JP4422362B2 - Method for producing a slab having a fine solidified structure - Google Patents

Method for producing a slab having a fine solidified structure Download PDF

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JP4422362B2
JP4422362B2 JP2001127626A JP2001127626A JP4422362B2 JP 4422362 B2 JP4422362 B2 JP 4422362B2 JP 2001127626 A JP2001127626 A JP 2001127626A JP 2001127626 A JP2001127626 A JP 2001127626A JP 4422362 B2 JP4422362 B2 JP 4422362B2
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molten steel
slab
zro
fine
concentration
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JP2002321043A (en
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隆 諸星
昌文 瀬々
浩至 菅野
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Nippon Steel Corp
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Nippon Steel Corp
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Description

【０００１】
【発明の属する技術分野】
本発明は、溶鋼中にＺｒ含有物を添加して接種核を形成させ、凝固組織を微細にする鋳片の製造方法に関する。
【０００２】
【従来の技術】
従来、溶鋼を凝固させた鋳片には、中心偏析やセンターポロシティ、内部割れ等の内部欠陥が発生する場合がある。この内部欠陥を抑制するため、鋳造時の溶鋼温度を低くする低温鋳造を行ったり、鋳型や支持セグメントに電磁攪拌装置を設置して凝固しつつある溶鋼を攪拌したりする等により、溶鋼が凝固した鋳片内の等軸晶の形成を促進し、凝固組織を微細化することが行われている。
しかし、低温鋳造を行う場合、溶鋼を鋳型に注湯する浸漬ノズルに詰まりが生じ易くなって、溶鋼湯面や鋳造速度が変動して操業が不安定になり、また、浸漬ノズルの極端な詰まりが発生すると、鋳造作業の中断を招く。
一方、電磁攪拌装置を用いて凝固しつつある溶鋼を攪拌する場合、攪拌力を付与した近傍の溶鋼の凝固組織を微細にできるが、鋳片の全体を等軸晶の微細な凝固組織にすることができない。しかも、鋳片の全体を等軸晶化するには、電磁攪拌装置を多段に配置する必要があるが、電磁攪拌装置の設置により、設備費用や電力コストが増大したり、設備制約により電磁攪拌装置の設置場所を確保することが困難になるという問題がある。
この対策として、特開平７−６２４１７号公報に記載されているように、金属ＭｇやＭｇ合金に、Ｓｉ、Ｍｎ、Ａｌ、Ｃの一種以上を混合した混合物を溶鋼に添加し、溶鋼内に生成する酸化物のサイズを小さくし、しかも、Ｍｇの添加コストを低減することにより、鋳片を加工した鋼材の品質を高める方法が行われている。
更に、特開平９−２８７０１５号公報に記載されているように、溶鋼に金属ＡｌあるいはＡｌ合金を添加してＡｌ脱酸を行った後、Ｚｒ、Ｃａ、Ｍｇの一種以上を添加し、溶鋼中の粗大なアルミナクラスター（酸化物）の個数を低減して製品の表面品質を改善する方法が行われている。
【０００３】
【発明が解決しようとする課題】
しかしながら、特開平７−６２４１７号公報に記載された方法では、Ｍｇを含む混合物を添加した際、溶鋼中に存在するＳｉＯ2 、ＭｎＯ、Ａｌ2 Ｏ3 等の酸化物が、脱酸過程でＭｇが酸化して生成するＭｇＯと結合して低融点の酸化物を形成するため、溶鋼の凝固時にＭｇＯが接種核として作用しなくなる。例え、ＭｇＯやＭｇＯ・Ａｌ2 Ｏ3 が生成した場合であっても、生成したＭｇＯやＭｇＯ・Ａｌ2 Ｏ3 が接種核として作用しない。
その結果、溶鋼を凝固させた鋳片は、等軸晶化せず、内部に、中心偏析やセンターポロシティ、内部割れ等の内部欠陥が発生し、鋳片の品質を著しく阻害する。更に、特開平９−２８７０１５号公報に記載された方法では、溶鋼をＡｌ脱酸した後、溶鋼にＺｒ、Ｃａ、Ｍｇの一種以上を添加しているので、脱酸によってＺｒＯ2 ・Ａｌ2 Ｏ3 、ＭｇＯ・Ａｌ2 Ｏ3 等が結合した複合の酸化物が生成するため、これ等の酸化物は、凝固初晶がγ−Ｆｅである溶鋼の接種核として作用しない。
更に、Ａｌ脱酸の後、溶鋼にＺｒを添加して脱酸を行うため、ＺｒＯ2 ・Ａｌ2 Ｏ3 、ＭｇＯ・Ａｌ2 Ｏ3 等が結合した複合型の酸化物が粗大化し、鋳片に、その一部が残存する。
そして、この鋳片を加熱して圧延加工を施して鋼材を製造した際、鋼材にヘゲ疵が発生したり、粗大な酸化物が鋼材の破壊起点になって鋼材の強度が低下する。更に、溶鋼中で、ＺｒＯ2 が粗大な酸化物を形成し、その一部が浮上してスラグに捕捉されるため、接種核として有効な溶鋼中のＺｒＯ2 が少なくなり、ＺｒＯ2 の絶対量が不足する。
