JP2004255457A

JP2004255457A - Composite roll for hot rolling and method for manufacturing it

Info

Publication number: JP2004255457A
Application number: JP2003052287A
Authority: JP
Inventors: Mitsuo Hashimoto; 光生橋本
Original assignee: NITTETSU HYPER METAL KK; Nippon Steel Corp
Current assignee: NITTETSU HYPER METAL KK; Nippon Steel Corp
Priority date: 2003-02-28
Filing date: 2003-02-28
Publication date: 2004-09-16

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite rolling roll with which a stable rolling operation is maintained for a long period of time without generating slip because of having large friction with rolled steel, without damaging the surface quality of a product caused by fine seizing and reducing wear when heavy reduction rolling is performed in a finish rolling mill train of hot continuous rolling of a steel strip, and to provide a manufacturing method of the roll. <P>SOLUTION: In this composite hot rolling roll, hot forging is applied to a composite roll after casting the composite roll by depositing the outer layer material made of a high-speed steel base material on the outer circumference of an iron-base core material by continuous tinkering. The metallic structure of the outer layer material has 5 to 30% carbides by an area rate, and the distribution of each carbide is adjusted to be ≤20 μm in the average gap between the adjacent carbides. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、鉄鋼の熱間圧延に用いられる圧延用ワークロールに関するもので、特に大圧下圧延に適したものである。
【０００２】
【従来の技術】
圧延用ワークロールに関する耐摩耗性に優れた複合ロール材として近年、その外層をハイス系ロール材と呼ばれるＶ、Ｃｒ、Ｍｏ、Ｗを多量に含有した白鋳鉄が採用されてきている。その従来の技術として、例えば特開平６−３１４３２号公報（特許文献１）、特開平７−２７８６５１号公報（特許文献２）等が開示されている。ここで、前記特許文献１に開示されている技術内容は、連続鋳掛け方法で製造した圧延用複合ロールを過酷な圧延条件で長期間、使用した場合、該製法によって外層と芯材との境界部に生じている鋳造欠陥を起点として外層の剥離が生ずる。これを防止するために前記連続鋳掛け方法で製造したロールを鍛錬成形比が１．１〜１．５程度の熱間鍛造を施すことによって、前記鋳造欠陥を圧着して解消させようとしたものである．
【０００３】
また、前記特許文献２に開示されている技術内容は、圧延用ロールの耐摩耗性、耐ヒートクラック性および耐肌荒性を向上させるために、連続鋳掛け方法で製造したロールを鍛錬成形比が１．４程度のＶ型アンビル加工を施し、当該肉盛り層における炭化物を微細に分布させるものである。一方、近年、連続圧延の特に仕上げ圧延機列の後段スタンドにおいて、大圧下圧延により高強度の微細粒鋼を製造することが望まれてきている。この圧延用のロールとして前記両特許文献に開示されているハイスロールの適用が考えられるが、長時間にわたり連続して圧延を行ったり、従来の圧延に比し極端に高い圧下率で安定して圧延を行うには従来の圧延方法と比し、以下に詳述しているとおり、特に耐スリップ性および耐焼付き性の観点から実用的でなく、実用的なロールの出現が望まれてきている。
【０００４】
前記大圧下圧延を行うにあたり、圧延用ロールの具備特性として、圧延鋼材と該ロールとの間で大きな摩擦を有しスリップを生じることなく（耐スリップ性）、かつ微小な焼付きによる製品の表面品質を損なわず（耐焼付き性）さらに摩耗が少なく、もって長時間にわたり安定した圧延操業を可能ならしめることが重要である。このための手段として、本発明者らは、種々のテスト・検討の結果、以下の技術思想によりそれを達成することを創出した。
