JP3806364B2 - Mold powder for continuous casting of steel and continuous casting method - Google Patents
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Description
【０００１】
【発明の属する技術分野】
鋼の連続鋳造用モールドパウダーおよびこのモールドパウダーを用いた鋼の連続鋳造方法に関する。
【０００２】
【従来の技術】
鋼の連続鋳造においては、鋳型内へ注入された溶鋼表面上にモールドパウダーを添加する。モールドパウダーは高温の溶鋼に加熱されて溶融し、鋳型と凝固シェルの間に流入する。モールドパウダーは主として、鋳型と凝固シェル間の潤滑、浮上してきた鋼中介在物の吸収除去、溶鋼の保温及び溶鋼の酸化防止の役割を果たす。このような機能を有するモールドパウダーは、連続鋳造プロセスにおいて重要な副資材であり、従来、ＣａＯ、Ａｌ2Ｏ3、ＳｉＯ2を基材とし、他の成分としてＮａ2Ｏ、ＮａＦ、ＣａＦ2、ＭｇＯ等を配合して、対象とする鋳片の鋳造速度等に応じた該パウダーの粘性、軟化温度、凝固温度に調整する構成になっている。
【０００３】
ところで、連続鋳造操業時には、操業条件の如何又はその変化による局所的な湯面変動に起因し、あるいは鋳型内に注入された溶鋼の注入流がメニスカス近傍で溶融パウダーと溶鋼との界面を乱し、パウダーを溶鋼中に巻き込み、凝固シェルヘ付着したりする場合がある。特に高速連続鋳造においては注入流の流速が大きくなるので、パウダーを巻き込みやすくなる。巻き込まれたパウダーが鋳片に付着したままの状態で圧延を行うと、伸展されて冷延鋼板の表面欠陥の原因となるため、連続鋳造用パウダーには、鋳造中にパウダー巻き込みを起こさない性質を有することが要請される。
【０００４】
高粘性パウダーを用いることにより、鋳型内溶鋼流動によるモールドパウダーの巻き込みを防止することができ、パウダー巻き込みに起因する欠陥発生が低減されることが知られている。特開平１０−２６３７６７号公報においては、極低炭素鋼用のモールドパウダーとして、１３００℃におけるパウダーの粘度を３ｐｏｉｓｅ以上とすることで、高速鋳造においてもモールドパウダーの巻き込みを防止できる点が記載されている。ただし、粘度が１５ｐｏｉｓｅを超えると、鋳型と凝固シェル間隙への適切なパウダーの流入ができなくなり、鋳片と鋳型との潤滑不良を生じ、ブレークアウト等の重大の操業トラブルの原因となるとし、モールドパウダーの粘度上限を１５ｐｏｉｓｅとしている。
【０００５】
特開２０００−２８００５１公報においては、パウダー粘性が増大することによる問題点として、パウダー消費量が減少し、鋳型の抜熱のばらつきが大きくなりかつ、スラグベアが出来やすいこと、パウダー流入が不均一となり、割れやブレークアウトが発生し易くなる等の欠点があり、鋳造速度に制限を設ける必要がある等の点を挙げている。同公報によると、鋳型内のメニスカス付近には溶鋼流速が遅い淀み部が存在し、パウダー流入不良が起きるのはこの淀み部であるとしている。そして、鋳型内の溶鋼に電磁力により外力を加えて流動を付与することにより淀み部を解消し、パウダーが高粘性であっても安定して鋳造可能であるとし、粘度が３〜２５ｐｏｉｓｅのパウダーを用いて電磁攪拌で流速８〜３０ｃｍ／ｓの流動を与えながらの鋳造する方法が開示されている。
【０００６】
特開平８−２６７１９５号公報においては、電磁ブレーキを用いながら鋳型内浸漬ノズルの溶鋼吐出孔角度を調整することにより、巻き込みの原因となる溶鋼流動を抑制して、粘度を高くしなくても巻き込みを抑制する等の試みが行われている。
【０００７】
【発明が解決しようとする課題】
鋳型内電磁攪拌や電磁ブレーキを適用した鋳造を行うためには設備投資が必要であり、場合によっては設備制約によってこれら設備を設置できない場合もある。従って、鋳型内電磁攪拌や電磁ブレーキを適用しなくても使用可能な高粘性パウダーが提供できれば、現状設備のままでパウダー巻き込みを防止することが可能となり、好ましい。
【０００８】
鋳型内で電磁攪拌を行うことによって溶鋼流動の淀み部は解消できるものの、高粘性パウダー使用時のパウダー流入不良は淀み部以外でも発生しており、粘度が１０ｐｏｉｓｅを超えるパウダーを使用する場合には、電磁攪拌適用時といえどもパウダー流入不良による問題を完全に解決するには至っていない。また、パウダー流入不良を起こさない程度の粘性を有するパウダーを使用した場合、電磁攪拌による溶鋼流速の増加によってかえってパウダー巻き込みが増加することが生じた。
【０００９】
電磁ブレーキ印加時にはパウダーの流入が不十分なために割れやブレークアウトが発生し易くなったり、渦や吹き込まれたガスが表面に上昇してきた際の気泡の破裂によって巻き込まれたパウダーの浮上が抑制されて欠陥が増加することが明らかになった。従って、高粘性パウダーと電磁ブレーキを同時に使用すると、ともにパウダーの流入性を悪化させる要因であるため、ブレークアウトなどのトラブルが一層発生しやすくなる。また、電磁ブレーキ使用時に粘性の低いパウダーを使用すると、パウダー巻き込みに起因する欠陥の増大を招くこととなる。
【００１０】
本発明は上述したような問題点を解消するものであって、流入性を確保しつつパウダー巻き込みが小さく、且つ、表面性状に優れた鋼材を製造するためのモールドパウダーおよび連続鋳造方法を提供することを目的とするものである。
【００１１】
【課題を解決するための手段】
即ち、本発明の要旨とするところは以下のとおりである。
（１）鋼の連続鋳造方法に使用するモールドパウダーであって、ＣａＯ、ＳｉＯ 2 及びＡｌ 2 Ｏ 3 を主成分として、ＺｒＯ 2 含有量が３質量％以上１５質量％以下で、１３００℃での粘度が１０ｐｏｉｓｅ以上１５０ｐｏｉｓｅ以下であり、かつ凝固時に固相率が４０％となる温度が９００℃以上１２００℃以下であることを特徴とする鋼の連続鋳造用モールドパウダー。
