JP3745689B2 - Manufacturing method for continuous cast slabs with excellent cleanliness - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、低コストで連々鋳の取鍋交換部の清浄性を向上させる連続鋳造鋳片の製造方法に関するものである。
【０００２】
【従来の技術】
連続鋳造において安定的に鋳造できる定常部では、これまでの種々の対策により清浄性の向上が図られてきた。しかしながら、鋼材の高清浄性のユーザー要求は高まる一方であり、連続鋳造時の取鍋交換部いわゆる非定常部については、清浄性の悪化のため、歩留ロスが問題となっている。
【０００３】
取鍋交換部の非定常部での清浄性悪化の原因としては、（１）取鍋内のスラグ流出、（２）取鍋の注入口の詰物、（３）空気、タンディッシュスラグ、耐火物による再酸化の影響が考えられる。（２）、（３）に関しては、種々の対策が行われているが、Ａｌキルド鋼の場合は再酸化によるアルミナの増加であり、表面欠陥が問題となる。このような表面欠陥については、特開平０５−１５４６２３号公報にあるように鋳型において電磁力を用いて溶鋼を攪拌することにより防止することが可能である。それに対して、（１）の取鍋内のスラグ流出に関しては、スラグ系介在物の鋳片内部への残存つまり内部欠陥の問題となる。表面欠陥と同様に、鋳型内の電磁力例えば電磁ブレーキなどによりノズルからの吐出流を抑制し、介在物の鋳片内部への侵入を抑制する技術がある。このような技術を利用しても、スラグ系介在物の鋳片内部への侵入は完全には抑制できない。
【０００４】
そのため、（１）の取鍋内のスラグ流出に関しては、抜本的に取鍋からのスラグ流出を防止するか、もしくはタンディッシュにおいてスラグ系介在物を浮上させることが必要である。これまでの技術としては、特開昭５５−１１７５４８号公報に代表されるように取鍋内に溶鋼を残し注入を停止することによりスラグの流出を防止する方法や特開平０８−２６７２２３号公報に代表されるスラグ流出防止用の物体をスラグとメタルの間に挿入する方法や特開昭５６−１２２６５６号公報に代表されるスラグ流出検知技術、さらに特開平０５−１６９２０７号公報に代表されるタンディッシュでの形状などの改善が挙げられる。特開平０８−２６７２２３号公報に代表されるようなスラグ流出防止用の物体をスラグとメタルの間に挿入する方法では、スラグ流出防止用の物体を挿入する際にスラグを巻き込むため、完全にスラグ流出を防止することが難しい。特開昭５６−１２２６５６号公報などにおけるにおけるスラグ流出検知技術については、既にタンディッシュへスラグが流出したことを検知しているため、完全な流出防止はできない。また、タンディッシュの形状を変更する場合、特にタンディッシュの浴深を深くする場合には、非常に多大なコストが必要となる。
【０００５】
いずれにしても、これまでの技術においては、操業性、スラグ流出の完全な防止ができない、多大なコスト増の問題があり、低コストで効果的な方法が求められる。
【０００６】
【発明が解決しようとする課題】
連続鋳造の取鍋交換部の清浄性を低コストで改善する製造方法を確立することである。
【０００９】
【課題を解決するための手段】
本発明の要旨は、以下の通りである。
【００１０】
手段１は、鋳造中に取鍋を交換して連続鋳造鋳片を製造するに当たり、取鍋からタンディッシュへ溶鋼注入開始前までに取鍋内の溶鋼中のＳ濃度を０．００６ｍａｓｓ％以下にし、取鍋からタンディッシュに溶鋼を注入開始後に溶鋼高さＨが下式（１）を満たすようにタンディッシュへの溶鋼の注入を停止し、取鍋交換して次の取鍋から溶鋼をタンディッシュ注入することを特徴とする清浄性に優れた連続鋳造鋳片の製造方法である。
１＜Ｈ／Ｄ≦１．４ ・ ・ ・（１）
Ｈ：取鍋内溶鋼高さ（ｍ）
Ｄ：取鍋の溶鋼注入口径（ｍ）
【００１１】
手段２は、鋳造中に取鍋を交換して連続鋳造鋳片を製造するに当たり、下式（２）を満たすようにタンディッシュ内の溶鋼にＳ、Ｓｅ、Ｔｅのいずれか１種類もしくは２種類以上添加することを特徴とする上記手段１記載の清浄性に優れた連続鋳造鋳片の製造方法である。
［％Ｓ］＋［％Ｓｅ］＋［％Ｔｅ］≧０．００８ ・ ・ ・（２）
［％Ｓ］：溶鋼中Ｓ濃度（ｍａｓｓ％）
［％Ｓｅ］：溶鋼中Ｓｅ濃度（ｍａｓｓ％）
