JP2004323961A

JP2004323961A - Outer layer material for hot rolling roll, and composite roll for hot rolling

Info

Publication number: JP2004323961A
Application number: JP2003124410A
Authority: JP
Inventors: Kenji Ichino; 健司市野; Hisashi Hiraoka; 久平岡
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-04-28
Filing date: 2003-04-28
Publication date: 2004-11-18
Anticipated expiration: 2023-04-28
Also published as: JP4311073B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide an outer layer material for a hot rolling roll superior in all of abrasion resistance, seizure resistance, surface roughening resistance and thermal shock resistance, and to provide a composite roll for hot rolling. <P>SOLUTION: The outer layer material has a composition comprising 2.6-3.5% C, 1.5-2.5% Si, 0.2-1.5% Mn, 1.0-2.5% Cr, 1.0-3.0% Mo, 2.0-7.0% Ni, 1.3-2.5% V, 0.1-0.8% Nb, 0.020-0.2% B, and one or two elements selected from 0.05% or less Ti and 0.1% or less Al, or further 4% or less Co, and the balance Fe with unavoidable impurities. The outer layer material has a structure containing 10 or more pieces/mm<SP>2</SP>graphite with a circle equivalent diameter of 20 μm or larger, and 15 to 75% spheroidal graphite in terms of a graphite spheroidized ratio (=ä(n<SB>V</SB>+n<SB>VI</SB>)/n}×100), (wherein n is the number (piece) of total graphite particles having the circle equivalent diameter of 20 μm or larger; and n<SB>V</SB>+n<SB>VI</SB>is the number (piece) of graphite particles having classified shapes V and VI prescribed in JIS G 5502-1995). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、熱間圧延用複合ロールに係り、とくに、耐摩耗性、耐焼付き性、耐肌荒れ性および耐熱衝撃性に優れ、熱間圧延仕上ミル用や継目無鋼管造管用として好適な熱間圧延用複合ロールに関する。
【０００２】
【従来の技術】
近年、鋼板や鋼管の熱間圧延技術の進歩はめざましく、それに伴い、使用される熱間圧延用ロールの特性、とくに耐摩耗性の向上が強く要求されてきた。このような耐摩耗性向上の要求に対し、外層組成を高速度工具鋼組成に類似した組成とし、硬質な炭化物を析出させて、耐摩耗性を格段に向上させた高性能ロール（以下、ハイス系ロールともいう）が開発され実用化されている。
【０００３】
一方、被圧延材の絞り圧延事故や、被圧延材の焼付きが生じやすい圧延スタンドやミルでは、黒鉛を含有し耐焼付き性に優れたＮｉグレン鋳鉄ロールが組み込まれて熱間圧延が行われてきた。熱間圧延用ロールに黒鉛を含有させることは、黒鉛が潤滑性に富むことから焼付き防止には有効である。しかし、従来のＮｉグレン鋳鉄ロールは、耐摩耗性が劣り、そのためロール寿命が短いという問題があった。一方、耐摩耗性に優れたハイス系ロールは、絞り事故や焼付きの発生によって粗大な熱衝撃亀裂が生成するなど、耐事故性の観点で問題がある。例えば、絞り圧延事故の発生頻度が高い鋼板の熱間圧延仕上後段スタンドでは、ハイス系ロールを安定して使用することはできず、依然として、多量のＮｉグレン鋳鉄ロールが使用されている。
【０００４】
このような問題に対し、例えば、特許文献１には、Ｎｉグレン鋳鉄に１．０〜５．０％のＶを添加し、耐摩耗性を向上させるとした熱間圧延用ロールが提案されている。また、特許文献２には、Ｎｉグレン鋳鉄に２．０〜８．０％のＶを添加し、０．５〜５％の黒鉛に加えて０．２〜１０％のＭＣ型炭化物を出現させて耐摩耗性を向上させるとした熱間圧延用ロールが提案されている。さらに特許文献３には、２〜１０％のＣｒと０．１〜１０％のＷと、Ｖ、Ｎｂのうちの１種または２種を合計で、１．５〜１０％を含み、黒鉛を有するハイス系鋳鉄材が提案されている。
【０００５】
また、鋼板圧延と同様に鋼管圧延においても、熱間圧延用ロールへの耐焼付き性と耐摩耗性の向上が強く要望されている。
最近では、圧延製品の品質向上と効率的生産のため、圧延速度の増加や連続圧延量の増加などが指向されており、熱間圧延用ロールの使用環境はますます過酷化している。さらに被圧延材の高合金化、圧延製品の表面品質要求の厳格化などにより、耐摩耗性、耐焼付き性に優れるとともに、耐肌荒れ性にも優れた熱間圧延作業ロールが要望されている。
【０００６】
例えば、特許文献４には、Ｎｂ、Ｗの含有量を制御して、炭化物の重力偏析を抑制し、肌荒れの発生を抑制した熱間圧延用ロールが提案されている。また、特許文献５には、Ｃｒ、Ｖ、Ｎｂの合計含有量を３％以下に制限し、２％以上の黒鉛を晶出させ、肌荒れの発生を抑制した耐摩耗性熱間圧延用ロールが提案されている。また、これらの耐摩耗性熱間圧延用ロールは、耐摩耗性と耐肌荒れ性向上の観点から、球状化した黒鉛粒を晶出させている。
【０００７】
【特許文献１】
特開平１−２８７２４８号公報
【特許文献２】
特開平６−３３５７１２号公報
【特許文献３】
特開平６−２５６８８９号公報
【特許文献４】
特願２００１−２０３３５号公報
【特許文献５】
特願２００１−１８１７８０号公報
【０００８】
【発明が解決しようとする課題】
しかしながら、特許文献４、特許文献５に記載された技術で製造された熱間圧延用ロールでは、耐熱衝撃性が低く、要求される耐摩耗性、耐焼付き性、耐肌荒れ性とともに、耐熱衝撃性を同時に満足することは不可能であった。
本発明は、上記した従来技術の問題を有利に解決し、耐摩耗性、耐焼付き性、耐肌荒れ性、および耐熱衝撃性がともに優れた、熱間圧延用ロール外層材及び熱間圧延用複合ロールを提案することを目的とする。
【０００９】
【課題を解決するための手段】
本発明者らは、上記した課題を達成するために、熱間圧延用ロールの耐摩耗性、耐焼付き性、耐肌荒れ性、耐熱衝撃性に影響する種々の要因について鋭意検討した。
Ｖ、Ｎｂ、Ｍｏ、ＣｒおよびＷの多量添加で耐摩耗性が向上することは、従来からよく知られている。しかし、本発明者らの検討によれば、耐摩耗性を向上させる目的で、ＣｒやＷを増量すると、肌荒れが一層顕著になる。また、Ｃｒ、ＷをＶ（Ｎｂ）と同時に増量すると、耐焼付き性、耐熱衝撃性が劣化する。これは、ＣｒやＷで形成される共晶炭化物が肌荒れや焼付きを発生させたり、熱衝撃亀裂の伝播を促進させるためと考えられる。
【００１０】
このように、Ｃｒ、Ｗは、耐摩耗性の向上作用があるものの、肌荒れ、焼付き、熱衝撃亀裂の発生を促進する傾向を有することから、本発明者らは、熱間圧延用ロールではＣｒは必要最小限の含有にとどめ、Ｗは無添加とすることが良いという知見を得た。なお、Ｃｒを必要最小限の含有とし、Ｗを無添加とすることは、黒鉛化の促進に有効であり、耐肌荒れ性の向上とともに耐焼付き性の向上にも寄与する。
【００１１】
また、本発明者らの検討によれば、熱間圧延用ロールとして必要最低限の耐摩耗性を確保するうえでは、Ｖは１．３質量％以上の含有を必要とする。しかし、Ｖを１．３質量％以上含有すると、粗大なＭＣ炭化物が晶出する。この粗大なＭＣ炭化物は熱間圧延においてロール表面に凸状に残存し、肌荒れを誘引する。本発明者らは、粗大なＭＣ炭化物の晶出抑制方法について、更なる検討を行い、Ｖと同時にＮｂを添加することがよいことを見出した。Ｎｂは、Ｎｂ炭化物を形成しそれを核としてＭＣ炭化物が晶出する。このため、ＭＣ炭化物の粗大化が一定レベル以下に抑制される。
【００１２】
また、ＶとＮｂに加え、さらに、ＡｌやＴｉを添加すると、ＡｌあるいはＴｉの極めて微細な化合物（酸化物、窒化物、炭化物等）がまず生成し母相中に微細分散する。このＡｌあるいはＴｉの微細な化合物は、ＮｂＣの生成核として、ＮｂＣの微細分散を著しく促進する。ＭＣ炭化物はこの微細分散したＮｂＣを核として成長するため、粗大化が顕著に防止でき、粗大ＭＣ炭化物を起因とした肌荒れを防止することができる。しかし、ＮｂとＡｌおよび／またはＴｉを含有した場合でも、Ｖを２．５質量％を超えて含有させると、ＭＣ炭化物が過度に増加して肌荒れが発生しやすくなる。
【００１３】
このように、ＶとＮｂに加え、さらに、Ａｌおよび／またはＴｉを適正量含有することにより、グレン鋳鉄系圧延ロールの耐摩耗性、耐肌荒れ性および耐焼付き性を同時に向上させることが可能となるが、この範囲内で、たとえば黒鉛を減少させたり、黒鉛形状を球状化して耐摩耗性をより向上させると、絞り事故に遭遇した場合に熱衝撃亀裂が粗大化する傾向となり、深さ４ｍｍを超える粗大な絞り亀裂が形成される場合があり、ロール寿命や圧延原単位を悪化させるという問題があった。
【００１４】
このような問題に対し、本発明者らは、熱間圧延用ロールの耐熱衝撃性を向上すべく、高耐摩耗型グレン鋳鉄系ロールでの耐熱衝撃性に及ぼす各種要因について鋭意検討した。その結果、本発明者らは、グレン鋳鉄の基地中に分散する黒鉛の形態を制御することにより、絞り事故時に発生する熱応力（熱歪）を分散させ、熱衝撃亀裂を分岐させて、熱衝撃亀裂深さを浅くすることが可能であることに思い至った。
【００１５】
まず、本発明者らが行った、基礎的実験結果について説明する。
