JP4035960B2 - Method of rolling a tube with a mandrel mill - Google Patents

Description

【００２２】
但し、
Ｔ：当該スタンド（第２スタンド）出側のロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該スタンド（第２スタンド）入側のロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
ｔｍａｘ：当該スタンド（第２スタンド）での肉厚圧下量の最大値（単位：ｍｍ）、
θ（ｔ）：当該スタンド（第２スタンド）において、肉厚圧下量がｔとなる周方向位置における溝底中央部からの角度の最大値（単位：度）。
【００２３】
図２は、ｔｍａｘとθ（ｔ）とを説明するための肉厚圧下量の周方向分布を模式的に示すグラフである。図２に示すように、ｔｍａｘは当該スタンドにおける肉厚圧下量の最大値であり、θ（ｔ）は、肉厚圧下量がｔとなる位置の溝底中央部からの角度で、肉厚圧下量がｔとなる位置が２箇所以上ある場合は、角度の最大値である。
【００２４】
なお、ここで定義した平均接触角ψは、管の変形の予測計算により、あるいは、素管を圧延した後のロール表面を観察し、ロール肌荒れ状況を調査することにより、求めることができる。
【００２５】
溝底部のスリップ発生有無は、溝底中央部の肉厚圧下率ｒと、平均接触角ψとで整理できる。スリップ発生の境界は、下記式（３）で表される。
【００２６】
ψ＝２１３ｒ−５２．０ （３）
したがって、溝底部における穴明き欠陥Ｃの発生を防止する条件は、下記式（４）で規定されるパラメータｓ１を用いて下記式（５）で与えられる。
【００２７】
ｓ１＝−２１３ｒ＋ψ （４）
ｓ１≧−５２．０ （５）
（ｖ）図１（ｄ）に示す肉厚圧下量の分布となる第４スタンドでのスリップの発生に及ぼす第４スタンドにおける溝底中央部の肉厚圧下率ｒと肉厚圧下量の最大部における肉厚圧下率ｒ_ＱＰ（以下、最大肉厚圧下率ともいう）との関係を調べた。なお、肉厚圧下率ｒと最大肉厚圧下率ｒ_ＱＰは下記式（６）、（７）で定義される。
【００２８】
ｒ＝（Ｔ０−Ｔ）／Ｔ０ （６）
ｒ_ＱＰ＝（Ｔ_ＱＰ−Ｔ）／Ｔ_ＱＰ （７）
但し、
Ｔ：当該スタンド（第４スタンド）出側のロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該スタンド（第４スタンド）入側のロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ_ＱＰ：当該スタンド（第４スタンド）で肉厚圧下量が最大となる部分の管の肉厚（単位：ｍｍ）、
例えば、第４スタンドのように中間部の肉厚圧下量が大きいときは、穴明き欠陥Ｃの発生有無を、溝底中央部の肉厚圧下率ｒと最大肉厚圧下率ｒ_ＱＰとで整理できる。スリップ発生の境界は、下記式（８）で表される。
【００２９】
ｒ_ＱＰ＝−３．６ｒ＋１．４９ （８）
したがって、中間部での穴明き欠陥Ｃの発生を防止する条件は、下記式（９）で規定されるパラメータｓ２を用いて下記式（１０）で与えられる。
【００３０】
ｓ２＝３．６ｒ＋ｒ_ＱＰ （９）
ｓ２≦１．４９ （１０）
（ｖｉ）上記穴明き欠陥Ｃの発生は、管の肉厚と外径との比と関係する。すなわち、マンドレルミル圧延後の管の肉厚ｔ_Ｆと外径Ｄ_Ｆとの比（ｔ_Ｆ／Ｄ_Ｆ：肉厚外径比ともいう）が３％以下のときに穴明き欠陥が発生しやすい。特に、炭素鋼では肉厚外径比（ｔ_Ｆ／Ｄ_Ｆ）が１．７％以下の場合に顕著となる。また、１％程度のクロムを含む普通鋼では肉厚外径比（ｔ_Ｆ／Ｄ_Ｆ）が２．１％以下の場合に、合金鋼では肉厚外径比（ｔ_Ｆ／Ｄ_Ｆ）が２．４％以下の場合に顕著となる。
【００３１】
本発明は、上記知見に基づき完成されたもので、その要旨は以下の通りである。
（１）複数の孔型ロールスタンドを有し、該孔型ロールスタンドが形成するロール孔型内にマンドレルバーを配置したマンドレルミルによる、前記マンドレルミルによる圧延後の管の肉厚ｔ_Ｆと管の外径Ｄ_Ｆとの比（ｔ_Ｆ／Ｄ_Ｆ）が０．０３以下である管の圧延方法において、上流から下流に向かって第１番目の孔型ロールスタンドと第２番目の孔型ロールスタンドの内の少なくとも第２番目の孔型ロールスタンドのロール孔型の溝底中央部の肉厚圧下率ｒと当該ロール孔型の平均接触角ψとにより下記式で規定されるパラメータｓ１が−５２．０以上の条件下で管の圧延を行うことを特徴とするマンドレルミルによる管の圧延方法。
【００３２】
ｓ１＝−２１３ｒ＋ψ
但し、
ｒ＝（Ｔ０−Ｔ）／Ｔ０、
【００３３】
【数４】

