JP3975600B2 - Outer layer material for rolling roll made by centrifugal casting, rolling roll and manufacturing method thereof - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、遠心鋳造製圧延ロール用外層材、圧延ロールおよびその製造方法に関し、とくに、組織偏析が無く、かつ、耐肌荒れ性と通板性に優れる遠心鋳造製圧延ロール用外層材、圧延ロールおよびその製造方法に関する。
【０００２】
【従来の技術】
鉄鋼圧延用のロール（圧延ロール）は、被圧延材と接触する外殻層を遠心鋳造法により製造するのが一般的である。これは、遠心鋳造法が、製造コスト低減や省エネルギーの観点から非常に有利であるためである。
一方、近年、熱間圧延用ロールの耐久性向上要求に応じて、適用材の主流が従来の高Cr系ロールから、耐摩耗性、耐肌荒れ性のより優れたハイス系ロールへと切り換えられている。ハイス系ロールが優れた耐摩耗性、耐肌荒れ性を持つのは、硬質なMC型炭化物（ＶやNbが主体）あるいはM2C やM6C 型炭化物（Mo、Ｗが主体）あるいはM7C3型炭化物(Cr 、Moが主体）が多量に出現した金属組織を持つためである。
【０００３】
ハイス系ロールの耐摩耗性、耐肌荒れ性をさらに向上させるために、特開平８−73977 号公報では、前段圧延用ロールの耐肌荒れ性、耐摩耗性の向上と摩擦係数低減を目的として、高Ｃ高Cr高Mo化による共晶炭化物の強化と増量を行い、10＜6.5[%C]-1.3[%V]-0.7[%Nb]≦2[%Cr]-2、を満たす成分系とすることが提案されている。
【０００４】
しかし、遠心鋳造では、溶湯中に析出した炭化物が溶湯との比重差によって遠心分離して偏析することがあり、特にＣ、Crの量が多くなると多量のCr炭化物が溶湯中に晶出し、それが遠心分離して著しい炭化物偏析が形成される。炭化物が偏析すると、その偏析層は硬く脆くなるため、圧延中の熱衝撃によって粗大なクラックが形成され、また不均一摩耗を誘発してロール表面の肌荒れをもたらす。
【０００５】
これに対し、特開平10−183289号公報では、耐肌荒れ性、耐摩耗性の著しい向上と炭化物偏析の抑制を目的とし、Ｃ、Cr、Moの最適調整（Cr≧12% ）を行い、[%C]+0.2[%Cr] ≦6.2 、0.27≦[%Mo]/[%Cr] ＜0.7 、を満たす成分系とすることが提案され、これにより、炭化物の著しい偏析は大幅に軽減された。
一方、遠心鋳造法で製造したロール（とくに外殻層）には組織偏析が生成し、その偏析模様が被圧延材（鋼板）表面に転写して鋼板の表面品質を低下させたり、耐肌荒れ性や耐クラック性などのロール特性を劣化させる問題がつきまとっている。その主たる原因は、硬質な炭化物が多量に存在する層と炭化物が少ない層とがロール径方向にバンド状（層状）あるいは斑状に偏在するためである。図５に示すようにロール径方向にバンド状（層状）をなす偏析をラミネーション偏析と呼ぶ。なお、図５は熱延用ロールの外殻層１内にデンドライト（基地）の濃化部（基地濃化層２）と炭化物の濃化部（炭化物濃化層３）とがロール径方向に交互に積層して形成されたラミネーション偏析(30 〜100mm 程度の肉厚内に２〜６本程度形成）の例を示す。
【０００６】
ハイス系ロールは高合金組成であるがゆえに遠心鋳造過程で顕著な炭化物偏析やラミネーション偏析が発生しやすい。これらの偏析抑制を重視すると合金設計の自由度が小さくなってロール特性の改善がままならず、逆にロール特性を重視して高合金化すると顕著な偏析が出現する。そのため、最近の表面品質に厳格な鋼板を圧延する場合、ロールの使用が制限（とくに最終スタンドへの使用が忌避）されてしまうという憂いがあった。
【０００７】
遠心鋳造方法の改良によりラミネーション偏析等の組織偏析を無くそうとする実用技術は、本発明者らが調査した限りではほとんど開示がなく、従来の遠心鋳造法では、なるべく静かに凝固させた方がよいという基本的思想から、外力がなるべく作用しないように、鋳型回転数を一定に精度よく保持し、かつ鋳型振動を抑えて鋳込むことが通例であった。一方、特許第2778896 号公報に、遠心鋳造製ロール外殻層の組織を微細かつ均一にして耐肌荒れ性、耐クラック性を向上させるために、鋳型への溶融金属（溶湯）の供給温度（鋳込み温度）を初晶生成温度Tc（℃）からTc＋90（℃）にかけての温度域に保って平均積層速度（鋳込み速度）を２〜40mm／分に管理する遠心鋳造方法が開示されている。このようなロール外殻層の組織均一化を狙った発明でも、鋳型回転数は、溶湯表面の遠心力の重力倍数が一例では140G、他例では120Gとなるよう、一定の値に設定されている。
【０００８】
しかし、従来の遠心鋳造法では、上記炭化物偏析やラミネーション偏析を無くすことはできず、特に、ラミネーション偏析は遠心鋳造にとって宿命的なもので回避不可能と考えられていた。
さらに、ロールは鋼板の熱間圧延中に熱膨張するのであるが、ハイスロールは耐摩耗性に優れるがゆえに摩耗量よりも熱膨張量の方が大きくなってサーマルクラウンが増大する。このため、鋼板の通板性が劣化して鋼板の噛込み不良や絞り等の圧延トラブルを誘発しやすいという問題があった。
【０００９】
【発明が解決しようとする課題】
上記のように、従来、圧延ロールは、経済的に有利な遠心鋳造法で製造され、ロール材質は耐摩耗性、耐肌荒れ性等のロール特性に優れるハイス系ロールが主流であるが、遠心鋳造法に特有のラミネーション偏析があって鋼板に偏析模様を付与するため表面厳格鋼板の圧延には使用が憚られている。また、従来のハイス系ロールではサーマルクラウンが大きくなって圧延トラブルを引き起こしやすいという問題がある。
【００１０】
本発明の目的は、これら従来技術の問題点を一挙に解決し、ロール特性に優れしかも鋼板に偏析模様を生じさせず、圧延トラブルを惹起することもない遠心鋳造製圧延ロール用外層材、圧延ロールおよびその製造方法を提供することにある。
【００１１】
【課題を解決するための手段】
金属材料を遠心鋳造で製造しようとすると、凝固の過程で溶湯中に晶出したデンドライトあるいは炭化物が、溶湯との比重差によって遠心分離する（溶湯より重い相は外周側、軽い相は中心側に移動する）現象が起こる。一方、ラミネーション偏析はデンドライト濃化層と炭化物濃化層が交互に重なってバンド状に偏析した形態を呈している。バンド状偏析の形成原因は、遠心鋳造での凝固過程における固相−液相界面（固液共存相）の剪断的流動にあると考えられ、大中ら（例えば、鋳造工学第69巻(1997)第３号第240 〜246 頁）は横型遠心鋳造でのバンド状偏析の発生には重力（1G）が影響していると報告している。この考え方によれば、重力が鋳型回転方向に作用する横型あるいは斜め型遠心鋳造を行う限りラミネーション偏析を回避することは困難であり、現に経験している通りである。なお、従来の遠心鋳造では、前記したように、溶湯に振動や剪断力がなるべくかからないようにするとの基本思想から、鋳造機と基礎の剛性を高め、鋳型回転数を極力一定に制御する操業がなされていたが、この方法ではラミネーション偏析の回避は不可能であった。また、前記特許第2778896 号公報に開示された方法では、鋳込み速度が著しく小さいため、凝固が不安定となって外層表面に２枚皮欠陥やスパッタ状欠陥が生じやすい。また、鋳込み速度が非常に小さいことに加え鋳込み温度もTc〜 Tc+90℃と低く管理範囲も狭いことから、溶湯の流動性を確保することは困難であり、かつ管理範囲を逸脱しやすく安定した操業を行うことも難しい。
【００１２】
