JP4065099B2 - Method for continuous casting of molten steel and continuous cast slab - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は鋼の連続鋳造方法に関する。本発明は更に詳しくは、鋳片横断面の金属組織を微細な等軸晶にすることができる溶鋼の連続鋳造方法およびそれを用いて鋳造した微細な凝固組織を有する連続鋳造鋳片に関するものである。
【０００２】
【従来の技術】
通常の連続鋳造鋳片の横断面には、その中心にポロシティを伴う最終凝固部と、この最終凝固部を取り囲むように配された中心近傍の粗い粒状晶部と、粗い粒状晶部を取り囲む粗い柱状晶部とが観察される。この粗い粒状晶と粗い柱状晶とを微細な等軸晶にすることができると、例えばスラブを薄板にした際には成形加工性が顕著に優れた薄板になり、また例えば厚板にした際には低温靱性に優れた厚板になる。
【０００３】
「鉄鋼便覧」第３版、II 製銑・製鋼、ｐ．６５３には、等軸晶は溶鋼過熱度が低いと増加することから、等軸晶化には低温鋳造が有効であることが示されている。しかし、低温鋳造では、溶融金属の過熱度を液相線に近い温度にし、これを浸漬ノズルから鋳型内に注入する必要があるため、浸漬ノズルの閉塞や鋳型内でのディッケル生成等の凝固異常を招く場合がある。
【０００４】
特開昭５０−２３３３８号公報は、誘導電磁攪拌装置を用いて、凝固界面近傍の溶鋼に流速が変化しない一方向に流れる旋回流を与え、柱状デンドライトを分断することにより柱状晶を等軸晶にする技術を記載している。しかし、本発明者らの知見では、この方法は等軸晶化する力が小さく、例えば等軸晶が生成し難いＣ含有率が０．１％以下の溶鋼の場合には、柱状晶を十分に等軸晶化する事が難しい。
【０００５】
特開平３−４４８５８号公報は、円柱または角柱ビレットの際に品質上の問題点となるポロシティを伴う最終凝固部を改善する方法で、例えば鋳型下１６ｍ〜２７ｍの最終凝固部近傍に誘導電磁攪拌装置を配し、半サイクルの攪拌時間が５〜３０秒の向きが反転する旋回流を用いる。しかし、この方法は最終凝固部を改善する方法であるため、誘導電磁攪拌装置を配する場所や旋回流の反転周期は本発明とは異なる。
【０００６】
【発明が解決しようとする課題】
本発明は、中心近傍の粗い粒状晶とそれを取り囲む粗い柱状晶とが共に微細に等軸晶化した鋳片を製造できる連続鋳造方法およびそれを用いて鋳造した微細な凝固組織を有する連続鋳造鋳片の提供を課題としている。
【０００７】
【課題を解決するための手段】
本発明は、（１）鋳型内メニスカスから鋳型下１０ｍの間に電磁コイルを有する連続鋳造装置において、取鍋、タンディッシュあるいは鋳型内でＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物を添加し、溶鋼を該電磁コイルにて発生する移動磁界により順逆方向に振動させながら鋳造することを特徴とする溶鋼の連続鋳造方法である。また、（２）請求項１において、順逆方向に振動させる振動波の順方向および逆方向の加速度を１０ｃｍ／ｓ2以上、且つ電磁コイルによる１周期の振動時間を０．２秒以上１０秒未満にすることを特徴とする溶鋼の連続鋳造方法である。また、（３）請求項２において、１周期の振動時間の間に０．０３秒以上、０．３秒以下の加速停止時間、或いは電源停止時間を設けることを特徴とする溶鋼の連続鋳造方法である。また、（４）請求項１から請求３の何れか１項において、溶鋼を順逆方向に振動させるとともに、順方向もしくは逆方向に旋回流を付与することを特徴とする溶鋼の連続鋳造方法である。また、（５）ＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物におけるＭｇＯ含有率を３０質量％以上、Ａｌ含有率を２０質量％以上、Ｔｉ含有率を２０質量％以上にしたことを特徴とする請求項１から４記載の溶鋼の連続鋳造方法である。また、（６）溶鋼量に対してＭｇＯ量が０．０１質量％から１質量％の範囲になるようにＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物を添加することを特徴とする請求項１から５記載の溶鋼の連続鋳造方法である。また、（７）請求項１から請求項６の何れかの連続鋳造方法により凝固組織を微細にしたことを特徴とする連続鋳造鋳片である。また、（８）請求項１から請求項６の何れかの連続鋳造方法により鋳片断面の平均等軸晶粒径で０．５〜３．０ｍｍの微細組織にしたことを特徴とする連続鋳造鋳片である。
【０００８】
【発明の実施の形態】