【００３４】
Ｔ：当該孔型ロールスタンド出側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該孔型ロールスタンド入側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
ｔ_ｍａｘ：当該孔型ロールスタンドでの肉厚圧下量の最大値（単位：ｍｍ）、
θ（ｔ）：当該孔型ロールスタンドにおいて、肉厚圧下量がｔとなる周方向位置における溝底中央部からの角度の最大値（単位：度）。
【００３６】
（２）複数の孔型ロールスタンドを有し、該孔型ロールスタンドが形成するロール孔型内にマンドレルバーを配置したマンドレルミルによる管の圧延方法において、上流から下流に向かって第３番目の孔型ロールスタンドと第４番目の孔型ロールスタンドの内の少なくとも一つの孔型ロールスタンドのロール孔型の溝底中央部の肉厚圧下率ｒと当該ロール孔型の肉厚圧下量の最大部における肉厚圧下率ｒ_ＱＰとにより下記式で規定されるパラメータｓ２が１．４９以下の条件下で管の圧延を行うことを特徴とするマンドレルミルによる管の圧延方法。
【００３７】
ｓ２＝３．６ｒ＋ｒ_ＱＰ
但し、
ｒ＝（Ｔ０−Ｔ）／Ｔ０、
ｒ_ＱＰ＝（Ｔ_ＱＰ−Ｔ）／Ｔ_ＱＰ
Ｔ：当該孔型ロールスタンド出側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該孔型ロールスタンド入側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ_ＱＰ：当該孔型ロールスタンドで肉厚圧下量が最大となる部分の当該孔型ロールスタンド入側の管の肉厚。
【００３８】
（３）複数の孔型ロールスタンドを有し、該孔型ロールスタンドが形成するロール孔型内にマンドレルバーを配置したマンドレルミルによる管の圧延方法において、上流から下流に向かって第１番目の孔型ロールスタンドと第２番目の孔型ロールスタンドとの、ロール孔型の溝底中央部の肉厚圧下率ｒとロール孔型の平均接触角ψとにより下記式で規定されるパラメータｓ１が−５２．０以上で、かつ、上流から下流に向かって第３番目の孔型ロールスタンドと第４番目の孔型ロールスタンドとの、ロール孔型の溝底の肉厚圧下率ｒとロール孔型の肉厚圧下量の最大部における肉厚圧下率ｒ_ＱＰとにより下記式で規定されるパラメータｓ２が１．４９以下の条件下で管の圧延を行うことを特徴とするマンドレルミルによる管の圧延方法。
【００３９】
ｓ１＝−２１３ｒ＋ψ
ｓ２＝３．６ｒ＋ｒ_ＱＰ
但し、
ｒ＝（Ｔ０−Ｔ）／Ｔ０
【００４０】
【数５】

【００４１】
ｒ_ＱＰ＝（Ｔ_ＱＰ−Ｔ）／Ｔ_ＱＰ
Ｔ：当該孔型ロールスタンド出側におけるロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該孔型ロールスタンド入側におけるロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
ｔ_ｍａｘ：当該孔型ロールスタンドでの肉厚圧下量の最大値（単位：ｍｍ）、
θ（ｔ）：当該孔型ロールスタンドにおいて、肉厚圧下量がｔとなる周方向位置における溝底中央部からの角度の最大値（単位：度）、
Ｔ_ＱＰ：当該孔型ロールスタンドで肉厚圧下量が最大となる部分の当該孔型ロールスタンド入側の管の肉厚（単位：ｍｍ）。
【００４２】
（４）前記マンドレルミルによる圧延後の管の肉厚ｔ_Ｆと管の外径Ｄ_Ｆとの比（ｔ_Ｆ／Ｄ_Ｆ）が０．０３以下であることを特徴とする上記（１）項〜（３）項のいずれかに記載のマンドレルミルによる管の圧延方法。
【００４３】
【発明の実施の形態】
本発明の実施の形態を図面を参照して説明する。
図３は、本発明に係るマンドレルミルの構成例を示す模式図である。図３において、符号１１はマンドレルミル、１２は中空の素管、１３はマンドレルバー、２２は圧延ロールである。なお、マンドレルミルの圧延では、マンドレルミルを構成する複数のスタンドの内、管のサイズや管の材質に基づき、一部のスタンドのみを用いる場合があり、図３では、圧延に使用されるスタンドのみを示す。
【００４４】
マンドレルミル１１は、上流から下流に向かって＃１（第１スタンド）〜＃８（第８スタンド）の８基の孔型ロールスタンドを備える。各孔型ロールスタンドは一対の圧延ロールを有し、隣接する孔型ロールスタンドのロール軸は圧延軸に垂直な面内で相互に９０度ずらして配置される。
【００４５】
このマンドレルミルにおいて、マンドレルバー１３を挿入された素管１２は、各孔型ロールスタンド＃１〜＃８の圧延ロール２２とマンドレルバー１３とにより肉厚圧下を受け、所定の外径と肉厚の管に圧延される。
【００４６】
最初に、本発明の一の実施形態を説明する。この説明では第２スタンドを例にとる。
図４は、本発明の一の実施形態に係る第２スタンドのロール孔型例を示す模式図である。図４において、符号１３は、マンドレルバー、２２は圧延ロールで、２３はロール孔型、２４は溝底中央部、２５は溝底部、２６はフランジ部、２７は被圧延材、Ｓは孔型オフセット量、φは溝底部を形成する第１曲率半径Ｒ_１に対する角度（以下、第１角度ともいう）である。
【００４７】
図４に示すように、圧延ロール対２２、２２は、溝底部を形成する第１曲率半径Ｒ１とこのＲ１に続く第２曲率半径Ｒ２をそれぞれが有し、中央部に溝底中央部２４を有する溝底部２５とフランジ部２６とからなるロール孔型２３を形成する。
【００４８】
本実施形態では、第２スタンドにおいて、ロール孔型の溝底中央部の肉厚圧下率ｒと平均接触角ψにより下記式（１１）で規定されるパラメータｓ１が−５２．０以上である。パラメータｓ１を−５２．０以上とすることにより、溝底部でのスリップの発生が抑制され、穴明き欠陥Ｃを防止する事が出来る。
【００４９】
ｓ１＝−２１３ｒ＋ψ （１１）
但し、
ｒ＝（Ｔ０−Ｔ）／Ｔ０、
【００５０】
【数６】