主としてＳｉ量を変化した、質量％で、２．９〜３．１％Ｃ−１．２〜２．８％Ｓｉ−０．５〜０．７％Ｍｎ−４．４〜４．５％Ｎｉ−１．９〜２．１％Ｃｒ−１．９〜２．０％Ｖ−２．０〜２．２％Ｍｏ−０．３〜０．４％Ｎｂ−０．０３〜０．０４％Ｔｉ−０．０５〜０．０６％Ｂ−残部Ｆｅからなる組成の溶湯を溶製し、市販の接種剤（Ｃａ−Ｓｉ、Ｆｅ−Ｓｉ系）でＳｉを０．１〜０．５％接種した後、厚さ３５ｍｍのＹ型キールブロックに鋳造した。得られたキールブロックに４００〜５００ ℃の温度で焼戻しする焼戻処理を行って、供試材とした。
【００１６】
これら供試材について、熱衝撃試験を実施し、耐熱衝撃性を評価した。また、これら供試材について、黒鉛組織（黒鉛粒の大きさ、形状、数）を測定し、ＪＩＳＧ５５０２−１９９５に準拠して黒鉛球状化率を算出した。黒鉛組織の測定方法、熱衝撃試験方法はつぎのとおりとした。
（１）黒鉛組織の測定
供試材表面を鏡面研磨したのち、ノーエッチで光学顕微鏡観察し、黒鉛粒の大きさ、形状、数を、画像解析装置を用いて定量した。なお、画像解析装置の解析画像の倍率は２００倍（顕微鏡倍率５０倍）とした。測定結果から、ＪＩＳＧ５５０２−１９９５に準拠して次式で定義される黒鉛球状化率を算出した。なお、ＪＩＳＧ５５０２−１９９５では長さ２０μｍ以上の黒鉛粒を対象とするが、ここでは、黒鉛粒の面積から求められる円相当直径で２０μｍ以上の黒鉛粒を対象としている。
【００１７】
黒鉛球状化率（％）＝｛（ｎ_Ｖ＋ｎ_ＶＩ）／ｎ｝×１００
（ここで、ｎ：円相当直径２０μｍ以上の黒鉛粒の総粒数（個）、ｎ_Ｖ：ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖの黒鉛粒数（個）、ｎ_ＶＩ：ＪＩＳＧ５５０２−１９９５に規定される形状分類ＶＩの黒鉛粒数（個））
なお、ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖ、形状分類ＶＩの黒鉛粒は、各黒鉛粒の面積比｛（黒鉛粒面積）／（黒鉛粒の最大長さを直径とする円の面積）｝を用いて、分類した。形状分類Ｖおよび形状分類ＶＩに分類される黒鉛粒は、面積比が０．５４以上の黒鉛粒とした（井川克也：素形材、ｖｏｌ．３７、Ｎｏ．９（１９９６．９）ｐ．５、表６参照）。なお、黒鉛粒が真円のとき面積比は１．０となる。
（２）熱衝撃試験
供試材から２５ｍｍ厚の板状試験片を採取し、円盤状の相手材（材質：Ｓ４５Ｃ、大きさ：１９０ｍｍ φ）を用いて、図５に示す構成の試験機で実施した。
【００１８】
高周波加熱コイルにより８００ ℃に加熱され、１００ｒｐｍで回転する円盤状の相手片（材質：Ｓ４５Ｃ、大きさ：１９０ｍｍ φ）を２５ｍｍ厚の板状試験片に荷重１００ｋｇｆ（９８０Ｎ）で１０ｓ間圧接して、板状試験片に，急加熱し除荷と同時に水冷する熱衝撃を印加した。試験後、開口の大きな亀裂が存在する位置２ヶ所で切断し断面で亀裂の最大深さを測定した。
【００１９】
得られた結果を、亀裂最大深さと黒鉛球状化率の関係で図１に示す。
図１から、溶湯中のＳｉ含有量と接種剤の種類、量との組み合わせにより、黒鉛球状化率を１５〜７５％に調整すると、熱衝撃時の亀裂最大深さが浅くなることがわかる。しかし、円相当直径で２０μｍ以上の黒鉛粒数が１０個／ｍｍ^２未満では、黒鉛球状化率が１５〜７５％であっても、熱衝撃時の亀裂最大深さは浅くならないことを知見した。このことから、亀裂最大深さを浅くし耐熱衝撃性を向上するためには、円相当直径で２０μｍ以上の黒鉛粒数を所定個数（１０個／ｍｍ^２）以上存在させるとともに、黒鉛球状化率を適正範囲（１５〜７５％）内に調整することが重要になることがわかる。
【００２０】
従来の熱延ロールでは、強度・延性の向上、耐摩耗性向上のために、黒鉛を球状黒鉛とすることが一般的であった。しかし、本発明者らは、優れた耐摩耗性と耐熱衝撃性を両立させるためには、黒鉛を完全な球状黒鉛とするのではなく、適正範囲内の黒鉛球状化率に調整することが重要であることを見出した。
上記した耐熱衝撃性向上の機構については、詳細についてはまだ明らかとなってはいないが、本発明者らは次のように考えている。
【００２１】
グレン鋳鉄中に分散する黒鉛は、基地（母材）に比べて強度が著しく低いため、力学的には基地（母材）中の空孔とみなせる。基地（母材）中に空孔（黒鉛）が多数あれば、ロール表層部に熱歪が発生しても多数の空孔が少しずつ歪むことになり、熱歪が分散し、熱衝撃亀裂の形成・成長のための応力集中が避けられる。応力集中が避けられれば、亀裂の形成・成長が少なくなり、亀裂最大深さが浅くなると考えられる。また、熱衝撃亀裂の先端が空孔（黒鉛）に突き当たると、その空孔に沿って亀裂が伝播する。黒鉛が分岐した形状をもつ場合にはその分岐形状に沿って亀裂が分岐・分散するため、亀裂最大深さが浅くなると考えられる。
【００２２】
例えば、黒鉛球状化率が１５〜７５％の範囲内では、黒鉛が適正に分岐した形状をもつ（図２参照）ため、亀裂の分岐・分散が適正に生じるが、黒鉛球状化率が１５％未満では、黒鉛の形状がＪＩＳＧ５５０２−１９９５に規定される形状分類でＩに近ずき、分岐が少なく細長く伸びた形状の黒鉛が多くなる。このような形状の黒鉛では、熱衝撃亀裂はその細長く伸びた形状の黒鉛に沿って成長し、亀裂最大深さが深くなると考えられる。また、黒鉛球状化率が７５％を超える（図３参照）と、ほとんどの黒鉛が球状に近く分岐の少ない形状の黒鉛となり、熱応力の分散や亀裂の分岐が起こりにくくなり、亀裂最大深さが深くなると考えられる。なお、円相当直径２０μｍ未満の黒鉛粒は、微細すぎて熱衝撃亀裂を抑制する効果が小さいため、本発明でいう黒鉛粒数としては数えていない。
【００２３】
円相当直径２０μｍ以上の黒鉛粒が所定個数以上存在することにより（図２参照）、上記した熱応力や亀裂の分岐・分散効果が顕著となるが、所定個数未満（図４参照）では、黒鉛粒間隔が広くなりすぎて、熱衝撃亀裂と黒鉛粒との衝突の確率が低下し上記した熱応力の分散や亀裂の分岐が期待できなくなり、亀裂最大深さが深くなると考えられる。
【００２４】
本発明者らは、更なる検討を行い、ＶとＢを含有し、さらにＡｌおよび／またはＴｉを含有したうえ、適正Ｓｉ量としたグレン鋳鉄組成の溶湯に適切な接種を行い、鋳造すると、基地（母材）中に上記した適正に分岐した形状の黒鉛粒を分散させるた黒鉛組織とするとができることを知見した。
本発明は、上記した知見に基づいて、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
（１）熱間圧延用複合ロールの外層に用いられるロール外層材であって、質量％で、Ｃ：２．６〜３．５％、Ｓｉ：１．５〜２．５％、Ｍｎ：０．２〜１．５％、Ｃｒ：１．０〜２．５％、Ｍｏ：１．０〜３．０％、Ｎｉ：２．０〜７．０％、Ｖ：１．３〜２．５％、Ｎｂ：０．１〜０．８％、Ｂ：０．０２０〜０．２％を含み、かつ、Ｔｉ：０．０５％以下およびＡｌ：０．１％以下のうちから選ばれた１種または２種を含み、残部Ｆｅおよび不可避的不純物からなる組成と、円相当直径２０μｍ以上の黒鉛が１０個／ｍｍ^２以上で、円相当直径２０μｍ以上の黒鉛粒についての下記に定義される黒鉛球状化率
黒鉛球状化率（％）＝｛（ｎ_Ｖ＋ｎ_ＶＩ）／ｎ｝×１００
（ここで、ｎ：円相当直径２０μｍ以上の黒鉛粒の総粒数（個）、ｎ_Ｖ：ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖの黒鉛粒数（個）、ｎ_ＶＩ：ＪＩＳＧ５５０２−１９９５に規定される形状分類ＶＩの黒鉛粒数（個））
が１５〜７５％である組織と、を有することを特徴とする熱間圧延用ロール外層材。
（２）（１）において、前記組成に加えてさらに、質量％で、Ｃｏ：４％以下を含むことを特徴とする熱間圧延用ロール外層材。
（３）外層と内層が溶着一体化してなる熱間圧延用複合ロールであって、前記外層が、質量％で、Ｃ：２．６〜３．５％、Ｓｉ：１．５〜２．５％、Ｍｎ：０．２〜１．５％、Ｃｒ：１．０〜２．５％、Ｍｏ：１．０〜３．０％、Ｎｉ：２．０〜７．０％、Ｖ：１．３〜２．５％、Ｎｂ：０．１〜０．８％、Ｂ：０．０２０〜０．２％を含み、かつ、Ｔｉ：０．０５％以下、Ａｌ：０．１％以下のうち選ばれた１種または２種を含み、残部Ｆｅおよび不可避的不純物からなる組成と、円相当直径２０μｍ以上の黒鉛粒数が１０個／ｍｍ^２以上で、円相当直径２０μｍ以上の黒鉛粒についての下記に定義される黒鉛球状化率
黒鉛球状化率（％）＝｛（ｎ_Ｖ＋ｎ_ＶＩ）／ｎ｝×１００
（ここで、ｎ：円相当直径２０μｍ以上の黒鉛粒の総粒数（個）、ｎ_Ｖ：ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖの黒鉛粒数（個）、ｎ_ＶＩ：ＪＩＳＧ５５０２−１９９５に規定される形状分類ＶＩの黒鉛粒数（個））
が１５〜７５％である組織と、を有することを特徴とする熱間圧延用複合ロール。
（４）（３）において、前記外層が、前記組成に加えて、さらに質量％で、Ｃｏ：４％以下を含むことを特徴とする熱間圧延用複合ロール。
【００２５】
【発明の実施の形態】
まず、本発明の熱間圧延用複合ロールの外層（外層材）の組成限定理由について説明する。なお、組成における質量％は単に％と記す。
Ｃ：２．６〜３．５％
Ｃは、Ｖ、Ｎｂ、Ｃｒ、Ｍｏと結合して、ロールの耐摩耗性を向上するための硬質炭化物形成に必須な元素であるとともに、黒鉛の晶出ならびに耐焼付き性、耐熱衝撃性を確保するために必須な元素であり、本発明では２．６％以上の含有を必要とする。一方、３．５％を超えて含有すると、共晶炭化物が多量に出現し、耐熱衝撃性を劣化させるとともに、ＭＣ型炭化物が粗大化して、耐肌荒れ性が低下する。このため、Ｃは２．６〜３．５％の範囲に限定した。なお、好ましくは、２．８〜３．３％である。
【００２６】
Ｓｉ：１．５〜２．５％
Ｓｉは、脱酸剤として作用するとともに、Ｃの活量を増加し黒鉛を晶出させやすくする元素であり、本発明では１．５％以上の含有を必要とする。一方、２．５％を超えて含有すると、黒鉛の粗大化及び組織の粗大化を生じ、ロールの耐肌荒れ性や耐摩耗性が著しく低下する。なお、Ｓｉ量を低目にすると黒鉛量が減少し、黒鉛球状化率が高くなり、一方、Ｓｉ量を高目にすると黒鉛量が増加し黒鉛球状化率が低くなる傾向がある。この現象は他の合金元素とのバランスに影響されるが、好適な黒鉛形態を得るためには、１．５〜２．５％の範囲で調整する必要がある。このようなことから、Ｓｉは１．５〜２．５％の範囲に限定した。
【００２７】
Ｍｎ：０．２〜１．５％
Ｍｎは、溶鋼中のＳをＭｎＳとして固定し、耐摩耗性を阻害するＳを無害、安定化するために有効である。また、焼入れ性を向上させ硬さを増加させるという効果もある。このような効果を得るためには、０．２％以上の含有が必要である。しかし、１．５％を超えて含有すると凝固界面に偏析し材料を脆化させる。このため、Ｍｎは０．２〜１．５％の範囲に含有した。なお、好ましくは０．３〜１．０％である。
【００２８】
Ｃｒ：１．０〜２．５％