［％Ｔｅ］：溶鋼中Ｔｅ濃度（ｍａｓｓ％）
【００１２】
手段３は、鋳造中に取鍋を交換して連続鋳造鋳片を製造するに当たり、取鍋交換時のタンディッシュ内の溶鋼高さが低下している間にタンディッシュから鋳型へ注入される溶鋼に、下式（２）を満たすようにタンディッシュ内でＳ、Ｓｅ、Ｔｅのいずれか１種類もしくはＳ、Ｓｅ、Ｔｅの２種類以上を添加することを特徴とする上記手段１記載の清浄性に優れた連続鋳造鋳片の製造方法である。
［％Ｓ］＋［％Ｓｅ］＋［％Ｔｅ］≧０．００８ ・ ・ ・（２）
［％Ｓ］：溶鋼中Ｓ濃度（ｍａｓｓ％）
［％Ｓｅ］：溶鋼中Ｓｅ濃度（ｍａｓｓ％）
［％Ｔｅ］：溶鋼中Ｔｅ濃度（ｍａｓｓ％）
【００１３】
【発明の実施の形態】
連続鋳造の取鍋交換部における取鍋スラグ流出に伴う鋳片の清浄性改善のために、取鍋内に溶鋼を残しスラグ流出を防止することをまず検討した。供試鋼の成分は表１のＡであり、低炭アルミキルド鋼である。取鍋内に溶鋼を残し、鋳造後に取鍋内に残存した溶鋼量を秤量して残溶鋼量を求めた。残溶鋼量は、取鍋における溶鋼高さＨに換算し、取鍋の溶鋼注入口Ｄで規格化（Ｈ／Ｄ）した。取鍋の溶鋼注入口Ｄは、１００ｍｍである。鋳片の清浄性については、取鍋交換部前後のスラブの介在物をスライム法で調査し、得られた５０μｍ超の介在物の組成をＥＰＭＡで定量分析してＣａＯ＞２０％を含むＣａＯ−Ａｌ2Ｏ3系の介在物をスラグ系介在物として個数測定し、定常部で得られるスラグ系介在物個数の平均値と標準偏差を足しあわせた値を１として規格化した。規格化した残溶鋼量（Ｈ／Ｄ）と規格化したスラグ系介在物個数との関係を図１に示す。図１に示すように、Ｈ／Ｄ＞１でほぼ定常部と同等のスラグ系介在物個数となることがわかった。ただし、図１に示すように、Ｈ／Ｄ＞１であっても、スラグ系介在物個数が多い場合があり、この原因について検討した。
【００１４】
【表１】
残溶鋼量の指標Ｈ／Ｄ＞１であってもスラグ系介在物が増加する要因として、（１）残溶鋼量が多い場合でも、溶鋼中に渦が発生しスラグが巻き込まれる、（２）二次精錬後からタンディッシュに注入するまでの間にスラグ系介在物が浮上し取鍋溶鋼の上部に濃化し、介在物が濃化した溶鋼が注入されるという２点が考えられる。ここで、スラグ巻き込みやスラグ系介在物の浮上現象に対して、スラグ／メタル界面エネルギーの影響が考えられるため、スラグ／メタル界面エネルギーに影響を与える溶質元素について検討した。スラグ／メタル界面エネルギーを低下させる元素としては、酸素と硫黄が考えられるが、今回検討しているものはＡｌキルド鋼であるために溶鋼中の硫黄濃度について検討した。
【００１６】
まず、図１に示したＨ／Ｄ＞１の領域について、Ｓ濃度とスラグ系介在物個数との関係を調査した。図２に溶鋼中Ｓ濃度と規格化したスラグ系介在物個数の関係を示す。図２に示すように、Ｓ濃度が０．００６％以下では定常部のスラグ系介在物個数と同等以下であることがわかった。逆にＳ濃度が０．００７％以上では、定常部よりも明らかにスラグ系介在物個数が増加する場合があることがわかった。Ｓ濃度が高くなると、スラグ／メタル界面エネルギーが低下し、スラグの巻き込みを助長するためと考えられる。
【００１７】
次に、Ｈ／Ｄ＞１の条件を満たし、鋳片の規格化したスラグ系介在物個数が１以下のものについて、冷延まで実施し、漏洩磁束を用いた内部欠陥探傷を行った。内部欠陥探傷の値は、定常部で得られる平均値と標準偏差を足しあわせた値を１として規格化し、Ｓ濃度との関係を調査した。図３に規格化した内部欠陥探傷結果とＳ濃度との関係を示す。図３に示すように、鋳片においてスラグ系介在物個数が定常部と同等であったにも関わらず、Ｓ濃度が０．００７％以下において内部欠陥探傷結果が定常部よりも高めになっていることがわかった。逆に、Ｓ濃度が０．００８％以上では、鋳片の規格化したスラグ系介在物個数が１以下であれば、内部欠陥探傷結果も定常部と同等の結果となった。この原因として、スラグ系介在物以外の連鋳パウダー系介在物、浸漬ノズル内に吹き込んでいる不活性ガスの残留気泡の影響が考えられる。取鍋交換部の非定常部においては、タンディッシュの溶鋼高さが低下するために、浸漬ノズル内の溶鋼流動状態が変化し、不活性ガスのボイリングなどが生じている可能性がある。特に、Ｓ濃度が低い場合には、ガス／メタル界面エネルギーが増加し、気泡が溶鋼中に安定して存在しにくくなるために、ボイリングなどを起こしやすい状態であると考えられる。
