JP3990917B2

JP3990917B2 - Oil quenching method for steel parts

Info

Publication number: JP3990917B2
Application number: JP2002021142A
Authority: JP
Inventors: 吉計下里; 三郎小川; 清浩宮本
Original assignee: Chugai Ro Co Ltd
Current assignee: Chugai Ro Co Ltd
Priority date: 2001-02-08
Filing date: 2002-01-30
Publication date: 2007-10-17
Anticipated expiration: 2022-01-30
Also published as: JP2002309314A

Description

【０００１】
【発明の属する技術分野】
本発明は鋼材部品の油焼入方法、特に、鋼材部品をマルクエンチ処理する方法に関するものである。
【０００２】
【従来の技術】
一般に、焼入温度に保持された鋼材部品を焼入油に浸漬すると、鋼材部品は蒸気膜段階（高温域）、沸騰段階（中温域）及び対流段階（低温域）の三段階を経て冷却される。蒸気膜段階での冷却速度は遅く、沸騰段階での冷却速度はそれよりも３〜１０倍早いことが知られている。また、高温焼入油（ホットクエンチオイル）は、低温焼入油（コールドクエンチオイル）と比べて中温及び低温域での冷却速度が遅いため、焼入れ変態に起因する歪みを軽減できるが、蒸気膜段階の時間が短く、蒸気膜段階の終了温度が高いため、高温域での温度差に起因する熱歪みが生じ易いことが知られている。一方、焼入油を減圧下におくと、図２に示すように、沸点が下降することによって蒸気膜段階の時間が長くなると共に、蒸気膜段階の終了温度が低くなる。そこで、このような現象を利用して、焼入れに起因する変形を軽減する方法として、焼入温度に保持された鋼材部品を高温焼入油中に浸漬して焼入れを行う方法、或いは減圧下の焼入油中に浸漬して焼入れを行う方法が実施されている。
【０００３】
他方、歯車等の鋼材部品の油焼入方法として、所定の焼入温度に保持した鋼材部品をマルテンサイト変態開始点(Ｍｓ点)より少し高い高温冷却剤に浸漬してマルテンサイト変態開始点(Ｍｓ点)より少し高い温度まで急速冷却し、鋼材部品全体がほぼ同一の温度になった時点で高温冷却剤から取り出して大気中で放冷することによりマルテンサイト変態させる方法（マルクエンチ処理）がある。この方法は、焼入歪みや焼入れのバラツキを軽減することができるが、大気中の放冷であるため、冷却速度が処理材のトレイ内での載置位置や被処理部材の部位間に温度差を生じ、その温度差に起因する焼入歪や焼入れバラツキが避けられないという問題がある。
【０００４】
この問題を解決する方法として、所定の焼入温度に保持した鋼材部品をマルテンサイト変態開始温度以上の高温冷却剤中に浸漬して急冷し、鋼材部品全体がほぼ同一の温度になった時点でマルテンサイト変態開始温度以下の低温冷却剤中に浸漬する方法(特開平２‐１０１１１３号公報)、或いは焼入油を循環させる循環系を備え、かつ、被処理材を包囲するフードを有する焼入槽を用い、前記循環系を停止させた状態でフード内に被処理材を浸漬し、その熱でフード内の焼入油をマルテンサイト変態開始点(Ｍｓ点)近傍まで上昇させ、被処理材全体がほぼ同一の温度になった時点で前記循環系で焼入油を循環させてマルテンサイト変態点以下に急冷する方法(特開平６‐２７９３８号公報)などが提案されている。
【０００５】
【発明が解決しようとする課題】
しかしながら、焼入装置の構成及び構造の観点から見れば、前者は高温冷却剤と低温冷却剤とを必要とするばかりでなく、高温冷却剤用の焼入槽と低温冷却剤用の焼入槽とが必要であるため、焼入装置の大型化及び複雑化が避けられずメンテナンス性が悪いという問題がある。後者は、前者における問題を解決するものではあるが、焼入槽自体が複雑化するという問題がある。また、焼入歪及びバラツキの観点から見れば、両者はいずれも冷却剤中に浸漬して鋼材部品全体をほぼ同一温度にする方式であるため、熱媒体としての冷却剤をトレイ上の各被処理材及び被処理材全体に均一に、かつ、十分に接触させるのは困難で、均熱時に鋼材部品間若しくは鋼材部品の部位間に温度差が生じ、焼入歪及びバラツキの軽減に効果はあるが、満足できるものではない。
【０００６】
前記課題を解決するため縷々研究した結果、これらの焼入歪及びバラツキはマルテンサイト変態時における鋼材部品間若しくは鋼材部品の部位間の温度差に起因すると同時に、高温域（約５５０℃以上）での冷却速度が速すぎることに起因することが明らかとなった。
