JP3395749B2 - Steel continuous casting method - Google Patents

Steel continuous casting method

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Publication number
JP3395749B2
JP3395749B2 JP2000032325A JP2000032325A JP3395749B2 JP 3395749 B2 JP3395749 B2 JP 3395749B2 JP 2000032325 A JP2000032325 A JP 2000032325A JP 2000032325 A JP2000032325 A JP 2000032325A JP 3395749 B2 JP3395749 B2 JP 3395749B2
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JP
Japan
Prior art keywords
molten steel
immersion nozzle
slab
casting
degree
Prior art date
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JP2001219253A (en
Inventor
祐久 菊地
昌司 原
方史 花尾
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Nippon Steel Corp
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Sumitomo Metal Industries Ltd
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Description

【発明の詳細な説明】 【0001】 【発明の属する技術分野】本発明は、浸漬ノズルの閉塞
を防止でき、良好な内部品質の鋳片を得ることができる
鋼の連続鋳造方法に関する。 【0002】 【従来の技術】連続鋳造される溶鋼は、通常、Al等で
脱酸処理されている。取鍋内の溶鋼は、いったんタンデ
ィッシュ内に注入された後、浸漬ノズルを介して、鋳型
内に注入される。 【0003】Alで脱酸した溶鋼を鋳造する際、浸漬ノ
ズルが閉塞する場合があり、極端な場合には鋳造作業の
継続が困難になる。浸漬ノズルが閉塞するのは、溶鋼中
のAlの酸化物などが浸漬ノズル内面に付着し、堆積す
ることが原因である。 【0004】浸漬ノズルが閉塞した際には、溶鋼が入っ
ているタンディッシュの上方からランスを浸漬ノズルの
近傍に挿入し、酸素ガスを吹き込むことにより、浸漬ノ
ズル内面に付着したAlの酸化物などを除去することが
行われる。しかし、この方法では、溶鋼中に酸素を吹き
込むので、溶鋼中に酸化物が発生するため、鋳片に残存
する非金属介在物が増加する。そのため、内部品質の良
好な鋳片が得られない。 【0005】浸漬ノズルの閉塞を防止する方法として、
タンディッシュの下部に配置する上ノズル(浸漬ノズル
を取り付ける部分)、または浸漬ノズルへの溶鋼の供給
量を制御するスライディングゲートから、浸漬ノズル内
を通過する溶鋼中にArガス等の不活性ガスを吹き込む
方法が採られている。しかし、吹き込まれたArガスが
溶鋼中で気泡となるので、その気泡が凝固殻に捕捉さ
れ、鋳片表皮下に気泡性欠陥が発生したり、極端な場合
には、その鋳片を素材として熱間圧延した製品表面に表
面疵が発生する場合がある。 【0006】浸漬ノズル内を通過する溶鋼中にArガス
などを吹き込まずに、浸漬ノズルの閉塞を防止する方法
として、溶鋼中にCaを含有させる方法が採られてい
る。たとえば、特開平9−192799号公報では、A
lで脱酸した溶鋼中にCaを1〜5ppm含有させる方
法が提案されている。この方法は、Caの添加によっ
て、溶鋼中に高融点であるAlの酸化物の凝集合体が形
成されるのを防止し、浸漬ノズルの閉塞を防止する方法
である。 【0007】また、特開平11−90597号公報で
は、タンデイッシュ内の溶鋼過熱度が20℃以下になっ
た場合に、タンディッシュ内の溶鋼中にCaを添加し、
溶鋼中のCa含有率を10〜50ppmとする方法が提
案されている。タンデイッシュ内の溶鋼過熱度が20℃
以下となり、溶鋼の温度が低温になった場合に浸漬ノズ
ル内面に付着する地金を、Ca添加により溶解するとさ
れている。 【0008】しかし、これら特開平9−192799号
公報および特開平11−90597号公報で提案された
方法を用いても、タンディッシュ内の溶鋼過熱度、鋳造
速度などの条件によっては、溶鋼中にCaを含有させて
も、浸漬ノズルが閉塞する場合がある。 【0009】 【発明が解決しようとする課題】本発明は、Al等で脱
酸された溶鋼を鋳造する際、浸漬ノズルの閉塞を防止で