【００５１】
Ｔ：当該スタンド（第２スタンド）出側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該スタンド（第２スタンド）入側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
ｔ_ｍａｘ：当該スタンド（第２スタンド）での肉厚圧下量の最大値（単位：ｍｍ）、
θ（ｔ）：当該スタンド（第２スタンド）において、肉厚圧下量がｔとなる周方向位置における溝底中央部からの角度の最大値（単位：度）
圧延時には、マンドレルバーの潤滑状態の変動あるいは素管の寸法変動などの圧延条件外乱があると、肉厚圧下率ｒや平均接触角ψが変化し、穴明き欠陥Ｃが発生し易い。したがって、上記圧延条件外乱を考慮し、ｓ１は−４０以上にするのが望ましい。
【００５２】
次に、ロール孔型の設計手法を第２スタンドを例に説明する。最初に各スタンドの孔型形状の初期値および圧下量を与え、第２スタンドの平均接触角ψと肉厚圧下率ｒを求める。次いで、求めた平均接触角ψと肉厚圧下率ｒとに基づき、前式（１１）からパラメータｓ１を算出し、パラメータｓ１が−５２．０より小さい場合は、ｓ１が−５２．０以上となるように第２スタンドの孔型オフセットＳや第１角度φなどの孔型パラメータを調整する。なお、ｓ１が−５２．０以上となるように第１スタンドの孔型パラメータを調整してもよい。
【００５３】
本実施形態では第２スタンドに適用する場合を示したが、第１スタンドと第２スタンドの内の少なくとも一つのスタンドに適用することにより穴明き欠陥Ｃの発生を抑制することができる。しかし、第２スタンドでは、平均接触角ψが他のスタンドに比べて小さく、溝底の肉厚圧下率ｒも他のスタンドに比べて大きくなる傾向にあるため、穴明き欠陥が発生しやすい。したがって、好ましくは、第２スタンドへの適用であり、更に好ましくは、第１スタンドと第２スタンドへの適用であり、なお更に好ましくは全スタンドへの適用である。
【００５４】
次に、本発明の別の実施形態を説明する。この実施形態の説明では第３スタンドへの適用を例にとる。
この実施形態では、第３スタンドにおいて、ロール孔型の溝底中央部の肉厚圧下率ｒとロール孔型の肉厚圧下量の最大部における肉厚圧下率ｒ_ＱＰとにより下記式で規定されるパラメータｓ２が１．４９以下である。パラメータｓ２を１．４９以下とすることにより、中間部におけるスリップの発生が抑制され、穴明き欠陥Ｃを防止することができる。
【００５５】
ｓ２＝３．６ｒ＋ｒ_ＱＰ （１２）
但し、
ｒ＝（Ｔ０−Ｔ）／Ｔ０、
ｒ_ＱＰ＝（Ｔ_ＱＰ−Ｔ）／Ｔ_ＱＰ、
Ｔ：当該スタンド（第３スタンド）出側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ０：当該スタンド（第３スタンド）入側でのロール孔型溝底中央部の管の肉厚（単位：ｍｍ）、
Ｔ_ＱＰ：当該スタンド（第３スタンド）で肉厚圧下量が最大となる部分の当該スタンド（第３スタンド）入側の管の肉厚（ｍｍ）、
圧延時に、マンドレルバーの潤滑状態の変動や素管の寸法変動などの圧延条件外乱があると、溝底中央部の肉厚圧下率ｒや最大肉厚圧下率ｒ_ＱＰが変化し、穴明き欠陥Ｃが発生し易い。したがって、圧延条件外乱を考慮して、ｓ２は１．２５以下にすることが望ましい。
【００５６】
次に、この実施形態に係るロール孔型の設計手法を第３スタンドを例に説明する。最初に、各スタンドの孔型形状の初期値および圧下量を与え、第３スタンドのロール孔型の溝底中央部の肉厚圧下率ｒと、最大肉厚圧下率ｒ_ＱＰを求める。次いで、求めた肉厚圧下率ｒと、最大肉厚圧下率ｒ_ＱＰに基づき、式（１２）でｓ２を算出し、パラメータｓ２が１．４９より大きい場合は、ｓ２が１．４９以下となるように、第１スタンド、第２スタンドおよび第３スタンドの内の少なくとも一つのスタンドの孔型形状のパラメータである孔型オフセットＳや第１角度φを調整する。例えば、第３スタンドのパラメータｓ２が１．４９より大きい場合は、第３スタンドの孔型オフセットＳを大きくする、および／または、第１角度φを小さくするように調整し、あるいは、第１スタンドや第２スタンドの孔型オフセットＳを小さく、および／または、第１角度φを大きくするように調整すればよい。
【００５７】
上記本実施形態では第３スタンドに適用する場合を示したが、第３スタンドと第４スタンドの内の少なくとも一つのスタンドに適用することにより中間部での穴明き欠陥Ｃの発生を抑制することができる。好ましくは、第３スタンドと第４スタンドの両方への適用であり、更に、好ましくは、全スタンドへの適用である。
【００５８】
一般的に、マンドレルミルの圧延においては、第１スタンドと第２スタンドでは、溝底部の肉厚圧下率が大きく、第３スタンドと第４スタンドでは、中間部の肉厚圧下率が大きい。したがって、第１スタンドと第２スタンドではｓ１を−５２．０以上とし、第３スタンドと第４スタンドではｓ２を１．４９以下とするのがよい。
【００５９】
マンドレルミル圧延後の管の肉厚外径比（ｔ_Ｆ／Ｄ_Ｆ）が小さくなると穴明き欠陥Ｃが発生しやすい。従って、本発明の方法は、管の肉厚外径比ｔ／Ｄが３％以下の圧延に適用すると好適である。
【００６０】
本発明に係る方法により製造される管の鋼種は特に、限定されるものでなく、炭素鋼や合金鋼とすることができる。但し、炭素鋼では肉厚外径比ｔ／Ｄが１．７％以下の範囲で、また、１％程度のクロムを含む炭素鋼では肉厚外径比ｔ／Ｄが２．１％以下の範囲で、合金鋼では肉厚外径比ｔ／Ｄが２．４％以下の範囲で、穴明き欠陥の発生が大きくなるので、肉厚外径比が上記範囲の管に適用すると穴明き欠陥の防止効果が大きいので好ましい。
【００６１】
【実施例】
（実施例１）
５基のスタンド（第１〜第５スタンド）を有するマンドレルミルを用い、各スタンドのロール孔型形状を種々変更した３種類の孔型列で、外径：３６８ｍｍのマンドレルバーを挿入した外径：４２０ｍｍで肉厚：１６ｍｍの炭素鋼製の素管を外径：３８０ｍｍ、肉厚：６ｍｍ：長さ：１０ｍの管に圧延し、本発明の効果を確認した。表１にロール孔型形状の特性値を示す。
【００６２】
【表１】

【００６３】
表１に示すように、孔型列Ｉは、第１、第２スタンドのｓ１を−５２．０以上、第３、第４スタンドのｓ２を１．４９以下とし、孔型列ＩＩは、第２スタンドのｓ１を−５２．０以上、第４スタンドのｓ２を１．４９以下とした。なお、比較のため、第１，第２スタンドのｓ１を−５２．０未満、第３、第４スタンドのｓ２を１．４９超とした孔型列ＩＩＩも準備した。
【００６４】
表１に示す孔型列毎に３０本の管を圧延し、穴明き欠陥Ｃが発生した管の本数を調査した。表２に穴明き欠陥Ｃの発生率を本数比率（％：発生本数／３０×１００）で示す。
【００６５】
【表２】

【００６６】
表２に示すように、試験Ｎｏ．３に比べ、試験Ｎｏ．１、２では穴明き欠陥Ｃの発生率が減少した。特に、試験Ｎｏ．１は穴明き欠陥Ｃの発生が無く、極めて良好であった。なお、穴明き欠陥Ａ、Ｂに関しては、試験Ｎｏ．１〜３のいずれにも発生しなかった。
（実施例２）
５基の孔型ロールスタンドを有するマンドレルミルを用い、肉厚外径比の異なる炭素鋼製の４種類の管を圧延し、穴明き欠陥の発生状況を調査した。表３に圧延条件を示す。
【００６７】
【表３】

