JP2005029825A

JP2005029825A - Method for producing connecting rod and connecting rod

Info

Publication number: JP2005029825A
Application number: JP2003195145A
Authority: JP
Inventors: Koichiro Inoue; 幸一郎井上; Shohei Shintani; 昌平新谷
Original assignee: Daido Steel Co Ltd; Fuji Heavy Industries Ltd
Current assignee: Daido Steel Co Ltd; Subaru Corp
Priority date: 2003-07-10
Filing date: 2003-07-10
Publication date: 2005-02-03

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a connecting rod with which higher proof stress can be obtained by further optimizing the forming state of V-base precipitation while using a non-heat treated steel for hot forging, containing V and further, effectively making good use of the added V. <P>SOLUTION: The hot-forged non-heat treated steel formed as precipitation-strengthened state based on the V-base precipitation in the final blank state, is used for steel material constituting the connecting rod. This steel material is hot-forged in the temperature zone from ≥1,100°C to ≤1,300°C to obtain the forged body having the basic shape of connecting rod. Successively, the forged body is intermediately cooled so that the average cooling speed in a first temperature zone from 800°C to 500°C becomes ≥1°C/sec. Then, after completing the intermediate-cooling process, aging precipitation of the V-base precipitation in the forged body is carried out in a second temperature zone from ≥500°C to ≤700°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、Ｖ炭窒化物等のＶ系析出物により析出強化した非調質鋼からなるコンロッドの製造方法と、それにより得られるコンロッドに関する。
【０００２】
【従来の技術】
【特許文献１】
特開２００３−２７１７８号公報
【特許文献２】
特開２００２−３５６７４３号公報
【特許文献３】
特開２００２−２７５５７８号公報
【特許文献４】
特開２００２−２５６３９４号公報
【特許文献５】
特開平１１−１９９９２４号公報
【特許文献６】
特開平９−１７６７８７号公報
【特許文献７】
特開平９−３５８９号公報
【特許文献８】
特開平８−２９１３７３号公報
【特許文献９】
特開平５−３３１５３６号公報
【特許文献１０】
特開平９−３１０１４６号公報
【特許文献１１】
特開２００２−３１６２３１号公報
【特許文献１２】
特開平７−２３３４３５号公報
【０００３】
従来、自動車エンジン用等に使用されるコンロッドは、特許文献１〜特許文献８に開示されているごとく、鍛造後、焼入れ焼き戻しを省略しても目的とした硬さが得られる熱間鍛造用非調質鋼により製造されている。熱間鍛造用非調質鋼としては、中炭素鋼に微量のＶを添加することにより、熱間鍛造後の冷却時にＶ炭窒化物等からなる析出物を鋼中に微細に析出させ、目的とする硬さを得るようにしている（特許文献１〜１１）。該熱間鍛造用非調質鋼は、鍛造後の組織がフェライト・パーライト組織とになる。フェライト・パーライト組織は、軟質相である初析フェライト部の強度が弱いため、ソルバイト単一組織となる調質鋼に比べると、同程度の硬さレベルであっても、耐力や疲労強度は相対的に不足する傾向にある。エンジン部品であるコンロッドは高耐力ひいては高疲労強度が要求される部品であるが、上記従来の非調質鋼により調質鋼と同じ耐力を得るには硬さを相当高くしななければならず、そうすると材料の被削性が大きく低下し、生産性が大幅に低下してしまう問題があった。
【０００４】
そこで、上記問題を解決するために、特許文献１２には、Ｖ含有量を従来の非調質鋼よりも高め、初析フェライト部へのＶ系析出部の析出量を高めてより高耐力化を図った非調質鋼が提案されている。
【０００５】
また、このようにＶ、Ｐ、Ｓｉを多量に含有する熱間鍛造用非調質鋼は高強度化のみならず、やわらかくて靭性の高い初析フェライト部が強化されて靭性が適度に低下しているため、これを利用して「かち割りコンロッド」と通称される連結部構造が採用されている（特許文献１〜８）。クランクシャフトが連結されるコンロッドの一方の連結部は、通常、コンロッド側の受け部と、これと分離されたキャップ部とのキャップ部との２部分からなるが、かち割りコンロッドでは、この２部分を熱間鍛造時点では一体の予備部として形成しておき、該予備部に適当な切り欠きを付与して衝撃力を加えることにより、キャップ部と受け部との２部分に破断分離する（つまり、「かち割る」）。このため、従来の材質では、連結部を一体成形するにしても、その分離は機械加工によって切断する必要があったが、かち割りコンロッドでは分離処理が衝撃力付与により一瞬で終わるためコストメリットが大きく、しかも、脆性破壊的な破面の嵌合もよく、連結部の機械的精度の確保も容易である。このため、近年では上記かち割りコンロッドの適用が拡大しつつある。
【０００６】
ところで、Ｖ含有量を増加させた非調質鋼は、鍛造後の冷却中に通常の非調質鋼よりも多量のＶ系析出物（例えばＶ炭窒化物）が析出することにより、高耐力あるいは良好な破断分離特性が実現する。しかし、こうしたＶ含有非調質鋼は従来、鍛造が終了した後は空冷などにより室温まで単純に冷却されるだけであり（例えば特許文献９）、Ｖ系析出物が十分に形成される前に室温付近まで冷却される結果、添加したＶの全てが析出物形成に必ずしも有効に活用されず、過飽和固溶のまま室温まで持ちきたされる無駄なＶ成分が多くなってしまう難点があった。このような過飽和固溶Ｖ成分は、鋼中の総Ｖ添加量が高くなればより増加する傾向にあり、せっかく多量に添加した高価なＶの多くが、期待した高耐力化あるいは破断分離特性向上に結びつくことなく、無駄に消費されてしまうというのが実状であった。そこで、特許文献１１及び１２では、図３に示すように、熱間鍛造後に６００〜８００℃の温度まで冷却した後、そのまま５００〜７００℃の炉内に３０〜６０ｍｉｎ保持する時効熱処理を行ってＶ系析出物の形成を促進し、添加したＶをより有効に活用して高耐力化を図る手法が提案されている。
【０００７】
【発明が解決しようとする課題】
Ｖ系析出物による耐力向上効果は、Ｖ系析出物の量だけではなく、Ｖ析出物の寸法や分布状態によっても大きな影響を受ける。耐力向上効果は、Ｖ系析出物の分布状態が高密度であるほど（つまり、単位堆積あたりの析出物の形成個数（数密度）が多いほど）高められる。他方、析出物が同じ数密度で形成されている場合、図４に示すように、析出物の寸法がある臨界寸法に到達するまでは、材料の耐力は析出物寸法とともに増加するが、ある臨界寸法を超えると耐力は逆に低下する。これは、析出物が小さい間は、析出物の結晶がマトリックスの結晶と格子整合し、転位移動を妨げる整合歪が析出物寸法の増大とともに大きくなるが、臨界寸法を超えると、析出物とマトリックスとの界面にミスフィット転位が導入されて整合歪が開放されてしまうからである（整合歪が開放される寸法にまで析出物成長が進行する時効熱処理状態を「過時効」と称している）。