Ｃｒは、共晶炭化物を増加させ、硬質化させる元素であり、Ｍｏとともに含有させることにより、ロール圧延時の炭化物破壊を抑制し耐摩耗性を向上させる効果を有する。このような効果を得るためには、１．０％以上の含有を必要とする。一方、２．５％を超えて含有すると、共晶炭化物が増加し、耐肌荒れ性が低下するとともに、黒鉛量が減少して耐焼付き性、耐熱衝撃性も低下する。このため、Ｃｒは１．０〜２．５％に限定した。なお、好ましくは１．２〜２．３％である。
【００２９】
Ｍｏ：１．０〜３．０％
Ｍｏは、共晶炭化物を過度に増量させることなく、炭化物や基地を強化する作用を有し、優れた耐肌荒れ性を維持しつつ耐摩耗性を向上させる効果を有する。特に、Ｎｂと複合して含有することにより、硬質なＭＣ型炭化物を強化し、耐摩耗性を顕著に向上させる重要な効果を発揮するが、このような効果を得るためには、１．０％以上の含有を必要とする。一方、３．０％を超えて含有すると、Ｍｏ主体の硬く脆弱な炭化物が多量に形成され、耐肌荒れ性、耐熱衝撃性が劣化する。このため、Ｍｏは１．０〜３．０％に限定した。なお、好ましくは１．２〜２．５％である。
【００３０】
Ｎｉ：２．０〜７．０％
Ｎｉは、焼入れ性を向上し、材料の硬さを増加させて耐摩耗性を向上させる効果を有する。また、Ｎｉは黒鉛の晶出を促進させる効果も有する。このような効果は２．０％以上の含有で認められるが、７．０％を超えて含有すると、オーステナイトが著しく安定化し、残留オーステナイト量が増加し耐焼付き性を低下させる。このため、Ｎｉは２．０〜７．０％の範囲に限定した。なお、好ましくは３．０〜６．０％である。
【００３１】
Ｖ：１．３〜２．５％
Ｖは、硬質なＭＣ型炭化物を形成し、耐摩耗性を向上させる効果を有する元素である。本発明では、一定レベル以上の耐摩耗性を得るために、１．３％以上の含有を必要とする。一方、２．５％を超えると、ＭＣ型炭化物が粗大化して肌荒れが発生する。さらに、焼付きが発生しやすくなったり、耐熱衝撃性が劣化する等の弊害もある。このため、Ｖは１．３〜２．５％の範囲に限定した。
【００３２】
Ｎｂ：０．１〜０．８％
Ｎｂは、ＭＣ型炭化物に固溶して炭化物を強化する作用を有する元素であり、とくに、所定範囲のＣｒ、Ｍｏ、Ｖと複合して含有することにより、炭化物を著しく強化して耐摩耗性を顕著に向上させる重要な効果を有している。また、Ｖのみの含有ではＭＣ型炭化物が羽毛状に成長して組織が粗大化し、ロールの肌荒れを誘引する。ＮｂとＶを複合して含有することにより、ＭＣ型炭化物の羽毛状化を抑制することができ、さらにＴｉおよび／またはＡｌと複合添加することにより、Ｔｉおよび／またはＡｌの酸化物、窒化物、炭化物等がＮｂＣの晶出核となって、ＮｂＣを微細分散させるとともに、次にＮｂＣを核としてＭＣ型炭化物が微細分散して、肌荒れが抑制されるという重要な効果がある。このような効果を得るためには、Ｎｂは０．１％以上の含有を必要とする。なお、Ｎｂが０．１％未満では、ＭＣ型炭化物の偏析や粗大化が発生し、肌荒れを防止できない。一方、Ｎｂを０．８％を超えて含有すると、ＭＣ型炭化物がデンドライト状に粗大化するため、却って肌荒れが発生するようになる。また、遠心鋳造法で製造すると、ＭＣ型炭化物の偏析が生じる。このようなことから、Ｎｂは０．１〜０．８％の範囲に限定した。なお、ＭＣ型炭化物粗大化の観点から、Ｎｂは０．１〜０．５％とすることが好ましい。
【００３３】
Ｂ：０．０２０〜０．２％
Ｂは、Ａｌおよび／またはＴｉと共存して含有することにより、黒鉛を分岐した形状とすると共に黒鉛粒数を増加させる重要な作用を有している。またさらにＢは耐焼付き性、耐熱衝撃性を向上させる効果も有している。このような効果はＡｌおよび／またはＴｉの共存下で０．０２０％以上の含有で認められるが、０．２％を超えて含有すると、炭化物が脆弱化し、耐摩耗性が低下するとともに、黒鉛量も減少する。このため、Ｂは０．０２０〜０．２％の範囲に限定した。なお、好ましくは０．０２〜０．１０％である。
【００３４】
Ｔｉ：０．０５％以下、Ａｌ：０．１％以下のうちから選ばれた１種または２種
Ｔｉおよび／またはＡｌは、Ｂと共存して含有することにより、前記したＢの作用を発揮させる重要な元素である。
Ｔｉ、Ａｌは、いずれも、Ｂと共存して含有することにより、黒鉛化を促進するとともに、黒鉛形状を分岐形状および／またはずんぐりとした面長形状とすることを助長し、耐焼付き性、耐熱衝撃性を向上させる重要な効果を有する。また、Ｔｉおよび／またはＡｌの酸化物、窒化物、炭化物等がＮｂ炭化物の核となり、ＭＣ炭化物を微細分散させる効果も有する。このような効果は、Ｔｉが０．００５％以上、Ａｌが０．００３％以上の含有で顕著に認められる。一方、Ｔｉを０．０５％を超えて含有すると、微細ＭＣ型炭化物が密集し、その部分で大きな肌荒れを発生させる。また、Ａｌを０．１％を超えて含有すると、溶湯の流動性が低下して鋳造欠陥が発生しやすくなる。このため、Ｔｉは０．０５％以下、Ａｌは０．１％以下に限定した。なお、Ａｌ、Ｔｉのいずれも含有しない場合には、耐焼付き性、耐熱衝撃性が著しく劣化すると共に、ＭＣ型炭化物を粗大化し、顕著な肌荒れ性が発生する。なお、Ｔｉは０．０１〜０．０４％とすることが好ましい。また、Ａｌは０．０１〜０．０７％とすることが好ましい。
【００３５】
つぎに、本発明の熱間圧延用複合ロールの外層（外層材）の組織限定理由について説明する。
本発明の熱間圧延用複合ロールの外層（外層材）では、上記した組成範囲内としたうえで、さらに円相当径が２０μｍ以上の黒鉛が１０個／ｍｍ^２以上で、かつその円相当直径が２０μｍ以上の黒鉛についての黒鉛球状化率が１５〜７５％となる組織を有する。
【００３６】
円相当直径が２０μｍ以上の黒鉛粒数：１０個／ｍｍ^２以上
円相当直径が２０μｍ以上の黒鉛は、絞り事故などで発生する熱応力や熱歪を分散・吸収する作用があり、また、熱衝撃亀裂の発生・成長を抑制する作用を有する。円相当直径が２０μｍ未満の微細な黒鉛ではこのような作用は小さい。なお、円相当直径が２０μｍ未満の微細黒鉛は、過度に存在すると耐摩耗性や耐肌荒れ性を劣化させる原因になるため、５００個／ｍｍ^２以下とすることが好ましい。より好ましくは２００個／ｍｍ^２以下である。
【００３７】
また、円相当直径が２０μｍ以上の黒鉛粒数が１０個／ｍｍ^２未満では、熱応力や熱歪の分散効果が不十分となり、さらに、熱衝撃亀裂が黒鉛に突き当たる確立が減少して亀裂を分岐・分散させる効果が減少する。このため、本発明では円相当直径が２０μｍ以上の黒鉛粒数を１０個／ｍｍ^２以上に限定した。なお、好ましくは、耐熱衝撃性と耐摩耗性を両立させる観点から円相当直径が２０μｍ以上の黒鉛粒数１２個／ｍｍ^２以上３５個／ｍｍ^２以下である。
【００３８】
円相当直径が２０μｍ以上の黒鉛についての黒鉛球状化率：１５〜７５％
本発明における黒鉛球状化率は、下記に定義される黒鉛球状化率を用いるものとする。
黒鉛球状化率（％）＝｛（ｎ_Ｖ＋ｎ_ＶＩ）／ｎ｝×１００
ここで、ｎ：円相当直径２０μｍ以上の黒鉛粒の総粒数（個）、
ｎ_Ｖ：ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖの黒鉛粒数（個）、
ｎ_ＶＩ：ＪＩＳＧ５５０２−１９９５に規定される形状分類ＶＩの黒鉛粒数（個）
なお、ＪＩＳＧ５５０２−１９９５に規定される形状分類Ｖ、形状分類ＶＩの黒鉛粒は、各黒鉛粒の面積比｛（黒鉛粒面積）／（黒鉛粒の最大長さを直径とする円の面積）｝を用いて、分類し、形状分類ＶおよびＶＩに分類される黒鉛粒は、面積比が０．５４以上の黒鉛粒とする（井川克也：素形材、ｖｏｌ．３７、Ｎｏ．９（１９９６．９）ｐ．５、表６参照）。
【００３９】
上記した黒鉛球状化率を１５〜７５％の範囲内、すなわち黒鉛を、完全な球状ではなく、ズングリとした面長形状および／または分岐した形状とすることにより、熱衝撃時に熱応力や熱歪が作用した際に、黒鉛が少しづつ変形し熱応力や熱歪を吸収・分散したり、あるいは黒鉛形状に沿って熱亀裂を分岐・分散させて、耐熱衝撃性を向上させる。図１に示すように、黒鉛球状化率が１５％未満では、細長く伸びた片状黒鉛が増大し、その片状黒鉛に沿って亀裂が容易に伝播・成長し、熱衝撃亀裂最大深さが深くなるため、耐熱衝撃性が低下する。さらに、片状黒鉛に沿って摩擦面の欠落が発生しやすくなり、耐摩耗性および耐肌荒れ性が低下する。また、黒鉛球状化率が７５％超えでは、球状に近い黒鉛が増えて、熱亀裂を分岐・分散させる効果が減少し、図１に示すように熱衝撃亀裂最大深さが深くなり、耐熱衝撃性が低下する。また、黒鉛が多すぎても摩擦面の有効面積が減少するため、耐摩耗性、耐肌荒れ性が劣化する。
【００４０】
本発明の最大の特徴は、優れた耐摩耗性と耐熱衝撃性を両立するために、黒鉛形態を特定した組織とする点にある。黒鉛組織を上記したように特定することにより、優れた耐摩耗性、耐肌荒れ性、耐焼付き性および耐熱衝撃性を兼備した熱間圧延作業用ロール外層（外層材）となる。
つぎに、本発明の熱間圧延用複合ロールの好ましい製造方法について説明する。
【００４１】
本発明では、ロール外層材の製造方法は特に限定されないが、上記した組成の溶湯を、遠心鋳造法で所定の寸法形状のロール外層材とすることが製造コストの観点から好ましい。なお、一般的に合金鋳鉄系材料中に生成する黒鉛は、母溶鉄に含まれる不可避的不純物（例えば、０．０１％未満のＣｕ、Ｐ、Ｓ、Ｚｎ、Ｐｂ、Ａｓ、Ｓｎ、Ｓｂ、Ｃｅ、Ｒａ、Ｗ等や５００ｐｐｍ以下のＮ、あるいは２００ｐｐｍ以下のＯ）の量や、溶解温度、溶解時間などが影響して、量や形状が変化するため、成分限定のみでは好適黒鉛形状を得ることは困難であり、また、好適黒鉛形状を得るための統一的手法を開示することも困難である。
【００４２】
しかし、本発明成分範囲であれば、Ｃａ−Ｓｉ系接種剤やＦｅ−Ｓｉ系接種剤の添加を適切に行うことによって、本発明の好適黒鉛形状を持つ黒鉛を出現させることが可能となる。ここで、接種中のＳｉとＣａは黒鉛を増量する作用を持ち、とくにＣａは黒鉛量の増加と黒鉛の先端に丸みを持たせる効果が大きく、Ｓｉは片状型の黒鉛を出現させる作用を有する。
【００４３】
本発明の成分範囲において、黒鉛が少ない、あるいは球状化率が高い場合、Ｃａ−Ｓｉ系接種剤かＦｅ−Ｓｉ系接種剤の添加量を増加することが推奨され、逆に黒鉛球状化率が低すぎるか黒鉛が多すぎる場合は接種剤添加量を減ずることが推奨される。なお、接種剤の添加量はＳｉ量で０．１〜０．６質量％とすることが好ましい。
なお、市販のＦｅ−Ｓｉ系接種剤はＣａを０〜３％程度含有するものが多く、Ｃａ−Ｓｉ系接種剤はＣａを２０〜４０％含有するものが多い。また、これらの接種剤にはＡｌ、Ｂａ、Ｓｒ等を含んだものも存在するが、本発明における好適な黒鉛組織の形成を阻害するものではなく、接種材の種類を限定する必要はない。
【００４４】
遠心鋳造法で所定の寸法形状のロール外層材を鋳造する場合、ロール外層材の凝固途中あるいは完全凝固後に、鋳型の回転を停止し内層材を静置鋳造して、複合ロールとすることが好ましい。これによりロール外層材の内面側が再溶解され外層と内層が溶着一体化した、複合ロールとなる。
静置鋳造される内層は、鋳造性と機械的性質に優れた球状黒鉛鋳鉄やいも虫状黒鉛鋳鉄あるいは黒鉛鋼などを用いるのが好ましい。また、外層と内層の間に、黒鉛鋼や高炭素鋼からなる中間層を設けてもよい。遠心鋳造法でロールを製造する場合、中間層は、外層の遠心鋳造に引きつづいて、遠心鋳造すれば良い。