【００６８】
表１に示す孔型列Ｉで、表３に示すそれぞれの圧延条件毎に、３０本の管を圧延し、穴明き欠陥Ｃが発生した管の本数を調査した。なお、比較のために、表１に示す孔型列ＩＩＩで上記と同様の試験も実施した。表４に穴明き欠陥Ｃの発生率を本数比率（％：発生本数／３０×１００）で示す。
【００６９】
【表４】

【００７０】
表４に示すように、試験Ｎｏ．１〜４は、穴明き欠陥の発生が無く、したがって、本発明は肉厚外径比が１．６〜３．０の管の圧延において有効であることが判った。特に、肉厚外径比が、２．３以下の薄肉の管の圧延において、著しい改善効果があった。
【００７１】
【発明の効果】
本発明によれば、スリップの発生を伴う穴明き欠陥の発生を防止することができる。特に、肉厚の薄い管材や変形能の劣る高合金鋼の圧延の際の穴明き欠陥の防止が可能となり、高強度管や薄肉管の製造範囲が拡大する。
【図面の簡単な説明】
【図１】 マンドレルミルで管を圧延する際の第１〜第４スタンドにおける肉厚圧下量の周方向分布の例を模式的に示すグラフで、同図（ａ）は第１スタンド、（ｂ）は第２スタンド、（ｃ）は第３スタンド、（ｄ）は第４スタンドの場合である。
【図２】 ｔｍａｘとθ（ｔ）とを説明するための肉厚圧下量の周方向分布を模式的に示すグラフである。
【図３】 本発明に係るマンドレルミルの構成例を示す模式図である。
【図４】 本発明の一の実施形態に係る第２スタンドのロール孔型例を示す模式図である。
【符号の説明】
１１：マンドレルミル、
１２：素管、
１３：マンドレルバー、
２２：圧延ロール、
２３：ロール孔型、
２４：溝底、
２５：溝部
２６：フランジ部、
２７：被圧延材。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of rolling a tube by a mandrel mill.
[0002]
[Prior art]
A mandrel mill widely adopted as a continuous rolling mill for hot rolling of small-diameter seamless steel pipes has a hollow base pipe with a mandrel bar inserted therein, and 4 to 8 hole types comprising a pair of upper and lower rolling rolls. It is configured to be rolled through a roll stand and formed into a tube having a target wall thickness. The roll shafts of adjacent perforated roll stands are arranged so as to be shifted from each other in a plane perpendicular to the rolling axis.
[0003]
By the way, when rolling is performed using such a mandrel mill, drilling may occur in the tube material being rolled. Drilling is a serious quality defect. If the pipe steel grade and the pipe outer diameter are the same, the thinner the finished wall thickness of the pipe material, the easier it is for drilling defects, and in high alloy steels that are inferior in deformability compared to ordinary steel. Is prone to occur.
[0004]
Conventionally, it has been considered that the cause of occurrence of this hole-deficient defect is that the flange portion not subjected to rolling reduction during rolling is pulled in the longitudinal direction and is broken due to insufficient deformability. Hereinafter, a drilling defect generated in the flange portion due to this cause is referred to as a drilling defect A.
[0005]
Various methods have been proposed to prevent the perforation defect A. For example, Japanese Patent Application Laid-Open No. 63-84720 discloses a method for reducing the rolling reduction rate of a stand in which a perforation defect A occurs and distributing the load to other stands, A method of reducing the rolling reduction rate of each stand by reducing the number is disclosed.
[0006]
On the other hand, in Japanese Patent Publication No. 7-102369, when the wall thickness reduction ratio is larger on both sides than the groove bottom portion, the groove bottom portion is extended in the longitudinal direction and necked, so that a hole defect occurs in the groove bottom portion. As a countermeasure against a perforated defect (hereinafter, this defect is referred to as a perforated defect B), there is a method of determining the radius of curvature of the roll groove bottom center portion of the first stand and the second stand based on the roll groove bottom interval. It is disclosed.
[0007]
[Problems to be solved by the invention]
In the method disclosed in Japanese Patent Laid-Open No. 63-84720, it is possible to prevent the perforation defect A, but there is a problem that the rollable range by the mandrel mill is limited. That is, if there are stands where the rolling load exceeds the reference value even if the rolling load is distributed to each stand, a thin walled tube is required to reduce the rolling reduction in the entire mandrel mill. However, there is a limit to the reduction in the thickness of the raw tube because the rolling reduction in a punch for manufacturing the raw tube is limited, and it may be difficult to roll the thin tube with the mandrel mill.
[0008]
According to the inventor's test, a hole defect different from the hole defects A and B is generated particularly in the rolling of a thin tube. According to the method disclosed in Japanese Patent Publication No. 7-102369. It has been found that the prevention of perforation defects different from perforation defects A and B is insufficient.
[0009]
An object of the present invention is to solve the above-described problems in rolling a tube by a mandrel mill, and to provide a method for rolling a tube by a mandrel mill that enables prevention of drilling defects different from the drilling defects A and B. It is to provide.
[0010]
[Means for Solving the Problems]
According to the present invention, a hole defect different from the hole defects A and B is prevented by appropriately designing a roll hole mold of a hole roll stand array in which mandrel mills are continuously arranged.
[0011]
As a result of earnest investigation on the cause of the occurrence of the hole-deficient defect in mandrel mill rolling, the present inventor has obtained the following new knowledge regarding the mechanism of the occurrence of the hole-deficient defect. In the following description, the perforated roll stand is also simply referred to as a stand, and is referred to as a first stand, a second stand,... From upstream to downstream of the mandrel mill.
[0012]
(I) FIG. 1 is a graph schematically showing an example of the circumferential distribution of thickness reduction in the first to fourth stands when a tube is rolled by a mandrel mill having five stands. a) is the case of the first stand, (b) is the second stand, (c) is the third stand, and (d) is the case of the fourth stand. In FIGS. 4A to 4D, the horizontal axis represents the angle from the center of the groove bottom of the roll hole type, and the vertical axis represents the thickness reduction amount.
[0013]
The roll hole type of the mandrel mill is usually designed to be elliptical in the front side stand of a plurality of stands and approach a perfect circle as the stand on the rear side becomes closer. Accordingly, as shown in FIGS. 1A and 1B, the circumferential distribution of the thickness reduction amount in the first stand and the second stand is a trapezoidal shape or a semicircular shape in which the thickness reduction amount in the groove bottom is large. Become. On the other hand, as shown in FIGS. 1C and 1D, the circumferential distribution of the thickness reduction amount in the third stand and the fourth stand is the middle portion (intermediate portion) between the groove bottom portion and the flange portion. A square shape with a large thickness reduction.
[0014]
(Ii) For example, in the case of a trapezoidal shape or semicircular shape in which the circumferential direction distribution of the thickness reduction ratio is high at the groove bottom as in rolling at the first stand or the second stand, the material at the groove bottom is stretched in the longitudinal direction. In the process, the material at the bottom of the groove undulates in the longitudinal direction due to restraint from surrounding materials, and in the extreme case, it becomes a folded state, causing a slip and generating a perforated defect at the bottom of the groove. This hole defect is accompanied by slip, and is different from the hole defects A and B.
[0015]
(Iii) Also, for example, when the distribution in the circumferential direction of the wall thickness reduction ratio is higher in the middle part than in the groove bottom part as in rolling in the third stand or the fourth stand, the material in the middle part is in the longitudinal direction. In the process of being stretched, the material of the intermediate part undulates in the longitudinal direction due to the restraint from the material of the groove bottom part and the material of the flange part, and in an extreme case, it becomes a folded state, causing slipping and drilling defects at the groove bottom part appear. This hole defect is accompanied by slip, and is different from the hole defects A and B.
[0016]
As described above, the present inventor newly introduced that a phenomenon in which the material of the groove bottom part or the material of the intermediate part between the groove bottom part and the flange part undulates in the longitudinal direction occurs to cause a slip and a perforation defect. I found out. Hereinafter, this hole defect is also referred to as a hole defect C.
[0017]
In order to avoid the undulation phenomenon that causes the perforation defect C, it is effective to make the circumferential distribution of the rolling reduction uniform in each stand. That is, for example, when the thickness reduction amount of the groove bottom portion is large as in the first stand or the second stand, the width of the reduction region may be widened in the circumferential direction and the reduction amount of the groove bottom portion may be reduced.
[0018]
Further, for example, when the thickness reduction amount of the intermediate portion is larger than the groove bottom portion as in the third stand or the fourth stand, the reduction amount of the intermediate portion may be reduced. However, the wall thickness reduction ratio at the bottom of the groove also affects the perforation defect C generated in the intermediate portion. Therefore, in order to prevent the perforation defect C of the stand having a large thickness reduction rate at the intermediate portion, such as the third stand and the fourth stand, the thickness reduction amount at the bottom of the groove and the thickness reduction amount at the intermediate portion. It is important to consider
[0019]
Based on the above basic study, the present inventor diligently studied a method for preventing a slip that leads to the occurrence of a perforated defect C, and obtained the following knowledge.
(Iv) Investigating the relationship between the thickness reduction ratio of the groove bottom of the second stand and the average contact angle on the occurrence of slip at the groove bottom of the second stand with the distribution of thickness reduction shown in FIG. It was. The wall thickness reduction ratio r and the average contact angle ψ in the second stand are defined by the following formulas (1) and (2).
[0020]
r = (T0−T) / T0 (1)
[0021]
[Equation 3]

