JP4750985B2 - Molten steel surface heat insulating material and continuous casting method of steel using the same - Google Patents
Molten steel surface heat insulating material and continuous casting method of steel using the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、連続鋳造用タンディッシュや取鍋などにより溶鋼を移送、又は精錬処理を行う際に、断熱・保温あるいは空気酸化防止を目的として溶鋼表面を被覆する溶鋼表面保温材に関するものである。
【０００２】
【従来の技術】
連続鋳造用タンディッシュや取鍋などにより溶鋼を移送、又は精錬処理を行う際、保温材を用いて溶鋼表面を被覆し溶鋼からの熱放散と外気の侵入を防止している。従来から保温材として、籾殻を蒸し焼きにした焼籾が主に用いられ、その主成分はＳｉＯ2とＣである。ＳｉＯ2は熱伝導率が低く保温効果に、Ｃは酸素をＣＯガスに変えるため酸素の遮断効果に優れている。このため、焼籾は保温効果及び空気遮断効果を有し、しかも安価であることを特徴とする保温材である。しかしながら、加工性向上の目的から鋼板中のＣ濃度を極力低下させた、例えばＣ濃度が５０ｐｐｍ以下の極低炭素鋼において、保温材中のＣ成分が溶鋼中にピックアップし鋼材の特性を低下させる欠点が知られている。また、保温材中のＳｉＯ2成分は溶鋼中のＡｌと反応してＡｌ2Ｏ3系の介在物を生成するため、表面欠陥を増大させるといった問題も生じる。従来、焼籾のこれらの欠点を解決するため、Ｃ及びＳｉＯ2成分の少ない保温材として、例えば特公平３−４８１５２号公報に記載されているように、ＭｇＯ系の保温材が使用されている。また、ＭｇＯ自体は熱伝導率が高いため、これに断熱性を付与した発泡ＭｇＯの製造方法についても種々検討され、特公昭４８−７４８５号公報等に記載されている。
【０００３】
【発明が解決しようとする課題】
しかしながら、ＭｇＯを主成分とする保温材は融点が高く、使用温度では主に粉末或いは焼結状の固相として存在しているため、溶鋼表面の均一な被覆状態が得られず、外気と溶鋼との反応によりＡｌ2Ｏ3系介在物を生成する。これに対し、ＭｇＯの一部をＳｉＯ2、Ｎａ2Ｏ、或いはＣａＦ2等の低融点化材に置き換えて保温材を溶融させ、溶鋼表面を均一に被覆する方法が考えられるが、この場合ＳｉＯ2やＮａ2Ｏは溶鋼中のＡｌと反応してＡｌ2Ｏ3系介在物を生成し、ＣａＦ2はタンディッシュ耐火物を溶損させることにより耐火物起因の介在物を増大させる。
【０００４】
これらの問題を鑑み、本発明は空気酸化と保温材の反応に起因する溶鋼汚染を確実に防止できる保温材と清浄性の優れた鋳片を得るための連続鋳造方法を提供することを目的とするものである。
【０００５】
【課題を解決するための手段】
上記課題を解決するために、本発明は以下に構成を要旨とする。即ち、（１）ＣａＯ、ＭｇＯ及びＡｌ、又は、ＳｉＯ 2 、ＣａＯ、ＭｇＯ及びＡｌからなる溶鋼表面保温材において、ＳｉＯ2含有率を１０質量％以下、ＭｇＯ含有率を１０〜３７質量％とし、且つＣａＯ含有率／ＭｇＯ含有率が１．５以上、Ａｌ含有率／ＭｇＯ含有率が０．２以上としたことを特徴とする溶鋼表面保温材である。また、（２）鋼の連続鋳造方法において請求項１記載の溶鋼表面保温材をタンディッシュ内の溶鋼表面上に添加することを特徴とする鋼の連続鋳造方法である。
【０００６】
【発明の実施の形態】
溶鋼表面を被覆する保温材として満足すべき条件は、溶鋼の保温性を確保した上で、空気酸化と保温材の反応に起因する溶鋼の汚染を確実に防止することである。発明者らはこれら基本条件を満足すべく保温材の検討を進めたきた結果、保温材と溶鋼の反応を防止するためには低ＳｉＯ2化が、空気酸化を抑制するためにはＭｇＯ、ＣａＯおよび金属Ａｌを同時に含有させることが有効であることを見いだした。すなわち、保温材中のＳｉＯ2は（１）式により溶鋼中のＡｌと反応するため、保温材の低ＳｉＯ2化はＡｌ2Ｏ3系介在物の生成防止の効果を有する。
３ＳｉＯ2＋４Ａｌ＝２Ａｌ2Ｏ3＋３Ｓｉ （１）
また、保温材中にＭｇＯと金属を含有させると、（２）式の反応が起こり、保温材層内部でＭｇガスが発生する。
３ＭｇＯ＋２Ａｌ＝３Ｍｇ（ガス）＋Ａｌ2Ｏ3 （２）
このＭｇガスは保温材内への酸素ガスの侵入を抑制すると共に、一部侵入してきた酸素ガスと（３）式のように反応し、酸素ガスをＭｇＯとして保温材中に固定する。
２Ｍｇ＋Ｏ2＝ＭｇＯ （３）