本発明の基本思想は、微細な酸化物を溶鋼中に分散させ、これに電磁コイルによる振動を加え溶鋼の過熱度を低下させることにより、微細に分散させた酸化物を等軸晶生成の核として効率的に活用し、鋳片内に微細な等軸晶を生成させることにある。この基本思想を実現するためには、▲１▼鋳片内で等軸晶の核となり得る微細な酸化物を生成させる方法と、▲２▼微細な酸化物を起点に生成した等軸晶核の再溶解を防止できるように溶鋼過熱度を低減させた上で、微細に分散させた酸化物が凝集・合体しない電磁コイルの振動条件を明らかにすることが重要である。
【０００９】
まず、▲１▼の方法について述べる。Ａｌ脱酸溶鋼には多数のＡｌ2Ｏ3系介在物が存在するが、この介在物は極めて凝集・合体し易く粗大な酸化物となるため、等軸晶の核として有効に作用しない。これに対し、本発明者らは、溶鋼中にＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物を添加し、Ａｌ2Ｏ3系介在物をＭｇＯ、或いはＭｇＯ・Ａｌ2Ｏ3に改質することにより、微細な酸化物を溶鋼中に均一に分散できること、さらにこれら酸化物が等軸晶生成の核になり易いことを見いだした。溶鋼中にＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物を添加すると、（１）式及び（２）式に示すようにＭｇＯはＡｌ、Ｔｉにより還元され、Ｍｇガスを発生する。
３ＭｇＯ＋２Ａｌ＝３Ｍｇ＋Ａｌ2Ｏ3 （１）
２ＭｇＯ＋Ｔｉ＝２Ｍｇ＋ＴｉＯ2 （２）
このＭｇガスは溶鋼中のＡｌ2Ｏ3系介在物を（３）式及び（４）式により還元し、ＭｇＯ又はＭｇＯ・Ａｌ2Ｏ3を生成する。
Ａｌ2Ｏ3＋３Ｍｇ＝３ＭｇＯ＋２Ａｌ （３）
４Ａｌ2Ｏ3＋３Ｍｇ＝３（ＭｇＯ・Ａｌ2Ｏ3）＋２Ａｌ （４）
ＭｇＯやＭｇＯ・Ａｌ2Ｏ3はＡｌ2Ｏ3と比べて溶鋼と濡れ易いため、これら酸化物は溶鋼中で微細に分散する。溶鋼中に微細なＭｇＯやＭｇＯ・Ａｌ2Ｏ3を分散させた溶鋼を連続鋳造すると、これら酸化物を核として微細な等軸晶が生成し、凝固組織を微細化できる。
【００１０】
本発明では、溶鋼中のＡｌ濃度は０．１％以下であり、これを超えるＡｌ濃度ではＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物を添加しても、Ａｌ2Ｏ3系介在物をＭｇＯ、或いはＭｇＯ・Ａｌ2Ｏ3に改質できず、微細な酸化物を溶鋼中に分散できない。溶鋼中Ａｌ濃度の下限値は特に規定するものではないが、０．００１％未満になると脱酸の効果が不安定になるため、０．００１％以上が望ましい。
【００１１】
また、ＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物の添加量は、ＭｇＯ分が溶鋼量に対して０．０１質量％から１質量％になるように添加する必要がある。ＭｇＯ分が０．０１質量％未満になると溶鋼中のＡｌ2Ｏ3系介在物を改質するに十分なＭｇガスが発生しないため、またＭｇＯ分が１質量％超になると溶鋼中のＭｇ濃度が高くなり、酸化物が粗大化し易くなることにより、何れも鋳片内の凝固組織を微細な等軸晶にする効果が失われるためである。
【００１２】
ＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物中のＭｇＯ含有率は３０質量％以上、Ａｌ及びＴｉ含有率は各々２０質量％以上にする必要があり、それ未満になると何れもＭｇガスの発生量が大きく低下し、Ａｌ2Ｏ3系介在物の改質効果が損なわれる。また、ＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物中には、ＭｇＯ、ＡｌやＴｉ以外の混入物、例えばＣａＯ、Ａｌ2Ｏ3、ＳｉＯ2等を混入させても良いが、相対的にＭｇＯ含有率、Ａｌ含有率及びＴｉ含有率が低下し、各々３０質量％、２０質量％未満にならないようにする必要がある。
【００１３】
なお、ＭｇＯ−Ａｌ混合物、或いはＭｇＯ−Ｔｉ混合物の添加は、取鍋、タンディッシュ或いは鋳型に限定されるものではなく、鋳型までの過程で溶鋼に添加すれば良い。
【００１４】
次に、▲２▼について述べる。一般に、電磁攪拌では、凝固界面の溶鋼に一方向の旋回流を付与するため、この旋回流が柱状デンドライトを分断し、等軸晶化を促進すると考えられており、溶鋼の過熱度を低下させる効果は比較的小さい。電磁攪拌による溶鋼過熱度の低減効果を高めていくためには、旋回流速を速くする必要があり、その場合微細な酸化物が凝集・合体により粗大化し、等軸晶の核として有効に機能しなくなる。
【００１５】
これに対し、本発明は凝固シェル前面の溶鋼を電磁コイルに基づく移動磁界により振動させ、凝固シェルと溶鋼間の熱伝達を促進し、溶鋼の過熱度を効果的に低減するものであり、微細な酸化物を起点に生成した等軸晶核の再溶解を防止できる。ここで、移動磁界とは、比較的低周波の磁場を移動させて印加することにより発生する磁界である。さらに、移動磁界による振動は凝固シェル前面に付与され、内部の溶鋼には流動が生じにくいため、酸化物の凝集・合体を抑制する効果も有している。
【００１６】
