JP2007111720A - Heat-retaining material for molten steel surface, and continuous casting method for steel using the same - Google Patents
Heat-retaining material for molten steel surface, and continuous casting method for steel using the same Download PDFInfo
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Abstract
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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およびそれを用いた連続鋳造方法に関するものである。 The present invention uses a molten steel surface heat insulating material that coats the 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, and the like. The present invention relates to a continuous casting method.
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溶鋼へ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され、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、鋼板の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、例えば特許文献１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、特許文献２等に記載されている。 When the molten steel is transferred or refined by a tundish for continuous casting or a 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 into the molten steel. 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 has been 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. There are known drawbacks. 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 that the surface defects of the steel plate are increased. Conventionally, in order to solve these drawbacks of shochu, an MgO-based heat insulating material is used as a heat insulating material having a small amount of C and SiO 2 components, as described in Patent Document 1, for example. In addition, since MgO itself has a high thermal conductivity, various methods for producing foamed MgO imparted with heat insulation properties have been studied and described in Patent Document 2 and the like.
しかしながら、特許文献１や特許文献２のような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。 However, since the heat insulating material mainly composed of MgO such as Patent Document 1 and Patent Document 2 has a high melting point and exists mainly as a powder or sintered solid phase at the operating temperature, the surface of the molten steel is uniform. A covering state cannot be obtained, and Al 2 O 3 inclusions are generated by the reaction between the outside air and molten steel. 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.
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。 In view of these problems, an object of the present invention is to provide a heat insulating material that can reliably prevent contamination of molten steel caused by the reaction between air oxidation and the heat insulating material, and a continuous casting method for obtaining a slab excellent in cleanliness. It is what.
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）溶鋼表面保温材において、ＳｉＯ2含有率を１０質量％以下、ＣａＯ／Ａｌ2Ｏ3を質量比で０．５〜２．０とし、且つＴｉを含有したことを特徴とする溶鋼表面保温材。
（２）溶鋼表面保温材において、ＳｉＯ2含有率を１０質量％以下、ＣａＯ／Ａｌ2Ｏ3を質量比で０．５〜２．０とし、且つＴｉとＴｉＯ2を含有したことを特徴とする溶鋼表面保温材。
（３）Ｔｉを０．１〜１０質量％含有していることを特徴とする請求項（１）記載の溶鋼表面保温材。
（４）Ｔｉを０．１〜１０質量％、ＴｉＯ2を２〜３０質量％含有していることを特徴とする請求項（２）記載の溶鋼表面保温材。
（５）鋼の連続鋳造方法において請求項（１）から（４）の何れかに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。
（６）鋼の連続鋳造方法において請求項（１）から（４）の何れかに記載の溶鋼表面保温材をタンディッシュ内のＴｉ脱酸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。
In order to solve the above problems, the present invention is summarized as follows. That is,
(1) Molten steel surface heat insulating material, wherein the SiO 2 content is 10% by mass or less, CaO / Al 2 O 3 is 0.5 to 2.0 by mass, and Ti is contained. Insulation material.
(2) The molten steel surface heat insulating material is characterized in that the SiO 2 content is 10% by mass or less, CaO / Al 2 O 3 is 0.5 to 2.0 by mass ratio, and Ti and TiO 2 are contained. The molten steel surface heat insulator.
(3) The molten steel surface heat insulating material according to claim (1), comprising 0.1 to 10% by mass of Ti.
(4) The molten steel surface heat insulating material according to claim (2), wherein 0.1 to 10% by mass of Ti and 2 to 30% by mass of TiO 2 are contained.
(5) A continuous casting method for steel, wherein the molten steel surface heat insulating material according to any one of claims (1) to (4) is added onto the molten steel surface in the tundish.
(6) In the continuous casting method of steel, the molten steel surface heat insulating material according to any one of claims (1) to (4) is added onto the Ti deoxidized molten steel surface in the tundish. Casting method.
本発明の溶鋼表面保温材およびこの保温材を用いた連続鋳造方法によれば、溶鋼の汚染は軽減され、鋳造品質は極めて向上する。また、耐火物の溶損もなく、保温材の排滓性も向上するため、操業面でも有効な連続鋳造方法を提供できる。 According to the molten steel surface heat insulating material and the continuous casting method using the heat insulating material of the present invention, contamination of the molten steel is reduced and the casting quality is greatly improved. In addition, since there is no refractory damage to the refractory and the heat-removing material is improved, the continuous casting method that is effective in terms of operation can be provided.