（２）炭素含有量が３質量％以下であることを特徴とする上記（１）に記載の鋼の連続鋳造用モールドパウダー。
【００１２】
（３）鋳型内電磁攪拌および、または鋳型内電磁ブレーキを用いた鋼の連続鋳造方法において上記（１）又は（２）に記載のパウダーを用いることを特徴とする鋼の連続鋳造方法。
（４）鋳型内の溶融パウダーが、ＣａＯ、ＳｉＯ 2 及びＡｌ 2 Ｏ 3 を主成分として、ＺｒＯ 2 含有量が３質量％以上１５質量％以下で、１３００℃での粘度が１０ｐｏｉｓｅ以上１５０ｐｏｉｓｅ以下であり、かつ凝固時に固相率が４０％となる温度が９００℃以上１２００℃以下であることを特徴とする鋼の連続鋳造方法。
（５）鋳型内の溶融パウダーの炭素含有量が３質量％以下であることを特徴とする上記（４）に記載の鋼の連続鋳造方法。
（６）鋳型内電磁攪拌および、または鋳型内電磁ブレーキを用いた鋼の連続鋳造方法において、鋳型内の溶融パウダーが上記（４）又は（５）に記載の溶融パウダーであることを特徴とする鋼の連続鋳造方法。
【００１３】
【発明の実施の形態】
溶鋼中へのパウダー巻き込み防止のためには、１３００℃におけるパウダー粘度が高いほど好ましい。一方、鋳型と凝固シェルの間の空間へのパウダー流入においては、水冷銅鋳型に熱を奪われるため、パウダーの温度は急速に低下する。パウダーの流入を促進するためには、パウダー温度が低下しても必要な流動性を保持している必要がある。一方、従来のパウダーにおいては、１３００℃における粘度が高いパウダーは当然のこととして１３００℃よりも低い温度における粘度が非常に高くなり、鋳型と凝固シェル間への流入性が悪化する。従来、高粘性パウダーにおいてパウダーの流入性が低下していた原因はこの点にあることが明らかになった。
【００１４】
十分に高温に加熱して溶融したパウダーの温度を低下していくと、溶融パウダー中に固相が発生する。そして、パウダーの固相率が４０％以上となると急激に粘度が上昇する。本発明においては、パウダーの固相率が４０％となる温度（固相率４０％温度）がパウダー流入性に大きな影響を及ぼすことを明らかにした。即ち、当該固相率４０％温度が１２００℃を超えるパウダーであると、鋳型と凝固シェル間に流入して温度が低下したときに溶融パウダーの流動性が劣化してパウダー流入性が劣るのに対し、固相率４０％温度が１２００℃以下であると流動性が劣化せず、良好なパウダー流入性を発揮することができる。１３００℃におけるパウダーの粘度を１０ｐｏｉｓｅ以上とすると同時に、パウダーの固相率が４０％となる温度を１２００℃以下とすることにより、パウダー巻き込みを防止するとともにパウダーの流入性を確保することが可能になる。
【００１５】
パウダー中に含有したＺｒＯ2は１３００℃の温度でも溶融せずに固相として存在し粘度を上昇する働きを有するので、パウダー中にＺｒＯ2を含有させると１３００℃におけるパウダー粘度を増大させることができる。一方、１３００℃以下の温度域でＺｒＯ2の固相比率は変化せず、即ち温度を低下してもパウダーの固相率を上昇させる因子とはならないので、ＺｒＯ2を含有させることで１３００℃での粘度を上昇させたパウダーは固相率が４０％となる温度を１２００℃以下の低温にすることが可能である。本発明においては、パウダー中にＺｒＯ2を３％以上含有することにより、１３００℃でのパウダー粘度を１０ｐｏｉｓｅ以上とすると同時に固相率４０％となる温度を１２００℃以下とすることが可能になる。パウダー中のＺｒＯ2含有量を４％以上とするとより好ましい。一方、パウダー中のＺｒＯ2含有量が１５％を超えると固相率が上昇しすぎるので、上限を１５％とする。
【００１６】
さらに、パウダーの炭素含有量を３質量％以下にすることによりパウダーの溶融を早め、溶融パウダー厚みを厚くすることによって流入を促進し、ブレークアウト等のトラブル回避効果を一層向上することが可能となる。鋳型内に投入したパウダーの溶融は、パウダー中に含有する炭素がある程度燃焼してから進行する。パウダーの炭素含有量が３質量％以下の場合、鋳型内にパウダーを投入してから溶融するまでの所要時間を短くして迅速に溶融させることが可能になるからである。パウダーの炭素含有量は２．１質量％以下であるとより好ましい。
【００１７】
この発明において、モールドパウダーの１３００℃における粘度を１０〜１５０ｐｏｉｓｅとするのは次の理由による。鋳型短辺に衝突して上方へ向く溶鋼の流れや浸漬ノズルの詰まりを防止する目的で溶鋼内に吹き込まれたガスは、メニスカス上の溶融パウダーと溶鋼との界面を乱し、パウダーを溶鋼中に巻き込み、鋳片内へのパウダー性介在物欠陥を生起させる。この傾向は鋳造速度やガス吹き込み量の増大やモールドパウダーの溶融粘度が小さい程大きい。そこで、この発明ではモールドパウダーの粘度を１０ｐｏｉｓｅ以上とすることで、いかなる鋳造速度、ガス吹き込み量においても、上記のようなモールドパウダーの巻き込みを防止できる。もっとも、粘度が１５０ｐｏｉｓｅを超えると、鋳型と凝固シェル間隙への適正なパウダーの流入ができなくなり、鋳片と鋳型との潤滑不良を生じ、ブレークアウト等の重大な操業トラブルの発生原因となる。したがって、モールドパウダーの粘度は１０〜１５０ｐｏｉｓｅとする。パウダーの粘度は１５ｐｏｉｓｅ以上であればより好ましい。
【００１８】
凝固時に固相率が４０％となる温度を１２００℃以下とした第１の理由は、上述のとおり鋳型と凝固シェル間へのパウダー流入性を改善する点にある。また、固相率が４０％となる温度が１２００℃を超えると、鋳型と凝固シェル間の摩擦力が増大し、鋳型直下で凝固シェルが引き裂かれたり、破断したりする現象が発生するために好ましくない。固相率が４０％となる温度を１１６０℃以下とするとより好ましい。一方、固相率が４０％となる温度が９００℃未満であると、スラグフィルムの厚みが不均一になるために好ましくない。
【００１９】