【０００７】
従って、本発明は、マルテンサイト変態時における鋼材部品間若しくは鋼材部品の部位間での温度差を軽減すると共に、高温域（約５５０℃以上）での冷却速度を熱歪を抑制するに十分な緩慢な速度にし、もって、焼入歪及び焼入れのバラツキを軽減することを課題とするものである。
【０００８】
【課題を解決するための手段】
本発明は、前記課題を解決するための手段として、所定の焼入温度に保持された鋼材部品を高温焼入油中に浸漬して当該鋼材部品の所定部位がマルテンサイト変態開始点(Ｍｓ点)の直上温度に達するまで急冷した後、当該鋼材部品を前記高温焼入油中から取り出して当該鋼材部品の保有熱により均熱化し、引続き前記高温焼入油中に浸漬して冷却するようにしたものである。
【０００９】
本発明の実施態様においては、急冷工程において、鋼材部品は、その所定部位、即ち、マルテンサイト変態に起因する歪みによる変形量が最も大きい部位、具体的には、鋼材部品の単位表面に対して内部体積の小さい部位（例えば、歯車の歯や角柱部品のコーナ部など、以下、先鋭部という）がその部位のＭｓ点直上の温度に達するまで急冷することが行われる。
【００１０】
これは次の知見に基づくものである。即ち、本発明方法においては、前記鋼材部品は、まず、高温焼入油中に浸漬してマルテンサイト変態開始点(Ｍｓ点)の直上温度にまで急冷されるが、この場合、鋼材部品全体が前記Ｍｓ点の直上の温度にまで冷却された時点で高温焼入油から取り出すのが好適である。しかしながら、実際の鋼材部品は、一般に焼入れ直前に浸炭処理を施されるため、鋼材部品内部に拡散した炭素の濃度が鋼材部品全体にわたって均一ではなく、部位によってＭｓ点が異なる場合がある。例えば、鋼材部品の先鋭部は、その表層部の炭素濃度が当該先鋭部以外の部位（例えば、鋼材部品内部の浸炭されていない部位、即ち、非浸炭部）よりも高くなるため、前記先鋭部表層部のＭｓ点（Ｍｓ_２）が前記非浸炭部のＭｓ点（Ｍｓ_１）よりも低くなり、変形量にバラツキを生じる結果となる。ところが、鋼材部品の先鋭部以外の部位、即ち、前記非浸炭部については、通常焼入れした場合でも変形が小さく、その部位の温度が当該部位のＭｓ点（Ｍｓ_１）直下となっても鋼材部品としての特性への悪影響が少なく、変形量が最も大きい部位である先鋭部の温度がその部位のＭｓ点（Ｍｓ_２）直上であれば、均熱化により鋼材部品全体の変形量のバラツキを小さくできるという知見に基づくものである。
【００１１】
前記焼入油は、被処理材に応じて種類、温度及び量が設定されるが、通常、ＪＩＳＫ２２４２２種１号又は２号に相当する高温焼入油が採用される。また、焼入油の温度は、１００〜１７０ ℃の範囲内の温度に設定される。なお、焼入油の量は、１次焼き入れにより前記設定温度が大きく変動しない量に設定すれば良い。
【００１２】
また、急冷後、被処理材を均熱化するため油焼入室内の焼入油の上方空間に保持されるが、この上方空間の雰囲気温度は、通常、高温焼入油の温度とほぼ同温度である。１次焼入れされた被処理材は、油焼入室の上方空間内にそれ自体の保有熱により均熱されてマルテンサイト変態開始点(Ｍｓ点)の直上近傍の温度になるまで保持する時間は、被処理材の大きさ、材質等によって異なるため一義的に定めることは困難であるが、通常、３０〜３００秒に設定される。
【００１３】
均熱化後、再度高温焼入油に浸漬する、所謂マルクエンチ処理により、被処理材のトレイ内での載置位置や被処理材の部位間の温度差をさらに小さくすることができる。
【００１４】
本発明の実施態様においては、焼入れ時の油焼入室内の圧力が７〜７５ＫＰa、より好ましくは、８〜４０ＫＰaに設定される。これは、７ＫＰaより高真空にすると蒸気膜段階が長くなりすぎて十分な焼入れ硬度が得難くなり、また、７５ＫＰaより低真空では十分な減圧効果が得られず、熱歪みを抑制できなくなるからである。
【００１５】
【発明の実施の形態】
以下、添付の図面を参照して本発明の実施の形態について説明する。図１は本発明方法の実施に使用するバッチ式浸炭炉の構造を示し、当該炉は加熱室１と油焼入室８とで構成されている。加熱室１は、バルブ２ａを備えた窒素供給ライン３ａを介して窒素ガス供給源４ａに接続されると共に、バルブ２ｂを備えた排気ライン５ａを介して真空ポンプ６ａに接続され、また、図示しない浸炭性ガス供給ラインを接続され、加熱室１は加熱室用扉７を介在させて油焼入室８に連結されている。
【００１６】
