JP2004043862A - Heat treatment method of pearlitic rail for improving toughness of welded joint - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、種々のレール溶接方法によって接合された高炭素含有のパーライト系レールの溶接継ぎ手部において、初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させるパーライト系レールの溶接継ぎ手部の熱処理方法に関するものである。
【０００２】
【従来の技術】
近年、海外の石炭や鉄鉱石を輸送する重荷重鉄道や国内の貨物鉄道では、より一層の鉄道輸送の高効率化のために、貨物の高積載化を強力に進めており、特に急曲線のレールでは、Ｇ．Ｃ．部や頭側部の耐摩耗性が十分確保できず、摩耗によるレール寿命の低下が問題となってきた。このような背景から、現状の共析炭素含有の高強度レール以上の耐摩耗性を有するレールの開発が求められるようになってきた。この問題を解決するため、本発明者らは下記に示すようなレールを開発した。
▲１▼過共析鋼（Ｃ：０．８５超〜１．２０％）を用いて、パーライト組織中のラメラ中のセメンタイト密度を増加させた耐摩耗性に優れたレール（特開平８−１４４０１６号公報）。
▲２▼過共析鋼（Ｃ：０．８５超〜１．２０％）を用いて、パーライト組織中のラメラ中のセメンタイト密度を増加させ、同時に、硬さを制御した耐摩耗性に優れたレール（特開平８−２４６１００号公報）。
これらのレールの特徴は、鋼の炭素量を増加し、パーライトラメラ中のセメタイト相の体積密度を増加させ、さらに、硬さを制御することによりパーライト組織の耐摩耗性を向上させるものであった。
【０００３】
また、オーステナイト域まで加熱されるレール溶接継ぎ手部においては、溶接部の諸特性を改善する目的で、下記に示すようなレール熱処理方法が開発された。
▲３▼Ａｅ１変態点以上の温度にあるレール溶接部のレール全体またはレール頭部と底部を気体または液体でパーライト変態が終了するまで冷却し、その後、急速冷却方法（特開昭５９−９３８３８号公報）。
▲４▼レール溶接部終了後、または、外部から加熱されて表面が５００℃以上の温度を保有するレールの脚部を急冷する方法（特開平３−２７７７２０号公報）。これらの熱処理方法の特徴は、溶接継ぎ手の硬さや残留応力を制御し、溶接継ぎ手部の耐破壊性や耐疲労損傷性を高めるものであった。
【０００４】
【発明が解決しようとする課題】
上記の▲１▼，▲２▼に示された発明レールでは、高炭素化により耐摩耗性の向上は図れる。しかし、８００〜９００℃の範囲に加熱されたレール溶接継ぎ手部においては、溶接後に靭性に有害な粗大な初析セメンタイト組織が生成し、レールの靭性が低下するといった問題があった。
【０００５】
また、この問題を解決するため、上記の▲３▼，▲４▼に示されたレール溶接継ぎ手部の熱処理方法を適用しても、８００〜９００℃の範囲に加熱されたレール溶接継ぎ手部では、耐摩耗性や靭性に有害なマルテンサイト組織が生成し、レールの靭性がさらに低下するといった問題があった。
【０００６】
このような背景から、高炭素含有のパーライト組織のレールにおいて、８００〜９００℃の範囲に加熱されたレール溶接継ぎ手部に生成する初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させるレールの熱処理方法の開発が望まれるようになった。
【０００７】
すなわち、本発明は、種々のレール溶接方法によって接合された高炭素含有のレール溶接継ぎ手部において、初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させることを目的としたものである。
【０００８】
【課題を解決するための手段】
本発明は上記目的を達成するものであって、その要旨とするところは次の通りである。
（１）質量で、Ｃ：０．８５超〜１．２０％を含有する鋼レールを溶接のため８００〜９００℃の範囲に加熱し接合した直後の溶接継ぎ手部におけるレール頭部および／または底部を、７５０℃以上の温度域から冷却速度１〜１０℃／ｓｅｃで加速冷却し、前記鋼レールの頭部および／または底部の温度が６８０〜５５０℃達した時点で加速冷却を停止し、その後、前記鋼レールの頭部およびまたは底部の温度が６８０℃を超えないように放冷または緩冷却することを特徴とする溶接継ぎ手部の靭性を向上させるパーライト系レールの溶接継ぎ手部の熱処理方法。
（２）加速冷却に先立ち、接合後の溶接継手部を、熱処理する目的で８００〜９００℃の範囲に再加熱することを特徴とする前記（１）記載の溶接継ぎ手部の靭性を向上させるパーライト系レールの溶接継ぎ手部の熱処理方法。
【０００９】
【発明が解決しようとする課題】
以下に本発明について詳細に説明する。
まず、本発明者らは、種々のレール溶接方法によって接合された高炭素含有パーライト系レールの溶接継ぎ手部において、８００〜９００℃の範囲に加熱された部位に粗大な初析セメンタイト組織が生成する原因を調査した。その結果、８００〜９００℃の範囲に加熱された部位では、オーステナイト相とセメンタイト相が混在する２相状態となる。このため、オーステナイト相に炭素が濃縮し、オーステナイト相に含有する炭素量が過剰となり、溶接後に靭性に有害な粗大な初析セメンタイト組織が生成することがわかった。