【０００５】
すなわち、前記耐スリップ性の向上のためには、粒状炭化物の適正量を微細かつ各炭化物間の隣接する間隙を小さく分散させることが不可欠であることが新たに判明した。炭化物が多すぎたり大きすぎたり、また各炭化物間の隣接する間隙が大きくなると炭化物に比べて軟らかい基地部が大きくなり、この部分が圧延中に平滑に摩耗するためにロールの表面粗度が小さくなり、結果的に圧延鋼材との間で適正な摩擦力を確保できなくなる。また耐焼付き性のためには、前記各炭化物間の間隙が大きくロール地（基地）が大きいと、大圧下圧延ではロールと圧延鋼材との接触部での圧力が増大するために微小な焼付きが発生して製品の表面性状を損なうことになるため各炭化物間の隣接する間隙を小さく分散させることが不可欠であることが新たに判明した。
【０００６】
しかしながら、前記両特許文献に開示されている技術内容では、共に、以下の課題を有しており前記大圧延用のロールとして実用的でない。前記のとおり、両者は共に連続鋳掛け方法によって複合ロールを製造し、その後、該複合ロールに熱間鍛造を施すものである。しかしながら、両者と本発明とは、その目的が相違し、それに伴いその構成も以下の如く相違する。すなわち、前記の両特許文献には、前記本発明者らが創出した大圧下圧延における圧延用ロールでの重要な具備特性である本圧延ロールにおける炭化物の面積率、微細化した後の該各炭化物の分布、すなわち、隣接する各炭化物間の間隙について一切開示されてない。これにより前記両特許文献に開示されている圧延用ロールを大圧下圧延に適用すると、スリップ及び焼付きが頻繁に発生する。
【０００７】
【引用文献】
（１）特許文献１（特開平６−３１４３２号公報）
（２）特許文献２（特開平７−２７８６５１号公報）
【０００８】
【発明が解決しようとする課題】
以上の従来技術の課題に鑑み、本発明の目的は、熱間帯鋼連続圧延の仕上圧延機列において大圧下圧延を行うにあたり、圧延鋼材との間で大きな摩擦を有しスリップを生じることなく、かつ微小な焼付きによる製品の表面品質を損なわずさらに摩耗が少なく、長時間にわたり安定した圧延操業を可能ならしめる圧延用複合ロールおよびその製造方法を提供するものである。
【０００９】
【課題を解決するための手段】
前記の課題を達成するために、本発明の熱間圧延用複合ロールは、
（１）鉄系芯材の外周に、ハイス系材料の外層材を連続鋳掛け法にて溶着させ複合ロールを鋳造後、該複合ロールに熱間鍛造を施し、前記外層材の金属組織が面積率で５〜３０％の炭化物を有し、該各炭化物の分布を隣接する炭化物間の平均間隙が２０μｍ以下になしたことを特徴とする熱間圧延用複合ロール。
【００１０】
（２）前記（１）に記載の外層材が質量％で、Ｃ：０．８〜４．０％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜２．０％、Ｃｒ：３．０〜１５％、Ｖ：３．０〜１５％、Ｍｏ、Ｗの１種または２種：≧２％、かつ、Ｍｏ＋０．５Ｗ：≧６．１％からなることを特徴とする熱間圧延用複合ロール。
（３）前記（２）に、さらに加えてＮｉ：０．２〜５％、Ｃｏ：０．５〜１０％、Ｎｂ：０．５０〜５．０％、Ａｌ，Ｔｉ，Ｚｒの１種以上：≦０．５％の１種または２種以上を含有させたことを特徴とする熱間圧延用複合ロール。
【００１１】
（２）鉄系芯材の外周に、ハイス系材料の外層材を引き抜き速度を１５〜１００ｍｍ／分で連続鋳掛け法にて溶着させ複合ロールを鋳造後、該複合ロールに鍛錬成形比が１．２〜３．０の熱間鍛造を施し、金属組織が面積率で５〜３０％の炭化物を有し、該各炭化物の分布を隣接する炭化物間の平均間隙が２０μｍ以下からなる前記外層材としたことを特徴とする熱間圧延用複合ロールの製造方法である。
【００１２】
【発明の実施の形態】
以下、本発明について詳細に説明する。
ここで、まず、本発明における外層であるハイス材の化学成分を限定した理由を以下に述べる。
Ｃは、ロールの性能で最も重要な耐摩耗性を確保する硬さを得るとともに、本発明の目的である大圧下圧延を可能ならしめるために鋼材との接触部で適度の摩擦を確保する役割をはたす炭化物を適正量晶出ならびに析出するために重要な元素である。Ｃ量が０．８％より少ないと耐摩耗性および耐肌荒れ性を向上させるために有効な硬い炭化物の晶出が少なく、さらに基地に固溶するＣが不足し、焼き入れによっても十分な基地硬さが得られなくなると同時に、合金添加の効果を十分発揮できない。一方、４．０％を越えると晶出する炭化物の量が過大となり、かえって鋼材との接触部での摩擦を減少させて大圧下圧延を困難にするとともに、使用中に外層材の割損や表層剥離等が生じ使用に堪えないためこれを上限とした。
【００１３】
Ｓｉ，Ｍｎは、ともに脱酸効果ならびに溶湯の流動性を高める観点より、一般の高速度鋼に含まれている各々０．２〜２．０％の範囲を含有させることが望ましい。
Ｃｒは、Ｃと結合し主にＭ_７Ｃ_３型の硬い炭化物を結晶粒界に晶出生成し、耐摩耗性を向上させる。網目状に大きく凝集して晶出する。添加量が少ないと炭化物が少なくその効果が十分確保できず、一方多すぎると炭化物の晶出量が過大となり前述のとおり強度が損なわれる。そこで最適な範囲として３．０％以上１５％以下とした。
【００１４】
