JP2004332075A

JP2004332075A - Carburization control method and carburizing device using the method

Info

Publication number: JP2004332075A
Application number: JP2003132072A
Authority: JP
Inventors: Satoshi Haneki; 敏羽木; Moriyoshi Tamura; 守淑田村; Masahiro Okumiya; 正洋奥宮
Original assignee: Toho Gas Co Ltd
Current assignee: Toho Gas Co Ltd
Priority date: 2003-05-09
Filing date: 2003-05-09
Publication date: 2004-11-25

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carburization control method capable of introducing carburization gas of adequate volume into a carburizing furnace during the carburization under a reduced pressure state, and consistently feeding carburized products of high quality, and a carburizing device using the method. <P>SOLUTION: In the carburization control method, during carburization in which carburization gas is introduced into a carburizing furnace 1 with a steel W as a work set therein, and carburization gas is reacted with the steel W to allow carbon to be subjected to solid solution/diffusion in the steel W, the flow rate of the introduced carburization gas, the exhaust flow rate of the in-furnace atmospheric gas, and the total pressure in the carburizing furnace 1 are measured, the concentration of the specified gas in the in-furnace atmospheric gas is constantly measured during the carburization on the exhaust side of the in-furnace atmospheric gas by using a concentration sensor 8, and the flow rate of the carburization gas introduced into the furnace is adjusted and controlled in the real time basis based on the measured values. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、浸炭処理制御方法及びその方法を用いた浸炭処理装置に関し、更に詳しくは、鋼材表面を炭化水素系ガスを主成分とする浸炭ガスを使用して浸炭処理する浸炭処理工程において、その浸炭ガスの浸炭炉内への導入流量を常時制御するのに好適に用いられる浸炭処理制御方法及びその方法を用いた浸炭処理装置に関する。
【０００２】
【従来の技術】
鋼材表面の硬化技術として古くから知られている「浸炭」は、低炭素鋼を浸炭ガス中で加熱し、鋼材表面からカーボン（炭素）を拡散させ、その鋼鋼材表層部のカーボン濃度を高める手法である。この浸炭処理により鋼材表層部は高温安定相であるオーステナイトからの急冷によりマルテンサイト化されて硬化層が形成され、鋼材内部は依然低炭素のままで高い靭性を保持した浸炭処理製品が得られる。
【０００３】
このような浸炭処理技術として従来から知られている方法の１つに、常圧下でのガス浸炭法があるが、このガス浸炭法では、例えば、浸炭ガスとして一酸化炭素（ＣＯ）を用いる場合、高温常圧ガス雰囲気下において一酸化炭素と被浸炭処理材との反応によりカーボン（Ｃ）と二酸化炭素（ＣＯ_２）が生成し、生成したカーボンが被浸炭処理材である鋼材中に固溶し、さらに固溶したカーボンが鋼材表面から内部へと拡散することにより浸炭処理が行われる（例えば、特許文献１参照）。
【０００４】
このガス浸炭のメカニズムは、▲１▼まず、高温雰囲気下において浸炭ガスが鋼材表面に接触し、▲２▼鋼材表面において鋼材と浸炭ガスとが反応（浸炭反応）してカーボンを生成すると共に生成カーボンが鋼材中へと固溶し、▲３▼固溶したカーボンが鋼材表面から内部への拡散することにより鋼材表層部に浸炭層が形成されるというものである。また、鋼材に固溶したカーボンが鋼材表面において浸炭ガスへと脱炭する反応も同時に生じている。このため、式１に示す平衡関係が成立し、この関係式から鋼材中のカーボンポテンシャルが求められる（式２参照）。
【０００５】
このガス浸炭法では、炉内雰囲気ガス中の二酸化炭素（ＣＯ_２）分圧や酸素（Ｏ_２）分圧を二酸化炭素センサ、酸素センサを用いて測定し、あるいは炉内雰囲気ガス中の水蒸気圧を露点計により測定し、これら測定値に基づいて所定の演算式によりカーボンポテンシャル（Ｐ_Ｃ）を算出する方法（式２参照）を用いて浸炭雰囲気を制御するのが一般的である。なお、式２におけるカーボンポテンシャルの算出に際しては、浸炭反応を式１のように仮定した。
【０００６】
【式１】

