化学成分組成と鋼中に含まれる酸化物系介在物のヤング率及びアスペクト比を適切に制御しているため、早期剥離を抑制して転動疲労特性を大きく向上できる。このような軸受用鋼材は、コロ、ニードル、玉等、主にラジアル方向の荷重が繰り返し付与される軸受部品の素材として有用であるのみならず、レース等、ストラス方向の荷重も繰り返し付与される軸受部品の素材としても有用であり、荷重の付与される方向にかかわらず転動疲労特性を安定的に改善することができる。 According to the steel material for bearings of the present invention, the chemical composition of the steel material and the Young's modulus and aspect ratio of the oxide inclusions contained in the steel are appropriately controlled. The characteristics can be greatly improved. Such a steel material for bearings is not only useful as a material for bearing parts such as rollers, needles, balls, etc., in which radial loads are repeatedly applied, but is also repeatedly applied in struts, such as races. It is also useful as a material for bearing parts, and the rolling fatigue characteristics can be stably improved regardless of the direction in which the load is applied.
本発明者らは、Ａｌによる脱酸処理を行なわなくても、早期剥離を抑制でき、転動疲労特性を安定的に改善することができる軸受用のＳｉ脱酸鋼材を提供するため、検討を重ねてきた。そして従来のＳｉ脱酸鋼材で転動疲労特性が十分に向上しない原因を突き止めた。すなわちＳｉ脱酸で得られる酸化物系介在物は、熱間圧延などの高温域で延伸しやすく、分断に至らなかった部分がアスペクト比の大きな状態で鋼材中に残存している。このアスペクト比の大きな酸化物系介在物が転動疲労寿命を十分に高くすることができない本質であった。そして本発明者らは、酸化物系介在物のアスペクト比を小さくし、かつヤング率を母相に近づけることで、酸化物系介在物と母相との界面部分への応力集中を緩和でき、もって転動疲労特性を十分に向上できることを見出し、本発明を完成した。 In order to provide a Si deoxidized steel material for bearings that can suppress early peeling and stably improve rolling fatigue characteristics without performing deoxidation treatment with Al, the present inventors have studied. It has been repeated. And the cause which rolling fatigue characteristics did not fully improve with the conventional Si deoxidized steel material was discovered. That is, the oxide inclusions obtained by Si deoxidation are easily stretched in a high temperature region such as hot rolling, and a portion that has not been divided remains in the steel material in a state with a large aspect ratio. It was the essence that oxide inclusions with a large aspect ratio could not sufficiently increase the rolling fatigue life. And by reducing the aspect ratio of the oxide inclusions and making the Young's modulus close to the parent phase, the present inventors can relieve stress concentration on the interface portion between the oxide inclusions and the mother phase, Thus, the present inventors have found that the rolling fatigue characteristics can be sufficiently improved and completed the present invention.
酸化物系介在物のアスペクト比は、例えば、ＳｉＯ2を主体とする酸化物系介在物の熱間圧延時の変形抵抗を高め、その延伸を抑制することによって小さくできる。またＳｉＯ2を主体とする酸化物系介在物の変形抵抗を高めるには、Ｓｉ脱酸で得られる酸化物系介在物では従来含有されていなかったＴｉＯ2やＭｇＯを含有させるようにすればよい。これらを含有させると、介在物のアスペクト比を小さくできる。また介在物のヤング率を母相に近づけることもできる。 The aspect ratio of the oxide inclusions can be reduced, for example, by increasing the deformation resistance during hot rolling of the oxide inclusions mainly composed of SiO 2 and suppressing the stretching. Further, in order to increase the deformation resistance of oxide inclusions mainly composed of SiO 2 , TiO 2 and MgO that have not been conventionally contained in oxide inclusions obtained by Si deoxidation may be included. . When these are contained, the aspect ratio of inclusions can be reduced. In addition, the Young's modulus of inclusions can be brought close to the parent phase.
なおＴｉＯ2やＭｇＯの作用の詳細は不明であるが、以下のように考えられる。すなわち、Ｓｉ脱酸で得られるＳｉＯ2含有酸化物系介在物にＴｉＯ2が含まれるようにすると、Ｓｉ脱酸鋼において発生し易かったアノーサイト（Ａｎｏｒｔｈｉｔｅ、ＣａＯ・Ａｌ2Ｏ3・２ＳｉＯ2）の生成が抑制される。アノーサイトはヤング率が低く、母相とのヤング率比が小さく転動疲労特性を低下させる。また、ＳｉＯ2含有酸化物系介在物にＭｇＯが含まれるようにすると、Ｓｉ脱酸鋼において発生し易かったＳｉＯ2を主体とする非晶質相の生成が抑制される。非晶質相はヤング率が低く、母相とのヤング率比が小さく転動疲労特性を低下させる。更に、熱間圧延時に延伸しやすくアスペクト比が大きくなり転動疲労特性を低下させる。また、Ｓｉ脱酸で得られるＳｉＯ2含有酸化物系介在物にＴｉＯ2またはＭｇＯが所定量含まれた場合に生成する結晶のヤング率は母相に近く、またそれらの結晶相は熱間加工時における変形抵抗も高く、熱間加工後においても介在物のアスペクト比は小さく、転動疲労特性の向上において非常に有利な状態となる。 The details of the action of TiO 2 and MgO are unknown, but are considered as follows. That is, when TiO 2 is included in the SiO 2 -containing oxide inclusions obtained by Si deoxidation, anorthite (Canorth, CaO · Al 2 O 3 · 2SiO 2 ) that was easily generated in Si deoxidized steel. Generation is suppressed. Anorthite has a low Young's modulus and a small Young's modulus ratio with the parent phase, which lowers rolling fatigue characteristics. Further, when MgO is contained in the SiO 2 -containing oxide inclusions, generation of an amorphous phase mainly composed of SiO 2 that is easily generated in Si deoxidized steel is suppressed. The amorphous phase has a low Young's modulus, and the Young's modulus ratio with the parent phase is small, thereby reducing the rolling fatigue characteristics. Furthermore, it is easy to stretch at the time of hot rolling, and the aspect ratio is increased, and the rolling fatigue characteristics are lowered. In addition, the Young's modulus of crystals produced when SiO 2 -containing oxide inclusions obtained by Si deoxidation contain a predetermined amount of TiO 2 or MgO is close to the parent phase, and those crystal phases are hot-worked. The deformation resistance at the time is high, and the aspect ratio of the inclusion is small even after hot working, which is a very advantageous state in improving rolling fatigue characteristics.
これに対し、前述した特許文献１、２にはいずれも、本発明のように、ＴｉＯ2およびＭｇＯで酸化物系介在物組成を制御することは記載されておらず、従ってヤング率制御に加えてアスペクト比をも低減し転動疲労特性の向上を図るという本発明の技術的思想は開示されていない。単にヤング率を制御しただけでは、母相の鋼と酸化物系介在物の界面に応力が集中し、所望とする転動疲労特性を確保できないことが判明した（後記する実施例を参照）。 In contrast, Patent Documents 1 and 2 described above do not describe that the oxide inclusion composition is controlled by TiO 2 and MgO as in the present invention, and therefore, in addition to Young's modulus control. Thus, the technical idea of the present invention to reduce the aspect ratio and improve the rolling fatigue characteristics is not disclosed. It has been found that simply controlling the Young's modulus concentrates stress at the interface between the parent phase steel and oxide inclusions, and cannot secure the desired rolling fatigue characteristics (see Examples described later).
以下、本発明について詳しく説明する。
本発明は軸受用鋼材を対象とするものであり、その成分組成とその限定理由は以下の通りである。
［Ｃ：０．８〜１．１％］
Ｃは、焼入硬さを増大させ、室温、高温における強度を維持して耐磨耗性を付与するための必須の元素である。こうした効果を発揮させるためには、Ｃは少なくとも、０．８％以上含有させる必要がある。しかしながら、Ｃ含有量が１．１％を超えて過剰になると、軸受の芯部に巨大炭化物が生成し易くなり、転動疲労特性に悪影響を及ぼすようになる。Ｃ含有量の好ましい下限は０．８５％以上（より好ましくは０．９０％以上）であり、好ましい上限は１．０５％以下（より好ましくは１．０％以下）である。
The present invention will be described in detail below.
The present invention is intended for steel for bearings, and the component composition and the reasons for limitation are as follows.
[C: 0.8 to 1.1%]
C is an essential element for increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. In order to exert such effects, it is necessary to contain C at least 0.8% or more. However, if the C content exceeds 1.1% and becomes excessive, giant carbides are easily generated in the core of the bearing, which adversely affects rolling fatigue characteristics. The preferable lower limit of the C content is 0.85% or more (more preferably 0.90% or more), and the preferable upper limit is 1.05% or less (more preferably 1.0% or less).
［Ｓｉ：０．１５〜０．８％］
Ｓｉは、脱酸元素として有効に作用する他、焼入れ・焼戻し軟化抵抗を高めて硬さを高める作用を有している。こうした効果を有効に発揮させるためには、Ｓｉ含有量は、０．１５％以上とする必要がある。しかしながら、Ｓｉ含有量が過剰になって０．８％を超えると、鍛造時に金型寿命が低下するばかりか、コスト増加を招くことになる。Ｓｉ含有量の好ましい下限は０．２０％以上（より好ましくは０．２５％以上）であり、好ましい上限は０．７％以下（より好ましくは０．６％以下）である。
[Si: 0.15 to 0.8%]
In addition to effectively acting as a deoxidizing element, Si has an effect of increasing hardness by increasing resistance to quenching and tempering. In order to effectively exhibit these effects, the Si content needs to be 0.15% or more. However, if the Si content is excessive and exceeds 0.8%, not only the die life is reduced during forging, but also the cost is increased. The preferable lower limit of the Si content is 0.20% or more (more preferably 0.25% or more), and the preferable upper limit is 0.7% or less (more preferably 0.6% or less).
