JP4273462B2 - Self-lubricating sliding part material and piston ring wire - Google Patents

Description

【００２３】
ここで、Ｐは圧力、ｘは摺動方向、ｙは摺動方向と直角方向の距離を表している。また、流体潤滑の基礎方程式であるＲｅｉｎｏｌｄｓ方程式は以下のように表される。
【００２４】
【数２】

【００２５】
ここで、ρは流体の密度、ｈが流体膜の厚み、ηは粘性係数、ｔは時間、ｕは相対速度を表している。よって、（１）、（２）式より、流体膜に正の圧力が発生する必要条件は以下のようになる。
【００２６】
【数３】

【００２７】
（３）式には３つの項が発生しているが、第１、第２項は相対速度ｕを含み、先述したくさび作用、伸縮作用に相当する。そして、ｕを含まない第３項こそが、ピストンリング／シリンダ間で相対速度がゼロとなっても機能する可能性が残されている、スクイズ作用を示す項である。
【００２８】
この第３項が負になることがどのような物理的意味を持つかを説明すると、それは流体の密度が一定であれば、流体膜の膜厚が時間的に急に減少することで、結果、流体膜に正圧が発生するという意味である。現実的にこのような現象を起こそうとすれば、基板を滑る板に垂直荷重を急激に与え、流体膜を搾り出す必要がある。その結果、流体膜は搾り出されてゆくが、同時に高い正圧が発生し、なかなか固体同士の接触には至らない、すなわち、スクイズ作用が期待できるのである。
【００２９】
ここで、この効果を増幅させるには、その摺動する表面に多数の穴状構造を形成することでいっそう高まることを、本発明者らは知見した。すなわち、摺動面に微細かつ多数の空孔を設ければ、相対速度がゼロにある、流体膜の破れた状態からであっても、次の時間経過には空孔中の流体が瞬時に流体の枯渇面に搾り出され、その移動した流体の大きな膜厚減少によるスクイズ作用を得ることができるのである。これにより、上記往復運動の上下死点近傍での焼付きを抑制でき、さらには摩擦係数を低下できるのである。
【００３０】
本発明の黒鉛析出組織はこの作用・効果にも注目して決定されたものである。すなわち、本発明の黒鉛組織は、まずそれ自体が固体潤滑作用を有する潤滑相として作用するのであるが、加えて同時に重要となるのが、その黒鉛が脱落した時の空孔として油膜保持性を向上させる上記のスクイズ作用である。このスクイズ作用は、圧力変動に対して油膜が安定して形成され、摺動面の形状としては表面に穴を配置することでその効果が高まることは上記の通りである。この効果を得るために黒鉛析出こそ有効であって、通常のすべり軸受、コロ軸受、玉軸受部材はもちろんのこと、圧力変動が大きな、ピストンリング、シリンダライナー、バルブリフタのシム面、種々のカム、歯車、金型部材、もしくは切断刃などの、すべり軸受のような連続的な流体潤滑膜形成が困難な摺動部にも効果を発揮するものである。
【００３１】
また、本発明の黒鉛析出組織は、そのピストンリングに適用した場合の、最近問題となってきたアルミニウム製ピストンとの凝着摩耗を防止することにも効果を発揮する。すなわち、アルミニウムは炭素に対しほとんど固溶限を持たないため、凝着反応が抑制されるのである。
【００３２】
以上、本発明の摺動部品用材料は、その鋼組織中に黒鉛を分散させるものであるが、その黒鉛の平均粒径は３μｍ以下にすることが重要である。これは、平均粒径が３μｍを超えると、その摺動中に黒鉛周辺部が破損し、この破片が摺動面内に入り込む懸念があるからである。なお、本発明の黒鉛分散による効果を得るために、任意の鋼組織断面に存在する黒鉛の占める面積率を１％以上にすることが好ましい。さらに好ましくは、比較的粗大な粒径１μｍ以上の黒鉛について、その平均粒径を５μｍ以下とすること、あるいはその面積率にて５％未満とすることである。
【００３３】
空孔の形成による潤滑効果は、上記の作用によって流体膜の減少を招き、いずれは効果を失うため、主流の流体潤滑設計としてはあまり扱われなかった。しかし、特に内燃機関といった連続的な流体潤滑膜の形成が困難な往復運動にあってこそ本発明の手段が有効であり、例えば相対速度がゼロ近くになり流体膜が破断する上下死点近傍が一時的な状態であり、すぐに潤滑油の潤沢な環境に移行してゆくピストンリング／シリンダ間のような、非定常的な摩擦挙動を起こす環境の場合、黒鉛析出は威力を発揮する。
【００３４】
ピストンリング、シリンダライナーに代表される、構造上一時的に流体膜の破断が生じる条件下でこそ、流体膜の維持が可能な潤滑設計を行なうことは重要である。それであっても、エンジンの回転数や摺動部品の構造がかわると、固体接触の可能性が高まるのであって、これら非定常的な摺動部品には、黒鉛鋼のように、固体潤滑作用のある黒鉛を析出させた材料を適用することでより広範囲な摺動条件に対応可能となる。
【００３５】
次に、本発明の摺動部品を構成する鋼の成分組成について説明する。
黒鉛鋼自体については、古くから報告があり、これらは主にＳｉ、Ｎｉを添加した合金とするものであるが、６００℃以上の温度保持を行なうような黒鉛析出処理に数十時間以上を要するものであった。そこで、本発明では、その数時間レベルでの黒鉛析出を完了させるため、セメンタイトが分解し易く、Ｎｉの上に適量のＡｌを添加したところに特徴を有する。
【００３６】
一般的に鉄中の炭素は、黒鉛として析出する前段階として、一旦、準安定なセメンタイトとして析出する。このため、黒鉛析出処理では、セメンタイトを分解し、安定な黒鉛へと変化させる必要がある。従来の黒鉛鋼では、セメンタイトの分解が進み難く、非常に長い時間を要した。そこで、本発明では、Ｃｒ等のセメンタイト分解を妨げる元素を抑制することで、準安定なセメンタイトが析出してもすぐ分解する成分設計を狙い、実質的にはセメンタイトが生成することなく、一挙に黒鉛として析出させることができる。
【００３７】
また、Ａｌは鉄中での拡散速度が速く、Ａｌを添加したことで、空孔の拡散速度が速くなり、黒鉛の析出場所となる空孔の集合体を形成する時間が短くなる。このような二つの作用効果を併用することで、本発明では黒鉛析出を短時間で行なうことが可能となった。また、浸炭処理等により、表面のみに黒鉛を析出させることも容易である。
【００３８】
更に、Ａｌは窒化硬化能を有する元素でもあり、窒化硬化鋼の合金設計上、好都合である。なお、同じく窒化硬化元素として使用される元素にＣｒがあるが、Ｃｒは本発明の根幹技術となる黒鉛形成を阻害するだけでなく、耐硫酸腐食性をも著しく低下させることから、その使用を極力避けるものである。よって、本発明の採用するＡｌは重要な含有元素である。
【００３９】
加えて、従来、黒鉛鋼に窒化処理を適用した場合など、窒化層中に例えば粒径１０μｍ以上の黒鉛といった欠陥があると、窒化層が脆くなるという欠点が指摘されており、窒化鋼に黒鉛を析出させるには難点があった。そこで、本発明においては、その窒化鋼としても成立させるために、析出する黒鉛の微細化にも努めた。
【００４０】
この析出黒鉛の微細化に関しては、（１）加工歪みによる微細化、（２）Ａｌ_２Ｏ_３等介在物の添加、（３）ＢＮ、ＴｉＣ等を析出核とする、３つの方法が挙げられる。しかし、（１）はプロセス上の制約があり、（２）は分散化の製法に難がある、そして、（３）は微量成分を制御することで達成される効果であるため、高Ｃ鋼においては製法に難がある。これらに関しては、例えば特開平１１−２４６９４０号にはＴｉＣを分散させる方法が、岩本隆ら「鉄と鋼Ｖｏｌ．８４（１９９８）ｐ．５７」にはＢＮを核に黒鉛析出を行なう方法が示されているが、これらは１０００℃以上の拡散速度が高い高温で第２相が析出するので、黒鉛の微細均一性を保つことが難しく、成分偏析の激しい高合金への適用が困難である。
【００４１】
そこで、本発明者らは、この黒鉛の微細化を達成するべく種々検討した結果、組織中のＣｕ−Ａｌ金属析出相が有効に作用することを見いだした。これは前述した、第２相を析出黒鉛の核として作用させる現象であるが、このＣｕ−Ａｌ相は８００℃以下という低温で析出するため、微細な黒鉛組織が安定かつ迅速に形成できる。本発明においては、具体的には脆化現象が起こらないレベルでのＣｕとＡｌの添加を行なうことにより、強度劣化が少なくかつ潤滑相として作用する黒鉛組織を形成することができる。加えて、Ｃｕは耐硫酸腐食性の向上にも寄与している。
【００４２】
黒鉛の分散による潤滑技術については、従来、鋳鉄の分野にて多く提案されているが、各分野の摺動部品についてその使用環境が年々苛酷化しているため、鋳鉄から鋼の表面処理材へと変更が進んでいる。しかし、それであっても、例えば内燃機関のシリンダブロックの場合、そのほとんどはアルミ化されているもののシリンダライナー部内壁は、今でも前述した効果を狙った、黒鉛を晶出させた鋳鉄が使用されている状況である。本発明の摺動部品は、摺動特性は勿論のこと、その他、環境の苛酷化により必要とされる強度、耐摩耗性、耐硫酸腐食性をも兼備させるために、鋳鉄の特性も取り込んだスチール材としての成立を目的とし、そのための合金設計に特徴を有するものである。以下、成分組成について説明する。
【００４３】
Ｃは一部が基地中に固溶して強度を付与し、一部は炭化物、あとの残部は黒鉛を形成して耐摩耗性と耐焼付き性を高める重要な元素である。このためには少なくとも０．４％が必要である。しかしながら、１．５％以上になると融点が低下し、インゴットを１２００℃前後で数十時間加熱するような、凝固偏析解消のための拡散焼鈍処理による組織の均一化が困難になる。よって、Ｃは０．４％以上１．５％未満とする。好ましくは、０．５％以上１．３％未満である。
【００４４】
Ｓｉは通常脱酸剤として添加されるが、ここでは黒鉛析出の促進元素として添加しており、耐硫酸腐食性を向上させる効果もあるため、０．１％を下限とした。一方では鋼の焼戻し軟化挙動にも影響し、特に低合金鋼においてはＳｉの影響は重要である。焼戻し軟化を防ぎ耐熱強度を高めるために好ましいＳｉ量は１．０％以上である。しかしながら、過度に添加するとＡ_１点が上昇するので、Ｓｉの上限は３．０％に規定する。よって、Ｓｉは０．１〜３．０％とする。好ましくは、０．５〜３．０％、さらに好ましくは、１．０〜３．０％である。
【００４５】