【００１０】
そこで、本発明者らは、過剰な炭素を含有するオーステナイト相から生成する初析セメンタイト組織を防止する方法を実験室により検討した。その結果、オーステナイト相とセメンタイト相が混在する８００〜９００℃の範囲に加熱された部位を、初析セメンタイト組織が生成しないように、ある温度以上の範囲から、初析セメンタイト組織が生成する温度領域を、ある一定範囲の冷却速度で加速冷却し、その生成を防止する方法が有効であることを知見した。
【００１１】
しかし、高温度に加熱されたレール表面を加速冷却すると、レール内部からの復熱が発生し易い。特に、加速冷却速度が大きい場合は、その復熱量も大きくなり、結果的に６８０℃以上まで温度が上昇し、加速冷却後に初析セメンタイト組織が生成することが確認された。
【００１２】
次に、本発明者らは、復熱時の初析セメンタイト組織の防止方法を検討した。その結果、加速冷却時の冷却速度の上限を定め、さらに、加速冷却後に簡便な緩冷却を施すことにより、その復熱を十分防止でき、初析セメンタイト組織の生成を防止できることを見出した。
【００１３】
すなわち、本発明は、種々のレール溶接方法によって接合された高炭素含有パーライト系レールの溶接継ぎ手部において、初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させることを目的としたものである。
【００１４】
次に、本発明の限定理由について詳細に説明する。
（１）鋼レールの炭素量
鋼レールの炭素量を上記請求範囲に限定した理由について詳細に説明する。
高炭素含有のレール溶接継ぎ手部では、オーステナイト相とセメンタイト相が混在する２相状態となり、粗大な初析セメンタイト組織が生成する。しかし、Ｃ量が０．８５％以下では、部分的なオーステナイト相に濃縮する炭素量も少なく、加速冷却を行わなくても初析セメンタイト組織の生成量が非常に少なく、レールの靭性に悪影響をおよぼさない。また、Ｃ量が１．２０％を超えると、部分的なオーステナイト相に濃縮する炭素量が非常に多くなり、加速冷却を行っても初析セメンタイト組織の生成を完全に防止することが困難になり、レールの靭性低下を防止できない。このため、鋼レールのＣ量を０．８５超〜１．２０％に限定した。
【００１５】
（２）熱処理方法
次に、レール溶接継ぎ手部において、８００〜９００℃の範囲に加熱されたレール頭部または底部を、７５０℃以上の温度域から冷却速度１〜１０℃／ｓｅｃで加速冷却し、前記鋼レールの頭部または底部の温度が６８０〜５５０℃達した時点で加速冷却を停止し、その後、前記鋼レールの頭部または底部の温度が６８０℃を超えないように緩冷却または放冷する熱処理方法において、温度範囲や加速冷却速度を上記のように限定した理由を説明する。
【００１６】
（ａ）加速冷却を行うレール溶接継ぎ手部の温度範囲
レール溶接継ぎ手部の加熱温度が８００℃未満の領域では、本成分系では、オーステナイト相が存在しない、または、オーステナイト相が微量であるため、部分的なオーステナイト相への炭素の濃化が発生しない。その結果、レールの靭性に有害な初析セメンタイト組織が生成せず、レールの靭性低下が発生しない。また、レール溶接継ぎ手部の加熱温度が９００℃を超える領域では、本成分系では、オーステナイト相のみの単相領域となる。したがって、オーステナイト相への炭素の濃化が発生しない。また、本加熱領域では、溶接後の冷却速度も早く、レールの靭性に有害な初析セメンタイト組織の生成も非常に少ない。このため、加速冷却を行うレール溶接継ぎ手部の温度を、８００〜９００℃の範囲に限定した。
【００１７】
なお、実際のレール溶接継ぎ手部では、上記の温度範囲に加熱された領域のみを限定的に加速冷却することは困難である。したがって、上記の温度範囲に加熱される部位を十分に加速冷却するには、上記の温度範囲を超えた周辺の加熱領域を含めて、加速冷却を施すことが望ましい。
【００１８】
また、溶接方法や装置のため、温度が８００℃以上にならない場合や、後述する加速冷却開始温度を充足できない場合は、加速冷却に先立って溶接部を再加熱しても良い。
【００１９】
（ｂ）加速冷却速度範囲
加速冷却速度が１℃／ｓｅｃ未満では、オーステナイト相とセメンタイト相が混在する２相状態において、炭素が濃化したオーステナイト相から発生する初析セメンタイト組織の生成を防止することが困難になり、レールの靭性が低下する。また、加速冷却速度が１０℃／ｓｅｃを超えると、加速冷却後のレール内部からの復熱が過大となり、加速冷却後、緩冷却を行っても、溶接継ぎ手部の温度が６８０℃を超え、レールの靭性に有害な初析セメンタイト組織が生成する。このため、加速冷却速度の範囲を１〜１０℃／ｓｅｃに限定した。
【００２０】
なお、１〜１０℃／ｓｅｃの冷却速度を得る方法としては、空気や空気を主としミスト等を加えた冷却媒体およびこれらの組み合わせにより、所定冷却速度を得ることが可能である。
【００２１】
（ｃ）加速冷却開始温度