電磁コイルの電流を図１のパターンで変動させると、これに対応して凝固シェル前面の溶鋼の振動流速は若干なまりながら追従する。凝固シェル前面の振動流速が一定であるｔ１またはｔ３の領域では、振動流による熱伝達促進の効果は電磁攪拌並であるが、順方向の加速領域ｔ２または逆方向の加速領域ｔ４では、凝固シェル前面の振動流に加速度が生じており、一定速度の電磁攪拌流に比べて非常に大きな熱伝達促進効果を有することを見いだした。この振動に伴い誘起される加速度の効果により、凝固シェル−溶鋼間の熱伝達が促進され、溶鋼の過熱度を効率的に低下させることができる。
【００１７】
また、電磁コイルの電流を、図２のパターンのように１周期の振動時間の間に加速停止時間を設けたパターンとすることも可能であり、この場合加速停止時間により急加速が緩和され、メニスカスがより安定化する効果が得られる。加速停止時間を０．３秒超にすると加速時間が短くなり熱伝達促進効果が損なわれ、逆に加速停止時間を０．０３秒未満にするとメニスカス安定化効果が損なわれるため、加速停止時間を設ける場合には０．０３秒以上０．３秒以下にすることが好ましい。
【００１８】
図３はスラブの連続鋳造における本発明の説明図で、（Ａ）は装置の縦断面の説明図で（Ｂ）及び（Ｃ）は矢印イ−イの横断面の説明図である。図３中１は凝固シェル、２は未凝固溶鋼、３，３’は電磁コイルである。スラブの電磁コイルはスラブの幅広面の両側の対応する位置に配され、電磁コイルに基づく移動磁界により図３（Ｂ）の如くに未凝固溶鋼２を矢印４，４’方向に旋回振動させる。本発明では、凝固シェル前面の溶鋼を振動させ、凝固シェルと溶鋼間の熱伝達を促進させるものであるから、図３（Ｂ）の如く振動時に溶鋼を旋回させる必要はなく、図３（Ｃ）の如くに未凝固溶鋼２を矢印５，５’方向に振動させても良い。
【００１９】
上記説明はスラブを例に行ったが、本発明の実施はスラブに限られたものではなく、ブルーム、ビレット等でも同様の効果が得られる。また、鋳型下１０ｍよりも更に下方では、既に鋳片表層から数１０ｍｍまでの凝固が完了しているので、鋳片全面の凝固組織をできるだけ微細化するには電磁コイルは凝固の始まる鋳型内メニスカスから鋳型下１０ｍの位置に設置することが有効である。
【００２０】
【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。
【００２１】
（実施例１）
炭素含有量が０．１１％の溶鋼５０ｋｇを高周波溶解炉で溶製し、これに６０質量％ＭｇＯ−４０質量％Ａｌ混合物５０ｇを添加し本発明の溶鋼とした。この溶鋼を、温度１６００℃で横２００ｍｍ、縦１００ｍｍ、高さ３００ｍｍの水冷銅製鋳型に注入した。注入後直ちに所定の振動パターンで鋳型内の溶鋼を振動させながら凝固させた。振動パターンは、周波数１０Ｈｚの交流で移動磁界を形成し、図１で電磁コイルの電流を最大１００アンペア、最小−１００アンペアとした。順方向の加速時間であるコイル電流増加時間ｔ２、逆方向の加速時間であるコイル電流減少時間ｔ４を調整することによって加速度を調整した。ｔ２、ｔ４が短いほど加速度が高くなり、ｔ２、ｔ４が長いほど加速度が低くなる。さらに最小コイル電流保持時間ｔ１、最大コイル電流保持時間ｔ３を所定の値に設定することにより、振動周期を０．２秒から１０秒まで変化させた。
【００２２】
鋳造後の鋼塊は横断面で切断し、凝固組織を顕出した後、等軸晶の円相当径（面全体の平均値）を評価した。なお、柱状晶の場合の粒径も同時に評価できるように等軸晶粒径は２（ａ・ｂ）0.5と定義した（ａは結晶粒の長径、ｂは結晶粒の短径である。）。鋳片横断面の平均等軸晶粒径と電磁コイルの振動周期との関係を図４に示す。図４から分かるように、ＭｇＯ−Ａｌ混合物を添加した鋳片の平均等軸晶粒径は、振動波の加速度の絶対値（順・逆方向の加速度があるため。）が１０ｃｍ／ｓ2以上で、且つ振動周期が０．２秒以上１０秒未満の領域で小さくなることが分かる。なお、振動波の加速度は、鋳型に水銀を充満させ、電磁コイルにより水銀を振動させた場合の流速をプロペラ流速計で測定した結果から算出したものである。これは、振動波の加速度の絶対値を１０ｃｍ／ｓ2とすることにより溶鋼過熱度が低下し、微細な酸化物を起点に生成した等軸晶核の再溶解が抑制されたためである。また、振動周期が１０秒以上では、電磁攪拌による旋回流が単に旋回方向を周期的に変化させているのと同じ流動状態になるため、また０．２秒未満では凝固シェル前面の溶鋼が電磁コイルの電流変化に追従し難くなるため、何れも振動による過熱度低減効果が損なわれ、等軸晶核が再溶解した結果、等軸晶粒径が細かくならなかったものと考えられる。したがって、凝固組織を微細化するためには、振動の順・逆方向の加速度を１０ｃｍ／ｓ2以上とし、その上で振動の周期を０．２秒以上１０秒未満にする必要がある。
【００２３】
