JP3614113B2 - Steel material for bearing element parts with excellent machinability - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、軸受を構成するボール、コロ、ニードル、シャフト、レースなどの軸受要素部品の用途に好適な被削性に優れた軸受要素部品用鋼材に関する。
【０００２】
【従来の技術】
ボール、コロ、ニードル、シャフト、レースなどの軸受要素部品の素材鋼としては、一般に、ＪＩＳ Ｇ ４８０５に規定されているＳＵＪ２鋼などの高炭素クロム軸受鋼が多用されている。
【０００３】
上記の所謂「軸受用鋼」は、熱間圧延などの手段で加工された後、軟化を目的とした球状化焼鈍を受け、次いで冷間鍛造、冷間抽伸や切削などの加工を施され、さらに、焼入れと低温での焼戻しによる熱処理を受けて所望の機械的性質を付与される。
【０００４】
上記の各工程のうち、切削加工はコストが嵩むので、切削速度の向上や工具寿命の延長が可能となる被削性に優れた軸受用鋼に対する要求が極めて大きくなっている。
【０００５】
鋼にＰｂやＳなどの快削元素（被削性改善元素）を単独あるいは複合させて添加すれば、被削性が向上することはよく知られている。しかし、各種の産業機械や自動車などに使用される軸受には高い面圧が繰り返し作用する。このため、軸受用鋼に前記快削元素を添加すれば、軸受（要素部品）の転動疲労寿命が大幅に低下してしまう。さらに、前記快削元素は一般に熱間加工性を低下させる。したがって、熱間圧延などの熱間加工時に表面割れや疵が発生しやすくなるという問題もある。
【０００６】
このため、特開平１−２５５６５１号公報には、鋼中にＲＥＭ（希土類元素）を含有させた「被削性に優れた高Ｓｉ−低Ｃｒ軸受鋼」が開示されている。しかし、ＲＥＭは極めて酸化されやすいため、鋼中での歩留まりが不安定である。また、鋼中に生成しやすいＲＥＭ酸化物の粒径や分散状態を制御することは、工業的には難しい。粗大なＲＥＭ酸化物が生成したり、ＲＥＭ酸化物が多量に生成すると、転動疲労寿命が大幅に低下してしまう。
【０００７】
特開平３−５６６４１号公報には、鋼中にＢＮ化合物を生成させることで、転動疲労寿命を低下させることなく被削性を向上させる「被削性に優れた軸受鋼」が開示されている。しかし、Ｂは鋼中への溶解度が小さいため、鋼中での歩留まりが不安定であるし、偏析も生じやすい。さらに、Ｂは高炭素鋼の凝固開始温度を著しく低下させるので、Ｂの偏析と相まって、凝固偏析が助長されることになる。加えて、凝固開始温度の低下が熱間加工性の低下につながり、熱間加工時に表面割れや疵が生成しやすくなる。したがって、軸受用鋼のＢ含有量をたとえ前記公報で規定された値、つまり、質量％で、０．００４〜０．０２０％にしても、必ずしも工業的規模で安定して軸受要素部品が製造できるというものでもなかった。
【０００８】
特開平９−２２７９９１号公報には、特定の条件で熱処理して組織中の炭化物数と硬さを調整する「被削性および冷間加工性に優れる軸受鋼およびその製造方法」が開示されている。しかし、この公報で提案された焼鈍条件では、加熱工程の途中で徐熱または等温保持をおこなう必要がある。このため、焼鈍時間が長くなり生産性の低下をきたす。さらに、徐熱、急熱、徐冷など熱処理条件の変更が必要であるため、例えば、鋼線材（以下、「鋼線材」を単に「線材」という）の一般的な形状である巻取りコイルを対象とする場合、コイル全体を均一に熱処理（焼鈍処理）することが困難である。たとえ均一な熱処理ができたとしても、工業的規模で用いられる連続熱処理炉は、一般に各ゾーンの温度が決まっていて、ゾーンの数も限られているため、前記公報で規定された条件で焼鈍を実施することは難しいし、規定条件で焼鈍するためには連続熱処理炉の改造や更新が必要でコストが嵩んでしまう。
【０００９】
上記の各公報で提案された技術によれば、一応は被削性に優れた鋼材、具体的には線材、棒鋼および鋼管を得ることができる。しかし、既に述べたように、生産性、品質の点で大きな問題があった。
【００１０】
【発明が解決しようとする課題】
本発明は、上記現状に鑑みなされたもので、その目的は、快削元素を特別に添加含有させることなく、かつ、焼鈍時間も従来と同様の１０〜２０時間程度であるため生産性の低下をきたすこともなく、ボール、コロ、ニードル、シャフト、レースなどの軸受要素部品の用途に好適な被削性に優れた鋼材（線材、棒鋼または鋼管）を提供することである。
【００１１】
なお、既に述べたように、軸受には高い面圧が繰り返し作用するので、後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命を有することを目標とする。
【００１２】
【課題を解決するための手段】
本発明の要旨は、下記の被削性に優れた軸受要素部品用鋼材にある。
【００１３】
質量％で、Ｃ：０．８〜１．２％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜１．５％、Ｃｒ：０．６〜２．０％、Ａｌ：０．０５％以下、Ｃｕ：２．０％以下、Ｎｉ：４．０％以下、Ｍｏ：０．５％以下、Ｖ：０．２％以下、Ｎｂ：０．１０％以下、Ｃａ：０．００３％以下、Ｍｇ：０．００３％以下を含有し、残部はＦｅおよび不純物からなり、不純物中のＴｉは０．００２％以下、Ｐは０．０２％以下、Ｓは０．０１５％以下、Ｎは０．００９％以下、Ｏ（酸素）は０．００１５％以下で、Ｓｉ、Ｍｎ、ＣｒおよびＭｏの関係が式「５．０≦１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ≦９．０」を満たし、セメンタイト中のＣｒとＭｎの合計濃度が５．０％以上であることを特徴とする被削性に優れた軸受要素部品用鋼材。
【００１４】
上記の本発明においては、上記各元素のうち、Ｃｕ、Ｎｉ、Ｍｏ、Ｖ、Ｎｂ、ＣａおよびＭｇの７元素については、必ずしも添加含有させる必要はなく、不純物量レベルであってもよい。また、本発明にいう鋼材とは、線材、棒鋼または鋼管をいう。
【００１５】
本発明者らは、線材、棒鋼および鋼管の球状化焼鈍後の組織、セメンタイト中のＣｒ、Ｍｎ濃度、およびセメンタイト粒径が被削性に及ぼす影響について調査、研究を重ね、その結果、下記の知見を得た。
（ａ）軸受用鋼の切削加工においては、被切削材中の炭化物の硬さが工具寿命や上限切削速度に大きく影響する。
（ｂ）セメンタイト中にＣｒ、Ｍｎが濃化すると、セメンタイトが硬化することは知られているが、マトリックスであるフェライト中のＣｒ、Ｍｎが減少すると、フェライトは軟化し、セメンタイトとマトリックスであるフェライトの硬度差が大きくなればなるほど、被削性が向上する。
（ｃ）セメンタイト中のＣｒ、Ｍｎ濃度を高めるためには、球状化焼鈍中にオーステナイトからフェライトに変態した後も、徐冷または保定すればよい。
（ｄ）転動疲労寿命を確保するためには一定以上の焼入性の確保が必要で、そのためにはＳｉ、Ｍｎ、ＣｒおよびＭｏの含有量を制御すればよい。
【００１６】