加速冷却を開始する温度が７５０℃未満になると、加速冷却を行う前に、炭素が濃化したオーステナイト相から初析セメンタイト組織が生成し、レールの靭性が低下する。このため、加速冷却開始温度を７５０℃以上に限定した。
【００２２】
（ｄ）加速冷却の温度範囲
加速冷却を停止する温度が５５０℃未満では、オーステナイト相とセメンタイト相が混在する２相状態において、加速冷却後にパーライト変態完了せず、パーライト組織中にマルテンサイト組織が生成し、レールの靭性が低下する。また、加速冷却を停止する温度が６８０℃を超えると、加速冷却直後に初析セメンタイト組織が生成し、レールの靭性を低下させる。このため、加速冷却の温度範囲を６８０〜５５０℃の範囲に限定した。
【００２３】
なお、加速冷却停止後の冷却は、作業効率を向上させるため、放冷が望ましい。しかし、加速冷却速度が大きい場合は、レール内部からの復熱も大きくなることから、加速冷却停止後、緩冷却を行うことが望ましい。緩冷却を行う方法としては、加速冷却時に利用した空気や空気を主としミスト等を加えた冷却媒体の圧力およびその冷却時間を制御することにより可能である。
【００２４】
ここで、レール溶接継ぎ手部について説明する。図１はレール溶接継ぎ手部の概要を示したものである。レール溶接継ぎ手部は、９００〜１２００℃のオーステナイト域単相温度域に加熱される溶接中心部（符号：１）、５００〜９００℃の温度域に加熱され、パーライト組織やその硬さに変化が発生する熱影響部（符号：２）、５００℃未満の温度域に加熱されが、パーライト組織やその硬さに変化が発生しない母材部（符号：３）に分類される。レール溶接継ぎ手部において、８００〜９００℃の範囲に加熱され、オーステナイト相とセメンタイト相が混在する２相状態となる部分は、符号２の溶接中心部に近い部分である。
【００２５】
なお、レール溶接の方法としては、フラッシュバット溶接、ガス圧接、テルミット溶接、エンクローズアーク溶接等が一般的に行われている。溶接方法の違いにより入熱量が異なるため、図１に示した溶接中心部（符号：１）、熱影響部 （
符号：２）、母材部（符号：３）の位置や範囲が若干異なる。したがって、加速冷却を行う部位は、各溶接方法のレール溶接継ぎ手部において、オーステナイト相とセメンタイト相が混在する８００〜９００℃の温度域となるレール部位を正確に把握し、その部位を十分に含むように冷却部位を選択することが望ましい。
【００２６】
次に、レールの部位について説明する。図２はレール部位の呼称を示したものである。「レール頭部」とは、図２に示す頭頂部（符号：４）および頭部コーナー部（符号：５）を含む部分である。また、「レール底部」とは、図２に示す足裏部（符号：６）を含む部分である。上記に説明した加速冷却速度、加速冷却の温度範囲および加速冷却開始温度は、図２に示す頭頂部（符号：４）および頭部コーナー部（符号：５）の頭部表面、または、頭部表面から深さ５ｍｍの範囲、また、レール底部においては、図２に示す足裏部（符号：６）の足裏表面、または、足裏表面から深さ５ｍｍの範囲で測定すれば、レール頭部およびレール底部の全体を代表させることができ、この部分の温度や冷却速度を制御することにより、初析セメンタイト組織の生成を防止し、レール溶接継ぎ手部の靭性を向上させることができる。
【００２７】
なお、レール溶接継ぎ手部の８００〜９００℃の範囲に加熱されたレールの冷却については、軌道に要求れる特性に応じて、レール頭部のみ、レール底部のみ、レール頭部と底部を任意に選ぶことができる。
従って、高炭素含有のパーライト系レールにおいて、８００〜９００℃の範囲に加熱されたレール溶接継ぎ手部に生成する初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させるには、レール溶接継ぎ手部において、８００〜９００℃の範囲に加熱されたレール頭部または底部を、初析セメンタイト組織の生成を防止するため、７５０℃以上の温度域から冷却速度１〜１０℃／ｓｅｃで加速冷却し、前記鋼レールの頭部または底部の温度が６８０〜５５０℃達した時点で加速冷却を停止し、その後、レール内部から発生する復熱による温度上昇にともなう初析セメンタイト組織の生成を防止するため、前記鋼レールの頭部または底部の温度が６８０℃を超えないように放冷または緩冷却する必要がある。
【００２８】
なお、本発明熱処理方法においては、炭素量以外の鋼レールの成分組成については、特に限定するものではないが、レールとしての最低限度の強度や硬さを確保するため、Ｓｉ：０．１０〜２．００％、Ｍｎ：０．２０〜２．００％を含む成分を基本として、必要に応じて、Ｃｒ，Ｍｏ，Ｖ，Ｎｂ，Ｂ，Ｃｏ，Ｃｕ，Ｎｉ，Ｔｉ，Ｍｇ，Ｃａ，Ａｌ，Ｚｒ等の元素を１種または２種以上含み残部は鉄および不可避的不純物からなる成分系が望ましい。
【００２９】
また、本熱処理製造法によって製造されたパーライト系レールの頭部および底部の金属組織は、パーライト組織であることが望ましいが、成分系、さらには、加速冷却条件によっては、パーライト組織中に微量な初析フェライト組織、初析セメンタイト組織およびベイナイト組織が生成することがある。しかし、パーライト組織中にこれらの組織が微量に生成してもレール頭部や底部の靭性に大きな影響をおよぼさないため、本熱処理製造法によって製造されたパーライト系レールの頭部および底部の組織としては、若干の初析フェライト組織、初析セメンタイト組織およびベイナイト組織の混在も含んでいる。
