JP5217784B2

JP5217784B2 - Steel continuous casting method

Info

Publication number: JP5217784B2
Application number: JP2008217399A
Authority: JP
Inventors: 祐司三木; 康夫岸本
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2008-08-26
Filing date: 2008-08-26
Publication date: 2013-06-19
Anticipated expiration: 2028-08-26
Also published as: JP2010051984A

Description

本発明は、電磁力によって鋳型内の溶鋼流動を制御しながら溶鋼を鋳造して鋳片を製造する連続鋳造方法に関する。 The present invention relates to a continuous casting method for producing a cast piece by casting molten steel while controlling the flow of molten steel in a mold by electromagnetic force.

鋼の連続鋳造では、タンディッシュ内に入れられた溶鋼が、タンディッシュ底部に接続された浸漬ノズルを通じて連続鋳造用鋳型内に注入される。この場合、浸漬ノズルの吐出孔から鋳型内に吐出される溶鋼流に、非金属介在物（主にアルミナなどの脱酸生成物）や、浸漬ノズルの内壁面から吹き込まれた不活性ガス（アルミナなどの付着・堆積によるノズル閉塞を防止するために吹き込まれる不活性ガス）の気泡が随伴するが、これが凝固シェルに捕捉されると、製品欠陥（介在物性欠陥、気泡性欠陥）となる。また、メニスカスに達した溶鋼上昇流にモールドフラックス（モールドパウダー）が巻き込まれ、これも凝固シェルに捕捉されることにより製品欠陥となる。 In continuous casting of steel, molten steel placed in a tundish is poured into a continuous casting mold through an immersion nozzle connected to the bottom of the tundish. In this case, non-metallic inclusions (mainly deoxidation products such as alumina) or inert gas (alumina) blown from the inner wall surface of the immersion nozzle into the molten steel flow discharged from the discharge hole of the immersion nozzle into the mold. Inert gas blown in order to prevent nozzle clogging due to adhesion / deposition, etc.) accompanies, but if this is trapped by the solidified shell, it becomes a product defect (inclusion property defect, bubble defect). In addition, mold flux (mold powder) is caught in the upward flow of molten steel that has reached the meniscus, and this is also captured by the solidified shell, resulting in a product defect.

従来、溶鋼中の非金属介在物、モールドフラックス、気泡が凝固シェルに捕捉され、製品欠陥となることを防止するために、鋳型内で溶鋼流に磁界を印加し、磁界による電磁気力を利用して溶鋼の流動を制御することが行われており、この技術に関して数多くの提案がなされている。
例えば、特許文献１には、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動する方法が開示されている。この方法は、浸漬ノズルの吐出口から吐出された後、上昇流と下降流に分かれる溶鋼流のうち、下降流を下部の直流磁界で制動し、上昇流を上部の直流磁界で制動することで、溶鋼流に随伴する非金属介在物やモールドフラックスが凝固シェルに捕捉されないようするものである。 Conventionally, in order to prevent non-metallic inclusions, mold flux, and bubbles in molten steel from being trapped in the solidified shell and resulting in product defects, a magnetic field is applied to the molten steel flow in the mold and electromagnetic force generated by the magnetic field is used. The flow of molten steel has been controlled, and many proposals have been made regarding this technology.
For example, Patent Document 1 discloses a method of braking a molten steel flow by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with the long side of the mold interposed therebetween. In this method, after being discharged from the discharge port of the immersion nozzle, the downflow is braked by the lower direct current magnetic field and the upward flow is braked by the upper direct current magnetic field. The non-metallic inclusions and mold flux accompanying the molten steel flow are prevented from being captured by the solidified shell.

また、特許文献２には、特許文献１と同じく鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動するとともに、上部磁極または下部磁極に交流磁界を重畳して印加する方法が開示されている。この方法は、特許文献１と同様の直流磁界による溶鋼流の制動を行うとともに、交流磁界による溶鋼の撹拌により、凝固シェル界面での非金属介在物などの洗浄効果を得ようとするものである。
特開平３−１４２０４９号公報特開平１０−３０５３５３号公報 Further, in Patent Document 2, the molten steel flow is braked by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the mold long side portion as in Patent Document 1, and the upper magnetic pole Alternatively, a method of applying an alternating magnetic field on the lower magnetic pole is disclosed. In this method, the molten steel flow is braked by a DC magnetic field similar to Patent Document 1, and the cleaning effect of nonmetallic inclusions and the like at the solidified shell interface is obtained by stirring the molten steel by an AC magnetic field. .
Japanese Patent Laid-Open No. 3-142049 JP-A-10-305353

最近、自動車外板用鋼板の品質厳格化に伴い、これまで問題にならなかった微小な気泡やモールドフラックスの巻き込みに起因する欠陥が問題視されるようになりつつあり、上記従来技術のような連続鋳造方法では、そのような厳しい品質要求に十分に対応できない。特に、合金化溶融亜鉛めっき鋼板は、溶融めっき後、加熱して母材鋼板の鉄成分を亜鉛めっき層に拡散させるものであり、母材鋼板の表層性状が合金化溶融亜鉛めっき層の品質に大きく影響する。すなわち、母材鋼板の表層に気泡性やフラックス性の欠陥があると、小さな欠陥であってもめっき層の厚みにむらが生じ、それが表面に筋状の欠陥としてが現れ、自動車外板などのような品質要求の厳しい用途には使用できなくなる。 Recently, along with stricter quality of steel plates for automobile outer plates, defects caused by entrapment of minute bubbles and mold flux that have not been a problem until now are becoming a problem. The continuous casting method cannot sufficiently meet such strict quality requirements. In particular, alloyed hot-dip galvanized steel sheets are heated after hot-dip plating to diffuse the iron component of the base steel sheet into the galvanized layer, and the surface layer properties of the base steel sheet contribute to the quality of the alloyed hot-dip galvanized layer. A big influence. In other words, if there are bubbles or flux defects on the surface layer of the base steel plate, even if it is a small defect, unevenness in the thickness of the plating layer occurs, which appears as a streak defect on the surface, such as an automobile outer plate It cannot be used for applications with severe quality requirements.

したがって本発明の目的は、上記のような従来技術の課題を解決し、電磁力を利用して鋳型内の溶鋼流動を制御することにより、従来問題とされてきたような非金属介在物やモールドフラックスによる欠陥だけでなく、微小な気泡やモールドフラックスの巻き込みによる欠陥が少ない高品質の鋳片を得ることができる連続鋳造方法を提供することにある。 Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art and to control the flow of molten steel in the mold by using electromagnetic force. An object of the present invention is to provide a continuous casting method capable of obtaining a high-quality slab with few defects due to entrapment of minute bubbles and mold flux as well as defects due to flux.

本発明者らは、上記課題を解決するために、電磁力を利用して鋳型内の溶鋼流動を制御する際の諸々の鋳造条件を検討した結果、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う方法において、鋳造するスラブ幅と鋳造速度に応じて、上部磁極と下部磁極に各々印加する直流磁界の強度および両直流磁界の強度比を最適化することにより、従来問題とされてきたような非金属介在物やモールドフラックスによる欠陥だけでなく、微小な気泡やモールドフラックスによる欠陥が少ない高品質の鋳片が得られることを見出した。また、そのような連続鋳造においてより高品質な鋳片を得るために、浸漬ノズルのノズル浸漬深さやノズル内径、スラブ厚さなどに最適範囲があり、その範囲において発明の効果が最も発現しやすいことが判った。 In order to solve the above-mentioned problems, the present inventors have studied various casting conditions when controlling the flow of molten steel in a mold using electromagnetic force. In the method of continuously casting steel while braking the molten steel flow by a DC magnetic field applied to each of the upper magnetic pole and a pair of lower magnetic poles, the upper magnetic pole and the lower magnetic pole are applied according to the slab width and the casting speed. By optimizing the strength of the direct current magnetic field and the strength ratio of both direct current magnetic fields, not only defects caused by non-metallic inclusions and mold flux, which have been regarded as problems, but also defects caused by minute bubbles and mold flux. It has been found that a high quality slab can be obtained. Further, in order to obtain a higher quality slab in such continuous casting, there is an optimum range of nozzle immersion depth, nozzle inner diameter, slab thickness, etc. of the immersion nozzle, and the effect of the invention is most easily manifested in that range. I found out.