この発明の連続鋳造用モールドパウダーは、ＣａＯ、ＳｉＯ2及びＡｌ2Ｏ3を主成分とするものであり、凝固温度や粘度等を調整するために、例えばＢ2Ｏ3、ＭｇＯ、Ｆ、Ｎａ2Ｏ、Ｌｉ2Ｏ等を数％程度で添加する。これらの成分の添加量は、この発明で規定している要件を具備する範囲内で目的に応じ所望量を任意に選べばよい。なお、これらの添加物は、従来のモールドパウダーに含まれているものである。
【００２０】
以上のとおりであるから、鋼の連続鋳造において本発明のモールドパウダーを使用する場合には、鋳型内電磁攪拌や電磁ブレーキを使用しない場合であっても、パウダー巻き込みの防止とパウダー流入の適正化を同時に実現することができ、ブレークアウトなどのトラブルを防止することができる。
【００２１】
本発明においてはさらに、本発明のモールドパウダーを使用しつつ鋳型内の電磁攪拌を用いると好ましい。電磁攪拌によって、鋳型内への溶鋼の供給に伴う必然的に生じる溶鋼流動とは独立に鋳型内溶鋼に流動を付与することで、熱の供給不足の部位に熱を供給し均一化することによって、パウダーの流入を促進して、高粘度パウダーを用いてより高速での鋳造が可能となる。また、電磁攪拌によってメニスカス近傍の溶鋼流れの淀み部をなくすことができ、淀み部起因のパウダー流入不良をも解消することができる。本発明では十分に粘性の高いパウダーを使用することが可能なので、電磁攪拌によって溶鋼流動が増大してもパウダー巻き込みが増えるおそれはない。
【００２２】
電磁攪拌を行うとともに本発明の高粘度のパウダーを用いることで、パウダーの巻き込みなしに攪拌推力を増加させて溶鋼流速を上げることが可能となり、鋳型内に侵入してきた介在物の凝固シェルへの捕捉防止が可能となり、パウダー系以外の欠陥、例えばタンディッシュから持ち込まれたスラグやアルミナ等の介在物も低減する。
【００２３】
鋳型内に電磁攪拌用のコイルを設置し、溶鋼表面近傍、望ましくは溶鋼表面位置から凝固シェル厚みが５ｍｍとなる位置までの凝固シェル前面近傍の溶鋼流速を２０ｃｍ／ｓ以上、望ましくは４０ｃｍ／ｓ以上となるように攪拌を行う。一方向に連続あるいは断続させて攪拌しても良いし、攪拌方向を時間的に反転させて攪拌しても良い。
【００２４】
本発明では熱供給促進の手段として、鋳型内溶鋼攪拌装置を例にとって説明したが、加熱装置、ヒーターによって溶鋼あるいは連鋳パウダーを加熱することによっても可能である。
【００２５】
鋳型内電磁ブレーキ使用時に本発明の流入性のよい高粘度パウダーを用いることによって好ましい結果を得ることができる。本発明のパウダーは流入性が優れているので、電磁ブレーキの印加時のパウダーの流入不良の発生を抑制して割れやブレークアウトの発生を防止することが可能となり、より高速での鋳造が可能となる。また本発明のパウダーは高粘性であるため、渦や吹き込まれたガスが表面に上昇してきた際の気泡の破裂があってもパウダーが巻き込まれにくく、巻き込まれたパウダーの浮上が抑制されて欠陥が増加するという電磁ブレーキの不利な点を解消することができる。さらに電磁ブレーキによる浸漬ノズルからの下降流速の低減によって介在物の鋳型内への侵入を抑制して、パウダー系以外の欠陥も低減する。
【００２６】
鋳型内に電磁ブレーキ用のコイルを設置し、鋳型下部、望ましくは浸漬ノズルの吐出孔位置からモールド下端位置までの溶鋼に０．０５Ｔ以上、好ましくは０．１５Ｔ以上の磁場を印加する。磁場は幅方向の一部分でも良いが、全幅に渡って印加されていることが望ましい。
電磁攪拌と電磁ブレーキを併用しても良い。
【００２７】
パウダーは鋳造中に溶鋼との反応や溶鋼中から浮上してきた介在物の吸収によって変質する。従って、使用前のモールドパウダーの粘度、固相率が４０％になる温度、組成を所定の範囲に調整するのみならず、鋳型内の溶融パウダーの粘度、固相率が４０％になる温度、組成を所定の範囲とすることが望ましい。
【００２８】
固相率４０％温度を調整するためにパウダー中に含有する成分は、融点を下げずに溶融パウダー中に固相粒子として存在する物質であればＺｒＯ2以外の物質であってもよい。すなわち、融点を下げずに溶融パウダー中に固相粒子として存在する物質を含有し、１３００℃での粘度が１０ｐｏｉｓｅ以上１５０ｐｏｉｓｅ以下であり、かつ凝固時に固相率が４０％となる温度が９００℃以上１２００℃以下であることを特徴とする鋼の連続鋳造用モールドパウダーまたは鋳型内の溶融パウダーであればよい。
【００２９】
炭素濃度が０．０５質量％以下の低炭素鋼においては、特に鋳片や製品での欠陥が発生しやすいので、本発明のモールドパウダーの使用、特に鋳型内電磁攪拌や電磁ブレーキとの併用によって高い効果を得ることができる。
【００３０】
【実施例】
転炉にて溶製した溶鋼３００ｔｏｎを、ＲＨにて所定の成分濃度に調整した極低炭素鋼の溶鋼を、タンディッシュ、浸漬ノズルを介して垂直曲げ型の連続鋳造機で、厚み２５０ｍｍ、幅１２００ｍｍの鋳片に鋳造した。鋳造速度を１．５ｍ／ｍｉｎとし、鋳型内には電磁攪拌装置および電磁ブレーキを設置し、必要に応じてこれらを印加した。鋳造条件は表１に示した条件である。
【００３１】
【表１】
鋳造には１〜３００ｐｏｉｓｅの粘性のパウダーを使用した。パウダーの粘性は１３００℃での粘性を用いた。粘度測定にあたっては、回転円筒法を用いた。測定対象パウダーを７００℃にて６０分間脱炭処理した試料を黒鉛坩堝に挿入し１４００℃にて１０〜１５分間予備溶解した後鉄坩堝に移し、縦型管状炉（エレマ炉）に入れ、Ｅ型粘度計のローターをスラグ中に浸漬し、１３００℃で３０分間安定させた後、ローターを回転させ粘性抵抗によるトルクを測定し、粘度を求めた。なおＥ型粘度計は事前に標準粘度液にて較正しておく。
【００３３】
使用したモールドパウダーは、凝固時に固相率が４０％となる温度が７００℃〜１３００℃の範囲とした。固相率が４０％となる温度の測定に当たっては、まず石英るつぼ内で使用前あるいは鋳型内から採取したパウダーを１４００℃の温度で溶解し、その後所定の温度まで温度を下げて１０分間保持した後、急冷して凝固させる。凝固後の試料を切断し、断面を研磨して顕微鏡で観察する。急冷前に溶融していた部分はガラス質となっており、急冷前から固相だった部分はガラス質ではなく結晶質なので、ガラス質以外の部分の面積を測定すればそれが当該所定温度における固相率となる。固相率４０％付近における温度と固相率の測定結果から、固相率が４０％となる温度を求めることができる。
