JP2005002422A - Method for producing steel material with little alumina cluster - Google Patents
Method for producing steel material with little alumina cluster Download PDFInfo
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Abstract
Description
【0001】
【発明の属する技術分野】
本発明は、自動車用鋼板、構造用・耐摩耗鋼用厚板や油井管用鋼管等に適したアルミナクラスターの少ない鋼材の製造方法に関するものである。
【0002】
【従来の技術】
【特許文献1】特開昭52−70918号公報
【特許文献2】特開2001−26842号公報
【特許文献3】特開平11−323426号公報
【特許文献4】特許1150222 号公報
【特許文献5】特許1266834 号公報
【特許文献6】特開平9−192799号公報
【非特許文献1】城田ら、材料とプロセス, 4(1991), p.1214
【非特許文献2】安中ら、鉄と鋼,(1995), p.17
【非特許文献3】H. Yin et al. ISIJ Int., 37(1997), p.936
【0003】
鋼板などの圧延鋼材は、一般的に転炉で溶製された未脱酸の溶鋼をAlで脱酸するアルミキルド鋼として製造されている。脱酸時に生成するアルミナは硬質で、クラスター化しやすく、数100 μm以上の介在物として残留する。したがって、溶鋼からの除去が不十分な場合、薄板での熱延、冷延時のスリバー疵(線状疵)、構造用厚板での材質不良、耐摩耗鋼用厚板での低温靭性低下や油井管用鋼管での溶接部UST 欠陥不良等の原因となる。さらにアルミナは連続鋳造時に浸漬ノズル内壁に付着、堆積し、閉塞の原因となることが良く知られている。
【0004】
このアルミナを溶鋼から除去する方法として、(1) 脱酸後に、アルミナの凝集、合体による溶鋼からの浮上、分離時間をできるだけ長くとるように転炉での出鋼時に脱酸剤のAlを投入する方法や、(2) 二次精錬法のひとつであるCAS やRH処理で溶鋼の強攪拌を行い、アルミナの浮上、分離を促進する方法や、(3) 溶鋼中へのCaの添加によってアルミナを低融点介在物のCaO−Al2O3に形態制御し無害化する方法等が行われていた。
【0005】
ところが、前記(1) 、(2) の方法によるアルミナの浮上分離対策では限界があって、数100 μm 以上の介在物を完全に除去できないため、スリバー疵を防止できないという問題があった。(3) のCaによる酸化物系介在物の改質は、介在物の低融点化によってクラスター生成が防止でき微細化する。しかし、非特許文献1(城田ら・材料とプロセス, 4(1991), p.1214 )によれば、アルミナを溶鋼中で液相のカルシウムアルミネートにするためには[Ca]/[T.O] を0.7 〜1.2 の範囲に制御する必要がある。そのためには、例えばT.O が40ppm で28〜48ppm という多量のCaを添加する必要がある。一方、タイヤ用のスチールコードや弁バネ材では、介在物を圧延加工時に変形しやすい低融点のCaO−SiO2−Al2O3(−MnO)系に制御し、無害化することが一般的に良く知られている。しかしながら、これらの方法では通常Caを安価なCaSi合金で添加するため、Siの上限の厳しい自動車用鋼板や缶用冷延鋼板では実用化されていないのが現状である。
【0006】
CeやLa等のREM を利用した溶鋼の脱酸では、▲1▼Alキルドを前提とし、Al脱酸後にREM をアルミナの改質剤として使用する方法や▲2▼Alを使用しないでREM を単独、またはCa、Mg等と組み合わせて脱酸する方法が知られている。
【0007】
Alキルドを前提にした方法として、特許文献1(特開昭52−70918号公報)によれば、Al脱酸、またはAl−Si 脱酸後にSe、Sb、LaまたはCeの一種以上を0.001 〜0.05%添加することにより、またはこれと溶鋼攪拌と組み合わせることによって、溶鋼/アルミナクラスター間の界面張力を制御して溶鋼中のアルミナクラスターを浮上分離させて除去する非金属介在物の少ない清浄鋼の製造法が示されている。また、特許文献2(特開2001−26842号公報)では溶鋼をAlおよびTiで脱酸後、Caおよび/またはREM を添加することにより、酸化物系介在物の大きさを50μm以下で、組成をAl2O3:10〜30wt%、Caおよび/またはREM 酸化物:5〜30wt%、Ti酸化物:50〜90wt%とする表面性状および内質に優れる冷延鋼板ならびにその製造方法が開示されている。さらに、特許文献3(特開平11−323426号公報)ではAl、REM およびZrの複合脱酸によってアルミナクラスターがなく、欠陥の少ない清浄なAlキルド鋼の製造方法が提案されている。しかしながら、これらの方法では、アルミナクラスターを確実に浮上分離させることが困難で、介在物欠陥を要求される品質レベルまで低減することができなかった。
【0008】