本発明は、このような知見に基づきなされたもので、以下を要旨とするものである。
［1］鋳型外側に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備えるとともに、溶鋼吐出孔の水平方向から下向きの溶鋼吐出角度が１０°以上３０°未満の浸漬ノズルを備え、前記上部磁極の磁場のピーク位置と前記下部磁極の磁場のピーク位置の間に前記溶鋼吐出孔が位置する連続鋳造機を用い、前記１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う方法であって、
鋳造速度を１．０ｍ／分以上とし、且つ下記条件（イ）〜（ハ）に従って、スラブ幅が１０００〜１８００ｍｍのスラブの連続鋳造を行うことを特徴とする鋼の連続鋳造方法。
・条件（イ）：鋳造するスラブ幅と鋳造速度が下記（a）〜（e）の場合には、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを１．３〜２．５、上部磁極に印加する直流磁界の強度Ａを０．２０〜０．４０Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．１０〜０．２５Ｔとする。
（a）スラブ幅１２５０ｍｍ未満
（b）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分以上
（c）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分以上
（d）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分以上
（e）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分以上
・条件（ロ）：鋳造するスラブ幅と鋳造速度が下記（f）、（g）の場合には、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを０．７〜１．５、上部磁極に印加する直流磁界の強度Ａを０.２５〜０．３５Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．２０〜０．４０Ｔとする。
（f）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分以上
（g）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分以上
・条件（ハ）：鋳造するスラブ幅と鋳造速度が下記（h）〜（m）の場合には、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを０．５以下、上部磁極に印加する直流磁界の強度Ａを０〜０．０６Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．１０〜０．４０Ｔとする。
（h）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分未満
（i）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分未満
（j）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分未満
（k）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分未満
（l）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分未満
（m）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分未満 The present invention has been made on the basis of such knowledge and has the following gist.
[1] A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and the molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 ° Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting steel while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
A continuous casting method for steel, characterized in that continuous casting of a slab having a casting speed of 1.0 m / min or more and a slab width of 1000 to 1800 mm is performed in accordance with the following conditions (a) to (c).
Condition (A): Ratio of DC magnetic field strength A applied to the upper magnetic pole and DC magnetic field strength B applied to the lower magnetic pole when the cast slab width and casting speed are the following (a) to (e) A / B is 1.3 to 2.5, DC magnetic field strength A applied to the upper magnetic pole is 0.20 to 0.40 T, and DC magnetic field strength B applied to the lower magnetic pole is 0.10 to 0.25 T. To do.
(A) Slab width of less than 1250 mm (b) Slab width of 1250 mm or more and less than 1350 mm and casting speed of 1.4 m / min or more (c) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 1.9 m / min or more (d) Slab width (E) Slab width of 1550 mm or more and less than 1650 mm and casting speed of 2.6 m / min or more ・ Condition (b): The slab width to be cast and the casting speed are the following (f) In the case of (g), the ratio A / B of the intensity A of the DC magnetic field applied to the upper magnetic pole and the intensity B of the DC magnetic field applied to the lower magnetic pole is 0.7 to 1.5, and is applied to the upper magnetic pole. The intensity A of the DC magnetic field is 0.25 to 0.35 T, and the intensity B of the DC magnetic field applied to the lower magnetic pole is 0.20 to 0.40 T.
(F) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.8 m / min or more (g) Slab width of 1750 mm or more and casting speed of 2.9 m / min or more ・ Condition (C): Slab width to be cast and casting speed are In the following cases (h) to (m), the ratio A / B of the DC magnetic field strength A applied to the upper magnetic pole and the DC magnetic field strength B applied to the lower magnetic pole is 0.5 or less, and applied to the upper magnetic pole. The strength A of the DC magnetic field is 0 to 0.06 T, and the strength B of the DC magnetic field applied to the lower magnetic pole is 0.10 to 0.40 T.
(H) Slab width of 1250 mm or more and less than 1350 mm and casting speed of less than 1.4 m / min (i) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 1.9 m / min (j) Slab width of 1450 mm or more and less than 1550 mm and casting (K) Slab width of 1550 mm or more and less than 1650 mm and casting speed of less than 2.6 m / min (l) Slab width of 1650 mm or more and less than 1750 mm and casting speed of less than 2.8 m / min (m) Slab width 1750mm or more and casting speed less than 2.9m / min

［2］上記［1］の連続鋳造方法において、浸漬ノズルのノズル浸漬深さを２３０〜２９０ｍｍとすることを特徴とする鋼の連続鋳造方法。
［3］上記［1］または［2］の連続鋳造方法において、浸漬ノズルのノズル内径（但し、溶鋼吐出孔の位置でのノズル内径）を７０〜９０ｍｍとすることを特徴とする鋼の連続鋳造方法。
［4］上記［1］〜［3］のいずれかの連続鋳造方法において、浸漬ノズルの各溶鋼吐出孔の開口面積を３６００〜８２００ｍｍ^２とすることを特徴とする鋼の連続鋳造方法。 [2] The continuous casting method of [1], wherein the immersion depth of the immersion nozzle is 230 to 290 mm.
[3] In the continuous casting method according to [1] or [2], the inner diameter of the immersion nozzle (however, the inner diameter of the nozzle at the position of the molten steel discharge hole) is 70 to 90 mm. Method.
[4] In the continuous casting method according to any one of [1] to [3], the opening area of each molten steel discharge hole of the immersion nozzle is set to 3600 to 8200 mm ² .

本発明によれば、電磁力を利用して鋳型内の溶鋼流動を制御するに当たり、鋳造するスラブ幅と鋳造速度に応じて、上部磁極と下部磁極に各々印加する直流磁界の強度および両直流磁界の強度比を最適化することにより、従来問題とされなかったような微小な気泡性欠陥やフラックス性欠陥が非常に少ない高品質の鋳片を得ることができる。このため、従来にない高品質のめっき層を有する合金化溶融亜鉛めっき鋼板を製造することが可能となる。
また、特に、浸漬ノズルのノズル浸漬深さ、ノズル内径、溶鋼吐出孔の開口面積を最適化することにより、より高品質な鋳片を得ることができる。 According to the present invention, in controlling the molten steel flow in the mold using electromagnetic force, the strength of the DC magnetic field applied to the upper magnetic pole and the lower magnetic pole and the both DC magnetic fields according to the slab width to be cast and the casting speed, respectively. By optimizing the strength ratio, it is possible to obtain a high-quality slab having very few microbubble defects and flux defects, which have not been regarded as problems in the past. For this reason, it becomes possible to manufacture the galvannealed steel plate which has a high quality plating layer which has not existed conventionally.
In particular, a higher quality slab can be obtained by optimizing the nozzle immersion depth of the immersion nozzle, the nozzle inner diameter, and the opening area of the molten steel discharge hole.

本発明の連続鋳造方法は、鋳型外側（鋳型側壁の背面）に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備えるとともに、溶鋼吐出孔の水平方向から下向きの溶鋼吐出角度αが１０°以上３０°未満の浸漬ノズルを備え、前記上部磁極の磁場のピーク位置と前記下部磁極の磁場のピーク位置の間に前記溶鋼吐出孔が位置する連続鋳造機を用い、前記１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う。
図１および図２は、本発明の実施に供される連続鋳造機の鋳型および浸漬ノズルの一実施形態を示すもので、図１は鋳型および浸漬ノズルの縦断面図、図２は同じく水平断面図（図１のＩＩ−ＩＩ線に沿う断面図）である。 The continuous casting method of the present invention includes a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outer side of the mold (the back side of the mold side wall) and downward from the horizontal direction of the molten steel discharge hole Using a continuous casting machine including an immersion nozzle having a molten steel discharge angle α of 10 ° or more and less than 30 °, wherein the molten steel discharge hole is positioned between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole. The steel is continuously cast while the molten steel flow is braked by a DC magnetic field applied to each of the pair of upper magnetic poles and the pair of lower magnetic poles.
1 and 2 show an embodiment of a mold and an immersion nozzle of a continuous casting machine used for carrying out the present invention. FIG. 1 is a longitudinal sectional view of the mold and the immersion nozzle, and FIG. It is a figure (sectional drawing which follows the II-II line of FIG. 1).

図において、１は鋳型であり、この鋳型１は鋳型長辺部１０（鋳型側壁）と鋳型短辺部１１（鋳型側壁）とにより水平断面矩形状に構成されている。
２は浸漬ノズルであり、この浸漬ノズル２を通じて鋳型１の上方に設置されたタンディッシュ（図示せず）内の溶鋼を鋳型１内に注入する。この浸漬ノズル２は、筒状のノズル本体の下端に底部２１を有するとともに、この底部２１の直上の側壁部に、両鋳型短辺部１１と対向するように１対の溶鋼吐出孔２０が貫設されている。
溶鋼中のアルミナなどの非金属介在物が浸漬ノズル２の内壁面に付着・堆積してノズル閉塞を生じることを防止するため、浸漬ノズル２のノズル本体内部に設けられたガス流路にＡｒガスなどの不活性ガスが導入され、この不活性ガスがノズル内壁面からノズル内に吹き込まれる。 In the figure, reference numeral 1 denotes a mold, and the mold 1 is constituted by a mold long side portion 10 (mold side wall) and a mold short side portion 11 (mold side wall) in a rectangular shape in a horizontal section.
Reference numeral 2 denotes an immersion nozzle, and molten steel in a tundish (not shown) installed above the mold 1 is injected into the mold 1 through the immersion nozzle 2. The immersion nozzle 2 has a bottom portion 21 at the lower end of a cylindrical nozzle body, and a pair of molten steel discharge holes 20 penetrates the side wall portion directly above the bottom portion 21 so as to face both mold short side portions 11. It is installed.
In order to prevent non-metallic inclusions such as alumina in the molten steel from adhering to and accumulating on the inner wall surface of the immersion nozzle 2 and blocking the nozzle, Ar gas is provided in the gas flow path provided inside the nozzle body of the immersion nozzle 2. An inert gas such as is introduced, and this inert gas is blown into the nozzle from the inner wall surface of the nozzle.

タンディッシュから浸漬ノズル２に流入した溶鋼は、浸漬ノズル２の１対の溶鋼吐出孔２０から鋳型１内に吐出される。吐出された溶鋼は、鋳型１内で冷却されて凝固シェル５を形成し、鋳型１の下方に連続的に引き抜かれ鋳片となる。鋳型１内のメニスカス６には、溶鋼の保温剤および凝固シェル５と鋳型１との潤滑剤として、モールドフラックスが添加される。
また、浸漬ノズル２の内壁面から吹き込まれた不活性ガスの気泡は、溶鋼吐出孔２０から溶鋼とともに鋳型１内に吐出される。 Molten steel flowing into the immersion nozzle 2 from the tundish is discharged into the mold 1 from a pair of molten steel discharge holes 20 of the immersion nozzle 2. The discharged molten steel is cooled in the mold 1 to form a solidified shell 5 and is continuously drawn below the mold 1 to form a slab. A mold flux is added to the meniscus 6 in the mold 1 as a heat insulating agent for molten steel and a lubricant between the solidified shell 5 and the mold 1.
Further, the inert gas bubbles blown from the inner wall surface of the immersion nozzle 2 are discharged into the mold 1 from the molten steel discharge hole 20 together with the molten steel.