本発明者らは、前記従来の基本思想ではラミネーション偏析を解消できないことに鑑み、重力の影響でラミネーション偏析が生成するのなら、従来とは逆に溶湯に積極的に回転方向の加速度を付与すれば、そこで生まれた剪断力によって任意の本数のバンド状偏析を生成させることができるのではないかと考えた。この逆転の発想に基づき、ラミネーション偏析の抑制法を鋭意検討した結果、鋳型回転速度を連続的あるいは断続的に変更して鋳型回転方向に加速度を付与することにより、無数のバンド状偏析を生成させることも原理的に可能であり、しかるに無数のバンド状偏析が生成すれば、マクロ的には均一な組織が得られたことになり、実用上も全く問題がないという知見を得た。
【００１３】
一方、ロール特性を向上させる手段についても鋭意検討し、特定量のＶとNbを含有せしめたハイス系ロールにおいて、CrとMoをバランスよく増量すると、耐摩耗性と耐肌荒れ性をハイレベルに維持しながら、線膨張係数が著しく低下してサーマルクラウンが減じ、噛込み不良や絞り等の圧延トラブルを起こさない、すなわち通板性にも優れたロールとなることを見いだした。
【００１４】
本発明はこれらの知見に基づいて成されたものである。すなわち、本発明は、以下の項に記載の遠心鋳造製圧延ロール用外層材である。
(1) 重量％で、Ｃ：1.5 〜3.5 ％、Si：0.1 〜2.0 ％、Mn：0.1 〜2.0 ％、Cr：５〜25％、Mo：２〜12％、Ｖ：３〜10％、Nb：0.5 〜５％を含有し、かつ〔％Mo〕／〔％Cr〕：0.25〜0.7 で、残部Feおよび不可避的不純物からなり、さらに半径方向に表面から30mmまでの領域で隣り合う極大値と極小値の差が平均値の20％以下となる炭化物量分布を有することを特徴とする遠心鋳造製ロール用外層材。
(2) さらに、重量％で、Co：10％以下を含有する(1) に記載の遠心鋳造製圧延ロール用外層材。
(3) さらに、重量％で、Ni：５％以下を含有する(1) または(2) に記載の遠心鋳造製圧延ロール用外層材。
(4) さらに、重量％で、Ta：10％以下を含有する(1) 〜(3) のいずれかに記載の遠心鋳造製圧延ロール用外層材。
(5) さらに、重量％で、Ｗ：10％以下を含有する(1) 〜(4) のいずれかに記載の遠心鋳造製圧延ロール用外層材。
(6) さらに、重量％で、Ti：３％以下を含有する(1) 〜(5) のいずれかに記載の遠心鋳造製圧延ロール用外層材。
(7) さらに、重量％で、Ｂ：0.1 ％以下を含有する(1) 〜(6) のいずれかに記載の遠心鋳造製圧延ロール用外層材。
(8) さらに、重量％で、Bi：0.1 ％以下を含有を含有する(1) 〜(7) のいずれかに記載の遠心鋳造製圧延ロール用外層材。
【００１５】
また、本発明は、以下の項に記載の圧延ロールである。
(9) 外殻層、中間層、内層の３層構造、または外殻層、内層の２層構造をもつ圧延ロールにおいて、前記外殻層が(1) 〜(8) のいずれかに記載の遠心鋳造製圧延ロール用外層材からなることを特徴とする圧延ロール。
また、本発明は、以下の項に記載の遠心鋳造製圧延ロール用外層材の製造方法である。
(10)遠心鋳造鋳型に外層材溶湯を鋳込んで圧延ロール用外層材を製造するにあたり、前記外層材溶湯を、重量％で、Ｃ：1.5 〜3.5 ％、Si：0.1 〜2.0 ％、Mn：0.1 〜2.0 ％、Cr：５〜25％、Mo：２〜12％、Ｖ：３〜10％、Nb：0.5 〜５％を含有し、あるいはさらに下記第２成分の１種または２種以上を含有し、かつ〔％Mo〕／〔％Cr〕：0.25〜0.7 で、残部Feおよび不可避的不純物からなる溶湯とし、該溶湯の鋳込み中に前記鋳型の回転数を1G/s 以上の増減速度で２回以上増減させるパターンで変動させることを特徴とする遠心鋳造製圧延ロール用外層材の製造方法。
【００１６】
記
〔第２成分〕 Co：10％以下、Ni：５％以下、Ta：10％以下、Ｗ：10％以下、
Ti：３％以下、Ｂ：0.1 ％以下、Bi：0.1 ％以下
また、本発明は、以下の項に記載の圧延ロールの製造方法である。
(11)遠心鋳造鋳型に外層材溶湯を鋳込んで外殻層を形成し、次いで、中間材溶湯を鋳込んで中間層を形成後、あるいは該中間層の形成を行わず、軸材溶湯を鋳込んで内層を形成する圧延ロールの製造方法において、前記外層材溶湯を、重量％で、Ｃ：1.5 〜3.5 ％、Si：0.1 〜2.0 ％、Mn：0.1 〜2.0 ％、Cr：５〜25％、Mo：２〜12％、Ｖ：３〜10％、Nb：0.5 〜５％を含有し、あるいはさらに下記第２成分の１種または２種以上を含有し、かつ〔％Mo〕／〔％Cr〕：0.25〜0.7 で、残部Feおよび不可避的不純物からなる溶湯とし、該溶湯の鋳込み中に前記鋳型の回転数を1G/s 以上の増減速度で２回以上増減させるパターンで変動させることを特徴とする圧延ロールの製造方法。
【００１７】
記
〔第２成分〕 Co：10％以下、Ni：５％以下、Ta：10％以下、Ｗ：10％以下、
Ti：３％以下、Ｂ：0.1 ％以下、Bi：0.1 ％以下
【００１８】
【発明の実施の形態】
まず、本発明における化学組成（成分系）の限定理由を述べる。
Ｃ：1.5 〜3.5 ％
Ｃは、ロールの耐摩耗性を向上させるための炭化物形成に必須の元素であり、1.5 ％未満では炭化物量が不足して優れた耐摩耗性を得ることができない。一方、3.5 ％を超えても耐摩耗性は向上せず、炭化物量が過多となって肌荒れが促進される。
【００１９】
Si：0.1 〜2.0 ％
Siは、脱酸材として必要な元素であり、また、Crと共に基地に固溶して高温酸化への抵抗力を高める作用もあるが、0.1 ％未満では効果に乏しく、2.0 ％超えでは効果が飽和する。
Mn：0.1 〜2.0 ％
Mnは、溶湯中のＳをMnS として固定し、Ｓの悪影響を除去するために添加する。また、焼入れ性を向上させる効果もある。しかし0.1 ％未満では効果に乏しく、一方、2.0 ％を超える添加はオーステナイトが多量に残留してロール特性を劣化させる。
【００２０】
Cr：５〜25％
Crは、耐摩耗性と耐肌荒れ性を向上させるためのCr系炭化物を出現させる作用があり、さらにロールの熱膨張量を減少せしめ、ロールの通板性を向上させる非常に有用な効果があるが、５％未満ではかかる作用効果が不足し、一方、25％を超えると炭化物過多となって耐肌荒れ性が低下する。なお、より好ましくは７〜20％である。
【００２１】
Mo：２〜12％、〔％Mo〕／〔％Cr〕：0.25〜0.7
Moは、上述のCr系炭化物およびMC型炭化物中に濃化してそれらの炭化物を強化することでロールの耐肌荒れ性と耐摩耗性を著しく高める効果を持つ。この効果を得るためにMoは２％以上必要であり、かつ、〔％Mo〕／〔％Cr〕の値が0.25以上となるように添加されなければならない。一方、Moが12％を超えるかあるいは〔％Mo〕／〔％Cr〕の値が0.7 を超えると、上記強化されたCr系炭化物に比べて脆弱なMo系炭化物が多量に出現し、耐肌荒れ性と耐摩耗性が著しく劣化する。
【００２２】
Ｖ：３〜10％
Ｖは、硬質なMC型炭化物を形成し、ハイス系ロールとして一定レベルの耐摩耗性を得るために必須の元素である。その効果を得るためには３％以上必要であるが、10％を超えると溶湯の融点を上昇させるとともに溶湯の流動性を低下させ、遠心鋳造時に未凝固領域への十分な剪断力を付与できなくなる。なお、より好ましくは３〜７％である。
【００２３】
Nb：0.5 〜５％
NbもＶ同様MC型炭化物形成元素であるが、MC炭化物をより強靱な(V,Nb,Mo)C系組成の複合MC型炭化物に改質し、耐摩耗性を著しく向上させる。その効果を得るためにはNbは0.5 ％以上必要であるが、５％を超えて添加するとMC型炭化物の晶出温度が著しく上昇するとともにMC型炭化物の著しい粗大化を招いて炭化物の偏析を助長する。なお、より好ましくは0.5 〜３％である。
【００２４】
さらに、本発明では、遠心鋳造時の未凝固領域の攪拌作用により、高合金化しても成分偏析が起こりにくく均一かつ安定した組織が得られることから、Co、Ni、Ta、Ｗ、Ti、Ｂ、Biを、以下に述べるそれぞれの作用効果を勘案して必要に応じて任意に選択し、単独であるいは複合して添加することができる。
Co：10％以下