き、良好な内部品質の鋳片を得ることができる鋼の連続
鋳造方法を提供することを目的とする。 【0010】 【課題を解決するための手段】本発明の要旨は、Alを
含有する溶鋼を、タンディッシュから鋳型に浸漬ノズル
を用いて注入する連続鋳造方法であって、溶鋼中にCa
を0.001〜0.005質量%含有させ、かつ、下記
(A)式を満足する条件で鋳造する鋼の連続鋳造方法に
ある。 【0011】 ここで、Q:単位時間に1本当たりの浸漬ノズルを通過
する溶鋼量(t/分) ΔT:タンディッシュ内における溶鋼過熱度で10〜4
5℃の範囲の値 α:Ca含有率で、0.001〜0.005質量%の範
囲の値 タンディッシュ内の溶鋼過熱度とは、タンディッシュ内
の溶鋼の温度と鋳造する鋼の融点との差の温度を意味す
る。 【0012】本発明者らは、前述の課題を次のように解
決した。すなわち、溶鋼中に含有させるCa含有率およ
びタンディッシュ内の溶鋼過熱度ΔTに応じて、浸漬ノ
ズル内を通過する溶鋼量Qを適正範囲とすることによっ
て、溶鋼中のAlの酸化物などが浸漬ノズル内面に付着
することに起因する浸漬ノズルの閉塞を防止する。 【0013】浸漬ノズルの閉塞は、溶鋼中のAlの酸化
物などが浸漬ノズル内面に付着し、堆積することが原因
である。その閉塞の発生のしやすさは、浸漬ノズル内を
通過する溶鋼の流速に依存する。ただし、通常用いられ
る浸漬ノズルの内径は70〜90mm程度であり、この
ような浸漬ノズルでは、浸漬ノズルの閉塞の発生のしや
すさは、浸漬ノズル内を通過する溶鋼量Qに依存すると
しても差し支えない。したがって、本発明の方法では、
浸漬ノズル内を通過する溶鋼量Qの適正な範囲を選択し
た。 【0014】図1は、溶鋼中のCa含有率、タンディッ
シュ内の溶鋼過熱度および浸漬ノズル内を通過する溶鋼
量が、浸漬ノズルの閉塞および鋳片品質に及ぼす影響を
示す図である。図1に示す2本の曲線のうち、上方の曲
線は下記(B)式を示し、下方の曲線は下記(C)式を
示す。また破線で示す2本の直線のうち、左の直線は下
記(D)式を示し、右の直線は下記(E)式を示す。 【0015】 Q1=−0.6×ln(ΔT×α)+3.0 ・・・(B) Q2=−0.6×ln(ΔT×α)−0.5 ・・・(C) ΔT×α=0.01 ・・・(D) ΔT×α=0.225 ・・・(E) 図1は、垂直部長さ1m、機長20mで1ストランドの
垂直曲げ型連続鋳造機を用い、Al脱酸されたC含有率
が0.05質量%の低炭素鋼を速度0.3〜10m/分
の範囲で鋳造した試験結果をまとめたものである。鋳片
サイズは、厚さ100mm、幅1000mmである。こ
の試験に用いた浸漬ノズルの条件は、吐出孔が2つで、
それぞれの吐出孔の大きさは、縦100mm、横30m
mで、ノズルの向きは下向き45°である。また、溶鋼
が通過する浸漬ノズルの部分の内径は80mmである。 【0016】Ca含有量は、0.0008〜0.005
2質量%、タンディッシュ内の溶鋼過熱度は5〜50℃
の条件である。浸漬ノズルの閉塞状況は、後述する浸漬
ノズル閉塞度、鋳片の内部品質は、後述する方法で鋳片
表皮下の非金属介在物の個数を求めて評価した。 【0017】図1に示すように、前述の(B)および
(C)式の示す曲線、および(D)および(E)式の示
す直線で囲まれた領域、すなわち、前述の(A)式の条
件を満足する領域では、浸漬ノズルの閉塞は発生せず、
また良好な品質の鋳片が得られることがわかった。これ
以外の領域では、浸漬ノズルの閉塞、鋳片の非金属介在
物の増加、浸漬ノズルなどの耐火物の溶損、ブレークア
ウトなどが生じた。 【0018】前述の(A)式の条件を満足する領域で
は、溶鋼中のCa含有率が0.0010〜0.0050
質量%であれば、Alで脱酸した溶鋼を鋳造する際、浸
漬ノズルの閉塞の原因となる溶鋼中のAlの酸化物を、
融点の低いCaO−Al23系の複合酸化物とするこ
とができる。また、タンディッシュ内の溶鋼過熱度が1
0〜45℃、単位時間に浸漬ノズルを通過する溶鋼量Q
(t/分)の値が適正な範囲であれば、溶鋼中のAlの
酸化物、CaO−Al23 系の複合酸化物などの浸漬
ノズル内面の付着を防止できることが明らかである。本
発明は、上記の結果を基に完成された。 【0019】 【発明の実施の形態】本発明の方法では、Alを含有
し、Caを0.001〜0.005質量%含有する鋼を
対象とする。 【0020】溶鋼中にCaを含有させることにより、浸
漬ノズルの閉塞の原因となる溶鋼中のAlの酸化物を、
融点の低いCaO−Al23 系の複合酸化物とするこ
とができる。このようなCaの効果を得るために、Ca
含有率を0.0010質量%以上とする。しかし、Ca
含有率が0.0050質量%を超えると、タンディッシ
ュ自体の耐火物、上ノズル、スライディングゲート、浸
漬ノズルなどが溶損しやすくなる。また、CaS等が形
成されやすくなるため、鋳片の非金属介在物が多くなる
場合がある。 【0021】本発明の方法では、タンディッシュ内の溶