【０００７】
【式２】

【０００８】
平衡定数Ｋ_１は温度によって決まる定数で文献等で既知であり、オーステナイトの飽和カーボン濃度ａ_Ｓも既知であり、また、Ｐ_ＣＯは導入する浸炭ガスによって決まり、一定であるから、Ｐ_ＣＯ２を測定すればカーボンポテンシャルＰ_Ｃを算出することができる。
【０００９】
しかし、近年、環境問題や資源問題に対する関心の高まりを背景として、省エネルギー、省資源や公害防止に極めて有効な浸炭法として減圧雰囲気下での浸炭処理工程が注目を浴びている。
【００１０】
この浸炭処理は、メタン（ＣＨ_４）、エタン（Ｃ_２Ｈ_６）等の炭化水素系ガスを浸炭ガスとして使用し、これを高温・減圧雰囲気下で鋼材と反応（浸炭反応）させることにより行うものである。例えば、浸炭ガスをメタンとする場合には、メタンが被浸炭処理材表面で炭素（Ｃ）と水素（Ｈ_２）に分解し、生成した炭素が鋼材中へと固溶しさらに鋼材表面から内部へと拡散する。
【００１１】
この減圧雰囲気下における浸炭処理によれば、上記浸炭反応によって高品質の浸炭処理製品が得られ、浸炭処理時のガス消費量が常圧浸炭処理に比べて少なくて済む上、二酸化炭素の排出がないため環境特性にも優れるという利点を有する。
【００１２】
【特許文献】
特開２０００−１４４３７１号公報
【００１３】
【発明が解決しようとする課題】
しかしながら、減圧雰囲気下での浸炭処理においては、「ＣＨ_４→Ｃ＋２Ｈ_２」のように浸炭ガスから鋼材への浸炭反応が起こるのみで鋼材から浸炭ガスへの炭素移動（脱炭反応）は起こらないため、常圧雰囲気下でのガス浸炭法のように式１のような平衡関係が得られず、雰囲気ガスの特定成分の濃度を測定して鋼材中のカーボンポテンシャルを求めることができない。従って、炭化水素系ガスを直接注入する減圧雰囲気下での浸炭処理では、雰囲気ガスの特定成分の濃度を測定するのみで浸炭雰囲気を制御するができない。
【００１４】
一方、減圧雰囲気下での浸炭処理においては、浸炭雰囲気中のガス種の検知及び制御を行う技術が未だ確立されておらず、現状においては、浸炭処理後の製品の浸炭処理の程度を確認して試行錯誤により浸炭ガスの導入量等を決定せざるを得なかった。
【００１５】
したがって、現状においては減圧雰囲気下での適正な浸炭処理条件を確立するのには困難を伴う。具体的には、浸炭炉の形状や大きさ、あるいは処理品（ワーク、鋼材）の材質や炉内への投入量などによって浸炭ガスの適正導入量が異なってくるという問題があった。また浸炭ガスの導入量がフィードバック制御されていないため、ガス導入量が過剰になると煤が発生し少なすぎると十分に浸炭されず、その結果、製品の品質にばらつきが生じ、歩留まりが悪くなるという問題があった。
【００１６】
そこで、本発明者らは種々検討を重ねた結果、「ＣＨ_４→Ｃ＋２Ｈ_２」の反応ように導入された炭化水素系の浸炭ガスと鋼材との浸炭反応によって生じる水素（Ｈ_２）や浸炭反応に寄与しなかった未反応の炭化水素系ガスの濃度を検知すると共にその他浸炭ガスの導入条件等を検知・制御することによって浸炭処理時の浸炭雰囲気の制御が可能であるとの考えに至ったものである。
【００１７】
本発明の解決しようとする課題は、減圧雰囲気下で浸炭処理を行う際の、適正量の浸炭ガスを導入することを可能にするとともに、高品質の浸炭処理製品の安定生産を可能とし、また更には処理時間の短縮化、歩留まりの改善による生産性の向上、更には浸炭ガス消費量の抑制などによる生産性の向上をも可能とする浸炭ガス制御方法及びその方法を用いた浸炭処理装置を提供することである。
【００１８】
【課題を解決するための手段】
この課題を解決するための本発明の浸炭処理制御方法は、被浸炭処理材が載置される浸炭炉内に炭化水素系ガスを主成分とする浸炭ガスを導入し、この浸炭ガスと前記被浸炭処理材との浸炭反応によって生成する炭素を被浸炭処理材中へ固溶・拡散させることにより被浸炭処理材の浸炭処理を行うに際し、前記浸炭ガスの導入流量と炉内雰囲気ガスの排出流量と炉内雰囲気ガスの全圧とを計測すると共に炉内雰囲気中の特定ガス種の濃度を炉内雰囲気ガスの排出側にて浸炭処理中常時計測し、これらの計測値に基づいて炉内に導入される浸炭ガス導入流量をリアルタイムで調整制御するようにしたことを要旨とする。
【００１９】
また本発明の浸炭処理装置は、被浸炭処理材である鋼材が載置される浸炭炉に、浸炭ガスを炉内へ導入する浸炭ガス導入管路と、炉内雰囲気ガスを排出する浸炭雰囲気ガス排出管路と、炉内雰囲気の全圧を計測する圧力計測装置と、炉内を加熱する加熱手段とを備え、前記浸炭ガス導入管路に、この管路を流れる浸炭ガスの流量を調整する浸炭ガス流量調整器と、この浸炭ガス流量調節器により調節される浸炭ガスの導入流量を計測する導入流量計測装置とが設けられ、前記浸炭排ガス導出管路に、この管路を流れる炉内雰囲気ガスの流量を計測する排出流量計測装置が設けられ、この浸炭排ガス導出管路の排出口近傍に炉内雰囲気中の特定ガス種の濃度を検知するセンサが設けられており、前記導入流量計測装置により計測される浸炭ガスの導入流量値と、前記排出流量計測装置により計測される炉内雰囲気ガスの排出流量値と、炉内雰囲気ガスの全圧値と、前記ガス圧センサにより検知される炉内雰囲気中の特定ガス種の濃度値とに基づいて鋼材中に浸炭固溶された炭素量を演算により求める浸炭量演算器と、この浸炭量演算器に基づいて前記浸炭ガス流量調節器の浸炭ガス流量を調節する浸炭ガス流量制御手段とが備えられていることを要旨とする。
【００２０】
この浸炭処理制御方法及びその方法を用いた浸炭処理装置によれば、導入された浸炭ガスと被浸炭処理材である鋼材との反応（浸炭反応）により発生する反応ガス及び浸炭反応に寄与しない未反応ガスとが混在する炉内の雰囲気ガス中の特定ガス種のガス濃度を浸炭炉の排出側において検知すると共に、浸炭ガスの導入流量値、炉内ガスの排出流量値及び浸炭炉内の全圧を検知することにより、適正の浸炭処理材が得られるように浸炭ガスの導入流量を制御することができる。
【００２１】
また、炉内の雰囲気ガス中のガス濃度をリアルタイムで制御することによって品質の安定した浸炭処理製品が得られ、また、過不足のない浸炭ガスの安定供給が図られることによって鋼材表面の煤の発生も回避され、さらには浸炭ガスの消費も必要最小限に抑えることができる。
【００２２】
また、炉内雰囲気中のガス濃度を検知する濃度センサとしては、浸炭反応によって発生する水素のガス濃度を計測すべく、プロトン導電性セラミックスを用いることが好ましい。これにより炉内の雰囲気ガスを精度よく制御することができる。
【００２３】
また、プロトン導電性セラミックスを用いた水素検知用の濃度センサとしては、プロトン導電性セラミックス管の炉内挿入端内外面に白金電極を設け、プロトン導電性セラミックス管内の基準水素と、炉内雰囲気ガス中の水素との濃度差によって生じる水素イオンのプロトン導電性セラミックス管内の拡散に伴う両白金電極間の電位差を測定することにより炉内水素濃度を測定するものが好ましい。これにより、炉内の雰囲気ガス中の水素濃度の検知ならびに浸炭制御がさらに高精度のものとなり高品質の浸炭処理製品を得ることができる。
【００２４】
【発明の実施の形態】
以下に本発明の好適な実施の形態を図面を参照して詳細に説明する。
【００２５】
図１は、本発明に係る浸炭処理装置の概略構成を示している。この浸炭処理装置は、被浸炭処理材である鋼材Ｗが載置されると共にこの鋼材Ｗを浸炭処理温度に加熱保持するための浸炭炉１と、浸炭ガスを炉内に導入するための浸炭ガス導入管路２と、導入された炉内雰囲気ガスを排出する炉内雰囲気ガス排出管路３とから構成される。浸炭炉１には炉内を浸炭温度まで加熱するためのヒータ４が備えられ、また、浸炭炉１の炉壁には炉内雰囲気ガスの圧力（全圧）を検知する圧力計５が備え付けられている。
【００２６】
浸炭ガス導入管路２には、この管路を流れる浸炭ガスの流量を増減させる自動電磁弁タイプの流量調節機能を備えた流量調節器６が設けられ、さらにこの下流側には炉内に導入される浸炭ガスの導入流量値を計測する導入ガス流量計７が設けられている。
【００２７】
また、炉内雰囲気ガス排出管路３の排出口近傍の炉壁には、炉内雰囲気中の特定のガス種の濃度を検知するための濃度センサ８が備え付けられている。なお、本実施の形態では、この濃度センサ８として、浸炭ガスと鋼材との反応（浸炭反応）によって生じる水素の濃度を検知する水素センサ８０が備え付けられている。濃度センサ８としては、水素濃度を検知するものだけでなく、炉内雰囲気中の炭化水素系ガスの濃度を検知するものであっても良い。
【００２８】
また、炉内雰囲気ガス排出管路３には、炉内を減圧状態とするための吸引ポンプ９が設けられ、この排出管路３の途中には吸引ポンプ９による炉内の吸引排出のオン・オフを切換可能な減圧バルブ１０が設けられ、この減圧バルブ１０の上流側には、この管路を流れる排出ガスの排出流量値を計測する排出ガス流量計１１が設けられている。
【００２９】
上記水素センサ８０、導入ガス流量計７、排出ガス流量計１１及び圧力計５は、この浸炭炉１を運転制御する制御システムに設けられる炭素量演算器１２に電気的に接続される。さらにこの炭素量演算器１２は、浸炭ガス導入管路２に設けられる流量調節器６とも電気的に接続され、この演算器１２からの指令信号により流量調節器６の自動電磁弁の開度が自動的に調節されるようになっている。
【００３０】
図２に浸炭炉１の排出側の炉壁に備え付けられてなる水素センサ８０の概略構成を示している。この水素センサ８０は、一端が閉塞されたプロトン導電性セラミックス管８１（以下、「プロトン導電管８１」と称する）の筒底部分の内外壁面に白金電極Ｅｉ、Ｅｓを設けたセンサプローブを形成し、これを基準水素ガスが装填されるセラミックスチューブ８２の先端にシール剤８３を介して接合し、このセンサプローブのプロトン導電管８１をセラミックス製の保護管８４に挿通してこの保護管８４内にこのプロトン導電管８１の導体部を無機接着剤８５により接合してなるもので、この保護管８４の導体部分を前記した浸炭炉１の炉壁に挿通装着することによりこの水素センサ８０はプロトン導電管８１の先端部分が浸炭炉１内に挿入された状態で炉壁に備え付けられている。
【００３１】
以下に、浸炭炉１内の雰囲気ガス中の水素濃度（分圧）を水素センサ８０を用いて検知する原理について図３を参照しながら説明する。まず、浸炭炉１に備え付けられる水素センサ８０のプロトン導電管８１内に基準用水素として１％濃度水素とヘリウムとからなる混合ガスを充填した状態で、減圧された炉内雰囲気中に浸炭ガスを導入し鋼材の浸炭処理を開始する。浸炭ガスと鋼材との反応（浸炭反応）によって発生する浸炭炉１内の水素濃度（分圧）はプロトン導電管８１内の基準用（１％濃度）水素よりも高いため、プロトン導電管８１内の炉内側では、「Ｈ_２→２Ｈ^＋＋２ｅ⁻」の反応が起こり、プロトン導電管８１内の水素が水素イオン（Ｈ^＋）となってプロトン導電管８１の管壁内を炉外側に向けて透過し、炉内の低水素濃度側では、「２Ｈ^＋＋２ｅ⁻→Ｈ_２」の反応が起ころうとする。
【００３２】
このような反応機構に基づいてプロトン導電管８１の内外壁面白金電極Ｅｉ、Ｅｓ間に炉内側とセラミックス管８１内側の水素の濃度差に対応した起電力が発生することとなり、この電位差を測定することにより浸炭炉１内の水素分圧が測定される。なお、浸炭炉１内の水素分圧Ｐ_Ｈ２は、次のＮｅｒｎｓｔの式（式３）を用いて求められる。
【００３３】
【式３】