【０００８】
前述の特許文献１１及び特許文献１２に開示されている非調質鋼の製造方法では、Ｖ系析出物の量は確かに増加させることができるが、析出物の数密度が不足しがちであり、また、上記の過時効状態に陥りやすく、時効熱処理による耐力向上効果が必ずしも顕著でない欠点がある。
【０００９】
本発明の課題は、Ｖを含有する熱間鍛造用非調質鋼を用いつつ、Ｖ系析出物の形成状態をより最適化し、ひいては添加したＶを有効活用することにより、さらに高耐力化を図ることができるコンロッドの製造方法と、それにより得られるコンロッドとを提供することにある。
【００１０】
【課題を解決するための手段及び作用・効果】
上記の課題を解決するために本発明のコンロッドの製造方法は、
最終的な素材状態がＶ系析出物に基づく析出強化状態とされる熱間鍛造非調質鋼を、コンロッドを構成する鋼材として用い、
鋼材を１１００℃以上１３００℃以下の温度域で熱間鍛造することにより、コンロッドの母形状を有する鍛造体を得る熱間鍛造工程と、
該熱間鍛造工程が終了後、鍛造体を８００℃から５００℃までの第一温度域の平均冷却速度が１℃／秒以上となるように中間冷却する中間冷却工程と、
該中間冷却工程が終了後、鍛造体を５００℃以上７００℃以下の第二温度域でＶ系析出物を時効析出させる時効熱処理工程と、
を有することを特徴とする。
【００１１】
Ｖ系析出物により析出強化を行う熱間鍛造非調質鋼において、析出強化に有効なＶ系析出物を現実的な時間の範囲で顕著に形成できる時効熱処理の温度範囲は５００℃以上８００℃以下（第一温度域）である。本発明のコンロッドの製造方法においては、熱間鍛造後、該第一温度域を１℃／秒以上の冷却速度で一旦急冷通過させ（中間冷却処理）、その後再び５００℃以上７００℃以下の第二温度域に加熱して時効熱処理を行なうようにした。特許文献１１及び１２に開示されている従来の工程では、鍛造後６００〜８００℃まで冷却し、その後５００〜７００℃の炉内に保持するような工程になっており、熱間鍛造温度から第一温度域にいわば直接移行させるため、析出物が顕著に形成される第一温度域での冷却速度が遅くなり、Ｖを固溶したマトリックスの過冷度が不十分となる。図５に示すように、析出物の核生成数は過冷度が大きいほど増加するので、過冷度が小さいと、図７に示すようにＶ系析出物の核発生が不十分となり、そのまま時効熱処理しても、析出物サイズは大きくなるが数密度が低くなる。従って、析出量は増加しても耐力向上にはあまり寄与しなくなる。この場合、時効熱処理時間を延長しても、数密度が小さいまま個々の析出物が成長するだけなので、過時効状態を招きやすくなり、耐力の顕著な向上はますます望めなくなる。
【００１２】
これに対し本発明の方法によると、図２に示すように、熱間鍛造後に上記第一温度域を１℃／秒以上の冷却速度で中間冷却処理することによりマトリックスの過冷度が高められ、図６に示すようにＶ系析出物の核発生が促進される。そして、該中間冷却処理により十分な数の析出核を発生させた後、第二温度域で時効熱処理を行うことにより、Ｖ系析出物の数密度を増加させつつ、その数密度を維持したまま個々の析出物のサイズを、整合ひずみが開放されない範囲で十分に大きくすることができる。すなわち、時効熱処理により添加したＶを無駄なくＶ系析出物の形成に活用でき、かつ、中間冷却処理による核生成促進により析出物の数密度も十分に高めることができるので、極めて高耐力のコンロッドを得ることができる。
【００１３】
また、本発明のコンロッドは、最終的な素材状態がＶ系析出物に基づく析出強化状態とされる熱間鍛造非調質鋼にて構成されたコンロッドであって、コンロッドを構成する鋼材が、Ｖの含有率をＷＶ（質量％）、引張応力をσＵＴＳ、０．２％耐力をσ０．２として、σ０．２／σＵＴＳにて表される耐力比Ｒｐｓが、Ｒｐｓ０＝０．６８＋０．４×ＷＶにて表される基準値Ｒｐｓ０（より望ましくは、０．７＋０．４×ＷＶにて表される基準値Ｒｐｓ０’）よりも大きいことを特徴とする。該本発明のコンロッドは、上記本発明の製造方法により、添加したＶ量に応じて、当該Ｖ添加量において従来のいかなる非調質鋼系コンロッドも至り得なかった、上記高耐力比を有するものとして、はじめて実現可能となったものである。これにより、自動車用等の高負荷環境で使用されるコンロッドとして、疲労強度等に格段に優れた製品が得られるようになる。
【００１４】
Ｖの析出制御により分布状態を最適化するためには、鍛造時においてＶがマトリックスに固溶した状態であることが必要である。鍛造温度が１１００℃未満になると十分なＶの固溶状態が得られなくなり、時効熱処理時において材料強化に寄与するＶ析出物を十分に形成できなくなる。他方、鍛造温度が１３００℃を超えると材料の酸化が著しくなる。以上の理由により鍛造温度は１１００℃以上１３００℃以下に設定するのがよく、より望ましくは１１５０℃以上１２５０℃以下に設定するのがよい。
【００１５】
Ｖ系析出物の数密度を向上させるためには、鍛造後の冷却速度を大きくする必要があり、このため鍛造後、Ｖ系析出物の析出温度範囲である第一温度域（５００℃以上８００℃以下）を１℃／秒以上で冷却する必要がある。５００℃未満の温度域ではＶの拡散速度が非常に遅くなり、Ｖ系析出物がほとんど生じなくなるので、冷却速度は５００℃まで１℃／秒以上に制御する必要がある。他方、８００℃以上の温度域ではＶはマトリックスに固溶したままであり、冷却速度を大きくする意味があまりない。したがって、冷却速度を制御する第一温度域は５００℃以上８００℃以下に設定する。なお、第一温度域を通過後の冷却条件については特に制限はなく、例えば５００℃以下まで冷却した後はそのま第二温度域にて時効熱処理してもよいし、室温まで冷却した後に第二温度域まで再昇温して時効熱処理してもよい。
【００１６】
また、中間冷却工程における冷却速度の上限として、該中間冷却工程での第一温度域における冷却を、ベイナイト発生臨界冷却速度以下で行なうことが望ましい。すなわち、過剰に冷却速度を増加するとベイナイトが発生し、硬さが高くなりすぎて被削性等が大幅に劣化することにつながるためである。
【００１７】
また、使用する鋼材は、Ｃ、Ｖ、Ｓｉ、Ｍｎ、Ｃｕ、Ｎｉ、Ｃ、Ｎのうち、ＶとＣとＮとを必須とする２種以上を含有するものを使用するとよい。Ｖ以外にＣとＮとＣとを含有することにより、時効熱処理時にＶ系析出物として微細なＶ炭窒化物を析出させることができ、耐力比向上効果が特に顕著となる。この場合、ベイナイト発生臨界冷却速度ｖｃ（℃／秒）は、Ｃの含有率をＷＣ（質量％）、Ｓｉの含有率をＷＳｉ（質量％）、Ｍｎの含有率をＷＭｎ（質量％）、Ｃｕの含有率をＷＣｕ（質量％）、Ｎｉの含有率をＷＮｉ（質量％）、Ｃｒの含有率をＷＣｒ（質量％）として、
ｖｃ≡（４０９−３１９×ＷＣ）／ｅｘｐ（−１．２×ＷＣ＋０．０５×ＷＳｉ＋３．０×ＷＭｎ＋１．６×ＷＣｕ＋１．４×ＷＮｉ＋２．０×ＷＣｒ＋１．２）‥（１）
により、実験的に定まることがわかっており、該臨界冷却速度ｖｃ以上にて第一温度域を冷却することにより中間冷却処理を行なうことが、当該中間冷却処理におけるベイナイト発生を防止する上で望ましい（なお、ベイナイト発生臨界冷却速度ｖｃにＮ含有量はほとんど影響を及ぼさない）。
【００１８】
次に、時効熱処理工程は、鍛造後の冷却中に析出できず過飽和に固溶しているＶを強制的に析出させる処理である。本発明ではＶ系析出物の数密度を高めるため、鍛造後の冷却時において、前記の第一温度域での冷却速度を高めることを骨子としている。しかし、冷却後の状態では冷却速度が高いため、Ｖ系析出物が析出する温度での保持時間は当然短くなり、添加したＶの大半は過飽和固溶状態のままである。そこで、時効熱処理工程では、５００℃以上７００℃以下の第二温度域で適切な時間保持することにより、過飽和なＶをＶ系析出物として過時効にならない範囲で析出成長させ、添加したＶを最大限有効に用いるようにする。時効熱処理が５００℃未満ではＶ拡散速度が小さいため、Ｖ系析出物の形成を促進できない。また、７００℃を超えるとＶ系析出物の析出成長速度が大きくなりすぎ、過時効を招きやすくなる。
【００１９】
時効熱処理工程は、上記第二温度域内に設定された一定の保持温度で鍛造体を等温保持することにより行なうことが望ましい。時効熱処理工程における処理時間が過度に長くなると、前述した過時効により、析出物の形成量は増加するが析出物サイズが過剰に大きくなって、却って耐力比が低下することにつながる。この場合、時効熱処理工程を等温保持処理とすれば、過時効を生じないための保持時間の管理等がきわめて容易となる。析出物の形成にはＶの拡散が必要であり、Ｖ含有量と保持温度に応じて、過時効とならないための適正な保持時間は異なるものとなる。具体的には、鋼材のＶの含有率をＷＶ（質量％）、等温保持の保持温度をΘ（℃）、保持時間をｔ（秒）として、