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結果、保温材と溶鋼の反応を防止するためには低ＳｉＯ2化が、また空気酸化を抑制するためにはＣａＯ／Ａｌ2Ｏ3の質量比を適正化した上でＴｉを少量含有させることが有効であることを見いだした。 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 CaO / Al to suppress air oxidation. It was found that it is effective to contain a small amount of Ti after optimizing the mass ratio of 2 O 3 .
保温材中のＳｉＯ2は［式１］と［式２］により溶鋼中のＡｌやＴｉと反応するため、保温材の低ＳｉＯ2化はＡｌ2Ｏ3系介在物やチタニア系介在物の生成防止の効果を有する。
３ＳｉＯ2＋４Ａｌ＝２Ａｌ2Ｏ3＋３Ｓｉ ［式１］
ＳｉＯ2＋Ｔｉ＝ＴｉＯ2＋Ｓｉ ［式２］
Since SiO 2 in the heat insulating material reacts with Al and Ti in the molten steel according to [Formula 1] and [Formula 2], reducing the heat insulating material to SiO 2 generates Al 2 O 3 inclusions and titania inclusions. Has the effect of prevention.
3SiO 2 + 4Al = 2Al 2 O 3 + 3Si [Formula 1]
SiO 2 + Ti = TiO 2 + Si [Formula 2]
本発明で規定される範囲の成分系において保温材中のＳｉＯ2含有率が１０質量％以下になると、［式１］と［式２］の反応速度は急速に低下し、実質的に生成するＡｌ2Ｏ3系介在物やチタニア系介在物が減少するため、ＳｉＯ2含有率を１０質量％以下にする必要がある。ＳｉＯ2を全く含まなければ、［式１］と［式２］の反応は起こらないので、当然その下限値は０質量％を含む。 When the SiO 2 content in the heat insulating material is 10% by mass or less in the component system within the range defined by the present invention, the reaction rates of [Formula 1] and [Formula 2] rapidly decrease and are substantially formed. Since Al 2 O 3 -based inclusions and titania-based inclusions decrease, the SiO 2 content needs to be 10% by mass or less. If SiO 2 is not included at all, the reaction of [Formula 1] and [Formula 2] does not occur, so the lower limit value naturally includes 0% by mass.
また、タンディッシュ内溶鋼の空気酸化を防止するためには、保温材を低融点化し、溶鋼表面を保温材で均一に被覆することにより、溶鋼中への空気の侵入を遮断することが重要である。保温材を低融点化するためには、ＣａＯ／Ａｌ2Ｏ3を質量比で０．５〜２．０の範囲にする必要があり、ＣａＯ／Ａｌ2Ｏ3が０．５未満および２．０超の範囲では保温材の溶融温度が鋼の融点１５３６℃以上となり溶融し難いためである。なお、保温材の溶融温度は、１０ｍｍ直径×１０ｍｍ高さの保温材タブレットを加熱炉中で昇温し、保温材タブレットの高さが１／２になった際の温度とした。 In order to prevent air oxidation of molten steel in the tundish, it is important to lower the melting point of the heat insulating material and to uniformly cover the surface of the molten steel with the heat insulating material, thereby blocking air intrusion into the molten steel. is there. To lower the melting point of the thermal insulation material, it must be in the range of 0.5 to 2.0 the CaO / Al 2 O 3 by mass ratio, CaO / Al 2 O 3 is less than 0.5 and 2. This is because if the temperature exceeds 0, the melting temperature of the heat insulating material is not less than 1536 ° C., which is the melting point of steel, and it is difficult to melt. The melting temperature of the heat insulating material was the temperature at which the temperature of the heat insulating material tablet was raised to half by heating the temperature of the heat insulating material tablet having a diameter of 10 mm × 10 mm in a heating furnace.