【００３４】
鋳型内電磁攪拌及び電磁ブレーキは電流値を変化させて、攪拌推力および磁場強度を変化させた。溶鋼の流速を歪みゲージを張り付けた耐火物を溶鋼中に浸漬することにより測定し、０〜１００ｃｍ／ｓまで変化させた。電磁ブレーキの磁場強度は０〜２Ｔまで変化させた。
【００３５】
鋳造中の鋳型内の溶融パウダーを採取し、組成を分析するとともに１３００℃における粘度、および固相率が４０％となる温度を測定した。
【００３６】
【表２】
鋳造して得られた鋳片を常法にて熱延・酸洗・冷延・焼鈍して自動車用の薄鋼板とし、表面を観察して表面疵を調査するとともに、プレス加工を行い割れの発生を検査した。
【００３８】
製造結果を表２に示す。表２において、表面疵およびプレス割れの発生率は、「１コイル内の表面疵あるいはプレス割れの個数÷コイル長（ｍ）×１００」を百分率で示した。また、表面疵とプレス割れそれぞれについて、欠陥部の介在物の組成を分析してパウダー起因とパウダー以外起因とに分類している。ブレークアウト予検知発生率は、鋳型内縦方向４箇所に熱電対を埋め込み、鋳型下部の熱電対温度が鋳型上部の熱電対温度より高くなった場合に、ブレークアウトに至る可能性があると判断して、鋳造を一時中断することが１チャージの鋳造の間に発生した回数を回／チャージで示した。鋳片表面疵発生率は、「表面疵が発生した鋳片数÷全鋳片数×１００」で示した。
【００３９】
表１、２のＮｏ．１〜８が本発明例である。モールドパウダー中のＺｒＯ2含有量を３〜１５質量％とすることにより、１３００℃での粘度１０〜１５０ｐｏｉｓｅと固相率４０％温度９００〜１２００℃とを両立させることができた。
【００４０】
本発明例Ｎｏ．１〜８においては、本発明の高粘性であってかつ固相率４０％温度が１２００℃以下のモールドパウダーを使用した結果として、製品板表面疵に占めるパウダー起因疵の発生率が従来の低粘性パウダーを用いた比較例Ｎｏ．９に比べ激減していると同時に、プレス加工時の割れの発生率が低減しており、さらにパウダー流入性が改善されているためにブレークアウト予検知がなかった。
【００４１】
本発明例Ｎｏ．１、３〜５、７は、電磁攪拌装置によりメニスカス近傍に浸漬ノズルからの溶鋼吐出流と独立に旋回流速を付与した結果、鋳片表層の介在物も低減されてパウダー以外の欠陥の発生率も低減している。
【００４２】
また、本発明例Ｎｏ．４〜８は本発明パウダーと電磁ブレーキとを併用しており、電磁ブレーキを印加してもブレークアウト予検知等のトラブルなく鋳造が可能であり、電磁ブレーキによって介在物の鋳型内部への侵入が抑制されて、パウダー以外の介在物が原因のプレス割れの発生率も低減している。
【００４３】
本発明例Ｎｏ．４、５、７は電磁攪拌と電磁ブレーキとを併用しつつ本発明パウダーを使用した例であり、最も良好な結果を得ることができた。
【００４４】
Ｎｏ．９〜１４が比較例である。比較例Ｎｏ．９はパウダー粘度が本発明範囲より低いので、ブレークアウト予検知の発生や鋳片表面疵発生はないものの、パウダー起因の表面疵やプレス割れが高い値を示している。比較例Ｎｏ．１０はパウダー粘度が高すぎ、パウダー潤滑不良に起因したブレークアウト予検知の発生が見られた。比較例Ｎｏ．１１はパウダー粘度は本発明範囲内にあるものの、固相率４０％温度が本発明範囲より高く、パウダー流入性不良に起因してブレークアウト予検知の発生が見られた。比較例Ｎｏ．１２は固相率４０％温度が低すぎ、スラグフィルム厚さ不均一に起因して鋳片表面疵の発生が見られると同時に、パウダー以外起因の表面疵やプレス割れが不良であった。比較例Ｎｏ．１３、１４は、１３００℃での粘性が２０ｐｏｉｓｅと高いものの固相率４０％温度が１２１０℃と本発明範囲上限を外れる例であり、パウダー巻き込みは低減しているもののパウダー流入特性が十分ではなく、鋳型内電磁攪拌を行っていないＮｏ．１４はブレークアウト予検知発生率が１．０回／チャージと高い値であり、鋳型内電磁攪拌を行っているＮｏ．１３でもブレークアウト予検知発生率が０．５回／チャージの値となっている。
【００４５】
本実施例においては垂直曲げ型連続鋳造設備を使用したが、湾曲型及び垂直型連続鋳造設備においても同様の効果が得られる。
【００４６】
また、本実施例では自動車用の薄鋼板用の鋳片製造にあたっての例で述べたが、本技術の本質とするところは、パウダーの巻き込みによって生じる欠陥を防止することであり、缶用鋼板、鋼管など他の鋼種の鋳片を製造する場合にも有効である。
【００４７】
【発明の効果】
本発明のモールドパウダーを使用して連続鋳造を行うことにより、パウダー流入性不良によるトラブルを防止しつつ、介在物やパウダーの巻き込みに起因する介在物によって発生する加工時の割れや線状疵の発生が少ない表面性状や加工性の優れた薄鋼板を製造するための鋼材を提供できる。そのようにして提供した鋼材を用いて冷延鋼板を製造できるのは勿論のこと、焼鈍後に電気亜鉛めっきや合金化電気亜鉛めっき鋼板として、またさらに、有機被覆鋼板の原板を製造することもできる。また、連続焼鈍条件が満たされる限り連続焼鈍溶融亜鉛めっき、合金化溶融亜鉛めっき用鋼板用鋼材としても使用可能である。従って、家庭電気製品や自動車、缶等の広い用途に適用できるため、産業上に与える効果は極めて大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a mold powder for continuous casting of steel and a continuous casting method of steel using this mold powder.