【００１８】
以上の結果から、取鍋からタンディッシュに溶鋼を注入する際のスラグ流出を抑制するために、取鍋内の溶鋼中Ｓ濃度を０．００６％以下とし、取鍋内の残溶鋼量がＨ／Ｄ＞１となる条件とし、タンディッシュから鋳型内に溶鋼を注入する際には、パウダー系欠陥、気泡系欠陥を防止するためにタンディッシュ内の溶鋼中Ｓ濃度が０．００８％以上とすることで、取鍋交換部の清浄性悪化を防止することが可能である。
【００１９】
次に、実操業において取鍋交換部の清浄性を向上させるための具体的な操業方法について検討した。まず、取鍋からタンディッシュに溶鋼を注入する際には、取鍋スラグの流出を防止する目的から、二次精錬までの工程で溶鋼中Ｓ濃度を０．００６％以下に低下させる必要がある。比較的グレードの低い鋼種においては、取鍋スラグの流出のみを防止することで問題はないが、極薄ブリキ用素材のようなグレードの高い鋼種においては、残留気泡や微量に存在するパウダー系介在物の影響が無視できないためにタンディッシュ内の溶鋼中Ｓ濃度を０．００８％以上とすることが必要である。そこで、取鍋内溶鋼のＳ濃度を０．００６％以下とし、タンディッシュ内の溶鋼にＳ濃度が０．００８％以上となるようにＳを添加することが必要である。タンディッシュ内にＳを添加する場合には、Ｓの歩留などを考慮してＦｅ−Ｓ合金をワイヤにより溶鋼中に添加することが好ましい。添加位置は、添加後の均一性を考慮して、取鍋から注入される注入流近傍に添加するのが好ましい。また、添加するタイミングについては、取鍋交換の際にタンディッシュの溶鋼高さが低くなっている状態の時にボイリングなどの影響が顕著となるために、取鍋交換部においてタンディッシュの溶鋼高さが低下する時期に鋳型へ注入する溶鋼について溶鋼中Ｓ濃度を０．００８％以上にする必要がある。Ｓを添加する際には、溶鋼のスループット量と必要なＳ濃度からワイヤ添加速度を決める。定常部においても、Ｓの添加により品質の安定化が図れるため、定常部、取鍋交換部を問わずＳを添加することは、品質上は好ましい。
【００２０】
さらに、スラグ／メタル及びガス／メタル界面エネルギーを制御する元素として、Ｓ以外にＳｅやＴｅも同等の効果があり、Ｓの代替としてＳｅやＴｅを添加しても構わず、Ｓ、Ｓｅ及びＴｅを２種以上混合して添加しても構わない。また、タンディッシュ内溶鋼にＳを添加する場合には鋳片の内部割れやＭｎＳ等の影響を考えて、できるだけＭｎ／Ｓ＞２０とすることが好ましい。
【００２１】
本発明の対象鋼種は特に限定するものではないが、キルド鋼、特にアルミキルド鋼である。
【００２２】
【実施例】
（実施例１）
自動車用冷延鋼板を対象に実施した。成分は前記した表１のＡである。製造工程は、溶銑予備処理、転炉、二次精錬（ＲＨ）、連鋳である。代表的な条件を表２に示す。評価の指標としては、前述の規格化したスラグ系介在物個数（１以下が良好）と製品段階でのスラグ系介在物起因の欠陥（加工時の鋼板の割れや表面キズ）を指標化した。介在物性欠陥については、定常部における介在物性欠陥の発生率の平均値に標準偏差を足しあわせた値を１として規格し、１以下を良好とした。本発明例では、溶銑予備処理において、溶鋼中Ｓ濃度を０．００５％以下とし、二次精錬の段階で復硫し溶鋼中Ｓ濃度が０．００６％を超えている場合には脱硫処理を行い、溶鋼中Ｓ濃度を０．００６％以下とした。さらに、取鍋からタンディッシュに溶鋼を注入する末期においては、取鍋内溶鋼高さ／取鍋の溶鋼注入口径＞１（Ｈ／Ｄ＞１）となる条件で溶鋼の注入を停止して次鍋の溶鋼注入を行う操業を行った。なお、取鍋の溶鋼注入口Ｄは、本発明例及び比較例とも１００ｍｍである。比較例１〜２では、溶鋼中Ｓ濃度の制御を行わず、取鍋の溶鋼注入末期の残溶鋼量の制御も実施しなかった。比較例３〜４では、溶鋼中Ｓ濃度の制御を行ったが、取鍋の溶鋼注入末期の残溶鋼量の制御は実施しなかった。比較例５〜６では、溶鋼中Ｓ濃度の制御を行わず、取鍋の溶鋼注入末期の残溶鋼量の制御のみを実施した。結果を表３に示す。
【００２３】
【表２】
【表３】
本発明例１〜５では、取鍋内の溶鋼中Ｓ濃度を０．００６％以下とし、かつ１．４≧Ｈ／Ｄ＞１となるように取鍋内に溶鋼を残しているために、スラグ流出がなくスラグ系介在物指標、介在物性欠陥の指標とも１以下となり良好である。
【００２６】