３×１０^−８≧（ｔ×ＷＶ×Ｄ）^１／２‥（２）
Ｄ＝３．１×１０^−４×ｅｘｐ（−２３９０００／（８．３１４×（Θ＋２７３））‥（３）
を充足するように、保持温度Θ（℃）と保持時間ｔ（秒）とを定めることが望ましい。（３）式は温度ΘにおけるＦｅ中のＶの拡散係数であり、（２）式において等号が成り立つときの保持時間ｔを限界保持時間ｔ０としたとき、保持時間ｔが該限界保持時間ｔ０を超えると過時効となって、耐力比の十分な向上が望めなくなる。
【００２０】
また、本発明の適用対象として、前述のかち割りコンロッドが好適である。図１はかち割りコンロッド１の一例を示すもので、両端部にコンロッドによる連結対象物の連結部３，４が形成されている。それら連結部３，４の少なくとも一方、本実施形態ではクランクシャフトが接続される大端部側をなす連結部４が、一体鍛造成形された予備体を破断することにより、コンロッド本体２側に一体的に残留する第一部分４ａと、該第一部分４ａから分離された第二部分４ｂとに分割形成されてなる。該連結部４は、該第一部分４ａと第二部分４ｂとの間に連結対象物を挟み込み、それら第一部分と第二部分とを破断面同士を密着させた形で、ボルトナット等の締結部材６，６により結合される分離連結部とされている。このようなかち割りコンロッドへの適用により、コンロッドの耐力比向上とともに、予備体の破断・分離工程がより容易となる利点も新たに生ずる。また、破断時においてカップコーン的な延性破断挙動が生じにくくなり、ひいては分離面（破断面）ＳＰの嵌合精度を高めることができる。予備体の破断は、予備体に形成された軸挿通用の貫通孔４ｈの内周面に切欠５，５を形成しておき、該貫通孔４ｈにこれよりも径大の割型７，７を嵌め込んで、楔８等を介して油圧プレス機等により割型７，７を貫通孔４ｈに圧入することにより、切欠５，５を起点として破断面ＳＰを形成することができる。なお、該かち割りコンロッドの基本構成自体は周知であるため、簡略な説明に留めた。
【００２１】
かち割りコンロッドに適した熱間鍛造非調質鋼としては、Ｖの他に、ＰとＳｉとを含有する鋼材を採用することが望ましい。熱間鍛造用非調質鋼においては、初析フェライト部が比較的硬度が低く靭性も大きいが、適量のＰとＳｉとを含有させておくと、初析フェライト部が強化されて靭性が適度に下がり、衝撃力による破断加工が容易となる上、カップコーン的な延性破壊も抑制される。具体的にな鋼材組成としては、
Ｆｅの含有率が９５質量％以上であり、
Ｃ：０．１５質量％以上０．４５質量％以下；
Ｓｉ：０．１質量％以上１．５質量％以下；
Ｍｎ：０．５質量％以上１．５質量％以下；
Ｐ：０．０３質量％以上０．１５質量％以下；
Ｃｕ：０．０１質量％以上０．５質量％以下；
Ｎｉ：０．０１質量％以上０．５質量％以下；
Ｃｒ：０．０１質量％以上１質量％以下；
Ｖ：０．１質量％以上０．４質量％以下；及び
Ｎ：０．００５質量％以上０．０３５質量％以下
を含有するものを使用することができる。
【００２２】
組成限定理由は以下の通りである。
（１）Ｃ：０．１５質量％以上０．４５質量％以下
Ｃは強度を確保するために必要な元素であり、Ｖ系炭窒化物の必須元素である。このため、０．１５質量％以上の添加が必要である。一方、０．４５質量％を超えて添加すると熱間鍛造後の硬さが過剰となり、被削性が低下するので０．４５質量％以下にする必要がある。
【００２３】
（２）Ｓｉ：０．１質量％以上１．５質量％以下
Ｓｉは鋼溶製時において脱酸作用を有しているとともに、破断分離時の塑性変形の主な原因である軟質相であるフェライトの強度を向上させて、耐力や疲労強度を向上させるとともに、破断分離時の変形（真円度変化）を抑制し、破断面の密着性を向上させる効果がある。このような効果を得るためにＳｉは０．１質量％以上含有させることが必要である。しかし、含有量が多すぎると不必要に硬さを増加させ被削性を劣化させるので１．５質量％以下とすることが必要である。
【００２４】
（３）Ｍｎ：０．５質量％以上１．８質量％以下、Ｃｒ：０．０１質量％以上１質量％以下
Ｍｎ及びＣｒは鍛造材の強度を高めるとともに焼入れ性を向上させ、炭素含有量が高い場合には、レーザー加工した切欠き底にもろい熱影響層を生成させ破断分離を容易にする。しかしながら多量に添加すると鍛造後にベイナイトが生成し、硬さが著しく増加して被削性を低下させるためそれぞれの範囲をＭｎ：０．５質量％〜１．８質量％、Ｃｒ：０．０１〜１質量％とした。
【００２５】
（４）Ｐ：０．０３質量％以上０．１５質量％以下
Ｐは不可避な不純物であるが、粒界への偏析により靭性を低下させる元素としてできるだけ低く抑えられるのが一般的である。しかしながら破断分離を行うかち割りコンロッドにおいては、破断時の変形を抑制し、破断面の密着性を向上させる元素として非常に有効に作用するため、０．０３質量％以上に積極添加を行っている。また、ＰはＳｉと同様にフェライト中に固溶しフェライトの強度を向上させるため耐力や疲労強度の向上に有効である。しかし、多量に添加してもその効果が飽和したり、熱間加工性を阻害するために、添加量は０．１５質量％以下に留める。
【００２６】
（５）Ｃｕ及びＮｉ：０．０１質量％以上０．５質量％以下
Ｃｕ、Ｎｉは不可避な不純物であり、０．０１質量％以下にすることは多大な努力を必要とし経済的に不利である。一方、Ｍｎ、Ｃｒと同様に強度を高めるためには有効な元素であるが多量の添加も同様に経済的に不利となるためその上限を０．５質量％以下にする。
【００２７】
（６）Ｖ：０．１質量％以上０．４質量％以下
ＶはＣ及びＮと微細な炭窒化物を生成し、鍛造後の強度を高める元素である。
特に耐力、疲労強度の向上には有効である。このような効果を得るためにも０．１質量％以上の添加が必要である。しかしながら０．４質量％以上添加しても高強度化の効果は飽和し、さらに被削性を低下させるので上限を０．４質量％にした。特にＶ添加量が０．１６質量％以上の組成を採用したとき、多量のＶが添加されているにもかかわらずこれを有効活用できる結果、耐力比が０．８以上に大幅に高められたコンロッドを実現することが可能となる。
【００２８】
（７）Ｎ：０．００５質量％以上０．０３５質量％以下
Ｎは不可避な不純物であるが、炭窒化物形成の必須元素として捉えることもできる。Ｎ含有量を０．００５質量％以下にすることは経済的に不利であり、また、多量に添加すると鋳造欠陥の原因になるため、その範囲を０．００５〜０．０３５質量％とする。
【００２９】
なお、脱酸剤としてＡｌが添加される場合は、Ｎの含有により、微細な窒化物が鋼中に分散形成され、熱間鍛造時の結晶粒粗大化を抑制する効果を発現する。この場合、Ａｌの含有量は０．００１質量％以上０．０１質量％以下とするのがよい。Ａｌを上記上限値を超えて多量に添加してもその効果が飽和するとともに、過度の結晶粒微細化作用により材料の延性が向上して破断分離後の破面の密着性を低下することにつながる。
【００３０】
また、鋼材としては、さらに、
Ｐｂ：０．００１質量％以上０．３質量％以下；
Ｂｉ：０．００１質量％以上０．３質量％以下；
Ｓ：０．００１質量％以上０．１５質量％以下；及び
Ｃａ：０．００１質量％以上０．０１質量％以下；
の１種以上を含有するものを使用することもできる。Ｐｂ、Ｂｉ、Ｓ、Ｃａはいずれも被削性を向上させるのに有効な元素であるので、鍛造品において被削性がさらに良好であることが要求される場合に、必要に応じてこれらのうちから選ばれる１種または２種以上を適量添加できる（いずれも、添加量が０．００１質量％未満では被削性向上効果が顕著でない）。なお、これら元素の添加量が多すぎると、強度や熱間加工性を低下させるので、各々上記下限値以下にて添加する必要がある。
【００３１】
また、鋼材には、
Ｔｉ：０．００１質量％以上０．０２質量％以下；
Ｚｒ：０．００１質量％以上０．０２質量％以下；及び
Ｍｇ：０．００１質量％以上０．０１質量％以下
の１種以上を含有するものを使用することもできる。Ｔｉ、Ｚｒ及びＭｇは炭窒化物を形成し鍛造後の結晶粒を微細化して強度を向上させるとともに、ＭｎＳの分布状態を微細化して、被削性を改善する効果がある（いずれも、添加量が０．００１質量％未満では被削性向上効果が顕著でない）。しかしながら多量に添加してもその効果が飽和するのでそれぞれ、各々上記上限値以下にて添加する必要がある。
【００３２】
【実施例】
以下、本発明の効果を確認するために行なった実験結果について説明する。
表１に示す組成が得られるように原料を配合し、電気炉で１５０ｋｇの鋼塊を溶製した。この鋼塊を、熱間加工シミュレータを用いて、表２に示す条件で熱間加工と等温保持を行った後、ＪＩＳ４号縮小引張試験片を加工して引張特性を調査した。その結果を表２に示す。
【００３３】
【表１】

