JP5723232B2 - Steel for bearings with excellent rolling fatigue life - Google Patents
Steel for bearings with excellent rolling fatigue life Download PDFInfo
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本発明は、軸受部品として用いたときに優れた転動疲労寿命を発揮する軸受用鋼材に関するものである。 The present invention relates to a steel material for a bearing that exhibits an excellent rolling fatigue life when used as a bearing component.
軸受用鋼として、従来からＪＩＳ Ｇ ４８０５（１９９９）に規定されるＳＵＪ２等の高炭素クロム軸受鋼が、自動車や各種産業機械等の種々の分野で用いられている軸受の材料として使用されている。しかし軸受は、接触面圧が非常に高い玉軸受やころ軸受等の内・外輪や転動体等、過酷な環境で用いられるため、非常に微細な欠陥（介在物等）から疲労破壊が生じ易いといった問題がある。この問題に対し、転動疲労寿命そのものを高めて上記保守の回数を低減させるべく、軸受用鋼材の改善が試みられている。 Conventionally, high carbon chromium bearing steel such as SUJ2 defined in JIS G 4805 (1999) has been used as a bearing material used in various fields such as automobiles and various industrial machines. . However, since bearings are used in harsh environments such as inner and outer rings and rolling elements such as ball bearings and roller bearings with extremely high contact surface pressure, fatigue failure is likely to occur due to very fine defects (inclusions, etc.). There is a problem. In order to solve this problem, attempts have been made to improve the steel for bearings in order to increase the rolling fatigue life itself and reduce the number of maintenance operations.
例えば特許文献１には、軸受材料において、欠陥となる酸化物系非金属介在物の個数を厳密に規定することにより高寿命化を図っている。一方で、特許文献２では、上記特許文献１の評価面積よりもはるかに大きい３００００ｍｍ2の被検面積を観察することによって、特に、硫化物の最大サイズが転動疲労寿命に影響していることを見出した旨示されている。 For example, in Patent Document 1, the life of the bearing material is increased by strictly defining the number of oxide-based nonmetallic inclusions that become defects. On the other hand, in Patent Document 2, by observing a test area of 30000 mm 2 that is much larger than the evaluation area of Patent Document 1, in particular, the maximum size of the sulfide affects the rolling fatigue life. Is indicated.
しかしながら、現在、工業的に用いられている軸受用鋼材の介在物は非常に厳密に制御されたものが多く、こうした介在物の制御だけでは転動疲労寿命を更に向上させることは困難な状況になっている。 However, there are many bearing steel inclusions that are currently used industrially that are very strictly controlled, and it is difficult to further improve the rolling fatigue life by controlling these inclusions alone. It has become.
そこで、特許文献３、４に示されるように、縞状偏析を低減することによって転動疲労寿命を改善する技術が提案されている。このうち特許文献３では、圧延温度を低めにし、鍛圧比を大きく（６０以上）することで、縞状偏析に起因する硬さばらつきを低減するものである。また、特許文献４では、圧延温度は比較的高めとし、鍛圧速度を遅くすることによって、縞状偏析を改善し、その後の球状化熱処理後における炭化物の面積率のばらつきを低減して転動疲労寿命を向上するものである。 Therefore, as shown in Patent Documents 3 and 4, techniques for improving the rolling fatigue life by reducing stripe segregation have been proposed. Among these, in Patent Document 3, the rolling temperature is lowered and the forging pressure ratio is increased (60 or more) to reduce the hardness variation due to the striped segregation. Further, in Patent Document 4, the rolling temperature is relatively high, and the forging speed is slowed to improve the stripe segregation, thereby reducing the variation in the area ratio of carbides after the subsequent spheroidizing heat treatment, thereby rolling fatigue. It will improve the service life.
これら特許文献３、４の技術では、転動疲労寿命の改善効果は発揮されていると言える。しかしながら、いずれも圧延方法や圧延サイズに制約があって、工業的に自由度の高い方法とは言えない。また、転動疲労寿命の改善効果についても、更に高まる長寿命化要望に必ずしも十分に対応できているとは言えないものである。 In the techniques of Patent Documents 3 and 4, it can be said that the effect of improving the rolling fatigue life is exhibited. However, there are restrictions on the rolling method and the rolling size, and it cannot be said that the methods are industrially highly flexible. Further, the effect of improving the rolling fatigue life cannot always be said to sufficiently meet the increasing demand for longer life.
