JP3637895B2 - Continuous casting powder and continuous casting method using the same - Google Patents

Description

【００１４】
高塩基度スラグ中に最も一般的に析出するとして知られている結晶、 例えば、特開平10−58102 号、特開平10−216907号記載のカスピダイン (3CaO・2SiO₂・CaF₂)は、図２の３次元格子軸図において、β＝109.6 度と一方向に傾いた単斜晶（Monoclinic）であり、形態の似た結晶は他に発見されなかった。
【００１５】
一方で、結晶形態の非常に似通った２つの結晶が見つかった。ゲーレナイト( 2CaO・Al₂O₃・SiO₂)とアケルマナイト(2CaO・MgO・2SiO₂)である。両者はともに、α＝β＝γ＝90度の正方晶（Tetragonal）であり、格子定数も近い。また、ペロブスカイト(CaO・Ti0₂）も正方晶に近い斜方晶（Orthorhombic、α＝β＝γ＝90度）で、格子定数の組み合わせもゲーレナイト、およびアケルマナイトに近いことがわかった。
【００１６】
結晶形態が酷似している場合には、両者は互いに自由な割合で溶け込み合う全率固溶体を作ると予測される。調査の結果、ゲーレナイトとアケルマナイトとは、予測通りメリライトという全率固溶体を作ることが確認された。
このように見出されたゲーレナイトとアケルマナイトが「主な結晶」となるパウダー組成について検討した。検討に当っては、鋼を汚さないパウダーとして具備すべき２点を念頭に置いた。すなわち、溶融スラグとなった際に塩基度(%T・CaO)/(%SiO₂)が1.1 以上であること、1300℃において0.30Pa・S以上の高粘度を維持することである。
【００１７】
溶融スラグは、高塩基度になるとスラグ中のSiO₂の結びつきが分断され粘性が低下する。それを補うために、両性酸化物のAl₂O₃ 等を添加する必要がある。本発明において、主な結晶として選んだゲーレナイトとアケルマナイトとを比較すると、ゲーレナイトが高塩基度で高 Al₂O₃濃度、アケルマナイトが低塩基度で Al₂O₃レスとなっている。ゆえに、両者の組み合わせは、高塩基度化にしたがいAl₂O₃ 濃度が上昇する組成となる。これは、高塩基度化に伴う粘度低下を補うのに好都合な組み合わせである。
【００１８】
このように、２種の結晶ゲーレナイトとアケルマナイトとの組み合せは、結晶形態が酷似している点からも、また、高塩基度かつ高粘性という特性を実現する点からも、非常に好適な結晶の組み合わせであった。
さらに、ゲーレナイトは構成組成としてAl₂O₃ を含むので、溶鋼の清浄度が悪くて鋳型内に浮上してきたAl₂O₃ 不純物が溶融スラグ層内に多く吸収される場合にも、その結晶析出が阻害されるという悪影響が小さい利点もある。
【００１９】
第１の本発明に係る連続鋳造用パウダーの特徴は、パウダーが溶融した溶融スラグ凝固時に、結晶形態が酷似するゲーレナイト、アケルマナイト、あるいは両者の全率固溶体であるメリライトを、主な結晶組成として析出させることである。
ここで、「主な結晶」とは、Ｘ線回析においてピーク高さが、他の結晶の２倍以上あることを前提とし、似た回析ピーク高さの結晶が共存する場合には、主な結晶は存在しないこととする。
【００２０】
また、結晶形態の良く似たゲーレナイトとアケルマナイトとは、回析パターンが酷似しており、両者の識別は困難である。そこで、これらの結晶は全て全率固溶体であるメリライトの１種と判断した。ただし、パウダー中の MgO濃度が10％以上かつAl₂O₃ 濃度が４％未満の場合はアケルマナイトに近い組成であると判断し、結晶名をアケルマナイトとする。
【００２１】
前記ゲーレナイト、アケルマナイトの２種の結晶の他に、塩基度上昇や凝固温度調整の観点から、ペロブスカイト結晶の析出のためにTi0₂も適宜添加することとした。
具体的化学組成の決定に当たっては、Al₂O₃ を含有する結晶であるゲーレナイトと、MgO を含有する結晶であるアケルマナイトと、TiO₂を含有する結晶であるペロブスカイトとを適正な比率、例えば、25：65：10の比率で析出することを想定した純組成を求め、その純組成に対し各元素の含有量のバラツキを±５％以下に近づけるようにした。
【００２２】
よって、第２の本発明に係る連続鋳造用パウダーの特徴は、溶融スラグの化学組成としてTiO₂を３〜15％含有し、溶融スラグ凝固時に、ゲーレナイト、アケルマナイト、あるいは両者の全率固溶体であるメリライトを、主な結晶組成として析出しするとともに、ペロブスカイト結晶組成も析出させることである。
【００２３】
TiO₂の添加に際しては、その濃度を３〜15％に抑えることが望ましい。TiO₂の濃度が15％を超えると、溶融スラグが高融点となりその凝固温度調整が困難となるからである。また、TiO₂の濃度が３％未満では、溶融スラグの物性調整作用等が発揮されず、TiO₂の添加の効果が少ないからである。
【００２４】
上記検討の結果を基に、試作したパウダーを表２及び表３に示す。
表３において、「主な結晶」とは、前述の定義と同じＸ線回析におけるピーク高さにより規定する。ゲーレナイト結晶とアケルマナイト結晶の判定も前述の通りである。また、T・CaO 濃度は、総Ca量をCaO に換算した濃度であり、例えばCaF2中のCa分も含んでいる。
【００２５】
【表２】

【００２６】
【表３】

【００２７】