その結果、例え、凝固初晶がγ−Ｆｅである溶鋼の接種核にＺｒＯ2 を活用した場合であっても、接種核の個数が少なくなるため、前記した特開平７−６２４１７号公報に記載された方法と同様に、溶鋼を凝固させた鋳片の凝固組織が粗大化し、鋳片の内部に、中心偏析やセンターポロシティ、内部割れ等の内部欠陥が発生し、鋳片の品質を著しく阻害するという問題がある。
【０００４】
本発明はかかる事情に鑑みてなされたもので、凝固初晶がγ−Ｆｅである溶鋼にＺｒを添加して微細なＺｒＯ2を多量に生成させ、これを溶鋼の接種核として利用して鋳片の凝固組織を微細にし、鋳片の内部欠陥を防止することができる微細な凝固組織を備えた鋳片の製造方法を提供することを目的とする。
【０００５】
【課題を解決するための手段】
前記目的に沿う本発明に係る微細な凝固組織を備えた鋳片の製造方法は、凝固初晶がγ−Ｆｅである溶鋼に、Ｚｒ含有物を複数に分けて添加し、前記溶鋼中にＺｒＯ2を生成させてから連続鋳造する微細な凝固組織を備えた鋳片の製造方法において、前記Ｚｒ含有物の１回目の添加量は、前記Ｚｒの濃度が、ＺｒＯ 2 を生成するＺｒの平衡濃度の７５％以下となる量である。
この方法により、溶鋼に添加したＺｒ含有物中のＺｒは、溶鋼中のフリー酸素（Ｏ）や酸化物と反応してＺｒＯ2を生成し、溶鋼中に微細な粒のＺｒＯ2を分散させ、しかも、ＺｒＯ2を接種核として溶鋼の凝固を開始させることができるので、ピンニング作用により鋳片の凝固組織を微細な等軸晶にすることができる。しかも、ＺｒＯ2が溶鋼中で、添加した部位の近傍でＺｒＯ2濃度が高くなる所謂局部的に高密度分布になるのを抑制し、ＺｒＯ2同士の結合、あるいはＺｒＯ2とＡｌ2Ｏ3等との結合によって粗大な酸化物が形成されるのを防止することができる。
【０００６】
本発明に係る微細な凝固組織を備えた鋳片の製造方法において、前記Ｚｒ含有物の１回目の添加量を、前記Ｚｒの濃度が、ＺｒＯ2を生成するＺｒの平衡濃度の７５％以下となる量にすることにより、溶鋼中に添加したＺｒ含有物中のＺｒを安定して微細なＺｒＯ2にすることができ、溶鋼中へのＺｒＯ2の分散を良好にすることができる。
【０００７】
更に、本発明に係る微細な凝固組織を備えた鋳片の製造方法において、前記１回目に添加する前記Ｚｒ含有物の添加量は、前記溶鋼中のＺｒ濃度が１０ｐｐｍ以下となる量であることが好ましい。
溶鋼にＺｒ含有物を添加する際、溶鋼中で生成したＺｒＯ2 が添加した部位の近傍で局部的に高密度となるのを安定して防止でき、しかも、ＺｒＯ2 同士の結合を少なくして微細なＺｒＯ2 を溶鋼中に分散することができる。
【０００９】
【発明の実施の形態】
続いて、添付した図面を参照しつつ、本発明を具体化した実施の形態につき説明し、本発明の理解に供する。
図１は本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる溶鋼の処理装置の全体図、図２は本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる連続鋳造装置の全体図である。
図１に示すように、本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる溶鋼の処理装置１０は、溶鋼１１を貯留可能で、しかも、底部にポーラスプラグ１２を取付けた取鍋１３と、取鍋１３の上方に、Ｚｒ含有物の一例であるＺｒ合金の表面を薄鋼板で覆ったＺｒ合金ワイア１４を案内するガイドパイプ１５を備え、ガイドパイプ１５に沿ってＺｒ合金ワイア１４を送り出し、取鍋１３内の溶鋼１１を覆ったスラグ１６を貫通し、溶鋼１１に供給するドラム式供給装置１７とを有している。
更に、図２に示すように、本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる連続鋳造装置２０は、溶鋼１１を貯湯するタンディッシュ２１と鋳型２２を有し、タンディッシュ２１の低部に鋳型２２に注湯する浸漬ノズル２３と、鋳型２２から下流側へ運ばれると共に、溶鋼１１に図示しない冷却水ノズルから散水して溶鋼１１の凝固殻１１ａの成長を促進する支持セグメント２４を有しており、更に、凝固した鋳片２５を連続的に引き抜く一対のピンチロール２６を備えている。
【００１０】
次に、本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法について溶鋼の処理装置１０、連続鋳造装置２０を用いて説明する。
炭素濃度が０．５０重量％以上の炭素（Ｃ）を含有する高炭素鋼用の溶鋼１１を取鍋１３に１５０トン入れ、この溶鋼１１を覆うように厚み１００ｍｍのスラグ１６を形成し、取鍋１３の底部に設けたポーラスプラグ１２から溶鋼１１内にアルゴンガスを２〜５ＮＬ／分吹き込んで溶鋼１１の攪拌を行った。
この溶鋼１１は、サンプリングされ、一般に用いられている分析装置を用い、溶鋼１１中に含まれるＳｉＯ2 、ＭｎＯ、Ａｌ2 Ｏ3 等の酸化物中のトータル酸素量が測定される。次に、Ｚｒ合金ワイア１４による溶鋼１１へのＺｒの添加量を求めるため、トータル酸素量を用いてＺｒの平衡濃度を以下の式を用いて求める。
ｌｏｇＫ＝（５７０００／Ｔ）＋２１．８ ・・・・・（１）
Ｋ＝ａＺｒ×ａＯ2 ・・・・・（２）
なお、ａＺｒはＺｒの活量であり、ａＯ2 は酸素（Ｏ）の活量であり、Ｋは平衡定数、Ｔは絶対温度（Ｋ）を表す。
なお、Ｚｒの平衡濃度は、日本学術振興会製鋼第１９委員会編に記載された製鋼反応の推奨平衡値（昭和５９年刊行）を参照して求めることもできる。
【００１１】
