JP3689375B2 - Flux for preventing reoxidation and continuous casting method using the same - Google Patents

Flux for preventing reoxidation and continuous casting method using the same Download PDF

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JP3689375B2
JP3689375B2 JP2002039879A JP2002039879A JP3689375B2 JP 3689375 B2 JP3689375 B2 JP 3689375B2 JP 2002039879 A JP2002039879 A JP 2002039879A JP 2002039879 A JP2002039879 A JP 2002039879A JP 3689375 B2 JP3689375 B2 JP 3689375B2
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molten steel
tundish
flux
mgo
cao
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JP2003236648A (en
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潤二 中島
欣晃 木村
亘 山田
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Nippon Steel Corp
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    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description

【0001】
【発明の属する技術分野】
連鋳鋳片の鋳造初期の空気酸化防止において、連鋳フラックスから還元性ガスを発生し、かつタンディッシュ内で低融点組成となるようにフラックスの組成を制御する方法を提供し、もって連鋳鋳片の鋳造初期の再酸化を防止するフラックスおよび当該フラックスを用いた連続鋳造方法に関するものである。
【0002】
【従来の技術】
近年、鋼材に要求される品質はますます厳しく、かつ多様化してきており、より清浄で機能性に富む鋼を製造する技術の開発が強く望まれている。鋼材中の酸化物系介在物に関しても、鋼材中での悪影響度を軽減するために一層の低減が要求されている。特にアルミナ系介在物は自動車用や電気製品用の薄鋼板では表面疵の原因となったり、DI缶用の薄鋼板では製缶時の割れの原因となったり、タイヤコード等線材の断線の原因、軸受け鋼等の棒鋼では転動疲労特性の悪化原因となることが知られており、その量や大きさの低減が強く求められてきた。連鋳工程においては、鋳造開始直後のタンディッシュ内での空気酸化および取鍋珪砂による溶鋼の再酸化の影響が大きく、タンディッシュの大型化、タンディッシュのシール性向上が図られてきたが定常部位の鋳片清浄性に比べると、鋳片の清浄性は劣位であり、更なる清浄性向上が求められている。
【0003】
溶鋼の空気酸化防止方法としては特開2001−131628、タンディッシュ内溶鋼の空気酸化防止方法として特開2001−219254が報告されている。これらの発明の中には、塊状のMgO,MgCO3に還元剤のAlを反応させMgガスを発生させ、溶鋼上に浸入する酸素と反応させ方法が記載されているが、MgOとAlとが反応した場合には、反応終了後のフラックスはAl23とAl23・MgOとの混合物となり高融点の酸化物となることから、溶鋼上に固体として浮遊して残存する結果となる。一般に溶鋼上を溶融酸化物が覆っている場合には、溶鋼の再酸化が防止されることはよく知られているが、MgOとAlとを混合したフラックスを添加した場合には、Mgガス発生が終了した後は、反応生成物として残存する酸化物が高融点であるために、溶鋼表面に残存しても溶鋼の再酸化防止効果が小さいだけでなく、タンディッシュ内部に付着して残存する。本発明者等が研究を進めた結果、TDの再使用を続けると付着量が再使用毎に増加し、タンディッシュ使用後の整備作業時に除去する作業が必要となることからタンディッシュを再使用する場合に、大きな障害となることが分かった。
【0004】
また、取鍋のSNへの地金の焼き付き防止を目的に用いられている珪砂の主成分はSiO2であり溶鋼中に懸濁すると溶鋼中のAlにより還元され、下記(4)式の反応により、溶鋼中にSiが濃化しかつ溶鋼中にO(酸素)が供給され、Al23が生成することから溶鋼の清浄性を悪化させる要因となる。取鍋の珪砂の影響を排する方法としては、取鍋からタンディッシュに溶鋼を注入開始する前に排滓ポット上でSNを開き、一度珪砂を排出した後に、SNを閉じ、タンディッシュ上に取鍋を移動させた後に再度SNを開き溶鋼の注入を開始する方法があるが、排滓ポットを設置する必要があること、一度珪砂を排滓した後に、SNを閉じて再度タンディッシュ上でSNを開くので取鍋の溶鋼温度が低い場合には地金の焼き付きが生じ、SNを開いても溶鋼が出ない状態となり、酸素を吹き付けて地金を溶断して開孔させる酸素開孔を行う必要が生ずる場合がある。酸素開孔を行うと取鍋からタンディッシュに溶鋼を直接注入する、いわゆるオープン注入の状態となり、ロングノズルが使用できないために空気酸化が激しくなり、溶鋼の清浄性を著しく損なう結果となるだけでなく酸素開孔作業は非常に危険な作業なので安全上の問題が大きい。従って、珪砂を一度排滓した後に溶鋼の注入を開始する方法は一般的ではない。
【0005】