結晶調査に供した溶融スラグは、600gのパウダーを1350℃の電気炉中で溶解した後、黒鉛坩堝を用いて200 ℃/hr の冷却速度で炉冷したものである。結晶名は、固有名がある場合には固有名（アケルマナイト、メリライト、カスピダイン、ペロブスカイトなど）で表記し、固有名がないものは、組成（CaO・SiO₂、3Na₂0・2Al₂O₃・4SiO₂など）で示した。
【００２８】
表２、表３中の例Ａ〜例Ｈは、主な結晶としてメリライト（ゲーレナイトとアケルマナイトと全率固溶体）、もしくはアケルマナイトを析出し、パウダーフィルム中の結晶析出が安定している。
例Ａ〜例Ｆにおいて、TiO₂を添加しているのは、適量の添加により凝固温度を下げ、スラグ粘度調整に効果があること、及び、TiO₂をCaO・TiO₂組成として添加することによりTiO₂量に応じたCaO が増加され塩基度が上昇する効果を有することによる。
【００２９】
TiO₂添加により析出される結晶、ペロブスカイトは、前述のように結晶形態が主な結晶であるゲーレナイトやアケルマナイト、メリライトに近いので、これら主な結晶の析出を阻害する問題を生じなかった。加えて、例Ａ〜例Ｆの例は粘度が高いので鋳片への溶融スラグの巻き込みがなかった。
【００３０】
粘度は、1300℃における測定値が0.30Pa・Sよりも低いと鋼中への溶融スラグの巻き込みが発生しやすく、1.80Pa・Sよりも高いと鋳型内の潤滑が悪化し、凝固シェルの鋳型への焼き付きが発生しやすくなる。凝固温度が1100℃よりも低い場合には、パウダーフィルム中の結晶析出サイトである固相が不足し、溶鋼と鋳型壁間の固相フィルムが薄くなり所望の結晶が生成されず、また、凝固温度が1280℃よりも高い場合には、パウダーフィルム中の液相が不足するので、溶鋼と鋳型壁間への流入が悪く、鋳型内での潤滑が悪化し凝固シェルの鋳型への焼き付きが発生しやすくなる。
【００３１】
例Ａ〜例Ｈの場合は、溶融スラグの凝固温度は、適正範囲（1100〜1280℃）内であるので、潤滑性を損なわず、かつ、パウダーフィルム中に結晶析出サイトである固相が適度に存在するので鋳型内の凝固シェルを緩冷却でき、上記結晶析出の安定さと相まって均一な緩冷却が実現され、割れ性の鋳片欠陥を発生させることなく鋳造することができた。
【００３２】
また、例Ａ〜例Ｆは、塩基度(%T・CaO)/(%SiO₂)が適正範囲1.1 〜1.6 内であるので、SiO₂活量が低く、Al等の鋼中脱酸成分により還元され難く、溶鋼汚染が抑制される。塩基度が1.1 よりも低いと、SiO₂がAl等の鋼中脱酸成分により還元され溶鋼を汚染しやすくなる。また、塩基度が1.6 よりも高いと、凝固温度が上昇し上記適正温度範囲内（上限1280℃）に収めることが困難となる。
【００３３】
さらに、溶融スラグの粘度、ならびに凝固温度調整のため、結晶組成に大きな影響を及ばさない範囲で、フッ素（Ｆ）を添加した。フッ素含有量は５％以下に抑え、カスピダイン結晶の析出を抑制する。カスピダイン結晶が多く析出すると主な結晶であるアケルマナイト、ゲーレナイト、メリライトの析出を不安定にする。また、フッ素添加量の抑制は、連続鋳造機等金属部品の腐食軽減や工業排水の水質悪化防止に対しても有効である。
【００３４】
よって、第３の本発明に係る連続鋳造用パウダーの特徴は、溶融スラグの化学組成が、 (％T・CaO)/(％SiO₂) の値が1.1 〜1.6 、フッ素含有量が５％以下であり、凝固温度が1100〜1280℃、1300℃における粘度が0.30〜1.80Pa・Sであり、かつ、溶融スラグ凝固時に、ゲーレナイト、アケルマナイト、あるいは両者の全率固溶体であるメリライトを、主な結晶組成として析出させること、または、さらに、溶融スラグの化学組成としてTiO₂を３〜15％含有し、ペロブスカイト結晶組成も析出させることである。なお、（％T・CaO)の値は、含有Ca量をCaO 量に変換した量も含めた値である。
【００３５】
また、アルカリ金属酸化物（表２中のNa₂0およびLi₂0）の含有量を少なくしているので、アルカリ金属酸化物とSiO₂との反応による溶融スラグ直上の生パウダー中の約800 ℃の低温で生じる液相が少なく、鋳型内生パウダーのの焼結が少ない。ゆえに、保温性が良くスラグベアも成長しないで、鋳片表面に疵の発生しない良好な鋳片が得られる。アルカリ金属酸化物は、その総量が８％を超えると、鋳型内におけるパウダーの焼結が顕著になるとともに、アルカリ金属酸化物を含有する結晶の析出量が増大し、主な結晶の析出が不安定になる。よって、Na₂0、Li₂0等のアルカリ金属酸化物の含有量は、８％以下とする。これが第４の発明である。
【００３６】
上記第３、第４の本発明に記載の連続鋳造用パウダーは、溶鋼中の脱酸元素である[sol.Al]濃度が0.003 〜0.100 ％の鋼を連続鋳造する際に使用すると効果的である。これは、[sol.Al]濃度が0.003 ％よりも低いと、脱酸されず鋼中酸素濃度が高くなり、本発明パウダーによって溶鋼の汚染を防止する意味が薄れるからである。また、[sol.Al]濃度が0.100 ％よりも高いと、スラグ中の（SiO₂）が溶鋼中の[sol.Al]により還元される現象が顕著となり、溶鋼汚染防止効果が不十分となるためである。
【００３７】
よって、第５の本発明の特徴は、溶鋼中の[sol.Al]含有量が0.003 〜0.100 ％の鋼を連続鋳造する際に、第３の本発明または第４の本発明の連続鋳造用パウダーを使用する連続鋳造方法である。
【００３８】
