JP2004181466A - Method for detecting surface defect on continuously cast slab - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、鋼の連続鋳造中に、鋳片の表面に発生する縦割れの発生を検知するのに有効な、連続鋳造鋳片の表面欠陥検知方法に関する。
【０００２】
【従来の技術】
炭素含有量が０．０８〜０．１４ ｗｔ％の、いわゆる中炭素鋼を対象とする連続鋳造では、連続鋳造時に鋳片表面に縦割れが発生しやすく、従ってこの種の鋼ではかかる縦割れ発生の防止対策をとることが不可欠である。中炭素鋼の連続鋳造において縦割れが発生しやすい理由は、０．０８〜０．１４ ｗｔ％という炭素の含有量は、亜包晶変態域であることから、鋼の凝固過程において変態応力が加わるとき、凝固シェルの成長が遅い部分と速い部分とが発生し、凝固シェル成長速度の不均一を招くことが原因と言われている。こうした縦割れが発生した鋳片というのは、品質不良品として手入れが必要になるため、コストの増大や圧延工程での操業計画の遅延などの事態を招いて、生産性を著しく低下させるという問題があった。
【０００３】
これに対して従来、凝固シェル成長速度の不均一現象は、鋳型内初期の抜熱量と関係があることに着目し、緩冷却すること、エアギャップを解消すること、あるいは鋳型と凝固シェルとの間におけるモールドフラックスフィルムの厚みを調整すること、などの対策を行ってきた。
【０００４】
しかしながら、従来の上述した鋳片縦割れ発生対策というのは、その鋳片が鋳型を通過したのち１時間程度以上経過した後の庇検査工程において判定した結果に基づいて行われており、少なくとも鋳造中にその発生の有無を予知することはできない。特に２ｍ／ｍｉｎを超えるような高速時造条件の下での縦割れ防止を鋳造中に検知する技術は確立されていないのが実情である。それ故に、実際には、多くの欠陥鋳片を生産しており、その損害は大きい。
【０００５】
これに対し、最近、鋳型内の抜熱量の分布に着目し、鋳型表面の熱流束を測定してその振幅の大きさを制御することにより、縦割れを予測し防止する技術（特許文献１）が提案されている。しかし、この技術の場合、検出結果と縦割れ発生とが一致しないことが多く、解決課題を残したものとなっている。
また、各測温素子の平均温度と各々の測温素子の出力の差分に着目し、これらの出力のうち幾つかの条件が一致した時に縦割れと判定をする方法（特許文献２）などが提案されている。しかし、この方法は、縦割れ発生の閾値を設定することが困難であり、感度をあげるとほぼ正常な鋳片まで縦割れ有りと判定されてしまうという問題がある。
また、鋳型幅方向、鋳造方向における測温素子の温度低下を積算することにより、判定する技術（特許文献３）も提案されている。しかし、この方法では、一定の鋳造速度においては高い検出率を示すものの、縦割れ防止アクション、鍋交換などで鋳造速度が減少した場合には誤検知をしてしまうという問題がある。その回避のために複雑なロジックを組み立ててシステム化することは可能であるが、十分な誤検知対策になっていないのが実情である。
【０００６】
【特許文献１】特公昭６３−５３９０４号公報
【特許文献２】特開平３−１３８０５号公報
【特許文献３】特開平８−１１７９４４号公報
【０００７】
【発明が解決しようとする課題】
上述した各種従来技術においては、なお多くの解決課題があることは、上述したとおりである。とくに、これらの方法において検討すべき内容としては、測温素子の埋め込み位置、すなわち鋳型内壁面から冷却スリット（水冷空間）位置までの厚み方向のどの位置に測温素子を埋め込むのか、その伝熱的な感度がどの程度のものを用いるのか、あるいは鋳造速度の影響はどの程度まで考慮すればいいのかなどの点で検討課題が残されていた。
【０００８】
そこで、本発明の目的は、連続鋳造中に鋳片縦割れの前駆現象を確実に検出し、表面性状の良好な連続鋳造鋳片を得るための、表面欠陥検知方法を提案することにある。
【０００９】
【課題を解決するための手段】
本発明は、上記の課題を解決するためになされたもので、鋳型壁の内部に溶鋼面から等しい位置における鋳型幅方向に測温素子を埋設し、任意の周期毎に各点における鋳型温度の時間変化率（変化量）を求めてその絶対値を算出し、その絶対値と、鋳片温度値および冷却水温とから、鋳型厚み方向における水冷スリット（水冷空間）端面から測温素子までの距離および前記水冷スリット端面から鋳型壁面までの距離で定義される鋳型壁と鋳片（凝固シェル表面）との間隙の状態（程度）を示すパラメータＤを算出し、このパラメータＤが、鋳型の幅方向いずれかの位置において所定の値、即ちモールドフラックスフィルムの厚みのパラメータとして示される限界値Ｄｐを超えたときに、縦割れ発生と判断するようにしたことを特徴とする鋳片表面の縦割れ発生を検知する方法である。
【００１０】
即ち、本発明は、連続鋳造用鋳型壁の内部に埋設した測温素子により、溶鋼面からの距離が等しい位置の幅方向における各測温点についての、ある時刻ｔｉからΔｔを経た時刻（ｔｉ＋１）における、下記式（１）によって定義され、鋳片温度の変化量の絶対値と鋳造速度補正項（フラックスフィルム全厚補正項）の比として示される鋳型壁と鋳片間との間隙の程度を示すパラメータＤを算出し、このパラメータＤの値が鋳型幅方向における前記各測温点のいずれかの位置において所定の限界値を超えるかどうかによって、縦割れ発生の有無を判定するようにしたことを特徴とする連続鋳造鋳片の表面欠陥検知方法である。
Ｔｗ：鋳型内冷却水の温度
【００１１】