鋳型１の外側（鋳型側壁の背面）には、鋳型長辺部を挟んで対向する１対の上部磁極３a，３bと１対の下部磁極４a，４bが設けられ、これら上部磁極３a，３bと下部磁極４a，４bは、それぞれ鋳型長辺部１０の幅方向において、その全幅に沿うように配置されている。
上部磁極３a，３bと下部磁極４a，４bは、鋳型１の上下方向において、上部磁極３a，３bの磁場のピーク位置（上下方向でのピーク位置：通常は上部磁極３a，３bの上下方向中心位置）と下部磁極４a，４bの磁場のピーク位置（上下方向でのピーク位置：通常は下部磁極４a，４bの上下方向中心位置）の間に溶鋼吐出孔２０が位置するように、配置される。また、１対の上部磁極３a，３bは、通常、メニスカス６をカバーする位置に配置される。 A pair of upper magnetic poles 3a and 3b and a pair of lower magnetic poles 4a and 4b that are opposed to each other with the long side of the mold interposed therebetween are provided on the outer side of the mold 1 (the back side of the mold side wall). The lower magnetic poles 4a and 4b are arranged along the entire width in the width direction of the mold long side portion 10, respectively.
The upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b are, in the vertical direction of the mold 1, the peak positions of the magnetic fields of the upper magnetic poles 3a and 3b (peak positions in the vertical direction: usually the vertical center positions of the upper magnetic poles 3a and 3b). ) And the magnetic field peak position of the lower magnetic poles 4a and 4b (the peak position in the vertical direction: usually the vertical central position of the lower magnetic poles 4a and 4b). In addition, the pair of upper magnetic poles 3 a and 3 b is usually disposed at a position covering the meniscus 6.

浸漬ノズル２の溶鋼吐出孔２０から鋳型短辺部方向に吐出された溶鋼は、鋳型短辺部１１の前面に生成した凝固シェル５に衝突して下降流と上昇流に分かれる。前記１対の上部磁極３a，３bと１対の下部磁極４a，４bには、各々直流磁界が印加されるが、これら磁極による基本的な作用は、直流磁界中を移動する溶鋼に作用する電磁気力を利用して、上部磁極３a，３bに印加される直流磁界で溶鋼上昇流を制動（減速させる）し、下部磁極４a，４bに印加される直流磁界で溶鋼下降流を制動（減速させる）するものである。 The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 in the direction of the mold short side part collides with the solidified shell 5 generated on the front surface of the mold short side part 11 and is divided into a downward flow and an upward flow. A direct current magnetic field is applied to each of the pair of upper magnetic poles 3a and 3b and the pair of lower magnetic poles 4a and 4b. The basic action of these magnetic poles is electromagnetic acting on molten steel moving in the direct current magnetic field. Using force, the molten steel upward flow is braked (decelerated) by the DC magnetic field applied to the upper magnetic poles 3a and 3b, and the molten steel downward flow is braked (decelerated) by the DC magnetic field applied to the lower magnetic poles 4a and 4b. To do.

本発明法では、溶鋼吐出孔２０からの溶鋼吐出角度α、すなわち水平方向から下向きの溶鋼吐出角度αが１０°以上３０°未満の浸漬ノズルを用いる。溶鋼吐出角度αが１０°未満では、上部磁極３a，３bの直流磁界で溶鋼上昇流を制動しても、溶鋼表面の乱れを適切に制御できず、モールドフラックスの巻き込みを生じてしまう。これに対して、溶鋼吐出角度αが大きくなると、非金属介在物や気泡が溶鋼下降流によって鋳型下方に運ばれて凝固シェルに捕捉されやすくなり、一方において、溶鋼吐出角度αが３０°未満では、本発明法による直流磁場制御で溶鋼流を最適化できることが判ったので、本発明では溶鋼吐出角度αが３０°未満の浸漬ノズル２を用いる。 In the present invention method, an immersion nozzle having a molten steel discharge angle α from the molten steel discharge hole 20, that is, a molten steel discharge angle α downward from the horizontal direction of 10 ° or more and less than 30 ° is used. When the molten steel discharge angle α is less than 10 °, even if the molten steel upward flow is braked by the DC magnetic field of the upper magnetic poles 3a and 3b, the turbulence of the molten steel surface cannot be properly controlled, and mold flux is involved. On the other hand, when the molten steel discharge angle α is increased, non-metallic inclusions and bubbles are easily carried down the mold by the molten steel descending flow and are easily captured by the solidified shell. On the other hand, when the molten steel discharge angle α is less than 30 °, Since it has been found that the molten steel flow can be optimized by direct current magnetic field control according to the method of the present invention, the immersion nozzle 2 having a molten steel discharge angle α of less than 30 ° is used in the present invention.

本発明法では、生産性の観点から鋳造速度を１．０ｍ／分以上とするが、さらに、鋳造するスラブ幅と鋳造速度に応じて、上部磁極３a，３bと下部磁極４a，４bに各々印加する直流磁界の強度および両直流磁界の強度比を、下記条件（イ）〜（ハ）のように最適化することにより、フラックス性欠陥および気泡性欠陥の原因となる、凝固シェル５へのモールドフラックスの巻き込み捕捉と、同じく微小気泡（主に浸漬ノズル内壁面から吹き込まれた不活性ガスの気泡）の捕捉を抑制するものである。 In the method of the present invention, the casting speed is set to 1.0 m / min or more from the viewpoint of productivity. Further, the casting speed is applied to the upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b according to the slab width and the casting speed, respectively. The mold to the solidified shell 5 that causes flux defects and bubble defects by optimizing the strength of the direct current magnetic field and the strength ratio of the two direct current magnetic fields as in the following conditions (a) to (c) Similarly, the trapping of the entrainment of the flux and the trapping of microbubbles (mainly bubbles of the inert gas blown from the inner wall surface of the immersion nozzle) are suppressed.

条件（イ）：鋳造するスラブ幅と鋳造速度が下記（a）〜（e）の場合には、上部磁極３a，３bに印加する直流磁界の強度Ａと下部磁極４a，４bに印加する直流磁界の強度Ｂの比Ａ／Ｂを１．３〜２．５、上部磁極３a，３bに印加する直流磁界の強度Ａを０．２０〜０．４０Ｔ、下部磁極４a，４bに印加する直流磁界の強度Ｂを０．１０〜０．２５Ｔとする。
（a）スラブ幅１２５０ｍｍ未満
（b）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分以上
（c）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分以上
（d）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分以上
（e）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分以上 Condition (a): When the slab width to be cast and the casting speed are the following (a) to (e), the strength A of the DC magnetic field applied to the upper magnetic poles 3a and 3b and the DC magnetic field applied to the lower magnetic poles 4a and 4b. The ratio A / B of the strength B is 1.3 to 2.5, the strength A of the direct current magnetic field applied to the upper magnetic poles 3a and 3b is 0.20 to 0.40 T, and the direct current magnetic field applied to the lower magnetic poles 4a and 4b. Strength B is set to 0.10 to 0.25T.
(A) Slab width of less than 1250 mm (b) Slab width of 1250 mm or more and less than 1350 mm and casting speed of 1.4 m / min or more (c) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 1.9 m / min or more (d) Slab width 1450 mm or more and less than 1550 mm and casting speed of 2.3 m / min or more (e) Slab width of 1550 mm or more and less than 1650 mm and casting speed of 2.6 m / min or more

上記（a）のようにスラブ幅が小さい場合や、上記（b）〜（e）のようにスラブ幅が大きくても相対的に鋳造速度が大きい場合には、浸漬ノズル２から吐出した溶鋼流が鋳型短辺部側の凝固シェルに衝突し、上方側に反転することにより生じる溶鋼上昇流（反転流）が強くなるので、上部磁極３ａ，３ｂの直流磁界の強度が下部磁極４ａ，４ｂの直流磁界の強度よりも小さいと、溶鋼表面でモールドフラックスの巻き込みが生じやすくなる。このため上記（a）〜（e）の場合には、溶鋼上昇流の制動を主眼とした直流磁界の制御を行うことが必要である。
上記（a）〜（e）の場合に、上部磁極３a，３bの直流磁界と下部磁極４a，４bの直流磁界の強度比Ａ／Ｂが１．３未満では、上部磁極３a，３bの直流磁界の強度が相対的に弱くなりすぎるため、溶鋼上昇流（反転流）によるモールドフラックスの巻き込みが生じやすくなる。一方、強度比Ａ／Ｂが２．５を超えると、溶鋼上昇流（反転流）の制動効果はあるものの、浸漬ノズル２から吐出した溶鋼流が下方向への大きな流れになりやすく、この溶鋼流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。 When the slab width is small as in (a) above, or when the casting speed is relatively high even though the slab width is large as in (b) to (e) above, the molten steel flow discharged from the immersion nozzle 2 Collides with the solidified shell on the short side of the mold, and the molten steel upward flow (reversal flow) generated by reversing the upper side becomes stronger, so that the DC magnetic field strength of the upper magnetic poles 3a, 3b is lower than that of the lower magnetic poles 4a, 4b. When the strength is lower than the DC magnetic field, mold flux is likely to be caught on the surface of the molten steel. For this reason, in the cases (a) to (e) described above, it is necessary to control the DC magnetic field mainly for braking the molten steel upward flow.
In the above cases (a) to (e), when the intensity ratio A / B of the DC magnetic field of the upper magnetic poles 3a and 3b and the DC magnetic field of the lower magnetic poles 4a and 4b is less than 1.3, the DC magnetic field of the upper magnetic poles 3a and 3b. Since the strength of the steel becomes relatively weak, the mold flux is likely to be caught by the molten steel upward flow (reverse flow). On the other hand, if the strength ratio A / B exceeds 2.5, the molten steel flow discharged from the immersion nozzle 2 tends to become a large downward flow although there is a braking effect of the molten steel upward flow (reversed flow). Non-metallic inclusions and bubbles accompanying the flow will sink downward and become easily trapped by the solidified shell.