鋼過熱度を10〜45℃とする。10℃未満では、溶鋼
の温度が低すぎるため、タンディッシュから浸漬ノズル
内で溶鋼が凝固し、ノズルが閉塞気味になる場合があ
る。つまり、目標の鋳造速度で鋳造できなくなる場合が
ある。極端な場合には、浸漬ノズルが鋳造中に閉塞す
る。鋳造速度を確保するために、タンディッシュの上方
からランスを溶鋼中に挿入して酸素ガスを吹き付ける処
理が採られるが、溶鋼中に酸化物が発生し、鋳片の非金
属介在物が増加するので、鋼の清浄度が悪くなる。一
方、過熱度が45℃を超えると、とくに鋳造速度が速い
場合に、鋳型内の凝固殻が浸漬ノズルの吐出流によって
再溶解しやすくなるので、ブレークアウトが発生する場
合がある。 【0022】本発明の方法では、浸漬ノズルは通常用い
られるノズルを使えばよい。たとえば、吐出孔は下向き
10〜30°程度の角度で、2つの吐出孔を備える浸漬
ノズルが用いられる。溶鋼が通過する浸漬ノズルの内部
の径は、通常の70〜90mm程度でよい。浸漬ノズル
の材質も、通常のアルミナグラファイトなどで構わな
い。本発明の方法では、単位時間に浸漬ノズルを通過す
る溶鋼量Q(t/分)が前述の(A)式を満足する条件
で鋳造する。図1を用いて、その理由を説明する。 【0023】内径の相違する浸漬ノズルでは、同じ単位
時間に浸漬ノズルを通過する溶鋼量でも、浸漬ノズルを
通過する溶鋼の流速は相違する。図1は、前述のとおり
厚さ100mm、幅1000mmの鋳片を、内径が80
mmの浸漬ノズルを用いて鋳造した結果である。しか
し、事前の鋳造試験によって、通常用いられる程度の、
内径が70〜90mm程度の浸漬ノズルであれば、単位
時間に浸漬ノズルを通過する溶鋼量によって、浸漬ノズ
ルの閉塞の発生の有無を予測できることを確認した。 【0024】(C)式の示す曲線、(D)式の示す直
線、(E)式の示す直線、および図の横軸とで囲まれた
図中にで示す領域では、Ca含有率およびタンディッ
シュ内の溶鋼過熱度が適正な範囲の値であっても、単位
時間に浸漬ノズルを通過する溶鋼量Q(t/分)が少な
すぎるので、浸漬ノズルの閉塞が発生しやすい。浸漬ノ
ズルの閉塞を解消するために、浸漬ノズル内面近傍の溶
鋼中に酸素ガスを吹き込むと、溶鋼の清浄度が悪化し、
鋳片表皮下の品質が悪くなる。 【0025】(C)式の示す曲線、(D)式の示す直
線、および図の縦軸とで囲まれた図中にで示す領域、
および、(B)式と(C)式の示す曲線、(D)式の示
す直線、および図の縦軸とで囲まれた図中にで示す領
域では、Ca含有率が低すぎるか、または、タンディッ
シュ内の溶鋼過熱度が低すぎる。そのために、浸漬ノズ
ルの閉塞しやすく、浸漬ノズルの閉塞を解消するため
に、浸漬ノズル内面近傍の溶鋼中に酸素ガスを吹き込む
と、溶鋼の清浄度が悪化し、鋳片表皮下の品質が悪くな
る。 【0026】(B)式の示す曲線、(D)式の示す直
線、および図の縦軸とで囲まれた図中にで示す領域で
は、前述のおよびの領域におけるのと同じく、浸漬
ノズルの閉塞が発生しやすくなったり、鋳片表皮下の品
質が悪くなったりするのに加えて、単位時間に浸漬ノズ
ルを通過する溶鋼量が多すぎるため、鋳型内の凝固殻が
浸漬ノズルの吐出流によって再溶解する場合もあり、極
端な場合にはブレークアウトが発生する。 【0027】(B)式の示す曲線と、(D)式および
(E)式の示す直線とで囲まれた図中にで示す領域で
は、Ca含有率およびタンディッシュ内の溶鋼過熱度が
適正な範囲の値であっても、単位時間に浸漬ノズルを通
過する溶鋼量が多すぎるため、鋳型内の凝固殻が浸漬ノ
ズルの吐出流によって再溶解しやすく、極端な場合には
ブレークアウトが発生する。 【0028】(B)式の示す曲線と(E)式の示す直線
とで囲まれた図中にで示す領域、および、(B)と
(C)式の示す曲線と(E)式の示す直線とで囲まれた
図中にで示す領域、さらに、(C)式の示す曲線、
(E)式の示す直線、および図の横軸とで囲まれた図中
にで示す領域では、Ca含有率が多すぎる。そのため
に、タンディッシュの耐火物や浸漬ノズル等が溶損しや
すくなるか、CaS等が形成されやすくなるため、鋳片
の非金属介在物が多くなりやすい。このほか、タンディ
ッシュ内の溶鋼過熱度が高すぎるために、鋳型内の凝固
殻が浸漬ノズルの吐出流によって再溶解しやすく、極端
な場合にはブレークアウトが発生する。 【0029】したがって、本発明の方法では、(B)と
(C)式の示す曲線、および(D)と(E)式の示す直
線とで囲まれた領域の条件、すなわち、前述の(A)式
を満足する条件で鋳造する。 【0030】本発明の方法を実施する手順は、たとえ
ば、Alを含有する溶鋼中にCa含有率が0.001〜
0.005質量%の範囲内になるように添加する。その
後、溶鋼中のCa含有率を測定する。溶鋼中のCa含有
率の分析方法は、たとえば、次の方法を用いることがで
きる。取鍋内の溶鋼を、通常用いられている紙管サンプ
ルなどで採取し、通常の放電発光分光法により分析す