保温材層内で（２）式により生成したＡｌ2Ｏ3はＣａＯと反応して低融点のカルシュウムアルミネートを形成し、この液相が溶鋼表面を均一に被覆する。そのため、空気中の酸素は保温材層を拡散して、保温材・溶鋼界面まで到達できず、確実に溶鋼の空気酸化を防止できる。さらに、生成したカルシュウムアルミネート中には一部ＭｇＯが含まれており、従来のＣａＦ2等を添加した低融点保温材に比べてタンディッシュ耐火物の溶損は軽微で、耐火物溶損に起因した溶鋼の汚染も防止できる。また、本発明の保温材はＭｇガスを発生させつつ液相化するため、熱間でタンディッシュを繰り返し使用する場合には、保温材の排滓性が非常に向上するといった利点もある。
【０００７】
本発明の保温材におけるＳｉＯ2含有率は１０質量％以下であり、その理由はＳｉＯ2含有率が１０質量％を超えると保温材中ＳｉＯ2と溶鋼中Ａｌの反応が起こり、Ａｌ2Ｏ3系介在物を生成するためである。ＭｇＯの含有率は１０〜３７質量％にすることが望ましく、１０質量％未満ではＭｇＯが少な過ぎ、十分な酸化防止に必要なＭｇガスを生成させることができず、また３７質量％超ではＭｇＯが過剰で相対的にＣａＯ含有率やＡｌ含有率が低下し、何れの場合も空気酸化防止の効果が損なわれる。また、保温材中にはＣａＯ含有率／ＭｇＯ含有率が１．５以上になるようにＣａＯを含有させることが好ましい。ＣａＯ含有率／ＭｇＯ含有率が１．５未満となると、ＣａＯが不足して保温材を低融点化できないためである。保温材中の金属ＡｌはＡｌ含有率／ＭｇＯ含有率で０．２以上になるように添加することが有効であり、Ａｌ含有率／ＭｇＯ含有率が０．２未満となるとＡｌが不足し、ＭｇＯを還元して空気酸化防止に十分な量のＭｇガスを発生させることができない。
【０００８】
保温材の基本組成は以上に述べた通りであるが、本発明品の機能を低下させない範囲であれば、Ａｌ2Ｏ3等の他酸化物やＣａＣｌ2、ＣａＦ2等の非酸化物を添加することも可能である。
【０００９】
以上に示したように、本発明の保温材を用いることにより、空気酸化と保温材の反応に起因する溶鋼汚染を確実に防止でき、その上で耐火物の溶損がない溶鋼表面保温材を提供できる。
【００１０】
【実施例】
以下に、実施例及び比較例を挙げて、本発明について説明する。
【００１１】
（実施例１）
０質量％ＳｉＯ2、５６．５質量％ＣａＯ、２７．３質量％ＭｇＯ、１６．２質量％Ａｌの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、低炭アルミキルド鋼を４００分間鋳造した。鋳造寸法は、厚み２４５ｍｍ×幅１５００ｍｍで、８５００ｍｍ長さに切断して１コイル単位とした。このスラブを常法により熱間圧延、冷間圧延し、最終的に厚み０．７ｍｍ×幅１５００ｍｍコイルの冷延鋼板とした。保温材の空気酸化防止効果及び反応防止効果はタンディッシュ入側と出側の全酸素濃度の上昇量及び冷延鋼板に発生した表面欠陥の発生個数により評価した。本発明の保温材により空気酸化と保温材による溶鋼汚染が防止され、且つ耐火物の溶損も抑制された結果、タンディッシュ入側から出側の全酸素濃度の上昇及び表面欠陥の発生はなかった。
【００１２】
（実施例２）
７質量％ＳｉＯ2、５０質量％ＣａＯ、２０質量％ＭｇＯ、２３質量％Ａｌの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、低炭アルミキルド鋼を４００分間鋳造した。鋳造寸法は、厚み２４５ｍｍ×幅１５００ｍｍで、８５００ｍｍ長さに切断して１コイル単位とした。このスラブを常法により熱間圧延、冷間圧延し、最終的に厚み０．７ｍｍ×幅１５００ｍｍコイルの冷延鋼板とした。本発明の保温材により空気酸化と保温材による溶鋼汚染が防止され、且つ耐火物の溶損も抑制された結果、タンディッシュ入側から出側の全酸素濃度の上昇及び表面欠陥の発生はなかった。
【００１３】
（比較例１）
１００質量％ＭｇＯの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、低炭アルミキルド鋼を４００分間鋳造した。鋳造寸法は、厚み２４５ｍｍ×幅１５００ｍｍで、８５００ｍｍ長さに切断して１コイル単位とした。このスラブを常法により熱間圧延、冷間圧延し、最終的に厚み０．７ｍｍ×幅１５００ｍｍコイルの冷延鋼板とした。ＭｇＯ系の従来保温材を使用したため、保温材層を通して酸素が拡散し、溶鋼の再酸化が生じた結果、タンディッシュ入側から出側にかけて全酸素濃度が２５ｐｐｍ上昇し、表面欠陥が平均で６個／コイル発生した。
【００１４】
（比較例２）
２０質量％ＳｉＯ2、８０重量％ＭｇＯの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、低炭アルミキルド鋼を４００分間鋳造した。鋳造寸法は、厚み２４５ｍｍ×幅１５００ｍｍで、８５００ｍｍ長さに切断して１コイル単位とした。このスラブを常法により熱間圧延、冷間圧延し、最終的に厚み０．７ｍｍ×幅１５００ｍｍコイルの冷延鋼板とした。ＳｉＯ2を含有するＭｇＯ系の従来保温材を使用したため、空気による溶鋼の再酸化と保温材中ＳｉＯ2の反応により溶鋼汚染が生じた結果、タンディッシュ入側から出側にかけて全酸素濃度が３０ｐｐｍ上昇し、表面欠陥が平均で８個／コイル発生した。