また、上部磁極３a，３bの直流磁界の強度Ａが０．２０Ｔ未満では、その直流磁界による溶鋼上昇流の制動効果が不十分で湯面変動が大きく、モールドフラックスの巻き込みが生じやすい。一方、強度Ａが０．４０Ｔを超えると、溶鋼上昇流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。
また、下部磁極４a，４bの直流磁界の強度Ｂが０．１０Ｔ未満では、その直流磁界による溶鋼下降流の制動効果が不十分であるため、溶鋼下降流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。一方、強度Ｂが０．２５Ｔを超えると、溶鋼下降流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。 If the strength A of the DC magnetic field of the upper magnetic poles 3a and 3b is less than 0.20T, the effect of braking the molten steel upward flow by the DC magnetic field is insufficient, the fluctuation of the molten metal surface is large, and mold flux is likely to be involved. On the other hand, when the strength A exceeds 0.40 T, the cleaning effect due to the molten steel ascending flow is reduced, so that nonmetallic inclusions and bubbles are easily captured by the solidified shell.
In addition, when the strength B of the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.10 T, the braking effect of the molten steel descending flow due to the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles accompanying the molten steel descending flow are generated. It sinks downward and is easily trapped by the solidified shell. On the other hand, when the strength B exceeds 0.25T, the cleaning effect due to the molten steel descending flow is reduced, so that nonmetallic inclusions and bubbles are easily trapped by the solidified shell.

また、上記（a）〜（e）の各場合には、それぞれ特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。すなわち、（a）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１２〜０．１６Ｔ、両直流磁界の強度比Ａ／Ｂを２．００〜２．２５とすることが好ましい。また、（b）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１３〜０．１８Ｔ、両直流磁界の強度比Ａ／Ｂを１．７８〜２．０８とすることが好ましい。また、（c）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１５〜０．２０Ｔ、両直流磁界の強度比Ａ／Ｂを１．６０〜１．８０とすることが好ましい。また、（d）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１６〜０．２１Ｔ、両直流磁界の強度比Ａ／Ｂを１．５２〜１．６９とすることが好ましい。また、（e）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１８〜０．２４Ｔ、両直流磁界の強度比Ａ／Ｂを１．３３〜１．５０とすることが好ましい。 In each of the cases (a) to (e), the optimum DC magnetic field strength A of the upper magnetic poles 3a and 3b, the DC magnetic field strength B of the lower magnetic poles 4a and 4b, and the strengths of both DC magnetic fields, respectively. There is a ratio A / B. That is, in the case of (a), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.12 to 0.16T. It is preferable that the intensity ratio A / B of both DC magnetic fields is 2.00 to 2.25. In the case of (b), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.13 to 0.18T. The intensity ratio A / B of both DC magnetic fields is preferably 1.78 to 2.08. In the case of (c), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.15 to 0.20T. It is preferable that the intensity ratio A / B of both DC magnetic fields is 1.60 to 1.80. In the case of (d), the DC magnetic field intensity A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field intensity B of the lower magnetic poles 4a and 4b is 0.16 to 0.21T. The intensity ratio A / B of both DC magnetic fields is preferably 1.52 to 1.69. In the case of (e), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.18 to 0.24T. It is preferable that the intensity ratio A / B of both DC magnetic fields is 1.33 to 1.50.

条件（ロ）：鋳造するスラブ幅と鋳造速度が下記（f）、（g）の場合には、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを０．７〜１．５、上部磁極に印加する直流磁界の強度Ａを０.２５〜０．３５Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．２０〜０．４０Ｔとする。
（f）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分以上
（g）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分以上 Condition (b): When the casting slab width and casting speed are the following (f) and (g), the ratio A of the DC magnetic field strength A applied to the upper magnetic pole and the DC magnetic field strength B applied to the lower magnetic pole / B is 0.7 to 1.5, the DC magnetic field strength A applied to the upper magnetic pole is 0.25 to 0.35 T, and the DC magnetic field strength B applied to the lower magnetic pole is 0.20 to 0.40 T. .
(F) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.8 m / min or more (g) Slab width of 1750 mm or more and casting speed of 2.9 m / min or more

上記（f）、（g）のようにスラブ幅が大きくても鋳造速度が極めて大きい場合には、浸漬ノズル２から吐出した溶鋼流が鋳型短辺部側の凝固シェルに衝突し、上方側に反転することにより生じる溶鋼上昇流（反転流）が非常に強くなるので、上部磁極３ａ，３ｂと下部磁極４ａ，４ｂの両方の直流磁界の強度を大きくしないと、溶鋼表面でのモールドフラックスの巻き込みと非金属介在物や気泡の下方向への潜り込みの両方が生じやすくなる。このため上記（f）、（g）の場合には、溶鋼上昇流と下降流の両方を制動する直流磁界の制御を行うことが必要である。 When the casting speed is extremely high even if the slab width is large as in (f) and (g) above, the molten steel flow discharged from the immersion nozzle 2 collides with the solidified shell on the mold short side, and on the upper side The molten steel upward flow (reversal flow) generated by reversal becomes very strong. Therefore, unless the strength of the DC magnetic field of both the upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b is increased, the mold flux is caught on the molten steel surface. Both non-metallic inclusions and bubbles are likely to sink downward. For this reason, in the case of the above (f) and (g), it is necessary to control the DC magnetic field that brakes both the upward and downward flow of the molten steel.

上記（f）、（g）の場合に、上部磁極３a，３bの直流磁界と下部磁極４a，４bの直流磁界の強度比Ａ／Ｂが０．７未満では、上部磁極３a，３bの直流磁界の強度が相対的に弱くなりすぎるため、溶鋼上昇流（反転流）によるモールドフラックスの巻き込みが生じやすくなる。一方、強度比Ａ／Ｂが１．５を超えると、溶鋼上昇流（反転流）の制動効果はあるものの、浸漬ノズル２から吐出した溶鋼流が下方向への大きな流れになりやすく、この溶鋼流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。 In the above cases (f) and (g), when the intensity ratio A / B of the DC magnetic field of the upper magnetic poles 3a and 3b and the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.7, the DC magnetic field of the upper magnetic poles 3a and 3b. Since the strength of the steel becomes relatively weak, the mold flux is likely to be caught by the molten steel upward flow (reverse flow). On the other hand, when the strength ratio A / B exceeds 1.5, the molten steel flow discharged from the immersion nozzle 2 tends to become a large downward flow although there is a braking effect of the molten steel upward flow (reversed flow). Non-metallic inclusions and bubbles accompanying the flow will sink downward and become easily trapped by the solidified shell.

また、上部磁極３a，３bの直流磁界の強度Ａが０．２５Ｔ未満では、その直流磁界による溶鋼上昇流の制動効果が不十分で湯面変動が大きく、モールドフラックスの巻き込みが生じやすい。一方、強度Ａが０．３５Ｔを超えると、溶鋼上昇流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。
また、下部磁極４a，４bの直流磁界の強度Ｂが０．２０Ｔ未満では、その直流磁界による溶鋼下降流の制動効果が不十分であるため、溶鋼下降流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。一方、強度Ｂが０．４０Ｔを超えると、溶鋼下降流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。 If the strength A of the DC magnetic field of the upper magnetic poles 3a and 3b is less than 0.25T, the effect of braking the molten steel upward flow by the DC magnetic field is insufficient, the fluctuation of the molten metal surface is large, and mold flux is likely to be involved. On the other hand, when the strength A exceeds 0.35 T, the cleaning effect due to the molten steel ascending flow is reduced, so that nonmetallic inclusions and bubbles are easily captured by the solidified shell.
In addition, when the strength B of the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.20T, the braking effect of the molten steel descending flow by the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles associated with the molten steel descending flow are generated. It sinks downward and is easily trapped by the solidified shell. On the other hand, when the strength B exceeds 0.40 T, the cleaning effect due to the molten steel descending flow is reduced, so that non-metallic inclusions and bubbles are easily captured by the solidified shell.

また、上記（f）,（g）の各場合には、それぞれ特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。すなわち、（f）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．３０〜０．３６Ｔ、両直流磁界の強度比Ａ／Ｂを０．８９〜０．９０とすることが好ましい。また、（g）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０．２７〜０．３２Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．３０〜０．３６Ｔ、両直流磁界の強度比Ａ／Ｂを０．８９〜０．９０とすることが好ましい。 In each of the cases (f) and (g), the optimum DC magnetic field strength A of the upper magnetic poles 3a and 3b, the DC magnetic field strength B of the lower magnetic poles 4a and 4b, and the strengths of both DC magnetic fields, respectively. There is a ratio A / B. That is, in the case of (f), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.30 to 0.36T. The intensity ratio A / B of both DC magnetic fields is preferably 0.89 to 0.90. In the case of (g), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0.27 to 0.32T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is 0.30 to 0.36T. The intensity ratio A / B of both DC magnetic fields is preferably 0.89 to 0.90.

条件（ハ）：鋳造するスラブ幅と鋳造速度が下記（h）〜（m）の場合には、上部磁極３a，３bに印加する直流磁界の強度Ａと下部磁極４a，４bに印加する直流磁界の強度Ｂの比Ａ／Ｂを０．５以下、上部磁極３a，３bに印加する直流磁界の強度Ａを０〜０．０６Ｔ、下部磁極４a，４bに印加する直流磁界の強度Ｂを０．１０〜０．４０Ｔとする。
（h）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分未満
（i）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分未満
（j）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分未満
（k）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分未満
（l）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分未満
（m）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分未満 Condition (c): When the casting slab width and casting speed are the following (h) to (m), the DC magnetic field strength A applied to the upper magnetic poles 3a and 3b and the DC magnetic field applied to the lower magnetic poles 4a and 4b. The ratio A / B of the magnetic field strength B is 0.5 or less, the direct current magnetic field strength A applied to the upper magnetic poles 3a and 3b is 0 to 0.06T, and the direct current magnetic field strength B applied to the lower magnetic poles 4a and 4b is 0. 10 to 0.40T.
(H) Slab width of 1250 mm or more and less than 1350 mm and casting speed of less than 1.4 m / min (i) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 1.9 m / min (j) Slab width of 1450 mm or more and less than 1550 mm and casting (K) Slab width of 1550 mm or more and less than 1650 mm and casting speed of less than 2.6 m / min (l) Slab width of 1650 mm or more and less than 1750 mm and casting speed of less than 2.8 m / min (m) Slab width 1750mm or more and casting speed less than 2.9m / min