［Ｍｎ：０．１〜１．０％］
Ｍｎは、鋼材マトリックスの固溶強化および焼入れ性を向上させる元素である。Ｍｎ含有量が０．１％を下回るとその効果が発揮されず、１．０％を上回ると加工性や被削性が著しく低下する。Ｍｎ含有量の好ましい下限は０．２％以上（より好ましくは０．３％以上）であり、好ましい上限は０．８％以下（より好ましくは０．６％以下）である。
[Mn: 0.1 to 1.0%]
Mn is an element that improves the solid solution strengthening and hardenability of the steel matrix. If the Mn content is less than 0.1%, the effect is not exhibited, and if it exceeds 1.0%, the workability and machinability are significantly reduced. The minimum with preferable Mn content is 0.2% or more (more preferably 0.3% or more), and a preferable upper limit is 0.8% or less (more preferably 0.6% or less).
［Ｃｒ：１．３〜１．８％］
Ｃｒは、焼入れ性の向上と安定な炭化物の形成によって、強度および耐摩耗性を向上させ、これによって転動疲労特性の改善に有効な元素である。こうした効果を発揮させるためには、Ｃｒ含有量は、１．３％以上とする必要がある。しかしながら、Ｃｒ含有量が過剰になって１．８％を超えると、炭化物が粗大化して、転動疲労特性および切削性を低下させる。Ｃｒ含有量の好ましい下限は１．４％以上（より好ましくは１．５％以上）であり、好ましい上限は１．７％以下（より好ましくは１．６％以下）である。
[Cr: 1.3-1.8%]
Cr is an element effective in improving rolling fatigue characteristics by improving strength and wear resistance by improving hardenability and forming stable carbides. In order to exert such effects, the Cr content needs to be 1.3% or more. However, when the Cr content is excessive and exceeds 1.8%, the carbides are coarsened, and the rolling fatigue characteristics and the machinability are deteriorated. The preferable lower limit of the Cr content is 1.4% or more (more preferably 1.5% or more), and the preferable upper limit is 1.7% or less (more preferably 1.6% or less).
［Ｐ：０．０５％以下（０％を含まない）］
Ｐは、結晶粒界に偏析して転動疲労特性に悪影響を及ぼす不純物元素である。特に、Ｐ含有量が０．０５％を超えると、転動疲労特性の低下が著しくなる。従って、Ｐ含有量は０．０５％以下に抑制する必要がある。好ましくは０．０３％以下、より好ましくは０．０２％以下とするのが良い。尚、Ｐは鋼材に不可避的に含まれる不純物であり、その量を０％にすることは、工業生産上、困難である。
[P: 0.05% or less (excluding 0%)]
P is an impurity element that segregates at the grain boundaries and adversely affects the rolling fatigue characteristics. In particular, when the P content exceeds 0.05%, the rolling fatigue characteristics are significantly deteriorated. Therefore, it is necessary to suppress the P content to 0.05% or less. Preferably it is 0.03% or less, more preferably 0.02% or less. In addition, P is an impurity inevitably contained in the steel material, and it is difficult to make the amount 0% in industrial production.
［Ｓ：０．０１５％以下（０％を含まない）］
Ｓは、硫化物を形成する元素であり、その含有量が０．０１５％を超えると、粗大な硫化物が残存するため、転動疲労特性が劣化する。従って、Ｓの含有量は０．０１５％以下に抑制する必要がある。転動疲労特性の向上という観点からは、Ｓ含有量は低ければ低いほど望ましく、好ましくは０．００７％以下、より好ましくは０．００５％以下とするのが良い。尚、Ｓは鋼材に不可避的に含まれる不純物であり、その量を０％にすることは、工業生産上、困難である。
[S: 0.015% or less (excluding 0%)]
S is an element that forms sulfides. If the content exceeds 0.015%, coarse sulfides remain, and therefore rolling fatigue characteristics deteriorate. Therefore, it is necessary to suppress the S content to 0.015% or less. From the viewpoint of improving rolling fatigue characteristics, the lower the S content, the more desirable, preferably 0.007% or less, and more preferably 0.005% or less. In addition, S is an impurity inevitably contained in the steel material, and it is difficult to make the amount 0% in industrial production.
［Ａｌ：０．０００２〜０．００５％］
Ａｌは、好ましくない元素であり、本発明の鋼材においては、Ａｌは極力少なくする必要がある。従って、酸化精錬後のＡｌ添加による脱酸処理は行わない。Ａｌ含有量が多くなり、特に０．００５％を超えてしまうと、Ａｌ2Ｏ3を主体とする硬質な酸化物の生成量が多くなり、しかも圧延した後も粗大な酸化物として残存するので、転動疲労特性が劣化する。従って、Ａｌの含有量を０．００５％以下とした。尚、Ａｌ含有量は、０．００２％以下とすることが好ましく、より好ましくは０．００１５％以下である。但し、Ａｌ含有量を０．０００２％未満にすると、酸化物中のＡｌ2Ｏ3含有量が少なくなり過ぎ、介在物がＳｉＯ2を多く含むようになって非晶質相のまま残存しやすくなるため、ヤング率が低下し転動疲労特性が低下する。また、Ａｌ含有量を所定量０．０００２％未満に制御するためには、Ａｌの混入を抑制するために、鋼中成分のみならず、フラックス中のＡｌ2Ｏ3含有量も少なくする必要があるが、高炭素鋼である軸受鋼においてＡｌ2Ｏ3含有量の少ないフラックスは非常に高価であり、経済的でない。従って、Ａｌ含有量の下限は０．０００２％以上（好ましくは０．０００５％以上）である。
[Al: 0.0002 to 0.005%]
Al is an undesirable element, and in the steel material of the present invention, it is necessary to reduce Al as much as possible. Therefore, deoxidation treatment by addition of Al after oxidative refining is not performed. If the Al content increases and exceeds 0.005% in particular, the amount of hard oxide mainly composed of Al 2 O 3 increases, and it remains as a coarse oxide after rolling. , Rolling fatigue characteristics deteriorate. Therefore, the Al content is set to 0.005% or less. Note that the Al content is preferably 0.002% or less, and more preferably 0.0015% or less. However, when the Al content is less than 0.0002%, the Al 2 O 3 content in the oxide becomes too small, and the inclusions contain a large amount of SiO 2 and remain in an amorphous phase. As a result, the Young's modulus decreases and the rolling fatigue characteristics decrease. Further, in order to control the Al content to be less than the predetermined amount of 0.0002%, it is necessary to reduce not only the components in the steel but also the Al 2 O 3 content in the flux in order to suppress the mixing of Al. However, a flux having a low Al 2 O 3 content in a bearing steel that is a high carbon steel is very expensive and not economical. Therefore, the lower limit of the Al content is 0.0002% or more (preferably 0.0005% or more).
［Ｃａ：０．０００２〜０．００２０％］
Ｃａは、酸化物中のＣａＯ含有量を制御し、転動疲労特性を改善するのに有効である。このような効果を発揮させるためには、Ｃａ含有量は０．０００２％以上とすることが好ましい。しかしながら、Ｃａ含有量を０．０００２％未満にすると、酸化物中のＣａＯ含有量が少なくなり過ぎ、介在物がＳｉＯ2を多く含むようになって非晶質相のまま残存しやすくなるため、ヤング率が低下し転動疲労特性が低下する。一方、Ｃａ含有量が過剰になって０．００２０％を超えると、酸化物組成におけるＣａＯの割合が高くなり過ぎて、生成する酸化物の結晶相のヤング率が低下し転動疲労特性が低下する。従って、Ｃａ含有量は０．００２０％以下とした。Ｃａ含有量の好ましい下限は０．０００３％以上（より好ましくは０．０００５％以上）であり、好ましい上限は０．００１％以下（より好ましくは０．０００８％以下）である。
[Ca: 0.0002 to 0.0020%]
Ca is effective in controlling the CaO content in the oxide and improving the rolling fatigue characteristics. In order to exhibit such an effect, the Ca content is preferably 0.0002% or more. However, if the Ca content is less than 0.0002%, the CaO content in the oxide becomes too small, and inclusions contain a large amount of SiO 2, which tends to remain in an amorphous phase. Young's modulus decreases and rolling fatigue characteristics decrease. On the other hand, if the Ca content becomes excessive and exceeds 0.0020%, the proportion of CaO in the oxide composition becomes too high, and the Young's modulus of the crystal phase of the oxide to be produced decreases and the rolling fatigue characteristics deteriorate. To do. Therefore, the Ca content is set to 0.0020% or less. The preferable lower limit of the Ca content is 0.0003% or more (more preferably 0.0005% or more), and the preferable upper limit is 0.001% or less (more preferably 0.0008% or less).