比較例１、２では、取鍋内の溶鋼中Ｓ濃度が０．００８％以上であり、かつ残溶鋼量の指標Ｈ／Ｄ＜１であるために、スラグ系介在物指標、介在物性欠陥指標とも大幅に１を超えている。
【００２７】
比較例３、４では、取鍋内の溶鋼中Ｓ濃度が０．００６％以下であるが、残溶鋼量の指標Ｈ／Ｄ＜１であり、スラグ系介在物指標、介在物性欠陥指標とも１を超えている。
【００２８】
比較例５、６では、残溶鋼量の指標Ｈ／Ｄ＞１であるが、取鍋内の溶鋼中Ｓ濃度が０．００７％以上であり、スラグ系介在物指標、介在物性欠陥指標とも１を超えている。
【００２９】
（実施例２）
ブリキを対象に実施した。供試鋼の成分は、前記した表１のＢである。製造工程は、溶銑予備処理、転炉、二次精錬（ＲＨ）、連鋳である。取鍋の溶鋼注入口Ｄは、１００ｍｍである。タンディッシュの溶鋼量は、定常部において６０ｔｏｎである。代表的な条件を表４に示す。評価の指標としては、前述の規格化したスラグ系介在物個数と冷延後の規格化した内部欠陥探傷指数（ともに１以下が良好）、さらに製品での介在物起因の欠陥の発生率を指標化した。介在物起因の欠陥の発生率については、定常部における介在物性欠陥の発生率の平均値に標準偏差を足しあわせた値を１として規格し、１以下を良好とした。本発明例では、溶銑予備処理において、溶鋼中Ｓ濃度を０．００５％以下とし、二次精錬の段階で復硫し溶鋼中Ｓ濃度が０．００６％を超えている場合には脱硫処理を行い、溶鋼中Ｓ濃度を０．００６％以下とした。取鍋からタンディッシュに溶鋼を注入する末期においては、取鍋内溶鋼高さ／取鍋の溶鋼注入口径＞１（Ｈ／Ｄ＞１）となる条件で溶鋼の注入を停止して次鍋の溶鋼注入を行う操業を行った。さらにタンディッシュにおける溶鋼量が６０ｔｏｎであるために、取鍋の残溶鋼が６０ｔｏｎの時点から取鍋溶鋼注入流にワイヤ添加でＦｅ−Ｓを添加し、タンディッシュでの溶鋼中Ｓ濃度を０．００８％以上とした。本発明例５〜８においては、Ｆｅ−Ｓ、Ｓｅ、Ｔｅを種々変えて添加した。比較例１〜２では、取鍋溶鋼のＳ濃度を０．００６％とし、取鍋の溶鋼注入末期の残溶鋼量の制御も行ったが、タンディッシュへのＳ添加は実施しなかった。比較例３〜４では、溶鋼中Ｓ濃度の制御を行わなわず、取鍋の溶鋼注入末期の残溶鋼量の制御は本発明例と同様に行った。比較例５〜６では、取鍋の溶鋼注入末期の残溶鋼量の制御を行わず、溶鋼中Ｓ濃度の制御は本発明例と同様に行った。結果を表５に示す。
【００３０】
【表４】
【表５】
本発明例１〜５では、取鍋内の溶鋼中Ｓ濃度を０．００６％以下とし、かつ１．４≧Ｈ／Ｄ＞１となるように取鍋内に溶鋼を残し、さらにタンディッシュでの溶鋼中Ｓ濃度を０．００８％以上としたため、介在物性欠陥指標は１以下となり良好である。
【００３３】
本発明例５〜８では、タンディッシュでの溶鋼成分［％Ｓ］＋［％Ｓｅ］＋［％Ｔｅ］＞０．００８％となるように添加しているが、介在物性欠陥指標は１以下となり良好である。
【００３４】
比較例１〜２では、取鍋内溶鋼のS濃度の制御と取鍋内の残溶鋼量の制御も行っているため、スラグ系介在物個数指標は１以下であるが、タンディッシュへのＳ、Ｓｅ、Ｔｅ等の添加を行わなかったため、内部欠陥探傷指標が１を超え、介在物性欠陥指標も１を超えている。
【００３５】
比較例３〜４は、取鍋内の溶鋼中Ｓ濃度が０．００７％を超えていたために、スラグ系介在物個数指標が１以上であり、介在物性欠陥指標も１を超えている。
【００３６】
比較例５〜６は、取鍋内の溶鋼中Ｓ濃度が０．００６％以下で、タンディッシュ内の溶鋼中Ｓ濃度を０．００８％以上に制御したが、取鍋内の残溶鋼量の制御を行っていないため、スラグ系介在物個数指標が１を超え、介在物性欠陥指標も１を超えている。
【００３７】
【発明の効果】
以上の結果から、本発明法により取鍋交換部の清浄性を低コストで定常部並に向上させることができる。
【図面の簡単な説明】
【図１】取鍋内の残溶鋼量とスラグ系介在物個数の関係を示す図である。
【図２】溶鋼中Ｓ濃度とスラグ系介在物個数の関係を示す図である。
【図３】溶鋼中Ｓ濃度と内部欠陥探傷結果の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a continuous cast slab that improves the cleanliness of a continuous cast ladle replacement part at low cost.