Coは、基地中に固溶するとともに他の合金元素の基地への固溶量を高めて基地をより強化する作用をもつ。ただし、10％を超えて添加してもその効果が飽和する。
【００２５】
Ni：５％以下
Niは、焼入れ性の向上作用があり、熱処理での変態挙動を制御するのに有用な元素となる。ただし、耐摩耗性を劣化させる作用も併せ持ち、５％を超えて添加するとこの作用が顕著になるとともに、焼入れ性を高める作用も飽和する。
Ta：10％以下
Taは、ＶやNbと同様、硬質なMC型炭化物の形成元素であり、耐摩耗性を向上する作用があるため含有せしめることができる。ただし、10％を超えると溶湯の流動性が著しく低下し、鋳造を困難にする。
【００２６】
Ｗ：10％以下
Ｗは、硬質な炭化物を形成する元素であることから添加してもよい。ただし、10％を超える添加は炭化物過多となって耐肌荒れ性と靱性を劣化させる。
Ti：３％以下
Tiは、MC型炭化物の形成元素であるとともに組織を微細化する作用も持つため添加することができる。ただし、３％を超えて添加してもその効果が飽和するばかりか、溶湯の流動性を著しく劣化させる。
【００２７】
Ｂ：0.1 ％以下
本発明の外層材は、Ｎと親和力の強いCr、Ｖ、Nbなどの合金元素を多量に含有するため、通常の大気溶解で多量のＮが不可避的に溶湯に混入する。ＢはＮと結合してBNを生成せしめることで、Ｎを安定にしてガス欠陥の生成を抑制する作用、および切削性を向上させる作用を持つ。ただし、0.1 ％を超えると効果が飽和する。
【００２８】
Bi：0.1 ％以下
Biは、組織を微細化する作用を持つが、0.1 ％を超える添加では外層材が著しく脆化する。
本発明の外層材、外殻層、外層材溶湯の組成においては、上記成分元素以外の残部はFeおよび不可避的不純物からなる。不可避的不純物としてはＰ、Ｓ、Ｏ、Ｎ等があるが、Ｐは脆化防止の観点から0.1 ％以下、Ｓは介在物防止の観点から0.1 ％以下、Ｏはザク巣防止の観点から200ppm以下、Ｎはザク巣防止の観点から1500 ppm以下に、それぞれ制限することが望ましい。
【００２９】
本発明(9) の中間層、内層の材質は特に限定されない。中間層は例えば黒鉛鋼で形成でき、内層は例えば球状黒鉛鋳鉄で形成できる。
本発明(11)の中間材溶湯、軸材溶湯の成分系は特に限定されない。中間材溶湯には例えば黒鉛鋼を溶解したものを使用でき、軸材溶湯には例えば球状黒鉛鋳鉄を溶解したものを使用できる。
【００３０】
次に、本発明(1) において、上記成分系要件に加え、半径方向に表面から30mmまでの領域で隣り合う極大値と極小値の差が平均値の20％以下となる炭化物量分布を有するという金属組織要件を設けた理由を述べる。
遠心鋳造法で製造した圧延ロール用外層材にはラミネーション偏析等の組織偏析が存在し、その偏析模様が鋼板表面に転写して鋼板の表面品質を低下させたり、ロールの耐肌荒れ性や耐クラック性などのロール特性を劣化させることは前述した。本発明者らの鋭意研究結果によれば、偏析模様や肌荒れは、近接２層の炭化物量の差によるところが大きい。すなわち、この差が大きいと両層で摩耗量に大差が生じ、外殻層表面に小さな凹凸ができ、この凹凸が巨視的には偏析模様や肌荒れとなって現れる。
【００３１】
近接２層の炭化物量の差は、図１に示されるような半径方向の炭化物量分布を測定し、この分布内で隣り合う極大値と極小値の差（Δｘi ＝｜ｘi+1 −ｘi ｜）で評価することができる。この差と偏析模様、肌荒れ発生状況との関係を調査し、この差が当該分布の平均値の20％を超えると大きな偏析模様や肌荒れを生じるが、20％以下であれば問題になるほどの偏析模様や肌荒れは生じないことがわかった。そして、かかる分布条件が表面から30mmまでの領域で満足されると、圧延用ロールとして十分に使用できる。よって上記のように限定した。
【００３２】
なお、分布内に複数存在するΔｘi と分布全体の平均値の比を、炭化物量分布の「変化率」と定義する。この定義を用いれば、本発明圧延ロール用外層材は、表面から30mmまでの領域で変化率20％以下の炭化物量分布を有する圧延ロール用外層材であるということができる。
炭化物量の測定方法は特に限定されないが、本発明者らは、外層材の横断面（ロール軸に直交する断面）をビレラ液で腐食して現出させた金属組織の100 倍観察像から画像解析装置により炭化物面積率（炭化物量に相当）を測定した。また、半径方向の分布を求める際、測定位置は表面から半径方向に１mmピッチの位置とし同位置で円周方向４点の測定値を平均して同位置での炭化物量データとし、測定位置と炭化物量データの関係を図１のようにグラフ化して隣接関係にある極大値と極小値の組を求めた。また、分布の平均値は炭化物量データ全体を平均して求めた。なお、半径方向測定ピッチは上記１mmに限定されず、適宜選択することができる。
【００３３】
次に、本発明に係る製造方法において、外層材溶湯の鋳込み中に鋳型（遠心鋳造鋳型）の回転数を変動させる点について説明する。
一定鋳型回転速度で遠心鋳造する従来の横型あるいは斜め型遠心鋳造法では、重力（1G）の影響で固液共存領域に剪断力が作用してラミネーション偏析等の粗大な組織偏析が不可避的に生成する。これに対し、本発明では、図２に示すように鋳込み中に軸心Ｏを中心として回転する鋳型４の回転数ｎを強制的に変動させるようにした。この変動のパターンは、 1G/s 以上の増減速度で２回以上増減させるものとする。これにより、鋳込み材に負荷される遠心力Gnを変動させるとともに、鋳込み材に剪断力Gvを連続的あるいは断続的に付与する(Gv ≠０）ことができ、したがって、任意に固液共存相の剪断作用と未凝固領域の攪拌作用を付与することができ、それゆえに組織偏析の分散と偏析成分の攪拌均一化を有効に促進できるのである。
【００３４】
鋳型の回転数の変動パターンは、 1G/s 以上の増減速度で２回以上増減させるものであれば、特に限定されるものではなく、例えば、振幅、周期とも一定として連続的に変動させるパターン（図３（Ａ））、振幅、周期のいずれか一方または両方を変化させながら連続的に変動させるパターン（図３（Ｂ））、断続的に変動させるパターン（図３（Ｃ））など、剪断力Gvを付与できるものであればいかなるパターンであってもよい。
【００３５】
なお、鋳型の回転数変動制御は、鋳込み材の外表面に負荷される遠心力（重力倍数で表す。以下同じ）の増減速度が１G/s 以上となるように行うこととしたのは、これが1G/sに満たないと、重力によるラミネーション偏析形成作用に打ち勝つことが難しいからである。
また、本発明では、凝固過程での固液共存領域に剪断力を付与することで攪拌均一化が叶って偏析防止および組織微細化を達成できるので、これらを目的とした鋳込み速度や鋳込み温度の厳密な管理は不要であり、厳格な管理規制を緩和できて操業をより安定化させることができる。
【００３６】
【実施例】
（実施例１）
表１に示す本発明適合組成の材料Ａ〜Ｄを溶解してなる溶湯を、表２のＡ〜Ｄ欄に示す鋳造条件で横型遠心鋳造機の鋳型に供給し、外径250mm 、内径150mm のスリーブロール（圧延ロール用外層材に相当）を鋳造した。
【００３７】
鋳込み中、発明例１（Ａ）、発明例２（Ｂ）では本発明に従い鋳型回転数ｎを変動させた。変動パターンには周期Τ(s) の周期関数ｎ＝α＋βcos(２πt/Τ) [rpm]（ｔは鋳込み開始からの時間(s) ）を採用し、発明例１ではα＝970rpm、β＝30rpm （すなわちｎの変動範囲は1000〜940rpm）、Τ＝12s （すなわちｎの平均増減速度は60/(12/2)=10rpm/s ）とし、発明例２ではα＝925rpm、β＝25rpm （すなわちｎの変動範囲は950 〜900rpm）、Τ＝６s （すなわちｎの平均増減速度は50/(6/2)=17rpm/s）とした。このとき、スリーブロール表面の遠心力の増減速度は、発明例１ではｎ＝1000rpm,940rpmのときの遠心力がそれぞれ140G,124G であることから平均約2.7G/s、発明例２ではｎ＝950rpm,900rpm のときの遠心力がそれぞれ126G,113G であることから平均約4.3G/sである。