[0002]
[Prior art]
In continuous casting of steel, mold powder is added onto the surface of the molten steel poured into the mold. The mold powder is heated and melted by high-temperature molten steel and flows between the mold and the solidified shell. The mold powder mainly plays a role of lubrication between the mold and the solidified shell, absorbing and removing inclusions in the steel that have floated up, keeping the molten steel warm and preventing oxidation of the molten steel. Mold powder having such a function is an important auxiliary material in the continuous casting process. Conventionally, CaO, Al 2 O 3 and SiO 2 are used as base materials, and Na 2 O, NaF, CaF 2 , and the like are used as other components. MgO etc. are mix | blended and it is the structure adjusted to the viscosity of this powder according to the casting speed etc. of the target slab, the softening temperature, and the solidification temperature.
[0003]
By the way, during continuous casting operation, the molten steel injected flow into the mold is disturbed at the interface between the molten powder and molten steel due to local fluctuations in the molten metal surface due to any or changes in the operating conditions. In some cases, the powder is entangled in the molten steel and adheres to the solidified shell. In particular, in high-speed continuous casting, the flow rate of the injection flow increases, so that it becomes easy to entrain powder. If rolling is performed while the entrained powder is still attached to the slab, it will be stretched and cause surface defects in the cold-rolled steel sheet. Is required to have
[0004]
It is known that by using a highly viscous powder, it is possible to prevent mold powder from being entrained due to molten steel flow in the mold, and to reduce the occurrence of defects due to powder entrainment. Japanese Patent Application Laid-Open No. 10-263767 describes that, as a mold powder for ultra-low carbon steel, the powder powder at 1300 ° C. has a viscosity of 3 poise or more, so that entrainment of mold powder can be prevented even in high-speed casting. Yes. However, if the viscosity exceeds 15 poise, the proper powder cannot flow into the mold and the solidified shell gap, resulting in poor lubrication between the slab and the mold, causing serious operational troubles such as breakout, The upper limit of the viscosity of the mold powder is 15 poise.
[0005]
In Japanese Patent Laid-Open No. 2000-280051, the problems caused by the increase in powder viscosity are that the powder consumption is reduced, the variation in heat removal from the mold is increased, the slag bear is easily formed, and the powder inflow is uneven. However, there are drawbacks such that cracks and breakout are likely to occur, and it is necessary to limit the casting speed. According to the publication, there is a stagnation portion where the molten steel flow velocity is slow in the vicinity of the meniscus in the mold, and it is this stagnation portion where the powder inflow failure occurs. And, by applying an external force to the molten steel in the mold by applying an electromagnetic force to flow, the stagnation part is eliminated, and even if the powder is highly viscous, it can be stably cast, and the powder has a viscosity of 3 to 25 poise. Is used to cast while applying a flow rate of 8 to 30 cm / s by electromagnetic stirring.
[0006]
In JP-A-8-267195, by adjusting the molten steel discharge hole angle of the immersion nozzle in the mold while using an electromagnetic brake, the molten steel flow causing the entrainment is suppressed, and the entrainment is performed without increasing the viscosity. Attempts have been made to suppress this.
[0007]
[Problems to be solved by the invention]
In order to perform casting using electromagnetic stirring in the mold or electromagnetic brake, capital investment is required, and in some cases, these facilities may not be installed due to equipment constraints. Therefore, if a highly viscous powder that can be used without applying electromagnetic stirring or electromagnetic brake in the mold can be provided, it is possible to prevent entrainment of the powder with the existing equipment, which is preferable.
[0008]
Although the stagnation part of the molten steel flow can be eliminated by electromagnetic stirring in the mold, powder inflow failure when using highly viscous powder has occurred in other parts than the stagnation part, and when using powder with a viscosity exceeding 10 poise However, even when electromagnetic stirring is applied, the problem due to poor powder inflow has not been solved completely. In addition, when powder having a viscosity that does not cause poor powder inflow is used, powder entrainment is increased due to an increase in molten steel flow rate due to electromagnetic stirring.
[0009]
Insufficient flow of powder when electromagnetic brake is applied, breakage and breakout are likely to occur, and vortex and blown gas rising to the surface suppresses entrainment of powder entrained by bubble burst It has become clear that defects will increase. Therefore, if a high-viscosity powder and an electromagnetic brake are used at the same time, both of them are factors that deteriorate the inflow of the powder, so that troubles such as breakout are more likely to occur. In addition, if powder with low viscosity is used when using an electromagnetic brake, defects due to powder entrainment will be increased.
[0010]
The present invention solves the problems as described above, and provides a mold powder and a continuous casting method for producing a steel material having a small powder entrainment and excellent surface properties while ensuring inflow properties. It is for the purpose.