Alを使用しない方法として、特許文献4(特許1150222 号公報)では、溶鋼をCaO 含有フラックスで脱酸後、Ca、Mg、REM の一種以上を含む合金を例えば100 〜200ppm添加し、介在物を低融点、軟質化するスチール用鋼の製造方法が開示されている。また、特許文献5(特許1266834 号公報)ではMn、Si等のAl以外の脱酸剤でT.O ≦100ppmに調整後、空気酸化防止を目的にREM を50〜500ppm添加する極細伸線性の良好な線材の製造方法が示されている。しかしながら、これらの方法では、脱酸で安価なAlを使用しないため、脱酸剤のコストアップという問題があった。また、Siで脱酸する場合には、Si上限の厳しい薄板材への適用は困難であった。
【0009】
一方、アルミナ粒子のクラスター化にはいくつかの生成機構が提案されている。例えば、特許文献6(特開平9−192799号公報)では、溶鋼中のP2O5がAl2O3粒子の凝集合体を促進していると考え、Caを添加して、nCaO・mP2O5とし、Al2O3のバインダーであるP2O5の結合力を低下させることにより、浸漬ノズルへのAl2O3付着が防止できることが示されている。また、非特許文献2(安中ら・鉄と鋼,(1995), p.17)によれば、連続鋳造で浸漬ノズルの閉塞防止のために用いているArガスに捕捉されたアルミナ粒子が、冷延鋼板に発生するスリバー疵の原因であると推察している。さらに、非特許文献3(H. Yin et al. ISIJ Int., 37(1997), p.936)は、気泡に捕捉されたアルミナ粒子がキャピラリー効果により気泡表面で凝集合体するという観察結果を示している。このように、アルミナクラスターの微視的な生成機構についても解明されつつあるが、クラスター化防止のための具体的方法が明らかではなかった。
【0010】
そこで本発明者らは、研究を重ねた結果、特願2002−214160号として、微量REM添加でアルミナ中の介在物組成を0.5〜15%とすることにより、自動車、家電用途の薄板のスリバー疵、構造用厚板の材質不良、耐摩耗用厚板の低温靭性低下、油井管用鋼管の溶接部UST欠陥等の表面疵や内部欠陥の原因となる粗大アルミナクラスターの少ない鋼材を提案した。また、特願2002−214161号として、溶鋼中のREM/T.Oを0.05〜0.5の範囲とし、アルミナクラスターの生成を抑制することにより、製品での表面疵や内部欠陥を低減し、連続鋳造時の浸漬ノズル閉塞を防止する方法を提案した。しかしながら、0.1〜10ppmの極微量REMの添加ではアルミナ中のREM酸化物含有量がばらつくため、REM添加によるアルミナクラスター生成抑制効果のばらつきも大きかった。すなわち、粗大アルミナクラスター生成を低位に安定して防止できないため、結果として、アルミナクラスター径や個数、ノズル閉塞改善効果のばらつきも大きかった。
【0011】
【発明が解決しようとする課題】
本発明は上記のような従来の問題点を有利に解決するためになされたものであり、薄板、厚板、鋼管、形鋼、棒鋼等の鋼材において製品欠陥の原因となる粗大なアルミナクラスターの生成を溶鋼中およびAr気泡表面で防止することにより、自動車、家電用途の薄板のスリバー疵、構造用厚板の材質不良、耐摩耗用厚板の低温靭性低下、油井管用鋼管の溶接部UST 欠陥等の表面疵や内部欠陥が少ない鋼材の製造を可能とするアルミナクラスターの少ない鋼材の製造方法を提供するためになされたものである。また本発明の他の目的は、連続鋳造時の浸漬ノズル閉塞による操業トラブルの防止により、耐火物コスト低減や浸漬ノズル交換に伴う生産性低下防止を可能にするアルミナクラスターの少ない鋼材の製造方法を提供することである。
【0012】
【課題を解決するための手段】
本発明者らはこの課題を解決するため、実験および検討を重ね、その成果として、合金中REM濃度を調整することによって、得られる鋼材のアルミナ中REM酸化物含有量のばらつきを小さくできることを突き止めた。本発明はこの知見に基づいてなされたものであって、質量%でCe、La、PrまたはNd等の1種類以上の希土類金属(REM)を0.00001〜0.0010%を含有するAlキルド鋼またはAl−Siキルド鋼を製造するにあたり、Al脱酸後に、REM含有量が1〜50%のREM合金を添加することを特徴とするものである。なお本発明における希土類元素とは、原子番号57のLaから原子番号71のLuをさす。
【0013】
鋼の成分は質量%で、C:0.0005〜1.5%、Si:0.005〜1.2%、Mn:0.05〜3.0%、P:0.001〜0.1%、S:0.0001〜0.05%、Al:0.005〜1.5%とし、あるいはさらに(a) Cu:0.1〜1.5%、Ni:0.1〜10.0%、Cr:0.1〜10.0%、Mo:0.05〜1.5%の1種または2種以上、または(b) Nb:0.005〜0.1%、V:0.005〜0.3%、Ti:0.001〜0.25%の1種または2種以上、または(c) B:0.0005〜0.005%の(a) 、(b)、(c)の何れか一つまたは二つ以上を含有し、残部がFe及び不可避的不純物とすることが好ましい。
【0014】
さらに、鋳片のスライム抽出で得られるアルミナクラスターの最大径を100μm以下とすることが好ましく、また、鋳片のスライム抽出で得られる20μm以上のアルミナクラスターの個数を2個/kg以下とすることが好ましい。