さらに、ＣａＯとＡｌ2Ｏ3を主成分（合計で５０質量％以上を目安）とする保温材中にＴｉを添加すると、部分的に保温材が未溶融状態である領域に侵入してきた酸素をＴｉがＴｉＯ2として固定するため、保温材溶融のばらつきまで考慮した上で確実に空気遮断することが可能となる。タンディッシュへの保温材投入は、手投げて実施されることが殆どであり、タンディッシュ内の場所により保温材厚みは均一でなく、溶融状態も一定ではないため、保温材へのＴｉ添加による酸素固定効果は保温材の空気遮断効果を大きく向上させる。 Furthermore, when Ti is added to a heat insulating material mainly composed of CaO and Al 2 O 3 (total of about 50% by mass or more), oxygen that has partially penetrated into the region where the heat insulating material is in an unmelted state is added. Since Ti is fixed as TiO 2 , it is possible to reliably shut off the air in consideration of variations in heat insulation melting. The heat insulating material is thrown into the tundish by hand, and the heat insulating material thickness is not uniform depending on the location in the tundish, and the molten state is not constant. The oxygen fixation effect greatly improves the air blocking effect of the heat insulating material.
保温材中のＴｉは空気と反応してＴｉＯ2を生成するため、鋳造開始初期を除いて保温材中にはＴｉＯ2が含有される。ＴｉＯ2は保温材の粘性を低下させ、溶鋼表面上での保温材の広がり性を向上させるため、より均一な保温材被覆状態が得られ易くなる。このため、ＣａＯとＡｌ2Ｏ3を主成分とするＴｉ含有の保温材中に予めＴｉＯ2を含有させておくと、鋳造初期からＴｉＯ2の低粘性化効果により保温材の被覆性が向上するため好ましい。 Since Ti in the heat insulating material reacts with air to generate TiO 2 , TiO 2 is contained in the heat insulating material except at the beginning of casting. Since TiO 2 lowers the viscosity of the heat insulating material and improves the spreadability of the heat insulating material on the molten steel surface, it becomes easier to obtain a more uniform heat insulating material coating state. For this reason, if TiO 2 is previously contained in the Ti-containing heat insulating material mainly composed of CaO and Al 2 O 3 , the covering property of the heat insulating material is improved by the effect of reducing the viscosity of TiO 2 from the early stage of casting. Therefore, it is preferable.
ＣａＯとＡｌ2Ｏ3を主成分とする保温材へのＴｉの好ましい添加範囲は０．１〜１０質量％である。Ｔｉの含有率が０．１質量％未満では空気中の酸素と反応し難く、反対に１０質量％超では溶鋼中にＴｉが一部溶解し、溶鋼中でのＴｉ濃度上昇やチタニア系介在物生成を引き起こすためである。 The preferable addition range of Ti to the heat insulating material mainly composed of CaO and Al 2 O 3 is 0.1 to 10% by mass. If the Ti content is less than 0.1% by mass, it is difficult to react with oxygen in the air. Conversely, if it exceeds 10% by mass, Ti partially dissolves in the molten steel, and the Ti concentration in the molten steel increases and titania inclusions. This is to cause generation.
また、ＣａＯとＡｌ2Ｏ3を主成分とするＴｉ含有保温材へのＴｉＯ2の好ましい添加範囲は２．０〜３０質量％である。ＴｉＯ2の含有率が２質量％未満ではＴｉ−ＣａＯ−Ａｌ2Ｏ3系保温材の粘性が低下し難く、反対に３０質量％超では溶鋼中のＡｌとＴｉＯ2が反応してＡｌ2Ｏ3系介在物を少量ではあるが生成し易いためである。 Also preferred range of addition of the TiO 2 to Ti-containing heat insulating material composed mainly of CaO and Al 2 O 3 is 2.0 to 30 mass%. When the content of TiO 2 is less than 2% by mass, the viscosity of the Ti—CaO—Al 2 O 3 heat insulating material is difficult to decrease, whereas when it exceeds 30% by mass, Al in the molten steel reacts with TiO 2 to react with Al 2 O. This is because it is easy to produce a 3 system inclusion although it is a small amount.
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、ＭｇＯ、ＺｒＯ2等の他の酸化物やＣａＣｌ2等の非酸化物を添加することも可能である。 Although the basic composition of the heat insulating material is as described above, other oxides such as MgO and ZrO 2 and non-oxides such as CaCl 2 should be added as long as the function of the product of the present invention is not deteriorated. Is also possible.