他方、油焼入室８は、バルブ２ｃを備えた窒素供給ライン３bを介して窒素ガス供給源４ｂに接続されると共に、バルブ２ｄを備えた空気供給ライン９を介して大気に連通し、バルブ２ｅを備えた排気ライン５ｂを介して真空ポンプ６ｂに接続されている。また、油焼入室８は、その内部に焼入油槽１１を有し、被処理材Ｗを昇降させて焼入油中に浸漬保持する一方、前記焼入油槽１１の上方の空間１４に保持する昇降手段（図示せず）とを備え、加熱室１側に中間扉１５が、また、大気側に装入兼搬出用扉１６がそれぞれ配設されている。
【００１７】
本発明方法の実施に際しては、まず、加熱室１及び油焼入室８を約７〜７５ＫＰaの範囲内の所定の真空度にまで真空排気する。両室が所定の真空度に達した後、被処理材Ｗを加熱室１から油焼入室８内に移送し、高温焼入油１３中に浸漬して当該被処理材Ｗの表面がマルテンサイト変態開始点(Ｍｓ点)の直上近傍の温度になるまで急冷して１次焼入れを行う。この時、焼入油１３が高温焼入油であって、油焼入室８内が減圧された状態で浸漬冷却されるため、大気圧下での高温焼入油による焼入と比較して蒸気膜段階の冷却速度が遅く、しかも、蒸気膜段階の冷却時間が長くなるため、被処理材Ｗは高温域（８５０〜５５０℃）を緩慢に、かつ、均一に冷却される。
【００１８】
次いで、焼入れを継続した場合には歪等が許容範囲外となる部位、例えば、被処理材の先鋭部表面の温度が表面マルテンサイト変態開始点(Ｍs_２点)の直上近傍の温度に達した時点で、被処理材Ｗを高温焼入油１３中から取り出して油焼入室８内の上方空間１４に放置し、被処理材Ｗの表面及び内部が表面マルテンサイト変態開始点(Ｍs_２点)の直上近傍の温度になるまで均熱化する。
【００１９】
更に、均熱化後、被処理材Ｗを再度高温焼入油１３中に浸漬して冷却し２次焼入れを行う。この際、被処理材Ｗは均熱処理工程を経ており、かつ、高温焼入油中に浸漬されるため、空冷するよりも均一に冷却できることから、マルクエンチ処理によりマルテンサイト変態が均一に行われる。また、高温焼入油は、低温焼入油に比較して冷温域の冷却速度が遅いため、被処理材Ｗをより均一に冷却できる。
【００２０】
冷却が完了すると、前記被処理材Ｗを高温焼入油１３から取り出し、バルブ２ｄを開いて空気供給ライン９から空気を油焼入室８内に導入して大気圧状態に復圧した後、油焼入室８から装入兼排出用扉１６を経て炉外へ取り出す。
【００２１】
被処理材Ｗは、焼入れ中、各部位において別々の温度履歴を示し、その部位におけるマルテンサイト変態開始点（Ｍｓ点）も炭素濃度（Ｃ％）に応じて異なっている。この炭素濃度とマルテンサイト変態開始点（Ｍｓ点）との関係は、例えば、公知の次式で与えられる。
Ｍｓ（℃）＝５５０−３６１（％Ｃ）−３９（％Ｍｎ）−３５（％Ｃｒ）−１７（％Ｎｉ）−１０（％Ｃｕ）−５（％Ｍｏ＋％Ｗ）＋１５（％Ｃｏ）＋３０（％Ａｌ）
【００２２】
従って、本発明が最も効果的なのは、被処理材Ｗの総ての部位において前記式で求められるマルテンサイト変態開始点（Ｍｓ点）直上の温度にて高温焼入油１３から取り出すことである。しかしながら、図３には４点のみ示したが、被処理材Ｗの部位に応じて異なる一次冷却時の温度推移及びマルテンサイト変態開始点（Ｍｓ点）をあらゆる点に渡って求め、かつ、マルテンサイト変態開始点（Ｍｓ点）直上の温度で取り出すことは事実上不可能である。そこで、本発明においては、変形量が最も大きい部位である先鋭部の温度がその部位のＭｓ点（Ｍｓ_２）直上の温度に達したところで高温焼入油１３から取り出すようにしている。これは、通常焼入れした場合でも変形量の小さい部位については、その表層部の温度が当該部位のＭｓ点（Ｍｓ_１）直下となっても、変形量が最も大きい部位である先鋭部の温度がその部位のＭｓ点（Ｍｓ_２）直上であれば、均熱化により鋼材部品全体の変形量のバラツキを小さくできるからである。
【００２３】
なお、焼入れを継続した場合に歪等が許容範囲外になる部位（具体的には、鋼材部品表面の焼入れの必要な部位）が表面マルテンサイト変態開始点(Ｍｓ_２点)の直上近傍の温度に到達する時点、すなわち、被処理材Ｗを高温焼入油１３中から取り出すタイミングは浸漬時間により管理する。また、1次焼入れした被処理材Ｗがマルテンサイト変態開始点(Ｍs_２点)の直上近傍の温度に到達する時点、すなわち、再度、高温焼入油１３中に浸漬するタイミングは油焼入室８の上方空間１４での放置時間により管理する。
【００２４】
前記実施態様においては、油焼入室８を減圧状態にした場合を例に挙げて説明したが、油焼入室８を大気圧状態で実施しても良い。この場合、一次焼入れ後、焼入油から引き上げて均熱処理を行うことにより、その後の二次焼入れでのマルテンサイト変態を均一に行わせることができるため、歪みのバラツキの低減に満足すべき効果がある。また、減圧状態では、高温域での被処理材Ｗの冷却をより均一に行うことができるため、歪みのバラツキ低減に、より一層の効果が得られる。
【００２５】
【実施例１】