ＭｎもＳｉと同様、脱酸剤として使用され、最低０．１％は必要であるが、過度に添加すると黒鉛の析出を害する。そのため上限を３．０％に規定した。よって、Ｍｎは０．１〜３．０％とする。
【００４６】
Ｃｒは有効な窒化硬化元素であるが、準安定なセメンタイトの分解を抑制し、黒鉛の析出を強力に阻害するだけでなく、耐硫酸腐食性を著しく低下させる元素なので、０．５％以下に規制する必要がある。よって、本発明においては、０〜０．５％、好ましくは、０〜０．３％とする。
【００４７】
Ｎｉは黒鉛の形成促進元素であり、またＣｕ添加鋼に生じる赤熱脆性を抑えるのに有益な元素でもあるが、一方でＦｅ中へのＣの固溶限を上げるため、焼鈍状態での加工性を阻害する元素である。よって、０．０５〜３．０％とした。好ましくは、０．６〜１．５％である。
【００４８】
ＡｌはＣｒと同様、窒化硬さを上昇させる元素であるが、本発明ではＣｒを上げられない分、Ａｌを添加することで窒化硬さを確保している。そして、その特徴とするところは、黒鉛形成元素であり、空孔の拡散を助け、更にはＣｕと共にＣｕ−Ａｌ相として黒鉛析出の核となり、迅速かつ微細な析出を起こさせるのに有効な元素であるということから、０．３％以上添加する必要がある。ここで、ＡｌもＳｉと同様、Ａ_１点を上昇させるため、２．０％以下に規定する。よって、本発明では０．３〜２．０％の添加範囲に限定する。好ましくは、０．７〜２．０％である。
【００４９】
ＭｏはＣｒに比較して黒鉛化を阻害しない炭化物形成元素であり、さらには耐熱性を付与する元素である。炭化物は、例えばピストンリング製造工程での曲げ加工後に行なう熱成形処理においてマトリックスを拘束するため、寸法安定性を高める働きを持つ。しかし、過度のＭｏ添加はＣｒと同様にセメンタイトの分解を抑制する。
【００５０】
それであっても、Ｍｏは上記の阻害の影響が少なく、一方で耐熱性の向上に大きく寄与し、熱処理時の寸法安定性に貢献する。特にピストンリングの製造工程では細線状態での熱処理工程が入るため、この特性の重要度が高く、合口形状のバラツキ抑制に有効となるため、０．３％以上添加する。一方、黒鉛の析出を阻害するため、その上限は２０％とする。ここで、Ｖ、ＷであってもＭｏに同様の上記効果を持つことから、本発明では、Ｍｏ、ＷおよびＶから成る群から選ばれた少なくとも１種を合計量で０．３〜２０％とすることができる。
【００５１】
ただし、Ｖを添加する場合であっても、Ｖ炭化物が生じると黒鉛析出を著しく阻害するため、鋼組織断面にＶ炭化物が観察されないことが望ましい。
あるいは、Ｍｏ、Ｗの複合あるいは単独添加による（Ｍｏ＋Ｗ）を０．３〜５．０％とし、Ｖは０．１％未満であることが好ましい。また、Ｍｏは、黒鉛によるスクイズ作用を促進し、高圧下での流体膜形成を助けることで、耐焼付き性を高め、かつ動摩擦係数を下げる効果がある。加えて、耐硫酸腐食性を向上させる効果もあることから、Ｍｏ単独での添加量が１．５〜３．０％であることが好ましい。
【００５２】
ＣｕはＣｕ−Ａｌの金属相を析出させ、微細な黒鉛組織を安定かつ迅速に形成するのに有効であることから、Ａｌと共に本発明にとって重要な元素である。加えて、耐硫酸腐食性を向上させる効果もある。よって、Ａｌ量も兼ねた相互的な含有量調整を要し、その効果を得るためには０．０５％以上、望ましくは０．２％以上添加する必要がある。一方、Ｃｕは過剰に添加すると焼きなまし時の硬さが高くなり、加工性を阻害するため、３．０％以下にする必要がある。よって、本発明では０．０５〜３．０％の組成範囲とし、好ましくは０．２〜３．０％とする。
【００５３】
さらに、潤滑性を向上させる技術として、内燃機関の場合では、従来、硫黄（Ｓ）は有機化してエンジンオイル中に極圧添加剤として添加され、焼付きを防止する技術として知られている。その一方で、本発明者らは、鋼中にＭｎＳといった硫化物（サルファイド）を存在させることにより、その硫黄（Ｓ）が摩擦発熱により摩擦面にできた新生面にｉｎ ｓｉｔｕなサルファイド膜を形成し、これが潤滑性能を向上させることに効果があることを突きとめた。この手段によれば、材料内部に潤滑物質が散在しているので、必要な局所で潤滑性能を上げるために潤滑物質を多量に添加する必要もなく、極圧添加剤のようにオイル交換時に効能が消失しないため、半永久的に機能することが期待できる。
【００５４】
また、ピストンリングについてＣｒ系炭化物を鋼中に増やす従来の手段は、言い換えると、シリンダライナーとの接触面積が少なく、単位面積あたりの摺動エネルギーが高いピストンリング側の耐摩耗性を大きくして、シリンダライナーとのバランスをとることが狙いである。このことにより耐焼付き性も上がるが、本質的には接触の不均一によって生じる状況をシリンダライナー側の摩耗を促進させることにより、接触面積を増大化させることで、局部の異常な面圧の上昇を回避することが狙いである。つまり、ピストンリング装着初期のなじみを良くする技術であり、凝着摩耗のような耐久性が必要とされる摩耗特性としての機能は少ない。
【００５５】
しかも、過度な耐摩耗性向上はシリンダライナー側を攻撃する状況が発生し、極度に進行するとかえってクリアランスの増加等を生じ、排気ガス量に関係のあるブローバイ量を増加させてしまうが、硫黄（Ｓ）の効果は、材料の摩耗を促進させずに摩擦係数を下げることで耐焼付き性を向上させているので、エンジンの運転が進んでもクリアランスの変化の少ない状況を持続させる効果が高い。
【００５６】
よって、本発明の摺動部品用材料においては、硫黄（Ｓ）を適量添加することにより耐焼付き性を更に改善することができる。すなわち、硫黄（Ｓ）は大部分Ｍｎと結びついてＭｎＳを形成し、これがエンジンオイルといった潤滑剤に作用して潤滑効果を発揮するために摩擦係数が低くなり、耐焼付き性が向上する。
【００５７】
ここで焼付きとは、摩擦発熱により摩擦面温度が上がり、材料間の原子の移動が熱振動によって起こるために固着する現象であり、摩擦面温度は摩擦エネルギー（＝摩擦係数×面圧×滑り速度）の単調増加関数の関係となる。そのため、摩擦係数が減少すると昇温し難くなり、耐焼付き性が向上する。この効果を得るために硫黄（Ｓ）の含有は有効であるが、過度に添加すると機械的性質が劣化し、例えばスチール製ピストンリング用の鋼線の引抜き加工工程にて破断が懸念されるので、上限を０．３％とすることが好ましい。含有に望ましくは０．０１〜０．３％、さらに望ましくは０．０３〜０．３％である。
【００５８】
また、更に本発明者らは、硫黄（Ｓ）を０．３％にまで添加した材料とするには、機械的性質の面からその製造工程に施される鍛造比を大きくすることが望ましいことも見いだした。つまり、摺動部品としての機械的性質の向上に有効であり、特には鋼線を曲げ加工することで得られるスチール製ピストンリングにとって、その曲げ加工工程での破断・折損を抑制するのに有効な手段でもある。
【００５９】
なお、この場合の鍛造比とは、上記ピストンリング製造工程におけるインゴットを出発とするピストンリング製品までの減面率で定義される。つまり、その鋼の鍛・展伸されていく方向に垂直な断面、すなわち最終的なピストンリング製品における小口断面に比して、（鍛造前のインゴット断面積）／（曲げ加工後の製品断面積）である。但し、曲げ加工による鋼線からピストンリング製品への減面率は本発明の効果達成の上で無視できるものであり、（鍛造前のインゴット断面積）／（曲げ加工前（鍛・展伸後）の鋼線断面積）で評価してよい。これら鍛造比の数字が高いほど鍛造が進んでいることを示す。
【００６０】
ＭｎＳといったサルファイドを含有する鋼は、その出発組織となる鋳造状態では、球状もしくは紡錘状のサルファイドが凝固セル組織の粒界三重点に多く存在し、その配向はランダムな組織となっているが、施す鍛造比が上がるに従ってサルファイドの配向状態が変化し、機械的性質が改善される。
【００６１】
この鍛造比を大きくしていくことで鋼線長さ方向へのサルファイドの配向性が高まり、つまり、ピストンリングに主に作用する周方向応力に沿った形でサルファイドが伸びるため、機械的性質の劣化がほとんどなくなる。この効果は、特に伸びたサルファイド、すなわち、アスペクト比（最大径／最小径）が３以上のサルファイドに関して顕著であり、言わば、特にアスペクト比が３以上のサルファイドの周方向に対する配向性が悪いと、機械的性質の劣化に繋がる。
【００６２】
具体的には、ピストンリングの外周面と並行な組織面で観察されるサルファイド系介在物、特にはアスペクト比：３以上のサルファイド系介在物の分布状態を、それぞれのサルファイド系介在物の最大径上を通る直線同志のなす並行度（鋭角側の角度）にて３０度以内の状態とすることで、特にピストンリング用線材として有効な摺動部品用材料とでき、例えばこの鍛造比を５００以上とすることが達成の上で好ましい。
【００６３】
図５に鍛造比１（鋳造まま）および５００の鍛造を行なった鋼を無腐食で４００倍の倍率で光学顕微鏡観察したマイクロ組織のスケッチ図と、その時のサルファイド系介在物の並行度測定を行った模式図を示す。アスペクト比：３以上の任意のサルファイド系介在物を２つ選定し、それら各々の最大径を通る直線（Ａ，Ｂ線）同志がなす鋭角の角度を測り、これを観察視野内ですべて測定する。この測定を少なくとも１０視野にわたり測定し、その角度の最大値を並行度と定義している。視野内に交点が無い場合（例えば図５−鍛造比５００）は、Ａ線に平行な、Ａ’線を補助線として測定してもよい。なお、サルファイド系介在物を１個とみなす定義は、４００倍の光学顕微鏡観察で繋がっていると見なされるものを１個とし、その最大径を通る直線を測定線として用いる。
【００６４】
図５において、鍛造比が１のものは３０度を超える関係のサルファイド系介在物が存在しているが、鍛造比５００のものはすべて３０度以下になっていることがわかる。詳細には３０度という数字は、破壊力学的に設計した数字である。図６はG.R.Irwin（Analysis of Stresses and Strains Near the End of a Crack Transversing a Plate，Trans.ASME，Ser.E,J.Appl.Mech.，Vol.24，No.3（1957），pp.361-364参照）が解析的に算出した、応力方向とき裂進展方向に角度が発生した時、応力拡大係数の変化がどう現れるかを示した図であり、数式としては以下のように表わされる。
【００６５】
【数４】