そして、ドラム式供給装置１７を作動させ、この溶鋼１１に、スラグ１６を貫通して、Ｚｒ合金ワイア１４によるＺｒの濃度をＺｒの平衡濃度の７５％以下となるように添加し、Ｚｒ合金ワイア１４は、２回に分割して添加した。
更に、最初（１回目）のＺｒの添加量は、Ｚｒ合金ワイア１４をＺｒ換算で１．５ｋｇとした。これは、溶鋼中のＺｒ濃度で１０ｐｐｍに相当する量である。この１回目のＺｒの添加による溶鋼中のＺｒ濃度が１０ｐｐｍを超えると、添加時に局部的に濃度の高い部位が形成され易くなり、ＺｒＯ2 同士の結合が生じる。Ｚｒ合金ワイア１４中のＺｒの添加量は、溶鋼１１中のＺｒ濃度がＺｒＯ2 を生成する平衡濃度の７５％以下となる量であるため、平衡論的にＺｒＯ2 は生成しない。しかし、実際には、Ｚｒ合金ワイア１４を添加している近傍において、Ｚｒ濃度が部分的に高くなっているため、ＺｒＯ2 が生成し、この生成したＺｒＯ2 は溶鋼１１中で安定に存在する。
なお、平衡濃度の７５％を超えてＺｒを添加すると、生成したＺｒＯ2 同士が結合したり、ＺｒＯ2 ・Ａｌ2 Ｏ3 等の粗大な酸化物が形成され易くなる。
一方、平衡濃度の５％未満となるように、Ｚｒを添加すると生成するＺｒＯ2 が少なくなり、接種核としての働きが減少する。
【００１２】
Ｚｒは、強脱酸元素であるため、溶鋼１１に含まれるＳｉＯ2 、ＭｎＯ、Ａｌ2 Ｏ3 等の酸化物と下式の反応によって、ＺｒＯ2 を生成する。
Ｚｒ＋ＳｉＯ2 →ＺｒＯ2 ＋Ｓｉ ・・・・・（１）
Ｚｒ＋２ＭｎＯ→ＺｒＯ2 ＋２Ｍｎ ・・・・・（２）
３Ｚｒ＋２Ａｌ2 Ｏ3 →３ＺｒＯ2 ＋４Ａｌ ・・・・・（３）
１回目に添加したＺｒ合金ワイア１４のＺｒ量は、溶鋼１１におけるＺｒの平衡濃度の７５％以下としたので、生成したＺｒＯ2 が微細な粒子になり、溶鋼１１中への分散性が良好なＺｒＯ2 にすることができる。
そして、取鍋１３の底部に設けたポーラスプラグ１２から溶鋼１１内に吹き込まれたアルゴンガスによる攪拌によって、ＺｒＯ2 の微細な粒子が溶鋼１１中に良好に分散する。
【００１３】
１回目のＺｒ合金ワイア１４の添加から数分〜１０分間経過し、１回目に添加したＺｒ合金ワイア１４のＺｒが、溶鋼１１と十分に攪拌された後、２回目のＺｒ合金ワイア１４の添加を、１回目のＺｒ合金ワイア１４の添加の場合と同様にして、ドラム式供給装置１７を作動させ、スラグ１６を貫通させて、Ｚｒ合金ワイア１４を溶鋼１１内に供給し、溶鋼１１中にＺｒを添加する。
２回目に添加されたＺｒ合金ワイア１４中のＺｒは、１回目のＺｒ合金ワイア１４の添加時に未反応で溶鋼１１中に残存したＳｉＯ2 、ＭｎＯ、Ａｌ2 Ｏ3 等の酸化物と、前記した還元反応によって、ＺｒＯ2 の微細な粒子となり、ポーラスプラグ１２から吹き込まれたアルゴンガスによる攪拌によって、溶鋼１１中に分散する。
また、１回目に添加してから数分〜１０分間以内で２回目の添加を行うと、１回目の添加で生成した溶鋼１１中のＺｒＯ2 が核として作用し、この核を起点に、２回目のＺｒ添加でＺｒＯ2 を生成することができるため、ＺｒＯ2 粒子の極端な粗大化を抑制でき、平均粒子径で１３〜１５μｍのＺｒＯ2 にすることができる。
その結果、鋳片を微細な凝固組織にすることができ、しかも、この鋳片を加工した鋼材のヘゲ疵等の欠陥の発生を防止することができる。
更に、Ｚｒ含有物としては、金属Ｚｒの他に、Ｚｒ・Ｎ、Ｚｒ・Ｓｉ2 、Ｚｒ・Ｎｉ合金、等を心として使用し、薄鋼板で巻いたものを用いる。
このように処理された溶鋼１１内には、溶鋼１１が凝固する際、接種核として作用するＺｒＯ2 が生成されており、以下に説明する連続鋳造装置２０により鋳造が行われる。
【００１４】
前記した凝固時に接種核として作用するＺｒＯ2 を生成させた溶鋼１１は、タンディッシュ２１に貯湯され、タンディッシュ２１の底部に設けた浸漬ノズル２３から鋳型２２内に注湯される。
そして、溶鋼１１は、鋳型２２による冷却と支持セグメント２４に付設した冷却水ノズルからの散水によって冷却され、凝固核１１ａを形成し、下流側になるにつれてこの凝固殻１１ａが成長して完全に凝固し、ピンチロール２６によって０．５〜１．８ｍ／分の速度で鋳片２５として引き抜かれる。
この鋳片２５は、凝固初晶がγ−Ｆｅであるので、溶鋼１１中に分散したＺｒＯ2 が接種核として作用し、この接種核を起点に凝固が開始され、更に、ピンニング作用によって、凝固組織が微細な等軸晶となる。
その結果、鋳片２５が凝固する際、中心偏析やセンターポロシティ、内部割れ等が生じることなく凝固し、内部欠陥の無い良品質の鋳片２５を製造することができる。
【００１５】
次に、微細な凝固組織を用いた鋳片を加工した鋼材について前記連続鋳造装置で製造された鋳片を用いて説明する。
連続鋳造装置２０で製造された鋳片２５は、所定の大きさに切断された後、図示しない加熱炉によって加熱され、圧延加工が施されて、軌条や形鋼等の鋼材として製造される。
この鋼材は、表層部での、粗大なＺｒＯ2の集合、あるいはＺｒＯ2とＡｌ2Ｏ3等の酸化物が結合した粗大な酸化物の生成を抑制しているので、圧延加工中に、粗大な酸化物に起因するヘゲ疵の発生が無く、しかも、鋳片２５の段階で微細な凝固組織にして中心偏析やセンターポロシティ、内部割れ等を解消しているため、この鋳片２５を圧延加工した鋼材は、中心偏析やセンターポロシティ、内部割れ等の内部欠陥を防止でき、良品質の鋼材となる。
【００１６】
【実施例】
次に、微細な凝固組織を備えた鋳片の製造方法及びその鋳片を加工した鋼材の実施例について説明する。