また、図４から明らかなように、本発明の条件で鋳造を行った鋳片においては、鋳片断面の平均等軸晶粒径が０．５ｍｍ〜３ｍｍの範囲に改善されている。このことから、本発明の上記（８）においては、平均等軸晶粒径を０．５ｍｍ〜３ｍｍに規定した。
【００２４】
（実施例２）
炭素含有量が０．１２質量％でタンディッシュ内の温度が１５５０℃の溶鋼を鋳造速度１．８ｍ／分で、２５０ｍｍ×１５００ｍｍの鋳型に鋳造するに際して、鋳型下２ｍの位置に電磁攪拌装置を配し、この電磁コイルに５００Ａ、周波数２Ｈｚの電流を流して凝固界面に３０ｃｍ／ｓの一方向の旋回流を形成した。この従来法で得られたスラブを調査したが、横断面の平均等軸晶粒径は３．５ｍｍであり、凝固組織は微細化しなかった。
【００２５】
一方、炭素含有量が０．１２質量％の取鍋内の溶鋼に、６０質量％ＭｇＯ−４０質量％Ｔｉ混合物を添加し、この溶鋼をタンディッシュ内温度１５５０℃、鋳造速度１．８ｍ／分で、上記と同じ鋳型に鋳造した。その際、鋳型下２ｍの位置に電磁コイルを配し、この電磁コイル電流（周波数２Ｈｚ）の周期を２秒（最大コイル電流５００Ａ、最小コイル電流−５００Ａ、コイル電流増加時間０．７秒、コイル電流減少時間０．７秒、最大電流保持時間０．３秒、最小電流保持時間０．３秒）、順・逆方向の加速度を８０ｃｍ／ｓ2の条件で凝固シェル前面の溶鋼を振動させた。この本発明の方法で得られたスラブを調査すると、横断面の平均等軸晶粒径は１．３ｍｍであり、凝固組織は微細化していた。
【００２６】
【発明の効果】
以上に説明したように、本発明によると、鋳片の凝固組織を微細に等軸晶化した連続鋳造鋳片を製造することができるため、薄板では成形加工性に、厚板では低温靱性に優れた材料を製造することが可能となる。
【図面の簡単な説明】
【図１】本発明の電磁コイルに用いる電流の模式図。
【図２】本発明の電磁コイルに用いる電流の模式図（加速停止時間ありの場合）。
【図３】電磁コイルによる振動印加の説明図。
【図４】鋳片断面の平均等軸晶粒径と電磁コイルの振動周期との関係を示す図。
【符号の説明】
１：凝固シェル、 ２：未凝固溶鋼、 ３，３’：電磁コイル
４，４’，５，５’：振動方向[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous casting method for steel. More particularly, the present invention relates to a molten steel continuous casting method capable of making the metal structure of a slab cross-section into a fine equiaxed crystal, and a continuous cast slab having a fine solidified structure cast using the molten steel. is there.
[0002]
[Prior art]
The cross section of a normal continuous cast slab has a final solidified part with porosity at its center, a coarse granular crystal part near the center arranged to surround this final solidified part, and a coarse particle surrounding the coarse granular crystal part. Columnar crystal parts are observed. When this coarse granular crystal and coarse columnar crystal can be made into fine equiaxed crystals, for example, when a slab is made into a thin plate, it becomes a thin plate with remarkably excellent moldability, and for example, when it is made into a thick plate Is a thick plate with excellent low-temperature toughness.
[0003]