本発明は、上記の知見に基づいて完成されたものである。
【００１７】
【発明の実施の形態】
以下、本発明について詳しく説明する。なお、以下において、「％」は「質量％」を意味する。
（Ａ）化学組成
Ｃ：０．８〜１．２％
Ｃは、焼入れと低温での焼戻しによる熱処理をおこなって軸受用鋼材（軸受要素部品）に所望の機械的性質を付与させるが、その含有量が０．８％未満では焼入れ焼戻し後の硬度が低く、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られない。一方、Ｃの含有量が１．２％を超えると、鋼の凝固開始温度が低下して熱間加工時、なかでも鋼材が鋼管の場合、熱間製管時に割れや疵が多発する。また、鋼の凝固時に巨大な炭化物が生成しやすくなるので、長時間の均質化熱処理をおこなわない場合には目標とする転動疲労寿命が得られない。したがって、Ｃ含有量は０．８〜１．２％とした。好ましい範囲は０．８〜１．０％、より好ましい範囲は０．８〜０．９％である。
【００１８】
Ｓｉ：０．２〜２．０％
Ｓｉは、転動疲労寿命を高めるのに有効な元素であるほか、脱酸剤として必要な元素でもある。また、Ｓｉは鋼の焼入性を向上させる元素でもある。しかし、その含有量が０．２％未満では前記の効果が得難い。なお、Ｓｉの含有量が０．６％以上になると被削性向上効果も大きくなる。一方、Ｓｉの含有量が２．０％を超えると、熱間圧延後や球状化焼鈍後に脱スケールするために長時間を要するので生産性の大幅な低下を招く。したがって、Ｓｉ含有量は０．２〜２．０％とした。好ましい範囲は０．５〜１．５％、より好ましい範囲は０．５〜１．０％である。
【００１９】
Ｍｎ：０．２〜１．５％
Ｍｎは、鋼の焼入性を向上させると同時に、Ｓによる熱間脆性の防止に必要な元素である。これらの効果を発揮させるためにはＭｎを０．２％以上含有させる必要がある。一方、Ｍｎの含有量が１．０％を超えるとＭｎのみならずＣの中心偏析が生じるようになり、１．５％を超えるとＭｎ、Ｃの中心偏析が顕著になり、転動疲労寿命の低下を招く。したがって、Ｍｎ含有量は０．２〜１．５％とした。好ましい範囲は０．２〜１．０％、より好ましい範囲は０．２〜０．８％である。
【００２０】
Ｃｒ：０．６〜２．０％
Ｃｒは、鋼の焼入性を向上させると同時に、セメンタイト中に濃化しやすい元素で、セメンタイトを硬化させて被削性を向上させる。しかし、その含有量が０．６％未満では前記の効果が得難い。一方、１．６％を超えるとＣｒのみならずＣ元素の中心偏析が生じるようになり、２．０％を超えるとＣｒ、Ｃの中心偏析が顕著になり、転動疲労寿命の低下を招く。したがって、Ｃｒ含有量は０．６〜２．０％とした。好ましい範囲は０．６〜１．６％、より好ましい範囲は０．６〜１．３％である。
【００２１】
Ａｌ：０．０５％以下
Ａｌは脱酸剤として添加するが、過剰なＡｌは非金属系介在物を形成して転動疲労寿命を低下さる。特に、その含有量が０．０５％を超えると、粗大な非金属系介在物を形成して転動疲労寿命の著しい低下を招き、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られなくなる。したがって、Ａｌの含有量は０．０５％以下とした。好ましい上限は０．０４％、より好ましい上限は０．０３％である。一方、十分な脱酸効果を得るためには、その含有量を０．０００３％以上とするのがよい。なお、Ａｌは、上記のＳｉによって脱酸が十分になされる場合には必ずしも添加する必要はなく、その含有量は不純物量レベルであってもよい。
【００２２】
Ｃｕ：２．０％以下（添加時の望ましい範囲は０．０５〜２．０％）
Ｃｕは添加しなくてもよい。添加すれば耐食性を高める作用がある。この効果を確実に得るには、Ｃｕは０．０５％以上の含有量とすることが好ましい。しかし、その含有量が２．０％を超えると結晶粒界に偏析して鋼塊の分塊圧延や線材の熱間圧延などの熱間加工時における割れや疵の発生が顕著になる。したがって、Ｃｕの含有量は２．０％以下とした。好ましい上限は１．５％、より好ましい上限は１．０％である。
【００２３】
Ｎｉ：４．０％以下（添加時の望ましい範囲は０．２〜４．０％）
Ｎｉは添加しなくてもよい。添加すれば、焼入れ後のマルテンサイト中に固溶して靱性を高める作用を有する。この効果を確実に得るには、Ｎｉは０．２％以上の含有量とすることが好ましい。しかし、４．０％を超えて含有させても、前記の効果は飽和し、コストが嵩むばかりである。したがって、Ｎｉの含有量を４．０％以下とした。好ましい上限は３．０％、より好ましい上限は２．０％である。
【００２４】
Ｍｏ：０．５％以下（添加時の望ましい範囲は０．０５〜０．５％）
Ｍｏも添加しなくてもよい。添加すれば、焼入性を高め、転動疲労寿命を高める作用がある。この効果を確実に得るには、Ｍｏは０．０５％以上の含有量とすることが好ましい。しかし、その含有量が０．５％を超えると、焼入性が高くなり過ぎて熱間圧延後にマルテンサイトが生成しやすくなり、割れが発生しやすくなる。したがって、Ｍｏの含有量は０．５％以下とした。好ましい上限は０．３％、より好ましい上限は０．２％である。
【００２５】
Ｖ：０．２％以下（添加時の望ましい範囲は０．０３〜０．２％）
Ｖは添加しなくてもよい。添加すれば、オーステナイト結晶粒を微細化させ、靱性を高める作用を有する。この効果を確実に得るには、Ｖは０．０３％以上の含有量とすることが好ましい。しかし、０．２％を超えて含有させても前記の効果は飽和し、コストが嵩むばかりである。したがって、Ｖの含有量は０．２％以下とした。好ましい上限は０．１％である。
【００２６】
Ｎｂ：０．１０％以下（添加時の望ましい範囲は０．０１〜０．１０％）
Ｎｂは添加しなくてもよい。添加すれば、オーステナイト結晶粒を微細化させ、靱性を高める作用を有する。この効果を確実に得るには、Ｎｂは０．０１％以上の含有量とすることが好ましい。しかし、０．１０％を超えて含有させても前記の効果は飽和し、コストが嵩むばかりである。したがって、Ｎｂの含有量は０．１０％以下とした。好ましい上限は０．０８％、より好ましい上限は０．０５％である。
【００２７】
Ｃａ：０．００３％以下
Ｃａは添加しなくてもよい。添加すれば、熱間加工性を高める作用を有する。この効果を確実に得るには、Ｃａは０．０００１％以上の含有量とすることが好ましい。しかし、Ｃａを０．００３％を超えて含有させても前記の効果は飽和し、コストが嵩むばかりである。したがって、Ｃａの含有量は０．００３％以下とした。好ましい上限は０．００２％である。
【００２８】
Ｍｇ：０．００３％以下
Ｍｇも添加しなくてもよい。添加すれば、熱間加工性を高める作用を有する。この効果を確実に得るには、Ｍｇは０．０００１％以上の含有量とすることが好ましい。しかし、Ｍｇを０．００３％を超えて含有させても前記の効果は飽和し、コストが嵩むばかりである。したがって、Ｍｇの含有量は０．００３％以下とした。好ましい上限は０．００２％である。
【００２９】
本発明においては、不純物としてのＴｉ、Ｐ、Ｓ、ＮおよびＯ（酸素）の含有量を下記のとおりに制限することが重要である。その理由は以下の通りである。