表２、表３中の例Ｇ、例Ｈは、上記例Ａ〜例Ｆに対し塩基度が低いので溶鋼を汚染しやすい例、例Ｊ、Ｌも同様に、上記例Ａ〜例Ｆに対し塩基度が低いので溶鋼を汚染しやすく、主たる結晶も異なる例である。例Ｑは、主な結晶であるカスピダイン (3CaO・2Si0₂・CaF₂)とペロブスカイト(CaO・TiO₂）の結晶形態差が大きいこと、および結晶組成に含まれないAi₂O₃の濃度が高いので、カスピダイン析出が不安定な例である。その他の例は、いずれも主な結晶が存在せず、パウダーフィルム中の結晶析出が不安定な例である。
例Ｊ〜Ｔのパウダーが溶けた溶融スラグは、その粘度はいずれも適正範囲に入っているが、上記のように塩基度の低さや結晶析出の不安定さにおいて問題があった。
【００３９】
【実施例】
以下、表２及び表３に示したパウダーを実連続鋳造機に供した結果の一例を示す。
表４には、表２及び表３中の例Ｆ、例Ｈおよび例Ｒを用いて、鋳型サイズ：矩形で２辺が1250mmと230mm 、鋳造速度1.2m/minで、［Ｃ］＝0.10％、［Si］＝0.02％、［Mn］＝0.8 ％、［sol.Al］＝0.030 ％の炭素鋼を連続鋳造した際の鋳片品質を比較した結果を示したものである。以下、例Ａ〜例Ｈを発明例Ａ〜発明例Ｈ、例Ｊ〜例Ｔを比較例Ｊ〜比較例Ｔとする。
【００４０】
【表４】

【００４１】
比較例Ｒは、鋳片表面割れ性欠陥が多く発生した。これは、粘度、塩基度ともに問題は無いが、パウダーフィルム中に析出すべき主な結晶が存在せず、鋳型内抜熱が不安定であることに起因すると考えられる。
【００４２】
これに対し、発明例Ｈのパウダーを使用して製造した鋳片の品質は、鋳片介在物欠陥指数が他のパウダーを使用して製造した鋳片の品質に比べて若干劣るものの、割れ性欠陥はなく全体としては良好であった。鋳片介在物欠陥指数が若干劣ったのは、主な結晶であるメリライトが析出されて表面欠陥もなく、粘度が高いため溶融スラグ巻き込み欠陥は抑制できているものの、塩基度が0.95と他のパウダーの塩基度よりやや低いため鋳片を汚染したものと考えられる（鋳片介在物欠陥指数２）。
発明例Ｆは、高粘度かつ高塩基度ゆえ鋳片介在物欠陥が少なく、かつ、スラグフィルム中に主な結晶としてメリライトを安定して析出するので、鋳型内抜熱が安定しており、鋳片割れ性欠陥の発生も少なかった。このように、発明例Ｆは、介在物欠陥と割れ性欠陥がともに少なく、総合的に高品質の鋳片を製造することができた。
【００４３】
表５には、発明例Ｅ、比較例Ｊ、比較例Ｑ、および比較例Ｓを用いて、鋳型サイズ：内径310mm 、鋳造速度1.4m/minで、［Ｃ］＝0.24％、［Si］＝0.20％、［Mn］＝1.5 ％、［Sol・Al］＝0.035 ％の炭素鋼を連続鋳造し、シームレスパイプに圧延した後のパイプの品質を比較した。
【００４４】
【表５】

【００４５】
比較例Ｊは、パイプ介在物欠陥指数が他のパウダーに対し劣る。これは、粘度が所定値なので溶融スラグ（パウダー）の巻き込み欠陥は抑制できているが、低塩基度のため鋳片汚染が大きいことに起因すると考えられる。また、比較例ＱおよびＳは、鋳込中の鋳型内抜熱の変動が大きかった。本例においては、鋳造する材質の割れ感受性が低いため、鋳片表面の割れ性欠陥発生は認められなかったものの、潜在的に欠陥発生の危険性が高いと言える。
【００４６】
これは、比較例Ｑにおいては、パウダーフィルム中に析出する主たる結晶であるカスピダインともう一つの析出結晶であるペロブスカイトの結晶形態差が大きいこと、および結晶組成に含まれないAl₂O₃を多く含むことから、カスピダインの析出が安定しなかったためと考えられる。比較例Ｓは、パウダーフィルム中に析出する主たる結晶が存在しないので、鋳型内抜熱の不安定さが特に顕著であった。
【００４７】
これら比較例に対し発明例Ｅは、高粘度かつ高塩基度ゆえ介在物欠陥が少なく、かつ、パウダーフィルム中に主たる結晶としてメリライトを安定して析出するので、鋳型内抜熱が安定しており、鋳片割れ性欠陥の発生も少なかった。このように、発明例Ｅは、介在物欠陥の低減と鋳型内抜熱の安定さを両立できた、総合的に優れたパウダーである。
【００４８】
【発明の効果】
以上説明したように、本発明に係る連続鋳造用パウダーとそれを使用した連続鋳造方法は、パウダーが鋳型内で溶けて生成された溶融スラグの粘度、塩基度ともに高く、かつ、パウダーフィルム中に析出される結晶が安定して所望の結晶が得られるので、そのパウダーを使用して得られた鋳片は、内部欠陥、表面欠陥の無い優れた品質の鋳片を得ることができる。
【図面の簡単な説明】
【図１】鋳型内におけるパウダーの状態を示した図である。
【図２】結晶構造を示すための３次元格子軸図である。
【符号の説明】
１ 鋳型
２ 溶鋼
３ 凝固シェル
４ 溶融スラグ
５ 生パウダー
６ 固相フィルム
７ 液相フィルム[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a powder for continuous casting of steel and a method for continuous casting of steel using this powder.
[0002]
[Prior art]
In recent years, as the demand for steel cleaning has increased, the powder added to the upper surface of the molten steel in the mold during continuous casting has become a liquid when melted by the heat of the molten steel in the mold (hereinafter referred to as the liquid powder). Is called "molten slag"),