本発明はこの様な事情に鑑みてなされたものであって、その目的は、転動疲労寿命を更に向上させた軸受を得るための軸受用鋼材を提供することにある。 This invention is made | formed in view of such a situation, The objective is to provide the steel material for bearings for obtaining the bearing which further improved the rolling fatigue life.
本発明に係る転動疲労寿命に優れた軸受用鋼材とは、Ｃ：０．９５〜１．１０％（質量％の意味、以下同じ）、Ｓｉ：０．１５〜０．９０％、Ｍｎ：１．２％以下（０％を含まない）、Ｃｒ：０．９０〜１．６０％、Ｐ：０．０２５％以下（０％を含まない）、Ｓ：０．０２５％以下（０％を含まない）を夫々含み、残部が鉄および不可避不純物からなり、鋼材の圧延方向に平行な面において、圧延方向に垂直な方向にＥＰＭＡ（Ｅｌｅｃｔｒｏｎ Ｐｒｏｂｅ Ｍｉｃｒｏ Ａｎａｌｙｚｅｒ）ライン分析したとき、ＣｒのＸ線強度値の標準偏差と平均値が、下記（１）式の関係を満足する点に要旨を有するものである。
（ＣｒのＸ線強度値の標準偏差／ＣｒのＸ線強度値の平均値）≦０．２５…（１）
The steel materials for bearings having excellent rolling fatigue life according to the present invention are C: 0.95 to 1.10% (meaning of mass%, the same shall apply hereinafter), Si: 0.15 to 0.90%, Mn: 1.2% or less (excluding 0%), Cr: 0.90 to 1.60%, P: 0.025% or less (excluding 0%), S: 0.025% or less (0% X-ray strength of Cr when an EPMA (Electron Probe Micro Analyzer) line analysis is performed in a direction perpendicular to the rolling direction on the plane parallel to the rolling direction of the steel material. The standard deviation and the average value of the values have a gist in that the relationship of the following formula (1) is satisfied.
(Standard deviation of X-ray intensity value of Cr / Average value of X-ray intensity value of Cr) ≦ 0.25 (1)
本発明の軸受用鋼材においては、（１）ＰおよびＳの合計含有量を０．０２０％以下（０％を含まない）に抑制することや、（２）更に他の元素として、Ｎｉ：０．２５％未満（０％を含まない）、Ｃｕ：０．２５％未満（０％を含まない）、およびＭｏ：０．０８％未満（０％を含まない）よりなる群から選択される１種以上を含有させることも有用であり、抑制または含有される成分に応じて鋼材の特性が更に改善される。 In the steel material for bearings of the present invention, (1) the total content of P and S is suppressed to 0.020% or less (not including 0%), and (2) as another element, Ni: 0 1 selected from the group consisting of less than .25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) It is also useful to contain more than seeds, and the properties of the steel material are further improved depending on the components to be suppressed or contained.
本発明によれば、転動疲労寿命を更に向上させた軸受用鋼材が実現できるので、軸受用鋼材を軸受に適用したときに、過酷な環境で用いられても優れた転動疲労寿命を発揮でき、無駄な保守（交換、点検等）を低減することができる。 According to the present invention, it is possible to realize a bearing steel material having a further improved rolling fatigue life. Therefore, when the bearing steel material is applied to a bearing, it exhibits an excellent rolling fatigue life even when used in a harsh environment. It is possible to reduce unnecessary maintenance (replacement, inspection, etc.).
本発明者らは、転動疲労寿命の向上を目指し、介在物制御とは異なる観点として、縞状偏析の影響を更に詳細に検討した。その結果、鋼材の圧延方向に平行な面において、圧延方向に垂直な方向にＥＰＭＡライン分析したとき、ＣｒのＸ線強度値の標準偏差と平均値が、下記（１）式の関係を満足するようにすれば、転動疲労寿命が格段に向上し得ることを見出し、本発明を完成した。
（ＣｒのＸ線強度値の標準偏差／ＣｒのＸ線強度値の平均値）≦０．２５…（１）
The present inventors examined the influence of striped segregation in more detail as a viewpoint different from inclusion control with the aim of improving the rolling fatigue life. As a result, when EPMA line analysis is performed in a direction perpendicular to the rolling direction on a plane parallel to the rolling direction of the steel material, the standard deviation and average value of the X-ray intensity value of Cr satisfy the relationship of the following formula (1). By doing so, it was found that the rolling fatigue life could be remarkably improved, and the present invention was completed.
