JP4000787B2 - High-strength gear excellent in tooth root bending strength and pitting resistance and manufacturing method thereof - Google Patents

Description

【００７３】
ＡおよびＢ鋼は、表１に示すごとく、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏ、Ａｌ、Ｎの含有量を本発明範囲内に規制した鋼であり、かつ、１．５≦３×Ｓｉ（％）−Ｍｎ（％）＋Ｃｒ（％）／４＋Ｍｏ（％）の条件式を満足する鋼である。
【００７４】
そして、ＡおよびＢ鋼を用いて歯車(以下、歯車Ａ、Ｂと記す。)を作製するに当たっては、これをまず、モジュール２、圧力角１８°、ねじれ角２７°の仕様からなる歯数２９の歯車に切削加工により成形する。
次いで、成形された歯車を浸炭処理する。本例における浸炭処理は、ガス浸炭法を用い、具体的には以下に示すごとく行った。
【００７５】
即ち、まずカーボンポテンシャル（Ｃ．Ｐ）を０．８％に維持すると共に温度９５０℃にキープしたガス雰囲気中において４時間浸炭させた後、次いで、Ｃ．Ｐを維持したまま温度を８６０℃に下げて４５分間均熱処理をした後、１３０℃の油に焼入れた。その後、温度１６０℃、１時間の焼戻し処理を行った。なお、この条件は通常ＪＩＳ鋼であるＳＣＭ材で適用されている条件の中の一条件である。
【００７６】
なお、通常ＪＩＳ鋼では、浸炭処理温度を９２０〜９８０℃、時間を１〜１５時間、均熱の温度を８４０〜８７０℃、時間を０．５〜２時間程度の間にて行っており、本発明は優れた浸炭性が得られるよう成分調整がされているので、従来鋼の量産ラインで条件を変更することなく処理が可能である。
【００７７】
上記浸炭処理により得られた歯車ＡおよびＢは、図１に示すごとく、母相１０の上に浸炭層１２が形成され、さらにその上に浸炭異常層１１が形成された表面状態となる。
【００７８】
本例における浸炭層１２は、ＥＰＭＡによる分析の結果、歯車ＡはＣ濃度が０．７７％であり、Ｘ線回折法による調査の結果、残留オーステナイト量が１２％となっていた。また、浸炭層１２の硬度はＨｖ８１１と非常に高くなった。
また、歯車ＢはＣ濃度が０．６７％であり、Ｘ線回折法による調査の結果、残留オーステナイト量が１０％以下となっていた。また、浸炭層１２の硬度はＨｖ７６９と非常に高くなった。
【００７９】
また、浸炭異常層１１は、図１に示すごとく、歯車Ａおよび歯車Ｂはその最大深さＤが７μｍおよび１０μｍであって、かつ、最大深さ位置から表面までの断面Ａにおける浸炭異常層の占める面積率は８５％および７５％であった。
【００８０】
さらに、この歯車Ａおよび歯車Ｂにショットピーニング処理を、粒径φ０．６ｍｍのショット粒を用いアークハイトが０．５０ｍｍＡの条件施した。そして、ショットピーニング処理後に最大圧縮残留応力を測定した結果、歯車Ａおよび歯車Ｂ共にピッチ円の表面では、４５０ＭＰａ、表面から５０μｍの深さの範囲の最大値では、１２５０ＭＰａと高い値を示していた。
【００８１】
ショットピ−ニング処理後における浸炭異常層１１は、その最大深さＤが２および３μｍであって、かつ、最大深さ位置から表面までの断面Ａにおける浸炭異常層の占める面積は９０％以上及び８５％と浸炭処理直後に比べ異常層に厚みのばらつきが少ない状態になっていた。なお、浸炭層の炭素濃度はショットピーニング処理前と変化がなく、加えて残留オーステナイト量についてはショットピーニング後は共に１０％以下となった。また、歯面の表面粗さＲmaxは８．７μｍ及び９．２μｍと比較的小さい値を示していた。この歯車について歯面強度を測定（測定方法は後述の実施例で記載）したところ、歯車Ａは２５ｋｇｆ・ｍ，歯車Ｂは２３ｋｇｆ・ｍと高い値（従来鋼ＳＣＭ４２０Ｈでは１８ｋｇｆ・ｍ）を示した。
【００８２】
次に、本例の歯車１の作用につき説明する。
本例の歯車１は、上記のごとく、高硬度の浸炭層１２の外層に、さらに上記特定量の浸炭異常層１２を設けてある。
そのため、２つの歯車１をかみ合わせて駆動することにより優れた初期なじみ性が発揮される。
【００８３】
即ち、歯車においては、歯面に生じる不均一な応力分布を緩和すべく浸炭異常層１１が摩耗し、歯面の形状が自己修正される。そして、浸炭異常層１１が摩耗により除去された歯面においては、その下層の浸炭層１２が表面に露出した状態となる。
【００８４】
それ故、本例の歯車は、一定の初期なじみ運転後においては、歯面の均一な接触状態が得られて応力分布が均一となると共に、接触面が高硬度の浸炭層により構成されるようになる。
これにより、従来の浸炭異常層を極力少なくする対策を施した歯車に比べて接触時の応力状態を良好にすることができ、耐ピッチング性を向上させることができる。
【００８５】
さらに、本例の歯車は、上記特定組成のＡ鋼及びＢ鋼を素材として用いている。そのため、歯車の運転時における摩擦熱による硬度低下も少ない。
それ故、本例の歯車は、長期間の使用によっても耐ピッチング性があまり劣化せず、長寿命となる。
【００８６】
以上歯車Ａ、Ｂの実施例について説明したが、同じ仕様の歯車を全く同一の製造方法でＣ鋼を用いて製造した歯車（以下、歯車Ｃと記す。）の結果について説明する。
【００８７】
Ｃ鋼は、本発明の各成分のうち、前述したＭｏ、Ｃｒ量の最適化をしていない比較鋼であって、Ｍｏ、Ｃｒ以外の成分は、本発明の範囲内の鋼である。このＣ鋼を用いてＡ、Ｂ鋼の場合と同じ浸炭処理条件を施した結果、浸炭層の炭素濃度は、０．５７％と本願発明で規定した値より低くなり、残留オーステナイトは１０％以下となったが、浸炭硬さがＨｖ６９７とＡ鋼およびＢ鋼に比べ著しく低い値となっていた。そこで、浸炭処理条件のうちＣポテンシャルを０．８％から１．０％に変更し、Ｃポテンシャル以外の条件は同じ条件で浸炭処理をやり直した結果、浸炭層の炭素濃度、浸炭硬さは、それぞれ、０．７９％、Ｈｖ８００と前記したＡ鋼とほぼ同一の値となり、残留オーステナイト量が１６％となった。このＡ鋼、Ｂ鋼およびＣ鋼の結果から、Ｍｏ、Ｃｒ量の最適化が浸炭焼入性の向上に効果が大きいことを示している。
【００８８】
また、前記歯車Ａ、Ｂと同一の浸炭処理条件で処理した歯車Ｃについて浸炭異常層の最大深さ及び最大深さ位置から表面までの断面における浸炭異常層の面積率を測定した結果１４μｍ、７０％と、歯車Ａ、Ｂに比べ厚みが高い値を示した。これはＣｒの低減およびＭｏの増量が浸炭異常層の厚み低下及び前記面積率向上に寄与しているものと推定される。
【００８９】
次に歯車Ｃを前記歯車Ａ、Ｂと全く同一の浸炭条件（Ｃ．Ｐ．：０．８％Ｃ）とショットピーニング処理を施した。その結果歯面粗さが、１０．７μｍと歯車Ａ、Ｂに比べ粗くなっているのが認められた。これは、ショットピーニング処理前において、浸炭異常層の厚みが厚かったことによるものと思われる。この歯車Ｃを歯車Ａ、Ｂと同様に歯面強度を測定した結果、１６ｋｇｆ・ｍと低い値となった。これは表面粗さが大きいあるいは浸炭層Ｃ％が低いことによる浸炭焼入層の組織（０．６０％Ｃ濃度以上の場合は針状マルテンサイト組織とラスマルテンサイト組織の混合となるが、０．６０％Ｃ濃度以下の場合はラスマルテンサイト組織主体となる）が異なることが影響していること等が考えられる。
【００９０】
以上の説明から明らかなように、本発明の歯車にとってＣｒ、Ｍｏ量の最適化がその性能向上に大きく寄与していることがわかる。
【００９１】
実施形態例２
本例は、表１に示した成分からなる供試鋼を準備して歯車を作製し、その浸炭層の軟化抵抗性を定量的に評価した。この表１の供試鋼のうち、１〜７鋼は本発明で規定する成分範囲内の鋼であり、８〜１４鋼は一部の成分が本発明の条件を満足しない比較鋼、１５鋼は従来鋼であるＳＣＭ４２０Ｈである。