“Handbook of Iron and Steel” 3rd Edition, II Steelmaking and Steelmaking, p. No. 653 shows that low temperature casting is effective for equiaxed crystallization because equiaxed crystals increase when the degree of superheated molten steel is low. However, in low-temperature casting, it is necessary to set the superheating degree of the molten metal to a temperature close to the liquidus and to inject it into the mold from the immersion nozzle, so solidification abnormalities such as clogging of the immersion nozzle and deckle generation in the mold May be invited.
[0004]
Japanese Patent Laid-Open No. 50-23338 uses an induction magnetic stirrer to give a swirling flow flowing in one direction in which the flow velocity does not change to the molten steel near the solidification interface, and by dividing the columnar dendrite into equiaxed crystals. The technology to make is described. However, according to the knowledge of the present inventors, this method has a small force for equiaxed crystallization. For example, in the case of a molten steel having a C content of 0.1% or less in which equiaxed crystals are difficult to form, columnar crystals are sufficient. It is difficult to equiaxially crystallize.
[0005]
Japanese Patent Application Laid-Open No. 3-44858 is a method for improving the final solidified portion with porosity, which becomes a quality problem in the case of a cylindrical or prismatic billet, for example, induction electromagnetic stirring near the final solidified portion of 16 m to 27 m below the mold. An apparatus is provided and a swirl flow in which the direction of a half cycle stirring time is reversed for 5 to 30 seconds is used. However, since this method is a method for improving the final solidified portion, the place where the induction electromagnetic stirring device is disposed and the reversal cycle of the swirling flow are different from those of the present invention.
[0006]
[Problems to be solved by the invention]
The present invention relates to a continuous casting method capable of producing a slab in which both coarse granular crystals in the vicinity of the center and coarse columnar crystals surrounding them are finely equiaxed, and continuous casting having a fine solidified structure cast using the same. The challenge is to provide slabs.