【００１５】
【発明の効果】
以上の如く、本発明の溶鋼表面保温材によれば溶鋼の汚染は軽減され、鋳造品質は極めて向上する。また、保温材を低融点化する必要がないため、耐火物の溶損もなく、操業面でも有効な保温材を提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a molten steel surface heat insulating material for covering a molten steel surface for the purpose of heat insulation / heat retention or air oxidation prevention when the molten steel is transferred or refined by a tundish or ladle for continuous casting.
[0002]
[Prior art]
When molten steel is transferred or refined by a continuous casting tundish or ladle, the surface of the molten steel is covered with a heat insulating material to prevent heat dissipation from the molten steel and intrusion of outside air. Conventionally, shochu obtained by steaming rice husk is mainly used as a heat insulating material, and the main components are SiO 2 and C. SiO 2 has a low thermal conductivity and has a heat retention effect, and C has an excellent oxygen blocking effect because it converts oxygen to CO gas. For this reason, shochu is a heat insulating material characterized by having a heat insulating effect and an air blocking effect and being inexpensive. However, for the purpose of improving workability, the C concentration in the steel sheet is reduced as much as possible, for example, in an extremely low carbon steel having a C concentration of 50 ppm or less, the C component in the heat insulating material is picked up in the molten steel and the characteristics of the steel material are reduced. Disadvantages are known. Further, since the SiO 2 component in the heat insulating material reacts with Al in the molten steel to generate Al 2 O 3 -based inclusions, there is a problem of increasing surface defects. Conventionally, in order to solve these drawbacks of shochu, an MgO-based heat insulating material has been used as a heat insulating material with less C and SiO 2 components, as described in, for example, Japanese Patent Publication No. 3-48152. . In addition, since MgO itself has high thermal conductivity, various methods for producing foamed MgO imparted with heat insulation have been studied and described in Japanese Patent Publication No. 48-7485.