【００３０】
Ｔｉ：０．００２％以下
Ｔｉは、Ｎと結合してＴｉＮを形成し、転動疲労寿命を低下させてしまう。特にその含有量が０．００２％を超えると、転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られない。したがって、Ｔｉの含有量は０．００２％以下とした。なお、不純物としてのＴｉの含有量はできるだけ少なくすることが望ましい。
【００３１】
Ｐ：０．０２％以下
Ｐは、粒界に偏析して転動疲労寿命を低下させてしまう。特に、その含有量が０．０２％を超えると転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られなくなる。したがって、Ｐの含有量は０．０２％以下とした。
【００３２】
Ｓ：０．０１５％以下
Ｓは、Ｍｎと結合してＭｎＳを形成し、転動疲労寿命を低下させてしまう。特にその含有量が０．０１５％を超えると、粗大なＭｎＳを形成しやすくなるので転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られない。したがって、Ｓの含有量は０．０１５％以下とした。
【００３３】
Ｎ：０．００９％以下
Ｎは、ＴｉやＡｌと結合してＴｉＮやＡｌＮを形成しやすく、Ｎ含有量が多くなって粗大なＴｉＮやＡｌＮが形成されると、転動疲労寿命が低下してしまう。特にその含有量が０．００９％を超えると、転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られない。したがって、Ｎの含有量は０．００９％以下とした。
【００３４】
Ｏ（酸素）：０．００１５％以下
Ｏは、酸化物系介在物を形成し、転動疲労寿命を低下させてしまう。特にその含有量が０．００１５％を超えると転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 回以上の転動疲労寿命）が得られない。したがって、Ｏの含有量は０．００１５％以下とした。なお、不純物としてのＯ含有量はできる限り少なくすることが望ましい。
（Ｂ）Ｓｉ、Ｍｎ、ＣｒおよびＭｏの関係
本発明においては、Ｓｉ、ＭｎおよびＣｒの各含有量を（Ａ）で規定した範囲内において、式「５．０≦１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ≦９．０」を満たす量に規定する。これは、後の実施例に示すように、式「１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ」で求められ値が５．０未満では、焼入性が不足するために、焼入れ後の硬度が低く、転動疲労寿命の低下が著しくなり、所望の転動疲労寿命（後述の実施例における転動疲労試験で、１．０×１０７ 以上の転動疲労寿命）が得られない。一方、式「１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ」で求められる値が９．０を超えると、焼入性が過剰で、熱間圧延後の冷却中にマルテンサイト組織が生成して、焼割れやハンドリング時の割れが発生しやすくなる。したがって、Ｓｉ、ＭｎおよびＣｒの各含有量を式「５．０≦１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ≦９．０」満たす量とした。
【００３５】
ここで、上記の式「１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ」は、鋼の焼入性を表し、下記の実験結果に基づいて各元素の係数を最小二乗法を用いて求めることにより、本発明者が初めて定めた式である。
【００３６】
すなわち、一般に、鋼の焼入性は、例えばＡＩＳＩ規格にも規定されているように、鋼の化学組成と結晶粒度から計算によって求め得る。しかし、本発明のような高炭素で、かつセメンタイト中にＭｎ、Ｃｒが濃化している鋼の場合、マトリックス中のＭｎ、Ｃｒ濃度が低下しているため、その計算結果と実際の焼入性とが一致しない。このため、以下に述べる実験をおこない、高炭素で、かつセメンタイト中にＭｎ、Ｃｒが濃化している本発明鋼の焼入性を正確に表すパラメータ式として上記の式を定めたのである。
【００３７】
実験内容：
後述する表１に示す各鋼を用い、同じく後述する表２に示す「条件Ｙ」と同じ条件で球状化焼鈍をおこなった外径４０ｍｍの棒鋼から試験片を採取し、ＪＩＳＧ ０５６１に規定される方法に準拠してジョミニー試験をおこなった。その際、試験片の加熱条件は、軸受用部品の一般的な焼入温度である８２０℃×２０分間保持とした。次いで、ロックウェルＣ硬度５０を示す位置までの冷媒供給側の試験片端部からの距離を各鋼について求めた。そして、この求めた各鋼の距離と、当該鋼中に含まれる各元素中、鋼の焼入性に大きな影響を及ぼすといわれているＳｉ、Ｍｎ、ＣｒおよびＭｏの４元素の含有量とから、各元素の効果が加算できると仮定し、最小二乗法によって、各元素の係数を求めた。その結果、各元素の係数は上記の式中に示す通りであった。
（Ｃ）セメンタイト中のＣｒとＭｎの合計濃度
本発明においては、セメンタイト中のＣｒとＭｎの合計濃度（以下、Ｃｒ＋Ｍｎ濃度という）を５．０％以上に規定する。これは、後述の実施例に示すように、セメンタイト中のＣｒ＋Ｍｎ濃度が５％未満であると、セメンタイトの硬化が十分ではないために、切削試験での工具寿命が一般的な方法で製造された鋼材切削時の工具寿命の１．３倍以上にならないためである。さらに、セメンタイト中のＣｒ＋Ｍｎ濃度が６．０％以上の場合、その工具寿命が一般的な方法で製造された鋼材切削時の工具寿命の１．５倍以上になるので、セメンタイト中のＣｒ＋Ｍｎ濃度は６．０％以上であることが望ましい。一方、セメンタイト中のＣｒ＋Ｍｎ濃度の上限は特に制限しないが、セメンタイト中のＣｒ＋Ｍｎ濃度が１５％を超えると、セメンタイト以外の炭化物になる可能性があるため、上限は１５％以下にすることが望ましい。
【００３８】
セメンタイト中のＣｒ＋Ｍｎ濃度を５．０％以上にするためには、鋼中のＣｒとＭｎの含有量を増加させるとともに、球状化焼鈍中にマトリックスがオーステナイトからフェライトに変態した後に、徐冷または保定することが有効である。
【００３９】
例えば、後の実施例に示すように、鋼Ｂ（０．９０％Ｃ−０．５０％Ｍｎ−０．８２％Ｃｒ）を７７０℃で２時間保持した後、１０℃／時間で６８０℃まで冷却し、その後炉外で放冷すると、セメンタイト中のＣｒ＋Ｍｎ濃度は４．２％となる。また、７７０℃で２時間保持した後、１０℃／時間で７００℃まで冷却した後、６５０℃までを５℃／時間で冷却し、その後炉外で放冷すると５．８％となる。なお、６５０℃未満では、ＣｒおよびＭｎの拡散が極めて遅くなるため、６５０℃未満の冷却速度はセメンタイト中のＣｒおよびＭｎ濃度にほとんど影響しない。
【００４０】
セメンタイト中のＣｒとＭｎ濃度の測定は、次の方法によって決定した。すなわち、抽出レプリカ法によって、セメンタイトを取り出し、透過型電子顕微鏡（ＴＥＭ）に付属したエネルギー分散形Ｘ線分析（ＥＤＳ）によって、各試料につき５個のセメンタイト中のＣとＭｎの濃度を測定し、その各平均値をその試料のセメンタイト中のＣｒとＭｎの濃度とした。