炭素（Ｃ）を０．８重量％を含む溶鋼１５０トンを取鍋に入れ、この溶鋼を１００ｍｍの厚みのスラグが覆う状態で、溶鋼中のＺｒ濃度を、Ｚｒの平衡濃度の７５％以下の濃度となるように、Ｚｒ合金ワイアを分割（２回）して添加した。
そして、この溶鋼を連続鋳造装置を用いて鋳造し、鋳片を製造した。この鋳片を切断し、断面の凝固組織をピクリン酸液でエッチングして凝固組織を顕微鏡で観察し、等軸晶率（鋳片の等軸晶率）を測定した後、ＺｒＯ2 個数、ＺｒＯ2 の平均粒径、ＺｒＯ2 の最大粒径の調査と、この鋳片に圧延加工を施した鋼材の圧延疵発生数を調査した。
実施例１は、初回（１回目）に、溶鋼中のＺｒ濃度が５ｐｐｍとなるようにＺｒ合金ワイアを添加し、この溶鋼を攪拌した後、２回目で、Ｚｒ濃度が１５ｐｐｍとなるようにＺｒ合金ワイアを添加した場合であり、鋳片の断面の等軸晶率が７６％となり、鋳片の断面に分散しているＺｒＯ2 個数が１１５個／ｃｍ2 と多く、しかも、このＺｒＯ2 の平均粒径が２．３μｍ、最大粒径が１３μｍと小さく、鋳片の凝固組織を微細にすることができた。
更に、この鋳片を圧延加工した鋼材の圧延疵発生数を０．５個／１００ｍにすることができ、表面及び内部品質に優れた鋼材が得られた。
実施例２は、初回（１回目）に、溶鋼中のＺｒ濃度で１０ｐｐｍとなるようにＺｒ合金ワイアを添加し、この溶鋼を攪拌した後、２回目で、Ｚｒ濃度が１０ｐｐｍとなるようにＺｒ合金ワイアを添加した場合であり、鋳片の断面の等軸晶率が７５％となり、鋳片の断面に分散しているＺｒＯ2 個数が１０８個／ｃｍ2 と多く、しかも、このＺｒＯ2 の平均粒径が２．５μｍ、最大粒径が１５μｍと小さく、鋳片の凝固組織を微細にすることができた。
更に、この鋳片を圧延加工した鋼材の圧延疵発生数を０．６個／１００ｍにすることができ、表面及び内部品質に優れた鋼材が得られた。
【００１７】
【表１】

Figure 0004422362
【００１８】
これに対し、比較例は、溶鋼中のＺｒ濃度が２０ｐｐｍとなるように、Ｚｒ合金ワイアを一括して添加した場合であり、鋳片の断面の等軸晶率が７０％となり、鋳片の断面に分散しているＺｒＯ2 個数が９８個／ｃｍ2 と少なく、しかも、このＺｒＯ2 の平均粒径が４．７μｍ、最大粒径が３３μｍと大きくなり、鋳片の凝固組織を十分に微細化することができなかった。
更に、この鋳片を圧延加工した鋼材の表面には、欠陥である圧延疵発生数が５．０個／１００ｍと多く、良品質の鋼材の歩留りも低下した。
【００１９】
以上、本発明の実施の形態を説明したが、本発明は、上記した形態に限定されるものでなく、要旨を逸脱しない条件の変更等は全て本発明の適用範囲である。
例えば、取鍋内の溶鋼の攪拌は、ポーラスプラグの他に、浸漬ランスを溶鋼中に浸漬し、浸漬ランスの先端からアルゴンガスを吹き込んで行うことができる。
更に、Ｚｒ含有物としては、金属Ｚｒの他に、Ｚｒ・Ｎ、Ｚｒ・Ｓｉ2 、Ｚｒ・Ｎｉ合金等を直接に溶鋼に添加して、溶鋼中のＺｒ濃度を高めることができる。
【００２０】
【発明の効果】
請求項１、２記載の微細な凝固組織を備えた鋳片の製造方法においては、凝固初晶がγ−Ｆｅである溶鋼に、Ｚｒ含有物を複数に分けて添加し、溶鋼中にＺｒＯ2を生成させてから連続鋳造するので、凝固初晶がγ−Ｆｅである溶鋼にＺｒ含有物を添加して微細なＺｒＯ2を生成させ、このＺｒＯ2を溶鋼の接種核として利用し、鋳片の凝固組織を微細にして、鋳片の中心偏析やセンターポロシティ、内部割れ等の内部欠陥を防止することができる。
しかも、ＺｒＯ2同士やＺｒＯ2・Ａｌ2Ｏ3等が結合した粗大な酸化物の形成を防止し、鋼材に発生するヘゲ疵等の表面欠陥を抑制し、粗大な酸化物に起因する強度の低下を防止し、良製品歩留りを高めることができる。
【００２１】
特に、請求項１記載の微細な凝固組織を備えた鋳片の製造方法においては、Ｚｒ含有物の１回目の添加量は、Ｚｒの濃度が、ＺｒＯ2を生成するＺｒの平衡濃度の７５％以下となる量にするので、ＺｒＯ2同士やＺｒＯ2とＡｌ2Ｏ3等とが結合した粗大な酸化物の形成を安定して抑制でき、微細なＺｒＯ2を溶鋼中へ分散させることができ、鋳片を微細な凝固組織にすることができる。
【００２２】
請求項２記載の微細な凝固組織を備えた鋳片の製造方法においては、１回目に添加するＺｒ含有物の添加量は、溶鋼中のＺｒ濃度が１０ｐｐｍ以下となる量にするので、粗大な酸化物の形成をより安定して抑制し、微細なＺｒＯ2を生成させて溶鋼中への分散をより高め、鋳片を微細な凝固組織にすることができる。
【図面の簡単な説明】
【図１】本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる溶鋼の処理装置の全体図である。
【図２】本発明の一実施の形態に係る微細な凝固組織を備えた鋳片の製造方法に用いる連続鋳造装置の全体図である。
【符号の説明】
１０：溶鋼の処理装置、１１：溶鋼、１１ａ：凝固殻、１２：ポーラスプラグ、１３：取鍋、１４：Ｚｒ合金ワイア、１５：ガイドパイプ、１６：スラグ、１７：ドラム式供給装置、２０：連続鋳造装置、２１：タンディッシュ、２２：鋳型、２３：浸漬ノズル、２４：支持セグメント、２５：鋳片、２６：ピンチロール[0001]
BACKGROUND OF THE INVENTION
The present invention, to form a seeded nuclei adding Zr inclusions in the molten steel, relates to the production method of the slab to refine the solidification structure.