【００４５】
なお、以上の説明は、主として、鋼板の熱間圧延用ロールを対象に説明してきたが、本発明は、鋼板の熱間圧延用複合ロールに限定されることはなく、カリバー付き鋼管圧延用複合ロールに適用しても何ら問題ないことはいうまでもない。なお、鋼管圧延用のスリーブ式ロールを製造する場合は、外層を遠心鋳造後、球状黒鉛鋳鉄や、高炭素鋼を内層材として、遠心鋳造すれば良い。
【００４６】
【実施例】
（実施例１）
表１に示す組成の溶湯を溶解し、表１に示す接種剤でＳｉを０．１〜０．４質量％接種した後、遠心鋳造法によりリングロール（外径：２５０ｍｍ φ、肉厚：８０ｍｍ）を鋳造した。なお、鋳込み温度は１４５０℃、遠心力は、重力倍数で１６０Ｇとした。鋳造後、８５０〜９００ ℃から焼入れ４００ ℃〜５００ ℃で焼戻す熱処理を施した。焼戻し後の硬さは、Ｈｓ７７〜８４であった。なお、Ｎｉグレン鋳鉄を従来例（Ｎｏ．ＮｉＧ）とした。
【００４７】
得られたこれらリングロールから試験片を採取し、黒鉛組織の測定、摩耗試験、焼付き試験、熱衝撃試験を行った。黒鉛組織の測定は、前記した方法と同様とした。他の試験方法はつぎのとおりとした。
（１）摩耗試験
リングロールから試験片（大きさ：６０ｍｍφ）を採取し、相手材（材質：Ｓ４５Ｃ、大きさ：１９０ｍｍ φ）と試験片の２円盤すべり摩耗方式の摩耗試験機を用いて摩耗試験を実施した。回転数７００ｒｐｍで試験片を回転させ、荷重：１００ｋｇｆ（９８０Ｎ）、すべり率：１０％として相手材と１５０ｍｉｎ間転動させた。なお、相手材は高周波加熱コイルにより７８０ ℃に加熱し、試験片は水冷した。試験後、試験片の摩耗減量（摩耗量）を測定した。さらに試験片表面を目視で観察し、凹凸状の肌荒れが目立つ場合（おおよそ２０μｍＲｚ _ＪＩＳ以上（Ｒｚ _ＪＩＳ：十点平均粗さ）、２．５ μｍＲａ以上（Ｒａ：中心線平均粗さ）に相当）を、肌荒れ有り（×）とし、それ以外を肌荒れなし（○）として、耐肌荒れ性を評価した。なお、Ｒｚ _ＪＩＳ、ＲａはＪＩＳＢ０６０１−２００１に準拠した値である。
（２）焼付き試験
リングロールから試験片（２５ｍｍ厚板状）を採取し、図５に示す方式の試験機で焼付き試験を実施した。高周波加熱コイルにより、９００ ℃に加熱されて１５０ｒｐｍで回転する円板状の相手材（材料：ＳＵＳ４１０、大きさ：１９０ｍｍ φ）を荷重１００ｋｇｆ（９８０Ｎ）で１０ｓ間、試験片（２５ｍｍ厚板状）に圧接した。試験後の試験片表面に相手材（金属）のへばり付きがある場合を「焼付き有り」（×）、へばり付きがなく表面が摩耗している場合を「焼付きなし」（○）として、耐焼付き性を評価した。
（３）熱衝撃試験
リングロールから試験片（２５ｍｍ厚の板状）を採取し、円板状の相手材（材質：Ｓ４５Ｃ）、大きさ：１９０ｍｍ φ）を用いて、図５に示す構成の試験機で実施した。
【００４８】
高周波加熱コイルにより８２０ ℃に加熱され、１５０ｒｐｍで回転する円板状の相手材を荷重１００ｋｇｆ（９８０Ｎ）で１５ｓ間、試験片（２５ｍｍ厚板状）に圧接して、試験片を急加熱し、除荷と同時に水冷して試験片に熱衝撃を印加した。試験後、浸透探傷試験を実施し、現象液のしみが大きい２ヶ所で切断し、断面を観察して亀裂の最大深さ（ｍｍ）を測定した。従来例（亀裂最大深さ：２．４ｍｍ）以下を耐熱衝撃性良好として、耐熱衝撃性を評価した。
【００４９】
得られた結果を表２に示す。
【００５０】
【表１】

【００５１】
【表２】

【００５２】
本発明例（リングロールＮｏ．Ｂ〜Ｊ）はいずれも、従来例（リングロールＮｏ．ＮｉＧ）の１．５倍を超える（摩耗量が２／３未満）優れた耐摩耗性を有するとともに、肌荒れや焼付きの発生がなく、かつ、熱衝撃亀裂の深さが従来例（リングロールＮｏ．ＮｉＧ）より小さく、耐熱衝撃性にも優れたものとなっている。
一方、本発明の範囲から外れる比較例は、耐摩耗性、耐肌荒れ性、耐焼付き性および耐熱衝撃性のうち、いずれか一つ以上の特性が著しく劣化している。
【００５３】
Ｓｉ量が本発明範囲を低く外れ、黒鉛粒数が少なく、黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ａ）は、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。また、黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｋ）は、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。Ｓｉ量が本発明範囲を低く外れ、黒鉛粒数が極めて少なく、黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｌ）は、焼付きが生じ耐焼付き性が低下するとともに、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。
【００５４】
また、Ｎｂ量が本発明範囲を高く外れた比較例（Ｎｏ．Ｍ）は、肌荒れ性が生じ耐肌荒れ性が低下するとともに、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。Ｎｂが無添加で、Ａｌ、Ｔｉのいずれも含有されないことから黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｎ）は肌荒れが生じ耐肌荒れ性が低下するとともに、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。ＭｏとＶ量が本発明範囲を高く外れ、Ｎｂが無添加の比較例（Ｎｏ．Ｏ）は、肌荒れと焼付きが生じ耐肌荒れ性および耐焼付き性が低下するとともに、熱衝撃亀裂深さが深く、耐熱衝撃性が劣化している。
【００５５】
また、Ｃ量が本発明範囲を低く外れ、黒鉛粒数が少なく、さらに黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｐ）は、焼付きが生ずるとともに深い亀裂が発生し、耐焼付き性、耐熱衝撃性が劣化している。Ｗを含有し、ＢおよびＡｌ／Ｔｉのいずれも含有せず、黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｑ）は極めて深く粗大な亀裂が発生し、耐熱衝撃性が顕著に低下している。Ｗを含有し、Ｖ量が本発明範囲を低く外れ、Ｎｂ量とＴｉ量が本発明範囲を高く外れ、さらにＢを含有しない比較例（Ｎｏ．Ｒ）は、皺状の著しい肌荒れが発生し、深い亀裂が発生して耐肌荒れ性、耐熱衝撃性が低下している。また、耐摩耗性もほとんど向上していない。また、Ｓｉ量、Ｍｏ量、Ｖ量が本発明範囲を低く外れ、Ｔｉ量が本発明範囲を高く外れて、黒鉛球状化率が本発明範囲を高く外れた比較例（Ｎｏ．Ｓ）は、耐摩耗性の向上が不十分であり、かつ、皺状の肌荒れも発生し耐肌荒れ性が低下している。
【００５６】
このように、本発明例は、耐摩耗性、耐肌荒れ性、耐焼付き性に優れ、さらに従来例（リングロールＮｏ．ＮｉＧ）に勝る極めて優れた耐熱衝撃性、とを兼備したロール外層材となっている。
（実施例２）
表３に示す組成の外層材用溶湯を低周波炉で溶解し、該溶湯を取鍋に注湯した際にＦｅ−ＳｉでＳｉを０．２％接種し、さらに該取鍋から遠心鋳造機に鋳造する際に、さらにＣａ−ＳｉをＳｉで０．２％接種し、ロール外層の厚みが１２０ｍｍになるように遠心鋳造した。なお、遠心力は、重力倍数で１６０Ｇとした。外層が凝固した後に鋳型の回転を停止、鋳型を立てて内層材（球状黒鉛鋳鉄）を鋳造して、外層と内層を金属結合した複合ロールとした。ついで、この複合ロールに、８８０ ℃に加熱保持した後、空冷し、その後、４００〜５００ ℃で焼戻す熱処理を施し、外層の硬さを８０〜８２Ｈｓとした。
【００５７】
熱処理後、ロール外層の胴端部から化学成分分析用試験片と組織観察用試験片を採取した。なお、内層についてはロール軸端の中心部から化学成分分析用試験片を採取した。化学成分分析用試験片と組織観察用試験片を用いて、ロール外層および内層の化学成分、および外層の黒鉛組織を定量した。なお、黒鉛組織の定量方法は前記した方法と同じとした。
【００５８】
得られた複合ロールを、熱間仕上圧延ミルのＦ７スタンドの作業ロールとして投入し、炭素鋼１００本の試験圧延を実施した。なお、そのうちの２本で絞り圧延を再現し、焼付き状況と絞りスラックの深さを確認した。
得られた結果を表３に示す。
【００５９】
【表３】

【００６０】
本発明例（複合ロールＮｏ．１）は、従来ロールであるＮｉＧ鋳鉄ロールの１．７倍の極めて良好な耐摩耗性を示し、肌荒れを発生しないことが確認された。また、絞り圧延において焼付きの発生もなく、絞りスラックの深さは、いずれも０．４ｍｍ以下で、極めて優れた耐事故性を有することが確認された。
【００６１】
【発明の効果】
本発明によれば、鋼板の熱間圧延の後段圧延スタンド用ロールとして、優れた耐摩耗性、耐焼付き性、耐肌荒れ性および耐熱衝撃性を兼備した複合ロールが安価に製造でき、産業上格段の効果を奏する。また、本発明の複合ロールは、鋼板の熱間圧延において絞り事故の多発する、後段圧延スタンド用ロールとして安定して使用できる。また、本発明の複合ロールは、優れた耐焼付き性、耐肌荒れ性および耐摩耗性が要求される鋼管用圧延ロールとしても適用できる。
【図面の簡単な説明】
【図１】熱衝撃亀裂最大深さと黒鉛球状化率との関係を示すグラフである。
【図２】本発明範囲の黒鉛形状（黒鉛球状化率：２７．５％）、黒鉛粒数（２０μｍ以上の黒鉛粒数：２４．５個／ｍｍ^２）を有する組織の一例を示すエッチングなしの光学顕微鏡組織写真である。
【図３】本発明範囲を外れる黒鉛形状（黒鉛球状化率：８１．７％）、黒鉛粒数（２０μｍ以上の黒鉛粒数：８．６個／ｍｍ^２）を有する組織の一例を示すエッチングなしの光学顕微鏡組織写真である。
【図４】本発明範囲を外れる黒鉛形状（黒鉛球状化率：５３．０％）、黒鉛粒数（２０μｍ以上の黒鉛粒数：６．６個／ｍｍ^２）を有する組織の一例を示すエッチングなしの光学顕微鏡組織写真である。
【図５】焼付き試験、熱衝撃試験で使用した試験機の概要を模式的に示す説明図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a composite roll for hot rolling, and in particular, is excellent in wear resistance, seizure resistance, surface roughening resistance and thermal shock resistance, and is suitable for a hot rolling finish mill or a seamless steel pipe forming. The present invention relates to a composite roll for rolling.