【００４１】
また、セメンタイトの粒径については、特に規定しないが、その平均粒径が０．４μｍ未満になると、鋼材の強度が上昇して、切削加工での切削抵抗が増加して工具寿命が短くなる傾向が大きくなる。このため、セメンタイトの平均粒径は０．４μｍ以上であることが好ましい。また、その上限も特に規定しないが、セメンタイトの平均粒径が０．８μｍを上回ると、工具寿命の改善効果も飽和してくる傾向があり、球状化焼鈍に要する時間も長くなって生産性が低下するため、セメンタイトの平均粒径は０．８μｍ以下であることが好ましい。
【００４２】
セメンタイトの平均粒径は、次のように定義されるものである。すなわち、各セメンタイト粒の面積を求め、その面積と等価な面積の円の直径を求め、それを各セメンタイト粒の見かけの粒径とする。次いで、面積を測定したすべてのセメンタイト粒の見かけの粒径の平均値を見かけのセメンタイト平均粒径とし、上記の見かけのセメンタイトの平均粒径を１．１２倍したものをセメンタイト平均粒径と定義する。
【００４３】
前記（Ａ）、（Ｂ）および（Ｃ）に記載した構成要件からなる本発明の鋼材（鋼線材、棒鋼または鋼管）は、通常の方法で冷間鍛造、冷間抽伸や切削などの加工を施され、さらに、焼入れと低温での焼戻しによる熱処理を受けて所望の機械的性質を有する軸受要素部品に仕上げられてから、精密機械部品である最終製品としての軸受に組み立てられる。
【００４４】
以下、実施例により本発明をさらに詳しく説明する。
【００４５】
【実施例】
表１に示す化学組成を有する１８種類の鋼を容量３００ｋｇの真空溶解炉を用いて溶製した。表１中、代符Ｂ〜ＩおよびＯ〜Ｒの鋼は化学組成が本発明で規定する範囲内のものであり、代符ＡおよびＪ〜Ｎの鋼は成分のいずれかが本発明で規定する範囲から外れた比較例である。
【００４６】
【表１】
【００４７】
クラックの発生が認められなかった代符Ａ〜ＦおよびＩ〜Ｒの鋼からなる鍛造材は、長さ６００ｍｍに切断した後、電気炉を用いて表２に示す４通りの熱処理条件（ヒートパターン）のそれぞれによる球状化焼鈍をおこなった。なお、表２中の条件Ｗは、従来の焼鈍処理のヒートパターンに相当し、炉外放冷に至るまでの所要時間は１１時間である。また、他の条件の所要時間は、条件Ｘが１８時間、条件Ｙが１４時間、条件Ｚが１０時間である。
【００４８】
【表２】
【００４９】
すなわち、Ｃｒ＋Ｍｎ濃度は、各丸棒の横断面を鏡面研磨した後、抽出レプリカ法によってセメンタイトを取り出し、透過型電子顕微鏡（ＴＥＭ）に付属したエネルギー分散形Ｘ線分析（ＥＤＳ）によって各試料につき５個のセメンタイト中のＣとＭｎの濃度を測定し、その平均値をその試料のセメンタイト中のＣｒとＭｎの濃度とし、これを合計して求めた。
【００５０】
セメンタイトの平均粒径は、上記と同様に鏡面研磨した各丸棒の横断面をピクラールで腐食処理し、その腐食処理表面を走査型電子顕微鏡を用いて各試料の円中心から半径の１／２位置を倍率５０００倍で１０視野撮影し、この写真を通常の方法による画像解析によって各セメンタイト粒の面積を求め、それと等価な面積の円を計算して求められる直径を各セメンタイト粒の直径とし、その直径の平均値を１．１２倍した値をセメンタイトの平均粒径とした。
【００５１】
また、球状化焼鈍後、研削加工して仕上げた直径３８ｍｍの各丸棒を対象に、切削試験もおこなった。すなわち、切削工具としてＪＩＳ Ｇ ４４０３に規定されるＳＫＨ４からなる三角チップを用い、無潤滑、周速５０ｍ／ｍｉｎ、切り込み量０．７ｍｍ、送り０．２ｍｍ／ｒｅｖ．の条件で旋削加工して工具寿命を調査し、被削性の指標とした。
【００５２】
なお、工具寿命は、前記条件で丸棒を切削加工した場合に、切削時間１０分までは３０秒ごと、それ以降は１分ごとに工具の主切り刃摩耗量を測定して、主切り刃摩耗量が０．２０ｍｍになった時点とした。また、被削性の目標は下記（イ）の条件を満足することとした。
（イ）各鋼について、条件Ｗで球状化焼鈍した丸棒の工具寿命を基準とし、これよりも３０％以上工具寿命が長いこと。
【００５３】
さらに、上記の切削加工に供した各丸棒から、直径１２ｍｍ、長さ２２ｍｍの試験片を切り出し、この試験片を焼入れ焼戻し処理（８２０℃×３０分間保持後油焼入れ→１６０℃×１時間保持の焼戻し）して次の条件による転動疲労試験に供した。
【００５４】
すなわち、円筒型の転動疲労試験機を用いて、潤滑油にＪＩＳ規格に規定される＃６８タービン油を使用して、ヘルツ最大接触応力が５８８ＭＰａ、試験片負荷回数が４６０００回／分の条件で転動疲労試験をおこなった。試験片は各鋼について１０個ずつとし、１０個の試験片の中で最初に表面剥離をおこしたときの回転数を「転動疲労寿命」とした。転動疲労寿命が１．０×１０７ 以上の場合に転動疲労特性に優れていると評価した。
【００５５】
表３および表４に、球状化焼鈍後のセメンタイト中のＣｒ＋Ｍｎ濃度、セメンタイトの平均粒径、旋盤による切削加工での工具寿命、転動疲労寿命の各調査結果をまとめて示す。
【００５６】
【表３】
【００５７】
上記のうち、試番１、４、２５、２８、２９、３２、３３、３６、４１および４４〜４６は、セメンタイト中のＣｒ＋Ｍｎ濃度が５．０％未満であるため、工具寿命も目標の値に達していない。
【００５８】
また、試番５、８、９、１２、１３、１６、１７、２０、２１、２４、４９、５２、５３、５６、５７、６０、６１および６４は、鋼の化学組成は本発明で規定する範囲内であるが、セメンタイト中のＣｒ＋Ｍｎ濃度が５．０％未満であるため、工具寿命が目標の値に達していない。
【００５９】
これに対し、本発明で規定する条件を満たす代符Ｂ〜ＩおよびＯ〜Ｒ鋼を用いた試番６、７、１０、１１、１４、１５、１８、１９、２２、２３、５０、５１、５４、５５、５８、５９、６２および６３は、工具寿命が目標の値に達しており、しかも転動疲労寿命も目標の１．０×１０７ 回を上回っている。
【００６０】
特に、セメンタイト中のＣｒ＋Ｍｎ濃度が６．０％以上の試番１１、２２、２３、５０、５１、５４、５５、５７、６０、６１および６４の工具寿命は、条件Ｗで球状化焼鈍したものに比べて５０％以上と長く、一段と優れている。
【００６１】
【発明の効果】
本発明の軸受要素部品用鋼材は、被削性に優れるとともに、転動疲労寿命が長い。このため、ボール、コロ、ニードル、シャフト、レースなどの軸受要素部品の高寿命化が図れる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel material for bearing element parts that is excellent in machinability and is suitable for use in bearing element parts such as balls, rollers, needles, shafts, and races constituting a bearing.