【0015】
【発明の実施の形態】
本発明では、質量%でCe、La、PrまたはNd等の1種類以上の希土類金属(REM)を0.00001〜0.0010%を含有するAlキルド鋼またはAl−Siキルド鋼を製造するにあたり、Al脱酸後に、REM含有量が1〜50%のREM合金を添加する。このようにREM含有量が50%以下のREM合金を用いることにより溶鋼中のREM酸化物濃度のばらつきが小さくなり、粗大アルミナクラスターの生成を安定して抑制することができる。
【0016】
本発明において鋼材のREM含有量が0.00001〜0.0010%のAlキルド鋼またはAl−Siキルド鋼に限定するのは、0.00001%未満ではアルミナクラスター生成抑制効果がなく、0.0010%超ではどのようなREM合金を使用してもアルミナ中REM酸化物含有量のばらつきが小さいためである。また合金中のREM含有量を1〜50%に限定するのは、1%未満では所定のREM濃度にするために大量の合金が必要なため溶鋼の温度降下が大きくなりすぎることがあり、50%超では従来と同様にアルミナ中REM酸化物含有量のばらつきが大きくなるためである。
【0017】
なお、本発明の製造方法で用いられるAl脱酸、Al−Si 脱酸鋼とは、質量%でC:0.0005〜1.5%、Si:0.005〜1.2%、Mn:0.05 〜3.0%、P:0.001 〜0.1%、S:0.0001〜0.05%、Al:0.005〜1.5%とし、あるいはさらに(a) Cu:0.1〜1.5%、Ni:0.1〜10.0%、Cr:0.1〜10.0%、Mo:0.05 〜1.5%の1種または2種以上、または(b)Nb:0.005 〜0.1%、V:0.005 〜0.3%、Ti:0.001〜0.25%の1種または2種以上、または(c)B:0.0005 〜0.005%の(a) 、(b) 、(c) 何れか一つまたは二つ以上を含有し、残部がFe及び不可避的不純物からなる炭素鋼であり、鋼材に必要な圧延を加えることにより、薄板、厚板、鋼管、形鋼、棒鋼等へ適用できる。この範囲が好ましい理由は以下の通りである。
【0018】
Cは鋼の強度を最も安定して向上させる基本的な元素であるため、所望する材料の強度によって含有量を0.0005〜1.5 %の範囲で調整する。強度あるいは硬度確保のためには0.0005%以上含有させることが望ましいが、1.5 %より多いと加工性が悪くなるので1.5 %以下がよい。
【0019】
Siは0.005 〜1.2 %としたのは、0.005%未満では予備処理が必要となって精錬に大きなコスト負担をかけ経済性を損ねることとなり、1.2 %より多いとメッキ不良が発生し、表面性状や耐食性を劣化するためである。
【0020】
Mnを0.05〜3.0 %としたのは、0.05%未満では精錬時間が長くなって、経済性を損ねることになり、3.0 %より多いと鋼材の加工性が大きく劣化するためである。
【0021】
Pを0.001 〜0.1 %したのは、0.001%未満では溶銑予備処理に時間とコストがかかり経済性を損ねることとなり、0.1 %より多いと鋼材の加工性が大きく劣化するためである。
【0022】
Sを0.0001〜0.05%としたのは、0.0001%未満では溶銑予備処理に時間とコストがかかり経済性を損ねることとなり、0.05%より多いと鋼材の加工性と耐食性が大きく劣化するためである。
【0023】
Alを0.005 〜1.5 %としたのは、0.005%未満ではAlN としてNをトラップし、固溶Nを減少させることができない。また、1.5 %より多いと加工性が劣化するので1.5 %以下が良い。
【0024】
以上が基本成分系であるが、本発明ではこれらの他にそれぞれの用途に応じて、(a) Cu、Ni、Cr、Moの1種以上、 (b) Nb、V 、Tiの1種以上、 (c) B の、(a)、(b) 、(c) 何れか一つまたは二つ以上を含有させることができる。
【0025】
Cu、Ni、Cr、Moは何れも鋼の焼入れ性を向上させる元素であって、Cu、NiおよびCrは0.1%以上、Moは0.05%以上含有させることによって、強度向上効果を示すが、Cuは1.5 およびMoは1.5%、NiおよびCrは10%を超えて添加すると靭性および加工性を損なうおそれがあるため、Cuは0.1 〜1.5%、NiおよびCrはそれぞれ0.1〜10%、Moは0.05〜1.5%の範囲に限定する。
【0026】
Nb、V 、Tiはいずれも析出強化により鋼の強度を向上させる元素であって、NbおよびV は0.005%以上、Tiは0.001%以上含有させることによって、強度向上効果を示すが、Nbは0.1%、V は0.3%、Tiは0.25%を超えて添加すると靭性を損なうおそれがあるため、Nbは0.005 〜0.1%、V は0.005 〜0.3%、Tiは0.001 〜0.25%の範囲に限定する。
【0027】
Bは鋼の焼入れ性を向上させ、強度を高める元素であって、0.0005%以上含有させることによって、強度向上効果を示すが、0.005%を超えて添加するとB の析出物を増加させ靭性を損なうおそれがあるため、0.0005〜0.005%の範囲に限定する。