ＭｏおよびＷは、主として硬いＭ_２Ｃ型炭化物を形成し耐摩耗性を向上させ、Ｖとともに本発明材の重要な元素である。その効果が現れるためには少なくとも１種を２％以上含有することが必要である。このＭ_２Ｃ型炭化物は棒状で結晶粒界に晶出するが、その後の熱処理工程を経てＭ_６Ｃ型炭化物になる。なお、この炭化物の過剰な晶出による脆弱化を抑制する目的で従来はＭｏ＋０．５Ｗを６．０％以下にするとされていた。しかしながら、本発明においてはその懸念は少なく、かえって炭化物を積極的に晶出することが必要で、したがって、Ｍｏ＋０．５Ｗを６．１％以上にすることが望ましい。
【００１５】
本発明においては、従来靭性の観点から好ましくなかったり、鋼材との接触部の摩擦を確保するのに有用ではなかったこれらＭ_７Ｃ_３、Ｍ_２ＣもしくはＭ_６Ｃ炭化物を晶出させ、後述する鍛造効果により有用なものとして積極的に利用することができた。本発明における外層材であるハイス材の基本的な成分は、上記の通りであるが適用を対象とするロールのサイズ、要求されるロールの使用特性等により、その他の成分として前記の化学成分に加えて更に、以下の成分を適宜添加するとよい。
【００１６】
Ｖは、優先的にＣと結合し極めて硬く粒状のＭＣ型炭化物、すなわちＶＣ炭化物を形成し、耐摩耗性を向上させるために極めて有用な元素であると共に、鋼材との接触部で適度の摩擦を確保するのに効果的である。添加量について、３．０％以下であるとその効果は小さく、一方、１５％を超えるとＶＣ型炭化物は偏析して晶出するためこれを上限とした。
Ｎｉは、０．２％程度以上を添加すると焼入性を向上させる効果を有する。従って、直径の大きいロールなど大きい硬化深度が要求される場合等に適宜添加するとよい。しかし多量に添加すると残留オーステナイトが過剰となりかえって高硬度が得られなくなるため５％以下の範囲で用いるとより有効である。
【００１７】
Ｃｏは、特に高温使用下で基地の硬さと強度を向上させるものである。従って、特に、耐摩耗性及び熱き裂性が要求されるような過酷な圧延条件の場合に添加するとその効果がより向上する。０．５％程度以上添加すると効果があるが、経済性の点からその上限を１０％以下とする．
Ａｌ，Ｔｉ，Ｚｒは、いずれもＭＣ型炭化物の晶出核を増加させて、分散晶出させる効果がある。従って、ＭＣ炭化物を微細かつ分散させる目的で添加するとよい。その添加量としては、前記の内の１種以上添加すると効果があるが、経済性の点からその上限を０．５％とする．
【００１８】
Ｎｂは、Ｖと同様にＣと強く結合しＭＣ型のＮｂＣ炭化物を形成するため、Ｖと類似した効果を有する。従って、耐摩耗性を一段と要求される場合に添加するとよりその効果が向上する。０．５０％以上の添加によりその効果があるが、Ｎｂを添加しすぎると材料が過共晶域となりやすく脆弱化ならびに著しい不均一組織が生じ、その後の鍛造工程を経ても目標とする組織が得られず、また凝固温度が高くなり鋳造が困難になるため、その上限値として５．０％以下とする。
【００１９】
本発明の外層材の圧延時の性質について、二つの円盤状試験片を接触させて行う熱間摩擦試験機を用いて実際の圧延条件、具体的には圧延温度、接触応力およびすべり率を合わせてシミュレートして種々実施し、本発明の創出に至った。
以下、図１から図３を使用し、その結果及び限定の理由を説明する。
図１は、外層材における炭化物の量及び形状と圧延鋼材との摩擦係数（μ）の関係を示すものであって、詳しくは、炭化物の量が異なる試験片を種々製作し、摩擦係数（μ）のテスト結果を示す。
【００２０】
なお、本発明が対象とする炭化物は、その大きさが１μｍ以上の晶出炭化物とした。前記の通り、大圧下圧延において、特に、圧延用ロールと圧延鋼材とのスリップは望ましくなく、熱間圧延条件下ではロールと圧延鋼材間の摩擦係数μは０．３を確保することが必要である。従って、図から明らかなように、外層材における炭化物の面積率は、摩擦係数μが０．３の高い範囲である５％以上３０％の範囲が好ましい。また、この炭化物は、耐摩耗性を確保する上でも有用であり、前記の通りその量を面積率で５％以上とする必要がある。一方、脆弱な炭化物が過剰であると外層材そのものを脆化させるため望ましくない。その上限量は、前記のとおり、３０％とするとよい。
【００２１】
図２は、外層材における各炭化物間の間隙と圧延鋼材との摩擦係数μの関係を示す図であって、詳しくは、各炭化物間の平均間隙が異なる試験片を種々製作し、摩擦係数のテスト結果を示す。図から明らかなように、各炭化物間の平均間隙が２０μ以下においては、摩擦係数μが０．３の高い範囲である。従って、外層材における各炭化物の平均間隙は２０μ以下が好ましい。一方、本発明材の製造方法において各炭化物間の平均間隔は２μｍが達成可能であるため、これを下限とした。ここで、各炭化物間の平均間隙は図４に示すごとく２００〜５００倍程度の顕微鏡写真を撮影し、図５に示すようにできるかぎり遊離した晶出炭化物を選んで７個をサンプリングし、各々の炭化物の中心で直角に交差する二線状近傍、四方向の隣接炭化物との距離を測定してその平均値を各炭化物間の間隔とし、さらに７個のデータの最大及び最小値を除いて、５個のデータの平均値を採用した。
【００２２】
すなわち、図５は、図４の晶出炭化物と炭化物間の間隙を求める模式図である。この図５に示すように、符号１〜７は各炭化物を示し、炭化物１は間隙１６．５μｍ、炭化物２は間隙１７．０μｍ、炭化物３は間隙８．５μｍ（最小値）、炭化物４は間隙１７．５μｍ、炭化物５は間隙２０．０μｍ（最大値）、炭化物６は間隙１７．０μｍ、炭化物７は間隙１３．５μｍのそれぞれの間隙を示している。それらの平均間隙は最小値、最大値の場合を除いた５個の平均値として１６．３μｍとなる。