[0022]
However,
T: Wall thickness (unit: mm) of the tube at the center of the roll hole type groove bottom on the exit side of the stand (second stand),
T0: Thickness (unit: mm) of the tube at the center of the roll hole type groove bottom on the entrance side of the stand (second stand),
tmax: Maximum thickness reduction (unit: mm) at the stand (second stand),
θ (t): The maximum value (unit: degree) of the angle from the center of the groove bottom at the circumferential position where the thickness reduction amount is t in the stand (second stand).
[0023]
FIG. 2 is a graph schematically showing the circumferential distribution of the thickness reduction amount for explaining tmax and θ (t). As shown in FIG. 2, tmax is the maximum thickness reduction amount at the stand, and θ (t) is an angle from the center of the groove bottom where the thickness reduction amount is t, When there are two or more positions where the amount is t, the angle is the maximum value.
[0024]
The average contact angle ψ defined here can be obtained by predicting the deformation of the tube, or observing the roll surface after rolling the blank tube and investigating the rough surface of the roll.
[0025]
The presence or absence of slip occurrence at the groove bottom can be arranged by the thickness reduction ratio r at the center of the groove bottom and the average contact angle ψ. The boundary of occurrence of slip is expressed by the following formula (3).
[0026]
ψ = 213r-52.0 (3)
Therefore, the condition for preventing the occurrence of the perforation defect C at the bottom of the groove is given by the following formula (5) using the parameter s1 defined by the following formula (4).
[0027]
s1 = −213r + ψ (4)
s1 ≧ −52.0 (5)
(V) The thickness reduction ratio r at the center of the groove bottom in the fourth stand and the maximum part of the thickness reduction amount on the occurrence of slip in the fourth stand having the distribution of the thickness reduction amount shown in FIG. The relationship with the wall thickness reduction ratio r _QP (hereinafter also referred to as the maximum wall thickness reduction ratio) was investigated. The wall thickness reduction ratio r and the maximum wall thickness reduction ratio r _QP are defined by the following formulas (6) and (7).
[0028]
r = (T0−T) / T0 (6)
r _QP = (T _QP −T) / T _QP (7)
However,
T: Thickness (unit: mm) of the tube at the center of the roll hole type groove bottom on the exit side of the stand (fourth stand),
T0: thickness of the tube (unit: mm) at the center of the roll hole type groove bottom on the entrance side of the stand (fourth stand),
T _QP : the wall thickness (unit: mm) of the portion of the tube where the wall thickness reduction amount is maximum on the stand (fourth stand),
For example, when the thickness reduction amount at the intermediate portion is large as in the fourth stand, the presence or absence of the drilling defect C is determined by the thickness reduction rate r and the maximum thickness reduction rate r _QP at the center of the groove bottom. I can organize. The boundary of occurrence of slip is expressed by the following formula (8).
[0029]
r _QP = -3.6r + 1.49 (8)
Therefore, the condition for preventing the occurrence of the perforated defect C in the intermediate portion is given by the following formula (10) using the parameter s2 defined by the following formula (9).
[0030]
s2 = 3.6r + r _QP (9)
s2 ≦ 1.49 (10)
(Vi) The occurrence of the perforation defect C is related to the ratio of the tube thickness to the outer diameter. That is, when the ratio between the wall thickness t _F and the outer diameter D _F of the tube after mandrel mill rolling (t _F / D _F : also referred to as the wall thickness outer diameter ratio) is 3% or less, a drilling defect occurs. Cheap. In particular, carbon steel becomes prominent when the thickness-to-outer diameter ratio (t _F / D _F ) is 1.7% or less. In the case of meat Atsugai diameter ratio in ordinary steel comprising about 1 percent chromium _{(t F} / _D F) is 2.1% or less, meat Atsugai diameter ratio in alloy steel _{(t F} / _D F) is It becomes remarkable when it is 2.4% or less.
[0031]
The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) The tube thickness t _F and the tube after rolling by the mandrel mill by a mandrel mill having a plurality of hole type roll stands, and a mandrel bar arranged in the roll hole mold formed by the hole type roll stand. In the tube rolling method in which the ratio (t _F / D _F ) to the outer diameter D _F of the tube is 0.03 or less, the first perforated roll stand and the second perforated roll from upstream to downstream The parameter s1 defined by the following equation is expressed by the thickness reduction ratio r at the center of the groove bottom of the roll hole type of the roll hole type of the stand and the average contact angle ψ of the roll hole type. A method of rolling a tube by a mandrel mill, wherein the tube is rolled under a condition of 52.0 or more.
[0032]
s1 = −213r + ψ
However,
r = (T0−T) / T0,
[0033]
[Expression 4]