[0002]
[Prior art]
Conventionally, internal defects such as center segregation, center porosity, and internal cracks may occur in a slab solidified from molten steel. In order to suppress this internal defect, the molten steel is solidified by performing low temperature casting to lower the molten steel temperature at the time of casting, or stirring the molten steel that is solidifying by installing an electromagnetic stirring device in the mold or support segment. The formation of equiaxed crystals in the cast slab has been promoted, and the solidified structure has been refined.
However, when performing low temperature casting, the immersion nozzle that pours molten steel into the mold is likely to become clogged, the molten steel surface and casting speed fluctuate, and the operation becomes unstable, and the immersion nozzle is extremely clogged. If this occurs, the casting operation is interrupted.
On the other hand, when the molten steel being solidified is stirred using an electromagnetic stirrer, the solidified structure of the molten steel in the vicinity to which the stirring force is applied can be made fine, but the entire slab is made into a fine solidified structure of equiaxed crystals. I can't. Moreover, in order to equiaxially crystallize the entire slab, it is necessary to arrange electromagnetic stirrers in multiple stages. However, installation of the electromagnetic stirrer increases equipment costs and power costs, and electromagnetic stirrer due to equipment restrictions. There is a problem that it is difficult to secure the installation location of the apparatus.
As a countermeasure for this, as described in JP-A-7-62417, a mixture of one or more of Si, Mn, Al, and C is added to molten steel as metal Mg or an Mg alloy, and formed in the molten steel. A method for improving the quality of a steel material obtained by processing a slab by reducing the size of the oxide to be produced and reducing the addition cost of Mg has been performed.
Furthermore, as described in JP-A-9-287015, after adding metal Al or Al alloy to molten steel and performing Al deoxidation, one or more of Zr, Ca, and Mg are added, A method for improving the surface quality of products by reducing the number of coarse alumina clusters (oxides) is being carried out.
[0003]
[Problems to be solved by the invention]
However, in the method described in JP-A-7-62417, when a mixture containing Mg is added, oxides such as SiO 2 , MnO, and Al 2 O 3 present in the molten steel are converted into Mg during the deoxidation process. Since it combines with MgO produced by oxidation to form an oxide with a low melting point, MgO does not act as an inoculum nucleus during solidification of the molten steel. For example, even when MgO or MgO.Al 2 O 3 is generated, the generated MgO or MgO.Al 2 O 3 does not act as an inoculum nucleus.
As a result, the slab obtained by solidifying the molten steel does not become equiaxed, and internal defects such as center segregation, center porosity, and internal cracks are generated inside, and the quality of the slab is significantly impaired. Further, in the method disclosed in JP-A-9-287015, after the molten steel was Al deoxidation, Zr in the molten steel, Ca, since the addition of one or more kinds of Mg, ZrO 2 · Al 2 O by deoxidation 3 and composite oxides in which MgO.Al 2 O 3 and the like are combined are formed, so that these oxides do not act as seed nuclei for molten steel whose solidification primary crystal is γ-Fe.
Furthermore, after deoxidizing Al, Zr is added to the molten steel to perform deoxidation, so that the composite oxide in which ZrO 2 · Al 2 O 3 , MgO · Al 2 O 3, etc. are bonded becomes coarse, and the slab A part of it remains.
When this slab is heated and rolled to produce a steel material, scabs are generated in the steel material, or a coarse oxide is a starting point for fracture of the steel material, resulting in a decrease in strength of the steel material. Furthermore, in the molten steel, ZrO 2 forms a coarse oxide, and a part of it rises and is captured by the slag, so that the effective amount of ZrO 2 in the molten steel as an inoculation nucleus decreases, and the absolute amount of ZrO 2 Is lacking.
As a result, for example, even when ZrO 2 is used as an inoculation nucleus of molten steel whose solidification primary crystal is γ-Fe, the number of inoculation nuclei is reduced, so that it is described in JP-A-7-62417 described above. In the same way as the above method, the solidified structure of the slab solidified with molten steel becomes coarse, and internal defects such as center segregation, center porosity, and internal cracks occur inside the slab, which significantly impairs the quality of the slab. There is a problem of doing.
[0004]
The present invention has been made in view of such circumstances. Zr is added to molten steel whose solidification primary crystal is γ-Fe to produce a large amount of fine ZrO 2 , which is used as an inoculation nucleus for molten steel. It aims at providing the manufacturing method of the slab provided with the fine solidification structure which can make the solidification structure of a piece fine and can prevent the internal defect of a slab.
[0005]
[Means for Solving the Problems]
The method for producing a slab having a fine solidification structure according to the present invention in accordance with the above object is to add a Zr-containing material to a molten steel whose solidification primary crystal is γ-Fe, and add ZrO in the molten steel. In the method for producing a slab having a fine solidified structure that is continuously casted after forming 2 , the first addition amount of the Zr-containing material is such that the concentration of Zr is the equilibrium concentration of Zr that generates ZrO 2. Of 75% or less.