【００９２】
各供試鋼を用いた歯車の作製は、基本的に実施形態例１と同様の製造方法によって行った。ただし、浸炭処理状態の違いにより評価結果に差異が生じることを防ぐため、浸炭層の炭素濃度は全て０．６０％以上となるよう比較鋼の浸炭処理条件を調整した。
【００９３】
軟化抵抗性の評価のため、上記の浸炭焼入焼戻し後において、さらに温度２５０℃、４時間の再焼戻し処理を行った。そして、再焼戻し処理前と後の浸炭層の断面硬度を測定した。
測定結果を表１に示す。
【００９４】
表１より知られるごとく、本発明で規定した条件を満足する１〜７鋼は、いずれの供試鋼においても、再焼戻し前の浸炭層の硬度は、Ｈｖ７５０以上と非常に高い硬度を示し、再焼戻し後においても硬さ低下はＨｖ１００以下となり、優れた軟化抵抗性を有していた。また、その中でも関係式の値が、高い鋼ほど硬さ低下が小さくなっていた。それに対し、比較鋼である８鋼は、再焼戻し処理による硬度低下は小さいが浸炭焼入焼戻し後の硬さがＨｖ７００以下と低く、比較鋼１０〜１３鋼は浸炭層の硬度はＨｖ７５０以上と高いが、再焼戻し後においての硬さ低下がＨｖ１００以上となり、軟化抵抗性が劣っている。また、従来鋼である１５鋼は、再焼もどし処理によりＨｖ７８６からＨｖ６４８と非常に大幅に低下した。特に、比較鋼、従来鋼の中で関係式の値が１．５未満の鋼については、７００Ｈｖを切るような低い硬度まで大きく低下した。
【００９５】
この硬度低下と上記関係式の値との関係を明確にすべく、これを図２に示す。
図２は、横軸に関係式の値を、縦軸に硬度の低下値（Ｈｖ）をとったものである。
【００９６】
同図より知られるごとく、関係式の値と再焼戻しによる硬度の低下値には相関関係があることがわかる。そして、関係式の値が１．５を超える場合には、再焼戻しによる硬さ低下が１００Ｈｖ以下となることもわかる。
【００９７】
以上の結果から、上記関係式の値を１．５以上に規制した鋼を用いた本発明の歯車は、歯車使用時の摩擦熱による軟化を抑制することができ、歯車の耐ピッチング性を長期にわたって維持することができることが明確となった。
【００９８】
実施形態例３
本例は、実施形態例２において用いた表１に示す供試鋼を準備し、肌焼鋼の組成、浸炭異常層の深さ、残留オーステナイト量、圧縮残留応力等がピッチング強度、歯元曲げ強度等にどのように影響するかを定量的に評価した。
【００９９】
各供試鋼は、溶解、鍛伸鍛造、焼ならしを行った後に、前述した歯車に加工し、浸炭処理を行った。
浸炭処理は、通常のガス浸炭を行ったものについては、実施形態例１の浸炭方法で行い、浸炭異常層を全く生成させないものについては、真空ガス浸炭処理により行った。浸炭処理後さらに実施形態例１に記載と同様のショットピーニング処理を施した。
【０１００】
ショットピーニング処理後において、歯車の状態を観察した結果、残留オーステナイト量が多い従来鋼１５は、歯欠けの発生が認められた。これは、残留オーステナイト量が多くなると、歯欠け発生がしやすくなることを示している。それに対し、残留オーステナイトが本発明の範囲内のものは、歯欠けは認められなかった。
【０１０１】
また、表面粗さについても調査したが、浸炭異常層が深いものほど、粗さが大きくなる傾向がみられた。特に、浸炭処理直後において浸炭異常層の最大深さを１３μｍ以下としたものは、表面粗さは全てＲmaxで１０μｍ以下の範囲内に入っていた。
【０１０２】
次に、歯車における強度評価については、動力循環式歯車試験機（神鋼造機(株)製）を使用した。そして、モジュール２、圧力角１８°、ねじれ角２７°の仕様からなる歯数２９、４３の歯車を各２ケ準備し、歯元曲げ強度評価については負荷トルクを４０ｋｇｆ・ｍ、回転速度２０００ｒｐｍ、油温８０℃（強制潤滑）の条件で試験を実施し、１０⁷回まで回転させて歯車に破壊が生じないかどうかを調べた。対して、歯面強度評価については、負荷トルクを２３ｋｇｆ・ｍ、回転速度４０００ｒｐｍ、油温１２０℃（強制潤滑）の条件で試験を実施し、５×１０⁷ 回転後のピッチング面積率を測定した。なお、ピッチング面積率は前記試験後の歯車の歯面のうちピッチング破壊が生じている部分の総面積をマイクロスコープを用いて測定し、測定値から面積率を計算することにより求めた。また、前記した実施形態例１については、前記条件のうち負荷トルクのみを変化させて試験を行い、ピッチング面積率が０％となる最大負荷トルクを測定することにより評価したものである。試験結果を表２に示す。
【０１０３】
【表２】

【０１０４】
表２より知られるごとく、浸炭処理直後の浸炭異常層の最大深さが０のもの及び１３μｍを超えるもの（試料Ｎｏ．２、９、１０、１２、１６）については、すべてピッチング面積率が３％以上であり、試料Ｎｏ．２を除き、歯元曲げ強度も低下した。特に試料Ｎｏ．２は、真空浸炭して異常層の厚みを０とした実施例であり歯元曲げ強度については優れた特性を示したが、歯面強度試験については、試験途中に曇り帯の生成が激しく生じ、凝着摩耗起点のピッチング破壊が起きて歯面強度が低下し、ピッチング面積率は１０％以上と非常に大きくなった。また、浸炭異常層が深いものは、前記した通り、表面粗さが大きくなり、またショットピ−ニング後の表面圧縮残留応力が低下するため、その点が悪影響を及ぼしているものと推定される。
【０１０５】
特に、浸炭異常層の占有面積率が７０％以上の場合でも、上記のごとく浸炭直後の浸炭異常層の最大深さが１３μｍを越える場合（試料Ｎｏ．１０）には、６．３％のピッチング面積率と歯元曲げ強度が低下する値となった。これらの結果、浸炭異常層が１３μｍを超える場合は、浸炭異常層の占有面積率に関係なく、表面粗さが大きく、ショット後の圧縮残留応力も低い値となり、その影響が歯車強度評価結果に反映されていると考えられる。
【０１０６】
また、浸炭異常層の最大深さ及び占有面積率が良好であっても、鋼の成分範囲が本発明範囲外の場合（試料Ｎｏ．１１、１５）、及び浸炭異常層の最大深さ及び占有面積率が良好であり、かつ、鋼の成分範囲が本発明範囲内にあっても、上述した関係式の値が１．５未満の場合（試料Ｎｏ．１３、１４）については、ピッチング強度が低い結果となった。特に、Ｔｉ量の多い試料Ｎｏ．１５は鋼内部を起点とするピッチング破壊であるスポ−リング破壊が生じており、この結果から強力なショットピ−ニングを付与した歯車の場合、硬質介在物の低減は不可欠であることがわかる。
【０１０７】
【発明の効果】
上述のごとく、本発明によれば、従来鋼の条件で浸炭処理を施すことができ、かつ、歯元曲げ強度、耐ピッチング性に優れた歯車及びその製造方法を提供することができる。
【図面の簡単な説明】
【図１】実施形態例１における、浸炭異常層の形成状態を示す説明図。
【図２】実施形態例２における、再焼戻しによる硬度低下状態を示す説明図。
【符号の説明】
１．．．歯車
１０．．．母相
１１．．．浸炭異常層
１２．．．浸炭層[0001]
【Technical field】
The present invention relates to a gear widely used in automobiles, construction vehicles, construction equipment, and the like, and more particularly to a high-strength gear excellent in tooth root bending fatigue strength and pitting resistance and a method for manufacturing the same.