［Ｔｉ：０．０００５〜０．０１０％］
Ｔｉは、本発明を特徴付ける元素である。所定量のＴｉを添加し、酸化物中のＴｉＯ2含有量を適切に制御することにより、Ｓｉ脱酸鋼において発生し易かったアノーサイト（Ａｎｏｒｔｈｉｔｅ、ＣａＯ・Ａｌ2Ｏ3・２ＳｉＯ2）の生成が抑制される。アノーサイトはヤング率が低く、酸化物系介在物の母相に対するヤング率比が小さくなって転動疲労特性を低下させるため、これらを抑制することで転動疲労特性が向上する。このような効果を得るためには、Ｔｉ含有量は０．０００５％以上とする必要がある。ただし、Ｔｉの含有量が多くなり、０．０１０％を超えると、ヤング率の高いＴｉ系の結晶相が多く生成してしまうため、転動疲労特性を低下する。従って、Ｔｉ含有量は０．０１０％以下とした。Ｔｉ含有量の好ましい下限は０．０００８％以上（より好ましくは０．００１１％以上）であり、好ましい上限は０．００５０％以下（より好ましくは０．００３０％以下）である。
[Ti: 0.0005 to 0.010%]
Ti is an element that characterizes the present invention. By adding a predetermined amount of Ti and appropriately controlling the content of TiO 2 in the oxide, generation of anorthite (Anorthite, CaO.Al 2 O 3 .2SiO 2 ) that was likely to occur in Si deoxidized steel Is suppressed. Anorthite has a low Young's modulus, and the Young's modulus ratio of the oxide inclusions to the parent phase is reduced to lower the rolling fatigue characteristics. Therefore, suppressing these improves the rolling fatigue characteristics. In order to obtain such an effect, the Ti content needs to be 0.0005% or more. However, if the Ti content increases and exceeds 0.010%, a large amount of Ti-based crystal phase having a high Young's modulus is generated, and therefore rolling fatigue characteristics are deteriorated. Therefore, the Ti content is set to 0.010% or less. The preferable lower limit of the Ti content is 0.0008% or more (more preferably 0.0011% or more), and the preferable upper limit is 0.0050% or less (more preferably 0.0030% or less).
［Ｏ：０．００２５％以下（０％を含まない）］
Ｏは、好ましくない不純物元素である。Ｏの含有量が多くなって、特に０．００２５％を超えると、粗大な酸化物が生成し易くなり、熱間圧延および冷間圧延後においても粗大な酸化物として残存し、転動疲労特性に悪影響を及ぼすため、０．００２５％以下とすることが好ましい。より好ましい上限は０．００２３％以下（更に好ましくは０．００２０％以下）である。
[O: 0.0025% or less (excluding 0%)]
O is an undesirable impurity element. When the content of O increases, especially when it exceeds 0.0025%, a coarse oxide tends to be formed, and remains as a coarse oxide even after hot rolling and cold rolling. In order to have an adverse effect on the content, it is preferably 0.0025% or less. A more preferable upper limit is 0.0023% or less (more preferably 0.0020% or less).
本発明で規定する含有元素は上記の通りであって、残部は鉄および不可避的不純物であり、該不可避的不純物として、原料、資材、製造設備等の状況によって持ち込まれる元素（例えば、Ａｓ、Ｈ、Ｎ等）の混入が許容され得る。例えば、Ｎについては常法の製造方法で製造した場合、０．００４〜０．００８％の範囲で含有される。Ｎについては、ＴｉＮ等の窒化物を生成するため、例えば、真空脱ガス処理を施すことで０．００４％以下まで低減することができれば、転動疲労寿命の更なる向上が可能になると考えられるため、できる限り低減することが望ましい。 The contained elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities. Elements that are brought in depending on the situation of raw materials, materials, production facilities, etc. as the unavoidable impurities (for example, As, H , N, etc.) can be permitted. For example, N is contained in a range of 0.004 to 0.008% when manufactured by a conventional manufacturing method. For N, a nitride such as TiN is generated. For example, if it can be reduced to 0.004% or less by performing vacuum degassing, it is considered that the rolling fatigue life can be further improved. Therefore, it is desirable to reduce as much as possible.
本発明では以上の元素を含有する軸受用鋼材において酸化物系介在物のアスペクト比（長径／短径）とヤング率とを特定の範囲に制御している。これらを制御することによって転動疲労特性を十分に向上できる。 In the present invention, the aspect ratio (major axis / minor axis) and Young's modulus of the oxide inclusions are controlled in a specific range in the bearing steel material containing the above elements. By controlling these, the rolling fatigue characteristics can be sufficiently improved.
（１）アスペクト比（長径／短径）
アスペクト比は酸化物系介在物の形状を示す値で、アスペクト比が小さくなるほど、酸化物系介在物は球状に近い形状をとる。その結果、酸化物系介在物と鋼である母相との界面部分への応力集中が緩和され、転動疲労特性が改善する。転動疲労特性を十分に向上させるためには、アスペクト比を３．０以下にする必要がある。アスペクト比は小さい程よく、好ましくは２．５以下、より好ましくは２．０以下である。
(1) Aspect ratio (major axis / minor axis)
The aspect ratio is a value indicating the shape of oxide inclusions. The smaller the aspect ratio, the closer the oxide inclusions have a spherical shape. As a result, the stress concentration at the interface between the oxide inclusions and the parent phase, which is steel, is alleviated and the rolling fatigue characteristics are improved. In order to sufficiently improve the rolling fatigue characteristics, the aspect ratio needs to be 3.0 or less. The aspect ratio is preferably as small as possible, preferably 2.5 or less, more preferably 2.0 or less.
（２）ヤング率比
酸化物系介在物のヤング率を母相のヤング率に近づけるほど、酸化物系介在物と母相との界面部分への応力集中が緩和され、転動疲労特性が改善する。酸化物系介在物のヤング率（Ｅ1）は、母相のヤング率（Ｅ2）に対する比（ヤング率比＝Ｅ1／Ｅ2）として規定される。所定の効果を達成するため、このヤング率比は０．５超、好ましくは０．７以上、さらに好ましくは０．８以上である。またヤング率比の上限は１．５未満、好ましくは１．３以下、より好ましくは１．２以下である。
(2) Young's modulus ratio The closer the Young's modulus of oxide inclusions is to the Young's modulus of the matrix phase, the more stress concentration at the interface between the oxide inclusions and the matrix phase is relaxed, and the rolling fatigue characteristics are improved. To do. The Young's modulus (E 1 ) of the oxide inclusion is defined as a ratio (Young's modulus ratio = E 1 / E 2 ) to the Young's modulus (E 2 ) of the parent phase. In order to achieve a predetermined effect, this Young's modulus ratio is more than 0.5, preferably 0.7 or more, more preferably 0.8 or more. The upper limit of the Young's modulus ratio is less than 1.5, preferably 1.3 or less, more preferably 1.2 or less.
なお前記アスペクト比及びヤング率比は、熱間圧延後の線材を球状化焼鈍と、それに次ぐ焼入れ・焼戻しした後の鋼材の長手方向（熱間圧延方向）に平行する断面で決定できる。この断面は、鋼材の直径Ｄの１／４の位置が中心部となるようにすることが好ましい。球状化焼鈍条件、焼入れ条件、焼戻し条件は、例えば、以下の通りであってもよい。
［球状化焼鈍条件］
加熱条件：温度７６０〜８００℃で２〜８時間
冷却パターン：“Ａｒ1変態点−６０℃”の温度までを速度１０〜１５℃／時で冷却し、次いで大気放冷する
［焼入れ条件］
加熱条件：温度８４０℃で３０分間
冷却パターン：油焼入れ
［焼戻し条件］
加熱条件：１６０℃で２０分間
冷却パターン：空冷
The aspect ratio and Young's modulus ratio can be determined by a cross section parallel to the longitudinal direction (hot rolling direction) of the steel material after spheroidizing annealing of the wire material after hot rolling and subsequent quenching and tempering. This cross-section is preferably such that the position of 1/4 of the diameter D of the steel material is the center. The spheroidizing annealing condition, quenching condition, and tempering condition may be as follows, for example.
[Spheroidizing annealing conditions]
Heating condition: 2 to 8 hours at a temperature of 760 to 800 ° C. Cooling pattern: Cool to a temperature of “Ar 1 transformation point −60 ° C.” at a rate of 10 to 15 ° C./hour and then let cool to the atmosphere [Quenching conditions]
Heating condition: 30 minutes at a temperature of 840 ° C Cooling pattern: Oil quenching [Tempering condition]
Heating condition: 20 minutes at 160 ° C Cooling pattern: Air cooling
なおアスペクト比は短径が１μｍ以上の酸化物系介在物を対象に複数測定され、その平均値を対象鋼材でのアスペクト比とする。また酸化物系介在物のヤング率は、式：（長径×短径）1/2で求まる介在物寸法が５μｍ以上となる介在物を対象に複数測定され、その平均値を酸化物系介在物のヤング率（Ｅ1）とする。母相のヤング率も複数測定され、その平均値を母相のヤング率（Ｅ2）とする。 A plurality of aspect ratios are measured for oxide inclusions having a minor axis of 1 μm or more, and the average value is defined as the aspect ratio of the target steel material. The Young's modulus of oxide inclusions was measured for inclusions with an inclusion size of 5 μm or more determined by the formula: (major axis × minor axis) 1/2 , and the average value was determined for the oxide inclusions. Of Young's modulus (E 1 ). A plurality of Young's moduli of the mother phase are also measured, and the average value is defined as the Young's modulus (E 2 ) of the mother phase.