【００３４】
また、この水素分圧Ｐ_Ｈ２を水素濃度Ｃ_Ｈ２に換算するために、気体の状態方程式から炉内水素のモル数と炉の容積を算出する。これにより浸炭炉１内の水素濃度値が算出される。算出式を式４に示す。
【００３５】
【式４】

【００３６】
次に、上記算出式に基づく水素センサによって検知される浸炭炉１内の水素濃度値を用いて、浸炭処理時に鋼材中へ固溶される炭素量を計測する演算手法について説明する。上述の式３及び式４から浸炭炉１内の水素濃度Ｃ_Ｈ２が求められると、以下の演算手法により固溶炭素量が算出される。
【００３７】
浸炭処理からｔ分後における炉内雰囲気ガスの排出流量Ｘ（ｔ）（単位：Ｌ／時間）を排出ガス流量計１１により計測し、Δｔ間（時間ｔ_１からｔ_２まで）に導入された浸炭ガスの排出積算流量Ａ（ｔ）を算出すると共に、排出側の浸炭雰囲気ガス中の特定ガス種の濃度（この場合、水素濃度）Ｃ_Ｈ２（モル／Ｌ）を計測し、排出積算流量Ａ（ｔ）と水素濃度Ｃ_Ｈ２（モル／Ｌ）からΔｔ間に浸炭炉１内で固溶した炭素量ΔＣ（ｔ）を算出する。算出式を式５に示す。なお、算出に際しては、浸炭ガスはメタン（ＣＨ_４）とし、これが浸炭時に全て炭素と水素とに分解したとの仮定をおいている。
【００３８】
【式５】

【００３９】
なお、炭素量ΔＣ（ｔ）の算出に際しては、浸炭炉１内に導入された浸炭ガスは全て浸炭反応に寄与し、炭素と水素に分解するとの仮定をおいている。従って、上記計算手法では、排出時のガス流量Ｘ（ｔ）と排出側の特定ガス種（この場合水素）の濃度を検出すれば、浸炭時の炭素の固溶量（浸炭量）を特定することができる。但し、導入される浸炭ガスの一部分のみが浸炭反応に寄与する場合などには、浸炭ガスの導入ガス流量や浸炭炉内の全圧を測定し、これらの測定値と上記炉内雰囲気ガスの排出流量値と排出側の特定ガス種濃度値とから鋼材中に固溶される炭素量ΔＣ（ｔ）を算出する演算式を炭素量演算器１２中に入力しておく必要がある。
【００４０】
上述の算出式（式３〜式５）に基づいた炭素量演算器１２によって演算された鋼材中の炭素量ΔＣ（ｔ）値が変動すれば、炭素量演算器１２内に設けられるコントローラからの指令信号により浸炭ガス導入管路２の流量計測器６に設けられる図示しない電磁弁の開度が自動調節（フィードバック制御）される。この浸炭ガス流量のフィードバック制御は浸炭処理中常時行われる。最終的には、鋼材Ｗ中に固溶される炭素量が所定の値となったところで浸炭処理を終了する。なお、上述の算出式では、浸炭炉１内に存在するガスはメタンと水素のみであると仮定しているが、浸炭処理過程においてこれらガス種以外の他のガス種、例えば、エチレン（Ｃ_２Ｈ_４）やアセチレン（Ｃ_２Ｈ_２）等が発生する場合には、これらガス種の濃度も濃度センサによって検知して上述の演算式（式５）を変更して当てはめる必要がある。
【００４１】
図４は、この実施例において適用された浸炭処理工程の浸炭処理サイクルを経時的に示したものである。浸炭炉１内に被浸炭処理材である鋼材Ｗをセットした後、この炉を密閉状態とし、炉内に不活性ガスである窒素（Ｎ_２）ガスを導入しながら吸引ポンプ９により炉内雰囲気を吸引・炉外へ排出し炉内を減圧状態とする。この時、炉内圧力は６０Ｐａ程度とする。
【００４２】
次いで、この減圧状態を保持したまま炉内雰囲気及び浸炭炉１内に載置される鋼材Ｗをヒータ４により加熱し、およそ１時間で９３０℃程度まで昇温する。そして、所定温度（９３０℃）に達したところで浸炭処理を開始する。本実施の形態においては、浸炭ガスとしてメタンを主成分とする都市ガス（１３Ａ）を用いた。なお、浸炭ガスはパルス方式により浸炭炉１内に導入した。ここで、パルス方式とは、浸炭ガス（ここでは、都市ガス（１３Ａ））を一定のタイミングでパルス的（間歇的）に炉内へ導入する手法である。
【００４３】
図４にこのパルス方式による浸炭処理のタイミングチャートを拡大して示す。（１）まず、浸炭ガスを所定ガス流量に制御して浸炭炉１内に導入する。この時、炉内雰囲気ガス排出管路３の末端に設けられた減圧バルブ９を開から閉に切り替えておく。（２）浸炭炉１内に所定量の浸炭ガスが導入された時点で導入を一旦停止して浸炭ガスを浸炭炉１内に拡散させると共に鋼材と反応（浸炭反応）させる。この時、浸炭ガスと鋼材との反応により炭素が鋼材中に固溶される。（３）浸炭ガスを所定時間拡散保持した後、減圧バルブ９を閉から開に戻して炉内雰囲気ガスを排出する。以上を１サイクルとして、これを複数サイクル繰り返すことで浸炭処理を行う。なお、浸炭ガスの導入量等の諸条件は、上述の炭素量演算器１２により演算される鋼材Ｗ中への炭素固溶量値に基づいて制御される。
【００４４】
このように浸炭制御することにより、浸炭炉１内に導入された浸炭ガスは鋼材との反応により炭素を生成し、この生成炭素が鋼材Ｗ中に固溶し、さらに固溶された炭素が鋼材Ｗの表面から内部へと拡散される。パルス方式による浸炭処理における炉内雰囲気ガス圧（全圧）は、浸炭ガス導入時において３０００Ｐａ、炉内ガス排出時において約１０Ｐａとなるように制御した。パルス浸炭の全サイクルを通した処理時間は１時間とした。本実施の形態に係る浸炭条件を表１に示す。
【００４５】
【表１】