[0002]
[Prior art]
In the environment surrounding automobiles, construction vehicles and construction equipment, there is a social demand for energy saving and further performance improvement, and efforts to reduce the weight of the vehicle body and increase the engine output are increasingly being promoted. For this reason, the usage environment of gears used in automobiles and construction vehicles / equipment, especially gears used in drive train transmission parts, has become more severe, with excellent tooth root bending fatigue strength and pitting resistance. There is a need for gears with
[0003]
In conventional gears, case hardening steel such as JlS-SCr420 steel, which is chromium steel, or JlS-SCM420 steel, which is chromium molybdenum steel, is used as a gear steel for producing the gear. And after shape | molding these case hardening steel in the shape of a gear, carburizing, hardening, and tempering (it describes as a carburizing process also including the description of the said claim) was performed, and it was set as what is called a carburized gear.
[0004]
[Problems to be solved]
However, the conventional gear has the following problems.
That is, in recent years, demands for weight reduction of the vehicle body and high output of the engine, which are required for automobiles, construction vehicles, and construction equipment, are becoming increasingly high. When carburizing the conventional steel SCM420H, an abnormal carburizing layer is usually formed with a thickness of 15 μm or more, and the softening of the hardness occurs due to the increase in tooth surface temperature during use. The original bending fatigue strength and pitting resistance cannot be obtained. In addition, even if shot peening treatment is applied to improve fatigue strength, there is a soft carburized abnormal layer on the surface layer, so the tooth surface roughness is increased or the compressive residual stress of the surface layer is not increased. An effect cannot be acquired and the said request | requirement cannot fully be satisfied.
[0005]
In order to solve this problem, many proposals have been made on the steel for carburized gears with improved strength, and there have also been many proposals concerning manufacturing methods such as shot peening. And these proposals have shown a dramatic improvement especially in terms of tooth root strength.
[0006]
However, the improvement of the tooth surface strength is not so large compared to the improvement of the tooth root strength. For this reason, the failure mode of the gear changed from tooth root fatigue to tooth surface fatigue, that is, pitching failure. In particular, gears used at high surface pressures and high speed rotations cannot cope with the environment, causing a problem of insufficient pitting resistance.
[0007]
Further, in order to satisfy recent severe demands, it is necessary to withstand a high torque that is applied during use, and even better tooth base fatigue strength has been demanded.
[0008]
For such a lack of pitting resistance, the amount of oxygen in the steel is reduced, the form control of inclusions by adding trace elements, and the formation of abnormal carburization layers by reducing the carburizing abnormal layer generation elements Various methods have been proposed for suppressing the erosion or carburizing steel for gears imparted with temper softening resistance.
[0009]
For example, JP-A-1-52467 discloses an attempt to define the shape of non-metallic inclusions. However, such a proposal requires advanced processing in the steelmaking process, which increases the manufacturing cost and ultimately increases the cost of the steel material, which meets the needs of users who insist on cost reduction. It is gone.
[0010]
In addition, in each of JP-A-2-85343, JP-A-1-306521, JP-A-1-47838, and JP-A-6-306572, the occurrence of an abnormal carburizing layer is caused by optimizing the composition of components such as Si. There are proposals to suppress and improve gear performance such as tooth surface strength (pitting resistance). There are many other proposals similar to this one.
[0011]
However, according to detailed investigations and research conducted by the present developers, the method of suppressing the carburizing abnormal layer not only provides excellent pitting resistance, but also promotes variations in pitching life of individual gears. I found out. In addition, if the carburized abnormal layer is thin, when shot peening is applied to improve the strength, the surface roughness becomes small, but the formation of cloudy bands (adhesion wear, scoring wear) is fast and the pitching strength is as intended. It also turned out not to improve.
[0012]
Furthermore, Japanese Patent Application Laid-Open No. 9-59756 discloses a proposal for controlling the carburizing and quenching treatment conditions according to the component composition of the steel material, ensuring an appropriate surface hardness and retained austenite amount, and ensuring excellent pitting resistance. ing.
[0013]
This invention is characterized by optimization of carburizing and quenching conditions, but is the same as the above-mentioned application in that the presence of an abnormal carburizing layer (grain boundary oxide layer) is suppressed. In addition, it is an invention that allows the presence of a large amount of retained austenite (20 to 50%), and when the shot peening treatment in which the maximum compressive residual stress exceeds 1000 MPa, the edge portion of the tooth end face is chipped (hereinafter referred to as tooth chipping). The fine crack (hereinafter referred to as an internal microcrack) is generated in the vicinity of 30 to 50 μm from the surface where the compressive residual stress is maximized by the shot peening treatment, which may be a starting point for pitching fracture.
[0014]
The present inventors have developed a new technology for solving such a conventional problem, can be manufactured at low cost, and provide a gear excellent in pitching resistance, and have already applied for a patent (Japanese Patent Laid-Open No. 10-259470). No., hereinafter referred to as prior application). Unlike the above-described conventional invention, the present invention actively utilizes the carburized abnormal layer and attempts to suppress the residual austenite as much as possible by setting the amount of retained austenite in the carburized layer to 25% or less.
[0015]
The development of the present invention has made it possible to manufacture gears with improved pitting resistance compared to the prior art. However, sufficient investigation has not been made on the problem that the C concentration of the surface layer during the carburizing treatment of high-Si steel that has been conventional is difficult to increase, and the present invention has a higher Si content than conventional SCM materials and the like. Steel has a problem that the C concentration in the surface layer is unlikely to increase during carburizing. In addition, the initial conformability when used in a carburized state has been sufficiently studied, but the effects of changes in the carburized abnormal layer and the increase in tooth surface roughness caused by shot peening treatment, Measures for chipping and internal microcracks have not been sufficiently studied.
[0016]
The present invention has been made in view of such conventional problems, and can easily ensure the target C concentration of the surface layer by carburizing treatment under normal conditions, and can ensure good initial conformability even when shot peening is performed. An object of the present invention is to provide a high-strength gear excellent in gear manufacturability that can solve problems such as generation of chipped teeth and occurrence of internal microcracks, as well as a high-strength gear excellent in tooth root bending strength and pitting resistance, and a manufacturing method thereof. .
[0017]
[Means for solving problems]
The invention according to claim 1 is the gear obtained by carburizing and shot peening after forming the case-hardened steel into a gear shape, and the case-hardened steel has a weight ratio of C: 0.10 to 0.00. 30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.20 to 0.85 %, Mo: 0.35-0.80%, Al: 0.020-0.060%, N: 0.0080-0.0200%, Ti: 0.010% or less, and 1. 5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%) was satisfied, the balance was composed of Fe and inevitable impurities, and shot peening was applied after carburizing treatment. The gear is a carburized layer having a C concentration of 0.60% or more and a residual austenite amount of 20% or less. And the outer layer of the carburized layer has a carburized abnormal layer composed of an incompletely quenched structure, and the maximum depth of the carburized abnormal layer is 2 to 10 μm, and the maximum In the high strength gear excellent in tooth root bending strength and pitting resistance, the area ratio of the carburized abnormal layer in the cross section from the depth position to the surface is 80% or more.
[0018]
What should be noted in the present invention is that carburizing treatment and shot peening treatment are performed using the case-hardened steel having the above specific composition, and the C concentration and residual austenite amount of the carburized layer after the treatment are in the above specific range, The abnormal layer is actively used, and the maximum depth and the area ratio occupied by the abnormal layer are in the specific range.