【００６６】
ここで、Ｋ_Ｉはき裂の進展の駆動力となる応力拡大係数、βは応力方向とき裂方向とのなす角度、σは応力、πは円周率、ａはき裂長さである。き裂は応力と直角方向に存在する（β＝９０゜）と進展しやすく、応力方向に沿ってき裂が存在する（β＝０゜）と、き裂が進まなくなり、き裂が進みやすくなる（つまり、応力拡大係数が急に上昇する）のが３０度超に相当する。介在物は力学的結合に乏しいためき裂と見なすことができることを考えると、介在物分布の配向性のバラツキは３０度以下で制御することが重要であることがわかる。そして、伸びた介在物についてその配向性を揃えることが重要であることがわかる。
【００６７】
硫黄（Ｓ）は鋼の機械的性質を劣化させる代表的元素なので、この強度的対策を行なうことがスチール製ピストンリングとして成立させるに好ましい。例えば１％もの硫黄（Ｓ）添加を可能としている特開平０７−２５８７９２号は鍛造比を十分に稼げないようなシリンダライナーも対象とし、基本的には鋳鋼を対象としたものである。現実に、ピストンリングのスチール化をコスト的に成立させているのは引抜き、圧延、曲げ等の塑性加工技術である。つまり、この工程を用いて１％もの硫黄（Ｓ）を含む鋼をピストンリング用線材に仕上げようとすれば、その塑性加工に必要とされる材料強度に不足することから、引抜き工程での破断が起きかねず、確実にスチール製ピストンリングとしての完成にたどり着き難いのである。
【００６８】
以上のように、本発明においては、その耐焼付き性の更なる向上の上で、０．３％以下の硫黄（Ｓ）を含有した摺動部品用材料とすることが好ましく、特にはこの鍛造比が極めて高い摺動部品用材料として成立するピストンリング用線材にこそ有効な手段となる。
【００６９】
そして、上記Ｓの効果をさらに高めるには、Ｃａを硫黄（Ｓ）と共に添加することが有効である。ＣａはＭｎＳ中に内在するため、焼付き表面へ流出しやすい。しかも、強力な還元作用があるため、焼付き表面のオキサイド形成を防止し、サルファイド形成を容易にすることから、潤滑性を向上させる。しかしＣａは過度に添加すると熱間加工性を害するので０．０１％以下が好ましい。上記の効果を得るのに好ましくは０．０００１％〜０．０１％、さらに好ましくは０．０００５％〜０．０１％である。
【００７０】
なお、ＳおよびＣａの添加は耐焼付き性の他に、切削性や研削性の向上にも効果がある。特にＭｎＳの分散や黒鉛の析出は鋼の切削性を向上させるため、研削では形成が困難な、曲率半径の小さなコーナー部の形成が切削で容易にできることから、特に油掻き機能の向上したピストンリングの製作が容易となる。
【００７１】
また、本発明の摺動部品用材料およびピストンリング用線材は、耐食性、特に耐硫酸腐食性を高める効果の上でＣｏの含有も可能である。更には、Ｍｏと同様に、黒鉛によるスクイズ作用を促進し、高圧下での流体膜形成を助けることで、耐焼付き性を高め、かつ動摩擦係数を下げる効果がある。この場合、好ましくは０．５％以上であるが、一方、高価な元素であるだけでなく、過多に添加してもそれ以上の効果がみられないことから、１０％以下とすることが望ましい。より好ましくは２〜５％である。
【００７２】
この本発明の摺動部品用材料およびピストンリング用線材を構成する鋼は、例えば以上に述べた各元素種の含有量を満たした上で、残部を実質的にＦｅとする鋼とすることができ、上記以外の元素種については例えば合計で１０％以下、更には５％以下とすればよい。これについては、残部Ｆｅおよび不可避的不純物の鋼としたものを適用できる。
【００７３】
以下の元素は下記の範囲内であれば本発明鋼に含まれてもよく、好ましい範囲である。
Ｐ≦０．１％、Ｍｇ≦０．０１％、Ｂ≦０．０１％、Ｚｒ≦０．１％
【００７４】
加えて、本発明の好ましい条件として、組織面中に占める非金属介在物の面積率を２．０％以下とすることで、鋼線材へと加工する引抜き工程での破断防止と、その線材をコイル状に成形する際の折損の発生防止に有効である。特に細線材の作製・加工を伴なうピストンリングの製造に有効であり、稼働率の高い製造が可能な範囲として望ましい。
【００７５】
窒化処理は、本発明に加算する効果として、更なる耐焼付き性と耐摩耗性の向上に有効であるが、本発明の場合、表面処理の有無にかかわらず優れた耐焼付き性が達成されており、ＰＶＤやＣｒメッキ等の他の表面処理を組み合わせてもよい。また、ピストンリングを例に取れば、これらの表面処理は、従来、主な摺動面であるピストンリング／シリンダライナー間の接触面に対し施され、ピストンとの摩擦面に対して施されないため、凝着摩耗を防止できない。しかし、本発明の材料であれば表面処理を施さずとも十分な耐焼付き性が発揮され、凝着反応も抑制されるので、特にピストンリングに有効な材料となる。
【００７６】
また、本発明の材料では黒鉛相が存在するため、例えばＣｕＣｌ_２溶液に浸すこと等で黒鉛が有する層状分子構造の、その分子層間に別分子やイオンを挿入させる、いわゆるインターカレーション処理を行なうことができ、摺動特性を更に向上させることができる。更には、インターカレーション処理後の黒鉛はポリマー重合触媒ともなるため、ポリマーコート（高分子被覆処理）による潤滑性処理を行なうことや、予めインターカレーション処理のみを行ない、潤滑油を重合させやすい状態にすることで、摺動時に重合反応を起こして自己潤滑状態を維持する摺動部品とすることができる。
【００７７】
【発明の実施の形態】
［例］
以下、具体例により本発明の効果を説明する。
例１
大気中の高周波誘導溶解により、表１に示す残部Ｆｅおよび不可避的不純物の組成に調整した各試料のインゴット（断面寸法２２０ｍｍ×２２０ｍｍ）を得た。試料Ｎｏ．１〜６は本発明を満たす例であり、試料Ｎｏ．１１〜１６が比較例である。なお、試料Ｎｏ．１６はＪＩＳ−ＳＵＳ４４０Ｂ相当材であり、現用のピストンリング材料である。
【００７８】
【表１】