る。次に、タンデイッシュ内の溶鋼の過熱度を求める。
これら求めた溶鋼中のCa含有率と溶鋼の過熱度から、
単位時間に1本当たりの浸漬ノズルを通過する溶鋼量Q
が、前述の(A)式を満足するように、鋳造速度を選択
する。鋳造速度を選択すれば、この溶鋼量Qが求まるか
らである。 【0031】 【実施例】垂直部長さ1m、機長20m、1ストランド
の垂直曲げ型連続鋳造機を用い、C含有率が0.05質
量%の低炭素鋼(Al脱酸鋼)を速度1.1〜7.1m
/分で、厚さ100mm、幅1000mmの鋳片に鋳造
した。吐出孔が2つで、それぞれの吐出孔の大きさは、
縦100mm、横30mmで、下向き45°の浸漬ノズ
ルを用いた。また、浸漬ノズルの内径は80mmであ
る。1ヒート約150tの溶鋼を、5ヒート連続して鋳
造した。 【0032】Caを0.0008〜0.0052質量%
の範囲内で含有させ、また、タンディッシュ内の溶鋼過
熱度を5〜50℃の範囲内で変化させて鋳造し、浸漬ノ
ズル閉塞度、鋳片表皮下の非金属介在物の発生状況、お
よびこれら鋳片を素材とした製品コイルの品質状況を調
査した。 【0033】浸漬ノズルの閉塞度は、次のようにして調
査した。まず、鋳造前に浸漬ノズルの内径を測定した。
次に、鋳造終了後に浸漬ノズルを回収して、浸漬ノズル
高さ方向の中央部でノズルを切断し、内径を測定した。
浸漬ノズル内部に付着物があり、内部の形状が円形でな
い場合には、長い径と短い径を求めて平均の径を内径と
した。そこで、鋳造後の浸漬ノズルの内径を鋳造前の内
径で除した値を浸漬ノズル閉塞度とした。したがって、
浸漬ノズル閉塞度が1.0未満は、閉塞していること、
また1.0を超えると、浸漬ノズル内部が溶損している
ことを意味する。 【0034】鋳片表皮下の非金属介在物の発生状況は、
次のようにして測定した。得られた鋳片から、鋳造方向
長さ200mmの鋳片サンプルを切出し、鋳片表皮1m
mを削除し、表面から1mmの研磨面を、100倍の光
学顕微鏡を用い、50μm以上の大きさの非金属介在物
の発生個数を測定し、1cm2 当たり発生個数を求め
た。 【0035】得られた鋳片を素材として、熱間圧延を行
って厚さ2.5mmの鋼帯を製造し、コイルに巻き取っ
た。この製品コイルを酸洗した後、表面疵の発生状況を
調査した。製品コイルの表面を手入れしても、製品コイ
ルとして使用できない場合には、格落ちの製品コイルと
判断した。この格落ちの製品コイルを、対象の試験で得
られた鋳片を熱間圧延した合計の製品コイル数で除した
値を、製品コイル格落ち指数とした。表1に試験条件と
試験結果を示す。 【0036】 【表1】本発明例の試験No.1からNo.5では、本発明で規
定する条件の範囲内の浸漬ノズルを通過する溶鋼量とな
る鋳造速度で鋳造した。これらの試験では、浸漬ノズル
閉塞度は0.85〜1.00で、浸漬ノズル内部に極わ
ずか付着物が有ったが、問題にならない程度であった。
鋳片表皮下の非金属介在物の発生個数は、1〜3個/c
2 で良好な品質の鋳片であった。そのため、その鋳片
を素材とした製品コイルの格落ちは発生せず、良好な品
質の製品コイルが得られた。 【0037】比較例の試験No.6では、本発明で規定
する溶鋼量の条件の下限より小さい値0.8t/分で試
験した。浸漬ノズル閉塞度は0.35で、かなり閉塞が
進んでいた。鋳造中に鋳造速度を確保し、鋳造を継続す
るために、浸漬ノズル内面近傍の溶鋼中に酸素ガスを吹
き込んだ。そのために、溶鋼中の酸化物が生成し、鋳片
表皮下の非金属介在物が14個でかなり悪い品質の鋳片
となった。その鋳片を素材とする製品コイルの格落ち指
数も0.3で悪かった。 【0038】比較例の試験No.7では、本発明で規定
する溶鋼量の範囲より大きな溶鋼量の値4.5t/分で
試験した。浸漬ノズル閉塞度は0.97で、問題なかっ
た。しかし、3ヒート目の鋳造中に、ブレークアウトが
発生した。鋳造速度を速くして、浸漬ノズルを通過する
溶鋼量を大きくしたため、鋳型内の凝固殻が吐出流によ
って再溶解したためである。 【0039】比較例の試験No.8では、本発明で規定
する条件の範囲外で、低い溶鋼の過熱度5℃で試験し
た。浸漬ノズル閉塞度は0.25で、著しく閉塞が進ん
だ。鋳造中に鋳造速度を確保するために、浸漬ノズル内
面近傍の溶鋼中に酸素ガスを頻繁に吹き込んだ。そのた
めに、溶鋼中の酸化物が著しく生成し、鋳片表皮下の非
金属介在物が26個で著しく悪い品質の鋳片となった。
その鋳片を素材とする製品コイルの格落ち指数も0.6
で著しく悪かった。 【0040】比較例の試験No.9では、本発明で規定
する溶鋼の過熱度より高い50℃で試験した。1ヒート
目の鋳造中にブレークアウトが発生した。浸漬ノズルの
閉塞度は1.00で、内径は当初のままであった。 【0041】比較例の試験No.10では、本発明で規
定する条件の範囲外で、Ca含有率を0.0008質量
%と低くして試験した。Ca含有の効果が少なく、浸漬
ノズルが閉塞気味で、鋳造後の浸漬ノズル閉塞度は0.