[0002]
[Prior art]
In recent years, the hot rolling technology of steel sheets and steel pipes has been remarkably advanced, and accordingly, there has been a strong demand for improving the properties of the hot rolling rolls used, particularly the wear resistance. In response to such demands for improved wear resistance, a high-performance roll (hereinafter referred to as HSS) having an outer layer composition similar to that of a high-speed tool steel and precipitating hard carbide to significantly improve wear resistance. System roll) has been developed and put into practical use.
[0003]
On the other hand, in rolling stands and mills in which the material to be rolled is subjected to drawing rolling accidents and seizure of the material to be rolled, hot rolling is performed by incorporating a Ni-grain cast iron roll containing graphite and having excellent seizure resistance. Have been. The inclusion of graphite in the hot-rolling roll is effective in preventing seizure because graphite is rich in lubricity. However, the conventional Ni-grain cast iron roll has a problem in that the wear resistance is inferior and the roll life is short. On the other hand, a high-speed steel roll excellent in wear resistance has a problem in terms of accident resistance, such as a coarse thermal shock crack being generated due to a drawing accident or seizure. For example, a high-speed steel roll cannot be stably used in a hot-rolling finishing post-stand of a steel sheet having a high frequency of reduction rolling accidents, and a large amount of Ni-grain cast iron rolls are still used.
[0004]
To solve such a problem, for example, Patent Document 1 proposes a roll for hot rolling in which 1.0 to 5.0% of V is added to Ni grain cast iron to improve wear resistance. I have. In addition, Patent Document 2 discloses that 2.0 to 8.0% of V is added to Ni grain cast iron, and 0.2 to 10% of MC type carbide appears in addition to 0.5 to 5% of graphite. Rolls for hot rolling have been proposed which improve wear resistance. Further, Patent Literature 3 includes 1.5 to 10% in total of 2 to 10% of Cr, 0.1 to 10% of W, and one or two of V and Nb, and contains graphite. A high-speed cast iron material having the same has been proposed.
[0005]
In addition, in the case of steel pipe rolling as well as in the case of steel plate rolling, there is a strong demand for improvements in seizure resistance and wear resistance to hot rolling rolls.
In recent years, in order to improve the quality and efficient production of rolled products, an increase in a rolling speed and an increase in a continuous rolling amount have been aimed at, and the use environment of hot rolling rolls has become increasingly severe. In addition, due to the high alloying of the material to be rolled and the strict requirements for the surface quality of the rolled product, there is a demand for a hot rolling work roll which is excellent in abrasion resistance and seizure resistance and also excellent in surface roughening resistance.
[0006]
For example, Patent Document 4 proposes a hot-rolling roll in which the contents of Nb and W are controlled to suppress gravitational segregation of carbides and to suppress occurrence of surface roughness. Patent Document 5 discloses a wear-resistant hot-rolling roll in which the total content of Cr, V, and Nb is limited to 3% or less, graphite of 2% or more is crystallized, and occurrence of surface roughness is suppressed. Proposed. In addition, these wear-resistant hot-rolling rolls crystallize spheroidized graphite particles from the viewpoint of improving wear resistance and skin roughness.
[0007]
[Patent Document 1]
JP-A-1-287248
[Patent Document 2]
JP-A-6-335712
[Patent Document 3]
JP-A-6-256889
[Patent Document 4]
Japanese Patent Application No. 2001-20335
[Patent Document 5]
Japanese Patent Application No. 2001-181780
[0008]
[Problems to be solved by the invention]
However, the rolls for hot rolling manufactured by the techniques described in Patent Literatures 4 and 5 have low thermal shock resistance, and have the required thermal resistance as well as the required wear resistance, seizure resistance, and rough surface resistance. It was impossible to satisfy at the same time.
The present invention advantageously solves the above-mentioned problems of the prior art, and is excellent in both abrasion resistance, seizure resistance, surface roughening resistance, and thermal shock resistance, and is an outer layer material for hot rolling and a composite for hot rolling. The purpose is to propose a role.
[0009]
[Means for Solving the Problems]
The present inventors have intensively studied various factors affecting the abrasion resistance, seizure resistance, surface roughness resistance, and thermal shock resistance of a hot rolling roll in order to achieve the above object.
It is well known that wear resistance is improved by adding a large amount of V, Nb, Mo, Cr and W. However, according to the study of the present inventors, when Cr or W is increased for the purpose of improving the wear resistance, the surface roughness becomes more remarkable. If Cr and W are increased simultaneously with V (Nb), seizure resistance and thermal shock resistance deteriorate. This is probably because the eutectic carbide formed of Cr or W causes roughening or seizure or promotes the propagation of thermal shock cracks.
[0010]
As described above, although Cr and W have an effect of improving abrasion resistance, they have a tendency to promote roughening, seizure, and thermal shock cracking. It has been found that the content of Cr should be kept to the minimum necessary, and that W should not be added. It is to be noted that the use of Cr in the minimum necessary content and no addition of W is effective for promoting graphitization, and contributes to the improvement of the surface roughness resistance and the seizure resistance.
[0011]
According to the study of the present inventors, V must be contained in an amount of 1.3% by mass or more in order to secure the minimum required wear resistance as a hot rolling roll. However, when V is contained in an amount of 1.3% by mass or more, coarse MC carbides are crystallized. The coarse MC carbide remains on the roll surface in a convex shape in hot rolling, and induces roughening of the surface. The present inventors have further studied a method for suppressing crystallization of coarse MC carbides and have found that it is preferable to add Nb simultaneously with V. Nb forms Nb carbides, and MC carbides are crystallized from the nuclei. Therefore, the coarsening of the MC carbide is suppressed to a certain level or less.
[0012]
Further, when Al or Ti is added in addition to V and Nb, an extremely fine compound of Al or Ti (oxide, nitride, carbide, etc.) is first generated and finely dispersed in the matrix. The fine compound of Al or Ti, as a nucleus for generating NbC, significantly promotes fine dispersion of NbC. Since MC carbide grows with this finely dispersed NbC as a nucleus, coarsening can be remarkably prevented, and rough surface caused by coarse MC carbide can be prevented. However, even when Nb and Al and / or Ti are contained, if V is contained in an amount exceeding 2.5% by mass, MC carbides are excessively increased and skin roughness is likely to occur.
[0013]
As described above, by containing an appropriate amount of Al and / or Ti in addition to V and Nb, it is possible to simultaneously improve the wear resistance, the surface roughening resistance, and the seizure resistance of the grain cast iron-based rolling roll. However, within this range, if, for example, graphite is reduced or the graphite shape is spheroidized to further improve wear resistance, a thermal shock crack tends to become coarse when a drawing accident is encountered, resulting in a depth of 4 mm. In some cases, a coarse drawn crack exceeding that of the above may be formed, and there is a problem that the roll life and the rolling unit consumption are deteriorated.
[0014]
In order to improve the thermal shock resistance of the hot-rolling roll, the present inventors have intensively studied various factors affecting the thermal shock resistance of the high-wear-resistant Glen cast iron-based roll against such a problem. As a result, the present inventors disperse the thermal stress (thermal strain) generated during the drawing accident by controlling the form of graphite dispersed in the base of the gray cast iron, branch the thermal shock cracks, I realized that it is possible to reduce the depth of the impact crack.
[0015]
First, the results of basic experiments performed by the present inventors will be described.
2.9 to 3.1% C-1.2 to 2.8% Si-0.5 to 0.7% Mn-4.4 to 4.5% Ni by mass% mainly changing the amount of Si. -1.9 to 2.1% Cr-1.9 to 2.0% V-2.0 to 2.2% Mo-0.3 to 0.4% Nb-0.03 to 0.04% Ti A melt having a composition of -0.05 to 0.06% B and the balance of Fe was melted, and 0.1 to 0.5% of Si was inoculated with a commercially available inoculant (Ca-Si, Fe-Si system). Thereafter, it was cast into a 35 mm thick Y-type keel block. The obtained keel block was subjected to a tempering treatment of tempering at a temperature of 400 to 500 ° C. to obtain a test material.
[0016]
These test materials were subjected to a thermal shock test to evaluate thermal shock resistance. The graphite structure (size, shape and number of graphite particles) of these test materials was measured, and the graphite spheroidization ratio was calculated in accordance with JIS G5502-1995. The measuring method of the graphite structure and the thermal shock test method were as follows.
(1) Measurement of graphite structure
After the surface of the test material was mirror-polished, the size, shape and number of the graphite particles were quantified using an image analyzer, without optical microscope observation with no etching. The magnification of the analysis image of the image analyzer was 200 times (microscope magnification: 50 times). From the measurement results, a graphite spheroidization ratio defined by the following equation was calculated based on JIS G5502-1995. Note that JIS G5502-1995 covers graphite particles having a length of 20 μm or more, but here, graphite particles having a circle equivalent diameter of 20 μm or more determined from the area of the graphite particles are used.