酸化物系介在物のアスペクト比とヤング率を所定の範囲にするには、例えば、該酸化物系介在物が、平均組成の質量％（全酸化物系介在物に対する比率）で、ＣａＯ：２０〜５０％、Ａｌ2Ｏ3：２０〜５０％、ＳｉＯ2：２０〜５７％と、ＴｉＯ2：３〜１０％およびＭｇＯ：３〜１０％のいずれか１種または両方とを含有する様にすればよい。なお、これらＣａＯ、Ａｌ2Ｏ3、ＳｉＯ2、ＴｉＯ2、ＭｇＯ以外の酸化物を残部として含有していてもよく、それら残部は不純物である。さらにＴｉＯ2、ＭｇＯについても、上記有効成分量より少ない３％未満となる場合は、本明細書では不純物に分類する。以下、各酸化物の含有量の設定理由について説明する。 In order to set the aspect ratio and Young's modulus of the oxide inclusions within a predetermined range, for example, the oxide inclusions are in mass% of the average composition (ratio to the total oxide inclusions), and CaO: 20 ~50%, Al 2 O 3: 20~50%, SiO 2: 20~57% and, TiO 2: 3-10% and MgO: so as to contain and either one or both of 3-10% do it. Note that these CaO, may contain Al 2 O 3, SiO 2, TiO 2, MgO is not an oxide of the balance, they balance being impurities. Further, TiO 2 and MgO are also classified as impurities in this specification when the amount is less than 3%, which is less than the above-mentioned effective component amount. Hereinafter, the reason for setting the content of each oxide will be described.
［ＣａＯ：２０〜５０％］
ＣａＯはＳｉＯ2に含まれると、酸化物の結晶化を促進して圧延時の延伸を抑制し、酸化物系介在物のアスペクト比低減に重要な役割を果たす。更には生成する結晶相のヤング率を母相に近づける働きがある。このような効果は、酸化物の平均組成におけるＣａＯ含有量を２０％以上に制御することによって得られる。しかしながら、ＣａＯ含有量が少なくなり過ぎると、ＳｉＯ2を多く含む介在物は非晶質相のまま残存しやすくなるため、ヤング率が低下し転動疲労特性が低下する。一方で、ＣａＯ含有量が多すぎると、酸化物組成におけるＣａＯの割合が高くなり過ぎて、生成する酸化物の結晶相のヤング率が低下し転動疲労特性が低下するため、５０％以下とする必要がある。酸化物中におけるＣａＯ含有量の好ましい下限は２２％以上（より好ましくは２５％以上）であり、好ましい上限は４３％以下（より好ましくは４１％以下）である。
[CaO: 20 to 50%]
When CaO is contained in SiO 2, to promote the crystallization of the oxide suppresses the stretching during rolling, it plays an important role in reducing the aspect ratio of the oxide inclusions. Furthermore, it has a function of bringing the Young's modulus of the crystal phase to be generated closer to the parent phase. Such an effect is obtained by controlling the CaO content in the average composition of the oxide to 20% or more. However, if the CaO content is too low, inclusions containing a large amount of SiO 2 tend to remain in an amorphous phase, resulting in a decrease in Young's modulus and a decrease in rolling fatigue characteristics. On the other hand, if the content of CaO is too large, the proportion of CaO in the oxide composition becomes too high, the Young's modulus of the crystal phase of the oxide to be produced is lowered, and the rolling fatigue characteristics are lowered. There is a need to. The preferable lower limit of the CaO content in the oxide is 22% or more (more preferably 25% or more), and the preferable upper limit is 43% or less (more preferably 41% or less).
［Ａｌ2Ｏ3：２０〜５０％］
Ａｌ2Ｏ3はＳｉＯ2に含まれると、酸化物の結晶化を促進して圧延時の延伸を抑制し、酸化物系介在物のアスペクト比低減に重要な役割を果たす。更には生成する結晶相のヤング率を母相に近づける働きがある。このような効果は、酸化物の平均組成におけるＡｌ2Ｏ3含有量を２０％以上に制御することによって得られる。しかしながら、Ａｌ2Ｏ3含有量が少なくなり過ぎると、ＳｉＯ2を多く含む介在物は非晶質相のまま残存しやすくなるため、ヤング率が低下し転動疲労特性が低下する。一方、酸化物の平均組成における含有量が５０％を超えると、溶鋼中および凝固過程でＡｌ2Ｏ3（コランダム）結晶相が晶出したり、あるいは圧延温度域でこれらの結晶相が生成する。これらの固相は硬質であり、粗大な介在物として存在し、転動疲労特性を悪化させる。こうした観点から、酸化物の平均組成におけるＡｌ2Ｏ3含有量は５０％以下とする必要がある。酸化物におけるＡｌ2Ｏ3含有量の好ましい下限は２２％以上（より好ましくは２５％以上）であり、好ましい上限は４３％以下（より好ましくは４１％以下）である。
[Al 2 O 3 : 20 to 50%]
When Al 2 O 3 is contained in SiO 2 , it promotes crystallization of the oxide, suppresses stretching during rolling, and plays an important role in reducing the aspect ratio of oxide inclusions. Furthermore, it has a function of bringing the Young's modulus of the crystal phase to be generated closer to the parent phase. Such an effect can be obtained by controlling the Al 2 O 3 content in the average composition of the oxide to 20% or more. However, if the Al 2 O 3 content is too low, inclusions containing a large amount of SiO 2 tend to remain in an amorphous phase, so that the Young's modulus is lowered and the rolling fatigue characteristics are lowered. On the other hand, if the content of the oxide in the average composition exceeds 50%, an Al 2 O 3 (corundum) crystal phase is crystallized in the molten steel and in the solidification process, or these crystal phases are generated in the rolling temperature range. These solid phases are hard and exist as coarse inclusions, deteriorating rolling fatigue properties. From such a viewpoint, the Al 2 O 3 content in the average composition of the oxide needs to be 50% or less. The preferable lower limit of the Al 2 O 3 content in the oxide is 22% or more (more preferably 25% or more), and the preferable upper limit is 43% or less (more preferably 41% or less).
［ＳｉＯ2：２０〜５７％］
ＳｉＯ2はＡｌ2Ｏ3（コランダム）結晶相のような硬質で粗大な介在物を抑制するために必要不可欠である。更には生成する結晶相のヤング率を母相に近づける働きがある。このような効果を有効に発揮させるためには、酸化物中にＳｉＯ2を２０％以上含有させる必要がある。しかしながら、ＳｉＯ2含有量が５７％を超えると、ＳｉＯ2主体の非晶質相が形成されるため、ヤング率が低下すると共に、熱間加工により延伸しアスペクト比が大きくなり転動疲労特性が悪化する。酸化物中におけるＳｉＯ2含有量の好ましい下限は２５％以上（より好ましくは３０％以上）であり、好ましい上限は５０％以下（より好ましくは４５％以下）である。
[SiO 2 : 20 to 57%]
SiO 2 is indispensable for suppressing hard and coarse inclusions such as Al 2 O 3 (corundum) crystal phase. Furthermore, it has a function of bringing the Young's modulus of the crystal phase to be generated closer to the parent phase. In order to exhibit such an effect effectively, it is necessary to contain 20% or more of SiO 2 in the oxide. However, when the SiO 2 content exceeds 57%, an amorphous phase mainly composed of SiO 2 is formed, so that the Young's modulus is lowered, and the aspect ratio is increased by stretching due to hot working, resulting in rolling fatigue characteristics. Getting worse. The preferable lower limit of the SiO 2 content in the oxide is 25% or more (more preferably 30% or more), and the preferable upper limit is 50% or less (more preferably 45% or less).
［ＴｉＯ2：３〜１０％］
ＴｉＯ2は、ＭｇＯと選択的に用いられ、本発明を特徴付ける酸化物成分である。酸性酸化物であるＳｉＯ2にＴｉＯ2が含まれると、Ｓｉ脱酸鋼において発生し易かったアノーサイト（Ａｎｏｒｔｈｉｔｅ、ＣａＯ・Ａｌ2Ｏ3・２ＳｉＯ2）の生成が抑制される。アノーサイトはヤング率が低く、介在物の母相に対するヤング率比が小さくなって転動疲労特性を低下させる。このアノーサイトを抑制することで転動疲労特性が向上する。このような効果は、酸化物の平均組成におけるＴｉＯ2含有量を３％以上に制御することによって得られる。しかしながら、ＴｉＯ2含有量が高すぎると、ヤング率の高いＴｉ系の結晶相が多く生成して転動疲労特性が低下するため、１０％以下とする。酸化物中におけるＴｉＯ2含有量の好ましい下限は４％以上（より好ましくは５％以上）であり、好ましい上限は８％以下（より好ましくは７％以下）である。
[TiO 2 : 3 to 10%]
TiO 2 is an oxide component that is selectively used with MgO and characterizes the present invention. When TiO 2 is contained in SiO 2 that is an acidic oxide, the formation of anorthite (Anorthite, CaO.Al 2 O 3 .2SiO 2 ) that is easily generated in Si deoxidized steel is suppressed. Anorthite has a low Young's modulus, and the ratio of Young's modulus of inclusions to the parent phase is reduced, thereby reducing the rolling fatigue characteristics. By suppressing this anorthite, rolling fatigue characteristics are improved. Such an effect can be obtained by controlling the TiO 2 content in the average composition of the oxide to 3% or more. However, if the TiO 2 content is too high, a large amount of Ti-based crystal phase with a high Young's modulus is generated and the rolling fatigue characteristics deteriorate, so the content is made 10% or less. The preferable lower limit of the TiO 2 content in the oxide is 4% or more (more preferably 5% or more), and the preferable upper limit is 8% or less (more preferably 7% or less).