上記（h）〜（m）のようにスラブ幅がある程度大きく且つ相対的に鋳造速度が小さい場合には、浸漬ノズル２から吐出した溶鋼流が鋳型短辺部側の凝固シェルに衝突し、上方側に反転することにより生じる溶鋼上昇流（反転流）が弱くなるので、モールドフラックスの巻き込みは生じにくなる一方で、図６（ａ）に示すように下部磁極４ａ，４ｂの直流磁界の強度が上部磁極３ａ，３ｂの直流磁界の強度よりも小さいと、気泡や非金属介在物が下部磁極４ａ，４の下方側に流出し、欠陥が発生しやすくなる。これに対し、下部磁極４ａ，４ｂの直流磁界の強度が上部磁極３ａ，３ｂの直流磁界の強度よりも大きければ、図６に（ｂ）に示すように、気泡が上昇・浮上しやすくなり、欠陥が少なくなる。このため上記（h）〜（m）の場合には、溶鋼上昇流をあまり減速させないような制動を主眼とした直流磁界の制御を行うことが必要である。 When the slab width is large to some extent and the casting speed is relatively small as in (h) to (m) above, the molten steel flow discharged from the immersion nozzle 2 collides with the solidified shell on the short side of the mold and As the molten steel ascending flow (reversing flow) generated by reversing to the side becomes weak, the mold flux is hardly involved, while the DC magnetic field strength of the lower magnetic poles 4a and 4b as shown in FIG. Is smaller than the DC magnetic field strength of the upper magnetic poles 3a, 3b, bubbles and non-metallic inclusions flow out to the lower side of the lower magnetic poles 4a, 4 and defects are likely to occur. On the other hand, if the strength of the DC magnetic field of the lower magnetic poles 4a and 4b is greater than the strength of the DC magnetic field of the upper magnetic poles 3a and 3b, as shown in FIG. There are fewer defects. For this reason, in the cases (h) to (m) described above, it is necessary to control the DC magnetic field with a focus on braking so as not to slow down the molten steel upward flow.

上記（h）〜（m）の場合に、上部磁極３a，３bの直流磁界と下部磁極４a，４bの直流磁界の強度比Ａ／Ｂが０．５を超えると、浸漬ノズル２から吐出した溶鋼流が下方向への大きな流れになりやすく、この溶鋼流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。
また、上部磁極３a，３bには直流磁界を印加しなくてもよい。一方、上部磁極３a，３bの直流磁界の強度Ａが０．０６Ｔを超えると、溶鋼上昇流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。
また、下部磁極４a，４bの直流磁界の強度Ｂが０．１０Ｔ未満では、その直流磁界による溶鋼下降流の制動効果が不十分であるため、溶鋼下降流に随伴する非金属介在物や気泡が下方向に潜り込み、凝固シェルに捕捉されやすくなる。一方、強度Ｂが０．４０Ｔを超えると、溶鋼下降流による洗浄効果が低下するため非金属介在物や気泡が凝固シェルに捕捉されやすくなる。 In the above cases (h) to (m), when the intensity ratio A / B of the DC magnetic field of the upper magnetic poles 3a and 3b and the DC magnetic field of the lower magnetic poles 4a and 4b exceeds 0.5, the molten steel discharged from the immersion nozzle 2 The flow tends to become a large downward flow, and the non-metallic inclusions and bubbles accompanying the molten steel flow will sink in the downward direction and be easily captured by the solidified shell.
Further, it is not necessary to apply a DC magnetic field to the upper magnetic poles 3a and 3b. On the other hand, when the strength A of the DC magnetic field of the upper magnetic poles 3a and 3b exceeds 0.06T, the cleaning effect due to the molten steel ascending flow is reduced, so that nonmetallic inclusions and bubbles are easily trapped by the solidified shell.
In addition, when the strength B of the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.10 T, the braking effect of the molten steel descending flow due to the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles accompanying the molten steel descending flow are generated. It sinks downward and is easily trapped by the solidified shell. On the other hand, when the strength B exceeds 0.40 T, the cleaning effect due to the molten steel descending flow is reduced, so that non-metallic inclusions and bubbles are easily captured by the solidified shell.

また、上記（h）〜（m）の各場合には、それぞれ特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。すなわち、（h）,(i)の場合には、上部磁極３a，３bの直流磁界の強度Ａを０〜０．０３Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１０〜０．１５Ｔ、両直流磁界の強度比Ａ／Ｂを０〜０．２０とすることが好ましい。また、（j）の場合において鋳造速度が１．６ｍ／分以上の場合には、上部磁極３a，３bの直流磁界の強度Ａを０〜０．０３Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．１３〜０．１９Ｔ、両直流磁界の強度比Ａ／Ｂを０〜０．１６とすることが好ましい。また、（j）の場合において鋳造速度が１．６ｍ／分未満の場合には、上部磁極３a，３bの直流磁界の強度Ａを０〜０．０３Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．３２〜０．３８Ｔ、両直流磁界の強度比Ａ／Ｂを０〜０．０８とすることが好ましい。また、（k）〜（m）の場合には、上部磁極３a，３bの直流磁界の強度Ａを０〜０．０３Ｔ、下部磁極４a，４bの直流磁界の強度Ｂを０．３２〜０．３８Ｔ、両直流磁界の強度比Ａ／Ｂを０〜０．０８とすることが好ましい。 In each of the cases (h) to (m), the optimum DC magnetic field strength A of the upper magnetic poles 3a and 3b, the DC magnetic field strength B of the lower magnetic poles 4a and 4b, and the strengths of both DC magnetic fields, respectively. There is a ratio A / B. That is, in the case of (h) and (i), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is set to 0 to 0.03T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is set to 0.10 to 0.0. It is preferable that the intensity ratio A / B of both DC magnetic fields is 0 to 0.20. In the case of (j), when the casting speed is 1.6 m / min or more, the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0 to 0.03T, and the DC magnetic field strength of the lower magnetic poles 4a and 4b. It is preferable that B is 0.13 to 0.19 T and the strength ratio A / B of both DC magnetic fields is 0 to 0.16. In the case of (j), when the casting speed is less than 1.6 m / min, the DC magnetic field strength A of the upper magnetic poles 3a and 3b is 0 to 0.03T, and the DC magnetic field strength of the lower magnetic poles 4a and 4b. It is preferable that B is 0.32 to 0.38 T and the intensity ratio A / B of both DC magnetic fields is 0 to 0.08. In the case of (k) to (m), the DC magnetic field strength A of the upper magnetic poles 3a and 3b is set to 0 to 0.03T, and the DC magnetic field strength B of the lower magnetic poles 4a and 4b is set to 0.32 to. It is preferable that the intensity ratio A / B of both DC magnetic fields is 0 to 0.08.

なお、以上述べた本発明の連続鋳造方法は、スラブ幅と鋳造速度に応じて規定される、下記（ｉ）〜（iii）のような３つの連続鋳造方法として捉えることもできる。
（ｉ）鋳型外側に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備えるとともに、溶鋼吐出孔の水平方向から下向きの溶鋼吐出角度が１０°以上３０°未満の浸漬ノズルを備え、前記上部磁極の磁場のピーク位置と前記下部磁極の磁場のピーク位置の間に前記溶鋼吐出孔が位置する連続鋳造機を用い、前記１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う方法であって、
鋳造速度１．０ｍ／分以上であって、スラブ幅と鋳造速度を下記（a）〜（e）のいずれかの条件とし、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを１．３〜２．５、上部磁極に印加する直流磁界の強度Ａを０．２０〜０．４０Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．１０〜０．２５Ｔとして連続鋳造を行うことを特徴とする鋼の連続鋳造方法。
（a）スラブ幅１２５０ｍｍ未満
（b）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分以上
（c）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分以上
（d）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分以上
（e）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分以上
また、上記（a）〜（e）の各場合には、それぞれにさきに述べたような特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。 In addition, the continuous casting method of the present invention described above can also be regarded as three continuous casting methods such as the following (i) to (iii) defined according to the slab width and the casting speed.
(I) A pair of upper magnetic poles and a pair of lower magnetic poles facing each other with the long side of the mold interposed therebetween are provided outside the mold, and the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 °. Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting steel while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
The casting speed is 1.0 m / min or more, the slab width and the casting speed are any one of the following conditions (a) to (e), the direct current magnetic field strength A applied to the upper magnetic pole and the direct current applied to the lower magnetic pole The ratio A / B of the magnetic field strength B is 1.3 to 2.5, the DC magnetic field strength A applied to the upper magnetic pole is 0.20 to 0.40 T, and the DC magnetic field strength B applied to the lower magnetic pole is 0. A continuous casting method of steel, wherein continuous casting is performed at 10 to 0.25 T.
(A) Slab width of less than 1250 mm (b) Slab width of 1250 mm or more and less than 1350 mm and casting speed of 1.4 m / min or more (c) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 1.9 m / min or more (d) Slab width 1450 mm or more and less than 1550 mm and a casting speed of 2.3 m / min or more (e) Slab width of 1550 mm or more and less than 1650 mm and a casting speed of 2.6 m / min or more In each case of the above (a) to (e), There are the particularly optimal DC magnetic field strength A of the upper magnetic poles 3a and 3b, the DC magnetic field strength B of the lower magnetic poles 4a and 4b, and the strength ratio A / B of both DC magnetic fields, as described above.