[0017]
Graphite spheroidization rate (%) = ｛(n _V + N _VI ) / N｝ × 100
(Where, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces), n _V : Number of graphite particles (pieces) of shape classification V specified in JIS G 5502-1995, n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pieces)
The graphite particles of the shape class V and the shape class VI defined in JIS G 5502-1995 have an area ratio of each graphite particle ｛(graphite particle area) / (area of a circle whose diameter is the maximum length of the graphite particle). ) Classified using｝. Graphite particles classified into the shape classification V and the shape classification VI were graphite particles having an area ratio of 0.54 or more (Katsuya Igawa: Basic material, vol. 37, No. 9 (1996. 9) p.5) , Table 6). When the graphite particles have a perfect circle, the area ratio is 1.0.
(2) Thermal shock test
A 25 mm-thick plate-shaped test piece was sampled from the test material, and a disk-shaped counterpart material (material: S45C, size: 190 mmφ) was used to carry out the test using the test machine having the configuration shown in FIG.
[0018]
A disk-shaped counterpart (material: S45C, size: 190 mmφ) heated to 800 ° C by a high-frequency heating coil and rotated at 100 rpm is pressed against a 25-mm-thick plate-shaped test specimen under a load of 100 kgf (980 N) for 10 s. Then, a thermal shock was applied to the plate-shaped test piece by rapidly heating it and cooling it with water simultaneously with unloading. After the test, the specimen was cut at two positions where a large crack having an opening was present, and the maximum depth of the crack was measured in the cross section.
[0019]
FIG. 1 shows the obtained results in relation to the maximum crack depth and the spheroidization rate of graphite.
From FIG. 1, it can be seen that when the graphite spheroidization ratio is adjusted to 15 to 75% by the combination of the Si content in the molten metal and the type and amount of the inoculant, the maximum crack depth upon thermal shock becomes shallower. However, the number of graphite particles having a circle equivalent diameter of 20 μm or more is 10 / mm. ² It was found that when the ratio is less than 15, the maximum crack depth at the time of thermal shock does not become shallow even when the graphite spheroidization ratio is 15 to 75%. From this, in order to reduce the maximum crack depth and improve the thermal shock resistance, the number of graphite particles having a circle equivalent diameter of 20 μm or more should be a predetermined number (10 / mm). ² ) It is understood that it is important to adjust the graphite spheroidization ratio within an appropriate range (15 to 75%) while keeping the above-mentioned content.
[0020]
In conventional hot-rolled rolls, it has been general to use spheroidal graphite instead of graphite in order to improve strength / ductility and abrasion resistance. However, in order to achieve both excellent abrasion resistance and thermal shock resistance, the present inventors need to adjust the graphite spheroidization ratio within an appropriate range, instead of using perfect spheroidal graphite. Was found.
Although the details of the mechanism for improving the thermal shock resistance described above have not been elucidated yet, the present inventors consider as follows.
[0021]
Since the graphite dispersed in the gray cast iron has a significantly lower strength than the matrix (base material), it can be mechanically regarded as pores in the matrix (base material). If there are many holes (graphite) in the matrix (base material), many holes will be distorted little by little even if thermal strain occurs in the surface layer of the roll, the thermal strain will be dispersed, and thermal shock cracks will occur. Stress concentration for formation and growth is avoided. It is considered that if stress concentration is avoided, the formation and growth of cracks is reduced and the maximum crack depth is reduced. When the tip of a thermal shock crack strikes a hole (graphite), the crack propagates along the hole. When the graphite has a branched shape, the cracks are branched and dispersed along the branched shape, so that the maximum crack depth is considered to be shallow.
[0022]
For example, when the graphite spheroidization ratio is in the range of 15 to 75%, the graphite has an appropriately branched shape (see FIG. 2), so that the cracks branch and disperse properly, but the graphite spheroidization ratio is 15%. If it is less than 1, the shape of graphite approaches I in the shape classification specified in JIS G 5502-1995, and the number of elongated and elongated graphite with few branches increases. In the graphite having such a shape, it is considered that the thermal shock crack grows along the elongated graphite, and the maximum crack depth increases. On the other hand, when the graphite spheroidization ratio exceeds 75% (see FIG. 3), most of the graphite becomes a sphere-like graphite with a small number of branches, which makes it difficult for thermal stress dispersion and crack branching to occur. Is thought to be deeper. Note that graphite particles having a circle-equivalent diameter of less than 20 μm are not counted as the number of graphite particles in the present invention because they are too fine and have a small effect of suppressing thermal shock cracking.
[0023]
The presence of graphite particles having a circle equivalent diameter of 20 μm or more is more than a predetermined number (see FIG. 2), and the above-mentioned effects of thermal stress and crack branching / dispersion become remarkable. It is considered that the grain spacing becomes too wide, and the probability of collision between the thermal shock crack and the graphite grain decreases, so that the above-mentioned dispersion of thermal stress and crack branching cannot be expected, and the maximum crack depth is considered to be deep.
[0024]
The present inventors conducted further studies, containing V and B, and further containing Al and / or Ti, and then inoculating a molten metal of a Glen cast iron composition having an appropriate Si content and casting, It has been found that a graphite structure in which the above-described appropriately branched graphite particles are dispersed in a matrix (base material) can be obtained.
The present invention has been completed based on the above findings, with further investigations. That is, the gist of the present invention is as follows.
(1) A roll outer layer material used for an outer layer of a composite roll for hot rolling, in which, by mass%, C: 2.6 to 3.5%, Si: 1.5 to 2.5%, and Mn: 0 0.2-1.5%, Cr: 1.0-2.5%, Mo: 1.0-3.0%, Ni: 2.0-7.0%, V: 1.3-2.5. %, Nb: 0.1-0.8%, B: 0.020-0.2%, and 1 selected from Ti: 0.05% or less and Al: 0.1% or less. Containing two or more species, the balance being Fe and unavoidable impurities, and 10 graphite / mm having a circle equivalent diameter of 20 μm or more. ² Above, graphite spheroidization rate defined below for graphite particles having a circle equivalent diameter of 20 μm or more.
Graphite spheroidization rate (%) = ｛(n _V + N _VI ) / N｝ × 100
(Where, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces), n _V : Number of graphite particles (pieces) of shape classification V specified in JIS G 5502-1995, n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pieces)
The hot roll outer layer material for hot rolling, comprising:
(2) The roll outer layer material for hot rolling according to (1), further comprising Co: 4% or less by mass% in addition to the composition.
(3) A composite roll for hot rolling in which an outer layer and an inner layer are welded and integrated, wherein the outer layer is, by mass%, C: 2.6 to 3.5%, and Si: 1.5 to 2.5%. %, Mn: 0.2 to 1.5%, Cr: 1.0 to 2.5%, Mo: 1.0 to 3.0%, Ni: 2.0 to 7.0%, V: 1. 3 to 2.5%, Nb: 0.1 to 0.8%, B: 0.020 to 0.2%, and among Ti: 0.05% or less, Al: 0.1% or less A composition containing one or two selected elements, the balance being Fe and unavoidable impurities, and the number of graphite particles having a circle equivalent diameter of 20 μm or more being 10 / mm. ² Above, graphite spheroidization rate defined below for graphite particles having a circle equivalent diameter of 20 μm or more.
Graphite spheroidization rate (%) = ｛(n _V + N _VI ) / N｝ × 100
(Where, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces), n _V : Number of graphite particles (pieces) of shape classification V specified in JIS G 5502-1995, n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pieces)
A composite roll for hot rolling, wherein the composite roll has a texture of 15 to 75%.
(4) The composite roll for hot rolling according to (3), wherein the outer layer further contains Co: 4% or less by mass% in addition to the composition.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limiting the composition of the outer layer (outer layer material) of the composite roll for hot rolling of the present invention will be described. In addition, the mass% in the composition is simply described as%.
C: 2.6 to 3.5%
C combines with V, Nb, Cr, and Mo and is an essential element for forming a hard carbide for improving the wear resistance of the roll, and secures crystallization of graphite, seizure resistance, and thermal shock resistance. In the present invention, the content is required to be 2.6% or more. On the other hand, if the content exceeds 3.5%, a large amount of eutectic carbides appear, deteriorating the thermal shock resistance, and at the same time, the MC-type carbides are coarsened and the surface roughening resistance is reduced. For this reason, C was limited to the range of 2.6 to 3.5%. In addition, preferably, it is 2.8 to 3.3%.
[0026]
Si: 1.5 to 2.5%
Si is an element that acts as a deoxidizing agent and increases the activity of C to facilitate crystallization of graphite. In the present invention, the content of Si is required to be 1.5% or more. On the other hand, if the content exceeds 2.5%, coarsening of graphite and coarsening of the structure occurs, and the surface roughening resistance and abrasion resistance of the roll are significantly reduced. It should be noted that when the amount of Si is low, the amount of graphite decreases and the spheroidization rate of graphite increases, while when the amount of Si is high, the amount of graphite increases and the spheroidization rate of graphite tends to decrease. Although this phenomenon is affected by the balance with other alloying elements, it is necessary to adjust it in the range of 1.5 to 2.5% in order to obtain a suitable graphite morphology. For these reasons, Si is limited to the range of 1.5 to 2.5%.
[0027]
Mn: 0.2-1.5%
Mn is effective for fixing S in molten steel as MnS 2 and harmless and stabilizing S, which inhibits wear resistance. In addition, there is also an effect of improving hardenability and increasing hardness. In order to obtain such an effect, the content of 0.2% or more is required. However, if the content exceeds 1.5%, segregation occurs at the solidification interface and the material becomes brittle. For this reason, Mn was contained in the range of 0.2 to 1.5%. Incidentally, the content is preferably 0.3 to 1.0%.
[0028]
Cr: 1.0 to 2.5%
Cr is an element that increases eutectic carbides and hardens them. When Cr is contained together with Mo, it has an effect of suppressing carbide breakage during roll rolling and improving wear resistance. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, if the content exceeds 2.5%, eutectic carbides increase and the surface roughening resistance decreases, and the amount of graphite decreases, and the seizure resistance and thermal shock resistance also decrease. For this reason, Cr was limited to 1.0 to 2.5%. In addition, it is preferably 1.2 to 2.3%.
[0029]
Mo: 1.0 to 3.0%
Mo has the effect of strengthening carbides and matrix without excessively increasing the amount of eutectic carbides, and has the effect of improving wear resistance while maintaining excellent surface roughness resistance. In particular, by containing Nb in combination, the hard MC type carbide is strengthened and has an important effect of remarkably improving the abrasion resistance. % Or more is required. On the other hand, when the content exceeds 3.0%, a large amount of hard and brittle carbide mainly composed of Mo is formed, and the surface roughness resistance and the thermal shock resistance are deteriorated. For this reason, Mo was limited to 1.0 to 3.0%. Incidentally, the content is preferably 1.2 to 2.5%.
[0030]
Ni: 2.0 to 7.0%
Ni has the effect of improving the hardenability, increasing the hardness of the material, and improving the wear resistance. Ni also has the effect of promoting crystallization of graphite. Such an effect is observed at a content of 2.0% or more. However, when the content exceeds 7.0%, austenite is remarkably stabilized, the amount of retained austenite increases, and the seizure resistance decreases. For this reason, Ni is limited to the range of 2.0 to 7.0%. In addition, it is preferably 3.0 to 6.0%.
[0031]
V: 1.3 to 2.5%
V is an element that forms a hard MC-type carbide and has an effect of improving wear resistance. In the present invention, in order to obtain a certain level or more of abrasion resistance, the content of 1.3% or more is required. On the other hand, if it exceeds 2.5%, MC type carbides become coarse and rough skin occurs. Further, there are also adverse effects such as the occurrence of seizure and deterioration of thermal shock resistance. For this reason, V was limited to the range of 1.3 to 2.5%.