自動車変速機用ファイナルギヤ（ＳＣＭ４２０Ｈ、直径１８０mm）を被処理材Ｗとしてトレイに４０個積載し、加熱室１に入れて９５０ ℃の浸炭温度で浸炭処理（有効浸炭深さ：０.７ mm）した後、油焼入室８に移送し、下記の条件で焼入れ処理を行った。この時の冷却曲線の実測値を図３に示す。なお、図３中、▲１▼は上段歯部、▲２▼は上段肉厚部、▲３▼は下段歯部及び▲４▼は下段肉厚部をそれぞれ示す。
焼入条件
焼入開始温度：８５０ ℃
焼入油温度：１２０ ℃
油焼入室８内圧力：大気圧
一次焼入（浸漬時間）：６８秒
均熱（放置時間）：２分
二次焼入（浸漬時間）：７分
【００２６】
【比較例１】
自動車変速機用ファイナルギヤ（材質：ＳＣＭ４２０Ｈ、直径：１８０mm）を被処理材Ｗとし、均熱工程を省き連続浸漬とした以外は下記のように実施例１と同一条件下で焼入れ処理を行った。
焼入条件
焼入開始温度：８５０ ℃
焼入油温度：１２０ ℃
油焼入室８内圧力：大気圧
焼入（浸漬時間）：１０分
【００２７】
実施例１及び比較例１で得た各処理材について、歯形及び歯筋変形量のバラツキを比較した結果は下記の通りであった。これらの結果から本発明方法により油焼入れしたものは、マルテンサイト変態時における鋼材部品間若しくは鋼材部品の部位間での温度差が軽減されると共に、高温域（約５５０℃以上）での冷却速度が緩慢で熱歪が抑制されバラツキが軽減されていることが判る。
【００２８】

【００２９】
【実施例２】
自動車変速機用ハイポイドギヤ（材質：ＳＣr４２０Ｈ、直径２００mm）を被処理材Ｗとしてトレイに３２個積載し、加熱室１に入れて９５０ ℃の浸炭温度で浸炭処理（有効浸炭深さ：０.９ mm）した後、油焼入室８に移送し、下記の条件で焼入れ処理を行った。
焼入条件
焼入開始温度：８５０ ℃
焼入油温度：１２０ ℃
油焼入室８内圧力：１３ＫＰa
一次焼入（浸漬時間）：５８秒
均熱（放置時間）：２分
二次焼入（浸漬時間）：１０分
【００３０】
【比較例２】
自動車変速機用ハイポイドギヤ（材質：ＳＣr４２０Ｈ、直径２００mm）を被処理材Ｗとし、焼入れをプレス焼入れとし、その他は実施例２と同一条件で下記の焼入れ処理を行った。
焼入条件
焼入開始温度：８５０ ℃
焼入油温度：１２０ ℃
油焼入室８内圧力：大気圧
焼入（矯正型焼入れ）
焼入（浸漬時間）：１０分
【００３１】
実施例２及び比較例２で得た各処理材について、歯形及び歯筋変形量のバラツキを比較した結果は下記の通りであった。

【００３２】
これらの結果から本発明方法により油焼入れしたものは、マルテンサイト変態時における鋼材部品間若しくは鋼材部品の部位間での温度差が軽減されると共に、高温域（約５５０℃以上）での冷却速度が緩慢で熱歪が抑制されバラツキが軽減されていることが判る。
【００３３】
なお、前記実施例及び比較例では、浸炭処理された鋼材部品について焼入れ処理を例に挙げているが、本発明方法は工具鋼、軸受鋼或いは機械構造用鋼の焼入処理にも適用することができる。また、焼入槽は一槽に限らず、二槽であっても良い。
【００３４】
【発明の効果】
以上説明したように、本発明は、高温焼入油を用いて一次冷却を行うようにしたので高温域での冷却を緩慢な速度で行うことができ、高温域での熱歪を抑制すると共に、Ｍｓ点の直上温度で鋼材部品を一旦高温焼入油から取り出してその保有熱により均熱化し、その後、高温焼入油中に再浸漬して冷却するようにしたので、マルテンサイト変態時における鋼材部品間若しくは鋼材部品の部位間での温度差を軽減でき、焼入歪のバラツキを少なくすることができる。また、本発明方法は、油焼入室の内部を減圧状態にして焼入れを行うため、中温度域の冷却をある程度速く行うことができ、減圧による冷却能不足を補い、焼入れ時間が増大するのを防止できる。更に、一槽の焼入槽でマルクエンチ処理ができるとともに、油焼入室を簡単な構造とすることができる。
【図面の簡単な説明】
【図１】本発明方法の実施に使用する浸炭炉の構造を示す説明図
【図２】鋼材部品の各種雰囲気圧力下での冷却曲線を示すグラフ
【図３】鋼材部品の各部位の冷却曲線を示すグラフ
【符号の説明】
１…加熱室
２…バルブ
３…窒素供給ライン
４…窒素ガス供給源
５…排気ライン
６…真空ポンプ
７…加熱室用扉
８…油焼入室
９…空気供給ライン
１１…焼入油槽
１３…焼入油
１４…均熱空間
１５…中間扉
１６…装入兼搬出用扉
Ｗ…被処理材[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oil quenching method for steel parts, and more particularly, to a method for marqueching a steel part.