（ii）鋳型外側に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備えるとともに、溶鋼吐出孔の水平方向から下向きの溶鋼吐出角度が１０°以上３０°未満の浸漬ノズルを備え、前記上部磁極の磁場のピーク位置と前記下部磁極の磁場のピーク位置の間に前記溶鋼吐出孔が位置する連続鋳造機を用い、前記１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う方法であって、
鋳造速度１．０ｍ／分以上であって、スラブ幅と鋳造速度を下記（f）、（g）のいずれかの条件とし、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを０．７〜１．５、上部磁極に印加する直流磁界の強度Ａを０.２５〜０．３５Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．２０〜０．４０Ｔとして連続鋳造を行うことを特徴とする鋼の連続鋳造方法。
（f）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分以上
（g）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分以上
また、上記（f）,（g）の各場合には、それぞれにさきに述べたような特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。 (Ii) A pair of upper magnetic poles and a pair of lower magnetic poles facing each other with the long side of the mold interposed therebetween are provided outside the mold, and the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 °. Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting steel while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
The casting speed is 1.0 m / min or more, the slab width and the casting speed are any of the following conditions (f) and (g), and the direct current magnetic field strength A applied to the upper magnetic pole and the direct current applied to the lower magnetic pole. The ratio A / B of the magnetic field intensity B is 0.7 to 1.5, the DC magnetic field intensity A applied to the upper magnetic pole is 0.25 to 0.35 T, and the DC magnetic field intensity B applied to the lower magnetic pole is 0. A continuous casting method for steel, wherein continuous casting is performed at 20 to 0.40 T.
(F) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.8 m / min or more (g) Slab width of 1750 mm or more and casting speed of 2.9 m / min or more In each case of the above (f) and (g) Have the optimum DC magnetic field strength A of the upper magnetic poles 3a and 3b, the DC magnetic field strength B of the lower magnetic poles 4a and 4b, and the strength ratio A / B of both DC magnetic fields as described above.

（iii）鋳型外側に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備えるとともに、溶鋼吐出孔の水平方向から下向きの溶鋼吐出角度が１０°以上３０°未満の浸漬ノズルを備え、前記上部磁極の磁場のピーク位置と前記下部磁極の磁場のピーク位置の間に前記溶鋼吐出孔が位置する連続鋳造機を用い、前記１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動しつつ、鋼の連続鋳造を行う方法であって、
鋳造速度１．０ｍ／分以上であって、スラブ幅と鋳造速度を下記（h）〜（m）のいずれかの条件とし、上部磁極に印加する直流磁界の強度Ａと下部磁極に印加する直流磁界の強度Ｂの比Ａ／Ｂを０．５以下、上部磁極に印加する直流磁界の強度Ａを０〜０．０６Ｔ、下部磁極に印加する直流磁界の強度Ｂを０．１０〜０．４０Ｔとして連続鋳造を行うことを特徴とする鋼の連続鋳造方法。
（h）スラブ幅１２５０ｍｍ以上１３５０ｍｍ未満で且つ鋳造速度１．４ｍ／分未満
（i）スラブ幅１３５０ｍｍ以上１４５０ｍｍ未満で且つ鋳造速度１．９ｍ／分未満
（j）スラブ幅１４５０ｍｍ以上１５５０ｍｍ未満で且つ鋳造速度２．３ｍ／分未満
（k）スラブ幅１５５０ｍｍ以上１６５０ｍｍ未満で且つ鋳造速度２．６ｍ／分未満
（l）スラブ幅１６５０ｍｍ以上１７５０ｍｍ未満で且つ鋳造速度２．８ｍ／分未満
（m）スラブ幅１７５０ｍｍ以上で且つ鋳造速度２．９ｍ／分未満
また、上記（h）〜（m）の各場合には、それぞれにさきに述べたような特に最適な上部磁極３a，３bの直流磁界の強度Ａ、下部磁極４a，４bの直流磁界の強度Ｂ、および両直流磁界の強度比Ａ／Ｂがある。 (Iii) A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the mold long side portion are provided outside the mold, and the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 °. Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting steel while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
The casting speed is 1.0 m / min or more, the slab width and casting speed are any one of the following conditions (h) to (m), and the direct current magnetic field strength A applied to the upper magnetic pole and the direct current applied to the lower magnetic pole. The ratio A / B of the magnetic field strength B is 0.5 or less, the DC magnetic field strength A applied to the upper magnetic pole is 0 to 0.06 T, and the DC magnetic field strength B applied to the lower magnetic pole is 0.10 to 0.40 T. A continuous casting method of steel, characterized by performing continuous casting as
(H) Slab width of 1250 mm or more and less than 1350 mm and casting speed of less than 1.4 m / min (i) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 1.9 m / min (j) Slab width of 1450 mm or more and less than 1550 mm and casting (K) Slab width of 1550 mm or more and less than 1650 mm and casting speed of less than 2.6 m / min (l) Slab width of 1650 mm or more and less than 1750 mm and casting speed of less than 2.8 m / min (m) Slab width 1750 mm or more and casting speed of less than 2.9 m / min In each of the cases (h) to (m), the particularly optimal DC magnetic field strength A of the upper magnetic poles 3a and 3b as described above is used. , The DC magnetic field intensity B of the lower magnetic poles 4a and 4b, and the DC magnetic field intensity ratio A / B.

以下、本発明法において、発明の効果が最も発現しやすい、特に好ましい鋳造条件について説明する。
まず、浸漬ノズル２のノズル浸漬深さは２３０〜２９０ｍｍとすることが好ましい。ここで、ノズル浸漬深さとは、メニスカス６から溶鋼吐出孔２０上端までの距離をいう。
このノズル浸漬深さが、本発明の効果に影響を及ぼすのは、ノズル浸漬深さが大きすぎても、小さすぎても、浸漬ノズル２から吐出される溶鋼の流動量や流速が変化したときに、鋳型内での溶鋼の流動状態が大きく変化するため、溶鋼流の適切な制御が難しくなるためである。すなわち、ノズル浸漬深さが２３０ｍｍ未満では、浸漬ノズル２から吐出される溶鋼の流動量や流速が変化したときに、ダイレクトに溶鋼表面（メニスカス）が変動し、表面の乱れが大きくなってモールドフラックスの巻き込みが起こり易くなり、一方、２９０ｍｍを超えると、溶鋼の流動量などが変動したときに、下方への流速が大きくなって非金属系介在物や気泡の潜り込みが大きくなる傾向がある。 Hereinafter, particularly preferable casting conditions in which the effects of the invention are most easily manifested in the method of the present invention will be described.
First, the nozzle immersion depth of the immersion nozzle 2 is preferably 230 to 290 mm. Here, the nozzle immersion depth refers to the distance from the meniscus 6 to the upper end of the molten steel discharge hole 20.
This nozzle immersion depth affects the effect of the present invention when the flow rate and flow rate of the molten steel discharged from the immersion nozzle 2 change even if the nozzle immersion depth is too large or too small. Moreover, since the flow state of the molten steel in the mold is greatly changed, it is difficult to appropriately control the molten steel flow. That is, when the nozzle immersion depth is less than 230 mm, the molten steel surface (meniscus) directly fluctuates when the flow rate or flow velocity of the molten steel discharged from the immersion nozzle 2 changes, and the surface disturbance increases, and the mold flux On the other hand, if it exceeds 290 mm, when the flow rate of molten steel fluctuates, the flow rate downwards tends to increase, and the non-metallic inclusions and bubbles tend to become deeper.

図３は、本発明法において、浸漬ノズル２のノズル浸漬深さの影響（フラックス性欠陥および気泡性欠陥に及ぼす影響）を調べた結果を示すものであり、浸漬ノズルの溶鋼吐出孔の溶鋼吐出角度α：１５°、スラブ幅：１０００ｍｍ、スラブ厚さ：２６０ｍｍ、鋳造速度：２．８ｍ／分、上部磁極と下部磁極の直流磁界の強度比Ａ／Ｂ：２．３６、上部磁極の直流磁界の強度Ａ：０．３０Ｔ、下部磁極の直流磁界の強度Ｂ：０．１３Ｔの鋳造条件による試験結果を示している。その他の鋳造条件は、浸漬ノズル内径：８０ｍｍ、浸漬ノズルの各溶鋼吐出孔の開口面積：４９００ｍｍ^２（７０ｍｍ＊７０ｍｍ）、浸漬ノズル内壁面からの不活性ガス吹き込み量：１２Ｌ／ｍｉｎ、使用したモールドフラックスの粘度（１３００℃）：０．６ｃｐである。
鋳造されたスラブについて、超音波探傷装置を用い、スラブ表層２〜３ｍｍの深さ位置に存在する粒径が概ね８０μｍ以上の気泡性欠陥およびフラックス性欠陥の個数を測定し、欠陥発生の程度を指数化したものである。図３によれば、本発明法において、特に、浸漬ノズル２のノズル浸漬深さを２３０〜２９０ｍｍとすることにより、気泡性欠陥、フラックス性欠陥がより効果的に低減していることが判る。 FIG. 3 shows the results of investigating the influence of the nozzle immersion depth of the immersion nozzle 2 (influence on the flux defect and bubble defect) in the method of the present invention, and the molten steel discharge of the molten steel discharge hole of the immersion nozzle. Angle α: 15 °, slab width: 1000 mm, slab thickness: 260 mm, casting speed: 2.8 m / min, intensity ratio of DC magnetic field of upper magnetic pole and lower magnetic pole A / B: 2.36, DC magnetic field of upper magnetic pole The test results under the casting conditions of A: 0.30 T, and DC magnetic field strength B of the lower magnetic pole: 0.13 T are shown. Other casting conditions are: immersion nozzle inner diameter: 80 mm, opening area of each molten steel discharge hole of the immersion nozzle: 4900 mm ² (70 mm * 70 mm), amount of inert gas blown from the inner wall of the immersion nozzle: 12 L / min, mold used Flux viscosity (1300 ° C.): 0.6 cp.
For the cast slab, using an ultrasonic flaw detector, measure the number of bubble defects and flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and determine the degree of defect occurrence. It is an index. According to FIG. 3, it can be seen that, in the method of the present invention, in particular, by setting the nozzle immersion depth of the immersion nozzle 2 to 230 to 290 mm, bubble defects and flux defects are more effectively reduced.