[0032]
Nb: 0.1 to 0.8%
Nb is an element having a function of strengthening the carbide by forming a solid solution in the MC-type carbide. In particular, when Nb is contained in combination with a predetermined range of Cr, Mo, and V, the carbide is significantly strengthened and the wear resistance is improved. Has an important effect of improving significantly. In addition, when only V is contained, the MC-type carbide grows in a feather shape, the structure becomes coarse, and rough surface of the roll is induced. By containing Nb and V in combination, it is possible to suppress the formation of feathers in the MC-type carbide. Further, by adding in combination with Ti and / or Al, oxides and nitrides of Ti and / or Al In addition, carbides and the like serve as crystallization nuclei of NbC 2 to finely disperse NbC 2, and subsequently, MC-type carbides are finely dispersed with NbC 2 as nuclei, thereby providing an important effect of suppressing roughening of the surface. In order to obtain such effects, Nb needs to be contained at 0.1% or more. If Nb is less than 0.1%, segregation and coarsening of MC-type carbides occur, and it is not possible to prevent rough skin. On the other hand, when Nb is contained in excess of 0.8%, the MC-type carbide coarsens in a dendrite shape, so that the surface roughening occurs. Further, when manufactured by a centrifugal casting method, segregation of MC type carbide occurs. For these reasons, Nb is limited to the range of 0.1 to 0.8%. In addition, from the viewpoint of MC type carbide coarsening, Nb is preferably set to 0.1 to 0.5%.
[0033]
B: 0.020 to 0.2%
B has an important effect of containing graphite and coexisting with Al and / or Ti to form graphite into a branched shape and increase the number of graphite particles. B also has the effect of improving seizure resistance and thermal shock resistance. Such an effect is recognized at a content of 0.020% or more in the coexistence of Al and / or Ti. However, when the content exceeds 0.2%, carbides become brittle, wear resistance is reduced, and graphite is reduced. The amount also decreases. For this reason, B was limited to the range of 0.020 to 0.2%. In addition, it is preferably 0.02 to 0.10%.
[0034]
One or two kinds selected from Ti: 0.05% or less and Al: 0.1% or less
Ti and / or Al are important elements that exhibit the above-described action of B when contained together with B.
Both of Ti and Al promote the graphitization by containing coexisting with B, and promote the formation of the graphite shape into a branched shape and / or a stubby surface length shape, and have seizure resistance, It has an important effect of improving thermal shock resistance. Further, oxides, nitrides, carbides, and the like of Ti and / or Al serve as nuclei of Nb carbide, and also have an effect of finely dispersing MC carbide. Such an effect is remarkably recognized when the content of Ti is 0.005% or more and the content of Al is 0.003% or more. On the other hand, when the content of Ti exceeds 0.05%, fine MC type carbides are densely formed, and large roughening is generated at the portion. Further, when Al is contained in excess of 0.1%, the fluidity of the molten metal is reduced and casting defects are liable to occur. For this reason, Ti is limited to 0.05% or less and Al is limited to 0.1% or less. When neither Al nor Ti is contained, seizure resistance and thermal shock resistance are remarkably deteriorated, and at the same time, MC type carbides are coarsened and remarkable surface roughness occurs. In addition, it is preferable to set Ti to 0.01 to 0.04%. Further, Al is preferably set to 0.01 to 0.07%.
[0035]
Next, the reason for limiting the structure of the outer layer (outer layer material) of the composite roll for hot rolling of the present invention will be described.
In the outer layer (outer layer material) of the composite roll for hot rolling according to the present invention, in addition to the above-mentioned composition range, 10 or more graphites having an equivalent circle diameter of 20 μm or more are further formed. ² It has a structure in which the graphite spheroidization ratio of graphite having a circle equivalent diameter of 20 μm or more is 15 to 75%.
[0036]
Number of graphite particles with an equivalent circle diameter of 20 μm or more: 10 / mm ² that's all
Graphite having an equivalent circle diameter of 20 μm or more has an effect of dispersing and absorbing thermal stress and thermal strain generated in a drawing accident or the like, and an effect of suppressing the generation and growth of thermal shock cracks. Such an effect is small in fine graphite having an equivalent circle diameter of less than 20 μm. Note that fine graphite having an equivalent circle diameter of less than 20 μm, if present excessively, causes deterioration of abrasion resistance and surface roughening resistance. ² It is preferable to set the following. More preferably, 200 pieces / mm ² It is as follows.
[0037]
The number of graphite particles having a circle equivalent diameter of 20 μm or more is 10 / mm. ² If it is less than 3, the effect of dispersing thermal stress and thermal strain becomes insufficient, and the probability of thermal shock cracks hitting graphite decreases, and the effect of branching and dispersing cracks decreases. Therefore, in the present invention, the number of graphite particles having an equivalent circle diameter of 20 μm or more is 10 particles / mm. ² Limited to the above. Preferably, the number of graphite particles having an equivalent circle diameter of 20 μm or more is 12 / mm from the viewpoint of achieving both thermal shock resistance and wear resistance. ² More than 35 pieces / mm ² It is as follows.
[0038]
Graphite spheroidization ratio for graphite having an equivalent circle diameter of 20 μm or more: 15 to 75%
The graphite spheroidization rate in the present invention uses the graphite spheroidization rate defined below.
Graphite spheroidization rate (%) = ｛(n _V + N _VI ) / N｝ × 100
Here, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces),
n _V : Number of graphite grains (pieces) of shape classification V specified in JIS G 5502-1995,
n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pcs)
The graphite particles of the shape class V and the shape class VI defined in JIS G 5502-1995 have an area ratio of each graphite particle ｛(graphite particle area) / (area of a circle whose diameter is the maximum length of the graphite particle). ) The graphite particles classified by using｝ and classified into the shape classifications V and VI are graphite particles having an area ratio of 0.54 or more (Katsuya Igawa: Basic material, vol. 37, No. 9 ( 1996. 9) p.5, see Table 6).
[0039]
The above-mentioned graphite spheroidization ratio is in the range of 15 to 75%, that is, the graphite is not a perfect sphere, but has a long surface shape and / or a branched shape which is not completely spherical. When graphite acts, graphite is gradually deformed to absorb and disperse thermal stress and thermal strain, or to branch and disperse thermal cracks along the graphite shape to improve thermal shock resistance. As shown in FIG. 1, when the graphite spheroidization ratio is less than 15%, elongated flaky graphite increases, cracks easily propagate and grow along the flaky graphite, and the maximum thermal shock crack depth is reduced. Since it becomes deep, the thermal shock resistance decreases. Further, the friction surface is likely to be lost along the flaky graphite, and the wear resistance and the surface roughening resistance are reduced. On the other hand, when the graphite spheroidization ratio is more than 75%, the amount of graphite close to sphere increases, and the effect of branching and dispersing the thermal cracks decreases. As shown in FIG. Is reduced. Further, even if the amount of graphite is too large, the effective area of the friction surface is reduced, so that the wear resistance and the rough surface resistance are deteriorated.
[0040]
The greatest feature of the present invention lies in that the structure of the graphite is specified to achieve both excellent wear resistance and thermal shock resistance. By specifying the graphite structure as described above, a roll outer layer (outer layer material) for hot rolling work having excellent wear resistance, surface roughening resistance, seizure resistance and thermal shock resistance is obtained.
Next, a preferred method for producing the composite roll for hot rolling of the present invention will be described.
[0041]
In the present invention, the method for producing the roll outer layer material is not particularly limited, but it is preferable from the viewpoint of the production cost that the molten metal having the above-described composition be formed into a roll outer layer material having a predetermined shape by centrifugal casting. In general, graphite produced in the alloy cast iron-based material is inevitable impurities (for example, Cu, P, S, Zn, Pb, As, Sn, Sb, Ce less than 0.01%) contained in the mother molten iron. , Ra, W, etc., the amount of N of 500 ppm or less, or the amount of O) of 200 ppm or less, the dissolution temperature, the dissolution time, and the like, change the amount and shape. It is also difficult to disclose a unified method for obtaining a suitable graphite shape.
[0042]
However, within the component range of the present invention, it is possible to make the graphite having a preferred graphite shape of the present invention appear by appropriately adding a Ca-Si based inoculant or an Fe-Si based inoculant. Here, Si and Ca during the inoculation have an effect of increasing the amount of graphite, particularly Ca has a large effect of increasing the amount of graphite and giving a rounded tip of graphite, and Si has an effect of causing flaky graphite to appear. Have.
[0043]
In the component range of the present invention, when the amount of graphite is small or the spheroidization rate is high, it is recommended to increase the amount of Ca-Si based inoculant or Fe-Si based inoculant to be added. If too low or too much graphite, it is recommended to reduce the amount of inoculant added. The inoculant is preferably added in an amount of 0.1 to 0.6% by mass in terms of Si amount.
In addition, many commercially available Fe-Si inoculants contain about 0 to 3% of Ca, and many Ca-Si inoculants contain 20 to 40% of Ca. Although some of these inoculants contain Al, Ba, Sr, etc., they do not inhibit the formation of a suitable graphite structure in the present invention, and there is no need to limit the type of inoculant.
[0044]
When casting a roll outer layer material having a predetermined size and shape by centrifugal casting, it is preferable to stop the rotation of the mold and allow the inner layer material to stand still during the solidification or complete solidification of the roll outer layer material to form a composite roll. . Thereby, the inner surface side of the roll outer layer material is redissolved, and the outer layer and the inner layer are welded and integrated to form a composite roll.
The inner layer to be subjected to stationary casting is preferably made of spheroidal graphite cast iron, worm-like graphite cast iron or graphite steel having excellent castability and mechanical properties. Further, an intermediate layer made of graphite steel or high carbon steel may be provided between the outer layer and the inner layer. When the roll is manufactured by the centrifugal casting method, the intermediate layer may be subjected to the centrifugal casting following the centrifugal casting of the outer layer.
[0045]
Although the above description has been mainly directed to hot rolls for steel sheets, the present invention is not limited to composite rolls for hot rolling steel sheets, It goes without saying that there is no problem if applied to a role. When a sleeve type roll for rolling a steel pipe is manufactured, the outer layer may be subjected to centrifugal casting, followed by centrifugal casting using spheroidal graphite cast iron or high carbon steel as an inner layer material.
[0046]
【Example】
(Example 1)
A melt having the composition shown in Table 1 was dissolved, and 0.1 to 0.4% by mass of Si was inoculated with the inoculant shown in Table 1, and then a ring roll (outer diameter: 250 mm φ, wall thickness: 80 mm) was obtained by centrifugal casting. ). The casting temperature was 1450 ° C., and the centrifugal force was 160 G as a multiple of gravity. After the casting, a heat treatment of tempering from 850 to 900 ° C to 400 ° C to 500 ° C was performed. The hardness after tempering was Hs 77 to 84. In addition, Ni grain cast iron was used as a conventional example (No. NiG).
[0047]
Specimens were collected from the obtained ring rolls and subjected to measurement of graphite structure, abrasion test, seizure test, and thermal shock test. The measurement of the graphite structure was performed in the same manner as described above. Other test methods were as follows.