本発明の保温材はＣａＯ原料、Ａｌ2Ｏ3原料、ＴｉＯ2原料、Ｔｉ原料等を本発明で規定の成分となるように配合し、均一に混じるように混錬することにより製造される。 The heat insulating material of the present invention is produced by blending a CaO raw material, an Al 2 O 3 raw material, a TiO 2 raw material, a Ti raw material or the like so as to become the specified components in the present invention and kneading them so as to be uniformly mixed.
本発明の保温材をタンディッシュ内の溶鋼表面上に添加して鋼の連続鋳造を行うと、空気酸化と、保温材と溶鋼との反応に起因する溶鋼汚染を確実に防止できる。また、従来のようにＣａＦ2を添加して低融点化する必要もなく、ＣａＦ2によるタンディッシュ耐火物の急激な溶損も防止できる。さらに、本発明の保温材は液相で低粘性であるため、熱間でタンディッシュを繰り返し使用する場合にも、保温材の排滓性が非常に向上するといった利点もある。 When the heat insulating material of the present invention is added onto the surface of the molten steel in the tundish and the steel is continuously cast, contamination of the molten steel due to air oxidation and the reaction between the heat insulating material and the molten steel can be reliably prevented. Further, it is not necessary to lower the melting point by adding CaF 2 as in the prior art, and it is possible to prevent the tundish refractory from being rapidly melted by CaF 2 . Furthermore, since the heat insulating material of the present invention has a low viscosity in the liquid phase, there is an advantage that the heat-dissipating property of the heat insulating material is greatly improved even when the tundish is repeatedly used in the hot state.
また、本発明の保温材をタンディッシュ内に添加して、予め真空脱ガス装置等でＴｉ脱酸された溶鋼を鋳造すると、保温材中Ｔｉの溶解および保温材中ＴｉＯ2と溶鋼中Ｔｉとの反応は何れも起こり難いため、本発明の保温材はＴｉ脱酸溶鋼の鋳造に最適な組成となっている。 Further, when the heat insulating material of the present invention is added to the tundish and the molten steel that has been Ti-deoxidized in advance by a vacuum degassing apparatus or the like is cast, melting of Ti in the heat insulating material and TiO 2 in the heat insulating material and Ti in the molten steel Since these reactions hardly occur, the heat insulating material of the present invention has an optimum composition for casting of Ti deoxidized molten steel.
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。 Hereinafter, the present invention will be described with reference to examples and comparative examples.
（実施例１）
８質量％ＳｉＯ2、３０質量％ＣａＯ、５７質量％Ａｌ2Ｏ3、５質量％Ｔｉの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、炭素濃度０．０４質量％の低炭アルミキルド溶鋼を２００分間鋳造した。なお、タンディッシュ内の溶鋼温度は１５５８℃である。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。
Example 1
400kg of heat retention material of 8% by mass SiO 2 , 30% by mass CaO, 57% by mass Al 2 O 3 , 5% by mass Ti is added to the tundish with a capacity of 60t, and a low-carbon aluminum killed molten steel with a carbon concentration of 0.04% by mass is added. Cast for 200 minutes. The molten steel temperature in the tundish is 1558 ° C. 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 form a cold-rolled steel sheet having a thickness of 0.7 mm × width of 1500 mm coil.
保温材の空気酸化防止効果及び反応防止効果は、定常鋳造領域でタンディッシュ入側と出側の溶鋼をサンプリングし、ガス分析を行うことにより測定した溶鋼中の全酸素濃度の上昇量、及び冷延鋼板の１コイル当たりに発生した表面欠陥の目視観察による発生個数で評価した。 The thermal oxidation prevention effect and reaction prevention effect of the heat insulating material are determined by sampling the molten steel on the inlet side and outlet side of the tundish in the steady casting region, and measuring the increase in the total oxygen concentration in the molten steel measured by gas analysis. The number of surface defects generated per coil of the rolled steel sheet was evaluated based on the number of generated defects by visual observation.
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と出側の全酸素濃度は０．００３４質量％と０．００３０質量％で全酸素濃度の上昇はなかった。また、表面欠陥の発生もなかった。 As a result of the thermal insulation of the present invention preventing air oxidation and molten steel contamination by the thermal insulation, and suppressing the refractory melting, the total oxygen concentration on the tundish entry and exit sides is 0.0034% by mass and 0. There was no increase in the total oxygen concentration at 0030 mass%. Further, no surface defects were generated.