65であった。鋳造中に鋳造速度を確保するために、溶
鋼中の浸漬ノズル内面近傍に酸素ガスを吹き込んだ。そ
のために、溶鋼中の酸化物が生成し、鋳片表皮下の非金
属介在物が9個で悪い品質の鋳片となった。その鋳片を
素材とする製品コイルの格落ち指数も0.2で悪かっ
た。 【0042】比較例の試験No.11では、本発明で規
定する条件の範囲外で、Ca含有率を0.0052質量
%と高くして試験した。浸漬ノズルが溶損気味で、鋳造
後の浸漬ノズル閉塞度は1.40であった。 【0043】 【発明の効果】本発明の方法の適用により、Alで脱酸
した溶鋼を鋳造する際、浸漬ノズルの閉塞を防止でき、
良好な品質の鋳片を得ることができる。
Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of steel capable of preventing clogging of an immersion nozzle and obtaining a slab of good internal quality. [0002] Molten steel continuously cast is usually deoxidized with Al or the like. The molten steel in the ladle is once injected into the tundish and then into the mold via the immersion nozzle. [0003] When casting molten steel deoxidized with Al, the immersion nozzle may be clogged. In extreme cases, it is difficult to continue the casting operation. The reason why the immersion nozzle is closed is that Al oxide or the like in the molten steel adheres to and deposits on the inner surface of the immersion nozzle. [0004] When the immersion nozzle is closed, a lance is inserted into the vicinity of the immersion nozzle from above the tundish containing molten steel, and oxygen gas is blown into the lance, thereby causing Al oxide or the like adhering to the inner surface of the immersion nozzle. Is performed. However, in this method, since oxygen is blown into the molten steel, an oxide is generated in the molten steel, so that nonmetallic inclusions remaining in the slab increase. Therefore, a cast piece with good internal quality cannot be obtained. As a method for preventing the clogging of the immersion nozzle,
Inert gas, such as Ar gas, is introduced into the molten steel passing through the immersion nozzle from the upper nozzle (the part to which the immersion nozzle is attached) arranged at the lower part of the tundish, or the sliding gate that controls the supply of molten steel to the immersion nozzle. The method of blowing is adopted. However, since the blown Ar gas becomes bubbles in the molten steel, the bubbles are trapped in the solidified shell, and bubble defects occur under the surface of the slab, or in extreme cases, the slab is used as a material. Surface defects may occur on the hot-rolled product surface. As a method for preventing clogging of the immersion nozzle without blowing Ar gas or the like into the molten steel passing through the immersion nozzle, a method of containing Ca in the molten steel has been adopted. For example, in Japanese Patent Application Laid-Open No. 9-192799, A
A method has been proposed in which 1 to 5 ppm of Ca is contained in molten steel deoxidized by 1. This method is a method for preventing the formation of agglomerates of Al oxides having a high melting point in molten steel by the addition of Ca, thereby preventing clogging of a dipping nozzle. In Japanese Patent Application Laid-Open No. 11-90597, when the degree of superheat of molten steel in a tundish becomes 20 ° C. or less, Ca is added to the molten steel in the tundish,
A method has been proposed in which the Ca content in molten steel is adjusted to 10 to 50 ppm. Superheat degree of molten steel in tundish is 20 ℃
It is stated below that the metal that adheres to the inner surface of the immersion nozzle when the temperature of the molten steel becomes low is dissolved by adding Ca. However, even if the methods proposed in JP-A-9-192799 and JP-A-11-90597 are used, depending on conditions such as the degree of superheat of the molten steel in the tundish and the casting speed, the molten steel cannot be used. Even if Ca is contained, the immersion nozzle may be blocked. SUMMARY OF THE INVENTION The present invention is directed to a steel capable of preventing clogging of an immersion nozzle when casting molten steel deoxidized with Al or the like and obtaining a cast piece of good internal quality. It is an object of the present invention to provide a continuous casting method. [0010] The gist of the present invention is a continuous casting method in which molten steel containing Al is injected from a tundish into a mold using a submerged nozzle.
Is contained in a range of 0.001 to 0.005% by mass and cast under a condition satisfying the following formula (A). [0011] Here, Q: the amount of molten steel passing through one immersion nozzle per unit time (t / min) ΔT: 10 to 4 degrees of superheat of molten steel in a tundish
A value in the range of 5 ° C. α: Ca content, a value in the range of 0.001 to 0.005% by mass The degree of superheat of molten steel in the tundish is the temperature of the molten steel in the tundish and the melting point of the steel to be cast. Means the temperature of the difference. The present inventors have solved the above-mentioned problem as follows. That is, according to the Ca content contained in the molten steel and the degree of superheat ΔT of the molten steel in the tundish, by setting the amount of molten steel Q passing through the immersion nozzle to an appropriate range, the oxides of Al and the like in the molten steel are immersed. The blockage of the immersion nozzle caused by the adhesion to the inner surface of the nozzle is prevented. The blockage of the immersion nozzle is caused by the fact that oxides of Al and the like in the molten steel adhere to and deposit on the inner surface of the immersion nozzle. The likelihood of the occurrence of the blockage depends on the flow rate of the molten steel passing through the immersion nozzle. However, the inner diameter of a commonly used immersion nozzle is about 70 to 90 mm, and in such an immersion nozzle, the likelihood of occurrence of blockage of the immersion nozzle depends on the amount of molten steel Q passing through the immersion nozzle. No problem. Therefore, in the method of the present invention,
An appropriate range of the amount of molten steel Q passing through the immersion nozzle was selected. FIG. 1 is a diagram showing the effects of the Ca content in molten steel, the degree of superheat of molten steel in a tundish, and the amount of molten steel passing through the immersion nozzle on the blockage of the immersion nozzle and the quality of the slab. Of the two curves shown in FIG. 1, the upper curve shows the following equation (B), and the lower curve shows the following equation (C). Further, of the two straight lines indicated by broken lines, the left straight line shows the following formula (D), and the right straight line shows the following formula (E). Q 1 = −0.6 × ln (ΔT × α) +3.0 (B) Q 2 = −0.6 × ln (ΔT × α) -0.5 (C) ΔT × α = 0. 01... (D) ΔT × α = 0.225 (E) FIG. 1 shows C deoxidized using a vertical bending type continuous casting machine having a vertical length of 1 m and a machine length of 20 m and a single strand. It is a summary of test results obtained by casting low-carbon steel having a content of 0.05% by mass at a speed of 0.3 to 10 m / min. The slab size is 100 mm in thickness and 1000 mm in width. The conditions of the immersion nozzle used in this test were two discharge holes,
The size of each discharge hole is 100 mm long and 30 m wide
At m, the orientation of the nozzle is 45 ° downward. The inner diameter of the part of the immersion nozzle through which the molten steel passes is 80 mm. The Ca content is 0.0008 to 0.005.