【００３８】
一方、比較例１（Ｃ）、比較例２（Ｄ) では従来通りｎは一定（950rpm）とし、遠心力の増減はない。
鋳造後、スリーブロール横断面を研磨後硝酸水溶液でエッチングして現出させたマクロ組織を観察した。この観察結果の一例を図６に写真で示す。また、同断面半径方向に表面から30mmまでの領域の炭化物量分布を前述の要領で測定して図４に示すようなグラフに整理し、これら分布の変化率を求めた。その結果を表２に示す。なお、表２には分布内に複数ある変化率データのうち最大のものを、該最大データ算出に用いた隣り合う極大値、極小値、それらの差、および分布全体の平均値（全平均）と共に掲げた。
【００３９】
表２より、本発明の製造方法に則り鋳型回転数を変動させながら遠心鋳造した発明例１、２では、ラミネーション偏析はほとんど認められず（図６(a) 参照) 、炭化物量分布の変化率は20％以下であった。これに対し、比較例１、２では、ラミネーション偏析が顕著に認められ（図６(b) 参照) 、炭化物量分布の変化率は20％を超えていた。
【００４０】
このように、本発明圧延ロール用外層材はラミネーション偏析の軽微なものとなるから、表面品質厳格材の仕上げ圧延に憂いなく使用できる。
【００４１】
【表１】
【表２】
（実施例２）
表３に示す組成を有し、かつ変化率20％以下の炭化物量分布を有する遠心鋳造製圧延ロール用外層材を製造し、1050℃から焼入れ、500 ℃で焼戻しを施して得られた素材から試験片を作製し、摩耗試験を行った。なお、表３において、A1〜A11 は本発明適合組成の発明例、B1,B2 は本発明逸脱組成の比較例である。
【００４４】
摩耗試験は相手材（S45C）と試験片の２円盤のすべり摩耗方式で行い、相手材を800 ℃に加熱し、試験片を水冷しながら800rpmで回転させ、試験片と相手材とをすべり率10％、荷重100kg で30分間圧接した。この試験を、相手材を替えて４回行った後、試験片の摩耗減量を測定した。
また、発明例A1〜A7、比較例B1,B2 についてはφ５×20(mm)の丸棒試験片を作製し、熱膨張試験機を用いて25℃から100 ℃の間の線膨張係数を測定した。
【００４５】
これらの試験結果を表３に示す。
表３より、発明例A1〜A11 は、比較例B1,B2 に比べて優れた耐摩耗性を有しており、特に、高Cr組成とした例で耐摩耗性が著しく向上した。
また、発明例A1〜A7の線膨張係数は比較例B1,B2 に比べて小さく、特にCrが７％を超える例では11×10-6を下回る低線膨張係数が得られた。
【００４６】
このように、本発明圧延ロール用外層材は、優れた耐摩耗性を有し、しかも線膨張係数が小さいことから、長寿命であるうえ、鋼板熱間圧延中のロールの形状変化（熱膨張）が抑制されて、通板性にも優れるのである。
【００４７】
【表３】
【発明の効果】
かくして本発明によれば、ロール特性に優れしかも鋼板に偏析模様を生じさせず、圧延トラブルを惹起することもない遠心鋳造製圧延ロール用外層材および圧延ロールが得られるという優れた効果を奏する。
【図面の簡単な説明】
【図１】本発明に係る金属組織要件の説明図である。
【図２】本発明に係る製造方法の説明図である。
【図３】鋳型の回転数変更パターンの例を示す波形図である。
【図４】炭化物量分布の実例（(a) は発明例１、(b) は比較例１）を示すグラフである。
【図５】ラミネーション偏析の例を示す模式図である。
【図６】スリーブロールの横断面組織の一例を示す写真である（a:発明例２、b:比較例１）。
【符号の説明】
１ 外殻層
２ 基地濃化層
３ 炭化物濃化層
４ 鋳型[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to an outer layer material for a rolling roll made by centrifugal casting, a rolling roll, and a method for producing the same, and in particular, an outer layer material for a rolling roll made by centrifugal casting that has no structure segregation and is excellent in rough surface resistance and sheet-passability. And a manufacturing method thereof.
[0002]
[Prior art]
As for the roll for steel rolling (rolling roll), it is common to manufacture the outer shell layer in contact with the material to be rolled by centrifugal casting. This is because the centrifugal casting method is very advantageous from the viewpoint of manufacturing cost reduction and energy saving.
On the other hand, in recent years, in response to demands for improving the durability of hot rolling rolls, the mainstream of applicable materials has been switched from conventional high Cr rolls to high-speed rolls with superior wear resistance and rough skin resistance. Yes. High-speed rolls have excellent wear resistance and rough skin resistance because they are hard MC type carbide (mainly V and Nb) or M2C or M6C-type carbide (mainly Mo, W) or M7CThreeThis is because it has a metal structure in which a large amount of type carbides (mainly Cr and Mo) appear.
[0003]
In order to further improve the wear resistance and skin roughness resistance of high-speed rolls, Japanese Patent Application Laid-Open No. 8-73977 discloses a technique for improving the roughness resistance, wear resistance and reducing the friction coefficient of the pre-rolling roll. Strengthening and increasing the amount of eutectic carbide by increasing the amount of C, Cr and Mo, and a component system satisfying 10 <6.5 [% C] -1.3 [% V] -0.7 [% Nb] ≦ 2 [% Cr] -2 It has been proposed to do.