[0007]
[Means for Solving the Problems]
In the present invention, (1) in a continuous casting apparatus having an electromagnetic coil between a meniscus in a mold and 10 m below the mold, an MgO-Al mixture or an MgO-Ti mixture is added in a ladle, tundish or mold, Is a continuous casting method of molten steel characterized in that casting is performed while vibrating in a forward and reverse direction by a moving magnetic field generated by the electromagnetic coil. (2) In
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The basic idea of the present invention is that fine oxides are dispersed in molten steel, and vibrations by electromagnetic coils are added to this to lower the superheat of the molten steel. In order to produce fine equiaxed crystals in the slab. In order to realize this basic concept, (1) a method of generating a fine oxide that can become an equiaxed crystal nucleus in a slab, and (2) an equiaxed crystal nucleus formed from a fine oxide as a starting point. It is important to clarify the vibration conditions of the electromagnetic coil in which the finely dispersed oxide does not agglomerate and coalesce while reducing the degree of superheated molten steel so that remelting of the steel can be prevented.
[0009]
First, the method (1) will be described. Although many Al 2 O 3 inclusions exist in Al deoxidized molten steel, these inclusions are very easy to agglomerate and coalesce and become coarse oxides, and thus do not act effectively as equiaxed crystal nuclei. In contrast, the present inventors have, MgO-Al mixture in the molten steel, or MgO-Ti mixture is added, by modifying the Al 2 O 3 inclusions MgO, or MgO · Al 2 O 3 The inventors have found that fine oxides can be uniformly dispersed in molten steel, and that these oxides are likely to become nuclei for the formation of equiaxed crystals. When an MgO—Al mixture or an MgO—Ti mixture is added to the molten steel, MgO is reduced by Al and Ti as shown in the formulas (1) and (2) to generate Mg gas.
3MgO + 2Al = 3Mg + Al 2 O 3 (1)
2MgO + Ti = 2Mg + TiO 2 (2)
This Mg gas reduces Al 2 O 3 inclusions in the molten steel according to the formulas (3) and (4) to generate MgO or MgO.Al 2 O 3 .
Al 2 O 3 + 3Mg = 3MgO + 2Al (3)
4Al 2 O 3 + 3Mg = 3 (MgO · Al 2 O 3 ) + 2Al (4)
Since MgO and MgO.Al 2 O 3 are more easily wetted with molten steel than Al 2 O 3 , these oxides are finely dispersed in the molten steel. When a molten steel in which fine MgO or MgO.Al 2 O 3 is dispersed in the molten steel is continuously cast, fine equiaxed crystals are generated with these oxides as nuclei, and the solidification structure can be refined.
[0010]
In the present invention, the Al concentration in the molten steel is 0.1% or less, and if the Al concentration exceeds this, even if an MgO-Al mixture or an MgO-Ti mixture is added, the Al 2 O 3 inclusions are changed to MgO, Or, it cannot be modified to MgO.Al 2 O 3 and fine oxides cannot be dispersed in the molten steel. The lower limit of the Al concentration in the molten steel is not particularly specified, but if it is less than 0.001%, the effect of deoxidation becomes unstable, so 0.001% or more is desirable.
[0011]
Moreover, it is necessary to add the MgO-Al mixture or the MgO-Ti mixture so that the MgO content is 0.01% by mass to 1% by mass with respect to the molten steel amount. If the MgO content is less than 0.01% by mass, sufficient Mg gas will not be generated to modify the Al 2 O 3 inclusions in the molten steel. If the MgO content exceeds 1% by mass, the Mg concentration in the molten steel This is because the effect of making the solidified structure in the slab a fine equiaxed crystal is lost because the oxide becomes higher and the oxide becomes coarser.
[0012]
The MgO-Al mixture or the MgO-Ti mixture should have a MgO content of 30% by mass or more, and the Al and Ti contents must each be 20% by mass or more. This greatly reduces the effect of modifying the Al 2 O 3 inclusions. Further, MgO-Al mixture or MgO-Ti mixture may contain contaminants other than MgO, Al and Ti, such as CaO, Al 2 O 3 , SiO 2, etc. The ratio, the Al content, and the Ti content must be reduced so that they do not become less than 30% by mass and less than 20% by mass, respectively.
[0013]
The addition of the MgO—Al mixture or the MgO—Ti mixture is not limited to the ladle, tundish, or mold, but may be added to the molten steel in the process up to the mold.
[0014]
Next, (2) will be described. In general, in electromagnetic stirring, a unidirectional swirl flow is imparted to the molten steel at the solidification interface, and this swirl flow is considered to break up columnar dendrites and promote equiaxed crystallization, thereby reducing the degree of superheat of the molten steel. The effect is relatively small. In order to increase the effect of reducing the degree of superheated molten steel by electromagnetic stirring, it is necessary to increase the swirl flow velocity. In that case, fine oxides become coarse due to agglomeration and coalescence, and function effectively as nuclei of equiaxed crystals. Disappear.