(Standard deviation of X-ray intensity value of Cr / Average value of X-ray intensity value of Cr) ≦ 0.25 (1)
Ｃｒの偏析は、球状化処理したときの炭化物サイズや面積率の不均一を招き、その結果として軸受として用いたときに転動疲労寿命を低下させることになる。上記（１）式の関係を満足したとき、Ｃｒの偏析が著しく低減された状態となって、転動疲労寿命が極めて優れたものとなる。 The segregation of Cr leads to non-uniform carbide size and area ratio when spheroidized, and as a result, reduces rolling fatigue life when used as a bearing. When the relationship of the above expression (1) is satisfied, Cr segregation is remarkably reduced, and the rolling fatigue life is extremely excellent.
本発明の軸受用鋼材は圧延材（球状化熱処理、および焼入れ・焼戻し前の鋼材）を想定したものであるが、上記（１）式の関係は、球状化材、および焼入れ・焼戻し材で実質的に変化しないため、いずれにも本発明の規定を適用できる。尚、上記（１）式の右辺の値は、好ましくは０．２３以下であり、より好ましくは０．２０以下である。 The bearing steel material of the present invention assumes a rolled material (steel material before spheroidizing heat treatment and quenching / tempering), but the relationship of the above formula (1) is substantially the same for the spheroidizing material and quenching / tempering material. The provisions of the present invention can be applied to any of them. The value on the right side of the above formula (1) is preferably 0.23 or less, more preferably 0.20 or less.
本発明の鋼材は、ＪＩＳ Ｇ４８０５（１９９９）で規定するＳＵＪ２〜４の化学成分組成をベースとするものであり、Ｃ：０．９５〜１．１０％、Ｓｉ：０．１５〜０．９０％、Ｍｎ：１．２％以下（０％を含まない）、Ｃｒ：０．９０〜１．６０％を満たすものである。これらの元素の範囲限定理由は次の通りである。 The steel material of the present invention is based on the chemical composition of SUJ2-4 specified in JIS G4805 (1999), C: 0.95 to 1.10%, Si: 0.15 to 0.90% , Mn: 1.2% or less (excluding 0%), Cr: 0.90 to 1.60% is satisfied. The reasons for limiting the ranges of these elements are as follows.
［Ｃ：０．９５〜１．１０％］
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摩耗性を付与するために必須の元素である。従って、０．９５％以上含有させなければならず、好ましくは０．９８％以上のＣを含有させることが望ましい。しかしながら、Ｃ含有量が多くなり過ぎると巨大炭化物が生成し易くなり、転動疲労特性に却って悪影響を及ぼす様になるので、Ｃ含有量は１．１０％以下、好ましくは１．０５％以下に抑えるべきである。
[C: 0.95 to 1.10%]
C is an essential element for increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. Therefore, it must be contained in an amount of 0.95% or more, and preferably 0.98% or more of C is contained. However, if the C content is excessively large, giant carbides are likely to be generated, which adversely affects the rolling fatigue characteristics. Therefore, the C content is 1.10% or less, preferably 1.05% or less. Should be suppressed.
［Ｓｉ：０．１５〜０．９０％］
Ｓｉは固溶強化元素であり、最終的に軸受等の部品とするときに、焼入れ・焼戻し工程において、焼戻し軟化を抑制する効果を発揮する他、炭化物を微細にする効果も発揮する。Ｓｉ含有量が０．１５％未満では、これらの効果が発揮されず、０．９０％を超えると、冷間鍛造性および熱間加工性を劣化させる等の悪影響がでるため、０．９０％以下とする必要がある。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０．８％以下（より好ましくは０．７％以下）である。
[Si: 0.15-0.90%]
Si is a solid solution strengthening element, and when it is finally used as a component such as a bearing, it exhibits the effect of suppressing temper softening and also the effect of making carbide finer in the quenching / tempering step. If the Si content is less than 0.15%, these effects are not exhibited, and if it exceeds 0.90%, adverse effects such as deterioration of cold forgeability and hot workability occur, so 0.90% It is necessary to do the following. 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 0.8% or less (more preferably 0.7% or less).
［Ｍｎ：１．２％以下（０％を含まない）］
Ｍｎは軸受鋼の焼入れ性を高めるために有効な元素であるが、その含有量が過剰になると、熱間加工後の硬さが高くなる過ぎて工業生産に支障を来す他、最終製品としたときに、残留オーステナイト相を多量に生成して疲労寿命を低下させるため、１．２％以下とする必要がある。Ｍｎ含有量の好ましい下限は０．３％以上（より好ましくは０．３５％以上）であり、好ましい上限は１．０％以下（より好ましくは０．８％以下）である。
[Mn: 1.2% or less (excluding 0%)]
Mn is an effective element for improving the hardenability of bearing steel. However, if its content is excessive, the hardness after hot working becomes too high, which hinders industrial production and the final product. In order to reduce the fatigue life by generating a large amount of retained austenite phase, it is necessary to be 1.2% or less. A preferred lower limit of the Mn content is 0.3% or more (more preferably 0.35% or more), and a preferred upper limit is 1.0% or less (more preferably 0.8% or less).