▲ 1 ▼, difficult to get caught in molten steel,
(2) Does not contaminate the molten steel with oxygen contained in the molten slag.
Two characteristics are now required. The physical properties of the molten slag to satisfy these characteristics are as follows:
(1) High viscosity for prevention of entrainment
( ₂₎ High basicity (low SiO ₂ content or high CaO content) for preventing molten steel contamination.
The basicity is the ratio of the CaO content (% by mass) to the SiO ₂ content (% by mass) in the slag when the powder is melted to form molten slag, that is, (% CaO) / (% SiO ₂ ) represents the value of (Hereinafter, mass% is simply indicated by%.)
[0003]
FIG. 1 is a view showing a state of powder in a mold of a continuous casting machine.
The molten steel 2 adjusted to a predetermined component, a predetermined temperature, etc. is continuously supplied into the mold 1 through the immersion nozzle, and powder is also continuously added onto the surface of the molten steel 2 in the mold 1. Is done. The supplied molten steel 2 is cooled in the mold 1, a solidified shell 3 is formed on the side surface, and the inside is pulled out from the lower end of the mold 1 in an unsolidified state. Then it is cut and taken out as a slab.
[0004]
In the mold 1, the powder added on the surface of the molten steel 2 is melted by the heat of the molten steel 2 at the surface portion of the molten steel 2 to become molten slag 4, and the powder above the molten slag 4 melts. It exists as raw powder 5 without. On the other hand, at the side of the molten steel 2 in contact with the mold 1, the molten slag 4 flows from the wall surface side of the mold 1, and then is cooled by the mold 1 to form a solid phase film 6, between the solid phase film 6 and the solidified shell 3. The molten slag 4 flows into the liquid phase film 7. The solid phase film 6 and the liquid phase film 7 are collectively referred to as a powder film.
[0005]
In the configuration of FIG. 1, among the physical properties of the molten slag that is in direct contact with the molten steel, increasing the viscosity is an important item from the viewpoint of preventing entrainment in the molten steel, and conventionally reducing the basicity of the molten slag. Thus, a highly viscous molten slag was obtained.