【００２３】
図３は、外層材における炭化物間の間隙と圧延鋼材との焼付きとの関係を示す図である。詳しくは、各炭化物間の平均間隙が異なる試験片を種々製作し、圧延シミュレーターでの焼付性のテスト結果を示す。この図から明らかなように、各炭化物の平均間隙が２０μ以下においては、６０％の高圧下率に相当する試験条件にしても，圧延鋼材との焼付きは、皆無である。従って、この点からも外層材における各炭化物の平均間隙は２０μが好ましい。また、本発明における鉄系の芯材は鍛鋼が望ましいが、ダクタイル鋳鉄等でもよい。
【００２４】
次に、製造方法について述べる。本発明においてはまず連続鋳掛け法により前記ハイス系外層材を芯材の周囲に溶着し複合ロールの素材を鋳造する。前記のとおり各炭化物間の間隙を２０μ以下にするためには、鋳造後の外層の金属組織を、前記炭化物を伴った１００μ以下の結晶粒にしなければならない。すなわち、従来技術で開示した連続鋳掛け法を採用した場合も、前記化学成分においてさらに鋳造条件を特定するより初めて達成できるものである。具体的には鋳造時の引抜き速度を１５〜１００ｍｍ／分以上にする必要がある。なお、間歇的に引抜く場合の引抜き速度は引抜き量（ｍｍ）を引抜き間隔（分）で割った値とした。１５ｍｍ／分以下では、鋳造時の結晶粒径が１００μｍ以上となり本発明材において鍛造により炭化物を分断・分散させても目標とする炭化物間隔を２０μｍ以下とすることができない。一方、引抜き速度は大きい方が鋳造時の結晶粒は微細になり望ましいが、大きすぎると外内層の境界において健全な組織が得られないため実用的な限界として１００ｍｍ／分とした。
【００２５】
さて、連続鋳掛けの後の状態で、外層材は、ＭＣ炭化物は粒状で比較的分散されており、本発明材として望ましいが、Ｍ_７Ｃ_３炭化物は結晶粒界に多量に凝集し、また、Ｍ_２Ｃ炭化物は大きな板状で結晶粒界に晶出しているため大きさ、形状および分布を改善する必要がある。そこで、この素材に熱間鍛造を施すことにより炭化物が小さく分断され且つ均一に分散する。前記の炭化物間の間隙を２０μとするためには鍛錬成形比を１．２以上とする必要があり、また経済的に製造するために３．０以下とする。
【００２６】
【実施例】
本発明の実施例として表１に示す化学成分にて連続鋳掛け法にて鍛錬成形比を考慮した複合ロール素材を鋳造した後、１１００℃に加熱し９５０℃以上の温度範囲にて鍛造成形し、これを繰り返し、直径６５０ｍｍ、外層厚み約８０ｍｍ（鍛練成形比１．２〜２．０）とし、複合ロールを製造した。引続き硬化熱処理を施し、最終的に機械加工を実施して所定の新製形状として実際の圧延に供した。ロールとしての品質および使用状況を表１に併記して示す。この表１から明らかなように、本発明例であるＮｏ．１〜５のロールはいずれも、ロールの製造品質である炭化物の間隙は２０μｍと低く、従って、実際の圧延機での使用成績、すなわち、大圧下圧延においてもスリップおよび焼き付き等は皆無であった。
【００２７】
一方、Ｎｏ．６〜８は比較例であり、表１に各々示す化学成分で、以下に各々示す製法で、前記本発明例と同じサイズの圧延用ロールを製造し、実際の圧延に供したものである。以下、各々について説明する。Ｎｏ．６は、本発明例と同じ製法、すなわち、連続鋳掛法にて鋳造したハイス系ロールに、鍛造を施して製造したものであるが、前記鋳造後の炭化物の結晶粒径が約１５０μｍと大きく、後段で、鍛錬成形比が２．０の高い鍛造を施してもその炭化物の間隙が２７．５と目標品質を満たさなかった。その結果、実際の圧延に使用した際に稀に、特に圧下率が高い場合にスリップおよび焼付きが発生した。
【００２８】
Ｎｏ．７は遠心鋳造法により鋳造したハイス系ロールに、鍛造を施して製造したものである。前記鋳造後の炭化物の結晶粒径が約２００μｍと大きいために、前記Ｎｏ．６と同様に鍛錬成形比が大なる鍛造を施しても炭化物の間隙は３２．５μｍと大きく、使用時にはスリップおよび焼付きが頻発した。Ｎｏ．８はＥＳＲ法にて鋳造した後に鍛造したハイス系ロールであるが、前記Ｎｏ．７よりさらに炭化物の間隙が大きく、使用時にはスリップおよび焼付きが頻発した。
【００２９】
【表１】

【００３０】
【発明の効果】
以上述べたように、本発明によればホットストリップミルの仕上げ圧延機列での大圧下圧延が可能となり経済的で生産性の向上ができ、さらに圧延製品の品質向上がなされ、工業的に大きな価値を有するものである。
【図面の簡単な説明】
【図１】外層材における炭化物の量及び形状と圧延鋼材との摩擦係数の関係を示す図、
【図２】外層材における炭化物間の間隙と圧延鋼材との摩擦係数の関係を示す図、
【図３】外層材における炭化物間の間隙と圧延鋼材との焼付きとの関係を示す図、
【図４】実施例における外層材の顕微鏡組織を示す写真（５００倍）、
【図５】図４の晶出炭化物と炭化物間の間隙を求める模式図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a work roll for rolling used in hot rolling of steel, and is particularly suitable for large rolling.