［Ｃｒ：０．９０〜１．６０％］
Ｃｒは、Ｃと結びついて微細な炭化物を形成し、耐摩耗性を付与すると共に、焼入性の向上に寄与する元素である。この様な効果を発揮させるには、Ｃｒ含有量を０．９０％以上とする。好ましくは１．０％以上である。しかし、Ｃｒが過剰に存在すると、粗大な炭化物が生成し、転動疲労寿命が却って低下する。従ってＣｒ含有量は１．６０％以下とする。好ましくは１．５％以下である。
[Cr: 0.90 to 1.60%]
Cr is an element that combines with C to form fine carbides, imparts wear resistance, and contributes to improving hardenability. In order to exert such an effect, the Cr content is set to 0.90% or more. Preferably it is 1.0% or more. However, if Cr is present excessively, coarse carbides are generated, and the rolling fatigue life is decreased. Therefore, the Cr content is 1.60% or less. Preferably it is 1.5% or less.
ＰやＳについては、ＪＩＳ Ｇ ４８０５（１９９９）で規定するＳＵＪ２〜４の化学成分組成に示される通り、夫々Ｐ：０．０２５％以下（０％を含まない）、Ｓ：０．０２５％以下（０％を含まない）とする必要があるが、これらの元素は縞状偏析を助長する傾向があり、その結果としてＣｒの偏析も助長する傾向がある。こうしたことから、ＰやＳについては合計含有量（Ｐ＋Ｓ）で０．０２０％以下（０％を含まない）とすることが好ましい。ＰおよびＳの合計含有量は、より好ましくは０．０１５％以下であり、更に好ましくは０．０１０％以下であるが、量産工程で製造する観点からは、その下限が０．００６％程度となる。 As for P and S, as indicated in the chemical composition of SUJ2-4 specified in JIS G 4805 (1999), P: 0.025% or less (excluding 0%), S: 0.025% or less However, these elements tend to promote stripe segregation, and as a result, also tend to promote Cr segregation. For these reasons, the total content (P + S) of P and S is preferably 0.020% or less (not including 0%). The total content of P and S is more preferably 0.015% or less, and still more preferably 0.010% or less. However, from the viewpoint of manufacturing in a mass production process, the lower limit is about 0.006%. Become.
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不純物であり、該不可避不純物として、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、Ａｌ、Ｏ、Ｎ等）の混入が許容され得る。尚、転動疲労寿命を高めるため、下記元素を規定範囲内で積極的に含有させることも可能である。 The contained elements defined in the present invention are as described above, and the balance is iron and inevitable impurities, and elements that are brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. (for example, Al, O, N) Etc.) can be allowed to be mixed. In order to increase the rolling fatigue life, the following elements can be positively contained within a specified range.
［Ｎｉ：０．２５％未満（０％を含まない）、Ｃｕ：０．２５％未満（０％を含まない）、およびＭｏ：０．０８％未満（０％を含まない）よりなる群から選択される１種以上］
Ｎｉ、Ｃｕ、Ｍｏは、いずれも母相の焼入性向上元素として作用し、硬さを高めて転動疲労特性の向上に寄与する元素である。これらの効果は、好ましくはＮｉで０．０３％以上、Ｃｕで０．０３％以上、Ｍｏで０．０１％以上含有させることによって有効に発揮される。しかし、Ｎｉ含有量が０．２５％以上、あるいはＭｏ含有量が０．０８％以上になると、加工性が劣化し、Ｃｕ含有量が０．２５％以上になると熱間圧延時の割れを助長するので、好ましくは夫々上記範囲内とするべきである。
[From the group consisting of Ni: less than 0.25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) One or more selected]
Ni, Cu, and Mo are all elements that act as a hardenability improving element of the parent phase and contribute to improving the rolling fatigue characteristics by increasing the hardness. These effects are preferably exhibited by containing 0.03% or more of Ni, 0.03% or more of Cu, and 0.01% or more of Mo. However, when the Ni content is 0.25% or more, or the Mo content is 0.08% or more, workability deteriorates, and when the Cu content is 0.25% or more, cracking during hot rolling is promoted. Therefore, each should preferably be within the above range.