【００３４】
【表２】

【００３５】
本発明の要件を満たす条件１〜４にて鍛造後の中間冷却及び時効熱処理を行うと、鍛造後冷却しただけの比較条件に比べて、いずれも高い耐力比が得られていることがわかる。他方、本発明の要件を満たさない条件５〜１２は、以下のような結果に終わっている。
条件５：鍛造温度が低く、加熱時のＶ固溶が不十分で耐力比低下。
条件６：冷却速度が遅いためＶ炭窒化物が粗大化し耐力比低下。
条件７：冷却速度が臨界冷却速度より大きいため、鍛造後にベイナイトが発生して耐力比低下。
条件８：冷却終了温度が高すぎるため、Ｖ炭窒化物が粗大化し耐力比低下。
条件９：等温保持温度が低すぎるために長時間保持しても耐力比の向上が見られない。
条件１０：等温保持温度が高すぎるため、等温保持中、一部オーステナイトが生成しＶ炭窒化物が再固溶、粗大化したため耐力比が大幅低下。
条件１１、１２：（２）で計算される臨界保持時間値ｔ０に対し、保持時間が長すぎるため過時効状態となり耐力比低下。
【００３６】
次に、表３に示す各種組成の鋼を同様に溶製し、これを用いて図１の形状を有するコンロッド実体の試作を実施した。このときの熱間加工条件は、加熱温度１２３０℃で鍛造した後、中間冷却処理として、８００℃から５００℃までの平均冷却速度を衝風冷却により１．８℃／秒で冷却した。その後、室温まで一旦冷却した後、５８０℃で３．６ｋｓの等温保持により時効熱処理を施した。そして、時効熱処理後は室温まで冷却し、本体部から平板引張試験を切り出して試験に供した。
【００３７】
【表３】

【００３８】
これによると、組成Ｇではベイナイト生成臨界冷却速度ｖｃが低くなりすぎ、鍛造後ベイナイトが生成して耐力比が低下しているが、それ以外の組成ではｖｃは十分に高く、良好な耐力比が得られていることがわかる。
【００３９】
また、表３のＡ〜Ｅ（及び比較例として、市販鋼材のＸＣ７０Ｓ）の各組成を用いてコンロッド実体を鍛造後、等温保持条件を表４に示すように変化させて時効熱処理を行った後機械加工し、さらにレーザー加工により予備体の貫通孔４ｈに深さ０．５ｍｍの切り欠きを施して破断分離し、貫通孔４ｈの破断分離後の真円度変化を測定した。以上の結果を表４に示す。
【００４０】
【表４】