【0028】
さらに、鋳片のスライム抽出で得られるアルミナクラスターの最大径が100 μm以下としたのは、100 μm より大きいと製品での表面欠陥や内部欠陥に繋がるためである。また、鋳片のスライム抽出で得られる20μm以上のアルミナクラスターの個数が2個/kg以下としたのは、2 個/kgより多いと圧延後に表面欠陥や内部欠陥に繋がるためである。
【0029】
溶鋼中へのREM合金の添加は、例えば二次精錬装置のCAS やRHを使って、溶鋼のAl脱酸後に行う。REM 添加量は極微量なので、溶鋼中REM濃度を均一にするため、RH槽内での還流溶鋼中への添加や取鍋添加後のArガス等での攪拌が望ましい。また、タンディッシュ、鋳型内溶鋼へREM合金を添加することもできる。
【0030】
【実施例】
以下に本発明の実施例を示す。
270tの転炉において吹錬後、所定の炭素濃度に調整して出鋼した。2次精錬で目標の溶鋼成分に調整し、Al脱酸後、REM合金 を添加した。REMはミッシュメタル(REM100%)、Fe−Si−REM合金として添加した。Fe−Si−REM合金の濃度は、Fe−45%Si−2%REM、Fe−40%Si−10%REM、Fe−30%Si−33%REM、Fe−20%Si−49%REM、Fe−10%Si−59%REM、Fe−5%Si−78%REMである。REM合金の添加1分後に溶鋼を採取し、介在物中REM酸化物濃度をSEM/EDSで分析した。各サンプル10個ずつ、サンプル断面からアルミナを任意抽出し、Fe分を除いた酸化物の合計が100%になるように換算した。最大と最小REM酸化物濃度の差を介在物組成のばらつきとした。REM酸化物濃度はCe2O3、La2O3、Pr2O3、Nd2O3の合計である。
【0031】
表中の溶鋼を垂直曲げ型連続鋳造機により、鋳片寸法が245mm 厚×1200〜2200mm幅、鋳造速度が1.0〜1.7m/min、タンディッシュ内溶鋼温度が1520〜1580℃の条件で鋳片を製造した。鋳片から採取したサンプルの最大クラスター径、クラスター個数、鋳造後の浸漬ノズル閉塞状況等は表2に示すとおりで、本発明がアルミナクラスター起因の製品欠陥を大幅に低減して優れた生産性を示すものであることが確認できた。
【0032】
なお、表1と表2における*1〜*6の意味は以下のとおりである。
*1 REMはCe、La、Pr、Ndの合計。
*2 MM:ミッシュメタル。質量%でCe:45%、La:35%、Pr:6%、Nd:9%、他不可避不純物からなる合金。
2REM:Fe−45%Si−2%REM合金、10REM:Fe−40%Si−10%REM合金、33REM:Fe−30%Si−33%REM合金、49REM:Fe−20%Si−49%REM合金、59REM:Fe−10%Si−59%REM合金、78REM:Fe−5%Si−78%REM合金。
*3 鋳片断面から任意抽出した10個の介在物組成の平均値。組成はEDS付きSEMで同定した。
*4 鋳片断面から任意抽出した10個の介在物中REM酸化物濃度の最大値と最小値の差。組成はEDS付きSEMで同定した。
*5 最大クラスター径の測定方法は、同一チャージ内で得られた5つの鋳片から、鋳片当たり質量1kg±0.1kgの鋼片を切り出してスライム抽出(最小メッシュ250μmを使用)した介在物を実体顕微鏡で写真撮影(40倍)し、写真撮影した介在物の長径と短径の平均値を全ての介在物で求めてその最大値を最大介在物径とした。クラスター個数は前述の総質量5kgの鋼片からスライム抽出した介在物であり、光学顕微鏡(100倍)で観察した20μm以上の全ての介在物個数を1kg単位個数に換算した。
*6 鋳造後に浸漬ノズル内壁の介在物付着厚みを測定。円周方向10点の平均値からノズル閉塞状況を以下の通りレベル分けした。付着厚さは、○:1mm未満、△:1〜5mm、×:5mm超。
【0033】
【表1】
【0034】
【表2】
【0035】
【発明の効果】
以上の説明から明らかなように、本発明によれば質量%でCe、La、PrまたはNd等の1種類以上の希土類金属(REM)を0.00001〜0.0010%を含有するAlキルド鋼またはAl−Siキルド鋼を製造するにあたり、Al脱酸後に、REM含有量が1〜50%のREM合金を添加することで、最終製品における表面疵や内部欠陥の原因となる粗大アルミナクラスターの生成を確実に防止できる。したがって、本発明は従来のAl脱酸鋼やAl−Si脱酸鋼における問題点を一掃したアルミナクラスターの少ない鋼材の製造方法として、産業の発展に寄与するところは極めて大である。さらに本発明によって、連続鋳造における溶鋼中アルミナの浸漬ノズルへの付着の確実に防止可能である。したがって、浸漬ノズル閉塞防止に対する効果も大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel material with few alumina clusters, which is suitable for automobile steel plates, structural and wear-resistant steel plate, oil well pipes and the like.