【００７９】
まず、これらのインゴットに熱間加工を施して、断面寸法が９ｍｍ×９ｍｍの線状素材を得た（鍛造比：約５９８）。なお、試料Ｎｏ．１３は鋳造こそ可能であったが、その後の熱間加工工程で鋼材が破壊したのでテストピースの作製ができなかった。
【００８０】
続いて、焼きなまし処理後に所定の焼入れ、焼戻し処理を行なって、硬さを４５ＨＲＣ前後に調整した。そして、その焼入れ、焼戻し後の線状素材の組織面を無腐食で観察して、黒鉛の分布状況（平均粒径、組織に占める面積率）を調べた。分布状況は、１０００倍の光学顕微鏡にて観察した１０視野の画像から、画像解析を用いて調査した。なお、黒鉛の直径は、測定した黒鉛の面積と同じ面積となる真円の直径を用いた。本発明鋼Ｎｏ．１〜６の黒鉛の分布状況は、その平均粒径は０．３〜２μｍ程度であり、面積率は０．５〜５％の品位であった。
【００８１】
図１〜図４は、試料Ｎｏ．３およびＮｏ．１４の黒鉛の分布状況を示す顕微鏡写真である。試料Ｎｏ．３のマトリックス組織には微細な黒鉛が析出しているが、試料Ｎｏ．１４の黒鉛は粗大である。これは適量のＣｕとＡｌを含む場合、黒鉛析出に先行して微細なＣｕ−Ａｌ相が析出し、それを核にして黒鉛が析出するため、微細な黒鉛分布となるが、ＣｕあるいはＡｌが不足するとこのメカニズムが働かないため、黒鉛が粗大に析出するものである。その他の試料も含め、黒鉛の分布状況を表２に示す。試料Ｎｏ．１１，１２，１５，１６の組織には黒鉛が観察されなかった。
【００８２】
そして、これらの試料にＨ_２：Ｎ_２＝１：１で５３０℃×５時間の条件によるイオン窒化処理を行ない、耐焼付き性および耐摩耗性の評価試料とした。耐焼付き性の評価に係る試験方法は、図７に示す超高圧摩擦摩耗試験機を用いて以下の条件で行ない、相手材の回転トルクが急激に立ち上がった時点を焼付き開始とし、そのときの荷重をスカッフ荷重として評価した。また、荷重が１０ＭＰａのときの、相手材の回転トルクより、動摩擦係数を求めた。図７中、符号１は試料を示し、２は相手材、Ｆは荷重を、それぞれ示す。
摺動面形状；５ｍｍ×５ｍｍの正方形
摩擦速度 ；２ｍ／秒
摩擦面圧力；初期圧１．５ＭＰａ，
１分毎に０．５ＭＰａずつ上昇
潤滑油 ；モータオイル＃３０
初期圧時にのみ滴下し(１０ｃｍ^３／分)、その後は供給を停止
相手材 ；ＪＩＳ ＦＣ２５０（ねずみ鋳鉄）（硬さ：100HRB）
【００８３】
耐摩耗性は往復動摩耗試験により評価した。これは別途作成した直径８ｍｍ、長さ２０ｍｍの試験片を直径２０ｍｍの相手材（ＦＣ２５０）と往復運動により擦り合わせることで、その時の摩耗幅を測定するものであり、試験方法の模式図を図８に、その他試験条件を以下に示す。図８中、符号１は試料を示し、２は相手材、Ｆは荷重、ＯＩＬは潤滑油を、それぞれ示す。
押し付け荷重 ；５００Ｎ
１回あたりの摺動距離；１３０ｍｍ
最大摺動速度 ；０．５ｍ／秒
潤滑油（滴下により供給）；モータオイル＃３０
相手材 ；ＪＩＳ ＦＣ２５０（ねずみ鋳鉄）（硬さ：100HRB）
以上、スカッフ荷重、動摩擦係数および摩耗幅の測定結果を、窒化硬さと共に、表２に示す。
【００８４】
【表２】

【００８５】
表２より、本発明を満たす試料Ｎｏ．１〜６は、いずれもスカッフ荷重が高く、摩耗幅が狭いことから、優れた耐焼付き性と耐摩耗性である。また、試料Ｎｏ．３〜６は、動摩擦係数も低い値であり、摺動部材として非常に優れた特性を示している。一方、本発明の黒鉛分布を満たさない比較試料は、全て耐焼付き性に劣っている。なお、試料Ｎｏ．１１、１５の耐摩耗性が不足しているのは、窒化硬化能を有する元素であるＣｒ，Ａｌのいずれの含有量もが低く、窒化硬さが低いためである。
【００８６】
また、実施例１で作製した試料につき、窒化処理を行なわない状態での同条件による焼付き試験を行った。結果を表３に示す。本発明の黒鉛分布を満たす試料Ｎｏ．１〜６の耐焼付き性が表面処理の有無によらず発揮されるところ、黒鉛が存在してもその粗大な比較試料Ｎｏ．１４のスカッフ荷重は低い。この試料Ｎｏ．１４の試験後の擦動面観察を行ったところ、黒鉛周辺部が破損しており、この破片が摩擦面内に入り込み摺動特性が悪化したものと考えられる。
【００８７】
【表３】

【００８８】
例２
表１の組成における試料Ｎｏ．１と試料Ｎｏ．１５を熱間圧延により直径５．５ｍｍのコイルとした後、引抜き−冷間圧延により断面寸法１．５ｍｍ×３．１ｍｍの平線形状に仕上げた。試料Ｎｏ．１は問題無く加工できたが、試料Ｎｏ．１５は冷間加工性が悪く、引抜き工程で破断した。両者の組織面中に占める非金属介在物の面積率を引抜き前のビレットままの状態で、次に行なう引抜き・圧延方向と直角な組織面にて画像解析したところ、試料Ｎｏ．１は１．８６％、試料Ｎｏ．１５は２．２３％であり、破断の原因はその硫黄（Ｓ）含有量の高さに加え、非金属介在物が２．０％を超える面積率になったことにある。
【００８９】
例３
表１における試料Ｎｏ．１〜６，１１，１２を実施例２に示す工程で断面寸法１．５ｍｍ×３．１ｍｍの平線形状に仕上げ、焼入れを１０００℃で３０分、焼戻しをそれぞれ硬さ５１０ＨＶ前後になるように行った。その後、砥石切断機で回転数１００００ｒｐｍ、送り速度１ｍｍ／秒による切断を１０回行ない、バリ発生頻度を調査した。表４に発生頻度を示す。
【００９０】
【表４】

【００９１】
試料Ｎｏ．１１，１２にはバリの発生が認められるが、適量の硫黄（Ｓ）添加をしている試料Ｎｏ．１〜５はバリの発生が見られず、本発明の硫黄（Ｓ）添加がバリ低減に効果が大きいことがわかる。これにより、特にピストンリングの製造における製造性も向上する。
【００９２】
例４
予め別に作製しておいた表１の試料Ｎｏ．１と同組成のインゴットを用い、鍛造比１〜１００００まで変化させた熱間加工工程にて、断面寸法３．０ｍｍ×３．０ｍｍの線材を作製した。そして、これらを焼入れ、焼戻しにより４００ＨＶの硬さに揃え、そのピストンリングとした時の外周面となる、線材の展伸・長手方向に並行な組織面でのサルファイド系介在物（アスペクト比≧３）の並行度を前記要領に従って測定した。
【００９３】
そして、これら硬さ調整後の線材にスパン３０ｍｍの三点曲げ試験を行ない、変位１０ｍｍまで曲げて破断しなかったものをＡ、破断したものをＢで評価した。これは、焼入れ、焼戻しを行なった線材をロール曲げ法によって所定の曲率のピストンリングに成形する際のその成形可能の可否を判定するものである。これらの結果を表５に示す。
【００９４】
【表５】