【００４１】
組成Ｄ及びＥに対する条件設定では、臨界保持時間を超えて時効熱処理されたため、過時効状態となり耐力が低下したと考えられる。また、破断分離後の真円度変化が大きくなり破断分離特性の低下が見られる。
【図面の簡単な説明】
【図１】本発明の適用対象となるコンロッドの一例を示す説明図。
【図２】本発明のコンロッドの製造方法に係る熱間鍛造後の熱履歴を模式的に示す図。
【図３】従来のコンロッドの製造方法に係る熱間鍛造後の熱履歴を模式的に示す図。
【図４】析出物寸法と耐力との関係を模式的に示す図。
【図５】析出物数密度と過冷度との関係を模式的に示す図。
【図６】本発明の中間冷却処理と時効熱処理とを関連付けて効果説明する概念図。
【図７】従来のコンロッドの製造方法の問題点を説明する図。
【符号の説明】
１かち割りコンロッド
２コンロッド本体
３，４連結部
４ａ第一部分
４ｂ第二部分
６，６締結部材[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a connecting rod made of non-tempered steel precipitation strengthened by V-based precipitates such as V carbonitride, and a connecting rod obtained thereby.
[0002]
[Prior art]
[Patent Document 1]
JP 2003-27178 A
[Patent Document 2]
JP 2002-356743 A
[Patent Document 3]
JP 2002-275578 A
[Patent Document 4]
JP 2002-256394 A
[Patent Document 5]
JP-A-11-199924
[Patent Document 6]
JP-A-9-176787
[Patent Document 7]
JP-A-9-3589
[Patent Document 8]
JP-A-8-291373
[Patent Document 9]
JP-A-5-331536
[Patent Document 10]
JP 9-310146 A
[Patent Document 11]
JP 2002-316231 A
[Patent Document 12]
JP-A-7-233435
[0003]
Conventionally, connecting rods used for automobile engines and the like are used for hot forging, as disclosed in Patent Documents 1 to 8, which can obtain the desired hardness even after omitting quenching and tempering after forging. Manufactured from non-tempered steel. As non-heat treated steel for hot forging, by adding a small amount of V to medium carbon steel, precipitates made of V carbonitrides and the like are finely precipitated in the steel during cooling after hot forging. Is obtained (Patent Documents 1 to 11). In the non-heat treated steel for hot forging, the structure after forging becomes a ferrite pearlite structure. The ferrite-pearlite structure has a weak strength in the pro-eutectoid ferrite part, which is a soft phase, so relative strength and fatigue strength are relatively high compared to tempered steel with a single sorbite structure even at the same hardness level. Tend to run short. Connecting rods, which are engine parts, are parts that require high yield strength and therefore high fatigue strength. However, in order to obtain the same yield strength as tempered steel by the conventional non-tempered steel, the hardness must be considerably increased. Then, there is a problem that the machinability of the material is greatly lowered and the productivity is greatly lowered.
[0004]
Therefore, in order to solve the above problem, Patent Document 12 describes that the V content is higher than that of the conventional non-tempered steel, and the precipitation amount of the V-based precipitation portion in the pro-eutectoid ferrite portion is increased to further increase the yield strength. Non-tempered steel has been proposed.
[0005]
In addition, non-heat treated steel for hot forging containing a large amount of V, P, and Si as described above is not only strengthened, but the soft and tough proeutectoid ferrite part is strengthened and the toughness is moderately lowered. For this reason, a connecting portion structure commonly referred to as a “split connecting rod” is employed (Patent Documents 1 to 8). One connecting part of the connecting rod to which the crankshaft is connected is usually composed of two parts, a receiving part on the connecting rod side and a cap part of the cap part separated from the connecting part. Is formed as an integral spare part at the time of hot forging, and an appropriate notch is applied to the spare part and an impact force is applied to break and separate the cap part and the receiving part into two parts (that is, , "Cabicil". For this reason, with the conventional material, even if the connecting portion is integrally formed, the separation has to be cut by machining, but with a split connecting rod, the separation process ends in an instant by applying impact force, so there is a cost merit. In addition, it is easy to fit a brittle fracture fracture surface, and it is easy to ensure the mechanical accuracy of the connecting portion. For this reason, in recent years, the application of the split connecting rod has been expanding.
[0006]
By the way, the non-tempered steel having an increased V content has a high yield strength because a larger amount of V-based precipitates (for example, V carbonitride) precipitate than the normal non-tempered steel during cooling after forging. Alternatively, good fracture separation characteristics are realized. However, such V-containing non-tempered steel is conventionally simply cooled to room temperature by air cooling or the like after forging is completed (for example, Patent Document 9), and before V-based precipitates are sufficiently formed. As a result of cooling to near room temperature, all of the added V is not always effectively used for the formation of precipitates, and there is a problem that the useless V component brought to room temperature in a supersaturated solid solution increases. Such supersaturated solid solution V component tends to increase as the total amount of V added in steel increases, and many of the expensive V added in large amounts are expected to have higher yield strength or improved fracture separation characteristics. The actual situation was that it was consumed in vain without being tied to the. Therefore, in Patent Documents 11 and 12, as shown in FIG. 3, after cooling to 600 to 800 ° C. after hot forging, an aging heat treatment is performed for 30 to 60 minutes in a furnace at 500 to 700 ° C. There has been proposed a method of promoting the formation of V-based precipitates and more effectively utilizing the added V to increase the yield strength.
[0007]
[Problems to be solved by the invention]
The effect of improving the yield strength by the V-based precipitate is greatly influenced not only by the amount of the V-based precipitate but also by the size and distribution state of the V precipitate. The effect of improving the yield strength is enhanced as the distribution state of the V-based precipitates is higher (that is, as the number of precipitates formed per unit deposition (number density) is larger). On the other hand, if the precipitates are formed with the same number density, the yield strength of the material increases with the precipitate size until the precipitate size reaches a critical dimension, as shown in FIG. If the size is exceeded, the proof stress is reduced. This is because while the precipitates are small, the crystals of the precipitates lattice match with the crystals of the matrix, and the alignment strain that hinders dislocation migration increases with the increase in the size of the precipitates. This is because misfit dislocations are introduced at the interface with the metal and the matching strain is released (the aging heat treatment state in which the precipitate growth proceeds to the dimension at which the matching strain is released is called “overaging”). .
[0008]
In the method for producing non-heat treated steel disclosed in Patent Document 11 and Patent Document 12 described above, the amount of V-based precipitates can surely be increased, but the number density of precipitates tends to be insufficient, However, there is a drawback that the above-described overaging state tends to occur and the effect of improving the yield strength by the aging heat treatment is not necessarily remarkable.
[0009]
The object of the present invention is to further improve the proof stress by optimizing the formation state of V-based precipitates and effectively using the added V while using non-heat treated steel for hot forging containing V. The object is to provide a connecting rod manufacturing method that can be achieved, and a connecting rod obtained thereby.
[0010]
[Means for solving the problems and actions / effects]
In order to solve the above-mentioned problem, the manufacturing method of the connecting rod of the present invention,
Hot forged non-tempered steel whose final material state is a precipitation strengthened state based on V-based precipitates is used as a steel material constituting the connecting rod,
A hot forging step of obtaining a forged body having a base shape of a connecting rod by hot forging a steel material in a temperature range of 1100 ° C. or higher and 1300 ° C. or lower;
After the hot forging step is finished, an intermediate cooling step of intermediate cooling the forged body so that the average cooling rate in the first temperature range from 800 ° C. to 500 ° C. is 1 ° C./second or more,
An aging heat treatment step of aging and precipitating a V-based precipitate in a second temperature range of 500 ° C. or more and 700 ° C. or less after the intermediate cooling step is completed;
It is characterized by having.
[0011]