[0015]
On the other hand, the present invention vibrates the molten steel in front of the solidified shell by a moving magnetic field based on an electromagnetic coil, promotes heat transfer between the solidified shell and the molten steel, and effectively reduces the degree of superheat of the molten steel. It is possible to prevent re-dissolution of equiaxed crystal nuclei generated from various oxides. Here, the moving magnetic field is a magnetic field generated by moving and applying a relatively low frequency magnetic field. Furthermore, vibration caused by the moving magnetic field is applied to the front surface of the solidified shell, and the internal molten steel is less likely to flow, so that it has an effect of suppressing oxide aggregation and coalescence.
[0016]
When the current of the electromagnetic coil is varied in the pattern of FIG. 1, the vibration flow velocity of the molten steel on the front surface of the solidified shell follows the current while gradually decreasing. In the region of t1 or t3 where the vibration flow velocity on the front surface of the solidified shell is constant, the effect of promoting heat transfer by the vibration flow is similar to that of electromagnetic stirring, but in the acceleration region t2 in the forward direction or the acceleration region t4 in the reverse direction, the solidified shell. It has been found that acceleration is generated in the vibration flow on the front surface, which has a much larger heat transfer promoting effect than the electromagnetic stirring flow at a constant speed. Due to the effect of acceleration induced by this vibration, heat transfer between the solidified shell and the molten steel is promoted, and the degree of superheat of the molten steel can be efficiently reduced.
[0017]
Further, the current of the electromagnetic coil may be a pattern in which an acceleration stop time is provided between vibration periods of one cycle as in the pattern of FIG. 2, in which case the rapid acceleration is mitigated by the acceleration stop time, An effect of stabilizing the meniscus can be obtained. If the acceleration stop time exceeds 0.3 seconds, the acceleration time is shortened and the heat transfer promotion effect is impaired. Conversely, if the acceleration stop time is less than 0.03 seconds, the meniscus stabilization effect is impaired. When providing, it is preferable to set it as 0.03 second or more and 0.3 second or less.
[0018]
FIG. 3 is an explanatory view of the present invention in continuous casting of a slab, (A) is an explanatory view of a longitudinal section of the apparatus, and (B) and (C) are explanatory views of a transverse section of an arrow II. In FIG. 3, 1 is a solidified shell, 2 is unsolidified molten steel, and 3 and 3 'are electromagnetic coils. The electromagnetic coils of the slab are arranged at corresponding positions on both sides of the wide surface of the slab, and the unsolidified
[0019]
Although the above description has been given by taking a slab as an example, the implementation of the present invention is not limited to a slab, and the same effect can be obtained with a bloom, billet, or the like. Further, solidification from the slab surface layer to several tens of millimeters has already been completed below 10 m below the mold, so the electromagnetic coil in the mold begins to solidify in order to make the solidified structure of the entire slab as fine as possible. It is effective to install it at a position 10 m below the mold.
[0020]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[0021]
Example 1
50 kg of molten steel having a carbon content of 0.11% was melted in a high-frequency melting furnace, and 50 g of a 60 mass% MgO-40 mass% Al mixture was added thereto to obtain the molten steel of the present invention. This molten steel was poured at a temperature of 1600 ° C. into a water-cooled copper mold having a width of 200 mm, a length of 100 mm, and a height of 300 mm. Immediately after the injection, the molten steel in the mold was solidified while vibrating with a predetermined vibration pattern. As the vibration pattern, a moving magnetic field was formed by an alternating current having a frequency of 10 Hz, and the current of the electromagnetic coil was 100 amperes at the maximum and -100 amperes at the minimum in FIG. The acceleration was adjusted by adjusting the coil current increase time t2 which is the acceleration time in the forward direction and the coil current decrease time t4 which is the acceleration time in the reverse direction. The shorter t2 and t4, the higher the acceleration, and the longer t2 and t4, the lower the acceleration. Furthermore, the vibration period was changed from 0.2 seconds to 10 seconds by setting the minimum coil current holding time t1 and the maximum coil current holding time t3 to predetermined values.