[0002]
[Prior art]
In the steady portion that can be stably cast in the continuous casting, the cleanliness has been improved by various measures so far. However, the user demand for high cleanliness of steel materials is increasing, and the yield loss becomes a problem due to the deterioration of cleanliness in the ladle replacement part during so-called continuous casting.
[0003]
Causes of deterioration of cleanliness in the unsteady part of the ladle exchange part are (1) slag outflow in the ladle, (2) filling of the ladle inlet, (3) air, tundish slag, refractory The effect of reoxidation due to. Various measures have been taken for (2) and (3), but in the case of Al killed steel, alumina is increased by reoxidation, and surface defects become a problem. Such surface defects can be prevented by stirring the molten steel using electromagnetic force in the mold as disclosed in JP-A No. 05-154623. On the other hand, regarding the slag outflow in the ladle of (1), the slag inclusions remain inside the slab, that is, a problem of internal defects. As with surface defects, there is a technique for suppressing the discharge flow from the nozzle by electromagnetic force in the mold, for example, electromagnetic brake, and suppressing the entry of inclusions into the slab. Even if such a technique is used, the penetration of slag inclusions into the slab cannot be completely suppressed.
[0004]
Therefore, regarding the slag outflow in the ladle of (1), it is necessary to drastically prevent the slag outflow from the ladle or to raise the slag inclusions in the tundish. As a conventional technique, as represented by Japanese Patent Laid-Open No. 55-117548, a method of preventing the outflow of slag by leaving molten steel in the ladle and stopping the injection, or Japanese Patent Laid-Open No. 08-267223 has been disclosed. A method of inserting a representative slag outflow prevention object between the slag and the metal, a slag outflow detection technique represented by Japanese Patent Laid-Open No. 56-122656, and a tongue represented by Japanese Patent Laid-Open No. 05-169207. Improvements such as the shape of the dish. In a method of inserting an object for preventing slag outflow between a slag and a metal as represented by Japanese Patent Application Laid-Open No. 08-267223, since the slag is caught when the object for preventing outflow of slag is inserted, the slag completely It is difficult to prevent spillage. With regard to the slag outflow detection technology in Japanese Patent Application Laid-Open No. 56-122656, etc., since it has already been detected that the slag has flowed out to the tundish, complete outflow prevention cannot be performed. Moreover, when changing the shape of a tundish, especially when making the bath depth of a tundish deep, very large cost is needed.
[0005]
In any case, the conventional techniques have a problem of significant cost increase that cannot completely prevent operability and slag outflow, and a low-cost and effective method is required.
[0006]
[Problems to be solved by the invention]
It is to establish a manufacturing method that improves the cleanliness of the ladle replacement part for continuous casting at low cost.
[0009]
[Means for Solving the Problems]
The gist of the present invention is as follows.
[0010]
1 <H / D ≦ 1.4 (1)
H: Molten steel height in the ladle (m)
D: Molten steel injection diameter of ladle (m)
[0011]
When manufacturing the continuous cast slab by exchanging the ladle during casting, means 2 is one or two of S, Se, and Te in the molten steel in the tundish so as to satisfy the following formula (2). The method for producing a continuous cast slab excellent in cleanliness as described in the
[% S] + [% Se] + [% Te] ≧ 0.008 (2)
[% S]: S concentration in molten steel (mass%)
[% Se]: Se concentration in molten steel (mass%)
[% Te]: Te concentration in molten steel (mass%)
[0012]
[% S] + [% Se] + [% Te] ≧ 0.008 (2)
[% S]: S concentration in molten steel (mass%)
[% Se]: Se concentration in molten steel (mass%)
[% Te]: Te concentration in molten steel (mass%)
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In order to improve the cleanliness of the slabs due to ladle slag outflow in the ladle replacement part of continuous casting, we first studied to prevent slag outflow by leaving molten steel in the ladle. The composition of the test steel is A in Table 1 and is a low-carbon aluminum killed steel. The molten steel was left in the ladle, and the amount of molten steel remaining in the ladle after casting was weighed to determine the amount of residual molten steel. The amount of residual molten steel was converted into the molten steel height H in the ladle and normalized (H / D) at the molten steel inlet D of the ladle. The ladle inlet D of the ladle is 100 mm. Regarding the cleanability of the slab, the inclusions in the slab before and after the ladle replacement part were investigated by the slime method, and the composition of the inclusions obtained exceeding 50 μm was quantitatively analyzed by EPMA, and CaO-containing CaO> 20%. The number of Al 2 O 3 inclusions was measured as slag inclusions, and the value obtained by adding the average value and the standard deviation of the number of slag inclusions obtained in the stationary part was normalized as 1. The relationship between the normalized residual molten steel amount (H / D) and the normalized number of slag inclusions is shown in FIG. As shown in FIG. 1, it was found that the number of slag inclusions was almost equal to that of the stationary part when H / D> 1. However, as shown in FIG. 1, even when H / D> 1, there are cases where the number of slag-based inclusions is large, and this cause was examined.