Mgガスが酸素と反応した後の反応生成物はMgO、Al23といった高融点の酸化物であるので、(1)溶鋼上に固体で残存し溶鋼に巻き込まれる場合があること、(2)溶鋼に巻き込まれない場合には、耐火物に付着し固着相を形成し、整備性が悪化すること、(3)還元剤が消費された場合、固体の酸化物となるので、溶鋼上に残存しても、空気酸化防止効果が無くなると、(4)スラグの上に添加された場合、スラグ中のMgO濃度、Al23濃度が上昇し、融点が上昇し、耐火物への付着量が増加し整備性が悪化するという問題点があった。
【0006】
【発明が解決しようとする課題】
これらの問題に鑑み、本発明は、鋳造初期の溶鋼の清浄性を低下させる原因となっているタンディッシュ内溶鋼の空気酸化および珪砂による再酸化を効率的に防止するフラックスおよび当該フラックスを用いた効率的な空気酸化および珪砂による再酸化を防止する手段を提供する。
【0007】
【課題を解決するための手段】
本発明は、下記(1),(2)を提供する。
(1)タンディッシュ内もしくはタンディッシュ溶鋼上に添加する再酸化防止用フラックスであって、CaO,MgO,Al23,Alを主成分とし、下式の組成範囲であることを特徴とするタンディッシュ再酸化防止用フラックス。
0.05≦MgO/(CaO+Al23+1.89×Al+MgO)≦0.3 (a)
0.15≦CaO/(CaO+Al23+1.89×Al+MgO)≦0.6 (b)
0.05≦(Al23+1.89×Al)/(CaO+Al23+1.89×Al+MgO)≦0.7 (c)
0.2≦MgO/Al≦2.24 (d)
但し、
MgO;フラックス中のMgO含有量(質量%)
CaO;フラックス中のCaO含有量(質量%)
Al23;フラックス中のAl23含有量(質量%)
Al;フラックス中のAl含有量(質量%)
(2)タンディッシュ内もしくはタンディッシュ溶鋼上に請求項1に記載する再酸化防止用フラックスを溶鋼1t当たり0.6kg以上の割合で添加することを特徴とする連続鋳造方法。
【0008】
【発明の実施の形態】
転炉等によって精錬し、取鍋に入れられた溶鋼はロングノズルを通じてArガス等の不活性ガスによってシールしたタンディッシュ内に注入されるが、タンディッシュの開口部より空気の侵入が激しく、侵入した空気による空気酸化により溶鋼中のAlと反応し、下記反応式に従って介在物が生成する。
4Al(溶鋼中)+3O2=2Al23(介在物) (1)
【0009】
一般的にタンディッッシュ内溶鋼の空気酸化防止対策として、タンディッシュ内部に不活性ガスを吹き込んで空気の侵入を防止する操業を実施しているが、タンディッッシュを覆う蓋を完全に密閉することは多大な設備費を要するとともに、整備性が悪化するので、現実的ではないことが分かった。従って、タンディッッシュ内に不活性ガスを吹き込んでシールを行っても、実際上タンディッシュ内への空気の侵入を完全に防止することは困難で、点検孔、試料採取孔、ロングノズル挿入孔等の開口部より、空気の侵入が見られる。特開2001−219254に示すように、タンディッシュ溶鋼上にMgOとAlを主成分とする塊状物を添加しMgガスを発生させることにより空気酸化を防止する方法が報告されている。TD内の溶鋼温度は、1500℃以上であり、その溶鋼上に添加された塊状物も1000℃以上の高温となっている。そのため(2)式の反応で示されるMgOのAlによる還元反応が進行しMgガスが発生する。発生したMgガスは雰囲気中の酸素と反応し、(3)式で示されるようにMgOを生成する。
3MgO+2Al=3Mg(ガス)+Al23 (2)
Mg(ガス)+O2(空気中)=MgO(固体) (3)
【0010】
従って、添加した塊状物中にAlが残存している間はMgガスにより、侵入してきた酸素による空気酸化は防止されるが、残存Al量が少なくなると効果は低下し、無くなるとMgガス発生が見られなくなるために再酸化防止効果が無くなってしまう。従って、継続的に溶鋼の空気酸化を防止するためには、一定時間毎に該塊状物を添加し続ける必要があった。また、(3)式に示されるように反応生成物であるMgOは、単体で残存するか、もしくは同じく反応生成物であるAl23と反応しMgO・Al23を形成するが、いずれも高融点の酸化物なので塊状の酸化物として溶鋼上に残存し、TD内壁の耐火物に付着して固い酸化物として残存して 整備性を悪化させることがわかった。
【0011】
また、取鍋のスライディングノズル(以下SNと称する)の開孔率確保のために珪砂を主体とする詰め砂が用いられているが、珪砂はSiO2が主成分であり、溶鋼中のAlと反応し、下記反応式に従って介在物が生成する。
3SiO2+2Al(溶鋼中)=2Al23(介在物)+Si(溶鋼中) (4)
【0012】
珪砂は図1に示すように鍋SNの上に充填されているので、取鍋からタンディッシュに溶鋼を注入する際にSNが開放されると先ず珪砂がタンディッッシュ中に落下し、その後溶鋼がタンディッシュ内に注入される。珪砂はSiO2主体で直径数百μm以上の粒状の物質なので、一度は溶鋼中に懸濁するものの大部分は浮上して溶鋼上に浮遊する。キャストスタート時に溶鋼上に低融点のフラックスが存在しないと、珪砂は時間の経過とともに(2)式の反応で徐々に溶鋼中のAlと反応し、溶鋼中にAl23介在物が供給されタンディッシュ溶鋼中の酸素濃度は長時間低下しない。また、固体の塊状物がタンディッシュ溶鋼上に存在していても固体同士では反応速度が遅く珪砂は塊状フラックスに吸収され難く、大部分の珪砂は(4)式に示すように溶鋼と反応することが分かった。しかるに、溶鋼上に低融点(TD溶鋼上で均一な溶融物となる程度に低融点ということで、融点として1100〜1500℃)のフラックスが存在すると、溶鋼上に浮上した粒状の珪砂は捕捉され、フラックス中に融解し、すみやかに吸収される。従って、低融点のフラックスがタンディッシュ溶鋼上に存在すると(4)式による珪砂による溶鋼の再酸化量が小さくなり図2に示すようになることが分かった。また、図3に示すようにフラックス中にAlが存在すると、(5)式の反応により珪砂が還元される反応も生じ、結果として珪砂による溶鋼の再酸化がさらに効率的に防止されることが分かった。