[0002]
[Prior art]
In recent years, white cast iron whose outer layer contains a large amount of V, Cr, Mo, and W, which is called a high-speed roll material, has been adopted as a composite roll material having excellent wear resistance related to a work roll for rolling. For example, JP-A-6-31432 (Patent Literature 1) and JP-A-7-278651 (Patent Literature 2) are disclosed as conventional techniques. Here, the technical content disclosed in Patent Document 1 is that when a composite roll for rolling manufactured by a continuous casting method is used for a long time under severe rolling conditions, a boundary portion between an outer layer and a core material is produced by the manufacturing method. The exfoliation of the outer layer occurs from the casting defect occurring at the starting point. To prevent this, the roll produced by the continuous casting method is subjected to hot forging with a forging ratio of about 1.1 to 1.5, so that the casting defect is pressed and eliminated. is there.
[0003]
In addition, the technical content disclosed in Patent Document 2 is to improve the abrasion resistance, heat crack resistance, and skin roughness resistance of the rolling roll, and to improve the forging ratio of the roll manufactured by the continuous casting method. About 1.4 V-shaped anvil processing is performed to finely distribute carbide in the build-up layer. On the other hand, in recent years, it has been desired to produce high-strength fine-grained steel by large reduction rolling in continuous rolling, particularly in the latter stage of the finishing mill train. The application of high-speed rolls disclosed in the above-mentioned patent documents is considered as a roll for this rolling.However, rolling is performed continuously for a long time or stably at an extremely high rolling reduction compared to conventional rolling. In order to perform rolling, as compared with the conventional rolling method, as described in detail below, it is not practical from the viewpoint of slip resistance and seizure resistance in particular, and the appearance of a practical roll has been desired. .
[0004]
In carrying out the above-mentioned large rolling, the rolls for rolling are characterized by having a large friction between the rolled steel material and the roll without causing slip (slip resistance), and the surface of the product due to minute seizure. It is important that stable rolling operation is possible over a long period of time without impairing quality (seizure resistance) and with less wear. As a means for achieving this, the present inventors have made various tests and studies and have achieved the following technical ideas.
[0005]
That is, it has been newly found that in order to improve the slip resistance, it is indispensable to disperse an appropriate amount of the granular carbide in a fine amount and to reduce an adjacent gap between the carbides. If the carbide is too large or too large, or if the adjacent gap between each carbide is large, the soft base part becomes large compared to the carbide, and this part wears smoothly during rolling, so the surface roughness of the roll is small. As a result, it becomes impossible to secure an appropriate frictional force with the rolled steel material. In order to prevent seizure, if the gap between the carbides is large and the roll base (base) is large, the pressure at the contact portion between the roll and the rolled steel material increases in large rolling reduction, so that small seizure occurs. It has been newly found that it is indispensable to disperse the adjacent gaps between the carbides to a small extent, since the surface properties of the product are spoiled due to the occurrence of cracks.
[0006]
However, both of the technical contents disclosed in the above-mentioned patent documents have the following problems, and are not practical as rolls for the large rolling. As described above, both manufacture a composite roll by a continuous casting method, and then perform hot forging on the composite roll. However, the objects of the present invention are different from those of the present invention, and accordingly, their configurations are also different as follows. That is, in the above-mentioned patent documents, the area ratio of carbides in the main rolling roll, which is an important feature of the rolling roll in the large rolling reduction created by the present inventors, the respective carbides after refinement. , Ie, the gap between adjacent carbides is not disclosed at all. As a result, when the rolling rolls disclosed in the above-mentioned patent documents are applied to large rolling, slip and seizure frequently occur.
[0007]
[References]
(1) Patent Document 1 (Japanese Patent Laid-Open No. 6-31432)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 7-278651)
[0008]
[Problems to be solved by the invention]
In view of the problems of the prior art described above, an object of the present invention is to perform a large reduction rolling in a finishing rolling mill train of hot strip continuous rolling, without causing a large friction between the rolled steel and slip. Another object of the present invention is to provide a composite roll for rolling which enables stable rolling operation for a long time without impairing the surface quality of the product due to minute seizure and further reducing abrasion, and a method for producing the same.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the composite roll for hot rolling of the present invention,
(1) An outer layer material of a high-speed material is welded to the outer periphery of an iron core material by a continuous casting method to cast a composite roll, and then the composite roll is subjected to hot forging. A composite roll for hot rolling, characterized in that the carbide has 5 to 30% of the carbide and the average gap between adjacent carbides is 20 μm or less.