[0006]
[Problems to be solved by the invention]
However, the reduced basicity of the molten slag sacrifices other physical properties to be provided. In other words, low basicity molten slag (hereinafter, simply referred to as “low basicity slag”) is produced by reducing SiO ₂ contained in the molten slag by a deoxidizing element in steel such as Al. There is a problem that ₂ O ₃ tends to contaminate molten steel.
[0007]
Therefore, the molten slag must be made highly viscous by a method other than a reduction in basicity. As the method, high basicity of molten slag (hereinafter, simply "high basicity slag" hereinafter.) May be added amphoteric oxides such as A1 ₂ 0 _3, when reducing the fluorine (F) density, viscosity increase However, on the other hand, crystal precipitation in the powder film formed when molten slag flows between the mold and the solidified shell becomes unstable. Crystals deposited in the powder film play an important role of suppressing heat removal to the mold and increasing the uniformity of solidification by slowly cooling the solidified shell. Therefore, unstable crystal precipitation causes uneven solidification of the shell and causes defects such as vertical cracks in the slab.
[0008]
Thus, in the prior art, a powder that simultaneously satisfies the conditions of high viscosity and high basicity at the time of melting required for powder for continuous casting and stable crystal precipitation in a powder film could not be realized. .
[0009]
The present invention provides a powder that satisfies the physical property of stable crystal precipitation in a powder film, which is required for a powder for casting, and a high viscosity and high basicity of molten slag as required, and a clean powder using the same. The purpose is to provide a continuous casting method for steel.
[0010]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the continuous casting powder according to the present invention precipitates melierite, which is a solid solution of gelenite, akermanite, or both, whose crystal morphology is very similar , as a main crystal composition during solidification of molten slag. To do.
And the molten slag formed with such powder suppresses heat removal to the mold, slowly cools the solidified shell to improve the uniformity of solidification generation, and defects such as vertical cracks occur in the slab. Can be prevented.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The matters to be examined by the inventors will be described in detail below.
In order to increase the viscosity of high basicity slag, when amphoteric oxide such as Al ₂ O ₃ is added or the fluorine (F) concentration is reduced, crystal precipitation in the powder film becomes unstable. The inventors of the present invention believe that a large number of crystals are competingly deposited on each other and that the melting point of the crystals is lowered because they contain a large amount of oxides other than the crystal composition (for example, Al ₂ O ₃ ). I guessed. The reason why the crystal precipitation becomes unstable when many crystals compete with each other is thought to be due to the fact that the melting point is greatly lowered because the interface free energy between different crystal phases is high.
[0012]
The inventors of the present invention consider that it is inevitable that many crystals precipitate when increasing the viscosity of high basicity slag, and seek a method for suppressing a decrease in melting point even when many crystals are present. I made it. In order to suppress the lowering of the melting point, it was considered effective to combine crystals having crystal forms (structure and lattice constant) close to each other.
Therefore, the structure of crystals actually precipitated in the powder film was examined by X-ray diffraction. The results are shown in Table 1.
[0013]
[Table 1]

[0014]
Crystals known to be most commonly precipitated in high basicity slag, for example, caspidine (3CaO · 2SiO ₂ · CaF ₂ ) described in JP-A-10-58102 and JP-A-10-216907 are shown in FIG. In the three-dimensional lattice axis diagram, β = 109.6 degrees and monoclinic crystal (Monoclinic) tilted in one direction, and no other crystals with similar morphology were found.
[0015]
On the other hand, two crystals with very similar crystal morphology were found. A gehlenite _{(2CaO · Al 2 O 3 ·} SiO 2) and Akerumanaito _{(2CaO · MgO · 2SiO 2)} . Both are tetragonal with α = β = γ = 90 degrees, and the lattice constants are close. It was also found that perovskite (CaO.Ti0 ₂ ) is orthorhombic (Orthorhombic, α = β = γ = 90 degrees) close to tetragonal, and the combination of lattice constants is close to that of gehlenite and akermanite.
[0016]
If the crystal forms are very similar, it is expected that they will form a full solid solution that melts at a free rate. As a result of the investigation, it was confirmed that Gaelenite and Akermanite form a solid solution called melilite as expected.
The powder composition in which the found galenite and akermanite were “main crystals” was examined. In consideration, two points that should be provided as a powder that does not pollute the steel were kept in mind. That is, when the molten slag is formed, the basicity (% T · CaO) / (% SiO ₂ ) is 1.1 or more, and a high viscosity of 0.30 Pa · S or more is maintained at 1300 ° C.
[0017]
When the molten slag has a high basicity, the SiO ₂ bond in the slag is broken and the viscosity decreases. In order to compensate for this, it is necessary to add amphoteric oxides such as Al ₂ O ₃ . In the present invention, comparing galenite and akermanite selected as main crystals, gehlenite has a high basicity and a high Al ₂ O ₃ concentration, and akermanite has a low basicity and is Al ₂ O ₃ -less. Therefore, the combination of both has a composition in which the Al ₂ O ₃ concentration increases as the basicity increases. This is a convenient combination to compensate for the viscosity drop associated with higher basicity.