【００３０】
【実施例】
次に、本発明の実施例について説明する。
表１に供試レール鋼の化学成分を示す。
表２は、表１に示す供試レール鋼を用いて、本発明法の熱処理を行った際のレール頭部と底部の熱処理条件（加速冷却を行う溶接継ぎ手部の温度範囲、加速冷却開始時の冷却部位の最低温度、加速冷却速度の範囲、加速冷却停止温度の範囲、その後の冷却条件と最大復熱温度）、さらに、レール頭部と底部のミクロ組織、初析セメンタイト組織の生成頻度を示す。
【００３１】
表３は、表１に示す供試レール鋼を用いて、比較法の熱処理を行った際のレール頭部と底部の熱処理条件（加速冷却を行う溶接継ぎ手部の温度範囲、加速冷却開始時の冷却部位の最低温度、加速冷却速度の範囲、加速冷却停止温度の範囲、その後の冷却条件と最大復熱温度）、さらに、レール頭部と底部のミクロ組織、初析セメンタイト組織の生成状況を示す。
【００３２】
ここで、本明細書中の図について説明する。図１はレール溶接継ぎ手部の概要を示したもの、図２はレール部位の呼称を示したものである。また、図３は初析セメンタイト組織の生成状況の評価方法を模式的に示したものであって、図中の線γは旧オーステナイト結晶粒界に沿って発生する初析セメンタイトを示し、軸線Ｘ，Ｙとの交点の数でセメンタイト組織の本数としている。
なお、図１において、１は９００〜１２００℃のオーステナイト域単相温度域に加熱される溶接中心部、２は５００〜９００℃の温度に加熱され、パーライト組織やその硬さに変化が発生する熱影響部、３は５００℃未満の温度に加熱され、パーライト組織やその硬さの変化が発生しない母材部である。また、図２において、４は頭頂部、５は頭部コーナー部、６は底部である。
【００３３】
表２，３に示す初析セメンタイト組織の生成状況の評価は次のとおりとした。
調査位置 ：レール溶接継ぎ手部において８２０〜８６０℃に加熱された領域で、かつ、頭部表面から深さ５ｍｍの範囲、足裏表面から深さ５ｍｍの範囲。
エッチング液 ：ピクリン酸カセイソーダ液
エッチング条件：８０℃×１２０ｍｉｎ
調査方法 ：光学顕微鏡により２００倍の視野で直交する３００μｍの線分と交差する初析セメンタイト組織の本数をカウント（図３参照）。
交差する初析セメンタイト組織の本数は、直交する３００μｍの各線分Ｘ，Ｙと交差した本数の合計とした。
調査視野 ：５視野以上調査し、その平均値をレールの代表値とした。
【００３４】
表２に示すように、表１に示す供試レール鋼において、８００〜９００℃の範囲に加熱されたレール頭部または底部、さらに、熱処理する目的で８００〜９００℃の範囲に再加熱されたレール頭部または底部を、種々の熱処理条件を制御した本発明法で製造したレール（符号：Ａ〜Ｆ）は、比較法で製造したレール（符号：Ｇ〜Ｋ）と比べて、加速冷却開始時の冷却部位の最低温度、加速冷却速度の範囲、加速冷却停止温度の範囲を制御することにより、種々のレール溶接方法によって接合された高炭素含有のパーライト系レールの溶接継ぎ手部において、初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させることができた。
【００３５】
【表１】
【表２】
【表３】
【発明の効果】
以上述べたように、種々のレール溶接方法によって接合された高炭素含有のパーライト系レールの溶接継ぎ手部において、加速冷却開始時の冷却部位の最低温度、加速冷却速度の範囲、加速冷却停止温度の範囲を制御することにより、初析セメンタイト組織の生成を防止し、溶接継ぎ手部の靭性を向上させることが可能となる。
【図面の簡単な説明】
【図１】レール溶接継ぎ手部の概要を示した図。
【図２】レール部位の呼称を示した図。
【図３】初析セメンタイト組織の生成状況の評価方法を模式的に示した図。
【符号の説明】
１：９００〜１２００℃のオーステナイト域単相温度域に加熱される溶接中心部。
２：５００〜９００℃の温度に加熱され、パーライト組織やその硬さに変化が発生する熱影響部。
３：５００℃未満の温度に加熱され、パーライト組織やその硬さの変化が発生しない母材部。
４：頭頂部
５：頭部コーナー部
６：足裏部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a pearlitic rail welding joint that prevents the formation of a proeutectoid cementite structure and improves the toughness of the welding joint in a high carbon content pearlite rail welding joint joined by various rail welding methods. The present invention relates to a heat treatment method for a part.