【００４６】
パルス方式による浸炭処理を繰り返した後、浸炭ガスの導入を停止し浸炭処理温度（９３０℃）に保ったまま浸炭炉１内に不活性ガス（Ｎ_２ガス）を導入すると共にこの状態を１時間保持することにより、鋼材Ｗの表面に固溶された炭素を鋼材内部へと拡散させる処理を行った。この拡散処理によって固溶炭素が鋼材Ｗの内部へと拡散し、鋼材Ｗの表層部に濃度が均一な炭素固溶層が形成される。
【００４７】
炭素が固溶された鋼材Ｗの表層部は、高温下（拡散処理温度である９３０℃）においては、オーステナイト相となっている。拡散処理後、鋼材Ｗを浸炭炉１より取り出し、次いで８０℃の温度で油焼入れ（急冷処理）を行った。この急冷処理により、表層部の高濃度の炭素が固溶されたオーステナイト相は無拡散変態を経て高硬度のマルテンサイト相へと相変態し、表層部には硬化層が形成される。一方、鋼材Ｗの内部（ここでいう内部は、表層のマルテンサイト相からなる硬化層によりもさらに内部を意味する。）は、炭素の固溶がなく依然炭素濃度が低いままであり、高靭性を保持した状態の浸炭処理製品が得られる。
【００４８】
上記浸炭処理工程により得られた浸炭処理製品の評価として、ビッカース硬さＨ_Ｖの測定を行った。この測定により得られるビッカース硬さＨ_Ｖ値の大小は、マルテンサイト化された硬化層中の炭素濃度の高低に対応するもので、浸炭処理材中にカーボンがどの程度固溶されているかの指標となるものである。測定は、浸炭処理材の深さ方向に対して行った。ビッカース硬さＨ_Ｖの測定は、日本工業規格「ＪＩＳＧ０５５７」に準拠して行い、圧入荷重は４．９０３Ｎとした。
【００４９】
ここで浸炭処理製品の評価基準として、硬化層のビッカース硬さＨ_Ｖ値が５５０以上であり、かつ、このＨ_Ｖ≧５５０の条件を満たす硬化層（以下、「有効硬化層」という。）が表面から０．６ｍｍ以上の深さまで形成されているものを合格品として、実用化レベルにある浸炭処理製品であると判断した。
【００５０】
その結果、ビッカース硬さの最大値は約８５０であり、さらに鋼材の最表面から０．６ｍｍの深さまではビッカース硬さＨ_Ｖ値が５５０を超えており、浸炭処理製品としての目標値（有効硬化層が０．６ｍｍ以上）を十分に満たす結果が得られた。
【００５１】
また、浸炭処理製品の断面についてＸ線マイクロアナライザ（ＥＰＭＡ）を行い、表層部に形成された浸炭硬化層中の炭素の濃度分布を調べた結果、炭素濃度は深さ方向に行くに従って徐々に減少する傾向にあったが部分的な濃度ムラは一切無く、炭素がほぼ均一に固溶された状態となっていることがわかった。また、処理製品の表面観察を行った結果、浸炭反応による煤の発生は一切認められず良好な表面性状を有していた。
【００５２】
以上のように、減圧雰囲気下において鋼材の浸炭処理を行うに際し、浸炭ガスの導入流量、炉内雰囲気ガスの排出流量、炉内雰囲気ガスの全圧及びガス排出側の炉内雰囲気中の特定ガス種の濃度（本実施の形態では水素濃度）を常時計測すると共にこれらの計測値を炭素量演算器へと送り、この炭素量演算器により浸炭処理時に鋼材中に固溶される炭素量を算出し、さらにこの算出値を流量調節器にフィードバックして浸炭ガスの導入流量を常時制御することによって、適正量の炭素が鋼材中に固溶され、煤の発生や浸炭ムラが一切ない表面性状に優れた浸炭処理製品が得られる。
【００５３】
本発明は上記した実施の形態に何ら限定されるものではなく、本発明の趣旨を逸脱しない範囲で種々の改変が可能である。例えば、上記実施の形態では、浸炭ガスとしてメタンを主成分とする都市ガス（１３Ａ）を用いて説明したが、これは既存の都市ガスの配管経路を利用すれば、浸炭処理工場へ簡単に浸炭ガスを供給できるからで、ガスボンベを用いたメタン、プロパン、アセチレン、エチレン＋水素などの炭化水素系ガス若しくはこれらの混合ガスを用いても同様の効果が得られることは言うまでもない。
【００５４】
また、上記の実施の形態では、水素センサとしては、一端が閉塞された筒状のプロトン導電性セラミックス管を用い、管内側に基準水素ガスを充填し、この基準水素ガス中と真空浸炭炉内の水素ガスの濃度（分圧）差に対応して発生する電圧を測定することにより改質触媒層内の水素濃度（分圧）を検知するものを用いて説明した。しかし、使用する水素センサはこれに限定されるものではなく、種々のタイプのものを用いることができる。例えば、基準水素ガスを用いない、固体電解質型のプロトン導電性セラミックス水素センサであってもよい。
【００５５】
また、この真空浸炭処理プロセスにおいて用いられる水素センサのプロトン導電性セラミックス材料としては、現在幾つかのものが知られており、例えば、ＳｒＣｅ_０．９５Ｙｂ_０．０５Ｏ_３−α、ＢａＣｅ_０．９Ｎｄ_０．１Ｏ_３−α、ＳｒＺｒ_０．９５Ｙｂ_０．０５Ｏ_３−α、ＣａＺｒ_０．９６Ｉｎ_０．０４Ｏ_３−α、Ａｌ_２Ｏ_３等もプロトン導電性を有するものとして適用できるものである。
【００５６】
なお、上記実施の形態では、浸炭ガスを浸炭炉内に導入する際にパルス方式を用いたが、これは炉内雰囲気ガスが効率よく新しい浸炭ガスで置換されるからであり、浸炭ガスを常時導入する方法であっても構わない。また、本願発明に係る浸炭処理装置としては、浸炭炉内の浸炭雰囲気ガス中の特定ガス種を検知することにより浸炭ガスの導入量を制御することができるものであれば良く、浸炭炉の構造や具体的な浸炭条件（浸炭温度、浸炭時間等）については上記実施例にのものに限られず種々の選択が可能である。
【００５７】
【発明の効果】
本発明の浸炭ガス制御方法及びその方法を用いた浸炭処理装置によれば、導入された浸炭ガスと被浸炭処理材である鋼材との反応（浸炭反応）により発生する反応ガス及び浸炭反応に寄与しない未反応ガスとが混在する炉内雰囲気中の特定ガス種の濃度を浸炭炉の排出側において検知すると共に、浸炭ガスの導入流量値、炉内雰囲気ガスの排出流量値及び浸炭炉内の全圧を検知することにより、適正の浸炭処理材が得られるように浸炭ガスの導入流量を制御することができるという効果がある。そして、このように浸炭制御がなされることにより、鋼材表面に煤が発生したり浸炭硬化層中の炭素の濃度ムラが生じたりすることが一切無く、表面性状に優れた浸炭処理製品が提供でき、さらには浸炭ガスの消費を最小限に抑えることが可能となり浸炭コスト並びに浸炭処理製品のコストの低廉化をも図ることができるという効果がある。
【図面の簡単な説明】
【図１】本願発明の実施の形態に係る浸炭処理装置の概略構成図である。
【図２】図１に示した浸炭処理装置に備え付けられる水素センサの概略構成図である。
【図３】図２に示した水素センサを使用して浸炭炉の雰囲気ガス中の水素分圧を測定する原理を説明するための模式図である。
【図４】本願発明の実施の形態に係る浸炭処理工程を示した図である。
【符号の説明】
Ｗ被浸炭処理材（鋼材）
１浸炭炉
２浸炭ガス導入管路
３炉内雰囲気ガス排出管路
４ヒータ
５圧力計
６流量調節器
７導入ガス流量計
８ガス圧センサ
９吸引ポンプ
１０減圧バルブ
１１排出ガス流量計
１２炭素量演算器
８０水素センサ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a carburizing treatment control method and a carburizing apparatus using the method, and more particularly, in a carburizing treatment step of carburizing a steel material surface using a carburizing gas containing a hydrocarbon-based gas as a main component. The present invention relates to a carburizing control method and a carburizing apparatus using the method that are preferably used to constantly control the flow rate of carburizing gas introduced into a carburizing furnace.
[0002]
[Prior art]
"Carburizing", which has long been known as a steel surface hardening technology, is a method of heating low-carbon steel in carburizing gas to diffuse carbon from the steel surface and increase the carbon concentration in the surface layer of the steel. It is. By this carburizing treatment, the surface layer of the steel material is martensitized by quenching from austenite which is a high-temperature stable phase to form a hardened layer, and a carburized product which maintains high toughness while the inside of the steel remains low carbon is obtained.
[0003]
One of the methods conventionally known as such a carburizing treatment technique is a gas carburizing method under normal pressure. In this gas carburizing method, for example, when carbon monoxide (CO) is used as a carburizing gas The reaction between carbon monoxide and the material to be carburized in a high-temperature and normal-pressure gas atmosphere causes carbon (C) and carbon dioxide (CO ₂ ) Is produced, the produced carbon forms a solid solution in the steel material to be carburized, and the solid solution carbon diffuses from the surface of the steel material to the inside to perform carburizing treatment (for example, see Patent Document 1). ).
[0004]
The mechanism of this gas carburization is as follows: (1) First, the carburizing gas comes into contact with the steel surface in a high-temperature atmosphere, and (2) the steel reacts with the carburizing gas on the steel surface (carburizing reaction) to generate carbon and generate it. (3) The carbon forms a solid solution in the steel material, and (3) the solid solution carbon diffuses from the surface of the steel material to the inside to form a carburized layer on the surface layer of the steel material. In addition, a reaction occurs in which carbon dissolved in the steel material is decarburized into a carburizing gas on the surface of the steel material. Therefore, the equilibrium relationship shown in Equation 1 is established, and the carbon potential in the steel material is determined from this relationship (see Equation 2).
[0005]
In this gas carburizing method, carbon dioxide (CO ₂ ) Partial pressure and oxygen (O ₂ ) The partial pressure is measured using a carbon dioxide sensor or an oxygen sensor, or the water vapor pressure in the furnace atmosphere gas is measured using a dew point meter, and based on these measured values, the carbon potential (P _C ) Is generally controlled by using a method of calculating (2). In calculating the carbon potential in Equation 2, the carburizing reaction was assumed as in Equation 1.
[0006]
(Equation 1)