［ＭｇＯ：３〜１０％］
ＭｇＯは、ＴｉＯ2と選択的に用いられ、本発明を特徴付ける酸化物成分である。ＳｉＯ2含有酸化物系介在物にＭｇＯが含まれると、Ｓｉ脱酸鋼において発生し易かったＳｉＯ2を主体とする非晶質相の生成が抑制される。非晶質相はヤング率が低く、母相とのヤング率比が小さく転動疲労特性を低下させる。更に、熱間加工時に延伸しやすくアスペクト比が大きくなり転動疲労特性を低下させる。このような効果は、酸化物の平均組成におけるＭｇＯ含有量を３％以上に制御することによって得られる。しかしながら、ＭｇＯ含有量が高すぎると、溶鋼中および凝固過程でＭｇＯ・Ａｌ2Ｏ3（スピネル）などの結晶相が晶出したり、あるいは、圧延温度域でこれらの結晶相が生成する。これらの固相は硬質であり、粗大な介在物として存在し、転動疲労特性を悪化させる。この硬質粗大な介在物を抑制するため、ＭｇＯは１０％以下とする。酸化物中におけるＭｇＯ含有量の好ましい下限は４％以上（より好ましくは５％以上）であり、好ましい上限は８％以下（より好ましくは７％以下）である。
[MgO: 3 to 10%]
MgO is an oxide component that is selectively used with TiO 2 and characterizes the present invention. When MgO is contained in the SiO 2 -containing oxide inclusions, generation of an amorphous phase mainly composed of SiO 2 that is easily generated in Si deoxidized steel is suppressed. The amorphous phase has a low Young's modulus, and the Young's modulus ratio with the parent phase is small, thereby reducing the rolling fatigue characteristics. Furthermore, it is easy to stretch at the time of hot working, and the aspect ratio becomes large and the rolling fatigue characteristics are lowered. Such an effect is obtained by controlling the MgO content in the average composition of the oxide to 3% or more. However, if the MgO content is too high, crystal phases such as MgO.Al 2 O 3 (spinel) are crystallized in the molten steel and in the solidification process, or these crystal phases are generated in the rolling temperature range. These solid phases are hard and exist as coarse inclusions, deteriorating rolling fatigue properties. In order to suppress this hard coarse inclusion, MgO is made 10% or less. The preferable lower limit of the MgO content in the oxide is 4% or more (more preferably 5% or more), and the preferable upper limit is 8% or less (more preferably 7% or less).
前記酸化物系介在物は、ＣａＯ、Ａｌ2Ｏ3、ＳｉＯ2、ＴｉＯ2および／またはＭｇＯで構成され、残部は不純物である。不純物としては、製造過程などで不可避的に含まれる不純物が挙げられる。不純物は、酸化物系介在物の結晶化状態やアスペクト比などに悪影響を及ぼさず、所望の特性が得られる限度において含まれ得るが、不純物全体（合計量）として、おおむね、２０％以下、好ましくは１０％以下、さらに好ましくは５％以下である。具体的には、例えばＺｒＯ2は約１％以下、Ｎａ2Ｏは約５％以下、Ｃｒ2Ｏ3は約５％以下の範囲で含有することができる。また、ＭｎＯは比較的許容幅の広い酸化物であり、最大で約１５％以下の範囲で含有することができる。 The oxide inclusions are composed of CaO, Al 2 O 3 , SiO 2 , TiO 2 and / or MgO, and the balance is impurities. Examples of impurities include impurities inevitably included in the manufacturing process. Impurities do not adversely affect the crystallization state and aspect ratio of oxide inclusions, and may be included as long as desired characteristics are obtained. However, the total impurities (total amount) are generally 20% or less, preferably Is 10% or less, more preferably 5% or less. Specifically, for example, ZrO 2 can be contained in a range of about 1% or less, Na 2 O in a range of about 5% or less, and Cr 2 O 3 in a range of about 5% or less. Further, MnO is an oxide having a relatively wide allowable width, and can be contained in a range of up to about 15%.
なお前記酸化物系介在物の組成は、熱間圧延後の線材を球状化焼鈍と、それに次ぐ焼入れ・焼戻しした後の鋼材の長手方向（熱間圧延方向）に平行する断面で決定できる。この断面は、鋼材の直径Ｄの１／４の位置が中心部となるようにすることが好ましい。球状化焼鈍条件、焼入れ条件、焼戻し条件は、例えば、上記アスペクト比決定の場合と同様であってもよい。なお酸化物系介在物の組成は短径が１μｍ以上の酸化物系介在物を対象に複数測定され、その平均値を対象鋼材での組成とする。 The composition of the oxide inclusions can be determined by a cross section parallel to the longitudinal direction (hot rolling direction) of the steel material after spheroidizing annealing and then quenching and tempering the wire material after hot rolling. This cross-section is preferably such that the position of 1/4 of the diameter D of the steel material is the center. The spheroidizing annealing condition, quenching condition, and tempering condition may be the same as those in the above aspect ratio determination, for example. A plurality of oxide inclusions are measured for oxide inclusions having a minor axis of 1 μm or more, and the average value is defined as the composition of the target steel material.
酸化物系介在物の組成を上記範囲にするのに好適な溶製方法は以下のとおりである。まず鋼材を溶製する際に、通常実施されるＡｌ添加での脱酸処理を行なわずに、Ｓｉ添加による脱酸を実施する。この溶製時には、Ａｌ2Ｏ3の含有量を制御するために、鋼中に含まれるＡｌ含有量を上記のとおり、０．０００２〜０．００５％に制御する。また、ＣａＯ制御方法としては、特に限定するものではないが、例えば、常法に従って調整できる。具体的には鋼中に含まれるＣａ含有量が上記のとおり０．０００２〜０．００２０％の範囲内に制御されるように溶製時にＣａを添加すればよい。Ｃａの添加方法は特に限定されず、例えば、Ｃａを含有する合金（例えば、Ｎｉ−Ｃａ合金など）を添加して調製してもよいし、あるいは、スラグ組成の制御によって溶鋼中のＣａ濃度を制御してもかまわない。 A suitable melting method for setting the composition of the oxide inclusions within the above range is as follows. First, when steel material is melted, deoxidation by Si addition is performed without performing the deoxidation treatment by Al addition that is usually performed. At the time of this melting, in order to control the content of Al 2 O 3 , the Al content contained in the steel is controlled to 0.0002 to 0.005% as described above. Further, the CaO control method is not particularly limited, but can be adjusted according to a conventional method, for example. Specifically, Ca may be added during melting so that the Ca content contained in the steel is controlled within the range of 0.0002 to 0.0020% as described above. The addition method of Ca is not specifically limited, For example, you may prepare by adding the alloy containing Ca (for example, Ni-Ca alloy etc.), or control Ca concentration in molten steel by controlling slag composition. You can control it.
また、ＴｉＯ2やＭｇＯ制御方法としては、特に限定するものではないが、例えば、常法に従って調製できる。具体的には鋼中に含まれるＴｉ含有量が上記のとおり０．０００５〜０．０１０％の範囲内に制御されるように溶製時にＴｉを添加すればよい。Ｔｉの添加方法は特に限定されず、例えば、Ｔｉを含有する合金（例えば、Ｆｅ−Ｔｉ合金などの鉄系合金）を添加して調製してもよいし、あるいは、スラグ組成の制御によって溶鋼中のＴｉ濃度を制御してもかまわない。また、ＭｇＯの制御方法についても特に限定するものではなく、例えば、スラグ組成の制御によって調整しても良いし、ＭｇＯ含有耐火物との反応を活用して、溶鋼中に存在する介在物中のＭｇＯ濃度を制御してもかまわない。
なお、ＳｉＯ2は、他の酸化物を上記のようにコントロールすることによって所定の範囲に調製されるものである。
The method for controlling TiO 2 or MgO is not particularly limited, but can be prepared, for example, according to a conventional method. Specifically, Ti may be added during melting so that the Ti content contained in the steel is controlled within the range of 0.0005 to 0.010% as described above. The addition method of Ti is not particularly limited, and may be prepared, for example, by adding an alloy containing Ti (for example, an iron-based alloy such as an Fe-Ti alloy) or in the molten steel by controlling the slag composition. The Ti concentration may be controlled. Further, the MgO control method is not particularly limited. For example, it may be adjusted by controlling the slag composition, or by utilizing the reaction with the MgO-containing refractory, the inclusion in the inclusions present in the molten steel The MgO concentration may be controlled.
Note that SiO 2 is prepared within a predetermined range by controlling other oxides as described above.