[0002]
[Prior art]
In recent years, heavy-duty railways that transport coal and iron ore abroad and freight railways in Japan have been strongly increasing the load of cargo in order to further increase the efficiency of railway transportation, especially on sharp curves. In the rail, C. The abrasion resistance of the part and the head side cannot be sufficiently secured, and the reduction in rail life due to wear has become a problem. From such a background, development of a rail having wear resistance higher than that of the current high-strength rail containing eutectoid carbon has been required. In order to solve this problem, the present inventors have developed a rail as shown below.
{Circle around (1)} Hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the cementite density in the lamellae in the pearlite structure and to provide a rail with excellent wear resistance (Japanese Patent Laid-Open No. 8-144016). Publication).
{Circle around (2)} Using hypereutectoid steel (C: more than 0.85 to 1.20%), the cementite density in the lamella in the pearlite structure is increased, and at the same time, the hardness is controlled and the wear resistance is excellent. Rail (JP-A-8-246100).
The features of these rails were to increase the carbon content of the steel, increase the volume density of the cemetite phase in the pearlite lamella, and improve the wear resistance of the pearlite structure by controlling the hardness. .
[0003]
In the case of a rail welding joint heated to the austenitic region, a rail heat treatment method as described below has been developed in order to improve various properties of the welded portion.
{Circle around (3)} The entire rail or the rail head and bottom of the rail weld at a temperature equal to or higher than the Ae1 transformation point is cooled with gas or liquid until the pearlite transformation is completed, and then the rapid cooling method (Japanese Patent Laid-Open No. 59-93838). Gazette).
{Circle around (4)} A method of rapidly cooling the legs of the rail whose surface has a temperature of 500 ° C. or more after being heated from the outside of the rail welding portion or from the outside (JP-A-3-277720). The features of these heat treatment methods were to control the hardness and residual stress of the welding joint and to enhance the fracture resistance and fatigue damage resistance of the welding joint.
[0004]
[Problems to be solved by the invention]
In the invention rails described in the above (1) and (2), the wear resistance can be improved by increasing the carbon. However, in the rail welding joint heated to a temperature in the range of 800 to 900 ° C., there is a problem that a coarse proeutectoid cementite structure harmful to toughness is generated after welding, and the toughness of the rail is reduced.
[0005]
In order to solve this problem, even if the method for heat treatment of the rail welding joint shown in the above (3) and (4) is applied, the rail welding joint heated to the range of 800 to 900 ° C. However, there is a problem that a martensite structure harmful to wear resistance and toughness is generated, and the toughness of the rail is further reduced.
[0006]
From such a background, it is possible to prevent the formation of a pro-eutectoid cementite structure generated in a rail welding joint heated to a temperature of 800 to 900 ° C. in a high carbon content pearlite structure rail and improve the toughness of the welding joint. It has become desirable to develop a heat treatment method for the rail to be heated.
[0007]
That is, the present invention aims to prevent the formation of a proeutectoid cementite structure in a high carbon content rail welding joint joined by various rail welding methods and improve the toughness of the welding joint. is there.
[0008]
[Means for Solving the Problems]
The present invention achieves the above object, and the gist thereof is as follows.
(1) A steel head containing C: more than 0.85 to 1.20% by mass is heated to a temperature in the range of 800 to 900 ° C. for welding, and the rail head and / or bottom at the welding joint immediately after joining. Is accelerated and cooled at a cooling rate of 1 to 10 ° C./sec from a temperature range of 750 ° C. or more, and the accelerated cooling is stopped when the temperature of the head and / or bottom of the steel rail reaches 680 to 550 ° C. A method of heat-treating a weld joint of a pearlitic rail, wherein the steel joint is allowed to cool or slowly cool so that a temperature of a head and / or a bottom of the steel rail does not exceed 680 ° C.
(2) Prior to accelerated cooling, the welded joint after joining is reheated to a temperature in the range of 800 to 900 ° C. for the purpose of heat treatment. Heat treatment method for welding joints of system rail.
[0009]
[Problems to be solved by the invention]
Hereinafter, the present invention will be described in detail.
First, the present inventors generate a coarse pro-eutectoid cementite structure at a portion heated to a temperature in the range of 800 to 900 ° C. in a weld joint portion of a high carbon content pearlite-based rail joined by various rail welding methods. The cause was investigated. As a result, a portion heated to a temperature in the range of 800 to 900 ° C. becomes a two-phase state in which an austenite phase and a cementite phase are mixed. For this reason, it was found that carbon was concentrated in the austenite phase, the amount of carbon contained in the austenite phase became excessive, and a coarse pro-eutectoid cementite structure harmful to toughness was formed after welding.