[0019]
Further, the present invention is different from the invention previously filed by the present inventors in that conventional steel (JIS SCM material, etc.) is obtained by regulating the upper limit of Cr addition to a low level and adjusting Mo to a larger amount. Newly found that it is easy to adjust the C content and carburization hardness of the surface layer in the carburizing atmosphere (atmosphere adjusted to the normal carbon potential) that is normally implemented in Si, Mo, In addition to improving the softening resistance by optimizing the amount, and further optimizing the form and depth of the carburizing abnormal layer, the point of improving the pitting resistance and the root bending strength, It is the point which discovered newly that the optimization of a form and suppression of the production | generation of a retained austenite amount were effective in the prevention of the malfunction which arises after a shot peening process.
[0020]
The carburizing abnormal layer is a layer composed of an incompletely quenched structure as described above. The incompletely quenched structure is a structure made of troostite or bainite generated during quenching in a series of carburizing processes. This carburized abnormal layer can be easily observed when it is corroded black when it is corroded with a corrosive solution such as nital after the cross section of the treated product is mirror finished. The carburized abnormal layer is generated as follows.
[0021]
That is, for example, in the case of gas carburizing treatment, a certain amount of oxygen is contained in the carburizing atmosphere. When this oxygen enters from the surface of steel, among Si, Cr, Mn, Ni, Mo, etc. contained (solid solution) in the substrate near the grain boundary, Si, Cr, and Mn are crystal grains. Combines with oxygen that has diffused through the field to form oxides. For this reason, hardenability falls in the vicinity where the oxide is formed. Therefore, martensite is not generated during quenching, and troostite or bainite is generated. This layer of incompletely quenched structure made of troostite or bainite is a carburized abnormal layer.
[0022]
Conventionally, it is considered that the presence of this carburized abnormal layer has an adverse effect on fatigue characteristics, and it has long been common knowledge to reduce as much as possible, but in the present invention, it is actively used. However, the maximum depth is carburized, and in order to prevent defects after shot peening treatment while ensuring excellent root bending strength after shot peening treatment, the conditions are strictly regulated compared to the previous application, The maximum depth is 5 to 13 μm (described later) immediately after the carburizing process, and 2 to 10 μm after the shot peening process. Further, the area occupied by the abnormal carburizing layer in the cross section from the maximum depth position to the surface (hereinafter referred to as “occupied area ratio”) is 70% or more (to be described later) immediately after the carburizing process, as in the prior application. Later, 80% or more. As shown in FIG. 1, this carburized abnormal layer is usually formed with variations in depth. Therefore, in the present invention, the thickness of the carburizing abnormal layer is defined by the maximum depth, and the degree of the unevenness of the depth is defined by the occupation area ratio of the carburizing abnormal layer.
[0023]
The reason why the maximum depth of the carburized abnormal layer after the shot peening treatment is limited to the above-described range is that when it is less than 2 μm, there is a problem that the effect of initial conformability described later is not sufficiently exhibited. On the other hand, the upper limit of 10 μm is the thickness obtained as a result of setting the upper limit immediately after the carburizing process to 13 μm, and the reason for limitation is the same as the reason for limiting the upper limit immediately after the carburizing process described later.
[0024]
Further, the reason why the area ratio occupied by the carburized abnormal layer between the maximum depth position and the surface after the shot peening process is 80% or more is that the area ratio occupied by the carburized abnormal layer immediately after the carburizing process is 70% or more. Is exactly the same as
[0025]
On the other hand, the upper limit of the occupied area ratio of the carburized abnormal layer is ideally 100%. In other words, the carburized abnormal layer becomes a uniform layer with less unevenness in depth as the occupied area ratio is higher. Therefore, after wear of the carburizing abnormal layer, the carburized layer with high hardness is exposed on the gear surface, and the subsequent pitting resistance can be improved.
[0026]
Next, the carburized layer located below the abnormal carburized layer has a C (carbon) concentration of 0.60% or more and a residual austenite amount of 20% or less as described above.
The carburized layer defined here refers to a layer within a range of 100 μm in depth (excluding the carburized abnormal layer on the surface) after the carburizing treatment.
[0027]
When the C concentration is less than 0.60%, there is a problem that the hardness of the martensite structure obtained by carburizing and quenching does not become sufficiently high. However, the upper limit of the C concentration may be 1.00% because cementite is generated at the grain boundaries, which may reduce fatigue strength and pitting resistance, and a large amount of retained austenite precipitates. preferable.
[0028]
Further, when the amount of retained austenite in the carburized layer exceeds 20%, the effect of increasing the hardness of the carburized layer is saturated, and there is a problem that tooth chipping and internal microcracks are likely to occur due to shot peening treatment.
[0029]
Further, the shot peening treatment is performed so that the compressive residual stress is 400 Mpa or more on the tooth surface of the gear, and the maximum compressive residual stress in the range from the surface to a depth of 50 μm is 1000 MPa or more. Is preferably processed under conditions such that the thickness is 10 μm or less. By processing so as to satisfy this condition, a gear having better pitching resistance can be obtained.
[0030]
Next, as the gear material of the present invention, case-hardened steel having the above specific composition is used. Below, the reason for limitation of each chemical component range is demonstrated.
C: 0.10 to 0.30%
In order to sufficiently satisfy the strength required for the carburized gear part, that is, to obtain the internal hardness Hv200 to 500 of the carburized gear part, it is 0.10% or more, preferably 0.13% or more. It is necessary to contain C. However, if the content exceeds 0.30%, the internal toughness is deteriorated, the strength of the gear is reduced, and further the machinability and the cold forgeability are deteriorated. did. Preferably, the upper limit is 0.27% or less.
[0031]
Si: 0.50 to 1.50%
During the carburizing process, Si in the carburized layer reacts with oxygen in the carburizing atmosphere to form an oxide. For this reason, hardenability falls in the surface layer vicinity of a to-be-processed goods, and what is called a carburizing abnormal layer is formed. That is, Si is an element that has an important influence on the formation of the carburized abnormal layer, and is also an element that increases the temper softening resistance of the martensite structure. In the present invention, in combination with the suppression effect of the carburizing abnormal layer possessed by Mo, the desired shape (that is, the depth of the carburizing abnormal layer is 5 to 13 μm, and in the cross section from the maximum depth position to the surface). In order to obtain a carburized abnormal layer in which the area ratio of the carburized abnormal layer is 70% or more) and to improve the temper softening resistance, it is necessary to contain Si in an amount of 0.50% or more, preferably 0.70% or more. is there. However, if the content exceeds 1.50%, it becomes difficult to ensure the predetermined C concentration of the surface layer by carburizing treatment, and cold forgeability, machinability, and toughness are reduced, so the upper limit was made 1.50% . Preferably, the upper limit is 1.20% or less.
[0032]
Mn: 0.30 to 1.00%
Mn is an element having a remarkable effect on improving hardenability, and Mn of 0.30% or more is required to ensure the hardness (Hv 200 to 500) necessary to secure the strength to the inside of the gear. It is necessary to contain. In addition, since Mn is an element that generates an abnormal carburization layer, the amount of addition must be within the optimum range for controlling the form of the abnormal carburization layer. Thus, it becomes difficult to obtain a target structure after carburizing, and the possibility of occurrence of tooth chipping and internal microcracks due to shot peening increases, so the upper limit was made 1.00%.
[0033]
P: 0.035% or less
P is an impurity that cannot be avoided during production, but it is an element that reduces the strength of grain boundaries and deteriorates fatigue characteristics, so the upper limit was made 0.035%.
[0034]
S: 0.035% or less
S, like P, is an impurity that cannot be avoided when mixed in a small amount during production, and is present as a sulfide inclusion such as MnS. However, since this inclusion becomes the starting point of fatigue fracture, it is preferable to reduce it as much as possible, and the upper limit was made 0.035%.
[0035]
Cr: 0.20 to 0.85%
Cr is an element that improves hardenability. After carburizing and quenching, it is necessary to contain 0.20% or more in order to obtain the above internal hardness. Moreover, Cr forms an oxide in the surface layer of the object to be processed together with Si during the carburizing process, and lowers the carburizing property. Due to this point, in the case of high Si steel, it is necessary to set the carbon potential of the carburizing treatment atmosphere to be particularly high and treat it. In the present invention, the upper limit of Cr is set to 0.85% or less in order to enable treatment in a C% atmosphere similar to JIS steel SCM420H or the like.