[0002]
[Prior art]
[Patent Document 1] JP-A-52-70918 [Patent Document 2] JP-A-2001-26842 [Patent Document 3] JP-A-11-323426 [Patent Document 4] JP-A-1150222 [Patent Document 5] [Patent Document 6] Japanese Patent Laid-Open No. 9-192799 [Non-Patent Document 1] Shirota et al., Materials and Processes, 4 (1991), p. 1214
[Non-Patent Document 2] Annaka et al., Iron and Steel, (1995), p. 17
[Non-Patent Document 3] Yin et al. ISIJ Int. , 37 (1997), p. 936
[0003]
Rolled steel materials such as steel plates are generally manufactured as aluminum killed steel that deoxidizes undeoxidized molten steel melted in a converter with Al. Alumina produced during deoxidation is hard and easily clustered, and remains as inclusions of several hundred μm or more. Therefore, when removal from molten steel is insufficient, hot rolling with thin plates, sliver rods (wire rods) during cold rolling, poor material quality with structural planks, low temperature toughness reduction with wear-resistant steel planks, It causes weld defect UST defects in oil well pipe steel pipes. Furthermore, it is well known that alumina adheres and accumulates on the inner wall of the immersion nozzle during continuous casting, causing clogging.
[0004]
As a method of removing this alumina from molten steel, (1) after deoxidation, Al is added as a deoxidizing agent when leaving the steel in the converter so that the alumina aggregates, floats from the molten steel by coalescence, and the separation time is as long as possible. (2) A method in which molten steel is strongly stirred by CAS or RH treatment, which is one of the secondary refining methods, to promote the floating and separation of alumina, and (3) Alumina by adding Ca to the molten steel. Has been carried out, for example, by making the form of CaO—Al 2 O 3 of inclusions of low melting point controlled to be harmless.
[0005]
However, there is a limit to the measures for floating and separating alumina by the methods (1) and (2), and there is a problem that inclusions of several hundred μm or more cannot be completely removed, so that sliver flaws cannot be prevented. (3) The modification of oxide inclusions by Ca can be made fine by preventing the formation of clusters by lowering the melting point of the inclusions. However, according to Non-Patent Document 1 (Shirota et al., Materials and Processes, 4 (1991), p. 1214), in order to make alumina into a liquid phase calcium aluminate in molten steel, [Ca] / [T. O] must be controlled in the range of 0.7 to 1.2. For this purpose, for example, T.W. It is necessary to add a large amount of Ca of O 2 at 40 ppm and 28 to 48 ppm. On the other hand, in steel cords and valve spring materials for tires, it is common to make inclusions harmless by controlling the inclusions to a low melting point CaO—SiO 2 —Al 2 O 3 (—MnO) system that easily deforms during rolling. Well known. However, in these methods, since Ca is usually added as an inexpensive CaSi alloy, the present situation is that it has not been put to practical use in automotive steel plates and can cold-rolled steel plates with a strict upper limit of Si.
[0006]
In deoxidation of molten steel using REM such as Ce and La, (1) Al killing is premised, REM is used as an alumina modifier after Al deoxidation, and (2) REM is used without using Al. A method of deoxidizing alone or in combination with Ca, Mg or the like is known.
[0007]
As a method based on Al killing, according to Patent Document 1 (Japanese Patent Laid-Open No. 52-70918), at least one of Se, Sb, La, or Ce is reduced to 0.1% after Al deoxidation or Al-Si deoxidation. By adding 001 to 0.05%, or in combination with molten steel stirring, the interfacial tension between the molten steel / alumina clusters is controlled and the alumina clusters in the molten steel are separated by floating and removed. Less clean steel production methods are shown. In Patent Document 2 (Japanese Patent Laid-Open No. 2001-26842), the molten steel is deoxidized with Al and Ti, and then Ca and / or REM is added to reduce the size of oxide inclusions to 50 μm or less. Is a cold-rolled steel sheet having excellent surface properties and internal quality, and Al 2 O 3 : 10 to 30 wt%, Ca and / or REM oxide: 5 to 30 wt%, Ti oxide: 50 to 90 wt%, and a method for producing the same Has been. Further, Patent Document 3 (Japanese Patent Application Laid-Open No. 11-323426) proposes a method for producing clean Al killed steel having few defects and no alumina clusters by the combined deoxidation of Al, REM and Zr. However, with these methods, it is difficult to reliably float and separate alumina clusters, and inclusion defects cannot be reduced to the required quality level.