[0004]
However, in centrifugal casting, carbides precipitated in the molten metal may be segregated by centrifugal separation due to the difference in specific gravity with the molten metal. Especially when the amount of C and Cr increases, a large amount of Cr carbide crystallizes in the molten metal. Centrifuge to form significant carbide segregation. When the carbide segregates, the segregation layer becomes hard and brittle, so that coarse cracks are formed by thermal shock during rolling, and non-uniform wear is induced, resulting in rough skin on the roll surface.
[0005]
On the other hand, in Japanese Patent Laid-Open No. 10-183289, the optimum adjustment of C, Cr, and Mo (Cr ≧ 12%) is performed for the purpose of significantly improving rough skin resistance and wear resistance and suppressing carbide segregation, % C] +0.2 [% Cr] ≦ 6.2, 0.27 ≦ [% Mo] / [% Cr] <0.7 is proposed to satisfy the component system, which significantly reduces carbide segregation. .
On the other hand, structure segregation occurs in rolls manufactured by centrifugal casting (especially the outer shell layer), and the segregation pattern is transferred to the surface of the material to be rolled (steel plate) to deteriorate the surface quality of the steel plate, or to have rough skin resistance. There are problems associated with deterioration of roll properties such as crack resistance. The main reason is that a layer containing a large amount of hard carbide and a layer containing less carbide are unevenly distributed in the form of a band (layer) or spots in the roll radial direction. As shown in FIG. 5, the segregation that forms a band shape (layer shape) in the roll radial direction is called lamination segregation. FIG. 5 shows that the dendrite (base) thickening part (base thickening layer 2) and the carbide thickening part (carbide thickening layer 3) are arranged in the roll radial direction in the outer shell layer 1 of the hot rolling roll. An example of lamination segregation formed by alternately laminating (forming about 2 to 6 within a thickness of about 30 to 100 mm) is shown.
[0006]
Since the high-speed roll has a high alloy composition, remarkable carbide segregation and lamination segregation are likely to occur during the centrifugal casting process. If emphasis is placed on the suppression of segregation, the degree of freedom in alloy design is reduced, and the improvement of roll characteristics does not remain. Conversely, if the roll characteristics are emphasized and a high alloy is made, remarkable segregation appears. For this reason, when rolling steel sheets that are strict in recent surface quality, there is a concern that the use of rolls will be restricted (especially avoiding use on the final stand).
[0007]
Practical techniques to eliminate the structure segregation such as lamination segregation by improving the centrifugal casting method are hardly disclosed as far as the present inventors investigated, and the conventional centrifugal casting method should be solidified as quietly as possible. From the basic idea of good, it has been customary to perform casting while keeping the mold rotational speed constant and accurately and suppressing mold vibration so that external force does not act as much as possible. On the other hand, in Japanese Patent No. 2778896, in order to improve the roughness resistance and crack resistance by making the structure of the outer shell layer of the centrifugal casting roll fine and uniform, the supply temperature of the molten metal (molten metal) to the mold (casting) A centrifugal casting method is disclosed in which the average lamination speed (casting speed) is controlled to 2 to 40 mm / min while maintaining the temperature in the temperature range from the primary crystal formation temperature Tc (° C.) to Tc + 90 (° C.). Even in the invention aiming at homogenizing the structure of the roll outer shell layer, the rotational speed of the mold is set to a constant value so that the gravity multiple of the centrifugal force on the molten metal surface is 140 G in one example and 120 G in the other examples. Yes.
[0008]
However, the conventional centrifugal casting method cannot eliminate the above-mentioned carbide segregation and lamination segregation. In particular, the lamination segregation is fatal to centrifugal casting and is considered to be unavoidable.
Furthermore, the roll expands thermally during hot rolling of the steel sheet, but the high-speed roll is superior in wear resistance, so the amount of thermal expansion is greater than the amount of wear and the thermal crown increases. For this reason, there existed a problem that the plate | board permeability of a steel plate deteriorated and it was easy to induce rolling troubles, such as a biting failure of a steel plate and drawing.
[0009]
[Problems to be solved by the invention]
As described above, conventionally, rolling rolls are manufactured by economically advantageous centrifugal casting, and high-speed rolls that are excellent in roll properties such as wear resistance and rough skin resistance are the mainstream. Since there is a lamination segregation peculiar to the law, and a segregation pattern is imparted to the steel sheet, it has been used for rolling a surface-strict steel sheet. In addition, the conventional high-speed roll has a problem that the thermal crown becomes large and rolling trouble is likely to occur.
[0010]
The object of the present invention is to solve these problems of the prior art all at once, and to provide an outer layer material for rolling rolls made by centrifugal casting that has excellent roll characteristics, does not cause segregation patterns on the steel sheet, and does not cause rolling trouble. It is to provide a roll and a manufacturing method thereof.
[0011]
[Means for Solving the Problems]
When trying to manufacture a metal material by centrifugal casting, dendrites or carbides crystallized in the melt during the solidification process are centrifuged due to the difference in specific gravity from the melt (the heavier phase than the melt is on the outer side and the lighter phase is on the center side). (Moving) phenomenon occurs. On the other hand, lamination segregation has a form in which dendrite concentrated layers and carbide concentrated layers are alternately overlapped to form a band shape. The formation of band-like segregation is thought to be due to the shear flow at the solid-liquid interface (solid-liquid coexisting phase) during the solidification process in centrifugal casting. No. 3 pp. 240-246) reported that gravity (1G) affects the occurrence of band-like segregation in horizontal centrifugal casting. According to this concept, it is difficult to avoid lamination segregation as long as horizontal or oblique centrifugal casting in which gravity acts in the direction of mold rotation is performed, as is actually experienced. In the conventional centrifugal casting, as described above, from the basic idea that vibration and shearing force are not applied to the molten metal as much as possible, there is an operation that increases the rigidity of the casting machine and the foundation and controls the mold rotation speed as constant as possible. However, this method cannot avoid lamination segregation. Further, in the method disclosed in the above-mentioned Japanese Patent No. 2778896, since the casting speed is extremely low, solidification becomes unstable, and two-layer skin defects and sputter-like defects are likely to occur on the outer layer surface. In addition to the very low casting speed, the casting temperature is also low (Tc ~ Tc + 90 ° C) and the management range is narrow, so it is difficult to ensure the fluidity of the molten metal and it is easy to deviate from the management range and is stable. It is also difficult to carry out operations.
[0012]
In view of the fact that lamination segregation cannot be eliminated by the conventional basic idea, the present inventors will positively give the molten metal an acceleration in the rotational direction, contrary to the conventional case, if lamination segregation is generated under the influence of gravity. For example, it was thought that an arbitrary number of band-shaped segregations could be generated by the shear force generated there. Based on this reversal idea, as a result of diligent investigation on the method of suppressing lamination segregation, infinite band-like segregation is generated by changing the mold rotation speed continuously or intermittently to give acceleration in the mold rotation direction. This is also possible in principle, and if innumerable band-like segregation is generated, a macroscopically uniform structure is obtained, and the knowledge that there is no practical problem at all is obtained.
[0013]
On the other hand, we have also intensively studied ways to improve roll properties, and in a high-speed roll containing a specific amount of V and Nb, increasing the amount of Cr and Mo in a well-balanced manner will maintain high levels of wear resistance and rough skin resistance. However, it has been found that the linear expansion coefficient is remarkably lowered, the thermal crown is reduced, and rolling troubles such as poor biting and drawing are not caused, that is, the roll has excellent sheet passing properties.
[0014]
The present invention has been made based on these findings. That is, the present invention is an outer layer material for a centrifugally-casting roll described in the following section.