3SiO2+2Al(フラックス中もしくは上)=2Al23(フラックス)+Si(溶鋼中) (5)
【0013】
以下に、再酸化防止剤の成分の限定理由を述べる。各成分濃度はいずれも質量%である。
(1)MgO;MgOはAlと反応しMgガスを発生させるための反応物質なのでMgガス生成の観点からは濃度が高い方が望ましいが、組成割合が高すぎると反応後のフラックスの融点が高くなりタンディッシュ耐火物への付着量が多くなるので、上限を0.3とした。また、MgO濃度が低すぎると、Mgガスの発生持続時間が短くなるので下限を0.05とした。
0.05≦MgO/(CaO+Al23+1.89×Al+MgO)≦0.3 (a)
【0014】
(2)CaO;CaOはMgガス発生には直接的には関与しないが、MgOと反応したAlの酸化により生成するAl23と反応し低融点化する。組成割合が高すぎると、反応後のフラックスの融点が高くなりタンディッシュ耐火物への付着量が多くなるので、上限を0.6とした。また、CaO濃度が低すぎると、Al23濃度が高い高融点酸化物となるので、下限を0.15とした。
0.15≦CaO/(CaO+Al23+1.89×Al+MgO)≦0.6 (b)
【0015】
(3)Al;Alは最終的には反応生成物としてAl23となり、フラックス中のAl23濃度が高くする。Al量が多すぎると融点が高くなりタンディッシュ耐火物への付着量が多くなるので、上限を0.7とした。また、AlはMgOと反応しMgガスを生成するための反応物質なので濃度が低すぎるとMgガスの発生持続時間が短くなるので下限を0.05とした。
0.05≦(Al23+1.89×Al)/(CaO+Al23+1.89×Al+MgO)≦0.7 (c)
【0016】
(4)効率的なMgガス発生のためには、(2)式の反応物質であるMgOとAlの比が(2)式の化学量論的比率に近いことが望ましいが、Alは吸収した珪砂との反応にも消費され、Al23となるので、一般的には化学量論的に必要な量以上に存在する事が望ましい。しかしながら反応生成物であるAl23量が多すぎると、Al23濃度が高くなり融点が上昇しタンディッシュ耐火物への付着量が多くなるのでMgO/Alの下限を0.2とし、上限を2.24とした。
0.2≦MgO/Al≦2.24 (d)
【0017】
上記理由により、タンディッシュ溶鋼中に侵入する空気中酸素の侵入をMgガスを用いて効率的に防止するためには軽焼ドロマイトおよびAlを含む塊状物をタンディッシュ内に添加し、Mgガスを生成した後に、低融点の溶融酸化物となる本発明を用いることが有効である。本発明例では安価に入手できる材料として軽焼ドロマイトとAlを含む塊状物をタンディッシュ内に添加する例を述べたが、予め、CaO,MgOの粉体を準備して、Al粉末と混合し、圧縮することにより塊状物を製造して用いても同等の効果が期待できる。今回は、添加し易い塊状のフラックスを添加する例を述べているが、圧縮成型せず、粉末状体の混合物の形で添加しても、所定量を袋詰めにし添加すれば、特に問題ない。具体的には、タンディッシュ内に添加する塊状物の投入時の組成が式(a)〜(d)に示すような範囲内であれば同等の効果が期待できる。Al粉末は純Alでも良いが、必ず純Alである必要は無く、金属Alを70%以上含有すれば、Al灰でも良い。
【0018】
【実施例】
以下に、実施例及び比較例を挙げて、本発明について説明する。
【0019】
(実施例1)
転炉〜二次精錬工程を経て取鍋2に成分C:20ppm、Si:0.010%、Mn:0.10%、P:0.02%、S:0.008%、Al:0.035%、温度1600℃の溶鋼300tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、取鍋より溶鋼を20t/分で注入を開始し、直ちに溶鋼の上に表2の組成の軽焼ドロマイト100kg、Al100kgからなる本発明の酸化防止剤を乗せ、Mgガスを発生させた。その後、タンディッシュSNを開き鋳型内に3t/minで注入した。この時、タンディッシュ出側の溶鋼中全酸素量は注入初期から一定値を示し、安定して全酸素量20ppmを確保できた。これにより、溶鋼汚染は確実に防止でき、圧延後の成品には表面欠陥は全く発生しなかった。また、鋳造末期のタンディッシュフラックスは溶融状態で鋼浴表面を覆っており、鋳造後のタンディッシュ耐火物に顕著な付着も見られず、鋳造終了後のタンディッシュ整備作業に支障が生じなかった。
【0020】
(実施例2)
転炉〜二次精錬工程を経て取鍋2に成分C:15ppm、Si:0.010%、Mn:0.08%、P:0.01%、S:0.009%、Al:0.040%、温度1600℃の溶鋼305tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、取鍋より溶鋼を20t/分で注入を開始し、直ちに溶鋼の上に軽焼ドロマイトとAlとを混合し圧縮成型したブリケット200kgからなる表1の組成の本発明の酸化防止剤5を乗せ、Mgガスを発生させた。その後、タンディッシュSNを開き鋳型内に3t/minで注入した。表1にブリケットの組成を示す。この時、タンディッシュ出側の溶鋼中全酸素量は注入初期から一定値を示し、安定して全酸素量25ppmを確保できた。これにより、溶鋼汚染は確実に防止でき、圧延後の成品には表面欠陥は全く発生しなかった。また、鋳造末期のタンディッシュフラックスは溶融状態で鋼浴表面を覆っており、鋳造後のタンディッシュ耐火物に顕著な付着も見られず、鋳造終了後のタンディッシュ整備作業に支障が生じなかった。
【0021】
【表1】

Figure 0003689375
【0022】
【表2】
Figure 0003689375
【0023】
(実施例3)