[0010]
Therefore, the present inventors have studied in a laboratory how to prevent a pro-eutectoid cementite structure generated from an austenite phase containing excess carbon. As a result, a region heated to a temperature of 800 to 900 ° C. in which the austenite phase and the cementite phase coexist is moved from a certain temperature range or higher to a temperature range in which the proeutectoid cementite structure is generated so that the proeutectoid cementite structure is not generated. It has been found that a method of accelerating cooling at a certain range of cooling rate to prevent its generation is effective.
[0011]
However, when the rail surface heated to a high temperature is accelerated and cooled, reheating from inside the rail is likely to occur. In particular, it was confirmed that when the accelerated cooling rate was high, the amount of recuperation was also large, and as a result, the temperature was increased to 680 ° C. or higher, and a proeutectoid cementite structure was formed after accelerated cooling.
[0012]
Next, the present inventors studied a method for preventing a proeutectoid cementite structure during reheating. As a result, it was found that by setting the upper limit of the cooling rate during accelerated cooling and further performing simple slow cooling after accelerated cooling, reheating can be sufficiently prevented and the formation of a proeutectoid cementite structure can be prevented.
[0013]
That is, an object of the present invention is to prevent the formation of a pro-eutectoid cementite structure in a welding joint portion of a high carbon content pearlite-based rail joined by various rail welding methods and to improve the toughness of the welding joint portion. Things.
[0014]
Next, the reasons for limitation of the present invention will be described in detail.
(1) Carbon content of steel rail The reason why the carbon content of the steel rail is limited to the above-described claims will be described in detail.
In the high carbon content rail welding joint, a two-phase state in which an austenite phase and a cementite phase are mixed is formed, and a coarse pro-eutectoid cementite structure is generated. However, when the C content is 0.85% or less, the amount of carbon concentrated in the partial austenite phase is small, and the amount of proeutectoid cementite structure is extremely small even without performing accelerated cooling, which adversely affects the toughness of the rail. No effect. If the C content exceeds 1.20%, the amount of carbon concentrated in the partial austenite phase becomes extremely large, making it difficult to completely prevent the formation of a proeutectoid cementite structure even when accelerated cooling is performed. As a result, the rail toughness cannot be prevented from decreasing. For this reason, the C content of the steel rail is limited to more than 0.85 to 1.20%.
[0015]
(2) Heat treatment method Next, in the rail welding joint, the rail head or bottom heated to the range of 800 to 900 ° C is accelerated and cooled at a cooling rate of 1 to 10 ° C / sec from a temperature range of 750 ° C or more. When the temperature at the head or bottom of the steel rail reaches 680 to 550 ° C., the accelerated cooling is stopped. Thereafter, the temperature is slowly cooled or released so that the temperature at the head or bottom of the steel rail does not exceed 680 ° C. The reason why the temperature range and the accelerated cooling rate are limited as described above in the heat treatment method for cooling will be described.
[0016]
(A) Temperature range of the rail welding joint for performing accelerated cooling In a region where the heating temperature of the rail welding joint is less than 800 ° C., in the present component system, the austenite phase does not exist or the austenite phase is very small. No carbon enrichment in the partial austenite phase occurs. As a result, a pro-eutectoid cementite structure harmful to the toughness of the rail is not generated, and the toughness of the rail does not decrease. In the region where the heating temperature of the rail welding joint exceeds 900 ° C., in the present component system, the region is a single-phase region including only the austenite phase. Therefore, carbon does not concentrate in the austenite phase. In this heating region, the cooling rate after welding is high, and the formation of a proeutectoid cementite structure harmful to the toughness of the rail is very small. For this reason, the temperature of the rail welding joint for performing the accelerated cooling is limited to the range of 800 to 900 ° C.
[0017]
In an actual rail welding joint, it is difficult to perform limited accelerated cooling only in a region heated to the above temperature range. Therefore, in order to sufficiently accelerate and cool a portion heated to the above temperature range, it is desirable to perform accelerated cooling including the peripheral heating region beyond the above temperature range.
[0018]
Further, when the temperature does not rise to 800 ° C. or higher due to the welding method or apparatus, or when the accelerated cooling start temperature described later cannot be satisfied, the welded portion may be reheated before the accelerated cooling.
[0019]
(B) Accelerated cooling rate range When the accelerated cooling rate is less than 1 ° C./sec, in a two-phase state in which an austenite phase and a cementite phase are mixed, the formation of a pro-eutectoid cementite structure generated from an austenite phase in which carbon is concentrated is prevented. And the toughness of the rail is reduced. Further, if the accelerated cooling rate exceeds 10 ° C./sec, the recuperation from the inside of the rail after accelerated cooling becomes excessive, and even if slow cooling is performed after accelerated cooling, the temperature of the welding joint exceeds 680 ° C. A proeutectoid cementite structure harmful to the toughness of the rail is formed. For this reason, the range of the accelerated cooling rate was limited to 1 to 10 ° C./sec.