なお、本発明において、前記縦割れの発生は、前記パラメータＤが、鋳型壁と鋳片との間隙に介在するモールドフラックスフィルム厚みのパラメータを示す、実験値を示す分子と測温位置補正項を分母とする下記式（２）で表される限界値Ｄｐを超えるか否かによって、判定することが好ましいと言える。
λｐ：モールドフラックスの熱伝導率［Ｗ／ｍＫ］
λｍｏｌｄ：鋳型銅板の熱伝導率［Ｗ／ｍＫ］
ｗ：鋳型内壁面から測温素子先端までの鋳型厚み方向の距離［ｍ］
ｈｗ：鋳型冷却水による熱伝達率［Ｗ／ｍ２Ｋ］
【００１２】
また、本発明において、鋳型内壁幅方向に埋設する前記測温素子の最大間隔は１００ ｍｍ以下とすることが好ましい。
【００１３】
【発明の実施の形態】
さて、鋳造欠陥の一つに、前述した鋳型内での初期凝固中の局所的な凝固遅れに伴う縦割れ現象があるが、発明者らはその縦割れの前駆現象として、鋳片表面（凝固シェル表面）に顕われる僅かな凹み（以下、「デプレッション」と称する）に着目してこれを観察した。その結果、発明者らは、前記縦割れに結びつくようなデプレッションの深さや長さなどの程度（大きさ）を、鋳型内壁中に埋設した測温素子による温度測定値から推定することが可能になることを見い出し、しかもこの方法が前記縦割れ発生の検知あるいは予知方法として有効であることを、数多くの鋳造実験ならびに鋳型銅板温度の解析から知った。以下、その具体的内容について説明する。
【００１４】
上述したように、前記デプレッションの規模、特にその深さが大きい時は、鋳片と鋳片（凝固シェル）との間にモールドフラックスが侵入してモールドフラックフィルム層が形成され、その部分の熱抵抗が相対的に増大する。それに伴い鋳型（銅板）の温度が低下するので、基本的にはその金属型の温度変化を監視することによって、前記デフレツションの検知が一応は可能である。しかしながら、測温素子の埋め込み位置、すなわち鋳型壁面から冷却スリット（水冷空間）位置までの鋳型銅板の厚み方向のどの位置に測温素子があり、その伝熱的な感度がどの程度か、あるいはモールドフラックの特性を考慮した伝熱特性などについても検討しないと、鋳造速度の変化に対応して刻々と変化する熱伝導条件をそのまま検出してしまい、大きな誤差を伴う検出結果を得ることになってしまう。
【００１５】
そこで、本発明では、モデル実験、数値解析を含めた研究開発を行った結果、鋳造速度や測温素子、その埋設位置の影響などの影響因子を除去して、純粋に鋳片表面に顕われるデプレッションの大きさのみを精度よく検出する方法として、上記式（１）で示されるデプレッション評価式（パラメータＤの式）に到達した。以下、この（１）式について図１に示すモデルに基づき説明する。
【００１６】
まず、鋳造初期における鋳型の抜熱量に応じた熱流束の鋳造モデルを考えた場合、鋳型に埋設した測温素子の出力する温度は、鋳型壁と鋳片（凝固シェル）との間に流入するモールドフラックスフイルムの厚みの増加とともに減少することが明らかである。
【００１７】
このような鋳造モデルにおいて、鋳型抜熱量に対応する総括熱抵抗ＲＴは、下記（３）式のように、鋳型冷却水による抜熱量を示す熱抵抗、鋳型自身の熱抵抗、鋳型とモールドフラックスフィルム界面での熱抵抗、モールドフラックスフィルム層自体での熱抵抗、および鋳片（凝固シェル）の熱抵抗の合計値で示すことができる。
【００１８】
そして、上記抜熱量に対応する総括熱流束ｑＴは、下記（４）式に示すように、鋳造鋼の固相線温度と鋳型冷却水の温度との差と、および前記熱抵抗との比として示すことができる。なお、下記式（４）において、左辺は溶鋼から冷却水まで貫く熱流束を示し、右辺はある測定点までの熱流束であって、これらの熱流束は鋳片を貫くどの位置とも一定（定常伝熱）であり、（４）式が成立する。
上記（３）、（４）式を整理して、モールドフラックフィルムの厚み（ｄｐ）を算出すると、下記（５）（６）（７）式を導くことができる。
そして、モールドフラックスフィルムの厚みｄｐの変動分Δｄｐは、次式（１０）（１１）のように示すことができるから、これを整理すると式（１２）のようになる。
そして、上記式（１２）から上記（１）式を導くための下記（１３）式に示す関係式が得られる。
上記式（１３）から上記式（１）を導くことができる。即ち、（１３）式は、鋳造条件と設備条件で与えられる項と、デプレッションそのものに起因する項とに分離することができるが、本発明において縦割れ発生起因となるものは、（１３）式中のデプレッションそのものに起因する項で構成される部分であるから、この項を独立させると上記（１）式が得られることになる。
【００２３】
なお、上記（２）式は、実質的にモールドフラックスフィルムの厚みを示すものであり、分母は上記（１３）式中の測温素子による測温条件、鋳造条件および設備条件で与えられる項を用いて、Ｄｐ値を補正することにより、より正確な縦割れ発生を検知するようにしたものであり、また、分子の数値は、後述する図２および表１の記載から明らかなように、数多くの調査から発明者らが実験的に求めた値である。
【００２４】
以上の説明からも明らかなように、上記評価式（１）式、（２）式による縦割れ発生の検知方法によれば、鋳造速度の変化を温度Ｔｉの変化として認識しているため、異なる鋳造速度の場合についても、縦割れ発生の限界値Ｄｐを補正する必要はなく、有利に用いることができる。
【００２５】
また、（１）式において、温度の変化の微分値（Ｔｉ＋１−Ｔｉ）に絶対値を用いたのは、ひとたび前駆現象であるデプレッションが発生した時には、温度上昇・降下がランダムに発生するものの、デプレッションの最大深さが同じ程度であれば温度上昇幅および降下幅の最大値は同じになるため、温度上昇あるいは温度降下の両方を等価も扱うほうが合理的であり、先行技術にあるような複雑な制御は不要になると考えたからである。