[0002]
[Prior art]
In general, when steel parts maintained at the quenching temperature are immersed in quenching oil, the steel parts are cooled through three stages: a vapor film stage (high temperature area), a boiling stage (medium temperature area), and a convection stage (low temperature area). The It is known that the cooling rate in the vapor film stage is slow and the cooling rate in the boiling stage is 3 to 10 times faster than that. In addition, high-temperature quenching oil (hot quench oil) has a slower cooling rate at medium and low temperatures than low-temperature quenching oil (cold quench oil), so it can reduce distortion caused by quenching transformation, but it is a vapor film. It is known that the thermal strain caused by the temperature difference in the high temperature range is likely to occur because the stage time is short and the end temperature of the vapor film stage is high. On the other hand, when the quenching oil is placed under a reduced pressure, as shown in FIG. 2, the boiling point is lowered, so that the time of the vapor film stage becomes longer and the end temperature of the vapor film stage becomes lower. Therefore, using such a phenomenon, as a method of reducing deformation caused by quenching, a method of quenching by immersing a steel part held at a quenching temperature in high-temperature quenching oil, or under reduced pressure A method of quenching by immersing in quenching oil has been practiced.
[0003]
On the other hand, as a method of oil quenching of steel parts such as gears, the martensitic transformation start point (the martensitic transformation start point by immersing the steel part maintained at a predetermined quenching temperature in a high-temperature coolant slightly higher than the martensitic transformation start point (Ms point) ( There is a method of martensitic transformation (marquenching treatment) by rapidly cooling to a temperature slightly higher than the Ms point), and when the entire steel parts are at substantially the same temperature, taking out from the high-temperature coolant and allowing to cool in the atmosphere. . Although this method can reduce quenching distortion and quenching variation, since it is allowed to cool in the atmosphere, the cooling rate is the temperature between the placement position of the treatment material in the tray and the part of the treated member. There is a problem that a difference is generated, and quenching distortion and quenching variation due to the temperature difference cannot be avoided.
[0004]
As a method for solving this problem, a steel part maintained at a predetermined quenching temperature is immersed in a high-temperature coolant at a temperature higher than the martensite transformation start temperature and rapidly cooled, and when the entire steel part reaches almost the same temperature. A method of immersing in a low-temperature coolant below the martensitic transformation start temperature (Japanese Patent Laid-Open No. 2-101113), or a quenching system having a circulation system for circulating quenching oil and having a hood surrounding the material to be treated Using a tank, the material to be treated is immersed in the hood while the circulation system is stopped, and the heat is used to raise the quenching oil in the hood to the vicinity of the martensite transformation start point (Ms point). A method has been proposed in which quenching oil is circulated in the circulation system and rapidly cooled below the martensite transformation point (JP-A-6-27938) when the temperature reaches almost the same temperature as a whole.
[0005]
[Problems to be solved by the invention]
However, from the viewpoint of the structure and structure of the quenching apparatus, the former requires not only a high temperature coolant and a low temperature coolant, but also a quench bath for the high temperature coolant and a quench bath for the low temperature coolant. Therefore, there is a problem that the quenching apparatus cannot be increased in size and complexity, and maintenance is poor. Although the latter solves the problem in the former, there is a problem that the quenching tank itself becomes complicated. Further, from the viewpoint of quenching distortion and variation, both of them are immersed in a coolant so that the entire steel part is brought to substantially the same temperature. It is difficult to bring the treated material and treated material uniformly and sufficiently into contact with each other, and a temperature difference occurs between the steel parts or between the parts of the steel parts at the time of soaking, which is effective in reducing quenching distortion and variation. Yes, but not satisfactory.
[0006]
As a result of extensive research to solve the above problems, these quenching distortions and variations are caused by a temperature difference between steel parts or parts of steel parts during martensitic transformation, and at a high temperature range (about 550 ° C. or more). It was clarified that the cooling rate was too high.
[0007]
Therefore, the present invention reduces the temperature difference between the steel parts or the parts of the steel parts at the time of the martensitic transformation and is sufficient to suppress the thermal strain at the cooling rate in the high temperature range (about 550 ° C. or higher). The object is to reduce the quenching distortion and quenching variation by using a slow speed.
[0008]
[Means for Solving the Problems]
As a means for solving the above-mentioned problems, the present invention immerses a steel part maintained at a predetermined quenching temperature in high-temperature quenching oil, and the predetermined part of the steel part has a martensitic transformation start point (Ms point). The steel part is taken out of the high-temperature quenching oil and is soaked by the retained heat of the steel part, and subsequently immersed in the high-temperature quenching oil to be cooled. It is a thing.