2% by mass, superheat degree of molten steel in tundish is 5-50 ° C
Is the condition. The condition of the clogging of the immersion nozzle was evaluated by determining the degree of clogging of the immersion nozzle described below and the internal quality of the slab by obtaining the number of nonmetallic inclusions under the surface of the slab by the method described below. As shown in FIG. 1, the area surrounded by the curves shown by the above-mentioned equations (B) and (C) and the straight line shown by the equations (D) and (E), that is, the above-mentioned equation (A) In the area that satisfies the condition of, the clogging of the immersion nozzle does not occur,
In addition, it was found that a slab of good quality was obtained. In other areas, clogging of the immersion nozzle, increase of nonmetallic inclusions in the slab, melting of refractory such as the immersion nozzle, breakout, etc. occurred. In the region satisfying the above condition (A), the Ca content in the molten steel is 0.0010 to 0.0050.
If it is mass%, when casting the molten steel deoxidized with Al, the oxide of Al in the molten steel causing the clogging of the immersion nozzle,
May be lower CaO-Al 2 O 3 composite oxide of melting point. The degree of superheat of molten steel in the tundish is 1
0-45 ° C, quantity of molten steel Q passing through immersion nozzle per unit time
When the value of (t / min) is in an appropriate range, it is apparent that adhesion of the oxide of Al in the molten steel, the complex oxide of the CaO-Al 2 O 3 system, etc. on the inner surface of the immersion nozzle can be prevented. The present invention has been completed based on the above results. DETAILED DESCRIPTION OF THE INVENTION The method of the present invention is directed to a steel containing Al and 0.001 to 0.005% by mass of Ca. By adding Ca to the molten steel, the oxide of Al in the molten steel, which causes blockage of the immersion nozzle,
May be lower CaO-Al 2 O 3 composite oxide of melting point. In order to obtain such an effect of Ca, Ca
The content is set to 0.0010% by mass or more. However, Ca
When the content exceeds 0.0050% by mass, the refractory of the tundish itself, the upper nozzle, the sliding gate, the immersion nozzle, and the like are easily melted. Further, since CaS or the like is easily formed, non-metallic inclusions in the cast piece may increase. In the method of the present invention, the degree of superheat of molten steel in the tundish is set to 10 to 45 ° C. If the temperature is lower than 10 ° C., the temperature of the molten steel is too low, so that the molten steel solidifies from the tundish in the immersion nozzle, and the nozzle may become slightly clogged. That is, casting may not be performed at the target casting speed. In extreme cases, the immersion nozzle will block during casting. In order to secure the casting speed, a process of inserting a lance into the molten steel from above the tundish and blowing oxygen gas is adopted, but oxides are generated in the molten steel and nonmetallic inclusions in the slab increase. Therefore, the cleanliness of the steel deteriorates. On the other hand, when the degree of superheat exceeds 45 ° C., particularly when the casting speed is high, the solidified shell in the mold tends to be re-dissolved by the discharge flow of the immersion nozzle, so that breakout may occur. In the method of the present invention, a commonly used nozzle may be used as the immersion nozzle. For example, an immersion nozzle having two ejection holes is used, with the ejection holes having a downward angle of about 10 to 30 °. The diameter of the inside of the immersion nozzle through which the molten steel passes may be about 70 to 90 mm. The material of the immersion nozzle may be ordinary alumina graphite or the like. In the method of the present invention, casting is performed under the condition that the amount of molten steel Q (t / min) passing through the immersion nozzle per unit time satisfies the above-mentioned formula (A). The reason will be described with reference to FIG. With immersion nozzles having different inner diameters, the flow speed of molten steel passing through the immersion nozzle differs even if the amount of molten steel passes through the immersion nozzle in the same unit time. FIG. 1 shows a slab having a thickness of 100 mm and a width of 1000 mm as described above and an inner diameter of 80 mm.