[0007]
[Equation 2]

[0008]
Equilibrium constant K ₁ Is a constant determined by the temperature and is known in the literature and the like, and the saturated carbon concentration austenite a _S Is also known, and P _CO Is determined by the carburizing gas to be introduced and is constant. _CO2 Measuring the carbon potential P _C Can be calculated.
[0009]
However, in recent years, a carburizing process under a reduced-pressure atmosphere has attracted attention as a carburizing method that is extremely effective for energy saving, resource saving, and pollution control, with increasing interest in environmental issues and resource issues.
[0010]
This carburizing treatment is performed using methane (CH ₄ ), Ethane (C ₂ H ₆ ) Is used as a carburizing gas, and is reacted with a steel material (carburizing reaction) under a high temperature and reduced pressure atmosphere. For example, when methane is used as the carburizing gas, methane is carbon (C) and hydrogen (H) on the surface of the material to be carburized. ₂ ), And the produced carbon forms a solid solution in the steel material and further diffuses from the surface of the steel material to the inside.
[0011]
According to the carburizing process under the reduced pressure atmosphere, a high-quality carburized product can be obtained by the carburizing reaction, the gas consumption during the carburizing process can be reduced as compared with the normal pressure carburizing process, and the emission of carbon dioxide can be reduced. It has the advantage of being excellent in environmental characteristics because it does not have it.
[0012]
[Patent Document]
JP 2000-144371 A
[0013]
[Problems to be solved by the invention]
However, in carburizing treatment under reduced pressure atmosphere, "CH ₄ → C + 2H ₂ ], Only the carburization reaction from the carburizing gas to the steel material occurs, and the carbon transfer from the steel material to the carburizing gas (decarburization reaction) does not occur. Therefore, the carbon potential in the steel material cannot be determined by measuring the concentration of the specific component of the atmospheric gas. Therefore, in the carburizing process under a reduced pressure atmosphere in which the hydrocarbon-based gas is directly injected, the carburizing atmosphere cannot be controlled only by measuring the concentration of the specific component of the atmospheric gas.
[0014]
On the other hand, in carburizing treatment under reduced pressure atmosphere, technology to detect and control gas species in carburizing atmosphere has not yet been established, and at present, the degree of carburizing treatment of products after carburizing treatment has been confirmed. Therefore, the amount of carburizing gas introduced had to be determined by trial and error.
[0015]
Therefore, at present, it is difficult to establish appropriate carburizing conditions under a reduced pressure atmosphere. Specifically, there has been a problem that the appropriate amount of carburizing gas varies depending on the shape and size of the carburizing furnace, the material of the processed product (work, steel material), the amount charged into the furnace, and the like. In addition, since the amount of carburizing gas introduced is not feedback-controlled, soot is generated when the amount of introduced gas is excessive, and carburization is not sufficiently performed when the amount is too small, resulting in variations in product quality and a decrease in yield. There was a problem.
[0016]
The present inventors have conducted various studies and found that “CH ₄ → C + 2H ₂ ] Produced by the carburizing reaction between the hydrocarbon-based carburizing gas introduced in the reaction of ₂ ) And the concentration of unreacted hydrocarbon-based gas that did not contribute to the carburizing reaction, as well as detecting and controlling other carburizing gas introduction conditions, etc., could control the carburizing atmosphere during carburizing. That is the idea.
[0017]
The problem to be solved by the present invention is to allow the introduction of an appropriate amount of carburizing gas when carburizing under a reduced-pressure atmosphere, and to enable stable production of high-quality carburized products, Furthermore, a carburizing gas control method and a carburizing apparatus using the method that can shorten the processing time, improve the productivity by improving the yield, and further improve the productivity by suppressing the carburizing gas consumption, etc. To provide.
[0018]
[Means for Solving the Problems]
In order to solve this problem, the carburizing control method according to the present invention comprises introducing a carburizing gas containing a hydrocarbon-based gas as a main component into a carburizing furnace in which a material to be carburized is placed, and combining the carburizing gas with the carburizing gas. When the carburizing treatment of the carburized material is performed by dissolving and diffusing carbon generated by the carburizing reaction with the carburized material into the carburized material, the flow rate of the carburizing gas and the flow rate of the atmosphere gas in the furnace are reduced. And the total pressure of the atmosphere gas in the furnace, and the concentration of a specific gas species in the furnace atmosphere is constantly measured during the carburizing process on the discharge side of the atmosphere gas in the furnace. The gist is to adjust and control the introduced carburizing gas introduction flow rate in real time.
[0019]
Further, the carburizing apparatus of the present invention comprises a carburizing gas introduction pipe for introducing a carburizing gas into a carburizing furnace in which a steel material to be carburized is placed, and a carburizing atmosphere gas for discharging an atmosphere gas in the furnace. A discharge pipe, a pressure measuring device for measuring the total pressure of the furnace atmosphere, and a heating means for heating the furnace are provided, and the flow rate of the carburizing gas flowing through the pipe is adjusted to the carburizing gas introduction pipe. A carburizing gas flow controller and an introduction flow measuring device for measuring an introduction flow rate of the carburizing gas controlled by the carburizing gas flow controller are provided. An exhaust flow rate measuring device for measuring a gas flow rate is provided, and a sensor for detecting a concentration of a specific gas type in a furnace atmosphere is provided near an outlet of the carburized exhaust gas derivation pipe, and the introduction flow rate measuring device is provided. Carburized gas measured by Inlet flow rate value, discharge flow rate value of furnace atmosphere gas measured by the discharge flow rate measuring device, total pressure value of furnace atmosphere gas, and specific gas type in furnace atmosphere detected by the gas pressure sensor A carburizing amount calculator for calculating the amount of carbon dissolved in the steel material based on the concentration value of the carburizing gas, and a carburizing gas for adjusting the carburizing gas flow rate of the carburizing gas flow controller based on the carburizing amount calculator. The gist is that a flow control means is provided.
[0020]
According to the carburizing control method and the carburizing apparatus using the method, the reaction gas generated by the reaction (carburizing reaction) between the introduced carburizing gas and the steel material to be carburized and the carburizing reaction which does not contribute to the carburizing reaction. At the discharge side of the carburizing furnace, the gas concentration of the specific gas type in the atmosphere gas in the furnace where the reaction gas is mixed is detected at the discharge side of the carburizing gas, the flow rate value of the carburizing gas, the discharge flow rate of the furnace gas, and the total By detecting the pressure, the introduction flow rate of the carburizing gas can be controlled so that an appropriate carburizing material can be obtained.