[0020]
As a method for obtaining a cooling rate of 1 to 10 ° C./sec, it is possible to obtain a predetermined cooling rate by using air, a cooling medium mainly containing air, and a mist or the like, and a combination thereof.
[0021]
(C) Accelerated Cooling Start Temperature If the temperature at which accelerated cooling is started is less than 750 ° C., a proeutectoid cementite structure is generated from the carbon-enriched austenite phase before accelerated cooling, and the rail toughness is reduced. For this reason, the accelerated cooling start temperature was limited to 750 ° C. or higher.
[0022]
(D) Temperature range of accelerated cooling When the temperature at which accelerated cooling is stopped is lower than 550 ° C., in a two-phase state in which an austenite phase and a cementite phase are mixed, pearlite transformation is not completed after accelerated cooling, and a martensite structure is formed in the pearlite structure. It forms and reduces the toughness of the rail. If the temperature at which the accelerated cooling is stopped exceeds 680 ° C., a pro-eutectoid cementite structure is formed immediately after the accelerated cooling, and the toughness of the rail is reduced. For this reason, the temperature range of accelerated cooling was limited to the range of 680 to 550 ° C.
[0023]
The cooling after the stop of the accelerated cooling is desirably allowed to cool in order to improve work efficiency. However, when the accelerated cooling rate is high, the recuperation from the inside of the rail also becomes large. Therefore, it is desirable to perform slow cooling after stopping the accelerated cooling. As a method for performing slow cooling, it is possible to control the pressure of a cooling medium mainly containing air or air and mist or the like used during accelerated cooling and the cooling time thereof.
[0024]
Here, the rail welding joint will be described. FIG. 1 shows an outline of a rail welding joint. The rail weld joint is heated to a single-phase temperature range of 900 to 1200 ° C in the austenitic single phase temperature range (symbol: 1), and heated to a temperature range of 500 to 900 ° C, and the pearlite structure and its hardness change. The generated heat-affected zone (symbol: 2) is classified into a base material (symbol: 3) that is heated to a temperature range of less than 500 ° C., but does not change in pearlite structure or hardness. In the rail welding joint, a portion which is heated to a temperature in the range of 800 to 900 ° C. and is in a two-phase state in which an austenite phase and a cementite phase are mixed is a portion near a welding center indicated by
[0025]
As a method of rail welding, flash butt welding, gas pressure welding, thermite welding, enclosed arc welding, and the like are generally performed. Since the heat input differs depending on the welding method, the welding center (symbol: 1) shown in FIG.
Reference numeral: 2), and the position and range of the base material portion (reference: 3) are slightly different. Therefore, the part to be subjected to the accelerated cooling accurately grasps the rail part in the temperature range of 800 to 900 ° C. where the austenite phase and the cementite phase coexist, and sufficiently includes the part in the rail welding joint part of each welding method. It is desirable to select the cooling part in such a manner.
[0026]
Next, the parts of the rail will be described. FIG. 2 shows the names of the rail parts. The “rail head” is a portion including the top part (reference number: 4) and the head corner part (reference number: 5) shown in FIG. The “rail bottom” is a portion including the sole (symbol: 6) shown in FIG. 2. The accelerating cooling rate, the accelerating cooling temperature range, and the accelerating cooling start temperature described above are the top surface (reference number: 4) and the head surface or the head corner (reference number: 5) shown in FIG. In the range of a depth of 5 mm from the surface, and at the bottom of the rail, if measured in the sole surface of the sole portion (symbol: 6) shown in FIG. The entire portion and the bottom of the rail can be represented. By controlling the temperature and cooling rate of this portion, the formation of a proeutectoid cementite structure can be prevented, and the toughness of the rail welding joint can be improved.
[0027]
Regarding the cooling of the rail heated to a temperature in the range of 800 to 900 ° C. in the rail welding joint, the rail head only, the rail bottom only, the rail head and the bottom are arbitrarily selected according to the characteristics required for the track. be able to.
Therefore, in order to prevent the formation of a proeutectoid cementite structure generated in a rail welding joint heated to a temperature of 800 to 900 ° C. in a high carbon content pearlite-based rail and to improve the toughness of the welding joint, At the weld joint, the rail head or bottom heated to 800 to 900 ° C is accelerated at a cooling rate of 1 to 10 ° C / sec from a temperature range of 750 ° C or higher in order to prevent the formation of a proeutectoid cementite structure. After cooling, when the temperature of the head or bottom of the steel rail reaches 680-550 ° C., the accelerated cooling is stopped, and thereafter, the formation of a proeutectoid cementite structure due to a rise in temperature due to reheating generated inside the rail is prevented. Therefore, it is necessary to cool or slowly cool the steel rail so that the temperature at the head or bottom does not exceed 680 ° C.
[0028]
In the heat treatment method of the present invention, the composition of the steel rail other than the carbon content is not particularly limited, but in order to secure the minimum strength and hardness of the rail, Si: 0.10 to 0.10. 2.00%, based on a component containing Mn: 0.20 to 2.00%, if necessary, Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al , Zr or the like, one or more elements are contained, and the remainder is desirably a component system comprising iron and unavoidable impurities.