【００９５】
鍛造比が高く、サルファイド系介在物の並行度が３０度以下のものは機械的性質に優れ、線材からリング形状への曲げ加工時に懸念される破断の抑制に効果的である。そして、これら曲げ加工性に優れた線材を曲げ加工してピストンリング形状とした時の、その外周面に並行な組織面に観察される上記サルファイド系介在物の並行度やアスペクト比は、線材時より実質変化がなかった。
【００９６】
これら線材状態にて観察されるサルファイド系介在物の並行度は、曲げ加工後のピストンリング状態にも反映される。そして、このような断面積の小さなピストンリングがエンジンに搭載された場合に懸念される疲労破壊の不足についても、並行度が３０度以下といったサルファイド系介在物の形態は機械的性質の向上に効果的であり、特にピストンリング用線材に好ましい手段である。
【００９７】
例５
大気中の高周波誘導溶解により、表６に示す残部Ｆｅおよび不可避的不純物の組成に調整した、本発明を満たすＮｏ．２１〜２３のインゴット（断面寸法２２０ｍｍ×２２０ｍｍ）を得た。試料Ｎｏ．２１に対して、試料Ｎｏ．２２はＭｏを多く含有し、試料Ｎｏ．２３はＣｏを多く含有している。
【００９８】
【表６】

【００９９】
まず、これらのインゴットに熱間加工を施して、断面寸法が９ｍｍ×９ｍｍの線状素材を得た（鍛造比：約５９８）。続いて、焼きなまし処理後に所定の焼入れ、焼戻し処理を行なって、硬さを４０ＨＲＣ前後に調整した。そして、その焼入れ、焼戻し後の線状素材の組織面を無腐食で観察して、黒鉛の分布状況（平均粒径、組織に占める面積率）を調べた。分布状況は、１０００倍の光学顕微鏡にて観察した１０視野の画像から、画像解析を用いて調査した。
【０１００】
【表７】

【０１０１】
試料Ｎｏ．２１〜２３全ての黒鉛析出は微細であった。表７に黒鉛の分布状況を示すが、その平均粒径は１μｍ以下であり、面積率は１〜４％であった。また、１μｍ以上の比較的粗大な黒鉛は、いずれの試料においても平均粒径で１〜１．５μｍ、面積率で１％未満である。そして、各試料において、黒鉛全体の面積に占める１μｍ以上の比較的粗大な黒鉛の面積の割合は４分の１以下であり、微細な黒鉛が析出黒鉛の大部分を占めていることがわかる。
【０１０２】
そして、これらの試料を動摩擦係数の評価試験片とした。動摩擦係数の評価方法は、同じく図７に示す超高圧摩擦摩耗試験機を用いて（但し、摩擦速度は図７の表示に異なる）、以下の条件で行ない、相手材に生じるトルクと荷重より、荷重をステップアップする毎に、動摩擦係数を求めた。図９に荷重の逆数と動摩擦係数の関係を示す。
摺動面形状；５ｍｍ×５ｍｍの正方形
摩擦速度 ；１ｍ／秒 摩擦面圧力；初期圧１．５ＭＰａ，
１分毎に０．５ＭＰａずつ上昇
潤滑油 ；モータオイル＃３０
１０ｃｍ^３／分を常時滴下
相手材 ；ＦＣ２５０（ＪＩＳねずみ鋳鉄）
【０１０３】
図９は、ストライベック線図と呼ばれる、摩擦摺動部分に掛かる負荷の条件を示した「負荷特性」を横軸にとり、摩擦係数を縦軸にとった曲線である。これによって潤滑状態を知ることができる。本実施例の場合、摩擦速度が１ｍ／秒で一定であることから、横軸は荷重の逆数で示すことができる。図９の各曲線において、摩擦係数が最も低いプロット（極値）より右側（低荷重側：矢印で示す範囲）は、潤滑膜が破壊されず流体潤滑を起している領域であり、左側（高荷重側）は流体膜が破れ、固体同士の接触により流体潤滑に固体潤滑が混じった領域である。よって、摩擦係数が最も低いプロット（極値）がグラフ左側にシフトするほど、より高荷重でも流体膜が破れず、流体潤滑できることを示している。
【０１０４】
図９の結果より、試料Ｎｏ．２１に対してＭｏ含有量の多い試料Ｎｏ．２２では、より高荷重側でも流体膜が保持されている。そして、Ｃｏ含有量の多い試料Ｎｏ．２３では、更に高荷重側でも流体膜が保持されていることが分かる。これは、Ｍｏ、Ｃｏに先に述べたスクイズ作用を促進する効果があることを示すものである。
【０１０５】
また、各曲線の最も左側のプロットは、焼付きを起したため試験を中止した荷重に対応している。試料Ｎｏ．２２、２３はＭｏ、Ｃｏの添加により上記のプロットが左側（高荷重側）にシフトしており、耐焼付き性においても更なる向上がみられる。そして、動摩擦係数も全体的に低くなっており、Ｍｏ、Ｃｏの添加によって黒鉛析出による良好な摺動特性がより向上している。
【０１０６】
本発明によれば、自己潤滑性の付与により表面処理なしでも耐焼付き性に優れ、摩擦係数も低く摩擦によるエネルギー損失が少ないことから、各種の摺動部品に適用が可能である。そして、そのサルファイド系介在物の形態調整も合わせて行なうことで、特にピストンリングとして有効であり、シリンダライナー、ピストンへの攻撃性も低減され、内燃機関の環境性能や耐久性の向上に対しても大きな寄与を果たす。また、製造工程における被削性等の加工性にも優れ、製造コストとリードタイムを削減できることから、性能・製造の両面で優れた摺動部品用材料とでき、産業上極めて有益な技術となる。
【０１０７】
産業上の利用可能性
本発明材料は、すべり軸受、コロ軸受、玉軸受、歯車、金型、自動車エンジン等の内燃機関に装着されるピストンリング、シリンダライナー、ベーン等の摺動部品に用いられる。
【図面の簡単な説明】
【図１】 図１は、本発明例材料（試料Ｎｏ．３）の組織断面に観察される黒鉛の分布状況を示すマイクロ組織写真である。
【図２】 図２は、図１に示した材料と同一材料の組織断面（別視野）に観察される黒鉛の分布状況を示すマイクロ組織写真である。
【図３】 図３は、比較例材料（試料Ｎｏ．１４）の組織断面に観察される黒鉛の分布状況を示すマイクロ組織写真である。
【図４】 図４は、図３に示した材料と同一材料の組織断面（別視野）に観察される黒鉛の分布状況を示すマイクロ組織写真である。
【図５】 図５は、サルファイド系介在物の並行度を説明するマイクロ組織写真のスケッチ図およびその模式図である。
【図６】 図６は、応力拡大係数に及ぼす、応力方向とき裂進展方向とのなす角度の影響を説明する図である。
【図７】 図７は、超高圧摩擦摩耗試験方法を説明する模式図である。
【図８】 図８は、往復動摩耗試験方法を説明する模式図である。
【図９】 図９は、荷重の逆数と動摩擦係数の関係を示すストライベック線図であり、潤滑状態を説明する図である。[0001]
BACKGROUND OF THE INVENTION
  The present invention belongs to materials used for sliding parts such as ordinary sliding bearings, roller bearings, ball bearings, gears, dies, piston rings mounted on internal combustion engines such as automobile engines, cylinder liners, and vanes.
[0002]
[Prior art]
  Conventionally, materials having excellent wear resistance are provided for sliding parts such as cylinder liners and vanes. Among them, piston rings used in internal combustion engines, particularly automobile engines, have been shifting from conventional cast irons to so-called steel piston rings that are used by processing steel wires into a ring shape. In other words, the wire obtained by subjecting an ingot having a predetermined composition to hot working such as forging and hot rolling is further made into a steel wire suitable for the small cross-sectional shape of the piston ring by drawing, etc., and adjusting the hardness, and It is a piston ring that is generally manufactured by bending into a ring shape with a fixed curvature.
[0003]
  Currently, the top ring, the second ring, and the oil ring are generally attached to one piston from the combustion chamber side. In Japan, however, the top ring and oil ring corresponding to the severe part are steel. Higher functionality due to aging has permeated. In these backgrounds, in recent years, research results of post-internal combustion engines such as electric vehicles have become more visible, and efforts to further improve the internal combustion engine side have increased, If we improve the quality of diesel oil and improve the performance of exhaust gas filters, we have found that diesel vehicles with higher internal cylinder pressure have a lower environmental impact than gasoline vehicles, so sliding performance that can handle more severe diesel engines There are times when there is a shout of improvement.
[0004]
  Recently, a scalpel is also included in the internal phenomenon of the engine, and it has been pointed out that the wear of the second ring made of cast iron is the most severe among the three rings (see Non-Patent Document 1 below).
[0005]
  A further reason for the shift to steel is that when the frictional force loss is reduced by thinning the ring structure to meet the environmental performance improvement of the engine, the mechanical strength needs to be improved accordingly. The background is the improvement of wear resistance. Furthermore, the environmental regulations for mainstream Cr plating in cast iron are becoming stricter in terms of technology transfer, ease of expansion, and surface treatment of the ring manufacturing process. Another reason is that the process has been accelerated.
[0006]
  In steel piston rings, a method for imparting wear resistance and seizure resistance by performing a surface treatment such as nitriding on the contact surface with the cylinder liner has been proposed (see Patent Document 1 below). There has also been proposed a method for improving the wear resistance without using any one (see Patent Document 2 below).
[Non-Patent Document 1]
  Hideki Saito and two others, “Research on wear of diesel engines under severe driving conditions”, JSME Kyushu Chapter 1999 Research Presentation Lecture (1999) (Internet <URL:
http://www.ns.kogakuin.ac.jp/~wwa1013/EGR/nagasaki/nagasaki.html>)
[Patent Document 1]
  JP-A-10-030726
[Patent Document 2]
  Japanese Patent Publication No.58-046542
[0007]
  Although steel piston rings are overwhelmingly superior in mechanical properties and wear resistance to cast iron rings, they are inferior in seizure resistance, which is one of the reasons why the second ring does not make steel. ing. In response to this problem, as disclosed in Patent Document 1, the contact surface with the cylinder liner is being used for surface treatment such as nitriding treatment. However, there remains room for improvement in terms of surface treatment costs and prevention of aluminum adhesion occurring on the contact surface with the piston.
[0008]
  In addition, there is an attempt to solve without surface treatment, and Patent Document 2 etc. mainly propose a Cr addition region of 10% or more as a component design that increases Cr-based carbide in steel, which is superior in performance / cost. . However, although the wear resistance is drastically improved by increasing the amount of carbide, there is a concern that the effect of improving the seizure resistance is insignificant and that the manufacturability such as machinability is deteriorated. For this reason, surface treatment such as PVD is performed, but an increase in cost is unavoidable.
[0009]
  In addition, even when light oil is used as the fuel or when gasoline is used, the amount of sulfur (S) contained may be large depending on the quality. When fuel with a high S content is used, SO is contained due to the contained sulfur (S).₄ ^2-And the piston ring is exposed to a sulfuric acid dew point environment. Therefore, sulfuric acid corrosion resistance is also required, and strict characteristics that are unprecedented in terms of corrosion resistance are also required.
[0010]
Disclosure of the invention
  In view of the above problems, the present invention improves the seizure resistance, and is applied to the piston ring with its sliding part material having excellent wear resistance by using a nitriding treatment in combination. An object of the present invention is to provide a piston ring wire that has excellent sulfuric acid corrosion resistance and manufacturability in addition to properties, and has a low coefficient of friction.
[0011]
  The present inventors have detailed the sliding behavior of various applied sliding parts, in particular, sliding parts exposed to a sliding environment under fluid lubrication such as a piston ring. We investigated and examined. As a result, by finding the optimum structure for improving seizure resistance and reducing the coefficient of friction, in addition to considering the sulfuric acid corrosion resistance, we also found an effective component composition to achieve that structure. Invented.
[0012]
  Thus, according to the first aspect of the present invention, in mass%, C: 0.4% or more and less than 1.5%, Si: 0.1 to 3.0%, Mn: 0.1 to 3.0% , Cr: 0 to 0.5%, Ni: 0.05 to 3.0%, Al: 0.3 to 2.0%, at least one selected from the group consisting of Mo, W and V is a total amount (Mo + W + V) 0.3 to 20%, Cu: 0.05 to 3.0% includedThe remainder is made of steel consisting of Fe and inevitable impurities and exists in the steel structure.A material for a sliding part having self-lubricating properties is provided in which the average particle diameter of graphite is 3 μm or less.
[0013]
  According to a preferred embodiment of this sliding part material,Any steelOn tissue cross sectionExistsThe area ratio occupied by graphite is 1% or more, and the average particle size is 3 μm or less. Also,In steel structureV carbidesExistsPreferably not.
[0014]
  Further, preferably, the total amount of at least one selected from the group consisting of Mo and W is 0.3 to 5.0% and V: less than 0.1%. A preferable amount of Al is 0.7 to 2.0%. The steel preferably contains Mo: 1.5 to 3.0% and Co: 10% or less. Further, the amount of sulfur (S) and Ca in the steel is preferably S: 0.3% or less and Ca: 0.01% or less. The steel is subjected to nitriding treatment and used as a material for sliding parts. It is preferable to do.
[0015]
  According to the second aspect of the present invention, in mass%, C: 0.4% or more and less than 1.5%, Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, Cr: 0 to 0.5%, Ni: 0.05 to 3.0%, Al: 0.3 to 2.0%, at least one selected from the group consisting of Mo, W and V in a total amount ( Mo + W + V) 0.3 to 20%, Cu: 0.05 to 3.0%,The balance is formed of steel consisting of Fe and inevitable impurities and exists in the steel structure.There is provided a piston ring wire made of steel having an average particle diameter of graphite of 3 μm or less. The piston ring made of this wire is parallel to the outer peripheral surface.Any steelCross section of tissueExist inThe distribution state of sulfide inclusions is within 30 degrees in parallelism between straight lines passing through the maximum diameter of each sulfide inclusion. Preferably, the piston ring wireAny steelCross section of tissueExist inThe area ratio occupied by graphite is 1% or more and the average particle size is 3 μm or less.
[0016]
  The piston ring wire preferably contains Co: 10% or less, or further contains S: 0.3% or less and Ca: 0.01% or less, and is preferably used after being subjected to nitriding treatment.
[0017]
  An important feature of the present invention is that a sliding structure represented by a piston ring has realized a steel structure in which fine and moderate graphite is precipitated as an optimum means for improving the seizure resistance and reducing the friction coefficient. It is. In other words, the aim is to improve durability by compensating for the peculiarity of the friction behavior between the piston ring and the cylinder, and is advantageous in terms of non-surface treatment materials or surface treatment costs. Even if it is a nitriding material, sufficient effects can be achieved with respect to the seizure resistance and the reduction of the friction coefficient, which were insufficient with the conventional techniques.
[0018]
  The graphite precipitation structure of the present invention is a component that is capable of sufficiently rapid fine precipitation of graphite and is effective for improving workability and machinability in order to establish the industrial implementation means. As a result of finding the composition, in addition to the addition of sulfur (S) alone or the combined addition of sulfur (S) and Ca, further improvement of these effects can be achieved. It is what has.
[0019]
  First, the graphite precipitation structure of the present invention will be described.
  In a sliding part, a fluid lubrication design in which a fluid film such as oil or water is constantly formed between mechanical elements that normally rub severely is mainly used. This fluid film formation utilizes the fact that buoyancy is generated in a fluid that moves relative to an airplane, and when the viscosity and relative speed of the fluid increase, the fluid film interposed between the sliding parts becomes thick. Protects mechanical elements from friction.
[0020]
  However, in the case of internal combustion engines, since most of the current engines are reciprocating, the relative speed between the piston ring / cylinder is zero near the top and bottom dead center, the fluid film is also broken, and wear and seizure occur. Occurs and interferes with the normal operation of the internal combustion engine. Furthermore, since the goal of improving the performance of the piston ring is to improve the so-called oil scraping action that prevents the oil from entering the combustion chamber, it is increasingly difficult to maintain the fluid film described above. It has become.
[0021]
  Even so, various fluid lubrication modes are working in such a sliding motion, and the present inventors have tried to solve the problem by reviewing and utilizing these modes. That is, there are three fluid lubrication modes: (1) wedge action, (2) expansion and contraction action, and (3) squeeze action. The squeeze action of (3) works even when the relative speed is zero. There is. This will be described below. For example, it is assumed that a plate-like solid slides on the substrate while interposing a fluid. In this case, the pressure distribution generated on the surface of the plate facing the substrate has a boundary condition in which the pressure becomes 0 at the edge portion of the plate, and in order to maintain lubrication, that is, to generate a positive pressure distribution, The distribution must be a convex function and can be described by the following conditions.
[0022]
[Expression 1]