[0034]
T: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the outlet side of the hole-type roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the inlet side of the hole-type roll stand
t _max : Maximum value of the thickness reduction amount in the perforated roll stand (unit: mm),
θ (t): The maximum value (unit: degree) of the angle from the center of the groove bottom at the circumferential position where the thickness reduction amount is t in the perforated roll stand.
[0036]
( 2 ) In a tube rolling method using a mandrel mill having a plurality of perforated roll stands and having a mandrel bar disposed in a roll perforation formed by the perforated roll stand, The thickness reduction ratio r at the center of the groove bottom of the roll hole type of at least one of the hole type roll stand and the fourth hole type roll stand and the maximum thickness reduction amount of the roll hole type. A tube rolling method using a mandrel mill, wherein the tube is rolled under the condition that the parameter s2 defined by the following formula is 1.49 or less by the wall thickness reduction ratio r _QP in the section.
[0037]
s2 = 3.6r + r _QP
However,
r = (T0−T) / T0,
r _QP = (T _QP −T) / T _QP
T: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the outlet side of the hole-type roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the inlet side of the hole-type roll stand
T _QP : The thickness of the tube on the inlet side of the hole-type roll stand at the portion where the wall thickness reduction amount is maximum in the hole-type roll stand.
[0038]
( 3 ) In a tube rolling method using a mandrel mill having a plurality of perforated roll stands, and a mandrel bar arranged in a roll perforation formed by the perforated roll stand, the first one from upstream to downstream The parameter s1 defined by the following equation is determined by the wall thickness reduction ratio r at the center of the groove bottom of the roll hole mold and the average contact angle ψ of the roll hole mold of the hole roll stand and the second hole roll stand. −52.0 or more, and the thickness reduction ratio r and roll hole at the groove bottom of the roll hole type of the third hole type roll stand and the fourth hole type roll stand from upstream to downstream The tube is rolled by a mandrel mill characterized in that the tube is rolled under the condition that the parameter s2 defined by the following equation is 1.49 or less by the wall thickness reduction ratio r _{QP at} the maximum wall thickness reduction amount of the die. Rolling method.
[0039]
s1 = −213r + ψ
s2 = 3.6r + r _QP
However,
r = (T0−T) / T0
[0040]
[Equation 5]

[0041]
r _QP = (T _QP −T) / T _QP
T: Thickness (unit: mm) of the tube at the center of the roll hole groove bottom on the outlet side of the hole roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole type groove bottom at the inlet side of the hole type roll stand
t _max : Maximum value of the thickness reduction amount in the perforated roll stand (unit: mm),
θ (t): In the perforated roll stand, the maximum value (unit: degree) of the angle from the groove bottom central portion at the circumferential position where the thickness reduction amount is t,
T _QP : Thickness (unit: mm) of the tube on the inlet side of the perforated roll stand at the portion where the thickness reduction amount is maximum in the perforated roll stand.
[0042]
(4) The above ratio of the outer diameter _{D F} of the thickness _{t F} and the tube of the tube after rolling by a mandrel mill _(t F / _{D F)} is characterized in that 0.03 or less (1) term The method of rolling a tube by a mandrel mill according to any one of to ( 3 ).
[0043]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings.
FIG. 3 is a schematic diagram showing a configuration example of a mandrel mill according to the present invention. In FIG. 3, reference numeral 11 is a mandrel mill, 12 is a hollow shell, 13 is a mandrel bar, and 22 is a rolling roll. In the rolling of the mandrel mill, some of the stands constituting the mandrel mill may be used based on the size of the tube and the material of the tube. In FIG. 3, the stand used for rolling is used. Show only.
[0044]
The mandrel mill 11 includes eight perforated roll stands # 1 (first stand) to # 8 (eighth stand) from upstream to downstream. Each perforated roll stand has a pair of rolling rolls, and the roll axes of adjacent perforated roll stands are arranged 90 degrees apart from each other in a plane perpendicular to the rolling axis.
[0045]
In this mandrel mill, the raw tube 12 into which the mandrel bar 13 is inserted is subjected to thickness reduction by the rolling rolls 22 and the mandrel bar 13 of each of the perforated roll stands # 1 to # 8, and has a predetermined outer diameter and thickness. Rolled into a tube.
[0046]
First, one embodiment of the present invention will be described. In this description, the second stand is taken as an example.
FIG. 4 is a schematic view showing a roll hole type example of the second stand according to the embodiment of the present invention. In FIG. 4, reference numeral 13 is a mandrel bar, 22 is a rolling roll, 23 is a roll hole mold, 24 is a groove bottom central part, 25 is a groove bottom part, 26 is a flange part, 27 is a material to be rolled, and S is a hole type. offset, the φ angle relative to the first radius of curvature R ₁ to form the groove bottom portion (hereinafter, also referred to as a first angle).
[0047]
As shown in FIG. 4, each of the rolling roll pairs 22 and 22 has a first curvature radius R1 that forms a groove bottom portion and a second curvature radius R2 that follows this R1, and a groove bottom central portion 24 at the center portion. A roll hole mold 23 including a groove bottom portion 25 and a flange portion 26 is formed.
[0048]
In the present embodiment, in the second stand, the parameter s1 defined by the following formula (11) is −52.0 or more by the thickness reduction ratio r and the average contact angle ψ at the center of the groove bottom of the roll hole type. By setting the parameter s1 to −52.0 or more, the occurrence of slip at the bottom of the groove is suppressed, and the perforation defect C can be prevented.
[0049]
s1 = −213r + ψ (11)
However,
r = (T0−T) / T0,
[0050]
[Formula 6]