[0009]
In the embodiment of the present invention, in the rapid cooling process, the steel material part is a predetermined part thereof, that is, a part where the amount of deformation due to strain caused by martensitic transformation is the largest, specifically, a unit surface of the steel part. Quenching is performed until a portion having a small internal volume (for example, a sharp portion such as a gear tooth or a corner portion of a prismatic part hereinafter) reaches a temperature just above the Ms point of the portion.
[0010]
This is based on the following knowledge. That is, in the method of the present invention, the steel part is first immersed in high-temperature quenching oil and rapidly cooled to a temperature just above the martensite transformation start point (Ms point). It is preferable to take out from the high-temperature quenching oil when it is cooled to a temperature just above the Ms point. However, since actual steel parts are generally carburized immediately before quenching, the concentration of carbon diffused inside the steel parts is not uniform throughout the steel parts, and the Ms point may differ depending on the part. For example, the sharp part of the steel part has a higher carbon concentration in the surface layer part than a part other than the sharp part (for example, a part that is not carburized inside the steel part, that is, a non-carburized part). The Ms point (Ms ₂ ) of the surface layer part becomes lower than the Ms point (Ms ₁ ) of the non-carburized part, resulting in variations in the amount of deformation. However, the part other than the sharp part of the steel part, that is, the non-carburized part is less deformed even when normally quenched, and the steel part even when the temperature of the part is directly below the Ms point (Ms ₁ ) of the part. If the temperature of the sharpened portion, which is the portion with the largest deformation amount, is just above the Ms point (Ms ₂ ) of the portion, the variation in the deformation amount of the entire steel part is reduced by soaking. It is based on the knowledge that it can be done.
[0011]
Although the kind, temperature, and quantity of the quenching oil are set according to the material to be treated, a high-temperature quenching oil corresponding to JIS K2242 Type 2 No. 1 or 2 is usually employed. Moreover, the temperature of hardening oil is set to the temperature within the range of 100-170 degreeC. The amount of quenching oil may be set to an amount that does not cause the set temperature to fluctuate greatly due to primary quenching.
[0012]
In addition, after quenching, the material to be treated is kept in the upper space of the quenching oil in the oil quenching chamber in order to equalize the temperature, and the ambient temperature in this upper space is usually almost the same as the temperature of the high temperature quenching oil. Temperature. The material that has been subjected to the primary quenching is held in the upper space of the oil quenching chamber until it reaches a temperature immediately above the martensite transformation start point (Ms point) by the heat retained by itself. Since it differs depending on the size, material, etc. of the material to be processed, it is difficult to define it uniquely, but it is usually set to 30 to 300 seconds.
[0013]
After soaking, the so-called marquenching process in which the material is soaked again in high-temperature quenching oil can further reduce the temperature difference between the placement position of the material to be treated in the tray and the part of the material to be treated.
[0014]
In the embodiment of the present invention, the pressure in the oil quenching chamber during quenching is set to 7 to 75 KPa, more preferably 8 to 40 KPa. This is because when the vacuum is higher than 7 KPa, the vapor film stage becomes too long and it is difficult to obtain sufficient quenching hardness, and when the vacuum is lower than 75 KPa, a sufficient decompression effect cannot be obtained and thermal distortion cannot be suppressed. is there.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows the structure of a batch-type carburizing furnace used for carrying out the method of the present invention. The furnace comprises a heating chamber 1 and an oil quenching chamber 8. The heating chamber 1 is connected to a nitrogen gas supply source 4a via a nitrogen supply line 3a provided with a valve 2a, and is connected to a vacuum pump 6a via an exhaust line 5a provided with a valve 2b. A carburizing gas supply line is connected, and the heating chamber 1 is connected to an oil quenching chamber 8 with a heating chamber door 7 interposed therebetween.
[0016]
On the other hand, the oil quenching chamber 8 is connected to the nitrogen gas supply source 4b via a nitrogen supply line 3b provided with a valve 2c and communicates with the atmosphere via an air supply line 9 provided with a valve 2d. Is connected to a vacuum pump 6b through an exhaust line 5b. The oil quenching chamber 8 has a quenching oil tank 11 inside thereof, and moves the workpiece W up and down so as to be immersed and held in the quenching oil, while holding it in a space 14 above the quenching oil tank 11. Elevating means (not shown) is provided, an intermediate door 15 is disposed on the heating chamber 1 side, and a loading / unloading door 16 is disposed on the atmosphere side.
[0017]
In carrying out the method of the present invention, first, the heating chamber 1 and the oil quenching chamber 8 are evacuated to a predetermined degree of vacuum within a range of about 7 to 75 KPa. After both chambers reach a predetermined degree of vacuum, the workpiece W is transferred from the heating chamber 1 into the oil quenching chamber 8 and immersed in the high temperature quenching oil 13 so that the surface of the workpiece W is martensite. First quenching is performed by rapid cooling until the temperature is just above the transformation start point (Ms point). At this time, since the quenching oil 13 is a high-temperature quenching oil and is cooled by immersion in a state where the inside of the oil quenching chamber 8 is depressurized, the steam is compared with quenching by high-temperature quenching oil under atmospheric pressure. Since the cooling rate in the film stage is slow and the cooling time in the vapor film stage is long, the material W to be processed is slowly and uniformly cooled in the high temperature region (850 to 550 ° C.).