In the hot forged non-tempered steel that performs precipitation strengthening with V-based precipitates, the temperature range of aging heat treatment that can remarkably form V-based precipitates effective for precipitation strengthening within a realistic time range is 500 ° C or higher and 800 ° C. Below (first temperature range). In the manufacturing method of the connecting rod of the present invention, after hot forging, the first temperature region is once rapidly cooled (intermediate cooling treatment) at a cooling rate of 1 ° C./second or more, and then again at 500 ° C. or more and 700 ° C. or less. Aging heat treatment was performed by heating to two temperature ranges. In the conventional processes disclosed in Patent Documents 11 and 12, the process is cooled to 600 to 800 ° C. after forging and then held in a furnace at 500 to 700 ° C. Since the transition is made directly to one temperature range, the cooling rate in the first temperature range where precipitates are remarkably formed becomes slow, and the supercooling degree of the matrix in which V is dissolved is insufficient. As shown in FIG. 5, the number of nucleation of precipitates increases as the degree of supercooling increases. Therefore, when the degree of supercooling is small, nucleation of V-type precipitates becomes insufficient as shown in FIG. Even with aging heat treatment, the precipitate size increases, but the number density decreases. Therefore, even if the amount of precipitation increases, it does not contribute much to improving the yield strength. In this case, even if the aging heat treatment time is extended, individual precipitates grow only with a small number density, so that an overaging state is likely to occur, and a remarkable improvement in yield strength cannot be expected.
[0012]
On the other hand, according to the method of the present invention, as shown in FIG. 2, the subcooling degree of the matrix can be increased by performing an intermediate cooling treatment in the first temperature range at a cooling rate of 1 ° C./second or more after hot forging. As shown in FIG. 6, nucleation of V-based precipitates is promoted. Then, after generating a sufficient number of precipitation nuclei by the intermediate cooling treatment, an aging heat treatment is performed in the second temperature range, thereby increasing the number density of the V-based precipitates while maintaining the number density. The size of the precipitate can be made sufficiently large as long as the matching strain is not released. That is, V added by aging heat treatment can be used for the formation of V-based precipitates without waste, and the number density of precipitates can be sufficiently increased by promoting nucleation by intermediate cooling treatment, so that a connecting rod with extremely high yield strength is obtained. be able to.
[0013]
The connecting rod of the present invention is a connecting rod made of hot forged non-heat treated steel whose final material state is a precipitation strengthened state based on V-based precipitates, and the steel material constituting the connecting rod is: The yield ratio Rps represented by σ0.2 / σUTS, where V content is WV (mass%), tensile stress is σUTS, 0.2% proof stress is σ0.2, Rps0 = 0.68 + 0.4 × It is characterized by being larger than a reference value Rps0 represented by WV (more desirably, a reference value Rps0 ′ represented by 0.7 + 0.4 × WV). The connecting rod of the present invention has the above-mentioned high yield strength ratio, which could not reach any conventional non-tempered steel-based connecting rod in the amount of V added according to the amount of V added by the manufacturing method of the present invention. For the first time, it became feasible. As a result, as a connecting rod used in a high load environment such as for automobiles, a product that is remarkably excellent in fatigue strength can be obtained.
[0014]
In order to optimize the distribution state by controlling the precipitation of V, it is necessary that V is in a solid solution state in the matrix during forging. When the forging temperature is less than 1100 ° C., a sufficient V solid solution state cannot be obtained, and V precipitates contributing to material strengthening cannot be sufficiently formed during aging heat treatment. On the other hand, when the forging temperature exceeds 1300 ° C., the material is significantly oxidized. For the above reasons, the forging temperature is preferably set to 1100 ° C. or higher and 1300 ° C. or lower, and more preferably 1150 ° C. or higher and 1250 ° C. or lower.
[0015]
In order to improve the number density of V-based precipitates, it is necessary to increase the cooling rate after forging. For this reason, after forging, the first temperature range (500 ° C. or higher and 800 ° C. or lower) which is the precipitation temperature range of V-based precipitates. ) Must be cooled at 1 ° C./second or more. In the temperature range below 500 ° C., the diffusion rate of V becomes very slow, and almost no V-based precipitates are generated. On the other hand, in the temperature range of 800 ° C. or higher, V remains in a solid solution in the matrix, and there is little meaning to increase the cooling rate. Therefore, the first temperature range for controlling the cooling rate is set to 500 ° C. or higher and 800 ° C. or lower. In addition, there is no restriction | limiting in particular about the cooling conditions after passing 1st temperature range, for example, after cooling to 500 degrees C or less, you may carry out aging heat processing in the 2nd temperature range as it is, and after cooling to room temperature, An aging heat treatment may be performed by re-heating to two temperature ranges.
[0016]
Moreover, as an upper limit of the cooling rate in the intermediate cooling step, it is desirable that the cooling in the first temperature region in the intermediate cooling step is performed at a bainite generation critical cooling rate or less. That is, if the cooling rate is excessively increased, bainite is generated, the hardness becomes too high, and the machinability and the like are significantly deteriorated.
[0017]
Moreover, as for the steel materials to be used, what contains 2 or more types which make V, C, and N essential among C, V, Si, Mn, Cu, Ni, C, and N is good. By containing C, N, and C in addition to V, fine V carbonitrides can be deposited as V-based precipitates during the aging heat treatment, and the effect of improving the yield ratio becomes particularly significant. In this case, the bainite generation critical cooling rate vc (° C./second) is such that the C content is WC (mass%), the Si content is WSi (mass%), the Mn content is WMn (mass%), Cu The content of WCu (mass%), the content of Ni as WNi (mass%), and the content of Cr as WCr (mass%),
vc≡ (409-319 × WC) / exp (−1.2 × WC + 0.05 × WSi + 3.0 × WMn + 1.6 × WCu + 1.4 × WNi + 2.0 × WCr + 1.2) (1)
Therefore, it is desirable to perform the intermediate cooling process by cooling the first temperature region at the critical cooling rate vc or higher in order to prevent the occurrence of bainite in the intermediate cooling process. (Note that the N content has little effect on the bainite generation critical cooling rate vc).
[0018]
Next, the aging heat treatment step is a treatment for forcibly precipitating V which cannot be precipitated during cooling after forging and is dissolved in supersaturation. In the present invention, in order to increase the number density of the V-based precipitates, it is essential to increase the cooling rate in the first temperature range during cooling after forging. However, since the cooling rate is high in the state after cooling, the holding time at the temperature at which the V-based precipitates are naturally shortened, and most of the added V remains in a supersaturated solid solution state. Therefore, in the aging heat treatment step, by maintaining an appropriate time in a second temperature range of 500 ° C. or more and 700 ° C. or less, supersaturated V is precipitated and grown as a V-based precipitate in a range that does not become over-aged, and the added V is added. Make the most effective use. When the aging heat treatment is less than 500 ° C., the V diffusion rate is small, and thus the formation of V-based precipitates cannot be promoted. Moreover, when it exceeds 700 degreeC, the precipitation growth rate of a V type precipitate will become large too much, and it will become easy to invite overaging.
[0019]
The aging heat treatment step is desirably performed by isothermally holding the forged body at a constant holding temperature set in the second temperature range. If the treatment time in the aging heat treatment step is excessively long, the amount of precipitates formed increases due to the above-described overaging, but the precipitate size becomes excessively large, leading to a decrease in the yield strength ratio. In this case, if the aging heat treatment step is an isothermal holding treatment, it becomes very easy to manage the holding time for preventing overaging. The formation of precipitates requires the diffusion of V, and the appropriate holding time for preventing overaging varies depending on the V content and holding temperature. Specifically, the V content of the steel material is WV (mass%), the isothermal holding temperature is Θ (° C.), and the holding time is t (seconds).
3 × 10 ^-8 ≧ (t × WV × D) ^1/2 (2)
D = 3.1 × 10 ^-4 × exp (−239000 / (8.314 × (Θ + 273)) (3)
It is desirable to determine the holding temperature Θ (° C.) and the holding time t (seconds) so as to satisfy Equation (3) is a diffusion coefficient of V in Fe at a temperature Θ. When the holding time t when the equal sign holds in the equation (2) is the limit holding time t0, the holding time t is the limit holding time t0. Exceeding this will cause overaging, and a sufficient improvement in the yield strength ratio cannot be expected.
[0020]
Moreover, the above-mentioned split connecting rod is suitable as an application object of this invention. FIG. 1 shows an example of a split connecting rod 1, and connecting