[0008]
As a method not using Al, in Patent Document 4 (Patent No. 1150222), after deoxidizing molten steel with a CaO 2 -containing flux, an alloy containing one or more of Ca, Mg, REM is added, for example, 100 to 200 ppm, and inclusions are added. A method for producing steel for steel having a low melting point and softening is disclosed. In Patent Document 5 (Japanese Patent No. 1266834), a deoxidizer other than Al, such as Mn and Si, is used for T.P. A method for producing a wire with good ultrafine wire drawing, in which 50 to 500 ppm of REM is added for the purpose of preventing air oxidation after being adjusted to O ≦ 100 ppm is shown. However, these methods have a problem of increasing the cost of the deoxidizer because inexpensive Al is not used for deoxidation. In addition, when deoxidizing with Si, it has been difficult to apply it to a thin plate material having a severe Si upper limit.
[0009]
On the other hand, several generation mechanisms have been proposed for clustering alumina particles. For example, in Patent Document 6 (Japanese Patent Laid-Open No. 9-192799), it is considered that P 2 O 5 in molten steel promotes agglomeration and coalescence of Al 2 O 3 particles, Ca is added, and nCaO · mP 2 It has been shown that adhesion of Al 2 O 3 to the immersion nozzle can be prevented by reducing the bonding strength of P 2 O 5 which is O 5 and Al 2 O 3 binder. According to Non-Patent Document 2 (Annaka et al., Iron and Steel, (1995), p. 17), alumina particles captured by Ar gas used for preventing clogging of the immersion nozzle in continuous casting are obtained. It is presumed that this is the cause of sliver defects that occur in cold-rolled steel sheets. Furthermore, Non-Patent Document 3 (H. Yin et al. ISIJ Int., 37 (1997), p. 936) shows an observation result that alumina particles trapped in the bubbles are aggregated and combined on the surface of the bubbles due to the capillary effect. ing. Thus, although the microscopic formation mechanism of alumina clusters is being elucidated, a specific method for preventing clustering has not been clarified.
[0010]
Therefore, as a result of repeated research, the inventors of the present invention, as Japanese Patent Application No. 2002-214160, made inclusions in alumina 0.5 to 15% by adding a small amount of REM, thereby reducing the thickness of thin plates for automobiles and home appliances. We proposed a steel material with few coarse alumina clusters that causes surface defects and internal defects such as sliver rods, structural plate defects, low-temperature toughness of wear-resistant plates, and UST defects in welded steel pipes. In addition, as Japanese Patent Application No. 2002-214161, REM / T. A method has been proposed in which O is set in the range of 0.05 to 0.5 to suppress generation of alumina clusters, thereby reducing surface flaws and internal defects in the product and preventing clogging of the immersion nozzle during continuous casting. However, the addition of a very small amount of REM of 0.1 to 10 ppm varies the REM oxide content in the alumina, so that the dispersion of the alumina cluster formation suppressing effect due to the addition of REM is also large. That is, since the formation of coarse alumina clusters cannot be stably prevented at a low level, as a result, there are large variations in the diameter and number of alumina clusters and the effect of improving nozzle clogging.
[0011]
[Problems to be solved by the invention]
The present invention has been made in order to advantageously solve the above-described conventional problems, and it is possible to obtain coarse alumina clusters that cause product defects in steel materials such as thin plates, thick plates, steel pipes, shaped steels, and steel bars. By preventing formation at the surface of molten steel and Ar bubbles, thin sliver rods for automobiles and household appliances, poor structural plate materials, low-temperature toughness of wear-resistant thick plates, welded UST defects in oil well pipes The present invention was made in order to provide a method for producing a steel material with few alumina clusters, which makes it possible to produce a steel material with few surface defects and internal defects. Another object of the present invention is to provide a method for producing a steel material with less alumina clusters that can prevent refractory costs and prevent productivity decline due to replacement of the immersion nozzle by preventing operation troubles due to the closure of the immersion nozzle during continuous casting. Is to provide.
[0012]
[Means for Solving the Problems]
In order to solve this problem, the present inventors have repeated experiments and studies, and as a result, have found that by adjusting the REM concentration in the alloy, it is possible to reduce the variation in the REM oxide content in the alumina of the obtained steel. It was. The present invention has been made based on this finding, and is an Al killed metal containing 0.00001 to 0.0010% of one or more rare earth metals (REM) such as Ce, La, Pr, or Nd in mass%. In producing steel or Al—Si killed steel, a REM alloy having a REM content of 1 to 50% is added after Al deoxidation. In the present invention, the rare earth element refers to La having atomic number 57 to Lu having atomic number 71.
[0013]
The composition of steel is% by mass, C: 0.0005 to 1.5%, Si: 0.005 to 1.2%, Mn: 0.05 to 3.0%, P: 0.001 to 0.1. %, S: 0.0001 to 0.05%, Al: 0.005 to 1.5%, or (a) Cu: 0.1 to 1.5%, Ni: 0.1 to 10.0 %, Cr: 0.1 to 10.0%, Mo: 0.05% to 1.5%, or (b) Nb: 0.005 to 0.1%, V: 0.0. 005 to 0.3%, Ti: one or more of 0.001 to 0.25%, or (c) B: 0.0005 to 0.005% of (a), (b), (c 1) or two or more thereof, and the balance is preferably Fe and inevitable impurities.