酸化物系介在物のアスペクト比及びヤング率を特定の範囲に制御するには、上述した酸化物系介在物の組成制御の他、鋼材の製造条件を制御する必要がある。具体的には、鋼材を熱間圧延または熱間鍛造の為に加熱する時の在炉時間（均熱時間）を十分に確保する必要がある。在炉時間が短くなると、アスペクト比が大きくなりすぎたり、ヤング率比が小さくなりすぎたりする場合がある。適切な在炉時間は加熱温度に応じて設定されるが、例えば、３５分以上、好ましくは４０分以上、さらに好ましくは４５分以上である。在炉時間の上限は常識的な範囲で設定でき、例えば、１８０分である。 To control the aspect ratio and Young's modulus of the oxide inclusions in a specific range, in addition to the composition control of oxide inclusions as described above, it is necessary to control the production conditions of the steel. Specifically, it is necessary to ensure a sufficient in-furnace time (soaking time) when the steel material is heated for hot rolling or hot forging. If the in-furnace time is shortened, the aspect ratio may become too large, or the Young's modulus ratio may become too small. An appropriate in-furnace time is set according to the heating temperature, and is, for example, 35 minutes or more, preferably 40 minutes or more, more preferably 45 minutes or more. The upper limit of the in-furnace time can be set within a common sense range, for example, 180 minutes.
本発明の鋼材を製造するに当たっては、アスペクト比やヤング率を特定の範囲にするための上記制約（酸化物の成分調製、在炉時間など）を除いては常法に従って熱間圧延または熱間鍛造すればよい。この圧延材または鍛造材は、必要に応じて球状化焼鈍を行った後、熱間加工または冷間加工を行ってもよい。さらには焼入れ、焼戻しをしてもよい。所定の部品形状に加工した後で焼入れ・焼戻しすると、本発明の軸受部品が得られる。鋼材段階の形状については、こうした製造に適用できるような線状・棒状のいずれも含むものであり、そのサイズも、最終製品に応じて適宜決めることができる。
上記軸受部品としては、例えば、コロ、ニードル、玉、レース等が挙げられる。
In producing the steel material of the present invention, hot rolling or hot rolling is carried out in accordance with a conventional method except for the above-mentioned restrictions for adjusting the aspect ratio and Young's modulus (preparation of oxide components, furnace time, etc.). Forging is sufficient. This rolled material or forged material may be subjected to hot working or cold working after spheroidizing annealing as necessary. Further, quenching and tempering may be performed. The bearing component of the present invention can be obtained by quenching and tempering after processing into a predetermined component shape. The shape of the steel material stage includes both a linear shape and a rod shape applicable to such production, and the size can be appropriately determined according to the final product.
Examples of the bearing parts include rollers, needles, balls, and races.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
（１）鋳片の製造
小型溶解炉（容量１７０ｋｇ／１ｃｈ）を用い、下記表１に示す各種化学成分組成の供試鋼（残部は鉄および不可避的不純物）を溶製し、鋳片（鋳片上部の直径がφ２４５ｍｍ、鋳片下部の直径がφ２１０ｍｍで鋳片の高さが４８０ｍｍ）を作製した。この溶製時にはＭｇＯ系耐火物の取鍋を用い、通常実施されるＡｌ脱酸処理を行わず、Ｃ、Ｓｉ、Ｍｎ、およびＣｒを用いて溶鋼の溶存酸素量を調整した後（但し、鋼材Ｎｏ．１１はＡｌ添加による脱酸処理を実施）、Ｔｉ源、Ｃａ源をこの順序で投入し、Ｔｉ含有量、Ｃａ含有量を制御した。このとき、酸化物系介在物中のＭｇＯの含有量は、溶製時にＭｇＯを含む耐火物を溶解炉や精錬容器、搬送容器などに用いることによって調整した。例えば、合金投入後の溶製時間を調整することで酸化物系介在物中のＭｇＯ含有量を調整した。なお、本実施例では、前記Ｃａ源としてＮｉ−Ｃａ合金を、前記Ｔｉ源としてＦｅ−Ｔｉ合金を、それぞれ用いた。このようにして得られた鋳片の化学成分を表１に示す。
(1) Manufacture of slab Using a small melting furnace (capacity 170kg / 1ch), melt the test steels of various chemical composition shown in the following Table 1 (the balance is iron and inevitable impurities) The diameter of the upper part of the piece was 245 mm, the diameter of the lower part of the slab was 210 mm, and the height of the slab was 480 mm. At the time of melting, a ladle made of MgO-based refractory is used, and after the usual oxygen deoxidation treatment is not performed, the dissolved oxygen amount of the molten steel is adjusted using C, Si, Mn, and Cr (however, the steel material No. 11 was subjected to deoxidation treatment by addition of Al), Ti source and Ca source were added in this order to control Ti content and Ca content. At this time, the content of MgO in the oxide inclusions was adjusted by using a refractory containing MgO in a melting furnace, a refining vessel, a transport vessel or the like during melting. For example, the MgO content in the oxide inclusions was adjusted by adjusting the melting time after charging the alloy. In this example, Ni—Ca alloy was used as the Ca source, and Fe—Ti alloy was used as the Ti source. The chemical composition of the slab thus obtained is shown in Table 1.
（２）圧延材の製造
得られた鋳片を、加熱炉において温度１１００〜１３００℃に加熱し、所定時間の間この温度域で鋳片を保持した後、温度９００〜１２００℃で分塊圧延した。その後、加熱炉において温度８３０〜１２００℃に加熱し、所定時間（後述する表２の在炉時間の欄に示す）の間この温度域で鋼材を保持した後、温度８３０〜１１００℃で熱間圧延または熱間鍛造を実施し、所定の径（φ６５ｍｍ）の熱間圧延材または熱間鍛造材を得た。
(2) Manufacture of rolled material The obtained slab is heated to a temperature of 1100 to 1300 ° C in a heating furnace, and the slab is held in this temperature range for a predetermined time, and then rolled at a temperature of 900 to 1200 ° C. did. Then, after heating to a temperature of 830 to 1200 ° C. in a heating furnace and holding the steel material in this temperature range for a predetermined time (shown in the column of in-furnace time of Table 2 described later), hot at a temperature of 830 to 1100 ° C. Rolling or hot forging was performed to obtain a hot rolled material or hot forged material having a predetermined diameter (φ65 mm).
（３）酸化物系介在物の平均組成決定用試験片の製造と平均組成の決定
上記熱間圧延材または熱間鍛造材を、７６０〜８００℃の温度範囲で２〜８時間加熱した後、１０〜１５℃／時の冷却速度で“Ａｒ1変態点−６０℃”の温度まで冷却してから大気放冷することにより（球状化焼鈍）、球状化セメンタイトを分散させた球状化焼鈍材を得た。
この球状化焼鈍材からφ６０ｍｍ、厚さ３０ｍｍの試験片を切り出し、温度８４０℃で３０分間加熱した後に油焼入れをし、次いで温度１６０℃で１２０分間焼戻しを行った。各試験片について、直径Ｄの１／４の位置が中心となるよう直径方向に２０ｍｍ長さ、鋼材の長手方向（圧延方向に相当）に５ｍｍＬ（圧延方向長さ）のミクロ試料を１個切り出し、断面を研磨した。研磨面を日本電子データム社製の電子線マイクロプローブＸ線分析計（Ｅｌｅｃｔｒｏｎ Ｐｒｏｂｅ Ｘ−ｒａｙ Ｍｉｃｒｏ Ａｎａｌｙｚｅｒ：ＥＰＭＡ 商品名「ＪＸＡ−８５００Ｆ」）を用いて観察し、短径が１μｍ以上の酸化物系介在物について成分組成を定量分析した。このとき、観察面積を１００ｍｍ2（研磨面）とし、介在物の中央部での成分組成を特性Ｘ線の波長分散分光により定量分析した。分析対象元素は、Ｃａ、Ａｌ、Ｓｉ、Ｔｉ、Ｍｎ、Ｍｇ、Ｎａ、Ｃｒ、Ｚｒ、Ｏ（酸素）とし、既知物質を用いて各元素のＸ線強度と元素濃度の関係を予め検量線として求めておき、分析対象とする上記介在物から得られたＸ線強度と上記検量線から各試料に含まれる元素量を定量し、その結果を算術平均することで平均の介在物組成を求めた。
(3) Production of test piece for determining average composition of oxide inclusion and determination of average composition After heating the hot rolled material or hot forged material at a temperature range of 760 to 800 ° C for 2 to 8 hours, By cooling to a temperature of “Ar 1 transformation point −60 ° C.” at a cooling rate of 10 to 15 ° C./hour and then allowing to cool to the atmosphere (spheroidizing annealing), a spheroidized annealing material in which spheroidized cementite is dispersed is obtained. Obtained.
A test piece having a diameter of 60 mm and a thickness of 30 mm was cut out from the spheroidized annealed material, heated at a temperature of 840 ° C. for 30 minutes, then oil-quenched, and then tempered at a temperature of 160 ° C. for 120 minutes. For each test piece, one micro sample of 20 mm length in the diameter direction and 5 mm L (length in the rolling direction) in the longitudinal direction of the steel material (corresponding to the rolling direction) is cut out so that the position of 1/4 of the diameter D is the center. The cross section was polished. The polished surface was observed using an electron probe X-ray micro analyzer (EPMA trade name “JJA-8500F”) manufactured by JEOL Datum, and an oxide system having a minor axis of 1 μm or more. The component composition was quantitatively analyzed for inclusions. At this time, the observation area was set to 100 mm 2 (polished surface), and the component composition at the center of the inclusion was quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays. The analysis target elements are Ca, Al, Si, Ti, Mn, Mg, Na, Cr, Zr, and O (oxygen), and the relationship between the X-ray intensity and the element concentration of each element using a known substance as a calibration curve in advance. The average inclusion composition was determined by quantifying the amount of elements contained in each sample from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve, and arithmetically averaging the results. .