【００２６】
次に、図２は、本発明における上記パラメータＤとデプレッションならびに縦割れ発生の関係を調査した結果を示すものである。この図に示すようにパラメータＤの値が増大するに従い、その増大した測温個所においてデプレッション、縦割れの発生を観察されるが、その限界値は、２．１２２×１０−４となることが明らかとなった。この時の操業条件は、以下のとおりである。
【００２７】
【表１】
次に、図３は、測温素子の埋設間隔と縦割れ検出失敗指数の関係図である。縦割れ検出失敗指数の定義は、鋳片長さ１０００ ｍあたりの検知できなかった縦割れの数である。この図３に示す結果に伴い、測温素子の間隔は最大でも１００ ｍｍ以下であることが必要と考えられ、これ以上の間隔では縦割れを見逃すおそれがある。従って、本発明において、鋳型内壁幅方向に埋設する前記測温素子の最大間隔は１５０ ｍｍ以下とする。
【００２９】
【実施例】
図４は、本発明の実施例を示すもので、操業条件は上記表１に示すものを基準とし、熱電対の間隔を６６ ｍｍとし、溶鋼面からの位置を１９０ ｍｍとし、鋳造速度２．２ｍ／ｍｉｎとしたときの、横軸に時間、縦軸に熱電対幅方向位置をとり、パラメータＤの大小を濃淡で示した図である。なお、この図において、時間的あるいは空間的に隣接してＤの値が特定の範囲にある場合には、その領域を同じ濃さで図示している。この図に示すように、パラメータＤの値が、２．１を超えるか否かにより、縦割れ発生位置の特定が可能となり、しかもその周辺のデプレッションの模様をも図示することができ、縦割れの前駆現象の検出も可能になることがわかった。
【００３０】
【発明の効果】
以上説明したとおり本発明によれば、連続鋳造における縦割れの検出をオンラインにおいて高い精度で行うことができるようになる。実際の操業においてはほぼ１００ ％の縦割れあるいはその前駆現象を検知でき、本発明の効果が確かめられている。
【図面の簡単な説明】
【図１】本発明の表面欠陥検知方法において用いられるパラメータＤの考え方を示す鋳造環境の模式図である。
【図２】本発明における縦割れ発生の閾値を示すグラフである。
【図３】本発明における縦割れ検出失敗指数と測温素子間隔の関係を示すグラフである。
【図４】実施結果の鋳型幅方向および鋳造時間におけるパラメータＤの大小を濃淡で示したグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for detecting a surface defect of a continuously cast slab, which is effective for detecting the occurrence of a vertical crack generated on the surface of a slab during continuous casting of steel.
[0002]
[Prior art]
In continuous casting for a so-called medium carbon steel having a carbon content of 0.08 to 0.14 wt%, vertical cracks are likely to occur on the slab surface during continuous casting. It is essential to take preventive measures. The reason that longitudinal cracks are likely to occur in continuous casting of medium carbon steel is that the carbon content of 0.08 to 0.14 wt% is in the subperitectic transformation region, so that the transformation stress during the solidification process of the steel is low. It is said that when this occurs, a portion where the growth of the solidified shell is slow and a portion where the growth is fast are generated, leading to an uneven growth rate of the solidified shell. Such slabs with vertical cracks need to be repaired as inferior products, which leads to increased costs and delays in operation planning in the rolling process, resulting in a significant decrease in productivity. was there.
[0003]