[0010]
(2) C: 0.8 to 4.0%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.0% by mass% of the outer layer material according to (1). Cr: 3.0 to 15%, V: 3.0 to 15%, one or two of Mo and W: ≧ 2%, and Mo + 0.5W: ≧ 6.1%. Composite roll for hot rolling.
(3) In addition to the above (2), Ni: 0.2 to 5%, Co: 0.5 to 10%, Nb: 0.50 to 5.0%, and at least one of Al, Ti, and Zr : A composite roll for hot rolling, characterized by containing one or more kinds of ≦ 0.5%.
[0011]
(2) An outer layer of a high-speed material is welded to the outer periphery of the iron-based core material by a continuous casting method at a drawing speed of 15 to 100 mm / min to cast a composite roll. Hot forging of 2 to 3.0, the metal structure has carbides of 5 to 30% in area ratio, and the distribution of the carbides is such that the average gap between adjacent carbides is 20 μm or less. A method for manufacturing a composite roll for hot rolling, characterized in that:
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
Here, first, the reasons why the chemical components of the high-speed steel as the outer layer in the present invention are limited will be described below.
C has the role of securing hardness to secure the most important abrasion resistance in the performance of the roll, and ensuring a suitable friction at the contact portion with the steel material in order to enable the large reduction rolling which is the object of the present invention. Is an important element for crystallization and precipitation of a proper amount of carbide. If the C content is less than 0.8%, there is little crystallization of hard carbide effective for improving abrasion resistance and surface roughening resistance, and furthermore, C dissolved in the base is insufficient, and sufficient base can be obtained by quenching. At the same time that hardness cannot be obtained, the effect of alloy addition cannot be sufficiently exhibited. On the other hand, if it exceeds 4.0%, the amount of carbides to be crystallized becomes excessively large, so that the friction at the contact portion with the steel material is reduced to make the large rolling reduction difficult. The upper limit was set because the surface layer peeled off and the use was unbearable.
[0013]
Both Si and Mn are desirably contained in the range of 0.2 to 2.0% each contained in general high-speed steel from the viewpoint of enhancing the deoxidizing effect and the fluidity of the molten metal.
Cr combines with C to form mainly M ₇ C ₃ type hard carbides at the crystal grain boundaries, thereby improving wear resistance. Crystallizes in large meshes. If the amount is small, the amount of carbides is small and the effect cannot be sufficiently ensured. On the other hand, if the amount is too large, the crystallization amount of carbides becomes excessive and the strength is impaired as described above. Therefore, the optimum range is set to 3.0% or more and 15% or less.
[0014]
Mo and W mainly form hard M ₂ C-type carbides to improve wear resistance, and are important elements of the present invention together with V. In order for the effect to be exhibited, it is necessary to contain at least one kind in an amount of 2% or more. This M ₂ C-type carbide is crystallized in the form of a rod at a crystal grain boundary, but becomes M ₆ C-type carbide through a subsequent heat treatment step. In order to suppress brittleness due to excessive crystallization of carbides, Mo + 0.5W is conventionally set to 6.0% or less. However, in the present invention, the concern is small, and it is necessary to positively crystallize carbides. Therefore, it is desirable to set Mo + 0.5W to 6.1% or more.
[0015]
In the present invention, these M ₇ C ₃ , M ₂ C or M ₆ C carbides, which were not conventionally desirable from the viewpoint of toughness or were not useful for securing friction at the contact portion with the steel material, are crystallized and described below. It could be used positively as a useful thing due to the forging effect. The basic components of the high-speed steel, which is the outer layer material in the present invention, are as described above, but depending on the size of the roll to which the application is applied, the required use properties of the roll, etc., the above-mentioned chemical components as other components may be used. In addition, the following components may be appropriately added.
[0016]
V is an element that is preferentially bonded to C to form an extremely hard and granular MC-type carbide, that is, a VC carbide, and is an extremely useful element for improving wear resistance, and has an appropriate friction at a contact portion with a steel material. It is effective to secure. The effect is small when the addition amount is 3.0% or less, while when it exceeds 15%, the VC type carbide segregates and crystallizes, so the upper limit was set.
Ni has an effect of improving hardenability when added at about 0.2% or more. Therefore, when a large curing depth is required such as a roll having a large diameter, it may be appropriately added. However, if a large amount is added, the residual austenite becomes excessive and high hardness cannot be obtained, so that it is more effective to use it in the range of 5% or less.
[0017]
Co improves the hardness and strength of the matrix, especially under high temperature use. Therefore, the effect is further improved when added in severe rolling conditions where abrasion resistance and thermal cracking properties are required. It is effective to add about 0.5% or more, but the upper limit is made 10% or less from the viewpoint of economy.