It is the result of casting using an immersion nozzle of mm. However, prior casting tests have shown that
With an immersion nozzle having an inner diameter of about 70 to 90 mm, it was confirmed that the occurrence of blockage of the immersion nozzle can be predicted by the amount of molten steel passing through the immersion nozzle in a unit time. In the region surrounded by the curve shown by the formula (C), the straight line shown by the formula (D), the straight line shown by the formula (E), and the horizontal axis, the Ca content and the tan Even if the degree of superheat of the molten steel in the dish is within an appropriate range, the amount of molten steel Q (t / min) passing through the immersion nozzle per unit time is too small, so that the immersion nozzle is likely to be blocked. When oxygen gas is blown into the molten steel near the inner surface of the immersion nozzle to eliminate the blockage of the immersion nozzle, the cleanliness of the molten steel deteriorates,
The quality of the slab under the skin deteriorates. The area shown in the figure surrounded by the curve shown by the equation (C), the straight line shown by the equation (D), and the vertical axis of the figure,
In the region surrounded by the curves shown by the formulas (B) and (C), the straight line shown by the formula (D), and the vertical axis of the drawing, the Ca content is too low, or , The degree of superheat of molten steel in the tundish is too low. Therefore, when oxygen gas is blown into the molten steel near the inner surface of the immersion nozzle, the cleanliness of the molten steel deteriorates, and the quality under the surface of the slab deteriorates, when the oxygen gas is blown into the vicinity of the inner surface of the immersion nozzle in order to easily block the immersion nozzle. Become. In the region surrounded by the curve shown by the expression (B), the straight line shown by the expression (D), and the vertical axis of the figure, the region indicated by the symbol のIn addition to the fact that blockage is likely to occur and the quality of the surface of the slab under the skin deteriorates, the amount of molten steel that passes through the immersion nozzle per unit time is too large, so the solidified shell in the mold causes the discharge flow of the immersion nozzle. May cause re-dissolution, and in extreme cases, breakout occurs. In the region surrounded by the curve shown by the formula (B) and the straight lines shown by the formulas (D) and (E), the Ca content and the degree of superheat of the molten steel in the tundish are appropriate. Even in the above range, the amount of molten steel passing through the immersion nozzle per unit time is too large, so the solidified shell in the mold is easily re-dissolved by the discharge flow of the immersion nozzle, and in extreme cases breakout occurs I do. The area surrounded by the curve shown by the equation (B) and the straight line shown by the equation (E), and the curve shown by the equations (B) and (C) and the area shown by the equation (E) The area shown in the figure surrounded by a straight line, and the curve shown by the equation (C),
In the region surrounded by the straight line represented by the formula (E) and the horizontal axis in the figure, the Ca content is too large. Therefore, the refractory of the tundish, the immersion nozzle, and the like are easily melted or CaS or the like is easily formed, so that the nonmetallic inclusions in the cast slab tend to increase. In addition, since the degree of superheating of the molten steel in the tundish is too high, the solidified shell in the mold is easily re-dissolved by the discharge flow of the immersion nozzle, and in extreme cases, breakout occurs. Therefore, in the method of the present invention, the condition of the region surrounded by the curves shown by the equations (B) and (C) and the straight line shown by the equations (D) and (E), that is, the above-mentioned (A) ) Cast under conditions that satisfy the formula. The procedure for carrying out the method of the present invention is, for example, a method in which a Ca content in molten steel containing Al is 0.001 to 0.001.
It is added so as to be in the range of 0.005% by mass. Then, the Ca content in the molten steel is measured. As a method for analyzing the Ca content in the molten steel, for example, the following method can be used. The molten steel in the ladle is collected with a commonly used paper tube sample or the like and analyzed by ordinary discharge emission spectroscopy. Next, the degree of superheat of the molten steel in the tundish is determined.
From the obtained Ca content in the molten steel and the degree of superheat of the molten steel,
The amount of molten steel Q that passes through one immersion nozzle per unit time Q
However, the casting speed is selected so as to satisfy the above expression (A). This is because if the casting speed is selected, the molten steel amount Q can be obtained. EXAMPLE A low-carbon steel (Al deoxidized steel) having a C content of 0.05% by mass was produced using a vertical bending type continuous casting machine having a vertical portion length of 1 m, a machine length of 20 m and a single strand at a speed of 1. 1-7.1m
At a rate of / min. To a slab having a thickness of 100 mm and a width of 1000 mm. There are two discharge holes, and the size of each discharge hole is
An immersion nozzle having a length of 100 mm and a width of 30 mm and a downward angle of 45 ° was used. The inner diameter of the immersion nozzle is 80 mm. Approximately 150 tons of molten steel in one heat was cast continuously for 5 heats. 0.0008 to 0.0052% by mass of Ca
And cast while changing the degree of superheat of molten steel in the tundish within the range of 5 to 50 ° C, the degree of clogging of the immersion nozzle, the occurrence of nonmetallic inclusions under the surface of the slab, and The quality status of product coils made from these slabs was investigated. The degree of blockage of the immersion nozzle was investigated as follows. First, the inner diameter of the immersion nozzle was measured before casting.
Next, the immersion nozzle was recovered after the casting was completed, and the nozzle was cut at the center in the height direction of the immersion nozzle, and the inner diameter was measured.
When there was a deposit inside the immersion nozzle and the inside shape was not circular, the long diameter and the short diameter were determined, and the average diameter was defined as the inner diameter. Therefore, the value obtained by dividing the inner diameter of the immersion nozzle after casting by the inner diameter before casting was defined as the immersion nozzle closing degree. Therefore,
If the immersion nozzle occlusion degree is less than 1.0, it is occluded,
If it exceeds 1.0, it means that the inside of the immersion nozzle is melted. The occurrence of non-metallic inclusions under the slab surface is as follows:
The measurement was performed as follows. A slab sample having a length of 200 mm in the casting direction was cut out from the obtained slab, and the slab skin was 1 m in length.
m was removed, and the number of nonmetallic inclusions having a size of 50 μm or more was measured on a polished surface 1 mm from the surface using a 100-fold optical microscope, and the number generated per 1 cm 2 was obtained. Using the obtained cast slab as a raw material, a steel strip having a thickness of 2.5 mm was manufactured by hot rolling and wound around a coil. After pickling this product coil, the occurrence of surface flaws was investigated. If the product coil cannot be used as a product coil even after the surface of the product coil has been cleaned, it is determined that the product coil has been downgraded. A value obtained by dividing the product coil having the downgraded by the total number of product coils obtained by hot rolling the cast pieces obtained in the target test was defined as a product coil downgrade index. Table 1 shows test conditions and test results. [Table 1] Test No. of the present invention example. No. 1 to No. In No. 5, the casting was performed at a casting speed that resulted in the amount of molten steel passing through the immersion nozzle within the range of the conditions specified in the present invention. In these tests, the degree of clogging of the immersion nozzle was 0.85 to 1.00, and there was a slight amount of deposits inside the immersion nozzle, but this was not a problem.