On the other hand, conventionally, the non-uniform phenomenon of the growth rate of the solidified shell has focused on the fact that there is a relationship with the amount of heat removed in the initial stage in the mold.Slow cooling, eliminating the air gap, or the gap between the mold and the solidified shell Measures such as adjusting the thickness of the mold flux film between them.
[0004]
However, the conventional measures against the occurrence of vertical slab cracks described above are performed based on the results determined in the eaves inspection step after about one hour has passed after the slab passed the mold, and at least It is not possible to predict whether or not it has occurred. In particular, the technology for detecting the prevention of longitudinal cracking under high-speed aging conditions such as exceeding 2 m / min during casting has not been established. Therefore, in practice, many defective slabs are produced, and the damage is great.
[0005]
On the other hand, recently, a technique for predicting and preventing longitudinal cracks by focusing on the distribution of heat removal in a mold, measuring the heat flux on the mold surface, and controlling the magnitude of the amplitude (Patent Document 1). Has been proposed. However, in the case of this technique, the detection result often does not coincide with the occurrence of vertical cracks, and this leaves a problem to be solved.
Also, a method of focusing on the difference between the average temperature of each temperature measuring element and the output of each temperature measuring element, and judging a vertical crack when some conditions among these outputs match (Patent Document 2). Proposed. However, this method has a problem that it is difficult to set a threshold value for the occurrence of vertical cracks, and if sensitivity is increased, it is determined that almost normal slabs have vertical cracks.
In addition, a technique has been proposed in which a determination is made by integrating temperature drops of a temperature measuring element in a mold width direction and a casting direction (Patent Document 3). However, in this method, although a high detection rate is exhibited at a constant casting speed, there is a problem that an erroneous detection is performed when the casting speed is reduced due to a vertical crack prevention action, replacement of a pot, or the like. Although it is possible to assemble complex logic to avoid this, it is possible to systematize it, but in reality, it does not provide sufficient countermeasures against erroneous detection.