本発明の軸受用鋼材は、上記成分組成を満たす鋼材を、例えばソーキング炉で加熱した後、熱間圧延することにより得られるが、上記（１）式の関係を満足するようにＣｒの偏析を低減するためには、その製造条件もできるだけ厳密に制御することが好ましい。 The steel material for bearings of the present invention is obtained by heating a steel material satisfying the above component composition in, for example, a soaking furnace, and then hot rolling, but the segregation of Cr is performed so as to satisfy the relationship of the above formula (1). In order to reduce it, it is preferable to control the manufacturing conditions as closely as possible.
本発明の軸受用鋼材を得るためには、上記成分組成を満たす鋼材を、鋳造から軸受部品となるまでの或る段階で、適正な条件で加熱する必要があるが、その加熱条件は、鋼材が鋳造されてからの加工履歴に大きく依存する。即ち、本発明で問題としているのは、鋳造時に不可避的に生じるミクロ偏析がその後の圧延で縞状に伸びた縞状偏析であるが、圧延や鍛造での鍛圧比（後述する「一次鍛圧比」）によって、縞状偏析の幅が変化する。従って、縞状偏析を熱処理で改善するためには、縞状偏析の幅に応じた加熱条件を設定する必要がある。 In order to obtain the steel material for a bearing of the present invention, it is necessary to heat a steel material satisfying the above component composition at an appropriate condition in a certain stage from casting to a bearing part. Greatly depends on the processing history after the casting. That is, the problem in the present invention is the striped segregation in which microsegregation inevitably generated during casting extends in a striped manner in the subsequent rolling, but the forging ratio in rolling and forging (the “primary forging ratio” described later). ”) Changes the width of the striped segregation. Therefore, in order to improve striped segregation by heat treatment, it is necessary to set heating conditions according to the width of the striped segregation.
従来では、偏析を低減するために、鋳造後の鋳片を加熱して、ＣｒやＣの拡散を行ってきたのであるが、実用的な熱処理温度と時間では、偏析元素の均質化が不十分であった。そのため、その後の圧延処理で偏析を低減する必要があったが、圧延処理では加熱温度が低過ぎて、必ずしも偏析を低減することはできていなかった。 In the past, in order to reduce segregation, the cast slab was heated to diffuse Cr and C, but the segregation element was not sufficiently homogenized at a practical heat treatment temperature and time. Met. Therefore, it was necessary to reduce the segregation in the subsequent rolling process, but the heating temperature was too low in the rolling process, and the segregation could not always be reduced.
そこで、本発明では、鋳片のままでなく、或る程度圧延若しくは鍛造した後に、偏析を低減する熱処理を行うことによって、偏析を大幅に改善し、その後の圧延条件の制約を少なくして、一般的な加工条件を含めて加工条件によらず、その後の球状化熱処理、焼入れ・焼戻しを経て軸受部品としたときの転動疲労寿命を向上し得たのである。 Therefore, in the present invention, not only as a slab, but after rolling or forging to some extent, by performing a heat treatment to reduce segregation, the segregation is greatly improved, and the subsequent rolling conditions are reduced, Regardless of the processing conditions including general processing conditions, the rolling fatigue life when the bearing parts were made through the subsequent spheroidizing heat treatment, quenching and tempering could be improved.
具体的な条件として、鋳造後に一旦一次鍛圧比（「鋳片の鋳造方向に垂直な断面積／圧延材若しくは鍛造材の加工方向に垂直な断面積」を言う。以下同じ）で３以上まで圧延若しくは鍛造した段階で、１２００〜１３５０℃で加熱処理（拡散熱処理）を施し、ミクロ偏析を改善した後、任意の条件で圧延や鍛造を施すことで、夫々の部品に合わせたサイズの軸受用鋼材が実現できる。そのときの加熱時間は、Ｃｒの拡散に基づく、後述する（２）式の関係を満足することが目安となり、工業的に無駄なく効率的な製造条件が提供できることになる。 As specific conditions, after the casting, the primary forging pressure ratio ("cross-sectional area perpendicular to the casting direction of the slab / cross-sectional area perpendicular to the processing direction of the rolled material or forged material") is rolled to 3 or more. Alternatively, at the stage of forging, heat treatment (diffusion heat treatment) is performed at 1200 to 1350 ° C. to improve microsegregation, and then rolling and forging are performed under arbitrary conditions, so that the steel material for the bearing has a size suitable for each part. Can be realized. The heating time at that time is based on satisfying the relationship of the formula (2), which will be described later, based on the diffusion of Cr, and an efficient production condition can be provided industrially without waste.