[0051]
T: Wall thickness (unit: mm) of the tube at the center of the roll hole type groove bottom at the exit side of the stand (second stand)
T0: wall thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom on the entrance side of the stand (second stand),
t _max : Maximum value of the thickness reduction amount at the stand (second stand) (unit: mm),
θ (t): Maximum value (unit: degree) of the angle from the center of the groove bottom at the circumferential position where the thickness reduction amount is t in the stand (second stand)
During rolling, if there are disturbances in rolling conditions such as a change in the lubrication state of the mandrel bar or a change in the dimensions of the tube, the wall thickness reduction ratio r and the average contact angle ψ change, and a hole defect C is likely to occur. Therefore, in consideration of the rolling condition disturbance, it is desirable that s1 is −40 or more.
[0052]
Next, a roll hole type design method will be described using the second stand as an example. First, an initial value and a reduction amount of the hole shape of each stand are given, and an average contact angle ψ and a thickness reduction ratio r of the second stand are obtained. Next, based on the obtained average contact angle ψ and thickness reduction ratio r, the parameter s1 is calculated from the previous equation (11). When the parameter s1 is smaller than −52.0, s1 is −52.0 or more. Thus, the hole type parameters such as the hole type offset S of the second stand and the first angle φ are adjusted. In addition, you may adjust the hole type parameter of a 1st stand so that s1 may become -52.0 or more.
[0053]
In the present embodiment, the case where the present invention is applied to the second stand has been described. However, the occurrence of the perforation defect C can be suppressed by applying to at least one of the first stand and the second stand. However, in the second stand, the average contact angle ψ is smaller than that of other stands, and the thickness reduction ratio r of the groove bottom tends to be larger than that of other stands, so that a punching defect is likely to occur. . Therefore, it is preferably applied to the second stand, more preferably applied to the first stand and the second stand, and still more preferably applied to all the stands.
[0054]
Next, another embodiment of the present invention will be described. In the description of this embodiment, application to the third stand is taken as an example.
In this embodiment, in the third stand, the thickness reduction ratio r at the center of the groove bottom of the roll hole type and the thickness reduction ratio r _{QP at} the maximum portion of the thickness reduction amount of the roll hole type are defined by the following equation. The parameter s2 is 1.49 or less. By setting the parameter s2 to 1.49 or less, the occurrence of slip in the intermediate portion is suppressed, and the hole defect C can be prevented.
[0055]
s2 = 3.6r + r _QP (12)
However,
r = (T0−T) / T0,
r _QP = (T _QP −T) / T _QP ,
T: wall thickness (unit: mm) of the tube at the center of the roll hole type groove bottom at the exit side of the stand (third stand),
T0: Wall thickness (unit: mm) of the tube at the center of the roll hole type groove bottom on the entrance side of the stand (third stand),
T _QP : Thickness (mm) of the tube on the entrance side of the stand (third stand) where the thickness reduction amount is maximum in the stand (third stand),
During rolling, if there are disturbances in the rolling conditions such as the mandrel bar lubrication condition and the dimensional fluctuation of the pipe, the wall thickness reduction ratio r and the maximum wall thickness reduction ratio _{QP at the} center of the groove will change, resulting in drilling. Defect C is likely to occur. Therefore, s2 is preferably 1.25 or less in consideration of disturbances in rolling conditions.
[0056]
Next, a roll hole type design method according to this embodiment will be described using a third stand as an example. First, an initial value and a reduction amount of the hole shape of each stand are given, and a wall thickness reduction ratio r and a maximum wall thickness reduction ratio r _QP at the center of the groove bottom of the roll hole type of the third stand are obtained. Next, based on the obtained wall thickness reduction ratio r and the maximum wall thickness reduction ratio r _QP , s2 is calculated by Expression (12). When the parameter s2 is greater than 1.49, s2 is 1.49 or less. As described above, the hole offset S and the first angle φ, which are the parameters of the hole shape of at least one of the first stand, the second stand, and the third stand, are adjusted. For example, when the parameter s2 of the third stand is larger than 1.49, the third stand is adjusted so that the hole offset S of the third stand is increased and / or the first angle φ is decreased, or the first stand is adjusted. Alternatively, the hole type offset S of the second stand may be adjusted to be small and / or the first angle φ may be increased.
[0057]
In the present embodiment, the case where the present invention is applied to the third stand has been described. However, by applying to at least one of the third stand and the fourth stand, the occurrence of the punching defect C in the intermediate portion is suppressed. be able to. It is preferably applied to both the third stand and the fourth stand, and more preferably applied to all the stands.
[0058]
In general, in mandrel mill rolling, the thickness reduction rate at the bottom of the groove is large at the first stand and the second stand, and the thickness reduction rate at the intermediate portion is large at the third stand and the fourth stand. Therefore, s1 is preferably −52.0 or more for the first stand and the second stand, and s2 is preferably 1.49 or less for the third stand and the fourth stand.
[0059]
When the thickness-to-outer diameter ratio (t _F / D _F ) of the tube after mandrel mill rolling is small, a hole defect C is likely to occur. Therefore, the method of the present invention is preferably applied to rolling with a wall thickness outer diameter ratio t / D of 3% or less.
[0060]
The steel type of the pipe manufactured by the method according to the present invention is not particularly limited, and can be carbon steel or alloy steel. However, in the case of carbon steel, the wall thickness outer diameter ratio t / D is in the range of 1.7% or less, and in the case of carbon steel containing about 1% chromium, the wall thickness outer diameter ratio t / D is 2.1% or less. In the case of alloy steel, the occurrence of drilling defects increases when the wall thickness outer diameter ratio t / D is 2.4% or less. This is preferable because the effect of preventing cracking defects is great.
[0061]
【Example】
Example 1
Using a mandrel mill having five stands (first to fifth stands), three types of hole type rows in which the roll hole shape of each stand was variously changed, and an outer diameter in which a mandrel bar having an outer diameter of 368 mm was inserted A carbon steel base tube having a thickness of 420 mm and a thickness of 16 mm was rolled into a tube having an outer diameter of 380 mm, a thickness of 6 mm, and a length of 10 m, and the effects of the present invention were confirmed. Table 1 shows the characteristic values of the roll hole shape.
[0062]
[Table 1]

[0063]
As shown in Table 1, s1 of the first and second stands is −52.0 or more, s2 of the third and fourth stands is 1.49 or less, and The s1 of the two stands was set to −52.0 or more, and the s2 of the fourth stand was set to 1.49 or less. For comparison, a hole-type column III was also prepared in which s1 of the first and second stands was less than −52.0, and s2 of the third and fourth stands was more than 1.49.
[0064]
Thirty tubes were rolled for each hole type row shown in Table 1, and the number of tubes in which a hole-deficient defect C occurred was examined. Table 2 shows the occurrence rate of the perforated defect C in terms of the number ratio (%: number of occurrences / 30 × 100).
[0065]
[Table 2]