[0018]
As described above, the combination of the two types of crystal gehlenite and akermanite is a very suitable crystal from the point that the crystal morphology is very similar and also from the point of realizing the characteristics of high basicity and high viscosity. It was a combination.
Furthermore, because gehlenite contains Al ₂ O ₃ as a constituent composition, even when Al ₂ O ₃ impurities that have floated in the mold due to poor cleanliness of the molten steel are absorbed in the molten slag layer, its crystal precipitation There is also an advantage that the adverse effect of inhibiting is small.
[0019]
Features of the first powder for continuous casting according to the present invention is deposited, when the molten slag solidification powder is melted, gehlenite the crystalline form is very similar, Akerumanaito or melilite a complete solid solution of both, as the main crystal composition It is to let you.
Here, “main crystal” is based on the premise that the peak height in X-ray diffraction is twice or more that of other crystals, and when crystals having similar diffraction peak heights coexist, There are no main crystals.
[0020]
In addition, gerenite and akermanite, which have similar crystal forms, have very similar diffraction patterns and are difficult to distinguish. Therefore, these crystals were all judged to be one type of melilite, which is a solid solution in all proportions. However, when the MgO concentration in the powder is 10% or more and the Al ₂ O ₃ concentration is less than 4%, it is judged that the composition is close to akermanite, and the crystal name is akermanite.
[0021]
In addition to the two types of crystals of gehlenite and akermanite, TiO ₂ was appropriately added for precipitation of perovskite crystals from the viewpoint of increasing basicity and adjusting the solidification temperature.
In determining the specific chemical composition, an appropriate ratio of gelenite, which is a crystal containing Al ₂ O ₃ , akermanite, which is a crystal containing MgO, and perovskite, which is a crystal containing TiO ₂ , for example, 25 : A pure composition assumed to precipitate at a ratio of 65:10 was obtained, and the variation in the content of each element was made close to ± 5% or less with respect to the pure composition.
[0022]
Therefore, the feature of the powder for continuous casting according to the second aspect of the present invention is that it contains ₃ to 15% of TiO ₂ as a chemical composition of molten slag, and is a solid solution of gelenite, akermanite, or both at the time of molten slag solidification. It is to precipitate melilite as a main crystal composition and to precipitate a perovskite crystal composition.
[0023]
When adding TiO ₂ , the concentration is desirably suppressed to 3 to 15%. This is because if the concentration of TiO ₂ exceeds 15%, the molten slag has a high melting point and it is difficult to adjust the solidification temperature. Further, when the concentration of TiO ₂ is less than 3%, the physical property adjusting action of the molten slag is not exhibited, and the effect of adding TiO ₂ is small.
[0024]
Tables 2 and 3 show the powders made on the basis of the results of the above examination.
In Table 3, “main crystal” is defined by the peak height in the same X-ray diffraction as defined above. The determination of the gehlenite crystal and the akermanite crystal is also as described above. The T · CaO concentration is a concentration obtained by converting the total Ca amount into CaO. For example, the T · CaO concentration includes Ca in CaF2.
[0025]
[Table 2]

[0026]
[Table 3]

[0027]
The molten slag used for crystal investigation was obtained by melting 600 g of powder in an electric furnace at 1350 ° C. and then cooling the furnace at a cooling rate of 200 ° C./hr using a graphite crucible. When there is a proper name, the crystal name is indicated with a proper name (acermanite, melilite, caspodyne, perovskite, etc.), and those without proper name are compositions (CaO · SiO ₂ , 3Na ₂ 0 · 2Al ₂ O ₃ · 4SiO ₂ etc.).
[0028]
In Examples A to H in Tables 2 and 3, melilite (gerenite, akermanite, and a total solid solution) or akermanite is precipitated as the main crystal, and the crystal precipitation in the powder film is stable.
In the example A~ example F, are you adding TiO ₂ lowers the solidification temperature by an appropriate amount of the addition, to be effective in the slag viscosity control, and, by adding TiO ₂ as CaO · TiO ₂ composition This is because CaO increases in accordance with the amount of TiO ₂ and the basicity increases.
[0029]
The crystals perovskite precipitated by the addition of TiO ₂ have a crystal form close to that of the main crystals, gelenite, akermanite, and melilite, as described above, and thus did not cause a problem of inhibiting the precipitation of these main crystals. In addition, since the examples A to F had a high viscosity, the molten slag was not caught in the slab.
[0030]
When the measured value at 1300 ° C is lower than 0.30 Pa · S, the molten slag is likely to be caught in the steel, and when it is higher than 1.80 Pa · S, the lubrication in the mold deteriorates and the mold of the solidified shell deteriorates. Seizure is likely to occur. When the solidification temperature is lower than 1100 ° C, the solid phase that is the crystal precipitation site in the powder film is insufficient, the solid phase film between the molten steel and the mold wall becomes thin, and the desired crystals are not formed. When the temperature is higher than 1280 ° C, the liquid phase in the powder film is insufficient, so the flow between the molten steel and the mold wall is poor, the lubrication in the mold deteriorates, and the solidified shell is seized on the mold. It becomes easy to do.