Al, Ti, and Zr all have the effect of increasing the crystallization nuclei of MC-type carbides and dispersing crystallization. Therefore, it is preferable to add the MC carbide for the purpose of finely dispersing it. As the amount of addition, it is effective to add one or more of the above, but the upper limit is made 0.5% from the viewpoint of economy.
[0018]
Nb has an effect similar to that of V since it binds strongly to C like V and forms NbC carbide of MC type. Therefore, when the wear resistance is further required, its effect is further improved by adding it. The effect is obtained by adding 0.50% or more. However, when Nb is added too much, the material tends to be in a hypereutectic region, and the material becomes brittle and a remarkable non-uniform structure is generated. Since it is not obtained, and the solidification temperature becomes high and casting becomes difficult, the upper limit is set to 5.0% or less.
[0019]
Regarding the properties of the outer layer material of the present invention at the time of rolling, the actual rolling conditions, specifically the rolling temperature, the contact stress and the slip ratio were adjusted by using a hot friction tester in which two disc-shaped test pieces were brought into contact with each other. The present invention was created by performing various simulations.
Hereinafter, the results and the reason for the limitation will be described with reference to FIGS.
FIG. 1 shows the relationship between the amount and shape of the carbide in the outer layer material and the coefficient of friction (μ) between the rolled steel material. More specifically, various test pieces having different amounts of the carbide were manufactured, and the friction coefficient (μ) was measured. 3) shows the test results.
[0020]
The carbide targeted by the present invention was a crystallized carbide having a size of 1 μm or more. As described above, in the large reduction rolling, in particular, the slip between the rolling roll and the rolled steel material is not desirable, and it is necessary to secure a friction coefficient μ between the roll and the rolled steel material of 0.3 under hot rolling conditions. is there. Therefore, as is clear from the figure, the area ratio of the carbide in the outer layer material is preferably in the range of 5% to 30%, which is the high range where the friction coefficient μ is 0.3. Further, this carbide is also useful for securing wear resistance, and its amount needs to be 5% or more in area ratio as described above. On the other hand, if the brittle carbide is excessive, the outer layer material itself is embrittled, which is not desirable. The upper limit is preferably 30% as described above.
[0021]
FIG. 2 is a diagram showing the relationship between the gap between carbides in the outer layer material and the friction coefficient μ between the rolled steel material. Specifically, various test pieces having different average gaps between the carbides were manufactured, and the friction coefficient was measured. Shows test results. As is clear from the figure, when the average gap between the carbides is 20 μ or less, the friction coefficient μ is in a high range of 0.3. Therefore, the average gap of each carbide in the outer layer material is preferably 20 μm or less. On the other hand, in the method for producing the material of the present invention, the average distance between the carbides can be attained to 2 μm, so this was set as the lower limit. Here, as shown in FIG. 4, the average gap between the carbides was photographed by a microphotograph of about 200 to 500 times, and as shown in FIG. 5, crystallized carbides separated as much as possible were selected and seven were sampled. Measure the distance between adjacent carbides in two directions near the center of the carbide at right angles, and the adjacent carbides in the four directions, determine the average value as the interval between the carbides, and exclude the maximum and minimum values of the seven data The average value of five data was adopted.
[0022]
That is, FIG. 5 is a schematic diagram for finding the gap between the crystallized carbide and the carbide in FIG. As shown in FIG. 5, reference numerals 1 to 7 indicate carbides, carbide 1 has a gap of 16.5 μm, carbide 2 has a gap of 17.0 μm, carbide 3 has a gap of 8.5 μm (minimum value), and carbide 4 has a gap of 17.5 μm, carbide 5 shows a gap of 20.0 μm (maximum value), carbide 6 shows a gap of 17.0 μm, and carbide 7 shows a gap of 13.5 μm. The average gap between them is 16.3 μm as an average value of five pieces excluding the case of the minimum value and the case of the maximum value.
[0023]
FIG. 3 is a diagram showing the relationship between the gap between carbides in the outer layer material and the seizure of the rolled steel material. Specifically, various test pieces having different average gaps between the carbides were manufactured, and the results of the seizure test using a rolling simulator are shown. As is apparent from this figure, when the average gap between the carbides is 20 μm or less, there is no seizure with the rolled steel even under the test conditions corresponding to a high pressure reduction of 60%. Therefore, also from this point, the average gap between the carbides in the outer layer material is preferably 20 μm. The iron-based core material in the present invention is desirably forged steel, but may be ductile cast iron or the like.
[0024]
Next, a manufacturing method will be described. In the present invention, first, the high-speed outer layer material is welded around a core material by a continuous casting method to cast a composite roll material. As described above, in order to reduce the gap between the carbides to 20 μm or less, the metal structure of the outer layer after casting must be crystal grains of 100 μm or less with the carbides. That is, even when the continuous casting method disclosed in the prior art is adopted, it can be achieved for the first time after further specifying the casting conditions in the chemical components. Specifically, the drawing speed during casting needs to be 15 to 100 mm / min or more. Note that the drawing speed in the case of intermittent drawing is a value obtained by dividing the drawing amount (mm) by the drawing interval (minute). If it is 15 mm / min or less, the crystal grain size at the time of casting becomes 100 μm or more, and even if carbide is divided and dispersed by forging in the material of the present invention, the target carbide spacing cannot be made 20 μm or less. On the other hand, it is desirable that the drawing speed be higher, because the crystal grains during casting become finer, but if it is too large, a sound structure cannot be obtained at the boundary between the outer and inner layers, so that the practical limit is 100 mm / min.