（実施例２）
２質量％ＳｉＯ2、４８質量％ＣａＯ、２５質量％Ａｌ2Ｏ3、２０質量％ＴｉＯ2、５質量％Ｔｉ保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、炭素濃度０．００３質量％の極低炭Ｔｉ脱酸溶鋼を２００分間鋳造した。なお、タンディッシュ内の溶鋼温度は１５６０℃である。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。
(Example 2)
400 kg of 2% by mass SiO 2 , 48% by mass CaO, 25% by mass Al 2 O 3 , 20% by mass TiO 2 , 5% by mass Ti heat insulating material are added to a tundish with a capacity of 60 t, and the carbon concentration is 0.003% by mass. An ultra-low carbon Ti deoxidized molten steel was cast for 200 minutes. In addition, the molten steel temperature in a tundish is 1560 degreeC. 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.
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と出側の溶鋼中の全酸素濃度は０．００３６質量％と０．００３１質量％で全酸素濃度の上昇はなかった。また、表面欠陥の発生もなかった。 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 melting loss, the total oxygen concentration in the molten steel on the tundish inlet side and outlet side is 0.0036 mass% There was no increase in the total oxygen concentration at 0.0031 mass%. Further, no surface defects were generated.
（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、炭素濃度０．０４質量％の低炭アルミキルド溶鋼を２００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。
(Comparative Example 1)
400 kg of a heat insulating material of 100% by mass MgO was added to a tundish having a capacity of 60 t, and a low carbon aluminum killed molten steel having a carbon concentration of 0.04% by mass was cast for 200 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.
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侵入し、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溶鋼中の全酸素濃度が０．００３６質量％から０．００６０質量％まで上昇した。また、表面欠陥が平均で６個／コイル発生した。 Since MgO-based conventional heat insulating material is used, oxygen enters through the heat insulating material layer and re-oxidation of the molten steel occurs. It increased to 0060% by mass. In addition, an average of 6 surface defects / coil was generated.
（比較例２）
２０質量％ＳｉＯ2、８０質量％ＭｇＯの保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、炭素濃度０．００２８質量％の極低炭アルミキルド鋼を２００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。
(Comparative Example 2)
400 kg of a heat insulating material of 20% by mass SiO 2 and 80% by mass MgO was added to a tundish having a capacity of 60 t, and an ultra low carbon aluminum killed steel having a carbon concentration of 0.0028% by mass was cast for 200 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.
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と溶鋼と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溶鋼中の全酸素濃度が０．００３質量％から０．００５９質量％まで上昇した。また、表面欠陥が平均で８個／コイル発生した。 Because MgO-based conventional heat insulating material containing SiO 2 was used, molten steel was contaminated by reoxidation of molten steel by air and reaction between SiO 2 and molten steel in the heat insulating material, resulting in molten steel from the tundish inlet side to the outlet side. The total oxygen concentration inside increased from 0.003 mass% to 0.0059 mass%. In addition, an average of 8 surface defects / coil was generated.
（比較例３）
１６質量％ＳｉＯ2、５８質量％ＣａＯ、２６質量％Ａｌ2Ｏ3の保温材４００ｋｇを容量６０ｔのタンディッシュに添加し、炭素濃度０．００２８質量％の極低炭アルミキルド鋼を２００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。
(Comparative Example 3)
400 kg of heat insulating material of 16% by mass SiO 2 , 58% by mass CaO, 26% by mass Al 2 O 3 was added to a tundish having a capacity of 60 t, and an ultra low carbon aluminum killed steel having a carbon concentration of 0.0028% by mass was cast for 200 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.
ＳｉＯ2を含有する高ＣａＯ−低Ａｌ2Ｏ3系の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と溶鋼と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溶鋼中の全酸素濃度が０．００３２質量％から０．００６２質量％まで上昇した。また、表面欠陥が平均で８．２個／コイル発生した。
Since a high CaO-low Al 2 O 3 type heat insulating material containing SiO 2 was used, molten steel was contaminated by reoxidation of the molten steel by air and reaction between SiO 2 and the molten steel in the heat insulating material. The total oxygen concentration in the molten steel increased from 0.0032 mass% to 0.0062 mass% from the side to the outlet side. In addition, an average of 8.2 surface defects / coil was generated.
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