転炉〜二次精錬工程を経て取鍋に成分C:13ppm、Si:0.010%、Mn:0.08%、P:0.01%、S:0.009%、Al:0.040%、温度1600℃の溶鋼310tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、直ちに軽焼ドロマイトとAlとを混合し圧縮成型したブリケット200kgからなる表1に示す組成の本発明の酸化防止剤を投入し、Mgガスを発生させた。その後、取鍋より溶鋼を20t/分で注入を開始し、その後、タンディッシュSNを開き鋳型内に3t/minで注入した。
【0024】
この時、タンディッシュ出側の溶鋼中全酸素量は注入初期から一定値を示し、安定して全酸素量22ppmを確保できた。これにより、溶鋼汚染は確実に防止でき、圧延後の成品には表面欠陥は全く発生しなかった。また、鋳造末期のタンディッシュフラックスは溶融状態で鋼浴表面を覆っており、鋳造後のタンディッシュ耐火物に顕著な付着も見られず、鋳造終了後のタンディッシュ整備作業に支障が生じなかった。
【0025】
(比較例1)
容量315tの取鍋に成分C:20ppm、Si:0.011%、Mn:0.09%、P:0.02%、S:0.01%、Al:0.041%、温度1600℃の溶鋼320tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、取鍋より溶鋼を20t/分で注入を開始し、直ちに溶鋼の上に軽焼ドロマイトとAlとを混合し圧縮成型したブリケット150kgからなる表1に示す組成の本発明の酸化防止剤を乗せ、Mgガスを発生させた。その後、タンディッシュSNを開き鋳型内に3t/minで注入した。
【0026】
タンディッシュ出側の溶鋼中全酸素量は注入開始直後から150t注入するまでは全酸素量25ppmを確保できたが、150t以降徐々に増加し、取鍋溶鋼注入末期には30ppmに到達した。このため、鋳造時の溶鋼汚染を防止できず、注入末期のスラブから製造した圧延後の成品には表面欠陥が発生した。
【0027】
(比較例2)
容量305tの取鍋に成分C:18ppm、Si:0.009%、Mn:0.08%、P:0.01%、S:0.0091%、Al:0.041%、温度1600℃の溶鋼320tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、取鍋より溶鋼を20t/分で注入を開始し、直ちに溶鋼の上にMgOを100kgとAlを100kg混合したフラックスを酸化防止剤として乗せ、Mgガスを発生させた。その後、タンディッシュSNを開き鋳型内に3t/minで注入した。
【0028】
タンディッシュ出側の溶鋼中全酸素量は注入開始直後から150t注入するまでは全酸素量22ppmを確保できたが、250t以降徐々に増加し、取鍋溶鋼注入末期には29ppmに到達した。このため、鋳造時の溶鋼汚染を防止できず、取鍋注入量250t以降の注入末期のスラブから製造した圧延後の成品には表面欠陥が発生した。また、鋳造終了後のタンディッシュにはAl23およびMgO・Al23の反応生成物およびMgOが付着し、キャスト終了後、スラグライン近傍に付着した付着物を取り除く必要が生じた。
【0029】
(比較例3)
容量315tの取鍋に成分C:20ppm、Si:0.011%、Mn:0.09%、P:0.01%、S:0.0091%、Al:0.041%、温度1600℃の溶鋼320tを溶製しタンディッシュを介して連続鋳造するにあたって、予め予熱したタンディッシュにArガスを2000リットル/分の流量で吹き込みながら、取鍋より溶鋼を20t/分で注入を開始し酸化防止剤を用いない状態で、タンディッシュSNを開き鋳型内に3t/minで注入した。
【0030】
タンディッシュ出側の溶鋼中全酸素量は注入開始直後から著しく増加し95ppmから鋳造量の増大とともに減少し注入末期にて全酸素量35ppmまで低下した。このため、鋳造時の溶鋼汚染を防止できず、鋳造開始直後のスラブから製造した圧延後の成品には著しく表面欠陥が発生した。
【0031】
実施例(1)〜(3)、比較例(1),(2)における(a),(b),(c),(d)の値を表3に示す。
【0032】
【表3】
Figure 0003689375
【0033】
なお、軽焼ドロマイトの組成の一例を表2に示す。CaO,MgOを主成分とし、SiO2、酸化鉄、Al23を含みP、S、CO2等の不可避的不純物を含んでいる。また、原産地および焼成方法により多少の成分は異なる。
【0034】
【発明の効果】
以上のごとく、本発明の取鍋内溶鋼の空気酸化防止方法によれば、取鍋内の空気酸化を定常的に抑制し、溶鋼汚染を確実に防止できるだけでなく、タンディッシュ内にフラックスが付着して残存しないので、タンディッシュの整備性が良好で鋳片の品質も極めて向上する。
【図面の簡単な説明】
【図1】取鍋SN上の鍋珪砂の状態を示す図。
【図2】溶融スラグの存在による珪砂吸収係数の違いを示す図。
【図3】溶融スラグの存在状況による珪砂からの溶鋼再酸化状況の違いを示す図。
【符号の説明】
1 取鍋上ノズル
2 取鍋SNプレート
3 珪砂[0001]
BACKGROUND OF THE INVENTION
In order to prevent air oxidation at the initial casting stage of continuous cast slabs, a method for controlling the composition of the flux so as to generate reducing gas from the continuous casting flux and to have a low melting point composition in the tundish is provided. The present invention relates to a flux for preventing reoxidation at the initial casting of a slab and a continuous casting method using the flux.