(1) By weight, C: 1.5-3.5%, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 5-25%, Mo: 2-12%, V: 3-10%, Nb : 0.5-5% and [% Mo] / [% Cr]: 0.25-0.7, consisting of the balance Fe and unavoidable impurities, and further having a local maximum adjacent to the surface from the surface to 30 mm in the radial direction An outer layer material for rolls made of centrifugal cast, characterized in that it has a carbide content distribution in which the difference in local minimum values is 20% or less of the average value.
(2) The outer layer material for rolling rolls according to (1), further comprising Co: 10% or less by weight.
(3) Further, the outer layer material for rolling rolls made by centrifugal casting according to (1) or (2), wherein Ni: 5% or less is contained by weight%.
(4) The outer layer material for rolling rolls according to any one of (1) to (3), further containing Ta: 10% or less by weight.
(5) Furthermore, the outer layer material for a roll made by centrifugal casting according to any one of (1) to (4), further containing, by weight%, W: 10% or less.
(6) Furthermore, the outer layer material for a roll made by centrifugal casting according to any one of (1) to (5), further containing, by weight%, Ti: 3% or less.
(7) The outer layer material for a roll made by centrifugal casting according to any one of (1) to (6), further containing, by weight%, B: 0.1% or less.
(8) The outer layer material for a centrifugally-rolled rolling roll according to any one of (1) to (7), further containing, by weight%, Bi: 0.1% or less.
[0015]
Moreover, this invention is a rolling roll as described in the following terms.
(9) In a rolling roll having a three-layer structure of an outer shell layer, an intermediate layer, and an inner layer, or a two-layer structure of an outer shell layer and an inner layer, the outer shell layer according to any one of (1) to (8) A rolling roll comprising an outer layer material for a centrifugal casting roll.
Moreover, this invention is a manufacturing method of the outer-layer material for centrifugal-rolling rolls as described in the following items.
(10) In producing an outer layer material for a rolling roll by casting an outer layer material melt into a centrifugal casting mold, the outer layer material molten material is, by weight percent, C: 1.5 to 3.5%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 5 to 25%, Mo: 2 to 12%, V: 3 to 10%, Nb: 0.5 to 5%, or one or more of the following second components And [% Mo] / [% Cr]: 0.25 to 0.7, and a molten metal composed of the remaining Fe and unavoidable impurities, and the number of rotations of the mold during casting of the molten metal1G / s With a pattern to increase or decrease twice or more at the above increase / decrease speedA method for producing an outer layer material for a rolling roll made by centrifugal casting, wherein the outer layer material is varied.
[0016]
Record
[Second component] Co: 10% or less, Ni: 5% or less, Ta: 10% or less, W: 10% or less,
Ti: 3% or less, B: 0.1% or less, Bi: 0.1% or less
Moreover, this invention is a manufacturing method of the rolling roll as described in the following items.
(11) Cast the outer layer material melt into the centrifugal casting mold to form the outer shell layer, and then cast the intermediate material melt after casting the intermediate material melt or without forming the intermediate layer. In the manufacturing method of the rolling roll which casts and forms an inner layer, the said outer-layer material molten metal is C: 1.5-3.5%, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 5-25 by weight%. %, Mo: 2 to 12%, V: 3 to 10%, Nb: 0.5 to 5%, or further containing one or more of the following second components, and [% Mo] / [ % Cr]: 0.25 to 0.7, and a molten metal composed of the remaining Fe and inevitable impurities, and the number of rotations of the mold is set during casting of the molten metal.1G / s With a pattern to increase or decrease twice or more at the above increase / decrease speedA method for producing a rolling roll, characterized by being varied.
[0017]
Record
[Second component] Co: 10% or less, Ni: 5% or less, Ta: 10% or less, W: 10% or less,
Ti: 3% or less, B: 0.1% or less, Bi: 0.1% or less
[0018]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the chemical composition (component system) in the present invention will be described.
C: 1.5-3.5%
C is an element essential for carbide formation for improving the wear resistance of the roll. If it is less than 1.5%, the amount of carbide is insufficient and excellent wear resistance cannot be obtained. On the other hand, even if it exceeds 3.5%, the wear resistance is not improved, and the amount of carbide is excessive, and rough skin is promoted.
[0019]
Si: 0.1 to 2.0%
Si is an element required as a deoxidizing material, and also has the effect of increasing the resistance to high-temperature oxidation by dissolving in the matrix together with Cr. However, if it is less than 0.1%, the effect is poor, and if it exceeds 2.0%, it is not effective. Saturates.
Mn: 0.1 to 2.0%
Mn is added to fix S in the molten metal as MnS and remove the adverse effects of S. It also has the effect of improving hardenability. However, if it is less than 0.1%, the effect is poor. On the other hand, if it exceeds 2.0%, a large amount of austenite remains and roll properties deteriorate.
[0020]
Cr: 5-25%
Cr has the effect of causing the appearance of Cr-based carbides to improve wear resistance and rough skin resistance, and also has a very useful effect of reducing the thermal expansion of the roll and improving the roll threadability. However, if it is less than 5%, such an effect is insufficient. On the other hand, if it exceeds 25%, the carbide resistance becomes excessive and the rough skin resistance decreases. In addition, More preferably, it is 7 to 20%.
[0021]
Mo: 2 to 12%, [% Mo] / [% Cr]: 0.25 to 0.7
Mo has the effect of remarkably enhancing the rough skin resistance and wear resistance of the roll by concentrating in the above-mentioned Cr-based carbide and MC-type carbide to strengthen those carbides. In order to obtain this effect, Mo needs to be 2% or more, and it must be added so that the value of [% Mo] / [% Cr] is 0.25 or more. On the other hand, if Mo exceeds 12% or the value of [% Mo] / [% Cr] exceeds 0.7, a large amount of Mo-based carbides appear weaker than the above-mentioned strengthened Cr-based carbides, resulting in rough skin resistance. And wear resistance are significantly degraded.
[0022]
V: 3-10%
V is an essential element for forming a hard MC type carbide and obtaining a certain level of wear resistance as a high-speed roll. In order to obtain the effect, 3% or more is necessary, but if it exceeds 10%, the melting point of the molten metal is raised and the fluidity of the molten metal is lowered, and sufficient shearing force can be applied to the unsolidified region during centrifugal casting. Disappear. In addition, More preferably, it is 3 to 7%.
[0023]
Nb: 0.5-5%
Nb is also an MC type carbide forming element like V, but MC carbide is modified to a tougher (V, Nb, Mo) C-based composite MC type carbide to significantly improve wear resistance. In order to obtain the effect, Nb is required to be 0.5% or more. However, if it is added in excess of 5%, the crystallization temperature of the MC type carbide is remarkably increased and the MC type carbide is markedly coarsened to cause segregation of the carbide. To encourage. In addition, More preferably, it is 0.5 to 3%.
[0024]
Furthermore, in the present invention, a uniform and stable structure is obtained in which the component segregation hardly occurs even when the alloy is made high by the stirring action of the unsolidified region at the time of centrifugal casting, so that Co, Ni, Ta, W, Ti, B Bi can be arbitrarily selected as necessary in consideration of the following effects, and can be added alone or in combination.
Co: 10% or less
Co has a function of further strengthening the base by dissolving in the base and increasing the amount of other alloy elements in the base. However, the effect is saturated even if added over 10%.
[0025]
Ni: 5% or less
Ni has an effect of improving hardenability and is a useful element for controlling the transformation behavior during heat treatment. However, it also has the effect of deteriorating the wear resistance, and when added over 5%, this effect becomes remarkable and the effect of increasing the hardenability is saturated.
Ta: 10% or less
Ta, like V and Nb, is a hard MC-type carbide forming element and can be contained because it has an effect of improving wear resistance. However, if it exceeds 10%, the fluidity of the molten metal is remarkably lowered, making casting difficult.
[0026]
W: 10% or less
W may be added because it is an element that forms a hard carbide. However, addition exceeding 10% results in excessive carbide and deteriorates rough skin resistance and toughness.