By this method, Zr in the Zr-containing material added to the molten steel reacts with free oxygen (O) and oxides in the molten steel to generate ZrO 2 , and fine grain ZrO 2 is dispersed in the molten steel. Moreover, since solidification of molten steel can be started using ZrO 2 as an inoculum nucleus, the solidification structure of the slab can be made into fine equiaxed crystals by the pinning action. In addition, ZrO 2 suppresses the so-called local high density distribution in which ZrO 2 concentration increases in the vicinity of the added site in the molten steel, and bonds between ZrO 2 , ZrO 2 and Al 2 O 3, etc. It is possible to prevent the formation of a coarse oxide due to bonding with the.
[0006]
In slab manufacturing method for having a fine solidification structure according to the present invention, the addition amount of the first of said Zr-containing compound, the concentration of the Zr is a more than 75% of the equilibrium concentration of Zr to produce a ZrO 2 by the amount that the, the Zr and Zr-containing material which had been added in the molten steel can be stably fine ZrO 2, it is possible to improve the dispersion of ZrO 2 into the molten steel.
[0007]
Furthermore, in the method for producing a slab having a fine solidified structure according to the present invention, the amount of the Zr-containing material added in the first time is an amount such that the Zr concentration in the molten steel is 10 ppm or less. Is preferred.
When adding a Zr-containing material to molten steel, ZrO 2 produced in the molten steel can be stably prevented from becoming locally dense in the vicinity of the added site, and the bonding between ZrO 2 can be reduced. Fine ZrO 2 can be dispersed in the molten steel.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
FIG. 1 is an overall view of a processing apparatus for molten steel used in a method for producing a slab having a fine solidified structure according to an embodiment of the present invention, and FIG. 2 is a fine solidified structure according to an embodiment of the present invention. It is a general view of the continuous casting apparatus used for the manufacturing method of the slab provided with.
As shown in FIG. 1, the molten steel processing apparatus 10 used for the manufacturing method of the slab provided with the fine solidification structure | tissue which concerns on one embodiment of this invention can store the molten steel 11, and also has a porous plug in the bottom part. 12 is provided, and a guide pipe 15 is provided above the ladle 13 to guide a Zr alloy wire 14 in which a Zr alloy surface, which is an example of a Zr-containing material, is covered with a thin steel plate. A Zr alloy wire 14 is sent out along the slag 16 covering the molten steel 11 in the ladle 13, and a drum type supply device 17 for supplying the molten steel 11 to the slag 16 is provided.
Furthermore, as shown in FIG. 2, the continuous casting apparatus 20 used for the manufacturing method of the slab provided with the fine solidification structure | tissue which concerns on one embodiment of this invention has the tundish 21 and the casting_mold | template 22 which store the molten steel 11 hot water. A dipping nozzle 23 for pouring the mold 22 into the lower part of the tundish 21, and being carried downstream from the mold 22 and sprinkling water from a cooling water nozzle (not shown) into the molten steel 11 to form the solidified shell 11 a of the molten steel 11. It has a support segment 24 that promotes growth, and further includes a pair of pinch rolls 26 for continuously drawing out the solidified slab 25.
[0010]
Next, the manufacturing method of the slab provided with the fine solidification structure | tissue which concerns on one embodiment of this invention is demonstrated using the processing apparatus 10 and the continuous casting apparatus 20 of molten steel.
150 tons of molten steel 11 for high carbon steel containing carbon (C) having a carbon concentration of 0.50 wt% or more is placed in a ladle 13, and a slag 16 having a thickness of 100 mm is formed so as to cover the molten steel 11. Argon gas was blown into the molten steel 11 from the porous plug 12 provided at the bottom of the pan 13 to stir the molten steel 11.
The molten steel 11 is sampled and the total oxygen content in oxides such as SiO 2 , MnO, Al 2 O 3 and the like contained in the molten steel 11 is measured using a commonly used analyzer. Next, in order to obtain the amount of Zr added to the molten steel 11 by the Zr alloy wire 14, the equilibrium concentration of Zr is obtained using the following equation using the total oxygen amount.
log K = (57000 / T) +21.8 (1)
K = aZr × aO 2 (2)
AZr is the activity of Zr, aO 2 is the activity of oxygen (O), K is the equilibrium constant, and T is the absolute temperature (K).
The equilibrium concentration of Zr can also be obtained by referring to the recommended equilibrium value for steelmaking reaction (published in 1984) described in the 19th Committee of the Japan Society for the Promotion of Science.
[0011]
Then, the drum type supply device 17 is operated, and the Zr concentration of the Zr alloy wire 14 is added to the molten steel 11 through the slag 16 so that it is 75% or less of the Zr equilibrium concentration. 14 was added in two portions.
Furthermore, the first (first time) addition amount of Zr was 1.5 kg in terms of Zr in the Zr alloy wire 14. This is an amount corresponding to 10 ppm in terms of Zr concentration in molten steel. When the Zr concentration in the molten steel by the first Zr addition exceeds 10 ppm, a locally high concentration portion is easily formed at the time of addition, and ZrO 2 is bonded to each other. The amount of Zr added in the Zr alloy wire 14 is such that the Zr concentration in the molten steel 11 is 75% or less of the equilibrium concentration that generates ZrO 2, and therefore ZrO 2 is not generated in equilibrium. However, in reality, the Zr concentration is partially increased in the vicinity where the Zr alloy wire 14 is added, so that ZrO 2 is generated, and the generated ZrO 2 exists stably in the molten steel 11. .
In addition, when Zr is added exceeding 75% of the equilibrium concentration, the generated ZrO 2 is bonded to each other, and a coarse oxide such as ZrO 2 · Al 2 O 3 is easily formed.
On the other hand, when Zr is added so as to be less than 5% of the equilibrium concentration, the amount of ZrO 2 produced decreases and the function as an inoculum nucleus decreases.