[0002]
[Prior art]
As a material steel for bearing element parts such as balls, rollers, needles, shafts, and races, generally, high carbon chromium bearing steel such as SUJ2 steel defined in JIS G 4805 is frequently used.
[0003]
The so-called "bearing steel" is subjected to spheroidizing annealing for softening after being processed by means such as hot rolling, and then subjected to processing such as cold forging, cold drawing and cutting, Furthermore, the desired mechanical properties are imparted by heat treatment by quenching and tempering at a low temperature.
[0004]
Of the above processes, the cost of cutting is high, and therefore there is an increasing demand for bearing steel with excellent machinability that can improve cutting speed and extend tool life.
[0005]
It is well known that machinability is improved by adding a free-cutting element (machining-improving element) such as Pb or S alone or in combination to steel. However, high surface pressure repeatedly acts on bearings used in various industrial machines and automobiles. For this reason, if the said free-cutting element is added to the steel for bearings, the rolling fatigue life of a bearing (element part) will fall significantly. In addition, the free-cutting elements generally reduce hot workability. Therefore, there is a problem that surface cracks and wrinkles are likely to occur during hot working such as hot rolling.
[0006]
For this reason, JP-A-1-255651 discloses “high Si-low Cr bearing steel excellent in machinability” in which REM (rare earth element) is contained in the steel. However, since REM is very easily oxidized, the yield in steel is unstable. In addition, it is industrially difficult to control the particle size and dispersion state of REM oxides that are easily generated in steel. When a coarse REM oxide is generated or a large amount of REM oxide is generated, the rolling fatigue life is significantly reduced.
[0007]
Japanese Patent Laid-Open No. 3-56641 discloses a “bearing steel with excellent machinability” that improves machinability without reducing the rolling fatigue life by generating a BN compound in the steel. Yes. However, since B has a low solubility in steel, the yield in steel is unstable and segregation is likely to occur. Furthermore, since B significantly reduces the solidification start temperature of the high carbon steel, solidification segregation is promoted in combination with the segregation of B. In addition, a decrease in the solidification start temperature leads to a decrease in hot workability, and surface cracks and wrinkles are likely to be generated during hot working. Therefore, even if the B content of the bearing steel is the value specified in the above-mentioned publication, that is, mass%, 0.004 to 0.020%, the bearing element parts are always manufactured stably on an industrial scale. It wasn't even possible.
[0008]
Japanese Patent Application Laid-Open No. 9-227991 discloses a “bearing steel excellent in machinability and cold workability and its manufacturing method” in which the number of carbides and hardness in the structure are adjusted by heat treatment under specific conditions. Yes. However, under the annealing conditions proposed in this publication, it is necessary to carry out slow heating or isothermal holding during the heating process. For this reason, annealing time becomes long and productivity falls. Furthermore, since it is necessary to change heat treatment conditions such as slow heating, rapid heating, and slow cooling, for example, a winding coil having a general shape of a steel wire (hereinafter, “steel wire” is simply referred to as “wire”) is used. When the target is used, it is difficult to uniformly heat treat (anneal) the entire coil. Even if a uniform heat treatment can be performed, a continuous heat treatment furnace used on an industrial scale generally has a predetermined temperature and a limited number of zones. Therefore, annealing is performed under the conditions specified in the above publication. It is difficult to carry out the process, and in order to perform the annealing under the specified conditions, the continuous heat treatment furnace needs to be modified or renewed, resulting in an increase in cost.
[0009]
According to the techniques proposed in each of the above publications, it is possible to obtain a steel material excellent in machinability, specifically, a wire, a steel bar, and a steel pipe. However, as already mentioned, there were major problems in terms of productivity and quality.
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of the above-mentioned present situation, and the object thereof is to reduce productivity because the free cutting element is not specially added and the annealing time is about 10 to 20 hours as in the conventional case. It is to provide a steel material (wire material, bar steel or steel pipe) excellent in machinability suitable for use in bearing element parts such as balls, rollers, needles, shafts, and races.
[0011]
As already described, since high surface pressure repeatedly acts on the bearing, it is 1.0 × 10 6 in a rolling fatigue test in Examples described later. 7 The goal is to have a rolling fatigue life of at least times.
[0012]
[Means for Solving the Problems]
The gist of the present invention resides in the following steel material for bearing element parts having excellent machinability.
[0013]
In mass%, C: 0.8 to 1.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.5%, Cr: 0.6 to 2.0%, Al: 0.05% or less, Cu: 2.0% or less, Ni: 4.0% or less, Mo: 0.5% or less, V: 0.2% or less, Nb: 0.10% or less, Ca: 0.00%. 003% or less, Mg: 0.003% or less, the balance is made of Fe and impurities, Ti in the impurities is 0.002% or less, P is 0.02% or less, S is 0.015% or less, N is 0.009% or less, O (oxygen) is 0.0015% or less, and the relationship among Si, Mn, Cr and Mo is expressed by the formula “5.0 ≦ 1.6 ×% Si + 4.0 ×% Mn + 3.0 × % Cr + 5.0 ×% Mo ≦ 9.0 ”and the total concentration of Cr and Mn in the cementite is 5.0% or more, a bearing element with excellent machinability Goods for steel.
[0014]
In the present invention, among the above elements, the seven elements of Cu, Ni, Mo, V, Nb, Ca and Mg do not necessarily need to be added and may be at the impurity level. Moreover, the steel material as used in the field of this invention means a wire, a steel bar, or a steel pipe.
[0015]
The present inventors have repeatedly investigated and studied the influence of the structure after spheroidizing annealing of wire rods, steel bars and steel pipes, the concentration of Cr and Mn in cementite, and the cementite particle size on machinability. Obtained knowledge.
(A) In the cutting of bearing steel, the hardness of the carbide in the workpiece greatly affects the tool life and the upper limit cutting speed.
(B) It is known that when Cr and Mn are concentrated in cementite, cementite is hardened. However, when Cr and Mn in ferrite as a matrix decreases, ferrite softens, and cementite and ferrite as matrix. The machinability improves as the difference in hardness increases.
(C) In order to increase the Cr and Mn concentrations in the cementite, after the transformation from austenite to ferrite during spheroidizing annealing, it may be gradually cooled or maintained.
(D) In order to ensure the rolling fatigue life, it is necessary to ensure a certain level of hardenability. For that purpose, the contents of Si, Mn, Cr and Mo may be controlled.
[0016]
The present invention has been completed based on the above findings.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below. In the following, “%” means “mass%”.