[0022]
The steel ingot after casting was cut in a cross section, and after revealing a solidified structure, the equivalent circle diameter (average value of the entire surface) of equiaxed crystals was evaluated. The equiaxed crystal grain size was defined as 2 (a · b) 0.5 so that the grain size in the case of columnar crystals could be evaluated simultaneously (a is the major axis of the crystal grains and b is the minor axis of the crystal grains). . FIG. 4 shows the relationship between the average equiaxed grain size of the slab cross section and the vibration period of the electromagnetic coil. As can be seen from FIG. 4, the average equiaxed grain size of the slab added with the MgO-Al mixture has an absolute value of vibration wave acceleration (because there is acceleration in the forward and reverse directions) of 10 cm / s 2 or more. In addition, it can be seen that the vibration period becomes smaller in the region of 0.2 seconds or more and less than 10 seconds. The acceleration of the vibration wave is calculated from the result of measuring the flow velocity when the mold is filled with mercury and the mercury is vibrated by the electromagnetic coil with a propeller anemometer. This is because by setting the absolute value of the acceleration of the vibration wave to 10 cm / s 2 , the degree of superheated molten steel is reduced, and the remelting of equiaxed nuclei generated from fine oxides is suppressed. In addition, when the vibration period is 10 seconds or more, the swirling flow due to electromagnetic stirring is in the same flow state as when the swirling direction is periodically changed, and when the vibration period is less than 0.2 seconds, the molten steel in front of the solidified shell is electromagnetic. Since it becomes difficult to follow the current change of the coil, the effect of reducing the degree of superheat due to vibration is impaired, and it is considered that the equiaxed crystal grain size was not reduced as a result of remelting of the equiaxed crystal nucleus. Therefore, in order to refine the solidified structure, it is necessary to set the acceleration in the forward / reverse direction of vibration to 10 cm / s 2 or more, and further, to set the vibration period to 0.2 seconds or more and less than 10 seconds.
[0023]
Further, as apparent from FIG. 4, in the slab cast under the conditions of the present invention, the average equiaxed grain size of the cross section of the slab is improved to the range of 0.5 mm to 3 mm. For this reason, in the above (8) of the present invention, the average equiaxed grain size is regulated to 0.5 mm to 3 mm.
[0024]
(Example 2)
When casting a molten steel having a carbon content of 0.12% by mass and a temperature in the tundish of 1550 ° C. into a 250 mm × 1500 mm mold at a casting speed of 1.8 m / min, an electromagnetic stirrer was placed at a position 2 m below the mold. Then, a current of 500 A and a frequency of 2 Hz was passed through the electromagnetic coil to form a unidirectional swirling flow of 30 cm / s at the solidification interface. The slab obtained by this conventional method was investigated, but the average equiaxed grain size in the cross section was 3.5 mm, and the solidified structure was not refined.
[0025]
On the other hand, a 60 mass% MgO-40 mass% Ti mixture was added to the molten steel in the ladle having a carbon content of 0.12 mass%, and the molten steel was subjected to a tundish internal temperature of 1550 ° C. and a casting speed of 1.8 m / min. Then, it was cast into the same mold as above. At that time, an electromagnetic coil is arranged at a position 2 m below the mold, and the period of this electromagnetic coil current (
[0026]
【The invention's effect】
As described above, according to the present invention, it is possible to manufacture a continuous cast slab in which the solidified structure of the slab is finely equiaxed, so that it is possible to produce a thin plate with good formability and a thick plate with low temperature toughness. An excellent material can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a current used in an electromagnetic coil of the present invention.
FIG. 2 is a schematic diagram of a current used in the electromagnetic coil of the present invention (when there is an acceleration stop time).
FIG. 3 is an explanatory diagram of vibration application by an electromagnetic coil.
FIG. 4 is a diagram showing the relationship between the average equiaxed grain size of the slab cross section and the vibration period of the electromagnetic coil.
[Explanation of symbols]
1: Solidified shell, 2: Unsolidified molten steel, 3, 3 ':
Claims (8)
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| CN114485167A (en) * | 2022-02-24 | 2022-05-13 | 上海元定科技有限公司 | Working process of smelting chamber |
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| CN103341617B (en) * | 2013-07-29 | 2015-05-13 | 河北联合大学 | Method for refining metal structure in oxide metallurgy by utilizing pulsed magnet field |
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