[0021]
In addition, by controlling the gas concentration in the atmosphere gas in the furnace in real time, a carburized product of stable quality can be obtained, and the stable supply of carburizing gas without excess or shortage can be achieved, so that the soot on the steel surface can be reduced. Generation is also avoided and the consumption of carburizing gas can be minimized.
[0022]
As a concentration sensor for detecting the gas concentration in the furnace atmosphere, it is preferable to use proton conductive ceramics in order to measure the gas concentration of hydrogen generated by the carburizing reaction. Thereby, the atmosphere gas in the furnace can be controlled with high accuracy.
[0023]
In addition, as a concentration sensor for detecting hydrogen using proton conductive ceramics, platinum electrodes are provided on the inner and outer surfaces of the proton conductive ceramic tube inserted into the furnace, and reference hydrogen in the proton conductive ceramic tube and the furnace atmosphere gas are used. It is preferable to measure the hydrogen concentration in the furnace by measuring the potential difference between both platinum electrodes due to the diffusion of hydrogen ions generated in the proton-conductive ceramic tube due to the difference in concentration with hydrogen in the furnace. As a result, the detection of the hydrogen concentration in the atmosphere gas in the furnace and the control of carburization become more accurate, and a high-quality carburized product can be obtained.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
[0025]
FIG. 1 shows a schematic configuration of a carburizing apparatus according to the present invention. This carburizing apparatus includes a carburizing furnace 1 for mounting a steel material W as a material to be carburized and heating and maintaining the steel material W at a carburizing temperature, and a carburizing gas for introducing a carburizing gas into the furnace. It is composed of an introduction pipe 2 and a furnace atmosphere gas discharge pipe 3 for discharging the introduced furnace atmosphere gas. The carburizing furnace 1 is provided with a heater 4 for heating the inside of the furnace to the carburizing temperature, and a pressure gauge 5 for detecting the pressure (total pressure) of the atmosphere gas in the furnace is provided on the furnace wall of the carburizing furnace 1. ing.
[0026]
The carburizing gas introduction pipe 2 is provided with a flow controller 6 having an automatic solenoid valve type flow control function for increasing / decreasing the flow rate of the carburizing gas flowing through the pipe, and further introduced into the furnace downstream thereof. An introduction gas flow meter 7 for measuring the introduction flow value of the carburizing gas to be used is provided.
[0027]
Further, a concentration sensor 8 for detecting the concentration of a specific gas species in the furnace atmosphere is provided on the furnace wall near the discharge port of the furnace atmosphere gas discharge pipe 3. In the present embodiment, a hydrogen sensor 80 for detecting the concentration of hydrogen generated by the reaction between the carburizing gas and the steel material (carburizing reaction) is provided as the concentration sensor 8. As the concentration sensor 8, not only a sensor for detecting the hydrogen concentration, but also a sensor for detecting the concentration of the hydrocarbon-based gas in the furnace atmosphere may be used.
[0028]
Further, a suction pump 9 for reducing the pressure in the furnace is provided in the furnace atmosphere gas discharge pipe 3, and in the middle of the discharge pipe 3, the suction pump 9 is turned on and off by the suction pump 9. A pressure reducing valve 10 that can be switched off is provided, and an exhaust gas flow meter 11 that measures an exhaust flow value of exhaust gas flowing through the pipeline is provided upstream of the pressure reducing valve 10.
[0029]
The hydrogen sensor 80, the introduced gas flow meter 7, the exhaust gas flow meter 11, and the pressure gauge 5 are electrically connected to a carbon content calculator 12 provided in a control system for controlling the operation of the carburizing furnace 1. Further, the carbon amount calculator 12 is also electrically connected to a flow controller 6 provided in the carburizing gas introduction pipe 2, and the opening degree of the automatic solenoid valve of the flow controller 6 is controlled by a command signal from the calculator 12. It is adjusted automatically.
[0030]
FIG. 2 shows a schematic configuration of a hydrogen sensor 80 provided on a furnace wall on the discharge side of the carburizing furnace 1. This hydrogen sensor 80 forms a sensor probe in which platinum electrodes Ei and Es are provided on inner and outer wall surfaces of a cylindrical bottom portion of a proton conductive ceramic tube 81 (hereinafter, referred to as “proton conductive tube 81”) whose one end is closed. This is joined to the tip of a ceramic tube 82 loaded with a reference hydrogen gas via a sealant 83, and the proton conductive tube 81 of the sensor probe is inserted into a ceramic protective tube 84 to be inserted into the protective tube 84. The conductor portion of the proton conductive tube 81 is joined by an inorganic adhesive 85. By inserting the conductor portion of the protective tube 84 through the furnace wall of the carburizing furnace 1 and mounting the hydrogen sensor 80 on the proton conductive tube 81, The pipe 81 is provided on the furnace wall in a state where the tip of the pipe 81 is inserted into the carburizing furnace 1.
[0031]
Hereinafter, the principle of detecting the hydrogen concentration (partial pressure) in the atmospheric gas in the carburizing furnace 1 using the hydrogen sensor 80 will be described with reference to FIG. First, with the proton conductive tube 81 of the hydrogen sensor 80 provided in the carburizing furnace 1 filled with a mixed gas of 1% hydrogen and helium as reference hydrogen, the carburizing gas was introduced into the depressurized furnace atmosphere. Introduce and start carburizing of steel. Since the hydrogen concentration (partial pressure) in the carburizing furnace 1 generated by the reaction between the carburizing gas and the steel material (carburizing reaction) is higher than the reference (1% concentration) hydrogen in the proton conductive tube 81, the hydrogen concentration in the proton conductive tube 81 is reduced. Inside the furnace, "H ₂ → 2H ⁺ + 2e ⁻ Occurs, and hydrogen in the proton conductive tube 81 is converted to hydrogen ions (H ⁺ ) And permeate the inside of the wall of the proton conductive tube 81 toward the outside of the furnace, and on the low hydrogen concentration side in the furnace, “2H ⁺ + 2e ⁻ → H ₂ Is about to occur.
[0032]
Based on such a reaction mechanism, an electromotive force corresponding to the hydrogen concentration difference between the inside of the furnace and the inside of the ceramic tube 81 is generated between the platinum electrodes Ei and Es on the inner and outer wall surfaces of the proton conductive tube 81, and this potential difference is measured. Thereby, the hydrogen partial pressure in the carburizing furnace 1 is measured. The hydrogen partial pressure P in the carburizing furnace 1 _H2 Is obtained using the following Nernst equation (Equation 3).
[0033]
[Equation 3]

[0034]
Also, this hydrogen partial pressure P _H2 Is the hydrogen concentration C _H2 The number of moles of hydrogen in the furnace and the volume of the furnace are calculated from the equation of state of the gas in order to convert to the equation. Thereby, the hydrogen concentration value in the carburizing furnace 1 is calculated. Formula 4 shows the calculation formula.
[0035]
(Equation 4)