[0014]
Furthermore, it is preferable that the maximum diameter of the alumina cluster obtained by slime extraction of the slab is 100 μm or less, and the number of alumina clusters of 20 μm or more obtained by slime extraction of the slab is 2 pieces / kg or less. Is preferred.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, when manufacturing Al killed steel or Al-Si killed steel containing 0.00001 to 0.0010% of one or more rare earth metals (REM) such as Ce, La, Pr or Nd in mass%. After the Al deoxidation, a REM alloy having a REM content of 1 to 50% is added. Thus, by using a REM alloy having a REM content of 50% or less, variation in the REM oxide concentration in the molten steel is reduced, and the formation of coarse alumina clusters can be stably suppressed.
[0016]
In the present invention, the REM content of the steel material is limited to Al killed steel or Al-Si killed steel having a REM content of 0.00001 to 0.0010%. This is because when the content exceeds%, any variation of the REM oxide content in alumina is small no matter which REM alloy is used. Further, the REM content in the alloy is limited to 1 to 50% because if the amount is less than 1%, a large amount of alloy is required to obtain a predetermined REM concentration, so that the temperature drop of the molten steel may become too large. This is because the content of REM oxide in alumina increases as in the conventional case when the content exceeds%.
[0017]
In addition, Al deoxidation used by the manufacturing method of this invention, and Al-Si deoxidation steel are C: 0.0005-1.5% in mass%, Si: 0.005-1.2%, Mn: 0.05 to 3.0%, P: 0.001 to 0.1%, S: 0.0001 to 0.05%, Al: 0.005 to 1.5%, or (a) Cu: 0.1 to 1.5%, Ni: 0.1 to 10.0%, Cr: 0.1 to 10.0%, Mo: 0.05 to 1.5%, or two or more (B) Nb: 0.005 to 0.1%, V: 0.005 to 0.3%, Ti: 0.001 to 0.25%, or (c) B: 0 .0005 to 0.005% of (a), (b), (c) a carbon steel containing one or two or more, the balance being Fe and inevitable impurities, By adding a main rolling can be applied thin, thick, steel, shape steel, the steel bars and the like. The reason why this range is preferable is as follows.
[0018]
Since C is a basic element that improves the strength of steel most stably, the content is adjusted in the range of 0.0005 to 1.5% depending on the strength of the desired material. In order to ensure strength or hardness, it is desirable to contain 0.0005% or more. However, if it exceeds 1.5%, workability deteriorates, so 1.5% or less is preferable.
[0019]
The Si content is set to 0.005 to 1.2%. If it is less than 0.005%, a pretreatment is required, which imposes a large cost on refining and impairs the economy. This is because defects occur and surface properties and corrosion resistance deteriorate.
[0020]
The reason why Mn is set to 0.05 to 3.0% is that, if it is less than 0.05%, the refining time becomes long and the economic efficiency is impaired. If it exceeds 3.0%, the workability of the steel material is greatly deteriorated. It is to do.
[0021]
The reason why P is 0.001 to 0.1% is that if it is less than 0.001%, the hot metal pretreatment takes time and cost, and the economic efficiency is impaired. If it exceeds 0.1%, the workability of the steel material is greatly deteriorated. It is to do.
[0022]
If S is 0.0001 to 0.05%, if less than 0.0001%, the hot metal pretreatment takes time and cost, and the economic efficiency is impaired. If it exceeds 0.05%, the workability and corrosion resistance of the steel material are impaired. This is because of a significant deterioration.
[0023]
The reason why Al is 0.005 to 1.5% is that if it is less than 0.005%, N is trapped as AlN and solid solution N cannot be reduced. Further, if it exceeds 1.5%, workability deteriorates, so 1.5% or less is good.
[0024]
The above is the basic component system, but in the present invention, in addition to these, (a) one or more of Cu, Ni, Cr, and Mo, (b) one or more of Nb, V, and Ti (C) Any one or two or more of (a), (b) and (c) of B can be contained.
[0025]
Cu, Ni, Cr, and Mo are all elements that improve the hardenability of the steel, and Cu, Ni, and Cr contain 0.1% or more, and Mo contains 0.05% or more. As shown, Cu is 1.5 and Mo is 1.5%, and Ni and Cr are added in excess of 10%, so that the toughness and workability may be impaired. And Cr is limited to 0.1 to 10%, and Mo is limited to a range of 0.05 to 1.5%.
[0026]
Nb, V, and Ti are all elements that improve the strength of the steel by precipitation strengthening. Nb and V are 0.005% or more, and Ti is contained by 0.001% or more. , Nb is 0.1%, V is 0.3%, and if Ti is added over 0.25%, the toughness may be impaired, so Nb is 0.005 to 0.1% and V is 0.005. -0.3%, Ti is limited to the range of 0.001-0.25%.
[0027]
B is an element that improves the hardenability of the steel and enhances the strength. By adding 0.0005% or more, B shows an effect of improving the strength, but if added over 0.005%, the precipitate of B increases. Therefore, the toughness may be impaired, so the content is limited to 0.0005 to 0.005%.