（４）酸化物系介在物のアスペクト比の決定
前記酸化物系介在物の平均組成決定用試験片を用い、短径が１μｍ以上の任意の酸化物系介在物（分析対象元素は、Ｃａ、Ａｌ、Ｓｉ、Ｔｉ、Ｍｎ、Ｍｇ、Ｎａ、Ｃｒ、Ｚｒ、Ｏ（酸素））を１００個選び、各々の長径と短径を測定し、各々の酸化物系介在物のアスペクト比を算出した。その結果を算術平均することで酸化物系介在物の平均のアスペクト比を求めた。
(4) Determination of aspect ratio of oxide inclusions Using the test piece for determining the average composition of oxide inclusions, any oxide inclusion having a minor axis of 1 μm or more (analysis target element is Ca, 100 Al, Si, Ti, Mn, Mg, Na, Cr, Zr, and O (oxygen)) were selected, the major axis and the minor axis were measured, and the aspect ratio of each oxide inclusion was calculated. The average aspect ratio of the oxide inclusions was obtained by arithmetically averaging the results.
（５）ヤング率比決定用試験片の製造とヤング率比の決定
上記と同様にして得られた球状化焼鈍材からφ６０ｍｍ、厚さ３０ｍｍの試験片を切り出し、８４０℃で３０分間加熱した後に油焼入れをし、次いで温度１６０℃で１２０分間焼戻しを行った。各試験片について、直径Ｄの１／４の位置が中心となるよう直径方向に２０ｍｍ長さ、鋼材の長手方向（圧延方向に相当）に５ｍｍＬ（圧延方向長さ）のミクロ試料を１個切り出し、断面を研磨した。Ａｇｉｌｅｎｔ Ｔｅｃｈｎｏｌｏｇｉｅｓ社製の測定装置「Ｎａｎｏ Ｉｎｄｅｎｔｅｒ ＸＰ／ＤＣＭ」を用い、Ａｇｉｌｅｎｔ Ｔｅｃｈｎｏｌｏｇｉｅｓ社製の解析ソフト「Ｔｅｓｔ Ｗｏｒｋｓ ４」を用いて解析した。上記ミクロ試料中に存在する介在物寸法（（長径×短径）1/2）が５μｍ以上の介在物を任意に１０個選び、介在物の中央にダイヤモンド製バーコビッチ型の圧子を押し込むことで連続剛性測定を行った。励起振動周波数４５Ｈｚ、励起振動振幅２ｎｍ、歪速度２ｎｍ、押し込み深さ１００ｎｍとした。その結果を算術平均することで酸化物系介在物の平均ヤング率（Ｅ1）を求めた。
また上記ミクロ試料中の母相部分を任意に２ヵ所選び、ダイヤモンド製バーコビッチ型の圧子を押し込むことで連続剛性測定を行った。励起振動周波数４５Ｈｚ、励起振動振幅２ｎｍ、歪速度２ｎｍ、押し込み深さ５００ｎｍとした。その結果を算術平均することで母相の平均ヤング率（Ｅ2）を求めた。
酸化物系介在物の平均ヤング率及び母相の平均ヤング率からヤング率比（Ｅ1／Ｅ2）を求めた。
(5) Manufacture of test piece for determining Young's modulus ratio and determination of Young's modulus ratio A test piece having a diameter of 60 mm and a thickness of 30 mm was cut out from the spheroidized annealed material obtained in the same manner as described above and heated at 840 ° C. for 30 minutes. Oil quenching was followed by tempering at a temperature of 160 ° C. for 120 minutes. For each test piece, one micro sample of 20 mm length in the diameter direction and 5 mm L (length in the rolling direction) in the longitudinal direction of the steel material (corresponding to the rolling direction) is cut out so that the position of 1/4 of the diameter D is the center. The cross section was polished. Analysis was performed using a measurement apparatus “Nano Indenter XP / DCM” manufactured by Agilent Technologies, and analysis software “Test Works 4” manufactured by Agilent Technologies. Continuously by selecting 10 inclusions with an inclusion dimension ((major axis x minor axis) 1/2 ) of 5 μm or more present in the micro sample, and inserting a diamond barkovic indenter into the center of the inclusion. Stiffness measurements were made. The excitation vibration frequency was 45 Hz, the excitation vibration amplitude was 2 nm, the strain rate was 2 nm, and the indentation depth was 100 nm. The average Young's modulus (E 1 ) of the oxide inclusions was obtained by arithmetically averaging the results.
In addition, the matrix portion in the micro sample was arbitrarily selected at two locations, and continuous stiffness measurement was performed by pushing in a diamond Barkovic indenter. The excitation vibration frequency was 45 Hz, the excitation vibration amplitude was 2 nm, the strain rate was 2 nm, and the indentation depth was 500 nm. The average Young's modulus (E 2 ) of the mother phase was obtained by arithmetically averaging the results.
The Young's modulus ratio (E 1 / E 2 ) was determined from the average Young's modulus of the oxide inclusions and the average Young's modulus of the matrix.
（６）スラスト転動疲労試験片の製造と転動疲労試験
上記と同様にして得られた球状化焼鈍材からφ６０ｍｍ、厚さ６ｍｍの試験片を切り出し、温度８４０℃で３０分間加熱した後に油焼入れをし、次いで温度１６０℃で１２０分間焼戻しを行った。最後に仕上げ研磨を施して表面粗さ：０．０４μｍＲａ以下のスラスト転動疲労試験片を作製した。
上記で得られたスラスト転動疲労試験片を用い、スラスト疲労試験機（スラスト型転動疲労試験機（ＦＪ−５Ｔ）、富士試験機製作所製）にて、負荷速度：１２００ｒｐｍ、鋼球数３個、面圧：５．２４ＧＰａ、中止回数：２億回の条件でスラスト転動疲労試験を実施した。
(6) Manufacture of Thrust Rolling Fatigue Specimen and Rolling Fatigue Test A test piece having a diameter of 60 mm and a thickness of 6 mm was cut out from the spheroidized annealed material obtained in the same manner as described above, and heated at a temperature of 840 ° C. for 30 minutes. Quenching was followed by tempering at a temperature of 160 ° C. for 120 minutes. Finally, finish polishing was performed to prepare a thrust rolling fatigue test piece having a surface roughness of 0.04 μmRa or less.
Using the thrust rolling fatigue test piece obtained above, with a thrust fatigue testing machine (thrust type rolling fatigue testing machine (FJ-5T), manufactured by Fuji Testing Machine Co., Ltd.), load speed: 1200 rpm, number of steel balls 3 A thrust rolling fatigue test was performed under the conditions of individual, surface pressure: 5.24 GPa, and number of cancellations: 200 million times.
転動疲労寿命の尺度として、通常、疲労寿命Ｌ10（累積破損確率１０％における疲労破壊までの応力繰り返し数、以下「Ｌ10寿命」と呼ぶ場合がある。）が用いられる。詳細には、Ｌ10とは、試験結果をワイブル確率紙にプロットして得られる累積破損確率１０％における疲労破壊までの繰り返し数の意味である（「軸受」、岩波全書、曽田範宗著を参照）。各鋼材につき、１６個の試料を用いて上記の試験を行ってＬ10寿命を決定した。従来鋼（鋼材Ｎｏ．１１）のＬ10寿命（１．２×１０7回）に対する各鋼材のＬ10寿命の比（寿命比）を求め、（ア）Ｌ10寿命４．８×１０7回以上（４．０倍以上の寿命比）を示す鋼材を可（転動疲労寿命に優れる）と評価し、（イ）Ｌ10寿命５．４×１０7回以上（４．５倍以上の寿命比）を示す鋼材を良（転動疲労寿命に特に優れる）と評価し、（ウ）Ｌ10寿命６．０×１０7回以上（５．０倍以上の寿命比）を示す鋼材を優（転動疲労寿命に特段に優れる）と評価した。 As a rolling fatigue life scale, fatigue life L 10 (the number of stress repetitions until fatigue failure at a cumulative failure probability of 10%, hereinafter may be referred to as “L 10 life”) is usually used. Specifically, L 10 means the number of repetitions until fatigue failure with a cumulative failure probability of 10% obtained by plotting the test results on Weibull probability paper (“Bearing”, Iwanami Zensho and Nobunori Hamada reference). For each steel, it was determined L 10 life tested above using the 16 samples. The ratio of the conventional steel L 10 life of each steel material for L 10 life (1.2 × 10 7 times) of (steel No.11) a (life ratio) calculated, (A) L 10 life 4.8 × 10 7 times Steel materials exhibiting the above (a life ratio of 4.0 times or more) were evaluated as acceptable (excellent in rolling fatigue life), and (b) L 10 life 5.4 × 10 7 times or more (4.5 times or more life) Steel) showing good (particularly excellent in rolling fatigue life), and (c) steel material showing L 10 life 6.0 × 10 7 times or more (5.0 times or more life ratio). The rolling fatigue life was particularly excellent).
これらの結果を表２に記載する。なお表２の鋼材Ｎｏ．は、同じ数字の表１の鋼材Ｎｏ．を用いた事を示す。また表中、「Ｅ＋０７」は「×１０7」の意味であり、「Ｅ＋０６」は「×１０6」の意味である。 These results are listed in Table 2. In Table 2, the steel material No. Are the steel numbers of Table 1 with the same numbers. Indicates that the is used. In the table, “E + 07” means “× 10 7 ”, and “E + 06” means “× 10 6 ”.