[0002]
[Prior art]
In recent years, the quality required for steel materials has become increasingly severe and diversified, and the development of technology for producing cleaner and functional steel is strongly desired. With respect to oxide inclusions in steel materials, further reduction is required in order to reduce the adverse effect in steel materials. In particular, alumina inclusions cause surface flaws in thin steel sheets for automobiles and electrical products, cause cracks when making cans in thin steel sheets for DI cans, and cause for disconnection of wires such as tire cords. It is known that steel bars such as bearing steel cause deterioration of rolling fatigue characteristics, and reduction of the amount and size thereof has been strongly demanded. In the continuous casting process, the effect of air oxidation in the tundish immediately after the start of casting and the reoxidation of molten steel by ladle silica sand has been significant, and the tundish has become larger and the tundish sealability has been improved. Compared with the slab cleanability of the part, the cleanability of the slab is inferior, and further improvement in cleanliness is required.
[0003]
Japanese Patent Laid-Open No. 2001-131628 has been reported as a method for preventing air oxidation of molten steel, and Japanese Patent Laid-Open No. 2001-219254 as a method for preventing air oxidation of molten steel in a tundish. In these inventions, there is described a method in which bulk MgO, MgCO 3 is reacted with Al as a reducing agent to generate Mg gas and react with oxygen entering the molten steel. In the case of the reaction, the flux after completion of the reaction becomes a mixture of Al 2 O 3 and Al 2 O 3 .MgO and becomes a high melting point oxide, resulting in floating as a solid on the molten steel. . In general, it is well known that when molten oxide covers molten steel, re-oxidation of molten steel is prevented. However, when flux mixed with MgO and Al is added, Mg gas is generated. Since the oxide remaining as a reaction product has a high melting point after completion of the process, not only the effect of preventing reoxidation of the molten steel is small but also remains attached to the inside of the tundish even if it remains on the surface of the molten steel. . As a result of research by the present inventors, if the reuse of TD is continued, the amount of adhesion increases every time it is reused, and it is necessary to remove it during maintenance work after using the tundish. It turned out to be a big obstacle.
[0004]
The main component of silica sand used to prevent seizure of ingots to SN in the ladle is SiO 2 and when suspended in the molten steel, it is reduced by Al in the molten steel, and the reaction of the following formula (4) As a result, Si concentrates in the molten steel and O (oxygen) is supplied into the molten steel, and Al 2 O 3 is generated. This causes deterioration of the cleanliness of the molten steel. As a method of eliminating the influence of the ladle silica sand, open the SN on the draining pot before starting to inject molten steel from the ladle into the tundish, once drain the silica sand, close the SN, and on the tundish There is a method to open the SN again after moving the ladle and start pouring the molten steel, but it is necessary to install a draining pot, once the silica sand is drained, the SN is closed and again on the tundish If the molten steel temperature of the ladle is low because the SN is opened, the metal bar will be seized, and even if the SN is opened, the molten steel will not come out, and the oxygen hole that blows oxygen and blows the metal will be opened. You may need to do it. Oxygen drilling results in a state of so-called open pouring, in which molten steel is directly injected from the ladle into the tundish, and because the long nozzle cannot be used, air oxidation becomes intense, resulting in a significant loss of cleanliness of the molten steel. Oxygen drilling is a very dangerous task, so there are major safety issues. Therefore, it is not common to start pouring molten steel after the silica sand has been removed once.
[0005]
Since the reaction product after the Mg gas reacts with oxygen is a high melting point oxide such as MgO or Al 2 O 3 , (1) it may remain solid on the molten steel and be entrained in the molten steel (2 ) When not entrained in the molten steel, it adheres to the refractory and forms a fixed phase and deteriorates maintainability. (3) When the reducing agent is consumed, it becomes a solid oxide, so Even if it remains, if the effect of preventing air oxidation disappears, (4) When added on the slag, the MgO concentration and Al 2 O 3 concentration in the slag will rise, the melting point will rise, and it will adhere to the refractory There was a problem that the quantity increased and the maintainability deteriorated.
[0006]
[Problems to be solved by the invention]
In view of these problems, the present invention uses a flux that efficiently prevents air oxidation of the molten steel in the tundish and reoxidation due to silica sand, which is a cause of reducing the cleanliness of the molten steel at the initial casting, and the flux. Provide a means to prevent efficient air oxidation and re-oxidation by silica sand.
[0007]
[Means for Solving the Problems]
The present invention provides the following (1) and (2).
(1) A reoxidation-preventing flux to be added in or on tundish molten steel, characterized in that it has CaO, MgO, Al 2 O 3 , and Al as the main components and has a composition range of the following formula. Tundish reoxidation flux.
0.05 ≦ MgO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.3 (a)
0.15 ≦ CaO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.6 (b)
0.05 ≦ (Al 2 O 3 + 1.89 × Al) / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.7 (c)
0.2 ≦ MgO / Al ≦ 2.24 (d)
However,
MgO: MgO content in flux (mass%)
CaO; CaO content in flux (mass%)
Al 2 O 3 ; content of Al 2 O 3 in the flux (mass%)
Al: Al content in the flux (% by mass)
(2) A continuous casting method characterized by adding the reoxidation-preventing flux according to claim 1 in the tundish or on the tundish molten steel at a rate of 0.6 kg or more per 1 ton of molten steel.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The molten steel refined by a converter and put into the ladle is injected into the tundish sealed with an inert gas such as Ar gas through a long nozzle. It reacts with Al in the molten steel by air oxidation by the air, and inclusions are generated according to the following reaction formula.