[0006]
[Patent Literature 1] Japanese Patent Publication No. 63-53904 [Patent Literature 2] Japanese Patent Application Laid-Open No. Hei 3-13805 [Patent Literature 3] Japanese Patent Application Laid-Open No. Hei 8-117944
[Problems to be solved by the invention]
As described above, there are still many problems to be solved in the various conventional techniques described above. In particular, what should be considered in these methods is the position at which the temperature measuring element is embedded, that is, at what position in the thickness direction from the inner wall surface of the mold to the position of the cooling slit (water cooling space), the heat transfer There are still issues to be considered in terms of how much sensitivity should be used or how much the effect of casting speed should be considered.
[0008]
Therefore, an object of the present invention is to propose a surface defect detection method for reliably detecting a precursory phenomenon of vertical slab cracks during continuous casting and obtaining a continuous cast slab having good surface properties.
[0009]
[Means for Solving the Problems]
The present invention has been made in order to solve the above-described problems, and embeds a temperature measuring element in a mold width direction at an equal position from a molten steel surface inside a mold wall, and measures a mold temperature at each point at an arbitrary cycle. The time change rate (change amount) is obtained and its absolute value is calculated. From the absolute value, the slab temperature value and the cooling water temperature, the distance from the end face of the water cooling slit (water cooling space) in the mold thickness direction to the temperature measuring element. And calculating a parameter D indicating a state (degree) of a gap between the mold wall and the slab (solidified shell surface) defined by the distance from the end surface of the water-cooled slit to the mold wall surface. slab, characterized in that the predetermined value in any position, i.e. when exceeding the limit value D p which is shown as the parameter of the thickness of the mold flux film was to be determined that the vertical cracks occurred This is a method for detecting the occurrence of vertical cracks on the surface.
[0010]
That is, according to the present invention, the time (Δt ) from a certain time ti with respect to each temperature measuring point in the width direction at a position having the same distance from the molten steel surface is measured by the temperature measuring element embedded inside the continuous casting mold wall ( t i + 1 ), the gap between the mold wall and the slab, defined as the ratio of the absolute value of the variation of the slab temperature to the casting speed correction term (flux film total thickness correction term), defined by the following equation (1): Is calculated, and the presence or absence of the occurrence of a vertical crack is determined based on whether or not the value of the parameter D exceeds a predetermined limit value at any position of each of the temperature measuring points in the mold width direction. A method for detecting surface defects of a continuously cast slab characterized by the following.
Note that, in the present invention, the occurrence of the vertical cracks is caused by the parameter D indicating a parameter of a mold flux film thickness interposed in a gap between a mold wall and a slab, a molecule indicating an experimental value and a temperature measurement position correction term. depending on whether the limit is exceeded D p represented by the following formula for the denominator (2), it can be said that it is preferable to determine.
λ p : thermal conductivity of mold flux [W / mK]
λ mold : thermal conductivity of mold copper plate [W / mK]
w: distance [m] in the mold thickness direction from the inner wall surface of the mold to the tip of the temperature measuring element
h w : heat transfer coefficient by mold cooling water [W / m 2 K]
[0012]
In the present invention, the maximum interval between the temperature measuring elements embedded in the width direction of the inner wall of the mold is preferably 100 mm or less.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
One of the casting defects is a vertical cracking phenomenon caused by a local solidification delay during the initial solidification in the mold described above. This was observed by focusing on a slight dent (hereinafter, referred to as "depression") appearing on the shell surface). As a result, the inventors can estimate the degree (size) of the depression or the length, etc., of the depression that leads to the vertical crack from the temperature measured by the temperature measuring element embedded in the inner wall of the mold. It was found from numerous casting experiments and analysis of the temperature of the mold copper plate that this method was effective as a method for detecting or predicting the occurrence of vertical cracks. Hereinafter, the specific contents will be described.
[0014]
As described above, when the size of the depression, particularly the depth thereof, is large, the mold flux penetrates between the slabs and the slabs (solidified shells) to form a mold flux film layer. The resistance increases relatively. Accordingly, the temperature of the mold (copper plate) decreases. Therefore, basically, it is possible to detect the deflation by monitoring the temperature change of the metal mold. However, where the temperature measuring element is located in the thickness direction of the mold copper plate from the embedding position of the temperature measuring element, that is, from the mold wall surface to the cooling slit (water cooling space) position, and how much the heat transfer sensitivity is, If heat transfer characteristics are not considered in consideration of the characteristics of the flux, heat conduction conditions that change every moment in response to changes in casting speed will be detected as they are, resulting in detection results with large errors. I will.