[0011]
[Means for Solving the Problems]
That is, the gist of the present invention is as follows.
(1) A mold powder used in a continuous casting method of steel, comprising CaO, SiO 2 and Al 2 O 3 as main components, with a ZrO 2 content of 3% by mass to 15% by mass, at 1300 ° C. A mold powder for continuous casting of steel, having a viscosity of 10 poise to 150 poise, and a temperature at which the solid phase ratio becomes 40% during solidification is 900 ° C to 1200 ° C.
(2) The mold powder for continuous casting of steel according to (1) above, wherein the carbon content is 3% by mass or less.
[0012]
(3) A continuous casting method for steel, wherein the powder according to (1) or (2) is used in a continuous casting method for steel using in-mold electromagnetic stirring and / or in-mold electromagnetic brake.
(4) The molten powder in the mold is mainly composed of CaO, SiO 2 and Al 2 O 3 , the ZrO 2 content is 3 mass% or more and 15 mass% or less, and the viscosity at 1300 ° C. is 10 poise or more and 150 poise or less. A continuous casting method for steel, characterized in that the temperature at which the solid fraction becomes 40% during solidification is 900 ° C. or higher and 1200 ° C. or lower.
(5) The continuous casting method for steel as described in (4) above, wherein the carbon content of the molten powder in the mold is 3% by mass or less.
(6) In the continuous casting method of steel using in-mold electromagnetic stirring and / or in-mold electromagnetic brake, the molten powder in the mold is the molten powder described in (4) or (5) above. Steel continuous casting method.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In order to prevent powder entrainment in molten steel, the higher the powder viscosity at 1300 ° C., the better. On the other hand, when the powder flows into the space between the mold and the solidified shell, the temperature of the powder rapidly decreases because the water-cooled copper mold loses heat. In order to promote the inflow of the powder, it is necessary to maintain the necessary fluidity even when the powder temperature is lowered. On the other hand, in a conventional powder, a powder having a high viscosity at 1300 ° C. naturally has a very high viscosity at a temperature lower than 1300 ° C., and the inflow property between the mold and the solidified shell is deteriorated. It has been clarified that this is the reason why the flowability of the powder has decreased in the past.
[0014]
When the temperature of the melted powder is lowered by heating to a sufficiently high temperature, a solid phase is generated in the melted powder. And when the solid-phase rate of a powder will be 40% or more, a viscosity will rise rapidly. In the present invention, it has been clarified that the temperature at which the solid phase ratio of the powder reaches 40% (solid phase ratio 40% temperature) has a great influence on the powder inflow. That is, when the powder has a solid phase ratio of 40% and a temperature exceeding 1200 ° C., the flowability of the molten powder deteriorates when the temperature is lowered between the mold and the solidified shell and the powder flowability is poor. On the other hand, when the temperature of the solid phase ratio is 40% or less, the fluidity is not deteriorated and good powder inflow property can be exhibited. By setting the powder viscosity at 1300 ° C. to 10 poise or higher and simultaneously setting the temperature at which the solid phase ratio of the powder is 40% to 1200 ° C. or lower, it is possible to prevent powder entrainment and ensure the inflow of powder. Become.
[0015]
Since ZrO 2 contained in the powder does not melt even at a temperature of 1300 ° C. and exists as a solid phase and has a function of increasing the viscosity, inclusion of ZrO 2 in the powder increases the powder viscosity at 1300 ° C. it can. Meanwhile, the solid phase ratio of ZrO 2 in a temperature range of 1300 ° C. or less unchanged, that is, even when the temperature was reduced not a factor to increase the solid fraction of the powder, 1300 ° C. by the inclusion of ZrO 2 The powder having an increased viscosity at can make the temperature at which the solid phase ratio becomes 40% be a low temperature of 1200 ° C. or lower. In the present invention, by containing 3% or more of ZrO 2 in the powder, the powder viscosity at 1300 ° C. can be made 10 poise or more, and at the same time, the temperature at which the solid phase ratio becomes 40% can be made 1200 ° C. or less. . More preferably, the ZrO 2 content in the powder is 4% or more. On the other hand, if the ZrO 2 content in the powder exceeds 15%, the solid phase ratio increases too much, so the upper limit is made 15%.
[0016]
Furthermore, by making the carbon content of the powder 3% by mass or less, the melting of the powder is accelerated, and by increasing the thickness of the molten powder, the inflow is promoted and the trouble avoidance effect such as breakout can be further improved. Become. The melting of the powder put into the mold proceeds after the carbon contained in the powder burns to some extent. This is because, when the carbon content of the powder is 3% by mass or less, it is possible to shorten the time required from the introduction of the powder into the mold to the melting and to quickly melt the powder. The carbon content of the powder is more preferably 2.1% by mass or less.
[0017]
In the present invention, the viscosity of the mold powder at 1300 ° C. is set to 10 to 150 poise for the following reason. The gas blown into the molten steel to prevent clogging of the molten steel flowing upward and colliding with the mold short side disturbs the interface between the molten powder on the meniscus and the molten steel, causing the powder to enter the molten steel. Engulfed into the slab, causing powdery inclusion defects in the slab. This tendency increases as the casting speed and gas blowing amount increase and the melt viscosity of the mold powder decreases. Therefore, in the present invention, by setting the viscosity of the mold powder to 10 poise or more, it is possible to prevent the mold powder from being entrained at any casting speed and gas blowing amount. However, if the viscosity exceeds 150 poise, proper powder cannot flow into the gap between the mold and the solidified shell, resulting in poor lubrication between the slab and the mold, causing a serious operational trouble such as breakout. Therefore, the viscosity of the mold powder is 10 to 150 poise. The viscosity of the powder is more preferably 15 poise or more.
[0018]
The first reason for setting the temperature at which the solid fraction becomes 40% during solidification to 1200 ° C. or less is to improve the powder inflow property between the mold and the solidified shell as described above. In addition, when the temperature at which the solid fraction becomes 40% exceeds 1200 ° C., the frictional force between the mold and the solidified shell increases, and the solidified shell is torn or broken immediately below the mold. It is not preferable. More preferably, the temperature at which the solid fraction becomes 40% is 1160 ° C. or lower. On the other hand, if the temperature at which the solid phase ratio becomes 40% is less than 900 ° C., the thickness of the slag film becomes uneven, which is not preferable.