[0023]
  Here, P represents pressure, x represents a sliding direction, and y represents a distance perpendicular to the sliding direction. The Reinolds equation, which is a basic equation for fluid lubrication, is expressed as follows.
[0024]
[Expression 2]

[0025]
  Here, ρ is the density of the fluid, h is the thickness of the fluid film, η is the viscosity coefficient, t is the time, and u is the relative velocity. Therefore, from the equations (1) and (2), the necessary conditions for generating a positive pressure in the fluid film are as follows.
[0026]
[Equation 3]

[0027]
  Although three terms are generated in the expression (3), the first and second terms include the relative speed u and correspond to the above-described wedge action and expansion / contraction action. And the third term that does not include u is a term that indicates a squeeze action that has a possibility of functioning even if the relative speed between the piston ring / cylinder becomes zero.
[0028]
  The physical meaning of this third term being negative can be explained as follows: If the density of the fluid is constant, the film thickness of the fluid film suddenly decreases with time. This means that a positive pressure is generated in the fluid film. In order to actually cause such a phenomenon, it is necessary to apply a vertical load suddenly to the plate sliding on the substrate and squeeze out the fluid film. As a result, the fluid film is squeezed out, but at the same time, a high positive pressure is generated, and the solids do not readily come into contact with each other, that is, a squeeze action can be expected.
[0029]
  Here, the present inventors have found that in order to amplify this effect, it is further increased by forming a large number of hole-like structures on the sliding surface. In other words, if a small number of fine holes are provided on the sliding surface, even if the fluid film is broken, the relative velocity is zero. The squeeze action can be obtained by squeezing out to the depleted surface of the fluid and greatly reducing the film thickness of the moved fluid. Thereby, the seizure in the vicinity of the top and bottom dead center of the reciprocating motion can be suppressed, and the friction coefficient can be further reduced.
[0030]
  The graphite precipitation structure of the present invention was determined by paying attention to this action and effect. That is, the graphite structure of the present invention first acts as a lubricating phase having a solid lubricating action, but in addition, at the same time, it becomes important that the oil film retainability as pores when the graphite falls off. It is said squeeze action to improve. As described above, the squeeze action is such that an oil film is stably formed against pressure fluctuation, and the effect of the squeeze action is enhanced by arranging holes on the surface. In order to obtain this effect, graphite precipitation is effective, not only ordinary sliding bearings, roller bearings, ball bearing members, but also large pressure fluctuations, piston rings, cylinder liners, shim surfaces of valve lifters, various cams, The present invention is also effective for a sliding portion such as a plain bearing such as a gear, a mold member, or a cutting blade, where it is difficult to form a continuous fluid lubricating film.
[0031]
  Moreover, the graphite precipitation structure of the present invention is also effective in preventing adhesive wear with an aluminum piston, which has recently become a problem when applied to the piston ring. That is, since aluminum has almost no solid solubility limit with respect to carbon, the adhesion reaction is suppressed.
[0032]
  As described above, the material for sliding parts of the present invention issteelOrganizationDuring ~In which graphite is dispersed.ThatIt is important that the average particle size of graphite is 3 μm or less. This is because if the average particle size exceeds 3 μm, the graphite peripheral part is damaged during the sliding, and there is a concern that the fragments enter the sliding surface. The effect of the graphite dispersion of the present invention is obtained.For any steelOrganizationRefusalOn the faceExists1% or more area ratio of graphiteInTo doIs preferred. More preferably, for graphite having a relatively coarse particle diameter of 1 μm or more, the average particle diameter is 5 μm or less, or the area ratio is less than 5%.
[0033]
  The lubrication effect due to the formation of pores caused a decrease in the fluid film due to the above-described action, and eventually lost its effect, so it was not treated as a mainstream fluid lubrication design. However, the means of the present invention is effective only in a reciprocating motion in which it is difficult to form a continuous fluid lubrication film such as an internal combustion engine. For example, the vicinity of the top and bottom dead center where the relative speed is close to zero and the fluid film breaks is present. In an environment that is in a temporary state and causes an unsteady frictional behavior, such as between a piston ring / cylinder that immediately shifts to an environment rich in lubricating oil, graphite precipitation is effective.
[0034]
  It is important to design a lubrication design that can maintain the fluid film only under conditions where the fluid film is temporarily broken due to the structure, such as a piston ring and a cylinder liner. Even so, if the engine speed or the structure of the sliding parts changes, the possibility of solid contact increases, and these non-stationary sliding parts have a solid lubricating action like graphite steel. A wider range of sliding conditions can be accommodated by applying a material with precipitated graphite.
[0035]
  Next, the component composition of steel constituting the sliding component of the present invention will be described.
  The graphite steel itself has been reported for a long time, and these are mainly alloys added with Si and Ni, but it takes several tens of hours or more for the graphite precipitation treatment to keep the temperature at 600 ° C. or higher. It was a thing. Therefore, the present invention is characterized in that cementite is easily decomposed in order to complete the graphite precipitation at the level of several hours, and an appropriate amount of Al is added on Ni.
[0036]
  In general, carbon in iron once precipitates as metastable cementite as a pre-stage of precipitation as graphite. For this reason, in the graphite precipitation treatment, it is necessary to decompose cementite and change it to stable graphite. In conventional graphite steel, it was difficult for decomposition of cementite, and it took a very long time. Therefore, in the present invention, by suppressing elements that interfere with the decomposition of cementite such as Cr, aiming at a component design that immediately decomposes even if metastable cementite is precipitated, substantially without producing cementite, all at once. It can be deposited as graphite.
[0037]
  Further, Al has a high diffusion rate in iron, and the addition of Al increases the diffusion rate of vacancies, and shortens the time for forming an aggregate of vacancies where graphite is deposited. By combining these two effects, it is possible to perform graphite precipitation in a short time in the present invention. It is also easy to deposit graphite only on the surface by carburizing treatment or the like.
[0038]
  Further, Al is an element having nitriding hardening ability, which is advantageous in designing an alloy of nitriding hardening steel. In addition, although there is Cr as an element similarly used as a nitriding hardening element, Cr not only inhibits graphite formation, which is the basic technology of the present invention, but also significantly reduces sulfuric acid corrosion resistance. Avoid as much as possible. Therefore, Al employed in the present invention is an important contained element.
[0039]
  In addition, it has been pointed out that when a nitriding treatment is applied to graphite steel, defects such as graphite having a particle size of 10 μm or more in the nitrided layer make the nitrided layer brittle. There was a difficulty in precipitating. Therefore, in the present invention, in order to establish the nitrided steel, efforts were made to refine the precipitated graphite.
[0040]
  Regarding the refinement of this precipitated graphite, (1) refinement by processing strain, (2) Al₂O₃There are three methods including addition of inclusions and the like, and (3) BN, TiC, etc. as precipitation nuclei. However, (1) has process limitations, (2) has difficulty in the process of dispersion, and (3) has the effect achieved by controlling trace components, so high C steel Is difficult to manufacture. With regard to these, for example, JP-A-11-246940 discloses a method of dispersing TiC, and Takashi Iwamoto et al. “Iron and Steel Vol. 84 (1998) p. 57” discloses a method of performing graphite precipitation using BN as a nucleus. However, since the second phase precipitates at a high temperature with a high diffusion rate of 1000 ° C. or higher, it is difficult to maintain the fine uniformity of graphite, and it is difficult to apply it to a high alloy with severe component segregation.
[0041]
  Thus, as a result of various studies to achieve the refinement of the graphite, the present inventors have found that the Cu—Al metal precipitation phase in the structure acts effectively. This is a phenomenon that causes the second phase to act as a nucleus of precipitated graphite as described above. Since this Cu—Al phase precipitates at a low temperature of 800 ° C. or less, a fine graphite structure can be formed stably and rapidly. In the present invention, specifically, by adding Cu and Al at a level at which the embrittlement phenomenon does not occur, it is possible to form a graphite structure that has less strength deterioration and acts as a lubricating phase. In addition, Cu contributes to the improvement of sulfuric acid corrosion resistance.
[0042]
  Conventionally, a lot of lubrication technology by dispersing graphite has been proposed in the field of cast iron. However, the use environment of sliding parts in each field is becoming severer year by year. Changes are progressing. Even so, for example, in the case of a cylinder block of an internal combustion engine, the inner wall of the cylinder liner part is still made of cast iron crystallized with graphite, aiming at the effects described above. It is a situation. The sliding component of the present invention incorporates the characteristics of cast iron in order to combine not only sliding characteristics but also strength, wear resistance, and sulfuric acid corrosion resistance required due to severe environmental conditions. It aims at the formation as a steel material, and has the characteristics in the alloy design for it. Hereinafter, the component composition will be described.
[0043]
  C is an important element that partly dissolves in the base to give strength, part is carbide, and the remaining part forms graphite to increase wear resistance and seizure resistance. This requires at least 0.4%. However, when it is 1.5% or more, the melting point is lowered, and it becomes difficult to make the structure uniform by diffusion annealing treatment for eliminating solidification segregation such as heating the ingot at around 1200 ° C. for several tens of hours. Therefore, C is 0.4% or more and less than 1.5%. Preferably, it is 0.5% or more and less than 1.3%.
[0044]
  Si is usually added as a deoxidizer, but here it is added as an element for promoting the precipitation of graphite and has the effect of improving the resistance to sulfuric acid corrosion, so 0.1% was made the lower limit. On the other hand, it affects the temper softening behavior of steel, and the influence of Si is important especially in low alloy steel. In order to prevent temper softening and increase the heat resistant strength, the preferable Si amount is 1.0% or more. However, if excessively added, A₁Since the point rises, the upper limit of Si is defined as 3.0%. Therefore, Si is made 0.1 to 3.0%. Preferably, it is 0.5 to 3.0%, more preferably 1.0 to 3.0%.
[0045]
  Mn, like Si, is used as a deoxidizer and needs to be at least 0.1%, but if added too much, it will damage the precipitation of graphite. Therefore, the upper limit is defined as 3.0%. Therefore, Mn is set to 0.1 to 3.0%.
[0046]
  Cr is an effective nitriding hardening element, but it suppresses metastable cementite decomposition and not only strongly inhibits precipitation of graphite, but also significantly reduces sulfuric acid corrosion resistance. It is necessary to regulate. Therefore, in the present invention, 0 to 0.5%, preferably 0 to 0.3%.
[0047]
  Ni is an element that promotes the formation of graphite, and is also an element that is useful for suppressing the red hot brittleness that occurs in Cu-added steels. On the other hand, in order to increase the solid solubility limit of C in Fe, workability in the annealed state is also important. It is an element that inhibits. Therefore, it was made 0.05 to 3.0%. Preferably, it is 0.6 to 1.5%.
[0048]
  Al, like Cr, is an element that increases the nitriding hardness, but in the present invention, the nitriding hardness is ensured by adding Al to the extent that Cr cannot be increased. And it is characterized by graphite-forming elements, elements that assist the diffusion of vacancies, and also become the core of graphite precipitation as Cu-Al phase together with Cu, and are effective in causing rapid and fine precipitation. Therefore, it is necessary to add 0.3% or more. Here, Al, like Si, is A₁In order to raise the point, it is specified to be 2.0% or less. Therefore, in this invention, it limits to the addition range of 0.3 to 2.0%. Preferably, it is 0.7 to 2.0%.
[0049]
  Mo is a carbide forming element that does not inhibit graphitization as compared with Cr, and is an element that imparts heat resistance. For example, the carbide restrains the matrix in the thermoforming process performed after the bending process in the piston ring manufacturing process, and thus has a function of improving the dimensional stability. However, excessive addition of Mo suppresses the decomposition of cementite like Cr.
[0050]
  Even so, Mo is less affected by the above-described inhibition, while contributing greatly to improvement in heat resistance and contributing to dimensional stability during heat treatment. Particularly in the manufacturing process of the piston ring, since a heat treatment process in a thin line state is included, the importance of this characteristic is high, and it becomes effective for suppressing variation in the joint shape, so 0.3% or more is added. On the other hand, in order to inhibit the precipitation of graphite, the upper limit is made 20%. Here, even if it is V and W, since it has the same effect as Mo, in the present invention, at least one selected from the group consisting of Mo, W and V is 0.3 to 20% in total amount. It can be.
[0051]
  However, even when V is added, if V carbide is generated, graphite precipitation is significantly inhibited.steelOrganizationRefusalIt is desirable that no V carbide is observed on the surface.
Or it is preferable that (Mo + W) by combined addition or single addition of Mo and W is 0.3 to 5.0%, and V is less than 0.1%. Further, Mo promotes the squeeze action by graphite and helps to form a fluid film under high pressure, thereby improving the seizure resistance and lowering the dynamic friction coefficient. In addition, since there is an effect of improving the sulfuric acid corrosion resistance, the addition amount of Mo alone is preferably 1.5 to 3.0%.
[0052]
  Cu is an important element for the present invention together with Al because it is effective for precipitating a Cu—Al metal phase and forming a fine graphite structure stably and rapidly. In addition, there is an effect of improving the resistance to sulfuric acid corrosion. Therefore, mutual content adjustment that also serves as the amount of Al is required, and in order to obtain the effect, it is necessary to add 0.05% or more, preferably 0.2% or more. On the other hand, if Cu is added excessively, the hardness during annealing becomes high and the workability is hindered, so it is necessary to make it 3.0% or less. Therefore, in the present invention, the composition range is 0.05 to 3.0%, preferably 0.2 to 3.0%.
[0053]
  Further, as a technique for improving lubricity, conventionally, in the case of an internal combustion engine, sulfur (S) has been known as a technique for preventing seizure by making it organic and adding it as an extreme pressure additive to engine oil. On the other hand, the present inventors, in the presence of sulfide (sulfide) such as MnS in the steel, formed an in situ sulfide film on the new surface where the sulfur (S) was made into a friction surface by frictional heat generation. It has been found that this is effective in improving the lubrication performance. According to this measure, since the lubricating substance is scattered inside the material, it is not necessary to add a large amount of the lubricating substance in order to improve the lubricating performance in the required area, and it is effective at the time of oil change like an extreme pressure additive. Can be expected to function semi-permanently.
[0054]
  In addition, the conventional means of increasing the Cr-based carbide in the steel for the piston ring is, in other words, increasing the wear resistance on the piston ring side where the contact area with the cylinder liner is small and the sliding energy per unit area is high. The aim is to balance the cylinder liner. This also increases the seizure resistance, but by increasing the contact area by promoting wear on the cylinder liner side, which is essentially caused by uneven contact, the abnormal surface pressure increases locally. The aim is to avoid. In other words, this is a technique for improving the familiarity at the initial stage of piston ring mounting, and has few functions as wear characteristics that require durability such as adhesive wear.
[0055]
  In addition, excessive wear resistance may cause a situation where the cylinder liner side is attacked. If it progresses extremely, it will increase clearance and increase the blow-by amount related to the amount of exhaust gas. The effect of S) is that the seizure resistance is improved by reducing the coefficient of friction without promoting the wear of the material, so that the effect of sustaining a state in which there is little change in the clearance even when the engine is advanced is high.
[0056]
  Therefore, in the sliding part material of the present invention, the seizure resistance can be further improved by adding an appropriate amount of sulfur (S). That is, sulfur (S) is mostly combined with Mn to form MnS, which acts on a lubricant such as engine oil to exert a lubricating effect, so that the friction coefficient is lowered and seizure resistance is improved.
[0057]
  Here, seizure is a phenomenon in which the friction surface temperature rises due to frictional heat generation, and the movement of atoms between materials occurs due to thermal vibration, and the friction surface temperature is fixed. Friction surface temperature is the friction energy (= friction coefficient × surface pressure × slip Speed) is a monotonically increasing function. Therefore, when the friction coefficient decreases, it becomes difficult to raise the temperature and seizure resistance is improved. In order to obtain this effect, the inclusion of sulfur (S) is effective, but if added excessively, the mechanical properties deteriorate, for example, there is a concern about breakage in the drawing process of a steel wire for a steel piston ring. The upper limit is preferably set to 0.3%. The content is desirably 0.01 to 0.3%, and more desirably 0.03 to 0.3%.
[0058]
  Furthermore, in order to obtain a material in which sulfur (S) is added to 0.3%, the present inventors desirably increase the forging ratio applied to the manufacturing process from the viewpoint of mechanical properties. I also found. In other words, it is effective in improving the mechanical properties as a sliding part, especially for steel piston rings obtained by bending steel wires, and effective in suppressing breakage and breakage in the bending process. It is also a simple means.
[0059]
  The forging ratio in this case is defined by the area reduction rate from the ingot in the piston ring manufacturing process to the piston ring product. In other words, compared to the cross-section perpendicular to the direction in which the steel is forged and extended, that is, the cross-sectional area of the final piston ring product, (the cross-sectional area of the ingot before forging) / (the cross-sectional area of the product after bending) ). However, the area reduction rate from the steel wire to the piston ring product by bending is negligible for achieving the effect of the present invention, (ingot cross-sectional area before forging) / (before bending (after forging and stretching) ) Steel wire cross-sectional area). A higher number of forging ratios indicates that forging is progressing.
[0060]
  Steel containing MnS such as MnS has a spherical or spindle-shaped sulfide in the triple point of the grain boundary of the solidified cell structure in the casting state as its starting structure, and its orientation is a random structure. As the forging ratio applied increases, the orientation state of sulfide changes, and the mechanical properties are improved.
[0061]
  Increasing this forging ratio increases the orientation of sulfide in the length direction of the steel wire, that is, the sulfide expands in a form that follows the circumferential stress that mainly acts on the piston ring. Almost no deterioration. This effect is particularly noticeable with respect to elongated sulfide, that is, sulfide with an aspect ratio (maximum diameter / minimum diameter) of 3 or more. In other words, when the orientation of the sulfide with an aspect ratio of 3 or more is poor in the circumferential direction, It leads to deterioration of mechanical properties.
[0062]
  Specifically, the distribution of sulfide inclusions observed on the structure surface parallel to the outer peripheral surface of the piston ring, in particular, the sulfide inclusions having an aspect ratio of 3 or more, and the maximum diameter of each sulfide inclusion. By setting the parallelism (angle on the acute angle side) between the straight lines passing above to within 30 degrees, it is possible to make the material for sliding parts particularly effective as a piston ring wire. For example, this forging ratio is 500 or more. It is preferable to achieve this.
[0063]
  Fig. 5 shows a sketch of a micro-structure of a forged steel with a forging ratio of 1 (as cast) and 500 and observed with an optical microscope at a magnification of 400 times without corrosion, and a parallelism measurement of sulfide inclusions at that time. A schematic diagram is shown. Aspect ratio: Select two arbitrary sulfide inclusions with an aspect ratio of 3 or more, measure the acute angle formed by the straight lines (lines A and B) passing through the maximum diameter of each, and measure all of them within the observation field. . This measurement is measured over at least 10 fields, and the maximum value of the angle is defined as parallelism. When there is no intersection in the field of view (for example, FIG. 5-forging ratio 500), the A ′ line parallel to the A line may be measured as an auxiliary line. In addition, the definition which considers a sulfide type inclusion as one is taken as what is considered to be connected by optical microscope observation of 400 times, and the straight line which passes through the maximum diameter is used as a measurement line.
[0064]
  In FIG. 5, sulfide inclusions with a forging ratio of 1 exceeding 30 degrees are present, but it can be seen that all of the forging ratios of 500 are 30 degrees or less. Specifically, the number of 30 degrees is a number designed from a fracture mechanics. Fig. 6 shows GRIrwin (Analysis of Stresses and Strains Near the End of a Crack Transversing a Plate, Trans. ASME, Ser. E, J. Appl. Mech., Vol. 24, No. 3 (1957), pp.361. -364) analytically calculated, and shows how the stress intensity factor changes when an angle is generated in the stress direction and in the crack propagation direction.
[0065]
[Expression 4]