The number of non-metallic inclusions under the slab surface is 1 to 3 / c
m 2 was a cast of good quality. Therefore, the product coil made of the slab was not degraded, and a good quality product coil was obtained. Test No. of Comparative Example In No. 6, the test was performed at a value of 0.8 t / min, which is smaller than the lower limit of the molten steel amount condition specified in the present invention. The degree of clogging of the immersion nozzle was 0.35, and the clogging was considerably advanced. Oxygen gas was blown into molten steel near the inner surface of the immersion nozzle in order to secure the casting speed during casting and continue casting. As a result, oxides in the molten steel were generated, and 14 nonmetallic inclusions under the surface of the cast slab resulted in a cast of considerably poor quality. The degraded index of the product coil using the slab was 0.3, which was bad. Test No. of Comparative Example In No. 7, the test was performed with a value of the molten steel amount of 4.5 t / min which was larger than the range of the molten steel amount specified in the present invention. The immersion nozzle clogging degree was 0.97, which was no problem. However, a breakout occurred during the third heat casting. This is because the casting speed was increased and the amount of molten steel passing through the immersion nozzle was increased, so that the solidified shell in the mold was re-melted by the discharge flow. Test No. of Comparative Example In No. 8, the test was performed at a low superheat degree of 5 ° C. of the molten steel outside the range specified in the present invention. The degree of clogging of the immersion nozzle was 0.25, and the clogging was remarkably advanced. Oxygen gas was frequently blown into the molten steel near the inner surface of the immersion nozzle to secure the casting speed during casting. As a result, oxides in the molten steel were remarkably generated, and 26 nonmetallic inclusions under the surface of the slab resulted in a slab of extremely poor quality.
The degraded index of the product coil using the slab is 0.6
Was remarkably bad. Test No. of Comparative Example In No. 9, the test was performed at 50 ° C. which was higher than the degree of superheat of the molten steel specified in the present invention. A breakout occurred during the first heat casting. The degree of clogging of the immersion nozzle was 1.00, and the inner diameter remained at the initial value. Test No. of Comparative Example In the test No. 10, the Ca content was reduced to 0.0008% by mass outside the range defined by the present invention. The effect of Ca content is small, the immersion nozzle is slightly closed, and the degree of immersion nozzle closure after casting is 0.
65. Oxygen gas was blown into the vicinity of the inner surface of the immersion nozzle in the molten steel to secure the casting speed during casting. As a result, oxides in the molten steel were generated, and the non-metallic inclusions under the surface of the slab were 9 pieces, resulting in a slab of poor quality. The degraded index of the product coil using the slab as a material was also poor at 0.2. Test No. of Comparative Example In No. 11, the test was performed by increasing the Ca content to 0.0052% by mass outside the range defined by the present invention. The immersion nozzle was slightly melted, and the degree of clogging of the immersion nozzle after casting was 1.40. By applying the method of the present invention, it is possible to prevent clogging of the immersion nozzle when casting molten steel deoxidized with Al.
Good quality slabs can be obtained.

【図面の簡単な説明】 【図1】Ca含有率、タンディッシュ内の溶鋼過熱度お
よび浸漬ノズル内を通過する溶鋼量が、浸漬ノズルの閉
塞および鋳片品質に及ぼす影響を示す図である。
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the effects of the Ca content, the degree of superheat of molten steel in a tundish, and the amount of molten steel passing through an immersion nozzle, on the clogging of the immersion nozzle and the quality of slab.

───────────────────────────────────────────────────── フロントページの続き (56)参考文献 特開 平7−116792(JP,A) 特開 平11−90597(JP,A) (58)調査した分野(Int.Cl.7,DB名) B22D 11/108 B22D 11/10 B22D 11/18 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-116792 (JP, A) JP-A-11-90597 (JP, A) (58) Fields investigated (Int. Cl. 7 , DB name) B22D 11/108 B22D 11/10 B22D 11/18

Claims (1)

(57)【特許請求の範囲】 【請求項1】Alを含有する溶鋼を、タンディッシュか
ら鋳型に浸漬ノズルを用いて注入する連続鋳造方法であ
って、溶鋼中にCaを0.001〜0.005質量%含
有させ、かつ、下記(A)式を満足する条件で鋳造する
ことを特徴とする鋼の連続鋳造方法。 −0.5−0.6×ln(ΔT×α)≦Q≦3.0−0.6× ln(ΔT×α) ・・・(A) ここで、Q:単位時間に1本当たりの浸漬ノズルを通過
する溶鋼量(t/分) ΔT:タンディッシュ内における溶鋼過熱度で10〜4
5℃の範囲の値 α:Ca含有率で、0.001〜0.005 質量%の範囲の値
(57) Claims 1. A continuous casting method in which molten steel containing Al is injected from a tundish into a mold by using an immersion nozzle, wherein 0.001 to 0 Ca is contained in the molten steel. A continuous casting method for steel, characterized by containing 0.005% by mass and casting under conditions satisfying the following formula (A). −0.5−0.6 × ln (ΔT × α) ≦ Q ≦ 3.0−0.6 × ln (ΔT × α) (A) where Q: amount of molten steel passing through the immersion nozzle per unit time ( t / min) ΔT: 10 to 4 in the degree of superheat of molten steel in the tundish
Value α in the range of 5 ° C .: Ca content, value in the range of 0.001 to 0.005% by mass
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