[0036]
Mo: 0.35-0.80%
Mo has the effect of improving hardenability and toughness and suppressing the carburizing abnormal layer, and by using it together with the effect of forming the carburizing abnormal layer possessed by Si, the desired form (that is, the carburizing abnormal layer) An abnormal carburization layer having a depth of 5 to 13 μm and an area ratio occupied by the abnormal carburization layer in the cross section from the maximum depth position to the surface of the abnormal carburization layer can be obtained. In addition, there is an effect of improving the carburizability of high-Si steel by regulating the upper limit of the Cr addition amount and adding an appropriate amount of Mo. In order to sufficiently obtain this effect, the lower limit was made 0.35%. However, if added in a large amount, not only a carburized abnormal layer having a desired form cannot be obtained, but also the cost is increased, and further, cold forgeability and machinability are deteriorated. It was.
[0037]
Al: 0.020 to 0.060%,
Al combines with N in the steel, and as AlN, has the effect of refining crystal grains after carburizing and quenching to improve toughness. In order to acquire this effect, it is necessary to contain 0.020% or more of Al. However, if the content exceeds 0.060%, excessive Al in the steel. ₂ O _Three Is generated and the strength decreases, so the upper limit was made 0.060%.
[0038]
N: 0.0080 to 0.0200%
As described above, N combines with Al to refine crystal grains as AlN. In order to acquire such an effect, it is necessary to contain 0.0080% or more of N. On the other hand, even if the content exceeds 0.0200%, the above effect is saturated and N is gasified during steelmaking, which may make it difficult to produce steel. Therefore, the upper limit was made 0.0200%.
[0039]
Ti: 0.010% or less
Ti binds to N to form TiN. When N is contained in an amount of 80 ppm or more, TiN is easily generated as a nonmetallic inclusion (about 20 μm or more). If this TiN is present at a depth of 30 to 100 μm from the surface, when strong shot peening with a maximum compressive residual stress of 1000 MPa or more is applied, it may cause internal microcracks from TiN. In addition, since the gear surface is extremely strengthened, the fracture starting point causes the tooth surface fracture of the internal fracture starting point due to the influence of shear stress, so the upper limit was made 0.010%.
[0040]
Next, the following relational expression for regulating chemical components in the case-hardened steel will be described.
[0041]
The relational expression 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%) for regulating the chemical component is a parameter for regulating the tempering resistance of martensite. That is, the tooth surface of the gear is exposed to an environment of 200 to 500 ° C. due to heat generated by friction, and the surface is tempered. As a result, when the decrease in the hardness of the tooth surface is large, that is, when the tempering resistance is low, it becomes a factor of pitching fracture.
[0042]
In the above relational expression, when the value is 1.5 or more, the tempering resistance of martensite is improved, and the decrease in martensite hardness on the tooth surface can be suppressed to Hv100 or less during use of the gear. Pitting resistance can be improved in terms of tooth surface hardness. The upper limit of the value of the relational expression is preferably 3.0 for reasons of deterioration of workability (machinability) due to an increase in material hardness and cost increase due to an increase in the amount of alloy elements.
[0043]
Next, the operation of the present invention will be described.
As described above, the gear of the present invention is provided with the specific amount of carburized abnormal layer further on the outer layer of the carburized layer having a specific C concentration and retained austenite. This carburized abnormal layer is worn and thinned to some extent by the shot peening process after the carburizing process, but the above-mentioned thickness (2 to 10 μm) remains, and the presence of this abnormal layer has excellent initial conformability at the stage of actual use. Demonstrates excellent pitting resistance.
[0044]
That is, in a carburized gear, when polishing is not performed after carburizing, it is inevitable that the shape of the tooth surface includes a certain amount of error (strain). Needless to say, individual gears have slightly different shapes for each tooth in one gear. This error (strain) greatly affects the distribution of contact pressure applied to the tooth surface.
[0045]
As a result, the maximum contact pressure generated on the tooth surface at the time of meshing driving of the conventional gear is extremely high, and often reaches the limit value of the load capacity. These have a great influence on the pitching life, in which not only the gear strength but also the influence of the surface pressure is the dominant factor.
[0046]
In this respect, the gear of the present invention has the carburizing abnormal layer of the specific thickness in the outer layer of the high-hardness carburizing layer. Therefore, the adverse effect of errors existing on the tooth surface can be greatly mitigated by the running-in operation of the gear incorporated in the apparatus.
[0047]
That is, the carburized abnormal layer is a soft structure made of an incompletely quenched structure. Therefore, it has the characteristic that it is easy to wear. This property greatly improves the initial conformability of the gear. Specifically, when the gear is actually engaged and driven, the carburized abnormal layer wears to alleviate the uneven stress distribution generated on the tooth surface, and the shape of the tooth surface is self-corrected.
[0048]
And the maximum thickness after the shot peening process of the carburizing abnormal layer in this invention is 2-10 micrometers. Therefore, the carburized abnormal layer is sufficiently removed by the initial familiarity.
[0049]
Further, in the tooth surface from which the carburized abnormal layer has been removed, the lower carburized layer is exposed on the surface. This carburized layer is composed of a martensite structure with a C concentration of 0.60% or more as described above, and the amount of retained austenite contained is 20% or less, and has a very high hardness.
[0050]
For this reason, in the gear that has been adapted to the initial stage, the tooth surface has a shape capable of obtaining a uniform contact state and a uniform high hardness.
Therefore, the gear of the present invention exhibits a very excellent pitching resistance after the running-in operation.
[0051]
On the other hand, the gear is tempered by being exposed to an environment of 200 to 500 ° C. by frictional heat during use. When the hardness of the tooth surface, that is, the hardness of the carburized layer is reduced by this tempering, the above-mentioned excellent pitting resistance is impaired.
[0052]
In this respect, the gear according to the present invention uses the case-hardened steel having the specific component range as a material. Therefore, it is excellent in resistance to softening by tempering. Therefore, the tooth surface strength can be kept high even during operation exposed to high temperatures, and the above-mentioned excellent pitting resistance can be exhibited.
[0053]
Further, in the present invention, in order to improve the carburizability in high Si steel, Mo is slightly increased and the upper limit of Cr is kept low compared with the previous application (Japanese Patent Laid-Open No. 10-259470). As a result, despite the high Si steel as in the present invention, there is a novel effect that the carburizing process can be performed in the same atmosphere as the conventional steel without adjusting to a high carbon potential atmosphere. Furthermore, by adjusting the addition range of Si and Mo, which have the greatest effect on the form of the carburizing abnormal layer, to an appropriate amount, the carburizing abnormal layer is controlled to have an appropriate form and depth, and the amount of retained austenite is reduced to that of ordinary steel By making it 20% or less, which is lower than in the case of the above, it becomes possible to prevent the occurrence of tooth chipping or the like which becomes a problem during the shot peening process. Further, as a result of adjusting the Si and Mo components, excellent softening resistance could be ensured.
[0054]
In addition, the case-hardened steel in the present invention has not undergone a composition change that increases the cost compared to conventional materials. Further, the cost of forming into a gear shape and carburizing treatment can be the same as the conventional one.
Therefore, in the present invention, the gear having the excellent pitching resistance can be obtained at low cost.
[0055]
The carburizing abnormal layer itself can be formed even when a conventional gear steel such as JlS-SCr420 steel or SCM420 steel is used. However, these carburized abnormal layers of the conventional steel have a thickness of 15 μm or more, which is deeper than that of the present invention, and the occupied area ratio is easily reduced to less than 70%, which is lower than when the steel of the present invention is processed. As a result, when the surface layer structure is viewed microscopically, a layer in which the incompletely hardened structure and the martensite structure are mixed is present on the tooth surface, and the form thereof has a large number of protrusions.