[0003]
[Problems to be solved by the invention]
However, the heat insulating material mainly composed of MgO has a high melting point, and since it exists mainly as a powder or sintered solid phase at the operating temperature, a uniform coating state on the surface of the molten steel cannot be obtained. To produce Al 2 O 3 inclusions. On the other hand, a method in which a part of MgO is replaced with a low melting point material such as SiO 2 , Na 2 O or CaF 2 to melt the heat insulating material and coat the molten steel surface uniformly is considered. 2 and Na 2 O react with Al in the molten steel to produce Al 2 O 3 inclusions, and CaF 2 increases the inclusion due to the refractory by melting the tundish refractory.
[0004]
In view of these problems, an object of the present invention is to provide a heat insulation material that can reliably prevent molten steel contamination caused by the reaction between air oxidation and the heat insulation material, and a continuous casting method for obtaining a slab excellent in cleanliness. To do.
[0005]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is summarized as follows. That, (1) CaO, MgO and Al, or, SiO 2, CaO, in the molten steel surface thermal insulation material consisting of MgO and Al, a SiO 2 content of 10 wt% or less, the MgO content as 10-37 mass%, The molten steel surface heat insulating material is characterized in that the CaO content / MgO content is 1.5 or more and the Al content / MgO content is 0.2 or more. Further, (2) a continuous casting method of steel, wherein the molten steel surface heat insulating material according to claim 1 is added onto the molten steel surface in the tundish.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Conditions that should be satisfied as a heat insulating material covering the surface of the molten steel are to reliably prevent contamination of the molten steel due to the reaction between the air oxidation and the heat insulating material while ensuring the heat retaining property of the molten steel. As a result of studying the heat insulating material to satisfy these basic conditions, the inventors have reduced SiO 2 to prevent the reaction between the heat insulating material and molten steel, and MgO and CaO to suppress air oxidation. And it has been found that it is effective to simultaneously contain metal Al. That is, since SiO 2 in the heat insulating material reacts with Al in the molten steel according to the formula (1), the reduction of the SiO 2 in the heat insulating material has an effect of preventing the formation of Al 2 O 3 -based inclusions.
3SiO 2 + 4Al = 2Al 2 O 3 + 3Si (1)
Further, when MgO and a metal are contained in the heat insulating material, the reaction of the formula (2) occurs, and Mg gas is generated inside the heat insulating material layer.
3MgO + 2Al = 3Mg (gas) + Al 2 O 3 (2)
This Mg gas suppresses the intrusion of oxygen gas into the heat insulating material and reacts with the partially invading oxygen gas as shown in the equation (3) to fix the oxygen gas as MgO in the heat insulating material.
2Mg + O 2 = MgO (3)
Al 2 O 3 produced by the equation (2) in the heat insulating material layer reacts with CaO to form low melting point calcium aluminate, and this liquid phase uniformly coats the surface of the molten steel. For this reason, oxygen in the air diffuses in the heat insulating material layer and cannot reach the heat insulating material / molten steel interface, thereby reliably preventing air oxidation of the molten steel. Furthermore, the generated calcium aluminate contains a part of MgO, and the tundish refractory has a slight erosion loss compared to conventional low melting point heat insulating materials to which CaF 2 or the like is added. The resulting contamination of molten steel can also be prevented. In addition, since the heat insulating material of the present invention turns into a liquid phase while generating Mg gas, there is an advantage that when the tundish is repeatedly used while hot, the heat insulating material is greatly improved.
[0007]
The SiO 2 content in the heat insulating material of the present invention is 10% by mass or less, and the reason is that when the SiO 2 content exceeds 10% by mass, a reaction between SiO 2 in the heat insulating material and Al in the molten steel occurs, and Al 2 O 3 This is to produce system inclusions. The content of MgO is preferably 10 to 37% by mass. If the content is less than 10% by mass, the amount of MgO is too small to produce sufficient Mg gas necessary for preventing oxidation. However, the CaO content and the Al content are relatively lowered, and in any case, the effect of preventing air oxidation is impaired. Further, CaO is preferably contained in the heat insulating material so that the CaO content / MgO content is 1.5 or more. This is because when the CaO content / MgO content is less than 1.5, CaO is insufficient and the temperature of the heat insulating material cannot be lowered. It is effective to add the metal Al in the heat insulating material so that the Al content / MgO content is 0.2 or more. When the Al content / MgO content is less than 0.2, Al is insufficient. MgO cannot be reduced to generate a sufficient amount of Mg gas to prevent air oxidation.