[0028]
Further, the reason that the maximum diameter of the alumina cluster obtained by slime extraction of the slab is 100 μm or less is that when it is larger than 100 μm, it leads to surface defects and internal defects in the product. The reason why the number of alumina clusters of 20 μm or more obtained by slime extraction of the slab was 2 / kg or less is that when it exceeds 2 / kg, it leads to surface defects and internal defects after rolling.
[0029]
Addition of the REM alloy into the molten steel is performed after Al deoxidation of the molten steel using, for example, CAS or RH of a secondary refining apparatus. Since the amount of REM added is extremely small, in order to make the REM concentration in the molten steel uniform, addition to the refluxing molten steel in the RH tank or stirring with Ar gas after addition of the ladle is desirable. Moreover, a REM alloy can also be added to tundish and molten steel in a mold.
[0030]
【Example】
Examples of the present invention are shown below.
After blowing in a 270 t converter, the steel was adjusted to a predetermined carbon concentration and produced. The REM alloy was added after adjusting to the target molten steel component by secondary refining and deoxidizing Al. REM was added as misch metal (REM 100%), Fe-Si-REM alloy. The concentration of the Fe-Si-REM alloy is Fe-45% Si-2% REM, Fe-40% Si-10% REM, Fe-30% Si-3% REM, Fe-20% Si-49% REM, Fe-10% Si-59% REM, Fe-5% Si-78% REM. One minute after the addition of the REM alloy, the molten steel was collected, and the REM oxide concentration in the inclusions was analyzed by SEM / EDS. For each 10 samples, alumina was arbitrarily extracted from the sample cross section and converted so that the total of oxides excluding Fe was 100%. The difference between the maximum and minimum REM oxide concentrations was the variation in inclusion composition. The REM oxide concentration is the sum of Ce 2 O 3 , La 2 O 3 , Pr 2 O 3 , and Nd 2 O 3 .
[0031]
The molten steel shown in the table is subjected to a vertical bending type continuous casting machine, and the slab size is 245 mm thick × 1200 to 2200 mm wide, the casting speed is 1.0 to 1.7 m / min, and the molten steel temperature in the tundish is 1520 to 1580 ° C. The slab was manufactured. Table 2 shows the maximum cluster diameter, the number of clusters, the clogging condition of the immersion nozzle after casting, etc., as shown in Table 2. The present invention significantly reduces product defects caused by alumina clusters and has excellent productivity. It was confirmed that this was shown.
[0032]
The meanings of * 1 to * 6 in Tables 1 and 2 are as follows.
* 1 REM is the sum of Ce, La, Pr, and Nd.
* 2 MM: Misch metal. An alloy composed of Ce: 45%, La: 35%, Pr: 6%, Nd: 9%, and other inevitable impurities in mass%.
2REM: Fe-45% Si-2% REM alloy, 10REM: Fe-40% Si-10% REM alloy, 33REM: Fe-30% Si-33% REM alloy, 49REM: Fe-20% Si-49% REM Alloy, 59REM: Fe-10% Si-59% REM alloy, 78REM: Fe-5% Si-78% REM alloy.
* 3 Average value of 10 inclusions arbitrarily extracted from the slab cross section. The composition was identified by SEM with EDS.
* 4 Difference between the maximum and minimum values of REM oxide concentration in 10 inclusions arbitrarily extracted from the slab cross section. The composition was identified by SEM with EDS.
* 5 The measurement method of the maximum cluster diameter is an inclusion in which a steel piece with a mass of 1 kg ± 0.1 kg per piece is cut out from five pieces obtained within the same charge and slime extracted (using a minimum mesh of 250 μm). Was photographed with a stereomicroscope (40 times), the average value of the major axis and minor axis of the photographed inclusions was determined for all the inclusions, and the maximum value was taken as the maximum inclusion diameter. The number of clusters is inclusions extracted from the above-mentioned steel pieces having a total mass of 5 kg, which are slime-extracted. The number of all inclusions of 20 μm or more observed with an optical microscope (100 times) was converted to 1 kg unit number.
* 6 Measure the thickness of inclusions on the inner wall of the immersion nozzle after casting. The nozzle clogging situation was classified into the following levels from the average value of 10 points in the circumferential direction. Adhesion thickness is less than (circle): 1mm, (triangle | delta): 1-5mm, x: more than 5mm.
[0033]
[Table 1]
[0034]
[Table 2]
[0035]
【The invention's effect】
As is clear from the above description, according to the present invention, Al killed steel containing 0.00001 to 0.0010% of one or more rare earth metals (REM) such as Ce, La, Pr or Nd by mass%. Or, when producing Al-Si killed steel, by adding REM alloy with 1 to 50% REM content after Al deoxidation, formation of coarse alumina clusters causing surface defects and internal defects in the final product Can be reliably prevented. Therefore, the present invention contributes greatly to the development of the industry as a method for producing a steel material with few alumina clusters that eliminates the problems in conventional Al deoxidized steel and Al—Si deoxidized steel. Further, according to the present invention, it is possible to reliably prevent the alumina in molten steel from adhering to the immersion nozzle in continuous casting. Accordingly, the effect of preventing the immersion nozzle from being blocked is great.
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