[0036]
Next, a description will be given of a calculation method for measuring the amount of carbon dissolved in steel at the time of carburizing, using the hydrogen concentration value in the carburizing furnace 1 detected by the hydrogen sensor based on the above calculation formula. From the

above formulas

3 and 4, the hydrogen concentration C in the carburizing furnace 1 is calculated. _H2 Is calculated, the amount of solute carbon is calculated by the following calculation method.
[0037]
The exhaust gas flow rate X (t) (unit: L / hour) of the atmosphere gas in the furnace after t minutes from the carburizing treatment was measured by the exhaust gas flow meter 11, and during Δt (time t ₁ To t ₂ ), The integrated discharge flow rate A (t) of the carburized gas introduced into the gas is calculated, and the concentration (in this case, the hydrogen concentration) C of the specific gas type in the carburized atmosphere gas on the discharge side is calculated. _H2 (Mol / L), and the integrated discharge flow rate A (t) and hydrogen concentration C _H2 From (mol / L), the amount of carbon ΔC (t) dissolved in the carburizing furnace 1 during Δt is calculated. Formula 5 shows the calculation formula. In the calculation, the carburizing gas was methane (CH ₄ ), And it is assumed that this was completely decomposed into carbon and hydrogen during carburization.
[0038]
(Equation 5)

[0039]
When calculating the carbon amount ΔC (t), it is assumed that all the carburizing gas introduced into the carburizing furnace 1 contributes to the carburizing reaction and is decomposed into carbon and hydrogen. Therefore, in the above calculation method, if the gas flow rate X (t) at the time of discharge and the concentration of the specific gas type (hydrogen in this case) on the discharge side are detected, the amount of solid solution of carbon (the amount of carburization) at the time of carburization is specified. be able to. However, when only a part of the carburizing gas introduced contributes to the carburizing reaction, the flow rate of the carburizing gas introduced and the total pressure in the carburizing furnace are measured, and these measured values and the exhaust gas of the above atmosphere gas in the furnace are measured. It is necessary to input into the carbon amount calculator 12 an arithmetic expression for calculating the carbon amount ΔC (t) dissolved in the steel material from the flow rate value and the specific gas concentration value on the discharge side.
[0040]
If the value of the carbon content ΔC (t) in the steel material calculated by the carbon content calculator 12 based on the above calculation formulas (Formulas 3 to 5) fluctuates, a controller provided in the carbon content calculator 12 sends The opening of a solenoid valve (not shown) provided in the flow rate measuring device 6 of the carburizing gas introduction pipe 2 is automatically adjusted (feedback control) by the command signal. This feedback control of the carburizing gas flow rate is always performed during the carburizing process. Finally, when the amount of carbon dissolved in the steel material W reaches a predetermined value, the carburizing process ends. In the above calculation formula, it is assumed that the gas existing in the carburizing furnace 1 is only methane and hydrogen. However, in the carburizing process, other gas types other than these gas types, for example, ethylene (C ₂ H ₄ ) And acetylene (C ₂ H ₂ ) And the like, it is necessary to detect the concentration of these gas species by a concentration sensor and modify and apply the above equation (Equation 5).
[0041]
FIG. 4 shows the carburizing cycle of the carburizing process applied in this embodiment over time. After the steel material W which is the material to be carburized is set in the carburizing furnace 1, the furnace is closed, and the inert gas nitrogen (N ₂ ) While the gas is being introduced, the atmosphere in the furnace is suctioned by the suction pump 9 and exhausted to the outside of the furnace to reduce the pressure inside the furnace. At this time, the furnace pressure is about 60 Pa.
[0042]
Next, the steel material W placed in the furnace atmosphere and the carburizing furnace 1 is heated by the heater 4 while maintaining the reduced pressure state, and the temperature is raised to about 930 ° C. in about one hour. Then, when the temperature reaches a predetermined temperature (930 ° C.), the carburizing process is started. In the present embodiment, city gas (13A) containing methane as a main component is used as the carburizing gas. Note that the carburizing gas was introduced into the carburizing furnace 1 by a pulse method. Here, the pulse method is a method of introducing a carburizing gas (here, city gas (13A)) into the furnace in a pulsed manner (intermittently) at a certain timing.
[0043]
FIG. 4 is an enlarged timing chart of the carburizing process by the pulse method. (1) First, the carburizing gas is introduced into the carburizing furnace 1 at a controlled gas flow rate. At this time, the pressure reducing valve 9 provided at the end of the furnace atmosphere gas discharge pipe 3 is switched from open to closed. (2) When a predetermined amount of carburizing gas is introduced into the carburizing furnace 1, the introduction is temporarily stopped, and the carburizing gas is diffused into the carburizing furnace 1 and reacted with a steel material (carburizing reaction). At this time, carbon is dissolved in the steel by the reaction between the carburizing gas and the steel. (3) After the carburizing gas is diffused and held for a predetermined time, the pressure reducing valve 9 is returned from the closed state to the open state, and the furnace atmosphere gas is discharged. The above is regarded as one cycle, and the carburizing process is performed by repeating this multiple times. Various conditions such as the amount of carburizing gas introduced are controlled based on the amount of carbon solid solution in the steel material W calculated by the above-described carbon content calculator 12.
[0044]
By controlling the carburization in this way, the carburizing gas introduced into the carburizing furnace 1 generates carbon by reacting with the steel material, and the generated carbon dissolves in the steel material W, and the solid solution carbon is further dissolved in the steel material. W is diffused from the surface to the inside. The furnace atmosphere gas pressure (total pressure) in the carburizing treatment by the pulse method was controlled to be 3000 Pa when the carburizing gas was introduced and to be about 10 Pa when the furnace gas was discharged. The processing time throughout the entire pulse carburizing cycle was 1 hour. Table 1 shows carburizing conditions according to the present embodiment.
[0045]
[Table 1]