[0008]
Although the basic composition of the heat insulating material is as described above, other oxides such as Al 2 O 3 and non-oxides such as CaCl 2 and CaF 2 are added as long as the function of the present invention is not deteriorated. It is also possible to do.
[0009]
As described above, by using the heat insulating material of the present invention, it is possible to reliably prevent molten steel contamination caused by the reaction between the air oxidation and the heat insulating material, and further on the molten steel surface heat insulating material that does not cause refractory melting. Can be provided.
[0010]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[0011]
Example 1
400 kg of a heat insulating material of 0% by mass SiO 2 , 56.5% by mass CaO, 27.3% by mass MgO, 16.2% by mass Al was added to a tundish having a capacity of 60 t, and low-carbon aluminum killed steel was cast for 400 minutes. Casting dimensions were 245 mm thickness x 1500 mm width, and were cut into 8500 mm lengths to make one coil unit. This slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm. The air oxidation prevention effect and reaction prevention effect of the heat insulating material were evaluated by the amount of increase in the total oxygen concentration on the entry side and the exit side of the tundish and the number of surface defects generated in the cold rolled steel sheet. As a result of preventing air oxidation and molten steel contamination due to the heat insulating material by the heat insulating material of the present invention and suppressing the refractory melt damage, there is no increase in the total oxygen concentration from the tundish inlet side to the outlet side and the occurrence of surface defects. It was.
[0012]
(Example 2)
400 kg of a heat insulating material of 7% by mass SiO 2 , 50% by mass CaO, 20% by mass MgO and 23% by mass Al was added to a tundish having a capacity of 60 t, and low-carbon aluminum killed steel was cast for 400 minutes. Casting dimensions were 245 mm thickness x 1500 mm width, and were cut into 8500 mm lengths to make one coil unit. This slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm. As a result of preventing air oxidation and molten steel contamination due to the heat insulating material by the heat insulating material of the present invention and suppressing the refractory melt damage, there is no increase in the total oxygen concentration from the tundish inlet side to the outlet side and the occurrence of surface defects. It was.
[0013]
(Comparative Example 1)
400 kg of heat retaining material of 100% by mass MgO was added to a tundish with a capacity of 60 t, and low-carbon aluminum killed steel was cast for 400 minutes. Casting dimensions were 245 mm thickness x 1500 mm width, and were cut into 8500 mm lengths to make one coil unit. This slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm. Since MgO-based conventional heat insulating material was used, oxygen diffused through the heat insulating material layer, resulting in reoxidation of the molten steel. As a result, the total oxygen concentration increased by 25 ppm from the inlet side to the outlet side of the tundish, resulting in an average of 6 surface defects. Pieces / coil were generated.
[0014]
(Comparative Example 2)
400 kg of a heat insulating material of 20 mass% SiO 2 and 80 wt% MgO was added to a tundish having a capacity of 60 t, and a low-carbon aluminum killed steel was cast for 400 minutes. Casting dimensions were 245 mm thickness x 1500 mm width, and were cut into 8500 mm lengths to make one coil unit. This slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm. Since MgO-based conventional heat insulating material containing SiO 2 was used, contamination of molten steel was caused by reoxidation of molten steel with air and reaction of SiO 2 in the heat insulating material. The average number of surface defects was 8 / coil.
[0015]
【The invention's effect】
As described above, according to the molten steel surface heat insulating material of the present invention, contamination of the molten steel is reduced, and the casting quality is greatly improved. In addition, since it is not necessary to lower the melting point of the heat insulating material, it is possible to provide a heat insulating material that is effective in terms of operation without melting of the refractory.
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Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS51145430A (en) * | 1975-03-31 | 1976-12-14 | Sumitomo Metal Ind | Keeping warm material for molten metal |
| JPS5296931A (en) * | 1976-02-10 | 1977-08-15 | Fuoseko Japan Rimitetsudo Yuug | Lagging plate couering molten steel |
| JPS57184563A (en) * | 1981-05-06 | 1982-11-13 | Kawasaki Steel Corp | Powder for surface coating of molten metal in continuous casting |
| JPH04105757A (en) * | 1990-08-24 | 1992-04-07 | Sumitomo Metal Ind Ltd | Heating start powder for dead soft steel |
| JPH06179059A (en) * | 1992-12-10 | 1994-06-28 | Nippon Steel Corp | Surface insulating material for molten steel |
| JPH07124741A (en) * | 1993-10-29 | 1995-05-16 | Nippon Steel Corp | Molten steel surface heat retaining agent |
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