[0056]
Therefore, in a gear using conventional steel, even if a carburized abnormal layer is provided, not only a good initial running-in state can be obtained, but also a pitting failure occurs due to the protruding portion of the remaining incompletely hardened structure. There are many cases. The martensitic structure of the steel is also inferior in softening resistance.
[0057]
Next, as in the invention of claim 2, in addition to the case-hardened steel used in the gear of claim 1, it is preferable to use steel containing Nb: 0.20% or less. Thereby, the strength of the gear can be further increased. The reasons for limitation are described below.
[0058]
Nb: 0.20% or less
Nb improves toughness and improves fatigue strength, for example, by refining crystal grains after carburizing. However, even if it is added in a large amount, not only these effects are saturated but also coarse precipitates are formed and the strength is lowered, so the upper limit was made 0.20%. In addition, in order to fully exhibit the said effect, it is preferable to make it contain 0.01% or more at least.
[0059]
Next, as in the invention of claim 3, by weight ratio, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.20 to 0.85%, Mo: 0.35 to 0.80%, Al: 0.020 to 0.060%, N : 0.0080 to 0.0200%, Ti: 0.005% or less and 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%) Then, using the case-hardened steel with the balance consisting of Fe and inevitable impurities, the case-hardened steel is formed into a gear shape, and then carburized to have a C concentration of 0.60% or more and a residual A carburized layer with an austenite content of 20% or less is formed, and an outer carburized layer is formed with an abnormal carburized layer composed of an incompletely quenched structure. The carburized abnormal layer has a maximum depth of 5 to 13 μm, the area ratio occupied by the carburized abnormal layer in the cross section from the maximum depth position to the surface is 70% or more, and is further subjected to shot peening treatment to obtain teeth. A compressive residual stress of the surface surface is 400 MPa or more, a residual stress that gives a maximum compressive residual stress of 1000 MPa or more in the range from the surface to a depth of 50 μm is applied to the surface, and the maximum depth of the carburizing abnormal layer is 2 to 10 μm, the maximum There is a method for producing a high-strength gear excellent in tooth root bending strength and pitting resistance, characterized in that the area ratio of the carburized abnormal layer in the cross section from the depth position to the surface is 80% or more.
[0060]
The carburizing process refers to a series of processes including carburizing, quenching, and tempering as described above.
In addition, carburizing can be performed using various treatment methods similar to those in the past. In particular, in the present invention, since the Cr and Mo ranges are regulated to an appropriate range in consideration of carburizing hardenability compared to the previous application, it is easier to obtain the target surface C concentration than the previous application. Therefore, it is possible to obtain the target C concentration and carburization hardness in the same carburizing atmosphere as conventional steel such as JIS SCM material.
[0061]
Moreover, after carburizing and adjusting C density | concentration, it quenches immediately after that or after cooling. Quenching may be performed by combining primary quenching and secondary quenching, or may be performed directly. Further, in order to adjust the amount of retained austenite, subzero treatment can be performed after quenching.
[0062]
Further, tempering after quenching is performed at about 130 to 180 ° C. as is usually done.
[0063]
Such a series of carburizing processes is performed on the case-hardened steel having the specific component range formed into a gear shape. Thereby, a gear having the specific carburized layer and the specific carburized abnormal layer can be easily manufactured.
[0064]
Next, the reason for limiting the tissue state immediately after the carburizing process will be described.
The reason why the maximum depth of the carburized abnormal layer is 5 to 13 μm is that when the maximum depth is less than 5 μm, the abnormal layer is almost worn and disappeared by the shot peening process, and the above-mentioned initial conformability effect is obtained. This is because when the thickness exceeds 13 μm, the tooth surface roughness after the shot peening process becomes large, and the effect of improving the strength by the shot peening process cannot be obtained sufficiently.
[0065]
Further, the area ratio of the carburized abnormal layer between the maximum depth position and the surface immediately after the carburizing treatment is set to 70% or more. If it is less than 70%, the carburized abnormal layer obtained after the subsequent shot peening treatment is used. The variation in the depth of the steel also increases, and a large number of soft incompletely quenched structures remain in a wedge shape on the surface of the carburized layer. Therefore, there is a problem that a crack starting from this tends to occur.
[0066]
After a series of carburizing processes, a shot peening process is further performed. The conditions for this treatment are not particularly limited, but the compressive residual stress on the tooth surface (excluding the abnormal layer) is 400 MPa or more, and the maximum compressive residual stress in the range from the surface (excluding the abnormal layer) to a depth of 50 μm is 1000 MPa or more. The shot peening process is performed under the following conditions. This treatment condition varies depending on conditions such as the component composition of the steel and the C concentration of the surface layer, but is generally 0.30 mmA or more (when the shot particle diameter is φ0.5 or more) or 0.10 mmN or more at the arc height ( The treatment can be performed in combination or independently under the condition of the case where the shot particle diameter is 0.3 or less. In particular, when it is performed in combination, it exhibits an excellent effect on the improvement of the root bending strength and the tooth surface strength. The surface roughness Rmax of the tooth surface after the shot peening treatment is preferably 10 μm or less. This is because when it becomes rough, the pitting resistance deteriorates.
[0067]
By this shot peening treatment, the abnormal layer existing on the surface is slightly worn, the maximum depth of the carburized abnormal layer is 5 to 13 μm to 2 to 10 μm, and the area occupied by the carburized abnormal layer between the maximum depth position and the surface The rate changes from 70% or more to 80% or more in a state where the irregularities of the abnormal layer are slightly less. Further, the carburized layer, which is a layer below the abnormal layer, remains as it is without being changed by shot peening.
[0068]
The reasons for limiting the maximum depth of the carburized abnormal layer after the shot peening process and the area ratio occupied by the carburized abnormal layer between the maximum depth position of the carburized abnormal layer and the surface are as described above.
The other reasons for limiting the numerical values are as described in the first aspect of the invention.
[0069]
Further, as in the invention of claim 4, in addition to the composition of the steel used in the manufacturing method of the gear of claim 3, the steel containing Nb: 0.20% or less is used, and the manufacturing method of claim 3 is used. It is preferable to apply.
In these cases, the same effect as described above can be obtained. The reason for limiting the numerical value is also as described in the description of claim 2.
[0070]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
A high-strength gear excellent in tooth root bending strength and pitting resistance according to an embodiment of the present invention and a manufacturing method thereof will be described with reference to FIG.
The gear 1 of this example is an example of a general helical gear. The helical gear is an example, and can be applied to a spur gear, a helical gear, a bevel gear, a screw gear, a worm gear, and other various gears.
[0071]
In manufacturing this helical gear, first, A, B, and C steels (same as steels 1, 2, and 8) shown in Table 1 were prepared as the case-hardened steel. Among these, steels A and B are steels that satisfy the conditions of the component composition of the present invention, and steel C is a comparative steel that is outside the scope of the present invention and whose Mo and Cr contents are not optimized. First, an embodiment of a gear manufactured using A and B steel will be described.
[0072]
[Table 1]

[0073]
As shown in Table 1, A and B steels are steels in which the contents of C, Si, Mn, Cr, Mo, Al, and N are regulated within the scope of the present invention, and 1.5 ≦ 3 × Si ( %)-Mn (%) + Cr (%) / 4 + Mo (%).
[0074]
In producing a gear (hereinafter, referred to as gears A and B) using A and B steels, first, the number of teeth having a specification of module 2, a pressure angle of 18 °, and a helix angle of 27 ° is 29. The gear is formed by cutting.
Next, the formed gear is carburized. The carburizing process in this example was performed as shown below using a gas carburizing method.
[0075]
That is, the carbon potential (CP) was first maintained at 0.8% and carburized in a gas atmosphere kept at a temperature of 950 ° C. for 4 hours. While maintaining P, the temperature was lowered to 860 ° C., soaked for 45 minutes, and then quenched into 130 ° C. oil. Thereafter, a tempering treatment was performed at 160 ° C. for 1 hour. This condition is one of the conditions normally applied to SCM materials that are JIS steel.