[0029]
Further, the metal structure of the head and bottom of the pearlite-based rail manufactured by the present heat treatment manufacturing method is desirably a pearlite structure. A proeutectoid ferrite structure, a proeutectoid cementite structure and a bainite structure may be formed. However, even if these structures are formed in trace amounts in the pearlite structure, they do not significantly affect the toughness of the rail head and bottom, so the head and bottom of the pearlitic rail manufactured by this heat treatment manufacturing method. The structure includes a slight mixture of a proeutectoid ferrite structure, a proeutectoid cementite structure and a bainite structure.
[0030]
【Example】
Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the test rail steel.
Table 2 shows the heat treatment conditions of the rail head and bottom when the heat treatment of the present invention was performed using the test rail steels shown in Table 1 (the temperature range of the weld joint for performing accelerated cooling, the time of starting accelerated cooling). (The minimum temperature of the cooling part, the range of the accelerated cooling rate, the range of the accelerated cooling stop temperature, the subsequent cooling conditions and the maximum recuperation temperature), the microstructure of the rail head and bottom, and the frequency of the proeutectoid cementite structure Show.
[0031]
Table 3 shows the heat treatment conditions of the rail head and bottom when performing the heat treatment of the comparative method using the test rail steels shown in Table 1 (the temperature range of the welding joint for performing accelerated cooling, the temperature at the start of accelerated cooling). It shows the minimum temperature of the cooling part, the range of the accelerated cooling rate, the range of the accelerated cooling stop temperature, the subsequent cooling conditions and the maximum recuperation temperature), the microstructure of the rail head and bottom, and the state of formation of the proeutectoid cementite structure. .
[0032]
Here, the drawings in this specification will be described. FIG. 1 shows an outline of a rail welding joint, and FIG. 2 shows names of rail parts. FIG. 3 schematically shows a method for evaluating the state of formation of the pro-eutectoid cementite structure. The line γ in the figure indicates the pro-eutectoid cementite generated along the prior austenite grain boundary, and the axis X , Y is the number of intersections with the number of cementite structures.
In addition, in FIG. 1, 1 is a welding center part which is heated to the austenite region single phase temperature region of 900 to 1200 ° C., 2 is heated to a temperature of 500 to 900 ° C., and a change occurs in the pearlite structure and its hardness. The heat-affected
[0033]
The evaluation of the state of formation of the proeutectoid cementite structure shown in Tables 2 and 3 was as follows.
Investigation position: A region heated to 820 to 860 ° C. in the rail welding joint, a range of 5 mm deep from the head surface, and a range of 5 mm deep from the sole surface.
Etching solution: Picric acid sodium hydroxide solution Etching condition: 80 ° C × 120 min
Investigation method: The number of proeutectoid cementite structures intersecting with orthogonal 300 μm line segments in a 200-fold visual field was counted using an optical microscope (see FIG. 3).
The number of intersecting pro-eutectoid cementite structures was the sum of the number of intersecting orthogonal 300 μm line segments X and Y.
Survey field: Five or more fields were surveyed, and the average value was used as the representative value of the rail.
[0034]
As shown in Table 2, in the test rail steels shown in Table 1, the rail head or bottom was heated to a range of 800 to 900 ° C, and further reheated to a range of 800 to 900 ° C for the purpose of heat treatment. Rails manufactured by the method of the present invention in which various heat treatment conditions are controlled for the head or bottom of the rails (signs: A to F) start accelerated cooling compared to rails manufactured by the comparison method (signs: G to K). By controlling the minimum temperature of the cooling part at the time, the range of the accelerated cooling rate, and the range of the accelerated cooling stop temperature, primary precipitation occurs at the weld joint of the high carbon content pearlite rail joined by various rail welding methods. The formation of the cementite structure was prevented, and the toughness of the weld joint was improved.
[0035]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
As described above, in the welding joint portion of the high carbon content pearlite rail joined by various rail welding methods, the minimum temperature of the cooling part at the start of accelerated cooling, the range of the accelerated cooling speed, the accelerated cooling stop temperature By controlling the range, the formation of a proeutectoid cementite structure can be prevented, and the toughness of the weld joint can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of a rail welding joint portion.
FIG. 2 is a diagram showing names of rail parts.
FIG. 3 is a view schematically showing a method for evaluating the state of formation of a proeutectoid cementite structure.
[Explanation of symbols]
1: A welding center heated to an austenitic single-phase temperature range of 900 to 1200 ° C.
2: A heat-affected zone where the pearlite structure and its hardness change when heated to a temperature of 500 to 900 ° C.
3: A base material portion heated to a temperature of less than 500 ° C. and having no change in pearlite structure or its hardness.
4: Top 5: Head corner 6: Sole
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| JP2002201204A JP4105909B2 (en) | 2002-07-10 | 2002-07-10 | Heat treatment method for pearlitic rails to improve toughness of welded joints |
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| JP4105909B2 JP4105909B2 (en) | 2008-06-25 |
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