上記の考え方は、一次鍛圧比によって変わる縞状偏析の幅に応じて、Ｃｒの拡散が十分となる加熱温度と時間に設定することがポイントとなる。一次鍛圧比をＲｆ、加熱温度をＴ（Ｋ）、加熱時間をｔ（時）とすると、下記の関係がある。
縞状偏析の幅 ∝ √（１／Ｒｆ）
拡散距離 ∝ √（拡散係数×ｔ）
拡散係数 ∝ ｅｘｐ（−Ｑ／ＲＴ）
但し、Ｑ：拡散の活性化エネルギー
Ｒ：気体定数
必要な加熱時間ｔ ∝ （１／Ｒｆ）×（ｅｘｐ（−Ｑ／ＲＴ））
The point of the above idea is to set the heating temperature and time at which Cr is sufficiently diffused according to the width of the stripe segregation that varies depending on the primary forging pressure ratio. When the primary forging pressure ratio is Rf, the heating temperature is T (K), and the heating time is t (hours), the following relationship is established.
Stripe segregation width ∝ √ (1 / Rf)
Diffusion distance ∝ √ (Diffusion coefficient xt)
Diffusion coefficient ∝ exp (-Q / RT)
However, Q: Activation energy of diffusion R: Gas constant Necessary heating time t ∝ (1 / Rf) × (exp (−Q / RT))
本発明者らは、上記の考え方に基づいて、Ｃｒのミクロ偏析を低減するためには、拡散熱処理条件を実験によって確かめ、推奨する熱処理条件を得た。即ち、拡散熱処理時間ｔを、下記（２）式を満足するように設定し、その時間よりも２０％程度以上長くしても改善幅は飽和するため、上限は工業的観点から決定した。尚、[（２）式の右辺]×１．２程度が推奨される。
ｔ＞８×１０-9×（１／Ｒｆ）×ｅｘｐ[６９．７／(０．００１９８６２４２×Ｔ)]
…（２）
Based on the above concept, the inventors of the present invention have confirmed the diffusion heat treatment conditions through experiments and obtained recommended heat treatment conditions in order to reduce Cr microsegregation. That is, the diffusion heat treatment time t is set so as to satisfy the following formula (2), and even if the diffusion heat treatment time t is longer by about 20% or more than that time, the improvement range is saturated, so the upper limit was determined from an industrial viewpoint. In addition, [Right side of formula (2)] × 1.2 is recommended.
t> 8 × 10 −9 × (1 / Rf) × exp [69.7 / (0.00198242 × T)]
... (2)
尚、拡散熱処理温度を１２００℃以上としているのは、工業的に合理的な時間内で処理を終えるために設定した下限であり、１３５０℃以下としているのは、この温度を超えて加熱すると、工業的に加熱設備コストが増大し、鋼材の表面に分厚い酸化膜が生成して次の圧延工程のための酸化皮膜除去工程コストが増大するためである。また、一次鍛圧比を大きくすればするほど、拡散熱処理時間は短くてすむが、工業的に連続炉で熱処理する場合は良いが、バッチ炉で熱処理を行う場合、炉の大きさに合わせて切断する必要が生じるため、生産性が著しく低下する。従って、一次鍛圧比は３以上（より好ましくは５以上）、１０以下（より好ましくは８以下）程度が好ましい。 The diffusion heat treatment temperature is set to 1200 ° C. or higher, which is a lower limit set for finishing the treatment within an industrially reasonable time, and 1350 ° C. or lower is heated above this temperature. industrial heating equipment cost is increased, because the thick oxide film on the surface of the steel material is generated by the oxide film removal process costs for the next rolling step increases. In addition, the larger the primary forging pressure ratio, the shorter the diffusion heat treatment time is. However, it is better to industrially heat treat in a continuous furnace, but when performing heat treatment in a batch furnace, cut according to the size of the furnace. Therefore, productivity is significantly reduced. Accordingly , the primary forging pressure ratio is preferably about 3 or more (more preferably 5 or more) and 10 or less (more preferably 8 or less).
本発明の軸受用鋼材は、所定の部品形状にされた後、球状化熱処理、および焼入れ・焼戻しされて軸受部品を製造するものであるが、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。 The bearing steel material of the present invention is a bearing component produced by spheroidizing heat treatment and quenching and tempering after being formed into a predetermined component shape. The shape of the steel material stage can be applied to such production. Both linear and rod-like shapes are included, and the size can be appropriately determined according to the final product.