[0066]
  Where K_IThe stress intensity factor that is the driving force for crack propagation, β is the angle between the stress direction and the crack direction, σ is the stress, π is the circumference, and a is the crack length. If a crack exists in a direction perpendicular to the stress (β = 90 °), it will easily propagate. If a crack exists along the stress direction (β = 0 °), the crack will not progress and the crack will easily progress. (That is, the stress intensity factor suddenly increases) corresponds to more than 30 degrees. Considering that inclusions are poor in mechanical coupling and can be regarded as cracks, it is understood that it is important to control the variation in orientation distribution of inclusions at 30 degrees or less. And it turns out that it is important to align the orientation about the extended inclusion.
[0067]
  Since sulfur (S) is a typical element that deteriorates the mechanical properties of steel, it is preferable to take this strength measure to establish a steel piston ring. For example, Japanese Patent Application Laid-Open No. 07-258792 that enables addition of 1% of sulfur (S) is intended for cylinder liners that cannot achieve a sufficient forging ratio, and basically for cast steel. Actually, it is a plastic working technique such as drawing, rolling, bending, etc. that makes the piston ring steel in terms of cost. In other words, if this process is used to finish steel containing 1% sulfur (S) into a piston ring wire, the material strength required for the plastic working is insufficient, so the fracture in the drawing process It is hard to reach the completion as a steel piston ring.
[0068]
  As described above, in the present invention, in order to further improve the seizure resistance, it is preferable to use a sliding part material containing 0.3% or less of sulfur (S), and particularly this forging. It is an effective means only for the wire material for piston rings, which is established as a material for sliding parts with an extremely high ratio.
[0069]
  In order to further enhance the effect of S, it is effective to add Ca together with sulfur (S). Since Ca is inherent in MnS, it tends to flow out to the seized surface. Moreover, since it has a strong reducing action, it prevents the formation of oxide on the seized surface and facilitates the formation of sulfide, thereby improving the lubricity. However, if Ca is added excessively, hot workability is impaired, so 0.01% or less is preferable. In order to acquire said effect, Preferably it is 0.0001%-0.01%, More preferably, it is 0.0005%-0.01%.
[0070]
  The addition of S and Ca is effective in improving machinability and grindability in addition to seizure resistance. In particular, the dispersion of MnS and the precipitation of graphite improve the machinability of steel, so it is difficult to form by grinding, and it is easy to form corners with a small radius of curvature by cutting. Is easy to manufacture.
[0071]
  In addition, the sliding part material and the piston ring wire of the present invention can contain Co in order to improve the corrosion resistance, particularly the sulfuric acid corrosion resistance. Furthermore, like Mo, the squeeze action by graphite is promoted, and the formation of a fluid film under high pressure helps to increase the seizure resistance and lower the dynamic friction coefficient. In this case, it is preferably 0.5% or more. On the other hand, not only is it an expensive element, but even if it is added excessively, no further effect is observed, so it is desirable to make it 10% or less. . More preferably, it is 2 to 5%.
[0072]
  The steel constituting the sliding part material and the piston ring wire of the present invention, for example, may satisfy the above-mentioned content of each element type, and the remaining part may be substantially Fe. The element types other than the above may be, for example, 10% or less in total, and further 5% or less. About this, what was made into steel of remainder Fe and an unavoidable impurity is applicable.
[0073]
  The following elements may be included in the steel of the present invention as long as they are within the following ranges, and are preferable ranges.
  P ≦ 0.1%, Mg ≦ 0.01%, B ≦ 0.01%, Zr ≦ 0.1%
[0074]
  In addition, as a preferable condition of the present invention, by making the area ratio of the nonmetallic inclusions in the structure surface 2.0% or less, it is possible to prevent breakage in the drawing process to be processed into a steel wire, and the wire This is effective in preventing the occurrence of breakage when forming into a coil shape. In particular, it is effective for the production of a piston ring that involves the production and processing of a fine wire, and is desirable as a range where production with a high operation rate is possible.
[0075]
  The nitriding treatment is effective to further improve seizure resistance and wear resistance as an effect to be added to the present invention, but in the case of the present invention, excellent seizure resistance is achieved regardless of the presence or absence of surface treatment. And other surface treatments such as PVD and Cr plating may be combined. Taking a piston ring as an example, these surface treatments are conventionally applied to the contact surface between the piston ring / cylinder liner, which is the main sliding surface, and not to the friction surface with the piston. Adhesive wear cannot be prevented. However, if the material of the present invention is used, sufficient seizure resistance is exhibited without surface treatment, and the adhesion reaction is also suppressed, so that the material is particularly effective for the piston ring.
[0076]
  Moreover, since the graphite phase is present in the material of the present invention, for example, CuCl₂A so-called intercalation treatment in which another molecule or ion is inserted between the molecular layers of the layered molecular structure of graphite by dipping in a solution or the like can be performed, and the sliding characteristics can be further improved. Furthermore, since the graphite after the intercalation treatment is also a polymer polymerization catalyst, it is easy to polymerize the lubricating oil by performing a lubricity treatment by polymer coating (polymer coating treatment) or by performing only the intercalation treatment in advance. By setting it in a state, it is possible to provide a sliding component that causes a polymerization reaction during sliding and maintains a self-lubricating state.
[0077]
DETAILED DESCRIPTION OF THE INVENTION
[Example]
  Hereinafter, the effect of the present invention will be described with reference to specific examples.
  Example 1
  Ingots (cross-sectional dimensions 220 mm × 220 mm) of the respective samples adjusted to the composition of the remaining Fe and inevitable impurities shown in Table 1 were obtained by high-frequency induction dissolution in the atmosphere. Sample No. 1 to 6 are examples satisfying the present invention. 11 to 16 are comparative examples. Sample No. Reference numeral 16 denotes a JIS-SUS440B equivalent material, which is an existing piston ring material.
[0078]
[Table 1]

[0079]
  First, these ingots were hot-worked to obtain a linear material having a cross-sectional dimension of 9 mm × 9 mm (forging ratio: about 598). Sample No. No. 13 could be cast, but the test piece could not be produced because the steel material was destroyed in the subsequent hot working process.
[0080]
  Subsequently, after the annealing process, predetermined quenching and tempering processes were performed, and the hardness was adjusted to around 45 HRC. Then, the textured surface of the linear material after quenching and tempering was observed without corrosion, and the graphite distribution state (average particle diameter, area ratio in the structure) was examined. The distribution situation was investigated using image analysis from images of 10 fields observed with a 1000 × optical microscope. In addition, the diameter of the circle used the diameter of the perfect circle used as the area of the measured graphite. Invention Steel No. As for the distribution status of graphite Nos. 1 to 6, the average particle size was about 0.3 to 2 μm, and the area ratio was a grade of 0.5 to 5%.
[0081]
  1 to 4 show sample Nos. 3 and no. It is a microscope picture which shows the distribution condition of 14 graphite. Sample No. In the matrix structure of No. 3, fine graphite is deposited. 14 graphite is coarse. This is because when a suitable amount of Cu and Al is contained, a fine Cu-Al phase is precipitated prior to the precipitation of graphite, and graphite is precipitated using this as a nucleus, resulting in a fine graphite distribution. If this amount is insufficient, this mechanism does not work, so that graphite precipitates coarsely. Table 2 shows the distribution of graphite, including other samples. Sample No. No graphite was observed in the 11, 12, 15 and 16 structures.
[0082]
  And these samples are H₂: N₂= 1: 1 and ion nitriding treatment was performed under the conditions of 530 ° C. × 5 hours to obtain seizure resistance and abrasion resistance evaluation samples. The test method related to the evaluation of seizure resistance is performed under the following conditions using the ultrahigh pressure frictional wear tester shown in FIG. 7, and when the rotational torque of the mating material suddenly rises, seizure start is started. The load was evaluated as a scuff load. Moreover, the dynamic friction coefficient was calculated | required from the rotational torque of the other party material when a load was 10 MPa. In FIG. 7, reference numeral 1 denotes a sample, 2 denotes a counterpart material, and F denotes a load.
    Sliding surface shape: 5mm x 5mm square
    Friction speed: 2m / sec
    Frictional surface pressure; initial pressure 1.5 MPa,
                Rise by 0.5 MPa every minute
    Lubricating oil; Motor oil # 30
                Dripping only at initial pressure (10 cm³/ Minute), then stop supplying
    Counterpart material: JIS FC250 (Gray cast iron) (Hardness: 100HRB)
[0083]
  The wear resistance was evaluated by a reciprocating wear test. This is to measure the wear width at that time by rubbing a separately prepared test piece having a diameter of 8 mm and a length of 20 mm with a counterpart material (FC250) having a diameter of 20 mm by reciprocating motion. 8 shows other test conditions. In FIG. 8, reference numeral 1 denotes a sample, 2 denotes a counterpart material, F denotes a load, and OIL denotes a lubricating oil.
    Pressing load: 500N
    Sliding distance per time: 130mm
    Maximum sliding speed: 0.5m / sec
    Lubricating oil (supplied by dripping); Motor oil # 30
    Counterpart material: JIS FC250 (Gray cast iron) (Hardness: 100HRB)
  The measurement results of the scuff load, dynamic friction coefficient, and wear width are shown in Table 2 together with the nitriding hardness.
[0084]
[Table 2]

[0085]
  From Table 2, sample No. 1 satisfying the present invention is obtained. Since Nos. 1 to 6 all have a high scuffing load and a narrow wear width, they have excellent seizure resistance and wear resistance. Sample No. Nos. 3 to 6 have a low coefficient of dynamic friction and show very excellent characteristics as sliding members. On the other hand, all the comparative samples that do not satisfy the graphite distribution of the present invention are inferior in seizure resistance. Sample No. The reason why the wear resistance of 11 and 15 is insufficient is that the contents of Cr and Al, which are elements having nitriding hardening ability, are low and the nitriding hardness is low.
[0086]
  In addition, the sample manufactured in Example 1 was subjected to a seizure test under the same conditions without performing nitriding. The results are shown in Table 3. Sample No. 1 satisfying the graphite distribution of the present invention. The seizure resistance of 1 to 6 is exhibited regardless of the presence or absence of surface treatment. The 14 scuff load is low. This sample No. When the rubbing surface was observed after test No. 14, it was considered that the graphite peripheral portion was damaged, and the fragments entered the friction surface and the sliding characteristics deteriorated.
[0087]
[Table 3]