[0031]
In the case of Examples A to H, the solidification temperature of the molten slag is within an appropriate range (1100 to 1280 ° C), so that the lubricity is not impaired and the solid phase that is the crystal precipitation site is appropriate in the powder film. Therefore, the solidified shell in the mold can be slowly cooled, and in combination with the stability of the crystal precipitation, uniform slow cooling can be realized, and casting can be performed without generating cracking slab defects.
[0032]
In Examples A to F, the basicity (% T · CaO) / (% SiO ₂ ) is within the appropriate range of 1.1 to 1.6, so the SiO ₂ activity is low, and due to deoxidizing components in steel such as Al. It is difficult to reduce and contamination of molten steel is suppressed. When the basicity is lower than 1.1, SiO ₂ is easily reduced by the deoxidizing component in the steel such as Al and contaminates the molten steel. On the other hand, if the basicity is higher than 1.6, the solidification temperature rises and it is difficult to keep the temperature within the above-mentioned appropriate temperature range (upper limit of 1280 ° C).
[0033]
Furthermore, in order to adjust the viscosity of the molten slag and the solidification temperature, fluorine (F) was added to the extent that the crystal composition was not greatly affected. The fluorine content is suppressed to 5% or less, and the precipitation of cuspidyne crystals is suppressed. When a large amount of caspidine crystals are precipitated, the main crystals of akermanite, gehlenite and melilite are unstable. Moreover, the suppression of the amount of fluorine added is also effective for reducing corrosion of metal parts such as continuous casting machines and preventing deterioration of water quality of industrial wastewater.
[0034]
Therefore, the feature of the powder for continuous casting according to the third aspect of the present invention is that the chemical composition of the molten slag has a value of (% T · CaO) / (% SiO ₂ ) of 1.1 to 1.6 and a fluorine content of 5% or less. The solidification temperature is 1100 to 1280 ° C, the viscosity at 1300 ° C is 0.30 to 1.80 Pa · S, and at the time of molten slag solidification, mellite, the total solid solution of gehlenite, akermanite, or both, is mainly crystallized. It is to deposit as a composition, or to contain ₃ to 15% of TiO ₂ as a chemical composition of the molten slag and to precipitate a perovskite crystal composition. The value of (% T · CaO) is a value including the amount of Ca content converted to CaO amount.
[0035]
In addition, since the content of alkali metal oxides (Na ₂ 0 and Li ₂ 0 in Table 2) is reduced, about 800 in the raw powder just above the molten slag due to the reaction between the alkali metal oxides and SiO _2. There is little liquid phase generated at a low temperature of ℃, and there is little sintering of green powder in the mold. Therefore, a good slab having good heat retention and no slag bear growth and no flaws on the slab surface can be obtained. When the total amount of the alkali metal oxide exceeds 8%, the sintering of the powder in the mold becomes remarkable, the amount of crystals containing the alkali metal oxide increases, and the precipitation of the main crystals does not occur. Become stable. Therefore, the content of alkali metal oxides such as Na ₂ 0 and Li ₂ 0 is 8% or less. This is the fourth invention.
[0036]
The powder for continuous casting described in the third and fourth aspects of the present invention is effective when used in continuous casting of steel having a concentration of [sol.Al], which is a deoxidizing element in molten steel, of 0.003 to 0.100%. is there. This is because if the [sol.Al] concentration is lower than 0.003%, the oxygen concentration in the steel is increased without being deoxidized, and the meaning of preventing contamination of the molten steel by the powder of the present invention is diminished. Moreover, when the [sol.Al] concentration is higher than 0.100%, the phenomenon that (SiO ₂ ) in the slag is reduced by [sol.Al] in the molten steel becomes insufficient, and the effect of preventing contamination of the molten steel becomes insufficient. Because.
[0037]
Therefore, the fifth feature of the present invention is that when the steel having a [sol.Al] content of 0.003 to 0.100% in the molten steel is continuously cast, it is used for the continuous casting of the third invention or the fourth invention. This is a continuous casting method using powder.
[0038]
Examples G and H in Table 2 and Table 3 have a lower basicity than Examples A to F, and thus easily contaminate molten steel, and Examples J and L are similar to Examples A to F. Since the basicity is low, the molten steel is easily contaminated, and the main crystals are also different examples. In Example Q, the main crystal caspidine (3CaO · 2Si0 ₂ · CaF ₂ ) and perovskite (CaO · TiO ₂ ) have a large crystal morphology difference, and the concentration of Ai ₂ O ₃ not included in the crystal composition is high. Therefore, caspidyne precipitation is an unstable example. In other examples, no main crystals exist, and crystal precipitation in the powder film is unstable.
Although the viscosity of the molten slag in which the powders of Examples J to T were all within the appropriate range, there was a problem with low basicity and unstable crystal precipitation as described above.
[0039]
【Example】
Hereinafter, an example of the result of using the powder shown in Table 2 and Table 3 for an actual continuous casting machine is shown.