[0012]
Since Zr is a strong deoxidizing element, ZrO 2 is generated by a reaction of the following formula with an oxide such as SiO 2 , MnO, Al 2 O 3 contained in the molten steel 11.
Zr + SiO 2 → ZrO 2 + Si (1)
Zr + 2MnO → ZrO 2 + 2Mn (2)
3Zr + 2Al 2 O 3 → 3ZrO 2 + 4Al (3)
Since the Zr amount of the Zr alloy wire 14 added for the first time is 75% or less of the equilibrium concentration of Zr in the molten steel 11, the generated ZrO 2 becomes fine particles and the dispersibility in the molten steel 11 is good. ZrO 2 can be used.
Then, fine particles of ZrO 2 are well dispersed in the molten steel 11 by stirring with argon gas blown into the molten steel 11 from the porous plug 12 provided at the bottom of the ladle 13.
[0013]
After several minutes to 10 minutes have passed since the first addition of the Zr alloy wire 14, the Zr alloy wire 14 added in the first time is sufficiently stirred with the molten steel 11, and then the second addition of the Zr alloy wire 14 is performed. In the same manner as in the case of the first addition of the Zr alloy wire 14, the drum type supply device 17 is operated, the slag 16 is penetrated, and the Zr alloy wire 14 is supplied into the molten steel 11. Add Zr.
Zr in the Zr alloy wire 14 added for the second time is an oxide such as SiO 2 , MnO, Al 2 O 3 which has not been reacted and remains in the molten steel 11 when the first Zr alloy wire 14 is added. By the reduction reaction, fine particles of ZrO 2 are formed and dispersed in the molten steel 11 by stirring with argon gas blown from the porous plug 12.
When the second addition is performed within a few minutes to 10 minutes after the first addition, ZrO 2 in the molten steel 11 produced by the first addition acts as a nucleus, and this nucleus serves as a starting point. Since ZrO 2 can be generated by the addition of Zr for the second time , extreme coarsening of ZrO 2 particles can be suppressed, and ZrO 2 having an average particle diameter of 13 to 15 μm can be obtained.
As a result, the slab can be made into a fine solidified structure, and the occurrence of defects such as scabs in the steel material processed from the slab can be prevented.
Further, as the Zr-containing material, in addition to the metal Zr, Zr · N, Zr · Si 2 , Zr · Ni alloy, etc. are used as a core, and those wound with a thin steel plate are used.
In the molten steel 11 thus treated, ZrO 2 that acts as an inoculation nucleus is generated when the molten steel 11 is solidified, and casting is performed by the continuous casting apparatus 20 described below.
[0014]
The molten steel 11 in which ZrO 2 that acts as an inoculation nucleus during solidification is stored in the tundish 21 and poured into the mold 22 from the immersion nozzle 23 provided at the bottom of the tundish 21.
The molten steel 11 is cooled by cooling by the mold 22 and water sprayed from the cooling water nozzle attached to the support segment 24 to form a solidified core 11a. The solidified shell 11a grows toward the downstream side and is completely solidified. Then, the slab 25 is pulled out by the pinch roll 26 at a speed of 0.5 to 1.8 m / min.
In this slab 25, since the solidification primary crystal is γ-Fe, ZrO 2 dispersed in the molten steel 11 acts as an inoculation nucleus, and solidification starts from this inoculation nucleus, and further, solidification occurs by a pinning action. The structure becomes fine equiaxed crystals.
As a result, when the slab 25 is solidified, it is solidified without causing center segregation, center porosity, internal cracks, etc., and a good quality slab 25 having no internal defects can be manufactured.
[0015]
Next, a steel material obtained by processing a slab using a fine solidified structure will be described using the slab manufactured by the continuous casting apparatus.
The slab 25 manufactured by the continuous casting apparatus 20 is cut into a predetermined size, heated by a heating furnace (not shown), subjected to a rolling process, and manufactured as a steel material such as a rail or a shape steel.
This steel material suppresses the formation of coarse ZrO 2 aggregates or coarse oxides in which ZrO 2 and oxides such as Al 2 O 3 are combined in the surface layer portion. The slab 25 is rolled because there is no generation of baldness due to the oxides, and since a fine solidified structure is formed at the stage of the slab 25 to eliminate center segregation, center porosity, internal cracks, etc. The processed steel can prevent internal defects such as center segregation, center porosity, and internal cracks, and can be a high quality steel.
[0016]
【Example】
Next, a method for producing a slab having a fine solidified structure and an example of a steel material obtained by processing the slab will be described.
In a state where 150 tons of molten steel containing 0.8% by weight of carbon (C) is put in a ladle and this molten steel is covered with 100 mm thick slag, the Zr concentration in the molten steel is 75% or less of the equilibrium concentration of Zr. The Zr alloy wire was added in portions (twice) so as to obtain a concentration.
And this molten steel was cast using the continuous casting apparatus, and the slab was manufactured. The slab was cut, was observed by etching the solidified structure of the cross section of the solidification structure with picric acid solution under a microscope, measuring the equal JikuAkiraritsu (equal JikuAkiraritsu of the slab), ZrO 2 number, ZrO 2 having an average particle size, and investigation of the maximum particle size of ZrO 2, was examined the number of rolling flaw generation of steel subjected to rolling to the cast slab.
In Example 1, the Zr alloy wire was added so that the Zr concentration in the molten steel was 5 ppm at the first time (first time), and after the molten steel was stirred, the Zr concentration was 15 ppm at the second time. a case of adding alloy wire, adjacent equal JikuAkiraritsu of the cross section of the slab 76%, ZrO 2 number dispersed in cross-section of the slab many as 115 pieces / cm 2, moreover, the ZrO 2 The average particle size was as small as 2.3 μm and the maximum particle size was as small as 13 μm, and the solidified structure of the slab could be made fine.
Furthermore, the number of rolling defects generated in the steel material obtained by rolling this slab can be reduced to 0.5 pieces / 100 m, and a steel material excellent in surface and internal quality was obtained.