4Al (in molten steel) + 3O 2 = 2Al 2 O 3 (inclusions) (1)
[0009]
In general, as a measure to prevent air oxidation of molten steel in the tundish, an operation is performed to prevent the intrusion of air by blowing an inert gas inside the tundish. However, it is very important to completely seal the lid that covers the tundish. It was not practical because it required equipment costs and deteriorated maintainability. Therefore, even if an inert gas is blown into the tundish and sealed, it is actually difficult to completely prevent the air from entering the tundish, such as an inspection hole, a sampling hole, a long nozzle insertion hole, etc. Intrusion of air is seen from the opening. As shown in Japanese Patent Laid-Open No. 2001-219254, there has been reported a method for preventing air oxidation by adding a mass containing MgO and Al as main components to tundish molten steel and generating Mg gas. The molten steel temperature in TD is 1500 degreeC or more, and the lump added on the molten steel is also high temperature of 1000 degreeC or more. Therefore, the reduction reaction of MgO by Al shown by the reaction of the formula (2) proceeds and Mg gas is generated. The generated Mg gas reacts with oxygen in the atmosphere to generate MgO as shown by the formula (3).
3MgO + 2Al = 3Mg (gas) + Al 2 O 3 (2)
Mg (gas) + O 2 (in air) = MgO (solid) (3)
[0010]
Therefore, while the Al remains in the added lump, the Mg gas prevents air oxidation due to the invading oxygen, but the effect decreases when the amount of residual Al decreases, and when it disappears, Mg gas is generated. Since it cannot be seen, the reoxidation preventing effect is lost. Therefore, in order to continuously prevent air oxidation of the molten steel, it is necessary to continue adding the lump at regular intervals. Further, as shown in the formula (3), MgO as a reaction product remains alone or reacts with Al 2 O 3 which is also a reaction product to form MgO · Al 2 O 3 . Since both oxides have high melting points, they remain on the molten steel as massive oxides, adhere to the refractory on the inner wall of the TD, and remain as hard oxides, degrading maintainability.
[0011]
Although packed sand mainly composed of quartz sand for porosity ensuring ladle sliding nozzle (hereinafter referred to as SN) is used, silica sand SiO 2 is a major component, and Al in the molten steel The inclusions are produced according to the following reaction formula.
3SiO 2 + 2Al (in molten steel) = 2Al 2 O 3 (inclusions) + Si (in molten steel) (4)
[0012]
Since the silica sand is filled on the pan SN as shown in FIG. 1, when the molten steel is poured into the tundish from the ladle, when the SN is opened, the silica sand first falls into the tundish, and then the molten steel is tangled. It is injected into the dish. Since silica sand is a granular substance with a diameter of several hundreds μm or more mainly composed of SiO 2 , most of what is suspended in the molten steel floats up and floats on the molten steel. If a low melting point flux does not exist on the molten steel at the start of casting, the silica sand gradually reacts with Al in the molten steel by the reaction of formula (2) over time, and Al 2 O 3 inclusions are supplied into the molten steel. The oxygen concentration in the tundish molten steel does not decrease for a long time. Moreover, even if solid lump exists on the tundish molten steel, the reaction rate between the solids is slow and the silica sand is not easily absorbed by the lump flux, and most of the silica sand reacts with the molten steel as shown in equation (4). I understood that. However, when there is a flux having a low melting point on the molten steel (the melting point is 1100-1500 ° C. as the melting point is so low that it becomes a uniform melt on the TD molten steel), the granular silica sand floating on the molten steel is captured. It melts in the flux and is absorbed quickly. Accordingly, it has been found that when a low melting point flux is present on the tundish molten steel, the amount of reoxidation of the molten steel by the silica sand according to the equation (4) becomes small and becomes as shown in FIG. In addition, when Al is present in the flux as shown in FIG. 3, there is a reaction in which the silica sand is reduced by the reaction of the formula (5), and as a result, reoxidation of the molten steel by the silica sand can be prevented more efficiently. I understood.
3SiO 2 + 2Al (in flux or above) = 2Al 2 O 3 (flux) + Si (in molten steel) (5)
[0013]
The reasons for limiting the components of the reoxidation inhibitor are described below. Each component concentration is mass%.
(1) MgO; MgO is a reactant for reacting with Al to generate Mg gas, so a higher concentration is desirable from the viewpoint of Mg gas generation. However, if the composition ratio is too high, the melting point of the flux after the reaction is high. Since the amount of adhesion to the nari-tundish refractory increases, the upper limit is set to 0.3. Further, if the MgO concentration is too low, the generation duration of Mg gas becomes short, so the lower limit was made 0.05.
0.05 ≦ MgO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.3 (a)
[0014]
(2) CaO; CaO is not directly involved in the generation of Mg gas, but reacts with Al 2 O 3 produced by oxidation of Al reacted with MgO to lower the melting point. If the composition ratio is too high, the melting point of the flux after the reaction increases and the amount of adhesion to the tundish refractory increases, so the upper limit was set to 0.6. Further, if the CaO concentration is too low, a high melting point oxide having a high Al 2 O 3 concentration is obtained, so the lower limit was set to 0.15.
0.15 ≦ CaO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.6 (b)
[0015]
(3) Al; Al eventually becomes Al 2 O 3 as a reaction product, and the concentration of Al 2 O 3 in the flux is increased. If the amount of Al is too large, the melting point increases and the amount of adhesion to the tundish refractory increases, so the upper limit was set to 0.7. Since Al reacts with MgO to produce Mg gas, if the concentration is too low, the duration of generation of Mg gas is shortened, so the lower limit was made 0.05.
0.05 ≦ (Al 2 O 3 + 1.89 × Al) / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.7 (c)
[0016]
(4) For efficient Mg gas generation, it is desirable that the ratio of MgO to Al, which is the reactant of formula (2), is close to the stoichiometric ratio of formula (2), but Al absorbed Since it is also consumed in the reaction with silica sand and becomes Al 2 O 3 , it is generally desirable that it be present in an amount more than that required stoichiometrically. However, if the amount of Al 2 O 3 that is a reaction product is too large, the concentration of Al 2 O 3 increases, the melting point increases, and the amount of adhesion to the tundish refractory increases, so the lower limit of MgO / Al is set to 0.2. The upper limit was 2.24.