Table 4 uses Example F, Example H, and Example R in Tables 2 and 3 and mold size: rectangular, 2 sides 1250 mm and 230 mm, casting speed 1.2 m / min, [C] = 0.10% , [Si] = 0.02%, [Mn] = 0.8%, [sol.Al] = 0.030%, the results of comparing the slab quality when continuously casting carbon steel. Hereinafter, Examples A to H are referred to as Invention Examples A to H, and Examples J to T are referred to as Comparative Examples J to T.
[0040]
[Table 4]

[0041]
In Comparative Example R, many slab surface cracking defects occurred. This is considered to be due to the fact that there is no problem in both viscosity and basicity, but there is no main crystal to be precipitated in the powder film, and the heat removal from the mold is unstable.
[0042]
On the other hand, the quality of the slab manufactured using the powder of Invention Example H is slightly inferior to the quality of the slab manufactured using another powder, but the cracking property is low. There was no defect and the whole was good. The slab inclusion defect index was slightly inferior because the main crystal melilite was precipitated and there was no surface defect, and the viscosity was high, so the molten slag entrainment defect could be suppressed, but the basicity was 0.95. It is considered that the slab was contaminated because it was slightly lower than the basicity of the powder (slab inclusion defect index 2).
Invention Example F has a high viscosity and a high basicity, so that there are few defects in slab inclusions, and melilite is stably precipitated as a main crystal in the slag film. There were also few occurrences of half-cracking defects. Thus, Invention Example F had few inclusion defects and cracking defects, and was able to produce a high quality slab comprehensively.
[0043]
In Table 5, using Invention Example E, Comparative Example J, Comparative Example Q, and Comparative Example S, [C] = 0.24%, [Si] = with a mold size of 310 mm in inner diameter and a casting speed of 1.4 m / min. The quality of pipes after continuous casting of 0.20%, [Mn] = 1.5%, and [Sol · Al] = 0.035% carbon steel and rolling them into seamless pipes was compared.
[0044]
[Table 5]

[0045]
In Comparative Example J, the pipe inclusion defect index is inferior to other powders. This is because the viscosity is a predetermined value, so that entrainment defects of molten slag (powder) can be suppressed, but due to low basicity, slab contamination is large. Further, in Comparative Examples Q and S, the variation in heat extracted from the mold during casting was large. In this example, since the cracking susceptibility of the material to be cast is low, the occurrence of cracking defects on the surface of the slab was not recognized, but it can be said that the risk of defect generation is potentially high.
[0046]
This is because in Comparative Example Q, there is a large difference in crystal morphology between caspodyne, which is the main crystal precipitated in the powder film, and perovskite, which is another precipitated crystal, and a large amount of Al ₂ O ₃ not included in the crystal composition. From this, it is considered that the deposition of caspidyne was not stable. In Comparative Example S, since the main crystals precipitated in the powder film do not exist, the instability of heat removal from the mold was particularly remarkable.
[0047]
In contrast to these comparative examples, Invention Example E has low inclusion defects due to high viscosity and high basicity, and stably deposits melilite as the main crystal in the powder film, so the heat extracted from the mold is stable. There were few occurrences of slab cracking defects. Thus, Invention Example E is a comprehensively excellent powder that can achieve both reduction of inclusion defects and stability of heat removal from the mold.
[0048]
【The invention's effect】
As described above, the continuous casting powder and the continuous casting method using the same according to the present invention are high in both viscosity and basicity of the molten slag formed by melting the powder in the mold, and in the powder film. Since the crystals to be precipitated are stably obtained, the slab obtained by using the powder can obtain a slab of excellent quality free from internal defects and surface defects.
[Brief description of the drawings]
FIG. 1 is a view showing a state of powder in a mold.
FIG. 2 is a three-dimensional lattice axis diagram for showing a crystal structure.
[Explanation of symbols]
1 Mold 2 Molten Steel 3 Solidified Shell 4 Molten Slag 5 Raw Powder 6 Solid Film 7 Liquid Film

Claims (5)

A continuous casting powder characterized by precipitating melierite, a solid solution of gelenite, akermanite, or both of which has a very similar crystal form , as a main crystal composition during solidification of molten slag. The powder for continuous casting according to claim 1, wherein ₃ to 15% by mass of TiO ₂ is contained as a chemical composition of the molten slag, and perovskite is precipitated as a crystal composition during solidification. Of the chemical composition of the molten slag, the ratio of T · CaO mass% to SiO ₂ mass% when the total Ca content is converted to CaO content, (% T · CaO) / (% SiO ₂ ) is 1.1 to 1.6, fluorine 3. The powder for continuous casting according to claim 1 or 2, wherein the content is 5% by mass or less, the solidification temperature is 1100 to 1280 ° C, and the viscosity at 1300 ° C is 0.30 to 1.80 Pa · S. . 4. The powder for continuous casting according to claim 1, wherein the total amount of alkali metal oxides is 8% by mass or less. A continuous casting method using the powder for continuous casting according to claim 3 or 4 when continuously casting steel having a content of [sol.Al] in molten steel of 0.003 to 0.100 mass%.

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