(1) Wear test
A test piece (size: 60 mmφ) was sampled from the ring roll, and a wear test was performed using a two-disk sliding wear type wear tester of the mating material (material: S45C, size: 190 mmφ) and the test piece. The test piece was rotated at a rotation speed of 700 rpm, and was rolled with the mating material for 150 minutes at a load of 100 kgf (980 N) and a slip ratio of 10%. The mating material was heated to 780 ° C. by a high-frequency heating coil, and the test piece was water-cooled. After the test, the abrasion loss (abrasion amount) of the test piece was measured. Further, the surface of the test piece is visually observed, and when uneven surface roughness is conspicuous (approximately 20 μm Rz _JIS (Rz _JIS : 10-point average roughness), 2.5 μm Ra or more (equivalent to Ra: centerline average roughness) was evaluated as having rough skin (x), and the other was evaluated as having no rough skin (○), and the rough skin resistance was evaluated. did. Note that Rz _JIS , Ra are values based on JIS B 0601-2001.
(2) Seizure test
A test piece (25 mm thick plate) was sampled from the ring roll, and a seizure test was carried out using a tester of the type shown in FIG. A test piece (25 mm thick plate) is heated to 900 ° C. by a high frequency heating coil and rotated at 150 rpm for a disc-shaped counterpart material (material: SUS410, size: 190 mm φ) for 10 s under a load of 100 kgf (980 N) for 10 s. Was pressed against. If the mating material (metal) has burrs on the surface of the test piece after the test, "seizure"(x); if there is no burrs and the surface is worn, "no seizure" (o) The seizure resistance was evaluated.
(3) Thermal shock test
A test piece (plate having a thickness of 25 mm) was sampled from the ring roll, and a disk-shaped counterpart (material: S45C), size: 190 mm φ) was used to carry out the test using a test machine having the configuration shown in FIG.
[0048]
A disk-shaped counterpart material heated to 820 ° C. by a high-frequency heating coil and rotated at 150 rpm is pressed against a test piece (25 mm thick plate) with a load of 100 kgf (980 N) for 15 s to rapidly heat the test piece. The specimen was cooled with water simultaneously with the unloading, and a thermal shock was applied to the test piece. After the test, a penetrant test was carried out, the specimen was cut at two places where the stain of the phenomenon liquid was large, the cross section was observed, and the maximum depth (mm) of the crack was measured. The thermal shock resistance was evaluated as good thermal shock resistance below the conventional example (maximum crack depth: 2.4 mm).
[0049]
Table 2 shows the obtained results.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
All of the present invention examples (ring roll Nos. B to J) have excellent wear resistance exceeding 1.5 times (wear amount is less than 2/3) more than 1.5 times that of the conventional example (ring roll No. NiG). There is no occurrence of skin roughness or seizure, the depth of the thermal shock crack is smaller than that of the conventional example (ring roll No. NiG), and the thermal shock resistance is excellent.
On the other hand, in Comparative Examples outside the scope of the present invention, at least one of the abrasion resistance, rough surface resistance, seizure resistance and thermal shock resistance is significantly deteriorated.
[0053]
In Comparative Example (No. A), in which the amount of Si was outside the range of the present invention, the number of graphite particles was small, and the spheroidization ratio of graphite was outside the range of the present invention, the thermal shock crack depth was deep and the thermal shock resistance was deteriorated. are doing. Further, in the comparative example (No. K) in which the graphite spheroidization ratio was outside the range of the present invention, the thermal shock crack depth was deep and the thermal shock resistance was deteriorated. The comparative example (No. L) in which the amount of Si deviated from the range of the present invention, the number of graphite particles was extremely small, and the degree of graphite spheroidization deviated from the range of the present invention was high. Thermal shock crack depth is deep and thermal shock resistance is deteriorated.
[0054]
In the comparative example (No. M) in which the Nb content was outside the range of the present invention, the surface roughness was reduced and the surface roughening resistance was reduced, and the thermal shock crack depth was deep and the thermal shock resistance was deteriorated. Comparative Example (No. N), in which Nb was not added and neither Al nor Ti was contained and the spheroidization ratio of graphite was out of the range of the present invention, was roughened and the surface roughening resistance was reduced, and the thermal shock cracking was reduced. The depth is deep, and the thermal shock resistance is deteriorated. In the comparative example (No. O) in which the Mo and V contents were outside the range of the present invention and Nb was not added, the surface roughness and seizure occurred, the skin roughness resistance and the seizure resistance were reduced, and the thermal shock crack depth was reduced. Deep, thermal shock resistance has deteriorated.
[0055]
In Comparative Example (No. P), in which the C content was outside the range of the present invention, the number of graphite particles was small, and the spheroidization ratio of graphite was outside the range of the present invention, seizure occurred and deep cracks occurred. , Seizure resistance and thermal shock resistance are deteriorated. The comparative example (No. Q) containing W, containing neither B nor Al / Ti, and having a graphite spheroidization rate outside the range of the present invention was extremely deep and coarse, and the thermal shock resistance was poor. It has been significantly reduced. The comparative example (No. R) containing W, the V amount deviated from the range of the present invention low, the Nb amount and the Ti amount deviated from the range of the present invention high, and the wrinkle-like remarkable surface roughness occurred in the comparative example containing no B. , Deep cracks are generated and the rough surface resistance and thermal shock resistance are reduced. Further, the wear resistance is hardly improved. The comparative example (No. S) in which the Si content, the Mo content, and the V content were outside the range of the present invention, the Ti amount was outside the range of the present invention, and the graphite spheroidization ratio was outside the range of the present invention was high. The abrasion resistance is not sufficiently improved, and wrinkle-like surface roughness also occurs, resulting in a decrease in the surface roughness resistance.
[0056]
As described above, the example of the present invention provides a roll outer layer material having excellent abrasion resistance, rough surface resistance, and seizure resistance, and also having extremely excellent thermal shock resistance superior to the conventional example (ring roll No. NiG). Has become.
(Example 2)
The molten metal for the outer layer material having the composition shown in Table 3 was melted in a low frequency furnace, and when the molten metal was poured into the ladle, 0.2% of Si was inoculated with Fe-Si. During casting, Ca-Si was further inoculated with 0.2% of Si and centrifugally cast so that the outer layer of the roll had a thickness of 120 mm. Note that the centrifugal force was 160 G in terms of a multiple of gravity. After the outer layer was solidified, the rotation of the mold was stopped, the mold was erected, and the inner layer material (spheroidal graphite cast iron) was cast to obtain a composite roll in which the outer layer and the inner layer were metal-bonded. Next, the composite roll was heated and maintained at 880 ° C., air-cooled, and then subjected to a heat treatment of tempering at 400 to 500 ° C. to make the hardness of the outer layer 80 to 82 Hs.
[0057]
After the heat treatment, a test piece for chemical composition analysis and a test piece for structure observation were collected from the body end of the outer layer of the roll. As for the inner layer, a test piece for chemical component analysis was collected from the center of the roll shaft end. Using the test piece for chemical component analysis and the test piece for structure observation, the chemical components of the outer layer and the inner layer of the roll and the graphite structure of the outer layer were quantified. The method for quantifying the graphite structure was the same as the method described above.
[0058]
The obtained composite roll was thrown in as a work roll of an F7 stand of a hot finish rolling mill, and 100 carbon steels were subjected to test rolling. In addition, reduction rolling was reproduced by two of them, and the state of seizure and the depth of reduction slack were confirmed.
Table 3 shows the obtained results.
[0059]
[Table 3]

[0060]
It was confirmed that the present invention example (composite roll No. 1) exhibited extremely good abrasion resistance 1.7 times that of the conventional NiG cast iron roll, and did not cause roughening. Further, there was no occurrence of seizure in the reduction rolling, and the depth of the reduction slack was 0.4 mm or less in each case, and it was confirmed that the material had extremely excellent accident resistance.
[0061]
【The invention's effect】
According to the present invention, a composite roll having both excellent wear resistance, seizure resistance, surface roughening resistance and thermal shock resistance can be manufactured at low cost as a roll for a post-rolling stand for hot rolling of a steel sheet. Has the effect of Further, the composite roll of the present invention can be used stably as a roll for a post-rolling stand, in which frequent drawing accidents occur during hot rolling of a steel sheet. Further, the composite roll of the present invention can also be applied as a rolling roll for steel pipes requiring excellent seizure resistance, surface roughening resistance and wear resistance.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the maximum thermal shock crack depth and the spheroidization rate of graphite.
FIG. 2 Graphite shape (graphite spheroidization ratio: 27.5%), number of graphite particles (number of graphite particles of 20 μm or more: 24.5 / mm) within the scope of the present invention. ² 4 is a micrograph of an optical microstructure without etching showing an example of a structure having the following.
FIG. 3 shows a graphite shape (graphite spheroidization rate: 81.7%), the number of graphite particles (the number of graphite particles of 20 μm or more: 8.6 / mm), which is outside the range of the present invention. ² 4 is a micrograph of an optical microstructure without etching showing an example of a structure having the following.
FIG. 4 shows a graphite shape (graphite spheroidization rate: 53.0%) and a graphite particle number (number of graphite particles of 20 μm or more: 6.6 particles / mm) out of the range of the present invention. ² 4 is a micrograph of an optical microstructure without etching showing an example of a structure having the following.
FIG. 5 is an explanatory view schematically showing an outline of a test machine used in a seizure test and a thermal shock test.

Claims

It is a roll outer layer material used for the outer layer of the composite roll for hot rolling.
C: 2.6 to 3.5%, Si: 1.5 to 2.5%,
Mn: 0.2 to 1.5%, Cr: 1.0 to 2.5%,
Mo: 1.0 to 3.0%, Ni: 2.0 to 7.0%,
V: 1.3 to 2.5%, Nb: 0.1 to 0.8%,
B: 0.020 to 0.2%
And a composition comprising one or two selected from Ti: 0.05% or less and Al: 0.1% or less, the balance being Fe and unavoidable impurities, and a circle equivalent diameter of 20 μm or more. Hot-rolling characterized in that the graphite has a structure in which the graphite spheroidization ratio defined below is 15 to 75% for graphite grains having a circle equivalent diameter of 20 μm or more with 10 graphite / mm ² or more. Roll outer layer material.
Serial graphite spheroidization ratio _{_{(%) = {(n V}} + n VI) / n} × 100
Here, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces)
n _V : number of graphite particles of shape class V specified in JIS G 5502-1995 (pieces)
n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pieces)

The roll outer layer material for hot rolling according to claim 1, further comprising Co: 4% or less by mass% in addition to the composition.

A composite roll for hot rolling in which an outer layer and an inner layer are welded and integrated, wherein the outer layer is, in mass%,
C: 2.6 to 3.5%, Si: 1.5 to 2.5%,
Mn: 0.2 to 1.5%, Cr: 1.0 to 2.5%,
Mo: 1.0 to 3.0%, Ni: 2.0 to 7.0%,
V: 1.3 to 2.5%, Nb: 0.1 to 0.8%,
B: 0.020 to 0.2%
And a composition comprising one or two selected from Ti: 0.05% or less and Al: 0.1% or less, the balance being Fe and unavoidable impurities, and having a circle equivalent diameter of 20 μm or more. A structure in which the number of graphite particles is 10 / mm ² or more and the graphite spheroidization ratio defined below for graphite particles having a circle equivalent diameter of 20 μm or more is 15 to 75%. Composite roll for rolling.
Serial graphite spheroidization ratio _{_{(%) = {(n V}} + n VI) / n} × 100
Here, n: the total number of graphite particles having a circle equivalent diameter of 20 μm or more (pieces)
n _V : number of graphite particles of shape class V specified in JIS G 5502-1995 (pieces)
n _VI : Number of graphite particles of shape classification VI specified in JIS G 5502-1995 (pieces)

The composite roll for hot rolling according to claim 3, wherein the outer layer further contains Co: 4% or less by mass% in addition to the composition.