[0014]
[Table 1]
Even if the index H / D> 1 of the residual molten steel, as a factor of increasing the slag inclusions, (1) even when the residual molten steel amount is large, vortex is generated in the molten steel and slag is involved (2) There are two possible points that the slag inclusions float and concentrate at the top of the ladle molten steel after the secondary refining until the molten steel is injected. Here, since the influence of slag / metal interface energy is considered on the slag entrainment and the slag-based inclusion floating phenomenon, solute elements that affect the slag / metal interface energy were examined. Oxygen and sulfur are conceivable as elements for lowering the slag / metal interface energy. However, since what is currently being examined is Al killed steel, the sulfur concentration in the molten steel was examined.
[0016]
First, the relationship between the S concentration and the number of slag inclusions was investigated in the region of H / D> 1 shown in FIG. FIG. 2 shows the relationship between the S concentration in molten steel and the number of standardized slag inclusions. As shown in FIG. 2, it was found that when the S concentration was 0.006% or less, it was equal to or less than the number of slag inclusions in the stationary part. On the contrary, it was found that when the S concentration is 0.007% or more, the number of slag inclusions may be clearly increased as compared with the steady portion. It is considered that when the S concentration is increased, the slag / metal interface energy is decreased, and the entrainment of the slag is promoted.
[0017]
Next, when the condition of H / D> 1 was satisfied and the standardized number of slag inclusions of the slab was 1 or less, cold rolling was performed, and internal defect inspection using leakage magnetic flux was performed. The value of the internal defect inspection was normalized by setting the value obtained by adding the average value and the standard deviation obtained in the stationary part to 1, and the relationship with the S concentration was investigated. FIG. 3 shows the relationship between the normalized internal defect inspection result and the S concentration. As shown in FIG. 3, although the number of slag inclusions in the slab was equal to that of the steady portion, the internal defect inspection result was higher than that of the steady portion when the S concentration was 0.007% or less. I found out. On the contrary, when the S concentration is 0.008% or more, if the number of standardized slag inclusions in the slab is 1 or less, the internal defect inspection result is the same as that of the stationary part. The cause of this is considered to be the influence of continuous casting powder type inclusions other than the slag type inclusions and residual bubbles of the inert gas blown into the immersion nozzle. In the unsteady part of the ladle exchange part, since the molten steel height of the tundish is lowered, there is a possibility that the molten steel flow state in the submerged nozzle changes and the boiling of the inert gas occurs. In particular, when the S concentration is low, the gas / metal interface energy is increased, and bubbles are less likely to exist stably in the molten steel, so that it is considered that boiling is likely to occur.
[0018]
From the above results, in order to suppress the slag outflow when pouring molten steel from the ladle into the tundish, the S concentration in the molten steel in the ladle is 0.006% or less, and the amount of residual molten steel in the ladle is H. When the molten steel is injected from the tundish into the mold under the condition of / D> 1, the S concentration in the molten steel in the tundish is 0.008% or more in order to prevent powder-type defects and bubble-type defects. By doing so, it is possible to prevent the cleanliness deterioration of the ladle replacement part.
[0019]
Next, the specific operation method for improving the cleanliness of the ladle exchange part in actual operation was examined. First, when pouring molten steel from the ladle into the tundish, it is necessary to reduce the S concentration in the molten steel to 0.006% or less in the process up to secondary refining in order to prevent the ladle slag from flowing out. . For steel grades with relatively low grades, there is no problem by preventing only ladle slag from flowing out, but for grades with high grade grades such as ultra-thin tinplate materials, residual air bubbles and small amounts of powder intervening exist. Since the influence of objects cannot be ignored, the S concentration in the molten steel in the tundish must be 0.008% or more. Therefore, it is necessary that the S concentration of the molten steel in the ladle is 0.006% or less, and S is added to the molten steel in the tundish so that the S concentration is 0.008% or more. When S is added to the tundish, it is preferable to add an Fe—S alloy to the molten steel by a wire in consideration of the yield of S and the like. In consideration of the uniformity after the addition, the addition position is preferably added in the vicinity of the injection flow injected from the ladle. Also, regarding the timing of addition, since the influence of boilering, etc. becomes significant when the molten steel height of the tundish is low when the ladle is replaced, the molten steel height of the tundish is changed at the ladle replacing part. It is necessary to make the S concentration in molten steel 0.008% or more with respect to the molten steel injected into the mold at the time when the temperature decreases. When S is added, the wire addition speed is determined from the throughput of molten steel and the required S concentration. Even in the stationary part, since the quality can be stabilized by adding S, it is preferable in terms of quality to add S regardless of the stationary part or the ladle changing part.