(A) Chemical composition
C: 0.8 to 1.2%
C performs heat treatment by quenching and tempering at a low temperature to impart desired mechanical properties to the bearing steel (bearing element parts), but if the content is less than 0.8%, the hardness after quenching and tempering is low. , Desired rolling fatigue life (1.0 × 10 in rolling fatigue test in examples described later) 7 More than the rolling fatigue life). On the other hand, when the content of C exceeds 1.2%, the solidification start temperature of the steel is lowered, and when hot working, especially when the steel is a steel pipe, cracks and wrinkles frequently occur during hot pipe making. In addition, since huge carbides are easily generated during solidification of the steel, the target rolling fatigue life cannot be obtained unless a long-time homogenizing heat treatment is performed. Therefore, the C content is set to 0.8 to 1.2%. A preferable range is 0.8 to 1.0%, and a more preferable range is 0.8 to 0.9%.
[0018]
Si: 0.2-2.0%
Si is an element effective for increasing the rolling fatigue life, and is also an element necessary as a deoxidizer. Si is also an element that improves the hardenability of steel. However, if the content is less than 0.2%, it is difficult to obtain the above effect. In addition, when the Si content is 0.6% or more, the machinability improving effect is also increased. On the other hand, if the Si content exceeds 2.0%, it takes a long time to descal after hot rolling or after spheroidizing annealing, resulting in a significant reduction in productivity. Therefore, the Si content is set to 0.2 to 2.0%. A preferable range is 0.5 to 1.5%, and a more preferable range is 0.5 to 1.0%.
[0019]
Mn: 0.2 to 1.5%
Mn is an element necessary for improving the hardenability of steel and at the same time preventing hot brittleness due to S. In order to exhibit these effects, it is necessary to contain 0.2% or more of Mn. On the other hand, if the Mn content exceeds 1.0%, not only Mn but also C center segregation occurs, and if it exceeds 1.5%, center segregation of Mn and C becomes prominent, and the rolling fatigue life is increased. Cause a decline. Therefore, the Mn content is set to 0.2 to 1.5%. A preferable range is 0.2 to 1.0%, and a more preferable range is 0.2 to 0.8%.
[0020]
Cr: 0.6-2.0%
Cr improves the hardenability of steel, and at the same time, is an element that is easily concentrated in cementite, and hardens cementite to improve machinability. However, if the content is less than 0.6%, it is difficult to obtain the above effect. On the other hand, if it exceeds 1.6%, center segregation of not only Cr but also C element will occur, and if it exceeds 2.0%, center segregation of Cr and C will become prominent, leading to a decrease in rolling fatigue life. . Therefore, the Cr content is set to 0.6 to 2.0%. A preferable range is 0.6 to 1.6%, and a more preferable range is 0.6 to 1.3%.
[0021]
Al: 0.05% or less
Al is added as a deoxidizer, but excess Al forms non-metallic inclusions and reduces the rolling fatigue life. In particular, if its content exceeds 0.05%, coarse non-metallic inclusions are formed and the rolling fatigue life is significantly reduced. The desired rolling fatigue life (rolling fatigue in the examples described later) 1.0 × 10 in the test 7 More than the rolling fatigue life). Therefore, the Al content is set to 0.05% or less. A preferable upper limit is 0.04%, and a more preferable upper limit is 0.03%. On the other hand, in order to obtain a sufficient deoxidation effect, the content is preferably 0.0003% or more. Note that Al is not necessarily added when deoxidation is sufficiently performed by the above-described Si, and the content thereof may be an impurity amount level.
[0022]
Cu: 2.0% or less (the desirable range at the time of addition is 0.05 to 2.0%)
Cu may not be added. Addition has the effect of increasing corrosion resistance. In order to reliably obtain this effect, the Cu content is preferably 0.05% or more. However, if the content exceeds 2.0%, segregation occurs at the grain boundaries, and cracks and wrinkles occur during hot working such as ingot rolling of steel ingots and hot rolling of wire rods. Therefore, the Cu content is set to 2.0% or less. A preferable upper limit is 1.5%, and a more preferable upper limit is 1.0%.
[0023]
Ni: 4.0% or less (desirable range when added is 0.2 to 4.0%)
Ni need not be added. If added, it has the effect of increasing the toughness by solid solution in the martensite after quenching. In order to reliably obtain this effect, the Ni content is preferably 0.2% or more. However, even if the content exceeds 4.0%, the above effect is saturated and the cost is increased. Therefore, the Ni content is set to 4.0% or less. A preferable upper limit is 3.0%, and a more preferable upper limit is 2.0%.
[0024]
Mo: 0.5% or less (desirable range at the time of addition is 0.05 to 0.5%)
Mo may not be added. If added, it has the effect of increasing hardenability and increasing the rolling fatigue life. In order to reliably obtain this effect, the Mo content is preferably 0.05% or more. However, if its content exceeds 0.5%, the hardenability becomes too high, and martensite is likely to be generated after hot rolling, and cracks are likely to occur. Therefore, the Mo content is set to 0.5% or less. A preferable upper limit is 0.3%, and a more preferable upper limit is 0.2%.
[0025]
V: 0.2% or less (desired range at the time of addition is 0.03-0.2%)
V may not be added. If added, the austenite crystal grains are refined and the toughness is increased. To obtain this effect with certainty, it is preferable that V is 0.03% or more. However, even if the content exceeds 0.2%, the above effects are saturated and the cost is increased. Therefore, the content of V is set to 0.2% or less. A preferable upper limit is 0.1%.
[0026]
Nb: 0.10% or less (desirable range at the time of addition is 0.01 to 0.10%)
Nb may not be added. If added, the austenite crystal grains are refined and the toughness is increased. In order to reliably obtain this effect, the Nb content is preferably 0.01% or more. However, even if the content exceeds 0.10%, the above effects are saturated and the cost is increased. Therefore, the Nb content is set to 0.10% or less. A preferable upper limit is 0.08%, and a more preferable upper limit is 0.05%.
[0027]
Ca: 0.003% or less
Ca need not be added. If added, it has the effect of improving hot workability. In order to reliably obtain this effect, the Ca content is preferably 0.0001% or more. However, even if Ca is contained in excess of 0.003%, the above effect is saturated and the cost is increased. Therefore, the Ca content is set to 0.003% or less. A preferable upper limit is 0.002%.
[0028]
Mg: 0.003% or less
Mg may not be added. If added, it has the effect of improving hot workability. In order to reliably obtain this effect, the Mg content is preferably 0.0001% or more. However, even if Mg is contained in excess of 0.003%, the above effects are saturated and the cost is increased. Therefore, the content of Mg is set to 0.003% or less. A preferable upper limit is 0.002%.
[0029]
In the present invention, it is important to limit the contents of Ti, P, S, N and O (oxygen) as impurities as follows. The reason is as follows.
[0030]
Ti: 0.002% or less
Ti combines with N to form TiN, reducing the rolling fatigue life. In particular, when its content exceeds 0.002%, the rolling fatigue life is significantly lowered, and the desired rolling fatigue life (1.0 × 10 in the rolling fatigue test in the examples described later). 7 More than the rolling fatigue life). Therefore, the Ti content is set to 0.002% or less. Note that it is desirable to reduce the content of Ti as an impurity as much as possible.
[0031]
P: 0.02% or less
P segregates at the grain boundaries and reduces the rolling fatigue life. In particular, when the content exceeds 0.02%, the rolling fatigue life is significantly reduced, and a desired rolling fatigue life (1.0 × 10 in rolling fatigue test in Examples described later). 7 More than the rolling fatigue life). Therefore, the content of P is set to 0.02% or less.