また、浸漬ノズル２のノズル内径、すなわち溶鋼吐出孔２０の位置でのノズル内径は７０〜９０ｍｍとすることが好ましい。浸漬ノズル２の内側にアルミナなどが部分的に付着した場合に、浸漬ノズル２から吐出する溶鋼に偏流（幅方向での流速の対称性が悪くなる）が生じることがあり、ノズル内径が７０ｍｍ未満では、そのような場合に偏流が極端に大きくなる恐れがある。このような極端な偏流が生じると、鋳型内での溶鋼流の制御が適切に行えなくなる。一方、浸漬ノズル２に流れる溶鋼量の調整は、浸漬ノズル２の上方のスライディングノズルの開度調整により行われるが、ノズル内径が９０ｍｍを超えるとノズル内部に溶鋼が充填されない部分が生じる恐れがあり、この場合も上記と同じような極端な偏流が生じ、鋳型内での溶鋼流の制御が適切に行えなくなる恐れがある。 The nozzle inner diameter of the immersion nozzle 2, that is, the nozzle inner diameter at the position of the molten steel discharge hole 20 is preferably 70 to 90 mm. When alumina or the like partially adheres to the inner side of the immersion nozzle 2, drift may occur in the molten steel discharged from the immersion nozzle 2 (symmetry of the flow velocity in the width direction is worse), and the inner diameter of the nozzle is less than 70 mm. Then, in such a case, there is a possibility that the drift becomes extremely large. When such an extreme drift occurs, the molten steel flow in the mold cannot be properly controlled. On the other hand, the amount of molten steel flowing through the immersion nozzle 2 is adjusted by adjusting the opening of the sliding nozzle above the immersion nozzle 2, but if the nozzle inner diameter exceeds 90 mm, there may be a portion where the molten steel is not filled inside the nozzle. In this case, too, an extreme drift similar to the above occurs, and there is a possibility that the molten steel flow in the mold cannot be properly controlled.

図４は、本発明法において、浸漬ノズル２のノズル内径の影響（フラックス性欠陥に及ぼす影響）を調べた結果を示すものであり、浸漬ノズルの溶鋼吐出孔の溶鋼吐出角度α：１５°、スラブ幅：１３００ｍｍ、スラブ厚さ：２６０ｍｍ、鋳造速度：２．３ｍ／分、上部磁極と下部磁極の直流磁界の強度比Ａ／Ｂ：２．３６、上部磁極の直流磁界の強度Ａ：０．３０Ｔ、下部磁極の直流磁界の強度Ｂ：０．１３Ｔの鋳造条件による試験結果を示している。その他の鋳造条件は、浸漬ノズルのノズル浸漬深さ：２６０ｍｍ、浸漬ノズルの各溶鋼吐出孔の開口面積：４９００ｍｍ^２（７０ｍｍ＊７０ｍｍ）、浸漬ノズル内壁面からの不活性ガス吹き込み量：１２Ｌ／ｍｉｎ、使用したモールドフラックスの粘度（１３００℃）：０．６ｃｐである。
鋳造されたスラブについて、超音波探傷装置を用い、スラブ表層２〜３ｍｍの深さ位置に存在する粒径が概ね８０μｍ以上のフラックス性欠陥の個数を測定し、欠陥発生の程度を指数化したものである。図４によれば、本発明法において、特に、浸漬ノズル２のノズル内径を７０〜９０ｍｍとすることにより、フラックス性欠陥がより効果的に低減していることが判る。 FIG. 4 shows the results of investigating the influence of the nozzle inner diameter of the immersion nozzle 2 (the influence on the flux defect) in the method of the present invention. The molten steel discharge angle α of the molten steel discharge hole of the immersion nozzle is 15 °. Slab width: 1300 mm, slab thickness: 260 mm, casting speed: 2.3 m / min, DC magnetic field strength ratio of upper magnetic pole and lower magnetic pole: A / B: 2.36, DC magnetic field strength of upper magnetic pole: A. The test results under the casting conditions of 30T and the DC magnetic field strength B of the lower magnetic pole of 0.13T are shown. Other casting conditions are: nozzle immersion depth of the immersion nozzle: 260 mm, opening area of each molten steel discharge hole of the immersion nozzle: 4900 mm ² (70 mm * 70 mm), amount of inert gas blown from the inner wall surface of the immersion nozzle: 12 L / min The viscosity of the mold flux used (1300 ° C.): 0.6 cp.
For the cast slab, an ultrasonic flaw detector was used to measure the number of flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and indexing the degree of defect occurrence. It is. According to FIG. 4, it can be seen that in the method of the present invention, the flux defect is more effectively reduced by setting the inner diameter of the immersion nozzle 2 to 70 to 90 mm.

また、浸漬ノズル２の各溶鋼吐出孔２０の開口面積は３６００〜８２００ｍｍ^２とすることが好ましい。この溶鋼吐出孔２０の開口面積が、本発明の効果に影響を及ぼすのは、溶鋼吐出孔２０の開口面積が小さすぎると溶鋼吐出孔２０から吐出される溶鋼流速が大きくなりすぎ、逆に開口面積が大きすぎると溶鋼流速が小さすぎ、いずれの場合も鋳型内の溶鋼流の流速を適正化しにくくなるからである。 The opening area of each molten steel discharge hole 20 of the immersion nozzle 2 is preferable to be 3600～8200Mm ^2. The opening area of the molten steel discharge hole 20 affects the effect of the present invention. If the opening area of the molten steel discharge hole 20 is too small, the flow velocity of the molten steel discharged from the molten steel discharge hole 20 becomes too large and the opening is reversed. This is because if the area is too large, the molten steel flow velocity is too small, and in any case, it becomes difficult to optimize the flow velocity of the molten steel flow in the mold.

図５は、本発明法において、浸漬ノズル２の各溶鋼吐出孔の開口面積の影響（フラックス性欠陥および気泡性欠陥に及ぼす影響）を調べた結果を示すものであり、浸漬ノズルの溶鋼吐出孔の溶鋼吐出角度α：１５°、スラブ幅：１３００ｍｍ、スラブ厚さ：２６０ｍｍ、鋳造速度：２．３ｍ／分、上部磁極と下部磁極の直流磁界の強度比Ａ／Ｂ：２．３６、上部磁極の直流磁界の強度Ａ：０．３０Ｔ、下部磁極の直流磁界の強度Ｂ：０．１３Ｔの鋳造条件による試験結果を示している。その他の鋳造条件は、浸漬ノズルのノズル浸漬深さ：２６０ｍｍ、浸漬ノズル内径：８０ｍｍ、浸漬ノズル内壁面からの不活性ガス吹き込み量：１２Ｌ／ｍｉｎ、使用したモールドフラックスの粘度（１３００℃）：０．６ｃｐである。
鋳造されたスラブについて、超音波探傷装置を用い、スラブ表層２〜３ｍｍの深さ位置に存在する粒径が概ね８０μｍ以上の気泡性欠陥およびフラックス性欠陥の個数を測定し、欠陥発生の程度を指数化したものである。図５によれば、本発明法において、特に、浸漬ノズル２の各溶鋼吐出孔２０の開口面積を３６００〜８２００ｍｍ^２とすることにより、気泡性欠陥、フラックス性欠陥がより効果的に低減していることが判る。 FIG. 5 shows the result of investigating the influence of the opening area of each molten steel discharge hole of the immersion nozzle 2 (influence on the flux defect and bubble defect) in the method of the present invention. Molten steel discharge angle α: 15 °, slab width: 1300 mm, slab thickness: 260 mm, casting speed: 2.3 m / min, DC magnetic field strength ratio A / B: 2.36, upper magnetic pole, upper magnetic pole The test results under the casting conditions of DC magnetic field strength A: 0.30 T and DC magnetic field strength B of the lower magnetic pole: 0.13 T are shown. The other casting conditions were: nozzle immersion depth of the immersion nozzle: 260 mm, immersion nozzle inner diameter: 80 mm, amount of inert gas blown from the inner wall surface of the immersion nozzle: 12 L / min, viscosity of the mold flux used (1300 ° C.): 0 .6 cp.
For the cast slab, using an ultrasonic flaw detector, measure the number of bubble defects and flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and determine the degree of defect occurrence. It is an index. According to FIG. 5, in the method of the present invention, in particular, by setting the opening area of each molten steel discharge hole 20 of the immersion nozzle 2 to 3600 to 8200 mm ² , bubble defects and flux defects are more effectively reduced. I know that.

また、その他の好ましい鋳造条件は以下のとおりである。
鋳造されるスラブ厚さは２２０〜３００ｍｍとすることが好ましい。浸漬ノズル２の溶鋼吐出孔２０から吐出される溶鋼は気泡を随伴しており、スラブ厚さが小さすぎると、溶鋼吐出孔２０から吐出される溶鋼流が鋳型長辺部側の凝固シェル５に近づき、凝固シェル界面に気泡が捕捉されやすくなる。特に、スラブ厚さが２２０ｍｍ未満では、本発明のような溶鋼流の電磁流動制御を実施しても、上記のような理由により気泡分布の制御が難しくなる。一方、スラブ厚さが３００ｍｍを超えると、熱延工程の生産性が低くなる難点がある。
浸漬ノズル２の内壁面からの不活性ガス吹き込み量は５〜２０Ｌ／ｍｉｎとすることが好ましい。気泡性欠陥を低減するには、不活性ガス吹き込み量が少ない方が好ましく、一方、不活性ガス吹き込み量が少なすぎるとノズル閉塞を起こしやすく、かえって偏流を大きくするために流速の制御が困難となる。
使用するモールドフラックスは、１３００℃での粘度が０．４〜１０ｃｐのものが好ましい。モールドフラックスの粘度が高すぎると、円滑な鋳造に支障をきたす恐れがあり、一方、モールドフラックスの粘度が低すぎるとモールドフラックスの巻き込みが生じやすくなる。 Other preferable casting conditions are as follows.
The slab thickness to be cast is preferably 220 to 300 mm. The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 is accompanied by bubbles, and if the slab thickness is too small, the molten steel flow discharged from the molten steel discharge hole 20 is directed to the solidified shell 5 on the long side of the mold. As a result, bubbles are easily trapped at the solidified shell interface. In particular, when the slab thickness is less than 220 mm, even if the electromagnetic flow control of the molten steel flow as in the present invention is performed, it is difficult to control the bubble distribution for the reasons described above. On the other hand, when the slab thickness exceeds 300 mm, the productivity of the hot rolling process is lowered.
The amount of inert gas blown from the inner wall surface of the immersion nozzle 2 is preferably 5 to 20 L / min. In order to reduce bubble defects, it is preferable that the amount of inert gas blown is small. On the other hand, if the amount of inert gas blown is too small, nozzle clogging is likely to occur, and on the contrary, control of the flow rate is difficult to increase drift. Become.
The mold flux used preferably has a viscosity at 1300 ° C. of 0.4 to 10 cp. If the viscosity of the mold flux is too high, smooth casting may be hindered. On the other hand, if the viscosity of the mold flux is too low, the mold flux is likely to be caught.

図１および図２に示すような連続鋳造機、すなわち、鋳型外側（鋳型側壁の背面側）に、鋳型長辺部を挟んで対向する１対の上部磁極と１対の下部磁極を備え、上部磁極の磁場のピーク位置と下部磁極の磁場のピーク位置の間に浸漬ノズルの溶鋼吐出孔が位置する連続鋳造機を用い、１対の上部磁極と１対の下部磁極に各々印加される直流磁界により溶鋼流を制動する連続鋳造方法により、表１〜表８に示す条件で約３００トンのアルミキルド溶鋼を鋳造した。
浸漬ノズルからの吹き込み不活性ガスにはＡｒガスを使用し、このＡｒガスの吹き込み量は、ノズル閉塞が起こらないようにスライディングノズルの開度に応じて、５〜１２ＮＬ／ｍｉｎの範囲内で調整した。連続鋳造機の仕様および他の鋳造条件は以下のとおりである。
・浸漬ノズルの溶鋼吐出孔の溶鋼吐出角度α：１５°
・浸漬ノズルの溶鋼吐出孔の形状：１辺の長さが８０ｍｍの正方形状
・浸漬ノズルの浸漬深さ：２６０ｍｍ
・浸漬ノズル内径：８０ｍｍ
・浸漬ノズルの各溶鋼吐出孔の開口面積：４９００ｍｍ^２
・使用したモールドフラックスの粘度（１３００℃）：０．６ｃｐ A continuous casting machine as shown in FIG. 1 and FIG. 2, that is, a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outside of the mold (on the back side of the mold side wall) Using a continuous casting machine in which the molten steel discharge hole of the immersion nozzle is located between the peak position of the magnetic field of the magnetic pole and the peak position of the magnetic field of the lower magnetic pole, a direct current magnetic field applied to each of the pair of upper magnetic poles and the pair of lower magnetic poles About 300 tons of aluminum killed molten steel was cast under the conditions shown in Tables 1 to 8 by the continuous casting method that brakes the molten steel flow.
Ar gas is used as the inert gas blown from the submerged nozzle, and the amount of Ar gas blown is adjusted within a range of 5 to 12 NL / min according to the opening of the sliding nozzle so that the nozzle is not blocked. did. The specifications of the continuous casting machine and other casting conditions are as follows.
-Molten steel discharge angle α of the molten steel discharge hole of the immersion nozzle: 15 °
-Shape of molten steel discharge hole of immersion nozzle: square shape with side length of 80 mm-Immersion depth of immersion nozzle: 260 mm
・ Immersion nozzle inner diameter: 80 mm
-Opening area of each molten steel discharge hole of the immersion nozzle: 4900 mm ²
-Viscosity of mold flux used (1300 ° C): 0.6 cp

鋳造されたスラブに対して熱間圧延、冷間圧延、合金化溶融亜鉛めっき処理を順次施し、この合金化溶融亜鉛めっき鋼板の表面欠陥をオンライン表面欠陥計で表面欠陥を連続的に測定し、コイル長さ１００ｍ当たりの欠陥個数に基づき下記基準で評価した（◎，○が合格レベル）。さらに、その内のサンプル切断可能な複数の鋼板について、フラックス性欠陥と気泡性欠陥を欠陥の外観及び断面分析調査により区分し、その結果から欠陥起因比率を推定した。それらの結果を、鋳造条件とともに表１〜表８に示す。
◎：欠陥個数０．２０個以下
○：欠陥個数０．２０個超、０．５０個以下
×：欠陥個数０．５０個超 The cast slab is sequentially subjected to hot rolling, cold rolling and alloying hot dip galvanizing treatment, and the surface defects of this alloyed hot dip galvanized steel sheet are continuously measured with an online surface defect meter, Based on the number of defects per 100 m of coil length, the following criteria were used for evaluation (◎ and ○ are acceptable levels). Furthermore, about the some steel plate in which the sample can be cut | disconnected, the flux defect and the bubble defect were divided by the external appearance and cross-sectional analysis investigation of the defect, and the defect origin ratio was estimated from the result. The results are shown in Tables 1 to 8 together with the casting conditions.
◎: Number of defects 0.20 or less ○: Number of defects more than 0.20, 0.50 or less ×: Number of defects more than 0.50

本発明の実施に供される連続鋳造機の鋳型および浸漬ノズルの一実施形態を示す縦断面図The longitudinal cross-sectional view which shows one Embodiment of the casting_mold | template and immersion nozzle of a continuous casting machine with which implementation of this invention is carried out 図１の実施形態における鋳型および浸漬ノズルの水平断面図Horizontal sectional view of the mold and the immersion nozzle in the embodiment of FIG. 本発明法において、浸漬ノズルのノズル浸漬深さの影響（フラックス性欠陥および気泡性欠陥に及ぼす影響）を示すグラフIn the method of the present invention, a graph showing the influence of the immersion depth of the immersion nozzle (effect on the flux defect and bubble defect) 本発明法において、浸漬ノズルのノズル内径の影響（フラックス性欠陥に及ぼす影響）を示すグラフIn this invention method, the graph which shows the influence (influence on a flux property defect) of the nozzle inner diameter of an immersion nozzle 本発明法において、浸漬ノズルの各溶鋼吐出孔の開口面積の影響（フラックス性欠陥および気泡性欠陥に及ぼす影響）を示すグラフIn this invention method, the graph which shows the influence (influence on a flux defect and a bubble defect) of the opening area of each molten steel discharge hole of an immersion nozzle 上部磁極の直流磁界の強度と下部磁極の直流磁界の強度とが異なる場合における、溶鋼流およびこれに随伴する気泡や非金属介在物の挙動を示す説明図Explanatory drawing showing the behavior of molten steel flow and the accompanying bubbles and non-metallic inclusions when the DC magnetic field strength of the upper magnetic pole and the DC magnetic field strength of the lower magnetic pole are different

Explanation of symbols

１鋳型
２浸漬ノズル
３a，３b 上部磁極
４a，４b 下部磁極
５凝固シェル
６メニスカス
１０鋳型長辺部
１１鋳型短辺部
２１底部
２０溶鋼吐出孔 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3a, 3b Upper magnetic pole 4a, 4b Lower magnetic pole 5 Solidified shell 6 Meniscus 10 Mold long side 11 Mold short side 21 Bottom 20 Molten steel discharge hole

Claims

A submerged nozzle having a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outside of the mold, and having a molten steel discharge angle of 10 ° or more and less than 30 ° downward from the horizontal direction of the molten steel discharge hole Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, and each of the pair of upper magnetic poles and the pair of lower magnetic poles. A method of continuously casting steel while braking a molten steel flow by an applied DC magnetic field,
A continuous casting method for steel, characterized in that continuous casting of a slab having a casting speed of 1.0 m / min or more and a slab width of 1000 to 1800 mm is performed in accordance with the following conditions (a) to (c).
Condition (A): Ratio of DC magnetic field strength A applied to the upper magnetic pole and DC magnetic field strength B applied to the lower magnetic pole when the cast slab width and casting speed are the following (a) to (e) A / B is 1.3 to 2.5, DC magnetic field strength A applied to the upper magnetic pole is 0.20 to 0.40 T, and DC magnetic field strength B applied to the lower magnetic pole is 0.10 to 0.25 T. To do.
(A) Slab width of less than 1250 mm (b) Slab width of 1250 mm or more and less than 1350 mm and casting speed of 1.4 m / min or more (c) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 1.9 m / min or more (d) Slab width (E) Slab width of 1550 mm or more and less than 1650 mm and casting speed of 2.6 m / min or more ・ Condition (b): The slab width to be cast and the casting speed are the following (f) In the case of (g), the ratio A / B of the intensity A of the DC magnetic field applied to the upper magnetic pole and the intensity B of the DC magnetic field applied to the lower magnetic pole is 0.7 to 1.5, and is applied to the upper magnetic pole. The intensity A of the DC magnetic field is 0.25 to 0.35 T, and the intensity B of the DC magnetic field applied to the lower magnetic pole is 0.20 to 0.40 T.
(F) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.8 m / min or more (g) Slab width of 1750 mm or more and casting speed of 2.9 m / min or more ・ Condition (C): Slab width to be cast and casting speed are In the following cases (h) to (m), the ratio A / B of the DC magnetic field strength A applied to the upper magnetic pole and the DC magnetic field strength B applied to the lower magnetic pole is 0.5 or less, and applied to the upper magnetic pole. The strength A of the DC magnetic field is 0 to 0.06 T, and the strength B of the DC magnetic field applied to the lower magnetic pole is 0.10 to 0.40 T.
(H) Slab width of 1250 mm to less than 1350 mm and casting speed of less than 1.4 m / min (i) Slab width of 1350 mm to less than 1450 mm and casting speed of less than 1.9 m / min (j) Slab width of 1450 mm to less than 1550 mm and casting (K) Slab width of 1550 mm or more and less than 1650 mm and casting speed of less than 2.6 m / min (l) Slab width of 1650 mm or more and less than 1750 mm and casting speed of less than 2.8 m / min (m) Slab width 1750mm or more and casting speed less than 2.9m / min

The continuous casting method of steel according to claim 1, wherein the immersion depth of the immersion nozzle is 230 to 290 mm.

The continuous casting method of steel according to claim 1 or 2, wherein the nozzle inner diameter of the immersion nozzle (however, the nozzle inner diameter at the position of the molten steel discharge hole) is 70 to 90 mm.

The method continuous casting of steel according to any one of claims 1 to 3, characterized in that the opening area of each molten steel discharge hole of the immersion nozzle and 3600~8200mm ^2.