portions

3 and 4 of connecting objects by connecting rods are formed at both ends. At least one of the connecting

portions

3 and 4, in this embodiment, the connecting portion 4 that forms the large end side to which the crankshaft is connected is integrated with the connecting rod body 2 side by breaking the integrally forged preform. The first portion 4a that remains as a result and the second portion 4b that is separated from the first portion 4a are divided. The connecting portion 4 is a fastening member such as a bolt and nut in a form in which a connection object is sandwiched between the first portion 4a and the second portion 4b and the first portion and the second portion are brought into close contact with each other. 6 and 6 are separated connection portions. The application to such a split connecting rod has the advantage that the strength ratio of the connecting rod is improved, and the process of breaking and separating the spare body becomes easier. Moreover, it becomes difficult to produce a cup cone-like ductile fracture behavior at the time of fracture, and as a result, the fitting accuracy of the separation surface (fracture surface) SP can be increased. For the breakage of the preliminary body,

notches

5 and 5 are formed in the inner peripheral surface of the through-hole 4h for shaft insertion formed in the preliminary body, and the

split molds

7 and 7 having a larger diameter than this are formed in the through hole 4h. Is inserted into the through-hole 4h by a hydraulic press or the like through the wedge 8 or the like, so that the fracture surface SP can be formed starting from the

notches

5 and 5. Since the basic structure of the split connecting rod itself is well known, only a brief description is given.
[0021]
As the hot forged non-heat treated steel suitable for the split connecting rod, it is desirable to adopt a steel material containing P and Si in addition to V. In non-heat treated steel for hot forging, the pro-eutectoid ferrite part has a relatively low hardness and high toughness, but if it contains an appropriate amount of P and Si, the pro-eutectoid ferrite part is strengthened and the toughness is moderate. This makes it easy to break by impact force and suppresses cup cone-like ductile fracture. As a concrete steel material composition,
Fe content is 95% by mass or more,
C: 0.15 mass% or more and 0.45 mass% or less;
Si: 0.1% by mass or more and 1.5% by mass or less;
Mn: 0.5% by mass or more and 1.5% by mass or less;
P: 0.03 mass% or more and 0.15 mass% or less;
Cu: 0.01 mass% or more and 0.5 mass% or less;
Ni: 0.01 mass% or more and 0.5 mass% or less;
Cr: 0.01 mass% or more and 1 mass% or less;
V: 0.1 mass% or more and 0.4 mass% or less; and
N: 0.005 mass% or more and 0.035 mass% or less
Can be used.
[0022]
The reasons for limiting the composition are as follows.
(1) C: 0.15 mass% or more and 0.45 mass% or less
C is an element necessary for ensuring the strength, and is an essential element of the V-based carbonitride. For this reason, addition of 0.15 mass% or more is required. On the other hand, if added over 0.45% by mass, the hardness after hot forging becomes excessive and the machinability deteriorates, so it is necessary to make it 0.45% by mass or less.
[0023]
(2) Si: 0.1% by mass or more and 1.5% by mass or less
Si has a deoxidizing action during steel melting, improves the strength of ferrite, which is a soft phase, which is the main cause of plastic deformation during fracture separation, and improves proof stress and fatigue strength, This has the effect of suppressing deformation (change in roundness) during break separation and improving the adhesion of the fracture surface. In order to obtain such effects, it is necessary to contain Si by 0.1% by mass or more. However, if the content is too large, the hardness is unnecessarily increased and the machinability is deteriorated.
[0024]
(3) Mn: 0.5% to 1.8% by mass, Cr: 0.01% to 1% by mass
Mn and Cr increase the strength of the forged material and improve the hardenability. When the carbon content is high, a brittle heat-affected layer is formed on the laser-processed notch bottom to facilitate break separation. However, when added in a large amount, bainite is formed after forging, and the hardness is remarkably increased to lower the machinability, so that the respective ranges are Mn: 0.5 mass% to 1.8 mass%, Cr: 0.01 to It was 1 mass%.
[0025]
(4) P: 0.03 mass% or more and 0.15 mass% or less
P is an inevitable impurity, but is generally kept as low as possible as an element that lowers toughness due to segregation at grain boundaries. However, in the split connecting rod that performs fracture separation, it acts as an element that suppresses deformation at the time of fracture and improves the adhesion of the fracture surface, so it is positively added to 0.03% by mass or more. . Further, P is effective in improving the yield strength and fatigue strength because it dissolves in the ferrite and improves the strength of the ferrite like Si. However, even if it is added in a large amount, the effect is saturated or the hot workability is hindered, so the addition amount is limited to 0.15% by mass or less.
[0026]
(5) Cu and Ni: 0.01% by mass or more and 0.5% by mass or less
Cu and Ni are inevitable impurities, and making it 0.01% by mass or less requires great efforts and is economically disadvantageous. On the other hand, although it is an element effective for increasing the strength as in Mn and Cr, addition of a large amount is also economically disadvantageous, so the upper limit is made 0.5% by mass or less.
[0027]
(6) V: 0.1 mass% or more and 0.4 mass% or less
V is an element that produces fine carbonitride with C and N, and increases the strength after forging.
This is particularly effective for improving the proof stress and fatigue strength. In order to obtain such an effect, addition of 0.1% by mass or more is necessary. However, even if 0.4% by mass or more is added, the effect of increasing the strength is saturated and further the machinability is lowered, so the upper limit was made 0.4% by mass. In particular, when a composition in which the amount of V added is 0.16% by mass or more is adopted, the yield strength ratio is greatly increased to 0.8 or more as a result of being able to effectively use this even though a large amount of V is added. A connecting rod can be realized.
[0028]
(7) N: 0.005 mass% or more and 0.035 mass% or less
N is an inevitable impurity, but can also be regarded as an essential element for carbonitride formation. It is economically disadvantageous to make the N content 0.005% by mass or less, and if it is added in a large amount, it causes casting defects, so the range is made 0.005 to 0.035% by mass.
[0029]
In addition, when Al is added as a deoxidizer, the inclusion of N causes fine nitrides to be dispersed and formed in the steel, thereby exhibiting the effect of suppressing grain coarsening during hot forging. In this case, the content of Al is preferably 0.001% by mass or more and 0.01% by mass or less. Even if Al is added in a large amount exceeding the above upper limit, the effect is saturated, and the ductility of the material is improved by excessive grain refining action and the adhesion of the fracture surface after fracture separation is reduced. Connected.
[0030]
In addition, as a steel material,
Pb: 0.001% by mass to 0.3% by mass;
Bi: 0.001 mass% or more and 0.3 mass% or less;
S: 0.001 mass% or more and 0.15 mass% or less; and
Ca: 0.001% by mass or more and 0.01% by mass or less;
What contains 1 or more types of these can also be used. Pb, Bi, S, and Ca are all effective elements for improving the machinability. Therefore, when it is required that the machinability is further improved in the forged product, these are necessary as necessary. An appropriate amount of one or more selected from among them can be added (in any case, if the added amount is less than 0.001% by mass, the machinability improving effect is not remarkable). In addition, since the intensity | strength and hot workability will fall when there is too much addition amount of these elements, it is necessary to add in below the said lower limit, respectively.
[0031]
In addition,
Ti: 0.001% by mass or more and 0.02% by mass or less;
Zr: 0.001% by mass or more and 0.02% by mass or less; and
Mg: 0.001 mass% or more and 0.01 mass% or less
What contains 1 or more types of these can also be used. Ti, Zr and Mg form carbonitrides and refine crystal grains after forging to improve strength and refine the distribution state of MnS to improve machinability (all added) If the amount is less than 0.001% by mass, the machinability improvement effect is not remarkable). However, even if added in a large amount, the effect is saturated.
[0032]
【Example】
Hereinafter, experimental results performed to confirm the effects of the present invention will be described.
The raw materials were blended so that the composition shown in Table 1 was obtained, and a 150 kg steel ingot was melted in an electric furnace. This steel ingot was subjected to hot working and isothermal holding under the conditions shown in Table 2 using a hot working simulator, and then a JIS No. 4 reduced tensile test piece was worked to examine the tensile characteristics. The results are shown in Table 2.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
It can be seen that when the intermediate cooling and aging heat treatment after forging are performed under conditions 1 to 4 that satisfy the requirements of the present invention, a high yield strength ratio is obtained in all cases compared to the comparative conditions that are only cooled after forging. On the other hand, Conditions 5 to 12 that do not satisfy the requirements of the present invention result in the following results.
Condition 5: The forging temperature is low, the V solid solution at the time of heating is insufficient, and the yield ratio is reduced.
Condition 6: Since the cooling rate is slow, the V carbonitride is coarsened and the yield ratio is reduced.
Condition 7: Since the cooling rate is higher than the critical cooling rate, bainite is generated after forging and the yield ratio is reduced.
Condition 8: Since the cooling end temperature is too high, the V carbonitride is coarsened and the yield strength ratio is reduced.
Condition 9: Since the isothermal holding temperature is too low, no improvement in the yield strength ratio is observed even if held for a long time.
Condition 10: Since the isothermal holding temperature is too high, a part of austenite is generated during the isothermal holding, and the V carbonitride is re-dissolved and coarsened, so that the yield strength ratio is greatly reduced.
Conditions 11 and 12: With respect to the critical holding time value t0 calculated in (2), since the holding time is too long, it becomes an over-aged state and the yield ratio decreases.
[0036]
Next, steels having various compositions shown in Table 3 were melted in the same manner, and a connecting rod body having the shape shown in FIG. The hot working conditions at this time were forging at a heating temperature of 1230 ° C. and then cooling at an average cooling rate from 800 ° C. to 500 ° C. at 1.8 ° C./second by blast cooling as an intermediate cooling treatment. Then, after cooling to room temperature, an aging heat treatment was performed by isothermal holding at 580 ° C. for 3.6 ks. And after aging heat processing, it cooled to room temperature, cut out the flat plate tension test from the main-body part, and used for the test.
[0037]
[Table 3]