[0076]
Normally, in JIS steel, the carburizing temperature is 920 to 980 ° C., the time is 1 to 15 hours, the soaking temperature is 840 to 870 ° C., and the time is about 0.5 to 2 hours. In the present invention, the components are adjusted so that excellent carburizing properties can be obtained, so that the processing can be performed without changing the conditions in the conventional mass production line of steel.
[0077]
As shown in FIG. 1, the gears A and B obtained by the carburizing process have a surface state in which the carburized layer 12 is formed on the parent phase 10 and the carburized abnormal layer 11 is further formed thereon.
[0078]
As a result of analysis by EPMA, the carburized layer 12 in this example has a C concentration of 0.77%, and as a result of investigation by X-ray diffraction, the amount of retained austenite is 12%. Moreover, the hardness of the carburized layer 12 was as high as Hv811.
Further, the gear B has a C concentration of 0.67%, and as a result of investigation by an X-ray diffraction method, the amount of retained austenite is 10% or less. Moreover, the hardness of the carburized layer 12 was as high as Hv769.
[0079]
Further, as shown in FIG. 1, the carburizing abnormal layer 11 has a maximum depth D of 7 μm and 10 μm for the gear A and the gear B, and a carburizing abnormal layer in the cross section A from the maximum depth position to the surface. The area ratio occupied was 85% and 75%.
[0080]
Further, shot peening treatment was applied to the gear A and the gear B using shot grains having a particle diameter of 0.6 mm and an arc height of 0.50 mmA. As a result of measuring the maximum compressive residual stress after the shot peening treatment, both the gear A and the gear B showed a high value of 450 MPa on the surface of the pitch circle and 1250 MPa on the maximum value in the depth range of 50 μm from the surface. .
[0081]
The carburizing abnormal layer 11 after the shot peening treatment has a maximum depth D of 2 and 3 μm, and the area occupied by the carburizing abnormal layer in the cross section A from the maximum depth position to the surface is 90% or more and 85 %, The abnormal layer had less variation in thickness than immediately after the carburizing treatment. The carbon concentration of the carburized layer was not changed from that before the shot peening treatment. In addition, the amount of retained austenite was 10% or less after the shot peening. Further, the surface roughness Rmax of the tooth surface was a relatively small value of 8.7 μm and 9.2 μm. When the tooth surface strength of this gear was measured (measurement method will be described in Examples described later), the gear A showed a high value of 25 kgf · m and the gear B 23 kgf · m (18 kgf · m in the conventional steel SCM420H). .
[0082]
Next, the operation of the gear 1 of this example will be described.
As described above, the gear 1 of the present example is provided with the specific amount of the carburizing abnormality layer 12 on the outer layer of the high-hardness carburizing layer 12.
Therefore, excellent initial conformability is exhibited by engaging and driving the two gears 1.
[0083]
That is, in the gear, the carburizing abnormality layer 11 is worn to alleviate the uneven stress distribution generated on the tooth surface, and the shape of the tooth surface is self-corrected. And in the tooth surface from which the carburizing abnormal layer 11 was removed by abrasion, the lower carburizing layer 12 will be in the state exposed to the surface.
[0084]
Therefore, in the gear of this example, after a certain initial running-in operation, a uniform contact state of the tooth surface is obtained, the stress distribution is uniform, and the contact surface is constituted by a hardened carburized layer. become.
As a result, the stress state at the time of contact can be improved and the pitting resistance can be improved as compared with a gear that has been provided with a countermeasure for reducing the carburized abnormal layer as much as possible.
[0085]
Further, the gear of this example uses A steel and B steel having the above specific composition as materials. Therefore, there is little decrease in hardness due to frictional heat during operation of the gear.
Therefore, the gear of this example does not deteriorate the pitting resistance so much even after long-term use, and has a long life.
[0086]
Although the embodiments of the gears A and B have been described above, the results of a gear (hereinafter referred to as the gear C) manufactured by using the C steel with the same specification and the same manufacturing method will be described.
[0087]
Steel C is a comparative steel in which the aforementioned Mo and Cr amounts are not optimized among the components of the present invention, and components other than Mo and Cr are steels within the scope of the present invention. As a result of performing the same carburizing treatment conditions as in the case of A and B steel using this C steel, the carbon concentration of the carburized layer is 0.57%, which is lower than the value specified in the present invention, and the residual austenite is 10% or less. However, the carburization hardness was significantly lower than Hv697 and steels A and B. Therefore, as a result of changing the C potential among the carburizing treatment conditions from 0.8% to 1.0% and performing the carburizing treatment again under the same conditions other than the C potential, the carbon concentration and carburization hardness of the carburized layer are: In each case, 0.79%, Hv800 and the above steel A were almost the same values, and the amount of retained austenite was 16%. From the results of steel A, steel B and steel C, it is shown that optimization of the amounts of Mo and Cr has a great effect on improving the carburizing hardenability.
[0088]
Moreover, as a result of measuring the maximum depth of the carburizing abnormal layer and the area ratio of the carburizing abnormal layer in the cross section from the maximum depth position to the surface of the gear C processed under the same carburizing processing conditions as the gears A and B, 14 μm, 70 %, And the thickness was higher than that of the gears A and B. This is presumed that the reduction of Cr and the increase of Mo contribute to the thickness reduction of the abnormal carburizing layer and the improvement of the area ratio.
[0089]
Next, the gear C was subjected to the same carburizing conditions (CP: 0.8% C) and shot peening treatment as the gears A and B. As a result, it was recognized that the tooth surface roughness was 10.7 μm, which was rougher than that of the gears A and B. This seems to be because the thickness of the carburized abnormal layer was thick before the shot peening treatment. As a result of measuring the tooth surface strength of the gear C in the same manner as the gears A and B, the value was as low as 16 kgf · m. This is the structure of the carburized hardened layer due to the large surface roughness or low C% of the carburized layer (in the case of 0.60% C concentration or more, it becomes a mixture of acicular martensite structure and lath martensite structure. It is conceivable that the difference is that the main component of the lath martensite structure is less than 60% C concentration.
[0090]
As is apparent from the above description, it can be understood that the optimization of the Cr and Mo amounts greatly contributes to the performance improvement of the gear of the present invention.
[0091]
Embodiment 2
In this example, a test steel having the components shown in Table 1 was prepared to produce a gear, and the softening resistance of the carburized layer was quantitatively evaluated. Among the test steels shown in Table 1, steels 1 to 7 are steels within the component range specified in the present invention, and steels 8 to 14 are comparative steels and 15 steels, some of which do not satisfy the conditions of the present invention. Is SCM420H, which is a conventional steel.
[0092]
Production of gears using each test steel was basically performed by the same manufacturing method as in the first embodiment. However, in order to prevent the difference in the evaluation results due to the difference in the carburizing state, the carburizing conditions of the comparative steel were adjusted so that the carbon concentration of the carburized layer was all 0.60% or more.
[0093]
In order to evaluate the softening resistance, after the carburizing quenching and tempering, a re-tempering treatment at a temperature of 250 ° C. for 4 hours was further performed. And the cross-sectional hardness of the carburized layer before and after the re-tempering treatment was measured.
The measurement results are shown in Table 1.
[0094]
As is known from Table 1, the steels 1 to 7 that satisfy the conditions specified in the present invention, in any of the test steels, the hardness of the carburized layer before re-tempering shows a very high hardness of Hv750 or more, Even after re-tempering, the hardness decreased to Hv100 or less, and had excellent softening resistance. In addition, among them, the lower the hardness, the smaller the steel having a higher relational expression value. On the other hand, the steel No. 8 which is a comparative steel has a small hardness decrease due to the re-tempering treatment, but the hardness after carburizing quenching and tempering is as low as Hv 700 or less, and the steels of the comparative steels 10 to 13 have a high hardness of Hv 750 or more. However, the hardness reduction after re-tempering becomes Hv100 or more, and the softening resistance is inferior. In addition, 15 steel, which is a conventional steel, was greatly reduced from Hv786 to Hv648 by re-tempering treatment. In particular, among the comparative steel and the conventional steel, the steel having a relational expression value of less than 1.5 was greatly reduced to a low hardness of less than 700 Hv.
[0095]
In order to clarify the relationship between this decrease in hardness and the value of the above relational expression, this is shown in FIG.