0.2 ≦ MgO / Al ≦ 2.24 (d)
[0017]
For the above reasons, in order to efficiently prevent the intrusion of oxygen in the air entering the tundish molten steel using Mg gas, a lump of light-burning dolomite and Al is added to the tundish, and Mg gas is added. It is effective to use the present invention which becomes a low melting point molten oxide after it is formed. In the example of the present invention, an example in which a light-burned dolomite and an agglomerate containing Al are added to the tundish as materials that can be obtained at low cost, CaO, MgO powder is prepared in advance and mixed with Al powder. The same effect can be expected even if a lump is manufactured and used by compression. This time, an example of adding a bulky flux that is easy to add is described, but even if it is added in the form of a powder mixture without compression molding, there is no particular problem if a predetermined amount is added in a bag. . Specifically, the same effect can be expected if the composition at the time of charging the lump added to the tundish is within the range as shown in the formulas (a) to (d). The Al powder may be pure Al, but is not necessarily pure Al, and may be Al ash if it contains 70% or more of metallic Al.
[0018]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[0019]
(Example 1)
Ingredient C: 20 ppm, Si: 0.010%, Mn: 0.10%, P: 0.02%, S: 0.008%, Al: 0.0. When melting 300t of molten steel at 035% and temperature of 1600 ° C and continuously casting it through a tundish, Ar gas was blown into the preheated tundish at a flow rate of 2000 liters / minute, and the molten steel was transferred from the ladle at 20t / minute. Then, immediately, the antioxidant of the present invention consisting of 100 kg of lightly burned dolomite having a composition shown in Table 2 and 100 kg of Al was placed on the molten steel to generate Mg gas. Thereafter, the tundish SN was opened and injected into the mold at 3 t / min. At this time, the total oxygen content in the molten steel on the tundish delivery side showed a constant value from the initial stage of injection, and a total oxygen content of 20 ppm could be secured stably. As a result, contamination of the molten steel could be reliably prevented, and no surface defects occurred in the product after rolling. In addition, the tundish flux at the end of casting covered the steel bath surface in a molten state, and no noticeable adhesion was observed on the tundish refractory after casting, which did not hinder the tundish maintenance work after casting. .
[0020]
(Example 2)
Ingredient C: 15 ppm, Si: 0.010%, Mn: 0.08%, P: 0.01%, S: 0.009%, Al: 0.0. When melting 305t of molten steel 040% at a temperature of 1600 ° C and continuously casting it through a tundish, Ar gas was blown into the preheated tundish at a flow rate of 2000 liters / min. Then, the antioxidant 5 of the present invention having the composition shown in Table 1 consisting of 200 kg of briquettes in which light-burned dolomite and Al were mixed and compression-molded on the molten steel was immediately put on, and Mg gas was generated. Thereafter, the tundish SN was opened and injected into the mold at 3 t / min. Table 1 shows the composition of briquettes. At this time, the total oxygen content in the molten steel on the tundish delivery side showed a constant value from the initial stage of injection, and a total oxygen content of 25 ppm could be secured stably. As a result, contamination of the molten steel could be reliably prevented, and no surface defects occurred in the product after rolling. In addition, the tundish flux at the end of casting covered the steel bath surface in a molten state, and no noticeable adhesion was observed on the tundish refractory after casting, which did not hinder the tundish maintenance work after casting. .
[0021]
[Table 1]
Figure 0003689375
[0022]
[Table 2]
Figure 0003689375
[0023]
(Example 3)
Ingredient C: 13 ppm, Si: 0.010%, Mn: 0.08%, P: 0.01%, S: 0.009%, Al: 0.040 When melting 310t of molten steel with a temperature of 1600 ° C and continuously casting it through a tundish, lightly burned dolomite and Al are immediately mixed while blowing Ar gas into the preheated tundish at a flow rate of 2000 liters / minute. Then, the antioxidant of the present invention having the composition shown in Table 1 consisting of 200 kg of compressed and molded briquettes was added to generate Mg gas. Thereafter, pouring of molten steel from the ladle was started at 20 t / min, and then the tundish SN was opened and poured into the mold at 3 t / min.
[0024]
At this time, the total oxygen content in the molten steel on the tundish delivery side showed a constant value from the initial stage of injection, and a total oxygen content of 22 ppm could be secured stably. As a result, contamination of the molten steel could be reliably prevented, and no surface defects occurred in the product after rolling. In addition, the tundish flux at the end of casting covered the steel bath surface in a molten state, and no noticeable adhesion was observed on the tundish refractory after casting, which did not hinder the tundish maintenance work after casting. .
[0025]
(Comparative Example 1)
In a ladle with a capacity of 315 t, component C: 20 ppm, Si: 0.011%, Mn: 0.09%, P: 0.02%, S: 0.01%, Al: 0.041%, temperature 1600 ° C When molten steel 320t was melted and continuously cast through the tundish, the injection of molten steel from the ladle at 20 t / min was started while Ar gas was blown into the preheated tundish at a flow rate of 2000 l / min. The antioxidant of the present invention having the composition shown in Table 1 consisting of 150 kg of briquettes formed by mixing lightly burned dolomite and Al on the molten steel and compression-molding was placed to generate Mg gas. Thereafter, the tundish SN was opened and injected into the mold at 3 t / min.
[0026]
The total oxygen content in the molten steel on the tundish delivery side was able to ensure a total oxygen content of 25 ppm from immediately after the start of injection until 150 t was injected, but gradually increased after 150 t and reached 30 ppm at the end of the ladle molten steel injection. For this reason, molten steel contamination at the time of casting could not be prevented, and surface defects occurred in the product after rolling manufactured from the slab at the end of pouring.