[0032]
S: 0.015% or less
S combines with Mn to form MnS, reducing the rolling fatigue life. In particular, when the content exceeds 0.015%, coarse MnS is easily formed, so that the rolling fatigue life is significantly reduced, and the desired rolling fatigue life (in the rolling fatigue test in Examples described later, 1.0 × 10 7 More than the rolling fatigue life). Therefore, the content of S is set to 0.015% or less.
[0033]
N: 0.009% or less
N easily bonds to Ti or Al to form TiN or AlN. When the N content increases and coarse TiN or AlN is formed, the rolling fatigue life is reduced. In particular, when the content exceeds 0.009%, the rolling fatigue life is remarkably lowered, and a desired rolling fatigue life (1.0 × 10 × in a rolling fatigue test in Examples described later). 7 More than the rolling fatigue life). Therefore, the N content is set to 0.009% or less.
[0034]
O (oxygen): 0.0015% or less
O forms oxide inclusions and decreases the rolling fatigue life. In particular, when the content exceeds 0.0015%, the rolling fatigue life is remarkably lowered, and a desired rolling fatigue life (1.0 × 10 × in a rolling fatigue test in Examples described later). 7 More than the rolling fatigue life). Therefore, the content of O is set to 0.0015% or less. In addition, it is desirable to reduce the O content as an impurity as much as possible.
(B) Relationship between Si, Mn, Cr and Mo
In the present invention, the content of each of Si, Mn and Cr is within the range defined by (A), and the formula “5.0 ≦ 1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 × % Mo ≦ 9.0 ”. This is determined by the formula “1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 ×% Mo” as shown in the following examples. Therefore, the hardness after quenching is low, and the rolling fatigue life is remarkably lowered. The desired rolling fatigue life (1.0 × 10 in the rolling fatigue test in Examples described later) is obtained. 7 The above rolling fatigue life) cannot be obtained. On the other hand, if the value obtained by the formula “1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 ×% Mo” exceeds 9.0, the hardenability is excessive, and after hot rolling A martensite structure is formed during cooling, and cracks during handling and cracking are likely to occur. Therefore, each content of Si, Mn, and Cr is set to satisfy the formula “5.0 ≦ 1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 ×% Mo ≦ 9.0”.
[0035]
Here, the above formula “1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 ×% Mo” represents the hardenability of the steel, and the coefficient of each element based on the following experimental results. Is a formula determined by the present inventor for the first time by using the least square method.
[0036]
That is, generally, the hardenability of steel can be obtained by calculation from the chemical composition and grain size of the steel, as defined in, for example, the AISI standard. However, in the case of steel with high carbon as in the present invention and Mn and Cr concentrated in cementite, since the Mn and Cr concentrations in the matrix are reduced, the calculation results and actual hardenability Does not match. For this reason, the following experiment was conducted, and the above formula was determined as a parameter formula that accurately represents the hardenability of the steel of the present invention that is high carbon and has Mn and Cr concentrated in cementite.
[0037]
Experiment contents:
Using each steel shown in Table 1 which will be described later, a test piece was taken from a steel bar having an outer diameter of 40 mm which was subjected to spheroidizing annealing under the same conditions as “Condition Y” shown in Table 2 which will be described later, and specified in JIS G 0561. The Jominy test was conducted according to the method. At that time, the heating condition of the test piece was held at 820 ° C. for 20 minutes, which is a general quenching temperature of the bearing component. Next, the distance from the end of the test piece on the refrigerant supply side to the position showing Rockwell C hardness 50 was determined for each steel. And from the distance of each obtained steel and the contents of the four elements of Si, Mn, Cr and Mo, which are said to have a great influence on the hardenability of the steel in each element contained in the steel. Assuming that the effect of each element can be added, the coefficient of each element was obtained by the least square method. As a result, the coefficient of each element was as shown in the above formula.
(C) Total concentration of Cr and Mn in cementite
In the present invention, the total concentration of Cr and Mn in cementite (hereinafter referred to as Cr + Mn concentration) is specified to be 5.0% or more. This is because, as shown in the examples described later, when the Cr + Mn concentration in the cementite is less than 5%, since the cementite is not sufficiently cured, the tool life in the cutting test was manufactured by a general method. This is because the tool life at the time of steel cutting is not 1.3 times or more. Furthermore, when the Cr + Mn concentration in the cementite is 6.0% or more, the tool life is 1.5 times or more of the tool life at the time of cutting a steel material manufactured by a general method, so the Cr + Mn concentration in the cementite is It is desirable to be 6.0% or more. On the other hand, the upper limit of the Cr + Mn concentration in the cementite is not particularly limited, but if the Cr + Mn concentration in the cementite exceeds 15%, there is a possibility of becoming a carbide other than cementite. Therefore, the upper limit is desirably 15% or less.
[0038]
In order to increase the Cr + Mn concentration in the cementite to 5.0% or more, the Cr and Mn contents in the steel are increased, and the matrix is transformed from austenite to ferrite during spheroidizing annealing, and then slowly cooled or retained. It is effective to do.
[0039]
For example, as shown in a later example, steel B (0.90% C-0.50% Mn-0.82% Cr) is held at 770 ° C for 2 hours, and then up to 680 ° C at 10 ° C / hour. When cooled and then allowed to cool outside the furnace, the Cr + Mn concentration in the cementite becomes 4.2%. Further, after holding at 770 ° C. for 2 hours, cooling to 700 ° C. at 10 ° C./hour, cooling to 650 ° C. at 5 ° C./hour, and then allowing to cool outside the furnace results in 5.8%. In addition, since the diffusion of Cr and Mn is extremely slow below 650 ° C., the cooling rate below 650 ° C. hardly affects the Cr and Mn concentration in cementite.
[0040]
The measurement of Cr and Mn concentration in cementite was determined by the following method. That is, the cementite is extracted by the extraction replica method, and the concentration of C and Mn in five cementites is measured for each sample by the energy dispersive X-ray analysis (EDS) attached to the transmission electron microscope (TEM). Each average value was made into the density | concentration of Cr and Mn in the cementite of the sample.
[0041]
The particle size of cementite is not particularly specified, but when the average particle size is less than 0.4 μm, the strength of the steel material increases, the cutting resistance in cutting tends to increase, and the tool life tends to be shortened. Becomes larger. For this reason, it is preferable that the average particle diameter of cementite is 0.4 micrometer or more. Although the upper limit is not particularly specified, if the average particle size of cementite exceeds 0.8 μm, the effect of improving the tool life tends to be saturated, and the time required for spheroidizing annealing becomes longer and the productivity is increased. In order to decrease, the average particle diameter of cementite is preferably 0.8 μm or less.
[0042]
The average particle diameter of cementite is defined as follows. That is, the area of each cementite grain is obtained, the diameter of a circle having an area equivalent to that area is obtained, and this is used as the apparent grain diameter of each cementite grain. Next, the average value of the apparent particle sizes of all cementite grains whose areas were measured was defined as the apparent cementite average particle size, and the average particle size of the above apparent cementite multiplied by 1.12 was defined as the cementite average particle size. To do.