[0015]
Therefore, in the present invention, model experiments, as a result of research and development including numerical analysis, as a result of removing the influence factors such as the influence of casting speed and temperature measuring element, its burying position, purely appear on the slab surface As a method of accurately detecting only the magnitude of depletion, a depletion evaluation formula (formula of parameter D) represented by the above formula (1) has been reached. Hereinafter, the equation (1) will be described based on the model shown in FIG.
[0016]
First, when considering a casting model of a heat flux corresponding to a heat removal amount of a mold in an early stage of casting, a temperature output from a temperature measuring element embedded in the mold flows between a mold wall and a slab (solidified shell). It is clear that the thickness decreases with an increase in the thickness of the mold flux film.
[0017]
In such a casting model, the overall thermal resistance RT corresponding to the amount of heat removed from the mold is, as shown in the following equation (3), the thermal resistance indicating the amount of heat removed by the mold cooling water, the thermal resistance of the mold itself, the mold and the mold flux. It can be represented by the total value of the thermal resistance at the film interface, the thermal resistance of the mold flux film layer itself, and the thermal resistance of the slab (solidified shell).
[0018]
Then, the ratio of the overall heat flux q T corresponding to the heat extraction amount, as shown in the following equation (4), the difference between the temperature of the solidus temperature and the mold coolant in cast steel, and said thermal resistance Can be shown as In the following equation (4), the left side shows the heat flux penetrating from the molten steel to the cooling water, and the right side shows the heat flux up to a certain measurement point, and these heat fluxes are constant (steady state) at any position penetrating the slab. Heat transfer), and equation (4) holds.
When the thicknesses (d p ) of the mold flux film are calculated by rearranging the expressions (3) and (4), the following expressions (5), (6), and (7) can be derived.
The variation Δd p of the thickness d p of the mold flux film can be represented by the following equations (10) and (11).
Then, a relational expression shown in the following expression (13) for deriving the expression (1) from the expression (12) is obtained.
The above equation (1) can be derived from the above equation (13). That is, equation (13) can be separated into terms given by casting conditions and equipment conditions, and terms caused by depletion itself. Since this is a part composed of a term due to the depression itself, if this term is made independent, the above equation (1) will be obtained.
[0023]
The above equation (2) substantially indicates the thickness of the mold flux film, and the denominator is a term given by the temperature measuring condition by the temperature measuring element, the casting condition and the equipment condition in the above equation (13). By correcting the Dp value, a more accurate detection of the occurrence of a vertical crack was performed. Further, as is clear from the description of FIG. It is a value experimentally obtained by the inventors from many studies.
[0024]
As apparent from the above description, the evaluation formula (1), because of the recognition according to the detection method of the vertical cracks occurred, a change in the casting speed as a change in the temperature T i by (2), for the case of different casting speed, it is not necessary to correct the limiting value D p of the vertical cracking can advantageously be used.
[0025]
Further, in Expression (1), the absolute value is used as the differential value (T i + 1 −T i ) of the change in temperature because once the depression, which is a precursor phenomenon, occurs, the temperature rises and falls randomly. However, if the maximum depth of depletion is the same, the maximum value of the temperature rise and fall is the same, so it is more reasonable to treat both the temperature rise and the temperature drop equivalently, as in the prior art. This is because complicated complicated control is unnecessary.
[0026]
Next, FIG. 2 shows the result of investigation of the relationship between the parameter D and the depression and the occurrence of vertical cracks in the present invention. As shown in this figure, as the value of the parameter D increases, depletion and occurrence of vertical cracks are observed at the increased temperature measuring points, but the limit value is 2.122 × 10 −4. It became clear. The operating conditions at this time are as follows.
[0027]
[Table 1]
Next, FIG. 3 is a diagram showing the relationship between the buried intervals of the temperature measuring elements and the vertical crack detection failure index. The definition of the vertical crack detection failure index is the number of undetectable vertical cracks per 1000 m of slab length. According to the results shown in FIG. 3, it is considered necessary that the interval between the temperature measuring elements is at most 100 mm or less, and if the interval is longer than this, there is a possibility that a vertical crack may be missed. Therefore, in the present invention, the maximum distance between the temperature measuring elements embedded in the width direction of the inner wall of the mold is set to 150 mm or less.