In FIG. 2, the horizontal axis represents the value of the relational expression, and the vertical axis represents the hardness decrease value (Hv).
[0096]
As can be seen from the figure, there is a correlation between the value of the relational expression and the decrease in hardness due to re-tempering. And when the value of a relational expression exceeds 1.5, it turns out that the hardness fall by re-tempering becomes 100 Hv or less.
[0097]
From the above results, the gear of the present invention using the steel in which the value of the above relational expression is regulated to 1.5 or more can suppress softening due to frictional heat when the gear is used, and can improve the pitting resistance of the gear for a long time. It became clear that can be maintained over.
[0098]
Embodiment 3
This example prepares the test steel shown in Table 1 used in Embodiment Example 2, and the composition of the case-hardened steel, the depth of the carburized abnormal layer, the amount of retained austenite, the compressive residual stress, and the like are pitching strength, tooth root bending. It was evaluated quantitatively how it affects the strength.
[0099]
Each test steel was melted, forged and forged, and normalized, then processed into the gears described above and carburized.
The carburizing process was performed by the carburizing method of Example 1 for those that were subjected to normal gas carburizing, and the vacuum carburizing process was performed for those that did not generate any carburizing abnormal layer. After the carburizing treatment, the same shot peening treatment as described in Example 1 was further performed.
[0100]
As a result of observing the state of the gears after the shot peening treatment, the conventional steel 15 having a large amount of retained austenite was found to have chipped teeth. This indicates that tooth loss tends to occur as the amount of retained austenite increases. On the other hand, tooth loss was not observed when the retained austenite was within the scope of the present invention.
[0101]
In addition, the surface roughness was also examined, but the deeper the carburized abnormal layer, the greater the roughness. Particularly, when the maximum depth of the carburized abnormal layer was 13 μm or less immediately after the carburizing treatment, the surface roughness was all in the range of 10 μm or less in terms of Rmax.
[0102]
Next, a power circulation type gear testing machine (manufactured by Shinko Engineering Co., Ltd.) was used for strength evaluation of the gears. Then, two gears each having 29 and 43 teeth with specifications of a module 2, a pressure angle of 18 °, and a torsion angle of 27 ° are prepared, and a load torque of 40 kgf · m, a rotational speed of 2000 rpm, The test was conducted at an oil temperature of 80 ° C. (forced lubrication). ⁷ It was investigated whether the gears were broken by rotating them up to once. On the other hand, for the evaluation of tooth surface strength, a test was carried out under the conditions of a load torque of 23 kgf · m, a rotation speed of 4000 rpm, and an oil temperature of 120 ° C. (forced lubrication). ⁷ The pitching area ratio after rotation was measured. The pitching area ratio was obtained by measuring the total area of the portion of the tooth surface of the gear after the test where pitching failure occurred using a microscope and calculating the area ratio from the measured value. In addition, the above-described first embodiment is evaluated by performing a test while changing only the load torque among the above conditions and measuring the maximum load torque at which the pitching area ratio is 0%. The test results are shown in Table 2.
[0103]
[Table 2]

[0104]
As is known from Table 2, the pitching area ratio is 3 for the cases where the maximum depth of the carburized abnormal layer immediately after the carburizing treatment is 0 and those exceeding 13 μm (sample Nos. 2, 9, 10, 12, 16). % Or more, and sample No. Except for 2, the root bending strength also decreased. In particular, sample no. No. 2 is an example in which the thickness of the abnormal layer was reduced to 0 by vacuum carburization, and showed excellent characteristics in terms of the root bending strength. As a result, pitting fracture at the adhesive wear starting point occurred, and the tooth surface strength decreased, and the pitching area ratio became very large at 10% or more. In addition, a deep carburized abnormal layer has a large surface roughness as described above, and the surface compressive residual stress after shot peening is reduced, which is presumed to have an adverse effect.
[0105]
In particular, even when the occupied area ratio of the carburized abnormal layer is 70% or more, when the maximum depth of the carburized abnormal layer immediately after carburizing exceeds 13 μm as described above (sample No. 10), the pitching is 6.3%. The area ratio and root bending strength decreased. As a result, when the carburized abnormal layer exceeds 13 μm, the surface roughness is large and the compressive residual stress after the shot is low regardless of the occupied area ratio of the carburized abnormal layer. It is thought that it is reflected.
[0106]
Further, even when the maximum depth and occupation ratio of the carburized abnormal layer are good, when the steel component range is outside the scope of the present invention (Sample Nos. 11 and 15), and the maximum depth and occupation of the carburized abnormal layer Even when the area ratio is good and the steel component range is within the range of the present invention, the pitching strength is low when the value of the relational expression described above is less than 1.5 (sample Nos. 13 and 14). The result was low. In particular, sample No. No. 15 shows a spalling failure that is a pitching failure starting from the inside of the steel. From this result, it is found that reduction of hard inclusions is indispensable in the case of a gear imparted with strong shot pinning.
[0107]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a gear that can be carburized under the conditions of conventional steel and that is excellent in tooth root bending strength and pitting resistance, and a manufacturing method thereof.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a formation state of an abnormal carburization layer in Embodiment 1;
FIG. 2 is an explanatory view showing a hardness reduction state due to re-tempering in Embodiment 2;
[Explanation of symbols]
1. . . gear
10. . . Mother phase
11. . . Carburized abnormal layer
12 . . Carburized layer

Claims (4)

In the gear obtained by carburizing and shot peening after forming the case-hardened steel into a gear shape, the case-hardened steel has a weight ratio of C: 0.10 to 0.30%, Si: 0.00. 50 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.20 to 0.85%, Mo: 0.35 -0.80%, Al: 0.020-0.060%, N: 0.0080-0.0200%, Ti: 0.010% or less, and 1.5 ≦ 3 × Si (% ) -Mn (%) + Cr (%) / 4 + Mo (%) satisfying the composition of which the balance is composed of Fe and inevitable impurities and to which shot peening is applied after carburizing treatment has a C concentration of 0. It has a carburized layer of 60% or more and a residual austenite amount of 20% or less. The outer layer of the carburized layer has a carburized abnormal layer composed of an incompletely quenched structure, and the maximum depth of the carburized abnormal layer is 2 to 10 μm, and from the position of the maximum depth to the surface A high-strength gear excellent in tooth root bending strength and pitting resistance, wherein the area ratio occupied by the carburized abnormal layer in the cross section is 80% or more. In addition to the composition of the case-hardened steel used in the gear according to claim 1, steel having Nb: 0.20% or less is further used, and the tooth root bending strength and pitting resistance are excellent. High strength gear. By weight ratio, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.00. 035% or less, Cr: 0.20 to 0.85%, Mo: 0.35 to 0.80%, Al: 0.020 to 0.060%, N: 0.0080 to 0.0200%, Ti: A composition containing 0.010% or less and satisfying 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%), the balance being Fe and inevitable impurities Using case-hardened steel, the case-hardened steel is formed into a gear shape, and then carburized to form a carburized layer having a C concentration of 0.60% or more and a residual austenite amount of 20% or less. In addition, an abnormal carburization layer composed of an incompletely quenched structure is formed on the outer layer of the carburized layer, and the maximum depth of the carburized abnormal layer is 5 The area ratio occupied by the carburized abnormal layer in the cross section from the maximum depth position to the surface is set to 70% or more, and further the shot peening treatment is performed so that the compressive residual stress on the tooth surface is 400 MPa or more. Giving a residual stress that gives a maximum compressive residual stress of 1000 MPa or more in a range up to a depth of 50 μm, and a maximum depth of the carburizing abnormal layer is 2 to 10 μm, and the carburizing abnormal layer occupies a section from the maximum depth position to the surface A method for producing a high-strength gear excellent in tooth base bending strength and pitting resistance, wherein the area ratio is 80% or more. In addition to the composition of the case-hardened steel used in the gear manufacturing method according to claim 3, the steel containing Nb: 0.20% or less is used, and the manufacturing method according to claim 3 is applied. The manufacturing method of the high-strength gear excellent in the root bending strength and pitting resistance.

Images