[0046]
After repeating the carburizing treatment by the pulse method, the introduction of the carburizing gas was stopped and the inert gas (N) was introduced into the carburizing furnace 1 while maintaining the carburizing temperature (930 ° C.). ₂ Gas) was introduced, and this state was maintained for one hour, thereby performing a process of diffusing carbon dissolved in the surface of the steel material W into the steel material. By this diffusion treatment, the solid solution carbon diffuses into the steel material W, and a carbon solid solution layer having a uniform concentration is formed on the surface layer of the steel material W.
[0047]
The surface portion of the steel material W in which carbon is dissolved is in an austenitic phase at a high temperature (diffusion processing temperature of 930 ° C.). After the diffusion treatment, the steel material W was taken out of the carburizing furnace 1 and then subjected to oil quenching (rapid cooling treatment) at a temperature of 80 ° C. By this quenching treatment, the austenite phase in which carbon at a high concentration in the surface layer is solid-dissolved undergoes a non-diffusion transformation to a high hardness martensite phase, and a hardened layer is formed on the surface layer. On the other hand, the inside of the steel material W (here, the inside means more inside than the hardened layer composed of the surface martensite phase) has no solid solution of carbon and still has a low carbon concentration, and has high toughness. Thus, a carburized product is obtained in a state where
[0048]
As the evaluation of the carburized product obtained by the above carburizing process, Vickers hardness H _V Was measured. Vickers hardness H obtained by this measurement _V The magnitude of the value corresponds to the level of carbon concentration in the martensitized hardened layer, and is an index of how much carbon is dissolved in the carburized material. The measurement was performed in the depth direction of the carburized material. Vickers hardness H _V Was measured in accordance with Japanese Industrial Standards “JIS G 0557” and the press-fit load was 4.903N.
[0049]
Here, the Vickers hardness H of the hardened layer is used as an evaluation standard for carburized products. _V The value is 550 or more and this H _V A product having a hardened layer satisfying the condition of ≧ 550 (hereinafter referred to as “effective hardened layer”) formed to a depth of 0.6 mm or more from the surface is regarded as an acceptable product, and is considered to be a carburized product at a practical use level. It was judged.
[0050]
As a result, the maximum value of the Vickers hardness is about 850, and the Vickers hardness H at a depth of 0.6 mm from the outermost surface of the steel material. _V The value exceeded 550, and a result sufficiently satisfying the target value (effective hardened layer of 0.6 mm or more) as a carburized product was obtained.
[0051]
X-ray microanalyzer (EPMA) was performed on the cross-section of the carburized product to examine the carbon concentration distribution in the carburized hardened layer formed on the surface layer. As a result, the carbon concentration gradually decreased in the depth direction. However, there was no partial concentration unevenness, and it was found that carbon was almost uniformly dissolved. Further, as a result of observing the surface of the treated product, no soot was generated at all by the carburizing reaction, and the treated product had good surface properties.
[0052]
As described above, when carburizing a steel material under a reduced-pressure atmosphere, the flow rate of the carburizing gas introduced, the flow rate of the furnace atmosphere gas, the total pressure of the furnace atmosphere gas, and the specific gas in the furnace atmosphere on the gas discharge side are determined. The concentration of the species (hydrogen concentration in the present embodiment) is constantly measured, and the measured values are sent to a carbon content calculator, which calculates the amount of carbon dissolved in the steel material during carburizing. The calculated value is fed back to the flow controller to constantly control the flow rate of the carburizing gas, so that an appropriate amount of carbon is dissolved in the steel material and the surface properties are free from soot and carburizing unevenness. Excellent carburized products are obtained.
[0053]
The present invention is not limited to the above embodiment at all, and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the description has been made using the city gas (13A) mainly composed of methane as the carburizing gas. However, if the existing city gas piping route is used, the carburizing plant can be easily carburized. Since a gas can be supplied, it goes without saying that the same effect can be obtained by using a hydrocarbon-based gas such as methane, propane, acetylene, ethylene + hydrogen, or a mixed gas thereof using a gas cylinder.
[0054]
Further, in the above embodiment, as the hydrogen sensor, a cylindrical proton conductive ceramic tube whose one end is closed is used, and the inside of the tube is filled with a reference hydrogen gas, and the inside of the reference hydrogen gas and the inside of the vacuum carburizing furnace are used. The description has been made using the method of detecting the hydrogen concentration (partial pressure) in the reforming catalyst layer by measuring the voltage generated corresponding to the difference in the hydrogen gas concentration (partial pressure). However, the hydrogen sensor to be used is not limited to this, and various types can be used. For example, a solid electrolyte type proton conductive ceramics hydrogen sensor that does not use a reference hydrogen gas may be used.
[0055]
Further, as the proton conductive ceramic material of the hydrogen sensor used in the vacuum carburizing process, several materials are currently known, for example, SrCe. _0.95 Yb _0.05 O _3-α , BaCe _0.9 Nd _0.1 O _3-α , SrZr _0.95 Yb _0.05 O _3-α , CaZr _0.96 In _0.04 O _3-α , Al ₂ O ₃ And the like can be applied as those having proton conductivity.
[0056]
In the above embodiment, the pulse method was used when introducing the carburizing gas into the carburizing furnace. This is because the atmosphere gas in the furnace is efficiently replaced with new carburizing gas. It may be a method of introducing. Further, the carburizing apparatus according to the present invention may be any apparatus that can control the amount of carburizing gas introduced by detecting a specific gas type in a carburizing atmosphere gas in a carburizing furnace. The specific carburizing conditions (carburizing temperature, carburizing time, etc.) are not limited to those in the above embodiment, and various selections are possible.
[0057]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the carburizing gas control method of this invention, and the carburizing apparatus using the method, the reaction gas and the carburizing reaction which generate | occur | produce by the reaction (carburizing reaction) of the introduced carburizing gas and the steel material which is to be carburized are contributed. At the discharge side of the carburizing furnace, the concentration of a specific gas species in the furnace atmosphere where unreacted gas is mixed is detected at the discharge side of the carburizing gas, the flow rate value of the carburizing gas, the discharge flow rate of the furnace atmosphere gas, and the total By detecting the pressure, there is an effect that the introduction flow rate of the carburizing gas can be controlled so that an appropriate carburizing material can be obtained. By performing the carburizing control in this way, there is no occurrence of soot on the steel material surface and non-uniformity of carbon concentration in the carburized hardened layer, and a carburized product having excellent surface properties can be provided. In addition, it is possible to minimize the consumption of carburizing gas, so that the carburizing cost and the cost of the carburized product can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a carburizing apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of a hydrogen sensor provided in the carburizing apparatus shown in FIG.
FIG. 3 is a schematic diagram for explaining a principle of measuring a hydrogen partial pressure in an atmosphere gas of a carburizing furnace using the hydrogen sensor shown in FIG.
FIG. 4 is a view showing a carburizing process according to the embodiment of the present invention.
[Explanation of symbols]
W Carburized material (steel)
1 carburizing furnace
2 Carburizing gas introduction pipeline
3 Atmosphere gas exhaust pipe
4 heater
5 Pressure gauge
6 Flow controller
7 Introduced gas flow meter
8 Gas pressure sensor
9 Suction pump
10 Pressure reducing valve
11 Exhaust gas flow meter
12 Carbon content calculator
80 Hydrogen sensor

Claims

A carburizing gas containing a hydrocarbon-based gas as a main component is introduced into a carburizing furnace on which the carburizing material is placed, and carbon generated by a carburizing reaction between the carburizing gas and the carburizing material is converted into a carburizing material. When carburizing the material to be carburized by dissolving and diffusing into it,
While measuring the introduction flow rate of the carburizing gas, the discharge flow rate of the furnace atmosphere gas, and the total pressure of the furnace atmosphere gas, the concentration of a specific gas species in the furnace atmosphere is being carburized on the discharge side of the furnace atmosphere gas. A carburizing treatment control method characterized by constantly measuring and adjusting and controlling a carburizing gas introduction flow rate introduced into a furnace in real time based on these measured values.

A carburizing gas introduction pipe for introducing a carburizing gas into the furnace, a carburizing atmosphere gas discharging pipe for discharging the furnace atmosphere gas, and a A pressure measuring device for measuring the total pressure, and a heating means for heating the inside of the furnace,
A carburizing gas flow controller that adjusts a flow rate of the carburizing gas flowing through the carburizing gas introduction pipe, and an introduction flow measuring device that measures an introduction flow rate of the carburizing gas adjusted by the carburizing gas flow controller. Is provided,
In the carburizing exhaust gas derivation pipeline, a discharge flow measurement device that measures a flow rate of the furnace atmosphere gas flowing through the pipeline is provided,
A concentration sensor for detecting the concentration of a specific gas species in the furnace atmosphere is provided in the vicinity of the discharge port of the carburizing exhaust gas outlet pipe,
The introduction flow rate value of the carburizing gas measured by the introduction flow rate measurement device, the discharge flow value of the furnace atmosphere gas measured by the discharge flow rate measurement device, the total pressure value of the furnace atmosphere gas, and the gas pressure sensor A carburizing amount calculator for calculating the amount of carbon dissolved and dissolved in the steel based on the concentration value of the specific gas type in the furnace atmosphere detected by the carburizing amount calculator; and the carburizing gas based on the carburizing amount calculator. A carburizing gas flow control device for controlling a carburizing gas flow rate of the flow rate controller.

3. The gas pressure sensor according to claim 2, wherein a proton conductive ceramic is used for measuring only a concentration of hydrogen generated by the carburizing reaction as a specific gas species in the furnace atmosphere. 3. The carburizing apparatus according to item 1.

The gas pressure sensor using the proton conductive ceramics, provided with platinum electrodes on the inner and outer surfaces of the proton conductive ceramics tube inserted into the furnace, the reference hydrogen gas in the proton conductive ceramics tube and hydrogen in the furnace atmosphere. 3. The method according to claim 2, wherein the concentration of hydrogen in the furnace atmosphere is measured by measuring a potential difference between the platinum electrodes due to diffusion of hydrogen ions generated in the proton conductive ceramic tube due to the concentration difference. 3. The carburizing apparatus according to 3.