[0038]
According to this, in composition G, the bainite formation critical cooling rate vc becomes too low and bainite is formed after forging and the yield strength ratio is lowered, but in other compositions, vc is sufficiently high and a good yield strength ratio is obtained. It turns out that it is obtained.
[0039]
Moreover, after forging a connecting rod substance using each composition of AE of Table 3 (and XC70S of a commercial steel material as a comparative example), after changing the isothermal holding conditions as shown in Table 4 and performing an aging heat treatment After machining, laser cutting was applied to the through hole 4h of the preliminary body to make a notch with a depth of 0.5 mm to break and separate, and the change in roundness after breaking and separating the through hole 4h was measured. The results are shown in Table 4.
[0040]
[Table 4]

[0041]
In the condition setting for the compositions D and E, it was considered that the aging heat treatment was performed beyond the critical holding time, so that the overaging state was reached and the yield strength was reduced. Moreover, the change in roundness after break separation becomes large, and the break separation characteristics are lowered.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an example of a connecting rod to which the present invention is applied.
FIG. 2 is a view schematically showing a thermal history after hot forging according to the connecting rod manufacturing method of the present invention.
FIG. 3 is a view schematically showing a heat history after hot forging according to a conventional connecting rod manufacturing method.
FIG. 4 is a diagram schematically showing the relationship between precipitate size and yield strength.
FIG. 5 is a diagram schematically showing the relationship between the number density of precipitates and the degree of supercooling.
FIG. 6 is a conceptual diagram for explaining the effect in association with the intercooling treatment and the aging heat treatment of the present invention.
FIG. 7 is a view for explaining problems of a conventional connecting rod manufacturing method.
[Explanation of symbols]
1 Split connecting rod
2 Connecting rod body
3, 4 connecting part
4a Part 1
4b Second part
6,6 Fastening member

Claims

Hot forged non-tempered steel whose final material state is a precipitation strengthened state based on V-based precipitates is used as a steel material constituting the connecting rod,
A hot forging step of obtaining a forged body having a base shape of a connecting rod by hot forging the steel material in a temperature range of 1100 ° C. or more and 1300 ° C. or less;
After the hot forging step is completed, an intermediate cooling step in which the forged body is intermediate cooled so that the average cooling rate in the first temperature range from 800 ° C. to 500 ° C. is 1 ° C./second or more,
And a aging heat treatment step of aging precipitation of the V-based precipitates in the second temperature range of 500 ° C. or more and 700 ° C. or less after the intermediate cooling step is completed.

The manufacturing method of the connecting rod of Claim 1 which cools in the said 1st temperature range in the said intermediate cooling process at a bainite generation critical cooling rate or less.

The steel material contains at least two of C, V, Si, Mn, Cu, Ni, C, and N, which essentially require V, C, and N, and the content of C is WC (mass %), The Si content is WSi (mass%), the Mn content is WMn (mass%), the Cu content is WCu (mass%), the Ni content is WNi (mass%),
When the Cr content is WCr (mass%), the bainite generation critical cooling rate vc (° C./sec) is set as follows:
vc≡ (409-319 × WC) / exp (−1.2 × WC + 0.05 × WSi + 3.0 × WMn + 1.6 × WCu + 1.4 × WNi + 2.0 × WCr + 1.2)
The manufacturing method of the connecting rod of Claim 2 defined by these.

The method for manufacturing a connecting rod according to any one of claims 1 to 3, wherein the aging heat treatment step is performed by isothermally holding the forged body at a constant holding temperature set in the second temperature range.

When the V content of the steel material is WV (mass%), the holding temperature of the isothermal holding is Θ (° C.), and the holding time is t (seconds).
3 × 10 ⁻⁸ ≧ (t × WV × D) ^1/2
D = 3.1 × 10 ⁻⁴ × exp (−239000 / (8.314 × (Θ + 273))
The connecting rod manufacturing method according to claim 4, wherein the holding time t is determined so as to satisfy

As the steel material, the Fe content is 95% by mass or more,
C: 0.15 mass% or more and 0.45 mass% or less;
Si: 0.1% by mass or more and 1.5% by mass or less;
Mn: 0.5% by mass or more and 1.5% by mass or less;
P: 0.03 mass% or more and 0.15 mass% or less;
Cu: 0.01 mass% or more and 0.5 mass% or less;
Ni: 0.01 mass% or more and 0.5 mass% or less;
Cr: 0.01 mass% or more and 1 mass% or less;
6. The method according to claim 3, wherein V: 0.1% by mass or more and 0.4% by mass or less; and N: 0.005% by mass or more and 0.035% by mass or less are used. The manufacturing method of the connecting rod as described in any one of.

As the steel material,
Pb: 0.001% by mass to 0.3% by mass;
Bi: 0.001 mass% or more and 0.3 mass% or less;
S: 0.001% by mass or more and 0.15% by mass or less;
Ca: 0.001% by mass or more and 0.01% by mass or less;
Ti: 0.001% by mass or more and 0.02% by mass or less;
The manufacturing method of a connecting rod according to claim 6, wherein one containing at least one of Zr: 0.001 mass% or more and 0.02 mass% or less; and Mg: 0.001 mass% or more and 0.01 mass% or less is used. Method.

A connecting rod composed of hot forged non-heat treated steel whose final material state is a precipitation strengthened state based on V-based precipitates, and the steel material constituting the connecting rod has a V content of WV (mass% ), Where the tensile stress is σUTS and the 0.2% proof stress is σ0.2, the proof stress ratio Rps expressed by σ0.2 / σUTS is the reference value Rps0 expressed by Rps0 = 0.68 + 0.4 × WV Connecting rod characterized by being larger than.

A spare body configured by using the steel material having the composition described in claim 6 or 7, wherein a connecting portion of a connecting object by connecting rods is formed at both ends, and at least one of the connecting portions is integrally forged. Is formed by dividing into a first part that remains integrally on the connecting rod body side and a second part separated from the first part, and the connection between the first part and the second part. The connecting rod according to claim 8, wherein the connecting object is formed as a separate connecting part that is sandwiched by a fastening member in such a manner that the object is sandwiched and the first part and the second part are brought into close contact with each other.