[0019]
The mold powder for continuous casting according to the present invention is mainly composed of CaO, SiO 2 and Al 2 O 3. For adjusting the solidification temperature, viscosity, etc., for example, B 2 O 3 , MgO, F, Na 2 O, Li 2 O or the like is added at about several percent. The amount of these components to be added may be arbitrarily selected according to the purpose within the range satisfying the requirements defined in the present invention. These additives are included in conventional mold powders.
[0020]
As described above, when using the mold powder of the present invention in continuous casting of steel, prevention of powder entrainment and optimization of powder inflow even when electromagnetic stirring and electromagnetic brake in the mold are not used. Can be realized simultaneously, and troubles such as breakout can be prevented.
[0021]
In the present invention, it is further preferable to use electromagnetic stirring in the mold while using the mold powder of the present invention. By applying magnetic flow to the molten steel in the mold independently of the inevitably generated molten steel flow accompanying the supply of molten steel into the mold by electromagnetic stirring, by supplying heat to the insufficient heat supply site and making it uniform By promoting the inflow of the powder, it becomes possible to perform casting at a higher speed using the high viscosity powder. Moreover, the stagnation part of the molten steel flow in the vicinity of the meniscus can be eliminated by electromagnetic stirring, and the powder inflow failure caused by the stagnation part can be eliminated. In the present invention, a sufficiently viscous powder can be used. Therefore, even if the molten steel flow is increased by electromagnetic stirring, there is no possibility that the powder entrainment will increase.
[0022]
By performing the magnetic stirring and using the high viscosity powder of the present invention, it becomes possible to increase the stirring thrust without entraining the powder and increase the flow velocity of the molten steel, and the inclusions that have penetrated into the mold to the solidified shell Capturing can be prevented, and defects other than the powder system, for example, inclusions such as slag and alumina brought in from the tundish are reduced.
[0023]
A coil for electromagnetic stirring is installed in the mold, and the molten steel flow velocity in the vicinity of the molten steel surface, preferably from the surface of the molten steel to the position where the thickness of the solidified shell reaches 5 mm, is 20 cm / s or more, preferably 40 cm / s. Stirring is carried out so as to achieve the above. The stirring may be performed continuously or intermittently in one direction, or the stirring direction may be reversed in time.
[0024]
In the present invention, the molten steel stirring device in the mold has been described as an example of the means for promoting heat supply. However, it is also possible to heat molten steel or continuous cast powder with a heating device and a heater.
[0025]
A favorable result can be obtained by using the high-viscosity powder of the present invention when the electromagnetic brake in the mold is used. Since the powder of the present invention has excellent inflow properties, it is possible to prevent the occurrence of cracks and breakouts by suppressing the occurrence of powder inflow failure when applying an electromagnetic brake, enabling higher speed casting It becomes. In addition, since the powder of the present invention is highly viscous, even if there is a burst of bubbles when the vortex or the blown gas rises on the surface, the powder is difficult to be caught, and the floating of the caught powder is suppressed and the defect is suppressed. It is possible to eliminate the disadvantage of the electromagnetic brake that increases. Further, by reducing the descending flow velocity from the immersion nozzle by the electromagnetic brake, the inclusions are prevented from entering the mold, and defects other than the powder system are also reduced.
[0026]
A coil for electromagnetic brake is installed in the mold, and a magnetic field of 0.05 T or more, preferably 0.15 T or more is applied to the molten steel from the lower part of the mold, desirably from the discharge hole position of the immersion nozzle to the lower end position of the mold. The magnetic field may be a part in the width direction, but it is desirable that the magnetic field be applied over the entire width.
You may use electromagnetic stirring and an electromagnetic brake together.
[0027]
Powder changes in quality due to reaction with molten steel during casting and absorption of inclusions floating from the molten steel. Therefore, the viscosity of the mold powder before use, the temperature at which the solid fraction becomes 40%, not only the composition is adjusted to a predetermined range, but also the viscosity of the molten powder in the mold, the temperature at which the solid fraction becomes 40%, It is desirable to keep the composition within a predetermined range.
[0028]
The component contained in the powder for adjusting the temperature of the solid phase rate of 40% may be a substance other than ZrO 2 as long as it is a substance that exists as solid phase particles in the molten powder without lowering the melting point. That is, it contains a substance present as solid phase particles in the molten powder without lowering the melting point, the viscosity at 1300 ° C. is 10 poise or more and 150 poise or less, and the temperature at which the solid phase rate becomes 40% during solidification is 900 ° C. It may be a mold powder for continuous casting of steel or a molten powder in a mold characterized by being 1200 ° C. or lower.
[0029]
In low-carbon steel with a carbon concentration of 0.05% by mass or less, defects in slabs and products are particularly likely to occur. Therefore, by using the mold powder of the present invention, particularly in combination with electromagnetic stirring and electromagnetic brake in the mold. High effect can be obtained.
[0030]
【Example】
300ton of molten steel melted in a converter, adjusted to a predetermined component concentration with RH, is a continuous casting machine of vertical bending type through a tundish and immersion nozzle, with a thickness of 250mm, width Cast into 1200 mm slabs. The casting speed was 1.5 m / min, an electromagnetic stirrer and an electromagnetic brake were installed in the mold, and these were applied as necessary. The casting conditions are those shown in Table 1.
[0031]
[Table 1]
For casting, a viscous powder of 1 to 300 poise was used. The viscosity at 1300 ° C. was used as the viscosity of the powder. In measuring the viscosity, the rotating cylinder method was used. A sample obtained by decarburizing the powder to be measured at 700 ° C. for 60 minutes was inserted into a graphite crucible, preliminarily melted at 1400 ° C. for 10 to 15 minutes, transferred to an iron crucible, placed in a vertical tubular furnace (Elema furnace), and E The rotor of the type viscometer was immersed in slag and stabilized at 1300 ° C. for 30 minutes, and then the rotor was rotated to measure the torque due to viscous resistance to determine the viscosity. The E-type viscometer is calibrated with a standard viscosity solution in advance.