[0029]
【Example】
FIG. 4 shows an embodiment of the present invention. The operating conditions are based on those shown in Table 1 above, the distance between the thermocouples is 66 mm, the position from the molten steel surface is 190 mm, and the casting speed is 2. FIG. 4 is a diagram showing the magnitude of the parameter D by shading, with the horizontal axis representing time and the vertical axis representing the position in the thermocouple width direction at 2 m / min. In this figure, when the value of D is temporally or spatially adjacent and the value of D is in a specific range, the area is shown with the same density. As shown in this figure, the location of the vertical crack can be specified depending on whether or not the value of the parameter D exceeds 2.1, and the depletion pattern around the location can be illustrated. It has been found that the detection of precursory phenomena becomes possible.
[0030]
【The invention's effect】
As described above, according to the present invention, it is possible to detect a vertical crack in continuous casting with high accuracy online. In actual operation, almost 100% of vertical cracks or their precursor phenomena can be detected, and the effect of the present invention has been confirmed.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a casting environment showing a concept of a parameter D used in a surface defect detection method of the present invention.
FIG. 2 is a graph showing a threshold value of occurrence of a vertical crack in the present invention.
FIG. 3 is a graph showing a relationship between a vertical crack detection failure index and a temperature measuring element interval according to the present invention.
FIG. 4 is a graph showing the magnitude of a parameter D in a mold width direction and a casting time as a result of shading by shading.
Claims (3)
Ｔｗ：鋳型内冷却水の温度By the temperature measuring element embedded in the mold wall for continuous casting, for each temperature measuring point in the width direction at a position at the same distance from the molten steel surface, at the time (t i + 1 ) after Δt from a certain time t i , A parameter D indicating the degree of the gap between the mold wall and the slab defined by the equation (1) is calculated, and the value of this parameter D is determined at a predetermined position at any one of the temperature measuring points in the mold width direction. A method for detecting a surface defect of a continuous cast slab, wherein the presence or absence of a vertical crack is determined depending on whether or not a limit value is exceeded.
λｐ：モールドフラックスの熱伝導率［Ｗ／ｍＫ］
λｍｏｌｄ：鋳型銅板の熱伝導率［Ｗ／ｍＫ］
ｗ：鋳型内壁面から測温素子先端までの鋳型厚み方向の距離［ｍ］
ｈｗ：鋳型冷却水による熱伝達率［Ｗ／ｍ２Ｋ］Generation of the vertical cracks, the parameter D is depending on whether the limit is exceeded D p represented by the following formula that shows the parameters of the mold flux film thickness interposed in a gap between the mold wall and the slab (2) The method for detecting a surface defect of a continuously cast slab according to claim 1, wherein the determination is made.
λ p : thermal conductivity of mold flux [W / mK]
λ mold : thermal conductivity of mold copper plate [W / mK]
w: distance [m] in the mold thickness direction from the inner wall surface of the mold to the tip of the temperature measuring element
h w : heat transfer coefficient by mold cooling water [W / m 2 K]
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| Application Number | Priority Date | Filing Date | Title |
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| JP2002348242A JP2004181466A (en) | 2002-11-29 | 2002-11-29 | Method for detecting surface defect on continuously cast slab |
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| JP2002348242A JP2004181466A (en) | 2002-11-29 | 2002-11-29 | Method for detecting surface defect on continuously cast slab |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2010172930A (en) * | 2009-01-29 | 2010-08-12 | Kobe Steel Ltd | Continuous casting method for medium carbon steel by means of change in casting speed and molten metal surface level |
| JP7469623B2 (en) | 2020-04-06 | 2024-04-17 | 日本製鉄株式会社 | Detection method for defects in slab during continuous casting |
-
2002
- 2002-11-29 JP JP2002348242A patent/JP2004181466A/en active Pending
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2010172930A (en) * | 2009-01-29 | 2010-08-12 | Kobe Steel Ltd | Continuous casting method for medium carbon steel by means of change in casting speed and molten metal surface level |
| JP7469623B2 (en) | 2020-04-06 | 2024-04-17 | 日本製鉄株式会社 | Detection method for defects in slab during continuous casting |
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