[0018]
Next, when quenching is continued, the temperature of the surface where the strain or the like is outside the allowable range, for example, the surface of the sharpened portion of the material to be processed has reached a temperature immediately above the surface martensite transformation start point (Ms ₂ point). At that time, the material to be treated W is taken out from the high-temperature quenching oil 13 and is left in the upper space 14 in the oil quenching chamber 8 so that the surface and the inside of the material to be treated W are surface martensite transformation start points (Ms ₂ points). Soak until it reaches a temperature just above.
[0019]
Further, after soaking, the material W to be treated is again immersed in the high-temperature quenching oil 13 and cooled to perform secondary quenching. At this time, since the material W to be processed has undergone a soaking process and is immersed in the high-temperature quenching oil, it can be cooled more uniformly than air cooling, so that the martensitic transformation is uniformly performed by the marquenching process. Moreover, since the high temperature quenching oil has a slower cooling rate in the cold temperature region than the low temperature quenching oil, the workpiece W can be cooled more uniformly.
[0020]
When the cooling is completed, the material W to be treated is taken out from the high-temperature quenching oil 13, the valve 2d is opened, air is introduced into the oil quenching chamber 8 from the air supply line 9, and the pressure is restored to the atmospheric pressure. The material is taken out of the furnace through the charging / discharging door 16 from the quenching chamber 8.
[0021]
The to-be-processed material W shows separate temperature history in each site | part during hardening, and the martensitic transformation start point (Ms point) in the site | part also differs according to carbon concentration (C%). The relationship between the carbon concentration and the martensite transformation start point (Ms point) is given by, for example, the following known formula.
Ms (° C.) = 550-361 (% C) −39 (% Mn) −35 (% Cr) −17 (% Ni) −10 (% Cu) −5 (% Mo +% W) +15 (% Co) +30 (% Al)
[0022]
Therefore, the present invention is most effective when it is taken out from the high-temperature quenching oil 13 at a temperature immediately above the martensite transformation start point (Ms point) obtained by the above formula in all the parts of the material to be treated W. However, although only four points are shown in FIG. 3, the temperature transition and the martensitic transformation start point (Ms point) at the time of primary cooling, which differ depending on the part of the material to be processed W, are obtained over all points, and the martensite It is virtually impossible to take out at a temperature just above the site transformation start point (Ms point). Therefore, in the present invention, when the temperature of the sharpened portion, which is the portion having the largest deformation amount, reaches a temperature just above the Ms point (Ms ₂ ) of the portion, the high temperature quenching oil 13 is taken out. This is because, even in the case of normal quenching, for a portion with a small amount of deformation, even if the temperature of the surface layer portion is just below the Ms point (Ms ₁ ) of the portion, the temperature of the sharp portion that is the portion with the largest amount of deformation is This is because if the portion is just above the Ms point (Ms ₂ ), the variation in the deformation amount of the entire steel part can be reduced by soaking.
[0023]
Note that the temperature at which the part where the strain, etc. is outside the allowable range when quenching is continued (specifically, the part that requires quenching on the surface of the steel part) is just above the surface martensite transformation start point (Ms ₂ point). time to reach, i.e., the timing at which to eject the workpiece W from a high-temperature quenching oil 13 is controlled by the immersion time. Further, when the first-quenched workpiece W reaches a temperature immediately above the martensitic transformation start point (Ms ₂ point), that is, when it is immersed again in the high-temperature quenching oil 13, the oil quenching chamber 8 It is managed by the leaving time in the upper space 14.
[0024]
In the said embodiment, although the case where the oil quenching chamber 8 was made into the pressure reduction state was mentioned as an example, you may implement the oil quenching chamber 8 in an atmospheric pressure state. In this case, after the primary quenching, the martensite transformation in the subsequent secondary quenching can be performed uniformly by pulling up from the quenching oil and performing a soaking treatment, so that the effect that should satisfy the reduction in distortion variation There is. Further, in the reduced pressure state, the material W to be processed can be more uniformly cooled in the high temperature region, and thus a further effect can be obtained in reducing the variation in distortion.
[0025]
[Example 1]
40 final gears for automobile transmission (SCM420H, diameter 180mm) are loaded on tray as material W to be processed, put into heating chamber 1 and carburized at carburizing temperature of 950 ° C (effective carburizing depth: 0.7 mm) After that, it was transferred to the oil quenching chamber 8 and subjected to quenching treatment under the following conditions. The measured value of the cooling curve at this time is shown in FIG. In FIG. 3, (1) indicates the upper tooth portion, (2) indicates the upper thick portion, (3) indicates the lower tooth portion, and (4) indicates the lower thick portion.