Ti: 3% or less
Ti can be added because it is a forming element of MC type carbide and has an effect of refining the structure. However, adding over 3% not only saturates the effect but also significantly deteriorates the fluidity of the melt.
[0027]
B: 0.1% or less
Since the outer layer material of the present invention contains a large amount of alloying elements such as Cr, V, and Nb having a strong affinity for N, a large amount of N is inevitably mixed in the molten metal by normal atmospheric melting. B combines with N to generate BN, thereby stabilizing N and suppressing the generation of gas defects, and improving machinability. However, if it exceeds 0.1%, the effect is saturated.
[0028]
Bi: 0.1% or less
Bi has the effect of refining the structure, but if it exceeds 0.1%, the outer layer material becomes extremely brittle.
In the composition of the outer layer material, outer shell layer, and outer layer material melt of the present invention, the remainder other than the above component elements is composed of Fe and inevitable impurities. Inevitable impurities include P, S, O, N, etc. P is 0.1% or less from the viewpoint of preventing embrittlement, S is 0.1% or less from the viewpoint of preventing inclusions, and O is 200 ppm from the viewpoint of preventing nests. In the following, it is desirable to limit N to 1500 ppm or less from the viewpoint of preventing zest nests.
[0029]
The material of the intermediate layer and inner layer of the present invention (9) is not particularly limited. The intermediate layer can be formed of, for example, graphite steel, and the inner layer can be formed of, for example, spheroidal graphite cast iron.
The component system of the molten intermediate material and the molten shaft material of the present invention (11) is not particularly limited. For example, a melt of graphite steel can be used as the molten intermediate material, and a melt of spheroidal graphite cast iron can be used as the molten shaft material, for example.
[0030]
Next, in the present invention (1), in addition to the above-mentioned component system requirements, it has a carbide amount distribution in which the difference between the local maximum value and the local minimum value in the region from the surface to 30 mm in the radial direction is 20% or less of the average value. The reason for establishing the metal structure requirement is described.
The outer layer material for rolling rolls manufactured by centrifugal casting has structure segregation such as lamination segregation, and the segregation pattern is transferred to the surface of the steel sheet to deteriorate the surface quality of the steel sheet. As described above, the roll characteristics such as the property are deteriorated. According to the results of earnest research by the present inventors, the segregation pattern and rough skin are largely due to the difference in the amount of carbide in the two adjacent layers. That is, if this difference is large, there is a large difference in the amount of wear between the two layers, and small irregularities are formed on the surface of the outer shell layer, and these irregularities appear macroscopically as segregated patterns or rough skin.
[0031]
The difference in the amount of carbide in the adjacent two layers is obtained by measuring a distribution of carbide amount in the radial direction as shown in FIG. 1, and the difference between the maximum value and the minimum value adjacent to each other in this distribution (Δxi= | Xi + 1-Xi|). Investigate the relationship between this difference, segregation pattern, and rough skin, and if this difference exceeds 20% of the average value of the distribution, a large segregation pattern or rough skin occurs. It was found that there was no pattern or rough skin. And when such distribution conditions are satisfied in the region from the surface to 30 mm, it can be sufficiently used as a roll for rolling. Therefore, it was limited as described above.
[0032]
Note that there are multiple Δx in the distribution.iIs defined as the “change rate” of the carbide distribution. If this definition is used, it can be said that the outer layer material for rolling rolls of the present invention is an outer layer material for rolling rolls having a carbide amount distribution with a change rate of 20% or less in a region from the surface to 30 mm.
The method for measuring the amount of carbide is not particularly limited. The carbide area ratio (corresponding to the amount of carbide) was measured by an analyzer. When obtaining the distribution in the radial direction, the measurement position is a 1 mm pitch position from the surface in the radial direction, and averages the measured values at four points in the circumferential direction at the same position to obtain carbide amount data at the same position. The relationship of the carbide amount data was graphed as shown in FIG. 1 to find a set of local maximum and local minimum values that are adjacent to each other. The average value of the distribution was obtained by averaging the entire carbide amount data. The radial measurement pitch is not limited to 1 mm, and can be selected as appropriate.
[0033]
Next, in the manufacturing method according to the present invention, the point that the rotational speed of the mold (centrifugal casting mold) is changed during casting of the outer layer material molten metal will be described.
In the conventional horizontal or oblique centrifugal casting method in which centrifugal casting is performed at a constant mold rotation speed, a shearing force acts on the solid-liquid coexistence region due to the influence of gravity (1G), and coarse structure segregation such as lamination segregation is inevitably generated. To do. On the other hand, in the present invention, as shown in FIG. 2, the rotational speed n of the mold 4 that rotates about the axis O during casting is forcibly changed.This variation pattern is 1G / s It shall be increased or decreased twice or more at the above increase / decrease speed.As a result, the centrifugal force Gn applied to the casting material can be varied, and a shearing force Gv can be applied to the casting material continuously or intermittently (Gv ≠ 0). The shearing action and the stirring action in the unsolidified region can be imparted, and therefore, the dispersion of the segregation of the structure and the uniform stirring of the segregation component can be effectively promoted.
[0034]
The fluctuation pattern of mold rotation speed is, 1G / s If you can increase or decrease twice or more at the above increase or decrease speed,There is no particular limitation, and for example, a pattern that continuously fluctuates with the amplitude and period being constant (FIG. 3A), a pattern that fluctuates continuously while changing one or both of the amplitude and period ( As long as the shearing force Gv can be applied, such as an intermittently changing pattern (FIG. 3C), any pattern may be used.
[0035]
Note that the rotational speed fluctuation control of the mold is performed so that the increase / decrease speed of the centrifugal force (expressed in gravity multiples; the same applies hereinafter) applied to the outer surface of the casting material is 1 G / s or more.It was decided thatIf this is less than 1 G / s, it is difficult to overcome the effect of gravity-induced lamination segregation.Is from.
Further, in the present invention, by applying shear force to the solid-liquid coexistence region in the solidification process, uniform stirring can be achieved and segregation prevention and microstructure refinement can be achieved. Strict management is not necessary, and strict management regulations can be relaxed, and operations can be stabilized.
[0036]
【Example】
Example 1
The molten metal obtained by melting the materials A to D having the composition suitable for the present invention shown in Table 1 is supplied to the mold of the horizontal centrifugal casting machine under the casting conditions shown in the columns A to D of Table 2, and the outer diameter is 250 mm and the inner diameter is 150 mm. A sleeve roll (corresponding to an outer layer material for a rolling roll) was cast.
[0037]
During casting, the mold rotation speed n was varied according to the present invention in Invention Example 1 (A) and Invention Example 2 (B). For the variation pattern, a periodic function n = α + βcos (2πt / Τ) [rpm] (t is the time (s) from the start of casting) is adopted. In Invention Example 1, α = 970 rpm, β = 30 rpm (That is, the fluctuation range of n is 1000 to 940 rpm), Τ = 12 s (that is, the average increase / decrease speed of n is 60 / (12/2) = 10 rpm / s), and in Invention Example 2, α = 925 rpm, β = 25 rpm (ie The fluctuation range of n was 950 to 900 rpm) and Τ = 6 s (that is, the average increase / decrease speed of n was 50 / (6/2) = 17 rpm / s). At this time, the increase / decrease speed of the centrifugal force on the sleeve roll surface is about 2.7 G / s on average because the centrifugal force is 140 G and 124 G at Inventive Example 1 at n = 1000 rpm and 940 rpm, respectively. Since the centrifugal forces at 950 rpm and 900 rpm are 126 G and 113 G, respectively, the average is about 4.3 G / s.
[0038]
On the other hand, in Comparative Example 1 (C) and Comparative Example 2 (D), n is constant (950 rpm) as before, and the centrifugal force does not increase or decrease.