[0025]
By the way, in the state after the continuous casting, the outer layer material is preferable as the material of the present invention, in which MC carbide is relatively dispersed in a granular form, but M ₇ C ₃ carbide is agglomerated in a large amount at the crystal grain boundary, M ₂ C carbides are large plates and are crystallized at crystal grain boundaries, so that their size, shape and distribution need to be improved. Then, by subjecting this material to hot forging, carbides are divided into small pieces and uniformly dispersed. The forging ratio must be 1.2 or more in order to make the gap between the carbides 20 μm, and 3.0 or less for economical production.
[0026]
【Example】
As an example of the present invention, after casting a composite roll material in consideration of the forging ratio by the continuous casting method with the chemical components shown in Table 1, it was heated to 1100 ° C and forged in a temperature range of 950 ° C or more, This was repeated to produce a composite roll having a diameter of 650 mm and an outer layer thickness of about 80 mm (forging ratio 1.2 to 2.0). Subsequently, a hardening heat treatment was performed, and finally, machining was performed to provide a predetermined new shape for actual rolling. Table 1 also shows the quality and usage status of the roll. As is evident from Table 1, the sample No. 1 of the present invention was not used. In all of the rolls 1 to 5, the gap between the carbides, which is the roll production quality, was as low as 20 μm, and therefore, the use results in actual rolling mills, that is, there was no slip or seizure even in large rolling reduction. .
[0027]
On the other hand, No. Reference numerals 6 to 8 are comparative examples, in which the chemical components shown in Table 1 were used. Rolls having the same size as those of the examples of the present invention were produced by the following production methods, and were subjected to actual rolling. Hereinafter, each will be described. No. No. 6 is manufactured by forging a high-speed steel roll cast by the continuous casting method in the same manufacturing method as that of the example of the present invention, and has a large crystal grain size of about 150 μm after casting. Even in the latter stage, even if forging with a high forging ratio of 2.0 was performed, the gap between the carbides was 27.5, which did not satisfy the target quality. As a result, slip and seizure occurred rarely when used in actual rolling, particularly when the rolling reduction was high.
[0028]
No. No. 7 is manufactured by subjecting a high-speed roll cast by a centrifugal casting method to forging. Since the crystal grain size of the cast carbide is as large as about 200 μm, Even when forging with a large forging ratio was performed in the same manner as in No. 6, the gap between carbides was as large as 32.5 μm, and slip and seizure occurred frequently during use. No. No. 8 is a high-speed roll forged after casting by the ESR method. The carbide gap was larger than that of No. 7, and slip and seizure occurred frequently during use.
[0029]
[Table 1]

[0030]
【The invention's effect】
As described above, according to the present invention, it is possible to perform large reduction rolling in a finishing rolling mill train of a hot strip mill, thereby improving economical productivity and further improving the quality of a rolled product. It has value.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the amount and shape of carbide in an outer layer material and the coefficient of friction with a rolled steel material;
FIG. 2 is a diagram showing a relationship between a gap between carbides in an outer layer material and a friction coefficient between the steel material and a rolled steel material;
FIG. 3 is a view showing a relationship between a gap between carbides in an outer layer material and seizure with a rolled steel material;
FIG. 4 is a photograph (× 500) showing a microscopic structure of an outer layer material in an example;
FIG. 5 is a schematic diagram for obtaining a gap between the crystallized carbide and the carbide in FIG. 4;

Claims

An outer layer material of a high-speed material is welded to the outer periphery of the iron-based material by a continuous casting method to cast a composite roll, and then the composite roll is subjected to hot forging, and the metal structure of the outer layer material has an area ratio of 5 to 5%. A composite roll for hot rolling, comprising 30% of carbides, wherein each carbide is distributed such that the average gap between adjacent carbides is 20 μm or less.

The outer layer material according to claim 1 is in mass%,
C: 0.8-4.0%,
Si: 0.2 to 2.0%,
Mn: 0.2-2.0%,
Cr: 3.0 to 15%,
V: 3.0 to 15%,
One or two of Mo and W: ≧ 2%,
And Mo + 0.5W: ≧ 6.1%
A composite roll for hot rolling, comprising:

In addition to claim 2,
Ni: 0.2-5%,
Co: 0.5 to 10%,
Nb: 0.50 to 5.0%,
One or more of Al, Ti, and Zr: ≤0.5%
A composite roll for hot rolling, characterized by containing one or more of the following.

The outer layer material of the high-speed material is welded to the outer periphery of the iron-based material by a continuous casting method at a drawing speed of 15 to 100 mm / min to cast a composite roll. 0.0 hot forging, and the metal structure has 5 to 30% carbide in area ratio, and the distribution of each carbide is the outer layer material having an average gap between adjacent carbides of 20 µm or less. A method for producing a composite roll for hot rolling, which is characterized in that: