JP4571930B2 - Continuous casting powder and steel continuous casting method - Google Patents
Continuous casting powder and steel continuous casting method Download PDFInfo
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本発明は、鋼の連続鋳造において鋳型内に添加する連続鋳造用パウダーとそれを用いた鋼の連続鋳造方法に関する。 The present invention relates to a powder for continuous casting added to a mold in continuous casting of steel and a method for continuous casting of steel using the same.
鋼の連続鋳造において、鋳型内に連続鋳造用パウダーが添加される。連続鋳造用パウダーは鋳型内の溶鋼表面で溶融し、鋳型壁と凝固シェルの間に流入する。パウダーには、鋳型内溶鋼表面の酸化防止、保温、鋳型内溶鋼中を浮上した介在物の吸収、さらには鋳型と鋳片間の潤滑、鋳型と鋳片間の伝熱制御といった役割がある。 In continuous casting of steel, powder for continuous casting is added into the mold. The powder for continuous casting melts on the surface of the molten steel in the mold and flows between the mold wall and the solidified shell. The powder plays a role of preventing oxidation of the molten steel surface in the mold, keeping the temperature, absorbing the inclusions floating in the molten steel in the mold, further lubricating between the mold and the slab, and controlling heat transfer between the mold and the slab.
このうち鋳型と鋳片間の伝熱制御は、特に鋳片表面性状に対して大きな影響を与える。鋳型湯面近傍での凝固シェルから鋳型への伝熱挙動が変動すると、鋳片の縦割れやブリードといった表面欠陥の生成を招く。すなわち、伝熱のばらつきにより凝固シェルの発達が不均一となり、凝固の遅れた部分に歪が集中し、デンドライト樹間でシェルが破断する。このような表面欠陥は連鋳鋳片の品質上の問題となるだけでなく、場合によっては破断部から溶鋼が漏れ、ブレークアウトにもつながる。したがって、鋳型と鋳片間の伝熱制御は連続鋳造用パウダーの機能の中でも特に重要なものといえる。 Of these, the heat transfer control between the mold and the slab particularly has a great influence on the slab surface properties. Variations in the heat transfer behavior from the solidified shell to the mold near the mold surface will cause surface defects such as vertical cracks and bleed in the slab. That is, the development of the solidified shell becomes non-uniform due to variations in heat transfer, strain concentrates on the delayed solidification portion, and the shell breaks between dendritic trees. Such surface defects not only cause problems in the quality of continuous cast slabs, but in some cases, molten steel leaks from the fractured part, leading to breakout. Therefore, it can be said that heat transfer control between the mold and the slab is particularly important among the functions of the powder for continuous casting.
パウダーによる伝熱制御に関する従来の方法として、もっぱら結晶化しやすい連続鋳造用パウダーの使用が行われている。連続鋳造用パウダーは、ＣａＯ、ＳｉＯ2、Ａｌ2Ｏ3、Ｎａ2Ｏ、Ｆ等を含有する合成スラグであるが、３ＣａＯ・２ＳｉＯ2・ＣａＦ2（カスピダイン）が晶出しやすい組成に調整し、鋳型と鋳片間に流入したパウダーフィルム内にカスピダインからなる固体の層を形成させることで、鋳型内の伝熱は安定化される。すなわちカスピダインからなる固体の層が鋳型表面に安定的に形成され、これが輻射伝熱を制御する、あるいはパウダーと鋳型間の界面の熱抵抗を増大させることによって、安定的な緩冷却を実現できる。 As a conventional method relating to heat transfer control using powder, continuous casting powder that is easy to crystallize is used. The powder for continuous casting is a synthetic slag containing CaO, SiO 2 , Al 2 O 3 , Na 2 O, F, etc., but 3CaO · 2SiO 2 · CaF 2 (Caspodyne) is adjusted to a composition that is easy to crystallize, Heat formation in the mold is stabilized by forming a solid layer made of cuspidine in the powder film flowing between the mold and the slab. That is, a solid layer made of caspodyne is stably formed on the mold surface, and this can control stable radiant cooling by increasing the thermal resistance of the interface between the powder and the mold.
特許文献１には、アルカリ金属とＦとの親和性を考慮した上で、カスピダインの析出を促進させることができるように化学組成を選択したモールドパウダーが記載されている。これにより、包晶域を含むＣ含有量範囲の鋼を高速で鋳造しても鋳片表面に縦割れが発生するのを防止できるとしている。 Patent Document 1 describes a mold powder in which a chemical composition is selected so that precipitation of caspodyne can be promoted in consideration of the affinity between an alkali metal and F. Thereby, even if steel with a C content range including a peritectic region is cast at a high speed, it is possible to prevent vertical cracks from occurring on the surface of the slab.
そのため、パウダー中のＣａＯとＳｉＯ2の質量比（ＣａＯ／ＳｉＯ2）を１．２〜１．８程度としたパウダーが特に前述の表面疵の発生しやすい中炭素鋼などでは使用されている。またカスピダインの結晶化のしやすさの指標として、凝固温度（ブレーキングポイントとも呼ばれ、パウダーを冷却したときに粘度が急激に増大する温度をいう。）が知られ、凝固温度の高温化により鋳型抜熱は安定化し、鋳片表面の縦割れは抑制される。 Therefore, a powder in which the mass ratio of CaO to SiO 2 in the powder (CaO / SiO 2 ) is about 1.2 to 1.8 is used particularly in the above-described medium carbon steel and the like where surface defects are likely to occur. In addition, as an index of the ease of crystallization of caspidyne, the solidification temperature (also called the braking point, which means the temperature at which the viscosity rapidly increases when the powder is cooled) is known. Mold heat removal is stabilized and vertical cracks on the slab surface are suppressed.
一方、鋳型湯面で溶融したパウダーが溶鋼に巻き込まれると、鋳片表面や内部の欠陥となる。上述のＣａＯ／ＳｉＯ2を１．２〜１．８程度とした連続鋳造用パウダーは一般に粘度が低く、鋳型内で溶鋼に巻き込まれやすい。したがって、安定的な緩冷却と巻き込み防止とを両立することは、これまでの技術では不十分である。 On the other hand, if the powder melted on the mold surface is caught in the molten steel, it becomes a defect on the surface of the slab or inside. The above-mentioned powder for continuous casting in which CaO / SiO 2 is about 1.2 to 1.8 generally has a low viscosity and is easily caught in molten steel in a mold. Therefore, it is insufficient with conventional techniques to achieve both stable slow cooling and prevention of entrainment.
本発明は、Ａｌ含有量が０．０１５質量％未満の鋼を対象とする。このような成分の鋼は、脱炭吹錬によって溶鋼中にＳｉが残存し、Ｓｉを０．０５質量％以上含有するので、ここではＳｉキルド鋼と呼ぶ。鋳片の形状は特に限定しないが、主な対象はビレットであり、一辺が１００〜２５０ｍｍの角状の鋳片を指す。またブルームも対象となる。 The present invention is directed to a steel having an Al content of less than 0.015% by mass. The steel having such a component is referred to herein as Si killed steel because Si remains in the molten steel by decarburization blowing and contains 0.05 mass% or more of Si. The shape of the slab is not particularly limited, but the main object is a billet, which refers to a square slab having a side of 100 to 250 mm. Bloom is also covered.
Ａｌ含有量が０．０１５質量％未満のＳｉキルド鋼を連続鋳造するに際して、塩基度０．９〜１．２程度のパウダーが一般的に用いられてきた。特に塩基度を１．２程度まで上げると鋳型内の緩冷却化が実現され、縦割れ等の表面疵が低減された。しかし特にビレットの場合には、鋳型の断面が小さいために溶鋼からの熱供給が不十分で、塩基度を高くしたパウダーの凝固温度を高くすると、パウダーの溶融が不十分となりブレークアウトを招くという問題があった。また塩基度が高いとパウダーの粘度が低下するため、パウダー巻き込みによる欠陥も発生した。 When continuously casting Si killed steel having an Al content of less than 0.015% by mass, powder having a basicity of about 0.9 to 1.2 has been generally used. In particular, when the basicity was raised to about 1.2, the mold was slowly cooled, and surface defects such as vertical cracks were reduced. However, especially in the case of billets, heat supply from the molten steel is insufficient due to the small cross section of the mold. There was a problem. Moreover, since the viscosity of powder fell when basicity was high, the defect by powder entrainment also occurred.
即ち、従来のように塩基度を１．２程度まで上げてカスピダインを晶出させて伝熱特性を制御し、それによって緩冷却を行うパウダーにおいては、溶融パウダーの粘度を上げることができず、パウダー巻き込みを抑制することが困難である。逆にパウダー巻き込み抑制のためにパウダー粘度を上げると、カスピダインを利用した鋳型冷却の緩冷却化が困難となる。 In other words, in conventional powders, the basicity is raised to about 1.2 to crystallize caspidine and the heat transfer characteristics are controlled, whereby the powder that is slowly cooled cannot increase the viscosity of the molten powder. It is difficult to suppress powder entrainment. On the other hand, if the powder viscosity is increased in order to suppress the entrainment of powder, it becomes difficult to slowly cool the mold using cuspidyne.
本発明は、Ｓｉキルド鋼の連続鋳造において、鋳型内の緩冷却による表面性状の良好な鋳片を得ることとパウダー巻き込みに伴う介在物性の欠陥を防止するという二つの要求に応えるため、パウダーの粘度を上げてパウダー巻き込みを防止しつつ、鋳型内の緩冷却を実現することのできる連続鋳造用パウダー及びそれを用いた連続鋳造方法を提供することを目的とする。 In the present invention, in order to meet the two requirements of obtaining a slab having a good surface property by slow cooling in a mold and preventing defects in inclusion physical properties due to powder entrainment in continuous casting of Si killed steel, An object of the present invention is to provide a powder for continuous casting capable of realizing slow cooling in a mold while increasing the viscosity to prevent powder entrainment and a continuous casting method using the powder.
非特許文献１に記載のように、溶融スラグと気相との界面において、スラグ中の水酸イオンと気相中の水蒸気との間には、
２（ＯＨ-）←→Ｈ2Ｏ＋（Ｏ2-） ［２］
の関係がある。ここで（ＯＨ-）と（Ｏ2-）はそれぞれ溶融スラグ中の水酸イオンと酸素イオンを表す。
As described in Non-Patent Document 1, at the interface between the molten slag and the gas phase, between the hydroxide ions in the slag and the water vapor in the gas phase,
2 (OH − ) ← → H 2 O + (O 2− ) [2]
There is a relationship. Here, (OH − ) and (O 2− ) represent a hydroxide ion and an oxygen ion in the molten slag, respectively.
スラグが高温の溶融状態においては、大気中の水蒸気がスラグ中に溶け込んで上記［２］式の右辺から左辺への反応が進行し、スラグ中に水酸イオンが形成される。スラグ温度が低下すると、［２］式の左辺から右辺への反応が起こり、水蒸気が気相として生成する。非特許文献１はＣａＯ−ＳｉＯ2二元系のスラグでの知見であるが、さらにＮａ2Ｏ、Ａｌ2Ｏ3、Ｆ等を含むパウダーでも原理的に同様であると考えられる。 When the slag is in a high-temperature molten state, water vapor in the atmosphere dissolves in the slag, and the reaction from the right side to the left side of the formula [2] proceeds to form hydroxide ions in the slag. When the slag temperature decreases, a reaction occurs from the left side to the right side of the formula [2], and water vapor is generated as a gas phase. Non-Patent Document 1 is knowledge of CaO—SiO 2 binary slag, but it is considered that the same principle is applicable to powders containing Na 2 O, Al 2 O 3 , F, and the like.
連続鋳造に供する鋼がＡｌを含有するＡｌキルド鋼あるいはＡｌ−Ｓｉキルド鋼である場合は、溶融パウダーと溶鋼との間で
３（ＯＨ-）＋２［Ａｌ］→（Ａｌ2Ｏ3）＋３［Ｈ］＋３ｅ- ［３］
の反応が起こる。その結果、溶融パウダー中の水酸イオンが溶鋼中のＡｌで還元されて溶鋼中に水素として移動する。従って、大気中の水蒸気が溶融パウダー中に水酸イオンとして取り込まれても、Ａｌによって還元されてしまうので、溶融パウダー中水酸イオン濃度が十分に上昇せず、溶融パウダーの温度が下がっても水蒸気気泡が発生するには至らない。
When the steel used for continuous casting is Al killed steel containing Al or Al—Si killed steel, 3 (OH − ) +2 [Al] → (Al 2 O 3 ) +3 [between the molten powder and the molten steel. H] + 3e - [3]
The reaction occurs. As a result, the hydroxide ions in the molten powder are reduced by Al in the molten steel and move as hydrogen into the molten steel. Therefore, even if water vapor in the atmosphere is taken into the molten powder as hydroxide ions, it is reduced by Al, so the hydroxide ion concentration in the molten powder does not rise sufficiently and the temperature of the molten powder decreases. Water vapor bubbles are not generated.
連続鋳造に供する鋼がＡｌ含有量の少ないＳｉキルド鋼である場合には、上記［３］式の反応による水酸イオンの減少は起こらない。一方、Ｓｉキルド鋼中のＳｉによって、
２（ＯＨ-）＋［Ｓｉ］→（ＳｉＯ2）＋２［Ｈ］＋２ｅ- ［４］
の反応が起こりえる。しかし、Ｓｉの還元力はＡｌに比較して著しく小さく、さらに溶融パウダー中の（ＳｉＯ2）の活量を大きくすれば、この反応はさらに抑制される。そして、連続鋳造用パウダーの塩基度を下げることにより、溶融パウダーの（ＳｉＯ2）活量を大きくすることができる。その結果、大気中の水蒸気から溶融パウダーに供給された水酸イオンは還元されず、溶融パウダーが鋳型と鋳片凝固シェルとの間に流入して温度が低下すると［２］式の左辺から右辺への反応が進行して水蒸気気泡が発生することとなる。
When the steel to be subjected to continuous casting is Si killed steel with a low Al content, there is no reduction of hydroxide ions due to the reaction of the above formula [3]. On the other hand, due to Si in Si killed steel,
2 (OH − ) + [Si] → (SiO 2 ) +2 [H] + 2e − [4]
The reaction can occur. However, the reducing power of Si is significantly smaller than that of Al, and this reaction can be further suppressed by increasing the activity of (SiO 2 ) in the molten powder. Then, by lowering the basicity of the continuous casting powder, it can be increased (SiO 2) activity of the molten powder. As a result, the hydroxide ions supplied from the water vapor in the atmosphere to the molten powder are not reduced, and when the molten powder flows between the mold and the slab solidified shell and the temperature decreases, the left side to the right side of the formula [2] As a result, the water vapor bubbles are generated.
そこで、Ａｌ含有量が０．０１５質量％未満のＳｉキルド鋼の連続鋳造において、連続鋳造用パウダーに下記［１］式の塩基度Ｂが０．７以下のパウダーを用いたところ、鋳型と凝固シェル間に流入したパウダーに微細かつ均一な気泡を生成させることができ、それによって低塩基度であるにもかかわらず緩冷却を実現することが可能となった。
Ｂ＝Ｔ．ＣａＯ／ＳｉＯ2 ［１］
Therefore, in continuous casting of Si killed steel having an Al content of less than 0.015% by mass, a powder having a basicity B of the following formula [1] of 0.7 or less is used as a powder for continuous casting. Fine and uniform air bubbles can be generated in the powder flowing between the shells, and thus it is possible to realize slow cooling despite the low basicity.
B = T. CaO / SiO 2 [1]
本発明は上記知見に基づいてなされたものであり、その要旨とするところは以下のとおりである。
（１）下記［１］式で示す塩基度Ｂが０．７以下で、かつ、Ｆを３．６〜８．５質量％含有し、１３００℃における粘度が１１〜２５ｐｏｉｓｅであり、Ａｌ含有量が０．０１５質量％未満の鋼のビレット連続鋳造に用いることを特徴とする連続鋳造用パウダー。
Ｂ＝Ｔ．ＣａＯ／ＳｉＯ2 ［１］
ここでＴ．ＣａＯはパウダー中のＣａがすべてＣａＯであるとしたときのＣａＯの含有量（質量％）、ＳｉＯ2はパウダー中のＳｉＯ2含有量（質量％）を表す。
（２）Ａｌ含有量が０．０１５質量％未満の鋼をビレット連続鋳造するに際し、上記（１）に記載の連続鋳造用パウダーを用いることを特徴とする鋼の連続鋳造方法。
This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) Basicity B represented by the following formula [1] is 0.7 or less , F is contained in 3.6 to 8.5% by mass, viscosity at 1300 ° C. is 11 to 25 poise, and Al content Is used for continuous billet casting of steel having a content of less than 0.015% by mass.
B = T. CaO / SiO 2 [1]
T. CaO represents the content (mass%) of CaO when all the Ca in the powder is CaO, and SiO 2 represents the SiO 2 content (mass%) in the powder .
(2 ) A continuous casting method for steel, characterized in that the powder for continuous casting described in (1) above is used for continuous billet casting of steel having an Al content of less than 0.015% by mass .
本発明により、Ａｌ含有量が少ないＳｉキルド鋼の連続鋳造において、パウダーフィルム中に安定的に気泡を発生し、鋳型内の緩冷却を実現するとともに、さらにパウダー巻き込みも低減させることで、割れやブリードなど鋳片表面欠陥がなく、かつパウダー巻き込みによる介在物欠陥が少ない鋳片を製造することが可能となった。 According to the present invention, in continuous casting of Si killed steel with a low Al content, bubbles are stably generated in the powder film, realizing slow cooling in the mold, and further reducing powder entrainment, It has become possible to produce a slab that is free from defects on the surface of the slab such as bleed and has few inclusion defects due to powder entrainment.
本発明は、Ａｌ含有量が０．０１５質量％未満の鋼を対象とする。このような成分の鋼は、脱炭吹錬によって溶鋼中にＳｉが残存し、Ｓｉを０．０５質量％以上含有するので、上述のとおりＳｉキルド鋼と呼ぶ。鋼の炭素濃度は規定しないが、０．０９〜０．８０質量％の中炭素〜高炭素鋼が対象である。鋳片の形状についても特に限定はしないが、主な対象はビレットであり、一辺が１００〜２５０ｍｍの角状の鋳片を指す。またブルームも本発明の対象となる。しかしパウダーフィルム中の気泡による緩冷却の効果は著しいので、大断面のスラブ連続鋳造ではかえって抜熱不足となりブレークアウトを招く場合もある。 The present invention is directed to a steel having an Al content of less than 0.015% by mass. Steel having such components is called Si killed steel as described above because Si remains in the molten steel by decarburization blowing and contains 0.05 mass% or more of Si. Although the carbon concentration of the steel is not specified, 0.09 to 0.80 mass% of medium carbon to high carbon steel is an object. The shape of the slab is not particularly limited, but the main object is a billet, which refers to a square slab having a side of 100 to 250 mm. Bloom is also an object of the present invention. However, the effect of slow cooling due to air bubbles in the powder film is remarkable, and in some cases, slab continuous casting with a large cross section may cause insufficient heat removal and cause breakout.
前述のとおり、溶融スラグと気相との界面において、スラグ中の水酸イオンと気相中の水蒸気との間には、
２（ＯＨ-）←→Ｈ2Ｏ＋（Ｏ2-） ［２］
の関係がある。ここで（ＯＨ-）と（Ｏ2-）はそれぞれ溶融スラグ中の水酸イオンと酸素イオンを表す。スラグが高温の溶融状態においては、大気中の水蒸気がスラグ中に溶け込んで上記［２］式の右辺から左辺への反応が進行し、スラグ中に水酸イオンが形成される。スラグ温度が低下すると、［２］式の左辺から右辺への反応が起こり、水蒸気が気相として生成する。
As described above, at the interface between the molten slag and the gas phase, between the hydroxide ions in the slag and the water vapor in the gas phase,
2 (OH − ) ← → H 2 O + (O 2− ) [2]
There is a relationship. Here, (OH − ) and (O 2− ) represent a hydroxide ion and an oxygen ion in the molten slag, respectively. When the slag is in a high-temperature molten state, water vapor in the atmosphere dissolves in the slag, and the reaction from the right side to the left side of the formula [2] proceeds to form hydroxide ions in the slag. When the slag temperature decreases, a reaction occurs from the left side to the right side of the formula [2], and water vapor is generated as a gas phase.
本発明は、Ａｌ含有量が０．０１５質量％未満の鋼の連続鋳造を対象にしており、
３（ＯＨ-）＋２［Ａｌ］→（Ａｌ2Ｏ3）＋３［Ｈ］＋３ｅ- ［３］
の反応による水酸イオンの減少は起こらない。
The present invention is directed to continuous casting of steel having an Al content of less than 0.015% by mass,
3 (OH − ) +2 [Al] → (Al 2 O 3 ) +3 [H] + 3e − [3]
There is no reduction of hydroxide ions due to the reaction of.
一方、本発明はＳｉキルド鋼を対象としており、Ｓｉキルド鋼中のＳｉによって、
２（ＯＨ-）＋［Ｓｉ］→（ＳｉＯ2）＋２［Ｈ］＋２ｅ- ［４］
の反応が起こりえる。しかし、Ｓｉの還元力はＡｌに比較して著しく小さく、さらに溶融パウダー中の（ＳｉＯ2）の活量を大きくすれば、この反応はさらに抑制される。そして、連続鋳造用パウダーの塩基度を下げることにより、溶融パウダーの（ＳｉＯ2）活量を大きくすることができる。その結果、大気中の水蒸気から溶融パウダーに供給された水酸イオンは還元されず、溶融パウダーが鋳型と鋳片凝固シェルとの間に流入して温度が低下すると［２］式の左辺から右辺への反応が進行して水蒸気気泡が発生することとなる。なお、上記［２］〜［４］式で、（ ）でくくった成分は溶融パウダー中成分を意味し、［ ］でくくった成分は溶鋼中成分を意味している。
On the other hand, the present invention is directed to Si killed steel, and by Si in Si killed steel,
2 (OH − ) + [Si] → (SiO 2 ) +2 [H] + 2e − [4]
The reaction can occur. However, the reducing power of Si is significantly smaller than that of Al, and this reaction can be further suppressed by increasing the activity of (SiO 2 ) in the molten powder. Then, by lowering the basicity of the continuous casting powder, it can be increased (SiO 2) activity of the molten powder. As a result, the hydroxide ions supplied from the water vapor in the atmosphere to the molten powder are not reduced, and when the molten powder flows between the mold and the slab solidified shell and the temperature decreases, the left side to the right side of the formula [2] As a result, the water vapor bubbles are generated. In the above formulas [2] to [4], the component enclosed in () means the component in the molten powder, and the component enclosed in [] means the component in the molten steel.
そこで、Ａｌ含有量が０．０１５質量％未満のＳｉキルド鋼のビレット連続鋳造において、低塩基度のパウダーを用い、鋳型の抜熱量の調査とパウダーフィルムの調査を行った。第１に、鋳型内に埋設した熱電対によって鋳型銅板内の温度を計測した。熱電対は鋳型湯面から１００ｍｍの位置に埋設してある。測定の結果、図１に示すように塩基度Ｂが０．７以下のパウダーを用いた場合に、鋳型内の温度は著しく低下し、鋳片から鋳型への抜熱が低下することが分かった。
Therefore, in the billet continuous casting of Si killed steel having an Al content of less than 0.015% by mass, the heat removal amount of the mold and the powder film were investigated using the low basicity powder. First, the temperature in the mold copper plate was measured by a thermocouple embedded in the mold. The thermocouple is embedded at a
さらに鋳造後鋳型壁面に付着したパウダーのフィルムを回収し、断面の観察を行った。その結果、低塩基度のパウダーを用いた場合、パウダーフィルム中に微細な気泡が多数存在し、均一に分散していた。その気泡の面積率（空隙率）を測定した。図２に示すように、気泡の面積率は塩基度Ｂの低下とともに増加することが分かった。 Furthermore, the powder film adhering to the mold wall surface was recovered after casting, and the cross section was observed. As a result, when a low basicity powder was used, many fine bubbles existed in the powder film and were uniformly dispersed. The area ratio (void ratio) of the bubbles was measured. As shown in FIG. 2, it was found that the area ratio of bubbles increased with a decrease in basicity B.
次にパウダーフィルム中の気泡のガス分析を行ったところ、水素成分が検出された。このことから、パウダーフィルム中の気泡は、溶融バウダー中に溶けた水酸イオンが水蒸気ガスとなって生成した気泡であると推定した。 Next, when a gas analysis of bubbles in the powder film was performed, a hydrogen component was detected. From this, it was estimated that the air bubbles in the powder film were air bubbles generated by the hydroxide ions dissolved in the molten powder as water vapor gas.
そこで、パウダーフィルム中の水酸イオン濃度の分析を試みた。まずパウダーフィルム中にＯＨ-として存在する水素の定量化法について述べる。これまでスラグ中の水酸イオン（ＯＨ-）の分析は溶融Ａｌ還元法（非特許文献１）で行われていた。この方法ではスラグ中のＯＨ-をＡｌでＨ2ガスに還元し、Ｈ2ガス量をガス質量分析計で測定することでＯＨ-を定量化する。しかしＦやＮａ2Ｏを含む連続鋳造用パウダーでは、加熱時にＮａＦ、ＳｉＦ4ガスが生成するため、この方法を用いることができない。そこで、核磁気共鳴（固体ＮＭＲ）を用いてＯＨ-の分析を行った。鋳造後採取したパウダーフィルムに対して、ＨのＮＭＲスペクトルを測定した。また標準試料としてカオリナイト（Ａｌ2Ｓｉ2Ｏ5（ＯＨ）4）を用いた。カオリナイトはＯＨ-としてＨを１．５６質量％含むことが既知なので、パウダー試料のＯＨ-に相当するＮＭＲスペクトルの面積値と標準試料のＯＨ-に相当するＮＭＲスペクトルの面積値の比を知ることで、パウダー中にＯＨ-として存在するＨの質量を定量化した。 Therefore, an attempt was made to analyze the hydroxide ion concentration in the powder film. First OH in powder film - describes a method for quantifying hydrogen present as. So far, analysis of hydroxide ions (OH − ) in slag has been performed by the molten Al reduction method (Non-patent Document 1). In this method, OH − in the slag is reduced to H 2 gas with Al, and the amount of H 2 gas is measured with a gas mass spectrometer to quantify OH − . However, a continuous casting powder containing F or Na 2 O cannot produce this method because NaF and SiF 4 gases are generated during heating. Therefore, OH - was analyzed using nuclear magnetic resonance (solid NMR). The NMR spectrum of H was measured for the powder film collected after casting. Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) was used as a standard sample. Since it is known to contain 1.56% by weight of H as, powder samples OH - - kaolinite OH know the ratio of the area value of the NMR spectrum corresponding to the - OH of the area value and the standard samples of the NMR spectrum corresponding to Thus, the mass of H present as OH − in the powder was quantified.
鋳造前（溶融前）のパウダーのＯＨ-として存在するＨは８０〜９０ｐｐｍであり、パウダー塩基度による差は小さかった。次に、Ｓｉキルド鋼を鋳造した後に鋳型壁から採取したパウダーフィルムについて、測定結果を図３に示す。ＯＨ-として存在するＨは、塩基度が１前後の場合には４０〜１１０ｐｐｍとばらつくのに対して、塩基度が低いと安定的に１２０〜１４０ｐｐｍと増大した。この結果より、塩基度が高いと溶鋼中のＳｉが溶融プールの溶融パウダーと反応して［４］式の反応が進行し、パウダー中ＯＨ-イオン濃度を低下させるので気泡が発生しないのに対して、塩基度が低く即ちパウダー中のＳｉＯ2の活量が高い場合には、［４］式の反応が進行せず、溶鋼中のＳｉが溶融パウダー中の水酸イオンを還元しにくくなるので、水酸イオンが高い濃度でパウダーフィルム中に残存する。その結果、パウダーフィルム中の水酸イオン濃度が冷却過程で飽和溶解度を超え、パウダーフィルム中に水蒸気気泡が微細かつ均一に発生することとなる。 The H present as OH − of the powder before casting (before melting) was 80 to 90 ppm, and the difference due to powder basicity was small. Next, a measurement result is shown in FIG. 3 for the powder film collected from the mold wall after casting the Si killed steel. H present as OH − varies from 40 to 110 ppm when the basicity is around 1, whereas it stably increases from 120 to 140 ppm when the basicity is low. From this result, when the basicity is high, the Si in the molten steel reacts with the molten powder in the molten pool and the reaction of the formula [4] proceeds to reduce the OH − ion concentration in the powder, so that no bubbles are generated. When the basicity is low, that is, when the activity of SiO 2 in the powder is high, the reaction of the formula [4] does not proceed, and Si in the molten steel is difficult to reduce the hydroxide ions in the molten powder. The hydroxide ions remain in the powder film at a high concentration. As a result, the hydroxide ion concentration in the powder film exceeds the saturation solubility in the cooling process, and water vapor bubbles are generated finely and uniformly in the powder film.
以上の結果から、Ａｌ含有量が０．０１５質量％未満のＳｉキルド鋼の連続鋳造において、塩基度Ｂが０．７以下のパウダーを用いたときに、パウダーフィルム中の水酸イオンの濃度が上昇し（図３）、パウダーフィルム冷却時に水酸イオンが水蒸気気泡となって微細かつ均一に発生し、パウダーフィルム中の空隙率を増大させることとなる（図２）。そしてその結果として、鋳型と凝固シェル間のパウダーフィルムが断熱性を増し、緩冷却化し、鋳型熱電対温度が低下することとなる（図１）。 From the above results, in the continuous casting of Si killed steel having an Al content of less than 0.015% by mass, when a powder having a basicity B of 0.7 or less is used, the concentration of hydroxide ions in the powder film is It rises (FIG. 3), and when the powder film is cooled, hydroxide ions are formed into water vapor bubbles in a fine and uniform manner, increasing the porosity in the powder film (FIG. 2). As a result, the powder film between the mold and the solidified shell increases the heat insulation and is slowly cooled, and the mold thermocouple temperature decreases (FIG. 1).
即ち、Ｓｉキルド鋼の連続鋳造用パウダーに塩基度Ｂが０．７以下のパウダーを用いることにより、鋳型と凝固シェル間に流入したパウダーに微細かつ均一な気泡を生成させることができ、それによって低塩基度であるにもかかわらず緩冷却を実現することが可能となった。そしてその結果、縦割れ等の表面疵の発生を防止することができる。 That is, by using a powder having a basicity B of 0.7 or less as powder for continuous casting of Si killed steel, fine and uniform bubbles can be generated in the powder flowing between the mold and the solidified shell. Despite the low basicity, it was possible to achieve slow cooling. As a result, generation of surface flaws such as vertical cracks can be prevented.
また、塩基度Ｂが０．７以下と低塩基度であるため、溶融パウダーの粘度を高粘度とすることができ、連続鋳造中におけるパウダーの巻き込みを防止することができる。 Moreover, since basicity B is 0.7 or less and low basicity, the viscosity of molten powder can be made high viscosity and the entrainment of the powder during continuous casting can be prevented.
本発明の連続鋳造用パウダーは、１３００℃における粘度が１０ｐｏｉｓｅ以上であることとすると好ましい。１３００℃における粘度が１０ｐｏｉｓｅ以上であれば、鋳型内溶鋼メニスカス近傍でのパウダー巻き込みを防止することができるので、パウダー巻き込み起因の介在物欠陥を防止することが可能となる。本発明の連続鋳造用パウダーは塩基度Ｂが０．７以下であるため、容易に１３００℃における粘度を１０ｐｏｉｓｅ以上とすることができる。 The powder for continuous casting of the present invention preferably has a viscosity at 1300 ° C. of 10 poise or more. If the viscosity at 1300 ° C. is 10 poise or more, powder entrainment in the vicinity of the molten steel meniscus in the mold can be prevented, and inclusion defects due to powder entrainment can be prevented. Since the powder for continuous casting of the present invention has a basicity B of 0.7 or less, the viscosity at 1300 ° C. can easily be 10 poise or more.
本発明の連続鋳造用パウダーは、ビレット連続鋳造に用いることとすると好ましい。パウダーフィルム中の気泡による緩冷却の効果は著しいので、大断面のスラブ連続鋳造ではかえって抜熱不足となりブレークアウトを招く場合があるが、ビレットであればブレークアウトが発生することはなく、気泡による緩冷却効果を十分に享受することができるからである。 The powder for continuous casting of the present invention is preferably used for billet continuous casting. Since the effect of slow cooling due to air bubbles in the powder film is remarkable, continuous slab casting with a large cross section may cause insufficient heat removal and breakout, but if it is a billet, breakout will not occur, This is because the slow cooling effect can be fully enjoyed.
表１に示す成分のＳｉキルド鋼を転炉、二次精錬設備にて溶製し、湾曲型のビレット連続鋳造機で連続鋳造した。鋳片の断面サイズは１３０ｍｍ角であった。鋳造速度は３ｍ／ｍｉｎとした。鋳型内で電磁攪拌をかけながら鋳造を行った。鋳造に当たり、表２に示す成分の連続鋳造用パウダーを使用した。表２のパウダー成分中、Ｔ．ＣａＯはパウダー中の含有ＣａがすべてＣａＯであるとしてカウントした値である。表２には各パウダーの１３００℃における粘度を記載している。粘度は、高温の回転粘度計にて、１３００℃でパウダーを溶融した後、ローターを回転させ、そのときのトルクから粘度を算出した。 Si killed steel having the components shown in Table 1 was melted in a converter and secondary refining equipment, and continuously cast using a curved billet continuous casting machine. The cross-sectional size of the slab was 130 mm square. The casting speed was 3 m / min. Casting was performed while applying electromagnetic stirring in the mold. In casting, powders for continuous casting having the components shown in Table 2 were used. Among the powder components in Table 2, T.P. CaO is a value counted assuming that all the Ca contained in the powder is CaO. Table 2 lists the viscosity of each powder at 1300 ° C. The viscosity was calculated from the torque at that time by rotating the rotor after melting the powder at 1300 ° C. with a high-temperature rotational viscometer.
鋳型内に埋設した熱電対によって、鋳造中の鋳型銅板内の温度を計測した。熱電対は鋳型湯面から１００ｍｍの位置に埋設してある。測定の結果、図１に示すように塩基度Ｂが０．７以下のパウダーを用いた場合に、鋳型内の温度は著しく低下し、鋳片から鋳型への抜熱が低下することが分かった。
The temperature inside the mold copper plate during casting was measured by a thermocouple embedded in the mold. The thermocouple is embedded at a
さらに鋳造後鋳型壁面に付着したパウダーのフィルムを回収し、断面の観察を行った。その結果、低塩基度のパウダーを用いた場合、パウダーフィルム中に微細な気泡が多数存在し、均一に分散していた。その気泡の面積率（空隙率）を測定した。図２に示すように、気泡の面積率は塩基度Ｂの低下とともに増加することが分かった。 Furthermore, the powder film adhering to the mold wall surface was recovered after casting, and the cross section was observed. As a result, when a low basicity powder was used, many fine bubbles existed in the powder film and were uniformly dispersed. The area ratio (void ratio) of the bubbles was measured. As shown in FIG. 2, it was found that the area ratio of bubbles increased with a decrease in basicity B.
パウダーフィルム中の水酸イオン濃度を、前述の核磁気共鳴（固体ＮＭＲ）を用いて分析した。図３に示すように、塩基度Ｂが０．７以下のパウダーを用いたときにパウダーフィルム中の水酸イオンの濃度が高くなっている。 The hydroxide ion concentration in the powder film was analyzed using the aforementioned nuclear magnetic resonance (solid NMR). As shown in FIG. 3, when a powder having a basicity B of 0.7 or less is used, the concentration of hydroxide ions in the powder film is high.
鋳片表面のブリード疵（溶鋼がシェルからこぼれだし凝固したもの）と割れの発生状況を調査した。結果を表２に示す。本発明の実施例である塩基度Ｂが０．７以下のパウダーを使用した場合には、ブリードと割れともに発生が非常に少なかった。パウダーフィルム中の気泡による均一な緩冷却化により、凝固シェルの発生が均一化し、シェルに加わる歪も低下したためである。 The bleed surface of the slab surface (the molten steel spilled from the shell and solidified) and cracks were investigated. The results are shown in Table 2. In the case of using a powder having a basicity B of 0.7 or less, which is an example of the present invention, both bleed and cracks were very little generated. This is because uniform slow cooling due to air bubbles in the powder film makes the generation of the solidified shell uniform and reduces the strain applied to the shell.
また鋳片内に巻き込まれたパウダーに起因した介在物の個数も測定した。鋳片をスライム法で電解し、介在物を抽出した。このうち１００μｍ以上の介在物であって、パウダー成分の検出されたものの個数を表２に示した。塩基度Ｂの低下とともにパウダー起因の介在物の個数は大きく低減した。これは、塩基度の低下とともにパウダーの粘度が増加するため、パウダーの巻き込みが抑制されたためである。 In addition, the number of inclusions resulting from the powder caught in the slab was also measured. The slab was electrolyzed by the slime method to extract inclusions. Table 2 shows the number of inclusions of 100 μm or more in which the powder component was detected. As the basicity B decreased, the number of inclusions derived from the powder greatly decreased. This is because the entrainment of the powder is suppressed because the viscosity of the powder increases as the basicity decreases.
Claims (2)
かつ、Ｆを３．６〜８．５質量％含有し、
１３００℃における粘度が１１〜２５ｐｏｉｓｅであり、
Ａｌ含有量が０．０１５質量％未満の鋼のビレット連続鋳造に用いることを特徴とする連続鋳造用パウダー。
Ｂ＝Ｔ．ＣａＯ／ＳｉＯ2 ［１］
ここでＴ．ＣａＯはパウダー中のＣａがすべてＣａＯであるとしたときのＣａＯの含有量（質量％）、ＳｉＯ2はパウダー中のＳｉＯ2含有量（質量％）を表す。 The basicity B shown by the following [1] formula is 0.7 or less ,
And F contains 3.6-8.5 mass%,
The viscosity at 1300 ° C. is 11-25 poise,
A powder for continuous casting, which is used for billet continuous casting of steel having an Al content of less than 0.015% by mass.
B = T. CaO / SiO 2 [1]
T. CaO represents the content (mass%) of CaO when all the Ca in the powder is CaO, and SiO 2 represents the SiO 2 content (mass%) in the powder.
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