[0020]
In addition to S, Se and Te have the same effect as elements for controlling the slag / metal and gas / metal interface energy, and Se or Te may be added as an alternative to S. S, Se and Te Two or more of these may be mixed and added. In addition, when adding S to the molten steel in the tundish, it is preferable to set Mn / S> 20 as much as possible in consideration of the internal crack of the slab and MnS.
[0021]
The target steel type of the present invention is not particularly limited, but is killed steel, particularly aluminum killed steel.
[0022]
【Example】
Example 1
The test was conducted on cold-rolled steel sheets for automobiles. The component is A in Table 1 described above. The manufacturing process is hot metal pretreatment, converter, secondary refining (RH), and continuous casting. Typical conditions are shown in Table 2. As an evaluation index, the above-mentioned standardized number of slag inclusions (1 or less is good) and defects due to slag inclusions at the product stage (cracking or surface flaws during processing) were indexed. For the inclusion physical defect, a value obtained by adding the standard deviation to the average value of the incidence rate of the inclusion physical defect in the steady portion was standardized as 1, and 1 or less was regarded as good. In the present invention example, in the hot metal preliminary treatment, the sulfur concentration in the molten steel is set to 0.005% or less, and desulfurization treatment is performed when the sulfur concentration in the molten steel exceeds 0.006% by resulfurization in the secondary refining stage. The S concentration in the molten steel was made 0.006% or less. Furthermore, in the final stage of pouring molten steel from the ladle into the tundish, the molten steel injection is stopped under the condition that the molten steel height in the ladle / the molten steel inlet diameter of the ladle> 1 (H / D> 1) Operation to inject molten steel into the pan was performed. In addition, the molten steel inlet D of the ladle is 100 mm in both the present invention example and the comparative example. In Comparative Examples 1 and 2, the S concentration in the molten steel was not controlled, and the amount of residual molten steel at the end of the molten steel injection in the ladle was not performed. In Comparative Examples 3 and 4, the S concentration in the molten steel was controlled, but the amount of residual molten steel at the end of the molten steel injection in the ladle was not controlled. In Comparative Examples 5 to 6, only the control of the residual molten steel amount at the end of the molten steel injection in the ladle was performed without controlling the S concentration in the molten steel. The results are shown in Table 3.
[0023]
[Table 2]
[Table 3]
In the present invention examples 1 to 5, since the S concentration in the molten steel in the ladle is 0.006% or less, and the molten steel is left in the ladle so that 1.4 ≧ H / D> 1, There is no slag outflow, and both the slag inclusion index and the index of inclusion property defects are good, being 1 or less.
[0026]
In Comparative Examples 1 and 2, since the S concentration in the molten steel in the ladle is 0.008% or more and the index H / D <1 of the residual molten steel amount, the slag inclusion index, the inclusion property defect index Both greatly exceed 1.
[0027]
In Comparative Examples 3 and 4, the S concentration in the molten steel in the ladle is 0.006% or less, but the index H / D <1 of the amount of residual molten steel, and both the slag inclusion index and the inclusion property defect index are 1 Is over.
[0028]
In Comparative Examples 5 and 6, the residual molten steel amount index H / D> 1, but the S concentration in the molten steel in the ladle is 0.007% or more, and both the slag inclusion index and the inclusion property defect index are 1. Is over.
[0029]
(Example 2)
Conducted on tinplate. The composition of the test steel is B in Table 1 described above. The manufacturing process is hot metal pretreatment, converter, secondary refining (RH), and continuous casting. The ladle inlet D of the ladle is 100 mm. The amount of molten steel in the tundish is 60 tons in the stationary part. Typical conditions are shown in Table 4. As an evaluation index, the above-mentioned number of standardized slag inclusions and standardized internal defect inspection index after cold rolling (both are 1 or less are good), and the incidence of defects caused by inclusions in the product are also used. Turned into. Regarding the incidence of inclusion-induced defects, the value obtained by adding the standard deviation to the average value of the incidence of inclusion physical defects in the steady portion was standardized as 1, and 1 or less was considered good. In the present invention example, in the hot metal preliminary treatment, the sulfur concentration in the molten steel is set to 0.005% or less, and desulfurization treatment is performed when the sulfur concentration in the molten steel exceeds 0.006% by resulfurization in the secondary refining stage. The S concentration in the molten steel was made 0.006% or less. In the final stage of pouring molten steel from the ladle into the tundish, the molten steel injection is stopped under the condition that the molten steel height in the ladle / the molten steel inlet diameter of the ladle> 1 (H / D> 1) The operation of pouring molten steel was performed. Furthermore, since the amount of molten steel in the tundish is 60 tonnes, Fe-S is added to the ladle molten steel injection flow from the time when the residual molten steel in the ladle is 60 tonnes, and the S concentration in the molten steel in the tundish is set to 0. 008% or more. In Invention Examples 5 to 8, Fe-S, Se, and Te were added in various ways. In Comparative Examples 1 and 2, the S concentration of the ladle molten steel was set to 0.006%, and the amount of residual molten steel at the end of molten steel injection in the ladle was also controlled, but S addition to the tundish was not performed. In Comparative Examples 3 to 4, the S concentration in the molten steel was not controlled, and the amount of residual molten steel at the end of the molten steel injection in the ladle was controlled in the same manner as in the present invention example. In Comparative Examples 5 to 6, the amount of residual molten steel at the end of the ladle injection of the ladle was not controlled, and the control of the S concentration in the molten steel was performed in the same manner as in the present invention. The results are shown in Table 5.