Quenching conditions Quenching start temperature: 850 ° C
Quenching oil temperature: 120 ℃
Oil quenching chamber 8 pressure: Atmospheric pressure primary quenching (immersion time): 68 seconds soaking (standing time): 2 minutes Secondary quenching (immersion time): 7 minutes [0026]
[Comparative Example 1]
A final gear for automobile transmission (material: SCM420H, diameter: 180 mm) was treated as material W, and the quenching process was performed under the same conditions as in Example 1 except that the soaking process was omitted and continuous immersion was performed. .
Quenching conditions Quenching start temperature: 850 ° C
Quenching oil temperature: 120 ℃
Oil quenching chamber 8 pressure: Atmospheric pressure quenching (immersion time): 10 minutes [0027]
About each processing material obtained in Example 1 and the comparative example 1, the result of having compared the variation of a tooth profile and the amount of deformation of a tooth trace was as follows. From these results, oil quenching by the method of the present invention reduces the temperature difference between steel parts or parts of steel parts during martensitic transformation, and cooling rate in a high temperature range (about 550 ° C. or higher). It can be seen that is slow, thermal strain is suppressed, and variation is reduced.
[0028]

[0029]
[Example 2]
32 hypoid gears for automobile transmissions (material: SCr420H, diameter 200 mm) are loaded on the tray as material W to be processed, put into heating chamber 1 and carburized at a carburizing temperature of 950 ° C. (effective carburizing depth: 0.9 mm) ) And then transferred to the oil quenching chamber 8 for quenching treatment under the following conditions.
Quenching conditions Quenching start temperature: 850 ° C
Quenching oil temperature: 120 ℃
Pressure in oil quenching chamber 8: 13KPa
Primary quenching (immersion time): 58 seconds Soaking (standing time): 2 minutes Secondary quenching (immersion time): 10 minutes [0030]
[Comparative Example 2]
The hypoid gear for automobile transmission (material: SCr420H, diameter 200 mm) was treated material W, the quenching was press-quenched, and the following quenching treatment was performed under the same conditions as in Example 2.
Quenching conditions Quenching start temperature: 850 ° C
Quenching oil temperature: 120 ℃
Oil quenching chamber 8 pressure: Atmospheric pressure quenching (correction quenching)
Quenching (dipping time): 10 minutes [0031]
About each processing material obtained in Example 2 and the comparative example 2, the result of having compared the variation of a tooth profile and the amount of deformation of a tooth trace was as follows.

[0032]
From these results, oil quenching by the method of the present invention reduces the temperature difference between steel parts or parts of steel parts during martensitic transformation, and cooling rate in a high temperature range (about 550 ° C. or higher). It can be seen that is slow, thermal strain is suppressed, and variation is reduced.
[0033]
In the examples and comparative examples, the carburized steel parts are exemplified as the quenching process, but the method of the present invention is also applicable to the quenching process of tool steel, bearing steel or machine structural steel. Can do. Further, the quenching tank is not limited to one tank and may be two tanks.
[0034]
【The invention's effect】
As described above, in the present invention, the primary cooling is performed using the high-temperature quenching oil, so that the cooling in the high temperature range can be performed at a slow rate, and the thermal distortion in the high temperature range is suppressed. The steel part is once taken out from the high-temperature quenching oil at a temperature just above the Ms point, soaked by the retained heat, and then cooled by reimmersing in the high-temperature quenching oil. It is possible to reduce the temperature difference between the steel parts or between the parts of the steel parts, and to reduce the variation in quenching strain. In addition, since the method of the present invention performs quenching with the inside of the oil quenching chamber under reduced pressure, the middle temperature range can be cooled to some extent faster, compensating for the lack of cooling capacity due to reduced pressure, and increasing the quenching time. Can be prevented. In addition, a marquenching process can be performed in one quenching tank, and the oil quenching chamber can have a simple structure.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the structure of a carburizing furnace used for carrying out the method of the present invention. FIG. 2 is a graph showing cooling curves of steel parts under various atmospheric pressures. FIG. 3 is a cooling curve of each part of the steel parts. Graph [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Heating chamber 2 ... Valve 3 ... Nitrogen supply line 4 ... Nitrogen gas supply source 5 ... Exhaust line 6 ... Vacuum pump 7 ... Heating chamber door 8 ... Oil quenching chamber 9 ... Air supply line 11 ... Quenching oil tank 13 ... Burning Oil entry 14 ... Soaking space 15 ... Intermediate door 16 ... Door for loading and unloading W ... Material to be treated

Claims

The largest part of the amount of deformation due to distortion caused by immersing the predetermined baked steel parts held by the entry temperature into the high temperature quenching oil in martensitic transformation of the steel component is martensitic transformation start point (Ms point) after quenched to reach just above the temperature, and soaking by the steel components of potential heat removed the steel part from a high-temperature quenching oil, characterized by cooling and subsequently immersed in hot quenching oil carburizing Oil quenching method for processed steel parts.

The method according to claim 1 , wherein the pressure in the oil quenching chamber during quenching is 7 KPa to 75 KPa.

The method according to claim 1 or 2, wherein the temperature of the high-temperature quenching oil is a temperature within a range of 100 to 170 ° C.