鋼材Ｎｏ．１は鋼中のＣが過剰な為、軸受の芯部に巨大炭化物が生成してＬ10寿命が不十分となった。鋼材Ｎｏ．２は鋼中のＣｒが過剰な為、炭化物が粗大化してＬ10寿命が不十分となった。鋼材Ｎｏ．３は鋼中のＣｒが不足して軸受の強度及び耐摩耗性が不足し、Ｌ10寿命が不十分となった。鋼材Ｎｏ．４は鋼中のＰが過剰な為、Ｐが結晶粒界に偏析してＬ10寿命が不十分となった。鋼材Ｎｏ．５は鋼中のＯが過剰な為、粗大な酸化物が生成し易くなり、Ｌ10寿命が不十分となった。鋼材Ｎｏ．６は鋼中のＡｌが過剰であって酸化物系介在物中のＡｌ2Ｏ3も過剰となり、Ａｌ2Ｏ3を主体とする硬質で粗大な酸化物が多くなる。そして該酸化物系介在物ヤング率比が大きくなり過ぎる為、Ｌ10寿命が不十分となった。鋼材Ｎｏ．７は鋼中のＣａが過剰であって酸化物系介在物中のＣａＯも過剰となり、酸化物系介在物の結晶相のヤング率が低下する為、Ｌ10寿命が不十分となった。鋼材Ｎｏ．８は鋼中のＴｉが不足し、酸化物系介在物中のＴｉＯ2及びＭｇＯが不足してアノーサイトの生成を抑制できなかった。アノーサイトはヤング率が小さ過ぎて、Ｌ10寿命が不十分となった。鋼材Ｎｏ．９は鋼中のＴｉが過剰であり、酸化物系介在物中のＴｉＯ2が過剰となってＴｉ系結晶相が増えた。Ｔｉ系結晶相はヤング率が高すぎる為、Ｌ10寿命が不十分となった。鋼材Ｎｏ．１０は酸化物系介在物中のＭｇＯが過剰となってスピネル相が晶出・成長した。スピネル相はヤング率が高すぎる為、Ｌ10寿命が不十分となった。鋼材Ｎｏ．１１は従来鋼に相当し、鋼中のＡｌが過剰である一方でＣａを含有しておらず、酸化物系介在物中のＡｌ2Ｏ3が多くなった。Ａｌ2Ｏ3を主体とする硬質酸化物はヤング率が高すぎる為、Ｌ10寿命が不十分となった。鋼材Ｎｏ．１２は鋼中のＡｌとＣａが不足しており、酸化物系介在物中のＳｉＯ2が多くなりすぎた。ＳｉＯ2を過剰に含む酸化物系介在物は非晶質相のまま残存しやすくなり、アスペクト比が大きくなり過ぎると共にヤング率が小さくなりすぎてＬ10寿命が不十分となった。鋼材Ｎｏ．１３は鋼中のＳ量が多すぎてアスペクト比が大きくなりすぎると共に粗大な硫化物が残存し、Ｌ10寿命が不十分となった。鋼材Ｎｏ．１４及びＮｏ．１５は熱間圧延または熱間鍛造前の加熱時の在炉時間が短すぎるために酸化物系介在物のアスペクト比が大きくなり過ぎてＬ10寿命が低下した。
これらに対して鋼材Ｎｏ．１６〜２３は、鋼材成分が適切であり、酸化物系介在物の平均アスペクト比及びヤング率比が適切であるため、Ｌ10寿命が十分に向上した。
Steel No. 1 because of excess C in steel, L 10 life was insufficient to generate huge carbide core of the bearing. Steel No. 2 because of excessive Cr in the steel, carbide has become is insufficient coarsened by L 10 life. Steel No. 3 is insufficient Cr in the steel insufficient strength and wear resistance of the bearing, L 10 life was insufficient. Steel No. For No. 4, P in the steel was excessive, so that P segregated at the grain boundaries and the L 10 life was insufficient. Steel No. 5 because excess is O in the steel tends to generate coarse oxides, L 10 life was insufficient. Steel No. In No. 6, Al in the steel is excessive and Al 2 O 3 in the oxide inclusions is also excessive, so that a hard and coarse oxide mainly composed of Al 2 O 3 increases. And since the oxide inclusion inclusion Young's modulus ratio becomes too large, the L 10 life becomes insufficient. Steel No. 7 Ca also becomes excessive CaO in excess was in oxide-based inclusions in the steel, because the crystalline phase Young's modulus of oxide inclusions is decreased, L 10 life was insufficient. Steel No. In No. 8, Ti in the steel was insufficient, TiO 2 and MgO in the oxide inclusions were insufficient, and the formation of anorthite could not be suppressed. Anorthite is the Young's modulus is too small, L 10 life became insufficient. Steel No. In No. 9, Ti in the steel was excessive, TiO 2 in the oxide inclusions was excessive, and the Ti crystal phase increased. For Ti-based crystal phase is the Young's modulus is too high, L 10 life became insufficient. Steel No. In No. 10, MgO in the oxide inclusions was excessive and the spinel phase crystallized and grew. Since the spinel phase is the Young's modulus is too high, L 10 life became insufficient. Steel No. 11 corresponds to the conventional steel, and Al in the steel is excessive, while it does not contain Ca, and Al 2 O 3 in the oxide inclusions increases. Since the hard oxide mainly composed of Al 2 O 3 has a too high Young's modulus, the L 10 life is insufficient. Steel No. No. 12 had a shortage of Al and Ca in the steel, and too much SiO 2 in the oxide inclusions. The oxide inclusions containing excessive SiO 2 tend to remain in an amorphous phase, the aspect ratio becomes too large, the Young's modulus becomes too small, and the L 10 life becomes insufficient. Steel No. 13 is too large, the S content in steel is coarse sulfides with the aspect ratio is too large residual, L 10 life was insufficient. Steel No. 14 and no. 15 is the aspect ratio becomes too large L 10 life of oxide inclusions for standing furnace time during the heating before hot rolling or hot forging is too short is decreased.
In contrast, steel No. In Nos. 16 to 23, the steel material component was appropriate, and the average aspect ratio and Young's modulus ratio of the oxide inclusions were appropriate, so the L 10 life was sufficiently improved.
Claims (3)
Ｓｉ：０．１５〜０．８％、
Ｍｎ：０．１〜１．０％、
Ｃｒ：１．３〜１．８％、
Ｐ ：０．０５％以下（０％を含まない）、
Ｓ：０．０１５％以下（０％を含まない）、
Ａｌ：０．０００２〜０．００５％、
Ｃａ：０．０００２〜０．００２０％、
Ｔｉ：０．０００５〜０．０１０％、
Ｏ ：０．００２５％以下（０％を含まない）
を含有し、残部は鉄及び不可避的不純物からなり、
球状化焼鈍と、それに次ぐ焼入れ・焼戻し後の鋼材の圧延方向に平行する断面に存在する、式：（長径×短径）1/2で求まる介在物寸法が５μｍ以上となる酸化物系介在物のヤング率の平均値をＥ1とし、母相のヤング率の平均値をＥ2とした場合に式：Ｅ1／Ｅ2で求まるヤング率比が０．５超１．５未満であり、
球状化焼鈍と、それに次ぐ焼入れ・焼戻し後の鋼材の圧延方向に平行する断面に存在する、短径が１μｍ以上の酸化物系介在物のアスペクト比の平均値が３．０以下であることを特徴とする転動疲労特性に優れた軸受用鋼材。 C: 0.8 to 1.1% (% means mass%, hereinafter the same unless otherwise specified),
Si: 0.15 to 0.8%,
Mn: 0.1 to 1.0%,
Cr: 1.3-1.8%,
P: 0.05% or less (excluding 0%),
S: 0.015% or less (excluding 0%),
Al: 0.0002 to 0.005%,
Ca: 0.0002 to 0.0020%,
Ti: 0.0005 to 0.010%,
O: 0.0025% or less (excluding 0%)
The balance consists of iron and inevitable impurities,
Oxide inclusions with an inclusion size of 5 μm or more determined by the formula: (major axis × minor axis) 1/2 existing in the cross section parallel to the rolling direction of the steel material after spheroidizing annealing and quenching / tempering . the average value of the Young's modulus and E 1, wherein when the average value of the Young's modulus of the matrix phase and E 2: Young's modulus ratio determined by the E 1 / E 2 is less than 0.5 ultra 1.5,
The average value of the aspect ratio of the oxide inclusions having a minor axis of 1 μm or more, which is present in a section parallel to the rolling direction of the steel material after spheroidizing annealing and quenching / tempering, is 3.0 or less. Steel material for bearings with excellent rolling fatigue characteristics.
ＣａＯ：２０〜５０％、
Ａｌ2Ｏ3：２０〜５０％、
ＳｉＯ2：２０〜５７％と、
ＴｉＯ2：３〜１０％およびＭｇＯ：３〜１０％のいずれか１種または両方とを含有し、
残部は不純物からなることを特徴とする請求項１に記載の軸受用鋼材。 The oxide inclusions minor diameter is more than 1μm contained in the steel, by mass% of the average composition,
CaO: 20 to 50%,
Al 2 O 3 : 20 to 50%,
And 20~57%,: SiO 2
TiO 2 : 3 to 10% and MgO: 3 to 10% any one or both,
The bearing steel according to claim 1, wherein the balance is made of impurities.
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