After casting, the macro structure was revealed by polishing the cross section of the sleeve roll and etching it with an aqueous nitric acid solution. An example of the observation result is shown in FIG. In addition, the carbide amount distribution in the region from the surface to 30 mm in the radial direction of the cross section was measured as described above and organized into a graph as shown in FIG. 4, and the rate of change of these distributions was obtained. The results are shown in Table 2. In Table 2, the maximum change rate data in the distribution is the maximum value of adjacent maximum values, minimum values, the difference between them, and the average value of the entire distribution (total average). With.
[0039]
From Table 2, in Examples 1 and 2 that were centrifugally cast while changing the mold rotation speed in accordance with the production method of the present invention, almost no lamination segregation was observed (see FIG. 6 (a)), and the rate of change in the carbide content distribution. Was less than 20%. In contrast, in Comparative Examples 1 and 2, lamination segregation was remarkably observed (see FIG. 6 (b)), and the rate of change in the carbide amount distribution exceeded 20%.
[0040]
Thus, since the outer layer material for rolling rolls of the present invention has a slight segregation of lamination, it can be used without difficulty for finish rolling of strict surface quality materials.
[0041]
[Table 1]
[Table 2]
(Example 2)
From a material obtained by producing an outer layer material for a rolling roll made by centrifugal casting having the composition shown in Table 3 and having a carbide amount distribution with a change rate of 20% or less, quenching from 1050 ° C., and tempering at 500 ° C. Test pieces were prepared and subjected to wear tests. In Table 3, A1 to A11 are invention examples conforming to the present invention, and B1 and B2 are comparative examples of composition deviating from the present invention.
[0044]
The wear test is performed by the sliding wear method of the mating material (S45C) and the test piece, and the mating material is heated to 800 ° C and rotated at 800 rpm while cooling the test piece with water to slide the test piece to the mating material. Pressure welding was performed at 10% and a load of 100 kg for 30 minutes. After this test was performed four times while changing the counterpart material, the wear loss of the test piece was measured.
For Invention Examples A1 to A7 and Comparative Examples B1 and B2, φ5 × 20 (mm) round bar test pieces were prepared and the linear expansion coefficient between 25 ° C and 100 ° C was measured using a thermal expansion tester. did.
[0045]
These test results are shown in Table 3.
From Table 3, Invention Examples A1 to A11 have excellent wear resistance as compared with Comparative Examples B1 and B2, and the wear resistance is remarkably improved particularly in the case of a high Cr composition.
In addition, the linear expansion coefficients of Invention Examples A1 to A7 are smaller than those of Comparative Examples B1 and B2, and particularly 11 × 10 5 in the case where Cr exceeds 7%.-6A low linear expansion coefficient of less than 5 was obtained.
[0046]
As described above, the outer layer material for rolling rolls of the present invention has excellent wear resistance and a low coefficient of linear expansion, so that it has a long life and changes in the shape of the roll during hot rolling of the steel sheet (thermal expansion). ) Is suppressed, and the sheet passing property is also excellent.
[0047]
[Table 3]
【The invention's effect】
Thus, according to the present invention, there is an excellent effect that an outer layer material for a rolling roll made by centrifugal casting and a rolling roll are obtained which are excellent in roll characteristics and do not cause segregation patterns on the steel sheet and cause no rolling trouble.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of metal structure requirements according to the present invention.
FIG. 2 is an explanatory diagram of a manufacturing method according to the present invention.
FIG. 3 is a waveform diagram showing an example of a mold rotation speed change pattern.
FIG. 4 is a graph showing an example of carbide distribution ((a) is Invention Example 1, and (b) is Comparative Example 1).
FIG. 5 is a schematic diagram showing an example of lamination segregation.
6 is a photograph showing an example of a cross-sectional structure of a sleeve roll (a: Invention Example 2, b: Comparative Example 1).
[Explanation of symbols]
1 outer shell layer
2 Base concentrated layer
3 Carbide thickened layer
4 Mold
Claims (11)
記
〔第２成分〕 Co：10％以下、Ni：５％以下、Ta：10％以下、Ｗ：10％以下、
Ti：３％以下、Ｂ：0.1 ％以下、Bi：0.1 ％以下In the production of an outer layer material for a rolling roll by casting the outer layer material melt into a centrifugal casting mold, the outer layer material melt is, by weight percent, C: 1.5 to 3.5%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0. %, Cr: 5 to 25%, Mo: 2 to 12%, V: 3 to 10%, Nb: 0.5 to 5%, or further containing one or more of the following second components, And [% Mo] / [% Cr]: 0.25 to 0.7, and a molten metal consisting of the remaining Fe and unavoidable impurities, and the casting speed of the mold is increased or decreased twice or more at a speed of 1 G / s or more during casting of the molten metal. A method for producing an outer layer material for a centrifugal casting roll, wherein the outer layer material is varied in a pattern to be increased or decreased .
[Second component] Co: 10% or less, Ni: 5% or less, Ta: 10% or less, W: 10% or less,
Ti: 3% or less, B: 0.1% or less, Bi: 0.1% or less
記
〔第２成分〕 Co：10％以下、Ni：５％以下、Ta：10％以下、Ｗ：10％以下、
Ti：３％以下、Ｂ：0.1 ％以下、Bi：0.1 ％以下The outer layer material is cast into a centrifugal casting mold to form an outer shell layer, and then the intermediate material is formed by casting the intermediate material or after or without forming the intermediate layer. In the manufacturing method of the rolling roll which forms an inner layer, the said outer layer material molten metal is C: 1.5-3.5%, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 5-25%, Mo by weight%. : 2 to 12%, V: 3 to 10%, Nb: 0.5 to 5%, or one or more of the following second components, and [% Mo] / [% Cr] : 0.25 to 0.7, a molten metal composed of the remaining Fe and inevitable impurities, and the casting speed of the mold is varied in a pattern of increasing or decreasing twice or more at a speed of increase or decrease of 1 G / s or more during casting of the molten metal. A method for manufacturing a rolling roll.
[Second component] Co: 10% or less, Ni: 5% or less, Ta: 10% or less, W: 10% or less,
Ti: 3% or less, B: 0.1% or less, Bi: 0.1% or less
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| JP04217499A JP3975600B2 (en) | 1999-02-19 | 1999-02-19 | Outer layer material for rolling roll made by centrifugal casting, rolling roll and manufacturing method thereof |
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| JP04217499A JP3975600B2 (en) | 1999-02-19 | 1999-02-19 | Outer layer material for rolling roll made by centrifugal casting, rolling roll and manufacturing method thereof |
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| JP2006257507A (en) * | 2005-03-17 | 2006-09-28 | Hitachi Metals Ltd | Alloy for nonferrous molten metal |
| US20060249230A1 (en) * | 2005-05-09 | 2006-11-09 | Crucible Materials Corp. | Corrosion and wear resistant alloy |
| US8479700B2 (en) * | 2010-01-05 | 2013-07-09 | L. E. Jones Company | Iron-chromium alloy with improved compressive yield strength and method of making and use thereof |
| JP5598147B2 (en) * | 2010-08-05 | 2014-10-01 | 新日鐵住金株式会社 | Manufacturing method of forged steel roll |
| JP5672255B2 (en) * | 2012-02-21 | 2015-02-18 | 新日鐵住金株式会社 | Manufacturing method of forged steel roll |
| JP6459375B2 (en) * | 2014-10-15 | 2019-01-30 | 株式会社Ihi | Centrifugal casting method of metal material |
| JP6866958B2 (en) * | 2018-11-28 | 2021-04-28 | Ｊｆｅスチール株式会社 | Roll for hot rolling Outer layer material and composite roll for hot rolling |
| JP7158312B2 (en) * | 2019-02-28 | 2022-10-21 | Ｊｆｅスチール株式会社 | Hot rolling roll outer layer material, hot rolling composite roll, and method for manufacturing hot rolling roll outer layer material |
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