[0088]
Example 2
  Sample No. in the composition of Table 1 1 and sample no. 15 was formed into a coil having a diameter of 5.5 mm by hot rolling, and then finished into a flat wire shape having a cross-sectional dimension of 1.5 mm × 3.1 mm by drawing-cold rolling. Sample No. No. 1 could be processed without problems, but sample no. No. 15 had poor cold workability and was broken in the drawing process. When the area ratio of the nonmetallic inclusions in the texture planes of both of them was in the billet state before drawing, image analysis was performed on the texture plane perpendicular to the drawing / rolling direction to be performed next. 1 is 1.86%, sample no. 15 is 2.23%, and the cause of the breakage is that, in addition to the high sulfur (S) content, the non-metallic inclusions have an area ratio exceeding 2.0%.
[0089]
Example 3
  Sample No. in Table 1 1 to 6, 11 and 12 are finished in a flat wire shape with a cross-sectional dimension of 1.5 mm × 3.1 mm in the process shown in Example 2, quenched at 1000 ° C. for 30 minutes, and tempered to have a hardness of around 510 HV, respectively. went. Thereafter, cutting was performed 10 times with a grindstone cutting machine at a rotational speed of 10000 rpm and a feed rate of 1 mm / second, and the occurrence of burrs was investigated. Table 4 shows the frequency of occurrence.
[0090]
[Table 4]

[0091]
  Sample No. 11 and 12 show the occurrence of burrs, but Sample No. with an appropriate amount of sulfur (S) added. 1 to 5 show no occurrence of burrs, and it can be seen that the addition of sulfur (S) of the present invention is highly effective in reducing burrs. Thereby, the manufacturability in the manufacture of the piston ring is also improved.
[0092]
Example 4
  Sample No. in Table 1 prepared separately in advance. Using an ingot having the same composition as 1, a wire rod having a cross-sectional dimension of 3.0 mm × 3.0 mm was produced in a hot working process in which the forging ratio was changed from 1 to 10,000. These are quenched and tempered so as to have a hardness of 400 HV and become a peripheral surface when the piston ring is used. Sulfide inclusions on the textured surface parallel to the expansion and longitudinal direction of the wire (aspect ratio ≧ 3) ) Was measured according to the above procedure.
[0093]
  Then, a three-point bending test with a span of 30 mm was performed on these wires after the hardness adjustment, and A that was not broken by bending to a displacement of 10 mm was evaluated by B and B that was broken was evaluated by B. This is to determine whether or not the wire rod that has been quenched and tempered can be formed when it is formed into a piston ring having a predetermined curvature by a roll bending method. These results are shown in Table 5.
[0094]
[Table 5]

[0095]
  When the forging ratio is high and the parallelism of sulfide inclusions is 30 degrees or less, the mechanical properties are excellent, and it is effective in suppressing breakage that is a concern during bending from a wire to a ring shape. The parallelism and aspect ratio of the sulfide inclusions observed on the textured surface parallel to the outer peripheral surface when bending the wire material excellent in bendability into a piston ring shape is as follows. There was no more substantial change.
[0096]
  The parallelism of sulfide inclusions observed in these wire rod states is also reflected in the piston ring state after bending. In addition, with regard to the lack of fatigue failure that is a concern when such a piston ring with a small cross-sectional area is mounted on the engine, the form of sulfide inclusions with a parallelism of 30 degrees or less is effective in improving mechanical properties. In particular, it is a preferable means for a wire for a piston ring.
[0097]
Example 5
  No. 1 satisfying the present invention was adjusted to the composition of the remaining Fe and inevitable impurities shown in Table 6 by high-frequency induction dissolution in the atmosphere. 21 to 23 ingots (cross-sectional dimensions of 220 mm × 220 mm) were obtained. Sample No. For sample 21, sample no. 22 contains a large amount of Mo. 23 contains a large amount of Co.
[0098]
[Table 6]

[0099]
  First, these ingots were hot-worked to obtain a linear material having a cross-sectional dimension of 9 mm × 9 mm (forging ratio: about 598). Subsequently, after the annealing process, predetermined quenching and tempering processes were performed, and the hardness was adjusted to around 40 HRC. Then, the textured surface of the linear material after quenching and tempering was observed without corrosion, and the graphite distribution state (average particle diameter, area ratio in the structure) was examined. The distribution situation was investigated using image analysis from images of 10 fields observed with a 1000 × optical microscope.
[0100]
[Table 7]

[0101]
  Sample No. All graphite deposits from 21 to 23 were fine. Table 7 shows the distribution of graphite. The average particle size was 1 μm or less, and the area ratio was 1 to 4%. Moreover, the relatively coarse graphite of 1 μm or more has an average particle diameter of 1 to 1.5 μm and an area ratio of less than 1% in any sample. And in each sample, the ratio of the area of the comparatively coarse graphite of 1 micrometer or more which occupies for the area of the whole graphite is 1/4 or less, and it turns out that fine graphite occupies most of precipitation graphite.
[0102]
  These samples were used as test pieces for evaluating the dynamic friction coefficient. The evaluation method of the dynamic friction coefficient is performed under the following conditions using the same ultra high pressure friction and wear tester as shown in FIG. 7 (however, the friction speed is different from the display in FIG. 7). Each time the load was stepped up, the dynamic friction coefficient was obtained. FIG. 9 shows the relationship between the reciprocal of the load and the dynamic friction coefficient.
    Sliding surface shape: 5mm x 5mm square
    Friction speed: 1 m / sec Friction surface pressure; initial pressure 1.5 MPa,
                Rise by 0.5 MPa every minute
    Lubricating oil; Motor oil # 30
                10cm³/ Min constantly dripping
    Mating material: FC250 (JIS gray cast iron)
[0103]
  FIG. 9 is a curve referred to as a Stribeck diagram, in which the “load characteristic” indicating the load condition applied to the frictional sliding portion is plotted on the horizontal axis and the friction coefficient is plotted on the vertical axis. This makes it possible to know the lubrication state. In the case of the present embodiment, the friction speed is constant at 1 m / sec, so that the horizontal axis can be represented by the reciprocal of the load. In each curve of FIG. 9, the right side (low load side: range indicated by the arrow) of the plot (extreme value) with the lowest friction coefficient is a region where the lubricating film is not broken and fluid lubrication occurs, and the left side ( The high load side) is an area where the fluid film is broken and solid lubrication is mixed with fluid lubrication due to contact between solids. Therefore, as the plot (extreme value) with the lowest friction coefficient shifts to the left side of the graph, the fluid film does not break even at higher loads, and fluid lubrication is possible.
[0104]
  From the results of FIG. Sample no. In 22, the fluid film is held even on the higher load side. And sample No. with much Co content. In FIG. 23, it can be seen that the fluid film is held even on the higher load side. This indicates that Mo and Co have the effect of promoting the squeeze action described above.
[0105]
  Moreover, the leftmost plot of each curve corresponds to the load at which the test was stopped because seizure occurred. Sample No. In the case of Nos. 22 and 23, the above plot is shifted to the left side (high load side) by addition of Mo and Co, and further improvement in seizure resistance is observed. And the dynamic friction coefficient is also low as a whole, and the favorable sliding characteristics due to graphite precipitation are further improved by the addition of Mo and Co.
[0106]
  According to the present invention, the self-lubricating property provides excellent seizure resistance without surface treatment, the friction coefficient is low, and the energy loss due to friction is small, so that it can be applied to various sliding parts. And by adjusting the shape of the sulfide inclusions, it is particularly effective as a piston ring, and the aggressiveness to the cylinder liner and piston is reduced, and the environmental performance and durability of the internal combustion engine are improved. Also make a big contribution. In addition, it is excellent in workability such as machinability in the manufacturing process and can reduce manufacturing cost and lead time, so it can be a material for sliding parts that is excellent in both performance and manufacturing, and it is an extremely useful technology in industry. .
[0107]
Industrial applicability
  The material of the present invention is used for sliding parts such as a piston ring, a cylinder liner, and a vane mounted on an internal combustion engine such as a slide bearing, a roller bearing, a ball bearing, a gear, a mold, and an automobile engine.
[Brief description of the drawings]
1 is a microstructural photograph showing the distribution of graphite observed on the cross section of the structure of the material of the present invention (sample No. 3).
FIG. 2 is a microstructural photograph showing the distribution of graphite observed in the cross section (different visual field) of the same material as the material shown in FIG.
3 is a microstructural photograph showing the distribution of graphite observed on the cross section of the structure of the comparative material (sample No. 14).
4 is a microstructural photograph showing the distribution of graphite observed in the cross section (different visual field) of the same material as the material shown in FIG. 3. FIG.
FIG. 5 is a sketch diagram of a microstructural photograph explaining the parallelism of sulfide inclusions and a schematic diagram thereof.
FIG. 6 is a diagram for explaining the influence of the angle between the stress direction and the crack growth direction on the stress intensity factor.
FIG. 7 is a schematic diagram illustrating an ultra-high pressure friction wear test method.
FIG. 8 is a schematic diagram for explaining a reciprocating wear test method.
FIG. 9 is a Stribeck diagram showing the relationship between the reciprocal of the load and the dynamic friction coefficient, and is a diagram for explaining the lubrication state.

Claims (16)

In mass%, C: 0.4% or more and less than 1.5%, Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, Cr: 0 to 0.5%, Ni: 0.05 to 3.0%, Al: 0.3 to 2.0%, at least one selected from the group consisting of Mo, W and V is 0.3 to 20% in total amount (Mo + W + V), Cu : the 0.05 to 3.0% seen contains the balance being formed of steel consisting of Fe and unavoidable impurities, the sliding part having a self-lubricating average particle diameter of the graphite present in the steel structure is 3μm or less Materials. And an area ratio of 1% or more occupied by the graphite that exist in any steel structure cross section, a sliding component material having the self-lubricating property according to claim 1, wherein an average particle diameter of 3μm or less. The material for a sliding part having self-lubricating property according to claim 1 or 2, wherein no V carbide is present in the steel structure .   The total amount (Mo + W) of 0.3 to 5.0% and V: less than 0.1% of at least one selected from the group consisting of Mo and W by mass% is from 1 to 3. The material for sliding parts having self-lubricating property according to any one of the above.   The material for a sliding part having self-lubricating property according to any one of claims 1 to 4, wherein the material is made of steel containing Al: 0.7 to 2.0% by mass.   The material for a sliding part having self-lubricating property according to any one of claims 1 to 5, wherein the material is made of steel containing Mo: 1.5 to 3.0% by mass.   The material for a sliding part having self-lubricating properties according to any one of claims 1 to 6, wherein the material is made of steel containing 10% or less by mass of Co: 10% or less.   The material for a sliding part having self-lubricating property according to any one of claims 1 to 7, wherein the material is made of steel containing S: 0.3% or less by mass%.   The material for sliding parts having self-lubricating properties according to claim 8, wherein the material is made of steel containing, by mass%, Ca: 0.01% or less. Sliding component material having the self-lubricating property according to claim 1, nitriding treatment Ru subjected at the manufacturing stage of the sliding parts of the product to claim 9. In mass%, C: 0.4% or more and less than 1.5%, Si: 0.1 to 3.0%, Mn: 0.1 to 3.0%, Cr: 0 to 0.5%, Ni: 0.05 to 3.0%, Al: 0.3 to 2.0%, at least one selected from the group consisting of Mo, W and V is 0.3 to 20% in total amount (Mo + W + V), Cu : the 0.05 to 3.0% seen contains the balance being formed of steel consisting of Fe and inevitable impurities, an average particle diameter of the graphite present in the steel structure is a piston ring for wire is 3μm or less, distribution of sulfide inclusions outer peripheral surface and that exist in any steel structure cross section concurrent when the piston ring is at eggplant parallelism of the straight line comrades through the maximum diameter upper of each sulfide-based inclusions Piston ring wire that is within 30 degrees. And an area ratio of 1% or more occupied by the graphite that exist in any steel structure cross section, the piston ring for wire according to claim 11 average particle diameter of 3μm or less.   The wire material for piston rings according to claim 11 or 12, wherein the wire material is made of steel containing 10% or less of Co by mass%.   The wire material for piston rings according to any one of claims 11 to 13, wherein the wire material is made of steel containing S: 0.3% or less by mass%.   The wire material for piston rings according to claim 14, which is made of steel containing Ca: 0.01% or less by mass. Piston ring for wire according to any one of claims 11 to claim 15 in which nitriding treatment is facilities during manufacture of the piston ring formed from wire which is a material.

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