[0030]
[Table 4]
[Table 5]
In Invention Examples 1-5, the S concentration in the molten steel in the ladle is 0.006% or less, and the molten steel is left in the ladle so that 1.4 ≧ H / D> 1. Since the S concentration in the molten steel is 0.008% or more, the inclusion physical defect index is 1 or less, which is favorable.
[0033]
In Invention Examples 5 to 8, the molten steel component [% S] + [% Se] + [% Te]> is added to be 0.008% in the tundish, but the inclusion physical property defect index is 1 or less. It is good.
[0034]
In Comparative Examples 1 and 2, since the control of the S concentration of the molten steel in the ladle and the control of the amount of residual molten steel in the ladle are also performed, the slag inclusion number index is 1 or less, but the S to the tundish , Se, Te, etc. were not added, so the internal defect inspection index exceeded 1, and the inclusion property defect index also exceeded 1.
[0035]
In Comparative Examples 3 to 4, since the S concentration in the molten steel in the ladle exceeded 0.007%, the slag inclusion number index was 1 or more, and the inclusion property defect index also exceeded 1.
[0036]
In Comparative Examples 5 to 6, the S concentration in the molten steel in the ladle was 0.006% or less, and the S concentration in the molten steel in the tundish was controlled to 0.008% or more. Since control is not performed, the slag inclusion number index exceeds 1, and the inclusion property defect index also exceeds 1.
[0037]
【The invention's effect】
From the above results, according to the method of the present invention, the cleanliness of the ladle replacement part can be improved at the same cost as the steady part.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the amount of residual molten steel in a ladle and the number of slag inclusions.
FIG. 2 is a diagram showing the relationship between the S concentration in molten steel and the number of slag inclusions.
FIG. 3 is a diagram showing the relationship between S concentration in molten steel and flaw detection results.
Claims (3)
１＜Ｈ／Ｄ≦１．４ ・ ・ ・（１）
Ｈ：取鍋内溶鋼高さ（ｍ）
Ｄ：取鍋の溶鋼注入口径（ｍ）In producing a continuous cast slab by replacing the ladle during casting, the S concentration in the molten steel in the ladle is reduced to 0.006 mass% or less before the start of pouring molten steel from the ladle to the tundish. After injecting molten steel into the tundish, stop the injection of molten steel into the tundish so that the molten steel height H satisfies the following formula (1), replace the ladle and inject the molten steel from the next ladle. A method for producing a continuous cast slab excellent in cleanliness characterized by the above.
1 <H / D ≦ 1.4 (1)
H: Molten steel height in the ladle (m)
D: Molten steel injection diameter of ladle (m)
［％Ｓ］＋［％Ｓｅ］＋［％Ｔｅ］≧０．００８ ・ ・ ・（２）
［％Ｓ］：溶鋼中Ｓ濃度（ｍａｓｓ％）
［％Ｓｅ］：溶鋼中Ｓｅ濃度（ｍａｓｓ％）
［％Ｔｅ］：溶鋼中Ｔｅ濃度（ｍａｓｓ％）When producing a continuous cast slab by replacing the ladle during casting, add one or more of S, Se, or Te to the molten steel in the tundish so that the following formula (2) is satisfied. The method for producing a continuous cast slab excellent in cleanliness according to claim 1.
[% S] + [% Se] + [% Te] ≧ 0.008 (2)
[% S]: S concentration in molten steel (mass%)
[% Se]: Se concentration in molten steel (mass%)
[% Te]: Te concentration in molten steel (mass%)
［％Ｓ］＋［％Ｓｅ］＋［％Ｔｅ］≧０．００８ ・ ・ ・（２）
［％Ｓ］：溶鋼中Ｓ濃度（ｍａｓｓ％）
［％Ｓｅ］：溶鋼中Ｓｅ濃度（ｍａｓｓ％）
［％Ｔｅ］：溶鋼中Ｔｅ濃度（ｍａｓｓ％）When manufacturing the continuous cast slab by exchanging the ladle during casting, the following formula is applied to the molten steel injected from the tundish to the mold while the molten steel height in the tundish at the time of ladle replacement decreases. The continuous excellent in cleanliness according to claim 1, wherein any one of S, Se, Te or two or more of S, Se, Te is added in the tundish so as to satisfy (2). A method for producing cast slabs.
[% S] + [% Se] + [% Te] ≧ 0.008 (2)
[% S]: S concentration in molten steel (mass%)
[% Se]: Se concentration in molten steel (mass%)
[% Te]: Te concentration in molten steel (mass%)
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