[0043]
The steel material (steel wire rod, steel bar or steel pipe) of the present invention comprising the constituent elements described in the above (A), (B) and (C) is subjected to processing such as cold forging, cold drawing and cutting by a usual method. After being subjected to heat treatment by quenching and tempering at a low temperature, it is finished into a bearing element part having desired mechanical properties, and then assembled into a bearing as a final product which is a precision mechanical part.
[0044]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0045]
【Example】
Eighteen types of steel having chemical compositions shown in Table 1 were melted using a vacuum melting furnace with a capacity of 300 kg. In Table 1, steels with the symbols B to I and O to R have chemical compositions within the range defined by the present invention, and steels with the symbols A and J to N have any of the components defined by the present invention. It is a comparative example outside the range to be.
[0046]
[Table 1]
[0047]
Forgings made of steels of symbols A to F and I to R in which cracks were not observed were cut to a length of 600 mm, and then subjected to four heat treatment conditions (heat pattern) shown in Table 2 using an electric furnace. ) Was subjected to spheroidizing annealing. The condition W in Table 2 corresponds to the heat pattern of the conventional annealing process, and the required time to cool outside the furnace is 11 hours. The time required for the other conditions is 18 hours for condition X, 14 hours for condition Y, and 10 hours for condition Z.
[0048]
[Table 2]
[0049]
In other words, the Cr + Mn concentration was 5 for each sample by mirror-polishing the cross section of each round bar, extracting the cementite by the extraction replica method, and energy dispersive X-ray analysis (EDS) attached to the transmission electron microscope (TEM). The concentrations of C and Mn in each cementite were measured, and the average value was defined as the concentration of Cr and Mn in the cementite of the sample, and the total was obtained.
[0050]
The average particle diameter of cementite is obtained by corroding the cross section of each round bar mirror-polished in the same manner as described above with Picral, and using a scanning electron microscope, the corroded surface is ½ of the radius from the center of each sample. The field was photographed at 10 magnifications at a magnification of 5000 times, and the area of each cementite grain was determined by image analysis according to a normal method, and the diameter obtained by calculating a circle with an equivalent area was taken as the diameter of each cementite grain. A value obtained by multiplying the average value of the diameters by 1.12 was defined as the average particle diameter of cementite.
[0051]
In addition, a cutting test was performed on each round bar having a diameter of 38 mm, which was ground and finished after spheroidizing annealing. That is, a triangular tip made of SKH4 specified in JIS G 4403 is used as a cutting tool, and there is no lubrication, the peripheral speed is 50 m / min, the cutting depth is 0.7 mm, the feed is 0.2 mm / rev. The tool life was investigated by turning under the above conditions and used as an index of machinability.
[0052]
The tool life is determined by measuring the wear amount of the main cutting edge of the tool every 30 seconds until the cutting time is 10 minutes and thereafter every minute after cutting the round bar under the above conditions. The time point when the amount of wear reached 0.20 mm. The machinability target was to satisfy the following condition (a).
(A) For each steel, the tool life is 30% or more longer than this, based on the tool life of the round bar annealed under condition W.
[0053]
Further, a test piece having a diameter of 12 mm and a length of 22 mm was cut out from each round bar subjected to the above-described cutting process, and this test piece was subjected to quenching and tempering treatment (holding at 820 ° C. × 30 minutes, oil quenching → 160 ° C. × 1 hour holding) And subjected to a rolling fatigue test under the following conditions.
[0054]
That is, using a cylindrical rolling fatigue tester, using a # 68 turbine oil specified in JIS standards as a lubricating oil, the Hertz maximum contact stress is 588 MPa, and the number of test piece loads is 46000 times / minute. A rolling fatigue test was conducted. Ten test pieces were used for each steel, and the number of rotations when surface peeling was first performed among the ten test pieces was defined as “rolling fatigue life”. Rolling fatigue life is 1.0 × 10 7 In these cases, it was evaluated that the rolling fatigue characteristics were excellent.
[0055]
Tables 3 and 4 collectively show the results of investigations on the Cr + Mn concentration in cementite after spheroidizing annealing, the average particle size of cementite, the tool life in cutting with a lathe, and the rolling fatigue life.
[0056]
[Table 3]
[0057]
Among the above, the trial numbers 1, 4, 25, 28, 29, 32, 33, 36, 41 and 44 to 46 have a tool life of the target value because the Cr + Mn concentration in the cementite is less than 5.0%. Not reached.
[0058]
Moreover, as for the trial numbers 5, 8, 9, 12, 13, 16, 17, 20, 21, 24, 49, 52, 53, 56, 57, 60, 61 and 64, the chemical composition of steel is defined by the present invention. However, since the Cr + Mn concentration in the cementite is less than 5.0%, the tool life does not reach the target value.
[0059]
On the other hand, trial numbers 6, 7, 10, 11, 14, 15, 18, 19, 22, 23, 50, 51 using the symbols B to I and O to R steel satisfying the conditions defined in the present invention. , 54, 55, 58, 59, 62 and 63, the tool life has reached the target value, and the rolling fatigue life is also the target of 1.0 × 10. 7 It exceeds the number of times.
[0060]
In particular, the tool life of the test numbers 11, 22, 23, 50, 51, 54, 55, 57, 60, 61 and 64 in which Cr + Mn concentration in the cementite is 6.0% or more is obtained by spheroidizing annealing under the condition W It is 50% or more long compared to, which is even better.
[0061]
【The invention's effect】
The steel for bearing element parts of the present invention is excellent in machinability and has a long rolling fatigue life. For this reason, the life of bearing element parts such as balls, rollers, needles, shafts, and races can be extended.
Claims (1)
５．０≦１．６×％Ｓｉ＋４．０×％Ｍｎ＋３．０×％Ｃｒ＋５．０×％Ｍｏ≦９．０ ・・・（１）In mass%, C: 0.8 to 1.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.5%, Cr: 0.6 to 2.0%, Al: 0.05% or less, Cu: 2.0% or less, Ni: 4.0% or less, Mo: 0.5% or less, V: 0.2% or less, Nb: 0.10% or less, Ca: 0.00%. 003% or less, Mg: 0.003% or less, the balance is Fe and impurities, Ti in the impurity is 0.002% or less, P is 0.02% or less, S is 0.015% or less, N is 0.009% or less, O (oxygen) is 0.0015% or less, the relationship among Si, Mn, Cr and Mo satisfies the following formula (1), and the total concentration of Cr and Mn in cementite is 5 Steel material for bearing element parts having excellent machinability, characterized by being 0.0% or more.
5.0 ≦ 1.6 ×% Si + 4.0 ×% Mn + 3.0 ×% Cr + 5.0 ×% Mo ≦ 9.0 (1)
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| JP2005067430A (en) * | 2003-08-26 | 2005-03-17 | Nsk Ltd | Rolling bearing unit for supporting wheel |
| JP2005256897A (en) | 2004-03-10 | 2005-09-22 | Ntn Corp | Machine element and its manufacturing method |
| KR100832960B1 (en) | 2006-12-29 | 2008-05-27 | 주식회사 포스코 | The method for manufacturing the high carbon chromium bearing steel |
| JP2009242917A (en) * | 2008-03-31 | 2009-10-22 | Jfe Steel Corp | Heat-treatment method for bearing steel |
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| CN111763889A (en) * | 2020-06-02 | 2020-10-13 | 钢铁研究总院 | High-carbon bearing steel and preparation method thereof |
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