以下、実施例によっ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変更を加えて実施することは勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.
下記表１に示す各種化学成分組成の鋼を１５０ｋｇ真空熔解によって溶製し、直径：２３０ｍｍ（丸棒材）のインゴットを作製した。 Steels having various chemical composition shown in Table 1 below were melted by 150 kg vacuum melting to produce ingots having a diameter of 230 mm (round bar).
上記で得られたインゴットを用い、下記表２に示す条件（鍛造温度、一次鍛圧比）で一旦所定の一次鍛圧比まで熱間鍛造した後、種々の条件で加熱処理を行い[拡散熱処理温度、拡散熱処理時間、および（２）式の右辺の値(８×１０-9×（１／Ｒｆ）×ｅｘｐ[６９．７／(０．００１９８６２４２×Ｔ)]）、更に熱間鍛造（二次鍛造温度、トータル鍛圧比）して、直径：６５ｍｍの丸棒とした。 Using the ingot obtained above, after hot forging to a predetermined primary forging ratio once under the conditions shown in Table 2 below (forging temperature, primary forging pressure ratio), heat treatment is performed under various conditions [diffusion heat treatment temperature, Diffusion heat treatment time and the value on the right side of equation (2) (8 × 10 −9 × (1 / Rf) × exp [69.7 / (0.00186242 × T)]), and further hot forging (secondary forging Temperature, total forging pressure ratio) to obtain a round bar having a diameter of 65 mm.
得られた丸棒を球状化熱処理し、更に焼入れ・焼戻しを実施してスラスト転動疲労試験を実施した。このときの球状化熱処理の条件は、一般的な条件として、７６０℃×６時間加熱して、６８０℃まで８時間かけて（平均冷却速度１０℃／時）徐冷した。焼入れ・焼戻しの条件は、８４０℃×３０分で加熱後油焼入れし、１６０℃×１２０分で焼戻しした。 The obtained round bar was subjected to spheroidizing heat treatment, further quenched and tempered, and a thrust rolling fatigue test was performed. The conditions for the spheroidizing heat treatment at this time were, as general conditions, heating at 760 ° C. × 6 hours and slow cooling to 680 ° C. over 8 hours (average cooling rate 10 ° C./hour). The conditions for quenching and tempering were heating at 840 ° C. × 30 minutes followed by oil quenching and tempering at 160 ° C. × 120 minutes .
得られた各鋼材（焼入れ・焼戻し材）からスラスト試験片を作製し、面圧：５．３ＧＰａにてスラスト転動疲労試験を各１０回ずつ実施し、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）を評価し、疲労寿命Ｌ10が６×１０6回以上を合格基準とした。 A thrust test piece is prepared from each of the obtained steel materials (quenched / tempered material), and a thrust rolling fatigue test is performed 10 times each at a surface pressure of 5.3 GPa to obtain a fatigue life L 10 (cumulative failure probability 10%). The number of stress repetitions until fatigue failure was evaluated, and the fatigue life L 10 was 6 × 10 6 times or more as an acceptance criterion.
また、各鋼材について、圧延方向に平行な面（鋼材の縦断面）において、球状化焼鈍前の状態で、圧延方向に垂直な方向でのＥＰＭＡライン分析（加速電圧：１５ｋＶ）を、１ｍｍ長さ（２μｍ間隔で約５００点分析）で実施し、各データ点のＣｒのＸ線強度を用いて平均値と標準偏差を計算し、（ＣｒのＸ線強度の標準偏差／ＣｒのＸ線強度の平均値）を評価した。 Further, for each steel material, an EPMA line analysis (acceleration voltage: 15 kV) in a direction perpendicular to the rolling direction in a state before spheroidizing annealing on a plane parallel to the rolling direction (longitudinal section of the steel material) is 1 mm long. (Analysis of about 500 points at intervals of 2 μm), the average value and standard deviation are calculated using the X-ray intensity of Cr at each data point, and the standard deviation of the X-ray intensity of Cr / the X-ray intensity of Cr (Average value) was evaluated.
これらの結果[（ＣｒのＸ線強度の標準偏差／ＣｒのＸ線強度の平均値）、疲労寿命Ｌ10]を、下記表３に示す。 These results [(standard deviation of Cr X-ray intensity / average value of Cr X-ray intensity), fatigue life L 10 ] are shown in Table 3 below.