[0033]
The mold powder used was in the range of 700 ° C. to 1300 ° C. at which the solid fraction becomes 40% during solidification. In measuring the temperature at which the solid phase ratio becomes 40%, first, the powder collected from the quartz crucible before use or from the mold was melted at a temperature of 1400 ° C., then the temperature was lowered to a predetermined temperature and held for 10 minutes. Then, it is rapidly cooled and solidified. The sample after solidification is cut, the cross section is polished and observed with a microscope. The part melted before quenching is glassy, and the part that was solid before quenching is crystalline, not glassy, so if you measure the area of the part other than glassy, It becomes a solid phase rate. The temperature at which the solid phase ratio becomes 40% can be determined from the measurement result of the temperature and the solid phase ratio in the vicinity of the solid phase ratio of 40%.
[0034]
The electromagnetic stirring and electromagnetic brake in the mold changed the current value, and changed the stirring thrust and the magnetic field strength. The flow rate of the molten steel was measured by immersing a refractory with a strain gauge attached thereto in the molten steel, and was varied from 0 to 100 cm / s. The magnetic field intensity of the electromagnetic brake was changed from 0 to 2T.
[0035]
The molten powder in the casting mold was sampled, the composition was analyzed, and the viscosity at 1300 ° C. and the temperature at which the solid fraction was 40% were measured.
[0036]
[Table 2]
The slab obtained by casting is hot rolled, pickled, cold rolled, and annealed by a conventional method to form a thin steel sheet for automobiles. The occurrence was examined.
[0038]
The production results are shown in Table 2. In Table 2, the occurrence rate of surface flaws and press cracks was expressed as a percentage of “number of surface flaws or press cracks in one coil ÷ coil length (m) × 100”. In addition, the surface flaws and press cracks are classified into those caused by powder and those other than powder by analyzing the composition of inclusions in the defective portion. Breakout pre-detection rate is determined that there is a possibility of breakout when thermocouples are embedded in four vertical locations in the mold and the thermocouple temperature at the bottom of the mold is higher than the thermocouple temperature at the top of the mold. The number of times that the casting was temporarily interrupted during one charge casting was expressed in times / charge. The slab surface flaw occurrence rate is indicated by “the number of slabs with surface flaws divided by the total number of slabs × 100”.
[0039]
No. in Tables 1 and 2. 1-8 are examples of the present invention. By setting the ZrO 2 content in the mold powder to 3 to 15% by mass, it was possible to achieve both a viscosity of 10 to 150 poise at 1300 ° C. and a solid phase ratio of 40% and a temperature of 900 to 1200 ° C.
[0040]
Invention Example No. 1-8, as a result of using the mold powder of the present invention having a high viscosity and a solid phase rate of 40% and a temperature of 1200 ° C. or less, the rate of occurrence of powder-induced wrinkles in the product plate surface wrinkles is low Comparative Example No. using viscous powder At the same time, it was drastically reduced compared to 9, and the occurrence rate of cracks during press working was reduced, and further, the powder flowability was improved, so there was no breakout detection.
[0041]
Invention Example No. Nos. 1, 3 to 5 and 7 show that the rate of occurrence of defects other than powder is reduced as a result of imparting a swirling flow velocity independently of the molten steel discharge flow from the immersion nozzle in the vicinity of the meniscus using an electromagnetic stirrer. Has also been reduced.
[0042]
In addition, Invention Example No. Nos. 4-8 use the powder of the present invention in combination with an electromagnetic brake, and can be cast without any troubles such as breakout pre-detection even when the electromagnetic brake is applied. As a result, the incidence of press cracks caused by inclusions other than powder is also reduced.
[0043]
Invention Example No. Nos. 4, 5, and 7 are examples in which the powder of the present invention was used while using both electromagnetic stirring and electromagnetic braking, and the best results could be obtained.
[0044]
No. 9 to 14 are comparative examples. Comparative Example No. No. 9 shows a high value of powder-induced surface flaws and press cracks, although the powder viscosity is lower than the range of the present invention, so that breakout pre-detection and slab surface flaws do not occur. Comparative Example No. In No. 10, the powder viscosity was too high, and occurrence of breakout predetection due to poor powder lubrication was observed. Comparative Example No. No. 11 had a powder viscosity within the range of the present invention, but the solid phase ratio was 40% higher than the range of the present invention, and the occurrence of breakout pre-detection was observed due to poor powder inflow. Comparative Example No. No. 12 had a solid phase ratio of 40% and the temperature was too low, and the occurrence of slab surface flaws was observed due to non-uniform slag film thickness. Comparative Example No. 13 and 14 are examples in which the viscosity at 1300 ° C. is as high as 20 poise, but the solid phase rate is 40%, and the temperature is 1210 ° C., which is outside the upper limit of the present invention. No. No electromagnetic stirring in the mold. No. 14 has a high breakout pre-detection occurrence rate of 1.0 times / charge, and No. 14 performing electromagnetic stirring in the mold. 13 also has a breakout pre-detection rate of 0.5 times / charge.
[0045]
Although the vertical bending type continuous casting equipment is used in this embodiment, the same effect can be obtained in the curved type and vertical type continuous casting equipment.
[0046]
Further, in this example, the example of manufacturing a slab for a thin steel sheet for automobiles was described, but the essence of the present technology is to prevent defects caused by entrainment of powder, It is also effective when producing slabs of other steel types such as steel pipes.
[0047]
【The invention's effect】
By performing continuous casting using the mold powder of the present invention, while preventing troubles due to poor powder influx, cracks and line defects caused by inclusions and inclusions caused by the inclusion of powder It is possible to provide a steel material for producing a thin steel sheet having excellent surface properties and workability with less generation. It is possible to produce cold-rolled steel sheets using the steel materials thus provided, as well as electrogalvanized and alloyed electrogalvanized steel sheets after annealing, and also to produce organic coated steel sheets. . Moreover, as long as the continuous annealing conditions are satisfied, it can also be used as a steel material for steel plates for continuous annealing hot dip galvanizing and alloying hot dip galvanizing. Therefore, since it can be applied to a wide range of uses such as household electric appliances, automobiles, and cans, the effect on the industry is extremely large.
Claims (6)
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