[0066]
As shown in Table 2, test no. Compared to test No. 3, test no. In 1 and 2, the occurrence rate of the perforated defect C decreased. In particular, test no. No. 1 was very good with no perforation defect C. In addition, regarding the drilling defects A and B, the test No. It did not occur in any of 1-3.
(Example 2)
Using a mandrel mill having five perforated roll stands, four types of pipes made of carbon steel with different thickness-to-outer diameter ratios were rolled, and the occurrence of drilling defects was investigated. Table 3 shows the rolling conditions.
[0067]
[Table 3]

[0068]
In the hole type row I shown in Table 1, 30 pipes were rolled for each rolling condition shown in Table 3, and the number of pipes in which the hole defect C occurred was investigated. For comparison, a test similar to the above was also performed for the hole type column III shown in Table 1. Table 4 shows the occurrence rate of the perforated defect C in terms of the number ratio (%: number of occurrences / 30 × 100).
[0069]
[Table 4]

[0070]
As shown in Table 4, test no. No. 1-4 has no occurrence of drilling defects, and therefore the present invention has been found to be effective in rolling a tube having a wall thickness outer diameter ratio of 1.6 to 3.0. In particular, there was a significant improvement effect in the rolling of thin tubes with a wall thickness-outer diameter ratio of 2.3 or less.
[0071]
【The invention's effect】
According to the present invention, it is possible to prevent the occurrence of a punching defect accompanied by the occurrence of slip. In particular, it is possible to prevent drilling defects when rolling thin-walled pipe materials or high-alloy steels with inferior deformability, and the manufacturing range of high-strength pipes and thin-walled pipes is expanded.
[Brief description of the drawings]
FIG. 1 is a graph schematically showing an example of a circumferential distribution of wall thickness reduction in first to fourth stands when a tube is rolled by a mandrel mill, in which FIG. ) Is for the second stand, (c) is for the third stand, and (d) is for the fourth stand.
FIG. 2 is a graph schematically showing a circumferential distribution of a thickness reduction amount for explaining tmax and θ (t).
FIG. 3 is a schematic view showing a configuration example of a mandrel mill according to the present invention.
FIG. 4 is a schematic view showing a roll hole type example of a second stand according to an embodiment of the present invention.
[Explanation of symbols]
11: Mandrel mill,
12: Raw tube,
13: Mandrel bar,
22: rolling roll,
23: Roll hole type,
24: groove bottom,
25: Groove 26: Flange
27: Rolled material.

Claims (4)

Thickness t _F of the tube after rolling by the mandrel mill and the outer diameter of the tube by a mandrel mill having a plurality of hole type roll stands and having a mandrel bar disposed in the roll hole mold formed by the hole type roll stand in the rolling method of the ratio of the _{D F (t F / D F} ) is 0.03 or less tubular, 1st grooved roll stand and of the first second grooved rolls stand from the upstream toward the downstream The parameter s1 defined by the following equation is at least −52. By the thickness reduction ratio r at the center of the groove bottom of the roll hole type of the second hole type roll stand and the average contact angle ψ of the roll hole type. A method of rolling a tube by a mandrel mill, wherein the tube is rolled under a condition of 0 or more.
s1 = −213r + ψ
However,
r = (T0−T) / T0,

T: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the outlet side of the hole-type roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the inlet side of the hole-type roll stand
t _max : Maximum value of the thickness reduction amount in the perforated roll stand (unit: mm),
θ (t): The maximum value (unit: degree) of the angle from the center of the groove bottom at the circumferential position where the thickness reduction amount is t in the perforated roll stand. In a method of rolling a tube by a mandrel mill having a plurality of perforated roll stands and having a mandrel bar disposed in a roll perforation formed by the perforated roll stand, a third perforated roll from upstream to downstream The thickness reduction ratio r at the center of the groove bottom of the roll hole type of at least one hole type roll stand of the stand and the fourth hole type roll stand and the meat at the maximum portion of the thickness reduction amount of the roll hole type Thickness reduction r _QP A tube rolling method using a mandrel mill, characterized in that the tube is rolled under the condition that the parameter s2 defined by the following formula is 1.49 or less.
s2 = 3.6r + r _QP
However,
r = (T0−T) / T0,
r _QP = (T _QP -T) / T _QP
T: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the outlet side of the hole-type roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom at the inlet side of the hole-type roll stand
T _QP : Thickness of the tube on the inlet side of the perforated roll stand at the portion where the wall thickness reduction amount is maximum in the perforated roll stand. In a method of rolling a tube by a mandrel mill having a plurality of perforated roll stands, and a mandrel bar arranged in a roll perforation formed by the perforated roll stands, the first perforated roll from upstream to downstream stand and the second second grooved roll stand, parameters s1 defined by the following equation by the average contact angle ψ of wall thickness rolling reduction rate r and the roll caliber of the groove bottom center portion of the roll caliber is -52 0.0 or more, and the thickness reduction ratio r of the roll hole type groove bottom and the roll hole type of the third hole type roll stand and the fourth hole type roll stand from upstream to downstream. A method of rolling a tube by a mandrel mill, characterized in that the tube is rolled under the condition that the parameter s2 defined by the following equation is 1.49 or less by the wall thickness reduction ratio r _{QP at} the maximum thickness reduction amount : .
s1 = −213r + ψ
s2 = 3.6r + r _QP
However,
r = (T0−T) / T0

r _QP = (T _QP −T) / T _QP
T: Thickness (unit: mm) of the tube at the center of the roll hole groove bottom on the outlet side of the hole roll stand
T0: Thickness (unit: mm) of the tube at the center of the roll hole-type groove bottom on the inlet side of the hole-type roll stand
t _max : Maximum value of the thickness reduction amount in the perforated roll stand (unit: mm),
θ (t): In the perforated roll stand, the maximum value (unit: degree) of the angle from the groove bottom central portion at the circumferential position where the thickness reduction amount is t,
T _QP : Thickness (unit: mm) of the tube on the inlet side of the perforated roll stand at the portion where the thickness reduction amount is maximum in the perforated roll stand. Tube thickness t after rolling by the mandrel mill _F And pipe outer diameter D _F Ratio to (t _F / D _F ) Is 0.03 or less. The method of rolling a tube by a mandrel mill according to any one of claims 1 to 3.

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