これらの結果から、次のように考察することができる。即ち、試験Ｎｏ．１〜１２のものでは、本発明で規定する要件を満足しており、いずれも転動疲労寿命が優れていることがわかる。特に、ＰとＳの合計含有量（「Ｐ＋Ｓ」で表示）が０．０２０％以下の鋼材を用いたもの（試験Ｎｏ．９）や、所定量のＭｏを含有したもの（試験Ｎｏ．１１、１２）では、より優れた転動疲労寿命を発揮していることが分かる。 From these results, it can be considered as follows. That is, test no. 1 to 12 satisfy the requirements stipulated in the present invention, and all of them have excellent rolling fatigue life. In particular, a steel material having a total content of P and S (indicated by “P + S”) of 0.020% or less (test No. 9) or a material containing a predetermined amount of Mo (test No. 11, In 12), it can be seen that a more excellent rolling fatigue life is exhibited.
これに対し、試験Ｎｏ．１３〜１６のものでは、本発明で規定する要件を外れているため［（ＣｒのＸ線強度の標準偏差／ＣｒのＸ線強度の平均値）が大きい］、いずれも疲労寿命Ｌ10が低くなっている。即ち、試験Ｎｏ．１３、１４のものは、拡散熱処理条件が適正ではなく、疲労寿命Ｌ10が低くなっている。 In contrast, test no. Intended 13-16, since the out the requirements specified in the present invention [(mean value of X-ray intensity of the standard deviation / Cr of X-ray intensity of Cr) is large, either low fatigue life L 10 It has become. That is, test no. 13 and 14 ones of the diffusion heat treatment conditions are not appropriate, the fatigue life L 10 is low.
試験Ｎｏ．１５のものは、一次鍛造を行っていないため、拡散熱処理条件が不適切となっており、上記試験Ｎｏ．１３、１４と同様の結果となっている。試験Ｎｏ．１６のものは、拡散熱処理を行っていないため、偏析が改善されず、疲労寿命Ｌ10が低くなっている。 Test No. No. 15 was not subjected to primary forging, so the diffusion heat treatment conditions were inadequate. The results are the same as 13 and 14. Test No. 16 ones, because it does not go to diffusion heat treatment, the segregation is not improved, the fatigue life L 10 is low.
これらのデータに基づいて、（ＣｒのＸ線強度値の標準偏差／ＣｒのＸ線強度値の平均値）とＬ10寿命との関係を図１に示すが、（ＣｒのＸ線強度値の標準偏差／ＣｒのＸ線強度値の平均値）を適切は範囲に制御することによって、優れた疲労寿命（転動疲労寿命）が達成されることが分かる。 Based on these data, the relationship between (standard deviation of Cr X-ray intensity value / average value of Cr X-ray intensity value) and L 10 life is shown in FIG. It can be seen that an excellent fatigue life (rolling fatigue life) can be achieved by appropriately controlling the standard deviation / average value of the X-ray intensity values of Cr within a range.
Claims (3)
Ｓｉ：０．１５〜０．９０％、
Ｍｎ：１．２％以下（０％を含まない）、
Ｃｒ：０．９０〜１．６０％、
Ｐ：０．０２５％以下（０％を含まない）、
Ｓ：０．０２５％以下（０％を含まない）を夫々含み、
残部が鉄および不可避不純物からなり、
鋼材の圧延方向に平行な面において、圧延方向に垂直な方向にＥＰＭＡライン分析したとき、ＣｒのＸ線強度値の標準偏差と平均値が、下記（１）式の関係を満足することを特徴とする転動疲労寿命に優れた軸受用鋼材。
（ＣｒのＸ線強度値の標準偏差／ＣｒのＸ線強度値の平均値）≦０．２５…（１） C: 0.95 to 1.10% (meaning mass%, the same shall apply hereinafter)
Si: 0.15-0.90%,
Mn: 1.2% or less (excluding 0%),
Cr: 0.90 to 1.60%,
P: 0.025% or less (excluding 0%),
S: each including 0.025% or less (excluding 0%),
The balance consists of iron and inevitable impurities,
When EPMA line analysis is performed in a direction perpendicular to the rolling direction on a plane parallel to the rolling direction of the steel material, the standard deviation and average value of the X-ray intensity value of Cr satisfy the relationship of the following formula (1). Steel material for bearings with excellent rolling fatigue life.
(Standard deviation of X-ray intensity value of Cr / Average value of X-ray intensity value of Cr) ≦ 0.25 (1)
Further, as other elements, Ni: less than 0.25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) The steel for bearings according to claim 1 or 2, comprising one or more selected from the group consisting of:
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