In Example 2, the Zr alloy wire was added so that the Zr concentration in the molten steel was 10 ppm in the first time (first time), and after the molten steel was stirred, the Zr concentration was adjusted to 10 ppm in the second time. The alloy wire is added, the equiaxed crystal ratio of the cross section of the slab is 75%, the number of ZrO 2 dispersed in the cross section of the slab is as large as 108 pieces / cm 2 , and the ZrO 2 The average particle size was as small as 2.5 μm and the maximum particle size was as small as 15 μm, and the solidified structure of the slab could be made fine.
Furthermore, the number of rolling defects generated in the steel material obtained by rolling this slab could be 0.6 / 100 m, and a steel material excellent in surface and internal quality was obtained.
[0017]
[Table 1]
Figure 0004422362
[0018]
On the other hand, the comparative example is a case where Zr alloy wires are added all at once so that the Zr concentration in the molten steel is 20 ppm. The equiaxed crystal ratio of the cross section of the slab becomes 70%. The number of ZrO 2 dispersed in the cross section is as small as 98 / cm 2 , and the average particle size of ZrO 2 is as large as 4.7 μm and the maximum particle size is as large as 33 μm. Could not be converted.
Furthermore, the surface of the steel material obtained by rolling this slab has a large number of rolling flaws as defects of 5.0 pieces / 100 m, and the yield of high-quality steel materials also decreased.
[0019]
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and all changes in conditions and the like that do not depart from the gist are within the scope of the present invention.
For example, the stirring of the molten steel in the ladle can be performed by dipping an immersion lance in the molten steel in addition to the porous plug and blowing argon gas from the tip of the immersion lance.
Further, as a Zr-containing material, Zr · N, Zr · Si 2 , Zr · Ni alloy, etc. can be directly added to the molten steel in addition to the metal Zr to increase the Zr concentration in the molten steel.
[0020]
【The invention's effect】
In the manufacturing method of the slab provided with the fine solidification structure of Claims 1 and 2 , the Zr-containing material is added to the molten steel whose solidification primary crystal is γ-Fe in a plurality of portions, and ZrO 2 is added to the molten steel. Is produced and then continuously cast. Therefore, a Zr-containing material is added to molten steel whose solidification primary crystal is γ-Fe to produce fine ZrO 2 , and this ZrO 2 is used as an inoculation nucleus of the molten steel. The solidification structure of the slab can be made fine to prevent internal defects such as center segregation, center porosity, and internal cracks in the slab.
Moreover, to prevent the formation of coarse oxides such as ZrO 2 or between the ZrO 2 · Al 2 O 3 is bound to suppress surface defects of scab defects and the like generated in the steel material, due to the coarse oxides strength Can be prevented, and the yield of good products can be increased.
[0021]
In particular, in the method for producing a slab having a fine solidified structure according to claim 1, the first addition amount of the Zr-containing material is such that the concentration of Zr is 75% of the equilibrium concentration of Zr producing ZrO 2. since the amount equal to or less than, the formation of coarse oxides and ZrO 2 together and ZrO 2 and Al 2 O 3 or the like is attached can stably suppressed, it is possible to disperse the fine ZrO 2 into the molten steel The slab can be made into a fine solidified structure.
[0022]
In the method for producing a slab having a fine solidified structure according to claim 2 , the amount of the Zr-containing material added at the first time is such that the Zr concentration in the molten steel is 10 ppm or less. Oxide formation can be suppressed more stably, fine ZrO 2 can be generated to further increase dispersion in the molten steel, and the slab can have a fine solidified structure.
[Brief description of the drawings]
FIG. 1 is an overall view of a molten steel processing apparatus used in a method for producing a slab having a fine solidified structure according to an embodiment of the present invention.
FIG. 2 is an overall view of a continuous casting apparatus used in a method for producing a slab having a fine solidified structure according to an embodiment of the present invention.
[Explanation of symbols]
10: Molten steel processing apparatus, 11: Molten steel, 11a: Solidified shell, 12: Porous plug, 13: Ladle, 14: Zr alloy wire, 15: Guide pipe, 16: Slag, 17: Drum type feeder, 20: Continuous casting apparatus, 21: tundish, 22: mold, 23: immersion nozzle, 24: support segment, 25: cast slab, 26: pinch roll

Claims (2)

凝固初晶がγ−Ｆｅである溶鋼に、Ｚｒ含有物を複数に分けて添加し、前記溶鋼中にＺｒＯ2を生成させてから連続鋳造する微細な凝固組織を備えた鋳片の製造方法において、
前記Ｚｒ含有物の１回目の添加量は、前記Ｚｒの濃度が、ＺｒＯ 2 を生成するＺｒの平衡濃度の７５％以下となる量であることを特徴とする微細な凝固組織を備えた鋳片の製造方法。In a method for producing a slab comprising a finely solidified structure in which a Zr-containing material is added to a molten steel whose solidification primary crystal is γ-Fe in a plurality of portions and ZrO 2 is generated in the molten steel and then continuously cast . ,
The first addition amount of the Zr-containing material is a slab having a fine solidified structure , wherein the Zr concentration is an amount that is 75% or less of the equilibrium concentration of Zr that produces ZrO 2. Manufacturing method . 請求項１記載の微細な凝固組織を備えた鋳片の製造方法において、前記１回目に添加する前記Ｚｒ含有物の添加量は、前記溶鋼中のＺｒ濃度が１０ｐｐｍ以下となる量であることを特徴とする微細な凝固組織を備えた鋳片の製造方法。According to claim 1 slab manufacturing method having a fine solidification structure as claimed, the addition amount of the Zr-containing compound to be added to the first time is that Zr concentration in the molten steel is an amount to be 10ppm or less A method for producing a cast slab having a fine solidified structure.
JP2001127626A 2001-04-25 2001-04-25 Method for producing a slab having a fine solidified structure Expired - Fee Related JP4422362B2 (en)

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