[0027]
(Comparative Example 2)
In a ladle with a capacity of 305 t, component C: 18 ppm, Si: 0.009%, Mn: 0.08%, P: 0.01%, S: 0.0091%, Al: 0.041%, temperature 1600 ° C When molten steel 320t was melted and continuously cast through the tundish, the injection of molten steel from the ladle at 20 t / min was started while Ar gas was blown into the preheated tundish at a flow rate of 2000 l / min. A flux in which 100 kg of MgO and 100 kg of Al were mixed was placed on the molten steel as an antioxidant to generate Mg gas. Thereafter, the tundish SN was opened and injected into the mold at 3 t / min.
[0028]
The total amount of oxygen in the molten steel on the tundish delivery side was able to secure a total oxygen amount of 22 ppm immediately after the start of injection until 150 t was injected, but gradually increased after 250 t and reached 29 ppm at the end of the ladle molten steel injection. For this reason, molten steel contamination at the time of casting could not be prevented, and surface defects occurred in the rolled product manufactured from the slab at the end of pouring after the ladle pouring amount of 250 t or more. Further, the reaction product of Al 2 O 3 and MgO · Al 2 O 3 and MgO adhered to the tundish after completion of casting, and it was necessary to remove the adhering material adhering to the vicinity of the slag line after the completion of casting.
[0029]
(Comparative Example 3)
In a ladle with a capacity of 315 t, component C: 20 ppm, Si: 0.011%, Mn: 0.09%, P: 0.01%, S: 0.0091%, Al: 0.041%, temperature 1600 ° C When melting 320t of molten steel and continuously casting it through a tundish, while pouring Ar gas into the preheated tundish at a flow rate of 2000 liters / minute, injection of molten steel from the ladle is started at 20t / minute to prevent oxidation. The tundish SN was opened without using the agent, and injected into the mold at 3 t / min.
[0030]
The total oxygen content in the molten steel on the tundish delivery side increased remarkably immediately after the start of injection, decreased from 95 ppm as the casting amount increased, and decreased to the total oxygen amount of 35 ppm at the end of the injection. For this reason, the molten steel contamination at the time of casting could not be prevented, and surface defects were remarkably generated in the product after rolling manufactured from the slab immediately after the start of casting.
[0031]
Table 3 shows values of (a), (b), (c), and (d) in Examples (1) to (3) and Comparative Examples (1) and (2).
[0032]
[Table 3]
Figure 0003689375
[0033]
An example of the composition of lightly burned dolomite is shown in Table 2. It contains CaO and MgO as main components, contains SiO 2 , iron oxide, Al 2 O 3 and contains unavoidable impurities such as P, S, and CO 2 . Some components vary depending on the country of origin and the firing method.
[0034]
【The invention's effect】
As described above, according to the method for preventing air oxidation of molten steel in a ladle according to the present invention, air oxidation in a ladle can be steadily suppressed, and contamination of molten steel can be surely prevented, and flux adheres to the tundish. Therefore, the tundish is easy to maintain and the quality of the slab is greatly improved.
[Brief description of the drawings]
FIG. 1 is a view showing a state of hot pot quartz sand on a ladle SN.
FIG. 2 is a diagram showing a difference in silica sand absorption coefficient due to the presence of molten slag.
FIG. 3 is a diagram showing a difference in molten steel reoxidation status from quartz sand depending on the presence status of molten slag.
[Explanation of symbols]
1 Ladle upper nozzle 2 Ladle SN plate 3 Silica sand

Claims (2)

タンディッシュ内もしくはタンディッシュ溶鋼上に添加する再酸化防止用フラックスであって、CaO,MgO,Al23,Alを主成分とし、下式の組成範囲であることを特徴とするタンディッシュ再酸化防止用フラックス。
0.05≦MgO/(CaO+Al23+1.89×Al+MgO)≦0.3 (a)
0.15≦CaO/(CaO+Al23+1.89×Al+MgO)≦0.6 (b)
0.05≦(Al23+1.89×Al)/(CaO+Al23+1.89×Al+MgO)≦0.7 (c)
0.2≦MgO/Al≦2.24 (d)
但し、
MgO;フラックス中のMgO含有量(質量%)
CaO;フラックス中のCaO含有量(質量%)
Al23;フラックス中のAl23含有量(質量%)
Al;フラックス中のAl含有量(質量%)
A reoxidation-preventing flux added to or on the tundish molten steel, which is mainly composed of CaO, MgO, Al 2 O 3 , and Al, and has a composition range of the following formula. Antioxidant flux.
0.05 ≦ MgO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.3 (a)
0.15 ≦ CaO / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.6 (b)
0.05 ≦ (Al 2 O 3 + 1.89 × Al) / (CaO + Al 2 O 3 + 1.89 × Al + MgO) ≦ 0.7 (c)
0.2 ≦ MgO / Al ≦ 2.24 (d)
However,
MgO: MgO content in flux (mass%)
CaO; CaO content in flux (mass%)
Al 2 O 3 ; content of Al 2 O 3 in the flux (mass%)
Al: Al content in the flux (% by mass)
タンディッシュ内もしくはタンディッシュ溶鋼上に請求項1に記載する再酸化防止用フラックスを溶鋼1t当たり0.6kg以上の割合で添加することを特徴とする連続鋳造方法A continuous casting method characterized in that the reoxidation-preventing flux according to claim 1 is added at a rate of 0.6 kg or more per 1 ton of molten steel in or on the tundish molten steel.
JP2002039879A 2002-02-18 2002-02-18 Flux for preventing reoxidation and continuous casting method using the same Expired - Fee Related JP3689375B2 (en)

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